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Article

Influence of Processing Time in Hydrogen Plasma to Prepare Gallium and Aluminum Codoped Zinc Oxide Films for Low-Emissivity Glass

by
Shang-Chou Chang
1,2,*,
Jhih-Ciang Hu
1,
Huang-Tian Chan
2 and
Chuan-An Hsiao
1,2
1
Department of Electrical Engineering, Kun Shan University, No. 195, Kunda Rd., Yongkang Dist., Tainan 710, Taiwan
2
Green Energy Technology Research Center, Kun Shan University, No. 195, Kunda Rd., Yongkang Dist., Tainan 710, Taiwan
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(7), 945; https://doi.org/10.3390/coatings12070945
Submission received: 30 May 2022 / Revised: 29 June 2022 / Accepted: 29 June 2022 / Published: 3 July 2022
(This article belongs to the Special Issue Manufacturing and Properties of New Special Glass- and Ceramic-Based Composites)
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Abstract

:
Low-emissivity glass has high transmission in the visible region and high reflectivity in the infrared region. Gallium and aluminum codoped zinc oxide (GAZO) processed by hydrogen (H2) plasma treatment holds promise for fabricating good low-emissivity glass. The applied processing time is one of the key factors in plasma treatment. The GAZO films were prepared by in-line sputtering at room temperature, and the effect of using different plasma processing times on the structural, electrical and optical properties of the films were surveyed. Experimental results indicate that H2 plasma treatment of GAZO film samples indeed influenced the structure, optical and electrical properties. An appropriate processing time can improve the electrical properties and reduce the emissivity of GAZO films. The optimum processing time is 5 min for plasma treatment of GAZO films. The electrical resistivity and emissivity of plasma-annealed films for 5 min decrease by 59% and 55% compared to those of as-deposited GAZO films. Values of 5.3 × 10−4 Ω-cm in electrical resistivity, 0.13 in emissivity and 94% in average optical transmittance in the visible wavelength region could be obtained for GAZO films after H2 treatment of 5 min in this work for low-emissivity glass applications.

