Structures and Polarity of III‐Nitrides: Phase Diagram Calculations Using Absolute Surface and Interface Energies

The structures and polarity of group‐III nitride semiconductors are theoretically investigated by calculating individual energy of polar surfaces and interfaces using density functional calculations. Using the surface and interface energies, we demonstrate phase diagrams as functions of temperature and pressure for polar AlN on 6H‐SiC(0001) substrate as well as polarity inversion of GaN on N‐polar AlN substrate. The calculated phase diagram for AlN on Si‐face SiC under H‐rich conditions reveals that the H‐terminated Al‐polar surfaces with substitutional Al atoms at AlN/SiC interface are stabilized over the wide range of Al pressure below 1320 K while that with N adatom are favorable for temperature beyond 1320 K. In contrast, the calculated phase diagram for GaN on N‐polar AlN substrate clarifies that the H‐terminated Ga‐polar surface with on N‐polar substrate with two monolayers of Al olverlayers is favorable over entire growth conditions. These results suggest that the stability of interface between III‐nitride and substrate rather than surface stability is crucial for the polarity of III‐nitrides.