Performance of ZnO TFTs with AlN as Insulator
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Performance of ZnO TFTs with AlN as Insulator Maria Merlyne De Souza1, Richard B Cross2, Suhas Jejurikar3, and K P Adhi3 1 EE department, University of Sheffield, Mappin Street, Sheffield, S13JD, United Kingdom 2 Emerging Technologies Research Centre, De Montfort University, Gateway Street, Leicester, LE1 9BH, United Kingdom 3 Department of Physics, University of Pune, Pune, 411007, India ABSTRACT The performance of ZnO TFTs fabricated via RF sputtering at room temperature, with Aluminium Nitride (AlN) as the underlying insulator are reported for the first time. The resulting mobility of the device is significantly improved, in comparison to that of SiN insulator, with corresponding values of 3.0 cm2/Vs for AlN and 0.2-0.7 cm2/Vs for SiN cm2/Vs respectively. The AlN devices show a higher instability with respect to gate bias stress, despite the fact that the interface states are reduced due to an improved lattice match with ZnO. A possible cause is the interaction of ultra-fast interface states with bulk traps in the ZnO, which are also well known to contribute to excess background electron concentration in the material. INTRODUCTION ZnO, a Transparent Oxide Semiconductor (TOS), offers considerable potential for room temperature fabrication of Thin Film Transistors (TFTs) over large area flexible substrates. Unlike conventional Group IV semiconductors, these materials have the potential to attain the same mobility in the amorphous as-deposited state as their crystalline counterparts, the key reason being that in II-VI compounds, the conduction band minimum is largely localized on the metal ‘s’ states, with no contribution from the oxygen ‘p’ states. Since angular disorder affects ‘p’ but not the spherically symmetrical ‘s’ states, mobility of as-deposited material remains high [1]. Although higher mobility is a significant benefit for any technology, it is not the only criterion for display applications. In particular, the issue of stability remains critical because of the sensitivity of the human eye to the minutest degradation at the pixel level. Furthermore, positive turn-on voltages and on/off ratios in excess of 106 remain paramount. In this work the device characteristics of ZnO TFTs with AlN as the insulator are presented for the first time. The choice of this material as insulator arises mainly from its excellent lattice match (a= 3.1 Å and c = 4.98 Å) with ZnO (a=3.249Å and c=5.201Å) and transparency to visible light. In comparison, the lattice constants of the beta-crystobalite phase of SiO2 are (a=3.67 Å and c=3.67 Å) and for SiN are (a= 4.85 Å and c=5.5 Å). EXPERIMENTAL SETUP The gate insulator stack, grown on a silicon substrate of concentration (1018/cm3), consisted of 50 nm of SiO2 grown thermally at 1000 C followed by 50 nm of SiN deposited via chemical vapour deposition (CVD), followed by a high temperature anneal at 1000 C. The amorphic AlN buffer layer was then grown on the SiO2/SiN insulator layers using Pulsed Laser Deposition
(PLD). Radiation from KrF excimer laser (λ = 248 nm, p
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