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 films reveal both exponential bandtails and deep gap states corresponding to disorder-induced gap states and dangling-bond defect levels (D° 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° level at E C-0.65eV ± 0.05eV. It is shown that a hydrogen-plasma treatment at 500°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 Iow as 5 × 1016cm−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.