Carbon-Incorporated Amorphous Indium Zinc Oxide Thin-Film Transistors

Transcription

1 Journal of ELECTRONIC MATERIALS, Vol. 43, No. 11, 2014 DOI: /s Ó 2014 The Minerals, Metals & Materials Society Carbon-Incorporated Amorphous Indium Zinc Oxide Thin-Film Transistors S. PARTHIBAN, 1 K. PARK, 1 H.-J. KIM, 1 S. YANG, 1 and J.-Y. KWON 1,2 1. School of Integrated Technology and Yonsei Institute of Convergence Technology, Yonsei University, Yeonsu-gu, Incheon , Korea jangyeon@yonsei.ac.kr We propose the use of amorphous-carbon indium zinc oxide (a-cizo) as a channel material for thin-film transistor (TFT) fabrication. This study chose a carbon dopant as a carrier suppressor and strong oxygen binder in amorphousindium zinc oxide (a-izo) channel material. a-cizo thin films were deposited using radiofrequency (RF) sputtering and postannealed at 150 C. X-ray diffraction and transmission electron microscopy analysis revealed that the film remained amorphous even after postannealing. The a-cizo TFT postannealed at 150 C exhibited saturation field-effect mobility of 16.5 cm 2 V 1 s 1 and on off current ratio of Key words: Metal oxide, amorphous, thin-film transistor, sputtering INTRODUCTION Amorphous-oxide semiconductor (AOS) thin-film transistors (TFTs) have attracted attention for application in next-generation active-matrix liquidcrystal displays (AM-LCD) and organic light-emitting diode (AM-OLED) display panels because of their high field-effect mobility, low-temperature processing capability, fabrication of a larger-area uniform coating, and transparency to visible light. 1 Among the AOS TFTs, amorphous-indium gallium zinc oxide (a-igzo) TFTs have been most extensively investigated. The a-igzo TFT shows fieldeffect mobility >10 cm 2 V 1 s 1 and has been proposed for use in backplanes of AM-OLED displays. 2 However, fabrication of next-generation applications, such as superhigh-definition panels, and integration of driver circuits require higher mobility. 3 The fabrication process and device structure of a-igzo TFTs are nearly optimized, leaving little room to improve their electron mobility and stability, thus next-generation applications require new materials. Incorporating various dopants, such as zirconium (Zr), 4 hafnium (Hf), 5 titanium (Ti), 6 silicon (Si), 7 lanthanum (La), 8 gadolinium (Gd), 9 gallium (Ga), 10 tungsten (W), 11,12 tantalum (Ta), 13 germanium (Ge), and boron (B), 14 into the oxide (Received February 12, 2014; accepted July 15, 2014; published online August 22, 2014) channel layer can increase the mobility via control of the carrier concentrations and enhances the bonding strength (BS). Doping of several metal ions into indium zinc oxide (IZO) TFTs has been reported; however, the results have not been thoroughly discussed. Various dopants have been incorporated into IZO TFTs based on their cation electronegativity (v). 4 9,14 However, questions have been raised regarding whether improved mobility and stability result from incorporation of cations with lower or higher v values into IZO TFTs. Recently, the lower v of La 3+ (1.212) and higher v of Si 4+ (1.769) cations doped into IZO TFTs yielded saturation field-effect mobility (l sat ) values of 3.02 cm 2 V 1 s 1 and 21.6 cm 2 V 1 s 1, respectively. 7,8 Si-doped IZO (SIZO) TFTs exhibited l sat of 21.6 cm 2 V 1 s 1 with improved bias stability, and the study reported that the stronger BS between Si and oxygen suppresses the creation of V O (donorlike) in the SIZO system. 7 Various mechanisms have been suggested to improve the stability of oxide TFTs by doping with suitable carrier-suppressing cations to produce a strong BS between the carrier suppressor and oxygen or dissociation energy 4,7,12,13 or with dopants having smaller ionic radius to enhance the BS of the carrier suppressor and oxygen, 15 being key factors in improving the stability of oxide TFTs. However, a strong BS between the carrier suppressor and oxygen does not always yield an effective dopant; For 4224

