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Synthesis and characterization of nanostructured ZnO multilayer grown by DC magnetron sputtering
Journal of Alloys and Compounds 602 (2014) 108–116
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Synthesis and characterization of nanostructured ZnO multilayer grown by DC magnetron sputtering Marzieh Shirazi a,⇑, M.T. Hosseinnejad b, Akbar Zendehnam c, Mahmood Ghoranneviss b, G. Reza Etaati d a
Department of Material Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran c Thin Film Laboratory, Department of Physics, Arak University, Arak, Iran d Nuclear Engineering and Physics Department, Amir Kabir University of Technology, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 8 November 2013 Received in revised form 9 February 2014 Accepted 5 March 2014 Available online 15 March 2014 Keywords: Magnetron sputtering ZnO multilayer PL XRD AFM
a b s t r a c t A novel method is presented for fabrication of zinc oxide nanolayers (ZnO) by coating multiple layers of ZnO through thermal oxidation of zinc (Zn) films on a glass substrate in multiple steps. This technique reduces temperature and time of thermal oxidation, and can be used for coating oxidant materials on low melting point substrates. In several experiments aiming at depositing a ZnO thin film with 300 nm thickness, deposition took place in a single step, two steps and three steps. Each step involved thermal oxidation of the Zn layer with 300 nm, 200 nm, or 100 nm thickness. The structural, morphological, optical and mechanical properties of the ZnO samples deposited in each experiment are investigated. The results of photoluminescence (PL) analysis and Urbach energy measurements indicate that higher transmittance and lower structural defects can be achieved with the samples produced in three steps, each involving deposition and oxidation of a Zn layer with 100 nm thickness. X-ray diffraction (XRD) analysis demonstrates that the degree of crystallinity of the deposited ZnO thin films strongly depends on the deposition steps. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) image analyses show that the size of grains and surface roughness of ZnO multilayers are lower than ZnO monolayer. Moreover, the mechanical properties of samples improved with increasing the deposition steps. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Deposition of thin layers of transparent conductive oxides has potential applications in many optoelectronic devices such as solar cells, flat panel displays, LCDs, surface acoustic wave (SAW) devices, optical waveguides, photonic devices, blue or ultra-violet (UV) LEDs, laser diodes (LD), resonators in wireless communication, and gas sensors [1–4]. Among the transparent conductive oxides, zinc oxide (ZnO) is a promising material in the development of the emerging technology of optoelectronic devices, mainly due to its desirable physical properties such as high transparency in visible and near infrared spectral region, high band gap energy (3.26 eV) and large exciton binding energy (60 meV) at room temperature [5–7]. Doping suitable elements into ZnO films is well known to be an effective method to engineer the electrical and optical properties of the films. Doped ZnO has received considerable attention due to its
⇑ Corresponding author. Tel.: +98 9124118622; fax: +98 02144869624. E-mail address: [email protected] (M. Shirazi). http://dx.doi.org/10.1016/j.jallcom.2014.03.029 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
improved performance in electrical conductivity and optical transmittance in the visible region. Recently, many elements such as Al [8], Mg [9], Ga [10], Yb [11], Ag [12], Ti [13] and Cu [14] have been doped or alloyed into ZnO film and desirable properties have been obtained. Due to their desirable applications, deposition of ZnO thin films has attracted significant interest from the research community, and a large variety of methods have been proposed, including radio frequency (RF) magnetron sputtering [15,16], atomic laser deposition (ALD) [17], pulsed laser deposition (PLD) [18], chemical vapor deposition (CVD) [19], sol–gel [20,21] and plasma-assisted molecular beam epitaxy (MBE) [22], to name a few. Among the current deposition techniques, the RF magnetron sputtering method has recently become popular due to its high deposition rate, growing at relatively low substrate temperature, high stability for largearea films and a suitable adhesion of the thin films on the substrate. Although a high degree of development and control has been achieved with sputtering, the effect of sputtering parameters (e.g. work pressure and gas composition, substrate temperature, the distance between target and substrate, and DC or RF power) on deposited film properties is still under investigation. Any change in the conditions of vacuum-deposited ZnO films will cause a change in
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the stoichiometry of ZnO, influencing its crystalline structure and optical properties. By adjusting growth conditions of ZnO thin films, e.g. impurity doped, substrate temperature, oxygen partial pressure and annealing process (annealing temperature, ambient and procedure), the structural and optical properties of films can be improved. However, the effect of each sputtering parameter on the properties of the deposited film has not been fully investigated yet, and a clear methodology for choosing the optimal parameters for best optical transparency and electrical conductivity is yet to appear in the thin film literature. Recently, Zhou et al. [23] investigated the effects of oxygen/argon ratio and annealing on structural and optical properties of the ZnO thin films deposited using the RF magnetron sputtering technique. Besleagaa et al. [24], deposited transparent ZnO films by RFmagnetron sputtering (1.78 MHz) onto glass substrates, using a mild-pressed ZnO powder target. They deposited ZnO thin films as a double-layer prepared at 0.3 Pa and at a temperature of 400 °C. The result had a bottom layer consisting that was highly depleted of oxygen in its ZnO structure, and a 53 nm top layer of textured ZnO. This paper presents a novel extension to the method of Besleagaa et al. [24] that would prevent the depletion of oxygen in the bottom layer. A minor difference between the proposed technique and the method of [24] is in utilization of DC (instead of RF) magnetron sputtering and using a Zn target (instead of ZnO target). The major contribution is in the idea to repeatedly deposit multiple layers of ZnO in several steps, each involving a lower amount of Zn as compared to deposition of the whole layer thickness in one go. With a smaller amount of Zn in each layer, oxidation is expected to be achieved at a lower temperature (small enough to prevent the depletion of oxygen) and in a shorter period of time. This hypothesis was validated via a comparative experimental study. The thin films prepared by deposition in one step, two steps and three steps (but with the same total thickness) were investigated for their different characteristics including structural and optical properties, surface morphology and hardness. The results demonstrate that the multilayer thin films deposited in three steps had generally more desirable properties in terms of transmittance, structural defects, crystallinity, surface roughness and hardness. 2. Experimental details Fig. 1 shows a schematic diagram of the DC magnetron sputtering system used in our experiments. A vacuum system (Hind High Vacuum, H.H.V, 1200 MSPT) with base pressure of 106 mbar was used, and a circular flat disc (thickness 3 mm, and diameter 125 mm) of pure zinc (99.9%) was made and employed as the sputtering target. Research grade argon (purity 99.9%) was employed at pressures 103–102 mbar for plasma formation in a vacuum bell jar. Before Zn deposition, the plasma discharge was run for a few minutes, and to ensure a high purity coating, the produced plasma was checked by a spectrometer with high resolving power. Deposition conditions and discharge current were optimized to obtain continuous, uniform, and densely packed thin films with the different thicknesses. All samples were deposited under the same experimental conditions. Circular glass substrates (thickness 1 mm and diameter 20 mm) were cleaned both ultrasonically and using alcohol for a period of 5 min, then with acetone for 10 min. The coating rate and the film thickness were measured by a vibrating quartz thickness monitor (1.95 nm/s), and the substrate temperature was monitored by an exact digital thermocouple (namely 300 K). For thermal oxidation of the Zn thin films and annealing of ZnO nanostructure, a conventional oven in open air was used. For optical measurements, a double beam spectrophotometer (Camspec model M350) with wavelength range of 200–1100 nm was employed. The transmittance (T), absorption coefficient (a) and their variations with wavelength were measured, and variations of the optical band gap (Eg) and Urbach energy with temperature were investigated. Photoluminescence (PL) spectra (Stellar Net EPP-200) were recorded at room temperature. The excitation wavelength was 340 nm with a xenon lamp used as the excitation source. The crystalline structure of the films was investigated using a Philips diffractometer (Xpert pw3373) with Cu Ka radiation, angle step size of 0.01°, and count time of 1.0 s per step. Morphological properties of the deposited samples were investigated by scanning electron microscopy (SEM, Hitachi S-4160) and atomic force microscopy (AFM, Auto Probe Pc; in contact mode, with low stress silicon nitride tip of less than 200 Å radius and tip opening of
Fig. 1. Schematic of DC magnetron sputtering system.
