DEPENDENCE OF PHOTOCHEMICALLY ETCHED POROUS SILICON FORMATION ON PHOTOETCHING WAVELENGTH AND POWER

It is well known that porous silicon can be formed by the combined che mical action of HNO3 and HF and that this reaction with silicon can be initiated by light. With n-type silicon in a diluted etching solution in the dark, the etching process can be controlled by the exposure of selected areas of the sample to light. In this work, different lasers are used to investigate the wavelength and power dependence of this e tching process. It is found that the use of higher energy photons prod uces fluorescent porous silicon which emits light of a higher energy. It is also observed that increasing etching laser intensity also produ ces porous silicon which fluoresces at a higher energy. Studies to inv estigate the effect of etching laser intensity on the fluorescence int ensity were inconclusive, suggesting that it plays a minor role compar ed to other environmental factors. Fluorescence micrographs show that the etched regions, though irradiated by spatially symmetric radiation , were nevertheless irregular in shape, clearly influenced by the reac tion-diffusion dynamics of the chemical kinetics. As well, different r egions of the etch spots clearly emitted light of different wavelength s, indicating that narrower emission profiles than are normally associ ated with porous silicon are possible when examined on length scales o f tens of microns. Low duty cycle pulsed radiation was not effective i n producing porous silicon which provides some indication about the ti me frame for the etching process to be photoinitiated. (C) 1996 Americ an Vacuum Society.