Study of the density of states in a-Si:H using the Si/electrolyte system

Localized states in a-Si:H are studied by pulsed measurements on the a-Si:H/electrolyte (S/E) system. The S/E 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. Measurements in the depletion range under illumination yield directly the total density of occupied states in the entire energy gap. This is 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 S/E system 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.