Table 1 Characterization of deposition and oxidation of S1, S2 and S3 samples. Sample
Characterization
Zn thickness (nm)
Temperature of oxidation (°C)
S1 S2 S3
1 layer of Zn (1-step) 2 layer of Zn (2-step) 3 layer of Zn (3-step)
300 200 + 100 100 + 100 + 100
400 400 400
18°) analysis. Hardness was measured using nanoindenter XP (MTS) under the continuous stiffness measurement (CSM) mode. For more certitude, the indentation test was repeated for five times on each sample.
3. Results and discussion To investigate the effect of coating in multiple layers, on the optical and structural properties of the deposited ZnO films, three samples, named S1, S2 and S3, are produced. The sample properties are specified in Table 1. They are deposited under similar conditions in terms of deposition rates, Ar pressure, discharge current, and voltage but in different steps. More precisely, S1, S2 and S3 samples are produced by deposition of Zn layer in one step, two steps, and three steps, respectively, and oxidation process is carried out after every step of deposition of Zn layer. The steps of deposition and oxidation for all samples are graphically detailed in Fig. 2. After deposition of ZnO thin films, the structural properties of the deposited samples are investigated by XRD analysis. XRD patterns of the all deposited samples are presented in Fig. 3. The respective positions of ZnO diffraction peaks show that all the films are polycrystalline with ZnO hexagonal wurtzite structure. The diffraction peaks of ZnO (1 0 0), ZnO (0 0 2), ZnO (1 0 1) and ZnO (1 1 0) crystalline planes are observed at 2h values of 31.8°, 34.4°, 36.2° and 56.8°, respectively. The respective positions of the diffraction peaks are in agreement with joint committee for powder diffraction standards (JCPDS) standard data for ZnO powders (refer to JCPDS card No. 00-003-0888). XRD results reveal the successful growth of deposited crystalline ZnO thin films on glass substrates for all three samples.
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Fig. 2. Schematic of production steps of S1, S2 and S3 samples.
Fig. 3. XRD spectra of S1, S2 and S3 samples.
Fig. 4 shows the relative peak intensity of ZnO (1 0 0), ZnO (0 0 2), ZnO (1 0 1) and ZnO (1 1 0) crystalline planes for obtained samples with various deposition steps. From Fig. 4, it is observable that the degree of crystallinity of the deposited thin films depends on the number of deposition steps. Indeed, the degree of crystallinity of the ZnO deposited samples, for all crystalline planes, increases by increasing the number of deposition steps. The grain size of crystallites is estimated using Scherer formula [25]:
D¼
0:9k B cosðhÞ
ð1Þ
where D is the average grain size, k is the incident X-ray wavelength (k = 1.54 Å, Ka (Cu)), B is the full half width at maximum (FWHM),
Fig. 4. Variations of the relative intensities of various ZnO diffraction peaks for obtained samples with different deposition steps.
and h is the diffraction angle. The results are presented in Table 2. They indicate that average grain size of samples decreases for deposited thin films with more steps of coating and oxidation. Fig. 5 presents optical transmittance of the deposited ZnO samples, in the UV–Vis wavelength (200–1100 nm) range, before and after post annealing. The results shown in Fig. 5a demonstrate that larger optical transmittance is achieved for the samples deposited in more steps, before post annealing. To justify the increase in
M. Shirazi et al. / Journal of Alloys and Compounds 602 (2014) 108–116 Table 2 Variations of the crystallite size and band gap energy (Eg), for S1, S2 and S3 samples. Sample identity
Crystallite size (nm)
Eg (eV)
S1 S2 S3
34 26 23
3.251 3.253 3.254
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conditions. We calculated band gap energy (Eg) for all samples from their transmittance spectra. We first calculated the absorption coefficient, given by [28]:
1 d
a ¼ ln
T0 T
ð2Þ
where T and T0 are the transmittance of the ZnO thin film samples and the substrate (glass) without coating, respectively, and d is the thin film thickness. The Eg was then calculated using Tauc’s relationship [28]: 2
ðahmÞ ¼ Aðhm Eg Þ
ð3Þ
where A is a constant, hm is the photon energy, and Eg is band gap energy. To calculate Eg a graph of (ahm)2 against hm is plotted (see Fig. 6). Extrapolating the straight linear portion of this plot to the photon energy (hm) axis would give the optical band gap value. The band gap energy values, determined from the curves shown in Fig. 6 are presented in Table 2. They are in agreement with the values reported in the literature so far [29]. From presented values in Table 2 it is observable that Eg increases when the material is formed by crystallites sufficiently small which this behaviour can be attributed to some extent quantum confinement [30]. Another important property of the deposited thin layer of ZnO, in the context of semiconductor technology application, is the Urbach energy. During the last decades, Urbach energy has been attributed to the thermally induced structural disorder. Dutta et al. [31] investigated the relationship between structural defects and disorders and Urbach energy of ZnO thin films. They indicated that any defect or disorder in the system gives rise to localized energy levels within the band gap or discrete states. Ilicana et al. [32] showed that Urbach energy of a ZnO thin film varies with its annealing temperature. On the other hand, Pejova et al. [33,34] found out that a direct (but inverse) relationship between the structural disorder and Urbach energy. They made the observation that in terms of lattice strain relaxation, the lattice constant increases with decreasing of dislocation density. This was further verified by Cody et al. [35] who suggested that Urbach energy comprises two components: one that is temperature dependent and justifies Urbach energy variations with annealing temperature (and thermal disorder), and another component that mainly depends on the inherent structural disorder. Mathematically, Urbach energy is modeled as [35]: Fig. 5. Optical transmittance spectra of S1, S2 and S3 samples at (a) 400 °C and (b) 750 °C.
transmittance, we note that the film deposited in more steps receives more oxygen. Fig. 5b demonstrates that after post annealing of the ZnO thin films, optical transmittance of S1 and S2 samples were decreased, while optical transmittance of S3 sample did not vary. The reduction of optical transmittance can be due to escape or desorption of oxygen molecules from the grain boundaries at temperatures over 500 °C [26,27]. The cut-off behaviour at the blue end of the optical spectrum is determined by direct electronic transitions from the valance band to the conduction band. Therefore, the direct optical band gap (Eg) can be determined from transmission data measured at short wavelengths. An important material property, in the context of semiconductor technology and applications, is the band gap energy. It is exactly this point that allows the nanotechnologists to design a semiconducting material with predefined (i.e. designed) optical and electrical properties (such as suitable band gap energy) by simply controlling the crystal size. For example, the band gap energy of various semiconducting materials can nowadays be tuned rather precisely by an appropriate choice of the synthetic route, such as the controllable precipitation under strictly defined
Eu ðT; XÞ ¼
H 1þX
ro
2
þ
1 eH=T 1
ð4Þ
where ro is the Urbach edge parameter of the order of unity, X is a measure of the degree of structural disorder of the material, and H is the Einstein temperature. X is defined as:
X¼
hU 2X i hU 2 i0
ð5Þ
where hU 2X i is the mean square deviation of atomic displacements caused by the actual structural disorder and hU2i0 is the zero-point uncertainty in the atomic positions. The Einstein temperature H is related to the Debye temperature (hD) by the following relationship [36]:
3 4
H ¼ hD
ð6Þ
Based on the above mentioned model, for a given temperature, Urbach energy only depends on the level of structural disorder. It is important to note that a higher post annealing temperature not only contributes to Urbach energy directly (through the temperature-dependent term) but also causes higher structural disorders, especially in high temperatures (over 550 °C) at which evaporation
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Fig. 6. Variation of (ahm)2 with photon energy (hm) for S1, S2 and S3 samples at 400 °C. The inset shows the variation of ln(a) against hm at 400 °C for S1, S2 and S3 samples.