1. Introduction

Low-emissivity (low-e) glass shows not only high reflectivity in infrared light, but also high visible light transmission, providing energy savings [1,2]. Emissivity and optical visible transmittance are important reference indexes of low-e glass. The lower the emissivity of low-e glass, the better the infrared reflection [1,2,3,4]. Silver-based low-e glass is the most common structure. Silver films of conventional low-e glass tend to result in high reflectivity for infrared light but also easy oxidation. Moreover, the adhesive force of silver films is poor. To overcome the problems associated with Ag films, a seed layer and blocker layer are often added onto and below the silver films to avoid oxidation and changes in the optical properties of the low-e glass, and to aid in the film’s adhesion with the glass. However, this also increases the number of film layers and the production cost of the films and increases the processing complexity [5,6,7].
Hence, another alternative material with good visible light transmission and emissivity is needed. At present, transparent conductive oxides (TCOs) have been used in an extensive range of applications due to their high visible transmittance and low electrical resistivity. They are usually used for a low-e window, in organic light-emitting diodes, solar cells, etc. [8,9,10,11]. Several TCOs, such as indium tin oxide (ITO) and zinc oxide (ZnO), have been intensively studied. ITO is the mainstream material in TCOs because of its excellent optical and electrical properties. However, ITO has some problems, such as high costs and toxicity. Compared with ITO, ZnO is a low-cost material with nontoxicity [12,13]. The electrical resistivity of ZnO can be improved by further impurity doping of group III elements, such as Al3+ or Ga3+. Amongst them, aluminum-doped zinc oxide (AZO) and gallium-doped zinc oxide (GZO) and gallium and aluminum codoped zinc oxide (GAZO) have been the most widely used of the TCO materials. AZO has many advantages, including low cost, nontoxicity, good temperature stability and chemical stability. The electrical resistivity of GZO is lower than that of AZO, and Ga in GZO is more resistant to oxidation than Al in AZO [12,14,15,16]. Notably, GAZO can obtain the advantages of both AZO and GZO [12]. The combination of the two materials forms GAZO, with higher optical and electrical properties than the single materials [17,18,19].
E. Hagen and H. Rubens reported the Hagen–Rubens relationship between the electrical resistivity and emissivity of materials; when the electrical resistivity decreases, the emissivity will also decrease [20]. By changing the preparation method of ZnO series (such as GAZO), the optical and electrical properties of the films can also be changed [21,22,23,24,25,26]. More previous studies report that H2 plasma treatment and heat treatment can increase the electrical stability of GAZO or AZO and decrease the electrical resistivity [23,24,25,26,27,28]. The improvement in the electrical properties of GAZO or AZO thin films may due to the following reasons: (1) the substitution of Al and Ga into Zn sites in ZnO crystals, creating one more free electron, which can increase the carrier concentration and conductivity [21,22,28]; (2) the increase in carrier concentration is due to the H2 atoms that act as shallow donors by H2 plasma treatment [23,24,25]; (3) negatively charged oxygen species are desorbed from the surface of ZnO [23,24,25].
