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Stability enhancement of low temperature thin-film transistors with atomic-layer-deposited ZnO:Al channels
Microelectronic Engineering 167 (2017) 105–109
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Stability enhancement of low temperature thin-film transistors with atomic-layer-deposited ZnO:Al channels Wen-Jun Liu, You-Hang Wang, Li-Li Zheng, Hong-Liang Lu, Shi-Jin Ding ⁎ State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 13 May 2016 Received in revised form 6 November 2016 Accepted 9 November 2016 Available online 14 November 2016 Keywords: ZnO:Al Thin-film transistor Atomic layer deposition Stability
a b s t r a c t The electrical characteristics of TFTs with atomic-layer-deposited ZnO:Al (ZAO) channels have been studied in this work. By increasing Al doping concentration, the ZAO film changes from polycrystalline to amorphous, and its bandgap widens as well. With post-annealing at 200 °C, the superior electrical stabilities under illumination and gate bias stress were achieved in ZAO TFTs compared with ZnO TFTs. For the strong immunity to illumination in ZAO TFTs, it is attributed to the widening bandgap of channel material for the reduction of the carrier concentration. While for the improved electrical stability under positive bias stress, it is mainly due to the suppression of interactions between the amorphous channel and the surrounding ambient, which is verified by the observations in N2 ambient. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Recently, transparent oxide semiconductors have attracted widespread interests because of high mobility, high transparency for visible light, and low process temperature [1–3]. In particular, the ZnO thinfilm transistors (TFTs) using an oxide semiconductor as a channel layer have emerged for the next generation display application. However, the ZnO thin-film transistors (TFT) suffer from the instabilities under illumination and/or gate bias stress [4–6]. By doping with different chemical elements, the structural, optical, and electrical characteristics of ZnO films can be tuned properly. Ku et al. [7] reported that a small amount of Mg incorporated into the channel layer of ZnO TFTs by MOCVD at 400 °C exhibited a superior stability against negative stress bias. It was mainly attributed to the reduction of donor-like defects associated with ionized oxygen vacancies. Hafnium-doped zinc oxide (HZO) TFTs with post annealing at 400 °C have a smaller Vth shift of −1 V than that of − 8 V for ZnO TFTs [8]. Furthermore, Cheremisin et al. observed that Indium-doped ZnO TFTs exhibit highly stable operational characteristics under both negative and positive bias stresses [9]. Obviously, such post-annealing steps underwent a relatively high thermal budget of 400 °C. However, in terms of flexible electronic applications, the maximum processing temperature of TFTs should be as low as possible meanwhile maintaining good performance. Therefore, it is also indispensable to explore the effect of low temperature annealing on the electrical characteristics of the ZnO TFTs. In this work, the TFTs with Al-doped ZnO channels using atomic layer deposition were fabricated under the maximum thermal budget ⁎ Corresponding author. E-mail address: [email protected] (S.-J. Ding).
http://dx.doi.org/10.1016/j.mee.2016.11.010 0167-9317/© 2016 Elsevier B.V. All rights reserved.
