- Email: [email protected]
Influence of ZnO buffer layer on the electrical, optical and surface properties of Ga-doped ZnO films
Accepted Manuscript Influence of ZnO buffer layer on the electrical, optical and surface properties of Gadoped ZnO films Hui Cheng, Hong Deng, Yan Wang, Min Wei PII:
S0925-8388(17)30195-0
DOI:
10.1016/j.jallcom.2017.01.172
Reference:
JALCOM 40539
To appear in:
Journal of Alloys and Compounds
Received Date: 2 December 2016 Revised Date:
13 January 2017
Accepted Date: 18 January 2017
Please cite this article as: H. Cheng, H. Deng, Y. Wang, M. Wei, Influence of ZnO buffer layer on the electrical, optical and surface properties of Ga-doped ZnO films, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.01.172. 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.
ACCEPTED MANUSCRIPT
Influence of ZnO buffer layer on the electrical, optical and surface properties of Ga-doped ZnO films Hui Cheng, Hong Deng*, Yan Wang and Min Wei State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China; * Correspondence: [email protected]; Tel.: +86-028-8320-2551
Abstract
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[email protected]; [email protected]; [email protected]
The influence of the ZnO buffer layer thickness on the structural, electrical, optical
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and surface properties of Ga-doped ZnO (GZO) films deposited on glass substrates by RF magnetron sputtering were investigated. X-ray diffraction results showed the
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obtained films had highly c-axis oriented with hexagonal (002) structures and GZO film with 20 nm buffer layer had the best crystalline quality. The resistivity in GZO/ZnO bi-layer films decreased significantly than that in GZO film without a ZnO buffer layer, and GZO film with 20 nm buffer layer showed the lowest resistivity of 4.09×10-4 Ω·cm. The bi-layer films exhibited the highest transmittance of over 80% in the visible light range and displayed a low near infrared transmittance. The
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correlation between surface morphology and wettability was studied and GZO/ZnO bi-layer films exhibited hydrophobic property with contact angle of θ from 104° to 108.5°, indicating acceptable property of environmental durability.
Keywords
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thin films, oxide materials, vapor deposition, electronic properties, optical properties
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1. Introduction
Transparent conductive oxides (TCOs) are one of the potential candidates for heat
shielding coatings because of the high visible transparent and high concentration of free electrons [1]. Among many kinds of TCOs, Ga doped ZnO (GZO) has gained great attention as an alternative to ITO due to its promising optical and electrical properties as well as its low cost, non-toxicity and the abundance of elements [2,3]. The covalent bond lengths of Ga–O and Zn–O are estimated to be 1.92 Å and 1.97 Å [4], respectively, which means the lattice distortion of wurtzite structure ZnO is minimal even for a high concentration of Ga in ZnO compared with IZO and AZO. In addition, GZO has better conduction stability than AZO because Ga is relatively oxidation resistant [3].
ACCEPTED MANUSCRIPT In recent years, many researchers have prepared different kinds of buffer layers to improve the optical and electrical properties of GZO films, structures such as GZO/Ni [5], GZO/TiO2 [6] and GZO/SiOX [7] were deposited and investigated. Previously reported the effects of ZnO buffer layer in the process of preparation [8-12], for optical transmittance spectra, most researchers have been concerned about the impact
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of buffer layer on the visible region, however, there are few studies about the effect of the buffer layer on the near-infrared band. Since the solar energy is distributed in the wavelength range from 250 nm to 2500 nm [13], and the visible light from 300 nm to 800 nm needs to be transmitted for window applications, heat radiation above 800 nm
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wavelength should be cut for effective thermal insulation [1].
The surface performance is of the same importance since it is a critical characteristic for determining the feasibility of the TCOs for outdoor device
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application. Surface with low free energy can reduce the adhesion of airborne contaminants and then be effectively removed by the rolling drops due to the hydrophobic behavior [14], thereby improving the environmental durability of the film.
Based on the above observations, in this work, to improve the optoelectronic properties of GZO film, ZnO with the same hexagonal wurtzite structure was
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introduced as buffer layer. Then the effects of ZnO buffer layer thickness on structural, electrical, optical properties, especially the near-infrared band were systematically characterized and the surface performances were also investigated.
