Group-III nitrides exhibit polytypism, varying crystal structures composed of identical layers and differ only in their stacking sequences, each exhibiting very different electrical and physical properties. For example, 3C- and 2H-SiC have a bandgap of 2.3 and 3.2 eV, respectively. Heterostructures of polytypes of the same material can offer great advantages as incompatibility issues such as lattice mismatch can be eliminated. However, the controlled growth of polytypes has been elusive, mainly due to the lack of understanding in its origin, i.e., if it is determined by thermodynamics, or the kinetics during crystal growth.
In this work, we show the first selective growth of zinc-blende (111) or wurtzite (0001) GaN films on the same polar MgO(111) substrate, controlled by whether N or Ga is deposited first. The cubic stacking is enabled by nitrogen-induced polar surface stabilization, which yields a metallic MgO(111)-(1×1)-O-N surface. The atomically abrupt semiconducting GaN(111)/MgO(111) interface is found to have a Mg-O-N-Ga stacking, where the N atom is bonded to O at the top site. This specific atomic arrangement at the interface allows the cubic stacking to more effectively screen the substrate and film electric dipole moment than the hexagonal stacking, thus stabilizing the zinc-blende phase even though the wurtzite phase is the ground state in the bulk.
Published in Physical Review Letters:
“Selected Growth of Cubic and Hexagonal GaN Epitaxial Films on Polar MgO(111)“, V. K. Lazarov, J. Zimmermann, S. H. Cheung, L. Li, M. Weinert, and M. Gajdardziska-Josifovska, Phys. Rev. Lett. 94, 216101 (2005).