of 200 °C. The performance of ZnO TFTs with and without Al doping was compared quantitatively. Superior electrical stability under gate bias stress and illumination was achieved in the ZAO TFTs, and the mechanism behind was also analyzed. 2. Experimental details A low resistive p-type (100) silicon substrate was used as the back gate of TFTs. After standard RCA cleaning, a 50-nm Al2O3 layer and ZAO active layer were deposited in turn by atomic layer deposition (ALD) at 200 °C without breaking vacuum. Herein, the precursors for ALD Al2O3 and ZnO films were Al(CH3)3 (TMA)/H2O and Zn(C2H5)2 (DEZ)/H2O, respectively. The compositions of the ZAO films were modulated by the number of ZnO and Al2O3 deposition cycles. Here, n is the cycle ratio of the ZnO deposition cycles to the Al2O3 deposition cycles and thus the Al content increases as the cycle ratio decreases, as shown in Table 1. The elemental composition of Zn, Al and O in the ZAO films can also be roughly estimated from the cycle ratio, in agreement with the report [10]. Subsequently, the active layer of ZAO was defined by photolithography, and formed by wet etching with diluted HCl solution. Finally, the source/drain contacts of 100 nm Mo layer were formed by sputtering and a lift-off process. The schematic structure of the fabricated TFTs and process flow is illustrated in Fig. 1. After that, post-annealing at 200 °C in air for different annealing time was performed to improve the performance of the fabricated TFTs. The thicknesses of the deposited ZAO and Al2O3 films were determined by an ellipsometer (Sopra GES-SE, France). The crystallinity and crystal orientation of deposited films were characterized by X-ray diffraction (XRD) with Cu KR radiation. The electrical characteristics of the TFTs with channel length/width (10 μm/100 μm) were measured
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Table 1 Compositions and thicknesses of ZnO and various ZAO films. Sample
Cycle ratio (n)
Total cycles
Thickness (nm)
Al (at.%)
Zn (at.%)
O (at.%)
ZnO ZAO-1 ZAO-2 ZAO-3 ZAO-4
N/A 19:1 9:1 6:1 4:1
200 200 200 210 200
41.2 38.4 37.1 35.9 32.4
0 4.65 8.70 11.77 15.38
50 44.19 39.13 35.29 30.77
50 52.16 52.17 52.94 53.85
with a semiconductor device analyzer (B1500A, Keysight Technologies) at room temperature in a dark box. 3. Results and discussion With the additional Al2O3 growth cycles, the thickness of the ZAO film is observed to be thinner than that of the pure ZnO sample, shown in Table 1. Moreover, the thickness of ZAO film decreases with increasing Al doping concentration, which is attributed to the suppression of ZnO growth on Al2O3 [11]. Fig. 2 shows the XRD spectra of ZAO thin films with different Al concentration. It can be seen that ZAO exhibits a hexagonal wurtzite structure with the ZnO (100), ZnO (002), and ZnO (101) peaks. While with increasing Al contents, the diffraction angle shifts from 34.5° to 35.0°, indicating the substitution of Al3+ ions for Zn2 + ions in the ZnO lattice during the growth [12]. It is known that the ionic radius of Al3 + cation is 0.54 Å, which is smaller than that of Zn2+ cation (0.74 Å) [13]. The substitutional doping of Al3+ at the Zn2 + sites will lead to a reduction of the lattice parameter in the ZnO phase and consequently result in the peak shifting upward. In addition, it is also found that the intensity of diffraction peaks reduces gradually with increasing the amount of Al contents. As the deposition cycle ratio of ZnO:Al2O3 decreases to 6:1, the ZAO film (ZAO-3) becomes amorphous. The optical properties of the ZAO films with different Al doping concentrations are also examined, as shown in Fig. 3. In the visible region, the transparency of the ZAO films increases with increasing Al doping concentration. For example, the transparency increased from 85.1% to 91.9% at a measured wavelength of 650 nm. Further, there is an obvious optical absorption for the ZnO film in the UVlight region in comparison with that of the ZAO film. According to Tauc's model the absorption reduces as their optical bandgap (Eg)
Fig. 2. XRD patterns for pure ZnO and ZAO thin films.
increases [14]. The optical bandgaps for the ZAO films with different Al doping concentrations were summarized in the inset table in Fig. 3. And it augments from 3.2 to 3.54 eV as the deposition cycle ratio of Al2O3: ZnO increases from 0 to 1:6. Fig. 4 shows time-dependent transfer characteristics of TFTs with various channel compositions annealed at 200 °C in air. It is worthy noting that the as-fabricated TFTs behave like a resistor. Oxygen vacancies in ZnO could supply free electrons in the conduction band, and be passivated by the annealing in air, resulting in the reduction of carrier concentration in ZnO film. For the ZAO-1 TFT, it is known that ~ 5% Al incorporated into ZAO can act as an electron donor, contributing to an n-type conductivity of the ZnO films [15,16]. Such Al doped TFTs could maintain the high carrier concentration even it would decrease due to the passivation of oxygen vacancies by annealing in air. While for the ZAO-3 (Z:A = 6:1) TFTs, it was observed small on-current and on/off ratio due to the increase in the film resistivity. However, the ZAO-2 TFTs demonstrated a superior performance such as Vth of 0.8 V, on/off current ratio of ~107, field-effect mobility of 0.133 cm2/(V·s) and subthreshold swing of 750 mV/dec, when the annealing time increased to 10 h. As a matter of fact, the ZAO-2 TFT exhibits a better performance than both ZAO-1 and ZAO-3 TFTs. The output characteristics of the
Fig. 1. (a) Schematic structure, (b) optical image and (c) process flow of the fabricated TFTs.