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2. Experimental details
GZO film and GZO/ZnO bi-layer films were deposited on glass substrates by
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radio-frequency (RF) magnetron sputtering. The substrates were successively cleaned in an ultrasonic cleaner for 15 min with acetone, alcohol and deionized water, and then blow dry with nitrogen before they were introduced into the sputtering system. Before deposition, the chamber was evacuated to less than 3.0 × 10-3 Pa. The GZO films were prepared at 300
with 110 W RF power in pure Ar (0.36 Pa) by using a
GZO (ZnO:97 at.%, Ga2O3:3 at.%) target. The ZnO buffer layers were deposited at 450
with 100 W RF power in a mixture of O2 and Ar (0.36 Pa) by using a
high-purity ZnO (99.99%) target. In order to investigate the influence of ZnO buffer layers, all GZO films’ thickness was fixed at 1.85 µm. The thickness of the ZnO buffer layer was 20, 55 and 116 nm, respectively. The thicknesses of the films were determined by Dektak 150 profilometry. The
ACCEPTED MANUSCRIPT crystal structure of the prepared samples were investigated by θ-2θ X-ray diffraction (XRD) scan using Cu Kα radiation (SHIMADZU, MAXima_X XRD-7000). The electrical properties were measured by Hall effect measurement system in Van Der Pauw configuration (Ecopia, HMS-2100). The optical transmission spectra were recorded using a UV/VIS spectrophotometer (PerkinElmer, Lambda750). The surface
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morphology was observed by a scanning electron microscope (FEI, Inspect F). The measurement of the contact angle of water with the film was carried out using the contact angle measurement system (JC2000D1).
3. Results and discussion
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3.1. Structural properties
The X-ray diffraction spectra for GZO single layer and GZO/ZnO bi-layer films are
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presented in Fig.1. As can be seen in this figure, all the samples exhibit a strong (002) peak at 2θ of about 35° and a weak (004) peak of about 73°, which indicate that the films are hexagonal wurtzite crystal structure with a preferential c-axis orientation. This reveals that the Ga ion substituted the Zn ion in the ZnO crystal structure. The grain size can be calculated with the full width at half maximum (FWHM) of the (002) diffraction peak using the Scherrer formula:
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= ܦ0.9ߣ/()ߠݏܿ ∙ ܤ
(1)
Here λ, B and θ denote the X-ray wavelength, the FWHM of (002) peak, and the corresponding Bragg diffraction angle, respectively. The result shows the grain size of single layer GZO film is 32.69 nm, and the grain size of GZO film with 20 nm, 55 nm,
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116 nm buffer layer are 40.27 nm, 37.05 nm and 34.59 nm, respectively, illustrating the GZO films grown on ZnO buffered substrates have better crystallinity. It also
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demonstrates that the crystalline quality slightly decreases when the thickness of buffer layer above 20 nm.
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Fig.1 XRD spectra of the GZO film and GZO/ZnO bi-layer films
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3.2. Electrical properties
Table 1 shows the resistivity, carrier concentration and carrier mobility of the GZO single layer and GZO/ZnO bi-layer films. It can be found that the preparation of the buffer layer increases the carrier concentration and mobility and thus reduces the resistivity substantially. The resistivity of the GZO/ZnO buffer samples decreases at the 20 nm buffered sample and then increases with the increasing of the buffer layer
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thickness. Similar variation tendency has been reported previously [2,15]. The lowest resistivity is obtained with 20 nm buffer layer can be attributed to the largest GZO grain size, which is in accordance with the results of XRD. The increase in the grain size can cause a decrease in grain boundaries, which can reduce the capture traps of
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carriers and the ability of scattering, hence, resulting in a higher conductivity [16]. The increase in the resistivity of the GZO/ZnO samples with a buffer layer thicker
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than 20 nm can be explained in terms of the relationship between the effective resistivity ρ of the bi-layer structure and sheet resistance of the GZO single layer and ZnO layer [15]:
1/ ρ = 1/ RGZO tGZO + 1/ RZnO tZnO
(2)
where RGZO and RZnO are the sheet resistances of the GZO and ZnO layer, tGZO and
tZnO are the thicknesses of the above layers, respectively. Since ZnO with high resistivity, once the thickness of the ZnO layer increases, the resistivity of the bi-layer films increases.
