Dependence of macropore formation in n-Si on potential, temperature, and doping

Subjecting illuminated n-type silicon wafers to anodic bias in an HF contai ning electrolyte results in the formation of macropores under certain condi tions. In this paper the formation of randomly nucleated macropores is stud ied as a function of the applied potential, the temperature, and the doping levels of the samples. A large number of micrographs was evaluated by comp uterized image processing and the data obtained are compared to predictions of pore formation models. It was found that the formation of randomly nucl eated macropores involves a prolonged nucleation phase. Starting from a pol ished surface, first macropores occur after a certain amount of Si has been homogeneously dissolved. In this nucleation phase the thickness of the hom ogeneously dissolved Si depends strongly on the doping level and the temper ature, but only weakly on the applied bias. In a second phase of stable por e growth, the density of pores is investigated as a function of temperature and anodic potential. For low-doped material a strong influence of the spa ce-charge region on the average macropore density is observed in accordance with existing models; an increased anodic bias, e.g., decreases the densit y of pores. For highly doped silicon the situation reverses; increasing ano dic bias increases the pore density, in contrast to predictions. The pore g rowth in this region is not very sensitive to the space-charge region but s eems to be dominated by the chemical-transfer rate. (C) 2000 The Electroche mical Society. S0013-4651(99)06-155-8. All rights reserved.