- Email: [email protected]
Substrate temperature dependent structural, optical and electrical properties of amorphous InGaZnO thin films
Accepted Manuscript Substrate temperature dependent structural, optical and electrical properties of amorphous InGaZnO thin films X.F. Chen, G. He, J. Gao, J.W. Zhang, D.Q. Xiao, P. Jin, B. Deng PII: DOI: Reference:
S0925-8388(15)00216-9 http://dx.doi.org/10.1016/j.jallcom.2015.01.143 JALCOM 33193
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
20 November 2014 29 December 2014 4 January 2015
Please cite this article as: X.F. Chen, G. He, J. Gao, J.W. Zhang, D.Q. Xiao, P. Jin, B. Deng, Substrate temperature dependent structural, optical and electrical properties of amorphous InGaZnO thin films, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.01.143
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Substrate temperature dependent structural, optical and electrical properties of amorphous InGaZnO thin films X. F. Chena, G. Hea∗, J. Gaoa, J. W. Zhanga, D. Q. Xiaoa, P. Jina, and B. Denga a
School of physics and Materials Science, Radiation Detection Materials & Devices
Lab, Anhui University, Hefei 230601, China.
The effects of substrate temperature (Ts) on the electrical and optical properties of amorphous InGaZnO thin films deposited by sputtering have been investigated. As Ts increased from RT to 400 oC, all the films remained amorphous, the transmission in the visible region increased from 92.8% to 93.54%, and the band gap decreased from 3.42eV to 3.31eV. Based on Cauchy-Urbach model, the optical properties of all samples were analyzed by spectroscopy ellipsometry (SE) and increase in refractive index has been detected with the increase in Ts. Results of Hall measurement showed that substrate temperature have remarkable influence on the resistivity (ρ), carrier concentration (n), and carrier mobility (µ) of IGZO films. As Ts increased from RT to 400oC, ρ decreased from 46.6 to 0.24Ω.cm, and then increased to 1.11Ω.cm at Ts of 400oC, and n increase from 5.67×1015 to 7.33×10 18cm-3. Investigation of X-ray photoelectron spectroscopy (XPS) indicated that as Ts increased, an O 1s component representing the oxygen vacancies increased in amount and that the intensity ratio of In/Ga increased but that of Zn/Ga decreased. ∗
Email: [email protected]
The analysis suggests that the increase of oxygen vacancies could explain the increase in n and reduction in ρ and that the compositional change could explain the change of Eg. Keywords: Amorphous InGaZnO films; Substrate temperature; Optical and electrical properties; Sputtering
1. Introduction Transparent amorphous indium-gallium-zinc-oxide (α-IGZO) thin film transistors (TFTs) are being considered as a replacement for conventional TFTs based on hydrogenated amorphous silicon to be used in optoelectronic device, flat panel displays, like active matrix displays, organic
light-emitting
diodes,
and
other
optoelectronic
device
applications [1-3]. Advantages of α-IGZO TFTs include high electron mobility, good uniformity, high transparency in visible light, and moderate processing temperature [4-7]. a-IGZO is composed of elements that constitute well known n-type oxide semiconductors such as In2O 3, Ga2O 3, and ZnO. Here, the metal cations of In3+, Ga3+, and Zn2+ have ns orbitals that contribute to the bottom part of the conduction band. Among these, the In 5s orbital is considered to be a main cause of efficient electron transport, owing to its large radii and large overlap between adjacent 5s orbitals of spherical symmetry, which structure is less sensitive to the arrangement of atoms [8,9]. As we know, successful
fabrication of flat-panel display applications based on a-IGZO channel materials is determined by the physical properties of a-IGZO. Therefore, it is necessary to investigate the optical and electrical characteristics of a-IGZO. For the sputtering-derived a-IGZO, all the characteristics are affected by the radio-frequency (RF) power, working pressure, and gas flow ratio. Along with control of oxygen partial pressure during the deposition [10], substrate temperature (Ts) is one of the important parameter for optimization of different properties of radio-frequency (RF) sputtered thin films. It can affect surface roughness, mobility (µ), charge carrier concentrations (n), optical transparency, the absorption coefficient (α), the extinction coefficient (k) and the optical bandgap (Eg) [11]. Based on the observation from Yuan [11], it can be noted that t the electrical and optical properties of a-IGZO thin films can be optimized by increase of Ts. However, the exact mechanism for the enhancement of the electrical properties related with the Ts has not been given reasonable explanation. In this study, we investigate the effects of substrate temperature on the structural, optical and electrical properties of a-IGZO films. The structural properties including crystallization, surface morphology and chemical composition were measured by X-ray diffraction (XRD), Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). Changes in the optical properties were investigated by using UV-VIS spectrophotometer and spectroscopic ellipsometery (SE). The
electrical properties were carried out by Hall effects measurement. Based on our analysis, it can be concluded that an O 1s component, which is attributed to oxygen vacancies, can be related to the change of the charge carrier concentration or conductivity, and that the increase in the In/Ga content ratio could be the cause of effective band-gap decrease. 2. Experimental Details InGaZnO4 thin films were deposited onto n-type Si (100) wafers with a resistivity of 2-5Ω.cm and quartz substrates (for UV-VIS spectrophotometer and Hall Effect measurement) by radio frequency magnetron sputtering with substrate temperature changing from room temperature to 400oC. Before deposition, a standard chemical cleaning with removal of the native oxide has been performed on Si and quartz substrates. High purity InGaZnO 4 (In:Ga:Zn:O=1:1:1:4) ceramic disk with a 60 mm diameter and 5mm thickness was used as the sputtering target. High purity Ar (99.999%) was introduced during the deposition. Distance from target to substrate was kept at 60 mm for all depositions. The system was pumped to 3.8×10-4 pa before Ar were introduced, the target was pre-sputtered in an argon atmosphere for 5 minute to remove the surface oxide of the target. During sputtering process, the RF power and working pressure were fixed at 40 W and 0.5 Pa. The flow rate of Ar and the sputtering time were set to 30 sccm and 30 minute, respectively. InGaZnO4 thin films were analyzed by X-ray diffraction (XRD, MXP