of oxygen produces oxygen vacancies as well as desorption of oxygen molecules from the grain boundaries. In our experiments, we have computed Urbach energy and investigated its variations with the number of deposition steps and for different temperatures. According to Urbach–Martienssen rule, for the range of energies hm < Eg, the optical absorption coefficient of a semiconductor follows the exponential form [37]:
a ¼ ao exp
hm Eo Eu
ð7Þ
where ao and Eo are constant, and Eu is the Urbach energy. Rewriting Eq. (7) as:
Eo 1 þ hm lnðaÞ ¼ lnðao Þ Eu Eu
ð8Þ
we note that Urbach energy, Eu, can be calculated by computing the inverse slope of the graph of ln(a) plotted versus photon energy (hm) for hm < Eg. The plots of ln(a) against hm for ZnO thin films at 400 °C (temperature of oxidation) in the sub-band gap region of linear dependency of ln(a) on hm are shown in Fig. 6. The values of Eu (computed from the slope of the plots shown in Fig. 6) vary from 149 meV to 114 meV then to 80 meV for S1, S2 and S3 samples, respectively. Urbach energy is well known to mainly depend on temperature and the degree of structural disorder of the thin films [34,35,37]. A smaller Eu value indicates less structural defects. The calculated Urbach energy values show that the S3 sample (which is prepared in three steps of deposition) has lower Eu and less structural defect. We have also investigated the effect of annealing temperature on the structural quality of the thin film (via it Eu value). Fig. 7 shows Eu values computed for various post annealing temperatures for all samples. Those values are observed to generally rise with temperature. This is expected as the structural defects and thermal disorders in the ZnO film intensify by post annealing at higher temperatures [26]. Once again, the Eu value for S1 sample is observed to be higher than S2 sample which in turn higher than S3 sample, demonstrating that the structural quality advantage gained via depositing in multiple steps is still applicable after post annealing at high temperatures.
The results obtained from XRD analysis and Urbach energy measurements show that with increasing the average crystal size, the Urbach energy decreases (see Figs. 3 and 7). This is expected from Cody model [35] (see Eq. (4)) in which Eu linearly depends on the degree of structural disorder. Indeed, when an increase in the average crystal size is followed by the relaxation of the lattice strain and the degree of structural disorder and this decreases Urbach energy [34,35]. Fig. 8 shows room temperature photoluminescence spectra for all of the samples with temperature of oxidation at T = 400 °C. This plot indicates a UV emission at 380 nm (3.26 eV) that is known as near-band-edge emission (NBE) and originates from the recombination of free exciton through an exciton collision process, and a green emission at 497 nm (2.49 eV) which is known as deep level (DL) emission and appears due to the radial recombination of a photo-generated hole with ionized oxygen vacancies (Vo) [38,39]. Although most of the practical optoelectronic devices of ZnO thin films are devised and assessed commonly based on their stability in the excitonic emission process, violet and UV luminescence from ZnO thin films are usually endured from the DL
Fig. 7. Urbach energy as a function of post annealing temperature for S1, S2 and S3 samples.
M. Shirazi et al. / Journal of Alloys and Compounds 602 (2014) 108–116
Fig. 8. Room temperature PL spectra of the ZnO thin films.
emission. Some theoretical and experimental investigations indicate that low content of native donor defects in ZnO, e.g. interstitial zinc (Zni) and vacancy oxygen (Vo), would lead to shallow donor levels partially ionized and depress the DL emission at RT [40,41]. The DL emission could be attributed to the oxygen vacancies or zinc interstitials [42–44]. The DL emission leads to decreasing of carrier/exciton lifetime and emission efficiency in the UV light devices. The results obtained from PL analysis reveal that the intensity of the NBE and DL emission at the S3 sample, which is produced in three steps, is lower than S1 and S2 samples. This
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result indicates that increasing of deposition steps leads to a lower rate of defects and better crystal quality in ZnO thin films which is in agreement with the results from Urbach energy measurements. Fig. 9 shows the comparison of surface morphologies of the ZnO deposited thin films with various deposition steps. The images presented in Fig. 9 indicate that the magnitude of grain boundaries in the ZnO monolayer thin film reduces and thus grains become larger in this sample with respect to deposited samples with more layers. On the other hand, the cross-sectional image of S1 sample indicates that voids are generated in the bottom part of S1 sample, and grain size on top of this film is larger than that of the bottom of the film. In fact, the bottom part of S1 sample does not receive enough oxygen in one step of oxidation, while S2 and S3 samples receives more oxygen in two steps and three steps of oxidation, respectively. Therefore, they have lower voids and larger grain boundaries. The surface morphology of the S1, S2 and S3 samples has also been investigated using atomic force microscope (AFM). The 2D and 3D surface topography of the deposited thin films are presented in Fig. 10. All the images have been obtained with a scanning area of 5 lm 5 lm. The AFM images reveal that the size of the grains/clusters on the surface of thin films decrease with increasing the number of layers (see Fig. 10). This observation is perfectly consistent with the previous conclusion that the average crystallite size of ZnO phase decreases with increasing the deposition steps which was observed from XRD and SEM results. Fig. 10 also shows histograms of grain sizes on the surface of the S1, S2 and S3 deposited thin films. Fitting Gaussian curves to the histograms reveals that the full width at half maximum (FWHM) of curves decreases with increasing of deposition steps (see Table 3). The decreasing of FWHM of curves shows that distribu-
Fig. 9. SEM images of S1, S2 and S3 ZnO thin films.
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Fig. 10. 2D and 3D AFM micrographs and histograms of distribution of size of grains on the surface of S1, S2 and S3 samples.
Table 3 FWHM and peak point position of the Gaussian curves fitted on the histograms of distribution of grain sizes.
Table 4 Variation of average and rms roughness measured by AFM analysis, for S1, S2 and S3 samples.
Sample identity
FWHM (nm)
Xc (nm) (peak point)
Sample identity
Ave roughness (nm)
RMS roughness (nm)
S1 S2 S3
93.2 42.4 40.5
132.7 69.8 64.1
S1 S2 S3
33.7 15.9 14.5
42.5 20.4 18.4
tion of grain sizes on the surface of deposited samples is more homogeneous in sample deposited in more steps. Moreover, the histograms show that the grain size generally decreases with increasing the deposition steps. This is evident from the modes of the histograms being 133 nm, 70 nm and 64 nm for samples deposited in a single step, two steps and three steps, respectively. To analyze and compare the surface roughness of the S1, S2 and S3 samples, in each experiment, we measured the roughness of five random areas over the surface of the deposited sample and recorded the average and root mean square (rms) values of the roughness measurements as presented in Table 4. They demonstrate less roughness on the surface of the deposited samples with more deposition steps. As it was already mentioned in SEM results, with increasing the deposition steps, the size of the grains becomes smaller, hence less roughness is achieved on the surface of the film. Moreover, as it is observable from the histograms of distribution of grain sizes on the surface of S1, S2 and S3 samples, the size of most of the grains decrease with increasing the deposition steps. Similarly, this leads to less surface roughness (see Fig. 10). In order to assess the mechanical properties of the coated ZnO thin films, nanoindentation measurements were taken. Fig. 11 shows the hardness and modulus of the S1, S2 and S3 samples that were determined from 50 nm to 60 nm penetration depth. This indentation depth was selected to cause adequate plastic deformation during indentation but to avoid the substrate effect [45]. From Fig. 11 it is observable that the hardness of the samples increases when the deposition steps increase from one to three steps. As for the modulus of the ZnO samples, the trend of its variation with respect to the deposition steps is fairly consistent with that of the hardness. The increased hardness can also be speculated from the XRD patterns shown in Fig. 3 which shows the crystalline phase of the zinc oxide increases when the deposition steps
Fig. 11. Hardness and modulus of prepared ZnO thin films with different deposition steps.
