Microcrystalline silicon thin film transistors made by plasma enhanced chemical vapour deposition

• Main Author: Froggatt, M. W. D.
• Published: University of Cambridge 1998
• Subjects: 537.622
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599237

Currently the transistors required for active matrix liquid crystal displays (AMLCDs) are fabricated using hydrogenated amorphous silicon (a-Si:H) owing to its large area capability and compatibility with a wide range of low cost substrates. Future displays will however require a material with a higher field effect mobility than a-Si:H and while polycrystalline silicon (poly-Si) can meet these requirements it does so currently at the expense of large area or low temperature substrate compatibility. The thesis investigates the suitability of hydrogenated microcrystalline silicon (μc-Si:H) for channel layers in thin film transistors (TFTs). μc-Si:H is a biphasic material consisting of crystalline regions in an amorphous matrix, and potentially offers a large area, low temperature deposition process similar to that of a-Si:H while providing an enhanced field effect mobility. Using the hydrogen dilution method in a conventional plasma enhanced chemical vapour deposition (PECVD) system μc-Si:H films were deposited and characterised. Films deposited by this method exhibited only moderate crystallinity but a wide range of conductivities, suggesting that impurity incorporation may have a more significant effect on microcrystalline films than their amorphous counterparts. TFTs fabricated using μc-Si:H channel layers exhibited clear transistor action, but field effect mobilities were uniformly lower than for equivalent structure a-Si:H channel devices. Significantly, attempts to improve the crystallinity of the channel layer resulted in degraded TFT performance consistent with an increase in defect rich material. The temperature dependence of mobility of μc-Si:H channel devices suggests that the reduced performance is a consequence of an increased density of conduction band tail states in μc-Si:H compared to a-Si:H. This increased density is in turn proposed to be due to the introduction of crystallites into the amorphous matrix and the subsequent increase in the density of weak Si-Si bonds.