Atomic layer deposition ZnO:N flexible thin film transistors and the effects of bending on device properties Jae-Min Kim, Taewook Nam, S. J. Lim, Y. G. Seol, N.-E. Lee, Doyoung Kim, and Hyungjun Kim Citation: Applied Physics Letters 98, 142113 (2011); doi: 10.1063/1.3577607 View online: http://dx.doi.org/10.1063/1.3577607 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/98/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Compositional tuning of atomic layer deposited MgZnO for thin film transistors Appl. Phys. Lett. 105, 202109 (2014); 10.1063/1.4902389 Effect of Al concentration on Al-doped ZnO channels fabricated by atomic-layer deposition for top-gate oxide thin-film transistor applications J. Vac. Sci. Technol. B 32, 041202 (2014); 10.1116/1.4880823 Schottky barrier source-gated ZnO thin film transistors by low temperature atomic layer deposition Appl. Phys. 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Downloaded to IP: 129.105.215.146 On: Tue, 23 Dec 2014 00:16:59 http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://oasc12039.247realmedia.com/RealMedia/ads/click_lx.ads/www.aip.org/pt/adcenter/pdfcover_test/L-37/1940596036/x01/AIP-PT/Keysight_APLArticleDL_121714/en_keysight_728x90_3325-2Pico.png/47344656396c504a5a37344142416b75?x http://scitation.aip.org/search?value1=Jae-Min+Kim&option1=author http://scitation.aip.org/search?value1=Taewook+Nam&option1=author http://scitation.aip.org/search?value1=S.+J.+Lim&option1=author http://scitation.aip.org/search?value1=Y.+G.+Seol&option1=author http://scitation.aip.org/search?value1=N.-E.+Lee&option1=author http://scitation.aip.org/search?value1=Doyoung+Kim&option1=author http://scitation.aip.org/search?value1=Hyungjun+Kim&option1=author http://scitation.aip.org/content/aip/journal/apl?ver=pdfcov http://dx.doi.org/10.1063/1.3577607 http://scitation.aip.org/content/aip/journal/apl/98/14?ver=pdfcov http://scitation.aip.org/content/aip?ver=pdfcov http://scitation.aip.org/content/aip/journal/apl/105/20/10.1063/1.4902389?ver=pdfcov http://scitation.aip.org/content/avs/journal/jvstb/32/4/10.1116/1.4880823?ver=pdfcov http://scitation.aip.org/content/avs/journal/jvstb/32/4/10.1116/1.4880823?ver=pdfcov http://scitation.aip.org/content/aip/journal/apl/103/25/10.1063/1.4836955?ver=pdfcov http://scitation.aip.org/content/aip/journal/apl/92/19/10.1063/1.2924768?ver=pdfcov http://scitation.aip.org/content/aip/journal/apl/91/18/10.1063/1.2803219?ver=pdfcov Atomic layer deposition ZnO:N flexible thin film transistors and the effects of bending on device properties Jae-Min Kim,1 Taewook Nam,1 S. J. Lim,2 Y. G. Seol,3 N.-E. Lee,3 Doyoung Kim,1 and Hyungjun Kim1,a� 1School of Electrical and Electronic Engineering, Yonsei University, 262 Seongsanno, Seodaemun-Gu, Seoul 120-749, Republic of Korea 2Department of Materials Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea 3School of Advanced Materials Science and Engineering and Center for Advanced Plasma Surface Technology, Sungkyunkwan University, Suwon, Kyunggi-do 440-746, Republic of Korea �Received 9 December 2010; accepted 22 March 2011; published online 7 April 2011� ZnO:N flexible thin film transistors were fabricated by atomic layer deposition on polyethylene naphthalate substrates and the effects of bending on the device properties investigated. The threshold voltage and saturation mobility were observed to change with respect to the amount of substrate bending. These modulations can be explained in terms of piezoelectric nature of in ZnO. In comparison with the previously reported single crystal nanowires ZnO field effect transistors, the amount of the electrical property modulation under bent condition is significantly reduced and our report shows a much improved stability for ZnO:N as a flexible device material. © 2011 American Institute of Physics. �doi:10.1063/1.3577607� Recently, ZnO based semiconductors have attracted great interest as the channel layer in flexible displays.1,2 They have high mobility and high transparency with low growth temperature, which is a crucial prerequisite to be employed for flexible/transparent applications. To date, oxide semicon- ductor thin film transistors �TFTs� have been mostly depos- ited by physical vapor deposition but having problems re- lated with uniformity and resistivity control.3–5 In contrast, atomic layer deposition �ALD� has great advantages such as precise thickness controllability and large area uniformity at relatively low growth temperatures.6 Such characteristics make ALD very attractive for the TFT fabrication on large area flexible substrate. Regarding the reliability issues for flexible displays, the devices