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Effect of substrate temperature on sputtered indium-aluminum-zinc oxide films and thin film transistors
Journal of Alloys and Compounds 791 (2019) 773e778
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of substrate temperature on sputtered indium-aluminum-zinc oxide films and thin film transistors Weidong Xu a, Jianfeng Jiang a, Sanjin Xu a, Yu Zhang a, Huayong Xu a, Lin Han b, Xianjin Feng a, * a b
School of Microelectronics, Shandong University, Jinan 250100, China Institute of Marine Science and Technology, Shandong University, Qingdao 266200, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2019 Received in revised form 14 March 2019 Accepted 16 March 2019 Available online 31 March 2019
Indium-aluminum-zinc oxide (IAZO) films and thin film transistors (TFTs) were prepared at different substrate temperatures by RF magnetron sputtering. Both the unannealed and annealed IAZO films exhibited an amorphous state with very flat surface topographies. An increase in oxygen vacancies and a decrease in Hall mobility with the increase of substrate temperature were observed for the annealed IAZO films. The 25 C-deposited film after annealing possessed the highest Hall mobility of 67.7 cm2/V. A slight increase in transmittance was observed after annealing with super-high average transmittances over 93.8% in the visible range and large optical bandgaps around 4.10 eV obtained for all the IAZO films. The IAZO TFT fabricated at 25 C exhibited the best overall performance with the highest saturation mobility of 11.11 cm2/V, lowest subthreshold swing of 0.42 V/dec, lowest hysterisis of 0.31 V, and optimal threshold voltage of 5.12 V. © 2019 Elsevier B.V. All rights reserved.
Keywords: Amorphous oxide semiconductors IAZO Substrate temperature Thin film transistors
1. Introduction Recently, amorphous oxide semiconductors (AOSs) have been widely used as the channel layer of thin film transistors (TFTs) because of their high mobility, low preparation temperature and good uniformity etc. [1e4] as compared to amorphous silicon (low mobility) and polysilicon (high preparation temperature and poor uniformity) [5e7]. At present, amorphous IGZO TFTs have been extensively researched because of the high electrical performance and good repeatability [8]. However, due to the relatively narrow band gap of IGZO (Eg~3.2 eV) [9], it is not possible to apply to detectors with wider band gaps, especially the deep ultraviolet detectors [10]. At the same time, the electrical stability of IGZO TFTs deteriorates when they work under bias and illumination for a long time [11e14]. Therefore, it is necessary to find a new AOS to overcome the shortcomings of IGZO including narrow band gap and poor stability. It has been suggested that the replacement of Ga by Al i.e. the formation of In-Al-Zn oxide (IAZO) could not only increase the band gap of the AOS films, but also improve the stability of the fabricated TFTs [15e17]. Because Al2O3 has a large band gap
[18,19] and high metal-oxygen binding energy. At present, the research on IAZO films and TFTs is very rare and mainly focuses on the pulsed laser deposition (PLD) [16,20] and solgel methods [21,22]. The highest saturation mobility (msat) of the asfabricated IAZO TFTs was only 2.2 cm2/V. In 2018, T. H. Cheng et al. fabricated IAZO TFTs using sputtering process for the first time [17] and investigated the influence of oxygen/argon flow ratio. But the performance of the best device was still unsatisfactory with a msat of 5.67 cm2/V and an on-off current ratio (Ion/Ioff) of 3.37 106. Nevertheless, since sputtering is known to be suitable for largescale production and compatible with the flat-panel display processes, it is necessary to study the effects of sputtering conditions on the properties of IAZO films and TFTs more thoroughly. It is wellknown that the substrate temperature during sputtering can strongly affect the properties of deposited films and consequently the performance of fabricated devices. Therefore, in this paper, the IAZO films and TFTs were prepared at different substrate temperatures using RF magnetron sputtering for the first time. The influence of substrate temperature on the film properties and device performance was investigated in detail. 2. Experimental
* Corresponding author. E-mail address: [email protected] (X. Feng). https://doi.org/10.1016/j.jallcom.2019.03.245 0925-8388/© 2019 Elsevier B.V. All rights reserved.
The IAZO films (120 nm in thickness) were prepared at different
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substrate temperatures (25 i.e. room temperature, 100, 200, 300 and 400 C) by RF magnetron sputtering using a high-purity InAlZnO target (4 N purity, In:Al:Zn ¼ 2:1:1 at.%). Commercially available SiO2 (100 nm)/pþ-Si and sapphire were chosen as the substrates for film deposition after a four-step ultrasonic cleaning in 5% Decon 90 aqueous solution, deionized water, acetone and anhydrous ethanol. The SiO2 layer was formed on Si by thermal oxidation. During deposition, high purity Ar gas was introduced into the chamber, and the Ar flow rate and sputtering pressure were respectively set to 20 SCCM and 3.68 mTorr under the RF power of 90 W. Finally, in order to study the effect of annealing on the film properties, the as-prepared films were annealed in air at 235 C for 1 h. The TFTs employing IAZO active layer (30 nm in thickness) grown on SiO2 (100 nm)/pþ-Si substrates under the same conditions as above were fabricated to have a bottom-gate top-contact structure. The pþ-Si and SiO2 served as the gate electrode and dielectric layer, respectively. After the deposition of IAZO channel, the samples were annealed in air at 235 C for 1 h. In the end, the Ti source and drain electrodes (50 nm in thickness) were deposited on IAZO by electron-beam evaporation using a shadow mask (W/ L ¼ 2000/60 mm). The film structure and surface morphology were measured by a Rigaku X-ray diffractometer (XRD) with Cu Ka1 radiation (l ¼ 1.5406 Å) and a Benyuan CSPM5500 atomic force microscope (AFM), respectively. The chemical states of the films were analyzed by a Thermo ESCALAB 250XI X-ray photoelectron spectroscope (XPS). The electrical properties of IAZO films were examined by an East Changing ET9000 Hall measurement system with Van der
Pauw method. A TU-1901 double-beam UVeviseNIR spectrophotometer was employed to study the optical transmittance and optical band gap of the films. An Agilent B2900 semiconductor analyzer was used to measure the electrical performance of IAZO TFTs.
