http://dx.doi.org/10.5573/JSTS.2014.14.5.666 JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.14, NO.5, OCTOBER, 2014 High Performance p-type SnO thin-film Transistor with SiOx Gate Insulator Deposited by Low-Temperature PECVD Method Myeonghun U1, Young-Joon Han1, Sang-Hun Song1, In-Tak Cho2, Jong-Ho Lee2, and Hyuck-In Kwon1,* Abstract—We have investigated the gate insulator effects on the electrical performance of p-type tin monoxide (SnO) thin-film transistors (TFTs). Various SnO TFTs are fabricated with different gate insulators of a thermal SiO2, a plasma-enhanced chemical vapor deposition (PECVD) SiNx, a 150 oCdeposited PEVCD SiOx, and a 300 oC-deposited PECVD SiOx. Among the devices, the one with the 150oC-deposited PEVCD SiOx exhibits the best electrical performance including a high field-effect mobility (=4.86 cm2/Vs), a small subthreshold swing (=0.7 V/decade), and a turn-on voltage around 0 (V). Based on the X-ray diffraction data and the localizedtrap-states model, the reduced carrier concentration and the increased carrier mobility due to the small grain size of the SnO thin-film are considered as possible mechanisms, resulting in its high electrical performance. Index Terms—P-type SnO TFTs, gate insulator, PECVD SiOx, localized-trap-states model I. INTRODUCTION Oxide semiconductor-based thin-film transistors (TFTs) have attracted considerable attention due to their Manuscript received Jun. 12, 2014; accepted Aug. 11, 2014 1 School of Electrical and Electronics Engineering, Chung-Ang University, Seoul 156-756, Korea 2 Department of Electrical and Computer Engineering, Seoul National University, Seoul 151-742, Korea E-mail : [email protected] excellent electrical performances.  However, compared to the n-type oxide TFTs including zinc oxide (ZnO) and amorphous indium-gallium-zinc oxide (aIGZO) TFTs, the p-type oxide TFTs have suffered from the low hole mobility, which has prevented the implementation of complementary logic-based circuits with the n-type oxide TFTs. Since the work from Ogo et al. in 2008 , tin monoxide (SnO) has attracted special attention as a channel material of the p-type oxide TFT due to the possibility for high hole mobilities, and various research results have been reported about the SnO-based p-type oxide TFTs. Liang et al.  investigated the effects of vacuum annealing on the electrical performance of SnO TFTs, and Okamura et al.  reported the SnO TFT fabricated using the solution process. Recently, Frescas et al.  tried to optimize the oxygen partial pressure and total pressure during the channel deposition by DC magnetron sputtering, and successfully demonstrated the record high mobility (~ 6.75 cm2/Vs) for p-type SnO TFTs on glass substrates. Various dielectrics including Al2Ox, thermal SiO2, and HfO2 were used as gate insulators of SnO TFTs [2-6]. However, as of yet, no comparative report has been made on the effects of gate insulator in p-type SnO TFTs although the gate insulator can strongly affect the electrical performance of field-effect transistors including TFTs . In this article, for the first time to the best of our knowledge, we comparatively investigate the effects of gate insulators on the electrical performance of p-type SnO TFTs. The experimental results show that it is very important to optimize the gate insulator to JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.14, NO.5, OCTOBER, 2014 667 improve the electrical performance in p-type SnO TFTs. II. EXPERIMENTS The experiments were performed on the p-type bottom gate SnO TFTs, where a heavily doped n-type silicon wafer was used as a substrate and a gate electrode. Four different dielectric layers including a thermal SiO2 (= 116 nm), a SiNx (= 110 nm) deposited by plasma-enhanced chemical vapor deposition (PECVD) at 250oC, and two SiOxs deposited by PECVD at 150 (= 113 nm) and 300oC (= 99 nm) were incorporated into the devices as gate insulators. A thermal SiO2 was grown by dry oxidation at 1000oC, and two SiOx layers were formed in different commercial foundries under different deposition conditions, respectively. After the formation of gate insulators, a 10 