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Title: Hydrogenated tetrahedrally bonded amorphous carbon for electronic applications
Author: Conway, N. M. J.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1999
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This thesis examines the properties of hydrogenated tetrahedrally bonded amorphous carbon (ta-C:H) grown using a plasma beam source(PBS) and assesses its potential as a semiconductor material for electronic applications. Production of high quality ta-C:H requires optimisation of the deposition conditions and it is therefore of importance to analyse the behaviour of the PBS. It is found that the operation of the PBS agrees with its theoretical description as a capacitively coupled system and the plasma has the required properties to produce ta-C:H. Characterisation of the structural properties confirms the tetrahedral nature of the deposited films. Characterisation of the electronic and optical properties show that ta-C:H is a wide band gap semiconductor which conducts by a thermally activated localised hopping mechanism with a defined activation energy. Measurements of photoconductivity show that ta-C:H grown using methane gas has a photosensitivity greater than in any other form of amorphous carbon. A high density of mid-gap defects will degrade the electronic properties of an amorphous material, however defects can often be passivated by hydrogen. Hydrogen only has a weak passivation effect in ta-C:H and the defect density remains high. Improved passivation is shown to be achieved by post-deposition annealing of ta-C:H, although experimental results suggest that the presence of hydrogen increases localisation of the conduction states. The ability to dope ta-C:H is also important for electronic applications. Strong evidence is presented which suggests that ta-C:H is slightly p-type and can be doped n-type by the addition of nitrogen. This n-type behaviour is further verified by the manufacture of n-type ta-C:H:N thin film transistors (TFTs). The TFTs show a small on/off ratio and require a large turn-on voltage. This is attributed to the relatively high defect density in the mid-gap region, despite its reduction by annealing, illustrating the main problem at present with the use of ta-C:H in device applications.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available