ELSEVIER Thin Solid Films 304 (1997) 358-364 Changes in photoluminescence behaviour and structure of porous silicon related to preparation conditions and laser irradiation H. Elhouichet a, B. Bess,/is a, O. Ben younes b, H. Ezzaouia a, M. Oueslati b a lnstitut National de Recherche Scientifique et Technique, US1, Laboratoire de Photot,olta'ique et des Mat&iaux Semiconducteurs. BP 95, 2050 Hammam-Lif Tunisia b Laboratoire de Spectroscopie Raman, Facultd des Sciences de Tunis, D@artement de Physique, 1006 Le Beh,~dbre. Tunis, Tunisia Received 21 August 1996; accepted 6 February 1997 Abstract Photoluminescence (PL) spectra of porous silicon (PS) are fitted by a theoretical model based on quantum confinement of electrons in Si nanocrystallites having spherical and cylindrical forms. This model permits one to correlate the PL spectra with the PS structure. It was found that the PS structure is almost independent of the porosity of the PS samples when elaborated in tim same HF solution, but it depends on the composition of the electrolytic solution and post-anodisation treatments such as oxidation. The specific surface area (SSA) was estimated and was found to decrease linearly when the porosity increases. It was pointed out that the SSA plays a key role in the PL behaviour within laser irradiation. The effect of laser irradiation on the PL behaviour has been discussed according to the proposed model, and was shown to be dependent on ambient atmosphere. It was shown that the crystallite size decreases throughout photo-oxidation under laser irradiation in air. © 1997 Elsevier Science S.A. Keywords: Laser irradiation; Luminescence; Nanostructures; Silicon 1. Introduction The discovery of visible room temperature photolumi- nescence (PL) from porous silicon (PS) has attracted much interest owing to its possible application to optoelectronic devices. Although, a large number of works on PS devices have been published, most basic questions concerning the PL origin and the morphology of PS remain unclear. The essentials of mechanisms which have been proposed for the PL are quantum confinement of carriers in Si nanocrys- tallites [1] and specific molecular luminescence centres, such as potysilanes [2] or sitoxene [3]. One reason for such a variety in PL mechanisms [4] could be the complexity of PS structures which depends specifically on the experi- mental conditions. The latter may particularly change the form and the size of the crystallites and the porosity which may have an influence on the specific surface area (SSA) of PS, depending on the electrical resistivity [5,6]. In most works, the effect of interaction between the ambient atmosphere and the SSA of PS has been ignored. Indeed, due to the fact that PS has a very reactive surface, its SSA may play an important role in the PL behaviour within laser irradiation. Thereby, the effect of intrinsic PS 0040-6090/97/$17.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0040-6090(97)00091-6 parameters such as porosity, crystallite size, etc., should be measured in vacuum or in an inert ambient. We try in this paper to point out the effect of porosity, electrolytic solution, oxidation and laser irradiation on the PL behaviour and structure of PS. From theoretically calculated PL spectra, based on the quantum confinement formalism, we correlate PL behaviour to PS structure. 2. Experiment The PS layers are prepared by the electrochemical anodisation method at a constant current density in a HF solution. The substrate used is p-type Si (100) of 1-1.5 f~ cm resistivity; the back ohmic contact is made with eutec- tic In-Ga. The porosity is determined by the gravimetric method [6]. For the elaborated samples, the layer thickness varies from 3 to 10 ~m. PL and Raman spectra are measured using a "Dilor" triple monochromator, and a photomutiplier with a GaAs photocathode. The 514.5 nm line of a continuous Ar + laser, with a power density of 4 W cm -2, was used as an excitation source.