Electric Field Thermopower Modulation Analyses of the Operation Mechanism of Transparent Amorphous SnO2 Thin-Film Transistor

Abstract

Transparent amorphous oxide semiconductors (TAOSs) based transparent thin film transistors (TTFTs) with high field effect mobility (μ FE) are necessary to develop advanced flat panel displays. Currently, amorphous (a-) InGaZnO4 is widely applied as the TAOS of the TFT channel of the commercially available OLED displays with a μ FE is ~10 cm2 V− 1 s− 1[1]. Among TAOSs, amorphous (a-) SnO2 has several advantages against current a-InGaZnO4 such as higher μ FE >100 cm2 V–1 s–1[2], which is one order of magnitude higher than that of a-InGaZnO4 and indium free. Although there are several researches on a-SnO2 TTFT, the operation mechanism has not been clarified thus far due to the strong gas sensing characteristics of SnO2.We prepared a bottom-gate top-contact type TTFT by pulsed laser deposition technique using 4.2-nm-thick a-SnO2 as the channel without any surface passivation. We measured the transistor characteristics and thermopower(S) in air and vacuum. The resultant a-SnO2 TTFT showed clear transfer (I d–V g) characteristics (FIG. a) with the on-to-off current ratios of ~105. All the TTFTs show clear pinch-off in the output characteristics. The threshold voltage (V th) was −14 V in air whereas it shifted dramatically to −23 V in vacuum shows a strong gas sensing characteristic. An almost linear relationship with a slope of ~120 μV K−1 decade−1 was observed in the –S–log n s plot (FIG. b) when n s exceeded 2.5 × 1012 cm−2 in air and a slope of ~84 μV K−1 decade−1 was observed in the –S–log n s plot when n s exceeded 2.9 × 1012 cm−2 in vacuum indicating that the E−k relation at the bottom of the conduction band is parabolic shaped. The t eff ≡ n s/n 3D of the conducting a-SnO2 channel were always ~1.7 ± 0.4 nm, insensitive to the gas atmosphere (FIG. c). From the thickness of the a-SnO2 film (4.2 nm) and the t eff (1.7 ± 0.4 nm), the carrier depletion depth at the top surface was concluded to be 2.5 nm of the a-SnO2 film, which shows the similar depletion length as reported data.[4] The present results would provide a fundamental design concept to develop a-SnO2 TTFT.[1] K. Nomura et al., Nature, 432, 488 (2004).[2] C. W. Shin et al., Sci. Rep. 6, 19023 (2016).[3] N. Yamazoe, Sensor Actuat. B-Chem. 5, 7 (1991).[4] D. Liang et al., Appl. Phys. Lett. 116, 143503 (2020) Figure 1