Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P7: Dopants and Defects in Semiconductors: OxidesFocus
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Sponsoring Units: DMP FIAP Chair: Michael Stavola, Lehigh University Room: 303 |
Wednesday, March 16, 2016 2:30PM - 3:06PM |
P7.00001: Magnetic resonance studies of the Mg acceptor in thick free-standing and thin-film GaN Invited Speaker: Mary Ellen Zvanut Mg, the only effective p-type dopant for the nitrides, substitutes for Ga and forms an acceptor with a defect level of about 0.16 eV. The magnetic resonance of such a center should be highly anisotropic, yet early work employing both optically detected magnetic resonance (ODMR) and electron paramagnetic resonance (EPR) spectroscopies revealed a defect with a nearly isotropic g-tensor. The results were attributed to crystal fields caused by compensation and/or strain typical of the heteroepitaxially grown films. The theory was supported by observation of the expected highly anisotropic ODMR signature in homoepitaxially grown films in which dislocation-induced non-uniform strain and compensation are reduced. The talk will review EPR measurements of thin films and describe new work which takes advantage of the recently available thick free-standing GaN:Mg substrates grown by hydride vapor phase epitaxy (HVPE) and high nitrogen pressure solution growth (HNPS). Interestingly, the films and HVPE substrates exhibit characteristically different types of EPR signals, and no EPR response could be induced in the HNPS substrates, with or without illumination. In the heteroepitaxial films, a curious angular dependent line-shape is observed in addition to the nearly isotropic g-tensor characteristic of the Mg-related acceptor. On the other hand, the free-standing HVPE crystals reveal a clear signature of a highly anisotropic shallow acceptor center. Comparison with SIMS measurements implies a direct relation to the Mg impurity, and frequency-dependent EPR studies demonstrate the influence of the anisotropic crystal fields. Overall, the measurements of the thick free-standing crystals show that the Mg acceptor is strongly affected by the local environment. [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P7.00002: Unusual electronic phase transition in hydrogenated TiO2 thin layer Chunlei Yue, Jin Hu, Xue Liu, Zhiqiang Mao, Jiang Wei Hydrogenated TiO2 has been studied intensively in recently years for its effectiveness of engineering band gap by introducing the hydrogen doping level close to the conduction band edge. Consequently, significant improvement of solar absorption efficiency has been achieved and has been successfully showcased in the photovoltaic and photocatalytic applications. Although the room temperature optical enhancement is fascinating, the comprehensive electronic properties of such hydrogenated TiO2 have hardly been investigated. Here we report our electric transport measurement of hydrogenated TiO2 thin layer in the temperature range from 400K to 3.5K. We observed a stabilized metallic behavior of hydrogenated TiO2, which persists down to 50K, and then a surprising transition to an insulating phase between 50K and 20K. Furthermore, the insulating phase of hydrogenated TiO2 shows a photocurrent response up to 4 orders magnitude. We interpret the possible mechanism as the transition of O-H vibrational modes, which leads to the freezing of electrons donated by the intercalated hydrogen. [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P7.00003: \textbf{Dynamics of H}$_{\mathrm{\mathbf{i}}}^{\mathrm{\mathbf{+}}}$\textbf{ in Indium Oxide} W. B. Fowler, M. Stavola, Ying Qin, P. Weiser, Weikai Yin Studies of IR absorption[1] under uniaxial stress[2] and diffusion[3] of H$_{\mathrm{i}}^{\mathrm{+}}$ as well as the dynamics of positively charged muonium[4] in In$_{\mathrm{2}}$O$_{\mathrm{3}}$ provide an experimental framework to understand these processes in detail. While the bixbyite structure[5] of In$_{\mathrm{2}}$O$_{\mathrm{3}}$ has overall cubic symmetry, its remarkable internal asymmetries lead to a number of candidate locations for H$_{\mathrm{i}}^{\mathrm{+}}$. Furthermore, the unique topology of In$_{\mathrm{2}}$O$_{\mathrm{3}}$ leads to constraints on possible H diffusion paths. We have used the CRYSTAL06 code[6] with a hybridized DFT Hamiltonian to determine equilibrium positions and vibrational frequencies for possible sites for H$_{\mathrm{i}}^{\mathrm{+}}$ and have analyzed candidate diffusion paths and processes for H$_{\mathrm{i}}^{\mathrm{+}}$ and Mu$^{\mathrm{+}}$.[1] W. Yin \textit{et al.}, Phys. Rev. B \textbf{91}, 075208 (2015). [2] P Weiser \textit{et al.}, this meeting. [3] Ying Qin \textit{et al.}, this meeting. [4] B. B. Baker \textit{et al.