Erbium-doped GaAs/AlGaAs quantum wells and normal mode coupling of semiconductor microcavities

Abstract

This dissertation investigated two different types of semiconductor heterostructures grown by molecular beam epitaxy. The first were erbium-doped GaAs/AlGaAs quantum well structures with concentrations of Er in the range of 10¹⁷-2 x 10¹⁹ cm⁻³. Photoluminescence (PL) of Er³⁺ ions and Er-induced defects were studied at liquid helium and higher temperatures. A strong diffusion of erbium and interdiffusion of gallium and aluminum ions were observed at the boundary of the GaAs/AlGaAs quantum wells which led, at high erbium concentrations, to the degradation of the quantum wells and macroscopic (average) leveling of the erbium and aluminum concentrations over the whole semiconductor structure. From high-resolution PL spectra the existence of three types of Er centers was deduced that differ by the positions of fine structure lines, PL lifetimes, and PL temperature dependence. Our results indicate that these centers are accompanied by the appearance of three types of carrier traps. Our experiments give evidence that the carriers captured into these traps control the Auger excitation of Er ions. Besides erbium luminescence at 1.54 μm, we have observed luminescence of erbium ions from upper excited states at 0.82 and 0.98 μm which demonstrates the possibility of realizing a three-level scheme of light emission. The second structures investigated were GaAs/AlAs microcavities containing different numbers of identical and non-identical quantum wells. The normal mode coupling behavior of these structures was studied, including the differences from the atomic case. For the identical quantum well structures we examined their linear and nonlinear behavior and modeled this behavior using a transfer matrix method. For the non-identical quantum well case we measured three-dip reflectivity spectra characteristic of three coupled oscillators. A theoretical model based on a nonlocal dielectric response and a transfer matrix method was used to model the microcavities and yielded good agreement with experiment. A simpler model using dispersion theory was also described that gave good qualitative agreement to the measurements.