Thin-film transistors with amorphous oxide channel layers Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5m60qw89f

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  • In recent years, a new class of high-performance thin-film transistors (TFTs) has emerged comprising amorphous oxide channel materials composed of heavy-metal cations (HMCs) with (n-1)d¹⁰ns⁰ (n ≥ 4, where 'n' refers to the row of the periodic table) electronic configuration. This thesis is devoted to the fabrication and characterization of TFTs employing two such amorphous oxide channels: zinc indium tin oxide (ZITO) and zinc tin oxide (ZTO). ZITO is demonstrated as a transparent and amorphous quaternary channel material for TFTs. Optical transmission of ZITO channels is ~ 85% in the visible portion of the electromagnetic spectrum (~ 400 to 700 nm). Peak incremental mobilities of 5 – 19 cm²V⁻¹s⁻¹ and turn-on voltages of -4 to -17 V are obtained for TFTs post-deposition annealed at 100 – 300°C, respectively. Current-voltage measurements indicate n-channel, depletion-mode transistor operation with excellent drain current saturation, and a drain current on-to-off ratio greater than 10⁶. Additionally, two routes are explored to successfully fabricate enhancement-mode TFTs employing ZTO as the channel layer. Initially, Ba is added to the ZTO material system to obtain BZTO, a low cost quaternary amorphous material system similar to indium gallium zinc oxide. BZTO TFTs are enhancement-mode devices with positive near-zero turn-on voltages and BZTO channels remain amorphous at anneal temperatures as high as 700 – 800°C. However, the incremental mobilities of these devices are less than 5 cm²V⁻¹s⁻¹. Thus, Ba doping is found to be an unsuitable route for enhancement-mode TFT development. Subsequently, an experimental effort is undertaken to optimize the ZTO process for the 1:1 ZnO:SnO₂ stoichiometry. Primarily, the ambient O₂ partial pressure during channel deposition and the post-deposition annealing temperature are deduced to be the two primary processing parameters most significantly impacting TFT performance. Enhancement-mode TFTs are demonstrated for O₂ partial pressure percentages ≤ 5% of the ambient Ar/O₂ processing pressure and annealing temperatures of 300 – 600°C. The best devices realized exhibit incremental mobilities of 10 – 30 cm²V⁻¹s⁻¹ and turn-on voltages of 0 – 5 V.
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