Theoretical studies of the electro-optic properties of quantum-well structures

Abstract

In this thesis the electro-optic properties of III-V compound quantum-well structures for applications in optoelectronic devices are investigated with particular emphasis on a novel optical modulator based on the band-filling effect, called the barrier reservoir and quantum-well electron transfer structure (BRAQWETS).

In order to calculate the optical properties of compound semiconductor heterostructure devices, it is necessary to first determine the electronic properties for both electrons and holes. We take into account the effects of an external electric field and free carrier injection by using a self-consistent Poisson-Schrodinger (SCPS) scheme and a Transfer Matrix Technique (TMT). We also consider the effect of a biaxial strain due to lattice mismatch in the TMT.

In a preliminary study, the change of the refractive index with applied electric fields up to 100 kV/cm has been studied for different quantum-well structures at room temperature. Our results show a noticeable polarization effect and an oscillatory electro-optic effect due to a modulation of the overlap integral by the electric field. Both TE and TM mode refractive indices decrease monotonically in single and double quantum-well structures for large electric fields, in contrast to the very strong oscillatory effect in multiple quantum-well structures.

The absorption coefficient of a single quantum-well structure at large electric fields has been investigated in relation with the quantum unconfined Stark effect (QUSE). Our numerical calculations for electric fields ranging from 100 to 500 kV/cm are in reasonable agreement with the experimental data.

We have used our model as a CAD tool to investigate electro-optic phenomena for a novel optical modulator called BRAQWETS and compared our results with the experimental data. In addition, we have developed a physical model for the time response of the BRAQWETS. Comparing our results with the experimental data, we have identified quantum-well tunneling as the ultimate mechanism limiting the device speed, and have predicted a switching time of less than 10 ps. Finally we have implemented our self-consistent code for nonzero current density and a position-dependent electron quasi-Fermi function. With these refinements, we were able to calculate the I-V characteristics and compare them with experimental results.