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Study of ZnO properties applied to thin film transistors

Intilla, L; (2016) Study of ZnO properties applied to thin film transistors. Doctoral thesis , UCL (University College London).

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Abstract

The work presented in this thesis is motivated by the great commercial impact of ZnO for its peculiar properties useful especially in optoelectronics. In particular, I focused my attention on some of the current challenges in thin film transistors from the point of view of the characterization of properties of the aforementioned material and the study of the device physics. In view of the interest in thin film depositions and given the potential of ZnO for obtaining thin film transistors, based on what is known in the literature I propose two ways to produce uniform films of the material. My attempt is to provide precious information on the relative importance of the relevant physical and electrical properties of ZnO. Firstly, based on previous preliminary experiments in the group of Prof. Cacialli, I propose a wet chemical technique, employing colloidal solutions of K-rich ZnO nanoparticles to produce a thin film on top of the substrates via spin-coating depositions. Following this strategy, I obtained n-type ZnO thin-film transistors (TFTs) showing apparent values of charge carrier mobility comparable to or better than the best low-temperature-processed thin-film ZnO TFTs reported on in the literature (the highest previously reported value was ~85 cm2 /Vs). I believe that the results obtained provide valuable information to the scientific community, as they suggest material design criteria. The work nonetheless produced some interesting additional results. In particular, the effects of processing on the SiO2 layer on the substrate may constitute a previously unseen phenomenon of oxide growth at low temperature and the formation of a high-κ dielectric by chemical reaction. The results presented in this thesis show a combination of analyses used to obtain useful and complementary information regarding the surface and interface characteristics of ZnO films deposited on Si/SiO2 substrates. The major result regarding the films is that the ZnO composition/structure is dependent on the atmosphere conditions during deposition onto the substrate as well as during its storage. I-V investigation will show how the devices performance will change if the device is kept in air or inert atmosphere. SEM and STEM investigation will show structural modifications, as well as the presence of Zn and K, penetrated into the substrate in the XPS spectra, both are a strong indication that a chemical interaction between the ZnO film and the substrate. I will also show how the preparation technique has been improved in term of time and energy, leading to a promising simple widely available and low-cost preparation technique for TFTs. Secondly, I explored a physical method using the pulsed plasma deposition (PPD) for the same purpose. Besides providing an alternative method of deposition at room temperature (compared to similar techniques such as pulsed laser deposition (PLD) and the radio frequency magnetron sputter deposition, both requiring the annealing of the substrate at temperatures above 300˚C), I contributed to expand the knowledge about the possibilities of this deposition technique providing new details about the dependence of the final product on the deposition conditions. In addition, I have also contributed to the preliminary work to produce p-type ZnO using an extra target material, besides the ZnO one, as doping material.

Type: Thesis (Doctoral)
Title: Study of ZnO properties applied to thin film transistors
Event: UCL
Language: English
UCL classification: UCL > Provost and Vice Provost Offices > UCL BEAMS
UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences
URI: http://discovery.ucl.ac.uk/id/eprint/1530069
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