}, AIP Conf. Proc. \textbf{1583}, 323 (2014). [5] M. Marezio, Acta Crystallogr. \textbf{20}, 723 (1966). [6] R. Dovesi \textit{et al.}, \textit{Crystal06 User's Manual }(University of Torino, Torino, 2006). [Preview Abstract] |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P7.00004: Gd-Doped BaSnO$_{3}$ Thin Films: High Mobility in a Magnetically-Doped Transparent Conducting Oxide Urusa Alaan, Padraic Shafer, Alpha N'Diaye, Elke Arenholz, Yuri Suzuki It has recently been shown that when the perovskite-structure BaSnO$_{3}$ (BSO) is doped with La$_{Ba}$$^{\prime}$, the result is a transparent conducting oxide with room-temperature electron mobilities that are much higher than conventional ternary oxides. The ability to achieve high carrier mobilities in BSO is promising for future perovskite-structure devices. We have used pulsed laser deposition to grow epitaxial thin films of Ba$_{0.96}$Gd$_{0.04}$SnO$_{3}$ (Gd:BSO) and Ba$_{0.96}$La$_{0.04}$SnO$_{3}$ (La:BSO) on (001) SrTiO$_{3}$ and (001) MgO substrates. At 300 K, Gd:BSO films have $\rho$$\sim$2 m$\Omega$$\cdot$cm, $\mu$$_{e}$$\sim$28 cm$^{2}$/V$\cdot$s and n$\sim$1.0 $\times$ 10$^{20}$cm$^{-3}$. At the same temperature, La:BSO films have $\rho$$\sim$0.4 m$\Omega$$\cdot$cm, $\mu$$_{e}$$\sim$58 cm$^{2}$/V$\cdot$s and n$\sim$2.5 $\times$ 10$^{20}$ cm$^{-3}$. While La:BSO is diamagnetic, Gd:BSO is paramagnetic with a clear magnetic response that saturates at $\sim$7$\mu$$_{B}$/Gd$^{3+}$, and a negative ordinary magnetoresistance at low temperatures. Like La:BSO, Gd:BSO is transparent and colorless in the visible regime. Thus, we have shown that Gd is good dopant for BSO in order to achieve transparency and metallicity that is coincident with a magnetic response. [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 3:54PM |
P7.00005: Links in Superconducting, ferroelectric {\&} antiferrodistortive instabilities in pristine {\&} ion implanted SrTiO$_{\mathrm{3}}$~by studying JT lattice distortions, Cubic-Tetragonal phase transition, incoherent lattice fluctuations~{\&} Phonons using RBS-Ion Channeling, XPS {\&} Raman Scattering. Kalyan Sasmal, Viktor Hadjiev, Quark Chen, Wei-Kan Chu Perovskite SrTiO$_{\mathrm{3}}$ is quantum paraelectric.Cubic-tetragonal phase transition at 105K,driven by condensation of zone corner phonon involving rotation of oxygen octahedra.Jahn-Teller centers allows dynamical charge transfer {\&} polaron-bipolaron formation.Dynamical covalency elicit structural instability in layered superconductor approach ferroelectric.MeV He$^{\mathrm{+}}$ RBS-Axial Ion Channeling,ultrafast real-space probe of sub-picometre atomic displacement is used to probe JT effect {\&} incoherent lattice fluctuations as a function of temperature in ion implanted STO lattice. Critical channeling angle $\psi_{\mathrm{c}}$ {\&} ratio of minima of Angular ICh--RBS spectral yield $\chi_{\mathrm{min\thinspace \thinspace }}$for Sr {\&} Ti sub lattices determine lattice distortion.Cr {\&} Fe impurities mostly distort Ti sublattice.Displacements of Ti$^{\mathrm{4+}}$ are calculated. Similar values of $\psi _{\mathrm{1/2}}$ for Sr sublattice indicates no displacement of Sr.Actually Cr/Fe located in Ti positions.JT Cr$^{\mathrm{4+}}$,Cr$^{\mathrm{5+}}${\&} Fe$^{\mathrm{4+}}$~impurity could induce Raman-active localized oxygen vibrational mode, which doesn't involve motion of nearest Fe or Ti ions.Displacive phase transition provides direct evidence of changes with temperature in thermal vibrational amplitude of lattice atoms across structural phase transition.Interplay of ferroelectric, antiferrodistortive distortions {\&} superconducting order are discussed. [Preview Abstract] |
Wednesday, March 16, 2016 3:54PM - 4:06PM |
P7.00006: Collapse of electrons to a donor cluster in SrTiO$_3$ HAN FU, Konstantin Reich, Boris Shklovskii It is known that when a nucleus has charge $Ze$ where $Z>137$, electrons collapse onto the nucleus resulting in a net charge $Z_n < Z$. This effect is due to the relativistic dispersion law. Here a similar effect is found for a donor cluster in SrTiO$_3$ (STO), but with a different origin (see Phys. Rev. B 92, 035204 (2015)). At low temperatures, STO has an enormously large dielectric constant and the nonlinear dielectric response becomes dominant when the electric field is still small. This leads to the collapse of electrons into a charged spherical donor cluster with radius $R$ when its total charge number $Z$ exceeds a critical value $Z_c\simeq R/a$ where $a$ is the lattice constant. The net charge $Z_{n}e$ grows with $Z$ until $Z$ exceeds $Z^*\simeq(R/a)^{9/7}$. After this point, the charge of the compact core $Z_{n}$ remains $\simeq Z^*$, while the rest $Z^*$ electrons form a sparse Thomas-Fermi electron atmosphere around it. We show that the thermal ionization of such two-scale atoms easily strips the outer atmosphere while the inner core remains preserved. We extend our results to the case of long cylindrical clusters. We discuss how our predictions can be tested by measuring conductivity of chain of discs of charge on the STO surface. [Preview Abstract] |
