Strain effects in semiconductor quantum wells

Abstract

In this thesis the effect of the strain which is present in a lattice mismatched quantum well (QW) on the properties of the device is investigated. The k.p method is used within the envelope function framework to obtain the bandstructure and the wave functions of bound and unbound states in both lattice matched and strained quantum wells. The model includes spin and interband mixing effects. We show that the mixing of wave function character between adjacent subbands which occurs in a QW can be reduced in a strained structure, and that this can result in the ground state subband having a reduced effective mass. The effect of the reduction in mixing on the optical matrix elements for transitions between the conduction and valence bands is also investigated. A model is developed which enables the calculation of the gain and spontaneous emission spectra and threshold properties of a multiple quantum well (MQW) laser device. The model includes a full description of the non- parabolic subband dispersion and the variation of the optical matrix elements along the subbands, together with an energy dependent lifetime broadening of the spectrum. The model is used to compare the performance of strained and unstrained InGaAs/InP MQW devices operating at 1.3/µm and 1.55µm. The reduced valence band edge effective mass of the strained devices is shown to lead to a reduced threshold current, temperature dependence and linewidth enhancement factor and an enhanced gains lope. The unbound states of the well are used to investigate the bound- unbound intervalence band absorption rate in the above devices. The absorption coefficient for this process is found to be small (<2cm(^1)) in all the cases considered.