Characteristics of porous n-type silicon obtained by photoelectrochemical etching

Abstract

The term porous silicon includes many different and complex materials whose physical properties depend critically on many parameters, such as the initial silicon properties or the preparation parameters. Most of the research performed on porous silicon has been dedicated to the p-type material and is well documented [1–4]. However it should be mentioned that a possible application to light emitting diode technology would be based on n-type substrates and consequently control of porous n-type silicon is a major issue. Porous silicon obtained by anodization in the dark of n+-type Si, is a sponge like material with randomly-positioned pores and a sparse silicon network of columns in the nanometer dimension range. In the case of n−-type Si, due to the rectifying behaviour of the semiconductor/electrolyte junction, it is necessary to illuminate the electrode during the anodization process (i.e. photoelectrochemical etching, PEC-etching) [3, 5–9]. Similarly porous n+-type Si can be formed under illumination. Comparatively little is known about these latter two cases [3, 5–16]. An essential result is that the porous n-type (n−and n+) silicon, obtained under illumination, generally consists of a superficial nanoporous silicon layer formed of silicon fibers [15, 16]. It covers a macroporous silicon layer containing pores in the micron size range and also coats the interior walls of the macropores [10, 11]. The room temperature visible luminescence originates only from the upper nanoporous layer [12].