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Type-II CdSZnSe strained layer superlattice (SLS) grown on a CdS buffer on (1 1 1)A GaAs provide evidence for the intrinsic Stark effect at visible wavelengths. Shifts in photoluminescence (PL) peak energies are expected when photoexcited carriers screen the internally generated piezoelectric fields. Characteristic PL blue shifts under increasing excitation for CdSZnSe samples are observed. Transmission electron micrographs show that a superlattice, grown at 400°C, is mostly hexagonal in structure but a similar sample, grown at 300°C, adopts a cubic habit. However, both samples show similar shift factors, 70 meV per decade of excitation intensity, in the range of power densities from 50 mW cm −2 to 20 kW cm −2 . These shift factors are only slightly smaller than that found in CdSCdSe hexagonal superlattices of similar period. This result is not expected since the bulk piezoelectric coefficients of cubic ZnSe are thought to be much smaller than those for wurtzite phase II–VI semiconductors. A resolution of these apparent contradictions may be found by involving the recently reported nonlinearity of cubic piezoelectric coefficients as a function of strain by Andre et al. [Phys. Rev. B 53 (1996) 6951].

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.

We report a survey of samples containing ultrathin layers of CdSe embedded in ZnSe or ZnS0.06Se0.94 matrices. CdSe single quantum wells (SQW) with well widths in the range 0.3–3 ML show several bands in the photon energy range from 1.7 to 2.4 eV. Similar bands have been attributed recently to self-organised quantum dots. Temperature-dependent photoluminescence (PL) shows that the bands in our samples vanish, without significant peak shift, at moderately high temperatures. Preliminary time-resolved PL shows that they also have relatively long lifetimes (ca. 30−250 ns). In all samples, the SQW exciton band lying at higher energies shows line narrowing with increasing temperature. Variations due to quantum-well thickness induce a correlation between the luminescence peak energy and the optical bandwidth which may be used to systematise our observations.

The first evidence of the intrinsic Stark effect at visible wavelengths (∼ 600 nm) is reported for a CdSZnSe strained layer superlattice (SLS) grown on a (111)A GaAs substrate. At the same time, smaller but significant exciton peak shifts with excitation power occur for similar samples that were grown on (100)GaAs substrates and thus predicted not to exhibit the effect. We attribute these shifts to band filling in a relaxed type-II SLS. We also report the largest light-induced exciton peak shift ever observed, 600 meV, in a CdSCdSe piezoelectric type-II strained layer superlattice. A shift of 150 meV per decade of excitation power occurs in a superlattice of period 16 nm under both pulsed and CW excitation at low temperature. Finally we mention the prospect of producing optically activated modulators that work in the visible and near infrared spectral region (500–1100 nm) from such structures.