2 Carbon-Incorporated Amorphous Indium Zinc Oxide TFTs 4225 instance, the higher BS of yttrium oxygen (Y O) 16 and lanthanum oxygen (La O) 8 showed worse performance than the lower BS of tungsten oxide (W O), 11 titanium oxide (Ti O), 12 and doped AOS TFTs. The reason could be that, while adding different ionic charges and sizes effectively enhances the formation of an amorphous phase, it does not produce strong oxygen binders. Similarly, using only the ionic radius of the carrier suppressor is not sufficient for choosing an effective dopant. 15 The ionic radii of manganese (Mn 7+ ) and sulfur (S 6+ )are smaller than that of Si 4+, but the BS values of Mn O, 17 and S O 17 are lower than that of Si O, 17 and they do not improve TFT stability. Determining an effective dopant for improving TFT stability relies on both the ionic radius and the BS of the carrier suppressor. No clear mechanism has been suggested for improving AOS mobility; however, selecting dopants based on their Lewis acid strength (L) has produced high mobility in transparent conducting oxide (TCO) thin films Doping using a suitable carrier suppressor, based on L values, can improve AOS TFT mobility. In this study, we propose selecting dopants based on their L value and strong BS, with oxygen as a strong carrier suppressor and using an oxygen binder to improve the mobility and stability. The empirical relationship associated with the electronegativity (v), ionic radius (r), and effective charge (Z) of dopants proposed by Zhang 18 could be useful for choosing dopants that improve mobility: L ¼ Z 7:7v þ 8:0: (1) r2 The dopant r and v are associated with higher Z. Dopants with a high value of Z/r 2 will more strongly polarize the electron cloud of the oxygen 2p 6 valence band. Thus, it can screen its charge, weakening it as a scattering center and thereby enhancing the mobility. 19 Based on the high L value of dopants, several high-mobility TCO thin films have been developed. 20 AOS TFTs doped with Hf (L = 1.464), Ta (L = 1.734), Zr (L = 2.043), Ti (L = 3.064), W (L = 3.158), and Si (L = 8.096) can be explained in the same manner We chose carbon (C) to confirm the role of a high-l dopant in IZO TFTs. C has a high L of and Z/ r 2 of This high L of carbon (32.917) can screen its charge, weakening it as a scattering center and enhancing the TFT mobility. The carbon oxygen (C O) ( kj/mol) bonding is strong, much stronger than either zinc oxygen (Zn O) (159 kj/mol) or indium oxygen (In O) (320.2 kj/mol) bonding, and can enhance the TFT stability. 17 The stronger BS between C and O could suppress the creation of V O in the carbon indium zinc oxide (CIZO) system and carrier generation via the formation of V O. EXPERIMENTAL PROCEDURES a-izo and a-cizo thin films were deposited on SiO 2 (100 nm)/p-si substrates at room temperature by a radiofrequency cosputtering method using carbon (99.99%) and IZO (99.99%) (ZnO: In 2 O 3 = 1:1 wt.%) targets (LTS Chemicals, Orangeburg, NY). The 4-inch circular targets were placed 10 cm from the substrate. The RF power of the IZO target was fixed at 100 W, while the RF power of the C target was varied to 0 W, 60 W, 70 W, and 80 W (referred to hereinafter as IZO, 1CIZO, 2CIZO, and 3CIZO C-incorporated thin films, respectively). The chamber pressure was set to 5 mtorr with constant flows of Ar of 24 sccm and O 2 of 4 sccm. The amorphous structure was confirmed by x-ray diffraction (XRD) analysis (SmartLab, Rigaku) and high-resolution transmission electron microscopy (HR-TEM; JSM-ARM200F). For x-ray photoelectron spectroscopy (XPS) measurements, the film surface was argon-ion etched for 120 s under high vacuum to avoid surface contamination and exposure to environmental air. XPS (Thermo Scientific) measurements were used to analyze the composition of the CIZO thin films, and atomic force microscopy (Park-XE-100) was used to examine their surface microstructures. Figure 1 shows a schematic cross-section of a bottom-gate inverted staggered a-cizo TFT. The p-type silicon and thermally oxidized SiO 2 (100 nm) act as a gate electrode and insulator, respectively. Maskless lithography and the lift-off technique were used to pattern an active layer area of 1000 lm lm. Direct-current (DC) magnetron sputtering was used to deposit a 100-nm thickness of molybdenum metal for source and drain electrodes, which were then patterned by maskless lithography using a light source (wavelength 405 nm, DL-1000; Nano Systems Solutions) and a lift-off technique. The channel width and length were 100 lm and 50 lm, respectively. Finally, the TFT devices were subjected to thermal annealing at 150 C for 1 h under atmospheric conditions. The transfer and output characteristics of the TFTs were measured at room temperature under dark and ambient conditions using a semiconductor parameter analyzer (SCS 4200; Keithley). Fig. 1. Cross-sectional view of a TFT.

