Porous silicon based optoelectronics: Characterization, process integration, and device fabrication

The primary objective of our work has been to fabricate light emitting diodes and solar cells based on porous silicon. In order to achieve this goal it was necessary to apply standard microelectronic processes (diffusion, implantation, lithography, deposition, and etching) to porous silicon without causing any degradation to its properties. The thesis describes the different technological challenges that were involved, and the results of our efforts to overcome them. The first step was to standardize the anodization process, by determining the porosity and nanocrystallite size for different current densities, HF concentration, and dopant density of the substrate. The effect of porosity and crystallite size on the structural, optical, electrical, mechanical, and thermal properties of porous silicon was examined in detail. Our characterization data showed that for device applications it was necessary to significantly improve material stability. We have achieved this by (a) annealing porous silicon in oxygen to form silicon-rich silicon oxide (SRSO) and (b) infiltrating polymers into porous silicon to form nanocomposites. Another important goal was to attain large scale integration capability through selective anodization. Accordingly, we have developed multi-layer etch isolators and different etch enhancing and inhibiting techniques that provide sub-micron etch tolerances. The successful resolution of materials and process integration issues has led to a significant improvement in device performance. We have fabricated SRSO LEDs with an external quantum efficiency {dollar}ge{dollar}0.1%. The devices remained stable after 100 hours of continuous operation. The LEDs based on low porosity porous silicon emit efficient band-edge luminescence, which is detectable at very low threshold voltages (2 V). The selective anodization techniques have enabled the fabrication of the first silicon based integrated optoelectronic device. We have also developed solar cells based on porous microcrystalline silicon with efficiencies greater than 10%.