Amorphous Indium Gallium Zinc Oxide Thin-Film Transistors, Non-volatile Memory and Circuits for Transparent Electronics
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Date
2010-06-26
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Abstract
The ability to make electronic devices, that are transparent to visible and near infrared wavelength, is a relatively new field of research in the development of the next generation of optoelectronic devices. A new class of inorganic thin-film transistor (TFT) channel material based on amorphous oxide semiconductors, that show high carrier mobility and high visual transparency, is being researched actively. The purpose of this dissertation is to develop amorphous oxide semiconductors by pulsed laser deposition, show their suitability for TFT applications and demonstrate other classes of devices such as non-volatile memory elements and integrated circuits such as ring oscillators and active matrix pixel elements.
Indium gallium zinc oxide (IGZO) is discussed extensively in this dissertation. The influence of several deposition parameters is explored and oxygen partial pressure during deposition is found to have a profound effect on the electrical and optical characteristics of the IGZO films. By optimizing the deposition conditions, IGZO TFTs exhibit excellent electrical properties, even without any intentional annealing. This attribute along with the amorphous nature of the material also makes IGZO TFTs compatible with flexible substrates opening up various applications.
IGZO TFTs with saturation field effect mobility of 12 – 16 cm2 V-1 s-1 and subthreshold voltage swing of < 200 mV decade-1 have been fabricated. By varying the oxygen partial pressure during deposition the conductivity of the channel was controlled to give a low off-state current ~ 10 pA and a drain current on/off ratio of > 1 x108. Additionally, the effects of the oxygen partial pressure and the thickness of the semiconductor layer, the choice of the gate dielectric material and the device channel length on the electrical characteristics of the TFTs are explored.
To evaluate IGZO TFT electrical stability, constant voltage bias stress measurements were carried out. The observed logarithmic dependence of the threshold voltage shift to the stress duration was modeled using a charge trapping/tunneling mechanism at the semiconductor/dielectric interface. By incorporating platinum nanoparticles in the dielectric layer of the TFT, non-volatile memory characteristics were achieved. The devices exhibited good memory behavior and up to 10 % charge retention extrapolated over 10 years.
The potential application for IGZO TFTs is examined by fabricating and characterizing 5- and 7-stage ring oscillators. The 5-stage ring oscillators operate at more than 2 MHz and have a sub 50 ns propagation delay at a supply voltage of 25 V. To the best of our knowledge these are the fastest all-transparent ring oscillators reported to date. As a practical demonstration, we integrated IGZO TFTs with a novel thin film electroluminescent phosphor to form an active matrix pixel element. The output intensity of the phosphor was successfully modulated by the TFT. These results demonstrate that IGZO TFTs are viable candidates for transparent circuits and display applications.
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Amorphous oxide semiconductors, thin-film transistors, TFT, IGZO, Indium gallium zinc oxide, transparent circuits, bias stability, ring oscillator, non-volatile memory
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PhD
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Electrical Engineering
