Polymorphic stability of AlAs/GaAs superlattices at high pressure

The pressure-induced ± structural transitions in [001]-oriented AlAs/GaAs superlattices (SLs) are studied by Raman scattering using a 300-K diamond-anvil press. The threshold pressures of the forward and reverse transitions in the AlAs and GaAs constituents of each SL are accurately measured for layer thicknesses in the range 300 20, and comparison is made to the bulk transitions reported in the preceding paper. We obtain direct microscopic evidence that (i) the SL constituents transform separately (first AlAs, then GaAs) or simultaneously, depending on whether the AlAs layers are thicker or thinner than 50; (ii) overpressing of zinc-blende AlAs by 5 GPa above its bulk stability limit is not matched by GaAs underpressing: (iii) the postreversal condition of the SLs is marked by increasing signs of bulk and interface disorder as the constituent layer thickness decreases. The AlAs overpressing shows that the effective polymorphic stability of these SLs is GaAs controlled over a wide layer-thickness range. The thermodynamics of high-pressure SL transitions is discussed. We find that the -AlAs/±-GaAs sixfold-fourfold interface encountered at high pressure has the empirical energy density ±=0.12±0.02 eV/A2, and is best described by a disordered-interface model. Comparison to microscopic theory for a pseudomorphic sixfold-fourfold interface shows that the calculated geometry probably does not occur at the static interfaces of AlAs/GaAs SLs, but might exist during transformation at the kinetic boundary of small nuclei forming within an ± matrix. Alloylike pressure stability is predicted when the SL periods are substantially thinner than the smallest nuclei.