3. Results and discussion XRD measurements were performed to investigate the structure of IAZO films deposited at various substrate temperatures. Fig. 1(a) displays the XRD patterns of IAZO films grown at 25 and 300 C with and without annealing. The XRD pattern of SiO2 (100 nm)/pþSi substrate is used for reference. It can be found that only the Si(100) substrate peak at around 69.5 can be observed, implying an amorphous state for these films. The same results were obtained for the films deposited at other substrate temperatures, which are not shown here due to limited space. The AFM images of IAZO films prepared at 300 C without and with annealing are shown in Fig. 1(b) and (c), respectively. Both films exhibit a smooth and dense surface with low root mean square (RMS) roughnesses of 0.49 and 0.55 nm obtained respectively for the unannealed and annealed films. The influence of substrate temperature on RMS roughness is shown in Fig. 1(d). We can see that low RMS roughnesses in the range of 0.47e0.69 nm are obtained for our films. The RMS roughness of the unannealed IAZO films generally decreases with substrate temperature, which indicates that high growth temperature is beneficial to promote the surface diffusion of the atoms and therefore reduce the film surface roughness. Meanwhile, annealing at 235 C can effectively reduce
Fig. 1. (a) XRD patterns of IAZO films deposited at 25 and 300 C. (b) and (c) AFM images of the IAZO films grown at 300 C before and after annealing, respectively. (d) Influence of substrate temperature on the RMS roughness.
W. Xu et al. / Journal of Alloys and Compounds 791 (2019) 773e778
the surface roughness of the 25 and 100 C-grown IAZO films, but an opposite trend is observed when the growth temperature approaches or exceeds the annealing temperature. Fig. 2(a)-(d) show the XPS spectra of O 1s core level for the unannealed 25 C-deposited, annealed 25 C-deposited, unannealed 400 C-deposited, and annealed 400 C-deposited films, respectively. It can be seen that the fitted O 1s core level is separated into three main peaks, which are centered at around 529.9, 531.2 and 531.9 eV corresponding respectively to the oxygen ions combined with In, Al and Zn ions (OI), oxygen vacancies (OII) and adsorbed surface contaminations (OIII) such as eCO3, eOH and H2O species. As we all know, the generation of oxygen vacancies can be
775
described using the following process: 1 Oxo ¼ 1=2ðOÞ2ðgÞ [ þ V ,, o þ 2e
(1)
Here, oxygen (O2) easily escapes from the oxide sublattice (Oxo ), which produces a double-charged oxygen vacancy (V ,, o ) and two free electrons. The oxygen vacancies can act as shallow donors, which are positively related to the electron concentration. By comparing Fig. 2(a) (please also refer to our previous work [15]) and (b), and/or Fig. 2(c) and (d), it is found that the percentage of OI increases and that of OII decreases after annealing of the IAZO films, which may indicate a decrease in carrier concentration. The similar
Fig. 2. (a)e(d) XPS spectra of O 1s core level for the 25 C and 400 C-deposited films before and after annealing. (e) Electrical performance of the annealed IAZO films as a function of substrate temperature.