18AHF MAC Science, Yokohama, Japan） using Cu Kα =1.5418 Å radiation for crystal structure determination at 40 kV and 100 mA. UV-VIS spectrophotometer (Shimadzu, UV-2550) with wavelength ranging from 190 to 900 nm was used to get the transmission and optical band gaps of IGZO films. Additionally, spectroscopic ellipsometery (SC630, SANCO Co, Shanghai) of rotating analyzer type was used to obtain the thickness, refractive index and extinction coefficient of the samples. The measurements were carried out in air at room temperature in the wavelength range of 280–1100 nm with a step of 10 nm at an incident angle of 65oC and 75oC. Cauchy–Urbach model [12] was used to obtain the thicknesses and optical constant of the as-prepared IGZO thin films. The electrical properties including electrical resistivity, carrier concentration, and Hall mobility were studied by Hall effect measurement using four point van der Pauw method. The measurements were performed at room temperature using an Ecopia HMS-5000 Hall Effect Measurement System. The chemical bonding states and band alignment of the targeted InGaZnO films on Si have been investigated by thermo Scientific XPS (ESCALAB 250Xi) system, equipped with Al K α radiation source (1253.6eV) and hemispherical analyzer with pass energy of 20 eV. The C 1s line with a binding energy of 285 eV was used as a reference to eliminate the charging effect. 3. Results and discussion
Fig. 1 shows the XRD spectra of IGZO films grown at different substrate temperatures. No apparent scattering peak has been observed, indicating that all the films are amorphous independently of substrate temperature. Such phenomenon has been detected in our previous publication [10]. It is reported that crystallization has been detected for 600oC-derived InGaZnO sample, which is higher than our substrate temperature [13,14]. Based on our observation, it can be inferred that the IGZO thin films still keep amorphous even at relatively higher substrate temperature of 400oC. The optical properties of the IGZO films were analyzed by UV-Vis spectroscopy on quartz substrates. The spectrum was recorded in the region of 200-900nm. Fig. 2 shows the optical transmission spectra of deposited InGaZnO 4 films. The average transmission in the visible range increases mildly from 92.91% to 93.54% with increasing the substrate temperature. Heat treatment causes atoms to arrange and lead to more regulation surface morphology, and improve the average transmission of the samples. The low absorption in the visible regions indicates its potential application for display devices. Based on Fig.2, it can be seen that the transmission of the films shows a relatively sharp absorption edge near 340nm. A strong absorption normally occurs due to the electron transition from valence band to conduction band, the band gap energy of samples is obtained from change of the absorption coefficient (α) in the
absorption edge [11,15]. The absorption coefficient α can be calculated from Beer’s law, I=Ioe-αd
(1)
Where I, Io and d are the transmitted intensity, the incident intensity and the thickness of the thin film, respectively. The optical band gap (Eg) of a-IGZO films with different substrate temperatures was determined from the absorption coefficient (α) using Tauc relation [16] (αhυ)1/2=c(hυ-Eg), where hυ is the incident photon energy, c is the constant and Eg is the optical band gap of the material, respectively. A plot of (αhυ)1/2 vs (hυ) is shown in Fig. 3 and the linear portion of the curve extrapolated to hυ axis to determine the energy band gap for the samples deposited at different substrate temperature. The inset shows the calculated values of the band gap for the samples. The band gap of a-IGZO films was found in the range of 3.42-3.31eV, which is close to the previously reported value [17]. The inset in Fig.3 illustrates that the bandgap reduces with the increase in substrate temperature, which is in good agreement with the observation from Yuan [11]. To obtain the optical constants of a-IGZO thin films as a function of substrate temperature, SE has been carried out. To obtain the thickness of the films and related optical constants, a three-layer-structured optical model structure (a-IGZO/SiO2/Si) has been used during fitting process. The Cauchy–Urbach dispersion model was used to fit the data. The
refractive index of the films was modeled by a Cauchy dispersion equation [18]: n (λ) =no+A/λ2+B/λ4
(2)
Where no, A, and B are the index parameters that specify the index of refraction, relating to the refractive index and the wavelength of light, respectively. Fig. 4(a) shows the influence of substrate temperature on the refractive index of IGZO films. It has been noted that the refractive indices are closely related to the density of materials, being higher at higher density [19]. Refractive indices of all samples are in the range of 1.95-2.17, meaning that our IGZO4 films are at higher density. The low values of packing density due to loose arrangement can attribute to the lower refractive index of the as-deposited thin film. Besides, it has been found that the refractive index of IGZO films increases with the increase of substrate temperature. This phenomenon can be explained as follow: the high substrate temperature provides thermal energy that increases the mobility of the atoms of the films, increases the diffusion of the atoms or molecules of the films, and reduces the defect of the film, and favors the formation of more closely packed thin films, resulting in a higher refractive index [20]. High frequency dielectric constants can be obtained from the refractive index through the following simple classical dispersion relation [21-22]: 2
n -1=
λ λ λ
( )
so=(no2-1)/ λo2 The parameters λo, So, and no are an average oscillator position, an average oscillator strength, and the refractive index at infinite wavelength λo (average interband oscillator wavelength), respectively. The oscillator parameters can be obtained by extrapolating the straight line portion of 1/(n2−1) against 1/λ 2 , as shown in Fig. 4(b). Parameters S0 and λ0 can be obtained from the slope (−1/S0) of the resulting straight line and the infinite wavelength intercept (1/S0λ02), respectively. The refractive index dispersion parameter (E/S0) and long wavelength refractive index ε∞ depend on the characteristics of the various interband transitions, can be determined by formula ε∞=n 02, the estimated dispersion parameters (S0, λ0, E0,n∞) are shown in Table 1. Based on Table 1, it can be noted that high frequency dielectric constants (ε∞) increase with the increase in substrate temperature, which can be attributed to the increase in refractive index of the films. The energy dependence characteristic of indirect allowed transition as illustrated by [E(ε2)
1/2
] versus photon energy (E) were shown in Fig. 5.