Our research team previously applied AZO/ITO films to low-e glass [29]. The AZO/ITO films were treated by vacuum annealing and hydrogen plasma annealing, respectively. Post-annealing has been shown to improve the electrical and optical properties of as-deposited AZO/ITO films and reduce emissivity. In addition, our research team reported the effects of substrate temperature, vacuum annealing and hydrogen plasma annealing, respectively, on the structure and electrical and optical properties of GAZO films [28,30,31]. According to previous reports, the processing time of hydrogen plasma treatment can affect the structure of GAZO films and thus their physical properties [30,31]. The experimental results indicate that a sufficient H2 plasma processing time of GAZO samples indeed obtains the minimum electrical resistivity. Annealing treatment will provide enough thermal energy for GAZO thin films to obtain the best optoelectronic properties, such as low resistivity and high visible light transmittance.
This paper builds on previously published research. The influence of plasma treatment on GAZO films was investigated in detail. We also surveyed the feasibility of GAZO films as low-e glass. The GAZO films were deposited on glass substrates using in-line sputtering. After this, GAZO films were post-annealed in hydrogen plasma at different processing times. The objective of the H2 plasma treatment was to improve the electrical and optical properties in the GAZO films and thereby reduce their emissivity.

2. Materials and Methods

Borosilicate glass was applied as the substrate onto which GAZO films were deposited. Before deposition, the glass substrates were sequentially cleaned by ultrasonic treatment in acetone, isopropyl alcohol for 5 min and pure water sequentially, and dried with N2. The GAZO films with a thickness of 500 nm were prepared on the glass substrates by in-line sputtering using a GAZO (ZnO:Ga2O3:Al2O3 = 97:2:1 wt.%) target at room temperature. The sputtering target of GAZO was 950 × 125 mm2 in size. GAZO films were deposited onto the glass substrates by in-line sputtering using two targets. The two targets were kept at a sputtering power of 3 kW, giving a total power of 6 kW. The sputtering power density was 2.53 W/cm2. The GAZO films was sputtered using a pure Ar flow of 300 sccm and working pressure of 3 × 10−3 torr, with a bass pressure of 1 × 10−5 torr. After deposition, the as-deposited GAZO films were annealed under hydrogen (H2) plasma using a plasma system at a plasma power of 600 W for 1 min, 3 min, 5 min, 7 min and 10 min. The process pressure of the H2 plasma treatment was kept at 25 Torr, with H2 gas flow at 100 sccm.
The H2 plasma-annealed GAZO films were measured for their microstructure, electrical and optical properties and emissivity in sequence. The surface morphology of the GAZO films before and after the plasma treatment was obtained by field emission scanning electron microscopy (FESEM; JEOL JSM-6700 F, JEOL, Tokyo, Japan) measurements. The crystalline structures of GAZO films were characterized by an X-ray diffractometer (XRD, MO3xHF22, MacScience Co. Ltd., Kanagawa, Japan). The optical transmittance was measured by an ultraviolet–visible spectrophotometer (Hitachi U-2800A, Hitachi, Tokyo, Japan) from 400 nm to 800 nm. The carrier concentration, mobility and electrical resistivity were measured by Hall measurement (Ecopia HMS-3000, Ecopia, Gyeonggi-do, Korea). The emissivity of GAZO films was obtained by an emissivity meter (TSS-5X, Japan Sensor Corp., Tokyo, Japan).

