Zinc oxide thin film transistors
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Thin film transistors on ZnO directly deposited by RF magnetron sputtering, were fabricated, measured and studied. The fabrication processes are as follows; a p-type silicon substrate was chosen for ZnO film deposition. The samples went through Acetone-Methanol-DI water basic cleaning process, while the PECVD (Plasma Enhanced Chemical Vapor Deposition) reactor was being cleaned at 250°C, for 1800 sec. Then PECVD was used to deposit oxide on the substrate. Two gate patterns were developed through UV lithography process. A buffered hydrofluoric (BHF) acid was used for 50 sec to open oxide windows within patterns. Chromium was used for gate electrodes during the metal evaporation process at about 2×10 -7 Torr. After gate pattern contacts were covered by thin glass, gate insulation was performed by PECVD at 300°C, for 175 sec, which deposited about 165 nm thick TEOS oxide. Then the ZnO film was deposited by RF magnetron sputtering from a 1.0 inch diameter target. A base vacuum in the range of 8×10 -6 Torr was first achieved. The vacuum system is diffusion pumped with an added cold trap and a titanium pump to remove water vapor. Sputtering was done at a power of 100 W in an atmosphere of 15 mTorr O 2 and 5 mTorr Argon. Finally source and drain patterns were fabricated with chromium metal. Sample testing was conducted with a Keithley 230 voltage source and a Keithley 617 current meter. The characteristics of ZnO thin film transistor including output, transfer and ZnO film were measured and analyzed. Achieved transconductance g m at V GS = 11 V is approximately 1.94×10 -5 S, field effect mobility μ m at V D =7V is about 101 cm 2 /V-s, and switching on/off current ratio is about 2.8×10 4 .