2009. "Crystallographic Dependence of Visible-Light Photochemistry in Epitaxial TiO2-xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Art. No. 085401. Abstract All films were grown by plasma assisted molecular beam epitaxy (PAMBE) in a custom chamber described elsewhere (1). Epitaxial films of TiO2-xNx(001) (x ≤ ~0.02) anatase were grown by PAMBE on undoped or Nb-doped (0.02 at. %) SrTiO3(001) (STO) and undoped LaAlO3(001) (LAO). Similarly, TiO2-xNx(001) (x ≤ ~0.02) rutile epifilms were grown on rutile TiO2(110). The growth and physical properties of N-doped anatase on LAO(001) and N-doped rutile on TiO2(110) have been described in detail elsewhere (2-4). In what follows, we describe the growth details for N-doped anatase on STO(001). The PAMBE chamber is connected to an x-ray photoelectron spectrometer (XPS) chamber and a photodesorption chamber. The former is equipped with a Gamma Data/Scienta SES 200 analyzer and a monochromatic AlK x-ray source. The latter includes a molecular dosing apparatus for TMAA, a Hg arc lamp, and a quadrupole mass spectrometer. The STO substrates were etched in buffered HF and annealed in flowing O2 at 1 atm. at 950oC for 8 hours. The etch dissolved SrO terraces and the oxygen anneal resulted in mass transport of the discontinuous TiO2 microterraces, resulting in an atomically flat, TiO2 terminated surface with a minimum step height of 4 Å (5). This treatment left some residual fluorine on the surface which could not be removed by annealing. The measured F 1s binding energy was ~684.0 eV, which is close to that exhibited by SrF2 – 684.6 eV (6). Based on this binding energy and the high degree of thermal stability, we conclude that F substitutes for O in the lattice. Under this assumption and using atomic photoemission cross sections (7), the F mole fraction within the anion sublattice is estimated to be ~0.05 within the probe depth of XPS at normal emission (~45 Å).
2009. "Crystallographic Dependence of Visible-light Photoactivity in Epitaxial TiO2−xNx Anatase and Rutile." Physical Review. B, Condensed Matter and Materials Physics 79(8):Article number: 085401. doi:10.1103/PhysRevB.79.085401 Abstract Nitrogen-doped TiO2 materials have been shown to exhibit visible-light photoactivity, but the operative mechanism(s) are not well understood. Here we use structurally and compositionally well-defined epitaxial films of TiO2−xNx anatase (001) and rutile (110) (x~0.02) to show a qualitative difference between the visible-light activities for the two polymorphs. Holes generated by visible light at N sites in anatase (001) readily diffuse to the surface and oxidize adsorbed trimethyl acetate while the same in rutile (110) remain trapped in the bulk. In light of the low doping densities that can be achieved in phase-pure material, conventional wisdom suggests that holes should be trapped at N sites in both polymorphs. Although the detailed mechanism is not yet understood, these results suggest that the hole hopping probability is much higher along the [001] direction in N-doped anatase than along the [110] direction in N-doped rutile.
2009. "X-ray absorption fine structure and magnetization characterization of the metallic Co component in Co-doped ZnO thin films ." Physical Review. B, Condensed Matter 79(7):Art. No. 075202. Abstract X-ray absorption fine structure (XAFS) measurements have been used to characterize a series of Co doped ZnO films grown on sapphire substrates by pulsed laser deposition. The emphasis is on characterization of the fate of the Co dopant: metallic particles or substitutional Co2+. It is shown that analysis of both the near edge and extended fine structure can provide a measurement of the fraction of metallic Co. Any quantitative understanding of magnetism in this system needs to take account of both types of Co. Results are reported for two types of films from two different groups that show distinctly different behavior. Films grown with high concentrations of Co show varying amounts of metallic Co that could be identified as hcp or fcc Co. Another set of films were annealed in Zn vapor to induce magnetism. These also showed significant metallic Co, but of a different type similar to the CoZn intermetallic. The bulk forms of both metals are magnetic and should contribute to the magnetism. Using bulk magnetic values, there are some discrepancies with room temperature magnetic measurements. The 2 magnetic properties of the small metal particles are likely changed by their surroundings and by superparamagnetism. Low temperature magnetic measurements for one of the samples confirmed this with an estimated blocking temperature of 50K.
2009. "Cation dopant distributions in nanostructures of transition-metal doped ZnO:Monte Carlo simulations." Physical Review. B, Condensed Matter and Materials Physics 79(7):Art. No. 075324. Abstract The path from trace doping to solid solution formation involves an intermediate regime in which the doping level is a few to several atomic percent. In this regime, dopant-dopant interactions, which are driven by the spatial arrangement of dopants, are critical factors in determining the resulting properties. Conventional wisdom counts on simple probabilistic methods for predicting dopant distributions. Here, we use Monte Carlo simulations to show that widely used, straightforward statistical models, such as that of Behringer1, are accurate only in the limit of infinitesimally small surface–to-volume ratio. For epitaxial films and nanoparticles, where much of the current interest resides, dopant distributions depend strongly on the surface-to-volume ratio. We present empirical expressions that accurately predict dopant bonding configurations as a function of film or particle size, shape and dopant concentration for doped ZnO, a material of particular interest in semiconductor spintronics.
2009. "Comment on “Origin of Metallic States at the Heterointerface between the Band Insulators LaAlO3 and SrTiO3"." Physical Review Letters 102(19):Art No.: 199703. doi:10.1103/PhysRevLett.102.199703 Abstract In a recent Letter, Yoshimatsu et al. [1] used soft x-ray photoemission to probe electronic structure at the buried epitaxial LaAlO3/SrTiO3(001) interface (LAO/STO). This system has been of significant recent interest because of reports of two-dimensional electron gas (2-DEG) behavior at the interface of two band insulators. Although oxygen vacancies in the STO can result in itinerant electrons, an intrinsic conducting layer appears to form, possibly originating with interface charge from LAO which allegedly alleviates the so-called “polar catastrophe” at the interface. The principal conclusions from [1] are: (i) there is no partially-reduced Ti at the interface, as expected if there is electron transfer from the LAO, and, (ii) band bending at the interface occurs and results in a quantum well which is populated with carriers in the case of the TiO2-terminated substrate, but not for the SrO-terminated substrate. While the first of these conclusions is defensible, the second is not.
2009. "Band Offsets at the Epitaxial Anatase TiO2/n-SrTiO3(001) Interface." Surface Science 603(5):771-780. Abstract We have used high-energy-resolution x-ray photoelectron spectroscopy to measure valence band offsets at the epitaxial anatase TiO2(002)/n-SrTiO3(001) heterojunction prepared by molecular beam epitaxy, Within experimental error, the valance band offset is zero for anatase thicknesses between 1 and 7 monolayers. The conduction band offset is also zero by virtue of the fact that both anatase and SrTiO3 exhibit the same bandgap value (~3.2 eV). In one set of experiments, the interface included a partial monolayer of fluorine remaining from the HF etch that was used to prepare the substrate. The F could not be removed without Ar ion sputtering and annealing, which in turn resulted in ~0.15 eV of band bending, indicating the presence of interfacial defects. The band offsets were measured to be approximately 0 eV as well when the F was removed. Density functional theory predicts the valence band offset for the clean interface to be 0.5 eV. Inclusion of interfacial F reduces the theoretical band offset to 0.2 eV, much closer to experiment, and suggesting that the interface dipoles created by F and sputter defects have a major effect on the band offset.
2008. "Hole-mediated Photodecomposition of Trimehtyl Acetate on a TiO2(001) Anatase Epitaxial Thin Film Surface." Journal of Physical Chemistry C 112(50):20050-20056. doi:10.1021/jp8077997 Abstract Surfaces of titanium dioxide in both rutile and anatase polymorphs have attracted significant attention in catalysis and photochemistry. The (110) orientation of rutile, and to a lesser extent other rutile orientations, have been studied on an atomic scale, yielding information on surface structure and chemical reactivity. In contrast, the thermal and photochemistry of well-defined, single-crystal anatase surfaces had not been investigated, largely because of the metastable nature of anatase , as well as the lack of availability of high-quality surfaces. Here we describe a study of the adsorption and photoreactivity of an organic adlayer, trimethyl acetate (TMA), on structurally-excellent anatase (001) epitaxial thin films grown by oxygen plasma assisted molecular beam epitaxy (OPAMBE). High-resolution scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), and photodesorption spectrometry have been used to study the chemisorptions and ultraviolet (UV) light-induced photodecomposition of TMA in ultrahigh vacuum. UV light promotes hole-mediated photodecomposition of TMA, resulting in decarboxylation to yield tert-butyl radical and CO2. The photochemical rate constant is equal to that measured for OPAMBE grown rutile TiO2(110) surfaces.
2008. "Absence of Intrinsic Ferromagnetic Interactions of Isolated and Paired Co Dopant Atoms in Zn1−xCoxO with High Structural Perfection." Physical Review Letters 100(15):Art. No. 157201. doi:10.1103/PhysRevLett.100.157201 Abstract We report element specific structural and magnetic investigations on Zn1−xCoxO epitaxial films using synchrotron radiation. Co dopants exclusively occupy Zn sites as revealed by x-ray linear dichroism leading to a high degree of structural perfection. Comparative magnetic field dependent measurements by x-ray magnetic circular dichroism and conventional magnetometry consistently show purely paramagnetic behavior for isolated Co dopant atoms with a magnetic moment of 4.8μB. However, the total magnetization is reduced by ~30%, indicating that Co-O-Co pairs are antiferromagnetically coupled. We find no sign of intrinsic ferromagnetism in Co:ZnO films.
2008. "Electronic properties of H and D doped ZnO epitaxial films." Applied Physics Letters 92(15):Art. No. 152105. Abstract ZnO epitaxial films grown by pulsed laser deposition in an ambient of H2 or D2 exhibit qualitatively different electronic properties compared to films grown in vacuum or O2, or bulk single crystals annealed in H2. These include temperature-independent resistivities of ~0.1 -cm, carrier (electron) concentrations in the 1018 cm-3 range, mobilities of 20-40 cm2/V-sec, and negligible (a few meV) activation energies for conduction. These transport properties are consistent with H (D) forming an ultra-shallow donor or conduction band states not achievable by post-growth annealing in H2.
2008. "A Study of H and D doped ZnO epitaxial films grown by pulsed laser deposition." Journal of Applied Physics 104(5):Article no. 053711. doi:10.1063/1.2975219 Abstract We examine the crystal structure, electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. We compared with pure ZnO films grown in O2 and vacuum. N-type electrical conductivity is enhanced by two to three orders of magnitude as a result of growing in H2 or D2. Temperature dependent Hall effect measurements reveal small (a few meV) carrier activation energies, along with carrier concentrations of 2-7 x 1018 cm-3, and mobilities of 20-40 cm2/Vs in ZnO films doped with H or D in the 1018 cm-3 range. We have modeled the low-temperature electrical properties of H- and D-doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 or D2 promotes the formation of an exceedingly shallow or conduction-band degenerate donor state, possibly associated with H or D substitution at O sites in the lattice.
2008. "Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films." New Journal of Physics 10:Art. No. 055010. doi:10.1088/1367-2630/10/5/055010 Abstract Epitaxial thin films of cobalt-doped ZnO (Co:ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10-4 – 105 -cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co:ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co:ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.
2008. "Hidden Ferromagnetic Secondary Phases in Cobalt-doped ZnO Epitaxial Thin Films." Physical Review. B, Condensed Matter 77(20):201303. doi:10.1103/PhysRevB.77.201303 Abstract The quest to discover a dilute magnetic semiconductor which is ferromagnetic at room temperature has led to extensive research on doped semiconducting oxides. However, the wide range of reported properties has raised doubts regarding the presence of intrinsic ferromagnetism in these materials. Here we explore the origin of ferromagnetism in epitaxial Co:ZnO thin films, which are paramagnetic but become weakly ferromagnetic (~0.05 μB/Co) after annealing in Zn vapor to introduce interstitial Zn. Conventional bulk materials characterization techniques indicate no phase segregation or Co reduction has occurred. However, x-ray photoelectron spectroscopy sputter depth profiling clearly indicates the presence of Co(0) in the Zn-treated films; x-ray absorption spectroscopy is utilized to identify the secondary phase as ferromagnetic CoZn (1.5 μB/Co, TC ~ 400 – 450 K). This work demonstrates that the potential for ferromagnetic secondary phases in doped oxides must be thoroughly discounted, through painstaking materials characterization, before claims of intrinsic ferromagnetism can be made.
2008. "N incorporation, composition and electronic structure in N-doped TiO2(001) anatase epitaxial films grown on LaAlO3(001)." Surface Science 602(1):133-141. doi:10.1016/j.susc.2007.09.061 Abstract We have investigated the properties of N-doped TiO2 anatase grown by plasma-assisted molecular beam epitaxy on LaAlO3(001) substrates. Phase-pure epitaxial films in which N substitutes for O with no secondary phases formation occur only over a narrow range of fluxes. The N solubility is limited to ~0.2 at. % of the anions and is an order of magnitude lower than that found in N-doped rutile. N substitution for O results in N 2p derived states off the top of the anatase valence band and the associated red shift in the optical bandgap.
