Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session G63: Defects in Gallium Oxide and NitrideFocus
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Sponsoring Units: DMP DCMP FIAP Chair: Anderson Janotti, University of Delaware Room: Mile High Ballroom 4D |
Tuesday, March 3, 2020 11:15AM - 11:51AM |
G63.00001: Defect Spectroscopy of Ga2O3 Invited Speaker: Steven Ringel Gallium oxide is an emerging ultra-wide bandgap semiconductor that holds enormous promise for devices needed to operate in extreme conditions, including high electric fields, high temperatures and radiation environments. Attributes such as its ~ 4.8 eV bandgap, ease of n-type doping, ability to synthesize AlGaO alloys to enable heterostructure devices, availability of melt-grown large area Ga2O3 substrates, and device figures of merit that theoretically exceed more mature wide bandgap technologies based on SiC and GaN, all contribute to rapid acceleration of research in this area. To date, Ga2O3 materials and devices are being grown using a wide range of epitaxial methods, and identifying point defects that are associated with growth conditions, and discriminating between impurity and native physical sources are key to guiding the advancement of material quality so that Ga2O3 can reach its ultimate properties desired for applications in RF power electronics and UV optoelectronics. This presentation focuses on the ongoing investigation of defect states in the Ga2O3 bandgap using a range of defect spectroscopy methods that enable comprehensive characterization of deep level defects across the entire ~ 4.8 eV bandgap. Deep level optical spectroscopy, deep level transient (thermal) spectroscopy and admittance spectroscopy are applied to Ga2O3 materials and devices grown by MBE, MOCVD, LPCVD and EFG methods to enable a quantitative comparison of how defect introduction varies with growth method. High energy particle radiation is used to distinguish native from impurity-based defect states provides further differentiation regarding physical sources. Identification of which defects contribute to important properties for device transport, such as carrier compenstation and electron mobility, is discussed. |
Tuesday, March 3, 2020 11:51AM - 12:03PM |
G63.00002: Ab initio studies on segregation of n-type dopants and vacancies near beta-Ga2O3(010) surface Jingyang Wang, Paulette Clancy Beta-Ga2O3 is a wide-bandgap semiconductor with important electronic and optoelectronic applications. It has been observed that upon extended period of thermal annealing, certain types of n-type dopants such as Sn show strong tendency of segregation towards Ga2O3 surfaces, even forming a secondary SnO2 phase. Using density functional theory, we conducted a comprehensive study on the thermodynamic preferrence of common n-type dopants (Si_Ga, Ge_Ga, Sn_Ga) and intrinsic defects (V_Ga, V_O) both near the (010)-surface and in the bulk of Ga2O3. The key findings are: (1) in bulk Ga2O3, Si prefers to occupy tetrahedral over octahedral Ga site (by -0.60 eV), Sn shows the opposite preference (by -0.90 eV), and Ge have almost equal preferrence on either Ga site; (2) Si and Ge shows weaker likelihood of segregation towards Ga2O3(010) surface (-0.14 eV for Si, -0.11 eV for Ge) than Sn (-0.25 eV), while both V_Ga and V_O assume strong tendency of segregation (< -0.4 eV); (3) all dopant species (Si, Ge, Sn) show significantly decreased (increased) segregation energy with the presence of surface V_Ga (V_O); (4) co-doping with Si or Ge in the second-nearest-neighbor configuration can raise the segregation energy of Sn. The implications for experimental processing conditions will be discussed. |
Tuesday, March 3, 2020 12:03PM - 12:15PM |
G63.00003: Electronic properties of the Ga2O3-Fe2O3 system Shoaib Khalid, Fernando Sabino, Anderson Janotti Fe2O3 has attracted a lot of interest due to its interesting electronic, chemical and magnetic properties, with applications in photoelectrochemical cells, gas sensor, and Li ion batteries. It is readily available as one of the most abundant material found on earth. On the other hand Ga2O3 is a wide-band-gap semiconductor promising for high power electronic devices and UV blind photodetectors. Here we present results for the electronic structure of Fe2O3 and Ga2O3 in both monoclinic and corundum phases using different exchange correlation functionals. We show how the valence and conduction band edges change with respect to the vacuum level depending upon the choice of functional used for Fe2O3 and Ga2O3. We discuss their band alignments, and the electronic structure and stability of the Ga2O3-Fe2O3 alloys. Finally, we discuss the possibility of forming a p-n heterojunction with p-Fe2O3 and n-Ga2O3 and its characteristics. |
