Electronic transport in heavily doped Ag/n-Si composite films

Hall measurements characterized Ag/n-Si composite films 1 micron thick produced by magnetron co-sputtering onto high resistivity Si (111) substrates at 550°C. The targets were Ag and n-type Si doped with 3 × 1019/cm3 of antimony. Films were prepared with 13, 16 and 22 at. % Ag and measured over a temperature range 77–500°K. Conduction takes place at low temperatures by variable rang hopping in localized states at the Fermi level and by thermal activation over grain boundaries at higher temperatures. The Log Resistivity vs 1/kT curves for the three Ag concentrations vary in a similar manner, but decrease in magnitude with increasing Ag due to the smaller number of grain boundaries between Ag nanoparticles occurring with increasing Ag concentration. At low temperatures Hall mobilities are essentially independent of temperature as the carrier densities for the three Ag concentrations are constant from 77 to slightly under 300°K with resistivities varying by small amounts. The mobilities at all Ag concentrations increase with temperature and approach each other as the effects of grain boundaries become less important. This work presents for the first time the effects of metal particles embedded in a semiconductor on the transport properties of carriers in the semiconductor. Though these effects are for a given average particle size most of the results are expected to hold over a range of particle sizes. Free electrons produced in films containing 13 and 16 at. % Ag result in concentrations of 1.5 × 1019/cm3, one half the antimony doping, while those with 22 at. % Ag, the carrier concentrations are three orders of magnitude higher. These constant carrier concentrations are due to the metal-insulator transition that occurs in doped crystalline and polycrystalline silicon for carrier densities nc >3.9 × 1018/cm3. The three orders of magnitude higher carrier concentration produced in films with 22 at. % Ag is argued to be due to doping of the Si matrix by the Ag nanoparticles at this concentration, a doping effect previously observed in ErAs in InGaAs. A discussion of this doping effect is presented that is in keeping with the experimental observations though other factors such as particle size effects need further experimental confirmation. Thermal activation energies and grain boundary barrier heights were obtained from resistivity and Hall mobility data. The barrier heights of 0.360, 0.390 and 0.470 eV measured for films with Ag concentrations of 13, 16 and 22 at. % Ag respectively, are quite high compared to those obtained by other methods used for producing polycrystalline Si (10-100 mev) and need to be considerably reduced in order to increase the efficiency of the detectors for which these Ag/n-Si composite films will be used. Thermal annealing of as-deposited films is suggested as one means to accomplish this result.