2008. "Molecular beam epitaxial growth of doped oxide semiconductors." Journal of Physics. Condensed matter 20(26):Art. No. 264004. Abstract Molecular beam epitaxy coupled with the use of activated oxygen is shown to be a powerful tool for the growth of well-defined, structurally-excellent oxide semiconductor films. The basics of the methodology are discussed. Several case studies are presented to illustrate some of the physical phenomena that can be investigated. These include Cr- and Co-doped TiO2 anatase, Ti-doped -Fe2O3 hematite, and N-doped TiO2 rutile.
2007. "Photoemission Electron Microscopy of TiO2 Anatase Films Embedded with Rutile Nanocrystals." Advanced Functional Materials 17(13):2133-2138. doi:10.1002/adfm.200700146 Abstract Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.
2007. "Spin resolved photoelectron spectroscopy of Fe3O4: the case against half-metallicity." Journal of Physics. Condensed matter 19(31):Art. No. 315218. Abstract Many materials have been theoretically predicted to be half-metallic, and hence suitable for use as pure spin sources in spintronic devices. Yet to date, remarkably few of these predictions have been experimentally verified. We have used spin polarized photoelectron spectroscopy to study one candidate half-metallic system, Fe3O4. Such experiments are normally hampered by difficulties in producing clean stoichiometric surfaces with a polarization that is truly representative of that of the bulk. However, by utilizing higher photon energies than have traditionally been used for such experiments, we can study polarization in ‘as received’ samples, essentially ‘looking through’ the disrupted surface. High quality, strain relieved, ex situ prepared Fe3O4 films have been thoroughly characterized by diffraction, transport and magnetometry studies of their crystallographic, electronic and magnetic properties. The spectroscopic results are found to agree fairly closely with previously published experimental data on in situ grown thin films and cleaved single crystals. However, despite the higher photoelectron kinetic energies of the experiment, it has not been possible to observe 100% polarization at the Fermi level. Hence, our data do not support the claim of true half-metallicity for Fe3O4.
2007. "Growth and structure of MBE grown TiO2 anatase films with rutile nano-crystallites ." Surface Science 601(6):1582-1589. doi:10.1016/j.susc.2007.01.039 Abstract We have grown TiO2 anatase films with rutile nanocrystalline inclusions using molecular beam epitaxy under different growth conditions. This model system is important for investigating the role of rutile/anatase interfaces in heterogeneous photocatalysis. To control the film structure, we grew a pure anatase (001) layer at a slow rate and then increased the growth rate to drive the nucleation of rutile particles. Structure analysis indicates that the rutile phase has four preferred orientations in the anatase film.
2007. "Nucleation and Growth of MOCVD Grown (Cr, Zn)O Films – Uniform Doping vs. Secondary Phase Formation." Journal of the Electrochemical Society 154(3):D134-D138. doi:10.1149/1.2424422 Abstract We report a detailed study of chromium solubility and secondary phase formation in MOCVD grown (Cr, Zn)O-based films on silicon (100). Simultaneous deposition of 0.15M Cr(TMHD) and 0.025M Zn(TMHD) based precursors in an oxidizing environment with a flow ratio of 1:10 resulted in secondary phase formation rather than uniform Cr doping. Based on several surface and micro-structural techniques, we have identified nano-crystalline ZnCr2O4 and disordered Cr2O3 as the secondary Cr-containing phases that nucleate. Analysis suggests that ZnCr2O4 crystallites are dispersed throughout the film and that disordered Cr2O3 layer may form at the interface. These results reveal a strong tendency for Cr to exist in octahedral, rather than tetrahedral coordination.
2007. "Metalorganic chemical vapor deposition of carbon-free ZnO using the bis(2,2,6,6 tetramethyl-3,5-heptanedionato)zinc precursor." Journal of Materials Research 22(5):1230-1234. doi:10.1557/JMR.2007.0146 Abstract We report the growth of c-axis oriented ZnO films on silicon (100) single crystal substrates by MOCVD. A relatively uninvestigated precursor, Zn(TMHD)2, was used in a cold-wall MOCVD reactor. XPS and RBS analysis yielded a zinc-to-oxygen atom percent ratio of 0.98 and 1.00 at the surface and in the overall film, respectively indicative of stoichometric ZnO. Due to the presence of carbon at the surface, the excess oxygen at the surface was in the form of C-O bonding. The c-axis orientation was confirmed by HRTEM and XRD. We look at these results from a viewpoint of an ongoing effort to ensure cleaner decompositions using Zn(TMHD)2.
2007. "Structure, Magnetism and Conductivity in Epitaxial Ti-doped -Fe2O3 Hematite: Experiment and density functional theory calculations." Physical Review. B, Condensed Matter and Materials Physics 75(10):, doi:10.1103/PhysRevB.75.104412 Abstract We explore the feasibility of growing epitaxial Ti-doped -Fe2O3 in which Ti(IV) substitutes for Fe(III) preferentially in one magnetic sublattice, but not the other. Such a structure has been predicted by first-principles theory to be energetically likely, and is expected to yield interesting and useful magnetic and electronic properties. However, we find that a majority of Ti dopants disperse and occupy random cation sites in both magnetic sublattices. Density functional theory predicts that the magnetically ordered and magnetically random structures are nearly isoenergetic.
2007. "N incorporation and electronic structure in N-doped TiO2(110) rutile." Surface Science 601(7):1754-1762. doi:10.1016/j.susc.2007.01.051 Abstract Epitaxial TiO2-xNx film growth under anion-rich conditions is characterized by nearly balanced incorporation rates for substitutional N (NO) and interstitial Ti (Tii). Tii donors fully compensate and stabilize N3-, but preclude the formation of p-type material. Hybridization occurs between Tii(IV) and NO3-, but the value of x is limited to ~0.02 under these conditions. Tii(IV)-NO3- states occur above the valence band maximum of pure TiO2, riving rise to enhanced optical absorption in the visible up to ~2.5 eV. Much higher NO and Tii concentrations result from using cation-rich conditions.
2007. "Surface Science Opportunities in the Electronic Structure of ZnO (A Perspective on the Article, "Quantitative Analysis of Surface Donors in ZnO", by D.C. Look)." Surface Science 601(23):5313-5314. doi:10.1016/j.susc.2007.10.004 Abstract ZnO is a wide-gap oxide semiconductor of considerable current interest for electronics, optoelectronics, and, possibly, semiconductor spintronics. Through selective electronic doping, ZnO can be a transparent conducting oxide [1], a UV light emitter [2,3], and, when alloyed with a few to several atomic percent of Co, Mn or other transition metals with unpaired d electrons, is squarely at the center of controversy in the field of high-Tc ferromagnetic semiconductors [4]. Despite this range of interests, fundamental aspects of electronic doping in ZnO remain poorly understood. For instance, theoretical calculations suggest that H at either interstitial or substitutional sites is responsible for the persistent n-type conductivity that frustrates efforts to achieve p-type behavior, an essential requirement for the fabrication of pn light emitting diodes and hole-mediated ferromagnetic coupling of Mn dopants [5,6]. Yet, experimental studies do not in general align with this prediction. Another interesting aspect of n-type conduction in ZnO is the appearance of a near-surface conducting channel that appears to be present in bulk crystals [7,8]. Although it has been a few years since the original observation of this phenomenon was described, detailed understanding is lacking. In this paper, David Look presents a transport study aimed at gaining insight into the phenomenon of near-surface conductivity in bulk ZnO. All samples investigated were unintentionally n-type, as is typical. The measured Hall data were interpreted using a two-layer conduction model based on standard charge balance equations. Look fit the temperature dependence of the electron mobility and carrier concentration and extracted the thicknesses of the surface conducting layer, which dominates at lower temperatures, for bulk crystals synthesized by different methods and marketed by different companies. By making reasonable assumptions about the surface acceptor density, the lower limit of the surface conducting layer thickness was extracted from the transport data. From a surface science perspective, two aspects of the investigation are of particular interest: (i) the donor concentration was enhanced by annealing in forming gas, which is 5% H2, and, (ii) despite relatively weak variation in the sheet carrier concentration, the surface conducting layer thickness (dsurf) varied from as large as 28 nm to as small as 1.5 nm, depending on the method of preparation and supplier. The ongoing exploration of this phenomenon begs for the kind of experiments that practitioners of surface, interface and thin-film science can readily conduct. In bulk crystals, is H doping really the cause? Answering this question represents a major challenge because the donor concentration is in the 1017-1018 cm-3 range. Determining the donor identity in this concentration range is a major experimental hurdle. How does dsurf depend on crystallographic orientation? How does the phenomenon depend on the extent of band bending? Does persistent photoconductivity come into play? Can the effect be modified by band bending modification via surface photovoltage effects? All of the above questions pertain to epitaxial films as well, but other questions also arise. How do the film thickness and strain state affect dsurf? Can the surface conduction effect be enhanced by judicious design and growth of quantum well structures made from ZnO, MgxZn1-xO, and/or CdxZn1-xO? Answering these and other questions that arise will expand our understanding of this fascinating and potentially important material, as well as pave the way for device applications.
2007. "Properties of Structurally Excellent N-doped TiO2 Rutile." Chemical Physics 339(1-3):27-35. doi:10.1016/j.chemphys.2007.04.024 Abstract We have used plasma-assisted molecular beam epitaxy to synthesize structurally near-perfect crystalline films of TiO2-xNx rutile for the first time. These materials allow the properties of TiO2-xNx to be elucidated without the interfering effects of oxygen vacancy defects. In the absence of such defects, the extent of N incorporation in the lattice is limited to 2 ± 1 at. % of the anions. Substitutional N (NO) exhibits a -3 formal charge due to charge transfer from shallow-donor interstitial Ti(III), which forms during epitaxial growth. Hybridization between NO and adjacent lattice Ti ions occurs, resulting in new states off the top of the rutile valence band and an apparent band gap reduction of ~ 0.5 eV. It is not yet known if these new states result in mobile electron-hole pair creation upon irradiation, but experiments are planned to answer this important question.
2007. "Growth, Electronic and Magnetic Properties of Doped ZnO Epitaxial and Nanocrystalline Films." Applied Physics A, Materials Science and Processing 88(1):1-5. doi:10.1007/s00339-007-3948-2 Abstract We have used oxygen plasma assisted metal organic chemical vapor deposition along with wet chemical synthesis and spin coating to prepare CoxZn1-xO and MnxZn1-xO epitaxial and nanoparticle films. Co(II) and Mn(II) substitute for Zn(II) in the wurtzite lattice in materials synthesized by both methods. Room temperature ferromagnetism in epitaxial Co:ZnO films can be reversibly activated by diffusing in Zn, which occupies interstitial sites and makes the material n-type. O-capped Co:ZnO nanoparticles, which are paramagnetic as grown, become ferromagnetic upon being spin coated in air at elevated temperature. Likewise, spin-coated N-capped Mn:ZnO nanoparticle films also exhibit room temperature ferromagnetism. However, the inverse systems, N-capped Co:ZnO and O-capped Mn:ZnO, are entirely paramagnetic when spin coated into films in the same way. Analysis of optical absorption spectra reveal that the resonances Co(I) ↔ Co(II) + e-CB and Mn(III) ↔ Mn(II) + h+VB are energetically favorable, consistent with strong hybridization of Co (Mn) with the conduction (valence) band of ZnO. In contrast, the resonances Mn(I) ↔ Mn(II) + e-CB and Co(III) ↔ Co(II) + h+CB are not energetically favorable. These results strongly suggest that the observed ferromagnetism in Co:ZnO (Mn:ZnO) is mediated by electrons (holes).
2007. "Advances in the Surface Science of TiO2 – A Global Perspective." Hyomen Kagaku. Journal of the Surface Science Society of Japan 28(10):561-572. Abstract TiO2 rutile single-crystal surfaces have served has useful prototypical, well-defined specimens for fundamental investigations of oxide surface science for many years. As a result of both experimental and theoretical efforts, we have gained considerable insight into the structural, electronic, thermochemical and photochemical properties of pristine as well as defective surfaces. In this brief review, I summarize some of the recent advances that have been made in the laboratories of participants of the International Workshop of Oxide Surfaces (IWOX) series, principally on TiO2(110).
2006. "Synthesis of Room-Temperature Ferromagnetic Cr-doped TiO₂(110) Rutile Single Crystals using Ion Implantation." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 242(1-2):198-200. doi:10.1016/j.nimb.2005.08.149 Abstract Ferromagnetic Cr-doped rutile TiO₂ single crystals were synthesized by high-temperature ion implantation. The associated structural, compositional and magnetic properties were studied by x-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, proton induced x-ray emission, x-ray diffraction, Cr K- and L-shell near-edge x-ray absorption spectroscopy, and vibrating sample magnetometry. Cr was distributed uniformly to the depth of about 300 nm with an average concentration of ~1 at. %. The samples are semiconducting and ferromagnetic as implanted, with a saturation magnetization of 0.29B/Cr atom at room temperature. Cr is in a formal oxidation state of +3 throughout the implanted region, and no CrO₂ is detected.
