Characterization of Porous Silicon: Structural, Optical and Electrical Properties

The aim of this paper is to provide a better understanding of photoluminescent porous silicon (PS) microstructure in relation to their electronic properties: absorption band edge shift 1 and quantum confinement hypothesis 1,2 , dielectric constant evolution and electroluminescence characteristics. Results concerning the p type PS microstructure characterization by X ray diffraction and electron microscopy are presented showing a noticeable decrease in crystallite size and surface area with decreasing substrate doping and increasing porosity. The optical transmission of homogeneous free-standing PS layers of different porosities and substrate dopings is studied, showing no evidence of a direct energy gap in PS. On the contrary, a large blue shift of the optical absorption edge, taking into account the total Si mass content in the PS film, is demonstrated. This shift is well correlated with the crystallite size variations with porosity and substrate doping and is attributed to a quantum confinement of electronic wavefunctions in the nanocrystallites. On the other hand, ellipsometry measurements show the PS absorption to be little affected by the microcrystalline structure of the material in the 3.5–5 eV range, i.e. above the direct band gap of bulk Si. This indicates that, if confirmed, the quantum confinement strongly affects the PS joint density of states in the vicinity of the Si band edge and, as could be expected, to a much lesser degree near the edge of the confining potential. Capacitance voltage measurements of thin PS layers allow the determination of the dielectric constant which is shown to decrease with increasing porosity. This behavior is in reasonable agreement with thee values deduced from the transmission experiments in the near infrared. Furthermore, it is shown that this dependence on porosity is well accounted for by the Bruggeman effective medium approximation. Finally, recent results concerning visible light emission from solidstate porous silicon devices will be presented: I-V characteristics, electroluminescence intensity and dynamic, quantum efficiency and device ageing.