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Characterization of novel BaZnSnO thin films by solution process and applications in thin film transistors
Materials Research Bulletin 68 (2015) 22–26
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Characterization of novel BaZnSnO thin films by solution process and applications in thin film transistors Jun Li a,b, * , Chuan-Xin Huang a , Jian-Hua Zhang b, ** , Wen-Qing Zhu a , Xue-Yin Jiang a , Zhi-Lin Zhang a,b a b
School of Material Science and Engineering, Shanghai University, Jiading, Shanghai 201800, People’s Republic of China Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai 200072, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 November 2014 Received in revised form 5 February 2015 Accepted 13 March 2015 Available online 17 March 2015
A novel BaZnSnO semiconductor is fabricated using solution process and the influence of Ba addition on the structure, the chemical state of oxygen and electrical performance of BaZnSnO thin films are investigated. A high performance BaZnSnO-based thin film transistor with 15 mol% Ba is obtained, showing a saturation mobility of 1.94 cm2/V s, a threshold voltage of 3.6 V, an on/off current ratio of 6.2 106, a subthreshold swing of 0.94 V/decade, and a good bias stability. Transistors with solution processed BaZnSnO films are promising candidates for the development of future large-area, low-cost and high-performance electronic devices. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials B. Sol–gel chemistry D. Electrical properties
1. Introduction Oxide-based semiconductor thin film transistors (TFTs) such as InGaZnO [1,2], HfInZnO [3] and MgInZnO [4] were frequently reported as an alternative of amorphous silicon TFTs due to their high carrier mobility, good optical transparency in the visible region, and superior uniformity over a large area. Generally, oxide semiconductor thin films can be grown by various deposition techniques under vacuum, including radio frequency (rf) magnetron sputtering, and pulsed laser deposition [5,6]. However, the high fabrication cost of vacuum processes always hinder the application in low-cost, large-area electronics. Solution-processed oxide semiconductor as active layer of TFTs can overcome the limitations due to simple, low cost, and large area uniformity. Moreover, solution processing is easily to control the compositions of oxide semiconductor. InGaZnO semiconductor has been proved to be a semiconductor closest to commercialization. However, a rare-earth indium element is expensive, toxic, and unstable in hydrogen plasma, which will enhance the fabrication cost of transistor. Accordingly, several research groups have paid more attention on indium-free
* Corresponding author at: School of Material Science and Engineering, Shanghai University, Jiading, Shanghai 201800, People’s Republic of China. Tel.: +86 2169980337; fax: +86 2169980322. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (J.-H. Zhang). http://dx.doi.org/10.1016/j.materresbull.2015.03.036 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
semiconductor such as tin oxide (SnO2) [7], and zinc tin oxide (ZnSnO) [8,9]. Sn-based TFTs have been researched as a promising alternative, because Sn has similar electronic configuration with In. On the other hand, ZnSnO-TFT has a superior electrical performance comparable to that of InGaZnO-TFT. In order to improve the device performance and stability of ZnSnO-TFT, the addition of appropriate element for oxygen-vacancy suppression is necessary. In particular, oxygen-vacancy formation is related to their high standard electrode potential (SEP) or low oxygen-vacancy formation [10]. Ba has a considerably lower SEP (2.90 V) than Zn (0.76 V) and Sn (0.13 V). Moreover, the band gap of BaO is larger (4.8 eV) than ZnO (3.3 eV) and SnO2 (3.6 eV) [11,12]. Therefore, incorporation of Ba into ZnSnO is expected to reduce the number of oxygen vacancies and increase the optical band gap. In this work, a novel BaZnSnO thin film is prepared using solution process and their application in thin film transistors. Solution-processed BaZnSnO TFTs are fabricated for the first time and the effects of the Ba content on the electrical properties of BaZnSnO-TFTs are investigated. 2. Experimental details The precursors of the prepared ZnSnO and BaZnSnO solutions were 0.1 M of zinc acetate dehydrate (Zn(CH3COO)22H2O) and 0.1 M of tin chloride dehydrate (SnCl22H2O) and barium nitrate (Ba(NO3)2). These precursors were dissolved in 2-methoxyethanol. A 0.1 M monoethanolamine (MEA) was then added in the precursor solution as a sol–gel stabilizer. The solution was stirred at 50 C for
J. Li et al. / Materials Research Bulletin 68 (2015) 22–26
1 h and then aged for 24 h. The atomic ratio of Zn:Sn was fixed at 1:1 and that of Ba:Zn ranged from 0 to 20 mol% Zn (ie., atomic ratio of Ba:Zn:Sn = 0–0.2:1:1). A bottom-gate-type TFT was fabricated. Highly doped n-type silicon wafer with 0.01–0.025 V cm was used as substrate and gate electrode. A 300-nm thick SiO2 used as a gate dielectric was fabricated by thermal oxidation on the high-doped n-type silicon wafer. Prior to processing, the wafer was cleaned with a standard wet-cleaning procedure. The ZnSnO film and BaZnSnO film were prepared by spin-coating at 3000 rpm for 30 s and prebaked at 200 C for 20 min. Subsequently, the wafer was annealed at 500 C for 60 min in ambient air. The thickness of ZnSnO, BaZnSnO (0.1), BaZnSnO (0.15) and BaZnSnO (0.2) are about 48, 50, 51, and 52 nm, respectively. Finally, a 60-nm-thick Al film was sequentially vacuum deposited onto active layer using a metal mask to define transistor with channel width W = 1000 mm and channel length L = 200 mm. The current–voltage characteristics of the devices were measured using an Agilent 4155C semiconductor analyzer. The capacitance measurements were conducted with an Agilent E4980A Precision LCR meter. The thickness of thin film was determined by a surface profiler (Alpha-Step IQ). X-ray diffraction (XRD) scans were measured with a Rigaku D\ \Max-2200 X-ray diffractometer using Cu Ka radiation. Optical properties of thin films were measured using a UV–vis spectrophotometer. The chemical bonding states of films was carried out with the x-ray photoelectron spectroscopy (XPS) (Thermo-ESCALAB 250Xl). All measurements were carried out at room temperature.
