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 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.

Stability of microcrystalline silicon for thin film solar cell applications

“…The conductivity decreases again when the sample is put back into air. This cycle can be repeated many times without any sign of fatigue (compare [13]). …”

“…A very careful and detailed investigation on mc-Si:H by Vep$ rek et al [13] reports, for material prepared by the chemical transport deposition process, that atmospheric gas adsorption and/or oxidation affect surface states, electronic transport and electron spin density. It will be important to investigate and identify these effects in state-of-the-art material prepared by PECVD and HWCVD.…”

Evolution of the microstructure in microcrystalline silicon prepared by very high frequency glow-discharge using hydrogen dilution

“…Matsuda et al 17 and Veprek et al 18 have already shown that a variation of the deposition temperature allows one to change from a ͗111͘ to a ͗220͘ preferential growth direction. It is commonly assumed that those planes being etched away the fastest are also those growing the fastest.…”

Enhanced mobility of hydrogenated MO-LPCVD ZnO contacts for high performances thin film silicon solar cells

“…These improvements cannot be attributed to an improvement in ZnO conductivity alone, most probably to a combination of gain in conductivity in the ZnO back contact layer, as well as to an improvement in the Si material quality. Indeed, during the pH2, all the cell sub-layers are exposed to an annealing (under vacuum at 200°C), which is thought to be beneficial to the Si material mainly [14]. Such V oc increase could however not be reproduced by applying only the annealing step (only + 10mV increase).…”