increase from one to three steps. Moreover, the hardness results can be justified by the Hall–Petch formula [46]:
H ¼ Ho þ kd
1=2
ð9Þ
where Ho is the intrinsic hardness of a single crystal, d is the grain size, k is a constant that depends on the metal alloy composition and H is the hardness. The reduction in the size of grains leads to the reduction of grains boundary voids and hence an increase in the hardness of the coating [47]. AFM analysis showed that the size of grains decreases with more deposition steps. Thus, the results obtained from hardness measurements are in reasonable agreement with AFM results.
M. Shirazi et al. / Journal of Alloys and Compounds 602 (2014) 108–116
4. Conclusion ZnO thin films deposition with 300 nm thicknesses were carried out by multilayer method at one, two and three steps with repeated coating of the Zn film followed by thermal oxidation, with 100, 200, and 300 nm thicknesses. The optical and structural characteristics of the prepared samples were studied by XRD, SEM, AFM, PL and UV–Vis spectroscopy analysis. From morphological point of view, the SEM and AFM micrographs showed that grain size and surface roughness of the deposited samples decrease with increasing the deposition steps. On the other hand, cross-sectional view SEM images of ZnO thin films indicated that voids are generated in the bottom part of ZnO monolayer (S1 sample), and grain size on top of this film is larger than the bottom of the film while ZnO multilayers (S2 and S3 samples) have less voids and grain size on top and bottom of films are almost same. In addition, optical results suggested that ZnO multilayers have a good optical stability and lower defects because of less intensity of the NBE and DL emission at PL spectra and Urbach energy in comparison with ZnO monolayer. These optical results are in agreement with the results obtained from XRD, SEM and AFM analysis. Furthermore, we observed from the hardness measurements that with more deposition steps, ZnO thin films with better mechanical properties are achieved. The proposed multi-step deposition scheme can be applied for coating a ZnO multilayer on not only a glass substrate but also on substrates with lower melting point such as plastics, polycarbonate (PC) and plexiglass which are used in different applications, and for fabricating of optical devices such as optical waveguides with uniform surface, large optical transmission and lower structural defects. However, depending on the melting point of the substrate material, one may need to modify the method to involve more deposition steps each occurring at lower oxidation temperatures. Investigation of such modifications and the properties of the resulting thin films is a suitable topic for further research in the future. Acknowledgment The first author (M. Shirazi) gratefully acknowledges Dr. Reza Hoseinnezhad at RMIT University, Australia, for his valuable suggestions to improve the manuscript. References [1] H. Huang, G. Fang, S. Li, H. Long, X. Mo, H. Wang, et al., Ultraviolet/orange bicolor electroluminescence from an n-ZnO/n-GaN isotype heterojunction light emitting diode, Appl. Phys. Lett. 99 (2011) 263502. [2] J. Bao, M.A. Zimmler, F. Capasso, X. Wang, Z.F. Ren, Broadband ZnO singlenanowire light-emitting Diode, Nano Lett. 6 (2006) 1719–1722. [3] L. Aghli-Moghadam, A. Baghizadeh, G. Nabiyoni, A. Farashiani, A. Zendehnam, Zn-diffused LiNbO3 waveguides fabricated by DC magnetron sputtering, Appl. Phys. A 97 (2009) 805–810. [4] M. Girtan, G.G. Rusu, S. Dabos-Seignon, M. Rusu, Structural and electrical properties of zinc oxides thin films prepared by thermal oxidation, Appl. Surf. Sci. 254 (2008) 4179–4186. [5] K. Kim, Y.W. Song, S. Chang, I.H. Kim, S. Kim, S.Y. Lee, Fabrication and characterization of Ga-doped ZnO nanowire gas sensor for the detection of CO, Thin Solid Films 518 (2009) 1190–1193. [6] F.A. Mahmoud, G. Kiriakidis, Nanocrystalline ZnO thin film for gas sensor application, J. Ovonic Res. 5 (2009) 15–20. [7] R. Singh, M. Kumar, S. Chandra, Growth and characterization of high resistivity c-axis oriented ZnO films on different substrates by RF magnetron sputtering for MEMS applications, J. Mater. Sci. 42 (2007) 4675–4683. [8] Z. Pan, X. Tian, S. Wu, X. Yu, Z. Li, J. Deng, et al., Investigation of structural, optical and electronic properties in Al–Sn co-doped ZnO thin films, Appl. Surf. Sci. 265 (2013) 870–877. [9] Z.Q. Ma, W.G. Zhao, Y. Wang, Electrical properties of Na/Mg co-doped ZnO thin films, Thin Solid Films 515 (2007) 8611–8614. [10] V. Bhosle, A. Tiwari, J. Narayan, Metallic conductivity and metalsemiconductor transition in Ga-doped ZnO, Appl. Phys. Lett. 88 (2006) 032106.
