Strained layer superlattices of wurtzite CdS/CdSe have been grown on (111)A GaAs substrates by metalorganic chemical vapor deposition and their optical properties studied by photoluminescence spectroscopy. It is shown that the superlattice layers contain giant strain-induced piezoelectric fields exceeding 2×108 V m−1. These fields are similar to those reported for (111) orientated III–V superlattices, but an order of magnitude greater. The recombination energies from a series of samples provide evidence for a type II conduction band offset of 0.23±0.10 eV (the electron wells being in the CdS), with the band structure heavily modified by the internal electric fields. In addition, the photoluminescence peak emission energy shows a strong dependence on the excitation power. This is interpreted as further evidence for the effect of internal fields. We conclude that this system shows new effects not previously observed in II–VI compound superlattices. The large band-gap tunability and the space-charge effects offer possibilities for all-optical switching devices in the 700–1300-nm region of the spectrum.
A series of Zno.93Mno.07Te/ZnTe multiple-quantum well (MQW) structures of different quantum well widths has been studied at 77 K using the conventional modulation technique of photoreflectivity (PR) with above-bandgap light providing the modulation as well as a novel version of the technique which employs below-bandgap excitation. Photoluminescence (PL) spectra of the structures are also presented. The appearance of the PR spectra obtained with below-bandgap excitation differs from that of the PR spectra with above-bandgap excitation, but fitting the spectra with appropriate lineshapes gives the same spectroscopic information about the energy positions of the excitonic transitions in the well and the bandgaps of the ZnTe buffer layer and the Zno.93Mno.07Te barriers. The fits to the PR spectra yield energy positions of the lowest excitonic transitions which are consistent with the PL data. From the transition energies calculated with a one-dimensional transfer matrix model in which strain effects are taken into account, we conclude that the structures are to a good approximation strained to the ZnTe buffer layers.of Engineering Design and i"ianuiaciure, Universi?r of iiuii,
i . iniroauciionThe multiple-quantum well (MQW) structures investigated here are formed from the Znl-,Mn,Te system in which the excitonic bandgap, Eg, at 4.2 K is given approximately by the following expression: Eg = 2.381 + 0 . 6 8 ~ in electronvolts (Twardowski 1983, Twardowski er a1 1984, Lee er al 1986, Mertins et al 1993) though the dependence of Eg on x does show some nonlinearity (Mertins er a1 1993). This system thus offers the possibility of producing wideblue-green spectral region). The system is also of interest because as-grown Znl-,Mn,Te is normally p-type; ZnTe is
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