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Fig. 3. Transmittance and bandgap of ZnO and ZAO films on glass substrate.
ZAO-2 TFTs annealed at 200 °C for 8 h and 10 h were shown in Fig. 5. It is worthwhile to mention that the ZAO-4 TFT always maintains the offstate after the same annealing. It is also noted that the crystallinity of the ZAO channel does not changed after annealing at 200 °C (data not shown here), indicating that the transfer characteristics of ZAO TFTs mainly depend on the Al content rather than annealing induced structure crystallinity.
Fig. 5. Output characteristics of the ZAO-2 TFTs annealed at 200 °C for (a) 8 h and (b) 10 h.
Fig. 6 shows the transfer characteristics of the ZnO and ZAO-2 TFTs under illumination of a halogen tungsten lamp. A small ΔVth of 0.25 V for ZAO-2 TFTs is observed, much smaller than that of 3.42 V for ZnO
Fig. 4. Transfer characteristics of (a) ZnO and (b) ZAO-1, (c) ZAO-2 and (d) ZAO-3 TFTs annealed in air at 200 °C for various time.
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Fig. 6. The illumination time-dependent transfer characteristics of the (a) ZnO and (b) ZAO-2 TFTs.
TFTs under illumination. Moreover, the off current of ZnO TFTs even increases by around four orders of magnitude under illumination. These results reveal that doping Al into ZnO can significantly improve the stability under illumination. The light absorption can increase the carrier concentration in the ZnO channel likely in two ways. One is that, it is easily absorbed by ZnO with a minimum optical bandgap of 3.2 eV compared with the Al-doped counterpart, generating plentiful holes in its valence band and excited electrons to its conduction band [17,18]. The other one is that, the light absorption can excite fully occupied neutral oxygen vacancies (VO) near the valence band maximum (VBM) [19], usually results in the increment of carrier concentration. As a result, the ZAO-2 TFTs with larger bandgap can suppress the off-current and reduce the subthreshold swing under illumination. Fig. 7 shows the negative and positive gate bias stress instabilities of the ZnO and ZAO-2 TFTs in air and N2, respectively. After a constant voltage stress (CVS) of 10 V for 1200 s, the ΔVth of the ZAO-2 TFT is only 0.1 V, which is nearly one order of magnitude smaller than that of the ZnO TFT. It was reported that a positive gate bias induces the adsorption
of oxygen molecules in the channel by decreasing the activation energy, and the charge transfer may occur by the adsorption of oxygen molecules in the back channel without passivation layer [20,21]. The oxygen molecules adsorbed in the back channel could form a depletion layer [20] by the interaction between oxygen molecules and minority carrier holes near the back channel surface. This will effectively cause the electron trapping in the vicinity of the channel/Al2O3 interface. Then, the trapped electrons could affect the TFTs operation by increasing the applied gate voltage, resulting in a positive shift in Vth. It is worthwhile to note that grain boundaries in ZnO TFT provide many adsorption sites for oxygen molecules within the channel layer because of its nature of polycrystalline, which accelerates the positive ΔVth under PBS for ZnO TFTs [22]. Moreover, under PBS, the majority carrier electrons could also be trapped in the vicinity of the channel/Al2O3 interface, consequently leading to a positive shift in Vth. Under a negative gate bias stress, the ΔVth for ZnO and ZAO-2 TFTs is nearly the same, shown in Fig. 7. The negative gate bias induces a repulsive force against the majority carrier electrons in the channel, and polar H2O molecules simultaneously attract electrons at the surface [21,22]. This electrostatic attraction results in an asymmetric distribution of majority carrier electrons in the channel, leading to the generation of an internal electric potential [23]. Such an induced electric potential could act as a positive gate bias and is effective in enhancing the n-channel formation. To reduce the influence of O2 and H2O on the instability of ZnO and ZAO-2 TFTs, the same experiments were carried out in N2 ambient. It was found that positive and negative ΔVth produced in N2 ambient are more inappreciable than that under atmospheric air, indicative of environmental ambient is crucial to the electrical instability of ZAO TFTs. Therefore, in air ambient both charge transfer and electric stress will degrade the TFTs by increasing the Vth. However, PBS only induces a small positive ΔVth in our case, which is verified by the observations in N2 ambient. 4. Conclusion
Fig. 7. Dependences of ΔVth on gate stress time for the ZnO and ZAO-2 TFT in air and N2, respectively.
