Indium Phosphide Quantum Dots in GaP and in In0.48Ga0.52P

The growth and structural properties of self-assembled InP quantum dots are presented and discussed, together with their optical properties and associated carrier dynamics. The QDs are grown using gas-source molecular-beam epitaxy in and on the two materials In0.48Ga0.52P (lattice matched to GaAs) and GaP. Under the proper growth conditions, formation of InP dots via the StranskiKrastanow mechanism is observed. The critical InP coverage for 2D-3D transition is found to be 3 ML for the InP/In0.48Ga0.52P system and 1.8 ML for the InP/GaP system. The structural characterization indicates that the InP/GaP QDs are larger and, consequently, less dense compared to the InP/In0.48Ga0.52P QDs; hence, InP dots on GaP tend to be strain-relaxed. The InP/In0.48Ga0.52P QDs tend to form ordered arrays when InP coverage is increased. Intense photoluminescence from InP quantum dots in both material systems is observed. The PL from InP/GaP QDs peaks between 1.9 and 2 eV and is by about 200 meV higher in energy than the PL line from InP/In0.48Ga0.52P QDs. The optical emission from dots is attributed to direct transitions between the electrons and heavy-holes confined in the InP dots, whereas the photoluminescence from a two-dimensional InP layer embedded in GaP is explained as resulting from the spatially indirect recombination of electrons from the GaP X valleys with holes in InP and their phonon replicas. The type-II band alignment of InP/GaP two-dimensional structures is further confirmed by the carrier lifetime above 19 ns, which is much higher than in type-I systems. The observed carrier lifetimes of 100–500 ps for InP/In0.48Ga0.52P QDs and 2 ns for InP/GaP QDs support our band alignment modeling. 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.