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

Abstract

Real time spectroscopic ellipsometry (RTSE) has been applied to study the nucleation, coalescence, and growth processes for ∼100–200 Å thick microcrystalline silicon (μc-Si:H) p-layers prepared by radio frequency (rf) plasma-enhanced chemical vapor deposition at 200 °C on amorphous silicon (a-Si:H) i-layers in the substrate/(n-i-p) device configuration. Analysis of the RTSE data provides the bulk p-layer dielectric function (2.5–4.3 eV), whose amplitude and shape characterize the void and crystalline Si contents in the p-layer. Among the parameters varied in this study of the deposition processes include the underlying a-Si:H i-layer surface treatment, the p-layer H2-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, trimethyl boron, B(CH3)3, and boron trifluoride, BF3, dopant source gases. We find that it is easiest to nucleate μc-Si:H p-layers immediately on the 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 H2-plasma treatment of the i-layer is necessary for immediate nucleation of Si microcrystals; without pretreatment, the p-layer nucleates and grows as an amorphous film. For H2-plasma-treated i-layers, p-layer microcrystal nucleation at low plasma 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 Si-containing ions. Under high power plasma conditions using BF3, dense single-phase μc-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 obtained only over narrow flow ratio ranges owing to the relative ease of dissociation of these gases in the plasma.