ELECTRONIC PROPERTIES OF POROUS SILICON M. R. BELTRAN, J. TAGUiEINA-MARW INEZ, M. CRUZt, and C. WANG Instituto .de investigaciones en Materiales Universidad Nacional Aut6noma de Mexico Apdo. Postal 70-360, 04510, Mexico, D.F. ABSTRACT ,Porous silicon (PS) has been extensively studied in recent years. The origin of its lumines- cence has been a subject of debate. This work attempts to give some insight towards the 'understanding of this phenomenon, studying the behaviour of the energy band structure o6f PS as a function of the pore morphology and distribution, for a given porosity. The porous structure is modeled as empty columns of different sizes and shapes, produced into an otherwise silicon perfect crystal. The columns are passivated with hydrogen atoms. A t•ght-binding Hamiltonian on an sp3s* basis set is applied onto a supercell. Due to the :simplicity of the model, morpholqogy effects can be analyzed. The results show that the hand gap and the nature of the ,states at the top of the valence band depend on the mor- phnlogy. Furthermore, we also discuss the shift of the conduction band minimum towards the gamma point, producing an almost direct band gap, as the pore distribution changes. I.NTRODUCTION Efficient viible luminescence in porous silicon (PS) at room temperature has been reported in samples produced by electrochemical etching in a hydrofluoric solution [1]. This sur- prising optical property has stimulated a great interest, since it is well known that bulk silicon has an indirect band gap of 1.1 eV, which prevents efficient interband radiative recombination in the visible region. One of the main goals in the optoelectronic industry is to find a cheap material which combines electronic and optical responses. It is believed that PS could be a good candidate to achieve this dream, since silicon is very abundant on earth and its use is supported by three decades of semiconductor technology. The understanding of the electronic properties of PS is also important from the scientific point of view, because this new material presents interesting quantum phenomenon, such as electronic behaviour in low dimensional systems as well as electron localization. There are mainly two different points of view explaining the mechanisms involved in the luminescence process. One of them emphasizes the quantum confinement effect [1] and the other suggests the essential participation of the surface layer [2, 3, 4, 5]. However, the 31 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society