IRREGULARITIES IN CURRENT-VOLTAGE CHARACTERISTICS OF HYDROGENATED-AMORPHOUS-SILICON-BASED BARRIER STRUCTURES - RESONANT-TUNNELING AGAINST HOPPING AND FILAMENTARY CONDUCTION THROUGH THE BARRIERS

The question has been investigated whether resonant tunnelling is responsible for observed bump-like irregularities in current-voltage characteristics of double-barrier structures based on hydrogenate amorphous Si (a-Si:H) and its alloys with C or N. In this paper, general physical considerations, as well as elementary calculations taking into account special features of amorphous barrier structures, are compared with the results of a systematic experimental study of a-Si:H/a-Si1-xC(x):H heterostructures. Different series of samples, each consisting of a double and a single barrier, and a sample without any heterostructure barrier, have been investigated. A wide variety of experimental features from complete smoothness of the I-V curves, to bumps, noise and even accidental step-like switching behaviour, were recorded at different temperatures. From an overall inconsistency of our experimental data with the calculations, and in agreement with basic physical considerations, resonant tunnelling is discarded as a possible mechanism. Instead it is argued that a large density of defect states in the barriers is responsible for the anomalies, via hopping conduction of the carriers through the localized-states distribution in the barriers, and by the formation of unstable filamentary transport paths.