Photoelectrochemical etching of Si and porous Si in aqueous HF

The irradiation of n-type Si(111) submerged in HF(aq) with a UV, visible or IR laser can lead to the formation of photoluminescent porous silicon (por-Si) thin films. We demonstrate that two distinct photoelectrochemical etching processes are induced by illumination: anisotropic etching, which leads to pore formation and propagation, and isotropic etching, which removes por-Si. As a result, both the solution/por-Si and the por-Si/c-Si interfaces assume shapes that are determined by the laser intensity profile. A counter reaction occurs in a spatially separate region of the crystal. A dark reaction between the por-Si and HF(aq) has also been observed. The photoelectrochemical reaction rates, after an initial increase, approach a constant value. The porous film increases steadily in thickness, while it continues to descend deeper into the crystalline Si. We discuss a mechanism for the formation and dissolution of the por-Si which emphasizes the effects of quantum confinement within the por-Si.