An electrochemical process aiming at the grafting of methoxy groups on the hydrogenated porous silicon surface at room temperature has been designed. This takes place through partial anodic dissolution of porous silicon in anhydrous methanol. A dissolution mechanism is proposed by analogy with that of the anodic dissolution of ~ilicon in aqueous fluoride media. The methoxylated surface exhibits improved optical characteristics (increased photolun~nescence efficiency, blue shift of the emission), similar to porous silicon anodically oxidized in a nonfluoride aqueous electrolyte. Its stability against aging is also improved as compared to that of the hydrogenated surface, but without reaching the stability of anodically oxidized porous silicon. This residual sensitivity to aging is ascribed to the nonnegligible amount of Sill species which remains on the methoxylated surface upon the modification process.
Porous silicon layers are chemically grafted with trimethylsiloxy groups by exposure to hexamethyldisilazane (HMDSj vapors. After treatment, infrared spectroscopy reveals that part of the initially hydrogenated surface is actually modified. Luminescence of the porous layers appears to be little affected by the treatment. On strongly luminescent samples, the photocarriers exhibit the same infrared absorption as in hydrogenated or oxidized porous silicon, evidencing the similar involvement of localized states in the luminescence process. The treated porous silicon layers also exhibit an increased stability against aging in ambient, as compared to hydrogenated porous silicon. Furthermore, the modification can be removed by simple exposure to H F vapor, thereby restoring the initial hydrogenated surface.
La surface hydrogenee du silicium possede des proprietes remarquables, mais une stabilite a I'oxydation insuffisante. Pour y remedier, plusieurs groupes organiques ont ete substitues a I'hydrogene de surface. Ces greffages peuvent se fairc chimiquement en plusieurs etapes, mais la voie electrochimique permet des reactions directes a partir de la surface hydrogenee. On peut ainsi methoxyler partiellement la surface du silicium poreux par dissolution anodique controlec, mais surtout methyler sa surface de maniere non destructive, avec un rendement de 80% limite seulement par I'encombrernent sterique. La surface methylee du silicium poreux possede une stabilite accrue d'un ordre de grandeur.The hydrogentated surface of silicon exhibits remarkable properties, but poor resistance to oxidation. To improve its stability, surface hydrogen has been replaced by several organic groups. Such grafting can be carried out chemically by a multi-step reaction scheme. However, an electrochemical approach allows direct reaction with the hydrogenated surface. The porous-silicon surface has been partially methoxylated by a controlled anodic dissolution. If has also been methylated using a non-destructive anodic process, with a yield of 80%, limited only by steric hindrance. The methylated surface of porous silicon exhibits a stability against oxidation increased by an order of magnitude.Mots cles: silicium, silicium poreux, greffage, greffage electrochimique, chimie de surface.1 y a plus de 20 ans, Harrick et Beckmann (1974) ont I demontre que l'on pouvait, par simple rinqage dans I'acide fluorhydrique, preparer des surfaces de silicium vierges d'oxydes et passivees par une monocouche de liaisons Si-H. Cette monocouche de liaisons Si-H est relativement stable a l'echelle de quelques heures. Elle confere a la surface des proprietes electroniques remarquables, en particulier une vitesse de recombinaison de surface extr2me-ment faible (Yablonovitch et al., 1986).Le silicium peut 6tre rendu poreux par dissolution anodique dans l'acide fluorhydrique. Lorsque la porosite du materiau est suffisamment elevee, le silicium poreux acquiert des proprietes optiques etonnantes. En particulier, il devient luminescent a temperature ambiante, avec un rendement eleve (Canham, 1990). Le matkriau possede une surface specifique tres importante cjusqu'a 1000 m2/cm3), sa porosite est ajustable, en fonction des conditions d'electrolyse, entre 30 et 90%, et l'epaisseur des couches poreuses peut atteindre plusieurs centaines de microns (Bomchil et al., 1989). Toutes ces proprietes font du silicium poreux un materiau attractif pour differents types d'application.Au niveau moleculaire, apres fabrication, la surface du silicium poreux est couverte de liaisons Si-H, ce qui n'est pas surprenant puisque le materiau a ete obtenu par un traitement dans l'acide fluorhydrique. La passivation de la surface associee a ce recouvrement par l'hydrogene est cmciale pour les proprietes de luminescence. En effet, si on introduit quelques defauts de surface, ceux-ci se comporte...
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