Study of a-Si:H Using the Solid/Electrolyte System
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STUDY OF a-Si:H USING THE SOLID/ELECTROLYTE SYSTEM Y. GOLDSTEIN', A. MANY*, S.Z. WEISZ**, J. PENALBERT**, W. MUNOZ** and M. GOMEZ** *Racah Institute of Physics, The Hebrew University, Jerusalem 91904, Israel. "**Department of Physics, University of Puerto Rico, Rio Piedras, PR 00931.
ABSTRACT 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 aSi: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 3 1018 cm" . 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 [L-r, 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 p.T to be 5 x 10-8 cmVN.
INTRODUCTION Considerable effort, both theoretical and experimental, has been devoted to derive the density of states spectra in a-Si:H films [1]. In this paper use is made for this purpose of pulse measurements on the semiconductor/electrolyte (S/E) system. Such measurements, which proved to be very useful in the study of crystalline semiconductors [2-4], have been found to be equally effective when applied to a-Si:H films. The essentially blocking nature of the S/E interface allows one to induce by an applied bias space-charge layers at the a-Si:H surface, ranging from large depletion to very strong accumulation conditions. In this manner, the entire energy gap in the space charge region, together with its localized states, can be swung below and above the Fermi level. At the same time, one can measure the surface space-charge density Qsc as a function of the barrier height Vs. In a-Si:H, Qsc resides predominantly in the localized states (except in strong accumulation conditions), so that the measurements yield, at least in principle, the energy distribution of the density of states. High-grade, device-quality films are usually close to intrinsic (resistivity -1010 ohm-cm). Such high-resistivity films cannot be handled by our measurement technique because the surface space-charge capacitance Csc cannot be charged within the short-duration applied pulse biases. Accordingly, most of the results to be reported here have been obtained under
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