Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G23: Focus Session: Dopants and Defects in Semiconductors V |
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Sponsoring Units: DMP Chair: John Lyons, University of California at Santa Barbara Room: 325 |
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G23.00001: Modeling of Threading Dislocation Core Fields and of Point Defects in GaN Jennifer R. Snively, Stefan C. Badescu Point defects and dislocations in GaN are involved in failure mechanisms of GaN-based electronic devices. Compared to bulk material, the electronic states and diffusivities of point defects are modified by dislocation elastic fields. For accurate descriptions atomistic calculations have to take into account both the long (Volterra) and the short-range components of the latter. We present an atomistic picture of defect energy levels and diffusion barriers for vacancies, interstitials and impurities next to threading dislocations in GaN. We include the dislocation core field derived from stress calculations using periodic supercells. We show that this increases significantly the point defect-dislocation interaction by comparison to the Volterra field and that the diffusion energy barriers are changed by as much as 50{\%}. The dependence of charged energy levels on the Fermi level is also modified on many lattice spacings away from the dislocations. We discuss in more detail the charged N vacancies and the Ga and Al interstitials. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G23.00002: Carrier-Induced Lattice Instability in Amorphous Oxide Semiconductors Yong-Sung Kim, Ho-Hyun Nahm, Dae-Hwan Kim Amorphous oxide semiconductors (AOS's) have high electron mobility even in amorphous phase. The AOS-based thin film transistors (TFTs) are nowadays intensively pursued to be adopted into high-resolution flat-panel displays. However, a facing bottleneck of the AOS-based TFT display applications is the instability problem under bias and illumination stress conditions. Especially, by negative bias and illumination stress (NBIS) or only by illumination stress (IS), the threshold voltage of the AOS TFTs is largely negative-shifted. In this work, we study the instability mechanism of the AOS's based on first-principles calculations. The valence band tail states of the AOS's are found to be characterized by the O-O pp$\sigma $* anti-bonding state. The excited localized-holes thus give lattice instability and form O-O bonds through the pp$\sigma $-hybridized interaction. The pp$\sigma $* level is heightened up into the conduction bands along with the O-O bond formation, and two electrons left from the created O-vacancy (V$_{\mathrm{O}})$ occupy the delocalized conduction band states. The O-O and V$_{\mathrm{O}}$ complex (a peroxide defect) is found to be a meta-stable donor defect and suggested as an origin of the NBIS and IS instabilities in AOS TFTs. Based on the suggested mechanism, we propose a direction to improve the stability of AOS thin films with optimizing the cation compositions. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G23.00003: Shuffle-Glide Transition of Dislocations in Silicon Zhi Li, Nithin Mathew, Catalin Picu Dislocation motion in diamond cubic Si can take place either on the shuffle or glide set of \textbraceleft 111\textbraceright planes. It is commonly accepted that shuffle planes are active at low temperatures and high applied stresses, while the glide planes become active at high temperatures and lower stresses. The transition of dislocations from one plane to the other is still a matter of debate, with some authors suggesting that such transition is impossible, and others proposing intermediate metastable states and transition barriers. In this work we show a mechanism by which shuffle dislocations may move to the glide plane without any intermediate state and evaluate the activation barrier (and activation volume) for the transition. We also support the previously observed sensitivity of dislocation mobility in the shuffle plane to the stress acting normal to the glide plane, and link this sensitivity to the nature of the gamma surface. The role of the normal stress in the shuffle-glide transition is also discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G23.00004: Role of defects in resistive switching TiO$_{2}$- and SrTiO$_{3}$-based devices Invited Speaker: Marek Skowronski Oxide-based resistive switching devices hold promise of being the next generation