Photon enhanced diffusion model for α-Si:H photo-degradation

Abstract

A photon enhanced diffusion (PED) model to describe hydrogenated amorphous silicon (α-Si:H) photo-degradation is proposed in this paper. The model utilizes the hydrogen content naturally incorporated into plasma-enhanced chemical vapour deposition (PECVD) silane (SiH4) produced α-Si:H structures (or films) as an ionized diffusing dopant within the modelled structure. In this model, the mobile carrier charge state contribution of the electrically active included hydrogen at a given physical location is determined dynamically with respect to the position of the Fermi level. In this paper, hydrogenated α-Si:H fabricated without additional doping is defined as h-type, to represent the hydrogen content of the α-Si:H within the modelled structure. A modelled α-Si:H structure (or device) consisting of n-type, h-type (included hydrogen dopant only) and p-type layers of widths 50 nm, 400 nm and 50 nm, respectively, having a 4 Ω external series resistance is considered in this paper. The physical diffusion of the ionized included hydrogen across dopant boundaries with differing Fermi levels provides mobile dopant compensation for the charge carrier concentration within the simulated α-Si:H structure. A hydrogen mobile charge state transition energy term for the diffusion of hydrogen across dopant boundaries within the deposited α-Si:H structure is included in this model as a variable combination of the α-Si:H structure's thermal and incident absorbed photon energy. In this model, the included hydrogen diffusion process and ionic charge state are directly related to the modelled dopant profile, temperature and cumulative adsorbed incident photon radiation. The total dopant charge distribution within the modelled α-Si:H structure is used to dynamically calculate the internal electrostatic potential and field profiles within the structure, with the changing charge compensation effects of the diffusing electrically active included hydrogen. High electric fields calculated to be within the modelled α-Si:H structure are used to enable incident photons to assist in a quantum-mechanical tunnelling mechanism, producing PED of the included hydrogen. The PED model calculates the open-circuit terminal potential (voltage) and resistance profile character across the modelled α-Si:H structure to provide a representative output of the photo-degradation process in time, temperature and incident photon exposure. Results generated by the proposed PED model closely follow the character of observed photo-degradation effects in α-Si:H photo-voltaic structures due to cumulative illumination, annealing and thermal cycling reported in the literature. Extracted values for the diffusion coefficient and proportion of electrically activated included hydrogen within the simulated α-Si:H structure of 1.38 × 10−16 cm−2 s−1 and 2 × 10−8, respectively, were obtained from the PED model simulations.