Electronic Properties of a-SiN x :H Thin Film Diodes
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Electronic Properties of a-SiN.:H Thin Film Diodes J M Shannon, J N Sandoe, I D French, A D Annis Philips Research Laboratories Cross Oak Lane, Redhill, Surrey RHi 5HA United Kingdom Abstract Metal-semiconductor-metal diodes have been fabricated using a range of dilute aSiNx:H alloys. Current transport in unstressed devices was dominated by thermionic-field emission of electrons through the reverse biassed metal-semiconductor contacts with a tunnelling effective mass 0.1me. The barriers to electrons were strongly pinned by bulk states and scaled almost linearly with optical band gap. Under high current stress, defect states were created below the Fermi level forming a -ve charge that changed the effective barrier height and produced drift in the current voltage characteristic. Unipolar stress measurements showed that this effect was related to hole transport. After prolonged stress the electrical characteristics were dominated by bulk effects due to the high density of metastable defects in the band gap. Introduction There has been interest for a number of years in thin film diodes that use dilute silicon nitride alloys as the active medium[1-4]. Adding nitrogen to amorphous silicon increases the band gap which leads in principle to a reduction in diode leakage and an increase in reverse blocking voltage compared with devices made using a-Si alone. This feature is particularly important when considering the application of diode addressing to active matrix LCD and the requirement for a single diode per pixel which must be capable of charging a pixel when either polarity is applied[I-4]. Although liquid crystal displays have been made based on a-SiNx:H layers for the active matrix there is uncertainty as to just how these devices work. In particular there is a need to distinguish between bulk and contact controlled current flow and what determines the current-voltage characteristic and the device stability. It is shown in this paper that if devices are designed and operated correctly their performance is very well suited to applications not requiring high current densities. V Cr/Al
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Mat. Res. Soc. Symp. Proc. Vol. 297. @1993 Materials Research Society
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Current-Voltage Characteristics Thin film diode structures were made using a PECVD reactor to deposit hydrogenated amorphous silicon nitride at temperatures between 250 and 300°C. In most cases contacts were made using Cr or Cr/Al but in some devices the top contact was made via phosphorus doped a-Si. The two structures are shown in Fig. 1. The IN characteristic of an a-SiNx:H diode with a n+a-Si:H contact is shown in Fig. 2. In this case the silicon to nitrogen ratio in the silicon nitride layer was 2:1. The layer was undoped and had a thickness of Metal a-s;N :H Metal -800A. The characteristic of an identical structure but with no nitrogen in the a-Si qcoe layer is shown for
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