Electronic properties of ladder-structured oligomers and the fabrication of electronic devices
Dihydroquinoxalino [2,3b]phenazine or dihydrotetraazapentacene (L5H[sub]2) has a similar structure to the pentacene molecule, which has recently caused a great deal of interest in a number of research corporations, as well as in academia for potential applications in molecular electronics. This project is mainly concerned with a comparative study of electrical and electronic properties of vacuum evaporated undoped and doped thin films of L5H[sub]2 and pentacene. The vacuum-evaporated thin films of both L5H[sub]2 and pentacene give high-quality amorphous thin films on conductive (ITO) glass, highly doped silicon (n-Si) and silicon dioxide (SiO[sub]2) substrates. The conduction processes in both L5H[sub]2 and pentacene thin films, occurs via positive charge carriers and the layers exhibit p-type semiconducting properties. The electrical conductivities and charge carrier mobilities for these vacuum evaporated thin films were found to be low, indicatives of charge carrier transport in a narrow polaron band with thermally assisted intermolecular hopping or tunnelling. Schottky diode devices based on L5H[sub]2 and pentacene thin films have been successfully fabricated using a range of metal contacts (Al, Au, Cu, Pb, Sn). The electronic device characteristics were mathematically modelled to determine the Schottky barrier heights at the various metal-semiconductor interfaces. The measurement of Schottky barrier heights were carried out using conventional Current-Voltage (I-V) and Capacitance-Voltage (C-Y) techniques. The measured Schottky barrier heights for both L5H[sub]2 and pentacene thin films, are largely independent of metal workfunction, which suggests that the Fermi level in these devices are pinned at mid gap by a high density of states at the interfaces. Results are also presented for organic field effect transistors (OFETs) based on thin films of L5H[sub]2 and pentacene. For the first time operational OFET devices based on L5H[sub]2 thin films have been reported. The device characteristics for these OFETs based on L5H[sub]2 are compared to device characteristic for similarly fabricated device structures based on pentacene thin films. The OFET characteristics were investigated to determine the field effect charge carrier mobilities, on/off ratios and threshold voltages for both L5H[sub]2 and pentacene devices. The effects of thin film morphology, channel length and dielectric thickness on OFET device characteristics have also been considered. The research work presented here clearly shows that OFETs based on L5H[sub]2 thin film are possible and their operational device characteristic could be improved immensely by better molecular 'organisation in the thin film layers, which would result in higher charge carriers mobilities, and better charge injection at the source/drain metal contacts.