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Surface-induced time-dependent instability of ZnO based thin-film transistors
Thin Solid Films 517 (2009) 6345–6348
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Surface-induced time-dependent instability of ZnO based thin-film transistors Ki-tae Kim, Kimoon Lee, Min Suk Oh, C.H. Park, Seongil Im ⁎ Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749 Korea
a r t i c l e
i n f o
Available online 21 February 2009 Keywords: ZnO Thin-film transistor Instability Surface Interdigitated structure
a b s t r a c t We report on the surface-induced time-dependent instability of ZnO based thin-film transistors (ZnO-TFTs) with interdigitated source/drain (S/D) electrodes. As time elapsed, a considerable shift of threshold voltage (VT) was observed (by ~− 16 V) from our TFT. Contact angle of de-ionized water on ZnO surface also changed from 30° to 110°, revealing time-dependent surface state change. According to X-ray photoemission spectroscopy (XPS) measurements, the Zn 2p3/2 core-level peak and the valence band maximum (VBM) of aged ZnO surface shifted to the higher binding energy by 0.3 eV, which implies a downward energy band bending of the ZnO back channel-surface. We conclude that without passivation layer any bottom gate ZnOTFT meets the surface-induced electrical instabilities due to the time-dependent conductance of ZnO surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
ZnO based thin-film transistors (ZnO-TFTs) have been potentially applied to various kinds of electrical applications such as drivers for display (LCD or OLED) [1] and logic circuit [2] due to their superior electrical characteristics compared to amorphous Si (a-Si). In order to accomplish any practical device applications with the ZnO based TFTs, their device stabilities should be confirmed beforehand. In particular, since it is very important for ZnO devices to maintain the initial electrical properties in air ambient of normal humidity as a condition for real usage, the time-dependent device properties under those normal ambient conditions should be essentially studied. In fact, only a few studies on the time-dependent instability issues of ZnO-TFTs were reported [3], probably because those issues involve a timedependent variation of ZnO surface state which is difficult to precisely investigate. ZnO surface has a polar character which results from nonstoichiometry with native defects such as Zn interstitials (Zni) and oxygen vacancies (VO) [4–6] and polar surfaces cause relatively high surface energy, leading to unstable surface characteristics [7,8]. In order to reach to thermo-dynamically stable or meta-stable surface states, ZnO surface would react to favorable molecules in ambient air involving H2O, O2, H2 and CO2 etc, so that the surface chemistry of ZnO would be modified resulting in the change of surface conductivity [9]. In the present work, we fabricated ZnO-TFTs with interdigitated structure which has relatively large surface as a back-channel to study and more clearly observe the time-dependent instability of ZnO-TFTs related to the properties of ZnO back-channel surface.
A substrate of thermally grown 200 nm-thick SiO2 on p+-Si (~0.01 Ω cm) was adopted for a gate dielectric and gate electrode, respectively. Prior to deposition of ZnO thin film, the substrate was cleaned with acetone, methyl alcohol and de-ionized water, in that order. A 100 nm-thick undoped ZnO thin film (area of 2.8 × 2.3 mm2) was deposited through a shadow mask at a high substrate temperature of 400 °C by rf magnetron sputtering in a vacuum chamber (with basal pressure of ~ 1 × 10− 6 Torr and working pressure of 10 mTorr composed of the mixture of Ar:O2 = 5:2 and the physical properties of the ZnO films were shown elsewhere [10]). And then, through the second shadow mask with interdigitated structure, a 200 nm-thick aluminum (Al) was deposited for source/drain electrodes by thermal evaporation on the ZnO layer. Finally, a metallic indium paste was used as a back-gate electrode. Fig. 1a and b show a 3-dimensional scheme and photographic plan views of our TFT device, respectively. Electrical characteristics of our ZnO-TFTs were measured and analyzed by using a semiconductor parameter analyzer (HP4155C, Agilent Technologies) at room temperature (RT) in an ambient condition (relative humidity of ~45%). X-ray photoelectron spectroscopy (XPS) measurements were carried out by using PHI5800 ESCA system which operated at 250 W with a monochromatic Al Kα (1486.6 eV) source. De-ionized (DI) water contact angle measurements were performed by contact angle analyzer (Phoenix 300, SEO) at RT.
