EMSL: Scott Chambers's Publications

Chambers SA, T Ohsawa, CM Wang, I Lyubinetsky, and JE Jaffe. 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.

Chambers SA. 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.

Droubay T, TC Kaspar, BP Kaspar, and SA Chambers. 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.

Heald SM, TC Kaspar, T Droubay, V Shutthanandan, SA Chambers, A Mokhtari, AJ Behan, HJ Blythe, JR Neal, M Fox, and G Gehring. 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.

Ohsawa T, I Lyubinetsky, Y Du, MA Henderson, V Shutthanandan, and SA Chambers. 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.

Ohsawa T, I Lyubinetsky, Y Du, MA Henderson, V Shutthanandan, and SA Chambers. 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).

Chambers SA. 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.

Cheung SH, P Nachimuthu, MH Engelhard, CM Wang, and SA Chambers. 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.

Kaspar TC, T Droubay, SM Heald, MH Engelhard, P Nachimuthu, and SA Chambers. 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.

Kaspar TC, T Droubay, SM Heald, P Nachimuthu, CM Wang, V Shutthanandan, CA Johnson, DR Gamelin, and SA Chambers. 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.