Tuesday, March 3, 2020 12:15PM - 12:27PM |
G63.00004: Unusual Formation of Point Defects and Their Complexes in Ultra-wide Band Gap Beta-Ga2O3 Jared Johnson, Zhen Chen, Joel Varley, Christine Jackson, Esmat Farzana, Aaron Arehart, Hsien-Lien Huang, Steven Ringel, Chris Van de Walle, David Muller, Jinwoo Hwang Beta-Ga2O3 has unique advantages including high breakdown voltage and availability as bulk substrates, which make it a viable candidate for next-generation power device applications. We present the first direct microscopic observation of the unusual formation of interstitial-divacancy complexes within beta-Ga2O3 lattice using atomic resolution scanning transmission electron microscopy. We observed that cation atoms are present in multiple interstitial sites, and each interstitial atom is paired with two adjacent vacancies. The observed structure is consistent with density functional theory calculation , which predicts them to be compensating acceptors. The number of the complexes increase as a function of Sn doping, which matches with the increase of the trap state at Ec - 2.1 eV measured using deep level optical spectroscopy, strongly suggesting that the defects correspond to that trap level. We also show that two neighboring complexes can further relax the structure in between, creating more cation vacancies. Our finding provides new crucial information on the exact structural origin of the properties of beta-Ga2O3 that has been unobtainable using other methods. |
Tuesday, March 3, 2020 12:27PM - 12:39PM |
G63.00005: Donor and acceptor properties in Ga2O3 polymorphs John Lyons, Darshana Wickramaratne, Joel Varley An ultra-wide band gap and substrate availability make Ga2O3 an attractive power electronic material. But questions still remain as to how best control its electrical conductivity through impurity doping. In this work, we evaluate the properties of acceptor and donor dopants in Ga2O3 polymorphs using first-principles calculations based on hybrid density functional theory. Hole localization and low-lying valence bands are exhibited by all Ga2O3 phases, causing deep acceptor behavior for all impurities considered. These polymorphs also share other properties, such as similar positions of the deep oxygen vacancy defect levels. In contrast, cation-site donors (such as C, Si, Ge, and Sn) are found to be shallow. These donors do exhibit defect transition levels above the Ga2O3 conduction band minimum, which may become relevant if Ga2O3 is alloyed with aluminum and the band gap increases. |
Tuesday, March 3, 2020 12:39PM - 12:51PM |
G63.00006: Optical Properties of Acceptor Impurities in Ga2O3 Intuon Chatratin, Fernando Sabino, Pakpoom Reunchan, Anderson Janotti Ga2O3 has attracted great attention as a promising material for high power electronic applications due to a very large band gap, high breakdown voltage and ability to be doped n-type. The Ga2O3 Baliga’s figure of merit is only lower than that of diamond. Most importantly, large single crystals of Ga2O3 are available, facilitating epitaxial growth. Although p-type Ga2O3 have not been demonstrated as acceptor impurities tend to introduce deep acceptor levels, they can be used to make semi-insulating layers, and that can be useful in device designing. The deep levels of acceptor impurities in ultra-wide-band gap oxides are often difficult to probe experimentally. To facilitate the experimental characterization of the acceptor impurities, we employed hybrid density-functional calculations to investigate the optical transitions of acceptor impurities in monoclinic Ga2O3. We constructed configuration coordinate diagrams to determine the absorption and emission energies that can be compared with the optical absorption and photoluminescence measurements. The results show that all impurities exhibit deep acceptor transitions. The calculated emission energies are compared with the available experiment data. |
Tuesday, March 3, 2020 12:51PM - 1:03PM |
