Space-charge effects in type-II strained layer superlattices

The low-temperature photoluminescence (PL) spectra of a range of CdS-CdSe and CdS-ZnSe type-II superlattices was measured under both cw and quasi-cw excitation over the excitation range from mW cm^-2 to MW cm^-2. In all cases, a single PL band due to the recombination of separated electrons and holes in the type-II bandstructure was observed. As the excitation density increases from a low value, the PL peak shifts logarithmically towards the blue. Total shifts of up to 200 meV for a CdS-CdSe sample of 10 nm period are observed, with a corresponding shift-factor of 30 meV per decade of excitation density. The shift factor increases in a regular fashion with the superlattice period. A comparison with the literature shows that a logarithmic dependance of the blue shift with increasing excitation density appears to be a universal feature of type-II SL, even in the absence of strain.