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Effect of temperature on the deposition of ZnO thin films by successive ionic layer adsorption and reaction
Applied Surface Science 258 (2012) 8109–8116
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Effect of temperature on the deposition of ZnO thin films by successive ionic layer adsorption and reaction Shih-Chang Shei a,∗ , Pay-Yu Lee b , Shoou-Jinn Chang b a b
Department of Electrical Engineering, National University of Tainan, Tainan 700, Taiwan, ROC Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 6 April 2012 Accepted 1 May 2012 Available online 9 May 2012 Keywords: SILAR Temperature treatment Ethylene glycol ZnO thin films
a b s t r a c t In this study, ZnO thin films were deposited on glass substrates by the successive ionic layer adsorption and reaction (SILAR) method, and the effect of the temperature treatment in ethylene glycol on the crystal structure, surface morphology, and optical properties of the films were investigated. When the temperature was below 85 ◦ C, the ZnO films showed poor optical transmission and had a rough surface crystal structure. As the temperature was increased, dense polycrystalline films with uniform ZnO grain distribution were obtained. The optical transmittance of the ZnO thin films fabricated at temperatures greater than 95 ◦ C was very high (90%) in the visible-light region. Therefore, it could be concluded that increasing the temperature of treatment in ethylene glycol helps in obtaining fine-grained ZnO films with a high growth rate and a low concentration of oxygen vacancies. However, temperatures greater than 145 ◦ C led to shedding of ZnO from the surface and a reduction in the growth rate. Thus, temperature treatment was confirmed to play an important role in ZnO film deposition instead of post thermal annealing after the film growth. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction ZnO is an important semiconductor material with a large bandgap (Eg = 3.37 eV), and it crystallizes in the hexagonal wurtzite structure (c = 0.521 nm, a = 0.325 nm), with oxygen atoms at the hexagonal sites and zinc atoms at the tetrahedral sites. ZnO also has high stability, good optical characteristics, and excellent electrical properties. ZnO thin films have been widely used in various devices such as piezoelectric transducers, surface acoustic wave filters, thin-film solar cells, gas sensor electronics, and ultraviolet (UV) light-emitting diodes. In addition, ZnO films with high transmittance in the visible region and low resistivity are suitable for use as transparent electrodes in electronic displays. ZnO thin films have been prepared by various chemical and physical deposition techniques such as sputtering, pulsed laser ablation, successive ionic layer adsorption and reaction (SILAR) [1–10], sol-gel methods [11–13], chemical bath deposition (CBD) [14–16], and chemical vapor deposition (CVD) [17]. Among these, solution-based chemical deposition techniques have recently attracted considerable attention because they are highly reliable and inexpensive; further, these methods require a low temperature
∗ Corresponding author at: 33, sec. 2, Shu-Lin st., Tainan 70005, Taiwan, ROC. Tel.: +88 662606123; fax: +88 662602305. E-mail addresses: [email protected], [email protected] (S.-C. Shei).
and facilitate large-area deposition, as opposed to other methods. However, very less effort has been devoted to the fabrication of transparent ZnO films via SILAR, which is one such solution-based approach. Gao et al. [1] included an ultrasonic rinsing step in the SILAR process for the preparation of ZnO thin films. In a previous study [18], we investigated the differences between rinsing procedures involving the use of deionized (DI) water and ethylene glycol. However, because of the high boiling point of ethylene glycol (197 ◦ C), we could not investigate the impact of the temperature treatment in ethylene glycol on the deposition of ZnO films in detail. In the present study, we investigate the effect of the temperature treatment in ethylene glycol on the structure and optical properties of the ZnO films fabricated by SILAR. In addition, we compared with the temperature treatment in ethylene glycol and the post thermal annealing after the film growth. The advantage of this method is that it provides optimal fabrication conditions under which high-quality ZnO thin films can be obtained. Moreover, it is easier to control deposition processes in SILAR than in other chemical methods. Given these advantages, this method can be applied to the fabrication of large-area ZnO thin films, which have potential applications in solar cell devices, light-emitting diodes, and other optoelectronic devices. 2. Experimental ZnO thin films were grown on glass substrates by the SILAR method. ZnCl2 (0.1 M) and concentrated ammonium hydroxide
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.004
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(SEM) and atomic force microscopy (AFM). The optical properties of the ZnO thin films were characterized by ultraviolet-visible-near infrared (UV–vis-IR) spectrophotometry, photoluminescence (PL) spectroscopy, and X-ray photoemission spectroscopy (XPS). 3. Results and discussion
Fig. 1. Process schemes of rinsing procedure for the deposition of ZnO thin films.
(NH4 OH; 29 wt% NH3 ) were used to prepare tetraammonium zinc complex ([Zn(NH3 )4 ]2+ ) solution. NH4 OH was added to adjust the pH of the solution to 10. ZnO film growth was performed through 20 deposition cycles, and then, the crystallinity and microstructure of the films were studied. During film deposition, the temperature of ethylene glycol was set at 75, 85, 95, 105, 125, 145, 165, and 180 ◦ C to convert Zn(OH)2 to ZnO, and the corresponding samples were labeled 1, 2, 3, 4, 5, 6, and 7, respectively. Fig. 1 shows the rinsing procedure. The steps followed in a single cycle for obtaining ZnO films (samples 1–8) are as follows. a) Glass substrates were dipped in [Zn(NH3 )4 ]2+ solution for 20 s. b) The substrates were dipped in DI water for 20 s to allow for the precipitation of Zn(OH)2 on their surface. c) The glass substrates were then dipped for 30 s in ultrasonicassisted DI water for the removal of counter ions (Cl− ) and loosely attached Zn(OH)2 grains. d) The glass substrates were immersed in ethylene glycol solution (rinsing temperature: 75, 85, 95, 105, 125, 145, 165, and 180 ◦ C for samples 1, 2, 3, 4, 5, 6, 7 and 8, respectively) for 20 s to allow for ZnO film formation. e) The substrates were finally cleaned by ultrasonic treatment in DI water for 30 s, so that ethylene glycol, loosely attached ZnO grains, and unreacted Zn(OH)2 grains were removed from the substrate surface. The reactions occurring during the abovementioned steps are expressed by Eqs. (1)–(4): Zn2+ + 4NH4 OH → [Zn(NH3 )4 ]2+ + 4H2 O
