INTERFACE PROPERTIES AND THE EFFECT OF STRAIN OF ZNSE/ZNS STRAINED-LAYER SUPERLATTICES

The ZnSe/ZnS alternately layered system is one of the fundamental examples of the wide-gap II-VI compound strained-layer superlattice (SLS) used to understand the basic physics associated with optical, electronic and structural properties. Interface properties of ZnSe/ZnS SLSs have been extensively investigated by means of MeV transmission electron microscopy (TEM), ion Rutherford backscattering (RBS)/channeling, X-ray diffraction (XRD), X-ray photoelectron emission spectra (XPS) and cross-sectional TEM measurements. It is shown that the ZnSe/ZnS SLS can be grown coherently up to a layer thickness of about 100 angstrom and to exhibit good crystalline quality. The experimental X-ray diffraction patterns are analyzed in terms of the kinematic approximation from which the strains being induced in each layer can be quantatively evaluated; the ZnSe well is certainly under compressive strain, while the ZnS barrier is under tensile strain. Band off-sets in the conduction and valence bands in the strained layers are experimentally determined to be about 0.25 and 0.73 eV by XPS, respectively, and are theoretically considered as a function of the well width thickness using the 'model solid theory'. The [110] channeling in the SLS shows much higher dechanneling yields than along the [100] growth direction. This may be derived from the small angle change in the inclined crystal directions at each interface which result in significant [100] dechanneling, and can be discussed in terms of the bond relaxation model or beam steering effects. The periodic oscillatory structure of the superlattices in RBS/channeling spectra has been observed in this SLS system. The interface properties can be understood in terms of the excitonic linewidth broadening which is influenced by either the interface roughness or by well width fluctuation. Photoluminescene and absorption spectroscopies of the ZnSe/ZnS and ZnSe/ZnSSe SLSs are used to characterise the interface, and to clarify the energy separation of the degenerate hole bands due to a compressive strain in the ZnSe quantum well. The effects of ion irradiation and thermal stability during annealing in SLS are briefly introduced with a view to providing the fundamental physical properties of the creation of lattice defects and impurity diffusion. A Fibonacci SLS sequence with a quasi-periodic superlattice system demonstrates the possibility of new functional devices.