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

Abstract

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 O 2 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 10 16 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.