Mechanism and control of formation of porous silicon onp-type Si

Abstract

A simple extension of Beale's and Lehmann's models for the formation of porous silicon layer on p-type silicon is proposed with a view to explain the experimental conditions necessary for obtaining either uniform vertical pores or non-uniform pore branching, as desired. A uniformity parameter is defined and correlated with the measured porosity. The dependence of the porosity and the uniformity factor with the various formation parameters of porous layer are studied experimentally and explained qualitatively.

Conductivity Modulation of Porous Silicon by Formation Parameters

“…7 show the variation of porosity (P m ), layer thickness (h m ), uniformity (u) and effective conductivity (s eff ), respectively. The uniformity defined as u ¼ P u /P m is a measure of pore branching, where P u is the porosity of the PS layer for a uniform vertical growth with the same HF concentration and the same J [3]. This figure indicates that the layer thickness lies in the range of 1-4 mm; porosity was in the range of 50-85%.…”

“…The conditions leading to uniform vertical growth and also non-uniform pore growth due to pore branching have been previously reported in a paper by Saha et al [3]. It was shown that the porosity for uniform (P u ) and non-uniform (P m ) pore growth for the same applied current are related by…”

“…Porous silicon has been prepared on a p-type (1-2 W cm) Si wafer by electrochemical etching method in Teflon cell as described in earlier papers [3,4]. Before anodization, the wafers were polished by alkali etching technique (20% NaOH solution, 80 C, 5 min).…”

“…However, it must be emphasized that due to different pore branching during PS growth, 'identical porosity' PS layers might have different surface to volume ratio leading to a possible different effective conductivity. The extent of pore branching has been described suitably by the 'uniformity factor' introduced by Saha et al [3]. It is thus obvious that, in addition to the nature of transport mechanism in PS nanocrystals, the microstructure/morphology plays a vital role in the determination of the effective conductivity.…”

“…It is thus obvious that, in addition to the nature of transport mechanism in PS nanocrystals, the microstructure/morphology plays a vital role in the determination of the effective conductivity. The microstructure depends not only on a number of parameters like surface roughness, resistivity, doping type, and density of the substrate but also on the preparation conditions controlled by anodizing current density, concentration of HF solution in the formation bath [3,4]. These might be properly adjusted to give the resulting PS the desired range of effective conductivity.…”