Active layer thickness effects on the structural and electrical properties of p-type Cu2O thin-film transistors
(Color online) (a) Cross-sectional diagram and (b) a top view of the Cu2O TFTs fabricated in an inverted staggered structure. W and L in the figure represent the channel width and length of the fabricated devices.
(Color online) (a) SEM and (b) AFM images of the copper oxide thin films with different thicknesses. As the film thickness increases, the average grain size and RMS roughness also increase and the area voids disappear. The average grain sizes of 171, 242, 298, and 383 nm were extracted with RMS roughnesses of 4.74, 5.23, 6.09, and 10.9 nm for 45, 65, 85, and 155 nm-thick copper oxide films, respectively.
(Color online) XRD patterns of the copper oxide thin films with different thicknesses. In all copper oxide thin films, Cu2O is the predominant phase.
(Color online) Optical transmittance spectra of the copper oxide thin films with various thicknesses in the wavelength range between 200 and 2100 nm. The optical transmittance notably decreases with increasing film thickness. The optical band gaps were ∼2.70, ∼2.75, ∼2.75, and ∼2.75 eV for 45, 65, 85, and 155 nm-thick copper oxide films, respectively.
(Color online) Representative transfer curves of the fabricated p-type Cu2O TFTs (W/L: 1000 μm/100 μm) with different active layer thicknesses at a drain-to-source voltage (V DS) of −1.0 V. The p-type Cu2O TFT device exhibits the cleanest transfer function with only a small SS when the channel thickness is 45 nm, whereas a notable subthreshold slope hump is observed in the transfer curves for devices with thicker channels.
(Color online) (a) Transfer curves of the fabricated p-type Cu2O TFTs with a channel thickness of 45 nm measured after various durations of air exposure. The on-current slightly increases, but there is no significant variation in the transfer curve shapes after 4 weeks of air exposure. (b) Transfer curves obtained using a “double sweep” methodology with the arrows indicating the direction of the sweeping voltage. A counter-clockwise hysteresis of ∼2.2 V is observed, indicating represents that the hole trapping in the interface states and/or bulk trap states is the cause of the observed hysteresis.
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