1. Advanced Science and Technology Letters Vol.29 (IRTT 2013), pp.409-413 http://dx.doi.org/10.14257/astl.2013.29.83 Fabrication and Characteristics analysis of ZnO Thin Film Transistors with Vertical Structure Yue Shan, Dongxing Wang, Yue Zhang, Jiabin Chen, Jinghua Yin and Hong Zhao (Department of Electronic Science and Technology, College of Applied Science, Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education Harbin University of Science and Technology, Heilongjiang, Harbin 150080, China) College of Applied Science, integrated circuit engineering, Harbin, China,She712830@163.com Abstract. ZnO thin film was prepared by magnetron sputtering Zn target and reacting with oxygen on the glass substrate, at room temperature. The structure of transistors was a sandwich structure of Ag/ZnO/Al/ZnO/Ag. Ag and Al layers as the electrodes were prepared by DC magnetron and ZnO were prepared by magnetron sputtering. Schottky contacts were formed between Ag and ZnO; and ohmic contact between Al and ZnO. The TFTs show the advantage of low opening voltage, large operation electric current and high charge carriers mobility and so on. When the device works, while the gate voltage VGS is 0.2 V, VDS is 3 V, the drain could achieve at as large as 9.15 mA, the threshold voltage Vth is only about 1.35 V. Keywords: RF Magnetron Sputtering; ZnO-TFT; carriers mobility 1 Introduction Oxide semiconductor has the advantages of Wide band gap, low preparing temperature,high carrier mobility, low production costs, compatible with the flexible substrate, high thin film transparency and so on, therefore, oxide semiconductor can be widely used in transparent electronics [1-2]. For TOS-TFT has character of transparent, high-performance, preparing at low temperature and so on, TOS-TFTs are becoming hot topics in the study of the next generation thin film transistor[3-5]. 2010, Aixia Lu et al. prepared ZnO thin film transistor which carrier mobility is 14.9 s -1 cm 2 V - 1, threshold voltage is 1.5 V, biggest switch current ratio is 10 6 orders of magnitude [6]. 2011, J. Israel Ramirez after a large number of experiments of the preparation of ZnO thin film transistor and the switch current ratio is up to 10 7 orders of magnitude, the threshold voltage is 1.7 V, the carrier mobility is 18 s -1 cm 2 V -1 [7]. In recent years, however, reported in the literature by using ZnO transparent thin film material as conducting channel TFTs, mainly adopts the flat structure, and still need to use lithography technology in the production process, which lead to the difficulty of the preparation of ZnO thin film ISSN: 2287-1233 ASTL Copyright © 2013 SERSC http://www.mercubuana.ac.id

2. Advanced Science and Technology Letters Vol.29 (IRTT 2013) transistor, and the system components of the driving voltage is higher, the working current microamps level [8], didn't realize the good uniform characteristics. According to the paper have been reported, the structure form is the main factor to affect device performance finally, so we use ZnO as a device of active layer, through the device geometric structural transformation from the traditional metal insulator semiconductor (metal insulator - semiconductor, MIS) flat structure to static induction transistor (static induction - transistor, SIT) to improve device performance,vertical structure is used to reduce the conductive channel TFTs length, thus improve the working frequency working current and reduces the driving voltage. 2 Device Fabrication and Measurement Using vertical ZnO conductive channel TFTs laminated structure is shown in fig. 1. ZnO thin film was prepared by magnetron sputtering Zn target and reacting with oxygen on the glass substrate, at room temperature. The structure of transistors was a sandwich structure of Ag/ZnO/Al/ZnO/Ag. Ag and Al layers as the electrodes were prepared by DC magnetron and ZnO were prepared by magnetron sputtering. Schottky contacts were formed between Ag and ZnO; and ohmic contact between Al and ZnO. Glass substrate Fig. 1. The vertical structure of ZnO thin film transistor structure Fig. 2 shows the capacitor- voltage characteristic between Al/ZnO/Ag three layers. According to the ideal metal - semiconductor Schottky barrier theory, by solving the Poisson equation: q  N A  o n o  2(  V ) W B (1) 1 2 B (  +V ) C A q  o Nn 2 Ag ZnO Al ZnO Ag CACA (2) 410 Copyright © 2013 SERSC

