N-Polar GaN Nanowires Seeded by Al Droplets on Si(111)

The polarity of GaN nanowires grown on Si(111) by molecular beam epitaxy is determined by convergent beam electron diffraction. Thick specimens lead to unambiguous results showing their N-polarity. Complementary sample etching indicates that all the nanowires have the same N-polarity and are surrounded by a Ga-polar two-dimensional layer. A correlation with the initial growth stage which uses consecutive exposures of the Si substrate to Al and N is established. Al–Si liquid droplets are formed and produce AlN columnar pedestals playing the role of seeds for the GaN nanowires. On the surrounding Si surface, the GaN layer nucleates with the opposite Ga-polarity.