2006. "Quantification of Dopant Concentrations in Diluted Magnetic Semiconductors using Ion Beam Techniques." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):402-405. doi:10.1016/j.nimb.2006.04.038 Abstract It has recently been demonstrated that magnetically doped TiO2 and SnO2 show ferromagnetism at room-temperature and Curie temperatures above room temperature. However, accurate knowledge of dopant concentrations is necessary to quantify magnetic moments in these materials. Rutherford Backscattering spectrometry (RBS) is one of the powerful techniques to quantify magnetic transition metal dopant concentrations in these materials. However, in some cases, the interference of RBS signals for different dopants and substrate elements in these materials makes analysis difficult. In this work, we demonstrate that particle induced x-ray emission (PIXE) can be successfully used to quantify the magnetic transition element dopants in several room temperature ferromagnetic materials synthesized using three different synthesis methods: oxygen plasma assisted molecular beam epitaxy, ion implantation and wet chemical methods.
2006. "Direct kinetic correlation of carriers and ferromagnetism in Co2+ : ZnO." Physical Review Letters 97:037203-1 - 037203-4. doi:10.1103/PhysRevLett.97.037203 Abstract We report the use of controlled introduction and removal of Zni to test the hypothesis that high-Curie-temperature ferromagnetism in cobalt-doped ZnO (Co2+:ZnO) is mediated by carriers. Using oriented epitaxial Co2+:ZnO films grown by chemical vapor deposition, kinetics measurements were used to correlate the oxidative quenching of ferromagnetism with the diffusion and oxidation of interstitial zinc. These results demonstrate controlled systematic variation of a key parameter involved in the ferromagnetism of Co2+:ZnO, namely interstitial zinc, and in the process unambiguously reveal this ferromagnetism to be dependent upon carriers. The distinction between defect-bound and free carriers in Co2+:ZnO is discussed
2006. "Magnetic properties of epitaxial Co-doped anatase TiO2 thin films with excellent structural quality." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 24(4):2012-2017. doi:10.1116/1.2216723 Abstract The heteroepitaxy of Co-doped anatase TiO2 on LaAlO3(001) has been refined with the goal of determining the relationship between structural quality and magnetic ordering. By significantly reducing the deposition rate and substrate temperature, well-ordered Co:TiO2 films with unprecedented crystalline quality were obtained by oxygen-plasma-assisted molecular beam epitaxy, as characterized by x-ray diffraction. These films exhibit uniform Co doping, with no evidence of Co segregation or secondary phases throughout the film depth or on the surface. Despite the improvement in crystalline quality and Co distribution, the films exhibit negligible ferromagnetism, with saturation moments of only ~0.1 B/Co. This loss of ferromagnetism is in stark contrast to faster-grown Co:TiO2 films, where a higher growth rate and substrate temperature typically result in lower crystalline quality, a highly non-uniform Co distribution, and average saturation moments of ~1.2 B/Co. The presence of ferromagnetism in faster-grown Co:TiO2 does not appear to arise from intrinsic point defects present in the bulk material, such as charge-compensating oxygen vacancies, but is instead attributed to the presence of extended structural defects.
2006. "Ferromagnetism and structure of epitaxial Cr-doped anatase TiO2 thin films." Physical Review. B, Condensed Matter and Materials Physics 73(15):155327 (12 p.). doi:10.1103/PhysRevB.73.155327 Abstract The materials and magnetic properties of Cr-doped anatase TiO2 thin films deposited on LaAlO3(001) and SrTiO3(001) substrates by oxygen-plasma-assisted molecular beam epitaxy have been studied in detail to elucidate the origin of ferromagnetic ordering. Cr substitution for Ti in the anatase lattice, with no evidence of Cr interstitials, segregation, or secondary phases, was independently confirmed by transmission electron microscopy (TEM) with energy dispersive x-ray (EDX) spectroscopy, extended x-ray absorption fine structure (EXAFS), and Rutherford backscattering spectrometry (RBS) in the channeling geometry. Epitaxial films deposited at ~0.1 Å/s were found to have a highly defected crystalline structure, as quantified by high resolution x-ray diffraction (XRD). These films were also ferromagnetic at room temperature with a moment of ~0.5 B/Cr, Curie temperatures in the range of 400 – 700°C, and exhibited shape and in-plane magnetocrystalline anisotropy. However, no free carrier spin polarization was observed by Hall effect measurements, raising questions about the mechanism of magnetism. Films deposited slowly (~0.015 Å/s) possessed a nearly perfect crystalline structure as characterized by XRD. Contrary to expectations, these films exhibited negligible ferromagnetism at all Cr concentrations. Annealing in vacuum to generate additional oxygen defects and free carrier electrons did not significantly increase the ferromagnetic ordering in either fast- or slow-grown films. These results contradict both oxygen-vacancy-derived free-carrier-mediated exchange and F-center-mediated bound magnetic polaron exchange mechanisms, and instead indicate the primary role of extended structural defects in mediating the ferromagnetic ordering in doped anatase films.
2006. "Carrier Dynamics in a-Fe2O3 (0001) Thin Films and Single Crystals Probed by Femtosecond Transient Absorption and Reflectivity." Journal of Applied Physics 99(5):Article: 053521 (6 pages). Abstract Femtosecond transient reflectivity and absorption is used to measure the carrier lifetimes in α-Fe2O3 thin films and hematite single crystals. The results from the thin films show that initially excited hot electrons relax to the bandedge within 300 femtoseconds and then recombine with holes or trap within 5 pioseconds. The trapped electrons have a lifetime of hundreds of picoseconds. The trapped electrons have a lifetime of hundreds of picoseconds. Transient reflectivity measurements from hematite (α-Fe2O3) single crystals show similar but slightly faster dynamics. In hematite, the transient reflectivity displays oscillations due to the formation of longitudinal acoustic phonons generated following absorption of the ultrashort excitation pulse.
2006. "Ferromagnetism in Oxide Semiconductors ." Materials Today 9(11):28-35. Abstract In order to become a practical technology, semiconductor spintronics requires the discovery and utilization of ferromagnetic semiconductors which exhibit spin polarization in the majority carrier band at and above room temperature. Intrinsic remanent magnetization would allow spin polarized currents to be propagated in such materials without the need for a continuous magnetic field. However, the discovery and understanding of such materials is proving to be a grand challenge in solid-state science. Indeed, one of the 125 critical unanswered scientific questions recently posed in Science magazine asks, “Is it possible to create magnetic semiconductors that work at room temperature?”
2006. "Ferromagnetism in Doped Thin-Film Oxide and Nitride Semiconductors and Dielectrics." Surface Science Reports 61(8):345-381. doi:10.1016/j.surfrep.2006.05.001 Abstract The principal goal in the field of high-Tc ferromagnetic semiconductors is the synthesis, characterization and utilization of semiconductors which exhibit substantial carrier spin polarization at and above room temperature. Such materials are of critical importance in the emerging field of semiconductor spintronics. The interaction leading to carrier spin polarization, exchange coupling between the dopant spins and the valence or conduction band, is known to be sufficiently weak in conventional semiconductors, such as GaAs and Si, that magnetic ordering above cryogenic temperatures is essentially impossible. Since the provocative theoretical predictions of Tc above ambient in p-Mn:ZnO and p-Mn:GaN (T. Dietl et al., Science 287 1019 (2000)), and the observation of room-temperature ferromagnetism in Co:TiO2 anatase (Y. Matsumoto et al., Science 291 854 (2001)), there has been a flurry of work in oxides and nitrides doped with transition metals with unpaired d electrons. It has even been claimed that room-temperature ferromagnetism can be obtained in certain d0 transition metals oxides without a dopant. In this Report, the field of transition metal doped oxides and nitrides is critically reviewed and assessed from a materials science perspective. Since much of the field centers around thin film growth, this Report focuses on films prepared not only by conventional vacuum deposition methods, but also by spin coating colloidal nanoparticles.
2005. "Epitaxial Growth and Properties of Magnetically Doped TiO₂." Chapter 7 in Thin Films and Heterostructures for Oxide Electronics, ed. Satishchandra B. Ogale, pp. 219-247. Springer, New York, NY. Abstract Spin electronics and photonics represent exciting frontiers in which new paradigms are envisioned for signal processing and computing.1-4 Using quantum mechanical spin states associated with charged particles to propagate information offers potential advantages over the use of charge alone, as in present-day technology. These advantages include the ability to combine logic and memory on the same chip, lower power consumption, decrease switching times in device architectures similar to those used today, and incorporate entirely new functionalities.
2005. "Heteroepitaxial Growth and Structural Analysis of Epitaxial ⍺-Fe₂O₃(1010) on TiO₂(001)." Journal of Materials Research 20(5):1250-1256. Abstract We have grown epitaxial ⍺-Fe₂O₃(1010) on TiO₂(001) rutile by oxygen plasma assisted molecular beam epitaxy. High resolution transmission electron microscopy (HRTEM), reflection high energy electron diffraction (RHEED) and x-ray diffraction pole figures confirm that the film is composed of four different in-plane orientations rotated by 90º relative to one another. For a given Fe₂O₃ unit cell, the lattice mismatch along the parallel [0001]Fe₂O₃ and [100]TiO₂ directions is nominally +67%. However, due to a three-fold repetition of the slightly distorted square symmetry of anion positions within the Fe₂O₃ unit cell, there is a coincidental anion alignment along the [0001]Fe₂O₃ and [100]TiO₂ directions which results in an effective lattice mismatch of only -0.02% along this direction. The lattice mismatch is nearly 10% in the orthogonal [1120]Fe₂O₃ and [100]TiO₂ directions. The film is highly ordered and well registered to the substrate despite a large lattice mismatch in one direction. The film grows in registry with the substrate along the parallel [0001]Fe₂O₃ and [100]TiO₂ directions and nucleates dislocations along the orthogonal [1120]Fe₂O₃ [100]TiO₂ directions.
2005. "Microstructure of Co-doped TiO₂ (110) Rutile by Ion Implantation." Journal of Applied Physics 97(7):99-104. Abstract Co-doped rutile TiO₂ was synthesized by injecting Co ions into single crystal rutile TiO₂ using high energy ion implantation. Microstructures of the implanted specimens were studied in detail using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy (EDS), electron diffraction, and HRTEM image simulations. The spatial distribution and conglomeration behavior of the implanted Co ions, as well as the point defect distributions induced by ion implantation, show strong dependences on implantation conditions. Uniform distribution of Co ions in the rutile TiO₂ lattice was obtained by implanting at 1075 K with a Co ion fluence of 1.25x10¹⁶ Co/cm². Implanting at 875 K leads to the formation of Co metal clusters. The precipitated Co metal clusters and surrounding TiO₂ matrix exhibit the orientation relationships Co<110>//TiO₂[001] and Co{111}//TiO₂(110). A structural model representing the interface between Co metal clusters and TiO₂ is developed based on HRTEM imaging and image simulations.
2005. "Spin-Polarized Conduction in Localized Ferromagnetic Materials: the case of Fe3O4 on MgO(100)." Journal of Applied Physics 98(8):084507(5). Abstract The surface electronic structure of 1500 °A thick Fe3O4(100) films has been investigated by spinresolved photoemission spectroscopy with vacuum ultra-violet (VUV) synchrotron radiation. The films, epitaxially grown on MgO(100) substrates using plasma assisted molecular beam deposition, are briefly exposed to air during transfer to the photoemission chamber. It is shown that clean surfaces of Fe3O4 films after exposure to air can be recovered through mild annealing in an oxygen atmosphere. The presence of the characteristic Verwey transition, as detected in the valence band photoemission spectra, provides evidence for the excellent electronic quality at the surface of these films. The top of the valence band is found to be negatively spin-polarized, with a value of ~ −50%; this result strongly points towards the localized nature of the 3d states in this system.
2005. "Topography of Anatase TiO₂ Film Synthesized on LaAlO₃(001) ." Nanotechnology 16(3):S18–S21. Abstract The surface of an anatase titanium dioxide (TiO₂) film grown on LaAlO₃ was observed by noncontact atomic force microscopy (NC-AFM). After cleaning with a cycle of argon ion sputtering and annealing, (1x4) and (1x5) reconstructed terraces appeared. In addition to the terraces, the sputter-annealed surface included many agglomerations. X-ray photoelectron spectroscopy analysis showed the presence of Tiⁿ⁺ (n≤3). The low diffusivity of the Tiⁿ⁺(n≤3) species generated by argon ion sputtering is responsible for the generation of the agglomerations.
2005. "Growth of Cr-doped TiO₂ Films in the Rutile and Anatase Structures by Oxygen Plasma Assisted Molecular Beam Epitaxy ." Thin Solid Films 484(1-2):289-298. Abstract As part of a search for new spintronic materials, highly ordered films of CrxTi₁-xO₂ in both rutile and anatase structure and for several Cr concentrations ranging from x=0.02 to 0.16 were grown by oxygen-plasma assisted molecular beam epitaxy. X-ray photoelectron diffraction data of the Cr 2p level exhibit the same patterns and the same modulation amplitudes as those observed for Ti 2p, providing a strong indication that a large fraction of the Cr atoms occupy substitutional lattice sites in both structures. The Cr 2p core-level spectra as well as a Cr 3d related dopant signal above the valence band of TiO₂ are characteristic of Cr³⁺ ions. At room temperature, Cr-doped anatase films exhibit ferromagnetic order with a saturation magnetization of ~0.6 µB per Cr atom and strong in-lane anisotropy.
