Evolution of spirals during molecular beam epitaxy of GaN on 6H-SiC(0001)

Evolution of spirals during molecular beam epitaxy growth of GaN films on 6H-SiC(0001) was studied by in situ scanning tunneling microscopy. It was found that dislocations emerge at the film surface, creating straight steps with orientation along 〈1100〉 directions with a density of 10¹⁰cm^-2 for 40-nm-thick films. During subsequent growth, these straight steps wind around dislocations and develop into spirals with a density of 10⁹cm^-2 for 100-nm-thick films. The spirals can be classified into three types: single arm, interlocking double arm, and closed loop. The first two types originate from steps with one end pinned, and the third type results from steps with both ends pinned. At film thickness larger than 200 nm, these spirals further evolve into spiral mounds with a density of 10⁷cm^-2. Based on the Burton, Cabrera, and Frank theory, a model is proposed to explain the formation of different types of spirals and the reduction of their densities.