Surface Chemistry of Porous Silicon

As-prepared porous silicon comes out covered with covalently bonded hydrogen. This hydrogen coating provides a good electronic passivation of the surface, but it exhibits limited stability, being removed by thermal desorption or converted into an oxide upon prolonged storage in air. Starting from the hydrogenated surface, an oxide layer with good electronic properties is also obtained by anodic oxidation or rapid thermal oxidation. The hydrogenated surface may be nitridized using thermal treatments in nitrogen or ammonia. Fast halogenation of the surface may be obtained at room temperature, but the resulting coating is rapidly converted to an oxide in the presence of moisture. Many metals have been incorporated into the pores, using chemical or vacuum techniques, or even direct incorporation during porous silicon formation. More interestingly, organic derivatization may increase surface stability or provide chemical functionalities. The poor reactivity of the hydrogenated surface can be remedied by using various methods: thermal desorption of hydrogen, hydroxylation or halogenation of the surface, thermal or UV assisted reaction. However, most promising results have been obtained through either Lewis-acid catalyzed grafting or electrochemical activation of the surface. The latter method has been used for grafting formate, alkoxy, and recently methyl groups. In most of these methods, oxidation is present as a parallel path, and care must be taken if it is not desired. Also, 100% substitution of the hydrogens by organic groups has never been attained, due to steric hindrance problems. The electrochemical method appears especially fast, and has led to 80% substitution of the hydrogens by methyl groups, with no photoluminescence loss and a chemical stability increased by one order of magnitude.