Amorphous silicon/Langmuir-Blodgett film MIS devices
Metal-insulator-semiconductor (MIS) structures based on glow discharge produced hydrogenated amorphous silicon (a-Si:H), and incorporating Langmuir-Blodgett (LB) film insulating layers, have been investigated. Two distinct types of MIS diode have been considered: tunnelling diodes (insulator thickness < 5 nm) and non-tunnelling diodes (insulator thickness > 10 nm). A preliminary study of insulated-gate field-effect-transistors (igfet's) has also been undertaken. Simple Schottky barrier (MS) structures, which are effectively a first step towards tunnelling MIS devices, have been made and characterised; results showed that these were 'state-of-the-art' devices. A preparation procedure has been developed which facilitates the successful deposition of LB film materials onto a-Si:H. Tunnelling MIS diodes containing diacetylene polymer LB insulating films have been fabricated, and capacitance measurements showed that the films were of reasonable quality. The current-voltage characteristics of these diodes were rather non-ideal and, it is thought, were dominated by the effects of two distinct levels of surface states. The appearance of peaks in the illuminated conductance-voltage curves supports this interpretation. Solar cells with an MIS structure can show an enhanced efficiency compared with MS (Schottky) cells. By using varying numbers of LB monolayers, the effects of increasing insulator thickness on a-Si:H solar cell parameters have been ascertained. Though the results closely parallel those of other researchers, the presence of a surface 'oxide' layer of ~ 4 nm thickness (as a result of the pre-LB film deposition etch) prevented optimisation of the cell efficiency. Further work is necessary in order to capitalise on the effect which has been demonstrated e.g. the development of a more suitable etch treatment, the use of different LB film materials. Non-tunnelling MIS diodes were made using cadmium stearate/stearic acid LB films. The capacitance-voltage curves were very similar to those which have been reported for conventional MIS devices on single crystal semiconductors. The LB films, although of slightly poorer structural quality than is possible on single crystal substrates, were nonetheless reproducible. An unusual ln J v V(^½) current-voltage dependence was found, which, it is suggested, was due to image-force effects. Large hysteresis was observed in the device characteristics due to polarisation or ionic motion. The characteristics were also influenced by the presence, in the upper half of the a-Si:H mobility gap, of a band of surface or bulk states. These simple structures could possibly be used as gas detectors, since the penetration of a gas into the LB film may well result in measureable changes in the device parameters. For the first time, an igfet based upon the a-Si:H/LB film system has been produced. The device showed a change in source-drain current of almost three orders of magnitude for a gate voltage change of lOV. This compares well with the early results of other workers using more conventional insulators. It is thought that the device performance was limited by the poor semiconducting properties of the surface region of the a-Si:H, and that an alternative FET configuration would lead to improvements. Possible applications include the switching of large area liquid crystal displays (using FET arrays) or, more tentatively, the exploitation of the 'molecular tailoring' qualities of LB films to produce specific biological and chemical FET sensors.