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Chiou, Thickness effect on luminescent properties of sol–gel derived ZnO thin films, J. Lumin. 136 (2013) 160–164. [22] B. Zhang, B. Yao, S. Wang, Y. Li, C. Shan, J. Zhang, et al., Influence of Zn/O ratio on structural, electrical and optical properties of ZnO thin films fabricated by plasma-assisted molecular beam epitaxy, J. Alloys Comp. 503 (2010) 155–158. [23] B. Zhoua, A.V. Rogachev, Z. Liu, D.G. Piliptsoua, H. Ji, X. Jiang, Effects of oxygen/ argon ratio and annealing on structural and optical properties of ZnO thin films, Appl. Surf. Sci. 258 (2012) 5759–5764. [24] C. Besleaga, G.E. Stan, A.C. Galca, L. Ion, S. Antohe, Double layer structure of ZnO thin films deposited by RF-magnetron sputtering on glass substrate, Appl. Surf. Sci. 258 (2012) 8819–8824. [25] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, vol. 3, Prentice Hall, NewJersey, 2001. [26] S.Y. Chu, W. Water, J.T. Liaw, Influence of postdeposition annealing on the properties of ZnO films prepared by RF magnetron sputtering, J. Euro. Ceram. Soc. 23 (2003) 1593–1598. [27] N. Croitoru, A. Seidman, K. Yassin, Some physical properties of ZnO sputtered films, Thin Solid Films 150 (1987) 291–301. [28] Z.W. Li, W. Gao, R.J. Reeves, Zinc oxide films by thermal oxidation of zinc thin films, Surf. Coat. Technol. 198 (2005) 319–323. [29] N. Ekem, S. Korkmaz, S. Pat, M.Z. Balbag, E.N. Cetin, M. Ozmumcu, Some physical properties of ZnO thin films prepared by RF sputtering technique, Int. J. Hydrogen Energy 34 (2009) 5218–5222. [30] E.M. Wong, P.C. Searson, ZnO quantum particle thin films fabricated by electrophoretic deposition, Appl. Phys. Lett. 74 (1999) 2939–2941. [31] S. Dutta, S. Chattopadhyay, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Role of defects in tailoring structural, electrical and optical properties of ZnO, Prog. Mater. Sci. 54 (2009) 89–136. [32] S. Ilicana, Y. Caglar, M. Caglar, F. Yakuphanoglu, Electrical conductivity, optical and structural properties of indium-doped ZnO nanofiber thin film deposited by spray pyrolysis method, Physica E 35 (2006) 131–138. [33] B. Pejova, B. Abay, I. Bineva, Temperature dependence of the band-gap energy and sub-band-gap absorption tails in strongly quantized ZnSe nanocrystals deposited as thin films, J. Phys. Chem. C 114 (2010) 15280–15291. [34] B. Pejova, The Urbach–Martienssen absorption tails in the optical spectra of semiconducting variable-sized zinc selenide and cadmium selenide quantum dots in thin film form, Mater. Chem. Phys. 119 (2010) 367–376. [35] G.D. Cody, T. Tiedje, B. Brooks, Y. Goldestein, Disorder and the opticalabsorption edge of hydrogenated amorphous silicon, Phys. Rev. Lett. 47 (1981) 1480–1483. [36] B. Abay, H.S. Guder, H. Efeoglu, Y.H. Yogurtcu, Excitonic absorption and Urbach–Martienssen tails in Gd-doped and undoped p-type GaSe, Semicond. Sci. Technol. 15 (2000) 535. [37] F. Urbach, The long-wavelength edge of photographic sensitivity and of the electronic absorption of solids, Phys. Rev. B 92 (1953) 1324. [38] P.K. Ooi, S.S. Ng, M.J. Abdullah, Z. Hassan, Structural and optical properties of In-doped ZnO thin films under wet annealing, Mater. Lett. 116 (2014) 396– 398. [39] J. Lim, K. Shin, H.W. Kim, C. Lee, Effect of annealing on the photoluminescence characteristics of ZnO thin films grown on the sapphire substrate by atomic layer epitaxy, Mater. Sci. Eng. B 107 (2004) 301–304. [40] E. Vasco, O. Bohme, E. Roman, Chemical characterization of ZnO films pulsed laser deposited on InP, J. Phys. Chem. 111 (2007) 3505–3511. [41] R. Deng, B. Yao, Y.F. Li, T. Yang, B.H. Li, Z.Z. Zhang, et al., Influence of oxygen/ argon ratio on structural, electrical and optical properties of Ag-doped ZnO thin films, J. Cryst. Growth 312 (2010) 1813–1816.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Synthesis and characterization of nanostructured ZnO multilayer grown by DC magnetron sputtering Marzieh Shirazi a,⇑, M.T. Hosseinnejad b, Akbar Zendehnam c, Mahmood Ghoranneviss b, G. Reza Etaati d a
Department of Material Engineering, Isfahan University of Technology, 84156-83111 Isfahan, Iran Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran c Thin Film Laboratory, Department of Physics, Arak University, Arak, Iran d Nuclear Engineering and Physics Department, Amir Kabir University of Technology, Tehran, Iran b
a r t i c l e
i n f o
Article history: Received 8 November 2013 Received in revised form 9 February 2014 Accepted 5 March 2014 Available online 15 March 2014 Keywords: Magnetron sputtering ZnO multilayer PL XRD AFM
a b s t r a c t A novel method is presented for fabrication of zinc oxide nanolayers (ZnO) by coating multiple layers of ZnO through thermal oxidation of zinc (Zn) films on a glass substrate in multiple steps. This technique reduces temperature and time of thermal oxidation, and can be used for coating oxidant materials on low melting point substrates. In several experiments aiming at depositing a ZnO thin film with 300 nm thickness, deposition took place in a single step, two steps and three steps. Each step involved thermal oxidation of the Zn layer with 300 nm, 200 nm, or 100 nm thickness. The structural, morphological, optical and mechanical properties of the ZnO samples deposited in each experiment are investigated. The results of photoluminescence (PL) analysis and Urbach energy measurements indicate that higher transmittance and lower structural defects can be achieved with the samples produced in three steps, each involving deposition and oxidation of a Zn layer with 100 nm thickness. X-ray diffraction (XRD) analysis demonstrates that the degree of crystallinity of the deposited ZnO thin films strongly depends on the deposition steps. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) image analyses show that the size of grains and surface roughness of ZnO multilayers are lower than ZnO monolayer. Moreover, the mechanical properties of samples improved with increasing the deposition steps. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Deposition of thin layers of transparent conductive oxides has potential applications in many optoelectronic devices such as solar cells, flat panel displays, LCDs, surface acoustic wave (SAW) devices, optical waveguides, photonic devices, blue or ultra-violet (UV) LEDs, laser diodes (LD), resonators in wireless communication, and gas sensors [1–4]. Among the transparent conductive oxides, zinc oxide (ZnO) is a promising material in the development of the emerging technology of optoelectronic devices, mainly due to its desirable physical properties such as high transparency in visible and near infrared spectral region, high band gap energy (3.26 eV) and large exciton binding energy (60 meV) at room temperature [5–7]. Doping suitable elements into ZnO films is well known to be an effective method to engineer the electrical and optical properties of the films. Doped ZnO has received considerable attention due to its
⇑ Corresponding author. Tel.: +98 9124118622; fax: +98 02144869624. E-mail address: [email protected] (M. Shirazi). http://dx.doi.org/10.1016/j.jallcom.2014.03.029 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
improved performance in electrical conductivity and optical transmittance in the visible region. Recently, many elements such as Al [8], Mg [9], Ga [10], Yb [11], Ag [12], Ti [13] and Cu [14] have been doped or alloyed into ZnO film and desirable properties have been obtained. Due to their desirable applications, deposition of ZnO thin films has attracted significant interest from the research community, and a large variety of methods have been proposed, including radio frequency (RF) magnetron sputtering [15,16], atomic laser deposition (ALD) [17], pulsed laser deposition (PLD) [18], chemical vapor deposition (CVD) [19], sol–gel [20,21] and plasma-assisted molecular beam epitaxy (MBE) [22], to name a few. Among the current deposition techniques, the RF magnetron sputtering method has recently become popular due to its high deposition rate, growing at relatively low substrate temperature, high stability for largearea films and a suitable adhesion of the thin films on the substrate. Although a high degree of development and control has been achieved with sputtering, the effect of sputtering parameters (e.g. work pressure and gas composition, substrate temperature, the distance between target and substrate, and DC or RF power) on deposited film properties is still under investigation. Any change in the conditions of vacuum-deposited ZnO films will cause a change in
