Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M44: Focus Session: Defects in Semiconductors: Oxides |
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Sponsoring Units: DMP FIAP Chair: Zhengyu Zhang, China University of Science and Technology Room: Mile High Ballroom 4C |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M44.00001: OH centers and the conductivity of hydrogen doped In$_{2}$O$_{3}$ single crystals Michael Stavola, Weikai Yin, Kirby Smithe, W. Beall Fowler, Philip Weiser, Lynn Boatner Mechanisms for the n-type conductivity of In$_2$O$_3$ have been controversial. Recent experiments suggest that O vacancies are the cause of conductivity.\footnote{S. Lee and D.C. Paine, Appl. Phys. Lett. \textbf{102}, 052101 (2013).} However, other recent experiments find that the H-doping of thin films gives rise to shallow donors.\footnote{T. Koida \textit{et al.}, Jpn. J. Appl. Phys. \textbf{46}, L685 (2007).} Theory also finds that interstitial H and H at an O vacancy are shallow donors in In$_{2}$O$_{3}$.\footnote{S. Limpijumnong \textit{et al.}, Phys. Rev. B \textbf{80}, 193202 (2009).} We have performed a series of IR absorption experiments to determine the properties of OH and OD centers in In$_{2}$O$_{3}$ single crystals. Annealing In$_{2}$O$_{3}$ samples in H$_{2}$ or D$_{2}$ at temperatures near 450$^{\circ}$C (30 min) produces an n-type layer $\approx $0.05 mm thick with an n-type doping of 2x10$^{9}$ cm$^{-3}$. The resulting free-carrier absorption is correlated with an OH center with a vibrational frequency of 3306 cm$^{-3}$ that we associate with interstitial H.\footnote{M. Stavola, J. Appl. Phys., to be published.} [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M44.00002: Hydrogen dynamics in indium oxide W.B. Fowler, M. Stavola, Weikai Yin It has been recognized that hydrogen is a shallow donor in several transparent oxides, including In$_{2}$O$_{3}$ [1]. Both interstitial H and H at an O vacancy have been suggested as candidates [2]. We have used the CRYSTAL06 code[3] with a hybridized DFT Hamiltonian to determine equilibrium positions and vibrational frequencies for both of these cases as well as for H at an In vacancy. While the bixbyite structure [4] of In$_{2}$O$_{3}$ has overall cubic symmetry, its remarkable internal asymmetries lead to a number of candidate locations for the H, each of which has different vibrational frequencies. This enables potential assignments of the experimental IR results obtained by Yin \textit{et al} [5]. Furthermore, the unique topology of In$_{2}$O$_{3}$ leads to constraints on possible H diffusion paths, which we have also investigated. \\[4pt] [1] P. D. C. King \textit{et al.}, Phys. Rev. B \textbf{80}, 081201(R) (2009).\\[0pt] [2] S. Limpijumnong \textit{et al.}, Phys. Rev. B \textbf{80}, 193202 (2009).\\[0pt] [3] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual }(University of Torino, Torino, 2006). \\[0pt] [4] M. Marezio, Acta Crystallogr. \textbf{20}, 723 (1966).\\[0pt] [5] W. Yin \textit{et al.}, this meeting; M. Stavola \textit{et al.}, J. Appl. Phys. (to be published). [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M44.00003: Structural Properties of Amorphous Indium-Based Oxides Rabi Khanal, Julia Medvedeva Amorphous transparent conducting and semiconducting oxides exhibit similar or even superior properties to those observed in their crystalline counterparts. To understand how the structural properties change upon amorphization and how chemical composition affects the local and long-range structure of the amorphous oxides, we employ first-principles molecular dynamics to generate amorphous In-X-O with X$=$Zn, Ga, Sn, Ge, Y, or Sc, and compare their local structure features to those obtained for amorphous and crystalline indium oxide. The results reveal that the short-range structure of the Metal-O polyhedra is generally preserved in the amorphous oxides; however, different metals (In and X) show quantitatively or qualitatively different behavior. Some of the metals retain their natural distances and/or coordination; while others allow for a highly distorted environment and thus favor ``defect'' formation under variable oxygen conditions. At the same time, we find that the presence of X increases both the average In-O coordination and the number of the 6-coordinated In atoms as compared to those in IO. The improved In coordination may be responsible for the observed reduction in the carrier concentration as the substitution level in In-X-O increases. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:27PM |
