Study of the luminescence mechanism in porous silicon structures

Abstract

Measurements of n- and p-type porous silicon indicate no direct correlation between particle size and photoluminescence (PL) energy. Controlled continuous removal of silicon does not result in a continuous PL blueshift, which would be expected in the quantum confinement model. Also, high temperature (1200 °C) anneals of porous silicon lead to a material consisting of 100–200 nm silicon spheres, with very low dangling bond densities, similar to crystalline silicon. This material does not exhibit noticeable PL in the visible range but when dipped in hydrofluoric acid (HF) for 1 s, strong visible PL appears with no structural changes noted. Polysilane/hydride complexes appear with the HF treatment, leading to the conclusion that the visible PL may be the result of a surface phenomenon related to the polysilane/hydride complexes, and not to a bulk Si quantum confinement effect. Anneals of porous silicon to 690 °C also show a significant redshifting of the PL, exhibiting identical behavior to measurements of shrinking of the optical bandgap of hydrogenated amorphous silicon (a-Si:H). With these results in mind, a surface related polysilane luminescence mechanism in porous silicon will be discussed, in which the PL energy is a function of H content and the PL intensity is a function of total surface area.