nm-thick SnO thin-film was deposited onto the insulator/Si substrates by RF magnetron sputtering with a 3 '' diameter metallic Sn target. The sputtering condition is as follows: a RF power of 50 W, a working pressure of 3 mTorr, a substrate temperature of 90oC, a substrate-to-target distance of 140 mm, and a gas (Ar + O2) mixing ratio of Ar/O2 = 90/6 (sccm/sccm). For the source/drain electrodes, a 50-nm-thick Ni was deposited at RT using an e-gun evaporator. All the layers in this work were patterned using shadow masks. The dielectric constants of the gate insulators are extracted to be 3.8, 8.7, 8.3, and 6.5 for a thermal SiO2, a PECVD SiNx, a 150oC-deposited PEVCD SiOx, and a 300oCdeposited PECVD SiOx, respectively. The relatively high dielectric constants of PEVCD SiOxs are believed to be caused from the hydroxyl group inside the PEVCD SiOx . The hydroxyl group in the PECVD SiOx has been known to increase the dielectric constant of the PECVD SiOx, and the low-temperature deposited PECVD SiOx can have more hydroxyl group due to the silanol and water impurities contained in the SiOx . III. RESULTS AND DISCUSSION Fig. 1(a) shows the representative transfer curve of the p-type SnO TFTs fabricated with a 150oC-deposited PECVD SiOx gate insulator. Measurements were made for a device with a channel width/length (W/L) of 1000 μm/50 μm at a drain-to-source voltage (VDS) of -2 V. The curve clearly exhibits the p-type behavior with a field- Fig. 1. Representative (a) transfer, (b) output characteristics of the p-type SnO TFTs fabricated with a 150oC-deposited PECVD SiOx gate insulator. effect mobility (μFE) of 4.86 cm2/Vs, a subthreshold swing (SS) of 0.7 V/decade, a threshold voltage (Vth) of 2.3 V, a turn-on voltage (Von) of -0.1 V, and a current on/off ratio (Ion/off) of ~ 3 × 104, where μFE was determined by the maximum transconductance at a VDS of -2.0 V , and Vth was estimated by the intercept of the extrapolated curve with a voltage axis. When compared with p-type SnO TFTs reported in previous works, the extracted μFE is the second largest value, after that of the record mobility device fabricated by Frescas et al. , however, our device exhibits much steeper SS and a more negatively shifted Von compared to the device of Frescas et al. . Fig. 1(b) depicts the representative output curves of the fabricated TFTs using a 150oCdeposited PECVD SiOx gate insulator, which exhibit a clear pinch-off and a current saturation. Fig. 2 compares the transfer curves and gate leakage currents of the p-type SnO TFTs fabricated using various gate insulators. It shows that the devices with different gate insulators exhibit much different transfer curves, which implies that the electrical performance of p-type SnO TFT strongly depends on the gate insulator. As shown in Fig. 1(a), the transfer curve of the SnO TFT 668 MYEONGHUN U et al : HIGH PERFORMANCE P-TYPE SNO THIN-FILM TRANSISTOR WITH SIOX GATE INSULATOR … Fig. 2. Transfer curves and gate leakage currents of the p-type SnO TFTs fabricated using various gate insulators of a thermal SiO2, a PECVD SiNx, a 150oC-deposited PEVCD SiOx, and a 300oC-deposited PECVD SiOx. fabricated using a 150oC-deposited PECVD SiOx shows a small SS, a Von around 0 (V), and a relatively high Ion. However, the devices fabricated using different gate insulators of a thermal SiO2, a PECVD SiNx, and a 300oC-deposited PECVD SiOx exhibit a much larger SS, a more positively shifted Von, and a relatively lower Ion compared to the device with a 150oC-deposited PECVD SiOx even though the SnO channel layer is deposited under the same conditions in all devices. Fig. 2 also shows that the gate leakage currents of the device with a 150oC-deposited PECVD SiOx exhibit relatively larger values than those of the devices with other gate insulators. To investigate the physical and chemical mechanism of the observed phenomenon in Fig. 2, we compare the X-ray diffraction (XRD) patterns of 10 nm-thick SnO thin-films deposited on various gate insulators (Fig. 3). In Fig. 3, the crystalline peaks corresponding to the (001), (101), (002), (112), and (103) directions of the SnO phase are clearly