ELECTRICAL PROPERTIES OF RECRYSTALLISED SIC FILMS FROM PECVD PRECURSORS FOR SILICON QUANTUM DOT SOLAR CELL APPLICATIONS M. Schnabel 1 , A. Witzky 1 , P. Löper 1 , R. Gradmann 2 , M. Künle 1 , S. Janz 1 1 Fraunhofer Institute for Solar Energy Systems ISE Heidenhofstrasse 2, D-79110 Freiburg, Germany 2 now with Fraunhofer Institute for Silicate Research ISC Neunerplatz 2, D-97082 Würzburg, Germany ABSTRACT: Silicon carbide (SiC) is a promising host material for silicon quantum dots (Si QDs), which are being investigated as absorber materials for tandem solar cells based solely on crystalline silicon. Amorphous silicon carbide (a-SiC) films are deposited by plasma-enhanced chemical vapour deposition (PECVD) and annealed under the same conditions usually used to precipitate Si QDs. During annealing, the films shrink by 20%, and some a-SiC transforms into SiC nanocrystals (nc-SiC) about 3 nm in size. P-type doping with boron is found to inhibit SiC crystallisation and lower conductance as compared to intrinsic films. N-type doping with phosphorus on the other hand promotes SiC crystallisation and leads to a higher conductance. The trends in conductance are ascribed solely to the effect of the dopant on crystallisation. Al, Ti, Cr, Ni20%Cr, and ITO are all found to form Ohmic contacts to the SiC films on deposition, with no change in contact properties brought about by sintering at 425°C. Temperature-dependent conductivity measurements on intrinsic SiC films reveal two distinct activation energies; 65 meV below 200 K, and 158 meV above 200 K. Keywords: Silicon Carbide, Electrical Properties, Recrystallisation 1 INTRODUCTION Crystalline silicon solar cells dominate the PV market, reaching efficiencies up to 25%. However by virtue of conventional silicon cells having a single band gap, their maximum efficiency is limited to 29.4% [1] by transmission and thermalisation losses. Thermalisation losses can be reduced by adding a top cell with a higher band gap above a conventional silicon cell to create a tandem cell. Silicon quantum dots (Si QDs) are a promising candidate for the top cell of an all-Si tandem solar cell, with a band gap that is tuneable from 1.3-1.7 eV [2, 3] by adjusting QD size. They are readily produced within a Si-based dielectric matrix by precipitation from a stack of alternating Si-rich and stoichiometric dielectric layers [4]. In Si QD layers produced in this way, the properties of the dielectric matrix material play a key role. The conduction and valence band offsets between the matrix material and the Si QDs affect the tunnelling of carriers between Si QDs, while in the absence of inter-dot tunnelling, the resistance of the matrix material dominates the resistance of the Si QD layer. Furthermore, the p-n or p-i-n structure required for carrier separation is most conveniently produced by depositing doped layers of the dielectric matrix material immediately before or after the Si QD precursor multilayer in one deposition step. Of the matrix materials proposed so far, the one that is most promising electrically is silicon carbide (SiC) as it only creates a comparatively small transport barrier to crystalline Si [2] and is readily doped [5]. Much work has already been published on the electrical properties of SiC, and on contact formation, but most of it is geared towards the use of SiC as a semiconductor for high- temperature electronics and hence focuses on single crystals, and the less defect-rich hexagonal polytypes of SiC [6-13]. Our SiC thin films deposited by plasma- enhanced chemical vapour deposition (PECVD) however tend to crystallise in the cubic 3C polytype on annealing [14]. 3C-SiC has a band gap of 2.39 eV as compared to band gaps of ~3 eV for the hexagonal polytypes [5], and typically exhibits a higher defect density [13]. Furthermore, crystals less than 10 nm in size form under the annealing conditions typically chosen to precipitate Si QDs from Si-rich SiC [15]. This means that the electrical properties are likely to differ significantly from those of single crystals and be dominated by grain boundary effects and defects. In this work the electrical properties of intrinsic and doped partially recrystallised SiC films are investigated and compared to their structure. Electrical contacts to different metals are also investigated to determine which are suitable for the production of Ohmic and Schottky contacts. 2 EXPERIMENTAL 2.1 Sample Preparation Hydrogenated silicon carbide films (a-SiC:H) were deposited on oxidised silicon and fused silica substrates for electrical and optical measurements respectively. The oxidised silicon wafers had been produced through the dry oxidation of 200µm thick 1 Ωcm phosphorus-doped FZ-Si wafers using dichloroethene, yielding 300 nm SiO 2 and hence providing an effectively insulating substrate. The fused silica substrates consisted of Spectrosil 2000 glass. All substrates were pre-annealed at 1100°C for 3 h to drive out residual contaminants. Both intrinsic and n- and p-type a-SiC:H films were deposited using a Roth&Rau AK400 plasma enhanced chemical vapour deposition (PECVD) reactor. The process parameters were 65 W HF (13.56 MHz) with a plasma power density of 100 W/cm, 0.3 mbar pressure and a substrate temperature of 270°C. The reactant gas fluxes are summarised in Table I. Films nominally 260 nm thick were deposited for structural and optical characterisation, and transmission line model (TLM) structures, and films nominally 130 nm thick were deposited for Meier-Schroder (M-S) structures. Deposition times were kept the same for undoped and doped films.