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.
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.
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. "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.
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. "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. "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.
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. "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.
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. "The Role of O(1D) in the Oxidation of Si(100)." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 21(2):895-899. Abstract Oxidation of silicon with neutral atomic oxygen species generated in a rare gas plasma has recently been shown to produce high-quality thin oxides. It has been speculated that atomic oxygen in the first excited state, O(1D), is a dominant reactive species in the oxidation mechanism. In this study, we investigate the role of O(1D) in silicon oxidation in the absence of other oxidizing species. The O(1D) is generated by laser-induced photodissociation of N2O at 193 nm. We find that, at 400?C, O(1D) is effective in the initial stages of oxidation, but the oxide growth rate falls dramatically past 1.5 nm. Oxide films thicker than 2 nm were unachievable regardless of oxidation time or N2O partial pressure (0.5-90 mTorr), indicating O(1D) cannot be a dominant reactive species in thicker oxidation mechanisms. We suggest that quenching of O(1D) to O(3P) (ground state) during diffusion through thicker oxides results in drastically slower oxidation kinetics. In contrast, oxidation with a vacuum ultraviolet (VUV) excimer lamp operating at 172 nm resulted in oxide thicknesses up to 4 nm. Thus, other species produced in plasmas and excimer lamps, such as molecular and atomic ions, photons, and free and conduction band electrons, play a dominant role in the rapid oxidation mechanism of thicker oxides (> 2 nm).