Electron field emission from amorphous silicon

Abstract

Hydrogenated amorphous silicon (a-Si:H) is used commercially for large area device fabrication in active matrix flat panel displays as the switching element in thin film transistors. We have found that a-Si:H thin films can be conditioned to field emit electrons at relatively low electric fields. Once conditioned, emission from nonoptimized films at macroscopic vacuum fields below 20 V/μm is routinely obtained. The threshold fields for electron emission appears to be dependent on the film thickness and fields below 10 V/μm have also been recorded. The Schottky junction between the a-Si:H and Cr is important to the electron emission process at low fields and plays a major role in helping to fully deplete the a-Si:H film such that electrons that enter the conduction band of the film by thermionic emission over the barrier (or tunneling through the barrier) can become “hot.” If the emission is localized, it still appears to be controlled by space charge and the back contact barrier. We have examined the electron emission from multilayer structures that contain Schottky and ohmic back contacts as well as thin n+surface layers. Although the electron emission can be fitted to a Fowler–Nordheim type curve, the values obtained for the barrier and emission area for the nominally flat thin films are unrealistic. We explain the observed emission using a space charge induced hot electron model, where the a-Si:H acts as an interlayer in which the electrons are heated.