Optical properties of two-dimensional semiconductor heterostructures: Ion implantation and carrier diffusion.

For the most part of this work, we study the effects of focused ion beam implantation in InGaAs/GaAs quantum well structures. The technique of steady-state photoluminescence spectroscopy is utilized to study the compositional disordering occurring in InGaAs/GaAs quantum well structures having been implanted at different doses and subsequently annealed. Photoluminescence results of uniformly implanted regions of InGaAs/GaAs samples are along with a simple formalism permitting the calculation of the interdiffusion lengths. These results, along with SIMS measurements, show that channeling effects play a most important role in disordering quantum wells that lie deep beneath the sample's surface. The observations also show a photoluminescence shift saturation with dose at approximately 2 $\times$ 10$\sp $ Ga$\sp{+}$/cm$\sp2$. Results of pattern-implanted samples give rise to the question of the minimum dose required to induce uniform compositional disordering in InGaAs/GaAs structures. Studies of this "critical" dose, are presented for both AlGaAs/GaAs and InGaAs/GaAs quantum well structures and show that the minimum dose required to induce intermixing in InGaAs/GaAs is approximately two orders of magnitude lower than that observed for the AlGaAs/GaAs structures which was 2 $\times$ 10$\sp $ Bi$\sp{+}$/cm$\sp2$. As another major part of this work, we describe a novel, low light level optical detection system that can be easily configured for various modes of operation. These include (1) time-integrated (steady-state) photoluminescence spectroscopy, (2) transient, spectrally gated photoluminescence decay, (3) time-windowed photoluminescence spectroscopy, (4) two-dimensional, time-integrated photoluminescence mapping, and (5) time-resolved, two-dimensional photoluminescence mapping with time resolution of ${\sim}$100 ps. We use the system in mode (5) to study electron-hole pair expansion in GaAs quantum wells as a function of temperature for a fixed excitation intensity. The results show that the diffusion coefficient rises from an initial value of 26 cm$\sp2$/s at 80 K to 38 cm$\sp2$/s at ${\sim}$100 K and then monotonically decreases to a value of ${\sim}$10 cm$\sp2$/s at room temperature. The rise in the diffusion coefficient is attributable to the gradual decrease of interface roughness scattering with increasing temperature while the decrease of the diffusion coefficient is attributable to the increase of phonon scattering mechanisms with increasing temperature. Results of diffusion at fixed temperature, but as a function of excitation intensity, are also presented and discussed