M44.00004: Bandgap narrowing of TiO$_{2}$ via codoping for enhanced photocatalytic reactions Invited Speaker: Bing Wang We investigated the growth of Cr$-$N codoped single-crystal anatase TiO$_{2}$(001) (A-TiO$_{2})$ thin films using pulsed laser deposition with a target of Cr$_{2}$O$_{3}$ and TiN mixture. N concentrations were finely tuned under different growth temperatures and oxygen pressures, and high quality films with atomically flat terraces were obtained. UV$-$Vis absorption measurements showed that the band-gap of the codoped A-TiO$_{2}$ film is significantly narrowed in comparison with the undoped and monoelement doped films. We further systematically investigated the structures and the activity of the oxidized and reduced (1 $\times$ 4) reconstructed surfaces of A-TiO$_{2}$ epitaxially grown on SrTiO$_{3}$ using scanning tunneling microscopy/spectroscopy, X-ray/ultraviolet photoemission spectroscopy and first-principles calculations. Quite unexpectedly, it is found that the perfect (1 $\times$ 4) surface of A-TiO$_{2}$ is not even active for H$_{2}$O and O$_{2}$ adsorption at room temperature. Two types of intrinsic point defects are identified, among which only the Ti$^{3+}$ defect site on the reduced surface demonstrates considerable activity for H$_{2}$O and O$_{2}$ adsorption. The perfect surface itself should be fully oxidized, but shows no obvious activity. We thus propose an oxidized ridge model for the reconstructed (1 $\times$ 4) surface, where the Ti atoms at the normal ridge sites are sixfold coordinated. The Ti-rich point defects on reduced surface are fourfold-coordinated. This model provides consistent explanations for our experimental observations We have compared the results with those from rutile TiO$_{2}$(001)-(1 $\times$ 1) surface in our investigations. Our findings suggest that the activity of the A-TiO$_{2}$ surface should depend on its reduction status, similar to that of rutile TiO$_{2}$ surfaces. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M44.00005: Hole polaron-polaron interaction in transition metal oxides and its limit to p-type doping Shiyou Chen, Lin-Wang Wang Traditionally the origin of the poor p-type conductivity in some transition metal oxides (TMOs) was attributed to the limited hole concentration: the charge-compensating donor defects, such as oxygen vacancies and cation interstitials, can form spontaneously as the Fermi energy shifts down to near the valence band maximum. Besides the thermodynamic limit to the hole concentration, the limit to the hole mobility can be another possible reason, e.g., the hole carrier can form self-trapped polarons with very low carrier mobility. Although isolated hole polarons had been found in some TMOs, the polaron-polaron interaction is not well-studied. Here we show that in TMOs such as TiO$_2$ and V$_2$O$_5$, the hole polarons prefer to bind with each other to form bipolarons, which are more stable than free hole carriers or separated polarons. This pushes the hole states upward into the conduction band and traps the holes. The rise of the Fermi energy suppresses the spontaneous formation of the charge-compensating donor defects, so the conventional mechanism becomes ineffective. Since it can happen in the impurity-free TMO lattices, independent of any extrinsic dopant, it acts as an intrinsic and general limit to the p-type conductivity in these TMOs. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M44.00006: Electronic and Magnetic Structure of C and N doped rutile-TiO$_{2}$: an \textit{ab-initio} DFT study Jacqueline Atanelov, Christoph Gruber, Peter Mohn We study the electronic and magnetic structure of carbon and nitrogen impurities and interstitials in rutile TiO${_2}$. To this end we perform \textit{ab-initio} calculations of a 48-atom supercell employing the VASP code. In order to obtain a realistic description of the size of the band gap, exchange and correlation are treated with functionals beyond ordinary LSDA. Both, atomic positions and cell dimensions are fully relaxed. Substitutional carbon and nitrogen are found to have a magnetic moment of 2 and 1$\mu{_B}$, respectively, with a tendency for anti-ferromagnetic long range order. For C/N on interstitial sites we find that carbon is non-magnetic while nitrogen always possesses a magnetic moment of 1$\mu{_B}$. We find that these interstitial positions are on a saddle point of the total energy. The stable configuration is reached when both carbon and nitrogen form a CO and NO dimer with a bond length close to the double bond for CO and NO. This result is in agreement with earlier experimental investigations detecting such NO entities from XPS measurements. For all configurations investigated both C and N states are found inside the TiO${_2}$ gap. These new electronic states are discussed with respect to tuning doped TiO${_2}$ for the application in photocatalysis. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M44.00007: A Coupling Mechanism between Co-doped Acceptors and its Control on Optical Absorption Edge of TiO2 Wei Li, Shihao Wei, Xiangmei Duan A hole-strain-mediated coupling between dopants in anatase TiO$_2$ is revealed by first-principles calculations. When the dopant complex on neighboring oxygen sites contains a large radius atom, and the doped system has at least one net hole, the dopants will strongly couple to form a pair through the local lattice strain induced by the large dopant. The coupling results in bandgap narrowing due to the appearance of the fully occupied mid-gap states, leading to a much more effective band gap reduction than that induced by mono-doping or conventional donor-acceptor codoping. The calculated absorption spectra show that acceptor-acceptor codopings could shift the absorption edge to the visible light region. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:15PM |
M44.00008: Synthesis conditions and electronic structures of heavily N-doped TiO$_{2}$ Yuta Aoki, Naoto Umezawa, Susumu Saito TiO$_{2}$ has drawn a lot of attention for its notable photocatalytic properties. Unfortunately, however, only a small portion of solar spectrum is utilized for photocatalytic activities of TiO$_{2}$ because of its wide band gap. To harvest solar energy more efficiently, TiO$_{2}$ must be sensitized under the irradiation of visible light which accounts for nearly 50 {\%} of solar light reaching ground surface. Although N-doped TiO$_{2}$ is a well-known visible-light driven photocatalyst, its photoabsorption cross section is still limited. In order to enhance visible-light absorption, high-concentration doping of N should be a promising solution. Here, we propose the synthesis conditions of heavily N-doped TiO$_{2}$ both for rutile and anatase structures based on the density-functional theory. We use supercell models with several different N concentrations to clarify the concentration dependence of the synthesis conditions. To discuss the synthesis conditions, we enforce a constraint to avoid the precipitation of other compounds, e.g. TiN, TiO, Ti$_{2}$O$_{3}$, TiO$_{2}$, during the synthesis of heavily N-doped TiO$_{2}$, which is described as a set of inequalities with respect to chemical potentials of N and O ($\mu _{\mathrm{N}}$ and $\mu_{\mathrm{O}})$ The results show that $\mu _{\mathrm{N}}$ must be larger than zero, which should be the upper limit for chemical potentials, in order for heavily N-doped TiO$_{2}$ to deposit stably. This means that high-concentration N doping is energetically difficult to be realized. Also, we will discuss the local arrangement of N atoms in connection with O vacancies and the electronic structures of examined models. [Preview Abstract] |
Wednesday, March 5, 2014 1:15PM - 1:27PM |
M44.00009: Bandgap Modulation of CeO2 Nanoparticles by Codoping of Y and Co Impurities T.S. Wu, C.H. Lai, H.T. Jeng, S.L. Chang, Y.L. Soo Interplay of trivalent and divalent dopants in (Y, Co) codoped CeO2 nanoparticles with dominantly tetravalent host cations has been investigated using a variety of structural and optical techniques. The nanoparticle samples were prepared by a Polyol method. As revealed by the x-ray diffraction (XRD) data, all nanocrystal samples under investigation have similar average particle size. The concentration of O vacancies in the samples was found to increase with Y doping level as indicated by the Raman spectroscopy, extended x-ray absorption fine structure (EXAFS), and x-ray absorption near edge structure (XANES) data. As determined from diffuse reflectance spectra, the bandgap of the sample appears to decrease with increasing Y concentration. However, a series of Co-free samples measured for comparison show no dependence of bandgap on Y concentration. We have proposed a theoretical model and performed numerical simulation using the Vienna ab initio simulation package (VASP) to explain such bandgap modulation effect. [Preview Abstract] |
Wednesday, March 5, 2014 1:27PM - 1:39PM |
