Electronic characterisation of amorphous silicon-carbon alloy thin films

• Main Author: Bayley, P. A.
• Published: Swansea University 1995
• Subjects: 530.417
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.636060

Excess electron and hole drift mobilities in hydrogenated amorphous silicon-carbon alloy thin films have been investigated using the time-of-flight technique. A detailed study of the temperature and electric field dependence of these parameters has been carried out. This has permitted the computation of the energy distributions of conduction and valence band tail localised states as a function of carbon concentration in the films. It is apparent from these data that increased carbon incorporation is associated with an increase in the total number of both the conduction and valence band tail states. This is supported by the experimental behaviour of the pre-transit currents. The present range of materials were produced by conventional low power radio frequency glow discharge of silane and methane. Typical carbon contents are below 10at.% and optical measurements show that the samples have optical energy gaps <I>E</I><SUB>g</SUB> up to ˜2.0eV. Time-of-flight charge collection experiments have been performed to investigate the free carrier mobility-lifetime products as a function of carbon concentration in the films. Results show a considerable decrease in this parameter with increasing carbon content. This is likely to be primarily associated with an increase in density of recombination centres in the films. The density of localised states in the materials have also been computed using a Fourier transformation of transient photo-current data. This reveals the increase in density of the conduction band tail states, as detected using the drift mobility analysis, but also reveals an increase in density and energetic broadening of the deep state distribution as carbon content is increased. This is consistent with the reduced carrier lifetimes inferred by the charge collection experiments. Combining the present data with localised state energy distributions measured using other techniques, we are able to illustrate the effect of carbon alloying on the entire localised state population. Preliminary transient photoconductivity results for glow discharge deposited diamond films are also reported.