The optical band gap (Eg) values of the IGZO films can also be determined by Tauc relation as above. The band gaps of 3.58, 3.55, 3.51, 3.50, and 3.40 eV have been obtained for substrate temperature ranging from room temperature to 400oC, respectively. It can be concluded that the band gaps gradually shift toward lower energy sides with the increase
of substrate temperature, confirmed by previous UV-vis measurements. In order to check the film quality, we measured Urbach energy width E0 of samples. E0 is believed to be a function of structural disorder and it follows the empirical Urbach rule given by:
Where α0 is the Urbach absorption at the edge E1, both obtained from the exponential band-tail absorption. The exponential absorption dependence in the Urbach region (hν
6 and Table 1, the electrical properties of resistivity, carrier concentration, and carrier mobility are very sensitive to the substrate temperature. The resistivity decreases from 46.60Ω.cm to 0.24Ω.cm as substrate temperature increases from RT to 300oC, than increases slightly to 1.11Ω.cm as substrate temperature is 400oC. The carrier concentration increases from 5.673×1015cm-3 to 7.339×1018cm-3 when substrate temperature increases from RT to 300oC. The Hall mobility of IGZO films are 23.59, 7.337, 1.912, 58.41 and 0.763cm2/v-1s-1 when substrate temperature ranges from room temperature to 400oC. Trend of the resistivity of a-IGZO samples related with substrate temperature can be explained by the change of carrier concentration and mobility. The increase in either the carrier concentration or the carrier mobility may cause decrease in the resistivity. It has been suggested that oxygen vacancies are sources of free electrons in transparent semiconducting oxide materials. On the basis of first-principles calculations, that oxygen vacancies in amorphous oxide semiconductors form defect levels within the bandgap [23]. The oxygen vacancies can act as shallow donors and supply conduction electrons in a-IGZO films, and the formation of the oxygen-vacancy is closely related to the generation of charge carriers according to the equation, O =1/2(O)2(g)↑+V ··+2e-1 Here, O2 is lost from the oxide sublattice (O ), leads to a doubly charged
oxygen vacancy (V ·· ) and two free electrons [24-25]. Thus, an increase of V o can effectively lead to a decrease of resistivity, unless scattering process is prevailing. The increase of carrier concentration at higher substrate temperatures can be explained as follows: the high substrate temperature provides thermal energy thermally excited oxygen atoms which may have higher chance to leave their original sites to form stronger O 2 molecule and escape from the IGZO films, leave oxygen vacancies with the remaining free electrons at those sites [15]. The generation of additional oxygen vacancies or defect sites at higher substrate temperature can result in the increase of nonstoichiometric a-IGZO in the spectra. In fact, increase of nonstoichiometric a-IGZO or O 1s component representing oxygen vacancies was observed in the following XPS measurements. XPS measurement was used in order to investigate chemical-structure change caused by substrate temperature. The O 1s peaks of a-IGZO films deposited at various temperatures are shown in Fig. 7. Each O 1s spectrum could be deconvoluted by three components: The lower energy peak located at 530.1 eV (O1) attributed to O2− ions surrounded by Zn, Ga and In atoms in the IGZO compound system [26]. While the higher energy peak located at 531.5eV (O2) is associated with O 2− ions that are in oxygen-deficient regions. [27,28]. The high binding energy component (O3) located at 532.85 eV is usually taken for the presence of loosely
bound oxygen on the surface of film belonging to a specific specie, e.g. –CO 3 contaminant [27]. The calculated intensity ratio of O2/(O1+O2+O3) and In 4d/Ga 3ds for the IGZO thin films deposited at various substrate temperatures are listed in Table 3. O2 component represents nonstoichiometric or distorted a-IGZO structure. Therefore, change in the intensity of this component means variation in the concentration of oxygen vacancies in IGZO films. As shown in Fig. 7 and Table 3. With increase of substrate temperature, the O2/(O1+O2+O3) intensity ratio increases, indicating that oxygen vacancies increase as Ts increases. It is in accord with increased carrier concentrations and decreased resistivity in Fig. 6 and Table 2. The Ga 3d and In 4d XPS core-level spectra of a-IGZO films deposited at various substrate temperatures are shown in Fig. 8. In 4d/Ga 3d intensity ratio are also indicated in table 3. As can be seen in Fig. 8, the energy peak located at 18.1eV and 19.9 eV are In 4d and Ga 3d, respectively. As shown in Fig. 8 and Table. 3, In 4d/Ga 3d intensity ratio has a slight increase as substrate temperature increases. It indicates that the content of In in IGZO films increases. Considering the ZnO (~3.3eV), In2O3 (~3.6 eV), and Ga2O3 (~4.9 eV) optical band gaps [29], the increase of In-4d/Ga 3d intensity ratio in IGZO films may lead to the reduction in band gap with the increase of substrate temperature, which is consistent with Yuan’s report [11]. Besides, based on Fig. 8 and Fig. 9, it can be
noted that the samples are rich of indium atom under higher substrate temperatures. For In, it is difficult to form oxides with oxygen, so it is easy to produce oxygen vacancy and exist in the crystal lattice instead of zinc atoms. Therefore, a higher indium atomic levels will increase carrier concentration in IGZO films [19,30].Thus, it can be inferred that as substrate temperatures increase, oxygen vacancies increase, resulting in increased carrier concentrations, which then led to the reduced resistivity. 4. Conclusions InGaZnO 4 thin films were deposited by RF sputtering with substrate temperature ranging from RT to 400oC. The effect of substrate heat treatment on the optical and electrical properties of IGZO was investigated by XRD, UV-VIS spectrophotometer, SE, Hall Effect measurement system, and XPS. Based on analysis, it can be concluded that the films deposited at higher substrate temperatures have lower optical band gaps, high refractive index, lower resistivity and higher charge carrier concentrations. XPS measurements indicate that higher substrate temperatures effectively made the film more nonstoichiometric. This implies the generations of oxygen vacancies and defects, additional free charge carriers thereby, and, as a result, lower resistivity. And the increase of In 4d/Ga 3d intensity ratio in IGZO films may explain the reduction of optical band gap at higher substrate temperature. Acknowledgments
The authors acknowledge the support from National Key Project of Fundamental Research (2013CB632705), National Natural Science Foundation of China (11104269, 51272001), Provincial Natural Science Foundation
of
Anhui
Higher
Education
Institution
of
China
(KJ2012A023), Key Project of Chinese Ministry of Education (212082), and Outstanding Young Scientific Foundation of Anhui University (KJJQ1103) and ‘‘211 project’’ of Anhui University.