3. Results and Discussion

Figure 1 shows the SEM images of AZO films before and after plasma treatment at various processing times. The as-deposited films (Figure 1a) had a relatively uniform, dense structure. However, the grain size changed with increasing plasma processing time, as shown in Figure 1b–f. It is apparent that the grain sizes became smaller with increased processing time. The calculated results show that the average particle size of the as-prepared GAZO films and those annealed at the hydrogen plasma of 1 min, 3 min, 5 min, 7 min and 10 min were 64.1 nm, 61.7 nm, 55.3 nm, 53.4 nm, 51.6 nm and 49.2 nm, respectively. Our previous studies showed that the surface roughness increases and nanocrystals are grown for the GAZO films after H2 plasma annealing; the atomic force microscope (AFM) analysis has been reported previously [30].
The crystalline structure of the films was observed by XRD spectra for both plasma-treated and as-deposited films and is shown in Figure 2. According to the standard ZnO XRD pattern from the Inorganic Crystal Structure Database (ICSD) with card number 31052, the diffraction peak values of 34.50° can be indexed to (002) crystal planes. As shown in Figure 2a, only one peak was exhibited in the diffraction angle (2θ) range 30–60, corresponding to the (002) of ZnO. The corresponding (002) peak locations of the XRD spectra are listed in Table 1. The two theta angles between 33.5° and 35.5° in Figure 2a are enlarged and shown in Figure 2b.
From the XRD spectra (Figure 2b), it can be also noted that the peak position corresponding to the ZnO (002) shifts as the plasma treatment time varies. It is seen that with an increase in plasma treatment time, the (002) peak initially shifts to lower angles as compared to that for the as-deposited GAZO films, and then again to a higher angle being greater than that for the as-deposited films. The angular position of the (002) diffraction peak shifts towards a lower angle as the plasma treatment time increases above 7 min. The (002) peak shifts to a lower angle with rising H2 plasma treatment time, which means that the adjacent (002) interplanar distance of the GAZO films increases with treatment time. However, the (002) peak shifts to higher angles after the GAZO films are subjected to H2 plasma treatment. This indicates that the spacing between the (002) of GAZO films reduces after plasma treatment.
The variation in the diffraction angle position can be attributed to the substitution of Zn2+ ions by Al3+ and Ga3+ ions [21,28] and H2 occupied in the Zn-O bond center [32,33]. The (002) diffraction peaks shifted to low angles. This might be due to the H2 atoms situated in the Zn-O bond center, causing the lattice parameters of ZnO films to increase [32,33]. However, the decrease in the space distance may have resulted from the replacement of the Zn ions with the lattice sites of Ga and Al ions in the GAZO films for H2 plasma treatment samples, because the ionic and covalent radii of Ga and Al are smaller than those of Zn. This may be associated with thermal energy. Our research team reported [28] that in GAZO films deposited at different temperatures by in-line sputtering, the (002) peaks to shift towards high angles with increasing substrate temperature. This phenomenon could be related to thermal energy supplied by the heating substrate, as higher thermal energy is provided, leading to more Al and Ga atom impurities to substitute for the Zn sites in the GAZO films at higher temperatures.
Figure 3 shows the variation in the electrical resistivity, carrier concentration and Hall mobility of the GAZO films after H2 plasma treatment. The data are shown in Table 1. We analyzed the carrier concentration, Hall mobility and electrical resistivity using the Hall measurement system (Model: Ecopia HMS-3000, Ecopia, Gyeonggi-do, Korea) and obtained an error within ± 2.5%. At first, the electrical resistivity of the as-deposited samples was 1.3 × 10−3 Ω-cm. Moreover, the effects of hydrogen plasma on the GAZO films were evaluated. The H2 plasma treatment was performed at the power of 600 W with the duration of 1 min, 3 min, 5 min, 7 min and 10 min, respectively. As can be seen in Figure 3, the electrical resistivity was obviously reduced with the increase in plasma treatment time, and the best optimized treatment time was 5 min, at which the resistivity decreased from 1.3 × 10−3 Ω-cm to 5.3 × 10−4 Ω-cm. When the treatment time was over 5 min, the electrical resistivity was slightly improved. The electrical resistivity of plasma-treated samples (5 min samples) decreased by 59% compared with the as-deposited samples. As shown in Figure 3 and Table 1, the carrier concentration increased as the duration of H2 plasma treatment increased. At first, the as-deposited value of the carrier concentration was 1.8 × 1021 cm−3 and the maximum was 3.6 × 1021 cm−3, obtained after 5 min treatment. When the treatment time was over 5 min, the carrier concentration started to decline.
