Irregular Electron Transport Through a-Si:H Based Potential Barriers
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IRREGULAR ELECTRON TRANSPORT THROUGH a-Si:H BASED POTENTIAL BARRIERS Norbert BERNHARD, B. FRANK, B. MOVAGHAR* and G. H. BAUER Pfaffenwaldring 47, Institut ffir Physikalische Elektronik, Universitit Stuttgart, D-7000 Stuttgart 80, F.R.Germany *Institut ffir Theoretische Physik, Ruhr-Universitdt Bochum, D-4630 Bochum, F.R.Germany
ABSTRACT Irregularities in the current-voltage-characteristics of a-Si:H based potential barriers have been investigated experimentally, and are discussed theoretically with respect to different transport mechanisms. The investigated samples were different series of double and single barrier a-Si:H - a-Sil-xCx:H - heterostructures, as well as homogeneous samples without heterostructure barrier. Current-voltage-(I-V)-characteristics showing a wide variety of features, from complete smoothness of the curves, to bumps and even accidental step-like switching behaviour, as well as different forms of noise, were recorded at different temperatures. Resonant tunnelling as an explaining transport mechanism for the anomalies was excluded because 3f inconsistency between experiment and calculations partially including special amorphous features. Instead it is argued that all observed irregularities, i. e. bumps in I-V-curves, switching-like behaviour, and appearance of noise, are related to current transport via trap-assisted tunnelling through locally strongly confined transport paths, leading to the meta-stable formation, change and break-down of conductory filaments.
INTRODUCTION In the first report about periodic multilayers of a-Si:H and compositional alloys /1/, characteristic dependences of the optical bandgap and the photoluminescence on the electronic well widths were interpreted in terms of quantum size effects, an explanation many other authors were following later. After the report about resonant tunnelling in amorphous double barrier structures /2/, the quantization picture seemed to be well established. Evidence against
the consistent interpretation of the optical blue shift in quantization terms was brought forward independently by two groups in 1991 /3,4/. One of the groups presented also current-voltage anomalies in single barrier structures /5/ which cannot be explained by resonant tunnelling. Another group had previously already observed random telegraphic noise (RTN), but no bumps which could attributed to resonant tunnelling /6/. An inconsistency with the quantization picture of the conductivity data of multilayers, absolutely parallel to that of the optical shift, was presented in a systematic study last year /7/. By these findings increasing doubt on the existence of macroscopically visible quantum size effects in amorphous semiconductors was growing in the scientific community. In order to contribute to resolve this unclear situation we tried to gain a more complete picture of electron transport through amorphous potential barriers by incorporating in this study systematic experimental investigations, as well as elementary calculations and general physical considerations. GEN
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