Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P53: Gallium OxideFocus Live
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Sponsoring Units: DMP DCOMP FIAP Chair: Ramon Collazo, North Carolina State University |
Wednesday, March 17, 2021 3:00PM - 3:36PM Live |
P53.00001: Hydrogen-related defects in Ga2O3 Invited Speaker: Michael Stavola β-Ga2O3 is an ultra-wide bandgap semiconductor that has attracted much attention because of the applications it promises. Hydrogen centers have been found to be a cause of n-type conductivity and also to interact with cation vacancies in several transparent conducting oxides [1]. Ga2O3 shows similar behaviors and a family of hydrogenated Ga(1) vacancy centers based on shifted configurations of the vacancy [2-4]. Infrared spectroscopy coupled with theory have revealed a wealth of information about these defects. The combination of O-H vibrational spectra, their isotope dependence, and polarization properties with theoretical analysis has determined the microscopic properties of the hydrogen centers observed in this low-symmetry structure [5-7]. This work is the result of collaborations with W. B. Fowler, S. J. Pearton and the Ph.D students listed in the references below [5-7]. |
Wednesday, March 17, 2021 3:36PM - 3:48PM Live |
P53.00002: Hydrogen trapped at perturbed Ga(1) vacancies in β-Ga2O3 W Fowler, Amanda Portoff, Andrew Venzie, Ying Qin, Michael Stavola It was discovered several years ago that the primary O-H vibrational absorption in β-Ga2O3 arises from two H trapped at a shifted Ga(1) vacancy[1], and that other features arise from one or more H trapped at the same defect with a different shifted configuration[2]. Subsequent work has yielded methods to obtain the dipole directions of such defects[3,4] and is revealing additional versions of defects with these two shifted configurations, but perturbed by lattice defects or atomic impurities, as reported in this meeting[5,6]. We are using the hybrid DFT CRYSTAL17 code[7] to assess candidates for these perturbed configurations and the constraints on these configurations established by the experimental results. |
Wednesday, March 17, 2021 3:48PM - 4:00PM Live |
P53.00003: Vibrational properties of oxygen-hydrogen centers in H+- and D+-implanted Ga2O3 Amanda Portoff, Andrew Venzie, Ying Qin, Michael Stavola, W Fowler, Stephen John Pearton The ion implantation of H+ and D+ into Ga2O3 produces several O-H and O-D centers that have been investigated by vibrational spectroscopy. These defects include the dominant VGa(1)-2H and VGa(1)-2D centers studied previously [1] in addition to additional defects that can be converted into this structure by thermal annealing. The polarization dependence of the spectra has been analyzed [2] to determine the directions of the transition moments of the defects and to provide information about defect structure [3]. The interaction of VGa(1)-nD centers with other defects in the implanted samples has also been investigated to help explain the number of O-D lines seen and their reactions upon annealing. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P53.00004: Acceptors in gallium oxide Jani Jesenovec, Jacob Ritter, Christopher Pansegrau, John McCloy, Matthew McCluskey Monoclinic gallium oxide (β-Ga2O3) is an ultra-wide bandgap semiconductor with potential applications in power electronics. Semi-insulating substrates are required for most practical devices such as metal-oxide-semiconductor field effect transistors. This presentation will discuss recent experimental studies on Czochralski-grown β-Ga2O3 single crystals doped with Mg and Zn acceptors. These dopants result in semi-insulating material and are likely compensated by oxygen vacancies and shallow donors. Ir impurities originating from the crucible form deep donors that also compensate acceptors. The Ir4+ oxidation state gives rise to an absorption threshold in the visible/UV part of the spectrum and an IR absorption peak at 5150 cm-1. Acceptors are passivated by hydrogen, an omnipresent contaminant, resulting in IR absorption peaks corresponding to O-H vibrational modes near 3300 cm-1. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P53.00005: Evidence for an Fe-H complex in Ga2O3 Andrew Venzie, Amanda Portoff, Michael Stavola, W Fowler, Stephen John Pearton Ga2O3 is doped with Fe to make it semi-insulating [1]. We have annealed Ga2O3:Fe samples obtained from the Tamura Corp. in D2 at elevated temperatures. This treatment produces an O-D vibrational line at 2578 cm-1 that has not been seen previously in deuterated Ga2O3 samples not containing Fe [2]. A treatment in a mixture of H2 and D2 produces an additional line at 2585 cm-1 suggesting that this O-D center contains two identical D atoms. One of the coupled O-D modes is allowed, and a second is forbidden, similar to the VGa(1) -2D center studied previously [3]. Centers containing both H and D give an additional dynamically decoupled mode. These results provide evidence for an Fe-(OD)2 complex in semi-insulating Ga2O3. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P53.00006: Acceptor-donor complexes in Ga2O3 Intuon Chatratin, Fernando Pereira Sabino, Pakpoom Reunchan, Anderson Janotti Ga2O3 has attracted great attention as a promising material for high power electronic devices due to a very large band gap and high breakdown voltage, with high Baliga’s figure of merit. Ga2O3 can be easily doped n-type, with Si, Ge, or Sn as shallow donors, but difficult to dope p-type. All tested candidate acceptor impurities lead to deep acceptor levels, lying at ~1 eV above the valence band. These deep acceptors are still useful for making Ga2O3 semi-insulating, adding flexibility to device design, and the interactions between acceptors, such as nitrogen, and donors may play important role in making Ga2O3 semi-insulating. Using electronic structure calculations based on hybrid density functional theory, we investigate the interactions between acceptor and donor impurities in different possible configurations of acceptor-donor complexes considering all the inequivalent cation and anion sites of the β-Ga2O3 crystal structure. We calculate binding energies of the complexes, and discuss changes in transition levels compared to those of the isolated species. These results aim at facilitating the experimental characterization of acceptor impurities to further the development of Ga2O3-based electronic devices. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P53.00007: Mid-gap defect levels of gallium oxide Suman Bhandari, Mary Ellen Zvanut As Ga2O3 continues to grow as a material for power electronics, consideration of impurities as well as intrinsic defects becomes essential. Here, Fe- and Mg-doped Ga2O3 crystals are irradiated between 0.6 and 4.8 eV, and the effect on Fe3+ and Ir4+ is analyzed at 30 K using electron paramagnetic resonance (EPR). The amount of Fe3+ decreases above 2.3 eV which is unlikely a direct ionization of Fe since the Fe2+/3+ level lies ~0.7 eV below conduction band edge (EC). Indeed, in Ga2O3:Fe, Ir4+ increases when Fe3+ decreases, indicating that the electron excited from Ir3+ to EC is captured by Fe3+. However, the decrease for Fe3+ is more than the increase for Ir4+, suggesting Ir alone cannot account for the decrease. Furthermore, in Ga2O3:Mg, Fe3+ decreases with no change of Ir4+. These results indicate that other mid-gap defects, likely intrinsic, also contribute electrons to Fe3+. The photo-threshold of 2.3 eV suggests that the Ir3+/4+ and the intrinsic defect levels lie ~2.3 eV from band edges. Moreover, the intrinsic defects are likely oxygen vacancies, VO, since there are several levels ~2 eV below EC. Thus, we suggest that the role of intrinsic defects in charge trapping will be important in Ga2O3:Fe films grown for device development. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P53.00008: Measurement of Ultrafast Dynamics of Gallium Vacancy Charge States in β-Ga2O3 Arjan Singh, Okan Koksal, Nicholas Tanen, Jonathan McCandless, Debdeep Jena, Huili Grace Xing, Hartwin Peelaers, Farhan Rana The study of point defects in β-Ga2O3 is essential to establish it as a material platform for high-power electronics. We present results from ultrafast optical-pump supercontinuum-probe spectroscopy measurements on β-Ga2O3. Use of a supercontinuum probe allows us to obtain the time-resolved absorption spectra of defects under non-equilibrium conditions with picosecond resolution. The defect absorption spectra shows peaks at two energies, ~2.2 eV and ~1.63 eV, and is maximum for light polarized along the crystal c-axis. The strength of the absorption associated with each peak is time-dependent; the spectral weight shifts from the lower energy peak to the higher energy peak with pump-probe delay. We are able to fit the time and polarization dependent probe absorption spectra for all probe wavelengths, at all pump-probe delays with a single set of rate equations for a three-level defect, and thus attribute the observed absorption features to Gallium vacancies. The three levels of the defect correspond to the three charge states (-1, -2, and -3) of Gallium vacancies. Finally, our results demonstrate that ultrafast supercontinuum spectroscopy is a useful tool to explore defect states and their dynamics in semiconductors. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P53.00009: Computational Fermi level engineering and doping-type conversion of Ga2O3 via three-step processing Anuj Goyal, Andriy Zakutayev, Vladan Stevanovic, Stephan Lany Ga2O3 is being actively explored for power electronics, and other applications due to its ultra-wide bandgap and low projected fabrication cost of high-quality crystals. N-type doping of Ga2O3 can be achieved and tuned, but p-type doping faces fundamental obstacles. Successful engineering of Ga2O3 based devices requires critical control of doping density, Fermi level position, providing opportunities for predictive process simulation. We use first-principles defect equilibrium calculations to simulate a 3-step growth-annealing-quench protocol for hydrogen assisted Mg doping in Ga2O3, taking into account the hydrogen-oxygen-water gas phase equilibrium. We predict type conversion to a net p-type regime following O-rich annealing after growth under reducing conditions in the presence of H2. We show that there is an optimal temperature that maximizes the net acceptor density during the equilibrium annealing step for a given Mg doping level. Quenching of non-equilibrium annealed samples then results in a Fermi level EF below mid-gap down to about EV +1.5 eV, creating a significant number of uncompensated neutral MgGa0 acceptors. This type converted Ga2O3 can create significant built-in field in a p-n junction with an adjoining n-type material and could enable vertical MOSFETs. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P53.00010: Reversible control of electronic transport in ionic-gel gated β-Ga2O3 thin films Anil Rajapitamahuni, Anusha Kamath Manjeshwar, Praneeth Ranga, Sriram Krishnamoorthy, Bharat Jalan Motivated by applications in power electronics, rapid progress has been made in the doping studies of β – Ga2O3 (BGO), although questions remain on the doping dependence of electron transport and mobility optimization at high doping densities. In this talk, we present our observations on electron transport properties of ionic – gel gated BGO films, grown via metal-organic vapor-phase epitaxy. A wide reversible voltage window of ±3 V is obtained even at 300 K for the electrostatic doping of BGO, showing the robustness of BGO with regards to unwarranted redox reactions in electrolyte gating of oxides. Temperature dependent Hall effect measurements are performed to study the doping dependence of carrier densities and mobilities in both depletion and accumulation modes. Two channel conduction model is then combined with Thomas-Fermi modelling to extract the accumulation layer carrier densities and mobilities. We have achieved an order of magnitude increase in doping densities from ~1017 cm-3 at 0V to ~ 1018 cm-3 at 2.5 V. Correspondingly, the 300 K mobility improves from 98 cm2V-1s-1 to 140 cm2V-1s-1, with the increase of induced electron densities. We will further discuss the scattering mechanisms limiting the electron mobility in ionic-gel gated BGO thin films. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P53.00011: Incorporation of aluminum in (AlxGa1−x)2O3 alloys Mengen Wang, Sai Mu, Chris Van de Walle Monoclinic β-Ga2O3 is a wide-gap (4.8 eV) semiconductor that exhibits a high breakdown field. Alloying Ga2O3 with Al2O3 can further increase the band gap. Epitaxial growth of (AlxGa1−x)2O3 is important for forming heterojunctions to Ga2O3 for high power electronics applications. Density functional theory (DFT) studies have found that Al atoms prefer to occupy the octahedral sites in bulk (AlxGa1−x)2O3 alloys.1 However, recent experimental studies have demonstrated that Al can occupy tetrahedral sites in concentrations almost as high as the octahedral sites. In this work, we perform DFT calculations to explore the co-adsorption of Al, Ga, and O adatoms on the Ga2O3 (010) surface. We find that Al can adsorb on tetrahedral sites in most of the reconstructions. The migration barrier of an Al adatom escaping from a tetrahedral site to an octahedral site is 1.81 eV, indicating that Al diffusion is much more restricted than Ga diffusion on the surface. Our findings indicate that kinetic limitations are responsible for Al occupying both octahedral and tetrahedral sites in (AlxGa1−x)2O3 thin films. |
Wednesday, March 17, 2021 5:36PM - 5:48PM Live |
P53.00012: First-principles surface calculations for monoclinic Ga2O3 and Al2O3 and consequences for cracking of (AlxGa1-x)2O3 films Sai Mu, Mengen Wang, Hartwin Peelaers, Chris Van de Walle The wide-bandgap semiconductor Ga2O3 is a promising candidate for high-power electronics. Alloying with Al for (AlxGa1-x)2O3 films enables heterostructures that are essential for device applications. However, the limited thickness of (AlxGa1-x)2O3 films grown on Ga2O3 substrates is a serious obstacle. Here we employ first-principles calculations to determine the brittle fracture toughness of such films for three growth orientations of the monoclinic structure: [100], [010], and [001].1 Surface energies and elastic constants are computed for the end compounds—monoclinic Ga2O3 and Al2O3—and used to interpolate to (AlxGa1-x)2O3 alloys. The appropriate crack plane for each orientation is determined, and the corresponding critical thicknesses are calculated based on Griffith’s theory. Our in-depth analysis of surface energies for both relaxed and unrelaxed surfaces provides important insights into the factors that determine the relative stability of different surfaces. A key conclusion is that the critical thickness is largest for (AlxGa1-x)2O3 films grown along [100]. |
Wednesday, March 17, 2021 5:48PM - 6:00PM Live |
P53.00013: Fe3+ and Cr3+ Photoluminescence of Fe doped β-Ga2O3 Cassi Remple, Jesse Huso, Matthew McCluskey Semi-insulating single crystalline β-Ga2O3 is becoming increasingly useful as a substrate for device fabrication. Fe doping single crystalline β-Ga2O3 is a method for producing such substrates. Cr3+ Photoluminescence (PL) emission peaks at 690 nm (1.80 eV) 696 nm (1.78 eV), as well as a broad feature around 715 nm (1.74 eV), are observed in Fe-doped β-Ga2O3. PL mapping showed signatures of Cr3+ and Fe3+ defects. Intriguingly, Fe3+ defects showed a broad 950nm (1.3 eV) peak that was highly localized, occurring at discrete spots on the sample surface. This suggests either that there is not a homogenous distribution of Fe in the sample, or that the Fe3+ emissions observed are specific to localized regions of Fe3+ in α-Ga2O3. PL mapping also revealed bands of high and low intensity that could be due to Cr3+ impurity striations introduced during crystal growth. |
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