Electronic subband structure of InP/${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$P quantum islands from high-pressure photoluminescence and photoreflectance

We have measured low-temperature photoluminescence (PL) under hydrostatic pressure and photomodulated reflectivity (PR) at ambient conditions of nanoscale InP islands embedded in a ${\mathrm{In}}_{0.48}$ ${\mathrm{Ga}}_{0.52}$P matrix-grown lattice matched on a GaAs substrate. The pressure experiments cover the range of the pressure-induced Γ-X conduction-band crossover both in the InP islands and in the ${\mathrm{In}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$P barrier. Below the Γ-X crossings the PL emission is dominated by direct optical transitions in the islands and in the barrier, both shifting to higher energy with increasing pressure. The PL bands observed above the crossover are broad and weak, and their pressure dependence turns to negative. These bands are therefore attributed to the indirect optical transitions between X conduction-band states and the Γ heavy holes of the InP islands and the ${\mathrm{In}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$P matrix, respectively. PR spectra show two well-resolved features below the direct gap of ${\mathrm{In}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$P, which are assigned to optical transitions between heavy-hole subbands and electron levels of the InP islands. From the combined PL and PR data we derive a value of 80±15 meV for the valence-band offset in the strained InP/${\mathrm{In}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$P system. The interpretation of experimental results in terms of subband-structure calculations within the envelope-function approximation allows us to estimate the amount of strain relaxation in the islands.