Forward and reverse high-pressure transitions in bulklike AlAs and GaAs epilayers

We report Raman studies of the transformations between the zinc-blende (α) and high-pressure (β) phases of bulk GaAs and AlAs epitaxial films under increasing and decreasing hydrostatic pressure using a 300-K diamond-anvil press. The forward α-β thresholds, as measured by the simultaneous onset of opacity and loss of Raman signal, are P_a^t=12.4±0.4 GPa for AlAs and P_g^t=17.3±0.4 GPa for GaAs. On decompression from 20 GPa or less, reversal to the zinc-blende state occurs in both materials with a hysteresis of 6–8 GPa; otherwise, GaAs enters a metastable phase. After reversal, the returning optical-phonon peaks exhibit asymmetric broadening and negative frequency shifts. Analogy to ion-bombarded GaAs shows that postreversal material is comprised of zinc-blende microcrystallites with diameters ∼65 Å and ∼175 Å in GaAs and AlAs, respectively. Thermodynamic considerations based on the hysteresis and microcrystallite size suggest that the surface energy per unit area for a β nucleus in a pure α matrix is ∼0.04–0.15 eV/Ų, in rough agreement with previous microscopic calculations for a rocksalt–zinc-blende AlAs/GaAs heterointerface. We propose that the kinetic homointerface in the bulk nucleation transitions is similar to the static sixfold-fourfold heterointerface involved in the superlattice phase changes discussed in the second paper.