The microstructure of (0001) GaN films grown by molecular beam epitaxy from a nanocolumn precursor layer

A major challenge in the production of GaN devices is to reduce the densities of threading dislocations, typically up to 109 – 1011 cm−2, generated by the use of highly mismatched substrates such as (0001)sapphire. One method is to use epitaxial lateral overgrowth (ELO) where a mask is used to restrict the growth of the GaN to “seed” columns. Lateral growth leads to “wings” where dislocation densities may be several orders of magnitude less than in the seed. In this paper we consider a maskless ELO approach in which the GaN is grown on (0001)sapphire. Firstly, GaN is grown by molecular beam epitaxy under strongly N-rich conditions, leading to pronounced nanocolumn growth [1]. The growth is then continued under Ga-rich conditions, leading to lateral growth and coalescence of the nanocolumns. Eventually, continuous GaN layers are formed with threading dislocation densities in the range 108 – 109 cm−2.