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effect of annealing on O 1s core level is observed for the films deposited at other substrate temperatures. From Fig. 2(c) and (d), it can be seen that with the increase of substrate temperature to 400 C, the percentage of OI decreases and that of OII increases, implying the generation of more free electrons. The film composition was also calculated using the XPS data. It is found that with the increase of substrate temperature from 25 to 400 C, the In content decreases monotonically from 51.6% to 49.8%, the Al content increases slightly from 19.0% to 19.3%, and the Zn content increases monotonically from 29.4% to 30.9%. This indicates that the change of O 1s core level may be related to the different film compositions. Similar results have been reported for IGZO [23]. The resistivity, carrier concentration and Hall mobility of the annealed IAZO films as a function of substrate temperature are depicted in Fig. 2(e). We can see that with the increase of substrate temperature from 25 to 400 C, the carrier concentration increases monotonically from 5.26 1014 to 3.97 1015/cm3, which is in good agreement with the variation trend of oxygen vacancies as discussed above. However, the increased oxygen vacancies with the increase of substrate temperature will lead to the enhanced electron scattering and recombination, which together with the variation in film composition could result in a decrease in Hall mobility. As a result, the IAZO film grown at 25 C has the best electrical properties with the highest Hall mobility (67.7 cm2/V) and a suitable resistivity (1.45 102 U cm), implying its great potential to be used as a channel layer in high performance TFTs. Fig. 3(a) and (b) exhibit the transmittance spectra of the unannealed and annealed IAZO films deposited on sapphire substrates at various substrate temperatures, respectively. The average absolute transmittances of all the unannealed IAZO films are above 93.8% in the visible range, which are obviously improved after annealing. This is probably due to the decreased photon scattering and absorption caused by the structural defects in our films. In addition, all the annealed films have a higher transmittance in the long wavelength region, which is mainly due to the decrease of free carriers [24]. For amorphous semiconductors, electronic transitions are not limited by the conservation of quasi-momentum because they do not satisfy long-range order. Therefore, a direct transition can occur, and we can calculate the optical band gap (Eg) of IAZO using the relation between ahn and hn:
ðahnÞ2 ¼ A hn Eg
(2)
Here, a, h, n, A are respectively the absorption coefficient, the Planck constant, the frequency of the incident light, and a material-
dependent constant. The insets in Fig. 3(a) and (b) are plots of (ahn)2 versus hn for the films without and with annealing, respectively. The optical Eg values can be obtained from the intersections between the extension lines of the linear part of these plots and the hn axis. All the IAZO films exhibit large optical band gaps in the range of 4.10 ± 0.02 eV, which are independent on substrate temperature. The wide Eg of IAZO indicates high optical stability and the great promise to be used in transparent electronics. The transfer curves of IAZO TFTs fabricated at 25, 100, 200, 300, and 400 C are displayed respectively in Fig. 4(a)-(e), where VG, VD and ID represent respectively the gate voltage, drain voltage and drain current. The plots marked by VD ¼ 1 V and VD ¼ 25 V are transfer curves of the linear and saturation regions, respectively. All the IAZO TFTs operate in a depletion mode, and have ideal transfer curves and high on currents. We can calculate the msat according to the slope of the (ID)1/2-VG transfer curves (saturation region) using the following formula:
msat ¼
2L WCi
pffiffiffiffiffi!2 v ID vVG
(3)
where L, W, and Ci are the channel length, channel width, and capacitance per unit area of the gate dielectric layer, respectively. The specific performance of IAZO TFTs are shown in Table 1, where SS represents the subthreshold swing, and hysterisis equals to the difference between the threshold voltages of the reverse sweep (Vt2) and forward sweep (Vt1). We can see that msat of the IAZO TFTs in general decreases with substrate temperature, with the highest value of 11.11 cm2/V obtained for the device fabricated at room temperature. This result is in good accordance with the variation trend of Hall mobility as shown in Fig. 2. The SS first increases and then decreases with substrate temperature, with the maximum value obtained at 200 C. This is basically consistent with the variation trend of the RMS roughness of annealed IAZO films, indicating that SS can be characterized and affected by the surface roughness of IAZO channel. Note that the different surface conditions of SiO2 (100 nm)/pþ-Si substrates before the deposition of IAZO channel may be another cause for the different SS values. The hysterisis also has a close correlationship with Hall mobility and/or msat, with the minimum value of 0.31 V obtained at room temperature, indicating the fewest defects in the active layer of this device. Overall, the IAZO TFT fabricated at 25 C exhibits the best performance with the highest msat, the lowest hysterisis, SS, and Vt, as well as a moderate Ion/Ioff obtained among all the devices. These parameters are much improved compared to those of the IAZO TFTs
Fig. 3. Optical transmittance spectra of the (a) unannealed and (b) annealed IAZO films with the plots of (ahn)2 versus hn shown in the inset.
W. Xu et al. / Journal of Alloys and Compounds 791 (2019) 773e778
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Fig. 4. (a)e(e) Transfer characteristics of IAZO TFTs fabricated at 25, 100, 200, 300 and 400 C, respectively.
Table 1 Electrical properties of IAZO TFTs fabricated at various substrate temperatures. temperature ( C)
msat (cm2/Vs)
Vt1 (V)
Vt2 (V)
hysterisis (V)
SS (V/dec)
Ion/Ioff
25 100 200 300 400
11.11 8.14 7.28 6.04 8.65
5.12 10.69 16.51 12.51 8.21
5.43 11.04 16.95 12.96 8.63
0.31 0.35 0.44 0.45 0.42
0.42 0.50 0.60 0.45 0.43
2.92 107 4.52 107 5.25 107 1.14 107 2.38 107
reported previously [16,17,20e22], indicating the great application potential of our devices in the next-generation electronic and optoelectronic fields. The performance of the 25 C-fabricated IAZO TFT under positive bias stress (PBS) was measured at room temperature to study the device stability as shown in Fig. 5. During the measurement, the VG of 30 V was applied to the gate electrode for different durations. From Fig. 5(a), it can be seen that the transfer curve moves in the positive direction as the bias time increases. The specific variations of Vt and SS as a function of stress time are displayed in Fig. 5(b). In
general, the shifts of transfer curve and threshold voltage are mainly due to the electron traps at the insulator/channel interface, in the bulk of the channel [23], and/or near the upper surface of the active layer. Fortunately, we can see that the SS in our study did not change significantly with stress time. This indicates that no new defects are generated at the insulator/channel interface and inside the channel, and the change of Vt is mainly related to the electron traps near the upper surface of the active layer. In addition, the offcurrent mainly flows through the upper surface of the active layer [25]. We can see that the increase of bias time has almost no effect
Fig. 5. (a) Variations in the transfer characteristics with the evolution of stress time at a VG of 30 V for the TFT fabricated at 25 C. (b) The shift of Vt and SS as a function of bias time.
