Electronic properties of hydrogenated amorphous carbon thin films.
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