Characterization of nanostructured zinc oxide thin films synthesized at room temperature using low energy plasma focus device

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CERAMICS INTERNATIONAL

Ceramics International 41 (2015) 15024–15033 www.elsevier.com/locate/ceramint

Characterization of nanostructured zinc oxide thin films synthesized at room temperature using low energy plasma focus device Mohammad Taghi Hosseinnejada,n, Marzieh Shirazia, Mahmood Ghorannevissb, Mohammad Reza Hantehzadehb, Elham Darabib a

Young Researchers and Elites Club, Science and Research Branch, Islamic Azad University, Tehran, Iran b Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran Received 26 May 2015; received in revised form 6 August 2015; accepted 10 August 2015 Available online 15 August 2015

Abstract Nanocrystalline zinc oxide (ZnO) thin films were synthesized on glass substrates under extremely non-equilibrium conditions of energetic ion condensation during the focus phase in a low energy plasma focus (PF) device. The samples were deposited using multiple focus shots (5, 10 and 20), at constant axial and angular positions with respect to the tip of anode and anode axis (8 cm and 01), respectively. The argon:oxygen admixture (in 7:3 ratio) was used as a working gas. The structural, morphological, electrical and optical properties of the ZnO deposited thin films were investigated. The results obtained from XRD and SEM analyzes indicated that the size of nanoparticles/agglomerates and the thickness of the ZnO thin films strongly depend on number of shots. AFM analysis revealed that the size of grains and surface roughness of ZnO samples increase with the number of shots. Measurement of the electrical parameters indicated that electrical resistivity is reduced and carrier mobility enhanced with increasing the shots, with no noticeable variations in the carrier concentration. Moreover, the results from optical transmission patterns revealed that optical transmittance and band gap energy decrease with more shots. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Plasma focus; Transparent conducting oxide; XRD; SEM; AFM

1. Introduction In the past decades, zinc oxide (ZnO) based materials, nanostructures, thin layers and devices have received significant attention because of their suitable features [1–3]. ZnO is an n-type II–VI semiconductor with a wide band gap energy (Eg E3.3 eV) together with a high exciton binding energy (EB=60 meV) at room temperature. Owing to the wide Eg and high EB at room temperature, ZnO is a favorable candidate for optoelectronics devices of short wave length. ZnO-based structures have been applied in a variety of fields such as UV photo-detectors [4,5], laser diodes (LDs) [6], UV/blue LEDs [7], sensors[8], SAW filters [9] and solar cells [10].

n

Corresponding author. Tel.: þ98 912 411 8622; fax: þ 98 21 4486 9626. E-mail address: [email protected] (M.T. Hosseinnejad).

http://dx.doi.org/10.1016/j.ceramint.2015.08.051 0272-8842/& 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

In recent years, a large variety of methods have been used for fabrication of ZnO thin films, including magnetron sputtering [11,12], pulsed laser deposition (PLD) [13], atomic laser deposition (ALD) [14], chemical vapor deposition (CVD) [15], sol–gel [16,17], and plasma-assisted molecular beam epitaxy (MBE) [18]. Intensive research is being conducted on the influence of the fabrication method and growth parameters on the structural, morphological, optical and electrical properties of ZnO thin films. In this research, our aim is to synthesize the zinc oxide thin films at room temperature environment using the high energy density pulsed plasmas of plasma focus (PF) device as a novel method. The PF device [19,20] is a simple and cost efficient device that generates a magnetic field itself to compress the plasma to a high density of
1025–1026 m  3 and high temperature (
1–2 keV) in short duration (
10  7 s) [21]. This device is a potential candidate of neutrons [22,23], soft

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and hard X-rays [24,25], relativistic electrons and energetic ions irradiation [26,27]. A intricate mix containing high energy density plasmas, instability accelerated energetic ions and fast moving shock wave, that is emanated from dense and hot pinched plasma column during the radial collapse phase of PF device, is used for different material processes like thin film deposition [28– 30], phase change of thin films [31], ion implantation [32,33] and thermal surface treatment [34,35]. Valuable features such as energetic deposition, high deposition rates, deposition at room temperature environment, short deposition time and possibility of the deposition under a reactive background gas pressure demonstrate that PF device can be used for thin film synthesis [28–35]. In the present work, ZnO thin films are coated on glass substrates, using a low energy (1.3 kJ) PF device. The samples are deposited with different number of focus shots (5, 10 and 20 shots), at 01 angular position with respect to anode axis and same distance from the tip of anode (8 cm). X-ray diffraction (XRD) is used to characterize the deposited thin films, whereas scanning electron microscopy (SEM) is utilized for investigation of the surface morphology and thickness of the samples. Also the thickness of the deposited ZnO samples is tested with surface profiler. The surface topography, the surface roughness and the distribution of grain sizes on the surface of the samples are specially studied by atomic force microscopy (AFM) analysis. Electrical resistivity and Hall effect (mobility and concentration) are measured by the four square probes method and the potentiometric technique, respectively. Moreover, a double beam spectrophotometer is applied for optical measurements.