2005. "Structure of the Hydrogen Stabilized MgO(111)-(1x1) Polar Surface: Integrated Experimental and Theoretical Studies." Physical Review. B, Condensed Matter and Materials Physics 71:115434. Abstract The surface structure of MgO(111)-(1x1) bulk and thinned single crystals have been investigated by transmission and reflection high-energy electron diffraction (THEED and RHEED), low energy electron diffraction (LEED), and x-ray photoelectron and Auger electron diffraction (XPD). The (1x1) polar surface periodicity is observed both after 800⁰C annealing in air and also after oxygen plasma cleaning and annealing in ultrahigh vacuum. The x-ray photoelectron spectroscopy and diffraction (XPS and XPD) results were analyzed by simulations based on path-reversed LEED theory and by first-principles calculations to help distinguish between different mechanisms for the stabilization of this extremely polar oxide surface: 1) stabilization by adsorption of a hydrogen monolayer; maintaining the insulating nature of the surface; and 2) stabilization of the clean O or Mg terminated 1x1 surface by interlayer relaxations and 2-D surface metallization. The analysis favors stabilization by a single OH layer, where hydrogen sits on top of the O ions with O-H bond distance of 0.98Å. The in-plane O and Mg positions fit regular rock-salt sites, the distance between the topmost O and Mg plane is 1.04 Å, contracted by ~14% with respect to bulk MgO distance of 1.21 Å, while the interlayer separation of the deeper layers is close to that of bulk, contracted by less than 1%. The presence of a monolayer of H associated with the terminal layer of oxygen reduces significantly the surface dipole and stabilizes the surface.
2005. "Atomic and Electronic Structure of the Fe₃O₄ (111)/MgO(111) Model Polar Oxide Interface." Physical Review. B, Condensed Matter 72(19):195401. doi:10.1103/PhysRevB.72.195401 Abstract High resolution transmission electron microscopy (HRTEM) and density functional calculations are used to study the effect of interface polarity on the atomic and electronic structure of the prototype Fe₃O₄(111)/MgO(111) polar oxide interface. We show that atomically abrupt interfaces exist between the MgO(111)-substrate and magnetite(111) film in regions separated by Fe nanocrystals, and propose a solution for this oxide-oxide interface structure. Comparisons of through-focus/through-thickness experimental HRTEM images with calculated images for model interface structures suggest metal-oxygen-metal (i.e., Mg-O-Fe) interface bonding with octahedral (B) coordination of the first Fe monolayer, rather than the combination of tetrahedral-octahedral-tetrahedral (ABA) stacking also found in Fe₃O₄. First-principles calculations for all the different models find metal-induced gap states in the interface oxygen layer. Consistent with the HRTEM results, the MgO-Fe₃O₄ interface stacking ...Mg/O/Mg/O/3FeB/O/FeAFeBFeA… is calculated to be the energetically most favorable, and effectively screening the MgO(111) substrate surface polarity. The data and calculations exclude mixing of Mg and Fe across the interface, in contrast to the commonly invoked mechanism of cation mixing at compound semiconductor polar interfaces.
2005. "Negligible Magnetism in Excellent Structural Quality CrxTi₁-xO₂ Anatase: Contrast with High-Tc Ferromagnetism in Structurally Defective CrxTi₁-xO₂." Physical Review Letters 95:217203. doi:10.1103/PhysRevLett.95.217203 Abstract The mechanism of ferromagnetism in doped oxides is under active debate. We reexamine doped TiO₂ anatase, using epitaxial Cr:TiO₂ with excellent structural quality as a model system. In contrast to highly oriented but defective Cr:TiO₂ (~0.5 µB/Cr), these structurally superior single crystal films exhibit negligible ferromagnetism. We show for the first time that charge compensating oxygen vacancies alone, as predicted by F-center mediated exchange, are not sufficient to activate ferromagnetism. Instead, the onset of ferromagnetism correlates with the presence of structural defects.
2005. "Co-doped Anatase TiO₂ Heteroepitaxy on Si(001)." Journal of Applied Physics 97(7):073511-073511-10. doi:10.1063/1.1868854 Abstract Pure anatase TiO₂ and CoxTi₁-x O₂ (0.01 < x < 0.04) epitaxial thin films were deposited by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) on Si(001) for evaluation as a potential dilute magnetic semiconductor material suitable for Si-based spintronic devices. Epitaxial growth on Si(001) was facilitated by the deposition of ¼or ½ ML Sr metal on the clean Si(001) surface to form an oxidation resistant silicide layer, followed by deposition of a thin SrTiO₃ buffer layer. Using ½ ML Sr metal to form the silicide allowed the deposition of 10 ML SrTiO₃ without oxidation of the Si interface. Epitaxial anatase could be grown on this heterostructure, although use of the oxygen plasma during deposition resulted in significant SiO₂ formation. Pure anatase films consisted of epitaxial anatase surface particles on a continuous anatase film. For Co-doped films, Co segregation to surface particles of epitaxial anatase was observed by Auger electron spectroscopy and transmission electron microscopy (TEM); faceting of the particles was observed for low Co doping concentrations. Although no secondary phases containing Co were observed in Co-doped anatase films by x-ray diffraction or TEM, x-ray absorption near edge spectroscopy indicated Co was present in the films as a mixture of Co(0), Co(II), and Co(III). All samples were ferromagnetic at room temperature; for lower Co concentrations, the ferromagnetic remanence (9%) and coercive field (100 Oe) were similar to phase-pure Co:TiO₂ / LaAlO₃. However, the presence of Co(0) under strongly oxidizing growth conditions known to oxidize the Si interface implies that under no deposition conditions can Co metal be eliminated while simultaneously protecting the Si interface from oxidation.
2005. "Oxygen Vacancies and Ferromagnetism in CoxTi₁–xO₂–x–y." Journal of Applied Physics 97(7):073908 (6 p.). Abstract Abstract: Cobalt-doped titanium dioxide, or CTO, has emerged in the past two years as a semiconducting, transparent, room-temperature ferromagnet. Very recently it has been shown that the magnetism in CTO often originates in surface nanoparticles or Co-rich regions that have a much-enhanced substitutional Co content up to 40% of Ti sites, so that magnetic CTO is not a true dilute magnetic semiconductor (DMS), but rather a fairly high-density spin system. In this work we describe a computational study of Co-rich CTO using the Generalized Gradient Approximation (GGA) to density functional theory (DFT) within the supercell model. Our total energy calculations show a strong tendency for Co-atom clustering or segregation on Ti sites. There is also a strong tendency for the oxygen vacancies to form complexes with the Co atoms. In addition, we find that the oxygen stoichiometry plays an essential role in determining the system’s magnetic order. The largest ordered moments require at least enough oxygen vacancies to put all of the Co atoms in the +2 charge state, as they indeed appear to be experimentally, so that the conventional DMS mechanism could only apply via n-type carriers. We find a small but not negligible spin density associated with Ti atoms near the vacancy sites, suggesting an F-center-mediated interaction between the much larger Co moments. We also present experimental data showing that the ferromagnetic remanence and coercive field increase with the n-type conductivity.
2005. "XAFS study of CoxTi₁-xO₂-x-anatase." Physica Scripta T115:597-599. doi:10.1238/Physica.Topical.115a00597 Abstract Co doped TiO₂ anatase is a promising candidate dilute magnetic semiconductor (DMS). DMS materials have potential applications in spintronics devices. These utilize the spin of carriers, and offer the promise of new devices with enhanced functionality. The application of DMS materials to spintronics is as spin injectors. These materials can have spin-polarized states in their valence or conduction bands. At interfaces with nonmagnetic semiconductors they can then be used for spin polarized carrier injection, allowing for the fabrication of novel devices utilizing spin. Theory and measurements indicate that they can be much more efficient spin injectors than ferromagnetic metals. This report describes some recent XAFS studies of these materials using the PNC-CAT beamlines at the APS.
2005. "Cr-doped TiO₂ Anatase- A Ferromagnetic Insulator ." Journal of Applied Physics 97:046103. Abstract Epitaxial ferromagnetic films of Cr-doped TiO₂ anatase (CrxTi₁-xO₂-x/₂) were grown on LaA1O3(001) using oxygen-plasma assisted molecular beam epitaxy. Cr K-shell x-ray absorption near-edge spectroscopy shows that the formal oxidation state of Cr is +3 throughout the films with no evidence for either elemental Cr or half-metallic CrO₂. Cr is found to substitute for Ti in the lattice with uniform distribution throughout the doped region of the film. The Cr-doped anatase films exhibit room temperature ferromagnetism aligned in-plane with a saturation magnetization of ~0.6 µB/Cr atom.
2005. "Optical properties of La-based high-K dielectric films." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 23(6):1676-1680. doi:10.1116/1.205655 Abstract We have characterized thin films of LaScO3 and LaAIO3 which were grown by molecular beam deposition on Si substrates. Samples of LaScO3 were also grown by pulsed laser deposition of MgO substrates. Using transmission studies between 1.5 and 6 eV, we have established that low temperature deposition leads to a reduced band gap with respect to the bulk crystal. Furthermore, using spectroscopic ellipsometry form 5 to 9 eV we observe substantial difference in near-band gap absorption between thin and thicker films for both materials. We obtain a band gap of 5.84 eV for the thinner film of LaAIO3, shereas we find a band gap of 6.33 eV for the thicker film of LaAIO3. Similarly we find band gaps of 5.5 and 5.9 eV, respectively, for thin and thick films of LaScO3
2005. "Structure, Band Offsets and Photochemistry at Epitaxial ⍺-Cr₂O₃/⍺-Fe₂O₃ Heterojunctions." Surface Science 587(3):L197-L207. Abstract We test the hypothesis that electron-hole pair separation following light absorption enhances photochemistry at oxide/oxide heterojunctions which exhibit a type II or staggered band alignment. We have used hole-mediated photodecomposition of trimethyl acetic acid chemisorbed on surfaces of heterojunctions made from epitaxial ⍺-Cr₂O₃ on ⍺-Fe₂O₃(0001) to monitor the effect of UV light of wavelength 385 nm (3.2 eV) in promoting photodissociation. Absorption of photons of energies between the bandgaps of ⍺-Cr₂O₃ (Eg = 4.8 eV) and ⍺-Fe₂O₃ (Eg = 2.1 eV) is expected to be strong only in the ⍺-Fe₂O₃ layer. The staggered band alignment should then promote the segregation of holes (electrons) to the ⍺-Cr₂O₃ (⍺-Fe₂O₃) layer. Surprisingly, we find that the ⍺-Cr₂O₃ surface alone promotes photodissociation of the molecule at hv = 3.2 eV, and that any effect of the staggered band alignment, if present, is masked. We propose that the inherent photoactivity of the ⍺-Cr₂O₃ (0001) surface results from the creation of bound excitons in the surface which destabilize the chemisorption bond in the molecule, resulting in photodecomposition.
2004. "Epitaxial Growth and Properties of Cobalt-doped ZnO on α-Al₂O₃ Single-Crystal Substrates." Physical Review. B, Condensed Matter 70(5):054424. Abstract Co-doped ZnO (CoxZn₁-xO) is of potential interest for spintronics due to the prediction of room-temperature ferromagnetism. We have grown epitaxial CoxZn₁-xO films on Al₂O₃(012) substrates by metalorganic chemical vapor deposition using a liquid precursor delivery system. High concentrations of Co (x < 0.35) can be uniformly incorporated into the film without phase segregation. Co is found to be in the ⁺² oxidation state, independent of x. This material can be grown n type by the deliberate incorporation of oxygen vacancies, but not by inclusion of ~1 at. % Al. Semiconducting films remain ferromagnetic up to 350 K. In contrast films without oxygen vacancies are insulating and nonmagnetic, suggesting that exchange interaction is mediated by itinerant carriers. The saturation and remanent magnetization on a per Co basis was very small (< 0.1 B/Co), even in the best films. The dependence of saturation magnetization, as measured by optical magnetic circular dichroism, on magnetic field and temperature, agrees with the theoretical Brillouin function, demonstrating that the majority of the Co(II) ions behave as magnetically isolated S = 3/2 spins.
2004. "Room-Temperature Ferromagnetism in Ion-Implanted Co-Doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Interest in diluted magnetic semiconductros (DMS) is growing rapidly within the emerging field of spintronics. For example, the ability to efficiently inject spin-polarized carriers into nonmagnetic semiconductor heterostructures creates new and exciting possibilities for utilizing DMS materials in spin-based devices. Several III-V and II-VI semiconductor materials doped with magnetic transition metal elements have been explored. Although these materials show promising behavior in some cases, most exhibit Curie temperatures of ~170 K or less. It has recently been shown that certain oxide semiconductors doped with magnetic transition elements show room-temperature ferromagnetism.
