A photon enhanced diffusion (PED) model to describe hydrogenated amorphous Si (α-Si:H) photo-degradation was proposed here. The model utilizes the H 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 H at a given physical location was determined dynamically with respect to the position of the Fermi level. Here, hydrogenated α-Si:H fabricated without additional doping was defined as h-type, to represent the H 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 H dopant only) and p-type layers of widths of 50, 400 and 50nm, respectively, having a 4Ω external series resistance was considered here. The physical diffusion of the ionized included H across dopant boundaries with differing Fermi levels provided mobile dopant compensation for the charge carrier concentration within the simulated α-Si:H structure. A H mobile charge state transition energy term for the diffusion of H across dopant boundaries within the deposited α-Si:H structure was 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 H diffusion process and ionic charge state were 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 was 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 were 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 H within the simulated α-Si:H structure of 1.38 x 10-16 cm-2 /s and 2 x 10-8, respectively, were obtained from the PED model simulations.

Photon-Enhanced Diffusion Model for α-Si:H Photo-Degradation. G.J.Phelps: Semiconductor Science and Technology, 2006, 21, 91-103

Figure 7

Diffusivity of Hf in Si

(Upper line: fast component, lower line: slow component)