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the stoichiometry of ZnO, influencing its crystalline structure and optical properties. By adjusting growth conditions of ZnO thin films, e.g. impurity doped, substrate temperature, oxygen partial pressure and annealing process (annealing temperature, ambient and procedure), the structural and optical properties of films can be improved. However, the effect of each sputtering parameter on the properties of the deposited film has not been fully investigated yet, and a clear methodology for choosing the optimal parameters for best optical transparency and electrical conductivity is yet to appear in the thin film literature. Recently, Zhou et al. [23] investigated the effects of oxygen/argon ratio and annealing on structural and optical properties of the ZnO thin films deposited using the RF magnetron sputtering technique. Besleagaa et al. [24], deposited transparent ZnO films by RFmagnetron sputtering (1.78 MHz) onto glass substrates, using a mild-pressed ZnO powder target. They deposited ZnO thin films as a double-layer prepared at 0.3 Pa and at a temperature of 400 °C. The result had a bottom layer consisting that was highly depleted of oxygen in its ZnO structure, and a 53 nm top layer of textured ZnO. This paper presents a novel extension to the method of Besleagaa et al. [24] that would prevent the depletion of oxygen in the bottom layer. A minor difference between the proposed technique and the method of [24] is in utilization of DC (instead of RF) magnetron sputtering and using a Zn target (instead of ZnO target). The major contribution is in the idea to repeatedly deposit multiple layers of ZnO in several steps, each involving a lower amount of Zn as compared to deposition of the whole layer thickness in one go. With a smaller amount of Zn in each layer, oxidation is expected to be achieved at a lower temperature (small enough to prevent the depletion of oxygen) and in a shorter period of time. This hypothesis was validated via a comparative experimental study. The thin films prepared by deposition in one step, two steps and three steps (but with the same total thickness) were investigated for their different characteristics including structural and optical properties, surface morphology and hardness. The results demonstrate that the multilayer thin films deposited in three steps had generally more desirable properties in terms of transmittance, structural defects, crystallinity, surface roughness and hardness. 2. Experimental details Fig. 1 shows a schematic diagram of the DC magnetron sputtering system used in our experiments. A vacuum system (Hind High Vacuum, H.H.V, 1200 MSPT) with base pressure of 106 mbar was used, and a circular flat disc (thickness 3 mm, and diameter 125 mm) of pure zinc (99.9%) was made and employed as the sputtering target. Research grade argon (purity 99.9%) was employed at pressures 103–102 mbar for plasma formation in a vacuum bell jar. Before Zn deposition, the plasma discharge was run for a few minutes, and to ensure a high purity coating, the produced plasma was checked by a spectrometer with high resolving power. Deposition conditions and discharge current were optimized to obtain continuous, uniform, and densely packed thin films with the different thicknesses. All samples were deposited under the same experimental conditions. Circular glass substrates (thickness 1 mm and diameter 20 mm) were cleaned both ultrasonically and using alcohol for a period of 5 min, then with acetone for 10 min. The coating rate and the film thickness were measured by a vibrating quartz thickness monitor (1.95 nm/s), and the substrate temperature was monitored by an exact digital thermocouple (namely 300 K). For thermal oxidation of the Zn thin films and annealing of ZnO nanostructure, a conventional oven in open air was used. For optical measurements, a double beam spectrophotometer (Camspec model M350) with wavelength range of 200–1100 nm was employed. The transmittance (T), absorption coefficient (a) and their variations with wavelength were measured, and variations of the optical band gap (Eg) and Urbach energy with temperature were investigated. Photoluminescence (PL) spectra (Stellar Net EPP-200) were recorded at room temperature. The excitation wavelength was 340 nm with a xenon lamp used as the excitation source. The crystalline structure of the films was investigated using a Philips diffractometer (Xpert pw3373) with Cu Ka radiation, angle step size of 0.01°, and count time of 1.0 s per step. Morphological properties of the deposited samples were investigated by scanning electron microscopy (SEM, Hitachi S-4160) and atomic force microscopy (AFM, Auto Probe Pc; in contact mode, with low stress silicon nitride tip of less than 200 Å radius and tip opening of
Fig. 1. Schematic of DC magnetron sputtering system.
Table 1 Characterization of deposition and oxidation of S1, S2 and S3 samples. Sample
Characterization
Zn thickness (nm)
Temperature of oxidation (°C)
S1 S2 S3
1 layer of Zn (1-step) 2 layer of Zn (2-step) 3 layer of Zn (3-step)
300 200 + 100 100 + 100 + 100
400 400 400
18°) analysis. Hardness was measured using nanoindenter XP (MTS) under the continuous stiffness measurement (CSM) mode. For more certitude, the indentation test was repeated for five times on each sample.
3. Results and discussion To investigate the effect of coating in multiple layers, on the optical and structural properties of the deposited ZnO films, three samples, named S1, S2 and S3, are produced. The sample properties are specified in Table 1. They are deposited under similar conditions in terms of deposition rates, Ar pressure, discharge current, and voltage but in different steps. More precisely, S1, S2 and S3 samples are produced by deposition of Zn layer in one step, two steps, and three steps, respectively, and oxidation process is carried out after every step of deposition of Zn layer. The steps of deposition and oxidation for all samples are graphically detailed in Fig. 2. After deposition of ZnO thin films, the structural properties of the deposited samples are investigated by XRD analysis. XRD patterns of the all deposited samples are presented in Fig. 3. The respective positions of ZnO diffraction peaks show that all the films are polycrystalline with ZnO hexagonal wurtzite structure. The diffraction peaks of ZnO (1 0 0), ZnO (0 0 2), ZnO (1 0 1) and ZnO (1 1 0) crystalline planes are observed at 2h values of 31.8°, 34.4°, 36.2° and 56.8°, respectively. The respective positions of the diffraction peaks are in agreement with joint committee for powder diffraction standards (JCPDS) standard data for ZnO powders (refer to JCPDS card No. 00-003-0888). XRD results reveal the successful growth of deposited crystalline ZnO thin films on glass substrates for all three samples.
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Fig. 2. Schematic of production steps of S1, S2 and S3 samples.
Fig. 3. XRD spectra of S1, S2 and S3 samples.
Fig. 4 shows the relative peak intensity of ZnO (1 0 0), ZnO (0 0 2), ZnO (1 0 1) and ZnO (1 1 0) crystalline planes for obtained samples with various deposition steps. From Fig. 4, it is observable that the degree of crystallinity of the deposited thin films depends on the number of deposition steps. Indeed, the degree of crystallinity of the ZnO deposited samples, for all crystalline planes, increases by increasing the number of deposition steps. The grain size of crystallites is estimated using Scherer formula [25]:
D¼
0:9k B cosðhÞ
ð1Þ
where D is the average grain size, k is the incident X-ray wavelength (k = 1.54 Å, Ka (Cu)), B is the full half width at maximum (FWHM),
Fig. 4. Variations of the relative intensities of various ZnO diffraction peaks for obtained samples with different deposition steps.
and h is the diffraction angle. The results are presented in Table 2. They indicate that average grain size of samples decreases for deposited thin films with more steps of coating and oxidation. Fig. 5 presents optical transmittance of the deposited ZnO samples, in the UV–Vis wavelength (200–1100 nm) range, before and after post annealing. The results shown in Fig. 5a demonstrate that larger optical transmittance is achieved for the samples deposited in more steps, before post annealing. To justify the increase in
M. Shirazi et al. / Journal of Alloys and Compounds 602 (2014) 108–116 Table 2 Variations of the crystallite size and band gap energy (Eg), for S1, S2 and S3 samples. Sample identity
Crystallite size (nm)
Eg (eV)
S1 S2 S3
34 26 23
3.251 3.253 3.254
111
conditions. We calculated band gap energy (Eg) for all samples from their transmittance spectra. We first calculated the absorption coefficient, given by [28]:
1 d
a ¼ ln
T0 T
ð2Þ
where T and T0 are the transmittance of the ZnO thin film samples and the substrate (glass) without coating, respectively, and d is the thin film thickness. The Eg was then calculated using Tauc’s relationship [28]: 2
ðahmÞ ¼ Aðhm Eg Þ
ð3Þ
where A is a constant, hm is the photon energy, and Eg is band gap energy. To calculate Eg a graph of (ahm)2 against hm is plotted (see Fig. 6). Extrapolating the straight linear portion of this plot to the photon energy (hm) axis would give the optical band gap value. The band gap energy values, determined from the curves shown in Fig. 6 are presented in Table 2. They are in agreement with the values reported in the literature so far [29]. From presented values in Table 2 it is observable that Eg increases when the material is formed by crystallites sufficiently small which this behaviour can be attributed to some extent quantum confinement [30]. Another important property of the deposited thin layer of ZnO, in the context of semiconductor technology application, is the Urbach energy. During the last decades, Urbach energy has been attributed to the thermally induced structural disorder. Dutta et al. [31] investigated the relationship between structural defects and disorders and Urbach energy of ZnO thin films. They indicated that any defect or disorder in the system gives rise to localized energy levels within the band gap or discrete states. Ilicana et al. [32] showed that Urbach energy of a ZnO thin film varies with its annealing temperature. On the other hand, Pejova et al. [33,34] found out that a direct (but inverse) relationship between the structural disorder and Urbach energy. They made the observation that in terms of lattice strain relaxation, the lattice constant increases with decreasing of dislocation density. This was further verified by Cody et al. [35] who suggested that Urbach energy comprises two components: one that is temperature dependent and justifies Urbach energy variations with annealing temperature (and thermal disorder), and another component that mainly depends on the inherent structural disorder. Mathematically, Urbach energy is modeled as [35]: Fig. 5. Optical transmittance spectra of S1, S2 and S3 samples at (a) 400 °C and (b) 750 °C.