(1)
[Zn(NH3 )4 ]2+ + 4H2 O → Zn2+ + 4NH4 + + 4OH−
(2)
−
(3)
Zn(OH)2 → ZnO(s) + H2 O
(4)
2+
Zn
+ 2OH → Zn(OH)2(s)
The abovementioned five steps were repeated 20 times at each temperature in ethylene glycol to obtain the samples for structural and optical analysis. For a comparison with the temperature treatment in ethylene glycol and the post annealing after the film growth, the sample 3 and sample 7 were annealed at 450 ◦ C in air for 30 min. The structure and crystallite size of the deposited films were investigated by X-ray diffraction (XRD). The surface morphologies of the samples were examined by scanning electron microscopy
Fig. 2(a) shows the XRD patterns of the ZnO thin films deposited at 75, 85, 95, 105, 125, 145, 165, and 180 ◦ C over 20 cycles (samples 1, 2, 3, 4, 5, 6, 7 and 8). Fig. 2(b) shows the samples 3 and 7 without and with post thermal annealing after the films growth. No peaks were seen in the XRD spectrum of sample 1, which indicated that the temperature treatment during film deposition should be higher than 75 ◦ C. In the XRD patterns of samples 2–8, the highestintensity peak corresponding to the (0 0 2) plane of wurtzite ZnO appeared at around 2 = 34.42◦ . Peaks due to the (1 0 0), (0 0 2), (1 01), (1 0 2), (1 0 3), and (1 1 0) planes of wurtzite ZnO were also found. However, the fact that the (0 0 2) peak has the strongest intensity indicated that growth occurred along the c-axis in all the samples. The intensity of the (1 0 0) and (1 0 1) peaks decreased when the temperature was increased beyond 125 ◦ C, while that of the (0 0 2) peak increased drastically. In the XRD pattern of the film deposited at 145 ◦ C, the (0 0 2) peak had the maximum intensity, while the (1 0 0) and (1 0 1) peaks diminished significantly. From the XRD patterns, the 2 value corresponding to the position of (0 0 2) ZnO diffraction, as shown in Fig. 2(c), shifts toward the higher angle as temperature of treatment increases. Moreover, the diffraction peaks of the post-annealing ZnO film also were shifted toward a higher diffraction angle, too. The shift in position of (0 0 2) peak can be attributed to better crystallinity with more relaxation caused both by the temperature treatment in ethylene glycol and postannealing treatment. The full width at half maximum (FWHM) for the (0 0 2) peak of the XRD patterns was calculated and shown in Fig. 2(d). In addition, the crystal domain size was estimated from FWHM of the (0 0 2) peak by the Scherrer equation. D=
0.9 b cos
(5)
where D is the diameter of the crystallites forming the film, is the wavelength of the Cu K␣ line, b is the FWHM in radians and is Bragg’s angle. As shown in Fig. 2(e), the grain size for the as-grown sample 2–8 was 27, 41.9, 47.1, 56.6, 50.8, 45.7, 41.3 nm, respectively. The grain size of samples 3 and 7 with post annealed at 450 ◦ C for 30 min was 47.8 nm and 48.6 nm. It is noted that when the temperature of treatment in ethylene glycol was increased higher than 95 ◦ C, a decrease in the FWHM of the (0 0 2) peak and an increase in the grain size were observed. These observations indicated that the temperature treatment had a notable influence on the quality of the ZnO thin films and agreed with the post annealing treatment. Fig. 3 shows the thickness of the ZnO thin films as a function of the temperature of treatment in ethylene glycol. The film thickness increased when the rinsing temperature was in the range 85–145 ◦ C. The results demonstrated that a high rinsing temperature is beneficial for decomposing Zn(OH)2 to ZnO and water, as shown in Eq. (4). The thermal decomposition temperature of Zn(OH)2 was 125 ◦ C, and thermal decomposition occurred abruptly and simultaneously with crystallization. In addition, a high rinsing temperature not only provided a better decomposition environment but also enhanced the crystal structure, as shown in the XRD patterns. However, the thickness of the ZnO thin films decreased when the rinsing temperature was increased beyond 145 ◦ C. Very high rinsing temperatures might cause shedding of the thin-film surface and a consequent decrease in the film thickness and XRD peak intensity, as shown in Fig. 2 [19–21]. On the basis of the film thickness, the growth rate of dense ZnO films fabricated by SILAR was estimated to be 5–17 nm per cycle.
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Fig. 2. (a) XRD patterns of ZnO thin films grown at different temperatures of treatment. (b) XRD patterns of ZnO thin films after annealing. (c) The data evaluated from XRD analysis of ZnO thin films grown at different temperature treatments and thin film with post annealing.
Fig. 3. The thickness of ZnO thin films as a function of the temperature treatments.
Fig. 4 shows the SEM images of the surface of the ZnO thin films prepared by SILAR at rinsing temperatures of 75–180 ◦ C. In the case of sample 1 (Fig. 4(a)), the ZnO grains were not a compact arrangement but were scattered over the glass surface. Fig. 4(b) shows the image of sample 2. Although the film was formed at 85 ◦ C, the grain size was not uniform (about 100–300 nm) and the grains were not sufficiently compact; further, there were many crevices in the film. In contrast, the surfaces of samples 3–8 (shown in Fig. 4(c)–(h)) were smooth, and the films were dense (grain size: <100 nm). These results indicated that the rinsing temperature plays an important role in deciding the structural properties and growth rate of the films and that compact ZnO thin films can be obtained at a high rinsing temperature. The present method afforded denser ZnO films than did the SILAR method we reported in a previous study [18]. Fig. 4(i) and (j) shows the surface morphology of sample 3 and 7 with the post annealing. The grain size of sample 3 was larger than that of as-grown sample. Fig. 5 illustrates AFM images (2 m × 2 m scan) showing the surface morphologies of the deposited ZnO films. The
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Fig. 4. The SEM images of the surface of the ZnO thin films prepared by SILAR at temperatures of (a) 75 ◦ C, (b) 85 ◦ C, (c) 95 ◦ C, (d) 125 ◦ C, (e) 145 ◦ C, (f) 165 ◦ C, and (g) 180 ◦ C.