3. Advanced Science and Technology Letters Vol.29 (IRTT 2013) 1 d C ( ) 2 = According to equation (3), measured1/C 2 and voltage V curve, as shown in figure 3, can get the curve slope 8.32962x10 19. So: 19 A q ee o Nn 2 A=0.04cm 2, q=1.6x10 -19 C, ε=8.656F/m, ε o =8.8542x10 -12 F/m, calculated the ZnO Schottky contact space charge density n=1.22x10 10 cm -3 in the depletion layer. 2 dV A q ee Nn o 2 = 8.32962 10 150.0p 3x10 20 Capacltance(pF) 120.0p 1/C 2 ( 2 ) 1/F 2x10 20 90.0p 60.0p 1x10 20 0 0.0 0.5 1.0 1.5 2.0 2.5 Coltage(Vj Coltage (V) Fig. 2. The Ag-ZnO Schottky capacitance to voltage characteristic curve and the Ag-ZnO Schottky contact V - 1 / C 2 characteristic curve Static DC characteristics test results of ZnO - TFT has shown in fig.2. Through the calculation we can get the threshold voltage of the device at around 1.35 V. Can be found from the figure, when gate voltage VGS is 0.2 V, VDS is 3 V, the device work drain-source current I DS = 9.15 x10 -3 Al, can serve as organic light emitting diode drive unit and other practical devices. IDS and VDS take logarithm in fig. 4, and make curve fitting for the logarithmic graph, when V GS =0V, VDS ranged from 0V to 3V, get the IDS-VDS ohmic contact figure line, the green line, which is taken logarithm for the black line in fig.4, as shown in fig. 3.By a = ∆ I d DS ∆ V DS A a µ = qn ZnO carrier mobility μ of this experiment preparation can be got. With σ is electrical conductivity, n=1.22x10 10 cm -3 is carrier concentration. So from equation (5) (6), ZnO thin film conductivity and mobility of charge carriers available to be σ=6.4288×10 - 8 S/cm, μ=32.93cm 2 V -1 s -1. Copyright © 2013 SERSC 411

4. Advanced Science and Technology Letters Vol.29 (IRTT 2013) 3 Conclusion Using radio frequency (RF) magnetron sputtering ZnO active layer, using the Al film as the grid of TFT, Ag film for TFT drain-source, vertical stacked on the quartz glass substrate preparation double Schottky structure transistor. TFT won the good performance of static working characteristics and high carrier mobility. And the channel length is only about 200nm. In grid voltage VGS (< 1.0 V), control the number of carrier from Ag metal electrode film tunneling into the ZnO thin film, has realized the control of the leakage source electrode current I DS. When the grid bias V GS = 0.2 V, V DS = 3 V, the drain-source current I DS = 9.15×10 -3 A, threshold voltage Vth is around 1.35 V. Results show that the TFT design under the condition of low driving voltage, can obtain higher output current, which is beneficial to realizing large active light-emitting display driver. 0.00 0.75 1.50 2.25 3.00 V DS (V) logV DS I ES (mA) 8 6 4 2 0 VGS=0V VGS=0.2V VGS=0.4V VGS=0.6V VGS=0.8V VGS=1.0V -2 -3 -4 logI ES -5 -0.3 0.0 0.3 0.6 slope=1.00 Fig. 4. The current-voltage output characteristic curve of the ZnO thin film transistor and log (I DS ) - log (VDS) relationship fitting curve References 1.J. S. Park, J. K. Jeong, H. J. Chung, Y. G. Mo, and H. D. Kim, “Electronic transport properties of amorphous indium-gallium-zinc oxide semiconductor upon exposure to water,” Appl. Phys. Lett., vol. 92, pp.072104, February 2008. 2.P. Görrn, M. Lehnhardt, T. Riedl, and W. Kowalsky, “The influence of visible light on transparent zinc tin oxide thin film transistors,” Appl. Phys. Lett., vol.91, pp.193504 November 2007. 3.S. J. Pearton, D. P. Norton, K. Ip, and Y. W.Heo, “Recent advances in processing of ZnO” J.Vac. Sci. Technol.B,vol.22,pp. 932 – 948,May 2004. 4.P. F. Carcia, R. S. McLean, M. H. Reilly, and G. Nunes, “Transparent ZnO thin-film transistor fabricated by rf magnetron sputtering,”Appl. Phys. Lett., vol.82, pp. 1117-1119, February 2003. 5.S. Masuda, K. Kitamura, Y. Okumura, S. Miyatake, H. Tabata, and T.Kawai, “Transparent thin film transistors using ZnO as an active channel layer and their electrical properties” J. Appl. Phys.vol.93,pp. 1624 - 1630, February 2003. 412 Copyright © 2013 SERSC

5. Advanced Science and Technology Letters Vol.29 (IRTT 2013) 6.Aixia Lu, Jia Sun, Jie Jiang, “Low-voltage transparent electric-double-layer ZnO-based thin-film transistors for portable transparent electronics,” Appl. Phys. Lett. Vol.96, pp.043114, January 2010. 1.N. L. Dehuff, E. S. Kettenring, D. Hong, et al. “Transparent thin-film transistors with zinc indium oxide channel layer,” Appl. Phys.lett., vol.97,pp.064505-064506, March 2005. 2.Hua Xu, Linfeng Lan, Miao Xu, Jianhua Zou, Lei Wang et al. “High performance indium-zinc-oxide thin-film transistors fabricated with a back-channel-etch-technique,” Appl. Phys.lett., vol.99,pp.253501,December 2011. Copyright © 2013 SERSC 413