Dependence of electrical conductivity of nanocrystalline silicon on structural properties and the effect of substrate bias

Abstract

The dark electrical conductivity of undoped and phosphorus-doped nanocrystalline (nc) and amorphous (a-) thin films of silicon has been studied as a function of substrate bias between Vb∼0 (floating) and −1000 V under conditions in which all other deposition parameters, such as temperature, deposition rate, discharge current and chemical composition of the plasma, were held constant. With increasing bias from Vb∼0 to −100 V, the room-temperature conductivity σRT of undoped nc-Si decreases from ∼10-−4 to about 10−9 Ω−1 cm−1. A further increase in the bias results in the appearance of a second conductance path which could be attributed to grain boundaries. At a bias of V b < −600 V, a mixture of nc- and a-Si phases are formed with σRT showing a rather complex dependence on V b and approaching that of a-Si for V b∼ −1000 V, where no crystalline component is detectable by X-ray diffraction. Phosphorus-doped nc-Si films prepared under the same conditions and with a gas-doping ratio of [PH3]/[H2] = 10−4 exhibit an almost constant conductivity over the whole bias range between 0 and −850 V. An interpretation of these results is suggested.