Formation of voids and their role in the recovery of sputtered AlN during high-temperature annealing

Abstract

The structural recovery of AlN grown by reactive sputtering on a sapphire substrate during high-temperature annealing is studied by means of transmission electron microscopy and secondary ion mass spectrometry. The as-grown film shows high-density planar defects, such as basal and prismatic stacking faults, caused by the limited diffusion length of the adatoms and, thus, presents a columnar structure. The presence of high-density nanopipes is associated with the presence of unintentional oxygen impurities. Based on the atomic resolution transmission electron microscopy analysis, we show that basal and prismatic stacking faults vanish in the films via a climb mechanism and describe this process as the nucleation of jogs promoted by the diffusion of vacancies. The nanopipes present in the as-grown film transform into faceted voids and act as a beneficial source of excess vacancies that promote dislocation annihilation by climb. The transformation of nanopipes to faceted voids resembles the transition from open channel pores to close faceted pores, which has been observed in porous silicon and can be described in terms of a classical sintering theory.