References [1] S.U. Yang, S.H. Choi, J. Lee, J. Kim, W.S. Jung, H.Y. Yu, Y. Roh, J.H. Park, J. Alloys Comp. 561 (2013) 228–230. [2] J.L. Wu, H.Y. Lin, B.Y. Su, Y.C. Chen, S.Y. Chu, S.Y. Liu, C.C. Chang, C.J. Wu, J. Alloys Comp. 592 (2014) 35–41. [3] M.K. Dai, J.T. Lian, T.Y. Lin, Y.F. Chen, J. Mater. Chem. C. 1 (2013) 5064–5071. [4] B.Y. Su, S.Y. Chu, Y.D. Juang, S.Y. Liu, J. Alloys Comp. 580 (2013) 10–14. [5] T. Kamiya, H. Hosono, NPG Asia Mater. 2 (2010) 15–22. [6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature. 432 (2004) 488-492. [7] T.Y. Hsieh, T.C. Chang, T.C. Chen, Y.C. Chen, Y.T. Chen, P.Y. Liao, A.K. Chu, W.W. Tsai, W.J. Chiang, J.Y. Yan, Appl. Phys. Lett. 101 (2012) 212104-1-212104-3. [8] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, H. Hosono, Science. 23 (2003) 1269-1272. [9] N. Fukuda, Y. Watanabe, S. Uemura, Y. Yoshida, T. Nakamura, H. Ushijima, J. Mater. Chem. C. 2 (2014) 2448-2454. [10] X.F. Chen, G. He, M. Liu, J.W. Zhang, B. Deng, P.H. Wang, M. Zhang, J.G. Lv, Z.Q. Sun, J. Alloys Comp. 615 (2014) 636-642. [11] L. Yuan, G.J. Fang, X. Zou, H. Huang, H. Zou, X. Han, Y. Gao, S.
Xu, X. Zhao, J. Phys. D: Appl. Phys. 42 (2009) 215301-1-215301-6. [12] J.M. Khoshman, M.E. Kordesch, Surf. Coat. Technol. 201 (2006) 3530-3535. [13] B.D. Ahn, H.S. Shin, D.L. Kim, S.M. Lee, J.S. Park, G.H. Kim, H.J. Kim, Jpn. J. App. Phy. 51 (2011) 015601. [14] S.J. Seo, J.H. Cho, Y.H. Jang, C.H. Kim, J. Korean Phys. Soc. 60 (2012) 267-271. [15] A. Thakur, H. Yoo, S.J. Kang, J.Y. Baik, I.J. Lee, H.K. Lee, K. Kim, B. Kim, S. Jung, J. Park, H.J. Shin, ECS J. Solid State Sci. Technol. 1(1) (2012) Q11- Q15. [16] N.Y. Yuan, S.Y. Wang, C.B. Tan, X.Q. Wang, G.G. Chen, J.N. Ding , J. Cryst. Growth. 366 (2013) 43-46. [17] H.H. Güllü, Ö. Bayraklı, I. Candan, E. Coskun, M. Parlak, J. Alloys Comp. 566 (2013) 83-89. [18] M.F. Al-Kuhaili, S.M.A. Durrani, I.A. Bakhtiari, M.A. Dastageer, M.B. Mekki, Mater. Chem. Phys. 126 (2011) 515-523. [19] A. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, Thin Solid Films 486 (2005) 38-41. [20] G. He, L.Q. Zhu, M. Liu, Q. Fang, L.D. Zhang, Appl. Surf. Sci. 253 (2007) 3413-3418. [21] G. He, L.D. Zhang, M. Liu, J.P. Zhang, X.J. Wang, C.M. Zhen, J. Appl. Phys. 105 (2009) 014109-1-014109-4.
[22] F. Yakuphanoglu, A. Cukurovali, I. Yilmaz, Physica B 351 (2004) 53–58. [23] T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv. Mater. 11 (2010) 044305-1-044305-5. [24] T. Kamiya, K. Nomura, M. Hirano, H. Hosono, Phys. Status. Solidi 5 (2008) 3098–3100. [25] X.F. Li, E.L. Xin, J.F. Shi, C.Y. Li, J.H. Zhang, J. Vac. Sci. Technol. A 31 (2013) 031505-1-031505-5. [26] M. Chen, Z.L. Pei, C. Sun, L.S. Wen, X. Wang, J. Cryst. Growth. 220 (2000) 254-262. [27] G.H. Kim, H.S. Kim, H.S. Shin, B.D. Ahn, K.H. Kim, H.J. Kim, Thin Solid Films 517 (2009) 4007–4010. [28] Y.F. Lu, H.Q. Ni, Z.H. Mai, Z.M. Ren, J. Appl. Phys. 88 (2000) 498-502. [29] A. Thakur, S.J. Kang, J.Y. Baik, H. Yoo, I.J. Lee, H.K. Lee, S. Jung, J. Park, H.J. Shin, J. Alloys Comp. 525 (2012) 172– 174. [30] T. Kamiya, K. Nomura, H. Hosono, J. Disp. Technol. 5 (2009) 468-483.
Figures and Table Caption Fig. 1. XRD pattern for the InGaZnO4 thin films deposited at various substrate temperature. Fig. 2. Transmission spectra of the InGaZnO 4 thin films deposited at various substrate temperature. Fig. 3. Relationship between (αhυ)1/2 and hυ for a-IGZO thin films deposited at various substrate temperature. The inset shows the variation of the band gap with the substrate temperature. Fig. 4. (a) The refractive index of IGZO films deposited at various substrate temperature. (b) Plots of 1/(n2-1) vs 1/λ2 for IGZO films deposited at various substrate temperature. Fig. 5. (a ) The determination of the optical band gap and (b)logarithm of the absorption coefficient α vs hυ for IGZO films deposited at various substrate temperature. Fig. 6. Electrical properties including resistivity, carrier concentration, and mobility of IGZO thin films deposited at various substrate temperature. Fig. 7. O 1s XPS spectra of a-IGZO films deposited at various substrate temperature. Fig. 8. Ga 4d and In 4d spectra of a-IGZO films deposited at various substrate temperature. Fig. 9. The intensity ratio of O2/(O1+O2+O3) and In 4d/Ga 3d as a
function of substrate temperature. Table 1 Dispersion parameters of the IGZO films deposited at various substrate temperature. Table 2 Electrical data including resistivity, carrier concentration, and mobility of IGZO thin films deposited at various substrate temperature. Table 3 The intensity ratio of O2/(O1+O2+O3) and In 4d/Ga 3d calculated from XPS for the IGZO thin films deposited at various substrate temperatures.
Highlights 1. Amorphous IGZO films are obtained by sputtering at various substrate temperatures. 2. Higher substrate temperatures lead to lower band gaps and high refractive index. 3. High temperature results in lower resistivity and larger charge carrier content. 4. Increased oxygen vacancies attributes to the reduced band gap. 5. Increased In content in IGZO films leads to the improved electrical properties.
