VOLTAGE-INDUCED MODIFICATIONS OF POROUS SILICON LUMINESCENCE

The remarkable voltage-tunable electroluminescence (VTEL) observed on porous silicon-electrolyte junctions is investigated in relation to ma terial morphology and electrolysis parameters. The electroluminescence (EL) is obtained upon cathodic polarization of n-type porous silicon in contact with aqueous solutions containing the persulphate ion. The observed long-lived EL shows a reversible spectral shift as large as 3 00 nm for an external bias variation of about 0.6 V. The study of the EL behaviour as a function of the external voltage and the persulphate ion concentration shows that while the amplitude of the EL is proport ional to the intensity of the exchanged current, the spectral position is only determined by the applied voltage. A qualitative model, takin g into account the voltage dependence of the charge injection probabil ity into the size-distributed silicon crystallites, gives a good descr iption of the observed VTEL behaviour. In a similar manner, cathodic p olarization induces a dramatic change in the porous silicon photolumin escence. It leads to a reversible, highly contrasted and energy-select ive quenching of the photoluminescence (QPL) for a polarization variat ion of only about 500 mV. A spectral blue shift, along with a signific ant narrowing of the PL line accompanies the observed strong QPL. This results from selective quenching starting at the low luminescence ene rgy and reaching progressively the high luminescence energy as the cat hodic polarization is increased. Just as for VTEL, this selective char acter of the QPL can be explained by a voltage-induced enhancement of charge injection into the size-distributed silicon nanocrystallites.