ACCEPTED MANUSCRIPT Table 1 The electrical properties of the GZO film and GZO/ZnO bi-layer films Carrier concentration ( ×10
20
cm
−3
)
Hall mobility ( cm
2
/ Vs )
Resistivity
( ×10−4 Ω ⋅ cm )
5.37
9.19
12.77
GZO/ZnO(20 nm)
8.47
18.00
4.09
GZO/ZnO(55 nm)
6.90
20.64
GZO/ZnO(116 nm)
6.04
21.40
3.3. UV-visible-NIR transmission properties
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GZO
4.40
4.82
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The transmittance spectra from 300 nm to 2500 nm of the deposited GZO and GZO/ZnO samples are shown in Fig.2. As shown in the figure, the transmittance of all the samples shows a sharp fall from the visible region to the ultraviolet region
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because of the band edge absorption, thus blocking the ultraviolet radiation. Besides, the peak value of the transmittance spectra are above 80% within the visible spectrum and appears at near 550 nm, providing a window of high luminous transmittance. The sample of GZO film without ZnO buffer layer shows higher average visible transmittance owing to the thickness effect [17].
The transmittance decreases in the near-infrared region can be attributed to the
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dependence of plasma reflectivity on free carrier concentration in the films due to a coherent oscillation of conduction electrons with incident electromagnetic radiation [18]. The relationship between the plasma wavelength λ p and the carrier concentration ne is expressed as ߣଶ ∼ 1/݊ [19]. GZO/ZnO bi-layer samples have
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lower transmittance in the near-infrared region than the GZO single layer film, and the inset of Fig.2 shows the lowest transmittance in the near-IR range is obtained with
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the 20 nm ZnO buffered sample and the transmittance is below 20% at the wavelength of 1194 nm, this can be attributed to the high carrier concentration lead to a decrease of the plasma wavelength λ p . Besides, the low transmittance property of the bi-layer films in the near-infrared region is much better than that of the most reported single-layer Al doped ZnO [1][20,21]. As the untreated glass is a poor heat insulator, the part of solar energy in addition to visible light can cause unnecessary heat transfer. The above result suggests that GZO/ZnO bi-layer films have an advantage for simultaneous pursuit of high visible transmittance and effective near-infrared barrier.
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3.4. Surface performances
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Fig.2 Transmittance spectra of the prepared samples. The inset shows the near-infrared transmittance in 700-1400 nm
Fig.3 shows the surface performances of the deposited GZO and GZO/ZnO bi-layer samples. The SEM images in Fig.3 reveal the surfaces exhibit columnar structure with nanopores. The nanopores cover the entire GZO film surface, which can provide a proper roughness required for hydrophobicity. The wettability was then confirmed by
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the water contact angle using sessile drop method. A deionized water droplet of about 3 µL was dropped on the deposited samples’ surface with a micropipette and the shape of water droplet on the surface of the sample is shown in the inset of Fig.3. It can be found with the increase of ZnO thin film thickness, the droplet on the surface
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of the deposited GZO coating exactly exhibits hydrophobic property with a contact angle of θ from 104° to 108.5°. Since the crystal lattice of GZO film is few less than that of ZnO, GZO films grown on ZnO buffer layer suffer tensile stress.
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With the increase of ZnO buffer thickness, the tensile stress applied in GZO films will be easily maintained, resulting in different surface morphology. The surface roughness shown an increase with the thickness of ZnO buffer layer. The wettability behavior of a surface is strongly related to the surface morphology
of the surface [22]. As shown in Fig.3, the morphology and roughness of the film surfaces change with the increase of the buffer layer thickness. The flat surface (Fig. 3a) shows smaller contact angle than others due to lower surface roughness. With the increase of surface roughness (Fig. 3b-d), more air is trapped within the interstices of rough surface and greatly increased the contact area of the air water interface, thus preventing the penetration of water droplets into the air pockets and giving rise to
ACCEPTED MANUSCRIPT larger contact angle in these samples. Similar phenomenon is explained by V. Dave et
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al. using the Cassier–Baxter model [23].
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Fig. 3 Surface morphology of the GZO and GZO/ZnO bi-layer samples (a) 0 nm buffer layer, (b) 20 nm buffer layer, (c) 55 nm buffer layer, (d) 116 nm buffer layer. The inset shows the shape of water droplet on the surface of the sample and the contact angel of water.
The surface free energy was calculated by the software provided with the measuring instrument according to the contact angle and the lowest surface free energy of 25.98 mN/m was acquired by GZO/ZnO bi-layer films with 116 nm buffer
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layer. The low surface energy apparently has always been the pivotal issue for the performance of environmental durability, which lowered the adhesion thus reduce the
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accumulation of air borne contamination and ensured good mobility of water drops.
