ELSEVIER Thin Solid Films 276 (1996) 187-190 Charge transport in porous silicon: considerations for achievement of efficient electroluminescence J. Ko~.ka, J. Oswald, A. Fejfar, R. Sedla~.~, V. 7.elezn~, Ho The-Ha, K. Luterowi, I. Pelant Institute of Physics. Academy of Sciences of Czech Republic, Cukrovanzickd 10, 162 O0 Praha 6, Czech Republic Abstract We have critically evaluated the mechanism of charge transport, the understanding of which is crucial for construction of porous silicon electroluminescent devices. Multiple transport paths or space-charge-limited effects in porous silicon give rise to complex field dependence of the transport activation energy (EA = 0.38.-0.67 eV). Direct correlation between the chemical composition of the internal surface of silicon nanocrystallites and the charge transport properties is demonstrated and the role of the tail states is emphasized. Recommendations for efficient electroluminescence, based on the microstructure and drift length, are summarized. Keywords: Electrical properties and measurements; Silicon; Luminescence; Growth mechanisms 1. Introduction There is no doubt that the understanding of the transport properties of porous silicon (PS) is a critical step for the increase of the efficiency of PS-based electroluminescent (EL) devices. Transport properties of PS[ 1 ] are tightly related to the PS microstructure, which can change from a more or less ori- ented columnar structure (see Fig. I and Table I ) to partially connected passivated nanoparticles or to amorphous-like "tissue" with embedded nanoparticles. Moreover, PS microstructure is a function of Si substrate conductivity and can even change along the PS thickness. For thin PS (about 1 Ixm or less) the current-voltage (I- V) characteristics are usually rectifying and controlled by the contact properties [ 1,2]. In this paper we concentrate on transport properties of sufficiently thick ( 10 p.m or more) self-supporting p-type nanoporous or n-type mesoporous PS, controlled by PS volume properties. The influence of water vapour or other chemicals [ 3,4 ] is minimized by experimen- tal conditions. The purpose of this paper is to discuss the open questions, which are for example: what is the transport mechanism in PS?, at which energy there is the transport path?, what is the drift length, role and origin of the "intergrain" barriers? On the basis of this discussion we critically evaluate the conse- quences for design of EL devices. 0040-6090/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI0040-6090 ( 95 ) 08108-9 a b c d Fig. !. Scheme of the possible microstructural types of porous silicon. The crystalline core of the silicon particles is shown as the hatched area. 2. Experimental details The majority of samples used throughout this study were self-supporting PS films fabricated by standard wet anodi- zation in HF-based solutions. To detach the films from the substrates, a short current pulse in a weak HF solution was applied. The films' porosities and thicknesses were deter- mined by gravimetry and optical microscopy. Different tech- nology was used only to fabricate the samples for an anisotropy study of conductivity: the samples were prepared in a double Teflon electrochemical cell by etching the pores from both sides through the substrate. Basic samples char- acteristics are given in Table 1. To measure the dark current-voltage characteristics the samples were placed into a mechanical sample holder [5] which allows measurement both in "sandwich" and "copla- nar" geometries. The measurements of dark conductivity as a function of temperature were performed in a vacuum cham- ber using a Keithley 616 electrometer. Infrared transmission was measured by a Bruker IFS 113v FTIR spectrometer.