Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.341335
Title: Electronic properties of hydrogenated amorphous carbon thin films.
Author: Khan, Rizwan Uddin Ahmad.
Awarding Body: University of Surrey
Current Institution: University of Surrey
Date of Award: 2000
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Abstract:
This thesis is concerned with the growth, electronic properties and modification of hydrogenated amorphous carbon films of a thickess range of 50-300 nm, which have been deposited using r f plasma-enhanced chemical vapour deposition. These films may be subdivided into two types according to the electrode on which they are grown and the resulting film properties. These are polymer-like amorphous carbon or PAC, and diamond-like amorphous carbon or DAC. PAC possesses a wide optical band gap (2.7 eV), high resistivity (1014 - 1015 n em) and low density of paramagnetic defects (f'o.l 1017 spins cm-3). The dominant current transport mechanism at room temperature has been observed to be hopping conduction at low electric fields and space-charge-limited current at high electric fields. The addition of nitrogen gas to the plasma to incorporate nitrogen within the film has been shown to move the Fermi level by 1 eV, towards midgap. A mechanism of doping due to the introduction of aromatic nitrogen-containing sites has been postulated. The boron, carbon and nitrogen ion implantation of PAC has resulted in the controllable increase in conductivity from 1015 to 106 f2 em as a function of ion dose, from 2 X 1012 to 2 x 1016 ions cm-2 . At low ion doses (up to 6 x 1014 ions cm-2Jthis occurs without any change in band gap; however, at higher doses the band gap collapses as a result of graphitisation. The dependence on the implant ion shows that it is possible to move the Fermi level towards the valence band with the implantation of boron, and towards midgap with the implantation of nitrogen. A hysteresis effect is observed at intermediate ion doses, which is attributed to the trapping of holes resulting in an increase in electron current. Implanting part of the thickness of the film at this ion dose has resulted in rectification, which has not previously been reported for this type of structure in amorphous carbon. DAChas been shown to possess a smaller band gap (0.7 eV), higher density of defects ( f'o.l 1020 spins cm-3) and lower resistivity ( f'o.l 1013 n em) than PAC. The room-temperature current transport is governed by band-tail conduction at fields below 105 V cm-1 , and the Poole-Frenkel effect at higher fields. The addition of nitrogen of up to 8 at. % has been observed to increase the band gap from 0.7 to 1.0 eV and therefore decrease the magnitude of the Poole-Frenkel conductivity. The Fermi level remains pinned at midgap, however. Therefore, it appears that PAC shows advantages over DAC in terms of future device applications
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.341335  DOI: Not available
Keywords: THIN FILMS; CARBON; CHEMICAL VAPOR DEPOSITION; ELECTRIC CONDUCTIVITY; NITROGEN ADDITIONS; BORON ADDITIONS; ION IMPLANTATION; ENERGY GAP; POINT DEFECTS; FERMI LEVEL Solid state physics Chemistry, Physical and theoretical
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