4. Conclusion
The influence of ZnO buffer layers on GZO films were studied. The crystallinity,
resistivity and optical performances of GZO films were improved with the addition of ZnO buffer layers. GZO film with 20 nm ZnO buffer layer presented optimal optoelectronic performance with the lowest resistivity of 4.09×10-4 Ω·cm, the highest transmittance above 80% and the lowest transmittance in the near-infrared range, which is below 20% at the wavelength of 1194 nm, achieved a balance between high visible transmittance and effective thermal insulation. The contact angle
ACCEPTED MANUSCRIPT measurements showed that the surface of the GZO films presented hydrophobic characteristic. With the increase of the ZnO buffer layer, the contact angle increased from 104° to 108.5º, indicating the deposited films with environmental durability. In summary, GZO films with ZnO buffer layers will likely perform better in thermal
Acknowledgements
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insulation films and outdoor optoelectronic devices than GZO single layer films.
The authors extend their gratitude to National High-tech R&D Program [grant number G060103011AA8042017] for funding this study.
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Author Contributions
Hui Cheng and Hong Deng conceived and designed the experiments; Hui Cheng performed the experiments; Yan Wang and Min Wei participated in analyzing the check the paper.
Conflicts of Interest
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experimental data; Hui Cheng wrote the paper, Deng Hong and Min Wei helped with
The authors declare no conflict of interest.
References
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[1] Y. Okuhara, T. Kato, H. Matsubara, N. Isu, M. Takata, Near-infrared reflection from periodically aluminium-doped zinc oxide thin films, Thin Solid Films 519 (2011) 2280-2286. [2] J. Wu, H Lin, Y. Chen, S. Chu, C Chang, C. Wu, Y. Juang, Effects of ZnO Buffer Layer on Characteristics of ZnO:Ga Films Grown on Flexible Substrates: Investigation of Surface Energy, Electrical, Optical, and Structural Properties, ECS J. Solid State Sci. Technol. 2 (2013) 115-119. [3] J.L. Zhao, X.W. Sun, H. Ryu, Y.B. Moon, Thermally stable transparent conducting and highly infrared reflective Ga-doped ZnO thin films by metal organic chemical vapor deposition, Opt. Mater. 33 (2011) 768-772. [4] S.R. Jian, G.J. Chen, S.K. Wang, T.C. Lin, S.C. Jang, J.Y. Juang, Y.S. Lai, J.Y. Tseng, Rapid thermal annealing effects on the structural and nanomechanical properties of Ga-doped ZnO thin films, Surf. Coat. Technol. 231 (2013) 176-179. [5] J.H. Jeon, T.K. Gong, S.K. Kim, S.H. Kim, S.Y. Kim, D.H . Choi, D.I. Son, D. Kim, Influence of a Ni buffer layer on the optical and electrical properties of GZO/Ni bi-layered films, J. Alloy. Compd. 639 (2015) 1-4. [6] D. Kim, Improved electrical and optical properties of GZO films with a thin TiO2 buffer layer deposited by RF magnetron sputtering, Ceram. Int. 40 (2014) 1457-1460. [7] B.D. Ahn, Y.G. Ko, H.O. Sang, J.H. Song, H.J. Kim, Effect of oxygen pressure of SiOx buffer layer on the electrical properties of GZO film deposited on PET substrate, Thin Solid Films 517 (2009) 6414-6417. [8] S.K. Kim, S.H. Kim, S.Y. Kim, J.H. Jeon, T.K. Gong, D.H. Choi, D.I. Son, D. Kim, Effect of substrate temperature on the structural, electrical, and optical properties of GZO/ZnO films deposited by radio frequency magnetron sputtering, Ceram. Int. 40 (2014) 6673-6676.