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3. Results and discussion Fig. 1(a) shows the XRD patterns of BaZnSnO films on SiO2/Si substrates with different Ba contents annealed at 500 C for 1 h in ambient air. Excluding the Si (400) peak, no remarkable peaks are exhibited, indicating that BaZnSnO films have amorphous structure irrespective of the Ba content. It is reported that the amorphous structure enables the realization of uniform device properties over large areas [13]. That is to say that the solutionprocessed BaZnSnO thin film could be applicable to large-area displays. In order to investigate the optical properties of BaZnSnO thin films, the transmittance spectra of BaZnSnO thin films on glass substrate are measured, as shown in Fig. 1(b). The average transmittance values of the prepared thin films in the visible region are over 90%. The optical band energy (Eg) of BaZnSnO semiconductor is determined by the optical properties. The absorption coefficient (a) can be calculated from Beer’s law: T (1) ¼ lnð1 RÞ a d ln T0 where T is the measurable transmission of the film, d is the thickness of the thin film, T0 is the transmission through the setup without the sample and R is the reflectivity. According to the absorption coefficient, optical band gap (Eg) can be expressed by [14,15]: ðahg Þ2 ¼ bðhg Eg Þ
(2)
Where h is Planck’s constant, g is the frequency of incident photon, and b is energy independent constant. Eg can be obtained by
Fig. 1. (a) XRD patterns of the BaZnSnO on a SiO2/Si substrate as a function of the Ba content. (b) Optical transmittance spectra of BaZnSnO films with different Ba contents. (c) The relationship between the band gap of BaZnSnO and Ba content. (d) Electrical properties of BaZnSnO film measured by Hall-effect measurements as a function of the Ba content.
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J. Li et al. / Materials Research Bulletin 68 (2015) 22–26
extrapolating the linear portion of the (ahg )2 and hg plots to the energy axis. The band gap of ZnSnO film and BaZnSnO (0.15) were 3.42 and 3.64 eV, respectively. The relationship between the band gap of BaZnSnO and Ba content is shown in Fig. 1(c). The band gap increases as the Ba content increases. The result is reasonable due to the larger band gap BaO (4.8 eV). The broadening of the band gap leads to a reduction in the electron charge carrier. The result is verified by the Hall-effect measurement. Fig. 1(d) shows electrical properties measured by Hall-effect measurements as a function of the Ba concentration in ZnSnO film. As the Ba concentration increases from 0 to 20 mol%, the carrier concentration declines from 6.5 1017 to 1.2 1015 cm3, and Hall mobility decreases from 4.3 to 1.4 cm2/V s. High carrier concentration is unfavorable for the TFT devices because a high leakage current results in device instability [16]. It is easily seen that Ba leads to a remarkable change in electrical properties. The influence of Ba content on the oxygen vacancies in BaZnSnO films is investigated by X-ray photoelectron spectroscopy (XPS) analysis. In order to show a clean surface, the carbon contamination of thin films is removed by Ar+ sputtering. All the peaks are calibrated by taking C 1s reference at 284.8 eV to compensate for any charge-induced shifts in the analysis. Fig. 2 shows (a) Zn 2P3/2, (b) Sn 3d5/2 and (c) O 1s peaks of XPS spectra of the BaZnSnO films. The Zn 2P3/2 peak centered at 1022.9 eV, and the Sn 3d5/2 peak centered at 487.7 eV, originated from oxygen-bound Zn and Sn, respectively. The Zn 2P3/2 peak and Sn 3d5/2 peak shift to lower binding energy with increasing Ba content. The results indicate that the oxygen deficiency of BaZnSnO film decreases due to the effect of Ba element as carrier suppressor. Ba ions can form stronger chemical bonds with oxygen than Zn or Sn ions because of