root-mean-square (rms) surface roughness of samples as grown and with the post annealing treatment was shown in Fig. 6. which indicated that the roughness of the ZnO films decreased with an increase in the temperature of treatment. Comparison of the AFM results with the XRD results revealed that under low-temperature conditions, the film growth rate is the same along the (0 0 2) and (1 0 1) planes, while growth along the (1 0 0) plane is suppressed. Hence, the films grown under these conditions had high rms surface roughness. With an increase in the temperature of treatment, growth occurred along the c-axis; the growth rate was high in the (0 0 2) plane, while growth in the (1 0 0) and (1 0 1) planes was suppressed. Consequently, thin films with a smooth surface were obtained. These results reveal that the surface smoothness of ZnO films decreases with an increase in temperature of treatment due to better dispersion. The agglomeration of ZnO was discussed in early report [18] [22]. The molecular adsorption on surfaces forms a number of hydroxyl groups (i.e., Zn OH) on the ZnO surface,
ethylene glycol enhanced the dispersion and reduced the agglomerations through the hydrogen bonds between hydroxyl groups absorbed on ZnO surface and ethylene glycol. Highly uniform ZnO thin films were obtained through the different temperature rinsing procedures. However, in the temperature 85 ◦ C, ethylene glycol sample with poor dispersions, the surface image shows larger agglomerations and particle size. In the ethylene glycol hydrogenbond network, inter-molecular hydrogen bonds was easy to be broken as temperature increased, on the other hands, higher temperature is necessary to broken intra-molecular hydrogen-bonds, during ethylene glycol rinsing procedures, an opened monomeric structures is prefer to interact with hydroxyl groups absorbed on ZnO surface, the binding prevents from water molecular on the ZnO surface and results in the dispersion of ZnO particles. The proportion of intra- and inter-molecular hydrogen-bonds in ethylene glycol was reported by Crupi et al. [22], it means that higher ethylene glycol rinsing temperature is necessary to obtained
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Fig. 5. The AFM images of the deposited ZnO films at temperatures of (a) 75 ◦ C, (b) 85 ◦ C, (c) 95 ◦ C, (d) 105 ◦ C, (e) 125 ◦ C, (f) 145 ◦ C, (g) 165 ◦ C, (h) 180 ◦ C, (i) deposited at 95 ◦ C with post annealing and (j) deposited at 165 ◦ C with post annealing.
Fig. 6. The rms value of ZnO thin films as a function of the temperature treatments.
compacted films, in this work, as the ethylene glycol temperature increased, the thin films got more smoother and the rms decreased. This result is not in agreement with the analysis of XRD due to the ethylene glycol temperature treatment with dispersion effect. Compared with the results of temperature treatment and the results of post annealing process revealed that post annealing process provides more activation energy to atoms to grow larger grains and leads to the higher rms, This phenomenon is consistent with the results of XRD. Fig. 7(a) shows the optical transmittance spectra of the ZnO thin films prepared by SILAR at temperature treatments of 75–180 ◦ C, and Fig. 7(b) shows the results of samples 3 and 7 with and without the post annealing treatment. The optical transmittance of all the ZnO film samples was as high as 90% in the visible region, except for sample 2, and a sharp ultraviolet absorption edge was observed
Fig. 7. (a) The optical transmittance spectra of the deposited ZnO films at temperatures of 75 ◦ C, 85 ◦ C, 95 ◦ C, 105 ◦ C, 125 ◦ C, 145 ◦ C, 165 ◦ C, and 180 ◦ C, respectively. (b) The optical transmittance spectra of the deposited ZnO films at temperatures of 95 ◦ C and 165 ◦ C with post annealing.
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Fig. 8. (a) The band gap of the deposited ZnO films at temperatures of 75 ◦ C, 85 ◦ C, 95 ◦ C, 105 ◦ C, 125 ◦ C, 145 ◦ C, 165 ◦ C, and 180 ◦ C for 20-cycle deposition. (b) The band gap of ZnO films deposited ZnO films at temperatures of 95 ◦ C and 165 ◦ C with post annealing.
at around 350 nm and sample 1 shows no absorption due to no film grown on the glass substrate. However, sample 2 had an opaque, white surface, and the sharp ultraviolet absorption edge appeared at around 325 nm. Specifically, the optical transmittance in the wavelength range 400–500 nm was above 95% for the ZnO thin films prepared at temperatures of 125–180 ◦ C, and the transmittance increased with the temperature. The optical transmittance increased in the case of thin films showing highly preferred orientation along the c-axis. Upon light irradiation of a non-oriented film structure in which growth occurs along the (1 0 0), (0 0 2), and (1 0 1) directions, the transmittance decreases because of dispersion at the opaque-grain boundaries. Fig. 8(a) shows the bandgap of the ZnO thin films. The bandgaps were estimated by extrapolation of the linear portion of ˛2 versus h plots, using the relation (˛h)2 = A(h − Eg ), where ˛ is the absorption coefficient; h, the photon energy; and Eg , the optical bandgap. The Eg values of samples 2, 3, 4, 5, 6, 7 and 8 were 3.43, 3.39, 3.39, 3.39, 3.37, 3.39, and 3.39 eV, respectively. The Eg values for the ZnO thin films deposited in ethylene glycol were very close to that of crystalline ZnO (3.37 eV), whereas those of the films deposited at low temperatures were higher. This indicated that as the temperature of ethylene glycol exceeds 125 ◦ C, the rinsing process provides a better growth environment for the ZnO thin films. Fig. 8(b) shows the absorption edge of the film exhibits a red-shift with high annealing temperature in air, which is attributed to the well-known Burstein–Moss effect [23]. The red-shift phenomenon corresponds to the decrease of carrier concentration with the post annealing process in air. The Eg values of as grown samples 3 and 7 were about 3.39 eV, and the Eg values of samples 3 and 7 with post annealing treatment decreased to 3.23 eV and 3.31 eV.
Fig. 9. (a) The PL spectra of the deposited ZnO films at rinsing temperatures of 75 ◦ C, 85 ◦ C, 95 ◦ C, 105 ◦ C, 125 ◦ C, 145 ◦ C, 165 ◦ C, and 180 ◦ C, respectively. (b) The PL spectra of ZnO films deposited ZnO films at rinsing temperatures of 95 ◦ C and 165 ◦ C with post annealing.
