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Deep level transient spectroscopy in plasma-assisted molecular beam epitaxy grown Al 0.2 Ga 0.8 N / GaN interface and the rapid thermal annealing effectThe incorporation of deep level defects in n-type GaN grown by ammonia-based molecular beam epitaxy ͑MBE͒ is studied via systematic adjustment of the NH 3 / Ga flux ratio. Deep level optical and transient spectroscopies, which together enable deep level detection throughout the GaN bandgap, reveal defect states whose individual concentrations vary with the NH 3 / Ga flux ratio. A general trend of lower concentration for deep levels at E C − 3.28, E C − 1.28, E C − 0.62, and E C − 0.25 eV with higher NH 3 / Ga flux ratio was observed, with the strongest reduction at the E C − 0.25 eV level, consistent with expectations for a V N -related defect. The known C N impurity state at E C − 3.28 eV and suspected C I -related state at E C − 1.28 eV also showed a moderate decrease in concentration at the higher NH 3 / Ga flux ratio. In contrast, the V Ga -related defect at E C − 2.62 eV was insensitive to the NH 3 / Ga flux ratio over the range studied here. Taken together, ammonia-MBE GaN has deep level defects with different sensitivities in flux ratios suggestive of independent physical sources. However, the total trap concentrations were significantly reduced for higher NH 3 / Ga flux ratios in n-type GaN grown by ammonia-MBE under the range of growth conditions used in this study, suggesting that higher NH 3 / Ga flux ratios will generate higher electronic quality GaN material when using ammonia-based MBE for device applications.

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The impact of growth conditions on the surface morphology and structural properties of ammonia molecular beam epitaxy GaN buffers layers on SiC substrates was investigated. The threading dislocation (TD) density was found to decrease with decreasing NH3:Ga flux ratio, which corresponded to an increase in surface roughness and reduction in residual compressive lattice mismatch stress. Furthermore, the dislocation density and compressive stress decreased for increasing buffer thickness. TD inclination was proposed to account for these observations. Optimized surface morphologies were realized at high NH3:Ga flux ratios and were characterized by monolayer-high steps, spiral hillocks, and pyramidal mounds, with rms roughness of ∼1.0 nm over 2×2 μm2 atomic force microscopy images. Smooth surface morphologies were realized over a large range of growth temperatures and fluxes, and growth rates of up to 1 μm/h were achieved. TD densities in the buffers as low as 3×109 cm−2 were demonstrated. These buffers were highly insulating and were used in recently reported AlGaN/GaN HEMTs with power densities of >11 W/mm at 4 and 10 GHz.

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