Properties of amorphous Si:F:H alloys

Previously we have shown that amorphous silicon-based alloys containing F and H (1–7) possess desirable properties for photovoltaic application. Amorphous tetrahedral semiconductors ordinarily possess a very large density of states which act as traps leading to low values for drift mobility and low recombination lifetimes of free carriers. However, Spear and his group (8–10) reported that a-Si decomposed from SiH4 gas by r.f. glow discharge and deposited on a heated substrate produces a film which has a comparatively low density of states. However, materials prepared in this way possess a large concentration of H.(11) Because of the relatively low density of states, these types of films can be doped n type, albeit using relatively large concentration of the dopant. However, p-type doping using B2H6 is generally accompanied with the decrease in the band gap, indicating alloying action rather than conventional doping.(9) Furthermore, the material has apparently a sufficiently large carrier lifetime that efficiencies of photovoltaic devices exceeding 5% have been reported.(12)