Chemical Surface Modification of Porous Silicon

Abstract

Resistance to room temperature oxidation and control over wetting properties can be achieved by chemical modification of a porous-silicon surface. Fourier transform infrared spectroscopy was used in the transmission mode to monitor the surface chemistry of both treated and untreated porous-silicon samples before and after exposure to humid air at room temperature. Surface modification methods investigated include: (i) vapor-phase silation using either hexamethyldisilazane or trimethylchlorosilane, and (if) rapid thermal annealing in nitrogen, ammonia, or argon ambients. The silation treatments, carried out in the presence of trace moisture, were successful both in creating surface trimethylsilyl groups and in suppressing room temperature oxidation. Rapid thermal annealing at temperatures as low as 500~ for 30 s eliminates all silicon hydrides. Nitrided porous-silicon layers are formed at 1100~ in either ammonia or nitrogen; in both cases the silicon nitride infrared absorption peaks scale with the porous layer thickness, indicating that the compounds are distributed throughout the porous layer.Electrochemical etching of silicon in hydrofluoric acid forms a high-surface-area material commonly called porous silicon. The physical characteristics: pore size, porosity, and specific surface area are determined by the processing conditions such as hydrofluoric-acid concentration, current density, dopant type, and dopant density. ~' 2 After the porous silicon is formed, its large exposed surface area will typically oxidize even in air at room temperature. This spontaneous oxidation is enhanced by the presence of moisture in the air. The growth of a native oxide layer (typically 1.5 to 4 nm thick) 3 significantly alters the structure and chemical activity of the porous silicon. For many sensor or optoelectronic users of porous silicon, the surface must be stabilized. In the following, some methods to passivate the surface of porous silicon from continued room temperature oxidation are described. 4

Nanocrystalline Porous Silicon

“…For that purpose substituting surface hydrogen by another chemical species has appeared desirable. Oxidations (Rossi et al, 2001;Bsiesy, et al 1991;Petrova-Koch et al, 1992) nitradation (Anderson et al, 1993) and halogenetion (Lauerhaas & Sailor, 1993) are found to be useful for PS surface passivation. Derivatisation by organic groups and polymer (Lees et al 2003;Mandal et al 2006), offers an alternative possibility to stabilize the material.…”

Silicon Nanocrystals in Porous Silicon and Applications

“…Derivatization with long alkyl groups led to good stability of PL [89], EL [90], photonic properties [91], and biosensors [92]. A few reports deal with nitridation [93][94][95][96] and thermal carbonization [97] of PSi. The latter process was shown to stabilize passive photonic structures without inducing much property shift [98].…”

Preparation and Characterization of Surface‐Modified Luminescent Porous Silicon

“…In order to confirm this statement, it may yet be desirable to prepare PS samples with other surface terminations, and to compare their properties with those of more common samples. Anderson et al [6] have recently shown that the porous silicon surface may be modified using treatment with hexamethyldisilazane. The luminescence properties of such modified samples were not reported yet.…”