DOPING OF SEMIINSULATING POLYCRYSTALLINE SILICON BY B, P, AND AS IMPLANTATION AND DIFFUSION

The formation of p- and n-type layers of semi-insulating polycrystalli ne silicon has been investigated by implantation and diffusion of B, P , and As. At room temperature the material resistivity can be changed by more than six orders of magnitude for both p- and n-type doping. A dramatic decrease of resistivity is observed for dopant concentrations above a threshold level which corresponds to the complete filling of midgap grain-boundary traps. The determination of this critical concen tration allows the evaluation of the donor and acceptor grain-boundary trap densities which are found to lie in the range from 7 to 14X10(12 )/cm(2). The charged traps produce depletion layers and potential barr iers at the grain boundaries; these barriers drive the free-carrier mo tion under external fields. Above the threshold concentration, the bar rier height decreases with the increasing concentration of active dopa nt, going from approximate to 0.5 eV to nearly 0 and correspondingly t he resistivity drops down. Thermionic emission and tunneling of the ca rriers through these intergrain barriers are assumed to be the main co nduction mechanisms. This assumption fits the temperature dependence o f resistivity and the predominance of one mechanism over the other dep ends upon temperature, oxygen concentration, and doping.