P. Harrison

Exciton magnetic polarons in quantum wells

Theoretical calculations of free‐exciton magnetic polaron energies in semimagnetic quantum‐well structures have been performed. The dependence of the polaron energy on well width, magnetic field, and temperature has been calculated. Polaron energy calculations in diffused semimagnetic quantum wells show a strong dependence of the polaron energy on the amount of diffusion in the quantum well. Consequently, magnetic polaron energies could be used as a means of measuring the diffusion coefficient.

Theory of exciton energy levels in multiply periodic systems

A general theoretical formalism is developed for evaluating the lowest energy levels of an exciton in a system which is of finite size, but multiply periodic, in one dimension, whilst being of infinite extent in the other two directions. A particular application of the formalism to a Kronig-Penney type I multiquantum well-superlattice structure is made and the significance of the results described.

Double crystal x-ray diffraction simulations of diffusion in semiconductor microstructures

Diffusion in group IV, III-V and II-VI semiconductors is an interesting problem not only from a fundamental physics viewpoint but also in practical terms, since it could determine the useful lifetime of a device. Any attempt to control the amount of diffusion in a semiconductor device, whether it be a quantum well structure or not, requires an accurate determination of the diffusion coefficient. The present theoretical study shows that this could be achieved via x-ray diffraction studies in quantum well structures. It is demonstrated that the rocking curves of single quantum wells are not sensitive to diffusion. However the intensity of the first order satellite, which is characteristic of superlattice rocking curves, is strongly dependent upon diffusion and it is proposed that this technique could be used to measure the diffusion coefficient D.

Effect of interface disorder on the optical properties of CdTe-Cd1−xMnxTe microstructures

Abstract High quality CdTe / Cd 1− x Mn x Te quantum well structures can be routinely grown in the authors laboratory by molecular beam epitaxy (MBE). At temperatures of ∼ 2 K these structures have optical linewidths ≲ 3 meV for quantum well widths ≲ 20 A. The present paper shows how information on the nature and extent of the interface roughness in such systems can be deduced from studies of the optical linewidths and Stokes shifts.

Optical methods for determining diffusion in magnetic quantum‐well structures

It is shown that the occurrence of magnetic polaron formation can have an appreciable influence on the values of the diffusion constants deduced from observations of the photoluminescence in diffused magnetic quantum‐well structures. Arguments are presented that show that photoluminescence excitation spectroscopy provides several independent, more accurate measurements from which the diffusion constants can be more reliably estimated.

ALLOY NONRANDOMNESS IN DILUTED MAGNETIC SEMICONDUCTORS

A simple model of alloy nonrandomness is introduced within a framework where the effective concentration of spin singlets as a function of the nominal concentration of magnetic ions in a nonrandom alloy can be obtained by transformations of the corresponding function in random alloys. The theory shows that a given system that is appreciably nonrandom can have a magnetic response almost identical with that of a random distribution. Possible ways of identifying alloy nonrandomness in diluted magnetic semiconductor structures are described.

Dynamics of exciton relaxation and excitation transfer to donor‐bound excitons in CdTe/CdMnTe quantum wells

Using time‐resolved photoluminescence spectroscopy we investigate the dynamics of exciton relaxation in CdTe/Cd1−xMnxTe multiple quantum wells. Unbound excitons are photoexcited and the timescale of formation of the donor‐bound excitons varies with excitation intensity from 73 ps to <8 ps and depends on movement of the unbound exciton in the well and the capture process at the donor site. Estimates of the capture cross‐section of excitons by neutral donors and of the radiative recombination time of the donor‐bound exciton are made. The rate of kinetic energy relaxation of a photocreated exciton from K=0 of the electron‐light hole band to K=0 of the electron‐heavy hole band is found to be (2.2±0.8)× 108 eV s−1.

Excitonic relaxation channels in double quantum wells

The purpose of this paper is to give a theoretical interpretation of the experimental results of Goede et al. (Superlatt. Microstruct. 12 363 (1992)) describing the relaxation of heavy-hole excitons in asymmetric double quantum well systems based on the semimagnetic CdTe-CdMnTe system. In order to explain certain spectroscopic data, Goede et al. had to invoke excitonic tunnelling between the wells for systems with narrow (25 AA) inner barriers. In this work it is shown that the experimental results imply tunnelling via a crossed excitonic state with the electron and hole localized in different wells. The experimental observations are interpreted in terms of a model involving phonon scattering and calculations of intersubband relaxation rates via confined phonon modes are shown to be in agreement with experimental observations.

Band gap renormalization and observation of the type I–type II transition in quantum well systems

Quantum wells (or barriers) are of finite extent (≤100 A) along the growth direction. Hence, for at least one or two monolayers adjacent to the interface, the corresponding one‐electron potential in the conduction (or valence) band must differ from that appropriate to the bulk material. The present article demonstrates the pronounced effect such short‐range ‘‘interface potentials’’ can have on the transition energies and oscillator strengths in a dilute magnetic semiconductor system undergoing a type I–type II transition in an external magnetic field.

Stability of single free particles against spatial localization through magnetic polaron formation

Abstract We present a quantum-mechanical treatment of the spatial localization of single free carriers resulting from the large exchange interaction between them and the lattice magnetic ions in dilute magnetic semiconductors. The formalism is applied to the Cd 1− x Mn x Te system, and the implications for the observation of spatial localization of single free carriers described.