The effects of linear and non-linear diffusion on exciton energies in quantum wells

This paper considers the technique of investigating diffusion processes via monitoring spectroscopically the ground state energy of an exciton confined in a quantum well. It is shown that the change in the exciton energy E–E0 during linear diffusion, can be described by an empirical relationship E–E0=(E–E0)(1–exp{–Dt/lw}), where E is the band gap of the initial barrier material, D the diffusion constant and t the time. Detailed calculations accounting for the changes in the exciton binding energy have shown that the parameter ~1.5 for all wells of width lw40 Å regardless of the material system. It is proposed that this relationship could be used to determine the linear diffusion coefficient D. Once D has been determined the relationship could then be utilized as a predictive tool, e.g., to determine the annealing time necessary to produce a given energy shift for a particular quantum well width. The paper goes on to discuss the effects non-linear diffusion processes could have on exciton energies in quantum wells. In particular, it is shown how detailed spectroscopy and annealing experiments when coupled with accurate modelling could be used to distinguish between constant and concentration dependent diffusion coefficients. © 1996 American Institute of Physics

Empirical pseudopotential calculations of Cd1-xMnxTe

Empirical pseudopotential calculations for the entire range of alloy concentrations of cubic Cd(1-x)Mn(x)Te are presented. The atomic form factors have been deduced empirically by fitting the band structure to spectroscopic data available from the literature. The pseudopotential band structures indicate optical bowing may occur in the alloy Cd(1-x)Mn(x)Te and have been used to determine the effective masses of the electron and light, and heavy holes, which for CdTe are in agreement with accepted values. The effective masses for Cd(1-x)Mn(x)Te are given for the first time, acid are expressed as first- and second-order polynomials in x. The implications of these results for spectroscopic experiments are discussed. © 1996 American Institute of Physics

The structural dependence of the effective mass and Luttinger parameters in semiconductor quantum wells

A detailed comparison of the empirical pseudopotential method with single and multiple band calculations based on the envelope function and effective mass approximations are presented. It is shown that, in order to give agreement with the more rigorous microscopic approach of the pseudopotential method, structural dependent effective masses and Luttinger parameters must be invoked. The CdTe/Cd(1 – x)Mn(x)Te system has been employed as an example, and the first pseudopotential calculations of quantum wells and superlattices in this material are presented. It is shown that the electron, light- and heavy-hole effective masses tend towards twice their bulk values in the limit of narrow quantum wells. © 1997 American Institute of Physics

Stark ladders in strongly coupled finite superlattices

It is shown that the nature and extent of wavefunction localisation of miniband states in an external electric field can be markedly different for finite superlattice structures, compared with infinite and is dependent on the strength of interaction between the initial electron (and hole) miniband states. The question of blue shifts and excitonic effects is described and possible device applications for strongly interacting superlattices structures are discussed

Optical properties of single magnetic quantum wells in an external magnetic field

Photoluminescence measurement performed on magnetic Cd1-xMnxTe quantum wells surrounded by Cd1-yMny Te barriers ([MATH]), grown by M. B. E. and subjected to an external magnetic field, are described. The observed excitation spectra are shown to be in agreement with calculations of exciton energies based on the envelope function approximation. Characteristic features of the samples are either the large ([MATH]18meV) Stokes' shift and/or the width of the P. L. lines ([MATH]11meV). These are to be contrasted with the values in comparable non-magnetic wells of CdTe where the Stokes' shift is typically less than 1 meV and the halfwidth of the P. L. lines is less than 1 or 2 meV. Arguments are presented which show that these observations, together with their magnetic field dependence, are consistent with the formation of magnetic polarons. The photoluminescence emission also contains a band attributed to excitons bound to donors. It is observed experimentally that this donor-bound exciton emission disappears with increasing magnetic field. A theoretical interpretation of the observed properties of this band is given in terms of excitons bound to donors which are situated at different positions in the well and barrier region

Comparative studies of excitons in magnetic asymmetric double quantum well structures

Four different asymmetric double quantum well structures in the dilute magnetic semiconductor system Cd1-xMnx Te have been investigated. Theoretical calculations are presented which show that the observed photoluminescence (PL) and photoluminescence excitation (PLE) energies are consistent with an anticrossing of the lowest two hole states in one of the double quantum well structures when subjected to an external magnetic field