Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F14: Focus Session: Dopants and Defects in III-V Nitrides |
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Sponsoring Units: DMP FIAP Chair: Denis Demchenko, Virginia Commonwealth University Room: 008A |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F14.00001: Carbon defects as sources of the green and yellow luminescence bands in undoped GaN Denis Demchenko, Michael Reshchikov In high-purity GaN grown by hydride vapor-phase epitaxy (HVPE), the commonly observed yellow luminescence (YL) band gives way to a green luminescence (GL) band at high excitation intensity. Based on hybrid functional calculations and experimental photoluminescence measurements, we propose that the GL band with a maximum at 2.4 eV is caused by transitions of electrons from the conduction band to the 0/+ level of the isolated C$_N$ defect. The YL band with a maximum at 2.1 eV, related to the transitions via the -/0 level of the same defect can be observed only for some high-purity HVPE samples. However, in less pure GaN samples (HVPE samples with larger O and C concentrations, as well as all MOCVD grown samples), no GL band is observed and another YL band with a maximum at 2.2 eV dominates the PL spectrum. The latter is attributed to the C$_N$O$_N$ complex. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F14.00002: Density Functional Theory Calculations of Activation Energies for Carrier Capture by Defects in Semiconductors Normand Modine, Alan Wright, Stephen Lee Carrier recombination due to defects can have a major impact on device performance. The rate of defect-induced recombination is determined by both defect levels and carrier capture cross-sections. Density functional theory (DFT) has been widely and successfully used to predict defect levels, but only recently has work begun to focus on using DFT to determine carrier capture cross-sections. Lang and Henry worked out the fundamental theory of carrier-capture by multiphonon emission in the 1970s and showed that, above the Debye temperature, carrier-capture cross-sections differ between defects primarily due to differences in their carrier capture activation energies. We present an approach to using DFT to calculate carrier capture activation energies that does not depend on an assumed configuration coordinate and that fully accounts for anharmonic effects, which can substantially modify carrier activation energies. We demonstrate our approach for the -3/-2 level of the Ga vacancy in wurtzite GaN. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 9:00AM |
F14.00003: Defect identification in semiconductors with positron annihilation: experiment and theory Invited Speaker: Filip Tuomisto Positron annihilation spectroscopy is a very powerful technique for the detection, identification and quantification of vacancy-type defects in semiconductors. In the past decades, it has been used to reveal the relationship between opto-electronic properties and specific defects in a wide variety of materials - examples include parasitic yellow luminescence in GaN, dominant acceptor defects in ZnO and broad-band absorption causing brown coloration in natural diamond. In typical binary compound semiconductors, the selective sensitivity of the technique is rather strongly limited to cation vacancies that possess significant open volume and suitable charge (negative of neutral). On the other hand, oxygen vacancies in oxide semiconductors are a widely debated topic. The properties attributed to oxygen vacancies include the inherent n-type conduction, poor p-type dopability, coloration (absorption), deep level luminescence and non-radiative recombination, while the only direct experimental evidence of their existence has been obtained on the crystal surface. We will present recent advances in combining state-of-the-art positron annihilation experiments and ab initio computational approaches. The latter can be used to model both the positron lifetime and the electron-positron momentum distribution - quantities that can be directly compared with experimental results. We have applied these methods to study vacancy-type defects in III-nitride semiconductors (GaN, AlN, InN) and oxides such as ZnO, SnO$_2$, In$_2$O$_3$ and Ga$_2$O$_3$. We will show that cation-vacancy-related defects are important compensating centers in all these materials when they are n-type. In addition, we will show that anion (N, O) vacancies can be detected when they appear as complexes with cation vacancies. [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F14.00004: The role of surface kinetics on defect generation and propagation during epitaxy of WBG semiconductors Angel Yanguas-Gil One of the greatest challenges in the application of WBG semiconductors to power electronics and optoelectronic applications is how to mitigate the impact of electrically active defects on device performance. While this is an issue that traditionally has been tackled through process development, from a fundamental point of view we still have a limited knowledge on the interplay between epitaxial growth and defect generation. This interaction goes both ways, with surface kinetics promoting the formation surface defects that get incorporated into the bulk, and the surface defects and surface morphology driving the kinetics of the growth process. In this work we explore the interplay between surface kinetics and 0D and 1D defects during epitaxy. By combining molecular dynamics and kinetic Monte Carlo simulations we have focused on understanding how surface kinetics promotes the formation of defects, and identified the surface configurations that are more likely to lead to point defects incorporated in the bulk. Conversely, we have also studied the impact of existing defects, in particular the interaction of surface species and surface steps with threading dislocations. Our target materials are GaN, AlN, and SiC. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F14.00005: Dilute-P GaNP Semiconductor Alloy for Visible Light Emitter Chee-Keong Tan, Nelson Tansu Group III-Nitride semiconductor alloy in particular InGaN alloy is widely employed as the active media for the solid state lighting applications. In addition to the InGaN alloy, dilute-As GaNAs alloy has recently been suggested as the potential material for high efficiency solid state lighting devices. In conjunction with dilute-As GaNAs alloy, dilute-P GaNP alloy has the potential for its use in light emitting applications. The literature on dilute-P GaNP alloy is severely limited, thus the understanding of electronic properties of the material is of great importance in establishing the first step towards the device implementation. In this work we present the analysis of the electronic properties of dilute-P GaNP alloys through First-Principle Density Functional Theory (DFT). Our analysis shows that the replacement of N atoms with Phosphorus (P) atoms in the GaN alloy leads to significant changes in the band structure including the band gap and effective mass. In addition, our finding indicates minimal interband Auger recombination in the dilute-P GaNP alloys as compared to InGaN alloy, suggesting the potential of GaNP alloys as high efficiency visible light emitter. The electronic properties of the dilute-P GaNP alloys will be discussed in depth. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F14.00006: The donor-acceptor relationship in HVPE GaN:Fe Substrates Ustun Sunay GaN is a wide bandgap semiconductor plagued by a high concentration of residual donors, typically from unintentionally incorporated Si or O. The effect of these donors can be masked by doping with deep acceptors, such as Fe, which compensates the donors creating a semi-insulating material that can be used for RF applications. Compensation is thought to occur when the Fe$^{3+}$ acceptor captures a donor electron, creating Fe$^{2+}$ and a positively charged donor. Previously, an electron paramagnetic resonance (EPR) spectroscopy study of lightly Fe-doped (1x10$^{17}$ cm$^{-3})$ bulk GaN showed the existence of neutral donors and Fe$^{3+}$ simultaneously. The presence of both calls into question the current understanding of compensation. To further understand donor-acceptor compensation, bulk GaN grown via hydride vapor epitaxy was intentionally codoped with Si donors and Fe acceptors in ratios ranging from 0.01 to 1.55. Both species were present in 3.5 K EPR spectra, but the Fe$^{3+}$ acceptor signal decreased, and the neutral donor signal increased monotonically as the Si:Fe ratio increased. While this shows that Fe$^{3+}$ partially compensates the neutral donor, interpretation of photo-EPR experiments suggests that some donors and acceptors are not interacting and that there is a multi-step mechanism for compensation. Cathodoluminescence (CL) results showed striated regions of luminescence intensity which indicate defect concentration non-uniformity. The leading explanation for the EPR and CL results is a physical separation between the donors and acceptors, leading to local variations in the Si:Fe ratio. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F14.00007: The Role of Oxygen on the Nature and Stability of Eu Centers in Eu doped Gallium Nitride Brandon Mitchell, Dolf Timmerman, Zhu Wiaxing, Junichi Takatsu, Masaaki Matsuda, Katharina Lorenz, Eduardo Alves, Atsushi Koizumi, Yasufumi Fujiwara, Volkmar Dierolf The effects of intentional and unintentional co-doping of oxygen on the incorporation of Eu into GaN has been thoroughly investigated. A new Eu precursor that does not contain oxygen in its molecular structure was utilized, which allowed for external control of the oxygen concentration in the samples by co-doping. The optical properties of the Eu ions were found to be considerably influenced by the absence of oxygen. It was concluded that the oxygen played an integral role in the location, stability, and local defect structure around the Eu ions that were doped into the GaN host. Furthermore, there is evidence that the normally occurring O in GaN is ``recycled'' by the Eu ions forming stable Eu-O complexes. The formation of these Eu-O complexes appears to be more beneficial to the crystal quality and stability than either defect is on its own. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F14.00008: Computational nano-material design of exotic luminescent materials based upon europium doped gallium nitrides Akira Masago, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida Eu-doped GaN has attracted much attention, because the red light luminescence ability provides us with expectations to realize monolithic full-color LEDs, which work on seamless conditions such as substrates, electrodes, and operating bias voltages. Toward implementation of multifunctional activity into the luminescent materials using the spinodal nano-structures, we investigate atomic configurations and magnetic structures of the GaN crystal codoped with Eu, Mg, Si, O, and/or the vacancies using the density functional method (DFT) calculations. Our calculations show that the impurity clusterized distributions are energetically favorable more than the homogeneous distribution. Moreover, analyses of the formation energy and binding energy suggest that the clusterized distributions are spontaneously formed by the nano-spinodal decomposition. Though the host matrix has no magnetic moments, the cluster has finite magnetic moments, where Zener's p-f exchange interaction works between the Eu f-state and the nearby N p-states. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F14.00009: Stabilization of free-standing GaN foils by threading edge dislocations Roman Gr\"oger, Lucien Leconte Computational studies of core structures of $1/3[11\bar{2}0]$ threading edge dislocations in bulk GaN predict the existence of a 5/7-atom ring configuration when viewed along the [0001] direction. While this agrees with recent high-resolution electron microscopy observations, previous studies also reveal the existence of an 8-atom ring configuration. We employ molecular statics calculations to show that the core stability of the threading edge dislocation in free-standing GaN foils depends on the foil thickness and the terminations of their surfaces. For the foil thickness above 6 nm, the edge dislocation is predicted to possess the 5/7-atom ring core structure. However, with decreasing thickness and depending on surface terminations the minimum energy core structure may change to both 8-atom ring and 4-atom ring configurations. By quantifying the interaction energy of the dislocation with the surface of the foil, we show that there exist conditions for which the threading edge dislocation in the foil is more stable than in the bulk. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F14.00010: Atomic and electronic structures of (GaN)$_{1-x}$(ZnO)$_x$ alloys: the role of short-range order Jian Liu, Philip Allen (GaN)$_{1-x}$(ZnO)$_x$ solid solution is a promising photocatalyst for efficient water splitting under visible illumination. For theoretical modeling, the special quasirandom structure (SQS) method which assumes random site occupancy is widely used. We have previously shown, with density-functional theory (DFT) total energy calculations, cluster expansion, and Monte Carlo simulations, that short-range order (SRO) is significant due to the non-isovalency. Thus it is desirable to include SRO in the construction of supercells. Inspired by the SQS method, we construct the ``special quasi-ordered structure'' (SQoS) supercells. Subsequent DFT calculations show that the atomic and electronic structures of SQS and SQoS alloys differ significantly. The SRO and (x,T) dependence of the valence band maximum stem mainly from the anti-bonding hybrids of N2p and Zn3d states. This suggests the possibility of engineering the band gap by tuning SRO. We also explore bond length distribution and bond angle variation over the composition-temperature (x,T) phase space using bond valence method (BVM). The validity of our BVM model is tested by DFT total energy calculations. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F14.00011: EPR detected defect center in bulk GaN substrates grown by high pressure nitrogen solution method J. Dashdorj, M.E. Zvanut, M.M. Bockowski Growth of high quality, free-standing GaN substrates is essential for further improvement of nitride device performance. Unfortunately, many bulk growth methods introduce unacceptable amounts of O donors. Samples studied in this work were grown by the high-pressure nitrogen solution method and intentionally doped with Mg to compensate O. Undoped and Mg-doped GaN samples were characterized by 4 K electron paramagnetic resonance (EPR) spectroscopy. No EPR signal is detected in the undoped samples. However, a nearly isotropic spectrum with g-value of 1.984 and a line-width of 120 G is observed in the doped sample after illumination with 2.8 eV light. Concentration of the center was estimated to be 10$^{17}$ cm$^{-3}$ while secondary ion mass spectroscopy revealed Mg and O levels of 10$^{19}$ cm$^{-3}$; Si, C, and Be levels of 10$^{17}$ cm$^{-3}$. Time-dependent photo-EPR data were well fit by a single defect-to-band transition model with a defect level of 0.67 eV above the valence band. This value is close to that predicted for the Be acceptor. Together with the similar concentrations of the EPR center and Be, the data suggest that the signal may be due to the accidental Be; however, a review of the literature suggests additional possibilities, such as a vacancy-related defect. [Preview Abstract] |
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