The effect of temperature treatment and post annealing treatment on the PL properties of the ZnO thin films was investigated. Fig. 9(a) shows the PL spectra of the ZnO thin films deposited at different temperatures ranging from 75 to 180 ◦ C. The spectra of all the samples (except sample 1) exhibited two emission peaks: a near-band-edge (NBE) UV emission peak and a broad peak corresponding to emission in the green-yellow region (485–600 nm). The UV PL intensities increased with the temperature treatment, but the green-yellow PL intensities significantly decreased with an increase in the temperature of treatment. With an increase in the temperature, strong UV emission was observed from the highly c-axis oriented ZnO film. Fig. 10 presents the energy levels and wavelengths of the defects in the ZnO thin films [24–30]. Three UV emission peaks were observed between 300 nm and 425 nm. The peak at 375 nm was assigned to NBE UV transition, which is
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Fig. 10. The energy levels and wavelengths of defects in ZnO thin film.
directly related to the crystal quality of ZnO [25], and the UV emission peak at around 400 nm was possibly due to the energy interval from the bottom of the conduction band to the zinc vacancy (VZn ) level (3.06 eV) [26–28]. The weak peak at around 417 nm was possibly due to the energy interval from the top of the valence band to the interstitial zinc (Zni ) level (2.90 eV) [17–19]. The crystallinity of the ZnO thin films was enhanced when the rinsing temperature was increased. This could be confirmed from the XRD results shown in Fig. 2(a). The broad yellow-green luminescence peak is associated with oxygen defects such as interstitial oxygen (Oi ) and antisite oxygen (OZn ) [24]. The luminescence band in the range 442–620 nm is attributed to Zn(OH)2 present in the ZnO thin films since Zn(OH)2 gives the Oi emission band (543 nm) [29]. Because Zn(OH)2 is hydrolyzed to ZnO and H2 O at 125 ◦ C, high temperatures can enhance the hydrolysis process. In our case, the high-temperature process actually helped in decreasing the luminescence intensity of
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the Oi emission band (543 nm). PL spectra indicated that the concentration of Oi in the ZnO films increases during low-temperature deposition since Zn(OH)2 hydrolysis is difficult at low temperatures. On the contrary, at temperatures higher than 125 ◦ C, the Oi concentration decreases significantly. From these results, the PL characteristics of the SILAR-derived ZnO thin films are confirmed to be strongly dependent on the rinsing temperature. Fig. 9(b) and (c) shows the PL spectra of the sample 3 and sample 7 with post annealing at high temperature. The PL spectra of sample 3 indicated that the concentration of Oi in the ZnO films increases after post annealing, however, the results of sample 7 was not obvious. This is attributed to sample 7 prepared with high temperature treatment and significantly improved the oxygen defects in the ZnO film as shown in the PL spectra. This results indicated that most of oxygen defects had been improved by temperature treatment in ethylene glycol and in agreement with the results by post annealing treatment. To further study the effect of rinsing temperature on the hydrolysis of Zn(OH)2 to ZnO, the chemical state of the constituent elements of the ZnO thin films was determined by XPS analysis. XPS measurements were carried out after removing the surface layer of hydroxyl groups (Zn OH), which was more than 50 nm thick, by Ar ion etching [30]. Fig. 11 shows the O 1s multiplex spectra of the ZnO thin films grown at different rinsing temperatures. The O 1s bands were successfully deconvoluted into two components using Gaussian profiles, as shown in Fig. 11(a)–(j). The binding energy of the O 1s peak for the as-grown film, around 532.0 eV, is attributed to the opaque Zn(OH)2 phase, and the O 1s binding energies in the range 529.7–530.6 eV can be due to Zn O bond formation [31–33] and 533 eV due to Si O bond formation[34,35] In the spectrum of the ZnO films deposited at 75 ◦ C, the XPS spectra shows strong intensity of Si O bonds and very weak peak of
Fig. 11. ZnO XPS multiplex spectra of the O 1s of the deposited ZnO films at rinsing temperatures of (a) 75 ◦ C, (b) 85 ◦ C, (c) 95 ◦ C, (d) 105 ◦ C, (e) 125 ◦ C, (f) 145 ◦ C, (g) 165 ◦ C, (h) 180 ◦ C, (i) deposited at 95 ◦ C with post annealing and (j) deposited at 165 ◦ C with post annealing.
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can be used to optimize SILAR methods for fabricating high-quality semiconductor thin films that have potential applications in optoelectronics. Acknowledgments The authors would like to thank the Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC. for the financial support under contract no. 101-D0204-6 and the LED Lighting Research Center of NCKU for the assistance of device characterization, and the National Science Council of Taiwan, ROC., under contract no. NSC 99-2221E-024-009 and NSC 98-3114-E009-002-CC2. References Fig. 12. The atom ratio of O/Zn calculated from the XPS spectra as a function of the samples.
Zn OH bond, it indicates that thin films was difficult to deposited at low temperature, this result is in agreement with the above analysis, When the temperature at 85 and 95 ◦ C (Fig. 11), high-intensity peaks due to Zn OH bonds and relatively low intensity peaks due to Zn O bonds could be seen, confirming that excess Zn(OH)2 existed in the ZnO film deposited at lower temperatures. On the contrary, when the rinsing temperature exceeded 125 ◦ C, weak peaks due to Zn OH bonds and relatively stronger peaks due to Zn O bonds were observed; this could be explained by the decomposition of Zn(OH)2 at higher temperatures. In Fig. 11(i)–(j), the samples with post annealing treatment could enhance the Zn O bonds and decrease the Zn OH bonds. Furthermore, from the XPS spectra, the atom ratio of O/Zn could be obtained and shown in Fig. 12. The atom ratio of O/Zn decreased with increasing the temperature of treatment. This results indicated that the films were O deficient at lower temperature treatments due to the high content of Zn(OH)2 as show in Fig. 11. As samples with post annealing treatment, the atom ratio of O/Zn decreases to close to 1 due to the decomposition of Zn(OH)2 hydrolysis at high temperature annealing treatment and similar to the results of high-temperature treatments in ethylene glycol. The results of our XPS measurements were consistent with the previously discussed PL data. These observations indicated that the PL and XPS characteristics of the ZnO thin films prepared by SILAR are strongly dependent on the temperature treatments. 4. Conclusion ZnO thin films obtained by SILAR at different temperature treatments in ethylene glycol (75–180 ◦ C) were investigated. With an increase in the temperature, preferential growth occurred along the c-axis, and high-intensity peaks due to the (0 0 2) plane were observed in the XRD spectrum; further, the obtained films had a smooth surface and showed good transparency in the visible spectral range. The films deposited at temperatures below 95 ◦ C showed a rapid decrease in their transparency, eventually becoming white. At very high temperatures (>145 ◦ C), only slight enhancement of the crystal quality was observed and the growth rate decreased. PL and XPS data revealed that the temperature treatment significantly affects the distribution of defects in the films. At low temperatures, a large number of these defects, mostly oxygen defects attributable to Zn(OH)2 in the films, were observed. However, the concentration of these defects reduced at high temperatures. Thus, we concluded that the quality and the structural and optical properties of the thin films depended strongly on the temperature treatment. Furthermore, temperature treatment was confirmed to play an important role in ZnO film deposition instead of post thermal annealing after the film growth. We are confident that the results of our study
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Effect of temperature on the deposition of ZnO thin films by successive ionic layer adsorption and reaction Shih-Chang Shei a,∗ , Pay-Yu Lee b , Shoou-Jinn Chang b a b
Department of Electrical Engineering, National University of Tainan, Tainan 700, Taiwan, ROC Institute of Microelectronics and Department of Electrical Engineering, National Cheng Kung University, Tainan 701, Taiwan, ROC
a r t i c l e
i n f o
Article history: Received 5 October 2011 Received in revised form 6 April 2012 Accepted 1 May 2012 Available online 9 May 2012 Keywords: SILAR Temperature treatment Ethylene glycol ZnO thin films
a b s t r a c t In this study, ZnO thin films were deposited on glass substrates by the successive ionic layer adsorption and reaction (SILAR) method, and the effect of the temperature treatment in ethylene glycol on the crystal structure, surface morphology, and optical properties of the films were investigated. When the temperature was below 85 ◦ C, the ZnO films showed poor optical transmission and had a rough surface crystal structure. As the temperature was increased, dense polycrystalline films with uniform ZnO grain distribution were obtained. The optical transmittance of the ZnO thin films fabricated at temperatures greater than 95 ◦ C was very high (90%) in the visible-light region. Therefore, it could be concluded that increasing the temperature of treatment in ethylene glycol helps in obtaining fine-grained ZnO films with a high growth rate and a low concentration of oxygen vacancies. However, temperatures greater than 145 ◦ C led to shedding of ZnO from the surface and a reduction in the growth rate. Thus, temperature treatment was confirmed to play an important role in ZnO film deposition instead of post thermal annealing after the film growth. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved.
