From monosilane to crystalline silicon. Part III. Characterization of amorphous, hydrogen-containing silicon products

Thermal decomposition of pure monosilane at initial pressures between 0.1 and 6.6 MPa produces a black/gray powder of loosely agglomerated spherical silicon particles (diameter ca. 3 μm), which contain ∼0.07-0.19 wt.% (ca. 2-5 at.%) hydrogen. Hydrogen is residing as diluted and clustered mono- (SiH), di- (SiH2), and trihydride (SiH3) units. Except for the trihydrides these Si-H configurations were observed by FT-IR spectroscopy whereas qualitative 29Si-NMR analyses additionally revealed SiH3 configurations. Desorption of hydrogen from the different kinds of hydride groupings is studied by FT-IR, 29Si-NMR, and hydrogen element analyses. The results show that on increasing temperature the desorption, broadly speaking, follows the hydrogen content of the hydride groupings, viz. (clustered) trihydrides being first and diluted monohydrid last stripped off from a-Si:H materials. Hydrogen desorption from diluted monohydride locations show second-order kinetics with Ea = 160 ± 20 kJ mol−1, while hydrogen desorption from clustered monohydride units appears to exhibit a first-order behavior with an activation energy of 110 ± 25 kJ mol−1. Hydrogen residing in clustered polyhydride regions appears to desorb according to a second-order mechanism below ∼873 K (Ea = 90 ± 25 kJ mol−1; hydrogen content exceeding ca. 36% saturation) and first-order behavior above 873 K. The desorption of hydrogen from the trihydride units is more difficult to evaluate, but seems to be of first order with an activation energy of 10 ± 5 kJ mol−1. The present a-Si:H materials show clear signs of crystallinity after 5 h of heating at 873 K.