the relatively high bond strength of Ba—O (562 KJ/mol) comparing with Zn—O (395 KJ/mol) and Sn—O (531.8 KJ/mol). The O 1s region of the XPS spectrum can be divided into three regions: low binding energy (OI, 530.7 eV), medium binding energy (OII, 531.5 eV) and high binding energy (OIII, 532.5 eV). All peaks can be fitted by XPSPEAK 4.1. The inset of Fig. 2(c) shows the OI, OII and OIII components of O 1s peak for BaZnSnO (0.15). Similar fitting can also be done. The low-binding energy (OI) side of the O 1s spectrum at 530.7 eV is attributed to O2 ions surrounding by Ba, Zn, and Sn atoms. The OII is related to O2 ions in the oxygen-deficient regions in BaZnSnO. The OIII is attributed to the presence of loosely bound oxygen on the surface of BaZnSnO film such as H2O, O2 on the film surface or inside the BaZnSnO films. The change of Vo concentration can be depicted by the peak area variation of OII component. The integral areas of OI, OII and OIII can be obtained by fitting the XPS spectra of O 1s. S1,S2, and S3 represent the integral areas of OI, OII and OIII peak, respectively. Stot is the sum of the integral areas of OI, OII and OIII peak. Interestingly, although the intensity of OIII peak is almost unchanged, the other two peaks are changed considerably by increasing Ba content. As the Ba ratio is increased, S1/Stot value gradually increases, but S2/Stot value monotonically decreases from 35.4% to 26.8%. The detail is shown in Fig. 2(d). It is seen that the relative amount of oxygen vacancy decreases with increasing Ba content, suggesting that Ba acts as a carrier suppressor in the ZnSnO thin films. In addition, the loose metal-oxide lattice bonding is strengthened as Ba content increases. Similar result has been reported by Wu et al. [17]. Furthermore, the oxygen vacancies in BaZnSnO film act as doubly ionized donors and contribute a maximum of two electrons to the electrical conductivity according to the equation:
Fig. 2. (a) Zn 2p3/2, (b) Sn 3d5/2, (c) O 1s XPS spectra of the BaZnSnO thin films, (d) the relation between oxygen deficiency and Ba content.
J. Li et al. / Materials Research Bulletin 68 (2015) 22–26
OXO ¼ VO þ 2e þ 1=2O2 ðgÞ [18]. It suggests that the decreases of oxygen vacancies result in the reduction of electron carrier concentration and electrical conductivity with increasing the Ba content. The result confirms that the Ba atom is an effective suppressor of oxygen vacancies, which act as a carrier source. The actual Ba atomic concentration can be computed from the measured peak area together with sensitivity factor. The atomic percentage of Ba to the total amount of Ba, Zn and Sn (Ba/ (Ba + Sn + Zn)) for obtained by the XPS for BaZnSnO (0.1), BaZnSnO (0.15) and BaZnSnO (0.2) are approximately 4.32, 6.41 and 8.7%, respectively. The transfer characteristics of BaZnSnO-TFT at a VDS of 20 V for various Ba content levels are shown in Fig. 3(a). As the Ba content increases, the off current decreases and Von shifts to a positive voltage. The off current is a function of the resistance of the channel layer according to the following equation: IDS;off ¼
s Wtch L
V DS
(3)
where s denotes the electrical conductivity of channel layer, tch is the thickness of the channel layer, VDS is the source-drain voltage, W and L are the width and length of the channel layer, respectively. The off current decreases monotonically with increasing Ba
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content, which is attributed to the decreased conductivity of the channel layer. The saturation mobility (msat) is extracted from a linear fitting to the (IDS)1/2–VGS curve, based on the equation IDS = (msatCoxW/2L)(VGS Vth)2, where Vth is the threshold voltage estimated from the intercept of the extrapolated curve with the voltage axis. The subthreshold swing (SS = dVGS/dlog(IDS)) is extracted from the linear part of the log(IDS)–VGS plot. The saturation mobility of TFTs decreases monotonically from 2.63 to 0.72 cm2/V s with increasing Ba content. It indicates that the excess Ba addition results in the lower carrier concentration and carrier scattering effect. The BaZnSnO-TFT with 15 mol% Ba has the best performance among all the devices, with a msat of 1.94 cm2/V s, a Vth of 3.6 V, an Ion/off of 6.2 106, and a SS of 0.94 V/ decade. More details are shown in Fig. 3(b). In addition, SS is dependent on the trap density in the bulk BaZnSnO and at the interface between the BaZnSnO channel and gate insulator [19]. The maximum trap density (Nmax SS ) can be expressed as: SSlogðeÞ C Nmax 1 i (4) ¼ SS kT=q q Where k is the Boltzmann constant, q is the electron charge and T is the temperature. The calculated trap density for undoped, 10, 15, and 20 mol% are 1.5 1012, 1.07 1012, 9.1 1011 and
Fig. 3. (a) The transfer characteristics of BaZnSnO-TFT for various Ba content. (b) Variation of electrical performance of BaZnSnO-TFTs with the Ba content. (c) The evolutions of transfer curves of BaZnSnO-TFTs with 15 mol% Ba content under positive bias stress.