In summary, we have demonstrated the TFTs with Al doped ZnO channels using atomic layer deposition for enhancing the stability. With respect to the polycrystalline of ZnO film, the ZAO film can maintain an amorphous nature by increasing Al doping concentration. Compared with ZnO TFTs, the ZAO TFTs have significant improvement in the electrical stabilities under illumination and gate bias stress. Additionally, the ZAO TFTs also exhibited superior immunity to the surrounding
W.-J. Liu et al. / Microelectronic Engineering 167 (2017) 105–109
ambient. These findings suggest the Al-doped ZnO film may be a highly promising channel material for flexible electronic applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61474027, 61274088), start-up program JIH1233003 at Fudan University. This work was also sponsored by Shanghai Pujiang Program (No. 16PJ1400800). References [1] E. Fortunato, A. Pimentel, L. Pereira, A. Gonçalves, G. Lavareda, H. Aguas, I. Ferreira, C.N. Carvalho, R. Martins, J. Non-Cryst, Solids 338 (2004) 806–809. [2] Y. Geng, W. Yang, H.L. Lu, Y. Zhang, Q.Q. Sun, P. Zhou, P.F. Wang, S.J. Ding, D.W. Zhang, IEEE Electron Device Lett. 35 (2014) 1266–1268. [3] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Gonçalves, A.J.S. Marques, R.F.P. Martins, L.M.N. Pereira, Appl. Phys. Lett. 85 (2014) 2541. [4] R.B.M. Cross, M.M.D. Souza, Appl. Phys. Lett. 89 (2006) 263513. [5] J. Lee, J.S. Park, Y.S. Pyo, D.B. Lee, E.H. Kim, D. Stryakhilev, T.W. Kim, D.U. Jin, Y.G. Mo, Appl. Phys. Lett. 95 (2009) 123502. [6] R. Navamathavan, E.J. Yang, J.H. Lim, D.K. Hwang, J.Y. Oh, J.H. Yang, J.H. Jang, S.J. Park, J. Electrochem. Soc. 153 (2006) G385. [7] C.J. Ku, W.C. Hong, T. Mohsin, R. Li, Z. Duan, Y. Lu, IEEE Electron Device Lett. 36 (2015) 914–916.
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[8] D.S. Han, D.Y. Moon, Y.J. Kang, J.H. Park, J.W. Park, Curr. Appl. Phys. 13 (2013) S98–S102. [9] A.B. Cheremisin, S.N. Kuznetsov, G.B. Stefanovich, AIP Adv. 5 (2015) 117124. [10] F.L. Zhao, J.C. Dong, N.N. Zhao, J. Wu, D.D. Han, J.F. Kang, Y. Wang, Rare Metals 35 (2016) 509–512. [11] Y. Geng, L. Guo, S.S. Xu, Q.Q. Sun, S.J. Ding, H.L. Lu, D.W. Zhang, J. Phys. Chem. 115 (2011) 12317–12321. [12] N.P. Dasgupta, S. Neubert, W. Lee, O. Trejo, J.R. Lee, F.B. Prinz, Chem. Mater. 22 (2010) 4769–4775. [13] K.E. Lee, M. Wang, E.J. Kim, S.H. Hahn, Curr. Appl. Phys. 9 (2008) 683–687. [14] D.L. Wood, J. Tauc, Phys. Rev. B Condens. Matter 5 (1972) 3144–3151. [15] R.K. Shukla, A. Srivastava, A. Srivastava, K.C. Dubey, J. Cryst. Growth 294 (2006) 427–431. [16] M. Kodu, T. Arroval, T. Avarmaa, R. Jaaniso, I. Kink, S. Leinberg, et al., Appl.Surf. Sci. 320 (2014) 756–7634. [17] J.H. Kim, U.K. Kim, Y.J. Chung, C.S. Hwang, Appl. Phys. Lett. 98 (2011) 232102. [18] S. Chen, W.P. Zhang, X.M. Cui, S.J. Ding, Q.Q. Sun, W. Zhang, Appl. Phys. Lett. 104 (2014) 103504. [19] H. Oh, S.M. Yoon, M.K. Ryu, C.S. Hwang, S. Yang, S.K. Park, Appl. Phys. Lett. 97 (2010) 183502. [20] S.K. Park, C. Hwang, M. Ryu, S. Yang, C. Byun, J. Shin, et al., Adv. Mater. 17 (2005) 590–594. [21] J.K. Jeong, H.W. Yang, J.H. Jeong, Y.-G. Mo, H.D. Kim, Appl. Phys. Lett. 93 (2008) 123508. [22] C.H. Anh, M.G. Yun, S.Y. Lee, H.K. Cho, IEEE Trans. Ele. Dev. 61 (2014) 73–78. [23] X. Zhang, J.P. Ndabakurangye, D.W. Kim, J.S. Choi, J. Park, Electron. Mater. Lett. 11 (2015) 964–972.
