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Synthesis and Characterization of Solution Processed ZnO Thin Films

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  • Recently ZnO has drawn a lot of attention in semiconductor industry due to its interesting features. High exciton binding energy, high resistivity against radiation, high breakdown voltage, insensitivity to visible light, and easy wet chemical etching are some of the interesting features of this material. ZnO materials can be deposited using two ways: vacuum deposition and solution processing methods. The ZnO thin films deposited using vacuum deposition offer better crsytallinity and hence electrical performance. The main drawback of vacuum deposition is the requirement of expensive vacuum pumps. Additionally the throughput of this method is less compared to solution-processed methods. Solution processing offers a cheap easy method of ZnO thin film deposition. Currently two solution processing methods are used: pyrolysis of a coated film of a solution-based ZnO precursor (usually zinc acetate) and the spin-coating of a colloidal dispersion of ZnO nanoparticles, subsequently subjected to sintering. In both these methods, it is difficult to control the electrical parameters of the films such as the doping density and defect concentration from run to run, and p-type ZnO is not yet reproducibly obtainable. Therefore, better methods to control the electrical characteristics of the FETs incorporating ZnO films are much needed. The properties of the semiconductor film have a huge impact on the electrical performance of the ZnO TFTs. Hence by reducing the number of defects high performance ZnO TFTs will be obtained. Another method of improving the TFT performance is by modifying the transistor structure. One method of modifying the transistor structure is through schottky barrier thin film transistor (SB-TFT) or the source gated transistor (SGT). In the first part of the thesis we will study the effect of Schottky source contact on the enhancement of the electrical properties of thin film transistors. The sol stabilizer used in the solution processing of ZnO functions variously as a sol homogenizer, chelating agent, wettability improver and capping agent. In spite of its obvious importance to influencing ZnO film properties, the effect of the sol stabilizer has not been systematically studied and is generally unknown. Although there are a few papers examining different stabilizers, these studies have been mainly restricted to alkaline short chain ligand bearing species such as ethanolamine, diethanolamine, triethanolamine, etc. Furthermore, these prior reports did not examine the effect of the stabilizers on the performance of the resulting ZnO thin films in optoelectronic devices. Our study also examines longer chain and acidic stabilizers such as oleic acid, oleylamine and octadecene, which are used extensively in the synthesis of colloidal II-VI quantum dots, but have not been used to form ZnO thin films. We examined the effect of six different sol stabilizers -triethylamine, oleylamine, oleic acid, octadecene, triethanolamine and ethanolamine (along with a sol without any stabilizer), on the grain size, crystallographic texture, and resistivity of solution processed ZnO films on thermal oxide-coated silicon substrates, and found large variations in the structural and electrical properties as a consequence of the choice of sol stabilizer. We synthesized ZnO films using various sol stabilizers and studied the effect of the stabilizing agent on the morphology, orientation, optical, and electrical characteristics of the deposited films. The effect of different sol stabilizers on the crystal texture of the films was investigated by studying the XRD results of the films. Raman studies were preformed on the solutions and the films to understand the nucleation and growth of the ZnO films. Four point probe measurements were performed to compare the resistivity of the films. The ratio of the photocurrent to dark current was measured in steady state photoconductivity measurements. By measuring the transient photoconductivity, mobility−lifetime product for photogenerated charge carriers was measured for each film. By performing C-V measurements using impedance spectroscopy, the doping value of each of the films deposited with different stabilizers was measured. Thin film transistors were fabricated and the effect of different stabilizers on their parameters like mobility and threshold voltage was studied. Using the doping values extracted from C-V measurements and the field effect mobility of the TFTs the barrier height of the grain boundaries and the trapped charge density at grain boundaries was calculated.

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    Attribution 3.0 International