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[9] S.K. Kim, S.Y. Kim, S.H. Kim, J.H. Jeon, T.K. Gong, D. Kim, D.Y. Yoon, D.Y. Choi, Influence of ZnO Thickness on the Optical and Electrical Properties of GZO/ZnO Bi-layered Films, Trans. Electr. Electron. Mater. 15 (2014) 198-200. [10] F. Jia, Q. Wang, D. L. Zhu, S. Han, P. J. Cao, W. J. Liu, Y. X. Zeng, Y. M. Lu, Influence of ZnO buffer layers on the optoelectronic properties in Ga-doped ZnO thin films prepared by RF magnetron sputtering on PET substrates, J. Mater. Sci.-Mater. Electron. 25 (2014) 2934-2938. [11] Z. Zhang, C. Bao, S. Ma, S. Hou, Effect of crystallinity of ZnO buffer layer on the properties of epitaxial (ZnO:Al)/(ZnO:Ga) bi-layer films deposited on c-sapphire substrate, Appl. Surf. Sci. 257 (2011) 7893-7899. [12] G. Li, J. Lu, Z. Ye, Room-temperature growth and optoelectronic properties of GZO/ZnO bilayer films on polycarbonate substrates by magnetron sputtering, Sol. Energy Mater. Sol. Cells 94 (2010) 1282-1285. [13] D. Miao, S. Jiang, S. Shang, Z. Chen, Highly transparent and infrared reflective AZO/Ag/AZO multilayer film prepared on PET substrate by RF magnetron sputtering, Vacuum 106 (2014) 1-4. [14] J. Zimmermann, F.A. Reifler, U. Schrade, G.R.J. Artus, S. Seeger, Long term environmental durability of a superhydrophobic silicone nanofilament coating, Colloid Surf. A-Physicochem. Eng. Asp. 302 (2007) 234-240. [15] C. Lee, A. Park, Y. Cho, M. Parka, W.I. Lee, H.W. Kim, Influence of ZnO buffer layer thickness on the electrical and optical properties of indium zinc oxide thin films deposited on PET substrates, Ceram. Int. 34 (2008) 1093-1096. [16] X. Wang, X. Zeng, D. Huang, X. Zhang, Q. Li, The properties of Al doped ZnO thin films deposited on various substrate materials by RF magnetron sputtering, J. Mater. Sci.-Mater. Electron. 23 (2012) 1580-1586. [17] M.D.L.L. Olvera, H. Gómez, A. Maldonado, Doping, vacuum annealing, and thickness effect on the physical properties of zinc oxide films deposited by spray pyrolysis, Sol. Energy Mater. Sol. Cells 91 (2007) 1449-1453. [18] N. Rashidi, V.L. Kuznetsov, J.R. Dilworth, M. Pepper, P.J. Dobsonc, P.P. Edwards, Highly conducting and optically transparent Si-doped ZnO thin films prepared by spray pyrolysis, J. Mater. Chem. C 1 (2013) 6960-6969. [19] S. Chen, M.E.A. Warwick, R. Binions, Effects of film thickness and thermal treatment on the structural and opto-electronic properties of Ga-doped ZnO films deposited by sol–gel method, Sol. Energy Mater. Sol. Cells 137 (2015) 202-209. [20] J. Wang, H. Wang, W.L. Zhou, G.X. Li, J. Yu, Preparation and character of textured ZnO:Al thin films deposited on flexible substrates by RF magnetron sputtering, Mater. Lett. 65 (2011) 2039-2042. [21] L. Gong, Z. Ye, J. Lu, L. Zhu, J. Huang, X. Gu, B. Zhao, Highly transparent conductive and near-infrared reflective ZnO:Al thin films, Vacuum 84 (2010) 947-952. [22] C. Ottone, A. Lamberti, V. Cauda, M. Fontana, Wetting Behavior of Hierarchical Oxide Nanostructures: TiO2 Nanotubes from Anodic Oxidation Decorated with ZnO Nanostructures, J. Electrochem. Soc. 161 (2014) 484-488. [23] V. Dave, H.O. Gupta, R. Chandra, Nanostructured hydrophobic DC sputtered inorganic oxide coating for outdoor glass insulators, Appl. Surf. Sci. 295 (2014) 231-239.
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Highlights ·GZO films exhibit low resistivity and high visible light transmittance. ·ZnO buffer layers reduce the transmittance of the near infrared band.
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·GZO/ZnO bi-layer films show hydrophobic performance.
S0925-8388(17)30195-0
DOI:
10.1016/j.jallcom.2017.01.172
Reference:
JALCOM 40539
To appear in:
Journal of Alloys and Compounds
Received Date: 2 December 2016 Revised Date:
13 January 2017
Accepted Date: 18 January 2017
Please cite this article as: H. Cheng, H. Deng, Y. Wang, M. Wei, Influence of ZnO buffer layer on the electrical, optical and surface properties of Ga-doped ZnO films, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.01.172. 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.