Contents lists available at ScienceDirect
Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee
Research paper
Stability enhancement of low temperature thin-film transistors with atomic-layer-deposited ZnO:Al channels Wen-Jun Liu, You-Hang Wang, Li-Li Zheng, Hong-Liang Lu, Shi-Jin Ding ⁎ State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai 200433, China
a r t i c l e
i n f o
Article history: Received 13 May 2016 Received in revised form 6 November 2016 Accepted 9 November 2016 Available online 14 November 2016 Keywords: ZnO:Al Thin-film transistor Atomic layer deposition Stability
a b s t r a c t The electrical characteristics of TFTs with atomic-layer-deposited ZnO:Al (ZAO) channels have been studied in this work. By increasing Al doping concentration, the ZAO film changes from polycrystalline to amorphous, and its bandgap widens as well. With post-annealing at 200 °C, the superior electrical stabilities under illumination and gate bias stress were achieved in ZAO TFTs compared with ZnO TFTs. For the strong immunity to illumination in ZAO TFTs, it is attributed to the widening bandgap of channel material for the reduction of the carrier concentration. While for the improved electrical stability under positive bias stress, it is mainly due to the suppression of interactions between the amorphous channel and the surrounding ambient, which is verified by the observations in N2 ambient. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Recently, transparent oxide semiconductors have attracted widespread interests because of high mobility, high transparency for visible light, and low process temperature [1–3]. In particular, the ZnO thinfilm transistors (TFTs) using an oxide semiconductor as a channel layer have emerged for the next generation display application. However, the ZnO thin-film transistors (TFT) suffer from the instabilities under illumination and/or gate bias stress [4–6]. By doping with different chemical elements, the structural, optical, and electrical characteristics of ZnO films can be tuned properly. Ku et al. [7] reported that a small amount of Mg incorporated into the channel layer of ZnO TFTs by MOCVD at 400 °C exhibited a superior stability against negative stress bias. It was mainly attributed to the reduction of donor-like defects associated with ionized oxygen vacancies. Hafnium-doped zinc oxide (HZO) TFTs with post annealing at 400 °C have a smaller Vth shift of −1 V than that of − 8 V for ZnO TFTs [8]. Furthermore, Cheremisin et al. observed that Indium-doped ZnO TFTs exhibit highly stable operational characteristics under both negative and positive bias stresses [9]. Obviously, such post-annealing steps underwent a relatively high thermal budget of 400 °C. However, in terms of flexible electronic applications, the maximum processing temperature of TFTs should be as low as possible meanwhile maintaining good performance. Therefore, it is also indispensable to explore the effect of low temperature annealing on the electrical characteristics of the ZnO TFTs. In this work, the TFTs with Al-doped ZnO channels using atomic layer deposition were fabricated under the maximum thermal budget ⁎ Corresponding author. E-mail address: [email protected] (S.-J. Ding).
