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 resp onsible for observed bump-like irregularities in current-voltage chara cteristics of double-barrier structures based on hydrogenate amorphous Si (a-Si:H) and its alloys with C or N. In this paper, general physic al considerations, as well as elementary calculations taking into acco unt special features of amorphous barrier structures, are compared wit h 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 featu res from complete smoothness of the I-V curves, to bumps, noise and ev en accidental step-like switching behaviour, were recorded at differen t temperatures. From an overall inconsistency of our experimental data with the calculations, and in agreement with basic physical considera tions, resonant tunnelling is discarded as a possible mechanism. Inste ad it is argued that a large density of defect states in the barriers is responsible for the anomalies, via hopping conduction of the carrie rs through the localized-states distribution in the barriers, and by t he formation of unstable filamentary transport paths.