2004. "Room Temperature Ferromagnetism in Ion-implanted Co-doped TiO₂(110) Rutile." Applied Physics Letters 84(22):4466-4468. Abstract Ferromagnetic Co-doped rutile TiO₂ singel crystals were successfully synthesized by high temperature ion implantation and charaterized by a variety of techniques. Co is uniformly distrubuted to the depth of ~300nm with an average concentration of ~2 at. %, except in the near-surface region, where the concentration is ~3 at. %. Magnetic measurements reval ferromagnetic behavior at room temperature with an effective saturation magnetization of ~0.6 ub/Co atom. The Co formal oxidation state is found to be +2 throughouth the implanted region, and no Co(0) is detected.
2004. "Studies of Two-and Three-Dimensional ZnO:Co Structures Through Different Sythetic Routes." Journal of Applied Physics 95(11):7393-7395 Part 2. Abstract Multilayers of Co and ZnO, with nominal layer thicknesses on the atomic scale with up to 25 bilayers, were deposited by ion beam sputtering on silicon and glass substrates at ambient temperature. Thick epitaxial CoxZn₁-xO films on A1₂O₃(012) substrates were grown by metalorganic chemical vapor deposition using a liquid precursor delivery system. All were co-doped with A₁. Comparative analysis of magnetization, resistivity, and magnetoresistance measurements, performed in the temperature range 2.5-300K, is presented. At small thickness of Co layers in the multilayer samples, these structures are diluted magnetic semiconductor (DMS) superlattices, with properties close to the epitaxial films. A crossover from DMS to discontinuous magnetic metal/semiconductor multilayers is observed with increasing metal content in the multilayers. This leads to changes in conduction mechanisms, with increasing contribution of quasi-three-dimensional or quasi-two-dimensional inter-granular hopping, and superparamagnetism.
2004. "Growth and Properties of molecular beam epitaxially grown ferromagnetic Fe-doped TiO2 rutile films on TiO2." Applied Physics Letters 84(18):3531-3533. Abstract We have grown epitaxial Fe-doped TiO₂ rutile films on rutile TiO₂(110) substrates, and have explored the resulting compositional, structural, morphological and magnetic properties. Clusters of mixed TiO₂ rutile and Fe₃O₄ form on the surface of a continuous rutile epitaxial film during growth. Room temperature ferromagnetism is observed, and is associated with the formation of secondary phase Fe₃O₄ rather than a true diluted magnetic oxide semiconductor.
2004. "Suppression of Subcutaneous Oxidation during the Deposition of Amorphous Lanthanum Aluminate on Silicon." Applied Physics Letters 84(23):4629-4631. Abstract Amorphous LaAlO₃ thin films have been deposited by molecular beam deposition directly on silicon without detectable oxidation of the underlying substrate. We have studied these abrupt interfaces by Auger electron spectroscopy, high-resolution transmission electron microscopy, medium-energy ion scattering transmission infrared absorption spectroscopy, and x-ray photoelectron spectroscopy. Together these techniques indicate that the films are fully oxidated and have less than 0.1 A of SiO₂ at the interface between the amorphous LaAlO₃ and silicon. These heterostructures are being investigated for alternative gate dielectric applications and provide a unique opportunity to control the interface between the silicon and the gate dielectric.
2004. "Measurements of the Band Offsets Between Amorphous LaAIO₃ and Silicon." Applied Physics Letters 84(5):726-728. Abstract The conduction and valence band offsets between amorphous LaAlO₃ and silicon have been determined from X-ray photoelectron spectroscopy (XPS) measurements. These films, which are free of interfacial SiO₂, were made by molecular beam deposition (MBD). The band line-up is type I with measured band offsets of 1.86 ± 0.1 eV for electrons and 3.23 ± 0.1 eV for holes. The band offsets are independent of the doping concentration in the silicon substrate as well as the LaAlO₃ film thickness. These amorphous LaAlO₃ films have a bandgap of 6.2 ± 0.1 eV.
2004. "Measurement of the Band Offsets between Amorphous LaAlO₃ and Silicon." Applied Physics Letters 84(5):726-728. Abstract The conduction and valence band offsets between amorphous LaAlO₃ and silicon have been determined from X-ray photoelectron spectroscopy (XPS) measurements. These films, which are free of interfacial SiO₂, were made by molecular beam deposition (MBD). The band line-up is type I with measured band offsets of 1.86 ± 0.2 eV for electrons and 3.23 ± 0.1 eV for holes. The band offsets are independent of the doping concentration in the silicon substrate as well as the amorphous LaAlO3 film thickness. These amorphous LaAlO3 films have a bandgap of 6.2 ± 0.1 eV.
2004. "Experimental Determination of Valence Band Maxima for SrTiO3, TiO2, and SrO and the Associated Valence Band Offsets with Si(001)." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 22(4):2205-2215. Abstract Abstract We address the issue of accurate determination of the energy at the top of the valence band for SrTiO₃(001) single crystals, as well as TiO₂(001) anatase and SrO epitaxial films. These measurements are of critical importance in determining valence band offsets for oxides in heterojunctions involving these materials. Three different methods are presented and compared: (1) fitting Gaussian broadened theoretical densities of states to x-ray excited valence band spectra, (2) finding the intersection of a regression line that spans the linear portion of the x-ray excited valence band leading edge with the background between the valence band maximum and the Fermi level, and, (3) determining the energy at which high-resolution ultraviolet photoemission intensity of the leading edge goes to zero. We find that method 1 is not reliable due to limitations in the accuracy of density functional theory when applied to these oxides. In contrast, methods 2 and 3 give physically reasonable results that are in good mutual agreement. The difference in VBM between method 1 and methods 2 & 3 is 0.4 – 0.6 eV, depending on the oxide. The true valence band maximum can be directly and accurately measured using methods 2 & 3 provided the experiment is carried out with adequate energy resolution. PACS numbers: 79.60.Jv a)Electronic mail: sa.chambers@pnl.gov
2004. "Accurate Valence Band Maximum Determination for SrTiO₃(001) ." Surface Science 554(2-3):81-89. Abstract We reexamine a well-established method for determining valence band maxima in semiconductors based on fitting photoemission spectra to theoretical densities of states. This technique is inaccurate for certain oxides because electronic structure methods predict too sharp a leading edge rise, which in turn appears to stem from an underestimation of the extent of metal-oxygen hybridization at the top of the valence band. In contrast, extrapolating the x-ray excited leading edge to the energy axis, in combination with the energy at which the UV-excited leading edge intensity goes to zero, yields consistent and reliable results that are useful for accurate band offset determinations.
2004. "Strong Room-Temperature Ferromagnetism in Co2+-Doped TiO2 Made from Colloidal Nanocrystals." Journal of the American Chemical Society 126(37):11640-11647. Abstract Colloidal cobalt-doped TiO2 (anatase) nanocrystals were synthesized and studied by electronic absorption, magnetic circular dichroism, transmission electron microscopy, magnetic susceptibility, cobalt K-shell X-ray absorption spectroscopy, and extended X-ray absorption fine structure measurements. The nanocrystals were paramagnetic when isolated by surface-passivating ligands, weakly ferromagnetic (Ms * 1.5 * 10-3 íB/Co2+ at 300 K) when aggregated, and strongly ferromagnetic (up to Ms ) 1.9 íB/Co2+ at 300 K) when spin-coated into nanocrystalline films. X-ray absorption data reveal that cobalt is in the Co2+ oxidation state in all samples. In addition to providing strong experimental support for the existence of intrinsic ferromagnetism in cobalt-doped TiO2, these results demonstrate the possibility of using colloidal TiO2 diluted magnetic semiconductor nanocrystals as building blocks for assembly of ferromagnetic semiconductor nanostructures with potential spintronics applications.
2004. "In Situ Grazing-Incidence Extended X-ray Absorption Fine Structure Study of Pb(II) Chemisorption on Hematite (0001) and (1-102) Surfaces." Langmuir 20(5):1667-1673. Abstract The dominant mode of binding of aqueous Pb(II) to single crystal (0001) (C-cut) and (1-102) (R-cut) surfaces of a-Fe₂O₃ has been determined using grazing-incidence x-ray absorption fine structure (GI-XAFS) spectroscopy. Oligomeric Pb(II) complexes were found to bind in inner-sphere modes on both surfaces, which is in contrast with the binding of aqueous Pb(II) on a-Al₂O₃ (0001) surfaces, where Pb(II) was found in past GI-XAFS studies to bind in a dominantly outer-sphere mode, indicating a significant difference in reactivity of these two surfaces to Pb(II). This difference in reactivity to Pb(II) is explained on the basis of recent crystal truncation rod diffraction studies of the a-Fe₂O₃ and a-Al₂O₃ C-cut surfaces in contact with bulk water, which found major structural differences between the surfaces of these two metal oxides.
2003. "Interface Characteristics of Iso-Structural Thin Film and Substrate Pairs." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 207(1):1-9. Abstract Cubic-CeO₂ and -Fe₂O₃ thin films have been epitaxially grown on yttria-stabilized ZrO₂ and -Al₂O₃ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by Rutherford backscattering spectrometry (RBS), high resolution transmission electron microscopy (HRTEM), and x-ray diffraction (XRD). RBS channeling spectra for both CeO₂/ZrO₂ and Fe₂O₃/Al₂O₃ show interface disorder-related scattering peaks. It is believed that the observed interface disorder-related scattering peaks on RBS spectra are attributed to the interface misfit dislocations. Cross sectional HRTEM reveals that interfaces of both systems are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. The experimentally observed dislocation spacings are approximately consistent with those calculated from the lattice mismatch, implying that the lattice mismatch is accommodated mainly by interface misfit dislocations above the critical thickness.
2003. "Band Offsets for the Epitaxial TiO₂/SrTiO₃/Si(001) System." Applied Physics Letters 83(18):3734-3736. Abstract We have used x-ray photoelectron spectroscopy with high energy resolution to determine band discontinuities at the two buried interfaces of the epitaxial TiO₂ (anatase)/ SrTiO₃/Si(001) system. The valence band offsets are -2.1 +/- 0.1 eV and +0.2 +/- 0.1 eV at the SrTiO₃/Si and TiO₂/SrTiO₃ heterojunctions, respectively. Assuming bulk band gaps for the SrTiO₃ and TiO₂ epitaxial films, the associated conduction band offsets are +0.1 +/- 0.1 eV and +0.1 +/- 0.1 eV. Si at the interface is in a flat-band state, indicating a very low density of electronic states. These results suggest that spin polarized electron injection from ferromagnetic Co-doped TiO₂ anatase into Si should be facile. PACS numbers: 79.60.Jv, 72.25.Dc
2003. "Investigation of Copper(I) Oxide Quantum Dots by Near-Edge X-ray Absorption Fine Structure Spectroscopy." Chemistry of Materials 20(15):3939-3946. Abstract Copper oxide quantum dots (CuOQD) were grown in various thicknesses on different SrTiO₃(001) surfaces and were investigated by near edge x-ray absorption fine structure (NEXAFS) spectroscopy. The experimental growth conditions for the CuOQD were optimized to obtain Cu₂O as the major phase. The CuOQD grown on clean SrTiO₃(001) surfaces at 825 K or higher with p(O₂) of 9.0x10-7 Torr or above contain mostly CuO contrasting to CuOQD grown at 800 K with p(O₂) of ~7.0x10-7 Torr that contain primarily Cu₂O. Furthermore, it is established that there is a strong interaction between the SrTiO₃(001) surface and the first few monolayers of the CuOQD, which induces the formation of Cu(II). However, this interaction is mitigated with increasing thickness of CuOQD resulting in the exclusive formation of Cu₂O in the topmost layers. The influence of the SrTiO₃(001) substrate on the formation of CuOQD can be reduced by modifying the substrate surface using chemical treatment and/or energetic Au₂⁺ ion-beam irradiation, since the substrate effect results from the reaction between the substrate oxygen and the copper atoms from the CuOQD. Examination of the photochemical properties of these CuOQD shows that prolonged soft x-ray irradiation under vacuum reduces Cu(II), which is present as a minor impurity in the CuOQD.
2003. "Polar Oxide Interface Stabilization by Formation of Metallic Nanocrystals." Physical Review Letters 90(21):216108. Abstract In-situ X ray photo-electron spectroscopy and ex-situ transmission electron microscopy and diffraction studies of a model Fe₃O₄(111)/MgO(111) polar oxide interface exclude stabilization by interface faceting, and uncover stabilization by dominant formation of metallic Fe(110) nanocrystals. The iron nanocrystals nucleate both at the interface and with in the magnetite film and grow in a Nishiyama-Wasserman orientation relationship with a bimodal size distribution related to twinning. Minority magnetite nanocrystals were also observed, growing in the less polar (100) orientation than the magnetite (111) film.
2003. "New Possibilities for Ferromagnetic Semiconductors." MRS Bulletin 28(10):729-733. Abstract New candidate ferromagnetic semiconductors have recently been grown by doping semiconducting transition-metal oxides with magnetic impurities. Some of these exhibit ferromagnetism at and above room temperature. The critical question is why? In many cases, complex solid-state chemistry is involved in the synthesis and drives the resulting properties. The observation of room-temperature ferromagnetism in these materials must be accompanied by a careful identification of the phases and structures present in order to accurately identify the origin of the magnetism.
