Thickness dependence of the properties of plasma-deposited a-Si:H films — NMR studies

Abstract

Both the NMR spectrum and spin-lattice relaxation time, T 1, have been measured on plasma-deposited a-Si:H films of different thickness, t, varying from 0.1 μm to 10 μm, prepared on heated substrates under similar conditions. The NMR spectrum shows the standard two-component line in all samples with substantial differences in H content with thickness: ≈ 10 atomic % for films with t ⩾ 1 μm, the normal result, but ≈ 30% for t = 0.1 μm. This indicates that the thin sample is similar to films that have been prepared on room temperature substrates and have poor electronic properties. The NMR T 1 versus temperature exhibits the standard u-shaped curve with a minimum near 50K for all samples, due to the presence of a small amount of molecular H 2 trapped in voids. The thin sample (t = 0.1 μm) shows an additional relaxation mechanism, not present in the samples with t ⩾ 1 μm, that causes T 1 to drop exponentially as the temperature is raised from 200K to 300K. This is most likely due to an electronic relaxation mechanism present in the thin samples but not in the thicker materials.