⁎ Corresponding author. Tel.: +82 2 2123 2842; fax: +82 2 392 1592. E-mail address: [email protected] (S. Im). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.02.093
3. Results and discussion Our device shown in Fig. 1 has the channel length (L) of 100 μm and width/length (W/L) ratio of ~ 190. The interdigitated structure has advantages of a large channel width, enabling the TFT devices to earn higher drain current (ID) and also to sensitively detect external signals such as chemical [11], biological [12], optical [13] and surface acoustic
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K. Kim et al. / Thin Solid Films 517 (2009) 6345–6348
Fig. 1. (a) Three-dimensional schematic and (b) photographic plan views of our ZnOTFT with interdigitated structure which has a nominal channel length of 100 μm and W/ L ratio of ~ 190.
wave (SAW) [14] signals through the large surface window. For these reasons we adopt this structure to observe the time-dependent instability of devices, which can be induced by any change in the surface states of device. Fig. 2a and b indicate drain current–drain voltage (ID–VD) output curves and drain current-gate voltage (ID–VG) transfer ones obtained from pristine and long term-aged ZnO-TFTs, respectively. Since our ZnO-TFTs had been fabricated, their electrical characteristics were measured in daily basis until the properties of the devices became saturated or stabilized, while those devices were kept in a vacuum
Fig. 3. X-ray photoemission spectroscopy (XPS) narrow scan spectra for (a) Zn 2p3/2 core-level peaks and (b) valence band (VB) spectra for pristine and aged surface. The inset shows an enlarged scale of VB spectra near the Fermi-level.
desiccator of ~1 Torr in the dark at the time of no measurement. According to Fig. 2a, under VG of 80 V and VD of 40 V, the initial ID was 5.7 mA but three times higher ID (17.5 mA) was observed after 25 days when the device was stabilized. Key parameters of ZnO-TFT such as threshold voltage (VT), field effect mobility (μ), on/off current ratio, and sub-threshold slope (S.S) were determined from the transfer curves of Fig. 2b and its inset (√ ID–VG and log10ID–VG, respectively). Large VT shift (ΔVT) was clearly observed to be −16 V (from 30 V to 14 V) after 25 days while the other parameters did not vary
Fig. 2. Electrical properties for ZnO-TFTs with interdigitated structure; (a) drain current–drain voltage (ID–VD) output curves and (b) square root drain current–gate voltage (√ ID–VG) transfer ones obtained from pristine, aged and further aged under ambient light. The inset shows log10ID–VG transfer curves for the device.
K. Kim et al. / Thin Solid Films 517 (2009) 6345–6348
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Fig. 4. Schematic energy band diagrams for (a) pristine and (b) aged ZnO surface along with de-ionized water contact angle data.
significantly; μ and on/off current ratio were maintained to be ~ 0.66 cm2/Vs and of ~ 5 × 107, respectively. According to X-ray diffraction, our 400 °C deposited-ZnO film on SiO2 appears polycrystalline with relatively small grain size of ~32 nm as estimated by Scherrer's formula (data not show here) and this means that our ZnO film contains many grain boundaries which can result in relatively poor mobility values. However, our ZnO-TFTs did not show any observable gate-induced hysteresis behavior in both of pristine and aged cases. It is probably because SiO2 as a gate dielectric is strong enough to maintain good channel/dielectric interface not allowing any charge injection from ZnO channel and/or p+-Si as a gate electrode during 80 V device operations. Then, since we suspect the probable change of ZnO surface property in the controlled desiccator ambient as a main reason of the large VT shift, we exposed our saturated (aged) ZnO device to ambient light for a long time (under fluorescent lamp: OSRAM, 1377 l× for another week after aging experiment) to confirm if our ZnO surface can be further modified by another ambient factor (light). As a result, ID current more increased and VT further decreased to 9.7 V (see Fig. 2a and b). It means that under a specific ambient the surface state of our 400 °C-deposited ZnO changes with time, leading to the changes of device properties as well. To address the issue on the