G63.00007: Transition levels for impurities in β-Ga2O3 Suman Bhandari, Mary Zvanut Bulk Ga2O3 is of interest for both electronic and optical applications. However, the large number of different impurities often complicates understanding of the basic material properties. Confusion can be minimized by directly monitoring individual impurities using photo-induced electron paramagnetic resonance (EPR). Specifically, the present work tests the validity of several proposed defect levels in Fe-doped and Mg-doped β-Ga2O3 crystals by identifying each impurity, Fe3+, Mg0, and Ir4+ with an EPR spectrum. The EPR intensity for the impurity is monitored during illumination from 1500 to 300 nm at 30 K or 130 K, and changes are associated with ionization. A photo-threshold between 2.0 and 2.3 eV where Ir4+ increases and Fe3+ decreases is assigned to the Ir3+/4+ defect level, and one between1.2 and 1.5 eV where Fe3+ increases is assigned the Fe2+/3+ level after consideration of the lattice relaxation. Ongoing study of the Mg-doped samples indicates that Mg0 decreases between 1.6 and 1.9 eV, consistent with reported levels for Mg-/0; however the photon energy dependence for the generation of the neutral Mg0 is not yet clear. The presentation will describe procedure and analysis details, particularly for the curious case of neutral Mg. |
Tuesday, March 3, 2020 1:03PM - 1:15PM |
G63.00008: Bismuth-alloyed Ga2O3 as a novel p-type transparent conducting oxide Xuefen Cai, Fernando P. Sabino, Anderson Janotti, Suhuai Wei Ga2O3 is a wide-band-gap semiconductor that has attracted great attention for applications in power electronics and UV-blind detectors. It is easy to make it n-type by adding Si, Ge, or Sn, yet difficult (or impossible) to make it p-type unless its electronic structure is fundamentally changed. Here we show that adding a few percent of Bi to Ga2O3 leads to an intermediate valence band that is sufficiently high in energy to enable p-type doping. Using first-principles calculations we study the electronic structure of Bi-alloyed Ga2O3 and the doping efficiency of possible acceptor impurities in this system. Specifically, we discuss the results for Mg, Zn, and Cu acceptors in (BixGa1-x)2O3, including their most favorable incorporation sites, formation energies, and transition energy levels. We show that dilute (BixGa1-x)2O3 alloys could be good candidates for novel p-type transparent conducting oxide that will create new opportunities in Ga2O3-based device design. |
Tuesday, March 3, 2020 1:15PM - 1:27PM |
G63.00009: Direct Imaging on Strain Relaxation of MBE-grown Single Phase alpha-(Al,Ga)2O3 on m-sapphire Substrate in Atomic Resolution Using Scanning Transmission Electron Microscopy Celesta Chang, Riena Jinno, Debdeep Jena, Huili Grace Xing, David Anthony Muller MBE-grown corundum structured alpha-Ga2O3 on c-plane sapphire substrates often contains considerable amount of beta-Ga2O3 due to c-plane facets. Recently, a successful MBE-growth of pure alpha-(Al, Ga)2O3 on m-plane sapphire was reported. Here, we show the relaxation mechanism of such films by performing strain analysis with scanning transmission electron microscopy (STEM). The films are partially relaxed through dislocation cores at the interface, some of which tend to climb up into the film. High angle annular dark field (HAADF)-STEM images in plan-view shows the formation of very thin gamma-Ga2O3 at the interface occupying less than 1% area density. Owing to its defect spinel-type structure that requires two cation vacancies, gamma-Ga2O3 is believed to form naturally at the interface to accommodate the strain arising from lattice mismatch. |
Tuesday, March 3, 2020 1:27PM - 1:39PM |
G63.00010: Ultrafast Optical Measurement of Defect Dynamics in β-Ga2O3 using Supercontinuum Pump-Probe Spectroscopy. Arjan Singh, Okan Koksal, Nicholas Tanen, Debdeep Jena, Huili Xing, Farhan Rana Ga2O3 polymorphs have shown great promise for high power devices. β-Ga2O3 has a large number of intrinsic and extrinsic defects with poorly understood properties. Non-equilibrium optical spectroscopy has proven to be a valuable tool in studying midgap defects in materials. In this work, we use optical pump-probe spectroscopy that employs a supercontinuum pulse to probe the defects and their ultrafast dynamics in a wide energy interval within the material bandgap. Our results show transient absorption of the probe pulse by midgap defects that is highly polarization selective. Our results show that hole capture times of defects scale with their separation in energy from the valence band with defects furthest away from the valence band exhibiting the slowest hole capture rates. Temperature dependence of hole capture rates indicate a thermally activated capture process involving lattice relaxation (multi-phonon process) that can be fitted with a Mott-Seitz expression. We present models to explain the nature of these defects and the ultrafast dynamics associated with the capture of photoexcited carriers by these defects. |
