Quantitative analysis of tungsten, oxygen and carbon concentrations in the microcrystalline silicon films deposited by hot-wire CVD

Abstract

In order for hot-wire chemical vapor deposition to compete with the conventional plasma-enhanced chemical vapor deposition technique for the deposition of microcrystalline silicon, a number of key scientific problems should be cleared up. Among these points, the concentration of tungsten (nature of the filament), as well as the concentration of oxygen and carbon (elements issued when vacuum is broken between two runs), should not exceed threshold values, beyond which electronic properties of the films could be degraded, as in the case of monocrystalline silicon. Quantitative chemical analysis of these elements has been carried out using the secondary ion mass spectrometry technique through depth profiles. It has been shown that for a high effective filament surface area ( S f=27 cm 2), the W content increases steadily from 5×10 14 to 2×10 18 atoms cm −3 when the filament temperature T f increases from 1500 to 1800 °C. For a fixed T f, the W content increases with the effective surface area S f. Thus, considering our reactor geometry, the W content does not exceed the detection limit (5×10 14 atoms cm −3) when T f and S f are limited to 1600 °C and 4 cm 2, respectively. For O and C elements, under deposition conditions of high dilution of silane in hydrogen (96%), O and C concentrations approaching 10 20 atoms cm −3 have been obtained. The introduction of an inner vessel inside the reactor, the addition of a load-lock chamber and a decrease in substrate temperature to 300 °C have led to a drastic decrease in these contents down to 3×10 18 atoms cm −3, compatible with the realization of 6% efficiency HWCVD μc-Si:H solar cells.