time-dependent instability induced by the change of surface state, we carried out X-ray photoelectron spectroscopy (XPS) measurements and obtained some results as shown in Fig. 3a and b. All atomic spectra were calibrated by taking adventurous hydrocarbon C 1s peak at 284.6 eV as a reference. Fig. 3a indicates Zn 2p3/2 core-level spectra for our pristine and aged ZnO surface. According to the Fig. 3a, the Zn 2p3/2 core-level peak very much shifted toward higher binding energy (EB) by 0.3 eV after aging. The observed peak shift means a change in surface charge states [15], which is related to the chemisorption of some gas molecules in ambient air. It is because the binding energy of core-level electrons of surface Zn is influenced by Coulomb interactions from other electrons and nuclei in the environmental gas molecules. In order to confirm the effects on Zn valence electrons, the valence band (VB) of ZnO surface was investigated. Fig. 3b and its inset show VB spectra of pristine and aged ZnO surfaces. The VB is basically determined by O 2p, mixed of Zn 4s-O 2p, and Zn 3d orbital positions located at ~4 eV, ~ 7 and ~ 9.5 eV, respectively (see the indications in Fig. 3b) [16]. The zero energy and the leading edge of the VB spectra at 1.9 eV for pristine VB refer to Fermi-energy and valence band maximum (VBM), respectively [15,16]. As shown in the figure, the peak shift (ΔEB) of 0.3 eV occurred in the Zn 3d peak of VB, which well matched with the corresponding change in Zn 2p3/2 core-level energy. The inset of Fig. 3b displays an enlarged
scale of the VB spectra near the Fermi-energy of ZnO surface. Although it is difficult to estimate the exact position of Fermi-energy with respect to the vacuum level due to the zero energy-calibration belong with equipment conditions, it is assured that the pristine VB spectra near Fermi-energy shifted to the higher level by ~ 0.3 eV. It means that Fermi-energy at the surface increases to 0.3 eV from its original level. Fig. 4a and b show the schematic energy band diagrams for the pristine and aged ZnO surface, respectively, based on VB spectra in Fig. 3b. As time elapses, the energy band at the surface displays the downward bending of 0.3 eV, which means the formation of a more conductive n-type ZnO surface. The increase of surface conductivity results in the time-dependent instability of ZnO-TFT devices, the VT shift as shown in Fig. 2b. According to Park et al. [17] and Kang et al. [18], the surface conductivity, modified by the reaction with donor and acceptor such as H2O and O2 molecules, can cause VT shift of ZnObased TFT device. Our case of time-dependent VT instability is similar to their results associated with controlled ambient gas although our TFT samples were kept in a low vacuum desiccator (~1 Torr). Usual ZnO film has non-stoichiometric surface due to the easy formation of Zn interstitials (Zni) and oxygen vacancies (VO) [19]. Many defective sites including VO may cause the surface quite polar, reactive and unstable. Therefore, various kinds of chemical species in ambient air tend to make ZnO surface more conductive (playing as donors), being adsorbed on the surface [9]. Contact angle of de-ionized water was clearly changed with the aging from initial 30 °C to 110 °C. This simple measurement also indicates that the ZnO surface state has been considerably changed (from hydrophilic to hydrophobic) due to the chemical adsorption and accumulation of the ambient air molecules. We believe that this contact angle result firmly supports the electrical instability of aged device along with the XPS results. 4. Conclusion In summary, ZnO-TFTs with interdigitated S/D electrodes were fabricated on SiO2/p+-Si substrate to investigate the time-dependent instability of the device. As time elapsed, we observed a considerable shift of VT of ZnO-TFT from 30 to 14 V and simultaneously observed contact angle changes on ZnO surface. According to XPS measurements, the Zn 2p3/2 core-level peak and the VBM of aged ZnO surface shifted to higher energy levels by 0.3 eV from those of pristine ZnO surface. These indicate that the surface of ZnO channel becomes more conductive with time. We conclude that a reactive polar ZnO surface causes the unstable electrical characteristics of ZnO-TFTs attracting some donor molecules.