observed in the XRD patterns of SnO thin-films deposited on a thermal SiO2 and a PECVD SiNx gate insulators. However, only a small and broad peak corresponding to the (101) direction of the SnO phase is observed in a XRD pattern of the SnO thin-film deposited on a 150oC-deposited PECVD SiOx gate insulator, which implies that the SnO thin-film on a 150oC-deposited PECVD SiOx gate insulator has a small grain size and a low crystallinity. The XRD pattern of the SnO thin-film on a 300oC-deposited PECVD SiOx gate insulator exhibits a SnO (101) peak larger than that in the Fig. 3. XRD patterns of 10 nm-thick SnO thin-films deposited on various gate insulators of a thermal SiO2, a PECVD SiNx, a 150oC-deposited PEVCD SiOx, and a 300oC-deposited PECVD SiOx. XRD pattern of the thin-film on a 150oC-deposited PECVD SiOx gate insulator. From the XRD data in Fig. 3, a plausible mechanism which can explain the results in Fig. 2 can be suggested based on the Seto’s localized-trap-states model . Generally, the small grain size of the polycrystalline thinfilm has been known to decrease the carrier mobility due to the increased number of grain boundaries. However, several papers regarding the polycrystalline silicon thinfilm  and pentacene-based organic TFTs  reported the opposite trend between mobility and grain size (increasing mobility with a decreasing grain size), and this inconsistency has been explained based on the Seto’s localized-trap-states model. The localized-trapstates model correlates the grain size of the polycrystalline thin-film with the carrier concentration and carrier mobility based on the assumption that crystallites of the polycrystalline thin-films have very low trap state density inside the grain, and that most trap states are localized at the grain boundary. Figs. 4(a)-(c) schematically show the variation of the trap density and the resulting band diagram with respect to the grain size in polycrystalline p-type semiconductor materials [11, 12]. Here, Nt is the trap density per area, L is the average grain size, n is the carrier (hole) concentration, and Eb is the potential barrier height at the grain boundary. In the localized-trap-states model, the small grain size reduces the free carrier concentration inside the polycrystalline semiconductor thin-film, because the free JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.14, NO.5, OCTOBER, 2014 Fig. 4. Schematic diagram of localized-trap-states model and corresponding band structure in polycrystalline p-type semiconductor materials. Along with the grain size, the band structure varies from (a) to (c). carriers are trapped by the localized trap states at grain boundaries. Moreover, when a number of trap states per unit volume is larger than the free carrier concentration due to the large number of grain boundaries, the trap states are not completely filled by free carriers, which reduces the potential barrier height at the grain boundaries as depicted in Fig. 4(a) and increases the carrier mobility inside the polycrystalline thin-films due to the reduced grain boundary scattering . As well known, a reduced carrier concentration inside the thinfilm enhances the current modulation by the gate voltage in TFTs, and moves Von to the negative direction in ptype TFTs. In Fig. 2, the p-type SnO TFT fabricated using a 150oC-deposited PECVD SiOx exhibits a higher μFE, a smaller SS, and a negatively shifted Von compared to the devices with a thermal SiO2 and a PECVD SiNx gate insulators. Considering a small and broad peak in the XRD pattern of the SnO thin-film deposited on a 150oC-deposited PECVD SiOx gate insulator in Fig. 3, the results in Fig. 2 is well explained by the localizedtrap-states model. We made Hall measurements to investigate the electrical properties of SnO thin-films on different gate insulators, but good Hall data were not available due to the low conductivities in all samples. Fig. 5 shows the atomic force microscopy (AFM) images for different gate insulators deposited on a heavily doped n-type silicon wafer. It shows that a 150oC-deposited PECVD SiOx gate insulator exhibits the largest root mean square (RMS) roughness among various gate insulators. From the previous work , a roughening transition temperature of PECVD SiOx was reported as ~ 250oC. Above this temperature, the SiOx 669 Fig. 5. AFM images for different gate insulators of (a) a thermal SiO2 (= 116 nm), (b) a PECVD SiNx (= 110 nm), (c) a 150oC-deposited PEVCD SiOx (= 113 nm), (d) a 300oCdeposited PECVD SiOx (= 99 nm) deposited on a heavily doped n-type silicon wafer. thin-film is dense and smooth, but below this was reported as ~ 250oC. Above this temperature, the SiOx thin-film is dense and smooth, but below this deposition temperature, the SiOx thin-film becomes porous and rough. The rough surface of 150oC-deposited PECVD SiOx in Fig. 5 and the relatively large gate leakage current of the device with a 150oC-deposited PECVD SiOx gate insulator in Fig. 2 are consistent with the report in the previous work. The increased surface roughness of the gate insulator can decrease the grain size of the thinfilm, because it reduces the energy barrier for nucleation during the formation of the nuclei . The smallest grain size deduced from the XRD data in a SnO thin-film on a 150oC-deposited PECVD SiOx gate insulator is believed to be caused from the roughest surface of a 150oC-deposited PECVD SiOx gate insulator. IV. CONCLUSIONS In this article, we examine the effects of gate insulators on the electrical performance of p-type SnO TFTs. Various SnO TFTs are fabricated with different gate insulators of a thermal SiO2, a PECVD SiNx, a 150oCdeposited PEVCD SiOx, and a 300oC-deposited PECVD SiOx, and the device fabricated with a 150oC-deposited PEVCD SiOx exhibits a much higher μFE, a smaller SS, and a more negatively shifted Von compared to the devices with other gate insulators. From the XRD data, 670 MYEONGHUN U et al : HIGH PERFORMANCE P-TYPE SNO THIN-FILM TRANSISTOR WITH SIOX GATE INSULATOR … the peaks corresponding to the crystalline SnO phase are clearly observed in the SnO thin-films on a thermal SiO2 and a PECVD SiNx gate insulators, but only a small and broad peak corresponding to the (101) direction of the SnO phase is observed in the SnO thin-film deposited on a 150oC-deposited PECVD SiOx gate insulator. It implies that the SnO thin-film on a 150oC-deposited PECVD SiOx gate insulator has a small grain size and a low crystallinity. Based on the localized-trap-states model, the reduced carrier concentration and the increased carrier mobility due to the small grain size of the SnO thin-film are considered as plausible mechanisms resulting in the high electrical performance of the p-type SnO TFT fabricated with a 150oC-deposited PECVD SiOx gate insulator. ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIP) (No. 2014-005368) and by the ChungAng University Research Scholarship Grants in 2013. REFERENCES      M. Nakata, K. Takechi, K. Azuma, E. Tokumitsu, H. Yamaguchi, and S. Kaneco, “Improvement of InGaZnO4 Thin Film Transistors Characteristics Utilizing Excimer Laser Annealing,” Appl. Phys. Express, vol.2, pp.021102, 2009 Y. Ogo, H. Hiramatsu, K. Nomura, H. Yanagi, T. Kamiya, M. Hirano, and H. Hosono, “P-channel thin-film transistor using p-type oxide semiconductor, SnO,” Appl. Phys. Lett., vol.93, pp. 032113, 2008. L. Y. Liang, Z. M. Liu, H. T. Cao, Z. Yu, Y. Y. Shi, A. H. Chen, H. Z. Zhang, Y. Q. Fang, and X. L. Sun, “Phase and optical characterizations of annealed SnO thin films and their p-type TFT applications,” J. Electrochem. Soc., vol.157, pp.H598, 2010. K. Okamura, B. Nasr, R. A. Brand, and H. 