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J. Li et al. / Materials Research Bulletin 68 (2015) 22–26
1.0 1012 cm2, respectively. The results show that 15 mol% Ba doping improved the device characteristics due to less charge trapping compared to that of undoped ZnSnO-TFTs. The oxygen vacancy defects, related to the electron charge trapping model, have a strong influence on the positive gate voltage stress induced instability. Fig. 3(c) shows the positive gate voltage stress induced shift of the transfer curve. The transistors are performed under a gate bias stress of 10 V and a drain stress of 0 V. A positive displacement of transfer characteristics along the voltage axis is observed. The transistor with BaZnSnO (0.15) channel shows a threshold voltage shift of 6.7 V after 1200 s bias stress time. The threshold voltage shift of TFT based BaZnSnO (0.15) semiconductor is obviously smaller than that (15.2 V) of ZnSnO-TFT (not shown). It is attributed to a small amount of oxygen vacancy defects in BaZnSnO (0.15) semiconductor. Apparently, Ba ion can suppress the oxygen vacancy defects, prevent the formation of the high electron concentration layer and thus improve the bias stability and mobility of TFTs. However, further studies on bias stability will be done in the later work. 4. Conclusions In summary, BaZnSnO-TFTs with different Ba content are fabricated by a solution process and the influence of Ba addition on the structure, the chemical state of oxygen and electrical performance of BaZnSnO thin films are investigated. With increasing Ba content, the blue shift of optical bandgap of BaZnSnO films is attributed to the formation of Ba—O bonds in the BaZnSnO films, leading to the decrease of carrier concentration. The O 1s XPS spectra show that Ba can effectively suppress oxygen vacancies in BaZnSnO and improve the electrical performance and bias stability of BaZnSnO-TFT. The best electrical performance of BaZnSnO-TFT is obtained when Ba content is 15 mol%. Thus, the BaZnSnO film is a promising candidate for the channel layer of transistor.
Acknowledgements The authors would like to acknowledge the financial support given by the Natural Science Foundation of China (51302165, 61274082, 61077013), project (2010AA3A337, 2008AA03A336), Shanghai Municipal Education Commission (ZZSD13047), Innovation Fund of shanghai University and China Postdoctoral Science Special Fund (2012T50387). References [1] G.H. Kim, B.D. Ahn, H.S. Shin, W.H. Jeong, H.J. Kim, H.J. Kim, Appl. Phys. Lett. 94 (2009) 233501. [2] J.M. Kwon, J. Jung, Y.S. Rim, D.L. Kim, H.J. Kim, ACS Appl. Mater. Interfaces 6 (2014) 3371. [3] W.H. Jeong, G.H. Kim, H.S. Shin, B.D. Ahn, H.J. Kim, M.K. Ryu, K.B. Park, J.B. Seon, S.Y. Lee, Appl. Phys. Lett. 96 (2010) 093503. [4] G.H. Kim, W.H. Jeong, B.D. Ahn, H.S. Shin, H.J. Kim, H.J. Kim, M.K. Ryu, K.B. Park, J.B. Seon, S.Y. Lee, Appl. Phys. Lett. 96 (2010) 163506. [5] K.H. Liu, T.C. Chang, K.C. Chang, T.M. Tsai, T.Y. Hsieh, M.C. Chen, B.L. Yeh, W.C. Chou, Appl. Phys. Lett. 104 (2014) 103501. [6] Y. Hanyu, K. Domen, K. Normura, H. Hiramatsu, H. Kumomi, H. Hosono, T. Kamiya, Appl. Phys. Lett. 103 (2013) 202114. [7] J. Jang, R. Kitsomboonloha, S.L. Swisher, E.S. Park, H. Kang, V. Subramanian, Adv. Mater. 25 (2013) 1042. [8] Y.J. Kim, B.S. Yang, S. Oh, S.J. Han, H.W. Lee, J. Heo, J.K. Jeong, H.J. Kim, ACS Appl. Mater. Interfaces 5 (2013) 3255. [9] Y. Zhao, L. Duan, G. Dong, D. Zhang, J. Qiao, L. Wang, Y. Qiu, Langmuir 29 (2013) 151. [10] H.B. Kim, H.S. Lee, Thin Solid Films 550 (2014) 504. [11] P.T. Tue, T. Miyasako, J. Li, H.T.C. Tu, S. Inoue, E. Tokumitsu, T. Shimoda, IEEE Trans. Electron Devices 60 (2013) 320. [12] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [13] C.G. Lee, A. Dodabalapur, Appl. Phys. Lett. 96 (2010) 243501. [14] S.T. Tan, B.J. Chen, X.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98 (2005) 013505. [15] J. Li, J.H. Zhang, X.W. Ding, W.Q. Zhu, X.Y. Jiang, Z.L. Zhang, Thin Solid Films 562 (2014) 592–596. [16] B.Y. Su, S.Y. Chu, Y.D. Juang, IEEE Trans. Electron Devices 59 (2012) 700. [17] C. Wu, X. Li, J. Lu, Z. Ye, J. Zhang, T. Zhou, R. Sun, L. Chen, B. Lu, X. Pan, Appl. Phys. Lett. 103 (2013) 082109. [18] Y.S. Rim, B.D. Ahn, J.S. Park, H.J. Kim, J. Phys. D: Appl. Phys. 47 (2014) 045502. [19] C. Lee, D. Striakhilev, S. Tao, A. Nathan, IEEE Electron Device Lett. 26 (2005) 637.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Characterization of novel BaZnSnO thin films by solution process and applications in thin film transistors Jun Li a,b, * , Chuan-Xin Huang a , Jian-Hua Zhang b, ** , Wen-Qing Zhu a , Xue-Yin Jiang a , Zhi-Lin Zhang a,b a b
School of Material Science and Engineering, Shanghai University, Jiading, Shanghai 201800, People’s Republic of China Key Laboratory of Advanced Display and System Applications, Ministry of Education, Shanghai University, Shanghai 200072, People’s Republic of China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 7 November 2014 Received in revised form 5 February 2015 Accepted 13 March 2015 Available online 17 March 2015
