Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K11: Defects in Semiconductors -- Wide Band GapFocus
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Sponsoring Units: DMP DCOMP FIAP Chair: Anderson Janotti, University of Delaware Room: BCEC 152 |
Wednesday, March 6, 2019 8:00AM - 8:12AM |
K11.00001: Vibrational spectroscopy of O-H centers in Ga2O3 Michael Stavola, W Fowler, Ying Qin Hydrogen has a strong influence on the electrical properties of transparent conducting oxides where it can give rise to shallow donors and can passivate deep compensating defects. The ultra-wide bandgap semiconducting oxide Ga2O3 is no exception [1,2]. Vibrational spectroscopy shows that the introduction of H into Ga2O3 produces several O-H centers with strongly polarized vibrational lines and different thermal stabilities. Some of these defects appear to be shallow donors and others to be VGa-nH centers. Measurements made for samples that contain both H and D provide clues about the number of H atoms each of these defects contains. Our results suggest different O-H centers that contain a single H atom , two equivalent H atoms, and three or more H atoms. The polarization properties and mode coupling effects for these O-H centers provide structure-sensitive information for complementary theory that suggests specific defect structures [3]. |
Wednesday, March 6, 2019 8:12AM - 8:24AM |
K11.00002: Multiple O-H centers in β-Ga2O3 W Fowler, Michael Stavola, Ying Qin Hydrogen impurities play a crucial role in the electrical conductivity of β-Ga2O3 by acting as shallow donors and by passivating deep acceptors [1,2]. Polarized IR spectroscopy in conjunction with heat treatments of β-Ga2O3 single crystals treated in an H2 (D2) ambient reveals a number of vibrational lines originating from defects containing one or more O-H(D) species[3,4]. Theoretical calculations using the CRYSTAL06 code [5] with hybridized DFT Hamiltonian and the polarization properties of these vibrational lines are used to suggest structures involving one, two, or three O-H(D) species for these defects. We find strong evidence for one or more O-H(D) associated with Ga(1) vacancies and no evidence for any O-H(D) associated with Ga(2) vacancies in the crystals studied. |
Wednesday, March 6, 2019 8:24AM - 9:00AM |
K11.00003: Probing the Nanoscale Interplay of Native Defects and Doping in Oxide Semiconductors Invited Speaker: Leonard Brillson Nanoscale optical and electrostatic techniques can now directly measure the movement of native point defects inside oxide semiconductors and how they control space charge regions, tunneling, and contact rectification. Depth-resolved cathodoluminescence spectroscopy (DRCLS) with hyperspectral imaging measures 3-dimensional defect redistribution on a nanoscale for ZnO, Ga2O3, SrTiO3, and BaSrTiO3, revealing how intrinsic and applied electric fields drive defect movement. Defects at metal-ZnO diodes change carrier densities, tunneling, and trap-assisted hopping, altering Zn- vs. O-polar Schottky barriers. Native point defects are present inside, not only on the surfaces of ZnO nano- and microwires as commonly thought.1,2 Nanoscale 3D measurement and imaging reveal electrically-active defects that extend deep inside wires, introducing new donors or acceptors that alter depletion widths, conducting channel volumes, and metal-ZnO nano-contact rectification. Using electron and ion beams, we altered defect distributions to create rectifying, ohmic, or blocking contacts with the same metal on the same nanowire, demonstrating the interplay between the nature of native point defects, the intrinsic doping, and the physical dimensions of the nanostructure itself in determining the electronic properties of the oxide interface.3 DRCLS also enabled us to correlate the dominant luminescence features of Ga2O3 with the most thermodynamically stable O vacancy, Ga vacancy, and Ga vacancy-hydrogen defect states in the band gap predicted theoretically.4 As with ZnO, the combined depth-resolved detection and processing of Ga2O3 suggests new avenues for identifying and controlling native point defects in semiconductors. |
Wednesday, March 6, 2019 9:00AM - 9:12AM |
K11.00004: Deep acceptors in Ga2O3 Hartwin Peelaers, John Lyons, Joel Basile Varley, Chris Van de Walle β-Ga2O3 is a wide-band-gap semiconductor with promising applications in high-power electronics. While n-type doping is straightforward, p-type doping is elusive, with only deep acceptors available. We use hybrid density functional theory to explore the properties of possible acceptors, and discuss the viability of obtaining semi-insulating material. All dopants we considered lead to deep acceptor levels that are more than 1.3 eV above the valence-band maximum. N and Mg were identified as the most promising deep acceptors. We evaluated incorporation in different configurations, and also considered the effect of native defects as well as complexes. We find that both dopants will lead to Fermi-level positions that are at least 1.3 eV away from the band edges. We also predict diffusion activation energies, finding that Mg is significantly more mobile. The information obtained in this study can be used to analyze and explain ion implantation experiments, and to guide design of semi-insulating Ga2O3 layers. |
