Recent progress of high temperature vapor phase epitaxy for the growth of GaN layers - Controlled coalescence of nucleation layers

This paper describes recent developments of the high temperature vapor phase epitaxy (HTVPE) for the growth of GaN seed layers on sapphire substrates. The concept of a controlled coalescence of nucleation layers adopted from metal organic vapor phase epitaxy is used for the first time in HTVPE to reduce the density of threading dislocations of the seed layers. This concept implies a delay of coalescence by reducing the ammonia flow and increasing the admixture of hydrogen in the carrier gas. A series of nucleation and seed layers grown under different ammonia flows and hydrogen concentrations is presented. The layers are analyzed by X-ray diffraction as well as by optical and scanning electron microscopy. Microscopic investigations reveal that nucleation layers with large facetted growth islands are formed during the delayed coalescence. The measured rocking curves and estimated dislocation densities from the mosaicity model indicate a significant improvement of the crystal quality of the corresponding seed layers. The lowest dislocation densities are found to be in the range of 4-5 × 108 cm−2, compared with a dislocation density of about 1 × 109 cm−2 in a reference layer grown without an intentional delay of coalescence. A possible explanation is that the existence of large growth islands with extended pyramidal facets causes a preferred bunching and annihilation of dislocations.