A novel BaZnSnO semiconductor is fabricated using solution process and the influence of Ba addition on the structure, the chemical state of oxygen and electrical performance of BaZnSnO thin films are investigated. A high performance BaZnSnO-based thin film transistor with 15 mol% Ba is obtained, showing a saturation mobility of 1.94 cm2/V s, a threshold voltage of 3.6 V, an on/off current ratio of 6.2 106, a subthreshold swing of 0.94 V/decade, and a good bias stability. Transistors with solution processed BaZnSnO films are promising candidates for the development of future large-area, low-cost and high-performance electronic devices. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials B. Sol–gel chemistry D. Electrical properties
1. Introduction Oxide-based semiconductor thin film transistors (TFTs) such as InGaZnO [1,2], HfInZnO [3] and MgInZnO [4] were frequently reported as an alternative of amorphous silicon TFTs due to their high carrier mobility, good optical transparency in the visible region, and superior uniformity over a large area. Generally, oxide semiconductor thin films can be grown by various deposition techniques under vacuum, including radio frequency (rf) magnetron sputtering, and pulsed laser deposition [5,6]. However, the high fabrication cost of vacuum processes always hinder the application in low-cost, large-area electronics. Solution-processed oxide semiconductor as active layer of TFTs can overcome the limitations due to simple, low cost, and large area uniformity. Moreover, solution processing is easily to control the compositions of oxide semiconductor. InGaZnO semiconductor has been proved to be a semiconductor closest to commercialization. However, a rare-earth indium element is expensive, toxic, and unstable in hydrogen plasma, which will enhance the fabrication cost of transistor. Accordingly, several research groups have paid more attention on indium-free
* Corresponding author at: School of Material Science and Engineering, Shanghai University, Jiading, Shanghai 201800, People’s Republic of China. Tel.: +86 2169980337; fax: +86 2169980322. ** Corresponding author. E-mail addresses: [email protected] (J. Li), [email protected] (J.-H. Zhang). http://dx.doi.org/10.1016/j.materresbull.2015.03.036 0025-5408/ ã 2015 Elsevier Ltd. All rights reserved.
semiconductor such as tin oxide (SnO2) [7], and zinc tin oxide (ZnSnO) [8,9]. Sn-based TFTs have been researched as a promising alternative, because Sn has similar electronic configuration with In. On the other hand, ZnSnO-TFT has a superior electrical performance comparable to that of InGaZnO-TFT. In order to improve the device performance and stability of ZnSnO-TFT, the addition of appropriate element for oxygen-vacancy suppression is necessary. In particular, oxygen-vacancy formation is related to their high standard electrode potential (SEP) or low oxygen-vacancy formation [10]. Ba has a considerably lower SEP (2.90 V) than Zn (0.76 V) and Sn (0.13 V). Moreover, the band gap of BaO is larger (4.8 eV) than ZnO (3.3 eV) and SnO2 (3.6 eV) [11,12]. Therefore, incorporation of Ba into ZnSnO is expected to reduce the number of oxygen vacancies and increase the optical band gap. In this work, a novel BaZnSnO thin film is prepared using solution process and their application in thin film transistors. Solution-processed BaZnSnO TFTs are fabricated for the first time and the effects of the Ba content on the electrical properties of BaZnSnO-TFTs are investigated. 2. Experimental details The precursors of the prepared ZnSnO and BaZnSnO solutions were 0.1 M of zinc acetate dehydrate (Zn(CH3COO)22H2O) and 0.1 M of tin chloride dehydrate (SnCl22H2O) and barium nitrate (Ba(NO3)2). These precursors were dissolved in 2-methoxyethanol. A 0.1 M monoethanolamine (MEA) was then added in the precursor solution as a sol–gel stabilizer. The solution was stirred at 50 C for
J. Li et al. / Materials Research Bulletin 68 (2015) 22–26
1 h and then aged for 24 h. The atomic ratio of Zn:Sn was fixed at 1:1 and that of Ba:Zn ranged from 0 to 20 mol% Zn (ie., atomic ratio of Ba:Zn:Sn = 0–0.2:1:1). A bottom-gate-type TFT was fabricated. Highly doped n-type silicon wafer with 0.01–0.025 V cm was used as substrate and gate electrode. A 300-nm thick SiO2 used as a gate dielectric was fabricated by thermal oxidation on the high-doped n-type silicon wafer. Prior to processing, the wafer was cleaned with a standard wet-cleaning procedure. The ZnSnO film and BaZnSnO film were prepared by spin-coating at 3000 rpm for 30 s and prebaked at 200 C for 20 min. Subsequently, the wafer was annealed at 500 C for 60 min in ambient air. The thickness of ZnSnO, BaZnSnO (0.1), BaZnSnO (0.15) and BaZnSnO (0.2) are about 48, 50, 51, and 52 nm, respectively. Finally, a 60-nm-thick Al film was sequentially vacuum deposited onto active layer using a metal mask to define transistor with channel width W = 1000 mm and channel length L = 200 mm. The current–voltage characteristics of the devices were measured using an Agilent 4155C semiconductor analyzer. The capacitance measurements were conducted with an Agilent E4980A Precision LCR meter. The thickness of thin film was determined by a surface profiler (Alpha-Step IQ). X-ray diffraction (XRD) scans were measured with a Rigaku D\ \Max-2200 X-ray diffractometer using Cu Ka radiation. Optical properties of thin films were measured using a UV–vis spectrophotometer. The chemical bonding states of films was carried out with the x-ray photoelectron spectroscopy (XPS) (Thermo-ESCALAB 250Xl). All measurements were carried out at room temperature.