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on the on-current, but it severely reduces the off-current, which means that the electrons near the upper surface of the active layer are significantly reduced. Therefore, the electron traps near the upper surface of the active layer are the main factor affecting the reliability of our IAZO TFTs. 4. Conclusions IAZO films and TFTs were prepared by RF magnetron sputtering and the influence of substrate temperature (25e400 C) on the film properties and device performance was investigated. We found that both the as-deposited and annealed IAZO films were in amorphous state with smooth surfaces, high average visible transmittances over 93.8%, and large optical band gaps (4.10 ± 0.02 eV). The oxygen vacancies, metal-oxygen bonds, film composition and consequently the electrical properties of the films were found to be obviously influenced by substrate temperature. As a result, the film grown at 25 C after annealing had the highest Hall mobility of 67.7 cm2/V. The performance of IAZO TFTs was also strongly affected by substrate temperature. The device fabricated at 25 C had the best overall performance with the highest field-effect mobility of 11.11 cm2/V and optimal threshold voltage of 5.12 V. Meanwhile, this device also exhibited the lowest hysterisis (0.31 V) and subthreshold swing (0.42 V/dec), indicating the minimal defects both inside the channel and at the insulator/ channel interface. The PBS stability of our IAZO TFTs is mainly affected by the electron traps near the upper surface of the channel layer. Acknowledgements This work was supported by the National Key Research and Development Program of China (Grant No. 2017YFB0405400), Key Research and Development Program of Shandong Province, China (Grant No. 2017GGX201007), Fundamental Research Funds of Shandong University (Grant No. 2018JC034), and China Postdoctoral Science Foundation (Grant No. 2018T110685). References [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors, Nature 432 (2004) 488e492. [2] K. Takenaka, M. Endo, G. Uchida, A. Ebe, Y. Setsuhara, Influence of deposition condition on electrical properties of a-IGZO films deposited by plasmaenhanced reactive sputtering, J. Alloys Compd. 772 (2019) 642e649. [3] Y.C. Zhang, G. He, C. Zhang, L. Zhu, B. Yang, Q.B. Lin, X.S. Jiang, Oxygen partial pressure ratio modulated electrical performance of amorphous InGaZnO thin film transistor and inverter, J. Alloys Compd. 765 (2018) 791e799. [4] J. Su, Y. Wang, Y. Ma, Q. Wang, L. Tian, S. Dai, R. Li, X. Zhang, Y. Wang, Preparation and electrical characteristics of N-doped In-Zn-Sn-O thin film transistors by radio frequency magnetron sputtering, J. Alloys Compd. 750 (2018) 1003e1006.
[5] K.K. Banger, R.L. Peterson, K. Mori, Y. Yamashita, T. Leedham, H. Sirringhaus, High performance, low temperature solution-processed barium and strontium doped oxide thin film transistors, Chem. Mater. 26 (2014) 1195e1203. [6] B. Wang, L. Zeng, W. Huang, F.S. Melkonyan, W.C. Sheets, L. Chi, A. Facchetti, Carbohydrate-assisted combustion synthesis to realize high-performance oxide transistors, J. Am. Chem. Soc. 138 (2016) 7067e7074. [7] S. Dai, T. Wang, R. Li, Q. Wang, Y. Ma, L. Tian, J. Su, Y. Wang, D. Zhou, X. Zhang, Y. Wang, Preparation and electrical properties of N-doped ZnSnO thin film transistors, J. Alloys Compd. 745 (2018) 256e261. [8] H.W. Zan, C.C. Yeh, H.F. Meng, C.C. Tsai, L.H. Chen, Achieving high field-effect mobility in amorphous indium gallium zinc oxide by capping a strong reduction layer, Adv. Mater. 24 (2012) 3509e3514. [9] T. Kamiya, K. Nomura, H. Hosono, Present status of amorphous In-Ga-Zn-O thin-film transistors, Sci. Technol. Adv. Mater. 