ACCEPTED MANUSCRIPT
Influence of ZnO buffer layer on the electrical, optical and surface properties of Ga-doped ZnO films Hui Cheng, Hong Deng*, Yan Wang and Min Wei State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China; * Correspondence: [email protected]; Tel.: +86-028-8320-2551
Abstract
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[email protected]; [email protected]; [email protected]
The influence of the ZnO buffer layer thickness on the structural, electrical, optical
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and surface properties of Ga-doped ZnO (GZO) films deposited on glass substrates by RF magnetron sputtering were investigated. X-ray diffraction results showed the
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obtained films had highly c-axis oriented with hexagonal (002) structures and GZO film with 20 nm buffer layer had the best crystalline quality. The resistivity in GZO/ZnO bi-layer films decreased significantly than that in GZO film without a ZnO buffer layer, and GZO film with 20 nm buffer layer showed the lowest resistivity of 4.09×10-4 Ω·cm. The bi-layer films exhibited the highest transmittance of over 80% in the visible light range and displayed a low near infrared transmittance. The
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correlation between surface morphology and wettability was studied and GZO/ZnO bi-layer films exhibited hydrophobic property with contact angle of θ from 104° to 108.5°, indicating acceptable property of environmental durability.
Keywords
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thin films, oxide materials, vapor deposition, electronic properties, optical properties
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1. Introduction
Transparent conductive oxides (TCOs) are one of the potential candidates for heat
shielding coatings because of the high visible transparent and high concentration of free electrons [1]. Among many kinds of TCOs, Ga doped ZnO (GZO) has gained great attention as an alternative to ITO due to its promising optical and electrical properties as well as its low cost, non-toxicity and the abundance of elements [2,3]. The covalent bond lengths of Ga–O and Zn–O are estimated to be 1.92 Å and 1.97 Å [4], respectively, which means the lattice distortion of wurtzite structure ZnO is minimal even for a high concentration of Ga in ZnO compared with IZO and AZO. In addition, GZO has better conduction stability than AZO because Ga is relatively oxidation resistant [3].
ACCEPTED MANUSCRIPT In recent years, many researchers have prepared different kinds of buffer layers to improve the optical and electrical properties of GZO films, structures such as GZO/Ni [5], GZO/TiO2 [6] and GZO/SiOX [7] were deposited and investigated. Previously reported the effects of ZnO buffer layer in the process of preparation [8-12], for optical transmittance spectra, most researchers have been concerned about the impact
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of buffer layer on the visible region, however, there are few studies about the effect of the buffer layer on the near-infrared band. Since the solar energy is distributed in the wavelength range from 250 nm to 2500 nm [13], and the visible light from 300 nm to 800 nm needs to be transmitted for window applications, heat radiation above 800 nm
SC
wavelength should be cut for effective thermal insulation [1].
The surface performance is of the same importance since it is a critical characteristic for determining the feasibility of the TCOs for outdoor device
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application. Surface with low free energy can reduce the adhesion of airborne contaminants and then be effectively removed by the rolling drops due to the hydrophobic behavior [14], thereby improving the environmental durability of the film.
Based on the above observations, in this work, to improve the optoelectronic properties of GZO film, ZnO with the same hexagonal wurtzite structure was
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introduced as buffer layer. Then the effects of ZnO buffer layer thickness on structural, electrical, optical properties, especially the near-infrared band were systematically characterized and the surface performances were also investigated.
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2. Experimental details
GZO film and GZO/ZnO bi-layer films were deposited on glass substrates by
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radio-frequency (RF) magnetron sputtering. The substrates were successively cleaned in an ultrasonic cleaner for 15 min with acetone, alcohol and deionized water, and then blow dry with nitrogen before they were introduced into the sputtering system. Before deposition, the chamber was evacuated to less than 3.0 × 10-3 Pa. The GZO films were prepared at 300
with 110 W RF power in pure Ar (0.36 Pa) by using a
GZO (ZnO:97 at.%, Ga2O3:3 at.%) target. The ZnO buffer layers were deposited at 450
with 100 W RF power in a mixture of O2 and Ar (0.36 Pa) by using a
high-purity ZnO (99.99%) target. In order to investigate the influence of ZnO buffer layers, all GZO films’ thickness was fixed at 1.85 µm. The thickness of the ZnO buffer layer was 20, 55 and 116 nm, respectively. The thicknesses of the films were determined by Dektak 150 profilometry. The
ACCEPTED MANUSCRIPT crystal structure of the prepared samples were investigated by θ-2θ X-ray diffraction (XRD) scan using Cu Kα radiation (SHIMADZU, MAXima_X XRD-7000). The electrical properties were measured by Hall effect measurement system in Van Der Pauw configuration (Ecopia, HMS-2100). The optical transmission spectra were recorded using a UV/VIS spectrophotometer (PerkinElmer, Lambda750). The surface
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morphology was observed by a scanning electron microscope (FEI, Inspect F). The measurement of the contact angle of water with the film was carried out using the contact angle measurement system (JC2000D1).
