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

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Abstract

We report Raman studies of the transformations between the zinc-blende (\ensuremath{\alpha}) and high-pressure (\ensuremath{\beta}) phases of bulk GaAs and AlAs epitaxial films under increasing and decreasing hydrostatic pressure using a 300-K diamond-anvil press. The forward \ensuremath{\alpha}-\ensuremath{\beta} thresholds, as measured by the simultaneous onset of opacity and loss of Raman signal, are ${\mathit{P}}_{\mathit{a}}^{\mathit{t}}$=12.4\ifmmode\pm\else\textpm\fi{}0.4 GPa for AlAs and ${\mathit{P}}_{\mathit{g}}^{\mathit{t}}$=17.3\ifmmode\pm\else\textpm\fi{}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 \ensuremath{\sim}65 \AA{} and \ensuremath{\sim}175 \AA{} in GaAs and AlAs, respectively. Thermodynamic considerations based on the hysteresis and microcrystallite size suggest that the surface energy per unit area for a \ensuremath{\beta} nucleus in a pure \ensuremath{\alpha} matrix is \ensuremath{\sim}0.04--0.15 eV/A${\mathrm{\r{}}}^{2}$, 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.

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