Properties of microcrystalline silicon. IV. Electrical conductivity, electron spin resonance and the effect of gas adsorption

For pt.III see ibid., vol.16, p.2005 (1983). Measured values of the electrical conductivity, sigma , and electron spin density (g=2.0057) of microcrystalline silicon can be essentially determined by the extent of the contamination of the samples by oxygen unless special precautions are taken as regards the sample preparation and/or handling. For samples deposited at a floating potential, two kinds of oxygen incorporation are identified: irreversible formation of Si-O bonds on the grain boundaries (and on the sample surface) and a reversible absorption which is probably associated with a nondissociative O₂^{delta -} (ads) state. The latter results in a decrease of sigma _RT by up to five orders of magnitude, an increase of the activation energy, epsilon _a, and of the preexponential factor, sigma ₀, as well as in an increase of the electron spin density. A reversible desorption of oxygen leads to an increase of sigma _RT up to not less than about 10^-2 Omega ^-1 cm^-1 and a decrease of the EPR signal below the detection limit of less than 10¹⁶ cm^-3. In order to avoid such effects a negative bias has to be applied to the substrate during deposition. Samples of undoped mu c-Si deposited in this way show neither the incorporation of oxygen into the bulk nor significant changes in the dark conductivity even after long-term exposure to air.