Wednesday, March 16, 2016 4:06PM - 4:18PM |
P7.00007: Point defects and band alignment in strontium cerate Michael Swift, Chris G. Van de Walle Strontium cerate (SrCeO$_{\mathrm{3}})$ is a well-known ionic conductor of both hydrogen and oxygen. In applications, it is frequently doped (for instance with yttrium or neodymium) to increase stability and promote diffusion. However, the microscopic effects of doping and native defects are not fully understood. Building on previous computational work in barium cerate (BaCeO$_{\mathrm{3}})$, we use density functional theory with a hybrid functional to study impurities, electronic structure, and band alignments in these systems. We establish trends that we expect to hold across the perovskite cerates. We also discuss the alignment of the thermodynamic charge-state transition levels of hydrogen, and applications to this class of materials. [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P7.00008: Ferromagnetism in ZnO doped with alkaline elements Yiren Wang, Jingyuan Piao, Guozhong Xin, Yunhao Lu, Zhimin Ao, Nina Bao, Jun Ding, Sean Li, Jiabao Yi We have observed room temperature ferromagnetism (RTFM) in ZnO doped with alkaline elements Using first-principles calculations we found the magnetization in these systems is originated from the O2p hole states around Zn vacancies. Calculations indicate that the formation energy of Zn vacancies alone is rather high while further investigation indicates the formation can be much stabilized by the alkaline dopants in the form of defect complexes. By calculating the formation energy of concerned defects and complexes, we found the role of the dopants that under a certain doping concentration: Zn vacancy, substitutional and interstitial dopants can form a defect complex, which can lower formation energy, therefore stabilizing Zn vacancies. Moreover K dopants have shown unique functions on the ferromagnetism since the substitutional K can induce magnetic moments to the system by forming partial zinc vacancy via lattice distortion. Hence K doped ZnO can be magnetic at low doping concentrations. Experimentally, Li, Na doped ZnO films and K doped ZnO nanorods with different doping levels are synthesized, RTFM can be observed in all these systems. The magnetization is found to be greatly influenced by the doping concentrations. The experimental results have shown good consistence with our theoretical calculations. Our studies can inspire the defect induced ferromagnetism as a new route for the fabrication of new diluted magnetic semiconductors. [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P7.00009: Oxygen vacancies in amorphous-Ta$_{2}$O$_{5}$ from first-principles calculations Jihang Lee, Emmanouil Kioupakis, Wei Lu Oxygen vacancies are thought to play a crucial role in the electrical and optical properties of tantalum pentoxide (Ta$_{2}$O$_{5}$) devices. Even though numerous experimental studies on oxygen vacancies in Ta$_{2}$O$_{5}$ exist, experimentally detected defects are ambiguously identified due to the absence of an accurate and conclusive theoretical analysis. We investigate oxygen vacancies in amorphous Ta$_{2}$O$_{5}$ with first-principles calculations based on hybrid density functional theory. The calculated thermodynamic and optical transition levels of stable oxygen vacancies are in good agreement with measured values from a variety of experimental methods, providing conclusive clues for the identification of the defect states observed in experiments. We determine the concentration of oxygen vacancies and their dominant oxidation state as a function of growth conditions. We analyze the characteristics of extra electrons introduced by donor-like oxygen vacancies, which include the formation of polarons. Our results provide insight into the fundamental properties of oxygen vacancies in Ta$_{2}$O$_{5}$, which is essential to controlling the properties of films and optimize the performance of devices. [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P7.00010: Charge storage in oxygen deficient phases of TiO2: defect Physics without defects A. C. M. Padilha, H. Raebiger, A. R. Rocha, G. M. Dalpian Defects in semiconductors can exhibit multiple charge states, which can be used for charge storage applications. Here we consider such charge storage in a series of oxygen deficient phases of TiO$_2$, known as Magnéli phases. These Ti$_n$O$_{2n-1}$ Magnéli phases present well-defined crystalline structures, i. e., their deviation from stoichiometry is accommodated by changes in space group as opposed to point defects. We show that these phases exhibit intermediate bands with the same electronic transitions akin to interstitial Ti defect levels in TiO$_2$-rutile. Thus, the Magnéli phases behave as if they contained a very large pseudo-defect density: $\frac{1}{2}$ per formula unit Ti$_n$O$_{2n-1}$. Depending on the Fermi Energy the whole material will become charged. These crystals are natural charge storage materials with a storage capacity that rivals the best known supercapacitors. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:06PM |
