Abstract
Multilayer self-assembled InAs∕GaAs quantum dot (QD) structures with varying GaAs spacer layer (SL) thickness are systematically investigated using surface photovoltage spectroscopy (SPS) and photoluminescence (PL). The optical transitions are more clearly visible in the room-temperature SPS spectra and exhibit more features in comparison to PL. The enhanced electronic coupling effect for thinner SL results in the appearance of an additional excited state lying higher than the second excited quantum dot state. A peculiar feature denoted as QDX below the fundamental transition is tentatively attributed to the optical absorption from uncoupled dots of which the density is lower than that of vertically coupled ones. The transition blueshifted with a decrease of the SL thickness indicates that the materials intermixing between InAs QDs and GaAs SL are strongly driven by strain. The intensity ratio between the ground state and QDX in the temperature dependent PL spectra is found to be associated with the processes of enhanced thermal carrier transfer between QDs and tunneling of carriers between QDs in adjacent layers for a stack due to a reduction in SL thickness. The results demonstrate considerable diagnostic value of SPS for the nondestructive characterization of vertically coupled self-assembled QD structures.
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