Enhanced lifetime in porous silicon light-emitting diodes with fluorine doped tin oxide electrodes Andréia G. Macedo a , Elder A. de Vasconcelos c , Rogério Valaski b , Fábio Muchenski a , Eronides F. da Silva Jr. c , Antônio F. da Silva d , Lucimara S. Roman a, ⁎ a Departamento de Física, Universidade Federal do Paraná, 81531-990, Curitiba-PR, Brazil b Flexitec Eletrônica Orgânica Ltda, 81531-990 Curitiba-PR, Brazil c Departamento de Física, Universidade Federal de Pernambuco, Cidade Universitária, 50670-901 Recife-PE, Brazil d Instituto de Física, Universidade Federal da Bahia, Campus Ondina, 40210-340, Salvador-BA, Brazil abstract article info Article history: Received 17 July 2007 Received in revised form 5 June 2008 Accepted 13 July 2008 Available online 19 July 2008 Keywords: Porous silicon Photoluminescence Fluorine doped tin oxide Light-emitting diodes We investigated the electrical and optical properties of porous Si (PS) light-emitting diodes using fluorinated tin oxide (FTO) as transparent electrodes. At high forward bias, the current–voltage characteristic is space charge limited. At low forward bias, it follows an exponential law. Whereas the electroluminescence (EL) in devices with non-fluorinated indium–tin oxide electrodes degrades in few minutes, EL intensity in devices with FTO electrodes shows little degradation after 1300 min of operation. This result indicates that the well known beneficial effects of fluorinated species in the improvement of resistance to irradiation and carrier injection degradation in metal–oxide–semiconductor devices might be also observed in PS devices. © 2008 Published by Elsevier B.V. 1. Introduction Transparent or semi-transparent electrodes in porous silicon (PS) light- emitting diodes (LEDs) have usually been fabricated with thin-metal layers of Al [1] or Au [2]; conducting polymers, such as polyaniline [3] or polypyrrole [4]; or degenerated semiconductors such as tin oxide (TO) [5], indium tin oxide (ITO) [6] and poly-Si [7]. Degradation of the elec- troluminescence (EL) in these PS LEDs is generally ascribed to the loss of surface hydride passivation from the porous silicon by exposure to light, ambient air, high temperatures or high electric fields leading to the formation of dangling bonds and non-radiative channels. Various methods have been proposed to minimize EL degradation, including: (a) oxidation to replace Si–H by Si–O bonds (rapid thermal oxidation [7] or wet oxi- dation in a H 2 O 2 solution [8]), (b) treatments in organic solutions to replace Si–H bonds by Si–C bonds [9–12], and (c) indium/tin evaporation, where the stabilizing effect was attributed either to the replacement of Si surface atoms, or by forming oxide-related luminescence centers [8]. It has been reported that a NF3/UV photo-chemical surface treatment enhanced the photoluminescence (PL) of PS about 1–2 orders of mag- nitude [13,14]. The enhancement in PL was attributed to the cleaning and passivation of the PS surface with fluorine, followed by a rapid growth of a more stoichiometric oxide SiO x ( x ≈ 2) layer in air. Incorporation of fluorine at the Si–SiO 2 interface in metal–oxide–semiconductor (MOS) devices has been widely used to reduce interface traps and improve resistance to degradation either by radiation or carrier injection [15–18]. This improve- ment is related to the passivation of dangling bonds and replacement of Si–H bonds by stronger Si–F bonds, removal of electron and hole traps and strain relaxation. In this work, we further investigated the effects of fluorine incorpora- tion in PS devices. We will discuss the electrical and optical properties of PS LEDs using fluorinated tin oxide (FTO) as transparent electrodes. The PS layers were formed by two different methods: electrochemical and vapor phase stain etches. For each type of porous silicon layer, we either used a TO or a FTO film as the transparent electrode. In comparison with control samples using TO electrodes, LEDs with FTO electrodes showed an improvement of 3 orders of magnitude in lifetime. Whereas the EL of the control samples completely decayed in few minutes, the EL of the devices with FTO electrodes showed little degradation after 1300 min of continuous operation. This is a promising result that indicates that the well known beneficial effects of fluorinated species in the improvement of resistance to degradation due to irradiation and carrier injection in MOS devices might be also observed in PS devices. 2. Experimental details The device diagram is shown in Fig. 1 . The substrates were Si wafers, p-type, (100)-oriented, with resistivity in the range 0.017 to Thin Solid Films 517 (2008) 870–873 ⁎ Corresponding author. Tel.: +55 41 3361 3275; fax: +55 41 3361 9418. E-mail address: lsroman@fisica.ufpr.br (L.S. Roman). 0040-6090/$ – see front matter © 2008 Published by Elsevier B.V. doi:10.1016/j.tsf.2008.07.007 Contents lists available at ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf