**3.17. Dealloying**

Using dealloying of Al-Si eutectic system, thin layers of mesoporous silicon with ultrahigh density of cylindrical pores with an average diameter of 5 to 13 nm have been realized [66]. The Al-Si system is deposited by sputtering of an Al0.56Si0.44 target with Ar pressure of 0.1 Torr at low temperatures (below 100°C). Cylindrical pores are generated as a result of combination of nanoscale phase separation of the Al-Si system during deposition and subsequent removal of Al cylinders by chemical etching. Indeed, by appropriate choosing of deposition parameters, especially the deposition rate, it is possible to control the structural development of Al-Si system that occurs at the surface during the film growth at low temperatures. Accordingly, by controlling the nanoscale phase separation through deposition parameters, Fukutani et al. have successfully fabricated Al nanocylinders surrounded by an amorphous silicon matrix [66]. Removing the Al cylinders by immersing the specimen in concentrated sulfuric acid solution for 24 h, a porous amorphous silicon layer with cylindrical pores is obtained (**Figure 20**). A subsequent annealing step at 800°C in H2 atmosphere for 1 h crystallizes the porous material without any visible alteration in the average size and density of the pores [67]. Further studies are needed to determine the controllability over the shape and orientation of the pores generated by this method.