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3. Results and discussion Fig. 1(a) shows the XRD patterns of BaZnSnO films on SiO2/Si substrates with different Ba contents annealed at 500 C for 1 h in ambient air. Excluding the Si (400) peak, no remarkable peaks are exhibited, indicating that BaZnSnO films have amorphous structure irrespective of the Ba content. It is reported that the amorphous structure enables the realization of uniform device properties over large areas [13]. That is to say that the solutionprocessed BaZnSnO thin film could be applicable to large-area displays. In order to investigate the optical properties of BaZnSnO thin films, the transmittance spectra of BaZnSnO thin films on glass substrate are measured, as shown in Fig. 1(b). The average transmittance values of the prepared thin films in the visible region are over 90%. The optical band energy (Eg) of BaZnSnO semiconductor is determined by the optical properties. The absorption coefficient (a) can be calculated from Beer’s law: T (1) ¼ lnð1 RÞ a d ln T0 where T is the measurable transmission of the film, d is the thickness of the thin film, T0 is the transmission through the setup without the sample and R is the reflectivity. According to the absorption coefficient, optical band gap (Eg) can be expressed by [14,15]: ðahg Þ2 ¼ bðhg Eg Þ
(2)
Where h is Planck’s constant, g is the frequency of incident photon, and b is energy independent constant. Eg can be obtained by
Fig. 1. (a) XRD patterns of the BaZnSnO on a SiO2/Si substrate as a function of the Ba content. (b) Optical transmittance spectra of BaZnSnO films with different Ba contents. (c) The relationship between the band gap of BaZnSnO and Ba content. (d) Electrical properties of BaZnSnO film measured by Hall-effect measurements as a function of the Ba content.
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extrapolating the linear portion of the (ahg )2 and hg plots to the energy axis. The band gap of ZnSnO film and BaZnSnO (0.15) were 3.42 and 3.64 eV, respectively. The relationship between the band gap of BaZnSnO and Ba content is shown in Fig. 1(c). The band gap increases as the Ba content increases. The result is reasonable due to the larger band gap BaO (4.8 eV). The broadening of the band gap leads to a reduction in the electron charge carrier. The result is verified by the Hall-effect measurement. Fig. 1(d) shows electrical properties measured by Hall-effect measurements as a function of the Ba concentration in ZnSnO film. As the Ba concentration increases from 0 to 20 mol%, the carrier concentration declines from 6.5 1017 to 1.2 1015 cm3, and Hall mobility decreases from 4.3 to 1.4 cm2/V s. High carrier concentration is unfavorable for the TFT devices because a high leakage current results in device instability [16]. It is easily seen that Ba leads to a remarkable change in electrical properties. The influence of Ba content on the oxygen vacancies in BaZnSnO films is investigated by X-ray photoelectron spectroscopy (XPS) analysis. In order to show a clean surface, the carbon contamination of thin films is removed by Ar+ sputtering. All the peaks are calibrated by taking C 1s reference at 284.8 eV to compensate for any charge-induced shifts in the analysis. Fig. 2 shows (a) Zn 2P3/2, (b) Sn 3d5/2 and (c) O 1s peaks of XPS spectra of the BaZnSnO films. The Zn 2P3/2 peak centered at 1022.9 eV, and the Sn 3d5/2 peak centered at 487.7 eV, originated from oxygen-bound Zn and Sn, respectively. The Zn 2P3/2 peak and Sn 3d5/2 peak shift to lower binding energy with increasing Ba content. The results indicate that the oxygen deficiency of BaZnSnO film decreases due to the effect of Ba element as carrier suppressor. Ba ions can form stronger chemical bonds with oxygen than Zn or Sn ions because of
