The dependence of electron mobility on growth conditions and threading dislocation density (TDD) was studied for n−-GaN layers grown by ammonia-based molecular beam epitaxy. Electron mobility was found to strongly depend on TDD, growth temperature, and Si-doping concentration. Temperature-dependent Hall data were fit to established transport and charge-balance equations. Dislocation scattering was analyzed over a wide range of TDDs (∼2 × 106 cm−2 to ∼2 × 1010 cm−2) on GaN films grown under similar conditions. A correlation between TDD and fitted acceptor states was observed, corresponding to an acceptor state for almost every c lattice translation along each threading dislocation. Optimized GaN growth on free-standing GaN templates with a low TDD (∼2 × 106 cm−2) resulted in electron mobilities of 1265 cm2/Vs at 296 K and 3327 cm2/Vs at 113 K.

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AlxGa1−xN/GaN (x = 0.06, 0.12, 0.24) and AlGaN/AlN/GaN heterostructures were grown on 6 H-SiC, GaN-on-sapphire, and free-standing GaN, resulting in heterostructures with threading dislocation densities of ∼2 × 1010, ∼5 × 108, and ∼5 × 107 cm−2, respectively. All growths were performed under Ga-rich conditions by plasma-assisted molecular beam epitaxy. Dominant scattering mechanisms with variations in threading dislocation density and sheet concentration were indicated through temperature-dependent Hall measurements. The inclusion of an AlN interlayer was also considered. Dislocation scattering contributed to reduced mobility in these heterostructures, especially when sheet concentration was low or when an AlN interlayer was present.

Metal-polar In 0.17 Al 0.83 N barriers, lattice-matched to GaN, were grown under N-rich conditions by plasma-assisted molecular beam epitaxy. The compositional homogeneity of these barriers was confirmed by plan-view high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography. Metal-polar In 0.17 Al 0.83 N/(GaN)/(AlN)/GaN structures were grown with a range of AlN and GaN interlayer (IL) thicknesses to determine the optimal structure for achieving a low two-dimensional electron gas (2DEG) sheet resistance. It was determined that the presence of a GaN IL was necessary to yield a 2DEG sheet density above 2 × 10 13 cm −2 . By including AlN and GaN ILs with thicknesses of 3 nm and 2 nm, respectively, a metal-polar In 0.17 Al 0.83 N/GaN/AlN/GaN structure regrown on a GaN-on-sapphire template yielded a room temperature (RT) 2DEG sheet resistance of 163 / . This structure had a threading dislocation density (TDD) of ∼5 × 10 8 cm −2 . Through regrowth on a free-standing GaN template with low TDD (∼5 × 10 7 cm −2 ), an optimized metal-polar In 0.17 Al 0.83 N/GaN/AlN/GaN structure achieved a RT 2DEG sheet resistance of 145 / and mobility of 1822 cm 2 V −1 s −1 . High-electron-mobility transistors with output current densities above 1 A mm −1 were also demonstrated on the low-TDD structure.

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