Tab-1
Tab-2
Tab-3
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
Fig.8
Fig.9
S0925-8388(15)00216-9 http://dx.doi.org/10.1016/j.jallcom.2015.01.143 JALCOM 33193
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
20 November 2014 29 December 2014 4 January 2015
Please cite this article as: X.F. Chen, G. He, J. Gao, J.W. Zhang, D.Q. Xiao, P. Jin, B. Deng, Substrate temperature dependent structural, optical and electrical properties of amorphous InGaZnO thin films, Journal of Alloys and Compounds (2015), doi: http://dx.doi.org/10.1016/j.jallcom.2015.01.143
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Substrate temperature dependent structural, optical and electrical properties of amorphous InGaZnO thin films X. F. Chena, G. Hea∗, J. Gaoa, J. W. Zhanga, D. Q. Xiaoa, P. Jina, and B. Denga a
School of physics and Materials Science, Radiation Detection Materials & Devices
Lab, Anhui University, Hefei 230601, China.
The effects of substrate temperature (Ts) on the electrical and optical properties of amorphous InGaZnO thin films deposited by sputtering have been investigated. As Ts increased from RT to 400 oC, all the films remained amorphous, the transmission in the visible region increased from 92.8% to 93.54%, and the band gap decreased from 3.42eV to 3.31eV. Based on Cauchy-Urbach model, the optical properties of all samples were analyzed by spectroscopy ellipsometry (SE) and increase in refractive index has been detected with the increase in Ts. Results of Hall measurement showed that substrate temperature have remarkable influence on the resistivity (ρ), carrier concentration (n), and carrier mobility (µ) of IGZO films. As Ts increased from RT to 400oC, ρ decreased from 46.6 to 0.24Ω.cm, and then increased to 1.11Ω.cm at Ts of 400oC, and n increase from 5.67×1015 to 7.33×10 18cm-3. Investigation of X-ray photoelectron spectroscopy (XPS) indicated that as Ts increased, an O 1s component representing the oxygen vacancies increased in amount and that the intensity ratio of In/Ga increased but that of Zn/Ga decreased. ∗
Email: [email protected]
The analysis suggests that the increase of oxygen vacancies could explain the increase in n and reduction in ρ and that the compositional change could explain the change of Eg. Keywords: Amorphous InGaZnO films; Substrate temperature; Optical and electrical properties; Sputtering
1. Introduction Transparent amorphous indium-gallium-zinc-oxide (α-IGZO) thin film transistors (TFTs) are being considered as a replacement for conventional TFTs based on hydrogenated amorphous silicon to be used in optoelectronic device, flat panel displays, like active matrix displays, organic
light-emitting
diodes,
and
other
optoelectronic
device
applications [1-3]. Advantages of α-IGZO TFTs include high electron mobility, good uniformity, high transparency in visible light, and moderate processing temperature [4-7]. a-IGZO is composed of elements that constitute well known n-type oxide semiconductors such as In2O 3, Ga2O 3, and ZnO. Here, the metal cations of In3+, Ga3+, and Zn2+ have ns orbitals that contribute to the bottom part of the conduction band. Among these, the In 5s orbital is considered to be a main cause of efficient electron transport, owing to its large radii and large overlap between adjacent 5s orbitals of spherical symmetry, which structure is less sensitive to the arrangement of atoms [8,9]. As we know, successful
fabrication of flat-panel display applications based on a-IGZO channel materials is determined by the physical properties of a-IGZO. Therefore, it is necessary to investigate the optical and electrical characteristics of a-IGZO. For the sputtering-derived a-IGZO, all the characteristics are affected by the radio-frequency (RF) power, working pressure, and gas flow ratio. Along with control of oxygen partial pressure during the deposition [10], substrate temperature (Ts) is one of the important parameter for optimization of different properties of radio-frequency (RF) sputtered thin films. It can affect surface roughness, mobility (µ), charge carrier concentrations (n), optical transparency, the absorption coefficient (α), the extinction coefficient (k) and the optical bandgap (Eg) [11]. Based on the observation from Yuan [11], it can be noted that t the electrical and optical properties of a-IGZO thin films can be optimized by increase of Ts. However, the exact mechanism for the enhancement of the electrical properties related with the Ts has not been given reasonable explanation. In this study, we investigate the effects of substrate temperature on the structural, optical and electrical properties of a-IGZO films. The structural properties including crystallization, surface morphology and chemical composition were measured by X-ray diffraction (XRD), Atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). Changes in the optical properties were investigated by using UV-VIS spectrophotometer and spectroscopic ellipsometery (SE). The
electrical properties were carried out by Hall effects measurement. Based on our analysis, it can be concluded that an O 1s component, which is attributed to oxygen vacancies, can be related to the change of the charge carrier concentration or conductivity, and that the increase in the In/Ga content ratio could be the cause of effective band-gap decrease. 2. Experimental Details InGaZnO4 thin films were deposited onto n-type Si (100) wafers with a resistivity of 2-5Ω.cm and quartz substrates (for UV-VIS spectrophotometer and Hall Effect measurement) by radio frequency magnetron sputtering with substrate temperature changing from room temperature to 400oC. Before deposition, a standard chemical cleaning with removal of the native oxide has been performed on Si and quartz substrates. High purity InGaZnO 4 (In:Ga:Zn:O=1:1:1:4) ceramic disk with a 60 mm diameter and 5mm thickness was used as the sputtering target. High purity Ar (99.999%) was introduced during the deposition. Distance from target to substrate was kept at 60 mm for all depositions. The system was pumped to 3.8×10-4 pa before Ar were introduced, the target was pre-sputtered in an argon atmosphere for 5 minute to remove the surface oxide of the target. During sputtering process, the RF power and working pressure were fixed at 40 W and 0.5 Pa. The flow rate of Ar and the sputtering time were set to 30 sccm and 30 minute, respectively. InGaZnO4 thin films were analyzed by X-ray diffraction (XRD, MXP