M44.00010: Metastable defects are the origin of high conductivity in gallium doped zinc oxide Andriy Zakutayev, Nicola Perry, Thomas Mason, David Ginley, Stephan Lany Doping in wide-bandgap materials is often counteracted by formation of intrinsic compensating defects of opposite charge. One prototypical exception to this general rule is gallium doped zinc oxide (ZnO:Ga) used as transparent conductor in numerous applications. High conductivities (1,000-10,000 S/cm) in ZnO:Ga are typically achieved during the growth at 250 - 350C around 10\textasciicircum -8 atm. The corresponding electron concentration exceed by a factor of 100,000 the values expected in equilibrium from first principles calculations. In contrast at high temperature and high oxygen partial pressure conductivity of ZnO:Ga measured in-situ shows good agreement with the ab initio theoretical thermodynamic model. The results of this study indicate that degenerate levels of doping in ZnO:Ga transparent conductive oxide used in practical applications are enabled by non-equilibrium concentration of both extrinsic donors and compensating acceptors. [1] A. Zakutayev et al, Appl. Phys. Lett. [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M44.00011: Optical Properties of Aluminum Doped Zinc Oxide Thin Film Grown by Remote PEALD Xingye Wang, Manpuneet Kaur, Robert Nemanich Transparent conducting oxides (TCO) are applied in optoelectronic devices and solar cells due to their high transmittance and low resistivity. Al-doped zinc oxide (AZO) film have been considered as a promising alternative to ITO. In this research, we investigated the optical properties of ZnO and AZO thin films deposited by remote plasma-enhanced atomic layer deposition (PEALD) using dimethylaluminum (DMAI) and dimethylzinc (DMZ) precursors. The substrates were double side polished amorphous quartz cleaned with acetone, methanol and dried with UHP nitrogen. Remote PEALD was then employed to deposit $\sim$ 100 cycles of ZnO thin films or AZO thin films (with Al: Zn cycle ratios from 1:6 to 1:2). In situ XPS indicated Al:Zn atomic ratios that increased from 3.2{\%} to 15.9{\%}. Transmittances higher than 90{\%} were measured from $\sim$ 500nm to 800nm for a film thickness of $\sim$ 20nm. XRD showed the films were crystalline on the amorphous substrates. A blue shift in the transmittance cut off was observed, and the optical band gap increased from 3.1 eV to 3.6 eV with increasing aluminum concentration. The results indicate that PEALD-grown AZO thin films are potential candidates for applications in high transparency TCO-based devices. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M44.00012: Short-range order and its effects on electrons in (GaN)$_{(1-x)}$(ZnO)$_x$ alloys Jian Liu, Luana Pedroza, Carissa Misch, Maria Fernandez-Serra, Philip Allen Prior work by Li et al. gives ``cluster expansion" parameters for (GaN)$_{(1-x)}$(ZnO)$_x$ alloys. From these, by Monte-Carlo calculations, large representative unit cells can be generated at any chosen temperature. We choose mainly T=1200K, typical of the temperature at which experimental samples fall out of equilibrium. The atoms are distributed on the wurtzite anion and cation sublattices with significant short-range order. A periodic supercell with 432 atoms is chosen as a compromise between accurate self-averaging and fully self-consistent and relaxed density-functional (DFT) computation. Composition- and temperature-dependent short-range order (SRO) parameters of the alloys are discussed. Entropy is related to the SRO parameters. DFT relaxation finds significant bond-length alterations. Typical Zn-O distances are larger by 10$\%$ than Ga-N distances in the alloy, even though in pure ZnO and GaN, bond lengths are nearly equal. Electronic properties of the alloys, and in particular, the influences of short-range order and bond-length fluctuations, will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M44.00013: First-principles studies of n-type and p-type doping in hematite $\alpha$-Fe$_{2}$O$_{3}$ Qiming Zhang, Congxin Xia Spin-polarized density functional theory calculations are conducted to study the substitutional anionic doping in hematite $\alpha$-Fe$_{2}$O$_{3}$ crystal. Selective Group V and VII impurities, as p-type and n-type dopants, respectively, are investigated. The formation energies are lower under Fe-rich environment in general. The impurity levels are discussed both by the transition levels of the formation energies and the single-particle levels. And the local magnetic structure around an impurity is also analyzed. [Preview Abstract] |
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