Silicon on Insulator Structures Obtained by Epitaxial Growth of Silicon over Porous Silicon

Abstract

Epitaxial silicon films are obtained by low temperature chemical vapor deposition on porous silicon layers (PSL). The PSL are formed on lightly and highly boron doped silicon substrates. In the case of lightly doped substrates, the epitaxial films exhibit a large defect density (1010 cm−2). It is shown that this defect density can be reduced by a 102 to 103 factor through the use of silicon channeled implantation and subsequent thermal annealing. On the other hand, when the epitaxial growth is performed on PSL formed from highly doped boron substrates, the epitaxial quality of the resulting films is equivalent to the homoepitaxy of silicon on bare silicon. On both kinds of samples it is shown that during oxidation of the buried PSL, the back interface of the epitaxial layer is oxidized by rapid diffusion of oxygen through the porous network, and then a dense oxide is formed at the epitaxial layer/porous silicon interface. Furthermore this oxide, whose thickness can be controlled, acts as a barrier towards boron diffusion even at the high densification temperature (1300°C) necessary to obtain a good quality buried oxide. Very low boron doping level is then measured in the epitaxial film.