1. Introduction ZnO is an important semiconductor material with a large bandgap (Eg = 3.37 eV), and it crystallizes in the hexagonal wurtzite structure (c = 0.521 nm, a = 0.325 nm), with oxygen atoms at the hexagonal sites and zinc atoms at the tetrahedral sites. ZnO also has high stability, good optical characteristics, and excellent electrical properties. ZnO thin films have been widely used in various devices such as piezoelectric transducers, surface acoustic wave filters, thin-film solar cells, gas sensor electronics, and ultraviolet (UV) light-emitting diodes. In addition, ZnO films with high transmittance in the visible region and low resistivity are suitable for use as transparent electrodes in electronic displays. ZnO thin films have been prepared by various chemical and physical deposition techniques such as sputtering, pulsed laser ablation, successive ionic layer adsorption and reaction (SILAR) [1–10], sol-gel methods [11–13], chemical bath deposition (CBD) [14–16], and chemical vapor deposition (CVD) [17]. Among these, solution-based chemical deposition techniques have recently attracted considerable attention because they are highly reliable and inexpensive; further, these methods require a low temperature
∗ Corresponding author at: 33, sec. 2, Shu-Lin st., Tainan 70005, Taiwan, ROC. Tel.: +88 662606123; fax: +88 662602305. E-mail addresses: [email protected], [email protected] (S.-C. Shei).
and facilitate large-area deposition, as opposed to other methods. However, very less effort has been devoted to the fabrication of transparent ZnO films via SILAR, which is one such solution-based approach. Gao et al. [1] included an ultrasonic rinsing step in the SILAR process for the preparation of ZnO thin films. In a previous study [18], we investigated the differences between rinsing procedures involving the use of deionized (DI) water and ethylene glycol. However, because of the high boiling point of ethylene glycol (197 ◦ C), we could not investigate the impact of the temperature treatment in ethylene glycol on the deposition of ZnO films in detail. In the present study, we investigate the effect of the temperature treatment in ethylene glycol on the structure and optical properties of the ZnO films fabricated by SILAR. In addition, we compared with the temperature treatment in ethylene glycol and the post thermal annealing after the film growth. The advantage of this method is that it provides optimal fabrication conditions under which high-quality ZnO thin films can be obtained. Moreover, it is easier to control deposition processes in SILAR than in other chemical methods. Given these advantages, this method can be applied to the fabrication of large-area ZnO thin films, which have potential applications in solar cell devices, light-emitting diodes, and other optoelectronic devices. 2. Experimental ZnO thin films were grown on glass substrates by the SILAR method. ZnCl2 (0.1 M) and concentrated ammonium hydroxide
0169-4332/$ – see front matter. Crown Copyright © 2012 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.05.004
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(SEM) and atomic force microscopy (AFM). The optical properties of the ZnO thin films were characterized by ultraviolet-visible-near infrared (UV–vis-IR) spectrophotometry, photoluminescence (PL) spectroscopy, and X-ray photoemission spectroscopy (XPS). 3. Results and discussion
Fig. 1. Process schemes of rinsing procedure for the deposition of ZnO thin films.
(NH4 OH; 29 wt% NH3 ) were used to prepare tetraammonium zinc complex ([Zn(NH3 )4 ]2+ ) solution. NH4 OH was added to adjust the pH of the solution to 10. ZnO film growth was performed through 20 deposition cycles, and then, the crystallinity and microstructure of the films were studied. During film deposition, the temperature of ethylene glycol was set at 75, 85, 95, 105, 125, 145, 165, and 180 ◦ C to convert Zn(OH)2 to ZnO, and the corresponding samples were labeled 1, 2, 3, 4, 5, 6, and 7, respectively. Fig. 1 shows the rinsing procedure. The steps followed in a single cycle for obtaining ZnO films (samples 1–8) are as follows. a) Glass substrates were dipped in [Zn(NH3 )4 ]2+ solution for 20 s. b) The substrates were dipped in DI water for 20 s to allow for the precipitation of Zn(OH)2 on their surface. c) The glass substrates were then dipped for 30 s in ultrasonicassisted DI water for the removal of counter ions (Cl− ) and loosely attached Zn(OH)2 grains. d) The glass substrates were immersed in ethylene glycol solution (rinsing temperature: 75, 85, 95, 105, 125, 145, 165, and 180 ◦ C for samples 1, 2, 3, 4, 5, 6, 7 and 8, respectively) for 20 s to allow for ZnO film formation. e) The substrates were finally cleaned by ultrasonic treatment in DI water for 30 s, so that ethylene glycol, loosely attached ZnO grains, and unreacted Zn(OH)2 grains were removed from the substrate surface. The reactions occurring during the abovementioned steps are expressed by Eqs. (1)–(4): Zn2+ + 4NH4 OH → [Zn(NH3 )4 ]2+ + 4H2 O
(1)
[Zn(NH3 )4 ]2+ + 4H2 O → Zn2+ + 4NH4 + + 4OH−
(2)
−
(3)
Zn(OH)2 → ZnO(s) + H2 O
(4)
2+
Zn
+ 2OH → Zn(OH)2(s)
The abovementioned five steps were repeated 20 times at each temperature in ethylene glycol to obtain the samples for structural and optical analysis. For a comparison with the temperature treatment in ethylene glycol and the post annealing after the film growth, the sample 3 and sample 7 were annealed at 450 ◦ C in air for 30 min. The structure and crystallite size of the deposited films were investigated by X-ray diffraction (XRD). The surface morphologies of the samples were examined by scanning electron microscopy
