Transmission electron microscopy investigation of the crystallographic quality of silicon films grown epitaxially on porous silicon

Epitaxial growth of thin crystalline layers on porous silicon can provide opportunities for silicon-on-insulator applications and silicon-based solar cells provided that the epitaxial layer has a sufficient crystallographic quality. Transmission electron microscopy (TEM) is used in this work to study the microstructural properties of porous silicon (PS) and of epitaxial Si layers grown on top of the PS. A more dense silicon layer exists in the upper part of the as-prepared porous silicon. The quality of the epitaxial layers is found to depend strongly on the morphology of the initial porous Si layers, and on the deposition temperature of the epitaxial silicon growth. The porous structure is completely destroyed after thermal CVD deposition of Si at too high temperature, resulting in a highly defective epitaxial layer. A high-quality epi-Si layer is obtained when depositing on a low porosity layer at 725°C. A stacked porous layer with a low porosity in the upper-part and a high porosity in the bulk can be formed by changing the conditions during the formation of the layer. On such a dual porous layer, an epitaxial silicon layer with a low defect density can be grown.