3 4226 Parthiban, Park, Kim, Yang, and Kwon RESULTS AND DISCUSSION Figure 2a shows the XRD patterns of the a-izo and a-cizo films as a function of the carbon sputtering power. No diffraction peaks for the crystalline phase appear in the XRD spectra. Figure 2b shows the amorphous characteristics of the 3CIZO thin film, and the amorphous nature of the IZO system was confirmed by TEM (not shown here). The HR-TEM images and fast Fourier transform (FFT) pattern clearly indicate that the 3CIZO film grew uniformly and displayed an amorphous phase. The atomic force microscopy image of the 3CIZO thin-film surface in Fig. 2c shows a uniform and smooth morphology with root-mean-square (RMS) roughness of 0.5 nm, which is suitable for display applications. Figure 3a shows that the O 1s peak of 3CIZO thin film, which was deconvoluted using a Gaussian profile, exhibited a peak at around ev along with a broad shoulder peak at ev. The peak at ev is attributed to oxygen, whereas the peak at ev is attributed to oxygen deficiency Figure 3b shows that the C 1s peak of 3CIZO thin film, deconvoluted using a Gaussian profile, exhibited peaks at ev and ev. The peak at ev is attributed to carbon (C C) The peak at ev is assigned to carbonate, where carbon is bonded to oxygen atoms. 26 The elemental content of the 3CIZO thin film as estimated from the C 1s, In 3d 5,Zn2p 3, and O 1s regions was 1.14 at.%, at.%, at.%, and at.%. Figure 4a shows the transfer curves of a-izo and a-cizo (with different C ratios) TFTs, and Table I gives their properties. At drain-to-source voltage (V DS ) of 15 V, the a-3cizo TFTs exhibited good transfer characteristics, such as l sat of 16.5 cm 2 V 1 s 1, threshold voltage (V th ) of 2.47 V, subthreshold swing (ss) of 0.76 V/decade, and an I on / I off ratio of In addition, the forward and reverse sweep of the TFT s hysteresis was negligible, even for the process temperature of 150 C. The shift of the turn-on voltage after carbon incorporation into the a-izo TFT shows that carbon acts as an effective carrier suppressor. Figure 4b (drain-tosource current I DS versus V DS ) shows the output characteristics obtained from the a-3cizo TFT. The observed clear pinch-off and drain saturation indicate that the gate and drain voltages controlled the electron transportation in the active channel. In addition, the metallic and semiconductor conduction of the TFT showed no current-crowding phenomenon in the low-drain-voltage regime. The a-3cizo TFTs exhibited good transfer characteristics at drain to source voltage (V DS ) = 15 V, such as l sat of 16.5 cm 2 V 1 s 1, a threshold voltage Fig. 2. (a) XRD patterns of 70-nm-thick a-izo and a-cizo thin films deposited on Corning glass substrates. (b) HR-TEM image and corresponding FFT patterns of a-3cizo films (inset), clearly showing amorphous patterns. (c) Atomic force microscopy image of a-3cizo (15 nm)/sio 2 (100 nm)/p-si. Fig. 3. XPS results of 70-nm-thick 3CIZO film deposited on p-si substrate. The data show the peak position of (a) O 1s and (b) C 1s.

4 Carbon-Incorporated Amorphous Indium Zinc Oxide TFTs 4227 Fig. 4. (a) Transfer characteristics of a-izo and a-cizo TFTs, and (b) output curve of a-3cizo TFT. Table I. TFT properties of a-izo and a-cizo devices C Sputtering l sat Power (W) V th (V) I on /I off (cm 2 /V s) ss (V/dec) 0 (IZO) (1CIZO) (2CIZO) (3CIZO) (V th ) of 2.47 V, ss of 0.76 V/decade, and an I on /I off ratio of In addition, the forward and reverse sweep of the TFTs hysteresis is negligible, even at a process temperature of 150 C. The l sat was estimated using the following equation: W I D ¼ C i l sat L ðv G V th Þ 2 ; (2) where C i is the capacitance per unit area of the gate dielectric (estimated to be F/cm 2 based on a dielectric constant of 3.9 for SiO 2 ), and W and L are the channel width and length, respectively. CONCLUSIONS We demonstrated fabrication of high-performance TFTs using a-cizo as a new channel layer. The fabricated a-cizo TFT postannealed at 150 C exhibited good electrical properties, with l sat of 16.5 cm 2 V 1 s 1, I on /I off ratio of , and ss of 0.76 V/decade. We believe that a-cizo film is a strong candidate for next-generation flat-panel display applications as an alternative to current oxide TFT materials, and can also be applied to flexible electronics. In addition, incorporation of a dopant having a high Lewis acid strength and strong bonding strength into the AOS could be a useful method to produce high-performance oxide materials as channel layers for TFTs, which may enable the realization of next-generation flat-panel displays. ACKNOWLEDGEMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2013 R1A1A ). REFERENCES 1. T. Kamiya and H. Hosono, NPG Asia Mater. 2, 15 (2010). 2. R. Hayashi, A. Sato, M. Ofuji, K. Abe, H. Yabuta, M. Sano, H. Kumomi, K. Nomura, T. Kamiya, and M. Hirano, SID Symp. Digest. 39, 621 (2008). 3. T. Arai, J. Soc. Inf. Disp. 20, 156 (2012). 4. J.S. Park, K. Kim, Y.G. Park, Y.G. Mo, H.D. Kim, and J.K. Jeong, Adv. Mater. 21, 329 (2009). 5. J.-Y. Kwon, J.S. Jung, K.S. Son, K.-H. Lee, J.S. Park, T.S. Kim, J.-S. Park, R. Choi, J.K. Jeong, B. Koo, and S.Y. Lee, Appl. Phys. Lett. 97 (2010). 6. H. Yong Chong, K. Wan Han, Y. Soo No, and T. Whan Kim, Appl. Phys. Lett. 99, (2011). 7. E. Chong, Y.S. Chun, and S.Y. Lee, Appl. Phys. Lett. 97, (2010). 8. P. Jae-Chul, K. Sang-Wook, K. Chang-Jung, and L. Ho-Nyeon, IEEE Electron Device Lett. 33, 685 (2012). 9. J.C. Park, S.W. Kim, C.J. Kim, and H.-N. Lee, IEEE Electron Device Lett. 33, 809 (2012). 10. K. Ebata, S. Tomai, Y. Tsuruma, T. Iitsuka, S. Matsuzaki, and K. Yano, Appl. Phys. Express 5, 1102 (2012). 11. S. Aikawa, P. Darmawan, K. Yanagisawa, T. Nabatame, Y. Abe, and K. Tsukagoshi, Appl. Phys. Lett. 102, (2013). 12. S. Aikawa, T. Nabatame, and K. Tsukagoshi, Appl. Phys. Lett. 103, (2013). 13. L. Lan, N. Xiong, P. Xiao, M. Li, H. Xu, R. Yao, S. Wen, and J. Peng, Appl. Phys. Lett. 102, (2013). 14. H. Kumomi, S. Yaginuma, H. Omura, A. Goyal, A. Sato, M. Watanabe, M. Shimada, N. Kaji, K. Takahashi, and M. Ofuji, J. Display Technol. 5, 531 (2009). 15. J.W. Hennek, J. Smith, A. Yan, M.-G. Kim, W. Zhao, V.P. Dravid, A. Facchetti, and T.J. Marks, J. Am. Chem. Soc. 135, (2013). 16. H.S. Shin, G.H. Kim, W.H. Jeong, B. Du Ahn, and H.J. Kim, Jpn. J. Appl. Phys. 49, 03CB01 (2010). 17. J. Kerr, CRC Handbook of Chemistry and Physics, 81st ed. (Boca Raton, FL: CRC Press, 2000). 18. Y. Zhang, Inorg. Chem. 21, 3886 (1982). 19. G. Campet, S.D. Han, S.J. Wen, J.P. Manaud, J. Portier, Y. Xu, and J. Salardenne, Mater. Sci. Eng. B. 19, 285 (1993). 20. S. Calnan and A. Tiwari, Thin Solid Films 518, 1839 (2010).

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