LONG-RANGE STRUCTURAL RELAXATION IN THE STAEBLER-WRONSKI EFFECT

X-ray photoemission spectroscopy (XPS) has been used to study the structural changes of device-quality a-Si:H between the annealed state (A) and the light-soaked state (B). The XPS spectra show a reversible shift of about 0.1 eV of the Si 2p peak to lower binding energy without a corresponding shift of the Si 2s peak in going from state A to state B. Density functional calculations on a Si6H14 prototype molecule are also presented which suggest that the Si core energy levels may indeed be shifted by comparable amounts when small structural rearrangements are forced upon the structure. The change in the Si 2p peak is too large to be caused by a rearrangement of the amorphous lattice restricted to the immediate surroundings of the defects responsible for the Staebler-Wronski effect (defect density: < 10(17) cm(-3)). It is proposed instead that the formation of dangling bonds under exposure to light is also accompanied by long-range structural rearrangements of the amorphous network. Results obtained by other groups are discussed which, in our opinion, also indicate the presence of long-range structural rearrangement during the Staebler-Wronski effect.