Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session F67: Defects in Gallium Oxide and Wide Band Gap MaterialsFocus Recordings Available
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Sponsoring Units: DMP Chair: Lasse Vines, University of Oslo Room: Hyatt Regency Hotel -Hyde Park |
Tuesday, March 15, 2022 8:00AM - 8:36AM |
F67.00001: Defects in – and on – Gallium Oxide Invited Speaker: Matthew D 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. Defects in and on these substrates will affect the performance of device structures grown on them. This presentation will discuss recent experimental studies on Czochralski-grown β-Ga2O3 single crystals doped with Mg, Zn, and Cu acceptors. While Mg and Zn result in semi-insulating material, Cu exhibits the unexpected property of persistent photodarkening. In addition to defects in the bulk, spectral microscopy has revealed several specific defects on the surface. Some of these localized centers are very bright UV emitters. By combining photoluminescence and Raman spectroscopy, intriguing clues are being found about the identity of these mysterious surface defects. |
Tuesday, March 15, 2022 8:36AM - 8:48AM |
F67.00002: Vibrational spectroscopy of OH-shallow-donor complexes in Ga2O3 Andrew D Venzie, Amanda Portoff, Michael J Stavola, Stephen J Pearton, W B Fowler Si and Ge on a Ga(1) site are n-type dopants in Ga2O3 [1,2]. Si impurities are also responsible for the unintentional n-type doping of Ga2O3. Recent experiments show that the Si donor can be passivated by H [3]. A Si-doped epitaxial layer grown by MBE and a Ge-doped layer fabricated by ion implantation were obtained for our studies. The introduction of H and D by plasma treatment into these Ga2O3 samples gave rise to OH-Si and OH-Ge complexes and the corresponding OD complexes. The vibrational frequencies of the OH-Si and OH-Ge complexes are 3478 and 3425 cm-1 (77K), respectively, and show a strong dependence on the donor impurity. The polarization properties, temperature dependences, and annealing stabilities of these OH-donor complexes have also been investigated to obtain clues about the microscopic structures of the defects. |
Tuesday, March 15, 2022 8:48AM - 9:00AM |
F67.00003: Vibrational spectroscopy of OH-deep acceptor complexes in Ga2O3 Amanda Portoff, Andrew D Venzie, Michael J Stavola, W B Fowler, Stephen J Pearton Ga2O3 is doped with Fe to make it semi-insulating [1]. Other deep acceptors such as Mg also give rise to semi-insulating behavior [2]. The introduction of H (D) into Ga2O3 doped with Fe or Mg gives rise to O-H (O-D)vibrational lines near 3490 (2585) cm-1 for both dopants [2,3]. Because Fe is frequently an unintentional contaminant in Ga2O3, we have performed additional experiments to investigate whether separate OH-deep acceptor complexes that contain Fe and Mg do indeed exist. We have studied deep-acceptor doped samples with SIMS to determine the concentrations of Fe and Mg and have also probed the polarization properties, temperature dependences, and annealing stabilities of the OD-deep acceptor complexes for Fe and Mg doped samples. Our results support the formation of distinct OD-Fe and OD-Mg complexes with very similar vibrational properties. |
Tuesday, March 15, 2022 9:00AM - 9:12AM |
F67.00004: Vibrational properties and defect structures for hydrogen trapped near impurities in β-Ga2O3 W B Fowler, Amanda Portoff, Andrew D Venzie, Michael J Stavola Portoff et al.[1] and Venzie et al.[2] have extended earlier studies[3-4] of the vibrational properties and related structures of defects in β-Ga2O3 that involve hydrogen trapped at a Ga(1) vacancy to include impurity donor- and acceptor-related defects that contain hydrogen. In addition to their thermal properties and frequencies, the polarization properties of these defects in this monoclinic system provide important clues as to their structures[1,2,5,6]. We are using the hybrid DFT CRYSTAL17 code[7] to assess candidates for these donor (such as Si, Ge) and acceptor (such as Fe, Mg, Ca) complexes within the constraints established by their observed properties. |
Tuesday, March 15, 2022 9:12AM - 9:24AM |
F67.00005: Electrically Detected Magnetic Resonance and Near-Zero Magnetoresistance in 4H-SiC pin Diodes Ashton Purcell
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Tuesday, March 15, 2022 9:24AM - 9:36AM |
