Density-of-States Distribution in Disilane LPCVD Deposited Amorphous Silicon as Determinated by SCLC
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DENSITY-OF-STATES DISTRIBUTION IN DISILANE LPCVD DEPOSITED AMORPHOUS SILICON AS DETERMINATED BY SCLC
C.MANFREDOTTI,G.GERVINO,L.MONTALDI,R.VARESIO,A.ZANINI AND U.NASTASI Istituto di Fisica Superiore, C.so M.D'Azeglio 46, 10125 Torino,Italy and Gruppo Nazionale di Struttura della Materia,C.N.R.,Italy
ABSTRACT I-V characteristics have been measured in a-Si:H films deposited from Si 2 H6 by LPCVD and have been analyzed according to a direct method. DOS distribution is thickness-dependent, similarly to what has been found on SiH deposited films ,with values at Fermi level ranging from 3 1 3 below 1016 cm" eV up to0 1018 cm" eV" . Deposition temperature ,at least in the interval 450 - 500 C, does not seem to affect DOS distribution . Hydrogen content is pratically constant in all the films . The only possible conclusion is that only thickness influences DOS distribution and that only for thickness larger than 0.8 um it is possible to get densities 1 3 at Fermi level below 1016 crr eV"
INTRODUCTION Recently [1] ,we reported on the application of a new direct method for the analysis of SCLC curves of amorphous silicon. This method does not rely on a-priori models for DOS distribution and it does not make use of regional approximation, but makes only the assumption of an homogeneous mobility and of an electrical sample length 1 bias indipendent. The spatial homogeneity of trap density is needed only to extend the results from the anode region, in which DOS is evaluated, to the whole sample thickness. The method itself, in some respect, could be considered as a more general version, without parameters to be determinated by fitting, of the method proposed by Solomon [2], while in simplified version, practically coincides with previously used methods [3,4]. In the present work, this method has been applied to samples obtained from disilane pyrolysis with films deposited previously from SiH 4. EXPERIMENTAL Amorphous silicon films are deposited on c-Si n-type substrates with low resistivity (below 1 ohm-cm) which are used as electrons injecting contacts. Evaporated Al dots are used both as anodes and as contacts on c-Si . The depositions are carried out at different temperatures from 450 C to 500 C, corresponding to growth rates from 0.1 to 1 A/s, in the central 60 cm long region of an hot-wall furnace in a quartz 5" tube. Substrates are in a standing-up position. Gas flow is kept at 50 SCCM and the pressure is 0.2 Torr. No particular pumping-down is used : the tube is washed for 24 hours with electronic grade N before deposition and the samples are only cleaned-up with standard procedures before to be introduced into the oven. The I-V characteristics are carried out at room temperature: the presence of good ohmic regions at low applied bias is indicative of a well-behaving of contacts. In many cases, the ohmic region is followed by a long superquadratic region (Fig.l) up to relatively high current densities, a fact that can be considered also as a proof of the presence
Mat Res. Soc Symp Proc Vol 70.
1986 Materials Research
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