Dynamics of porous silicon formation by etching in HF + V2O5 solutions

Formation of porous silicon by etching of silicon wafers with vanadium pentoxide (V2O5) dissolved in hydrofluoric acid (HF) has been studied with infrared spectroscopy and electron microscopy. V2O5 creates VO2+ in solution, which initiates the reaction by injecting holes into the silicon valence band. Much is known about the mechanism of etching that leads to flat Si surfaces; however, the transition to pore formation is not well understood. The rate of film growth depends linearly on the V2O5 concentration in aqueous solutions but has a nonlinear dependence on the formal HF concentration. Addition of ethanol greatly decreases the etch rate and changes the pore morphology from a mixture of {100} + {110} planes to predominantly {100} planes. A plot of thickness versus etch time evolves from a quadratic to a linear dependence, whereas the surface area depends linearly on the etch depth. These observations are consistent with a model in which pores with a uniform diameter nucleate randomly then lengthen linearly in time. The pore density increases at short times and then reaches a saturation value. The probability that the collision of a VO2+ ion with the surface leads to etching of a Si atom (reactive sticking coefficient) is similar to 3 x 10(-8).