Study of a-Si:H using the solid/electrolyte system

Pulsed measurements on the solid-electrolyte system, which proved very useful in the study of crystalline semiconductors, have been found to be equally effective when applied to hydrogenated amorphous Si films. Here, as well, the a-Si:H/electrolyte interface is essentially blocking to current flow and, as a result, surface space-charge layers, ranging from large depletion to very strong accumulation conditions, can be induced and studied. In particular, valuable information can be gained on the density of the localized bulk states. Measurements in the depletion range under illumination yield directly the total density of occupied states in the entire energy gap. This is very useful in obtaining a quick and reliable assessment of the quality of the amorphous films. In high-grade films we find that the total density of occupied states is around 10¹⁸ cm^-3. The data in the accumulation range, on the other hand, provide useful information on unoccupied states near the conduction band edge. The blocking nature of the amorphous Si/electrolyte interface is utilized also to apply a sweep-out technique for an accurate determination of μτ, the product of the electron mobility and lifetime, even when this value is very low. In a rather poor-quality film, for example, we find μτ to be 5×10^-8 cm²/V.