Unique Porous Structures Supported by Anodic Treatments in Liquid Ammonia (-50°C, Patm)

Since the first anodic porosification of semiconductors in the 1950s, numerous studies have reported a wide range of porous structures. The conditions used during the growth of pores can explained the shapes obtained but the formation mechanism and especially the influence of surface chemistry during porosification still raise many questions. It is in such context that the contribution of liquid ammonia (-55°C) is crucial. Indeed, this remarkable non-aqueous solvent allows, on the one hand, to obtain novel porous structures on n-InP and, on the other hand, it allows for the first time a specific and original interfacial chemistry, governed by the chemistry of nitrogen. In fact, in liquid ammonia, the dissolution of III-V semiconductors (GaAs, InP) is systematically done after a surface passivation step. This passivation step is particularly stable on InP and revealed the formation of a phosphazene type film (monolayer) resulting from the concomitant oxidation of the solvent and the semiconductor. The "multi-material" approach is a key factor, as it underlines the determinant role of the surface chemistry. In fact, at higher current densities, the porous structures of n-InP and n-GaAs show contrasting structures, which give an original behaviour compared to the aqueous medium. In order to determine the efficiency of the dissolution mechanism (at lower and higher current density), the quantity of indium (or gallium) salt will be quantified by Inductively coupled plasma optical emission spectroscopy analysis after ammonia evaporation. Moreover, the remarkable reproducibility of the anodic behaviour allowed physico-chemical characterisations of the porous surface, supported by scanning electron microscopy and the variation of the interfacial potential, photoluminescence and X-ray photoelectron spectroscopy. More specifically on n-InP, where an original structure was revealed, these analyses, during the early stages of porosification, led to the identification of a porosification mechanism in this solvent. Furthermore, at higher current densities, an amorphous film becomes thicker on the surface of the porous structure. Further porosification requires the permeability of this amorphous structure which would most probably result from the polymerisation of phosphazene since the presence of nitrogen could be clearly identified on the pore walls by Auger spectroscopy.