of non-volatile high density memory. While small size, fast switching and long retention time have been demonstrated, many questions remain pertaining the switching mechanism. In particular, the role of point and extended defects in resistance switching remains to be elucidated. Most proposed interpretations invoke oxygen vacancy redistribution as the origin of the resistance change. However, the measurements of the vacancy drift in the Schottky barriers on SrTiO$_{3}$ indicated that mobility at room temperature is eight orders of magnitude too low to account for the reported switching times. This difficulty could be alleviated by faster vacancy motion along the dislocation lines and/or local Joule heating during switching. Careful mapping of the dislocation distribution before and after switching in nanoscale devices did not find a good correlation between dislocations and the I-V characteristics. Lateral SrTiO$_{3}$ devices with no dislocations have similar switching behavior to the ones fabricated on high dislocation density materials. The only correlation present was the generation of dislocations in devices with dissipated power level above 10 mW. The apparent mechanism is the thermal stress driven plastic deformation. While the Joule heating could speed up the defect motion, it could not explain the long retention times. This is frequently interpreted as due to formation of oxygen-deficient phases in TiO$_{2}$-based memristors. Transmission Electron Microscopy analysis of nanoscale vertical devices has revealed numerous physical changes with the extent strongly dependent on the level of dissipated power. Optimized device structures that switch with the power dissipation below 2 mW exhibited very limited degree of crystallization and no new phases. Many of the initially posed questions remain unanswered. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G23.00005: Oxygen vacancies at the surface of SrTiO$_3$ thin films Alexandre R. Silva, Gustavo M. Dalpian The 2-D electron gas at the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO), two band insulators, has been the subject of intense research owing to the fact that this interface can show metallic, superconducting, and magnetic effects, properties that are absent in the bulk counterparts. The metallic behavior has also been observed at the STO surface, without the need of the oxides' interface. Although the reason of this behavior is not well defined, there are three hypotheses for this: the polar catastrophe; the oxygen vacancies produced in the experiment, and cations intermixing. In this work, first principles calculations based on the density functional theory and using hybrid functionals were performed to reveal the atomic and the electronic structure of vacancies at the (001) surface of STO films. We have analyzed both the TiO$_2$ and SrO-terminated surfaces. For pure surfaces, we observed atomic relaxations up to the 5$^{th}$ atomic layer. The surface band structure of ideal STO slabs shows that the STO thin films are insulating in both terminations, but insert surface levels in the gap of bulk STO. Defective STO slabs are observed to be metallic, and we observe a strong tendency for the oxygen vacancies to migrate into the surface. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G23.00006: Persistent photoconductivity in strontium titanate Matthew McCluskey, Marianne Tarun Strontium titanate (SrTiO$_{\mathrm{3}})$ is often used as a substrate for oxide thin films such as high-temperature superconductors. It has the perovskite structure and an indirect band gap of 3.25 eV. Our prior work showed that hydrogen impurities form a defect complex that contains two hydrogen atoms. The complex was tentatively attributed to a passivated strontium vacancy. Alternatively, it could be a partially passivated titanium vacancy. In order to suppress strontium vacancies (and create titanium vacancies), we annealed samples in an evacuated ampoule with SrO powder. These samples show unexpected behavior. After illuminating with light (405 nm, 3.06 eV), the free-electron concentration increases significantly. After the light is turned off, the high conductivity persists at room temperature. We tentatively attribute this effect to the excitation of an electron from a titanium vacancy into the conduction band, with a high barrier for recapture. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G23.00007: A hybrid density functional study on energetics of native defects in anatase $\rm TiO_2$ Adisak Boonchun, Pakpoom Reunchan, Naoto Umezawa, Jinhua Ye The energetics and electronic structures of native defects in anatase $\rm TiO_2$ have been studied by means of hybrid density-functional calculations. Here, we show that oxygen vacancy ($V_{\rm O}$) and titanium interstitial ($\rm Ti_\textit{i}$) are both shallow donors and are likely to form with a substantial concentration in an oxygen poor condition. While titanium vacancies ($V_{\rm Ti}$) is a deep acceptor. The charge neutrality showed that Fermi level is pinned at the conduction band minimum in good agreement with the common observations of n-type conductivity in a reduced $\rm TiO_2$. Self-trap hole ($\rm O_O$) states are localized at oxygen anions. On the other hand the self-trapped electron ($\rm Ti_{Ti}$) cannot be produced in the bulk. Although, $\rm Ti_\textit{i}$ is the strongest candidate for the unintentional $n$-type conductivity owning to its low formation energy, we show that the post-growth of $V_{\rm O}$ in anatase is also possible under annealing temperature and pressure. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G23.00008: DFT-based first-principle calculation of the carrier activation ratio in the F-doped anatase TiO$_2$ and the thermodynamic analysis of the formation of TiOF$_2$ phase Hideyuki Kamisaka, Nanako Mizuguchi, Koichi Yamashita, Tetsuya Hasegawa The F-doped anatase TiO$_2$ (FTO) could be an alternative transparent conductive oxide, but the experimentalists have reported low carrier activation ratio of 20 - 30{\%}, and that the formation of TiOF$_2$ hampers its production when using the PLD method. We investigated this system using the standard DFT-based band structure method. The PBE functional was adapted with the Hubburd $+$U terms. The value of $+$U parameters was adjusted to meet the generalized Koopman's theorem (gKT). We found that the formation energy of F$_{\mathrm{O}}$ (F dopant substituting O) and F$_{\mathrm{O}}^{+}$ is quite close to each other, and the two crosses when the Fermi level is slightly above the conduction band minimum. Combining the Burstein-Moss effect and this crossing of the formation energies, a simple statistical analysis was made. The calculated activation ratio was about 10{\%} - 32{\%}, which agreed with the experimental data. The free energies of bulk TiO$_2$, 3{\%} FTO, 6{\%} FTO and TiOF$_2$ were compared using the DFT result. Contributions from the distribution entropy of the anions, lattice vibrations, free energy of conductive electrons, and the entropy from the spin state of trapped electrons was considered with relevant approximations. We found that the free energy of TiO$_2$, 3{\%} FTO, and TiOF$_2$ come close to each other under the condition of T$=$800K and P(O$_2)=$10$^{-5}$ Torr, which coincides the experimental report. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G23.00009: Origins of Persistent Photoconductivity in GaAsN Alloys R.L. Field III, Y.Q. Wang, C. Kurdak, R.S. Goldman In GaAs$_{\mathrm{1-x}}$N$_{x}$ alloys, we observe significant persistent photoconductivity (PPC) at cryogenic temperatures for $x$ \textgreater\ 0.006, with the PPC strength increasing with increasing $x$ and decreasing upon rapid-thermal annealing (RTA). Since the RTA-induced suppression is accompanied by a reduction of the interstitial N fraction, the N-induced donor state is likely associated with N pairs. PPC is attributed to the promotion of carriers from a ground N-pair state to the conduction band edge, inducing modifications in the N-pair molecular bond configuration. When illumination is terminated, an energy barrier hinders the return of carriers to the N-pair induced complex. With the addition of thermal energy, the original N-pair configuration is restored and the N-pair induced complex is then able to accept carriers. We use PPC at cryogenic temperatures to go through a metal-insulator transition in GaAsN by increasing the carrier density with illumination. For different illumination durations we determine the minimum metallic conductivity, giving us the critical carrier density, $n_{c}$, at the transition point. We then determine the effective mass, $m*$, using the Mott criterion $n_{c}^{1/3}a_{H} = $ 0.26 where $a_{H} = $ (4$\pi \varepsilon $h$^{2})$/($e^{2}m$*) is the Bohr radius. We use PPC to induce a metal-insulator transition in GaAsN. We will discuss the effective mass as a function of N concentration and compare to the predictions of the band anticrossing model. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G23.00010: Defect Related Magnetism and Conduction in As-grown and Annealed Pulsed Laser Deposited SnO$_2$:Co Thin Films Gratiela M. Stoian, P.A. Stampe, R.J. Kennedy, E. Lochner, Y. Xin, S. von Molnar Magnetic semiconductor SnO$_{2}$:Co films were grown on r-cut sapphire substrates via Pulsed Laser Deposition from a doped target with a nominal Co concentration of 5 at.{\%}. To study the role of oxygen vacancies and other defects in tuning the ferromagnetic (FM) and electrical properties of these materials, films were deposited at different growth rates, temperatures and oxygen pressures. In addition, some films were annealed at various conditions. Magnetometry data show that films grown at optimal conditions are FM at room temperature with a saturation magnetization of 20 emu/cm$^{3}$. The moment per unit area varies linearly with the film thickness, suggesting the magnetism in our materials is a volume property. Magnetization decreases monotonically with the growth rate. A transition from a semiconducting, magnetic material to an insulating, non-magnetic material was observed below a film thickness of 50 {\AA}. Annealing films grown at a higher than optimal deposition rate under the same conditions used for their growth, led to an initial rapid increase in the magnetization followed by constant magnetization after further annealing. We also report on the temperature dependence of the electro-magnetic properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G23.00011: Exploring nanoscale fluctuations and ferroelectric phase stabilization in S doped PbTe thermoelectrics Kevin Knox, Emil Bozin, Christos Malliakas, Mercouri Kanatzidis, Simon Billinge PbTe is one of the most important commercial thermoelectric materials for applications above room temperature. A paraelectric phase of fluctuating ferroelectric-like Pb structural dipoles emerges in PbTe at elevated temperatures, although it adopts an average rock-salt structure at all temperatures. These intrinsic nanoscale fluctuations are believed to improve the thermoelectric properties of PbTe by limiting the lattice thermal conductivity. Additionally, alloying and chemical substitution in PbTe appreciably improve the thermoelectric figure of merit, as is the case in PbTe$_{1-x}$S$_x$. However, the exact mechanism for this enhancement is not well understood. It has been shown that PbTe$_{1-x}$S$_x$ exhibits a peak in resistivity at a doping dependent temperature. By analogy with Ge doped PbTe, this anomalous resistivity may be the signature of a ferroelectric phase stabilization. In this talk, we explore this possibility by characterizing the average and the local structure of PbTe$_{1-x}$S$_x$ as a function of temperature and doping using a neutron based atomic pair distribution function (PDF) approach. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G23.00012: H$_2$ exposure induced structural and electrical modulation in MoS$_2$ Min Park, Sung Jin Chang, Hu Young Jung, Seung Jae Baek, Yongseok Jun, Byung Hoon Kim, Yung Woo Park We have demonstrated the structural modulation at the edge of MoS$_2$ due to H$_2$ exposure with spatially resolved Raman spectroscopy and the electrical characteristics of few-layer MoS$_2$ with respect to H$_2$ pressure from vacuum to 20 bars at 295 K \textless\ $T$ \textless\ 350 K. Upon H$_2$ exposure, the significant change of the edge in E$_{\mathrm{2g}}$ mode was observed. The conductance increases and threshold voltage ($V_{th})$ shifts toward a negative gate voltage region, indicating n-type doping. These behaviors are enhanced by high temperature and long exposure time ($t)$. The results reveal the creation of vacancy at the edge sites of MoS$_2$ in H$_2$ atmosphere causing the enhancement of $n$-type doping due to increase of metallic region. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G23.00013: Discovery of Burstein-Moss shift in Re-doped MoS$_2$ nanoparticles Qi Sun, Lena Yadgarov, Rita Rosentsveig, Gotthard Seifert, Reshef Tenne, Janice Musfeldt We investigated the optical properties of Re-doped MoS$_2$ nanoparticles and compared our findings with the pristine and bulk analogs. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling whereas doping has the opposite effect. We model the doping-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in model nanoscale materials. [Preview Abstract] |
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