T. Takagi, R. Hayashi, G. Ganguly*, M. Kondo and A. Matsuda

Thin Film Silicon Solar Cells Super Lab., Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba-shi, Ibaraki 305-0045, Japan
ttakagi@etl.go.jp

*Solarex, 826 Newtown-Yardley Rd., Newtown, PA 18940, USA

Mass spectrometry of silane (pure and H2-diluted) plasma was studied using a quadruple mass spectrometer under various deposition conditions of hydrogenated amorphous silicon (a-Si:H). Our aim was to correlate the gas-phase species with the film property after light-induced degradation and to find the ideal deposition conditions to achieve high-rate growth of stable a-Si:H films.

A parallel-plate plasma enhanced chemical vapour deposition ( PECVD ) system with an excitation frequency of 13.56 MHz was used. Both the film deposition and mass spectrometry were performed under deposition conditions of a-Si:H giving growth rates ranging from 2 A/s to 20 A/s. We observed the signal intensities of SiH2+ ( m/e=30 ) and Si2H4+ ( m/e=60 ) as the most abundant ions in the mass fragmentation related to monosilane and disilane, respectively.

The film property of a-Si:H films after light-induced degradation was monitored by the fill factor ( F.F. ) of photo current - voltage characteristics in n+ crystalline Si/a-Si:H/Ni Schottky cells after 6 hours of light soaking under 3-sun illumination at 60 C. The F.F. after light soaking deteriorated with the increase in the growth rate. The contribution of disilane related radicals to the film growth, represented as the disilane fraction ( [m/e=60] / [m/e=30] ), showed a good correspondence with the F.F. after light soaking.

We identified the contribution of higher-order silane-related radicals to the film growth as a cause of the degradation of a-Si:H film property. It is suggested from the results mentioned above that suppression of gas-phase higher-order silane-related radicals is a clue to obtain stable a-Si:H solar cells at high growth rate. We are considering to further investigate the higher-order silanes ( SinHx , n less than or equal to 3 ) to find a much clearer correlation between the gas-phase species and the film property.