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.