Real time spectroscopic ellipsometry studies of the nucleation and growth of p-type microcrystalline silicon films on amorphous silicon using B2H6, B(CH3)(3) and BF3 dopant source gases

Real time spectroscopic ellipsometry (RTSE) has been applied to study the n ucleation, coalescence, and growth processes for similar to 100-200 Angstro m thick microcrystalline silicon (mu c-Si:H) p-layers prepared by radio fre quency (rf) plasma-enhanced chemical vapor deposition at 200 degrees C on a morphous silicon (a-Si:H) i-layers in the substrate/(n-i-p) device configur ation. Analysis of the RTSE data provides the bulk p-layer dielectric funct ion (2.5-4.3 eV), whose amplitude and shape characterize the void and cryst alline Si contents in the p-layer. Among the parameters varied in this stud y of the deposition processes include the underlying a-Si:H i-layer surface treatment, the p-layer H-2-dilution flow ratio, the p-layer dopant source gas and flow ratio, and the p-layer rf plasma power flux. Here we emphasize the differences among p-layer deposition processes using diborane, B2H6, t rimethyl boron, B(CH3)(3), and boron trifluoride, BF3, dopant source gases. We find that it is easiest to nucleate mu c-Si:H p-layers immediately on t he i-layer without any surface pretreatment when B2H6 is used as the source gas. In contrast, when B(CH3)(3) or BF3 is used, a H-2-plasma treatment of the i-layer is necessary for immediate nucleation of Si microcrystals; wit hout pretreatment, the p-layer nucleates and grows as an amorphous film. Fo r H-2-plasma-treated i-layers, p-layer microcrystal nucleation at low plasm a power is controlled by the catalytic effects of B-containing radicals at the i-layer surface, irrespective of the dopant source, whereas nucleation at higher plasma power is controlled by the bombardment of the i-layer by S i-containing ions. Under high power plasma conditions using BF3, dense sing le-phase mu c-Si:H p-layers can be obtained over a wide range of the dopant gas flow ratio. In contrast, for B2H6 and B(CH3)(3), such properties are o btained only over narrow flow ratio ranges owing to the relative ease of di ssociation of these gases in the plasma. (C) 1999 American Institute of Phy sics. [S0021-8979(99)00208-X].