Monte Carlo simulations of electron transport in quantum well heterostructures
The parallel transport of electrons in pseudomorphic In(_z)Ga(_1-x)As/GaAs quantum wells is influenced by the degree of spacial confinement and by the effect of the indium concentration which determines, the amount of alloy scattering, the subband structure, and material parameters. The indium content changes the bandstructure and material parameters through both direct compositional and strain effects. We use the single particle and ensemble methods of Monte-Carlo simulation to investigate how the above phenomena influence the transport properties of electrons in In(_x)Ga(_1-x)As/GaAs quantum wells. To understand the effects of alloying and strain on the electron transport properties we first consider electrons in bulk In(_x)Ga(_1-x)As/GaAs. Alloying and strain are considered in artificial systems were the effect(s) of these factors on electron transport may be isolated. For a range of indium compositions, we consider independently the effects of alloying (with and without alloy scattering) and strain on the bandstructure and material parameters and, in turn, their effects on the electron transport properties. We show that increasing the indium concentration generally improves the carrier low field mobility and peak velocity of unstrained materials but has a detrimental effect on the saturation velocity. Strain reduces the low field mobility and peak velocity but gives a slightly higher saturation velocity when compared to GaAs, and the unstrained system. Comparison of transient and steady state transport phenomena is made for strained In(_0.15)Ga(_0.085)As/GaAs quantum well structures, at fields high enough for real- and reciprocal-space transfer to occur. An artificial case, called the unstrained system, where the strain effects on the bandstructure and material parameters are neglected is also considered. Differences between the strained and unstrained well results are small and mainly transient. At steady state, most of the electrons for almost all fields reside in unbound states. The strained and unstrained systems show higher low field mobilities when compared to bulk GaAs. Lattice vibrations are also affected by heterostructures and we have made a study of the effects on the low field transport of electrons in a 70Å Al(_0.3)Ga(_0.7)As/GaAs quantum well when the polar optical phonon modes which interact with the electrons are described by three phonon models which describe the lattice vibrations of the heterostructure; the Hydrodynamic Model (EDM), the Dielectric Continuum Model (DCM), and the Bulk Phonon Approximation (BPA). We show that the BPA and EDM predict similar transport effects and are in good agreement with experimental results. We conclude that, at present, the BPA is an adequate model to describe the phonon modes in heterostructure quantum wells for use in transport calculations.