Vibrational Properties of a-Si:H Films Containing Voids: Experiment and Modeling
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CABARROCAS**,G. LANDA*, M. DJAFARI-ROUHANI*'1 *Laboratoire de Physique des Solides, Universit6 Paul Sabatier, 118 route de Narbonne, 31062 Toulouse cedex-4, France, ESA5477 associ6e au CNRS * *Laboratoire de Physique des Interfaces et des Couches Minces, UMR 7647 associ6e au CNRS, Ecole Polytechnique, F-91128 Palaiseau Cedex, France IfLaboratoire d'Analyse et d'Architecture des Syst~mes-CNRS, 7 avenue du Colonel Roche, 31077 Toulouse cedex, France
ABSTRACT In this paper, we present results about the vibrational properties of hydrogenated amorphous silicon films. We expect to explain the slight differences observed in the Raman spectra using atomic-scale modeling. In particular, we focuse on the correlation of our results to the density of samples. This should give quantitative structural information which could be correlated to both macroscopic properties and elaboration conditions.
INTRODUCTION Improving the efficiency of a-Si:H-based solar cells is an actual technological matter. In order to optimize both elaboration conditions and electronic properties, the aim is still the structural characterization of a-Si:H films which are known to contain voids and/or clusters, both of nanometric size. Since these small domains cannot be easily evidenced by conventional structural characterization technique such as X-Ray diffraction or TEM, vibrational properties can be used to obtain a structural information. Nevertheless, Raman spectra generally exhibit only slight changes, that are difficult to attribute unambiguously to a specific structure. In the following, we show Raman scattering experiments of different a-Si:H films obtained by plasma-enhanced chemical vapor deposition (PECVD) and calculations of the density of vibrational states (DVS.) The calculations are carried out on highly disordered silicon with different structures, modeled through the size of voids and their distribution in the system. Therefore, we expect a quantitative analysis of the experimental Raman spectra.
EXPERIMENT AND MODELING Experiments 1 micron-thick a-Si:H films are obtained by PECVD from a silane / argon mixture. As explained elsewhere [ 1,2], the strong dilution of silane conducts to materials with improved electronic transport properties. It has been proposed that the structure of such materials is different from conventional a-Si:H films, and may contain small ordered domains formed by silicon clusters deposited from the plasma [ 1,2]. However, these clusters are very difficult to evidence experimentally by TEM or X-ray diffraction since their diameter should be less than 2 nm. During elaboration, pressure ( 60 mTorr), SiH4/Ar ratio and RF power are kept constant. The variable parameter is the substrate temperature (Tu) ranging from 100 'C to 250 °C. 451 Mat. Res. Soc. Symp. Proc. Vol. 557 01999 Materials Research Society
The electronic properties of the films are measured by dark conductivity, defect density and Urbach tail. The Raman scattering experiments were performed using a triple monochromator spectrometer (Coderg T800), cou
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