2. Experimental setup ZnO thin films are deposited on glass substrates using a Mather type PF device powered by a single Maxwell capacitor (10 μF, 25 kV). Fig. 1 shows a schematic diagram of the PF device along with the set-up for thin film synthesis. The technical parameters of the device, in these experiments, are as follows; the operating voltage is 16 kV with the maximum discharge current and the input energy of 130 kA and 1.3 kJ, respectively. Details of the plasma focus device are presented in [35]. For thin film deposition, the stainless steel focus chamber is evacuated up to 10  3 Torr by a rotary vane pump. Then the argon–oxygen admixture is used as a working gas, where the total pressure of gas mixture is kept at 1 Torr. Our investigations on the strong focusing indicated that the best outcome was achieved with the argon:oxygen ratio of 7:3 at a combined filling gas pressure of 1 Torr. Hence, all ZnO thin film depositions experiments are performed with this combination. The qualitative explanation of the process of ZnO thin film formation in plasma focus device being operated with argon: oxygen admixture as the working gas is as follows: during the radial collapse phase of the plasma focus operation, an extremely dense and hot focused plasma column, containing the ionic species of filling gas such as oxygen and argon, is

Fig. 1. 2D and 3D schematic diagrams of plasma focus device used for ZnO coatings.

formed at the tip of central electrode (anode). The plasma temperature reaches a sufficiently high level to cause the complete ionization of each of the filling gas species. In low energy plasma focus devices like the one used in this experiment, the sausage instabilities (m ¼ 0) are seen to set in dense and hot focused plasma column which accelerates the electrons towards the anode tip and ions (of the filling gas species) in opposite direction [36]. The bombardment of the substrate by instability accelerated high energy ions of argon and oxygen, axially decaying high energy density plasma and strong shock wave results in an enormous and rapid increase in the temperature of the substrate surface, causing its local melting and evaporation at the surface [31,37], and modifying the surface without damaging the substrate. Besides, the interaction of high energy density pinch plasma with anode material as well as the bombardment of

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instability accelerated relativistic electrons (which move towards the anode tip) on the top of anode ablate the solid Zn anode tip and result in the formation of Zn plasma which react chemically with the reactive oxygen ions of the filling gas species to form ZnO and then deposit on glass substrate. Hence the energetic ion and electron beams generated in dense plasma focus device [38,39] are used to synthesize ZnO thin films. In this study, glass substrates with 10  10  1 mm3 dimensions are used. These substrates are ultrasonically cleaned, first using alcohol for a period of 5 min, then acetone for 10 min. Subsequently, they are mounted on the sample holder along the anode axis at the distance of 80 mm from the tip of anode. Three different thin films are deposited using 5, 10 and 20 focus shots. A mobile shutter is placed between the glass substrate and the anode. The degree of focusing action in PF device is indicated by a sharp voltage peak in the high voltage waveform and a steep current dip in the Rogowski coil waveform. They are both recorded by a four-channel TDS 2014B (100 MHz) TEKTRONIX digital oscilloscope, as shown in Fig. 2. Shutter is removed after obtaining suitable focusing, and the ZnO thin film is deposited with various number of focus shots (5, 10 and 20 shots). Morphological properties of deposited samples are 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 181) analysis. The thickness of deposited ZnO thin films is investigated using cross-sectional SEM images and also surface profiler with the accuracy of 10 nm (Dektak 3030, Veeco Instruments Inc.). The crystalline structures of the films are determined using a Philips diffractometer (Xpert pw3373) with a step size of 0.021 and count time of 1.0 s per step. The diffraction patterns are recorded in detector scan mode at small grazing incident angle of 31. The electrical properties of the layers are investigated by measuring the electrical resistivity (ρ) and Hall effect (mobility (μ) and concentration (N)) at room temperature. The electrical resistivity is measured by four square probes technique and Hall effect by the potentiometric method. Moreover, optical

