Photoelectrochemistry of Strained-Layer and Lattice-Matched Supperlattice Electrodes: Effects Due to Buffer Layers

Strained-layer and lattice-matched superlattice electrodes have been studied and compared as photoelectrodes in photoelectrochemical cells, and the effects of the presence of buffer layers in the strained-layer systems have been established. Photocurrent spectroscopy and photomodulated reflectance spectroscopy of superlattice electrodes, etched superlattice electrodes, and buffer layer structures reveal that highly strained superlattices (1.8% mismatch) have poorly defined quantization effects in their quantum wells; strained-layer superlattices with less mismatch (0.9%) have better defined quantization, but this is not reflected in their photocurrent spectra. On the other hand, lattice-matched superlattice electrodes exhibit extremely well-defined quantization effects that are clearly exhibited in multiple photocurrent peaks that match theoretical predictions; photomodulated reflectance spectra exhibit 17 transitions that represent all the possible allowed transitions in the quantum wells, including all light and heavy hole transitions, as well as unconfined transitions above the well barriers. The present work indicates that previous results reported for the photoelectrochemistry of highly strained superlattices probably reflected a photoresponse that was influenced more by the buffer layers than by the superlattice layers. The occurrence of hot electron transfer from photoexcited superlattice electrodes remains to be demonstrated unequivocally.