185 EFFECT OF GRAIN BOUNDARY SEGREGATION ON THE TRANS- BARRIER CONDUCTIVITY OF POLYCRYSTALLINE SILICON S.PIZZINI AND M.ACCIARRI University of Milano, Dept. of Physical Chemistry and Electrochemistry, Via Golgi 19, 20133 Milano, Italy ABSTRACT The conductivity of single grain boundaries of electronic grade, Bridgman grown polycrystalline silicon samples was measured using the dc polarization technique with the aim of detecting any influence on the carrier transport regime resulting from the segregation of oxygen and carbon, which, by themselves, should behave as electrically inactive impurities. To this scope as grown samples and samples heat treated at 1123 and 1223 K, differing in their initial oxygen and carbon content, were used and the conductivity measured in the 298-100 K range. The results indicate that for both the carbon rich and for the oxygen rich samples the conductivity across the grain boundaries is of the thermally activated type and that it could be discussed in terms of carriers which are thermally emitted over the barrier and cross the barrier in the limit of the short mean free path. Samples which do not present a net excess of oxygen or carbon, apparently behave as single crystal specimens, instead. The deconvolution of the I-V curves is then used for obtaining the densitl of the interface states responsible of the set-up of the grain boundaries barrier, whose shape supports a completely new hypothesis about the configuration of the impurity cloud at the grain boundaries. INTRODUCTION In a earlier paper [1-] we have reported and discussed the results of a series of d.c. conductivity measurement carried out by biasing single grain boundaries (GB) of polycrystalline silicon samples, with the aim of investigating the influence of oxygen and carbon segregation at GB on the barrier transit probability. The implicit assumption was that a strongly disordered region of finite size extends across the geometrical position of the GB, but that the gap states associated to the dis- order could be considered as true interface states. Consequently, the behaviour of GB under dc bias could be discussed within the frame of the double Schottky barrier model and, then, also information on the density of states (DOS) function could be extracted from the results of the measurements, according to already existing models [2 - 4]. It was shown that samples containing a carbon excess Nc > No always presented a thermally activated conductivity behaviour in the 298-100 K range, while, apparently, both samples presenting an oxygen excess No > Nc or which were equiconcentrated No-Nc presented a behaviour which was substantially equivalent to that of the single crystal sample used as the reference. It is however well known that not only carbon segregation [5], but also oxygen segregation causes the set up of local disorder conditions, due to the large specific vol- ume differences between SiC, Si0 2 and silicon [6 - 8]. What we expect, therefore, on purely theoretical grounds, that both in the case of carbon and oxygen segregation Frenkel pairs are generated either at the SiO/Si or at the SiC ISi interface [8], where the star means that the oxygen or carbon segregation causes the formation of polyatomic Si-O or Si-C aggregates of variable size and stoichiometry, in order to minimize the strain associated to their presence. In the first case, cloud of interstitials propagates from the interface to the bulk of the silicon phase while vacancies are incorporated in the nucleus of the silica phase. Mat. Res. Soc. Symp. Proc. Vol. 182. @1990 Materials Research Society