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Acknowledgements This work was supported by the fundamental R&D Program for Core Technology of Materials funded by the Ministry of Commerce, Industry and Energy, KOSEF (R01-2006-000-11277-0), the IT R&D program of MKE/IITA [2006-S079-02, Smart window with transparent electronic devices], and Brain Korea 21 Program. References [1] Elvira M.C. Fortunato, Pedro M.C. Barquinha, Ana C.M.B.G. Pimentel, Alexandra M.F. Gonçalves, António J.S. Marques, Luís M.N. Pereira, Rodrigo F.P. Martins, Adv. Mater. 17 (2005) 590. [2] Min Suk Oh, D.K. Hwang, Kimoon Lee, Seongil Im, Appl. Phys. Lett. 90 (2007) 153511. [3] P. Barquinha, E. Fortunato, A.A. Gonçalves, A. Pimentel, A. Marques, L. Pereira, R. Martins, Superlattices Microstruct. 39 (2006) 319. [4] Wolfgang H. Hirschwald, Acc. Chem. Res. 18 (1985) 228. [5] Leonid V. Azároff, Introduction to solids, McGraw-Hill Book Company, New York, 1960, p. 371.
[6] K.H. Tam, C.K. Cheung, Y.H. Leung, A.B. Djurišić, C.C. Ling, C.D. Beling, S. Fung, W.M. Kwok, W.K. Chan, D.L. Phillips, L. Ding, W.K. Ge, J. Phys. Chem., B 110 (2006) 20865. [7] G. Kresse, Olga Dulub, Ulrike Diebold, Phys. Rev. B. 68 (2003) 245409. [8] A. Wander, N.M. Harrison, J. Chem. Phys. 115 (2001) 2312. [9] V.E. Henrich, P.A. Cox, The surface science of metal oxide, Cambridge University Press, Cambridge, 1994, pp. 255–269, pp. 296–301. [10] H.S. Bae, Seongil Im, J. Vac. Sci. Technol., B 22 (2004) 1191. [11] J. Frank, M. Fleischer, H. Meixner, Sens. Actuators, B, Chem. 48 (1998) 318. [12] C.A. Savran, T.P. Burg, J. Fritz, S.R. Manalis, Appl. Phys. Lett. 83 (2003) 1659. [13] Antonella Sciuto, Fabrizio Roccaforte, Salvatore Di Franco, Vito Raineri, Appl. Phys. Lett. 89 (2006) 081111. [14] Tsung-Tsong Wu, Wei-Shan Wang, J. Appl. Phys. 96 (2004) 5249. [15] B.J. Coppa, C.C. Fulton, P.J. Hartlieb, R.F. Davis, J. Appl. Phys. 95 (2004) 5856. [16] Z.Z. Ye, Y.J. Zeng, Y.F. Lu, S.S. Lin, L. Sun, L.P. Zhu, B.H. Zhao, Appl. Phys. Lett. 91 (2007) 112110. [17] Jin-Seong Park, Jae Kyeong Jeong, Hyun-Joong Chung, Yeon-Gon Mo, Hye Dong Kim, Appl. Phys. Lett. 92 (2008) 072104. [18] Donghun Kang, Hyuck Lim, Changjung Kim, Ihun Song, Jaechoel Park, Youngsoo Park, Appl. Phys. Lett. 90 (2007) 192101. [19] Yanfa Yan, M.M. Al-Kassim, Phys. Rev., B 72 (2005) 235406.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Surface-induced time-dependent instability of ZnO based thin-film transistors Ki-tae Kim, Kimoon Lee, Min Suk Oh, C.H. Park, Seongil Im ⁎ Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749 Korea
a r t i c l e
i n f o
Available online 21 February 2009 Keywords: ZnO Thin-film transistor Instability Surface Interdigitated structure
a b s t r a c t We report on the surface-induced time-dependent instability of ZnO based thin-film transistors (ZnO-TFTs) with interdigitated source/drain (S/D) electrodes. As time elapsed, a considerable shift of threshold voltage (VT) was observed (by ~− 16 V) from our TFT. Contact angle of de-ionized water on ZnO surface also changed from 30° to 110°, revealing time-dependent surface state change. According to X-ray photoemission spectroscopy (XPS) measurements, the Zn 2p3/2 core-level peak and the valence band maximum (VBM) of aged ZnO surface shifted to the higher binding energy by 0.3 eV, which implies a downward energy band bending of the ZnO back channel-surface. We conclude that without passivation layer any bottom gate ZnOTFT meets the surface-induced electrical instabilities due to the time-dependent conductance of ZnO surface. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
ZnO based thin-film transistors (ZnO-TFTs) have been potentially applied to various kinds of electrical applications such as drivers for display (LCD or OLED) [1] and logic circuit [2] due to their superior electrical characteristics compared to amorphous Si (a-Si). In order to accomplish any practical device applications with the ZnO based TFTs, their device stabilities should be confirmed beforehand. In particular, since it is very important for ZnO devices to maintain the initial electrical properties in air ambient of normal humidity as a condition for real usage, the time-dependent device properties under those normal ambient conditions should be essentially studied. In fact, only a few studies on the time-dependent instability issues of ZnO-TFTs were reported [3], probably because those issues involve a timedependent variation of ZnO surface state which is difficult to precisely investigate. ZnO surface has a polar character which results from nonstoichiometry with native defects such as Zn interstitials (Zni) and oxygen vacancies (VO) [4–6] and polar surfaces cause relatively high surface energy, leading to unstable surface characteristics [7,8]. In order to reach to thermo-dynamically stable or meta-stable surface states, ZnO surface would react to favorable molecules in ambient air involving H2O, O2, H2 and CO2 etc, so that the surface chemistry of ZnO would be modified resulting in the change of surface conductivity [9]. In the present work, we fabricated ZnO-TFTs with interdigitated structure which has relatively large surface as a back-channel to study and more clearly observe the time-dependent instability of ZnO-TFTs related to the properties of ZnO back-channel surface.
A substrate of thermally grown 200 nm-thick SiO2 on p+-Si (~0.01 Ω cm) was adopted for a gate dielectric and gate electrode, respectively. Prior to deposition of ZnO thin film, the substrate was cleaned with acetone, methyl alcohol and de-ionized water, in that order. A 100 nm-thick undoped ZnO thin film (area of 2.8 × 2.3 mm2) was deposited through a shadow mask at a high substrate temperature of 400 °C by rf magnetron sputtering in a vacuum chamber (with basal pressure of ~ 1 × 10− 6 Torr and working pressure of 10 mTorr composed of the mixture of Ar:O2 = 5:2 and the physical properties of the ZnO films were shown elsewhere [10]). And then, through the second shadow mask with interdigitated structure, a 200 nm-thick aluminum (Al) was deposited for source/drain electrodes by thermal evaporation on the ZnO layer. Finally, a metallic indium paste was used as a back-gate electrode. Fig. 1a and b show a 3-dimensional scheme and photographic plan views of our TFT device, respectively. Electrical characteristics of our ZnO-TFTs were measured and analyzed by using a semiconductor parameter analyzer (HP4155C, Agilent Technologies) at room temperature (RT) in an ambient condition (relative humidity of ~45%). X-ray photoelectron spectroscopy (XPS) measurements were carried out by using PHI5800 ESCA system which operated at 250 W with a monochromatic Al Kα (1486.6 eV) source. De-ionized (DI) water contact angle measurements were performed by contact angle analyzer (Phoenix 300, SEO) at RT.
⁎ Corresponding author. Tel.: +82 2 2123 2842; fax: +82 2 392 1592. E-mail address: [email protected] (S. Im). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.02.093
3. Results and discussion Our device shown in Fig. 1 has the channel length (L) of 100 μm and width/length (W/L) ratio of ~ 190. The interdigitated structure has advantages of a large channel width, enabling the TFT devices to earn higher drain current (ID) and also to sensitively detect external signals such as chemical [11], biological [12], optical [13] and surface acoustic
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K. Kim et al. / Thin Solid Films 517 (2009) 6345–6348
Fig. 1. (a) Three-dimensional schematic and (b) photographic plan views of our ZnOTFT with interdigitated structure which has a nominal channel length of 100 μm and W/ L ratio of ~ 190.