http://dx.doi.org/10.1016/j.mee.2016.11.010 0167-9317/© 2016 Elsevier B.V. All rights reserved.
of 200 °C. The performance of ZnO TFTs with and without Al doping was compared quantitatively. Superior electrical stability under gate bias stress and illumination was achieved in the ZAO TFTs, and the mechanism behind was also analyzed. 2. Experimental details A low resistive p-type (100) silicon substrate was used as the back gate of TFTs. After standard RCA cleaning, a 50-nm Al2O3 layer and ZAO active layer were deposited in turn by atomic layer deposition (ALD) at 200 °C without breaking vacuum. Herein, the precursors for ALD Al2O3 and ZnO films were Al(CH3)3 (TMA)/H2O and Zn(C2H5)2 (DEZ)/H2O, respectively. The compositions of the ZAO films were modulated by the number of ZnO and Al2O3 deposition cycles. Here, n is the cycle ratio of the ZnO deposition cycles to the Al2O3 deposition cycles and thus the Al content increases as the cycle ratio decreases, as shown in Table 1. The elemental composition of Zn, Al and O in the ZAO films can also be roughly estimated from the cycle ratio, in agreement with the report [10]. Subsequently, the active layer of ZAO was defined by photolithography, and formed by wet etching with diluted HCl solution. Finally, the source/drain contacts of 100 nm Mo layer were formed by sputtering and a lift-off process. The schematic structure of the fabricated TFTs and process flow is illustrated in Fig. 1. After that, post-annealing at 200 °C in air for different annealing time was performed to improve the performance of the fabricated TFTs. The thicknesses of the deposited ZAO and Al2O3 films were determined by an ellipsometer (Sopra GES-SE, France). The crystallinity and crystal orientation of deposited films were characterized by X-ray diffraction (XRD) with Cu KR radiation. The electrical characteristics of the TFTs with channel length/width (10 μm/100 μm) were measured
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Table 1 Compositions and thicknesses of ZnO and various ZAO films. Sample
Cycle ratio (n)
Total cycles
Thickness (nm)
Al (at.%)
Zn (at.%)
O (at.%)
ZnO ZAO-1 ZAO-2 ZAO-3 ZAO-4
N/A 19:1 9:1 6:1 4:1
200 200 200 210 200
41.2 38.4 37.1 35.9 32.4
0 4.65 8.70 11.77 15.38
50 44.19 39.13 35.29 30.77
50 52.16 52.17 52.94 53.85
with a semiconductor device analyzer (B1500A, Keysight Technologies) at room temperature in a dark box. 3. Results and discussion With the additional Al2O3 growth cycles, the thickness of the ZAO film is observed to be thinner than that of the pure ZnO sample, shown in Table 1. Moreover, the thickness of ZAO film decreases with increasing Al doping concentration, which is attributed to the suppression of ZnO growth on Al2O3 [11]. Fig. 2 shows the XRD spectra of ZAO thin films with different Al concentration. It can be seen that ZAO exhibits a hexagonal wurtzite structure with the ZnO (100), ZnO (002), and ZnO (101) peaks. While with increasing Al contents, the diffraction angle shifts from 34.5° to 35.0°, indicating the substitution of Al3+ ions for Zn2 + ions in the ZnO lattice during the growth [12]. It is known that the ionic radius of Al3 + cation is 0.54 Å, which is smaller than that of Zn2+ cation (0.74 Å) [13]. The substitutional doping of Al3+ at the Zn2 + sites will lead to a reduction of the lattice parameter in the ZnO phase and consequently result in the peak shifting upward. In addition, it is also found that the intensity of diffraction peaks reduces gradually with increasing the amount of Al contents. As the deposition cycle ratio of ZnO:Al2O3 decreases to 6:1, the ZAO film (ZAO-3) becomes amorphous. The optical properties of the ZAO films with different Al doping concentrations are also examined, as shown in Fig. 3. In the visible region, the transparency of the ZAO films increases with increasing Al doping concentration. For example, the transparency increased from 85.1% to 91.9% at a measured wavelength of 650 nm. Further, there is an obvious optical absorption for the ZnO film in the UVlight region in comparison with that of the ZAO film. According to Tauc's model the absorption reduces as their optical bandgap (Eg)
Fig. 2. XRD patterns for pure ZnO and ZAO thin films.
