Excitation mechanism of Tb3+ in a-Si3N4:H under sub-gap excitation

We studied a sample of Tb-doped a-Si3N4:H prepared by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR PECVD). The sample has an optical gap E04 = 4.7 ± 0.3 eV and refractive index n (at 632 nm) = 1.81 ± 0.01. Room temperature photoluminescence was measured under sub-gap excitation. Both characteristic a-Si3N4:H and Tb3+ photoluminescence peaks were detected in the sample as deposited. Annealing at 300 °C maximizes the Tb3+ photoluminescence lines. At higher annealing temperatures the Tb3+ photoluminescence decreases while the host photoluminescence increases. The Tb3+ photoluminescence is inversely correlated with the density of Si-H bonds in the sample. The results indicate that silicon dangling bonds are involved in the excitation of the Tb3+ ions. We propose a new efficient non-radiative recombination path to the static disorder model that explains the luminescence of amorphous silicon and alloys: the Auger excitation of a rare earth ion near a silicon dangling bond. The model provides a very good explanation of the excitation and does not require the presence of nanostructures.