By appropriately increasing the treatment time of hydrogen plasma annealing, the electrical resistivities of GAZO films were significantly reduced, and the carrier concentrations were increased. The improved electrical properties of the hydrogen-treated ZnO series materials may be attributed to (1) exchanges of ion sites between impurities and the matrix (e.g., Al ions or Ga ions replace Zn ions) in ZnO series materials during heat treatment [21,22,28]; (2) hydrogen atoms acting as shallow donor states [23,24,25,34].
When the substitution of Zn2+ ions with Al3+ (or Ga3+) in ZnO lattice causes an increase in the carrier concentration due to the supply of free electrons, this causes a significant reduction in the resistivity of GAZO films [21,28]. K. Zhu et al. reported [21] on GAZO films deposited at different substrate temperatures by magnetron sputtering; the substitution of Al3+ and Ga3+ ions and doping efficiency increased with increasing temperature, in turn improving the carrier concentration of the GAZO films. In addition, the H2 plasma treatment led to hydrogen atoms occupied in the Zn-O bond center of GAZO films, as well as acting as shallow donor states in GAZO, which may increase the carrier concentration [30]. H2 atoms situated in the Zn-O bond center may result in an increase in the interplanar distance for the GAZO films, similar to reports on AZO films [33].
As shown in Figure 2b, the ZnO (002) diffraction angle increased after plasma annealing treatment, which may be attributed to Al3+ or Ga3+ ions replacing the lattice positions of Zn2+ ions. However, the addition of H2 may increase the interplanar spacing; this resulted in a decrease in the diffraction angle. In short, the interaction of the two phenomena leads to a shift in the ZnO (002) diffraction angle.
On the other hand, the appropriate annealing time results in point defects and reduces surface defects, thus enhancing the carrier concentration and Hall mobility. However, a long annealing time may result in more point defects and thus reduce the Hall mobility. This could explain why the Hall mobility of the H2 plasma-annealed GAZO films reached a maximum at 3 min and not at 5 min.
The optical transmittance spectra of the GAZO films (as-deposited films) and the GAZO films treated by H2 plasma at a duration of 1~10 min are shown in Figure 4. The arithmetic mean value of transmittance in the wavelength from 400 nm to 800 nm was calculated as the average optical transmittance. The average optical transmittance in the visible range from 400 nm to 800 nm of the as-deposited and H2 plasma-treated GAZO films is also included in Table 1. It can be seen from the curves in Figure 4 that the optical transmittance obviously increases with the increase in plasma treatment time. The average optical transmittance of as-deposited samples is 85%. The average calculated optical transmittance of the hydrogen-plasma-annealed GAZO films with 1 min, 3 min, 5 min, 7 min and 10 min is 90%, 92%, 94%, 95% and 96%, respectively. The detailed data are shown in Table 1.
The GAZO films have high transmittance across the whole visible range, with oscillation fringes across the spectrum. The fluctuations seen in the spectra may have resulted from the interference effect of reflection at the interfaces [35]. The amplitude of oscillation in GAZO films is reduced with processing time for hydrogen-plasma-annealed GAZO films, as observed from Figure 4. These results are similar to previous studies. The H2 plasma treatment reduced the amplitude of oscillation. This can be attributed to the increased surface roughness of GAZO films by H2 plasma treatment [30].
The emissivity of the as-deposited and hydrogen-plasma-treated GAZO films is also included in Table 1. Emissivity was assessed using an emissivity meter and the standard error of the instrument was ±0.01. The emissivity of as-deposited samples was 0.29. The emissivity of all plasma treatment samples was under 0.18. The GAZO films with the plasma treatment for 5 min showed the lowest emissivity at 0.13. The emissivity of plasma-annealed GAZO films (5 min sample) decreased by 55% compared with that of the as-deposited GAZO films. The electrical resistivity of plasma treatment samples (5 min sample) was 59% less than that of as-deposited samples. According to the Hagen–Rubens relation [20], the emissivity of materials is accompanied by a decrease in electrical resistivity. The results are shown in Table 1; the decrease in emissivity is observed as the electrical resistivity is reduced, which means that their ability to reflect infrared waves is increased.

