Metal Oxide-based (ZnO and IZO) Thin Film Transistors and Circuits
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1109-B03-38
Metal Oxide-based (IZO and ZnO) TFTs for Flexible Electronics Shahrukh A. Khan and Miltiadis Hatalis Display Research Laboratory, Lehigh University, Bethlehem, PA 18015
ABSTRACT This work emphasizes room temperature deposition and fabrication of staggered bottom-gate ZnO and IZO TFTs. We synthesized these oxide thin films by RF sputtering in an Ar/Oxygen ambience with no intentional heating of the substrates. Bottom gate staggered structure ZnO TFTs were fabricated (Ti/Au/Ti gate and Au/Ti source/drain) and characterized. ZnO TFTs retained well-behaved transfer characteristics down to a channel length of 4 µm with field effect mobility of 5 cm2/V.s, on/off current ratio exceeding 106 and threshold voltage around -5V. The IZO TFTs, with ITO as gate metal layer and highly conducting amorphous IZO forming the source/drain material had reasonably high field effect mobility of 20 cm2/V.s and on/off current ratio exceeding 106, which are well suited for active matrix display applications. Finally, to demonstrate the viability of oxide-based device integration, simple circuits such as inverters and pseudologic circuits are designed, fabricated and tested.
INTRODUCTION Oxide-semiconductor based thin-film transistors (TFTs) have advanced tremendously off-late and provides an attractive alternative to silicon-based TFTs. Current industrial approaches to produce thin film transistors for display devices include hydrogenated amorphous silicon (a-Si:H) and low-temperature poly silicon (LTPS). The mainstay of today’s display devices based on a-Si:H although mature, is typically incompatible with plastic substrates and poses other limitations such as light-induced degradations and inherent low-mobility. On the other hand, LTPS offers high mobility but suffers from poor threshold voltage variation across display backplanes [1]. Oxide semiconductors composed of heavy-metal cations with (n-1)d10ns0 (n ≥ 4) electronic configurations [2, 3] have been widely investigated as they offer several key advantages. They are usually wide-gap materials, transparent in the visible spectrum and thus render possible ubiquitous transparent electronics. Furthermore, large carrier mobilities are achievable due to their unique electronic configuration. The preservation of relatively high mobility in the amorphous phase can be attributed to a high degree of localization [4, 5] and suggests that if the carrier concentration can be controlled, the properties of amorphous oxides are quite suitable for TFT applications. It is desirable to use very thin films of these metal-oxide materials to produce high electronic activity based either on amorphous or nanostructured layers inorder to reduce the role of interface and surface states on carrier transport as required in TFTs. In this study, we present room temperature fabrication and characterization of amorphous ZnO and IZO based TFTs.
Figure 1: Schematic cross-section of fabricated amorphous oxide-TFT
EXPERIMENTAL DETAILS In order to optimize the electrical, structural and optical propertie
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