transmittance, we note that the film deposited in more steps receives more oxygen. Fig. 5b demonstrates that after post annealing of the ZnO thin films, optical transmittance of S1 and S2 samples were decreased, while optical transmittance of S3 sample did not vary. The reduction of optical transmittance can be due to escape or desorption of oxygen molecules from the grain boundaries at temperatures over 500 °C [26,27]. The cut-off behaviour at the blue end of the optical spectrum is determined by direct electronic transitions from the valance band to the conduction band. Therefore, the direct optical band gap (Eg) can be determined from transmission data measured at short wavelengths. An important material property, in the context of semiconductor technology and applications, is the band gap energy. It is exactly this point that allows the nanotechnologists to design a semiconducting material with predefined (i.e. designed) optical and electrical properties (such as suitable band gap energy) by simply controlling the crystal size. For example, the band gap energy of various semiconducting materials can nowadays be tuned rather precisely by an appropriate choice of the synthetic route, such as the controllable precipitation under strictly defined
Eu ðT; XÞ ¼
H 1þX
ro
2
þ
1 eH=T 1
ð4Þ
where ro is the Urbach edge parameter of the order of unity, X is a measure of the degree of structural disorder of the material, and H is the Einstein temperature. X is defined as:
X¼
hU 2X i hU 2 i0
ð5Þ
where hU 2X i is the mean square deviation of atomic displacements caused by the actual structural disorder and hU2i0 is the zero-point uncertainty in the atomic positions. The Einstein temperature H is related to the Debye temperature (hD) by the following relationship [36]:
3 4
H ¼ hD
ð6Þ
Based on the above mentioned model, for a given temperature, Urbach energy only depends on the level of structural disorder. It is important to note that a higher post annealing temperature not only contributes to Urbach energy directly (through the temperature-dependent term) but also causes higher structural disorders, especially in high temperatures (over 550 °C) at which evaporation
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Fig. 6. Variation of (ahm)2 with photon energy (hm) for S1, S2 and S3 samples at 400 °C. The inset shows the variation of ln(a) against hm at 400 °C for S1, S2 and S3 samples.
of oxygen produces oxygen vacancies as well as desorption of oxygen molecules from the grain boundaries. In our experiments, we have computed Urbach energy and investigated its variations with the number of deposition steps and for different temperatures. According to Urbach–Martienssen rule, for the range of energies hm < Eg, the optical absorption coefficient of a semiconductor follows the exponential form [37]:
a ¼ ao exp
hm Eo Eu
ð7Þ
where ao and Eo are constant, and Eu is the Urbach energy. Rewriting Eq. (7) as:
Eo 1 þ hm lnðaÞ ¼ lnðao Þ Eu Eu
ð8Þ
we note that Urbach energy, Eu, can be calculated by computing the inverse slope of the graph of ln(a) plotted versus photon energy (hm) for hm < Eg. The plots of ln(a) against hm for ZnO thin films at 400 °C (temperature of oxidation) in the sub-band gap region of linear dependency of ln(a) on hm are shown in Fig. 6. The values of Eu (computed from the slope of the plots shown in Fig. 6) vary from 149 meV to 114 meV then to 80 meV for S1, S2 and S3 samples, respectively. Urbach energy is well known to mainly depend on temperature and the degree of structural disorder of the thin films [34,35,37]. A smaller Eu value indicates less structural defects. The calculated Urbach energy values show that the S3 sample (which is prepared in three steps of deposition) has lower Eu and less structural defect. We have also investigated the effect of annealing temperature on the structural quality of the thin film (via it Eu value). Fig. 7 shows Eu values computed for various post annealing temperatures for all samples. Those values are observed to generally rise with temperature. This is expected as the structural defects and thermal disorders in the ZnO film intensify by post annealing at higher temperatures [26]. Once again, the Eu value for S1 sample is observed to be higher than S2 sample which in turn higher than S3 sample, demonstrating that the structural quality advantage gained via depositing in multiple steps is still applicable after post annealing at high temperatures.
The results obtained from XRD analysis and Urbach energy measurements show that with increasing the average crystal size, the Urbach energy decreases (see Figs. 3 and 7). This is expected from Cody model [35] (see Eq. (4)) in which Eu linearly depends on the degree of structural disorder. Indeed, when an increase in the average crystal size is followed by the relaxation of the lattice strain and the degree of structural disorder and this decreases Urbach energy [34,35]. Fig. 8 shows room temperature photoluminescence spectra for all of the samples with temperature of oxidation at T = 400 °C. This plot indicates a UV emission at 380 nm (3.26 eV) that is known as near-band-edge emission (NBE) and originates from the recombination of free exciton through an exciton collision process, and a green emission at 497 nm (2.49 eV) which is known as deep level (DL) emission and appears due to the radial recombination of a photo-generated hole with ionized oxygen vacancies (Vo) [38,39]. Although most of the practical optoelectronic devices of ZnO thin films are devised and assessed commonly based on their stability in the excitonic emission process, violet and UV luminescence from ZnO thin films are usually endured from the DL
Fig. 7. Urbach energy as a function of post annealing temperature for S1, S2 and S3 samples.
M. Shirazi et al. / Journal of Alloys and Compounds 602 (2014) 108–116
Fig. 8. Room temperature PL spectra of the ZnO thin films.
emission. Some theoretical and experimental investigations indicate that low content of native donor defects in ZnO, e.g. interstitial zinc (Zni) and vacancy oxygen (Vo), would lead to shallow donor levels partially ionized and depress the DL emission at RT [40,41]. The DL emission could be attributed to the oxygen vacancies or zinc interstitials [42–44]. The DL emission leads to decreasing of carrier/exciton lifetime and emission efficiency in the UV light devices. The results obtained from PL analysis reveal that the intensity of the NBE and DL emission at the S3 sample, which is produced in three steps, is lower than S1 and S2 samples. This
113
result indicates that increasing of deposition steps leads to a lower rate of defects and better crystal quality in ZnO thin films which is in agreement with the results from Urbach energy measurements. Fig. 9 shows the comparison of surface morphologies of the ZnO deposited thin films with various deposition steps. The images presented in Fig. 9 indicate that the magnitude of grain boundaries in the ZnO monolayer thin film reduces and thus grains become larger in this sample with respect to deposited samples with more layers. On the other hand, the cross-sectional image of S1 sample indicates that voids are generated in the bottom part of S1 sample, and grain size on top of this film is larger than that of the bottom of the film. In fact, the bottom part of S1 sample does not receive enough oxygen in one step of oxidation, while S2 and S3 samples receives more oxygen in two steps and three steps of oxidation, respectively. Therefore, they have lower voids and larger grain boundaries. The surface morphology of the S1, S2 and S3 samples has also been investigated using atomic force microscope (AFM). The 2D and 3D surface topography of the deposited thin films are presented in Fig. 10. All the images have been obtained with a scanning area of 5 lm 5 lm. The AFM images reveal that the size of the grains/clusters on the surface of thin films decrease with increasing the number of layers (see Fig. 10). This observation is perfectly consistent with the previous conclusion that the average crystallite size of ZnO phase decreases with increasing the deposition steps which was observed from XRD and SEM results. Fig. 10 also shows histograms of grain sizes on the surface of the S1, S2 and S3 deposited thin films. Fitting Gaussian curves to the histograms reveals that the full width at half maximum (FWHM) of curves decreases with increasing of deposition steps (see Table 3). The decreasing of FWHM of curves shows that distribu-
Fig. 9. SEM images of S1, S2 and S3 ZnO thin films.
