Strategies to modulate Amorphous Oxide Semiconductor properties for low temperature transparent transistors

• Main Author: Kulkarni, Mohit Rameshchandra
• Other Authors: Nripan Mathews
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2019
• Subjects: Engineering; Engineering::Materials
• Online Access: https://hdl.handle.net/10356/136480
• Institution: Nanyang Technological University

Amorphous Metal Oxide Semiconductors (AMOS) with properties such as high optical transparency, flexibility, and high electron mobility have emerged as a promising candidate over a-Si:H, poly-Si and organic semiconductor in the field of transparent and flexible electronics. However, the high processing temperature of AMOS constrains the fabrication process, including the choice of substrates, as the plastic substrates used for flexible electronics impose a limited thermal budget. In addition, unique novel applications such as oxide semiconductor-based logic gates, non-von Neumann architecture based neuromorphic devices and RRAM, require post-fabrication control over conductance of oxide semiconductor. Consequently, to achieve both athermal activations and better control over oxide conductivity, modulation of the charge carriers becomes vital. Typically, the carrier concentration of metal oxides is critically determined by the oxygen vacancies/ defects and external doping, where the increase in oxygen vacancies gives rise to higher conductivity/mobility. Oxygen vacancies are typically controlled during the growth of the thin films by modulating the partial pressure of oxygen during sputtering/ other vacuum deposition processes or through high-temperature post-deposition annealing. Such approaches increase the complexity of fabrication as well as allow less control over device behaviors. Hence the need for new post-fabrication techniques is manifested. The principal motivation of the thesis is to find alternate post-fabrication strategies to achieve the on-demand transformation of an oxide semiconductor for flexible electronics in order to avoid high-temperature annealing. In this work, the novel approaches to control oxygen vacancies concentration, and doping achieved through surface modification, which will decide charge carrier densities and charge transport in oxide semiconductors were investigated. The feasibility of Ga/Zn-free Indium Tungsten Oxide (IWO) as a semiconducting channel layer for TFT was investigated. In-depth studies of various process parameters concluded that an IWO thickness of 20 nm, fabricated with an oxygen flow rate of 1sccm and annealed at 200 oC, yielded the best device performance. The work function of the electrode material had a significant impact on the performance of the TFT with ITO emerging as the best candidate. Flexible TFTs on PI substrates with non-degraded device performance for a bending radius of up to 3 mm were also demonstrated. The prospect of athermal annealing with the inclusion of a reducing oxide layer carefully chosen from the Ellingham diagram used as an overlayer on the semiconducting oxide layer was also explored. From these studies, it was discovered that the current through the channel was increased after the deposition of the overlayer, which can be linked to the increased number of oxygen vacancies in the channel. The on-state current of the TFT was shown to increase with the thickness of overlayer oxide from 1 nm to 9 nm. As the demand for oxygen from overlayer rises with thickness, more oxygen is removed from the bottom layer of the semiconductor oxide, thus making it more metallic. Experiments with different overlayer oxides and semiconducting oxides were conducted, which concluded that oxide reduction through overlayer deposition could provide a universal method for athermally activating or transforming TFTs from enhancement mode operation into depletion mode operation as well as improving the environmental stability of the devices. Another method for selective on-demand modification of TFT properties using chemical surface treatment was also explored in the dissertation. Herein, the possibilities of dipole induced and doping induced charge carrier modulation of oxide semiconductors were explored by grafting various self-assembled monolayers (SAMs) such as silane, thiol, polymers with amine groups and a few organic dopant molecules on IWO channel layer. Such surface modifications can achieve a change in the carrier concentration by change in work function or shifting- pinning of the Fermi level, or additional field generated due to dipoles created at the surface. This increase in carrier concentration can be utilized to achieve high-performance TFT. Hydrogen peroxide assisted wet chemical treatment was also explored as a route for controlled passivation of oxygen vacancies. Dynamic carrier modulation utilizing a novel field-driven athermal activation of AMOS channels via electrolyte gating was also implemented. The high electrostatic field provided by the ionic liquid gating facilitates reversible migration of the charged oxygen species. The study also gave critical insights into different parameters affecting oxygen vacancies generation. Various material characterization techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet photoelectron spectroscopy (UPS), Fourier-transform infrared spectroscopy (FTIR) and electrical characterizations were used to support these effects on oxide semiconductor. The above-mentioned techniques targeted the control of semiconductor conductance, thus altering the working mode of thin-film transistor from depletion-mode to enhancement mode and vice versa. This active programming of the operating mode of TFT is beneficial for the development of inverter logic gates. In addition, neuromorphic transistors, facilitated by field-induced activation, were also demonstrated. Hence, the ability to modify the electronic properties of amorphous metal oxide semiconductor precisely using oxygen vacancy modulation and surface chemical doping to unlock optimal electrical performances of electronic devices would be relevant for the development of the flexible and transparent electronics.