F67.00006: Role of carbon and hydrogen in limitingn-type doping of monoclinic (AlxGa1-x)2O3 Sai Mu, Mengen Wang, Joel B Varley, John L Lyons, Darshana Wickramaratne, Chris G Van de Walle Effective control of the carrier concentration is essential for Ga2O3-based high-power electronics. Using density functional theory, we explore the self-compensation of Si donors and the impact of carbon and hydrogen impurities on doping and compensation in (AlxGa1-x)2O3. We find that Si is an effective donor in (AlxGa1-x)2O3 up to 70% Al before forming a DX center. We further demonstrate that Si donors in (AlxGa1-x)2O3 can be compensated by interstitial H at Al concentrations exceeding 1% and by substitutional Ccation at 5% Al. The diffusivity of H and the likelihood of complex formation are also assessed. A stable Ccation-H is revealed, which is electrically neutral in n-type Ga2O3 but turns into a compensating center at 54% Al in (AlxGa1-x)2O3. CO-H incorporates easily in (AlxGa1-x)2O3, acting as an acceptor. This provides a possible source of C-related compensation in oxygen-poor Ga2O3 grown by metal-organic chemical vapor deposition. Our study highlights that, while Si is in principle a suitable shallow donor in (AlxGa1-x)2O3 alloys up to high Al compositions, control of unintentional impurities is essential to avoid compensation. |
Tuesday, March 15, 2022 9:36AM - 9:48AM |
F67.00007: Self-trapped holes and polaronic acceptors in ultrawide bandgap oxides John L Lyons Although Ga2O3 is widely believed to be the among most promising ultrawide-bandgap semiconductors, its inability to be p-type doped hampers future applications. Recently, other oxide materials have emerged as potential competitors to Ga2O3, but their propensity for hole conductivity is less well known. Here the stability of hole polarons in a set of ultrawide-bandgap oxides (Ga2O3, Al2O3, ZnGa2O4, MgGa2O4, LiGaO2 and GeO2) is examined and compared, both in pristine material and in the presence of acceptor impurities. Holes spontaneously self-trap in all oxides investigated, with varying stabilities. Acceptor impurities further stabilize these trapped holes, leading to large acceptor ionization energies. Hole trapping also leads to characteristic distortions and distinct optical transitions, which may explain some experimentally-observed signals. These results indicate that achieving p-type conductivity in any of these oxides is unlikely, with the possible exception of GeO2. |
Tuesday, March 15, 2022 9:48AM - 10:00AM |
F67.00008: Optical transition of neutral Mg in Mg-doped β-Ga2O3 Suman Bhandari, Mary E Zvanut, John L Lyons, Darshana Wickramaratne Mg-doped gallium oxide (β-Ga2O3) is semi-insulating and can be useful for power electronic devices. Understanding defects and their charge-transition levels is critical for the success of Ga2O3-based devices. Here, we investigate optical transitions of Mg0 using photoinduced electron paramagnetic resonance (photo-EPR), an absorption spectroscopy in which charge state changes are detected by the EPR intensity. Photo-EPR measurements are carried out at 130 K by illuminating Mg-doped Ga2O3 crystals, grown by Czochralski or floating zone method. Steady-state results show an onset of the charge transition of Mg0 near 1.6 eV, larger than the Mg-/0 level predicted by our density functional theory (DFT) calculations. However, the optical cross section spectrum derived from time-dependent measurements agrees well with results from a model using the DFT-calculated relaxation energy. We conclude that the observed neutral-to-negative transition of Mg involves an electron transition from the valence band to the Mg0 and the Mg-/0 level lies ~1.35 eV above the valence band maximum, with a relaxation energy of ~1.10 eV. |
Tuesday, March 15, 2022 10:00AM - 10:12AM |
F67.00009: Investigation of p-Type Doping in β-and κ-Ga2O3 Charles J Zeman, George C Schatz, Martin A Mosquera, Samuel M Kielar, Leighton O Jones We have systematically investigated the effects of all possible combinations of vacancies and silicon substitutions on the electronic structure of the β and κ phases of Ga2O3 using plane-wave density functional theory (DFT) methods. It was found that VGa defects are associated with a sufficient shift of the Fermi level to lower energy to induce p-type behavior, with formation energies in the range of 9.0 ± 0.2 eV. Calculations with single atom substitutions in the κ phase, including nitrogen, phosphorous, and silicon, did not show p-type character, although NO substitutions may lead to shallow acceptor states. In the pursuit of elucidating how MOCVD growth of Ga2O3 can result in p-type behavior, as indicated by experimental results in the literature, we examined the role of combining hydrogen and silicon substitutions. The results showed that p-type behavior is observable when gallium atoms are substituted for hydrogen within the coordination sphere of SiO substitutions. This shows that silicon can act as an amphoteric dopant for p-type Ga2O3 semiconducting materials when hydrogen is included with formation energies < 6.0 eV. |