18AHF MAC Science, Yokohama, Japan） using Cu Kα =1.5418 Å radiation for crystal structure determination at 40 kV and 100 mA. UV-VIS spectrophotometer (Shimadzu, UV-2550) with wavelength ranging from 190 to 900 nm was used to get the transmission and optical band gaps of IGZO films. Additionally, spectroscopic ellipsometery (SC630, SANCO Co, Shanghai) of rotating analyzer type was used to obtain the thickness, refractive index and extinction coefficient of the samples. The measurements were carried out in air at room temperature in the wavelength range of 280–1100 nm with a step of 10 nm at an incident angle of 65oC and 75oC. Cauchy–Urbach model [12] was used to obtain the thicknesses and optical constant of the as-prepared IGZO thin films. The electrical properties including electrical resistivity, carrier concentration, and Hall mobility were studied by Hall effect measurement using four point van der Pauw method. The measurements were performed at room temperature using an Ecopia HMS-5000 Hall Effect Measurement System. The chemical bonding states and band alignment of the targeted InGaZnO films on Si have been investigated by thermo Scientific XPS (ESCALAB 250Xi) system, equipped with Al K α radiation source (1253.6eV) and hemispherical analyzer with pass energy of 20 eV. The C 1s line with a binding energy of 285 eV was used as a reference to eliminate the charging effect. 3. Results and discussion
Fig. 1 shows the XRD spectra of IGZO films grown at different substrate temperatures. No apparent scattering peak has been observed, indicating that all the films are amorphous independently of substrate temperature. Such phenomenon has been detected in our previous publication [10]. It is reported that crystallization has been detected for 600oC-derived InGaZnO sample, which is higher than our substrate temperature [13,14]. Based on our observation, it can be inferred that the IGZO thin films still keep amorphous even at relatively higher substrate temperature of 400oC. The optical properties of the IGZO films were analyzed by UV-Vis spectroscopy on quartz substrates. The spectrum was recorded in the region of 200-900nm. Fig. 2 shows the optical transmission spectra of deposited InGaZnO 4 films. The average transmission in the visible range increases mildly from 92.91% to 93.54% with increasing the substrate temperature. Heat treatment causes atoms to arrange and lead to more regulation surface morphology, and improve the average transmission of the samples. The low absorption in the visible regions indicates its potential application for display devices. Based on Fig.2, it can be seen that the transmission of the films shows a relatively sharp absorption edge near 340nm. A strong absorption normally occurs due to the electron transition from valence band to conduction band, the band gap energy of samples is obtained from change of the absorption coefficient (α) in the
absorption edge [11,15]. The absorption coefficient α can be calculated from Beer’s law, I=Ioe-αd
(1)
Where I, Io and d are the transmitted intensity, the incident intensity and the thickness of the thin film, respectively. The optical band gap (Eg) of a-IGZO films with different substrate temperatures was determined from the absorption coefficient (α) using Tauc relation [16] (αhυ)1/2=c(hυ-Eg), where hυ is the incident photon energy, c is the constant and Eg is the optical band gap of the material, respectively. A plot of (αhυ)1/2 vs (hυ) is shown in Fig. 3 and the linear portion of the curve extrapolated to hυ axis to determine the energy band gap for the samples deposited at different substrate temperature. The inset shows the calculated values of the band gap for the samples. The band gap of a-IGZO films was found in the range of 3.42-3.31eV, which is close to the previously reported value [17]. The inset in Fig.3 illustrates that the bandgap reduces with the increase in substrate temperature, which is in good agreement with the observation from Yuan [11]. To obtain the optical constants of a-IGZO thin films as a function of substrate temperature, SE has been carried out. To obtain the thickness of the films and related optical constants, a three-layer-structured optical model structure (a-IGZO/SiO2/Si) has been used during fitting process. The Cauchy–Urbach dispersion model was used to fit the data. The
refractive index of the films was modeled by a Cauchy dispersion equation [18]: n (λ) =no+A/λ2+B/λ4
(2)
Where no, A, and B are the index parameters that specify the index of refraction, relating to the refractive index and the wavelength of light, respectively. Fig. 4(a) shows the influence of substrate temperature on the refractive index of IGZO films. It has been noted that the refractive indices are closely related to the density of materials, being higher at higher density [19]. Refractive indices of all samples are in the range of 1.95-2.17, meaning that our IGZO4 films are at higher density. The low values of packing density due to loose arrangement can attribute to the lower refractive index of the as-deposited thin film. Besides, it has been found that the refractive index of IGZO films increases with the increase of substrate temperature. This phenomenon can be explained as follow: the high substrate temperature provides thermal energy that increases the mobility of the atoms of the films, increases the diffusion of the atoms or molecules of the films, and reduces the defect of the film, and favors the formation of more closely packed thin films, resulting in a higher refractive index [20]. High frequency dielectric constants can be obtained from the refractive index through the following simple classical dispersion relation [21-22]: 2
n -1=
λ λ λ
( )
so=(no2-1)/ λo2 The parameters λo, So, and no are an average oscillator position, an average oscillator strength, and the refractive index at infinite wavelength λo (average interband oscillator wavelength), respectively. The oscillator parameters can be obtained by extrapolating the straight line portion of 1/(n2−1) against 1/λ 2 , as shown in Fig. 4(b). Parameters S0 and λ0 can be obtained from the slope (−1/S0) of the resulting straight line and the infinite wavelength intercept (1/S0λ02), respectively. The refractive index dispersion parameter (E/S0) and long wavelength refractive index ε∞ depend on the characteristics of the various interband transitions, can be determined by formula ε∞=n 02, the estimated dispersion parameters (S0, λ0, E0,n∞) are shown in Table 1. Based on Table 1, it can be noted that high frequency dielectric constants (ε∞) increase with the increase in substrate temperature, which can be attributed to the increase in refractive index of the films. The energy dependence characteristic of indirect allowed transition as illustrated by [E(ε2)
1/2
] versus photon energy (E) were shown in Fig. 5.