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Fig. 10. 2D and 3D AFM micrographs and histograms of distribution of size of grains on the surface of S1, S2 and S3 samples.
Table 3 FWHM and peak point position of the Gaussian curves fitted on the histograms of distribution of grain sizes.
Table 4 Variation of average and rms roughness measured by AFM analysis, for S1, S2 and S3 samples.
Sample identity
FWHM (nm)
Xc (nm) (peak point)
Sample identity
Ave roughness (nm)
RMS roughness (nm)
S1 S2 S3
93.2 42.4 40.5
132.7 69.8 64.1
S1 S2 S3
33.7 15.9 14.5
42.5 20.4 18.4
tion of grain sizes on the surface of deposited samples is more homogeneous in sample deposited in more steps. Moreover, the histograms show that the grain size generally decreases with increasing the deposition steps. This is evident from the modes of the histograms being 133 nm, 70 nm and 64 nm for samples deposited in a single step, two steps and three steps, respectively. To analyze and compare the surface roughness of the S1, S2 and S3 samples, in each experiment, we measured the roughness of five random areas over the surface of the deposited sample and recorded the average and root mean square (rms) values of the roughness measurements as presented in Table 4. They demonstrate less roughness on the surface of the deposited samples with more deposition steps. As it was already mentioned in SEM results, with increasing the deposition steps, the size of the grains becomes smaller, hence less roughness is achieved on the surface of the film. Moreover, as it is observable from the histograms of distribution of grain sizes on the surface of S1, S2 and S3 samples, the size of most of the grains decrease with increasing the deposition steps. Similarly, this leads to less surface roughness (see Fig. 10). In order to assess the mechanical properties of the coated ZnO thin films, nanoindentation measurements were taken. Fig. 11 shows the hardness and modulus of the S1, S2 and S3 samples that were determined from 50 nm to 60 nm penetration depth. This indentation depth was selected to cause adequate plastic deformation during indentation but to avoid the substrate effect [45]. From Fig. 11 it is observable that the hardness of the samples increases when the deposition steps increase from one to three steps. As for the modulus of the ZnO samples, the trend of its variation with respect to the deposition steps is fairly consistent with that of the hardness. The increased hardness can also be speculated from the XRD patterns shown in Fig. 3 which shows the crystalline phase of the zinc oxide increases when the deposition steps
Fig. 11. Hardness and modulus of prepared ZnO thin films with different deposition steps.
increase from one to three steps. Moreover, the hardness results can be justified by the Hall–Petch formula [46]:
H ¼ Ho þ kd
1=2
ð9Þ
where Ho is the intrinsic hardness of a single crystal, d is the grain size, k is a constant that depends on the metal alloy composition and H is the hardness. The reduction in the size of grains leads to the reduction of grains boundary voids and hence an increase in the hardness of the coating [47]. AFM analysis showed that the size of grains decreases with more deposition steps. Thus, the results obtained from hardness measurements are in reasonable agreement with AFM results.
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4. Conclusion ZnO thin films deposition with 300 nm thicknesses were carried out by multilayer method at one, two and three steps with repeated coating of the Zn film followed by thermal oxidation, with 100, 200, and 300 nm thicknesses. The optical and structural characteristics of the prepared samples were studied by XRD, SEM, AFM, PL and UV–Vis spectroscopy analysis. From morphological point of view, the SEM and AFM micrographs showed that grain size and surface roughness of the deposited samples decrease with increasing the deposition steps. On the other hand, cross-sectional view SEM images of ZnO thin films indicated that voids are generated in the bottom part of ZnO monolayer (S1 sample), and grain size on top of this film is larger than the bottom of the film while ZnO multilayers (S2 and S3 samples) have less voids and grain size on top and bottom of films are almost same. In addition, optical results suggested that ZnO multilayers have a good optical stability and lower defects because of less intensity of the NBE and DL emission at PL spectra and Urbach energy in comparison with ZnO monolayer. These optical results are in agreement with the results obtained from XRD, SEM and AFM analysis. Furthermore, we observed from the hardness measurements that with more deposition steps, ZnO thin films with better mechanical properties are achieved. The proposed multi-step deposition scheme can be applied for coating a ZnO multilayer on not only a glass substrate but also on substrates with lower melting point such as plastics, polycarbonate (PC) and plexiglass which are used in different applications, and for fabricating of optical devices such as optical waveguides with uniform surface, large optical transmission and lower structural defects. However, depending on the melting point of the substrate material, one may need to modify the method to involve more deposition steps each occurring at lower oxidation temperatures. Investigation of such modifications and the properties of the resulting thin films is a suitable topic for further research in the future. Acknowledgment The first author (M. Shirazi) gratefully acknowledges Dr. Reza Hoseinnezhad at RMIT University, Australia, for his valuable suggestions to improve the manuscript. References [1] H. Huang, G. Fang, S. Li, H. Long, X. Mo, H. Wang, et al., Ultraviolet/orange bicolor electroluminescence from an n-ZnO/n-GaN isotype heterojunction light emitting diode, Appl. Phys. Lett. 99 (2011) 263502. [2] J. Bao, M.A. Zimmler, F. Capasso, X. Wang, Z.F. Ren, Broadband ZnO singlenanowire light-emitting Diode, Nano Lett. 6 (2006) 1719–1722. [3] L. Aghli-Moghadam, A. Baghizadeh, G. Nabiyoni, A. Farashiani, A. Zendehnam, Zn-diffused LiNbO3 waveguides fabricated by DC magnetron sputtering, Appl. Phys. A 97 (2009) 805–810. [4] M. Girtan, G.G. Rusu, S. Dabos-Seignon, M. Rusu, Structural and electrical properties of zinc oxides thin films prepared by thermal oxidation, Appl. Surf. Sci. 254 (2008) 4179–4186. [5] K. Kim, Y.W. Song, S. Chang, I.H. Kim, S. Kim, S.Y. Lee, Fabrication and characterization of Ga-doped ZnO nanowire gas sensor for the detection of CO, Thin Solid Films 518 (2009) 1190–1193. [6] F.A. Mahmoud, G. Kiriakidis, Nanocrystalline ZnO thin film for gas sensor application, J. Ovonic Res. 5 (2009) 15–20. [7] R. Singh, M. Kumar, S. Chandra, Growth and characterization of high resistivity c-axis oriented ZnO films on different substrates by RF magnetron sputtering for MEMS applications, J. Mater. Sci. 42 (2007) 4675–4683. [8] Z. Pan, X. Tian, S. Wu, X. Yu, Z. Li, J. Deng, et al., Investigation of structural, optical and electronic properties in Al–Sn co-doped ZnO thin films, Appl. Surf. Sci. 265 (2013) 870–877. [9] Z.Q. Ma, W.G. Zhao, Y. Wang, Electrical properties of Na/Mg co-doped ZnO thin films, Thin Solid Films 515 (2007) 8611–8614. [10] V. Bhosle, A. Tiwari, J. Narayan, Metallic conductivity and metalsemiconductor transition in Ga-doped ZnO, Appl. Phys. Lett. 88 (2006) 032106.
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