Correlation Between Minority Carrier Diffusion Length and Microstructure in a-Si:H Thin Films
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CORRELATION BETWEEN MINORITY CARRIER DIFFUSION LENGTH AND MICROSTRUCTURE IN a-Si:H THIN FILMS G. Conte*, G. Fameli*, G. Nobile*, A. Rubino*, E. Terzini*, F. Villani*, D. Caputo**, G. de Cesare**, F. Irrera**, F. Palma** and M.C. Rossi** *ENEA - Centro Ricerche Fotovoltaiche, P.O.Box 32, 80055 Portici (Na), Italy ** UniversitA di Roma "La Sapienza", Dipartimento di Ingegneria Elettronica, Via Eudossiana 18, 00184 Roma, Italy ABSTRACT Aim of this work is to investigate the opto-electronic properties of amorphous hydrogenated silicon (a-Si:H). The deposition temperature has been used as a driving force to modify the morphology and bonded hydrogen distribution. The influence of the hydrogen microstructure on the carriers lt product has been examined. The majority and minority carrier pt have been evaluated from the diffusion length measurement, by using the Steady State Photocarrier Grating (SSPG) technique, and from the photoconductivity in the steady state condition (SSPC). The igt values have been correlated with the defect density and the Fermi level position. Some considerations are proposed to explain the carrier transport in terms of the compositional inhomogeneities in Si:H alloys due to the morphological variations. INTRODUCTION During the past few years many efforts have been devoted to the study of the influence of the hydrogen microstructure on the transport properties of a-Si:H thin films. It is generally accepted that this material does not consist of a homogeneous random network, but rather of the mixture of a well-developed three-dimensional Si network and of a highly-disordered low-dimensional Si:H alloy region including voids [1]. We correlate the two different morphological phases to the variation of the stretching modes at 2000-2100 cm-t in the IR spectra. The contribution at about 2000 cm-1 is related to the SiH mode, while the one at 2070 cm-1 is associated to the presence of a defected, hydrogen rich phase (SiHc) that influences the transport properties. Morphology may be very sensitive to the preparation conditions, so that the relative amount of the two phases can change as a function of the deposition temperature [2]. These inhomogeneities in the material can produce local potential fluctuations at the phase-boundary and, as a consequence, a not clear definition of the mobility gap can be given [3]. We have studied as physical parameters the minority carrier transport properties using the SSPG technique [4] in the ambipolar regime, and the majority ones by photoconductivity. As already reported in literature [5], these two complementary techniques give the opportunity to study the behaviour of both carriers as a function of the defect density and the Fermi level position. We report on the influence of the morphology induced defects on the transport properties of the two carriers, correlating the variation of the transport properties with the relative amount of the two morphological phases. EXPERIMENTAL Films were deposited on Coming 7059 glass and crystalline silicon substrates in a 13.56 MHz
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