increases [14]. The optical bandgaps for the ZAO films with different Al doping concentrations were summarized in the inset table in Fig. 3. And it augments from 3.2 to 3.54 eV as the deposition cycle ratio of Al2O3: ZnO increases from 0 to 1:6. Fig. 4 shows time-dependent transfer characteristics of TFTs with various channel compositions annealed at 200 °C in air. It is worthy noting that the as-fabricated TFTs behave like a resistor. Oxygen vacancies in ZnO could supply free electrons in the conduction band, and be passivated by the annealing in air, resulting in the reduction of carrier concentration in ZnO film. For the ZAO-1 TFT, it is known that ~ 5% Al incorporated into ZAO can act as an electron donor, contributing to an n-type conductivity of the ZnO films [15,16]. Such Al doped TFTs could maintain the high carrier concentration even it would decrease due to the passivation of oxygen vacancies by annealing in air. While for the ZAO-3 (Z:A = 6:1) TFTs, it was observed small on-current and on/off ratio due to the increase in the film resistivity. However, the ZAO-2 TFTs demonstrated a superior performance such as Vth of 0.8 V, on/off current ratio of ~107, field-effect mobility of 0.133 cm2/(V·s) and subthreshold swing of 750 mV/dec, when the annealing time increased to 10 h. As a matter of fact, the ZAO-2 TFT exhibits a better performance than both ZAO-1 and ZAO-3 TFTs. The output characteristics of the
Fig. 1. (a) Schematic structure, (b) optical image and (c) process flow of the fabricated TFTs.
W.-J. Liu et al. / Microelectronic Engineering 167 (2017) 105–109
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Fig. 3. Transmittance and bandgap of ZnO and ZAO films on glass substrate.
ZAO-2 TFTs annealed at 200 °C for 8 h and 10 h were shown in Fig. 5. It is worthwhile to mention that the ZAO-4 TFT always maintains the offstate after the same annealing. It is also noted that the crystallinity of the ZAO channel does not changed after annealing at 200 °C (data not shown here), indicating that the transfer characteristics of ZAO TFTs mainly depend on the Al content rather than annealing induced structure crystallinity.
Fig. 5. Output characteristics of the ZAO-2 TFTs annealed at 200 °C for (a) 8 h and (b) 10 h.
Fig. 6 shows the transfer characteristics of the ZnO and ZAO-2 TFTs under illumination of a halogen tungsten lamp. A small ΔVth of 0.25 V for ZAO-2 TFTs is observed, much smaller than that of 3.42 V for ZnO
Fig. 4. Transfer characteristics of (a) ZnO and (b) ZAO-1, (c) ZAO-2 and (d) ZAO-3 TFTs annealed in air at 200 °C for various time.
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Fig. 6. The illumination time-dependent transfer characteristics of the (a) ZnO and (b) ZAO-2 TFTs.