Fig. 2(a) shows the XRD patterns of the ZnO thin films deposited at 75, 85, 95, 105, 125, 145, 165, and 180 ◦ C over 20 cycles (samples 1, 2, 3, 4, 5, 6, 7 and 8). Fig. 2(b) shows the samples 3 and 7 without and with post thermal annealing after the films growth. No peaks were seen in the XRD spectrum of sample 1, which indicated that the temperature treatment during film deposition should be higher than 75 ◦ C. In the XRD patterns of samples 2–8, the highestintensity peak corresponding to the (0 0 2) plane of wurtzite ZnO appeared at around 2 = 34.42◦ . Peaks due to the (1 0 0), (0 0 2), (1 01), (1 0 2), (1 0 3), and (1 1 0) planes of wurtzite ZnO were also found. However, the fact that the (0 0 2) peak has the strongest intensity indicated that growth occurred along the c-axis in all the samples. The intensity of the (1 0 0) and (1 0 1) peaks decreased when the temperature was increased beyond 125 ◦ C, while that of the (0 0 2) peak increased drastically. In the XRD pattern of the film deposited at 145 ◦ C, the (0 0 2) peak had the maximum intensity, while the (1 0 0) and (1 0 1) peaks diminished significantly. From the XRD patterns, the 2 value corresponding to the position of (0 0 2) ZnO diffraction, as shown in Fig. 2(c), shifts toward the higher angle as temperature of treatment increases. Moreover, the diffraction peaks of the post-annealing ZnO film also were shifted toward a higher diffraction angle, too. The shift in position of (0 0 2) peak can be attributed to better crystallinity with more relaxation caused both by the temperature treatment in ethylene glycol and postannealing treatment. The full width at half maximum (FWHM) for the (0 0 2) peak of the XRD patterns was calculated and shown in Fig. 2(d). In addition, the crystal domain size was estimated from FWHM of the (0 0 2) peak by the Scherrer equation. D=
0.9 b cos
(5)
where D is the diameter of the crystallites forming the film, is the wavelength of the Cu K␣ line, b is the FWHM in radians and is Bragg’s angle. As shown in Fig. 2(e), the grain size for the as-grown sample 2–8 was 27, 41.9, 47.1, 56.6, 50.8, 45.7, 41.3 nm, respectively. The grain size of samples 3 and 7 with post annealed at 450 ◦ C for 30 min was 47.8 nm and 48.6 nm. It is noted that when the temperature of treatment in ethylene glycol was increased higher than 95 ◦ C, a decrease in the FWHM of the (0 0 2) peak and an increase in the grain size were observed. These observations indicated that the temperature treatment had a notable influence on the quality of the ZnO thin films and agreed with the post annealing treatment. Fig. 3 shows the thickness of the ZnO thin films as a function of the temperature of treatment in ethylene glycol. The film thickness increased when the rinsing temperature was in the range 85–145 ◦ C. The results demonstrated that a high rinsing temperature is beneficial for decomposing Zn(OH)2 to ZnO and water, as shown in Eq. (4). The thermal decomposition temperature of Zn(OH)2 was 125 ◦ C, and thermal decomposition occurred abruptly and simultaneously with crystallization. In addition, a high rinsing temperature not only provided a better decomposition environment but also enhanced the crystal structure, as shown in the XRD patterns. However, the thickness of the ZnO thin films decreased when the rinsing temperature was increased beyond 145 ◦ C. Very high rinsing temperatures might cause shedding of the thin-film surface and a consequent decrease in the film thickness and XRD peak intensity, as shown in Fig. 2 [19–21]. On the basis of the film thickness, the growth rate of dense ZnO films fabricated by SILAR was estimated to be 5–17 nm per cycle.
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Fig. 2. (a) XRD patterns of ZnO thin films grown at different temperatures of treatment. (b) XRD patterns of ZnO thin films after annealing. (c) The data evaluated from XRD analysis of ZnO thin films grown at different temperature treatments and thin film with post annealing.
Fig. 3. The thickness of ZnO thin films as a function of the temperature treatments.
Fig. 4 shows the SEM images of the surface of the ZnO thin films prepared by SILAR at rinsing temperatures of 75–180 ◦ C. In the case of sample 1 (Fig. 4(a)), the ZnO grains were not a compact arrangement but were scattered over the glass surface. Fig. 4(b) shows the image of sample 2. Although the film was formed at 85 ◦ C, the grain size was not uniform (about 100–300 nm) and the grains were not sufficiently compact; further, there were many crevices in the film. In contrast, the surfaces of samples 3–8 (shown in Fig. 4(c)–(h)) were smooth, and the films were dense (grain size: <100 nm). These results indicated that the rinsing temperature plays an important role in deciding the structural properties and growth rate of the films and that compact ZnO thin films can be obtained at a high rinsing temperature. The present method afforded denser ZnO films than did the SILAR method we reported in a previous study [18]. Fig. 4(i) and (j) shows the surface morphology of sample 3 and 7 with the post annealing. The grain size of sample 3 was larger than that of as-grown sample. Fig. 5 illustrates AFM images (2 m × 2 m scan) showing the surface morphologies of the deposited ZnO films. The
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Fig. 4. The SEM images of the surface of the ZnO thin films prepared by SILAR at temperatures of (a) 75 ◦ C, (b) 85 ◦ C, (c) 95 ◦ C, (d) 125 ◦ C, (e) 145 ◦ C, (f) 165 ◦ C, and (g) 180 ◦ C.