The optical band gap (Eg) values of the IGZO films can also be determined by Tauc relation as above. The band gaps of 3.58, 3.55, 3.51, 3.50, and 3.40 eV have been obtained for substrate temperature ranging from room temperature to 400oC, respectively. It can be concluded that the band gaps gradually shift toward lower energy sides with the increase
of substrate temperature, confirmed by previous UV-vis measurements. In order to check the film quality, we measured Urbach energy width E0 of samples. E0 is believed to be a function of structural disorder and it follows the empirical Urbach rule given by:
Where α0 is the Urbach absorption at the edge E1, both obtained from the exponential band-tail absorption. The exponential absorption dependence in the Urbach region (hν
6 and Table 1, the electrical properties of resistivity, carrier concentration, and carrier mobility are very sensitive to the substrate temperature. The resistivity decreases from 46.60Ω.cm to 0.24Ω.cm as substrate temperature increases from RT to 300oC, than increases slightly to 1.11Ω.cm as substrate temperature is 400oC. The carrier concentration increases from 5.673×1015cm-3 to 7.339×1018cm-3 when substrate temperature increases from RT to 300oC. The Hall mobility of IGZO films are 23.59, 7.337, 1.912, 58.41 and 0.763cm2/v-1s-1 when substrate temperature ranges from room temperature to 400oC. Trend of the resistivity of a-IGZO samples related with substrate temperature can be explained by the change of carrier concentration and mobility. The increase in either the carrier concentration or the carrier mobility may cause decrease in the resistivity. It has been suggested that oxygen vacancies are sources of free electrons in transparent semiconducting oxide materials. On the basis of first-principles calculations, that oxygen vacancies in amorphous oxide semiconductors form defect levels within the bandgap [23]. The oxygen vacancies can act as shallow donors and supply conduction electrons in a-IGZO films, and the formation of the oxygen-vacancy is closely related to the generation of charge carriers according to the equation, O =1/2(O)2(g)↑+V ··+2e-1 Here, O2 is lost from the oxide sublattice (O ), leads to a doubly charged
oxygen vacancy (V ·· ) and two free electrons [24-25]. Thus, an increase of V o can effectively lead to a decrease of resistivity, unless scattering process is prevailing. The increase of carrier concentration at higher substrate temperatures can be explained as follows: the high substrate temperature provides thermal energy thermally excited oxygen atoms which may have higher chance to leave their original sites to form stronger O 2 molecule and escape from the IGZO films, leave oxygen vacancies with the remaining free electrons at those sites [15]. The generation of additional oxygen vacancies or defect sites at higher substrate temperature can result in the increase of nonstoichiometric a-IGZO in the spectra. In fact, increase of nonstoichiometric a-IGZO or O 1s component representing oxygen vacancies was observed in the following XPS measurements. XPS measurement was used in order to investigate chemical-structure change caused by substrate temperature. The O 1s peaks of a-IGZO films deposited at various temperatures are shown in Fig. 7. Each O 1s spectrum could be deconvoluted by three components: The lower energy peak located at 530.1 eV (O1) attributed to O2− ions surrounded by Zn, Ga and In atoms in the IGZO compound system [26]. While the higher energy peak located at 531.5eV (O2) is associated with O 2− ions that are in oxygen-deficient regions. [27,28]. The high binding energy component (O3) located at 532.85 eV is usually taken for the presence of loosely
bound oxygen on the surface of film belonging to a specific specie, e.g. –CO 3 contaminant [27]. The calculated intensity ratio of O2/(O1+O2+O3) and In 4d/Ga 3ds for the IGZO thin films deposited at various substrate temperatures are listed in Table 3. O2 component represents nonstoichiometric or distorted a-IGZO structure. Therefore, change in the intensity of this component means variation in the concentration of oxygen vacancies in IGZO films. As shown in Fig. 7 and Table 3. With increase of substrate temperature, the O2/(O1+O2+O3) intensity ratio increases, indicating that oxygen vacancies increase as Ts increases. It is in accord with increased carrier concentrations and decreased resistivity in Fig. 6 and Table 2. The Ga 3d and In 4d XPS core-level spectra of a-IGZO films deposited at various substrate temperatures are shown in Fig. 8. In 4d/Ga 3d intensity ratio are also indicated in table 3. As can be seen in Fig. 8, the energy peak located at 18.1eV and 19.9 eV are In 4d and Ga 3d, respectively. As shown in Fig. 8 and Table. 3, In 4d/Ga 3d intensity ratio has a slight increase as substrate temperature increases. It indicates that the content of In in IGZO films increases. Considering the ZnO (~3.3eV), In2O3 (~3.6 eV), and Ga2O3 (~4.9 eV) optical band gaps [29], the increase of In-4d/Ga 3d intensity ratio in IGZO films may lead to the reduction in band gap with the increase of substrate temperature, which is consistent with Yuan’s report [11]. Besides, based on Fig. 8 and Fig. 9, it can be
noted that the samples are rich of indium atom under higher substrate temperatures. For In, it is difficult to form oxides with oxygen, so it is easy to produce oxygen vacancy and exist in the crystal lattice instead of zinc atoms. Therefore, a higher indium atomic levels will increase carrier concentration in IGZO films [19,30].Thus, it can be inferred that as substrate temperatures increase, oxygen vacancies increase, resulting in increased carrier concentrations, which then led to the reduced resistivity. 4. Conclusions InGaZnO 4 thin films were deposited by RF sputtering with substrate temperature ranging from RT to 400oC. The effect of substrate heat treatment on the optical and electrical properties of IGZO was investigated by XRD, UV-VIS spectrophotometer, SE, Hall Effect measurement system, and XPS. Based on analysis, it can be concluded that the films deposited at higher substrate temperatures have lower optical band gaps, high refractive index, lower resistivity and higher charge carrier concentrations. XPS measurements indicate that higher substrate temperatures effectively made the film more nonstoichiometric. This implies the generations of oxygen vacancies and defects, additional free charge carriers thereby, and, as a result, lower resistivity. And the increase of In 4d/Ga 3d intensity ratio in IGZO films may explain the reduction of optical band gap at higher substrate temperature. Acknowledgments
The authors acknowledge the support from National Key Project of Fundamental Research (2013CB632705), National Natural Science Foundation of China (11104269, 51272001), Provincial Natural Science Foundation
of
Anhui
Higher
Education
Institution
of
China
(KJ2012A023), Key Project of Chinese Ministry of Education (212082), and Outstanding Young Scientific Foundation of Anhui University (KJJQ1103) and ‘‘211 project’’ of Anhui University.
References [1] S.U. Yang, S.H. Choi, J. Lee, J. Kim, W.S. Jung, H.Y. Yu, Y. Roh, J.H. Park, J. Alloys Comp. 561 (2013) 228–230. [2] J.L. Wu, H.Y. Lin, B.Y. Su, Y.C. Chen, S.Y. Chu, S.Y. Liu, C.C. Chang, C.J. Wu, J. Alloys Comp. 592 (2014) 35–41. [3] M.K. Dai, J.T. Lian, T.Y. Lin, Y.F. Chen, J. Mater. Chem. C. 1 (2013) 5064–5071. [4] B.Y. Su, S.Y. Chu, Y.D. Juang, S.Y. Liu, J. Alloys Comp. 580 (2013) 10–14. [5] T. Kamiya, H. Hosono, NPG Asia Mater. 2 (2010) 15–22. [6] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, H. Hosono, Nature. 432 (2004) 488-492. [7] T.Y. Hsieh, T.C. Chang, T.C. Chen, Y.C. Chen, Y.T. Chen, P.Y. Liao, A.K. Chu, W.W. Tsai, W.J. Chiang, J.Y. Yan, Appl. Phys. Lett. 101 (2012) 212104-1-212104-3. [8] K. Nomura, H. Ohta, K. Ueda, T. Kamiya, M. Hirano, H. Hosono, Science. 23 (2003) 1269-1272. [9] N. Fukuda, Y. Watanabe, S. Uemura, Y. Yoshida, T. Nakamura, H. Ushijima, J. Mater. Chem. C. 2 (2014) 2448-2454. [10] X.F. Chen, G. He, M. Liu, J.W. Zhang, B. Deng, P.H. Wang, M. Zhang, J.G. Lv, Z.Q. Sun, J. Alloys Comp. 615 (2014) 636-642. [11] L. Yuan, G.J. Fang, X. Zou, H. Huang, H. Zou, X. Han, Y. Gao, S.
