Kinetic limitations of stress relaxation and generation in GaN/AlN and AlGaN: Si/AlN heterostructures grown on c-sapphire by plasma-assisted molecular beam epitaxy

The paper describes experimental study of stress relaxation and generation in (1-2)-μm-thick GaN and AlGaN layers grown on AlN/c-Al2O3 buffer layers by low temperature (<720 °C) metal-rich plasma-assisted molecular beam epitaxy (PA MBE). The atomically smooth undoped GaN layers demonstrate only gradual relaxation of the compressive stress, which is probably related to thermodynamically driven inclination of threading dislocations (TDs).The slower stress relaxation at the lower growth temperature is explained by kinetic limitation of this process. The switch of compressive to tensile stress in the less-strained undoped Al0.7Ga0.3N layers, attributed mostly to the same effect of TD inclination, occurs in the low-temperature PA MBE conditions at much larger thickness (~0.6 with) as compared to MOVPE ones. Introduction of high Si doping (n~1019cm-3) reduces noticeably the initial compressive stress in the AlGaN film due to substitution of small Si atoms in the group-III sublattice. At larger thickness, Si atoms seem to effect the TD propagation and suppress generation of tensile stress related to TD inclination, which makes possible to grow ~1μm-thick Al0.7Ga0.3N: Si films without cracking.