Nanoscale properties of molecular and oxide based thin film devices measured by SPM

Abdullah, Isam 2016. Nanoscale properties of molecular and oxide based thin film devices measured by SPM. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Organic and inorganic-metal oxide thin film transistors can be solution-processed, providing large area, low cost and low processing temperature leading to strong industrial and research interest. The nanoscale properties of each material system have been investigated using scanning probe techniques. Pentacene is a model small molecule organic semiconductor but it is an air sensitive material and insoluble. However, its derivative 6,13-bis(triisopropyl-silylethynyl) pentacene (TIPS-PEN) is amenable to solution processing. The motivation to study TIPSPEN is not to chase performance figures only but to investigate how the solution deposited film topography can be controlled by varying the solvent composition and the insulating polymer binder concentration. We report the effect of anisole / decane binary solvent mixture and the subsequent addition of low percentage weight of (atactic amorphous polystyrene) aPS on the surface morphology of TIPS-PEN thin films. It was found that addition of up to 20 wt% decane has little impact on micro-scale crystal morphology but has a significant influence on the growth mode, mean grain size and terrace roughness. The effect of the TIPS-PEN / aPS blend ratio up to 20 wt% of aPS in drop-cast thin film was similarly investigated on untreated SiO2 and on poly(4-vinylphenol) ( PVP) interlayers. It is found that addition of aPS has a vital effect on macroscopic crystal properties such as surface coverage, unity of orientation, long range order, and average field effect mobility. It also changes the surface morphology and layer ordering on the nano-scale. Mobility values obtained from PVP treated surface were higher than those from SiO2 surface by five times, which was consistent with preferred TIPS-PEN crystal growth on hydrophobic surface PVP of low surface energy. Indium oxide (In2O3) is an n-type (electron transport), optical transparent with wide bandgap values between 3.5 and 4 eV, and high performance semiconductor. Spin coating was used to prepare indium oxide thin film transistors (TFT) of two different thicknesses of the device channel. As the thickness of the In2O3 active layer is increased, the device mobility increases, the threshold voltage is shifted in the negative direction, off-drain current is increased, and on-drain current is less pronounced. A systematic study was carried out to investigate the transistor stability under bias stress. Applying a positive gate bias stress to indium oxide transparent TFTs was found to induce a parallel shifts of threshold voltage in positive direction without changing the device mobility or the subthreshold gate voltage swing. Scanning Kelvin Probe Microscopy (SKPM) was used to study the surface potential distribution of operating devices under a range of drain and gate biases. The potential profiles showed evidence of metal contact diffusion into the channel region which appeared as a flatting of the profile close to the drain electrode. The experimental data was confirmed and complemented with simulation results for contact material diffusion into the In2O3 channel. A new method of performing Electrostatic Force Microscopy (EFM) was developed and applied to the In2O3 TFT. The tip bias was modulated and the resulting phase difference between the cantilever response and the applied bias was recorded. The measurements were affected by a time delay, likely to be caused by capacitive charging of the semiconductor beneath the tip. The effects of the time delay were corrected by the use of an inverse filter in the analysis software developed. This new version of the EFM-phase method was applied to the operating biased In2O3 TFTs and the results compared with those obtained with SKPM.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Physics and Astronomy
Subjects:	Q Science > QC Physics
Uncontrolled Keywords:	SPM, AFM, EFM, SKpM, organic semiconductor, TIPS-PEN, Indium oxide, FET, TFT
Funders:	Iraqi Government Scholarship
Date of First Compliant Deposit:	30 August 2016
Last Modified:	08 Jul 2021 11:12
URI:	https://orca.cardiff.ac.uk/id/eprint/94080

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