P7.00011: Influence of the ``second gap'' on the optical absorption of transparent conducting oxides Viet-Anh Ha, David Waroquiers, Gian-Marco Rignanese, Geoffroy Hautier Transparent conducting oxides (TCOs) are critical to many technologies (e.g., thin-film solar cells, flat-panel displays or organic light-emitting diodes). TCOs are heavily doped (\emph{n} or \emph{p}-type) oxides that satisfy many design criteria such as high transparency to visible light (i.e., a band gap $>$ 3 eV), high concentration and mobility of carriers (leading to high conductivity), ... In such (highly doped) systems, optical transitions from the conduction band minimum to higher energy bands in \emph{n}-type or from lower energy bands to the valence band maximum in \emph{p}-type are possible and can degrade transparency. In fact, it has been claimed that a high energy ($>$ 3eV) for any of these transitions made possible by doping, commonly referred as a high ``second gap'', is a necessary design criterion for high performance TCOs. Here, we study the influence of this second gap on the transparency of doped TCOs by using \emph{ab initio} calculations within the random phase approximation (RPA) for several well-known \emph{p}-type and \emph{n}-type TCOs. Our work highlights how the second gap affects the transparency of doped TCOs, shining light on more accurate design criteria for high performance TCOs. [Preview Abstract] |
Wednesday, March 16, 2016 5:06PM - 5:18PM |
P7.00012: Small polarons and point defects in LaFeO$_3$ Zhen Zhu, Hartwin Peelaers, Chris G. Van de Walle The proton-conductive perovskite-type LaFeO$_3$ is a promising negative-electrode material for Ni/metal-hydride (Ni-MH) batteries. It has a discharge capacity up to 530 mAhg$^{-1}$ at 333~K, which is significantly higher than commercialized AB$_5$-type alloys. To elucidate the underlying mechanism of this performance, we have investigated the structural and electronic properties of bulk LaFeO$_3$, as well as the effect of point defects, using hybrid density functional methods. LaFeO$_3$ is antiferromagnetic in the ground state with a band gap of 3.54~eV. Small hole and electron polarons can form through self- or point-defect-assisted trapping. We find that La vacancies and Sr substitutional on La sites are shallow acceptors with the induced holes trapped as small polarons, while O and Fe vacancies are deep defect centers. Hydrogen interstitials behave like shallow donors, with the donor electrons localized on nearby iron sites as electron polarons. With a large trapping energy, these polarons can act as electron or hole traps and affect the electrical performance of LaFeO$_3$ as the negative electrode for Ni-MH batteries. We acknowledge DOE for financial support. [Preview Abstract] |
Wednesday, March 16, 2016 5:18PM - 5:30PM |
P7.00013: Time-resolved photoluminescence of SiOx encapsulated Si Seref Kalem, Amal Hannas, Tomas Österman, Villy Sundström Silicon and its oxide SiOx offer a number of exciting electrical and optical properties originating from defects and size reduction enabling engineering new electronic devices including resistive switching memories. Here we present the results of photoluminescence dynamics relevant to defects and quantum confinement effects. Time-resolved luminescence at room temperature exhibits an ultrafast decay component of less than 10 ps at around 480 nm and a slower component of around 60 ps as measured by streak camera. Red shift at the initial stages of the blue luminescence decay confirms the presence of a charge transfer to long lived states. Time-correlated single photon counting measurements revealed a life-time of about 5 ns for these states. The same quantum structures emit in near infrared close to optical communication wavelengths. Nature of the emission is described and modeling is provided for the luminescence dynamics. The electrical characteristics of metal-oxide-semiconductor devices were correlated with the optical and vibrational measurement results in order to have better insight into the switching mechanisms in such resistive devices as possible next generation RAM memory elements. [Preview Abstract] |
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