Characteristics of nucleation layer and epitaxy in GaN/sapphire heterostructures

Abstract

We present the details of GaN nucleation layer grown on (0001) sapphire substrates below 600°C by metal organic chemical vapor deposition. These films have cubic (c-GaN) zinc blende structure which starts to transform into a hexagonal (h-GaN) wurtzite structure upon annealing around 650°C and above. The films deposited above 700°C by pulsed laser deposition directly on sapphire substrate showed the wurtzite structure. Both c-GaN and h-GaN films grow epitaxially on (0001) sapphire substrates via domain matching epitaxy, where integral multiples of planes match across the film-substrate interface. The c-GaN has the following epitaxial relationship: ⟨111⟩c-GaN‖⟨0001⟩sap, ⟨110⟩c-GaN‖⟨10-10⟩sap, and ⟨211⟩c-GaN‖⟨−2110⟩sap. In terms of planar matching, (220) planes of c-GaN match with (30-30) planes of sapphire, and 1∕3(4¯2¯2¯) planes of c-GaN match with (−2110) planes of sapphire in the perpendicular direction. The transformation from c-GaN into h-GaN involves the transformation of (220) planes of c-GaN into (−2110) planes of h-GaN and 1∕3(422) planes of c-GaN into (30-30) planes of h-GaN, and the epitaxial relationship changes to ⟨0001⟩h-GaN‖⟨0001⟩sap and ⟨−2110⟩h-GaN‖⟨10-10⟩sap. In terms of planar matching epitaxy, (−2110) planes of h-GaN match with (30-30) planes of sapphire, and, in the perpendicular direction, (30-30) planes of h-GaN match with (−2110) planes of sapphire. This epitaxial relationship is known as 30° or 90° rotation. It is interesting to note that relative spacing for c-GaN as well as h-GaN planes remains the same during this transformation because of a(c-GaN)=√2a(h-GaN)=√3c(h-GaN)∕2 equivalence between lattice constants of cubic and hexagonal structures. The transformation from cubic to hexagonal structure can occur via insertion or removal of stacking faults in {111} planes of c-GaN and {0001} planes of h-GaN. The hexagonal structure is preferred as a template for higher-temperature growth, however, the cubic structure, which is a defective hexagonal with stacking faults in alternate layers, can also provide a template for epitaxy. The role of Shockley partials terminating at the island edges and the dislocations associated with subgrain boundaries in the generation of threading dislocations is discussed.