The density of states in amorphous silicon determined by space-charge-limited current measurements

Abstract

Abstract Space-charge-limited current (SCLC) flow has been investigated as a function of applied potential, temperature and specimen thickness in amorphous silicon films prepared by the glow-discharge technique. Specimens had symmetric n+-i-n+ or n+-n-n+ structures in which the thin n+ surface layers gave strong electron injection. From the analysis of the current—voltage characteristics in the SCLC regime it is shown that the current flow is controlled by localized states situated at the quasi-Fermi level. Specimens in a thickness range from 0·7–2 μm satisfy the scaling law for SCLCs, confirming that the measurements are representative of a bulk-controlled property. Three methods for the determination of the density-of-state distribution, g(ϵ), from the SCLC characteristics were investigated and gave results agreeing within a factor of two. g(ϵ) curves were determined in a range from 0·75–0·46 eV below ϵc. Their shape agrees with that of typical field-effect curves, but their magnitude lies below the field-effect results by a factor of 3 to 5, in the above range. This suggests that the density of states in the surface region probed by the field effect may exceed the volume density. However, the volume distribution deduced here from the SCLCs differs basically in shape and magnitude from that found by DLTS measurements. g(ϵ) curves obtained by the three experimental methods are compared and discussed in some detail.