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Sidorov, “Surface roughness of SiO2 from a remote microwave plasma enhanced chemical vapor deposition process,” J. Vac. Sci. Technol. B, vol.16, pp.2165, 1998. S. Steudel, S. D. Vusser, S. D. Jonge, D. Janssen, S. Verlaak, J. Genoe, and P. Heremans, “Influence of the dielectric roughness on the performance of pentacene transistors,” Appl. Phys. Lett., vol.85, pp.4400, 2004. JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.14, NO.5, OCTOBER, 2014 Myeonghun U received a bachelor’s degree in physics from Dongguk University, Seoul, Korea in 2012. He is currently a graduate school student in the School of Electrical and Electronics Engineering at ChungAng University in Seoul. His current research interests include oxide thin-film transistors. Young-Joon Han received a bachelor’s degree in Electrical and Electronics Engineering from ChungAng University, in 2013. He is currently a graduate school student in the School of Electrical and Electronics Engineering at Chung-Ang University in Seoul. His current research interests include Fabrication of ptype SnO Thin-Film Transistors. Sang-Hun Song received his BS degree in Electronics Engineering from Seoul National University in 1986 and his MA and Ph.D. degrees both from Princeton University in 1988 and 1997 respectively. His doctoral research studied on magneto optical and magneto transport properties of the 2 dimentional carriers in strained semiconductor layers. In 1997, he joined LG Semincon Co., Ltd. as a DRAM circuit designer. In 2001, he joined the School of Electrical and Electronics Engineering at Chung-Ang University in Seoul, where he is now a professor. In 2007, he was a visiting scholar to GLAM at Stanford University. His research interests include semiconductor physics and devices, memory devices, nanoelectronics and novel techniques in semiconductor characterization. Recently, he is working on sensors that can detect force and mass. 671 In-Tak Cho received B.S and M.S degrees in electrical engineering from Kyungpook National University, Seoul, Korea in 2008 and 2010, respectively. He is currently working toward the Ph.D. degree in electrical engineering at Seoul National University, Seoul, Korea. His research interest is metal oxide semiconductor TFT and TMDC FET. Jong-Ho Lee received the Ph.D. degree from Seoul National University, Seoul, in 1993, in electronic engineering. In 1994, he was with the School of Electrical Engineering, Wonkwang University, Iksan, Chonpuk, Korea. In 2002, he moved to Kyungpook National University, Daegu Korea, as a Professor of the School of Electrical Engineering and Computer Science. Since September 2009, he has been a Professor in the School of Electrical Engineering, Seoul National University, Seoul Korea. From August 1998 to July 1999, he was with Massachusetts Institute of Technology, Cambridge, as a postdoctoral fellow. He has authored or coauthored more than 160 papers published in refereed journals and over 280 conference papers related to his research and has been granted ~90 patents in this area. His research interests include CMOS technology, non-volatile memory devices, thin film transistors, sensors, bio interface, and neuromorphic technology. He has been served as a subcommittee member of IEDM, ITRS ERD member, a general chair of IPFA2011, and IEEE EDS Korea chapter chair. In 2006, he was a recipient of the “This Month’s Scientist Award” for his contribution in the development of practical highdensity/high-performance 3-dimensional nano-scale CMOS devices. He invented Saddle FinFET (or recess FinFET) for DRAM cell and NAND flash cell string with virtual source/drain, which have been applying for mass production. 672 MYEONGHUN U et al : HIGH PERFORMANCE P-TYPE SNO THIN-FILM TRANSISTOR WITH SIOX GATE INSULATOR … Hyuck-In Kwon received the B.S., M.S., and Ph.D. degrees in electrical engineering from Seoul National University, Seoul, in 1999, 2001, and 2005, respectively. From August 2004 to March 2006, he was a Research Associate with the University of Illinois, Urbana. In 2006, he joined the System LSI Division, Samsung Electronics Company, Korea, where he was a Senior Engineer with the Image Development Team. From September 2007 to February 2010, he worked for the School of Electronic Engineering in Daegu University as a Full-Time Lecturer and an assistant professor. Since 2010, he has been with Chung-Ang University, Seoul, Korea, where he is currently an associate professor in the School of Electrical and Electronics Engineering. His research interests include CMOS active pixel image sensors, oxide thin-film transistors, GaN-based power devices, and silicon nanotechnologies.
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