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

Abstract

A systematic study has been performed to determine the characteristics of an optimized nucleation layer for GaN growth on sapphire. The films were grown during GaN process development in a vertical close-spaced showerhead metalorganic chemical vapor deposition reactor. The relationship between growth process parameters and the resultant properties of low temperature GaN nucleation layers and high temperature epitaxial GaN films is detailed. In particular, we discuss the combined influence of nitridation conditions, V/III ratio, temperature and pressure on optimized nucleation layer formation required to achieve reproducible high mobility GaN epitaxy in this reactor geometry. Atomic force microscopy and transmission electron microscopy have been used to study improvements in grain size and orientation of initial epitaxial film growth as a function of varied nitridation and nucleation layer process parameters. Improvements in film morphology and structure are directly related to Hall transport measurements of silicon-doped GaN films. Reproducible growth of silicon-doped GaN films having mobilities of 550 cm 2 /Vs with electron concentrations of 3 × 10 17 cm -3 , and defect densities less than 10 8 cm -2 is reported. These represent the best reported results to date for GaN growth using a standard two-step process in this reactor geometry.

MOVPE growth of AlGaN/GaN superlattices on ZnO substrates for green emitter applications

“…Thin films grown on substrates with lattice mismatches usually exhibit a mosaic structure, meaning that the films consist of sub-grains slightly misoriented with respect to each other and the underlying substrate. In general, large grain size is desirable, because the majority of dislocations in the epilayer are formed at grain boundaries [14,15]. The RMS of sample ZA and ZB were measured to be 7.1 and 3.8 nm, respectively, indicating the smoothness of epilayer surface is remarkably improved by increasing the cover layer growth temperature, which is consistent with the XRD results.…”

Using optical reflectance to measure GaN nucleation layer decomposition kinetics

“…It is known that GaN nuclei are generated [5] and it is on these nuclei that the subsequent high T GaN growth occurs. Obviously, how the nuclei form and how the high T growth proceeds influence the microstructure and subsequent dislocation density [10,11]. Ideally, methods to measure and control the extent of NL decomposition and nuclei formation during each growth run are strongly desired.…”

MOCVD growth and optical properties of non-polar (11–20) a-plane GaN on (10–12) r-plane sapphire substrate

“…Moreover, mean size of the sub-grain for sample B was remarkable larger than that of sample A, which might be due to the enhancement of the lateral migration of Ga atoms compared to Al atoms on the growing surface. In general, large grain size is desirable for an epilayer with a mosaic structure, because the majority of dislocations in the epilayer are formed at grain boundaries [16,17]. The relatively smooth surface as well as large grain dimension of AlN/AlGaN buffer supplies a superior underlying layer for the subsequent growth of a-plane GaN film.…”