root-mean-square (rms) surface roughness of samples as grown and with the post annealing treatment was shown in Fig. 6. which indicated that the roughness of the ZnO films decreased with an increase in the temperature of treatment. Comparison of the AFM results with the XRD results revealed that under low-temperature conditions, the film growth rate is the same along the (0 0 2) and (1 0 1) planes, while growth along the (1 0 0) plane is suppressed. Hence, the films grown under these conditions had high rms surface roughness. With an increase in the temperature of treatment, growth occurred along the c-axis; the growth rate was high in the (0 0 2) plane, while growth in the (1 0 0) and (1 0 1) planes was suppressed. Consequently, thin films with a smooth surface were obtained. These results reveal that the surface smoothness of ZnO films decreases with an increase in temperature of treatment due to better dispersion. The agglomeration of ZnO was discussed in early report [18] [22]. The molecular adsorption on surfaces forms a number of hydroxyl groups (i.e., Zn OH) on the ZnO surface,
ethylene glycol enhanced the dispersion and reduced the agglomerations through the hydrogen bonds between hydroxyl groups absorbed on ZnO surface and ethylene glycol. Highly uniform ZnO thin films were obtained through the different temperature rinsing procedures. However, in the temperature 85 ◦ C, ethylene glycol sample with poor dispersions, the surface image shows larger agglomerations and particle size. In the ethylene glycol hydrogenbond network, inter-molecular hydrogen bonds was easy to be broken as temperature increased, on the other hands, higher temperature is necessary to broken intra-molecular hydrogen-bonds, during ethylene glycol rinsing procedures, an opened monomeric structures is prefer to interact with hydroxyl groups absorbed on ZnO surface, the binding prevents from water molecular on the ZnO surface and results in the dispersion of ZnO particles. The proportion of intra- and inter-molecular hydrogen-bonds in ethylene glycol was reported by Crupi et al. [22], it means that higher ethylene glycol rinsing temperature is necessary to obtained
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Fig. 5. The AFM images of the deposited ZnO films at temperatures of (a) 75 ◦ C, (b) 85 ◦ C, (c) 95 ◦ C, (d) 105 ◦ C, (e) 125 ◦ C, (f) 145 ◦ C, (g) 165 ◦ C, (h) 180 ◦ C, (i) deposited at 95 ◦ C with post annealing and (j) deposited at 165 ◦ C with post annealing.
Fig. 6. The rms value of ZnO thin films as a function of the temperature treatments.
compacted films, in this work, as the ethylene glycol temperature increased, the thin films got more smoother and the rms decreased. This result is not in agreement with the analysis of XRD due to the ethylene glycol temperature treatment with dispersion effect. Compared with the results of temperature treatment and the results of post annealing process revealed that post annealing process provides more activation energy to atoms to grow larger grains and leads to the higher rms, This phenomenon is consistent with the results of XRD. Fig. 7(a) shows the optical transmittance spectra of the ZnO thin films prepared by SILAR at temperature treatments of 75–180 ◦ C, and Fig. 7(b) shows the results of samples 3 and 7 with and without the post annealing treatment. The optical transmittance of all the ZnO film samples was as high as 90% in the visible region, except for sample 2, and a sharp ultraviolet absorption edge was observed
Fig. 7. (a) The optical transmittance spectra of the deposited ZnO films at temperatures of 75 ◦ C, 85 ◦ C, 95 ◦ C, 105 ◦ C, 125 ◦ C, 145 ◦ C, 165 ◦ C, and 180 ◦ C, respectively. (b) The optical transmittance spectra of the deposited ZnO films at temperatures of 95 ◦ C and 165 ◦ C with post annealing.
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Fig. 8. (a) The band gap of the deposited ZnO films at temperatures of 75 ◦ C, 85 ◦ C, 95 ◦ C, 105 ◦ C, 125 ◦ C, 145 ◦ C, 165 ◦ C, and 180 ◦ C for 20-cycle deposition. (b) The band gap of ZnO films deposited ZnO films at temperatures of 95 ◦ C and 165 ◦ C with post annealing.
at around 350 nm and sample 1 shows no absorption due to no film grown on the glass substrate. However, sample 2 had an opaque, white surface, and the sharp ultraviolet absorption edge appeared at around 325 nm. Specifically, the optical transmittance in the wavelength range 400–500 nm was above 95% for the ZnO thin films prepared at temperatures of 125–180 ◦ C, and the transmittance increased with the temperature. The optical transmittance increased in the case of thin films showing highly preferred orientation along the c-axis. Upon light irradiation of a non-oriented film structure in which growth occurs along the (1 0 0), (0 0 2), and (1 0 1) directions, the transmittance decreases because of dispersion at the opaque-grain boundaries. Fig. 8(a) shows the bandgap of the ZnO thin films. The bandgaps were estimated by extrapolation of the linear portion of ˛2 versus h plots, using the relation (˛h)2 = A(h − Eg ), where ˛ is the absorption coefficient; h, the photon energy; and Eg , the optical bandgap. The Eg values of samples 2, 3, 4, 5, 6, 7 and 8 were 3.43, 3.39, 3.39, 3.39, 3.37, 3.39, and 3.39 eV, respectively. The Eg values for the ZnO thin films deposited in ethylene glycol were very close to that of crystalline ZnO (3.37 eV), whereas those of the films deposited at low temperatures were higher. This indicated that as the temperature of ethylene glycol exceeds 125 ◦ C, the rinsing process provides a better growth environment for the ZnO thin films. Fig. 8(b) shows the absorption edge of the film exhibits a red-shift with high annealing temperature in air, which is attributed to the well-known Burstein–Moss effect [23]. The red-shift phenomenon corresponds to the decrease of carrier concentration with the post annealing process in air. The Eg values of as grown samples 3 and 7 were about 3.39 eV, and the Eg values of samples 3 and 7 with post annealing treatment decreased to 3.23 eV and 3.31 eV.
Fig. 9. (a) The PL spectra of the deposited ZnO films at rinsing temperatures of 75 ◦ C, 85 ◦ C, 95 ◦ C, 105 ◦ C, 125 ◦ C, 145 ◦ C, 165 ◦ C, and 180 ◦ C, respectively. (b) The PL spectra of ZnO films deposited ZnO films at rinsing temperatures of 95 ◦ C and 165 ◦ C with post annealing.