11 (2010), 044305. [10] X. Du, W. Wang, M. Wang, X. Feng, J. Ma, IneAleSn oxide semiconducting films with high mobility prepared by metal-organic chemical vapor deposition, Mater. Lett. 183 (2016) 359e361. [11] J.S. Seo, B.S. Bae, Improved electrical performance and bias stability of solution-processed active bilayer structure of indium zinc oxide based TFT, ACS Appl. Mater. Interfaces 6 (2014) 15335e15343. [12] J.S. Heo, J.W. Jo, J. Kang, C.Y. Jeong, H.Y. Jeong, S.K. Kim, K. Kim, H.I. Kwon, J. Kim, Y.H. Kim, M.G. Kim, S.K. Park, Water-mediated photochemical treatments for low-temperature passivation of metal-oxide thin-film transistors, ACS Appl. Mater. Interfaces 8 (2016) 10403e10412. [13] I.K. Lee, K.H. Lee, S. Lee, W.J. Cho, Microwave annealing effect for highly reliable biosensor: dual-gate ion-sensitive field-effect transistor using amorphous InGaZnO thin-film transistor, ACS Appl. Mater. Interfaces 6 (2014) 22680e22686. [14] J.Y. Bak, Y. Kang, S. Yang, H.J. Ryu, C.S. Hwang, S. Han, S.M. Yoon, Origin of degradation phenomenon under drain bias stress for oxide thin film transistors using IGZO and IGO channel layers, Sci. Rep. 5 (2015) 7884. [15] W. Xu, M. Xu, J. Jiang, C. Luan, L. Han, X. Feng, High performance thin film transistors with sputtered In-Al-Zn-O channel and different source/drain electrodes, IEEE Electron. Device Lett. 40 (2019) 247e250. [16] Z. Ye, S. Yue, J. Zhang, X. Li, L. Chen, J. Lu, Annealing treatment on amorphous InAlZnO films for thin-film transistors, IEEE Trans. Electron Devices 63 (2016) 3547e3551. [17] T.H. Cheng, S.P. Chang, S.J. Chang, Electrical properties of indium aluminum zinc oxide thin film transistors, J. Electron. Mater. 47 (2018) 6923e6928. [18] P. Ma, L. Du, Y. Wang, R. Jiang, Q. Xin, Y. Li, A. Song, Low voltage operation of IGZO thin film transistors enabled by ultrathin Al2O3 gate dielectric, Appl. Phys. Lett. 112 (2018), 023501. [19] B.P. Dhonge, T. Mathews, S.T. Sundari, C. Thinaharan, M. Kamruddin, S. Dash, A.K. Tyagi, Spray pyrolytic deposition of transparent aluminum oxide (Al2O3) films, Appl. Surf. Sci. 258 (2011) 1091e1096. [20] J. Zhang, J.G. Lu, Q.J. Jiang, B. Lu, X.H. Pan, L.X. Chen, Z.Z. Ye, X.F. Li, P.J. Guo, N.J. Zhou, Stability of amorphous InAlZnO thin-film transistors, J. Vac. Sci. Technol. B 32 (2014), 010602. [21] S.M. Park, D.H. Lee, Y.S. Lim, D.K. Kim, M. Yi, Effect of aluminum addition to solution-derived amorphous indium zinc oxide thin film for an oxide thin film transistors, Microelectron. Eng. 109 (2013) 189e192. [22] M.J. Park, J.Y. Bak, J.S. Choi, S.M. Yoon, Impact of aluminum incorporation into In-Zn-O active channel for highly-stable thin-film transistor using solution process, ECS Solid State Lett. 3 (2014) Q44eQ46. [23] S.M. Yoon, N.J. Seong, K. Choi, G.H. Seo, W.C. Shin, Effects of deposition temperature on the device characteristics of oxide thin-film transistors using ineGaeZneO active channels prepared by atomic-layer deposition, ACS Appl. Mater. Interfaces 9 (2017) 22676e22684. [24] M.J. Gadre, T.L. Alford, Highest transmittance and high-mobility amorphous indium gallium zinc oxide films on flexible substrate by room-temperature deposition and post-deposition anneals, Appl. Phys. Lett. 99 (2011), 051901. [25] J.W. Park, D. Lee, H. Kwon, S. Yoo, Improvement of on-off-current ratio in TiOx active channel TFTs using N2O plasma treatment, IEEE Electron. Device Lett. 30 (2009) 362e364.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effect of substrate temperature on sputtered indium-aluminum-zinc oxide films and thin film transistors Weidong Xu a, Jianfeng Jiang a, Sanjin Xu a, Yu Zhang a, Huayong Xu a, Lin Han b, Xianjin Feng a, * a b
School of Microelectronics, Shandong University, Jinan 250100, China Institute of Marine Science and Technology, Shandong University, Qingdao 266200, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 January 2019 Received in revised form 14 March 2019 Accepted 16 March 2019 Available online 31 March 2019
Indium-aluminum-zinc oxide (IAZO) films and thin film transistors (TFTs) were prepared at different substrate temperatures by RF magnetron sputtering. Both the unannealed and annealed IAZO films exhibited an amorphous state with very flat surface topographies. An increase in oxygen vacancies and a decrease in Hall mobility with the increase of substrate temperature were observed for the annealed IAZO films. The 25 C-deposited film after annealing possessed the highest Hall mobility of 67.7 cm2/V. A slight increase in transmittance was observed after annealing with super-high average transmittances over 93.8% in the visible range and large optical bandgaps around 4.10 eV obtained for all the IAZO films. The IAZO TFT fabricated at 25 C exhibited the best overall performance with the highest saturation mobility of 11.11 cm2/V, lowest subthreshold swing of 0.42 V/dec, lowest hysterisis of 0.31 V, and optimal threshold voltage of 5.12 V. © 2019 Elsevier B.V. All rights reserved.