2003. "New Possibilities for Ferromagnetic Semiconductors." MRS Bulletin 28(10):729-733. Abstract Some new candidate ferromagnetic semiconductors have recently been discovered by doping semiconducting transition metal oxides with magnetic impurities. Some of these exhibit ferromagnetism at and above room temperature. The critical question is why? In many cases, complex solid state chemistry is involved in the synthesis and drives the resulting properties. The observation of room temperature ferromagnetism in these materials must be accompanied by a careful identification of phases and structures present in order to accurately identify the origin of the magnetism.
2003. "New Materials for Spintronics." MRS Bulletin 28(10):706-708. Abstract One of the critical materials needs for the development of spin electronics is diluted magnetic semiconductors (DMS) which retain their ferromagnetism at and above room temperature.¹ Spin polarization in DMS materials leads to the possibility of spin-polarized current injection into nonmagnetic semiconductor heterostructures. Such transport is of critical importance in the development of devices that utilize spin (e.g. spin-LEDs and spin-FETs). New magnetically-doped semiconducting oxides that show promise because of Curie points which exceed room temperature are currently being investigated in our lab and elsewhere. However, the detailed materials properties and mechanism(s) of magnetism in these systems have been elusive. In this talk, I will present recent results from our laboratory focused on the MBE synthesis and properties of these ferromagnetic oxide semiconductors. This work was funded by the PNNL Nanoscience and Technology Initiative, the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Materials Science and Engineering Physics, and the DARPA Spins in Semiconductors (SPINS) Initiative.
2003. "Local Co Structure in Epitaxial Cox Ti₁-xO₂-x Anatase." Physical Review. B, Condensed Matter and Materials Physics 67(10):22-25, art. no. 100401. Abstract The quest for diluted magnetic semiconductors (DMS) which retain their magnetism at and above room temperature is spanning several classes of materials. Such materials are critically important in the development of spintronics as spin injectors for semiconductor heterostructures that can operate without cryogenic cooling. Group IV, III-V, and II-VI DMS materials typically exhibit Curie temperatures (Tc) well below ambient due to weak interaction of the magnetic impurities. Calculations based on the Zener model of magnetism suggest that the strongest interaction is that mediated by holes, and experimental studies carried out to date have borne out this prediciton. One notable exception is that of Mn-doped GaN, which grows n-type by gas-source molecular beam epitaxy under certain conditions, and appears to be ferromagnetic at room temperature. In addition, it has recently been shown that at least one oxide semiconductor - Co-doped TiO₂ anatase or CoxTi₁-xO₂-x - is ferromagnetic well above room temperature when doped n-type by oxygen vacancies for x < ~0.1, but the mechanism of magnetism remains unknown.
2003. "Clusters and Magnetism in Epitaxial Co-doped TiO₂ Anatase." Applied Physics Letters 82(8):1257-1259. Abstract We show that under certain conditions, highly Co-enriched Ti0₂ anatase clusters nucleate on epitaxial Ti0₂ anatase grown on LaA₁O₃(001) by oxygen plasma assisted molecular beam epitaxy. In the most extreme cases, virtually all incident Co segregates to the clusters, yielding a nanoscale ferromagnitic phase that is not ferromagnetic in homogeneous films of the same Co concentration. The nucleation of this phase simultaneous with continuous epitaxial film growth must be carefully monitored in order to avoid drawing false conclusions about the film structure
2002. "The Characteristics of Interface Misfit Dislocations for Epitaxial alpha-Fe₂O₃ on alpha-Al₂O₃(0001)." Thin Solid Films 414:31-38. Abstract Alpha-Fe₂O₃(0001) films of thickness equal to ~7 nm and ~70 nm were epitaxially grown on alpha-Al₂O₃(0001) by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interfaces were characterized using high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), and x-ray diffraction (XRD). The interface exhibited coherent regions separated by equally-spaced misfit dislocations. When imaged from the [2110] direction, the dislocation spacing is 7.0 +- 1.1 nm for the 70 nm thick speciman, and 7.2 +- 0.1 nm for the 7 nm thick speciman. When imaged from the [0110] direction, the disslocation spacing is 4.5 +- 0.1 nm for the 7 nm thick speciman. The experimentally observed dislocation spacings are appproximately consistent with those calculated from the lattice mismatch between alpha-Al₂O₃ and alpha-Fe₂O₃, implying that the lattice mismatch is accomodated mainly be interface misfit dislocations above the critical thickness, which is less than 7 nm. This conclusion is also corroborated by the measured reidual strainof ~0.5% determined from x-ray diffraction for the 70 nm film . EELS analysis reveals that the Fe L₂,3-edge shows no measurable chemical shift relative to the L₂,3-edge of structural Fe₃⁺, indicating complete oxidation of Fe in the as-grown film.
2002. "HRTEM Characterization of Interface Between Iso-Structural Thin Solid Film and Substrate." Microscopy & Microanalysis 8(Suppl S02):1160-1161. doi:10.1017/S1431927602107975 Abstract Alpha-Fe₂O₃ and c-CeO₂ thin films have been epitaxially grown on alpha-Al₂O₃ and yttria- stabilized c-ZrO₂ substrates, respectively, by oxygen plasma assisted molecular beam epitaxy (OPA-MBE). The interface structural features between the films and the substrates were characterized by high resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy (EELS), Rutherford backscattering spectrometry (RBS), and x-ray diffraction (XRD). For the two systems studied, the interfaces are similarly characterized by coherent regions that are separated by misfit dislocations periodically distributed along the interface. These results will be presented along with the results from molecular dynamics (MD) simulations of these interfaces.
2002. "Spin-resolved photoelectron spectroscopy of Fe₃O₄." Surface Science 513(3):L451-L457. Abstract The existence of a new class of magnetic materials displaying metallic character for one electron spin population and isulating character for the other was first populated by DeGroot et al. in 1983 based on theoretical band structure calculations of the ferromagnetic Heusler alloy NiMnSb. Since then such half metallic materials, which by definition possess 100% electron polarization at the Fermi energy, have attracted considerable theoretical, experimental, and technological interest as potential pure spin sources for use in spintronic devices. In addition to Heusler alloys (e.g. NiMnSb, PtMnSb), half metallic character has also been predicted to occur in a wide range of manganites (e.g. La₁-xCaxMnO₃, La₁-x-SrxMnO₃), metallic oxides (e.g. Fe₃O₄, CrO₂) and CMR systems. However, such predictions have proven to be extremely difficult to confirm experimentally. Possible reasons for this include the theoretical limitations arising from the complex crystallographic structure of many such materials and limitations in applying the single electron picture to materials where strong electron correlation may be present; this is compounded by experimental difficulties posed by their structural complexity and issues such as surface contamination, segregation, and reconstruction.
2002. "Growth and Structure of Epitaxial Ce₁-xZrxO₂ Thin Films on Yttria-Stabilized Zirconia (111)." Journal of Electron Spectroscopy and Related Phenomena 126(1-3):177-190. Abstract We describe here studies aimed at the identification of optimum parameters for the epitaxial growth of the mixed-oxides films, Ce₁ xZrxO₂ with x = 0.1, 0.2 and 0.3, by oxygen-plasma-assisted MBE on single crystal Y-stabilized ZrO₂ (YSZ) substrates. The resulting films were characterized by RHEED, LEED, XPS/XPD, XRD, and RBS/C in order to determine their bulk and surface structures and compositions. Pure-phase, epitaxial Ce₁ xZrxO₂ films readily grew on YSZ(111) without showing any contamination of yttria from the substrate. The resulting epitaxial film surfaces are unreconstructed and exhibit the structure of bulk CeO2(111). XPS data indicate that both Ce and Zr cations are formally in the ⁺⁴ oxidation state for all films prepared here. Small differences in the photoemission results for Zr-doped ceria films as compared to those obtained for pure ZrO₂ may be explained by changes in electronic structure when Zr is added to ceria that, in turn, results from longer Zr O bond distances in the mixed oxides. The minimum yields obtained from the random and channeling spectra of these films also provide evidence that high quality single crystal CeO₂ and Ce₀․₇Zr₀․₃O₂ materials were grown. For the Zr-doped films, Zr atoms are shown to occupy the lattice sites of Ce in the bulk structure of CeO₂ (111). Indeed, based on minimum yield values, the fraction of Zr substitution for Ce cations in the film was estimated to be 88%.
2002. "Molecular Beam Epitaxial Growth and Properties of CoFe₂O₄ on MgO(001)." Journal of Magnetism and Magnetic Materials 246(1-2):124-139. Abstract We have grown single-crystal Co ferrite (CoFe₂O₄) on MgO(001) by oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE), and have characterized the composition, structure, surface morphology, and magnetic properties by a number of methods The as-grown OPA-MBE material forms a perfect inverse spinel, in which all Co is in the ⁺² formal oxidation state, and occupies octahedral sites within the cation sublattice. The OPA-MBE film surfaces are very flat, with mean roughnesses of only a few A, and exhibit large, stable magnetic domains. The measured moment per unit volume is 250 emu/cm3, and the saturation magnetization for films in the 1000A film thickness range is ~60% of that of bulk Co ferrite. The material also exhibits strain-dependent magnetic anisotropy that can be understood by considering the various contributions to the total magnetic energy. The overall quality of epitaxial Co ferrite grown on MgO by OPA-MBE is superior in every respect to that obtained using pulsed laser deposition as the growth method.
2002. "Laminar Growth of Ultrathin Metal Films on Metal Oxides: Co on Hydroxylated alpha-Al₂O₃(0001)." Science 297(5582):827-831. Abstract We show that monolayer quantities of Co metal, deposited in ultrahigh vacuum, wet the alpha-Al₂O₃(0001) surface as prepared by oxygen plasma cleaning at room temperature. While this cleaning procedure removes adventitious carbon, it does not remove the monolayer of hydroxyls resulting from dissociative chemisorption of water vapor accompanying air exposure. O 1s core-level spectra taken at grazing emission establish the presence of the OH monolayer, which is manifested in the presence of a peak ~1.7eV to higher binding energy relative to the lattice oxygen peak. After 0.3 ML Co deposition at 300K, the OH O 1s peak intensity is greatly reduced. After interaction with the surface, ~2/3 of the Co in this dose is metallic, while the remainder is Co(II). A 0.8 ML Co deposition results in a larger metallic to ionic Co ratio. First-principles density functional theory has been used to investigate several possible surface reactions that give rise to wetting. All that emit water are endothermic. However, those that emit H₂ are exothermic. Initial Co adatoms promote the removal of two H atoms per Co, and each donates two electrons to the remaining O- ions as cation sites are occupied. Exothermicity is great for the isolated process (i.e. a single Co reacting with the surface). However, the presence of residual OH and metallic Co following 0.3 ML deposition suggests metallic Co must also be present to catalyze the reaction. Metallic Co adataoms exhibit strong d-d binding to oxidized Co(II), thus producing wetting.
2002. "Epitaxial Growth and Properties of MBE Grown Ferromagnetic Co-doped TiO₂ Anatase Films on SrTiO₃(001) and LaAlO₃(001)." Thin Solid Films 418(2):197-210. Abstract We have investigated the heteroepitaxial growth and materials properties of pure and Co-doped TiO₂ anatase on SrTiO₃(001) and LaAlO₃(001), grown by oxygen plasma assisted molecular beam epitaxy. This material is a promising new diluted magnetic semiconductor that shows large magnetization and a Curie temperature well above room temperature. We have found that epitaxial films with the highest crystalline quality and most uniform distribution of Co result when a rather slow growth rate (~0.01 nm/sec) is used over a substrate temperature range of 550 degrees Celcius to 600 degrees celcius. These conditions result in layer-by-layer growth of single-crystal films, and a very low density of extremely small nanocrystalline inclusions. In contrast, growth at a higher rate (~0.04 nm/sec) leads to extensive formation of secondary phase rutile nanocrystals to which Co diffuses and segregates. The rutile nanocrystals nucleate on the evolving anatase film surface in such a way that lattice strain between the two phases is minimized. Co appears to substitute for Ti in the lattice and exhibits a ⁺² formal oxidation state. Both pure and Co-doped films are typically n-type semiconductors despite the lack of intentional n-type doping, although a wide range of conductivities is observed.
2002. "A Potential Role In Spintronics." Materials Today 5(4):34-39. Abstract The essential aim of spin electronics, or spintronics, is to use the spin of quantum mechanical particles to carry signals and process information. Conventional electronics technology relies on the charges of electrons and holes for this purpose. Signals consist of voltage pulses, each of which is a bundle of charged carriers. Furthermore, conventional digital electronics is classical in nature in that bits are defined in terms of such discrete charge pulses. Here, a "1" bit may be represented by a negative voltage pulse whereas a "0" bit would be the absence of such a pulse. Devices continue to diminish in size in order to achieve higher speeds. As this shrinkage occurs, design parameters are impacted in such a way that the materials in current use are pushed to their limits. As an example from the digital arena, there is Moore's law, which states that the logic density, or amount of storable information per unit area in silicon based integrated circuits doubles every eighteen months. Attempting to keep up with Moore's law has led to the International Technology Roadmap for Semiconductors. This roadmap is aimed specifically at predicting, among other things, how thin the gate oxide must be in order to keep pace with the rate at which lateral dimensions are being reduced in complementary metal oxide semiconductor (CMOS) devices. For a given dielectric constant, the gate thickness must scale with the cross-sectional gate area in order to keep the capacitance constant. As a result of this trend, the main-stay gate oxide of the silicon industry, SiO2, will reach a fundamental limit within a few years because of unacceptably high gate leakage currents that result from the use of SiO2 films of thickness less than ~1.5 nm. Gate leakage results in power dissipation at times other than those when the transistor is changing state, which defeats one of the purposes of CMOS.
