Molecular beam epitaxy of AlAs/GaAs heterostructures and superlattices

Abstract

This review provides an outline of the main characteristics of a modern molecular beam epitaxy (MBE) system with particular reference to the growth of ultrathin crystalline films. The ability of MBE to achieve monolayer control is exploited in growing well-defined multiple quantum well (MQW) and superlattice structures in the AlAs/GaAs system and the main aim here is to discuss their optical properties as measured by photoluminescence, photoluminescence excitation and luminescence decay. Exciton luminescence dominates the behaviour of high quality samples and the binding energy of the two-dimensional exciton has been measured from observation of 1s and 2s recombination. Reducing the well width below about 30 Å results in Type II behaviour where the electrons and holes are spatially separate, the electrons thermalizing into X conduction band states in the barrier. Reduction of the barrier thickness below 13 Å causes the system to revert to a normal Type I behaviour where the electrons occupy Γ states in the well. These features aid our understanding of short-period superlattice properties, typified by a series of AlAs n /GaAs m structures with n = m = 6,5,4,3,2,1 whose spectra are briefly reviewed. Finally, the use of MQW and superlattice structures as components in injection lasers is described. Operating wavelengths down to 700 nm can be achieved by using narrow GaAs wells and the incorporation of AlAs/GaAs superlattices appears to give worthwhile improvements in performance compared with similar devices employing AlGaAs alloy layers.