Wednesday, March 6, 2019 9:12AM - 9:24AM |
K11.00005: A study of deep level defects in β-Ga2O3 using thermal admittance spectroscopy J Hendricks, Mo Ahoujja, Shin Mou, Adam T Neal The β-Ga2O3 semiconductor is receiving great interest due to its potential applications for high power and deep-ultraviolet devices. However, in spite of a promising future of β-Ga2O3 device technology, its electronic properties, in particular deep level defects, are still not well understood. In this paper we investigated deep level defects in unintentionally doped β-Ga2O3 Schottky diodes, edge-defined film fed grown, using thermal admittance spectroscopy (TAS). An Arrhenius analysis of the TAS measurements shows two deep levels with energies of E1=0.428 eV and E2=1.07 eV and cross sections of σ1=1.83x10-13 cm2 and σ2=4.64x10-13 cm2, respectively. |
Wednesday, March 6, 2019 9:24AM - 9:36AM |
K11.00006: QSGW calculation of the band structure of Ga2O3-Al2O3 alloys Amol Ratnaparkhe, Walter R L Lambrecht β-Ga2O3 has recently drawn attention as an ultra-wide bandgap semiconductor. The alloys between Ga2O3 and Al2O3 are of interest to obtain even higher band gap materials. The two materials have different ground state structures, monoclinic β-Ga2O3, and corundum α-Al2O3 respectively. We study the Ga2O3-Al2O3 alloys in both the structures using the linearized muffin-tin orbital approach and ABINIT pseudopotential approach for the relaxation of the structural parameters. We consider all possible mixed structures within the common 10 atom primitive unit cells, with compositions x = 0, 0.25, 0.5, 0.75, 1.0 in (Ga2O3)1−x(Al2O3)x. We find the relative ordering of the two structures in the end compounds in agreement with experiment and energy differences in good agreement with previous computational work. We find the Al atoms tend to preferentially occupy the octahedral site when substituting for Ga. The band structure of the alloy models in the two structures is evaluated using the QSGW method. |
Wednesday, March 6, 2019 9:36AM - 9:48AM |
K11.00007: Enabling p-type doping in In2O3 by a band engineering through alloying Fernando Sabino, Suhuai Wei, Anderson Janotti In2O3 is a wide-band gap semiconductor of great importance to the optoelectronic industry. It is often used as transparent contact for solar cells, LEDs, and liquid crystal displays. It has a highly dispersive conduction band, composed of the In s orbitals, that lie relatively low with respect to the vacuum level, making it easy to dope n-type, while the low-energy valence band, composed mostly of O p orbitals, making it difficult to achieve p-type doping because acceptor impurities introduce deep levels in the gap. One way to overcome this limitation is to raise its valence band. Using first-principles calculations we explore different approaches to lift the valence band in In2O3 by alloying. We discuss different alloying elements, both on oxygen and metal sites. We compute the formation enthalpy of these alloys, and their stability with respect to phase separation. We calculate band gaps, and analyze the effects of alloying on the position of the valence and conduction bands, determining the band alignment between the alloys and the parent compounds. Finally, we address their optical properties by calculating absorption coefficients as a function of alloy composition. |
Wednesday, March 6, 2019 9:48AM - 10:00AM |
K11.00008: Non-exponential decay of persistent photocarriers in an AlGaN/AlN/GaN heterostructure David Daughton, BoKuai Lai, Jeffrey Lindemuth We report on the kinetics of persistent photoconductivity (PPC) in AlGaN/AlN/GaN heterostructures grown on silicon substrates. Under sub-bandgap illumination (1.91, 2.10, and 2.73 eV), photocarriers from ionized, deep-level defects increase the 2DEG conductivity which persists for hours to days after the illumination has been removed. Using a novel Hall Effect characterization protocol, a logarithmic decay of the persistent carrier concentration, independent of carrier scattering, was observed. The measured logarithmic decay is consistent with a physical separation of defects from the conducting channel and is thought to result from a recombination front propagating through the heterostructure. Persistent photocarrier concentration kinetics were studied as a function of photon dose, wavelength, and temperature. At the shortest wavelength, temperature-dependent studies show the coexistence of two logarithmic decay channels and could indicate PPC in this heterostructure arises from multiple defect species or, perhaps, the defects have different spatial proximity to the conducting channel. |
Wednesday, March 6, 2019 10:00AM - 10:12AM |