wave (SAW) [14] signals through the large surface window. For these reasons we adopt this structure to observe the time-dependent instability of devices, which can be induced by any change in the surface states of device. Fig. 2a and b indicate drain current–drain voltage (ID–VD) output curves and drain current-gate voltage (ID–VG) transfer ones obtained from pristine and long term-aged ZnO-TFTs, respectively. Since our ZnO-TFTs had been fabricated, their electrical characteristics were measured in daily basis until the properties of the devices became saturated or stabilized, while those devices were kept in a vacuum
Fig. 3. X-ray photoemission spectroscopy (XPS) narrow scan spectra for (a) Zn 2p3/2 core-level peaks and (b) valence band (VB) spectra for pristine and aged surface. The inset shows an enlarged scale of VB spectra near the Fermi-level.
desiccator of ~1 Torr in the dark at the time of no measurement. According to Fig. 2a, under VG of 80 V and VD of 40 V, the initial ID was 5.7 mA but three times higher ID (17.5 mA) was observed after 25 days when the device was stabilized. Key parameters of ZnO-TFT such as threshold voltage (VT), field effect mobility (μ), on/off current ratio, and sub-threshold slope (S.S) were determined from the transfer curves of Fig. 2b and its inset (√ ID–VG and log10ID–VG, respectively). Large VT shift (ΔVT) was clearly observed to be −16 V (from 30 V to 14 V) after 25 days while the other parameters did not vary
Fig. 2. Electrical properties for ZnO-TFTs with interdigitated structure; (a) drain current–drain voltage (ID–VD) output curves and (b) square root drain current–gate voltage (√ ID–VG) transfer ones obtained from pristine, aged and further aged under ambient light. The inset shows log10ID–VG transfer curves for the device.
K. Kim et al. / Thin Solid Films 517 (2009) 6345–6348
6347
Fig. 4. Schematic energy band diagrams for (a) pristine and (b) aged ZnO surface along with de-ionized water contact angle data.
significantly; μ and on/off current ratio were maintained to be ~ 0.66 cm2/Vs and of ~ 5 × 107, respectively. According to X-ray diffraction, our 400 °C deposited-ZnO film on SiO2 appears polycrystalline with relatively small grain size of ~32 nm as estimated by Scherrer's formula (data not show here) and this means that our ZnO film contains many grain boundaries which can result in relatively poor mobility values. However, our ZnO-TFTs did not show any observable gate-induced hysteresis behavior in both of pristine and aged cases. It is probably because SiO2 as a gate dielectric is strong enough to maintain good channel/dielectric interface not allowing any charge injection from ZnO channel and/or p+-Si as a gate electrode during 80 V device operations. Then, since we suspect the probable change of ZnO surface property in the controlled desiccator ambient as a main reason of the large VT shift, we exposed our saturated (aged) ZnO device to ambient light for a long time (under fluorescent lamp: OSRAM, 1377 l× for another week after aging experiment) to confirm if our ZnO surface can be further modified by another ambient factor (light). As a result, ID current more increased and VT further decreased to 9.7 V (see Fig. 2a and b). It means that under a specific ambient the surface state of our 400 °C-deposited ZnO changes with time, leading to the changes of device properties as well. To address the issue on the time-dependent instability induced by the change of surface state, we carried out X-ray photoelectron spectroscopy (XPS) measurements and obtained some results as shown in Fig. 3a and b. All atomic spectra were calibrated by taking adventurous hydrocarbon C 1s peak at 284.6 eV as a reference. Fig. 3a indicates Zn 2p3/2 core-level spectra for our pristine and aged ZnO surface. According to the Fig. 3a, the Zn 2p3/2 core-level peak very much shifted toward higher binding energy (EB) by 0.3 eV after aging. The observed peak shift means a change in surface charge states [15], which is related to the chemisorption of some gas molecules in ambient air. It is because the binding energy of core-level electrons of surface Zn is influenced by Coulomb interactions from other electrons and nuclei in the environmental gas molecules. In order to confirm the effects on Zn valence electrons, the valence band (VB) of ZnO surface was investigated. Fig. 3b and its inset show VB spectra of pristine and aged ZnO surfaces. The VB is basically determined by O 2p, mixed of Zn 4s-O 2p, and Zn 3d orbital positions located at ~4 eV, ~ 7 and ~ 9.5 eV, respectively (see the indications in Fig. 3b) [16]. The zero energy and the leading edge of the VB spectra at 1.9 eV for pristine VB refer to Fermi-energy and valence band maximum (VBM), respectively [15,16]. As shown in the figure, the peak shift (ΔEB) of 0.3 eV occurred in the Zn 3d peak of VB, which well matched with the corresponding change in Zn 2p3/2 core-level energy. The inset of Fig. 3b displays an enlarged
