Influence of preparation conditions on forward-bias currents of amorphous silicon Schottky diodes

Abstract

We report on the open-circuit voltage Voc and the forward-current-voltage characteristic, J=Js(expqV/nKT-1), of Schottky diodes obtained by evaporation of platinum onto hydrogenated amorphous silicon (a-Si :H) fabricated by glow-discharge decomposition of silane. The diode characteristics were studied as a function of several preparation parameters, namely, silane pressure, substrate temperature, silane flow rate, annealing temperature. Rectifying behavior was obtained only for substrate temperatures between 200 and 300°C. Within this constraint, the preparation parameters have only a small effect on Voc but have large influences on n and Js. The factor n decreases towards ideality (n=1) as the silane pressure decreases, as the substrate temperature increases and with annealing of the Pt contact. The corresponding decrease of the concentration of polyhydride H sites with similar variations in the preparation parameters suggests that the recombination center responsible for nonideal value of n are associated with the polyhydride sites. We argue that the nearly constant value of Voc and therefore of the potential barrier probably originates from the crossing of the Fermi level with deep levels near the interface rather than from a previously proposed interface state. Although we were unable to use a sufficiently low temperature to choose between the thermionic emission and diffusion theories for the forward-current-voltage relationship, we point out that carrier injection in the metal base transistor structure favors a thermionic emission model. Assuming thermionic emission the value deduced for the Richardson constant is of the same order of magnitude as found for crystal Si Schottky barriers.