S. A. Telfer

Excitons with large binding energies in MgS/ZnSe/MgS and ZnMgS/ZnS/ZnMgS quantum wells

The wide bandgap II-VI semiconductors have unique properties which allow the possibility of suppressing the exciton-phonon scattering up to room temperature in quantum well structures designed so that the exciton excitation E1s?2s>h?LO. In particular, magnetic field and temperature dependent measurements are used to study the exciton binding energies and to investigate the exciton-LO phonon scattering processes of high quality ZnSe quantum wells in MgS grown by MBE. The small inhomogeneous broadening of the exciton transitions in these samples allows the observation of higher excited exciton states. Due to the large difference in band gap between ZnSe and MgS the exciton binding energy in a 5?nm well is found to be 43.9?meV, which is the largest reported for this material system. The FWHM of the heavy hole absorption transitions measured as a function of temperature shows that the scattering of the excitons by the LO?phonons is partially suppressed. These results are compared with ZnS quantum wells where the exciton g-values have been measured and the exciton binding energies have been deduced from the exciton diamagnetic shifts. The results show the possibility of suppressing exciton-LO phonon scattering in these structures.

Growth of ZnSe and ZnCdSe on (2 1 1)B GaAs substrates

Abstract This paper reports on the growth of ZnSe and ZnCdSe epilayers on GaAs(2 1 1)B substrates by molecular beam epitaxy. The layer quality, as determined by the PL spectra, depends critically on the reconstruction of the GaAs surface before growth. Two surface reconstructions are suggested for the (2 1 1)B surface which correspond to the two different corrugation heights that are observed by RHEED. For ZnCdSe there is a correlation between the surface reconstruction, growth temperature and epilayer quality, while for ZnSe the correlation is less clear. One of the surface reconstructions is clearly associated with twinning and the break-up of the surface into facets.

A spectroscopic study of the piezoelectric effect in ZnSe/ZnCdSe single quantum wells grown on (2 1 1)B GaAs

The piezoelectric effect has been studied in ZnSe/ZnCdSe single quantum wells grown on (2 1 1)B GaAs substrates by molecular beam epitaxy. The effect of the internal electric field on the exciton spectra has been investigated using photocurrent and photoluminescence spectroscopy on Schottky structures where an external reverse bias can be applied. The strength of the piezoelectric field has been determined from the flat band condition in each quantum well.

MEASUREMENT OF THE CRITICAL THICKNESS OF ZNCDSE QUANTUM WELLS IN ZNSE BARRIER LAYERS BY THE PIEZOELECTRIC EFFECT

The occurrence of the piezoelectric effect in a strained zincblende semiconductor layer grown on an (h11) plane results in an internal electric field which can be observed through the optical properties. We report a study of Zn0.85Cd0.15Se/ZnSe quantum wells grown on (211)B GaAs substrates where the quantum confined Stark effect due to the internal field shifts the luminescence to longer wavelengths provided that the quantum well layer is strained. When the well width is greater than the critical thickness the layer begins to relax and the internal field decreases. We have used these measurements to determine that the critical thickness for the onset of strain relaxation of the Zn0.85Cd0.15Se quantum wells grown on (211) oriented substrates is 20 nm. The method will be applicable to materials such as the nitride semiconductors with wurtzite symmetry.

Band structure parameters of Zn1-xCdxSe investigated by spin-flip Raman spectroscopy

Spin-flip Raman scattering spectroscopy has been applied to the study of the wide band-gap semiconductor material Zn 1 x Cd x Se in order to determine for the first time the dependence on the composition, x, of the gyromagnetic ratio of electrons in the Γ 6 conduction band. The experimental values for the Zn 1 -x Cd x Se, Zn 1 -x Mg x Se and ZnS x Se 1 -x alloy systems are discussed in terms of the k.p perturbation theory for the band structure near the direct band gap and it is found that the observed dependence on composition can be reproduced well only when the five-band level of approximation is used together with the addition of a third-order perturbation term. Simple interpolation schemes have been used to estimate the band structures between the binary end-members of each alloy, taking into account the bowing of the fundamental band gap and making reasonable assumptions about the behaviour of the spin-orbit coupling parameter.

Creating excitons in II-VI quantum wells with large binding energies

The wide bandgap II-VI semiconductors have unique properties which allow the possibility of suppressing the exciton-phonon scattering up to room temperature in quantum well structures designed so that the exciton excitation E/sub 1s/spl rarr/2s/>h/spl nu//sub LO/. High quality ZnSe quantum wells in MgS and ZnS quantum wells in ZnMgS have been grown by MBE and these have excellent optical properties. Magnetic field and linewidth temperature dependent measurements have been used to determine the exciton binding energies and to investigate the exciton-LO phonon scattering processes. The results show the possibility of suppressing exciton-LO phonon scattering in these structures.