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OPTIMIZATION OF OPTOELECTRONIC PROPERTIES OF a-SiC:H FILMS F.Demichelis,G.Crovini,C.F.Pir'i,E.Tresso, Dept. Physics, Politecnico, Torino, Italy R.Galloni,R.Rizzoli,C.Summonte, LAMEL-CNR, Bologna, Italy F.Zignani, Dip. Chimica Appl. e Scienza Mat. Fac. Ingegneria, Bologna, Italy G.Amato, IENGalileo Ferraris Torino, Italy P.Rava, Elettrorava, Savonera, Torino, Italy A.Madan, MVSystems Inc., Golden, Colorado. ABSTRACT Amorphous silicon carbide films have been deposited by PECVD in SiH 4 +CH4 +H 2 mixtures at different hydrogen dilutions. The optoelectronic properties of the films have been measured by transmittance-reflectance spectroscopy, photothermal deflection spectroscopy and photo and dark electrical conductivity. Structural properties have been obtained by FTIR spectroscopy. It was found that high hydrogen dilution leads to materials of improved quality. p-i-n device structures have been deposited with intrinsic layers at different hydrogen dilution levels. INTRODUCTION Amorphous silicon carbide (a-SiC:H) is normally deposited using the PECVD (plasma enhanced chemical vapor deposition) method. This generally employs gas mixtures of silane (SiH 4) and methane (CH4) in a capacitively coupled system using 13.56 MHz plasma excitation and power density of about 50 to 80 mW cm 2 . However it has been found [ 1-3] that as the band gap increases (with increasing C content) the optoelectronic properties degrade: specifically, as Eg> 2.0 eV, the density of defects and the Urbach energy increase and the majority and minority carrier transport properties of the material tend to degrade. This has been linked to changes in the structural properties such as the inclusion of Si-CH3 bonding configurations and the formation of microvoids within the material [4]. Attention has therefore focused on the use of other feedstock materials such as disilylmethane (DSM), trisilylmethane (TSM) and tetrasilylmethane (tetraSM) as these already possess network building Si-C bonds [3,5,6]. Further, as there are fewer C-H bonds in silylmethane than in CH4 produced films, so materials with improved properties could result. Previously [11, excellent quality a-SiC:H material was produced using gas mixtures of only SiH 4 and CH4 and manipulating the deposition pressure and excitation power in the plasma to control the extent of ion bombardment occuring at the surface. The parameter D = power density/deposition pressure was defined and it was concluded that at large values of D, more energetic ion bombardment of the growing film would occur which would have a tendency to break the C-H bonds (binding energy 3.2 eV) which are weaker than Si-C bonds (binding energy 4.3 eV) and therefore to allow C to bond to Si, increasing chemical order. By optimizing the ratio D, it was obtained for band gap of 2 eV, a density of states at the Fermi level of approximately 4x 101 6 cm 3 eV 1 . In order to improve the properties of a-SiC:H alloys even further, we have deposited a-SiC:H films using SiHl4 and CH4 gas mixtures and we have examined the difference with and