are required to have strong stability under bending conditions since the substrates are frequently subjected to large tensile or compressive strain, which would cause sub- stantial changes in the device properties. Oh et al.7 showed a complementary inverter using n-channel sputtered ZnO and p-channel pentacene channels on a polyethersulfone flexible substrate. Besides, a single crystal ZnO nanowire field effect transistor �FET� was fabricated on a flexible substrate by Kwon et al.8 They reported that the threshold voltage �VTH� and channel mobility was significantly changed by substrate bending, strongly dependent on the direction of bending and the radius of curvature. However, there has been no previous report on the bending reliability of ALD ZnO thin film based flexible TFT devices. In this letter, we fabricated ALD nitro- gen doped ZnO flexible TFTs on polyethylene naphthalate �PEN� substrates and investigated the change in device prop- erties with respect to the amount of substrate bending. To begin with, 100 nm thick Ti was patterned for a gate electrode. Subsequently, 130 nm thick ALD Al2O3 was de- posited as a gate insulator using trimethyl aluminum and water vapor at a growth temperature �Ts� of 150 °C. Then, a 60 nm thick ZnO:N active layer was prepared at Ts =125 °C using diethyl zinc as a precursor and diluted am- monium hydroxide solution �0.01%� as a single reactant source for oxygen and the nitrogen dopant. The channel area �width / length=40 �m /20 �m� was defined by standard lithographic process, followed by wet etching with a diluted HCl solution �HCl:H2O=1:40�. Finally, a 100 nm thick Ti source/drain layer was patterned by liftoff. The process tem- perature through the entire device fabrication did not exceed 150 °C, which satisfies plastic substrate compatibility. The bending equipment was a specially designed bend- ing tool equipped with a charge coupled device camera to measure the radius of curvature of the flexible device. Both ends of the sample were clamped by the probing pins con- nected to the parameter analyzer �HP 4156B� during mea- surement to obtain transfer and output curves with respect to the amount of bending. The corresponding strain values are calculated according to the following equation, assuming that the TFT layers are approximated by a single continuous film covering on PEN backplane and the elastic modulus of both PEN and ZnO channel layers have similar values,9 Strain �%� = Thickness of PEN + Thickness of TFT 2 � Rc . �1� Positive and negative values indicate upward and downward bending, respectively. We used nitrogen doped ZnO as an active layer since the electrical properties of ALD ZnO TFTs can be effectively controlled by in situ N doping.10 At first, we conducted the electrical property measurements without device bending to examine the feasibility of flexible TFT �FTFT� device operation on flexible PEN substrates. As a result, typical FET behavior was observed from the output and transfer curves showing source-drain current �IDS� satu- ration by drain voltage �VD� sweep and IDS modulation by gate voltage �VG� sweep with VTH=15.2 V as shown in Figs. 1�a� and 1�b�. The IOFF and ION/OFF are measured to be 30 pA and 8�106, and subthreshold voltage swing and satu- ration mobility ��sat� are 0.91 V/dec and 20.9 cm2 /V s, re-a�Electronic mail:
[email protected]. APPLIED PHYSICS LETTERS 98, 142113 �2011� 0003-6951/2011/98�14�/142113/3/$30.00 © 2011 American Institute of Physics98, 142113-1 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.105.215.146 On: Tue, 23 Dec 2014 00:16:59 http://dx.doi.org/10.1063/1.3577607 http://dx.doi.org/10.1063/1.3577607 http://dx.doi.org/10.1063/1.3577607 spectively. This result indicates that the operation of TFT fabricated by the current ALD processes is also extendable to flexible plastic substrate. Next, the bending effects on the FTFT characteristics were investigated according to the amount of substrate bend- ing. Since the path for electron current flow is along the channel/bottom oxide interface due to the formation of the lowest electron potential, the external stresses exerted to this interfacial layer would have the greatest effect on the elec- trical properties of TFT. It should be noted that upward bend- ing induces a compressive strain at the bottom side of active layer adjacent to the gate insulator, whereas a downward bending gives rise to a tensile strain. Thus, the influences caused by device deflection are expected to occur in an op- posite way depending on whether the substrate is bent up- ward or downward since the opposite kinds of strain are applied on the current path. In