2002. "A Potential Role in Spintronics." Materials Today 5(4):34-39. Abstract The essential aim of spin electronics, or spintronics, is to use the spin of quantum mechanical particles to carry signals and process information. Conventional electronics technology relies on the charges of electrons and holes for this purpose. Signals consist of voltage pulses, each of which is a bundle of charged carriers. Furthermore, conventional digital electronics is classical in nature in that bits are defined in terms of such discrete charge pulses. Here, a '1' bit may be represented by a negative voltage pulse whereas a '0' bit would be the absence of such a pulse.
2002. "Growth of MoO3 films by oxygen plasma assisted molecular beam epitaxy." Thin Solid Films 414(2):205-215. Abstract The growth of MoO₃ films on SrLaAlO₄(0 0 1), a substrate lattice-matched to b-MoO , by oxygen plasma assisted molecular beam epitaxy was characterized using reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy, Xray diffraction (XRD), and atomic force and scanning tunneling microscopies (AFM and STM).It was found that the flux of reactive oxygen species to the surface was not high enough to maintain the proper stoichiometry, even at the lowest measurable deposition rates. Therefore, the films were grown by depositing Mo in small increments and then allowing the Mo to oxidize. At 675 K, the films grew epitaxially but in a three-dimensional manner. XRD of films grown under these conditions revealed atetragonal structure that has not been previously observed in bulk MoO₃ samples.
2001. "Structure of thin epitaxial oxide films and their surfaces ." Chapter 7 in The Chemical Physics of Solid Surfaces, vol. 9, Oxide Surfaces, ed. D.P. Woodruff, pp. 301-323. Elsevier, New York, NY. Abstract Metal oxides possess many properties that make them unique and potentially important for a range of technologies. No other class of materials exhibits such a wide range of behavior: band gaps spanning the visible and UV; electronic properties ranging from superconducting to metallic to semiconducting to insulating; magnetic properties ranging from ferromagnetic to antiferromagnetic; and dielectric properties ranging from low-k to ferroelectric and piezoelectric. The next century will require new materials systems encompassing these properties, often in artificially-structured combinations, and fabricated with a high degree of control. For example, it is clear that UV-based optoelectronics, magnetic tunnel junction and ferroelectric nonvolatile memory, spin quantum devices, and highly effective photocatalytic materials will be of great importance in information, medical, and environmental applications. Metal oxides and their associated surfaces will play important roles in many of these functions. The rich chemical, electronic, optical, and magnetic behavior of metal oxides, along with superior thermal stability, uniquely suit these materials for a number of applications. The richness in properties of metal oxides is derived from the inherent chemical and physical complexities of these materials. These complexities include multiple valences, complex phase diagrams, and vastly different properties among isostructural oxides.
2001. "Surface Structures of Anatase TiO2(001): Reconstruction, Atomic Steps, and Domains." Physical Review. B, Condensed Matter 6323(23):5402. Abstract The surface structure of anatase TiO2(001) was investigated using scanning tunneling mircroscopy (STM), x-ray photoelectron spectroscopy (XPS), reflection high energy electron diffraction (RHEED), and low energy electron diffraction (LEED). A two-domain (1x4)/(4x1) reconstruction, similar to those previously reported in LEED and ion scattering studies, was observed by STM and RHEED. This reconstruction was found to be stable not only from room temperature up to 850 degrees Celcius in ultra-high vacuum and oxygen rich environments, but also during the anatase film growth. High-resolutions STM images obtianed at positive sample biases revealed two types of atomic rows within each surface unit cell, indicating different Ti-derived states at the surface. In contrast, XPS of the reconstructed surfaces showed no evidence of Ti3+ states. Based on the STM, XPS, RHEED, and LEED results, an "added" -and-"missing" -row model is proposed to account for the (1x4) reconstruction. Atomic steps and their relationship to the population of (1x4) and (4x1) domains were also investigated. Results showed that for vicinal surfaces, the domain population depended strongly on the overall surface step orientation. While populations of the (1x4) and the (4x1) domains were nearly equal on the flat (001) surfaces, they became signigicantly lopsided on the surface 2 degrees away from the (001) direction, demonstrating a strong correlation between surface steps and domain population on vicinal surfaces.
2001. "Surface Structure of Anatase TiO₂(001): Reconstruction, Atomic Steps, and Domains." Physical Review. B, Condensed Matter and Materials Physics 63(23):235402-235409. Abstract The surface structure of anatase TiO₂(001) was investigated using scanning tunneling microscopy (STM), x-ray photoelectron spectroscopy (XPS), reflection high energy electron diffraction (RHEED), and low energy electron diffraction (LEED). A two-domain (1x4)/(4x1) reconstruction, similar to those previously reported in LEED and ion scattering studies, was observed by STM and RHEED. This reconstruction was found to be stable not only from room temperature to 850 °C in ultra-high vacuum and oxygen rich environments, but also during the anatase film growth. High-resolution STM images obtained at positive sample biases revealed two types of atomic rows within each surface unit cell, indicating different Ti-derived states at the surface. At the same time, XPS of the reconstructed surfaces showed no evidence of Ti³⁺. Based on the STM, XPS, RHEED, and LEED results, an “added”-and-“missing”-row model is proposed to account for the (1x4) reconstruction. Atomic steps and their relationship to the population of (1x4) and (4x1) domains were also investigated. Results showed that for vicinal surfaces, the domain population depended strongly on the overall surface step orientation. While populations of the (1x4) and the (4x1) domains were nearly equal on the flat (001) surfaces, they became significantly lopsided on the surface with its normal 2° away from the (001) direction, demonstrating a strong correlation between surface steps and domain population on vicinal surfaces.
2001. "Surface Sensitive Fe 2p Photoemission Spectra for alpha-Fe₂O₃ (0001) - The Influence of Symmetry and Crystal-Field Strength." Physical Review. B, Condensed Matter 64(20):205414-205420. Abstract We have measured high-energy-resolution Fe 2p photoelectron spectra for ⍺-Fe₂O₃/⍺-Al₂O₃(0001) grown by oxygen-plasma-assisted molecular beam epitaxy at emission angles chosen to enhance bulk and surface contributions. We quantitatively reproduce the subtle lineshape differences in these spectra by taking into account the break in symmetry and change in crystal field splitting generated by the presence of the surface.
2001. "The Role of Oxide Ionicity in Electronic Screening at Oxide/Metal Interfaces." Physical Review. B, Condensed Matter 64:075410-1 - 075410-6. Abstract Metal and oxygen core-level Auger and photoemission spectra have been measured from ultra-thin epitaxial films of Cr₂O₃ and Fe₂O₃ on Pt(111). The binding energy shifts of monolayer films relative to thicker films reveal a much smaller extent of screening by the metal substrate than that reported earlier for MgO/Ag(001). This finding suggests that the higher degree of bond covalency in Cr₂O₃ and Fe₂O₃ relative to MgO results in more effective intrinsic screening of charge transfer processes, rendering the metal much less effective for extrinsic screening. As a result, the metal is expected to have a negligible effect on the optical, electronic and magnetic properties of the oxide.
2001. "Epitaxial Growth and Properties of Ferromagnetic Co-doped TiO2 Anatase." Applied Physics Letters 79(21):3467-3469. Abstract We have used oxygen-plasma-assisted molecular beam epitaxy (OPA-MBE) to grow CoxTi1-xO2 anatase on SrTiO3(001) for x= ~0.01-0.10, and have measured the structural, compositional, and magnetic properties of the resulting films. Whether epitaxial or disordered, these CoxTi1-xO2 films are ferromagnetic semiconductors at and above room temperature. However, the magnetic and structural properties depend critically on the Co distribution, which varies widely with growth conditions. Co is substitutional in the anatase lattice and in the +2 formal oxidation state in ferromagnetic CoxTi1-xO2. The magnetic properties of OPA-MBE grown material are significantly better than those of analogous PLD-grown material.
2001. "Band Offset and Structure of SrTiO3/Si(001) Heterojunctions." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 19(3):934-939. Abstract We have measured the band offsets and materials properties of epitaxial SrTiO3/Si(001) heterojunctions for both n- and p-substrates, with and without an interfacial SiO2 layer. The through-air transfer from the growth chamber to the photoemission system results in significant surface hydroxylation and roughening, although the SrTiO3-Si interface is undisturbed. Surface hydroxylation notwithstanding, the structural quality of 20A thick epitaxial SrTiO3 on Si(001) is comparable to that of bulk SrTiO2(001). We find valence and conduction band offsets of ~2.1eV and ~0.0 eV, respectively, independent of doping type and the presence of SiO2. These results are consistent with theoretical band offset predictions based on the electron affinity rule, modified by the presence of an interface dipole. While measured leakage currents through SrTiO3/SiO2/Si(001) heterostructures are significantly reduced relative to control SiO2/Si(001) specimens, the band offset measurements suggest that the related leakage current is not from an additional conduction band barrier.
2000. "Interaction of Aqueous Chromium Ions with Iron Oxide Surfaces." Chapter 14 in Nuclear Site Remediation: First Accomplishments of the Environmental Management Science Program. ACS Symposium Series, vol. 778, ed. Eller, P. Gary; Heineman, William R., pp. 212-246. American Chemical Society, Washington DC. Abstract To gain a more fundamental understanding of abiotic processes controlling reduction reactions of aqueous chromate and dichromate ions (Cr(VI)aq) in subsurface environments, we carried out molecular-level experimental and modeling studies of the interaction of water and Cr(VI)aq with well-characterized single crystal samples of synthetic and natural hematite and magnetite. A reductionist approach was adopted in which simplified model systems of increasing complexity were studied. Photoemission spectroscopy (PES), photo-electron diffraction, and vacuum STM were used to characterize the composition, atomic structure, and morpho-logy of clean surfaces of ⍺-Fe₂O₃(0001) and Fe₃O₄(100) grown by molecular beam epitaxy on single crystal substrates of ⍺-Al₂O₃(0001) and MgO(100), respectively.
2000. "Ion Beam Analysis of Interface Reactions in Magnetite and Maghemite Thin Films." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 161-163:510-514. Abstract We have investigated inteface reactions between expitaxilly-grown magnetite (Fe₃O₄) and maghemite (gamma-Fe₂O₃) films with MgO substrates using Rutherford backscattering (RBS), channeling, and x-ray diffraction(XRD). Annealing these films in 2.0x10 -6 Torr of oxygen at temperatures up to 970 K enhances Mg outdiffusion into the films and increases the film thickness depending on temperature. The magnetite film thickness reach a limiting value at 870 K anneal while the maghemite film thickness did not maximize after annealing at 970 K. After the annealing at 970 K, both films produced a compound with composition close to magnesioferrite (MgFe₂O₄). XRD results reveal the formatioin of magnesioferrite (MgFe₂O₄) films after annealing both films at 970 in oxygen.
2000. "Reaction of Water with the (100) and (111) Surfaces of Fe3O4." Surface Science 453(1-3):32-46. doi:10.1016/S0039-6028(00)00305-8 Abstract We have examined changes in the electronic structure of magnetite (100) and (111) surfaces after reaction with water vapor (p(H2O) ranging from 10-9 to 9 Torr) and liquid water at 298 K using chemical shifts in the O 1a core-level photoelectron spectra obtained with a synchrotron radiation source. The surfaces were prepared in ultra high vacuum from natural magnetite crystals. We found that the vapor pressure, p(H2O), at which first reacts with magnetite is similar for the two surfaces (<10-5 Torr for 3 min. exposures, corresponding to doses of < 1.8 x 103 langmuirs) and is consistent with a small sticking coefficient. This reaction is manifested in the O 1s spectra by the growth of a shoulder at about 1.5 eV lower kinetic energy than the main lattice oxygen feature. We attribute this new feature to hydroxyl groups resulting from dissociative chemisorption of water on the magnetite surfaces, initially on defect sites. The p(H2O) for the onset of an extensive hydroxylatoin reaction is = 10-3 Torr (3 min. dose or = 1.8 x 105 Langmuirs). Magnetite (100) and (111) surfaces exposed to higher p(H2O) react more extensively, with hydroxylation extending several layers (=8 A) deep into the bulk. A comparison of O KVV Auger K-edge adsorption spectra of water vapor-exposed magnetite (100) and (111) surfaces with the corresponding total yeild spectra of goethite (FeOOH), limonite (FeOOH . nH2O), and hematite (Fe2O3) clearly shows that the reaction product on the magnetite surfaces is not goethite, limonite, or hematite. In addition, similarity of the Fe L3M23M23 Auger yield L-edge absorption spectra before and after exposure of the magnetite (111) surface to liquid water indicates that the oxidation state of iron is unchanged. These annealing experiments indicate that once the magnetite surface is hydroxylated, it is extremely difficult to thermally clean without Ar+ sputtering.
