High-k inorganic dielectrics are attractive materials for use in semiconducting technology because of increasing pressures to reduce device size whilst maintaining high capacitance values and allowing low operating voltages. This report presents an investigation into the use of a high-k oxide material, aluminium titanium oxide (ATO) as the dielectric in organic semiconducting devices. This thesis set out to investigate the important roles that both the insulator and the interface it forms with the semiconductor play in device performance and threshold voltage instability. In particular, charge trapping at the interface or in insulator states was the main focus of this investigation due to the resultant instability it creates in organic devices.
Metal-insulator-metal (MIM) devices were investigated with the aim of determining the contribution of the ATO to metal-insulator-semiconductor (MIS) capacitor response. Poly (3-hexylthiophene) was used as the organic semiconductor in MIS capacitors and field-effect transistors, which were studied using electrical characterisation techniques employed so successfully in the development of silicon technology.
Capacitance and loss were measured as a function of gate voltage and frequency for both MIM and MIS devices for a range of experimental conditions, principally involving the use of different temperatures and voltage stressing. The experiments were repeated while irradiating the devices with light of energy in the region of the polymer band-gap, and the results analysed and compared to those obtained in the dark.
The series of experiments in this thesis found that the ATO capacitance vs. voltage response was controlled by the semiconducting titanium dioxide layers whilst the insulating layers of aluminium oxide controlled the leakage current. The DC resistance, determined by the good insulating properties of the aluminium oxide, was 7.1 x 1013 nm. The low frequency resistance, related to conduction losses in the titanium oxide, was ~ 40 Mn. The dielectric constant of the material was found to be variable, in the range ~ 16.5 to 21, depending on the distribution of the two oxides throughout the A TO layer. Electrical breakdown of the A TO layer in the MIS configuration took place at
~4.5 MVcm·1•
Majority charge (hole) trapping of 1013 cm·2 in the dark, at the interface or in insulator states, resulted in a large anti - clockwise hysteresis present in the admittance vs. voltage characteristics. Doping density in the P3HT layer was found to be~ 1015 cm·3. A density of interface states of 1011 eV-1/cm·2 was calculated from the interface loss peak. Illumination with light produced photogenerated electrons resulting in an increase in the depletion layer capacitance and strong minority charge (electron) trapping with concurrent increase in hysteresis. Two sets of traps were identified; slow states which responded to the DC gate voltage and which were responsible for the hysteresis, and fast states which were able to react to the AC signal.
Devices made with ATO as the insulator were far too unstable to be used in organic semiconducting technology due to the inherent charge trapping properties of th