Visible Electroluminescence from p-Type Porous Silicon in Electrolyte Solution

Abstract

Electroluminescence (EL) from p-type porous silicon (p-PS) prepared by anodic oxidation in HF solution for various oxidation times (preparation times) and at various current densities in Na 2 SO 4 aqueous solution was investigated galvanostatically. EL was observed as soon as constant current started to flow. It increased at the beginning, reached a maximum, and then decreased with anodic reaction time. The electrode potential gradually increased with oxidation time from ca. 0.8 V (vs Ag/AgCl) at the beginning to 1.2 V after several hundreds of seconds and increased steeply after EL ceased to be observed. How long EL lasted and the total integrated EL intensity were linearly related to the preparation time. EL spectra were blue-shifted during anodic oxidation, and the rate of the blue-shift was dependent on the preparation condition of the p-PS. The time dependence of the integrated EL intensity had a good correlation with that of the amount of Si-H n species, which was determined by in situ reflection FT-IR, indicating that the surface Si-H n acts as an electron injector. Cross sectional SEM observations showed that the thickness of the p-PS layer was linearly related to the preparation time. AFM measurements showed that the surface morphology of the p-PS depended not on the preparation time but on the current density for preparation. On the basis of these results, the mechanism of EL was discussed.

Infrared induced visible emission from porous silicon: the mechanism of anodic oxidation

“…We have also measured the time-dependence of the emission spectrum. Results similar to those found by Billat [12] and Uosaki et al [13] were obtained; the emission maximum shifts significantly towards shorter wavelengths as the oxidation time increases. This blue-shift was explained by assuming a size-distribution of the light-emitting particles; larger particles are oxidized first and the smaller particles become active as the oxidation proceeds.…”

“…As the oxidation proceeds, the emission at wavelengths below 830 nm first increases, while the maximum clearly shifts towards shorter wavelengths. This blue-shift is similar to that observed with p-type electrodes [12,13]. At the end of the current plateau in Fig.…”

“…The transient nature of the current and of the corresponding luminescence is attributed to the fact that Si-H species are consumed in the oxidation process and are not regenerated. Recently, Uosaki et al [ 13] reported in situ FT-IR measurements which clearly show that the anodic oxidation and the corresponding luminescence are accompanied by a strong decrease of the Si-H coverage. However, Hory et al [ 14] found the Si-H coverage to remain approximately constant, which might be due to a much smaller anodization current used in the experiments.…”

“…Bright visible luminescence has also been observed during the anodic oxidation of porous silicon in HF-free indifferent electrolyte solutions [5][6][7][8][9][10][11][12][13]. Most of the work has concentrated on the electrochemical oxidation of p-type material [5][6][7][8]10,12,13].…”

“…Most of the work has concentrated on the electrochemical oxidation of p-type material [5][6][7][8]10,12,13]. The process responsible for the minority carrier injection is not clear as there are no electroactive species in solution capable of donating electrons to the conduction band.…”