Keywords: Amorphous oxide semiconductors IAZO Substrate temperature Thin film transistors
1. Introduction Recently, amorphous oxide semiconductors (AOSs) have been widely used as the channel layer of thin film transistors (TFTs) because of their high mobility, low preparation temperature and good uniformity etc. [1e4] as compared to amorphous silicon (low mobility) and polysilicon (high preparation temperature and poor uniformity) [5e7]. At present, amorphous IGZO TFTs have been extensively researched because of the high electrical performance and good repeatability [8]. However, due to the relatively narrow band gap of IGZO (Eg~3.2 eV) [9], it is not possible to apply to detectors with wider band gaps, especially the deep ultraviolet detectors [10]. At the same time, the electrical stability of IGZO TFTs deteriorates when they work under bias and illumination for a long time [11e14]. Therefore, it is necessary to find a new AOS to overcome the shortcomings of IGZO including narrow band gap and poor stability. It has been suggested that the replacement of Ga by Al i.e. the formation of In-Al-Zn oxide (IAZO) could not only increase the band gap of the AOS films, but also improve the stability of the fabricated TFTs [15e17]. Because Al2O3 has a large band gap
[18,19] and high metal-oxygen binding energy. At present, the research on IAZO films and TFTs is very rare and mainly focuses on the pulsed laser deposition (PLD) [16,20] and solgel methods [21,22]. The highest saturation mobility (msat) of the asfabricated IAZO TFTs was only 2.2 cm2/V. In 2018, T. H. Cheng et al. fabricated IAZO TFTs using sputtering process for the first time [17] and investigated the influence of oxygen/argon flow ratio. But the performance of the best device was still unsatisfactory with a msat of 5.67 cm2/V and an on-off current ratio (Ion/Ioff) of 3.37 106. Nevertheless, since sputtering is known to be suitable for largescale production and compatible with the flat-panel display processes, it is necessary to study the effects of sputtering conditions on the properties of IAZO films and TFTs more thoroughly. It is wellknown that the substrate temperature during sputtering can strongly affect the properties of deposited films and consequently the performance of fabricated devices. Therefore, in this paper, the IAZO films and TFTs were prepared at different substrate temperatures using RF magnetron sputtering for the first time. The influence of substrate temperature on the film properties and device performance was investigated in detail. 2. Experimental
* Corresponding author. E-mail address: [email protected] (X. Feng). https://doi.org/10.1016/j.jallcom.2019.03.245 0925-8388/© 2019 Elsevier B.V. All rights reserved.
The IAZO films (120 nm in thickness) were prepared at different
774
W. Xu et al. / Journal of Alloys and Compounds 791 (2019) 773e778
substrate temperatures (25 i.e. room temperature, 100, 200, 300 and 400 C) by RF magnetron sputtering using a high-purity InAlZnO target (4 N purity, In:Al:Zn ¼ 2:1:1 at.%). Commercially available SiO2 (100 nm)/pþ-Si and sapphire were chosen as the substrates for film deposition after a four-step ultrasonic cleaning in 5% Decon 90 aqueous solution, deionized water, acetone and anhydrous ethanol. The SiO2 layer was formed on Si by thermal oxidation. During deposition, high purity Ar gas was introduced into the chamber, and the Ar flow rate and sputtering pressure were respectively set to 20 SCCM and 3.68 mTorr under the RF power of 90 W. Finally, in order to study the effect of annealing on the film properties, the as-prepared films were annealed in air at 235 C for 1 h. The TFTs employing IAZO active layer (30 nm in thickness) grown on SiO2 (100 nm)/pþ-Si substrates under the same conditions as above were fabricated to have a bottom-gate top-contact structure. The pþ-Si and SiO2 served as the gate electrode and dielectric layer, respectively. After the deposition of IAZO channel, the samples were annealed in air at 235 C for 1 h. In the end, the Ti source and drain electrodes (50 nm in thickness) were deposited on IAZO by electron-beam evaporation using a shadow mask (W/ L ¼ 2000/60 mm). The film structure and surface morphology were measured by a Rigaku X-ray diffractometer (XRD) with Cu Ka1 radiation (l ¼ 1.5406 Å) and a Benyuan CSPM5500 atomic force microscope (AFM), respectively. The chemical states of the films were analyzed by a Thermo ESCALAB 250XI X-ray photoelectron spectroscope (XPS). The electrical properties of IAZO films were examined by an East Changing ET9000 Hall measurement system with Van der
Pauw method. A TU-1901 double-beam UVeviseNIR spectrophotometer was employed to study the optical transmittance and optical band gap of the films. An Agilent B2900 semiconductor analyzer was used to measure the electrical performance of IAZO TFTs.
3. Results and discussion XRD measurements were performed to investigate the structure of IAZO films deposited at various substrate temperatures. Fig. 1(a) displays the XRD patterns of IAZO films grown at 25 and 300 C with and without annealing. The XRD pattern of SiO2 (100 nm)/pþSi substrate is used for reference. It can be found that only the Si(100) substrate peak at around 69.5 can be observed, implying an amorphous state for these films. The same results were obtained for the films deposited at other substrate temperatures, which are not shown here due to limited space. The AFM images of IAZO films prepared at 300 C without and with annealing are shown in Fig. 1(b) and (c), respectively. Both films exhibit a smooth and dense surface with low root mean square (RMS) roughnesses of 0.49 and 0.55 nm obtained respectively for the unannealed and annealed films. The influence of substrate temperature on RMS roughness is shown in Fig. 1(d). We can see that low RMS roughnesses in the range of 0.47e0.69 nm are obtained for our films. The RMS roughness of the unannealed IAZO films generally decreases with substrate temperature, which indicates that high growth temperature is beneficial to promote the surface diffusion of the atoms and therefore reduce the film surface roughness. Meanwhile, annealing at 235 C can effectively reduce
Fig. 1. (a) XRD patterns of IAZO films deposited at 25 and 300 C. (b) and (c) AFM images of the IAZO films grown at 300 C before and after annealing, respectively. (d) Influence of substrate temperature on the RMS roughness.
