EFFECT OF ANNEALING ON THE OPTICAL-PROPERTIES OF PLASMA DEPOSITED AMORPHOUS HYDROGENATED SILICON

Amorphous silicon films prepared by radiofrequency plasma decomposition of silane contain between 10 and 25 at % hydrogen depending on the potential and temperature of the substrate and plasma parameters. The way hydrogen is bonded in these films has been determined from the infrared stretching, bending, and rocking or wagging modes of the Si-H complexes. Monohydrides SiH, dihydrides SiH2 and short chains of polysilane (SiH2)n an be identified. The potential of the substrate with respect to the plasma and to a lesser degree the substrate temperature determine the prevalent Si-H complex. At substrate potentials close to that of the plasma(SiH2)n complexes are favored whereas isolated SiH and SiH2 groups are found in films prepared on strongly negative potential substrates. The films can be dehydrogenated by annealing. Substrate potential and temperature influence strongly the refractive index and the onset of interband optical transitions. The optical gap of these films lies between 1.6 and 1.85 eV which is appreciably higher than the values 1.2-1.5 eV of sputtered and evaporated amorphous Si films. As the hydrogen is driven out by annealing the optical gap decreases to 1.6-1.7 eV. At photon energies below the optical gap one observes a preparation sensitive absorption tail between α=102 and10 3 cm-1 which cannot be removed by annealing. The films crystallize between 700 and 780°C with a crystallization energy of (2±0.3) kcal/mol. © 1979.