On the Origin of the Electrically-Induced Spectral Shift of Porous Silicon Photo- and Electro- Luminescence
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A. BSIESY, M.A. HORY, F. GASPARD, R. HERINO, M. LIGEON, F. MULLER, R. ROMESTAIN, J.C. VIAL Laboratoire de Spectrometrie Physique (CNRS URA 08), Universit6 J. Fourier de Grenoble, B.P. 87, 38402 Saint Martin d'Hhres, France
ABSTRACT Experimental results showing two electrically-induced phenomena, namely the voltage-tunable electroluminescence (VTEL) and the voltage-induced quenching of porous silicon photoluminescence(QPL) are given. In both cases, a spectral shift as large as 300 nm can be recorded for an external bias variation of only 0.5V. This spectral shift is characterised by a blue-shift of the whole EL line in the case of the VTEL whereas it results from a progressive and selective quenching starting by the low-energy part of the luminescence line in the case of the QPL experiments. The origin of this spectral shift is discussed in relation with an electrically-induced selective carrier injection into the silicon nanocrystallites accompanied with an enhancement of the non-radiative recombination which might take place by an Auger relaxation process. Finally, it is shown that a partial oxidation of the porous silicon layer leads to a complete loss of the selectivity of these two phenomena. This result is qualitatively discussed by considering the voltage drop distribution between the substrate and the silicon nanocrystallites. The voltage drops are modified by the growth of the oxide layer on the nanocrystallite surface leading to a modification of the energy barriers at the crystallite boundaries.
INTRODUCTION It has been shown that while highly-porous silicon layers can only give a very weak electroluminescence (EL) signal if contacted by a solid electrode[l,2], a very efficient EL can be obtained if a liquid contact is used[3,4,5] allowing the electrical polarisation to be applied in the whole bulk of the porous material. This possibility has first been demonstrated during the anodic oxidation of p-type porous silicon where an intense emission of visible light can be observed[3]. It results from radiative recombination between holes and electrons injected on the confined electronic levels of the silicon nanocrystallites[6]. However, this electroluminescence is accompanied by an irreversible evolution of the material through its oxidation[7] which results in a rapidly vanishing of the EL emission. On the other hand, a much more permanent EL has been obtained by cathodically biasing n-type porous silicon in contact with an electrolyte containing the persulphate ion (S208-2)[4,51. Further, the energy of the emitted light can be easily tuned through the external polarisation [8]. In a similar manner, the cathodic polarisation induces a dramatic change of the porous silicon photoluminescence. It leads to a reversible, highly contrasted and energy selective quenching of the photoluminescence (QPL) for a polarisation variation of only about 0.5V[9]. This strong QPL is accompanied by a spectral blue-shift along with an important narrowing of the PL line. In this paper, the major experimental features of these t
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