Improving the Performance of Tin Oxide Thin-Film Transistors by Using Ultralow Pressure Sputtering

Abstract

Thin-film transistors ͑TFTs͒ are fabricated with a tin oxide channel deposited by using ultralow pressure sputtering ͑ULPS͒. The effect of sputtering pressure on the device performance of the tin oxide TFTs was investigated. The TFTs with tin oxide channel deposited by conventional sputtering pressure did not show a promising performance. However, the saturation mobility ͑ sat ͒ and the threshold voltage ͑V th ͒ of the ULPS-deposited SnO x TFTs were improved to ϳ3.9 cm 2 /V s and ϳ0.6 V, respectively. The better device performance of the ULPS-deposited SnO x TFT was attributed to the reduced free electron density ͑ϳ10 17 /cm 3 ͒ resulting from the formation of a nanocrystalline phase.Recently, transparent oxide thin-film transistors ͑TFTs͒ with channel layers comprised of ZnO, In 2 O 3 -ZnO, In 2 O 3 -Ga 2 O 3 -ZnO, or ZnO-SnO 2 have been intensively studied for potential application as the back-plane of active matrix organic light emitting diodes, active matrix liquid crystal displays, and flexible displays due to their relatively high mobility ͑Ͼ5 cm 2 /V s͒ and good transparency in the visible range compared to the counterpart of amorphous Si TFTs. 1-4 Among the various kinds of transparent oxide semiconductor, tin oxide has been extensively studied as a semiconductor material for the channel layer in TFTs due to its wide bandgap of 3-4 eV, high optical transparency ͑Ͼ85%͒ in the visible light range, and good chemical stability. However, all TFTs using tin oxide semiconductors were n-type depletion-mode devices, which indicates a relatively high net electron carrier density ͑N d Ͼ 10 18 /cm 3 ͒, leading to impractically high operation gate voltage and power consumption. 5-7 Therefore, the reduction and/or control of the N d of sputtered tin oxide films has been a major focus of research on TFT applications. The N d of the sputtered tin oxide film has been related to the crystalline structure, oxidation state, and oxygen deficiency. 8-10 Recently, Huh et al. reported the strong dependence of the electrical properties of indium-doped tin oxide ͑ITO͒ films on the sputtering pressure and thus the kinetic energy of the sputtered particles. 11 This study reports the fabrication of TFTs with a SnO x film as the channel layer by using ultralow pressure sputtering ͑ULPS͒, with a sputtering pressure lower than 1.3 ϫ 10 −1 Pa, compared to higher than 6.7 ϫ 10 −1 Pa for a conventional sputtering pressure ͑CSP͒. As the sputtering pressure was decreased from 6.7 ϫ 10 −1 to 6.7 ϫ 10 −2 Pa, the N d of the SnO x thin film was dramatically reduced 100-fold from 10 19 to 10 17 cm −3 , resulting in a transistor performance with a reasonable mobility and on-and off-current ratio ͑I on/off ͒. In addition, the origin of the modulation of appropriate N d is discussed in detail. ExperimentalMo ͑250 nm͒ was deposited as a gate electrode by sputtering on a SiO 2 /p-type Si ͑p-Si͒ substrate. A 400 nm thick SiO x N y film was grown as a gate dielectric by plasma-enhanced chemical vapor deposition at a substrate temperature of 330°C. The S...

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