Optical properties and carrier dynamics of InP quantum dots embedded in GaP

The optical properties and associated carrier dynamics of self-organized InP quantum dots embedded in a GaP matrix are presented and discussed, together with their growth and structural properties. InP deposited on GaP (001) using gas-source molecular-beam epitaxy forms Stranski-Krastanow quantum dots for an InP coverage greater than 1.8 monolayers. The size of dots is dependent on the growth conditions; supercritical InP deposition under a sufficiently high phoshine flux results in relatively small (≈20 nm) and dense (≈ 5 × 109 dots/cm2) dots with intense optical emission. The photoluminescence from the quantum dots is observed up to room temperature at about 2 eV; photoluminescence from the strained two-dimensional InP wetting layer peaks at about 2.2 eV. Modeling based on the “model-solid theory” as well as time-resolved photoluminescence indicate that the band alignment for the InP wetting layer is indirect and probably type II; this emission results from spatially indirect recombination of electrons in the GaP X valley with holes in the InP and their phonon replicas. The band alignment of InP quantum dots, however, is type I. Whereas low-temperature time-resolved photoluminescence measurements indicate a rather long carrier lifetime of about 25 ns for the wetting layer, the carrier lifetime in the quantum dots is about 2 ns, typical for type-I quantum dots. Pressure-dependent photoluminescence measurements provide further evidence for a type-I band alignment for InP/GaP QDs at normal pressure, but indicate that they become type II under hydrostatic pressures of about 1.2 GPa and are consistent with an energy difference between the lowest InP and GaP states of about 31 meV. Exploiting the visible direct-bandgap transition in the GaP system could lead to an increased efficiency of light emission in GaP-based light emitters.

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