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the surface roughness of the 25 and 100 C-grown IAZO films, but an opposite trend is observed when the growth temperature approaches or exceeds the annealing temperature. Fig. 2(a)-(d) show the XPS spectra of O 1s core level for the unannealed 25 C-deposited, annealed 25 C-deposited, unannealed 400 C-deposited, and annealed 400 C-deposited films, respectively. It can be seen that the fitted O 1s core level is separated into three main peaks, which are centered at around 529.9, 531.2 and 531.9 eV corresponding respectively to the oxygen ions combined with In, Al and Zn ions (OI), oxygen vacancies (OII) and adsorbed surface contaminations (OIII) such as eCO3, eOH and H2O species. As we all know, the generation of oxygen vacancies can be
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described using the following process: 1 Oxo ¼ 1=2ðOÞ2ðgÞ [ þ V ,, o þ 2e
(1)
Here, oxygen (O2) easily escapes from the oxide sublattice (Oxo ), which produces a double-charged oxygen vacancy (V ,, o ) and two free electrons. The oxygen vacancies can act as shallow donors, which are positively related to the electron concentration. By comparing Fig. 2(a) (please also refer to our previous work [15]) and (b), and/or Fig. 2(c) and (d), it is found that the percentage of OI increases and that of OII decreases after annealing of the IAZO films, which may indicate a decrease in carrier concentration. The similar
Fig. 2. (a)e(d) XPS spectra of O 1s core level for the 25 C and 400 C-deposited films before and after annealing. (e) Electrical performance of the annealed IAZO films as a function of substrate temperature.
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effect of annealing on O 1s core level is observed for the films deposited at other substrate temperatures. From Fig. 2(c) and (d), it can be seen that with the increase of substrate temperature to 400 C, the percentage of OI decreases and that of OII increases, implying the generation of more free electrons. The film composition was also calculated using the XPS data. It is found that with the increase of substrate temperature from 25 to 400 C, the In content decreases monotonically from 51.6% to 49.8%, the Al content increases slightly from 19.0% to 19.3%, and the Zn content increases monotonically from 29.4% to 30.9%. This indicates that the change of O 1s core level may be related to the different film compositions. Similar results have been reported for IGZO [23]. The resistivity, carrier concentration and Hall mobility of the annealed IAZO films as a function of substrate temperature are depicted in Fig. 2(e). We can see that with the increase of substrate temperature from 25 to 400 C, the carrier concentration increases monotonically from 5.26 1014 to 3.97 1015/cm3, which is in good agreement with the variation trend of oxygen vacancies as discussed above. However, the increased oxygen vacancies with the increase of substrate temperature will lead to the enhanced electron scattering and recombination, which together with the variation in film composition could result in a decrease in Hall mobility. As a result, the IAZO film grown at 25 C has the best electrical properties with the highest Hall mobility (67.7 cm2/V) and a suitable resistivity (1.45 102 U cm), implying its great potential to be used as a channel layer in high performance TFTs. Fig. 3(a) and (b) exhibit the transmittance spectra of the unannealed and annealed IAZO films deposited on sapphire substrates at various substrate temperatures, respectively. The average absolute transmittances of all the unannealed IAZO films are above 93.8% in the visible range, which are obviously improved after annealing. This is probably due to the decreased photon scattering and absorption caused by the structural defects in our films. In addition, all the annealed films have a higher transmittance in the long wavelength region, which is mainly due to the decrease of free carriers [24]. For amorphous semiconductors, electronic transitions are not limited by the conservation of quasi-momentum because they do not satisfy long-range order. Therefore, a direct transition can occur, and we can calculate the optical band gap (Eg) of IAZO using the relation between ahn and hn:
ðahnÞ2 ¼ A hn Eg
(2)
Here, a, h, n, A are respectively the absorption coefficient, the Planck constant, the frequency of the incident light, and a material-
dependent constant. The insets in Fig. 3(a) and (b) are plots of (ahn)2 versus hn for the films without and with annealing, respectively. The optical Eg values can be obtained from the intersections between the extension lines of the linear part of these plots and the hn axis. All the IAZO films exhibit large optical band gaps in the range of 4.10 ± 0.02 eV, which are independent on substrate temperature. The wide Eg of IAZO indicates high optical stability and the great promise to be used in transparent electronics. The transfer curves of IAZO TFTs fabricated at 25, 100, 200, 300, and 400 C are displayed respectively in Fig. 4(a)-(e), where VG, VD and ID represent respectively the gate voltage, drain voltage and drain current. The plots marked by VD ¼ 1 V and VD ¼ 25 V are transfer curves of the linear and saturation regions, respectively. All the IAZO TFTs operate in a depletion mode, and have ideal transfer curves and high on currents. We can calculate the msat according to the slope of the (ID)1/2-VG transfer curves (saturation region) using the following formula:
msat ¼
2L WCi
pffiffiffiffiffi!2 v ID vVG
(3)
where L, W, and Ci are the channel length, channel width, and capacitance per unit area of the gate dielectric layer, respectively. The specific performance of IAZO TFTs are shown in Table 1, where SS represents the subthreshold swing, and hysterisis equals to the difference between the threshold voltages of the reverse sweep (Vt2) and forward sweep (Vt1). We can see that msat of the IAZO TFTs in general decreases with substrate temperature, with the highest value of 11.11 cm2/V obtained for the device fabricated at room temperature. This result is in good accordance with the variation trend of Hall mobility as shown in Fig. 2. The SS first increases and then decreases with substrate temperature, with the maximum value obtained at 200 C. This is basically consistent with the variation trend of the RMS roughness of annealed IAZO films, indicating that SS can be characterized and affected by the surface roughness of IAZO channel. Note that the different surface conditions of SiO2 (100 nm)/pþ-Si substrates before the deposition of IAZO channel may be another cause for the different SS values. The hysterisis also has a close correlationship with Hall mobility and/or msat, with the minimum value of 0.31 V obtained at room temperature, indicating the fewest defects in the active layer of this device. Overall, the IAZO TFT fabricated at 25 C exhibits the best performance with the highest msat, the lowest hysterisis, SS, and Vt, as well as a moderate Ion/Ioff obtained among all the devices. These parameters are much improved compared to those of the IAZO TFTs
Fig. 3. Optical transmittance spectra of the (a) unannealed and (b) annealed IAZO films with the plots of (ahn)2 versus hn shown in the inset.
