MOLECULAR-BEAM EPITAXY INVESTIGATION OF INTERFACE DISORDER EFFECTS IN MAGNETIC II-VI QUANTUM-WELLS

The present article describes the results of a combined theoretical and experimental investigation into the quality of the interfaces of II-VI quantum well structures grown by molecular-beam epitaxy (MBE). Detailed information is presented on the dilute magnetic semiconductor system CdTe/Cd1-xMnxTe in which the CdTe forms the well and the Cd1-xMnxTe forms the barrier. Structures are grown routinely in which both the photoluminescence (PL) and photoluminescence excitation (PLE) linewidths are narrow (approximately 1-2 meV). This is indicative of high quality material, a feature which is confirmed by the x-ray data in which Pendellosung fringes can be seen. However, in spite of this, it is found that although the same growth conditions are nominally employed quantum well structures are obtained which show either (i) different discernible structures in the PL and the PLE spectra, or (ii) marked differences in the Stokes' shift between the PLE and PL from one sample to the next. A related feature concerns observations of an asymmetrical magnetic field splitting of the heavy-hole exciton states in the barriers. The results of theoretical calculations of the exciton energy levels and their associated linewidths are presented. A comparison of theory with the experimental observations shows that the above effects can be accounted for in terms of interface disorder and magnetic field dependent interface potentials. Thus in case (i) above, the relative intensity of the components in the PLE is different from that in the PL. This is consistent with large island growth in the plane of the well, large here meaning that the island diameters exceed those of the exciton diameter (approximately 140-150 angstrom). Observations of a Stokes' shift, even though the absorption and emission lines are narrow, can be accounted for theoretically if the concept of smaller scale disorder is introduced, i.e., island sizes that are small compared with the diameter of the exciton. Finally, results are presented which show a remarkable asymmetry in the Zeeman splitting of the heavy-hole exciton lines. It is shown that this can be accounted for by introducing deep, short-range interface potentials (approximately 1 or 2 monolayers). The latter are magnetic field dependent, the source of the asymmetry being attributed to band-gap renormalization effects. The implications of all these results for MBE growth conditions are described.