Near Field Optical Microscope Investigations Of Inmmiscibility Effects And Photoreflectance Contrast In Iii V Semiconductor Materials

This project elucidated and examined the unique science behind the spectral and temporal contrast of near field scanning optical microscopy, illuminated the technology this enabled, and considered the limits of simultaneous position, time and spectral resolution. Specifically, (1) High-resolution, quantitative measurements of excess carrier lifetime in silicon were acquired in a novel, all-optical technique. The contrast in the images was modeled and is of interest since the resolution is significantly shorter than the carrier diffusion length. (2) a Ti-sapphire laser has been constructed and modified to decrease pump power requirements so that measurements can be made before the carriers diffuse from under the probe. Novel electron-hole droplet effects are expected. (3) Spectroscopic nano-Raman images were acquired for the first time. The Raman data illustrated interesting manifestations of a near-field in comparisons with far-field spectroscopy. (4) The first and most thorough studies of the optical and thermal properties of near-field scanning optical microscope probes gave insights which led to the development of a new generation of high throughput probes. Other results reflect instrumentation advances: (5) A new constant linear motion system useful for NSOM or other probe microscope coarse approach. (6) A new, low cost force feedback system. (7) A nanometer-resolution, kHz bandwidth, millimeter range position sensor.

This grant supported a program for local spectroscopic studies of semiconductor nanostructure materials. Our work has looked at the various types of techniques of near field optics and pursued the options that are optimized for the optical spectroscopy of semiconductor nanostructures. The thrust of the research has involved the use of solid immersion lenses. This form of near field optics strikes a balance between the need for high spatial resolution and high optical throughput. We have demonstrated that these techniques can be implemented within the context of a cryogenic system and obtain spatial resolution of order lambda3. We have used these techniques to characterize naturally occurring quantum dots in thin GaAs quantum wells. Our studies reveal the surprising fact that these samples have not only zero dimensional excitons but also two dimensional excitons. In fact, most of the material plays host to the two dimensional species while the zero dimensional species occupies only 1-3% of the sample. This result is surprising because all of the light emission comes from the zero dimensional exciton. The two dimensional exciton is observed using photoluminescence excitation diffusion, a technique wherein we are able to generate a local optical excitation and watch it diffuse.

This monograph is concerned with the III-V bulk and low-dimensional semiconductors, with the emphasis on the implications of multi-valley bandstructures for the physical mechanisms essential for opto-electronic devices. The optical response of such semiconductor materials is determined by many-body effects such as screening, gap narrowing, Fermi-edge singularity, electron-hole plasma and liquid formation. Consequently, the discussion of these features reflects such interdependencies with the dynamics of excitons and carriers resulting from intervalley coupling.