TFTs under illumination. Moreover, the off current of ZnO TFTs even increases by around four orders of magnitude under illumination. These results reveal that doping Al into ZnO can significantly improve the stability under illumination. The light absorption can increase the carrier concentration in the ZnO channel likely in two ways. One is that, it is easily absorbed by ZnO with a minimum optical bandgap of 3.2 eV compared with the Al-doped counterpart, generating plentiful holes in its valence band and excited electrons to its conduction band [17,18]. The other one is that, the light absorption can excite fully occupied neutral oxygen vacancies (VO) near the valence band maximum (VBM) [19], usually results in the increment of carrier concentration. As a result, the ZAO-2 TFTs with larger bandgap can suppress the off-current and reduce the subthreshold swing under illumination. Fig. 7 shows the negative and positive gate bias stress instabilities of the ZnO and ZAO-2 TFTs in air and N2, respectively. After a constant voltage stress (CVS) of 10 V for 1200 s, the ΔVth of the ZAO-2 TFT is only 0.1 V, which is nearly one order of magnitude smaller than that of the ZnO TFT. It was reported that a positive gate bias induces the adsorption
of oxygen molecules in the channel by decreasing the activation energy, and the charge transfer may occur by the adsorption of oxygen molecules in the back channel without passivation layer [20,21]. The oxygen molecules adsorbed in the back channel could form a depletion layer [20] by the interaction between oxygen molecules and minority carrier holes near the back channel surface. This will effectively cause the electron trapping in the vicinity of the channel/Al2O3 interface. Then, the trapped electrons could affect the TFTs operation by increasing the applied gate voltage, resulting in a positive shift in Vth. It is worthwhile to note that grain boundaries in ZnO TFT provide many adsorption sites for oxygen molecules within the channel layer because of its nature of polycrystalline, which accelerates the positive ΔVth under PBS for ZnO TFTs [22]. Moreover, under PBS, the majority carrier electrons could also be trapped in the vicinity of the channel/Al2O3 interface, consequently leading to a positive shift in Vth. Under a negative gate bias stress, the ΔVth for ZnO and ZAO-2 TFTs is nearly the same, shown in Fig. 7. The negative gate bias induces a repulsive force against the majority carrier electrons in the channel, and polar H2O molecules simultaneously attract electrons at the surface [21,22]. This electrostatic attraction results in an asymmetric distribution of majority carrier electrons in the channel, leading to the generation of an internal electric potential [23]. Such an induced electric potential could act as a positive gate bias and is effective in enhancing the n-channel formation. To reduce the influence of O2 and H2O on the instability of ZnO and ZAO-2 TFTs, the same experiments were carried out in N2 ambient. It was found that positive and negative ΔVth produced in N2 ambient are more inappreciable than that under atmospheric air, indicative of environmental ambient is crucial to the electrical instability of ZAO TFTs. Therefore, in air ambient both charge transfer and electric stress will degrade the TFTs by increasing the Vth. However, PBS only induces a small positive ΔVth in our case, which is verified by the observations in N2 ambient. 4. Conclusion
Fig. 7. Dependences of ΔVth on gate stress time for the ZnO and ZAO-2 TFT in air and N2, respectively.
In summary, we have demonstrated the TFTs with Al doped ZnO channels using atomic layer deposition for enhancing the stability. With respect to the polycrystalline of ZnO film, the ZAO film can maintain an amorphous nature by increasing Al doping concentration. Compared with ZnO TFTs, the ZAO TFTs have significant improvement in the electrical stabilities under illumination and gate bias stress. Additionally, the ZAO TFTs also exhibited superior immunity to the surrounding
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ambient. These findings suggest the Al-doped ZnO film may be a highly promising channel material for flexible electronic applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61474027, 61274088), start-up program JIH1233003 at Fudan University. This work was also sponsored by Shanghai Pujiang Program (No. 16PJ1400800). References [1] E. Fortunato, A. Pimentel, L. Pereira, A. Gonçalves, G. Lavareda, H. Aguas, I. Ferreira, C.N. Carvalho, R. Martins, J. Non-Cryst, Solids 338 (2004) 806–809. [2] Y. Geng, W. Yang, H.L. Lu, Y. Zhang, Q.Q. Sun, P. Zhou, P.F. Wang, S.J. Ding, D.W. Zhang, IEEE Electron Device Lett. 35 (2014) 1266–1268. [3] E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Gonçalves, A.J.S. Marques, R.F.P. Martins, L.M.N. Pereira, Appl. Phys. Lett. 85 (2014) 2541. [4] R.B.M. Cross, M.M.D. Souza, Appl. Phys. Lett. 89 (2006) 263513. [5] J. Lee, J.S. Park, Y.S. Pyo, D.B. Lee, E.H. Kim, D. Stryakhilev, T.W. Kim, D.U. Jin, Y.G. Mo, Appl. Phys. Lett. 95 (2009) 123502. [6] R. Navamathavan, E.J. Yang, J.H. Lim, D.K. Hwang, J.Y. Oh, J.H. Yang, J.H. Jang, S.J. Park, J. Electrochem. Soc. 153 (2006) G385. [7] C.J. Ku, W.C. Hong, T. Mohsin, R. Li, Z. Duan, Y. Lu, IEEE Electron Device Lett. 36 (2015) 914–916.
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