K11.00009: Incorporation of Boron in Gallium Nitride Mark E. Turiansky, Jimmy Shen, Darshana Wickramaratne, Chris Van de Walle Ternary alloys of the group-III nitrides are integral in the design of electronic and optoelectronic devices. To extend the capabilities of these alloys, boron-containing nitride alloys, such as BxGa1-xN (BGaN), are being considered. Boron nitride, however, is most stable in the hexagonal phase, unlike GaN which is stable in the wurtzite phase. As such, there is a lack of experimental information on the properties of wurtzite-phase boron-containing alloys. Using first-principles calculations with a hybrid functional, we explore the nature of boron incorporation in GaN. In the dilute limit, we consider substitutional and interstitial incorporation of boron. We also study the change in the electronic structure of GaN with increasing boron concentration. Wurtzite BN has an indirect band gap, while GaN has a direct band gap. We investigate the direct and indirect nature of the alloy band gap using a projection scheme and by calculating dipole matrix elements. We find that the fundamental band gap of BGaN is direct for boron concentrations below 50%. The large band-gap bowing results in a fundamental band gap that is effectively unchanged from the GaN band gap for a wide range of B concentrations. |
Wednesday, March 6, 2019 10:12AM - 10:24AM |
K11.00010: Identifying Defects and their Electronic Signatures in Regrown GaN Heterostructures Jiaheng He, Guanjie Cheng, Davide Del Gaudio, Jordan M Occena, Fabian Naab, Rachel Goldman, Mohsen Nami, Bingjun Li, Jung Han Although silicon-based electronics are used to power light-emitting diodes and electric vehicles, their utility in high power applications is limited by a low breakdown voltage. The most promising alternative power devices consist of vertical GaN devices, which often require regrown active regions. Thus, advances in high power device performance require a detailed understanding of the influence of regrowth processing steps on interfacial defects and their electronic signatures. In this work, we examine a series of GaN p-i-n structures prepared with and without ex-situ ambient exposure and/or chemical etching. To quantify the concentration of various native and extrinsic point defects, we utilize a combination of ion beam analyses in conjunction with x-ray diffraction. For all samples, channeling Rutherford backscattering spectroscopy data reveals minimum yield values < 2%, with displaced atom densities ranging from 1 to 3 x 1020/cm3. For all samples, cathodoluminescence spectroscopy reveals the GaN near-bandedge and donor-acceptor pair luminescence. We discuss the influence of interface regrowth on variations in the density of displaced Ga atoms and the intensity of yellow and infrared luminescence. |
Wednesday, March 6, 2019 10:24AM - 10:36AM |
K11.00011: Spectrally resolved dynamics of energy transfer in GaN:Eu Ruoqiao Wei, Brandon Mitchell, Dolf Timmerman, Tom Gregorkiewicz, Wanxin Zhu, Yasufumi Fujiwara, Volkmar R G Dierolf Europium(Eu) doped Gallium Nitride has demonstrated great potential as the red-emitting active layer in nitride-based light emitting diodes(LEDs). External quantum efficiency of up to 9.2% have been achieved. In such optimized layers, we performed a systematic series of temperature dependent, spectrally and time resolved photoluminescence measurements with goal to understand the energy transfer processes between the host material and the various energy levels of the Eu ions. Eu ions are affected by their local environment resulting in at least eight different centers that can be spectrally distinguished. For the most dominant centers, we find that the energy transfer to the Eu ion takes place on a time scale faster than 5 ns. Depending on incorporation site, the energy transfer leads to the population of the 5D0 and/or the 5D2 state. Direct energy transfer to the 5D1 states is negligible. We further show that Eu ions in their long-lived 5D0 state can be excited further leading the creation of electron hole pairs which subsequently relax and transfer their energy back to the Eu ions. This leads to a change in the color of the total emitted light and enables color tunability of the LEDs. |
Wednesday, March 6, 2019 10:36AM - 10:48AM |
K11.00012: Photoluminescence quantum efficiency of Nd optical centers in GaN epilayers Yifei Wang, Ho Vinh, Vinh Q Nguyen
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Wednesday, March 6, 2019 10:48AM - 11:00AM |
K11.00013: Interface chemistry and electrical characteristics of 4H-SiC/SiO2 after nitridation in varying atmospheres Anna Regoutz SiC/SiO2 is a prototypical wide-bandgap semiconductor/dielectric interface, which represents the challenges faced by many such material systems. A multitude of different defects leads to unacceptably large defect densities near the SiC conduction band and management of interfacial defects still remains a topic of lively discussion and current interest. |
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