Figs. 1�a� and 1�b�, IDS in- creases when the device was bent in an upward direction under the same gate and drain voltages and vice versa. The output and transfer curves under device bending between �1.2 and +1.2 of 1 /Rc shows a gradual change in IDS de- pending on the amount of bending but only the extremes are shown for clarity. Figures 2�a� and 2�b� represent the change in VTH and �sat as a function of 1 /Rc, and their percentage changes ref- erenced to the value of normal condition. The value of VTH’s systematically decreased from downward to upward bending in the range of �5%, when 1 /Rc is adjusted from �1.2 to 1.2 cm−1. Meanwhile, the �sat values increased with upward bending from around �2% to 2% change. These results in- dicate that applied strain to the device causes systematic change in the device properties. ZnO is a well-known piezoelectric material having the highest piezoelectric tensor among the tetrahedral bonded semiconductors.11 Deflection of a ZnO thin film creates a relative displacement of Zn2+ cations with respect to O2− anions in the wurzite structure, inducing a strong piezoelec- tric potential field inside the channel area. He et al.12 re- ported that positive potential is induced at the stretched sur- face ���0� and negative potential is induced at the compressed surface ���0� of ZnO material. Likewise, the compressive strain ���0� on the region of channel layer adjacent to gate insulator in ZnO TFT produces positive pi- ezoelectric potential when the device is bent in an upward direction, and vice versa. Thus, the formation of two poten- tials of opposite signs, depending on the bending direction at the electrical path in channel area, can be the main reason for the opposite change in the electrical transport terms of VTH and �sat as shown in Figs. 2�a� and 2�b�. Figures 3�a� and 3�b� illustrate the piezoelectric effect on the band diagram of ZnO:N channel/Al2O3 gate insulator/Ti to explain the possible mechanism of the modulations on the electrical transport behavior in flexible TFTs with respect to substrate bending. As previously discussed, the compressed side of the ZnO channel that interfaces with the gate insula- tor is affected by the negative piezoelectric field when bended upwards �Fig. 3�a�� even without any gate biasing. It induces a lowering of the ZnO:N channel surface potential barrier with downward modulation of energy band structure leading to the negative shift of VTH since less VG is needed to reach the channel accumulation layer for the formation of an electrical current path. Whereas, a positive piezoelectric field is generated at the ZnO channel/gate insulator interface when bended downwards as shown in Fig. 3�b�, resulting in potential barrier highering, leading to the positive shift of VTH. This explanation is in excellent accordance with the change in electrical transport behavior measured in Fig. 2�a�. FIG. 1. �a� and �b� represent the modulations of output and transfer curves when the device is applied to external stress in condition of �1.2 cm−1 of 1 /Rc with reference to unbending condition �1 /Rc=0�. FIG. 2. The changes in �a� VTH and �b� �sat as a function of 1 /Rc, and their percentage variations. 142113-2 Kim et al. Appl. Phys. Lett. 98, 142113 �2011� This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.105.215.146 On: Tue, 23 Dec 2014 00:16:59 The �sat for ZnO TFTs can be calculated according to the following equation:13 �sat = 2Lm2 WCox , m = dIDS 1/2 dVG , �2� where L and W indicate channel length and width, respec- tively. COX defines the capacitance of the alumina dielectric layer with a dielectric constant of 8.8 and the value of which remains a constant value of 8.2 nF /cm2 regardless of sub- strate bending. It should be noted that the �sat is directly related to the value of transconductance �gm=dIDS /dVG� ex- tracted from the slope of IDS versus VG in the current satu- rated region �VG�20 V� of the transfer curve �Fig. 1�b��. The estimated gm for ZnO TFT under upward bending is higher, whereas that for TFT under downward bending is lower than the normal state. Therefore, the mobility changes are mainly due to the variations in gm with respect to the device deflection. This modulation tendency of the electrical transport characteristics under the device bending conditions corresponds to other reports on ZnO material based devices such as ZnO nanowire FETs and flexible ZnO strain sensor from other research groups.8,14 Also, similar reports on bend- ing experiments considering flexible FETs but using penta- cene �p-type� channel instead of oxide semiconductor have shown a similar mobility change depending