Interface properties and capacitance-voltage behaviour of diamond devices prepared by microwave-assisted CVD

Free standing diamond films were used to study the effect of diamond surface morphology and microstructure on the electrical properties of Schottky barrier diodes. By using free standing films both the rough top diamond surface and the very smooth bottom surface are available for post-metal deposition. Rectifying electrical contacts were then established either with the smooth or the rough surface. The estimate of doping density from the capacitance-voltage plots shows that the smooth surface has a lower doping density when compared with the top layers of the same film. The results also show that surface roughness does not contribute significantly to the frequency dispersion of the small signal capacitance. The electrical properties of an abrupt asymmetric n(+)(silicon)-p(diamond) junction have also been measured. The I-V curves exhibit at low temperatures a plateau near zero bias, and show inversion of rectification. Capacitance-voltage characteristics show a capacitance minimum with forward bias, which is dependent on the environment conditions. It is proposed that this anomalous effect arises from high level injection of minority carriers into the bulk.

Effect of oxygen on the electrical characteristics of field effect transistors formed from electrochemically deposited films of poly(3-methylthiophene)

Field effect devices have been formed in which the active layer is a thin film of poly(3-methylthiophene) grown electrochemically onto preformed source and drain electrodes. Although a field effect is present after electrochemical undoping, stable device characteristics with a high modulation ratio are obtained only after vacuum annealing at an elevated temperature, and only then if the devices are held in vacuo. The polymer is shown to be p type and the devices operate in accumulation only. The hole mobility in devices thermally annealed under vacuum is around 10 -3 cm 2 V -1 s -1. On exposure to ambient laboratory air, the device conductance increases by several orders of magnitude. This increase may be reversed by subjecting the device to a further high-temperature anneal under vacuum. Subsidiary experiments show that these effects are caused by the reversible doping of the polymer by gaseous oxygen.

Electronic transport in field-effect transistors of sexithiophene

The electronic conduction of thin-film field-effect-transistors (FETs) of sexithiophene was studied. In most cases the transfer curves deviate from standard FET theory; they are not linear, but follow a power law instead. These results are compared to conduction models of "variable-range hopping" and "multi-trap-and-release". The accompanying IV curves follow a Poole-Frenkel (exponential) dependence on the drain voltage. The results are explained assuming a huge density of traps. Below 200 K, the activation energy for conduction was found to be ca. 0.17 eV. The activation energies of the mobility follow the Meyer-Neldel rule. A sharp transition is seen in the behavior of the devices at around 200 K. The difference in behavior of a micro-FET and a submicron FET is shown. (C) 2004 American Institute of Physics.