Influence of pressure on AlN thick films prepared by epitaxial lateral overgrowth through hydride vapor phase epitaxy

Abstract

Smooth and crack-free (0002) AlN thick films (∼30 μm) were epitaxially grown on trench-patterned AlN/sapphire templates through epitaxial lateral overgrowth (ELO) using hydride vapor phase epitaxy. The trenches on AlN/sapphire templates were oriented along [10−10]_AlN, where the width of terraces and grooves was 8 μm and 4 μm, respectively. A comprehensive study was conducted on the influence of growth pressure on the morphology, crystal quality, and residual stress of the AlN thick films. As the growth pressure decreased from 40 torr to 20 torr, the lateral-to-vertical growth rate ratio of AlN increased, facilitating faster coalescence of separated AlN terraces to form a continuous film. Triangular voids were formed during the coalescence of AlN terraces, with the largest void size being observed at 20 torr due to the fastest coalescence. These triangular voids prevented the propagation of threading dislocations into the epitaxial layer, promoting their merging into loops and annihilation, while also releasing residual stress in the AlN film. As the growth pressure decreased, these triangular voids gradually became larger, accompanied by a reduction in etch pit density and residual stress in the AlN film. Compared to the initial AlN/sapphire template, the etch pit density of the AlN thick film grown by ELO at 20 torr decreased from 1.1 × 10⁸ cm⁻² to 3 × 10⁷ cm⁻², while the residual stress was reduced from 1.4 GPa to 0.4 GPa.