2000. "HREELS, TPD and XPS Study of the Interaction of Water with the Alpha-Cr₂O₃(001) Surface." Surface Science 449(1-3):135-150. Abstract The interaction of water with the (001) surface of alpha-Cr₂O₃ was examined with TPD, HREELS and XPS. Two alpha-Cr₂O₃(001) surfaces were examined, both of which were grown on alpha-Al₂O₃(001) substrates using oxygen-plasma assisted MBE. The two surfaces differed in that one was grown with alpha-Fe₂o3 interlayers whereas the other was grown directly on alpha-Al₂O₃(001). The in-plane lattice spacing of the alpha-Cr₂O₃(001) surface on alpha-Fe₂O3/alpha-Al₂O₃(001) was 2% expansively strained and unstrained alpha-Cr₂O₃(001) surface grown directly on alpha-Al₂O₃(001). Both the strained and unstained surfaces exhibited similar water TPD behavior with the possible exception that the desorption states of water on the strained surface were shifted slightly to lower temperatures relative to those on the unstained surface. Water adsorbs on alpha-Cr₂O₃(001) in both molecular and dissociative states, with the former desorbing in TPD at 295 K and the latter at 345 K. TPD uptake measurements and XPS data suggest that each surface Cr₃+ atom has the capacity to bind two water molecules, one in a molecular state and one in a dissociative state. Water in the dissociative state is comprised of two distinct OH groups based on HREELS, one of which is a terminal group with a v(OH) mode at 3600 cm -1 and the other of which is presumably bridging group with a v(OH) mode at 2885 cm-1. These losses shift to 2645 and 2120 cm-1 with D₂O adsorption. The low loss energy for the bridging OH/OD group indicates it involvement in a very strong hydrogen-bonded interaction with another species, presumably the oxygen atom of the terminal OH group. This pairing behavior is likely responsible for the first-order desorption kinetics observed for the recombinative desorption state at 345 K. The hydrogen-bonding interaction is unusually strong, as exemplified by the very low v(OH) stretch energy for the bridging OH group.
2000. "Surface Structure of MBE-Grown Fe3O4(001) by X-Ray Photoelectron Diffraction and Scanning Tunneling Microscopy." Surface Science 450(2000):L273-L279. Abstract We have investigated the surface termination and interlayer relaxations of Fe3O4(001) grown on MgO(001) by oxygen-plasma-assisted molecular beam epitaxy. Despite the fact that autocompensated surfaces can be constructed in principle by terminating with either a half layer of tetrahedral Fe, or a modified layer of octahedral Fe plus tetrahedral O, the combination of photoelectron diffraction and scanning tunneling microscopy suggest the former. The first four interlayer spacings are relaxed by-14%, -57%, -19%, and +29% of the respective bulk value. PACS numbers: 61.10Lx, 68.55.Bd.
2000. "Noncommutative Band Offset At Alpha-Cr2O3/Alpha-Fe2O3 (0001) Heterojunctions." Physical Review. B, Condensed Matter 61(19):13223-13229. Abstract We have measured the valence band discontinuity at artificially structured, epitaxial heterojunctions of alpha-Cr2O3(0001) and alpha-Fe2O3(0001). Layered film structures of these two materials maintain the in-plane lattice parameter of alpha-FeO3(0001). Thus, the alpha-Cr2O3(0001) layers are under a 2.3% tensile stress. The valence band offsets are 0.3+-0.1 eV and 0.7+-0.1 eV when the top layer is Fe2O3 and Cr2O3, repectively. The noncommumtativity in band offset appears to be due to a growth-sequence-dependent interface dipole.
2000. "Epitaxial Growth and Properties of Thin Film Oxides." Surface Science Reports 39(5-6):105-180. Abstract Oxide film growth by molecular beam epitaxy for the purpose of preparing metal oxides for surface science studies is discussed and reviewed. Critical issues such as the choice of oxidizing gas, the selection of substrates, crystal symmetry and lattice match, interface chemistry, and the relationship of these issues to the materials science of oxide-on-metal and oxide-on-oxide film growth are discussed. Recent work on the growth of select, representative oxides that span the ranges of crystal structure, metal oxidation state, and stoichiometry is reviewed and used to illustrate the basic materials science.
2000. "Band Discontinuities at Epitaxial SrTiO3/Si(001)Heterojunctions." Applied Physics Letters 77(11):1662-1664. Abstract We have used photoemission methods to directly measure the valence and conduction band offsets at SrTiO3/Si(001) interfaces, as prepared by molecular beam epitaxy. Within experimental error, the measured values are the same for growth on n- and p-Si, with the entire band discontinuity occurring at the valence band edge. In addition, band bending is much larger at the p-Si heterojunction than at the n-type heterojunction. Previously published threshold voltage behavior for these interfaces can now be understood in light of the present results.
1999. "Morphological and Structural Investigation of the Early Stages of Epitaxial Growth of Alpha-Fe2O3 (0001) on Alpha-Al2O3 (0001) by Oxygen-Plasma-Assisted MBE." Surface Science 443(3):212-220. Abstract We have investigated the early stages of Alpha-Fe2O3 (0001) film growth on Alpha-Al2O3 (0001) using oxygen-plasma-enhanced molecular beam epitaxy along with reflection high-energy electron diffraction, noncontact atomic force microscopy, and x-ray photoelectron spectroscopy and diffraction. A compressionally-strained, fully-stoichiometric Alpha-Fe2O3 film three monolayers thick forms prior to the onset of three-dimensional island formation and lattice relaxation. The surface of this film appears to buckle along <1120>, giving rise to a new set of inwardly-contracted diffraction spots which, if not resolved from the substrate spots, could be interpreted as a 12% in-plane lattice parameter expansion. Such an interpretation has led prior investigator to conclude that the interfacial layer consists of a disordered cation layer with an in-plane lattice parameter ~6% larger than that of Alpha-Fe2O3 [4]. Our interpretation of the diffraction data suggests that the interfacial layer is badly distorted, but commensurate with the substrate.
1999. "Surface Structure of MBE-Grown alpha-Fe2O3(0001) by Intermediate-Energy X-ray Photoelectron Diffraction." Surface Science 425(2-3):276-286. Abstract Uses intermediate-energy x-ray photoelectron diffraction to determine the surface structure of epitaxial alpha-Fe2O3(0001) grown on alpha-Al2O3(0001) and compares the experiment with quantum mechanical scattering theory. The results agree well with the predictions of molecular mechanics and spin-density functional theory previously reported in the literature for the Fe-terminated surface.
1999. "Rutherford Backscattering and Channeling Studies of Mg and Fe Diffusion at the Interface of gamma-Fe₂O₃(001)/MgO(001)." Surface and Interface Analysis 27(4):194-198. Abstract Investigates the crystalline quality of an epitaxially grown gamma-Fe₂O₃(001) film on Mg/O(001) substrate along with the Mg and Fe inter-diffusion using Rutherford Backscattering and channeling experiments.
1999. " Model Catalyst Studies with Single Crystals and Epitaxial Thin Oxide Films." Catalysis Today 51(3-4):513-519. Abstract The paper describes recent results from our relatively new program to perform detailed studies of the catalytic properites of metal-oxide materials; in particular, to effect a determination of the active catalytic site(s) and the mechanism for reactions over this especially important class of heterogeneous catalysts. Issues of structure-sensitivity, poisioning and promotion, and competing reaction mechanisms are critical questions that need to be addressed in a detailed manner for catalysis by oxides. As just one important example, both surface (Langmui-Hinhelwood) and direct (Eley-Rideal) reaction mechanism have been proposed for the selective catalytic reduction (SCR) reaction of nitrogen oxides (NOx) over vanadia/titania catalysts. For this program, we are using a number of unique, state-of-the-art capabilities available in the Environmental Molecular Sciences Laboratory (EMSL) at Pacific Northwest National Laboratory; for example, the first molecular beam epitaxy (MBE) system dedicated to the growth of model metal-oxide films, and a unique moderate-pressure catalytic reactor/surface science apparatus. We describe the growth, characterization, and water adsorption properties of a thin Fe3O4(001) film grown on a lattice-matched MgO(001) substrate. Because our moderate pressure catalysis studies are preliminary at this point, we instead describe our previous results on the CO oxidation reaction over a Ru(0001) model catalyst to demonstrate the utility of the experimental approach. We specifically discuss the possibility that this reaction occurs by an Eley-Rideal mechanism.
1999. "Growth and Structure of Epitaxial CeO₂ by Oxygen Plasma-Assisted Molecular Beam Epitaxy." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 17(3):926-935. Abstract Investigates the epitaxial growth of CeO₂ films on three different substrates Si(111), SrTiO₃(001) and MgO(001).
1999. "X-ray absorption and photoemission study of the adsorption of aqueous Cr(VI) on single crystal hematite and magnetite surfaces." Surface Science 424(2-3):219-231. Abstract Studies the speciation of chromium in overlayers on atomically clean surfaces of single crystal magnetite and hematite using Cr L-edge, Fe L-edge, and O K-edge x-ray absorption spectra collected with synchrotron radiation.
1999. "Interaction of D2O with CeO2(001) Investigated by Temperature Programmed Desorption and X-ray Photoelectron Spectroscopy." Langmuir 15(11):3993-3997. Abstract Studies the interaction of D2O with the CeO2(001). Finds that D2O desorption occurs in three states with temperature of 152, 200, and 275 K and defines them as multilayer D2O, weakly bound surface D2O, and hydroxyl recombination.
1999. "Epitaxial Growth and Characterization of Ce1-xZrxO2 Thin Films." Journal of Vacuum Science and Technology A--Vacuum, Surfaces and Films 17(3):961-969. Abstract Epitaxial films have been grown on SrTiO3(001) by oxygen-plasma-assisted molecular beam epitaxy. The film growth at 600 degrees C is predominantly nucleation and growth of 3-D islands. The films become much smoother after rapid thermal annealing at 700 degrees C for 30 seconds in the oxygen plasma. High-energy ion channeling reavelas that Zr atoms substitutionally incorporate at cation sites in the CeO2 lattice for all doping levels, leading to Ce1-xZrxO2 solid solutions. Analysis of Zr 3d and Ce 3d core-level binding energies shows that the oxidation state of both Zr and Ce is +4. Lattice distortion induced by incorporation of Zr in the CeO2 lattice beomes prevalent for high doping levels, and surfaces roughen accordingly.
1999. "Surface Termination, Composition and Reconstruction of Fe3O4(001) and gamma-Fe2O3(001)." Surface Science 420(2-3):111-122. Abstract Addresses a current controversy concerning the nature of the surfaces of Fe3O4(001) and gamma-Fe2O3(001) grown on MgO(001) by molecular beam epitaxy. Shows that gamma-Fe2O3(001) exhibits a (1x1) surface net and that the very-near-surface region is subsdtantially enriched with oxygen relative to the bulk.
1999. "Growth of beta-MnO2 Films on TiO2(110) by Oxygen-Plasma-Assisted Molecular Beam Epitaxy." Surface Science 420(2-3):123-133. Abstract Discusses the essential need to understand the heterogeneous chemistry of mineral surfaces at a molecular level for accurate modeling of surface complexion processes in natural environments. Describes the first MBE growth and characterization of ultrathin films of B-MnO2 on TiO2 (110).
1998. "Epitaxial Growth of fcc Ti Films on Al(001) Surfaces." Physical Review. B, Condensed Matter 56(15):9841-9847. Abstract Studies the growth of thin Ti films on Al(001) surfaces with high-energy ion scattering (HEIS), x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction (XPD). Observes that although there is a general similarity of fcc Ti growth on both Al(001) and Al(110), the submonolayer growth regime does show differences for the two surfaces.
1998. "Reaction of Water Vapor with alpha-Al203 and alpha-Fe203(0001) Surfaces: Synchrotron X-ray Photoemission Studies and Thermodynamic Calculations." Surface Science 417(1998):53-65. Abstract X-ray photoemission experiments were carried out to study the reaction of water vapor with clean (0001) surfaces of single-crystal corundum (alpha-Al2O3) and single-crystal, thin-film hematite (alpha-Fe2O3) at constant reaction time (3 min) as a function of water vapor pressure [p(H2O)] and, in separate experiments, at longer reactions times(=30 min, with total exposure ranging from 1.8 L to 1.8 x 10 (10) L at constant p(H2)). A two-stage reaction was observed for each oxide surface, with dissociative chemisorption of water occurring mainly at defect sites below threshold pressures of ~1 Torr and ~10 (-4) Torr for the corundum and hematite (0001) surfaces, respectively. Extensive hydroxylation of these surfaces was found to occur above the respective threshold pressures. Longer exposures to water vapor below these threshold pressures did not result in increased hydroxylation; however, longer exposures above these threshold pressures resulted in increased hydroxylation, as expected. The threshold pressure of alpha-Al2O3 was accurately predicted using a simple equilibrium thermodynamic model for the conversion of corundum to gibbsite (Al(OH)3). In contrast, the measured threshold pressure for hematite is more than five orders of magnitude lower than the predicted threshold pressure for the conversion of hematite to FeOOH or Fe(OH)3. This difference between observation and prediction of hematite (0001) is not completely understood, but may be due to the presence of a passivating layer of Fe-hydroxide that reduces the surface energy of the hydroxylated hematite (0001).