The effect of temperature treatment and post annealing treatment on the PL properties of the ZnO thin films was investigated. Fig. 9(a) shows the PL spectra of the ZnO thin films deposited at different temperatures ranging from 75 to 180 ◦ C. The spectra of all the samples (except sample 1) exhibited two emission peaks: a near-band-edge (NBE) UV emission peak and a broad peak corresponding to emission in the green-yellow region (485–600 nm). The UV PL intensities increased with the temperature treatment, but the green-yellow PL intensities significantly decreased with an increase in the temperature of treatment. With an increase in the temperature, strong UV emission was observed from the highly c-axis oriented ZnO film. Fig. 10 presents the energy levels and wavelengths of the defects in the ZnO thin films [24–30]. Three UV emission peaks were observed between 300 nm and 425 nm. The peak at 375 nm was assigned to NBE UV transition, which is
S.-C. Shei et al. / Applied Surface Science 258 (2012) 8109–8116
Fig. 10. The energy levels and wavelengths of defects in ZnO thin film.
directly related to the crystal quality of ZnO [25], and the UV emission peak at around 400 nm was possibly due to the energy interval from the bottom of the conduction band to the zinc vacancy (VZn ) level (3.06 eV) [26–28]. The weak peak at around 417 nm was possibly due to the energy interval from the top of the valence band to the interstitial zinc (Zni ) level (2.90 eV) [17–19]. The crystallinity of the ZnO thin films was enhanced when the rinsing temperature was increased. This could be confirmed from the XRD results shown in Fig. 2(a). The broad yellow-green luminescence peak is associated with oxygen defects such as interstitial oxygen (Oi ) and antisite oxygen (OZn ) [24]. The luminescence band in the range 442–620 nm is attributed to Zn(OH)2 present in the ZnO thin films since Zn(OH)2 gives the Oi emission band (543 nm) [29]. Because Zn(OH)2 is hydrolyzed to ZnO and H2 O at 125 ◦ C, high temperatures can enhance the hydrolysis process. In our case, the high-temperature process actually helped in decreasing the luminescence intensity of
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the Oi emission band (543 nm). PL spectra indicated that the concentration of Oi in the ZnO films increases during low-temperature deposition since Zn(OH)2 hydrolysis is difficult at low temperatures. On the contrary, at temperatures higher than 125 ◦ C, the Oi concentration decreases significantly. From these results, the PL characteristics of the SILAR-derived ZnO thin films are confirmed to be strongly dependent on the rinsing temperature. Fig. 9(b) and (c) shows the PL spectra of the sample 3 and sample 7 with post annealing at high temperature. The PL spectra of sample 3 indicated that the concentration of Oi in the ZnO films increases after post annealing, however, the results of sample 7 was not obvious. This is attributed to sample 7 prepared with high temperature treatment and significantly improved the oxygen defects in the ZnO film as shown in the PL spectra. This results indicated that most of oxygen defects had been improved by temperature treatment in ethylene glycol and in agreement with the results by post annealing treatment. To further study the effect of rinsing temperature on the hydrolysis of Zn(OH)2 to ZnO, the chemical state of the constituent elements of the ZnO thin films was determined by XPS analysis. XPS measurements were carried out after removing the surface layer of hydroxyl groups (Zn OH), which was more than 50 nm thick, by Ar ion etching [30]. Fig. 11 shows the O 1s multiplex spectra of the ZnO thin films grown at different rinsing temperatures. The O 1s bands were successfully deconvoluted into two components using Gaussian profiles, as shown in Fig. 11(a)–(j). The binding energy of the O 1s peak for the as-grown film, around 532.0 eV, is attributed to the opaque Zn(OH)2 phase, and the O 1s binding energies in the range 529.7–530.6 eV can be due to Zn O bond formation [31–33] and 533 eV due to Si O bond formation[34,35] In the spectrum of the ZnO films deposited at 75 ◦ C, the XPS spectra shows strong intensity of Si O bonds and very weak peak of
Fig. 11. ZnO XPS multiplex spectra of the O 1s of the deposited ZnO films at rinsing temperatures of (a) 75 ◦ C, (b) 85 ◦ C, (c) 95 ◦ C, (d) 105 ◦ C, (e) 125 ◦ C, (f) 145 ◦ C, (g) 165 ◦ C, (h) 180 ◦ C, (i) deposited at 95 ◦ C with post annealing and (j) deposited at 165 ◦ C with post annealing.
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can be used to optimize SILAR methods for fabricating high-quality semiconductor thin films that have potential applications in optoelectronics. Acknowledgments The authors would like to thank the Bureau of Energy, Ministry of Economic Affairs of Taiwan, ROC. for the financial support under contract no. 101-D0204-6 and the LED Lighting Research Center of NCKU for the assistance of device characterization, and the National Science Council of Taiwan, ROC., under contract no. NSC 99-2221E-024-009 and NSC 98-3114-E009-002-CC2. References Fig. 12. The atom ratio of O/Zn calculated from the XPS spectra as a function of the samples.
Zn OH bond, it indicates that thin films was difficult to deposited at low temperature, this result is in agreement with the above analysis, When the temperature at 85 and 95 ◦ C (Fig. 11), high-intensity peaks due to Zn OH bonds and relatively low intensity peaks due to Zn O bonds could be seen, confirming that excess Zn(OH)2 existed in the ZnO film deposited at lower temperatures. On the contrary, when the rinsing temperature exceeded 125 ◦ C, weak peaks due to Zn OH bonds and relatively stronger peaks due to Zn O bonds were observed; this could be explained by the decomposition of Zn(OH)2 at higher temperatures. In Fig. 11(i)–(j), the samples with post annealing treatment could enhance the Zn O bonds and decrease the Zn OH bonds. Furthermore, from the XPS spectra, the atom ratio of O/Zn could be obtained and shown in Fig. 12. The atom ratio of O/Zn decreased with increasing the temperature of treatment. This results indicated that the films were O deficient at lower temperature treatments due to the high content of Zn(OH)2 as show in Fig. 11. As samples with post annealing treatment, the atom ratio of O/Zn decreases to close to 1 due to the decomposition of Zn(OH)2 hydrolysis at high temperature annealing treatment and similar to the results of high-temperature treatments in ethylene glycol. The results of our XPS measurements were consistent with the previously discussed PL data. These observations indicated that the PL and XPS characteristics of the ZnO thin films prepared by SILAR are strongly dependent on the temperature treatments. 4. Conclusion ZnO thin films obtained by SILAR at different temperature treatments in ethylene glycol (75–180 ◦ C) were investigated. With an increase in the temperature, preferential growth occurred along the c-axis, and high-intensity peaks due to the (0 0 2) plane were observed in the XRD spectrum; further, the obtained films had a smooth surface and showed good transparency in the visible spectral range. The films deposited at temperatures below 95 ◦ C showed a rapid decrease in their transparency, eventually becoming white. At very high temperatures (>145 ◦ C), only slight enhancement of the crystal quality was observed and the growth rate decreased. PL and XPS data revealed that the temperature treatment significantly affects the distribution of defects in the films. At low temperatures, a large number of these defects, mostly oxygen defects attributable to Zn(OH)2 in the films, were observed. However, the concentration of these defects reduced at high temperatures. Thus, we concluded that the quality and the structural and optical properties of the thin films depended strongly on the temperature treatment. Furthermore, temperature treatment was confirmed to play an important role in ZnO film deposition instead of post thermal annealing after the film growth. We are confident that the results of our study
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