Chapter 7 Crystal Structure of Group III Nitrides

Abstract

This chapter discusses the crystal structure of group III nitrides. The elements B, Al, Ga, and In form compounds with N having the composition A III N. The chemical bond of these compounds is predominantly covalent—that is, the constituents develop four tetrahedral bonds for each atom. Because of the differences in electronegativity of the two constituents, there is a significant ionic contribution to the bond that determines the stability of the respective structural phase. The group III nitrides AlN, GaN, and InN can crystallize in the following three crystal structures: (1) wurtzite, (2) zinc-blende, and (3) rock-salt. In fact, crystal structures and related physical properties—such as the indirect bandgap—are analogous to the modifications of carbon. Thus, BN exists in tetrahedral-like and graphite-like modifications. At high external pressures, a phase transformation of the wurtzite towards the rock-salt (NaCl) structure takes place that is predominantly found in ionic bonded crystals. In this structure, anions form a face-centered cubic (fcc) lattice where open sites in between are occupied by ions of opposite charge.