T. Stirner

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/Cd1−xMnxTe 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.

SPECTROSCOPIC EVIDENCE FOR DIFFERENT LASER GAIN MECHANISMS IN OPTICALLY PUMPED ZNCDS/ZNS QUANTUM WELL STRUCTURES

The role of biexcitons has been studied in two optically pumped quantum well (QW) structures of Zn1−xCdxS/ZnS, one with 18% Cd concentration and the other with 3% in the wells. For the x=18% QW structure, high excitation photoluminescence and stimulated emission indicate that the laser gain mechanism involves biexcitons. For the x=3% QWs, even though biexcitons are clearly observed in the spontaneous emission, they are not responsible for laser gain in this structure. Instead exciton–exciton scattering may be the more likely mechanism responsible for laser gain close to threshold, while at higher densities an estimate of the carrier density indicates an electron–hole plasma as the likely source of optical gain. The different mechanisms in the two cases can, very likely, be attributed to one of differing degrees of localization both within the QW and at alloy fluctuations.

Magnetic localization of free exciton magnetic polarons in diluted magnetic semiconductors

A model of free exciton magnetic polarons (EMPs) in diluted magnetic semiconductors is proposed on the basis of the localization of the hole in the magnetic potential well created by its own exchange field. The corresponding calculations of the free EMP binding energies, as a function of Mn2+ ion concentration and temperature, are performed. The results are in good agreement with recent experimental measurements. It is found that free EMPs are stable up to relatively high temperatures (T≲30 K) over a wide range of Mn concentrations x without any other primary localization, e.g., alloy potential fluctuations. The dependence of the critical temperature of free EMP formation on the Mn concentration is determined for various diluted magnetic semiconductor materials, and is in good agreement with the experimental results for Cd1−xMnxTe. The suppression of free EMPs under the application of an external magnetic field is also considered.

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.

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.

Localized magnetic polarons in diluted magnetic semiconductors

A theory of localized magnetic polarons in diluted magnetic semiconductors is developed. The theory involves below band gap states and alloy fluctuations utilizing the optimal fluctuation method. A comparison of the theoretical calculations with published experimental results is performed. The implications of the comparison are discussed and the need for extending the theory to include an additional impurity/native lattice defect potential described. The magnetic field and temperature dependence of the polaron energies are also evaluated.

Quantum theory of infrared detectors based on intrasubband transitions in III–V quantum wells

One of the perceived drawbacks of multiquantum well (QW) infrared detectors based on intrasubband conduction band transitions in III–V materials (e.g., GaAs/AlxGa1−xAs) is that photons at normal incidence to the multiquantum well interfaces will either not be absorbed or, at best, will be absorbed very weakly. The present article reviews briefly certain key aspects of the experimental evidence and theoretical arguments that both support and challenge this view, prior to developing a completely different approach to the problem. It is shown that the questions at issue relate to basic concepts of quantum mechanics. On this basis it is argued that in QW structures of appropriate design normal incidence absorption should occur, thus enabling, in principle, two-dimensional infrared detector imaging arrays to be fabricated in a simple manner.

Simplified treatment of scattering processes in quantum well structures

Arguments are developed which show that, to a good approximation, the essential physics and many of the quantitative details pertaining to the relative rates of carrier–carrier, carrier–longitudinal optical phonon, and carrier–photon scattering in different quantum well structures can be understood in terms of a simplified approach. The latter is based on fundamental concepts of quantum theory and involves the separation of the matrix element in the scattering rate calculation into two simpler parts: an “energy effect” term and a “wave-function effect” term. As an application of the method, the question of attaining lasing action in the far-infrared (terahertz) region of the spectrum is discussed briefly.

A photonic crystal with dielectric insertion in macroporous silicon

A photonic crystal structure, designed with dielectric rods at the centres of the air holes in a triangular lattice in macroporous silicon, is investigated theoretically. The calculations show that this structure has several large photonic band gaps at successively higher photon energies. Potential applications of this structure, based on photonic and optoelectronic devices utilizing the higher-lying band gaps, such as optical filters, dual-mode and three-mode lasers, are discussed briefly.

Excimer laser induced diffusion in magnetic semiconductor quantum wells

Studies of pulsed laser annealing (PLA) of CdTe/CdMnTe quantum well structures are made in order to examine depth dependent effects in laser irradiated semiconductors. Since diffusion coefficients are strongly dependent on the temperature, depth resolution is achieved because the diffusion of Mn from the barriers into the quantum wells is depth dependent. Multiple quantum well (MQW) structures of CdTe/CdMnTe were annealed with single pulses from an XeCl laser at 308 nm. At a threshold of 90 mJ cm−2 two new emission bands are observed that are attributed to the diffusion of Mn from barrier layers to QWs. The diffusion associated with these bands, measured as the integrated product of the diffusion constant and time, is found to be 300 and 30 A2. Calculations of the temperature, reached within the surface following PLA, using an analytical solution of the heat diffusion equation coupled with known high temperature diffusion coefficients predict the diffusion to decrease by one order of magnitude within one ...