the relatively high bond strength of Ba—O (562 KJ/mol) comparing with Zn—O (395 KJ/mol) and Sn—O (531.8 KJ/mol). The O 1s region of the XPS spectrum can be divided into three regions: low binding energy (OI, 530.7 eV), medium binding energy (OII, 531.5 eV) and high binding energy (OIII, 532.5 eV). All peaks can be fitted by XPSPEAK 4.1. The inset of Fig. 2(c) shows the OI, OII and OIII components of O 1s peak for BaZnSnO (0.15). Similar fitting can also be done. The low-binding energy (OI) side of the O 1s spectrum at 530.7 eV is attributed to O2 ions surrounding by Ba, Zn, and Sn atoms. The OII is related to O2 ions in the oxygen-deficient regions in BaZnSnO. The OIII is attributed to the presence of loosely bound oxygen on the surface of BaZnSnO film such as H2O, O2 on the film surface or inside the BaZnSnO films. The change of Vo concentration can be depicted by the peak area variation of OII component. The integral areas of OI, OII and OIII can be obtained by fitting the XPS spectra of O 1s. S1,S2, and S3 represent the integral areas of OI, OII and OIII peak, respectively. Stot is the sum of the integral areas of OI, OII and OIII peak. Interestingly, although the intensity of OIII peak is almost unchanged, the other two peaks are changed considerably by increasing Ba content. As the Ba ratio is increased, S1/Stot value gradually increases, but S2/Stot value monotonically decreases from 35.4% to 26.8%. The detail is shown in Fig. 2(d). It is seen that the relative amount of oxygen vacancy decreases with increasing Ba content, suggesting that Ba acts as a carrier suppressor in the ZnSnO thin films. In addition, the loose metal-oxide lattice bonding is strengthened as Ba content increases. Similar result has been reported by Wu et al. [17]. Furthermore, the oxygen vacancies in BaZnSnO film act as doubly ionized donors and contribute a maximum of two electrons to the electrical conductivity according to the equation:
Fig. 2. (a) Zn 2p3/2, (b) Sn 3d5/2, (c) O 1s XPS spectra of the BaZnSnO thin films, (d) the relation between oxygen deficiency and Ba content.
J. Li et al. / Materials Research Bulletin 68 (2015) 22–26
OXO ¼ VO þ 2e þ 1=2O2 ðgÞ [18]. It suggests that the decreases of oxygen vacancies result in the reduction of electron carrier concentration and electrical conductivity with increasing the Ba content. The result confirms that the Ba atom is an effective suppressor of oxygen vacancies, which act as a carrier source. The actual Ba atomic concentration can be computed from the measured peak area together with sensitivity factor. The atomic percentage of Ba to the total amount of Ba, Zn and Sn (Ba/ (Ba + Sn + Zn)) for obtained by the XPS for BaZnSnO (0.1), BaZnSnO (0.15) and BaZnSnO (0.2) are approximately 4.32, 6.41 and 8.7%, respectively. The transfer characteristics of BaZnSnO-TFT at a VDS of 20 V for various Ba content levels are shown in Fig. 3(a). As the Ba content increases, the off current decreases and Von shifts to a positive voltage. The off current is a function of the resistance of the channel layer according to the following equation: IDS;off ¼
s Wtch L
V DS
(3)
where s denotes the electrical conductivity of channel layer, tch is the thickness of the channel layer, VDS is the source-drain voltage, W and L are the width and length of the channel layer, respectively. The off current decreases monotonically with increasing Ba
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content, which is attributed to the decreased conductivity of the channel layer. The saturation mobility (msat) is extracted from a linear fitting to the (IDS)1/2–VGS curve, based on the equation IDS = (msatCoxW/2L)(VGS Vth)2, where Vth is the threshold voltage estimated from the intercept of the extrapolated curve with the voltage axis. The subthreshold swing (SS = dVGS/dlog(IDS)) is extracted from the linear part of the log(IDS)–VGS plot. The saturation mobility of TFTs decreases monotonically from 2.63 to 0.72 cm2/V s with increasing Ba content. It indicates that the excess Ba addition results in the lower carrier concentration and carrier scattering effect. The BaZnSnO-TFT with 15 mol% Ba has the best performance among all the devices, with a msat of 1.94 cm2/V s, a Vth of 3.6 V, an Ion/off of 6.2 106, and a SS of 0.94 V/ decade. More details are shown in Fig. 3(b). In addition, SS is dependent on the trap density in the bulk BaZnSnO and at the interface between the BaZnSnO channel and gate insulator [19]. The maximum trap density (Nmax SS ) can be expressed as: SSlogðeÞ C Nmax 1 i (4) ¼ SS kT=q q Where k is the Boltzmann constant, q is the electron charge and T is the temperature. The calculated trap density for undoped, 10, 15, and 20 mol% are 1.5 1012, 1.07 1012, 9.1 1011 and
Fig. 3. (a) The transfer characteristics of BaZnSnO-TFT for various Ba content. (b) Variation of electrical performance of BaZnSnO-TFTs with the Ba content. (c) The evolutions of transfer curves of BaZnSnO-TFTs with 15 mol% Ba content under positive bias stress.
