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Reactive High Power Impulse Magnetron Sputtering of Zinc Oxide for Thin Film Transistor Applications

Reed, Amber Nicole
http://orcid.org/0000-0001-9833-4022
http://rave.ohiolink.edu/etdc/view?acc_num=dayton1429028518

Abstract Details

2015, Doctor of Philosophy (Ph.D.), University of Dayton, Materials Engineering.
Zinc oxide (ZnO) is an emerging thin film transistor (TFT) material for transparent flexible displays and sensor technologies, where low temperature synthesis of highly crystallographically ordered films over large areas is critically needed. This study maps plasma assisted synthesis characteristics, establishes polycrystalline ZnO growth mechanisms and demonstrates for the first time low-temperature and scalable deposition of semiconducting grade ZnO channels for TFT applications using reactive high power impulse magnetron sputtering (HiPIMS). Plasma parameters, including target currents, ion species and their energies were measured at the substrate surface location with mass spectroscopy as a function of pressure and applied voltage during HiPIMS of Zn and ZnO targets in O2/Ar. The results were correlated to film microstructure development investigated with x-ray diffraction, atomic force microscopy, scanning electron microscopy and transmission electron microscopy which helped establish film nucleation and growth mechanisms. Competition for nucleation by (100), (101) and (002) oriented crystallites was identified at the early stages of film growth, which can result in a layer of mixed crystal orientation at the substrate interface, a microstructural feature that is detrimental to TFT performance due to increased charge carrier scattering in back-gated TFT devices. The study revealed that nucleation of both (100) and (101) orientations can be suppressed by increasing the plasma density while decreasing ion energy. After the initial nucleation layer, the microstructure evolves to strongly textured with the (002) crystal plane oriented parallel to the substrate surface. The degree of (002) alignment was pressure-dependent with lower deposition pressures resulting in films with (002) alignment less than 3.3°, a trend attributed to less energy attenuation of the low energy (2- 6 eV) Ar+, O+, and O2+ ions observed with mass spectrometry measurements. At pressures of 7 mTorr and lower, a second population of ionized gas (Ar+, O+, and O2+) species with energies up to 50 eV appeared. The presence of higher energy ions corresponded with a bimodal distribution of ZnO grain sizes, confirming that high energy bombardment has significant implications on microstructural uniformity during large area growth. Based on the established correlations between process parameters, plasma characteristics, film structure and growth mechanisms, optimum deposition conditions for (002) oriented nanocrystalline ZnO synthesis at 150 °C were identified and demonstrated for both silicon oxide wafers of up to 4 inch diameter and on flexible polymer (Kapton) substrates. The feasibility of the low temperature processing of ZnO films for TFT applications was verified by preliminary tests with back-gated device prototypes. Directions of future research are outlined to further develop this low temperature growth method and apply results of this study for ZnO applications in semiconductor devices.
Andrey Voevodin, Ph.D. (Committee Chair)
Christopher Muratore, Ph.D. (Committee Member)
Paul Murray, Ph.D. (Committee Member)
Kevin Leedy , Ph.D. (Committee Member)
Charles Browning, Ph.D. (Committee Member)
187 p.
Materials Science
High Power Impulse Magnetron Sputtering; Zinc Oxide, Thin Film Growth; Plasma Processing;

Recommended Citations

Citations

  • Reed, A. N. (2015). Reactive High Power Impulse Magnetron Sputtering of Zinc Oxide for Thin Film Transistor Applications [Doctoral dissertation, University of Dayton]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1429028518

    APA Style (7th edition)

  • Reed, Amber. Reactive High Power Impulse Magnetron Sputtering of Zinc Oxide for Thin Film Transistor Applications. 2015. University of Dayton, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=dayton1429028518.

    MLA Style (8th edition)

  • Reed, Amber. "Reactive High Power Impulse Magnetron Sputtering of Zinc Oxide for Thin Film Transistor Applications." Doctoral dissertation, University of Dayton, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=dayton1429028518

    Chicago Manual of Style (17th edition)

dayton1429028518
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© 2015, all rights reserved.
This open access ETD is published by University of Dayton and OhioLINK.