FIRST-PRINCIPLES PSEUDOPOTENTIAL STUDY OF THE PHASE-STABILITY OF THE III-V SEMICONDUCTORS GAAS AND ALAS

A first-principles pseudopotential study of the phase stability of the III-V semiconductors GaAs and AlAs is reported. For both compounds we study the zinc-blende, wurtzite, NaCl, CsCl, beta-tin, NiAs, and sc16 structures, the latter being the binary analog of the metastable bc8 structure observed in Si and Ge. For GaAs we have also considered a st ructure of orthorhombic symmetry with four atoms per unit cell. This s tructure is the same as that recently discovered for ZnTe III [Phys. R ev. Lett. 73, 1805 (1994)], and includes as a special case the Pmm2 st ructure which had previously been suggested as the structure of GaAs I I. Comparison is made with the experimental transition pressures, equa tions of state, and internal parameters of these compounds. We investi gate the recent suggestion that the sc16 structure is very low in ener gy in III-V compounds. We find that sc16 GaAs is thermodynamically sta ble to all the other phases studied in a very small range of pressures , but sc16 AlAs is not thermodynamically stable at any pressure. For b oth GaAs and AlAs we find that the CsCl structure is thermodynamically stable at very high pressures.