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Fig. 4. (a)e(e) Transfer characteristics of IAZO TFTs fabricated at 25, 100, 200, 300 and 400 C, respectively.
Table 1 Electrical properties of IAZO TFTs fabricated at various substrate temperatures. temperature ( C)
msat (cm2/Vs)
Vt1 (V)
Vt2 (V)
hysterisis (V)
SS (V/dec)
Ion/Ioff
25 100 200 300 400
11.11 8.14 7.28 6.04 8.65
5.12 10.69 16.51 12.51 8.21
5.43 11.04 16.95 12.96 8.63
0.31 0.35 0.44 0.45 0.42
0.42 0.50 0.60 0.45 0.43
2.92 107 4.52 107 5.25 107 1.14 107 2.38 107
reported previously [16,17,20e22], indicating the great application potential of our devices in the next-generation electronic and optoelectronic fields. The performance of the 25 C-fabricated IAZO TFT under positive bias stress (PBS) was measured at room temperature to study the device stability as shown in Fig. 5. During the measurement, the VG of 30 V was applied to the gate electrode for different durations. From Fig. 5(a), it can be seen that the transfer curve moves in the positive direction as the bias time increases. The specific variations of Vt and SS as a function of stress time are displayed in Fig. 5(b). In
general, the shifts of transfer curve and threshold voltage are mainly due to the electron traps at the insulator/channel interface, in the bulk of the channel [23], and/or near the upper surface of the active layer. Fortunately, we can see that the SS in our study did not change significantly with stress time. This indicates that no new defects are generated at the insulator/channel interface and inside the channel, and the change of Vt is mainly related to the electron traps near the upper surface of the active layer. In addition, the offcurrent mainly flows through the upper surface of the active layer [25]. We can see that the increase of bias time has almost no effect
Fig. 5. (a) Variations in the transfer characteristics with the evolution of stress time at a VG of 30 V for the TFT fabricated at 25 C. (b) The shift of Vt and SS as a function of bias time.
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on the on-current, but it severely reduces the off-current, which means that the electrons near the upper surface of the active layer are significantly reduced. Therefore, the electron traps near the upper surface of the active layer are the main factor affecting the reliability of our IAZO TFTs. 4. Conclusions IAZO films and TFTs were prepared by RF magnetron sputtering and the influence of substrate temperature (25e400 C) on the film properties and device performance was investigated. We found that both the as-deposited and annealed IAZO films were in amorphous state with smooth surfaces, high average visible transmittances over 93.8%, and large optical band gaps (4.10 ± 0.02 eV). The oxygen vacancies, metal-oxygen bonds, film composition and consequently the electrical properties of the films were found to be obviously influenced by substrate temperature. As a result, the film grown at 25 C after annealing had the highest Hall mobility of 67.7 cm2/V. The performance of IAZO TFTs was also strongly affected by substrate temperature. The device fabricated at 25 C had the best overall performance with the highest field-effect mobility of 11.11 cm2/V and optimal threshold voltage of 5.12 V. Meanwhile, this device also exhibited the lowest hysterisis (0.31 V) and subthreshold swing (0.42 V/dec), indicating the minimal defects both inside the channel and at the insulator/ channel interface. The PBS stability of our IAZO TFTs is mainly affected by the electron traps near the upper surface of the channel layer. Acknowledgements This work was supported by the National Key Research and Development Program of China (Grant No. 2017YFB0405400), Key Research and Development Program of Shandong Province, China (Grant No. 2017GGX201007), Fundamental Research Funds of Shandong University (Grant No. 2018JC034), and China Postdoctoral Science Foundation (Grant No. 2018T110685). References [1] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Room-temperature fabrication of transparent flexible thin-film transistors using amorphous oxide semiconductors, Nature 432 (2004) 488e492. [2] K. Takenaka, M. Endo, G. Uchida, A. Ebe, Y. Setsuhara, Influence of deposition condition on electrical properties of a-IGZO films deposited by plasmaenhanced reactive sputtering, J. Alloys Compd. 772 (2019) 642e649. [3] Y.C. Zhang, G. He, C. Zhang, L. Zhu, B. Yang, Q.B. Lin, X.S. Jiang, Oxygen partial pressure ratio modulated electrical performance of amorphous InGaZnO thin film transistor and inverter, J. Alloys Compd. 765 (2018) 791e799. [4] J. Su, Y. Wang, Y. Ma, Q. Wang, L. Tian, S. Dai, R. Li, X. Zhang, Y. Wang, Preparation and electrical characteristics of N-doped In-Zn-Sn-O thin film transistors by radio frequency magnetron sputtering, J. Alloys Compd. 750 (2018) 1003e1006.
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