Tuesday, March 3, 2020 1:39PM - 1:51PM |
G63.00011: X-ray diffraction studies of GaN p-i-n structures for high power electronics Alexandra Zimmerman, Jiaheng He, GuanJie Cheng, Davide Del Gaudio, Jordan Occena, Fabian Naab, Mohsen Nami, Bingjun Li, Jung Han, Rachel Goldman 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. In this work, we examine the p-i interface of a series of GaN p-i-n structures prepared with and without ex-situ ambient exposure and/or chemical etching. For the “in-situ” GaN structure, elastic recoil detection analysis (ERDA) revealed the highest interfacial [H] and Rutherford backscattering shows the highest density of displaced Ga atoms, likely due to efficient incorporation of MgGa. Here, we report on x-ray diffraction studies of the crystallinity of the GaN p-i-n structures. The full width at half max (FWHM) of phi and omega scans were used to quantify the mosaicity and threading dislocation (TD) densities at the p-i interfaces. Interestingly, the lowest screw-type dislocation density and highest edge-type dislocation density are observed for the “in-situ” GaN structure. The relationship between interfacial [H], displaced Ga, and screw- and edge-type dislocations will be discussed. |
Tuesday, March 3, 2020 1:51PM - 2:03PM |
G63.00012: Influence of interfacial defects on the electronic states at GaN p-i-n diode interfaces GuanJie Cheng, Jiaheng He, Alexandra Zimmerman, Davide Del Gaudio, Fabian Naab, Mohsen Nami, Bingjun Li, Jung Han, Rachel Goldman Although silicon-based electronics are used for light-emitting diodes and electric vehicles, their utility in high power applications is limited by a low breakdown voltage. A promising alternative is vertical GaN devices, but these require regrown active regions. Here, we report on the influence of regrowth processing steps on interfacial defects and their electronic signatures. A comparison of GaN p-i-n structures prepared with and without ex-situ ambient exposure and/or chemical etching reveals the highest interfacial near-band edge (NBE) and donor-acceptor pair (DAP) cathodoluminescence (CL) emission from the in-situ structures. Interestingly, elastic recoil detection analysis (ERDA) and Rutherford backscattering spectroscopy reveal the lowest interfacial [H] but the highest fraction of displaced Ga atoms, suggesting efficient incorporation of MgGa. On the other hand, for the ex-situ structures, minimal interfacial [H] is observed, and the highest surface NBE and DAP CL emission is apparent. Finally, for the etched/regrown structures, ERDA reveals the highest interfacial [H], and significant yellow CL emission is observed. We will discuss relationships between interfacial [H], the fraction of displaced Ga atoms, CL emission features, and diode performance. |
Tuesday, March 3, 2020 2:03PM - 2:15PM |
G63.00013: Study of carbon-related point defects in C-doped GaN using photo-induced electron paramagnetic resonance spectroscopy Subash Paudel, Mary Zvanut, Michal Bockowski Incorporation of carbon impurities in GaN creates semi-insulating substrates. However, the role of carbon has not been fully understood. We used photo-induced electron paramagnetic resonance (EPR) to study point defects in mm thick free-standing 1017-1019 cm-3 C-doped GaN. An isotropic signal at g ~1.987 was observed at 3.5 K with an intensity which increases with carbon concentration, indicating the signal represents a C-related defect. In [C]< 6x1017 cm-3 samples, additional signals, a neutral donor with gpar=1.951 and gper=1.950, and an anisotropic signal with gpar~2.121, were observed. The intensity of the C-related and the donor signal start to increase at 2.75 eV and decrease at 0.95 eV. The same quenching threshold for C-related and donor signals suggests that 0.95 eV represents the energy required to excite an electron to C-related defects. The ionization threshold, 0.95 eV, is consistent with the predicted value for (-/0) transition level of CN. The decrease in donors during quenching is one order of magnitude less than the decrease in C-related defects, which suggests the change in C-related defects is not solely controlled by the donor but also by other defects such as gpar~2.121 defect. |
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