3. Results and discussion
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3.1. Structural properties
The X-ray diffraction spectra for GZO single layer and GZO/ZnO bi-layer films are
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presented in Fig.1. As can be seen in this figure, all the samples exhibit a strong (002) peak at 2θ of about 35° and a weak (004) peak of about 73°, which indicate that the films are hexagonal wurtzite crystal structure with a preferential c-axis orientation. This reveals that the Ga ion substituted the Zn ion in the ZnO crystal structure. The grain size can be calculated with the full width at half maximum (FWHM) of the (002) diffraction peak using the Scherrer formula:
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= ܦ0.9ߣ/()ߠݏܿ ∙ ܤ
(1)
Here λ, B and θ denote the X-ray wavelength, the FWHM of (002) peak, and the corresponding Bragg diffraction angle, respectively. The result shows the grain size of single layer GZO film is 32.69 nm, and the grain size of GZO film with 20 nm, 55 nm,
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116 nm buffer layer are 40.27 nm, 37.05 nm and 34.59 nm, respectively, illustrating the GZO films grown on ZnO buffered substrates have better crystallinity. It also
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demonstrates that the crystalline quality slightly decreases when the thickness of buffer layer above 20 nm.
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Fig.1 XRD spectra of the GZO film and GZO/ZnO bi-layer films
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3.2. Electrical properties
Table 1 shows the resistivity, carrier concentration and carrier mobility of the GZO single layer and GZO/ZnO bi-layer films. It can be found that the preparation of the buffer layer increases the carrier concentration and mobility and thus reduces the resistivity substantially. The resistivity of the GZO/ZnO buffer samples decreases at the 20 nm buffered sample and then increases with the increasing of the buffer layer
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thickness. Similar variation tendency has been reported previously [2,15]. The lowest resistivity is obtained with 20 nm buffer layer can be attributed to the largest GZO grain size, which is in accordance with the results of XRD. The increase in the grain size can cause a decrease in grain boundaries, which can reduce the capture traps of
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carriers and the ability of scattering, hence, resulting in a higher conductivity [16]. The increase in the resistivity of the GZO/ZnO samples with a buffer layer thicker
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than 20 nm can be explained in terms of the relationship between the effective resistivity ρ of the bi-layer structure and sheet resistance of the GZO single layer and ZnO layer [15]:
1/ ρ = 1/ RGZO tGZO + 1/ RZnO tZnO
(2)
where RGZO and RZnO are the sheet resistances of the GZO and ZnO layer, tGZO and
tZnO are the thicknesses of the above layers, respectively. Since ZnO with high resistivity, once the thickness of the ZnO layer increases, the resistivity of the bi-layer films increases.
ACCEPTED MANUSCRIPT Table 1 The electrical properties of the GZO film and GZO/ZnO bi-layer films Carrier concentration ( ×10
20
cm
−3
)
Hall mobility ( cm
2
/ Vs )
Resistivity
( ×10−4 Ω ⋅ cm )
5.37
9.19
12.77
GZO/ZnO(20 nm)
8.47
18.00
4.09
GZO/ZnO(55 nm)
6.90
20.64
GZO/ZnO(116 nm)
6.04
21.40
3.3. UV-visible-NIR transmission properties
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The transmittance spectra from 300 nm to 2500 nm of the deposited GZO and GZO/ZnO samples are shown in Fig.2. As shown in the figure, the transmittance of all the samples shows a sharp fall from the visible region to the ultraviolet region
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because of the band edge absorption, thus blocking the ultraviolet radiation. Besides, the peak value of the transmittance spectra are above 80% within the visible spectrum and appears at near 550 nm, providing a window of high luminous transmittance. The sample of GZO film without ZnO buffer layer shows higher average visible transmittance owing to the thickness effect [17].
