Metastable effects in hydrogeneated amorphous silicon based superlattices

Abstract

The metastable effects arising from a variety of external influences in hydrogenated amorphous silicon-silicon nitride(a-Si:H/a-$SiN_x$:H) compositional and n-p-n-p... doping-modulated a-Si:H superlattices heave been studied. In a-Si:H/a-$SiN_x$:H compositionally modulated superlattices, the coplanar conductance increases with bias voltage at moderately large biases and decreases after bias stressing even at room temperature. After a brief light illumination, upon which the Staebler-Wronski defect is hardly created, we observed persistent photoconductivity (PPC) and an increase of sub-band-gap absorption in the compositional multilayers. Annealing above the film depositing temperature decreases the PPC effect, but the increase of the sub-band-gap absorption for the PPC state is observed, indicating that the PPC effect is correlated with an increase of gap state near the interfaces. I also observed an excess conductivity in the intentionally nitrogen-contaminated a-Si:H film by light illumination at an elevated temperature. On the basis of experimental results, I propose a new model that the PPC effect in a-Si:H/a-$SiN_x$:H compositional superlattices is associated with the conversion of threeflold nitrogen into an active donor and accompanying generation of dangling bond defect at and/or near the interfaces. Thermal quenching from an elevated temperature increases the conductivity of a-Si:H/a-$SiN_x$:H compositional superlattices as well as of n-p-n-p... doping-modulated a-Si:H superlattices. The increase in coplanar conductivity of compositional multilayers might be explained by the conversion of N into an active donor near interfaces, similar to the PPC effect. The frozen-in excess conductance in both coplanar and transverse directions after thermal quenching is attributed to the increase of dangling bonds of p-layers and the rise of Fermi level of n-layers in a-Si:H doping-modulated superlattices.