Low temperature (<150 °C) hydrogenated amorphous silicon grown by PECVD with source gas heating
Hydrogenated amorphous silicon (a-Si:H) is a semiconductor that is widely used in a variety of applications. A particularly important development has been the incorporation of this material into thin film transistor (TFT) arrays for the active matrix addressing of liquid crystal displays. Plasma Enhanced Chemical Vapour Deposition (PECVD) is one of the most successful techniques currently in use for the deposition of device quality a-Si:H. However, there is an increasing desire to improve process compatibility with low cost, plastic substrates. This entails trying to reduce the deposition temperature from approximately 250 - 300°C to below 150°C, whilst maintaining material quality. This thesis describes the design of a novel, low temperature PECVD system incorporating the facility to pre-heat the deposition source gases. The physical and electronic properties of a-Si:H deposited at <150°C are investigated, and the performance of TFT structures incorporating optimised material as the active layer is described. It is shown that the physical properties of a-Si:H produced at a substrate temperature of 125°C with the source gas line heated to 400 °C are commensurate with films deposited at 250-300 QC. The hydrogen content of the optimised film was found to be 10.5 %, with a Tauc bandgap of 1.66 e V. Pre-heating of the source gases also leads to an increase in the proportion of hydrogen bonded in the monohydride configuration. It is suggested that the diffusion of the film-forming gaseous species is enhanced by this technique, resulting in a reduction in the degree of disorder within the film and hydrogen elimination. Consequently, the concentration of hydrogen and the Tauc bandgap also decrease, leading to an increase in photoconductivity of one order of magnitude. TFTs exhibit a switching ratio of 1 Os, which is approximately an order of magnitude smaller than high temperature a-Si:H TFTs, but a comparable OFF current of approximately 10.12 A. However, the field effect mobility of these devices is very poor (10.3 cm2V·l s·I). This is thought to be due to a high interface state density at the boundary between the low temperature, gas-heated a-Si:H layer and the high temperature silicon nitride gate insulator.
- PhD