Xu, X. Zhao, J. Phys. D: Appl. Phys. 42 (2009) 215301-1-215301-6. [12] J.M. Khoshman, M.E. Kordesch, Surf. Coat. Technol. 201 (2006) 3530-3535. [13] B.D. Ahn, H.S. Shin, D.L. Kim, S.M. Lee, J.S. Park, G.H. Kim, H.J. Kim, Jpn. J. App. Phy. 51 (2011) 015601. [14] S.J. Seo, J.H. Cho, Y.H. Jang, C.H. Kim, J. Korean Phys. Soc. 60 (2012) 267-271. [15] A. Thakur, H. Yoo, S.J. Kang, J.Y. Baik, I.J. Lee, H.K. Lee, K. Kim, B. Kim, S. Jung, J. Park, H.J. Shin, ECS J. Solid State Sci. Technol. 1(1) (2012) Q11- Q15. [16] N.Y. Yuan, S.Y. Wang, C.B. Tan, X.Q. Wang, G.G. Chen, J.N. Ding , J. Cryst. Growth. 366 (2013) 43-46. [17] H.H. Güllü, Ö. Bayraklı, I. Candan, E. Coskun, M. Parlak, J. Alloys Comp. 566 (2013) 83-89. [18] M.F. Al-Kuhaili, S.M.A. Durrani, I.A. Bakhtiari, M.A. Dastageer, M.B. Mekki, Mater. Chem. Phys. 126 (2011) 515-523. [19] A. Takagi, K. Nomura, H. Ohta, H. Yanagi, T. Kamiya, M. Hirano, H. Hosono, Thin Solid Films 486 (2005) 38-41. [20] G. He, L.Q. Zhu, M. Liu, Q. Fang, L.D. Zhang, Appl. Surf. Sci. 253 (2007) 3413-3418. [21] G. He, L.D. Zhang, M. Liu, J.P. Zhang, X.J. Wang, C.M. Zhen, J. Appl. Phys. 105 (2009) 014109-1-014109-4.
[22] F. Yakuphanoglu, A. Cukurovali, I. Yilmaz, Physica B 351 (2004) 53–58. [23] T. Kamiya, K. Nomura, and H. Hosono, Sci. Technol. Adv. Mater. 11 (2010) 044305-1-044305-5. [24] T. Kamiya, K. Nomura, M. Hirano, H. Hosono, Phys. Status. Solidi 5 (2008) 3098–3100. [25] X.F. Li, E.L. Xin, J.F. Shi, C.Y. Li, J.H. Zhang, J. Vac. Sci. Technol. A 31 (2013) 031505-1-031505-5. [26] M. Chen, Z.L. Pei, C. Sun, L.S. Wen, X. Wang, J. Cryst. Growth. 220 (2000) 254-262. [27] G.H. Kim, H.S. Kim, H.S. Shin, B.D. Ahn, K.H. Kim, H.J. Kim, Thin Solid Films 517 (2009) 4007–4010. [28] Y.F. Lu, H.Q. Ni, Z.H. Mai, Z.M. Ren, J. Appl. Phys. 88 (2000) 498-502. [29] A. Thakur, S.J. Kang, J.Y. Baik, H. Yoo, I.J. Lee, H.K. Lee, S. Jung, J. Park, H.J. Shin, J. Alloys Comp. 525 (2012) 172– 174. [30] T. Kamiya, K. Nomura, H. Hosono, J. Disp. Technol. 5 (2009) 468-483.
Figures and Table Caption Fig. 1. XRD pattern for the InGaZnO4 thin films deposited at various substrate temperature. Fig. 2. Transmission spectra of the InGaZnO 4 thin films deposited at various substrate temperature. Fig. 3. Relationship between (αhυ)1/2 and hυ for a-IGZO thin films deposited at various substrate temperature. The inset shows the variation of the band gap with the substrate temperature. Fig. 4. (a) The refractive index of IGZO films deposited at various substrate temperature. (b) Plots of 1/(n2-1) vs 1/λ2 for IGZO films deposited at various substrate temperature. Fig. 5. (a ) The determination of the optical band gap and (b)logarithm of the absorption coefficient α vs hυ for IGZO films deposited at various substrate temperature. Fig. 6. Electrical properties including resistivity, carrier concentration, and mobility of IGZO thin films deposited at various substrate temperature. Fig. 7. O 1s XPS spectra of a-IGZO films deposited at various substrate temperature. Fig. 8. Ga 4d and In 4d spectra of a-IGZO films deposited at various substrate temperature. Fig. 9. The intensity ratio of O2/(O1+O2+O3) and In 4d/Ga 3d as a
function of substrate temperature. Table 1 Dispersion parameters of the IGZO films deposited at various substrate temperature. Table 2 Electrical data including resistivity, carrier concentration, and mobility of IGZO thin films deposited at various substrate temperature. Table 3 The intensity ratio of O2/(O1+O2+O3) and In 4d/Ga 3d calculated from XPS for the IGZO thin films deposited at various substrate temperatures.
Highlights 1. Amorphous IGZO films are obtained by sputtering at various substrate temperatures. 2. Higher substrate temperatures lead to lower band gaps and high refractive index. 3. High temperature results in lower resistivity and larger charge carrier content. 4. Increased oxygen vacancies attributes to the reduced band gap. 5. Increased In content in IGZO films leads to the improved electrical properties.
Tab-1
Tab-2
Tab-3
Fig.1
Fig.2
Fig.3
Fig.4
Fig.5
Fig.6
Fig.7
Fig.8
Fig.9