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  1. The key obstacle toward realizing integrated gallium nitride (GaN) electronics is its low hole mobility. Here, we explore the possibility of improving the hole mobility of GaN via epitaxial matching to II–IV nitride materials that have recently become available, namely, ZnGeN 2 and MgSiN 2 . We perform state-of-the-art calculations of the hole mobility of GaN using the ab initio Boltzmann transport equation. We show that effective uniaxial compressive strain of GaN along the [Formula: see text] by lattice matching to ZnGeN 2 and MgSiN 2 results in the inversion of the heavy hole band and split-off hole band, thereby lowering the effective hole mass in the compression direction. We find that lattice matching to ZnGeN 2 and MgSiN 2 induces an increase in the room-temperature hole mobility by 50% and 260% as compared to unstrained GaN, respectively. Examining the trends as a function of strain, we find that the variation in mobility is highly nonlinear; lattice matching to a hypothetical solid solution of Zn 0.75 Ge 0.75 Mg 0.25 Si 0.25 N 2 would already increase the hole mobility by 160%. 
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  2. Abstract

    This study uses density functional theory calculations to explore the energetics and electronic structures of planar defects in monoclinicβ‐Ga2O3, including twin boundaries (TBs) and stacking faults (SFs). TBs on the (001)A, (001)B, (100)A, (100)B, and (−102) planes are examined; it is found that (100)A has a very low formation energy (0.01 Jm2), consistent with its observation in a number of experiments. For SFs, SFs on the (100) plane have much lower energy (0.03 Jm2) than SFs formed on the (010) and (001) planes. Growth on a (100) surface is thus expected to result in more planar‐defect formation, again consistent with experimental observations. In spite of their higher energies, TBs and SFs on planes other than (100) have been experimentally observed in epitaxial layers. Their origins are explained in terms of coalescence of different growth regions when the growth direction changes, or when low‐energy TBs on the growing surface lead to domains with different twinning orientation.

     
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  4. Abstract

    Spin qubits based on shallow donors in silicon are a promising quantum information technology with enormous potential scalability due to the existence of robust silicon-processing infrastructure. However, the most accurate theories of donor electronic structure lack predictive power because of their reliance on empirical fitting parameters, while predictive ab initio methods have so far been lacking in accuracy due to size of the donor wavefunction compared to typical simulation cells. We show that density functional theory with hybrid and traditional functionals working in tandem can bridge this gap. Our first-principles approach allows remarkable accuracy in binding energies (67 meV for bismuth and 54 meV for arsenic) without the use of empirical fitting. We also obtain reasonable hyperfine parameters (1263 MHz for Bi and 133 MHz for As) and superhyperfine parameters. We demonstrate the importance of a predictive model by showing that hydrostatic strain has much larger effect on the hyperfine structure than predicted by effective mass theory, and by elucidating the underlying mechanisms through symmetry analysis of the shallow donor charge density.

     
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