Electronic structure of self-assembled InP/GaP quantum dots from high-pressure photoluminescence

The electronic structure of self-organized InP/GaP quantum dots (QDs) has been studied by means of photoluminescence (PL) measurements as a function of hydrostatic pressure up to 8 GPa, temperature, and laser excitation power. At ambient pressure the PL emission of the sample arises from direct optical transitions between the lowest electron and hole {Γ}-point states confined in the QD's. At a very low pressure of about 0.15 GPa, the Γ-X conduction-band crossover occurs, after which the PL emission of the dots becomes roughly 20 times weaker in intensity and its energy exhibits the slight redshift typical of indirect recombination processes from the conduction-band X valleys. Our results indicate a type-I band alignment for the strained InP/GaP dot structure at low pressure and yield a value of 300±{}30 meV for the valence-band offset. Upon further increase in pressure above 1.2 GPa we observe the quenching of the dot emission, which is taken as evidence for a type-I--type-II transition.