Direct optical transitions in indirect-gap (Al0.5Ga0.5)0.51In0.49P by atomic ordering.

Abstract

(${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${)}_{\mathit{y}}$${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$P alloys naturally constitute monolayer superlattices on GaAs(001) substrates by atomic ordering when they are grown by organometallic vapor-phase epitaxy. The spontaneous long-range ordering causes the band-gap reduction and band splitting at the valence-band maximum. When the Al composition in a (${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${)}_{0.5}$${\mathrm{In}}_{0.5}$P-random alloy is larger than \ensuremath{\sim}0.4, the optical transition is indirect, and the conduction-band minimum is at ${\mathit{X}}_{6\mathit{c}}$. From electroreflectance and photoluminescence measurements, we have found for long-range ordered (${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$${)}_{0.51}$${\mathrm{In}}_{0.49}$P that the ${\mathrm{\ensuremath{\Gamma}}}_{6\mathit{c}}$ valley crosses the ${\mathit{X}}_{6\mathit{c}}$ by the band-gap reduction. The emission from the strongly ordered alloy is optically direct. The temperature dependence of photoluminescence intensity shows that statistical potential fluctuations and localized states in the low-energy part of the exciton resonance play an important role in the direct luminescence of the long-range ordered (${\mathrm{Al}}_{0.5}$${\mathrm{Ga}}_{0.5}$${)}_{0.51}$${\mathrm{In}}_{0.49}$P. \textcopyright{} 1996 The American Physical Society.