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1.0 1012 cm2, respectively. The results show that 15 mol% Ba doping improved the device characteristics due to less charge trapping compared to that of undoped ZnSnO-TFTs. The oxygen vacancy defects, related to the electron charge trapping model, have a strong influence on the positive gate voltage stress induced instability. Fig. 3(c) shows the positive gate voltage stress induced shift of the transfer curve. The transistors are performed under a gate bias stress of 10 V and a drain stress of 0 V. A positive displacement of transfer characteristics along the voltage axis is observed. The transistor with BaZnSnO (0.15) channel shows a threshold voltage shift of 6.7 V after 1200 s bias stress time. The threshold voltage shift of TFT based BaZnSnO (0.15) semiconductor is obviously smaller than that (15.2 V) of ZnSnO-TFT (not shown). It is attributed to a small amount of oxygen vacancy defects in BaZnSnO (0.15) semiconductor. Apparently, Ba ion can suppress the oxygen vacancy defects, prevent the formation of the high electron concentration layer and thus improve the bias stability and mobility of TFTs. However, further studies on bias stability will be done in the later work. 4. Conclusions In summary, BaZnSnO-TFTs with different Ba content are fabricated by a solution process and the influence of Ba addition on the structure, the chemical state of oxygen and electrical performance of BaZnSnO thin films are investigated. With increasing Ba content, the blue shift of optical bandgap of BaZnSnO films is attributed to the formation of Ba—O bonds in the BaZnSnO films, leading to the decrease of carrier concentration. The O 1s XPS spectra show that Ba can effectively suppress oxygen vacancies in BaZnSnO and improve the electrical performance and bias stability of BaZnSnO-TFT. The best electrical performance of BaZnSnO-TFT is obtained when Ba content is 15 mol%. Thus, the BaZnSnO film is a promising candidate for the channel layer of transistor.
Acknowledgements The authors would like to acknowledge the financial support given by the Natural Science Foundation of China (51302165, 61274082, 61077013), project (2010AA3A337, 2008AA03A336), Shanghai Municipal Education Commission (ZZSD13047), Innovation Fund of shanghai University and China Postdoctoral Science Special Fund (2012T50387). References [1] G.H. Kim, B.D. Ahn, H.S. Shin, W.H. Jeong, H.J. Kim, H.J. Kim, Appl. Phys. Lett. 94 (2009) 233501. [2] J.M. Kwon, J. Jung, Y.S. Rim, D.L. Kim, H.J. Kim, ACS Appl. Mater. Interfaces 6 (2014) 3371. [3] W.H. Jeong, G.H. Kim, H.S. Shin, B.D. Ahn, H.J. Kim, M.K. Ryu, K.B. Park, J.B. Seon, S.Y. Lee, Appl. Phys. Lett. 96 (2010) 093503. [4] G.H. Kim, W.H. Jeong, B.D. Ahn, H.S. Shin, H.J. Kim, H.J. Kim, M.K. Ryu, K.B. Park, J.B. Seon, S.Y. Lee, Appl. Phys. Lett. 96 (2010) 163506. [5] K.H. Liu, T.C. Chang, K.C. Chang, T.M. Tsai, T.Y. Hsieh, M.C. Chen, B.L. Yeh, W.C. Chou, Appl. Phys. Lett. 104 (2014) 103501. [6] Y. Hanyu, K. Domen, K. Normura, H. Hiramatsu, H. Kumomi, H. Hosono, T. Kamiya, Appl. Phys. Lett. 103 (2013) 202114. [7] J. Jang, R. Kitsomboonloha, S.L. Swisher, E.S. Park, H. Kang, V. Subramanian, Adv. Mater. 25 (2013) 1042. [8] Y.J. Kim, B.S. Yang, S. Oh, S.J. Han, H.W. Lee, J. Heo, J.K. Jeong, H.J. Kim, ACS Appl. Mater. Interfaces 5 (2013) 3255. [9] Y. Zhao, L. Duan, G. Dong, D. Zhang, J. Qiao, L. Wang, Y. Qiu, Langmuir 29 (2013) 151. [10] H.B. Kim, H.S. Lee, Thin Solid Films 550 (2014) 504. [11] P.T. Tue, T. Miyasako, J. Li, H.T.C. Tu, S. Inoue, E. Tokumitsu, T. Shimoda, IEEE Trans. Electron Devices 60 (2013) 320. [12] V. Srikant, D.R. Clarke, J. Appl. Phys. 83 (1998) 5447. [13] C.G. Lee, A. Dodabalapur, Appl. Phys. Lett. 96 (2010) 243501. [14] S.T. Tan, B.J. Chen, X.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98 (2005) 013505. [15] J. Li, J.H. Zhang, X.W. Ding, W.Q. Zhu, X.Y. Jiang, Z.L. Zhang, Thin Solid Films 562 (2014) 592–596. [16] B.Y. Su, S.Y. Chu, Y.D. Juang, IEEE Trans. Electron Devices 59 (2012) 700. [17] C. Wu, X. Li, J. Lu, Z. Ye, J. Zhang, T. Zhou, R. Sun, L. Chen, B. Lu, X. Pan, Appl. Phys. Lett. 103 (2013) 082109. [18] Y.S. Rim, B.D. Ahn, J.S. Park, H.J. Kim, J. Phys. D: Appl. Phys. 47 (2014) 045502. [19] C. Lee, D. Striakhilev, S. Tao, A. Nathan, IEEE Electron Device Lett. 26 (2005) 637.