Spontaneous polarization and piezoelectric constants of III-V nitrides

The spontaneous polarization, dynamical Born charges, and piezoelectric constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using the Berry-phase approach to polarization in solids. The piezoelectric constants are found to be up to ten times larger than in conventional III-V and II-VI semiconductor compounds, and comparable to those of ZnO. Further properties at variance with those of conventional III-V compounds are the sign of the piezoelectric constants ~positive as in II-VI compounds! and the very large spontaneous polarization. @S0163-1829~97!51740-1# In this paper we report an ab initio study of the spontaneous polarization, piezoelectric constants, and dynamical charges of the III-V nitride semiconductors AlN, GaN, and InN. 1 This class of polarization-related properties is of obvious importance for the study of nitride-based piezodevices 2 and multilayer structures. In particular, knowledge of these properties allows an insightful treatment of the polarization ~and ensuing electric fields! in strained and polarized nitride junctions and superlattices under any strain condition, as discussed elsewhere. 3 From the present study, one of the first applications of the modern theory of polarization in solids 4,5 to real and ‘‘difficult’’ materials of technological interest, the nitrides emerge as highly unusual III-V materials, resembling II-VI oxides and in some respects ferroelectric perovskites. The results we report here are of special interest in view of the scarcity of the data ~both experimental and theoretical! available at present for the nitrides. In the absence of external fields, the total macroscopic polarization P of a solid is the sum of the spontaneous polarization P eq in the equilibrium structure, and of the straininduced or piezoelectric polarization dP. In the linear regime, the piezoelectric polarization is related to the strain e by