Abstract
In this thesis the results are presented of a theoretical investigation into the effects of strain and quantum well disordering (interdiffusion across the well-barrier interface) on the carrier confinement profiles and subband structure of strained InGaAs/GaAs undoped, single quantum wells and the resultant changes in their optical properties, including the absorption coefficient and the refractive index, which are of importance in photonic applications. An error function distribution is used for the constituent atoms composition after interdiffusion. The subband structure is calculated using spatially dependent effective masses and strain, within the parabolic band approximation and neglecting valence subband mixing. Excitonic effects and the continuum states below the barrier band edge are considered. The results obtained show that disordering of InGaAs/GaAs quantum wells leads to graded strain and carrier confinement profiles. The ground state transition energy increases with interdiffusion so that the fundamental absorption edge shifts to shorter wavelengths, and can be tuned to wavelengths around 1 mum. For wavelengths > 1 mum the refractive index decreases as the extent of disordering increases, resulting in a positive refractive index step with respect to the as-grown region. This indicates the possibility of strong lateral confinement of photons in strained quantum wells. Results are also presented showing how disordering modifies the exciton Stark shift that occurs when an external electric field is applied. They show that disordering can increase the Stark shift in InGaAs/GaAs quantum wells, leading to enhanced electroabsorption effects, and thus demonstrating the potential of improving device performance, such as a higher on/off ratio and a lower operational voltage, in electroabsorption modulators. It is also shown that the disordering of In0.53Ga0.43As/InP quantum wells, combined with the strain that can arise, leads to confinement profiles which are quite distinct from those of both strained InGaAs/GaAs and lattice-matched AlGaAs/GaAs disordered quantum wells, and which could be of interest for photonic applications. An abrupt confinement profile is maintained after interdiffusion, with a potential build-up in the barrier near the interface, and miniwells at the bottom of the potential wells. Results show that the heavy hole well can support the ground state within the miniwells and that the potential build-up can result in quasi-bound states.