Tuesday, March 15, 2022 10:12AM - 10:24AM |
F67.00010: First-principles study of planar defects in β-Ga2O3 Mengen Wang, Sai Mu, Chris G Van de Walle β-Ga2O3 is a wide-bandgap semiconductor which is promising for high-power electronic device applications. Planar defects such as stacking faults and twin boundaries have been observed in β-Ga2O3. We use density functional theory to explore the energetics and electronic structure of these defects. We find that twin boundaries can form on the (001)A, (001)B, (100)A, (100)B and (102) planes. The (100)A twin boundary has the lowest formation energy, indicating that it is more likely to find twin-boundary structures during the (100)-oriented growth. Scanning transmission electron microscopy (STEM) studies show that a stacking fault forms at a (010) plane in samples in which the growth direction switches from [010] to [001] [APL Mater. 8, 031104 (2020)]. We explain the origin of this stacking fault and describe its structure and energetics. |
Tuesday, March 15, 2022 10:24AM - 10:36AM |
F67.00011: First principle theory and experimental evaluation of low dimensional β-Ga2O3 resistive switching memory Chandrasekar Sivakumar, Mon-Shu Ho Resistive switching (RS) memory is one of the prospective future generation nonvolatile memories, with potential advantages over other emerging nonvolatile memories such as DRAM, PCRAM, CBRAM, and so on. The scientific community has comprehensively investigated dielectric thin films as resistive switching material, but the feasibility of low-dimensional (LD) materials has not been broadly examined. One of the promising candidates investigated for diverse semiconductor device applications is beta phase gallium oxide (β-Ga2O3). The objective of this work was to perform a theoretical and experimental evaluation of the RS phenomenon in LD:β-Ga2O3 nanostructures and deliver a profound insight. The LD:β-Ga2O3 resistive memory devices were experimentally validated on nanostructures grown by the VLS method. The as-grown nanostructures were subjected to the physical and chemical characterizations, which revealed the presence of spontaneous defects, particularly oxygen vacancies, and further examinations were performed to validate the role of oxygen vacancies. Thus, the analogous structures consistent with the DFT framework were modeled, and the fundamental physical and electronic properties were examined to deliver a profound insight. |
Tuesday, March 15, 2022 10:36AM - 10:48AM |
F67.00012: Ga vacancy-mediated diffusion of Sn donors in β-Ga2O3 Ymir Kalmann Frodason, Patryk P Krzyzaniak, Lasse Vines, Joel B Varley, Chris G Van de Walle, Klaus Magnus H Johansen Diffusion of the n-type dopant Sn in β-Ga2O3 is studied using secondary-ion mass spectrometry combined with first-principles calculations. Diffusion of Sn from a Sn-doped bulk substrate with surface orientation (001) into an epitaxial layer is observed after heat treatments under O2 gas flow in the temperature range 1000-1250°C. Calculated formation energies of Sn-related and intrinsic defects suggest that the migration of Sn is primarily mediated by Ga vacancies (VGa), through the formation and dissociation of intermediate mobile VGaSnGa complexes. The Sn concentration-depth profiles reveal diffusion starting at 1050°C, and all profiles exhibit a plateau and a sharp drop in Sn concentration at the diffusion front. The evolution of the Sn diffusion profiles is well-described by a reaction-diffusion (RD) model, yielding diffusion parameters for the VGaSnGa complex. The effect of lattice anisotropy is studied by calculating migration barriers for the VGaSnGa complex, using the nudged elastic band method. Good agreement is found between the RD modelling results and the first-principles calculations. Notably, the energetics of the complex correlate with the favored coordination environment of Sn, and this trend is further explored by comparing with calculations for Si and Ge donors. |
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