Hydrogenated amorphous silicon p-doping with diborane, trimethylboron and trimethylgallium

The uses of diborane, trimethylboron and trimethylgallium gases have been systematically compared in order to obtain p-type hydrogenated amorphous silicon grown in silane rf glow discharges. The doping properties and the contamination effects due to the thermal CVD have been investigated by an in situ Kelvin probe. This study, which has been completed by electrical (dark conductivity, activation energy and fill factor of standard p-i-n devices), and optical (a combination of transmission and photothermal deflection spectroscopy) measurements, indicates that trimethylboron doped layers have semiconducting properties similar to those of diborane doped layers. When trimethylboron is used, the contamination is shown to be reduced by at least a factor 50. In contrast, trimethylgallium, despite its acceptable doping efficiency, produces a contamination intermediate between the diborane and the trimethylboron ones. The effects of C incorporation in the doped layers have also been studied, in particular by optical absorption measurements in the band-edge region.