The impact of nitridation and nucleation layer process conditions on morphology and electron transport in GaN epitaxial films

A systematic study has been performed to determine the characteristics of a n optimized nucleation layer for GaN growth on sapphire. The films were gro wn during GaN process development in a vertical close-spaced showerhead met alorganic chemical vapor deposition reactor. The relationship between growt h process parameters and the resultant properties of low temperature GaN nu cleation layers and high temperature epitaxial GaN films is detailed. In pa rticular, we discuss the combined influence of nitridation conditions, V/II I ratio, temperature and pressure on optimized nucleation layer formation r equired to achieve reproducible high mobility GaN epitaxy in this reactor g eometry. Atomic force microscopy and transmission electron microscopy have been used to study improvements in grain size and orientation of initial ep itaxial film growth as a function of varied nitridation and nucleation laye r process parameters. Improvements in film morphology and structure are dir ectly related to Hall transport measurements of silicon-doped GaN films. Re producible growth of silicon-doped GaN films having mobilities of 550 cm(2) /Vs with electron concentrations of 3 x 10(17) cm(-3), and defect densities less than 10(8) cm(-2) is reported. These represent the best reported resu lts to date for GaN growth using a standard two-step process in this reacto r geometry.