4. Conclusions

In this paper, we aimed to investigate the changes in the structure and optical and electrical properties of GAZO films caused by H2 plasma treatment with different times. We also investigated the feasibility of applying GAZO films to low-e glass. As the H2 plasma treatment processing time increased, the (002) diffraction peak shifted toward different positions with respect to the corresponding one for as-deposited samples. The surface morphology also changed with the treatment time. With the use of a suitable processing time, the carrier concentrations of GAZO films can be improved. The plasma treatment decreased the electrical resistivity and lowered the emissivity of the GAZO films. The average visible light transmittance of all hydrogen-plasma-treated GAZO films was higher than 90%. The optimum time of H2 plasma treatment is 5 min. The electric resistivity, emissivity and average optical transmittance in the visible range from 400 nm to 800 nm are 5.3 × 10−4 Ω-cm, 0.13 and 94%, respectively, for GAZO films after plasma treatment for 5 min. The electrical resistivity and emissivity of 5 min plasma-treated GAZO films decreases by 59% and 55% compared with that of as-deposited samples. The H2 plasma-treated GAZO films can be applied to low-e glass.

Author Contributions

Conceptualization, S.-C.C.; methodology, S.-C.C. and J.-C.H.; validation, J.-C.H., H.-T.C. and C.-A.H.; writing—original draft preparation, J.-C.H. and H.-T.C.; writing—review and editing, S.-C.C., J.-C.H., H.-T.C. and C.-A.H.; supervision, S.-C.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported in part by the Ministry of Science and Technology of Taiwan, ROC, under Contract No. MOST 110-2637-E-168-004, and in part by the Green Energy Technology Research Center, Department of Electrical Engineering, Kun Shan University, Tainan, Taiwan, through the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project of the Ministry of Education (MOE) in Taiwan.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We would like to thank the anonymous reviewers and the editor for their valuable comments and remarks, which helped us to improve the original manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Coatings 12 00945 g001a 550Coatings 12 00945 g001b 550
Figure 1. Top-view SEM images of the as-deposited and plasma-treated GAZO films: (a) as-deposited, hydrogen-plasma-treated for (b) 1 min, (c) 3 min, (d) 5 min, (e) 7 min and (f) 10 min.
Figure 1. Top-view SEM images of the as-deposited and plasma-treated GAZO films: (a) as-deposited, hydrogen-plasma-treated for (b) 1 min, (c) 3 min, (d) 5 min, (e) 7 min and (f) 10 min.
Coatings 12 00945 g001aCoatings 12 00945 g001b
Coatings 12 00945 g002 550
Figure 2. XRD spectra of the hydrogen-plasma-processed GAZO films. According to the standard ZnO XRD pattern of ICSD 31052, the black dotted lines represent the XRD peak position of (002) of ZnO. (a) The XRD spectra of the all samples is exhibited with the 2θ = 30–60°, (b) a magnified (002) peak.
Figure 2. XRD spectra of the hydrogen-plasma-processed GAZO films. According to the standard ZnO XRD pattern of ICSD 31052, the black dotted lines represent the XRD peak position of (002) of ZnO. (a) The XRD spectra of the all samples is exhibited with the 2θ = 30–60°, (b) a magnified (002) peak.
Coatings 12 00945 g002
Coatings 12 00945 g003 550
Figure 3. Electrical properties of the as-deposited and H2 plasma-processed GAZO films.
Figure 3. Electrical properties of the as-deposited and H2 plasma-processed GAZO films.
Coatings 12 00945 g003
Coatings 12 00945 g004 550
Figure 4. Optical transmittance spectra for the as-deposited and H2 plasma-processed GAZO films.
Figure 4. Optical transmittance spectra for the as-deposited and H2 plasma-processed GAZO films.
Coatings 12 00945 g004
Table
Table 1. Structural, electrical and optical properties and emissivity of the as-deposited and hydrogen-plasma-processed GAZO films.
Table 1. Structural, electrical and optical properties and emissivity of the as-deposited and hydrogen-plasma-processed GAZO films.
Plasma TimeNone1 min3 min5 min7 min10 min
ZnO Plane (002) 2θ (°)34.40°34.30°34.42°34.38°34.26°34.32°
Electrical Resistivity (10−4 Ω-cm)13.0 ± 0.23%7.4 ± 0.25%6.1 ± 0.22%5.3 ± 0.23%6.3 ± 0.23%6.6 ± 0.22%
Carrier Concentration (1021 cm−3)1.8 ± 0.22%2.1 ± 0.23%2.4 ± 0.22%3.6 ± 0.22%3.5 ± 0.24%3.1 ± 0.23%
Hall Mobility (cm2/vs)3.8 ± 0.25%4.7 ± 0.22%5.2 ± 0.24%3.9 ± 0.22%4.3 ± 0.22%4.9 ± 0.23%
Emissivity0.29 ± 0.020.18 ± 0.010.15 ± 0.010.13 ± 0.010.15 ± 0.010.16 ± 0.02
Average Transmittance in Visible (400–800 nm) Region (%)859092949596
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Chang, S.-C.; Hu, J.-C.; Chan, H.-T.; Hsiao, C.-A. Influence of Processing Time in Hydrogen Plasma to Prepare Gallium and Aluminum Codoped Zinc Oxide Films for Low-Emissivity Glass. Coatings 2022, 12, 945. https://doi.org/10.3390/coatings12070945

AMA Style

Chang S-C, Hu J-C, Chan H-T, Hsiao C-A. Influence of Processing Time in Hydrogen Plasma to Prepare Gallium and Aluminum Codoped Zinc Oxide Films for Low-Emissivity Glass. Coatings. 2022; 12(7):945. https://doi.org/10.3390/coatings12070945

Chicago/Turabian Style

Chang, Shang-Chou, Jhih-Ciang Hu, Huang-Tian Chan, and Chuan-An Hsiao. 2022. "Influence of Processing Time in Hydrogen Plasma to Prepare Gallium and Aluminum Codoped Zinc Oxide Films for Low-Emissivity Glass" Coatings 12, no. 7: 945. https://doi.org/10.3390/coatings12070945

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