GRAIN-BOUNDARY STATES AND HYDROGENATION OF FINE-GRAINED POLYCRYSTALLINE SILICON FILMS DEPOSITED BY MOLECULAR-BEAMS

The energy distribution of grain-boundary states is determined for polycrystalline silicon films grown under ultra-high vacuum conditions. Conductivity and electron spin resonance measurements on n-type film reveal both exponential bandtails and deep gap states corresponding to disorder-induced gap states and dangling-bond defect levels (D0 and D-). The latter are responsible for the pinning of the Fermi level observed at moderate doping. Both experimental techniques agree with a location of the D- level at E(c)-0.30 eV and the D-degrees level at E(c)-0.65 eV +/- 0.05 eV. It is shown that a hydrogen-plasma treatment at 500-degrees-C reduces the dangling-bond density by an order of magnitude and that it also yields a conduction bandtail twice as steep. The replacement of weak Si-Si bonds by more energetic Si-H bonds would explain the steeper bandtails. This view is supported by absolute measurements by nuclear reaction showing that the hydrogen content exceeds by two orders of magnitude the original dangling-bond density. A spin density as low as 5 x 10(16) cm-3 is measured for the first time in fine-grained polycrystalline silicon. The passivation of Si dangling bonds is accompanied by a shift of the pinned position of the Fermi level from 0.5 to 0.45 eV below E(c), which could mean that the remaining deep defects have a slightly different energy distribution.