on the induced substrate strain.15 Although the modulations of IDS, VTH, and �sat caused by bending in our experiment are similar to the previously reported results on ZnO nanowire FETs,8 the current ALD ZnO:N TFT have a much better stability against bending. For example, under 1.1% substrate strain, the VTH and �sat �changes in the values by bending� of the current TFT devices are measured to be 0.5 V of and 0.2 cm2 /V s, re- spectively, which are much smaller than 4.2 V and 1450 cm2 /V s for previously reported ZnO nanowire FET. We attribute this mainly to the difference in microstructure. While ZnO nanowires are single crystal, ALD ZnO:N thin films are composed of nanocrystalline grains. We have pre- viously shown that the incorporation of N in ZnO thin film significantly reduces the crystallinity of the �002� oriented crystal structure, resulting in the formation of a nanocrystal- line or amorphous phase.10 The piezoelectric field in ZnO is critically dependent upon the crystal orientation, showing the strongest piezoelectric response along the c-axis �001�. This implies that the �001� oriented single crystalline ZnO has the strongest piezoelectric field when subjected to strain. In con- trast, the piezoelectric coefficient in a polycrystalline ZnO thin film is degraded since the polar �001� orientation of each crystalline grain exists with some spread of angles, which does not align with the direction of the electric field.16 Thus, an amorphous microstructure for the active layer is expected to have a benefit in terms of device reliability under a bend- ing environment. This research was supported by Future-based Technol- ogy Development Program �Nano Fields� and Basic Re- search Program through the National Research Foundation of Korea �NRF� funded by the Ministry of Education, Science, and Technology �Grant Nos. 2010-0020230 and 2010- 0024066�. 1E. Fortunato, P. Barquinha, A. Pimentel, A. Gonçalves, A. Marques, L. Pereira, and R. Martins, Thin Solid Films 487, 205 �2005�. 2H. Hosono, Thin Solid Films 515, 6000 �2007�. 3Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Dogan, V. Avrutin, S. J. Cho, and H. Morkoc, J. Appl. Phys. 98, 041301 �2005�. 4H. Ohsaki, Y. Tachibana, A. Mitsui, T. Kamiyama, and Y. Hayashi, Thin Solid Films 392, 169 �2001�. 5P. F. Carcia, R. S. McLean, M. H. Reilly, and J. G. Nunes, Appl. Phys. Lett. 82, 1117 �2003�. 6H. Kim, H. Lee, and W. Maeng, Thin Solid Films 517, 2563 �2009�. 7M. S. Oh, W. Choi, K. Lee, D. K. Hwang, and S. Im, Appl. Phys. Lett. 93, 033510 �2008�. 8S. Kwon, W. Hong, G. Jo, J. Maeng, T. Kim, S. Song, and T. Lee, Adv. Mater. �Weinheim, Ger.� 20, 4557 �2008�. 9K. Lee, J. Lee, H. Hwang, Z. Reitmeier, R. Davis, J. Rogers, and R. Nuzzo, Small 1, 1164 �2005�. 10S. J. Lim, J.-M. Kim, D. Kim, S. Kwon, J.-S. Park, and H. Kim, J. Elec- trochem. Soc. 157, H214 �2010�. 11A. Dal Corso, M. Posternak, R. Resta, and A. Baldereschi, Phys. Rev. B 50, 10715 �1994�. 12J. He, C. Hsin, J. Liu, L. Chen, and Z. Wang, Adv. Mater. �Weinheim, Ger.� 19, 781 �2007�. 13D. Neamen, Semiconductor Physics and Devices �McGraw-Hill, New York, 1997�. 14J. Zhou, Y. Gu, P. Fei, W. Mai, Y. Gao, R. Yang, G. Bao, and Z. Wang, Nano Lett. 8, 3035 �2008�. 15T. Sekitani, Y. Kato, S. Iba, H. Shinaoka, T. Someya, T. Sakurai, and S. Takagi, Appl. Phys. Lett. 86, 073511 �2005�. 16J. Gardeniers, Z. Rittersma, and G. Burger, J. Appl. Phys. 83, 7844 �1998�. FIG. 3. ZnO:N channel/Al2O3 gate insulator/Ti gate band diagrams to ex- plain the results of modulation of electrical transport behaviors when �a� upward and �b� downward bending through the possible piezoelectricity mechanism occurred in flexible TFTs in response to external stresses. 142113-3 Kim et al. Appl. Phys. Lett. 98, 142113 �2011� This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 129.105.215.146 On: Tue, 23 Dec 2014 00:16:59 http://dx.doi.org/10.1016/j.tsf.2005.01.066 http://dx.doi.org/10.1016/j.tsf.2006.12.125 http://dx.doi.org/10.1063/1.1992666 http://dx.doi.org/10.1016/S0040-6090(01)01023-9 http://dx.doi.org/10.1016/S0040-6090(01)01023-9 http://dx.doi.org/10.1063/1.1553997 http://dx.doi.org/10.1063/1.1553997 http://dx.doi.org/10.1016/j.tsf.2008.09.007 http://dx.doi.org/10.1063/1.2956406 http://dx.doi.org/10.1002/adma.200800691 http://dx.doi.org/10.1002/adma.200800691 http://dx.doi.org/10.1002/smll.200500166 http://dx.doi.org/10.1149/1.3269973 http://dx.doi.org/10.1149/1.3269973 http://dx.doi.org/10.1103/PhysRevB.50.10715 http://dx.doi.org/10.1002/adma.200601908 http://dx.doi.org/10.1002/adma.200601908 http://dx.doi.org/10.1021/nl802367t http://dx.doi.org/10.1063/1.1868868 http://dx.doi.org/10.1063/1.367959