scale of the VB spectra near the Fermi-energy of ZnO surface. Although it is difficult to estimate the exact position of Fermi-energy with respect to the vacuum level due to the zero energy-calibration belong with equipment conditions, it is assured that the pristine VB spectra near Fermi-energy shifted to the higher level by ~ 0.3 eV. It means that Fermi-energy at the surface increases to 0.3 eV from its original level. Fig. 4a and b show the schematic energy band diagrams for the pristine and aged ZnO surface, respectively, based on VB spectra in Fig. 3b. As time elapses, the energy band at the surface displays the downward bending of 0.3 eV, which means the formation of a more conductive n-type ZnO surface. The increase of surface conductivity results in the time-dependent instability of ZnO-TFT devices, the VT shift as shown in Fig. 2b. According to Park et al. [17] and Kang et al. [18], the surface conductivity, modified by the reaction with donor and acceptor such as H2O and O2 molecules, can cause VT shift of ZnObased TFT device. Our case of time-dependent VT instability is similar to their results associated with controlled ambient gas although our TFT samples were kept in a low vacuum desiccator (~1 Torr). Usual ZnO film has non-stoichiometric surface due to the easy formation of Zn interstitials (Zni) and oxygen vacancies (VO) [19]. Many defective sites including VO may cause the surface quite polar, reactive and unstable. Therefore, various kinds of chemical species in ambient air tend to make ZnO surface more conductive (playing as donors), being adsorbed on the surface [9]. Contact angle of de-ionized water was clearly changed with the aging from initial 30 °C to 110 °C. This simple measurement also indicates that the ZnO surface state has been considerably changed (from hydrophilic to hydrophobic) due to the chemical adsorption and accumulation of the ambient air molecules. We believe that this contact angle result firmly supports the electrical instability of aged device along with the XPS results. 4. Conclusion In summary, ZnO-TFTs with interdigitated S/D electrodes were fabricated on SiO2/p+-Si substrate to investigate the time-dependent instability of the device. As time elapsed, we observed a considerable shift of VT of ZnO-TFT from 30 to 14 V and simultaneously observed contact angle changes on ZnO surface. According to XPS measurements, the Zn 2p3/2 core-level peak and the VBM of aged ZnO surface shifted to higher energy levels by 0.3 eV from those of pristine ZnO surface. These indicate that the surface of ZnO channel becomes more conductive with time. We conclude that a reactive polar ZnO surface causes the unstable electrical characteristics of ZnO-TFTs attracting some donor molecules.
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K. Kim et al. / Thin Solid Films 517 (2009) 6345–6348
Acknowledgements This work was supported by the fundamental R&D Program for Core Technology of Materials funded by the Ministry of Commerce, Industry and Energy, KOSEF (R01-2006-000-11277-0), the IT R&D program of MKE/IITA [2006-S079-02, Smart window with transparent electronic devices], and Brain Korea 21 Program. References [1] Elvira M.C. Fortunato, Pedro M.C. Barquinha, Ana C.M.B.G. Pimentel, Alexandra M.F. Gonçalves, António J.S. Marques, Luís M.N. Pereira, Rodrigo F.P. Martins, Adv. Mater. 17 (2005) 590. [2] Min Suk Oh, D.K. Hwang, Kimoon Lee, Seongil Im, Appl. Phys. Lett. 90 (2007) 153511. [3] P. Barquinha, E. Fortunato, A.A. Gonçalves, A. Pimentel, A. Marques, L. Pereira, R. Martins, Superlattices Microstruct. 39 (2006) 319. [4] Wolfgang H. Hirschwald, Acc. Chem. Res. 18 (1985) 228. [5] Leonid V. Azároff, Introduction to solids, McGraw-Hill Book Company, New York, 1960, p. 371.
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