The transmittance decreases in the near-infrared region can be attributed to the
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dependence of plasma reflectivity on free carrier concentration in the films due to a coherent oscillation of conduction electrons with incident electromagnetic radiation [18]. The relationship between the plasma wavelength λ p and the carrier concentration ne is expressed as ߣଶ ∼ 1/݊ [19]. GZO/ZnO bi-layer samples have
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lower transmittance in the near-infrared region than the GZO single layer film, and the inset of Fig.2 shows the lowest transmittance in the near-IR range is obtained with
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the 20 nm ZnO buffered sample and the transmittance is below 20% at the wavelength of 1194 nm, this can be attributed to the high carrier concentration lead to a decrease of the plasma wavelength λ p . Besides, the low transmittance property of the bi-layer films in the near-infrared region is much better than that of the most reported single-layer Al doped ZnO [1][20,21]. As the untreated glass is a poor heat insulator, the part of solar energy in addition to visible light can cause unnecessary heat transfer. The above result suggests that GZO/ZnO bi-layer films have an advantage for simultaneous pursuit of high visible transmittance and effective near-infrared barrier.
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3.4. Surface performances
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Fig.2 Transmittance spectra of the prepared samples. The inset shows the near-infrared transmittance in 700-1400 nm
Fig.3 shows the surface performances of the deposited GZO and GZO/ZnO bi-layer samples. The SEM images in Fig.3 reveal the surfaces exhibit columnar structure with nanopores. The nanopores cover the entire GZO film surface, which can provide a proper roughness required for hydrophobicity. The wettability was then confirmed by
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the water contact angle using sessile drop method. A deionized water droplet of about 3 µL was dropped on the deposited samples’ surface with a micropipette and the shape of water droplet on the surface of the sample is shown in the inset of Fig.3. It can be found with the increase of ZnO thin film thickness, the droplet on the surface
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of the deposited GZO coating exactly exhibits hydrophobic property with a contact angle of θ from 104° to 108.5°. Since the crystal lattice of GZO film is few less than that of ZnO, GZO films grown on ZnO buffer layer suffer tensile stress.
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With the increase of ZnO buffer thickness, the tensile stress applied in GZO films will be easily maintained, resulting in different surface morphology. The surface roughness shown an increase with the thickness of ZnO buffer layer. The wettability behavior of a surface is strongly related to the surface morphology
of the surface [22]. As shown in Fig.3, the morphology and roughness of the film surfaces change with the increase of the buffer layer thickness. The flat surface (Fig. 3a) shows smaller contact angle than others due to lower surface roughness. With the increase of surface roughness (Fig. 3b-d), more air is trapped within the interstices of rough surface and greatly increased the contact area of the air water interface, thus preventing the penetration of water droplets into the air pockets and giving rise to
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al. using the Cassier–Baxter model [23].
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Fig. 3 Surface morphology of the GZO and GZO/ZnO bi-layer samples (a) 0 nm buffer layer, (b) 20 nm buffer layer, (c) 55 nm buffer layer, (d) 116 nm buffer layer. The inset shows the shape of water droplet on the surface of the sample and the contact angel of water.
The surface free energy was calculated by the software provided with the measuring instrument according to the contact angle and the lowest surface free energy of 25.98 mN/m was acquired by GZO/ZnO bi-layer films with 116 nm buffer
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layer. The low surface energy apparently has always been the pivotal issue for the performance of environmental durability, which lowered the adhesion thus reduce the
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accumulation of air borne contamination and ensured good mobility of water drops.
4. Conclusion
The influence of ZnO buffer layers on GZO films were studied. The crystallinity,
resistivity and optical performances of GZO films were improved with the addition of ZnO buffer layers. GZO film with 20 nm ZnO buffer layer presented optimal optoelectronic performance with the lowest resistivity of 4.09×10-4 Ω·cm, the highest transmittance above 80% and the lowest transmittance in the near-infrared range, which is below 20% at the wavelength of 1194 nm, achieved a balance between high visible transmittance and effective thermal insulation. The contact angle
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Acknowledgements
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insulation films and outdoor optoelectronic devices than GZO single layer films.
The authors extend their gratitude to National High-tech R&D Program [grant number G060103011AA8042017] for funding this study.
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Author Contributions
Hui Cheng and Hong Deng conceived and designed the experiments; Hui Cheng performed the experiments; Yan Wang and Min Wei participated in analyzing the check the paper.
Conflicts of Interest
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experimental data; Hui Cheng wrote the paper, Deng Hong and Min Wei helped with
The authors declare no conflict of interest.
References
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Highlights ·GZO films exhibit low resistivity and high visible light transmittance. ·ZnO buffer layers reduce the transmittance of the near infrared band.
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·GZO/ZnO bi-layer films show hydrophobic performance.