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Low Temperature ZnO TFTs Fabrication with Al and AZO Contacts for Flexible Transparent Applications Gerardo Gutierrez-Heredia1,2, Israel Mejia1, Norberto Hernandez-Como1, Martha E. RivasAguilar1, Victor H. Martinez-Landeros1,2, Francisco S. Aguirre-Tostado2, Bruce E. Gnade2 and Manuel Quevedo1,C. 1 Department of Materials Science and Engineering, University of Texas at Dallas, USA. 2 Centro de Investigacion en Materiales Avanzados S. C., Campus Monterrey, Mexico. ABSTRACT Zinc Oxide (ZnO) Thin-Film Transistors (TFTs) using Aluminum (Al) and Aluminum-doped zinc Oxide (AZO) as Source-Drain (S-D) contacts are reported. The fabrication process was carried out using five photolithography steps with a maximum processing temperature of 100 ˚C, which makes the process compatible with flexible/transparent applications. The AZO and ZnO films were deposited using Pulsed Laser Deposition (PLD). Aluminum was deposited using ebeam. The devices showed mobilities >10 cm2/V-s, threshold voltage in the range of 7 V and On/Off current ratios >105. The resistance analysis showed that AZO is a better contact with lower contact resistance as identified in the TFTs. The AZO and ZnO stacks characterized by UV-V shows an optical transmission >80 %. INTRODUCTION Compatibility with low temperature processing is one of the requirements for metal-oxide based devices used for transparent applications such as displays, health sensors, photodetectors and solar cells [1,2,3,4]. Low temperature process enables large area applications while maintaining lower fabrication costs compared to Silicon (Si) technologies. Ideally, semiconductor materials with a wide bandgap (Eg > 3.0 eV) are required for transparent electronics [5,6,7]. Among those semiconductors, ZnO (Eg ~ 3.3 eV) has been proposed as a potential candidate given its compatibility with both requirements: low temperature processing and suitable with large area substrates, which are strict for flexible electronic applications. In addition, ZnO films can achieve Hall mobilities of ~200 cm2/V-s [8]. Besides the semiconductor, Transparent Conductive Oxides (TCO) are required as contacts for transparent electronics. A promising TCO is AZO, which is a candidate to replace Indium Tin Oxide (ITO). Replacing ITO with AZO can potentially result in inexpensive devices while eliminating the need for a scarce element, such as indium [9]. In this paper, we demonstrate a low temperature photolithography process to fabricate ZnObased TFTs. The entire fabrication process is carried out at a maximum temperature of 100 ˚C. In addition, detailed contact resistance analyses demonstrate that ZnO/AZO interface is more stable, resulting in lower contact resistance compared with the ZnO/Al system. EXPERIMENTAL DETAILS The ZnO TFTs were fabricated on 500 nm of thermally grown SiO2. 100 nm of Chrome (Cr) was deposited by e-beam and patterned using wet etching processing to pattern the gate contact. Next, 90 nm of Hafnium Oxide (HfO2) was deposited by Atomic Layer Deposition (ALD) at 100

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˚C. 45 nm of ZnO was then