properties of thin films are determined using a UV/VIS spectrophotometer (Varian, 500 scan vary) in spectral range of 200–1000 nm. 3. Results and discussion The XRD patterns of ZnO thin films deposited onto glass substrates are shown in Fig. 3(a). In this analysis, the material is scanned in the (2θ) range between 251 and 551. The respective positions of ZnO diffraction peaks show that all the films are polycrystalline with ZnO hexagonal wurtzite structure. The diffraction peaks of ZnO (100), ZnO (002) and ZnO (101) crystalline planes are observed at 2θ values of 31.81, 34.41and 36.21, respectively. For all ZnO diffraction peaks, the respective positions of peaks are in reasonable agreement with the Joint Committee for Powder Diffraction Standards (JCPDS) standard data for ZnO powder (See JCPDS Card no: 00-036-1451). These patterns reveal the successful growth of ZnO crystalline phase on glass substrates for all samples deposited with different number of focus shots. The EDX analysis is used to assess the elemental composition of the deposited ZnO thin films. The typical EDX spectra of the sample deposited by 20 focus shots is presented in Fig. 3 (b). All other spectra indicate similar pattern but with some alterations in the peaks intensities. From Fig. 3(b), it is observed that except the peaks supposed to be present in deposited film and substrate, no other peak related to any impurity element is detected in EDX spectra. XRD analysis shows that the respective positions of all diffraction peaks are the same for all deposited samples. But the peak intensities vary with the number of focus shots. Therefore, the degree of crystallinity of deposited thin film in each experiment strongly depends on number of focus shots. The variations of the relative intensities of ZnO (1 0 0), ZnO (0 0 2) and ZnO (1 0 1) diffraction peaks as a function of focus shots is presented in Fig. 4(a). Following this graph, the intensity of the ZnO (1 0 0), ZnO (0 0 2) and ZnO (1 0 1) peaks increase continuously as the number of focus shots is increased from 5 to 20 shots. With increasing the number of focus shots, more material with more energy is expected to be deposited on substrate and this makes the deposited films thicker. Therefore, the enhancement of the thickness of the deposited films with a larger number of focus shots can be relevant to the increase in the intensity of the ZnO diffraction peaks. The Scherrer formula is used for calculation of average crystallite sizes of ZnO deposited samples Crystallite size ¼ 0:93 λ=β cos ðθÞ

Fig. 2. (a) Current and (b) voltage signals of a typical plasma focus discharge.

ð1Þ

where λ, β and θ are the X-ray wavelength, full width at half maximum (FWHM) of the diffraction peak and Bragg's diffraction angle, respectively. Fig. 4(b) indicates the change of average crystallite sizes of ZnO (1 0 0), ZnO (0 0 2) and ZnO (1 0 1) planes as a function of focus shots. From Fig. 4(b), we observe that the average crystallite sizes of ZnO (1 0 0), ZnO (0 0 2) and ZnO (1 0 1) phases, increase from 26, 20 and 21 nm for 5 focus shots to 38, 27 and 34 nm for 20 focus shots, respectively. Presented typical average crystallite sizes are of the order of

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Fig. 3. (a) XRD patterns of the ZnO thin films synthesized using different number of focus shots. (b) The typical EDX pattern of thin film deposited with 20 focus shots.

Fig. 4. The variations of the (a) relative intensities and (b) crystallite size of all the ZnO diffraction peaks as a function of focus shots.

nanoscale features observed in the SEM image (refer to Fig. 5). Based on these results, the average crystallite size of ZnO phase increases with the number of focus shots. A larger number of focus shots results in higher ion dose. Thus, greater transient annealing of the zinc oxide sample surface takes place. The increased transient and local annealing of the deposited surface layer may lead to the average grain growth and crystallite size in the deposited Zinc oxide sample. Fig. 5(a)–(c) shows SEM images of the deposited samples for multiple numbers of focus shots. The SEM micrographs of the prepared samples demonstrate that crystalline structures grow in size by increasing the number of focus shots. The surface morphology of these deposited thin films is almost identical. However, conglomeration of grains is observed for samples deposited with various number of focus shots. The SEM images of the all deposited thin films show the presence of individual

nano-phase grains and the conglomeration of these grains to form the larger grains or particles distributed over the surface. With increasing the number of focus shots, a progress in the energetic ion irradiation causes more energy being transferred to the surface of sample leading to further mobility of nanoparticles and therefore resulting in bigger sized conglomerates. The results obtained from SEM analysis support our argument of the impressive role being played by the total energy deposited by irradiation of energetic ions on surface morphology of thin film deposited by plasma focus device. The thickness of deposited thin films is investigated by surface profiler with the accuracy of 10 nm (Dektak 3030, Veeco Instruments Inc.). The average thickness of the samples deposited with different number of focus shots is presented in Table 1. Thickness measurements indicate the expansion of thickness with increasing the number of focus shots from 5 to 20 shots which is well adapted with the XRD results. To achieve more reliable thickness measurements (compared to surface profiler), the cross-sectional SEM image of the deposited thin film by 20 focus shots was performed (see Fig. 5(d)). From this image, the thickness of sample deposited with 20 focus shots is found to be in perfect agreement with the result obtained from surface profiler. Also, from this crosssectional image it is observable that deposited thin film is packed and homogeneous in depth. Using AFM, we have investigated the surface topography of the ZnO thin films deposited with different number of focus shots. The two-dimensional (2D) and three-dimensional (3D) AFM images of thin films are shown in Fig. 6. For all samples, scanning area is 3 μm  3 μm. Also all AFM images are acquired in constant force mode and digitized into 512 pixels  512 pixels. As it can be seen from Fig. 6, with increasing the number of focus shots the distributions of the grains on the sample surfaces are observed to be more heterogeneous. Also, the growth of grains/clusters on the surface of thin films by increasing the number of shots is observable from AFM images. For all deposited samples, the average grain height as a function of focus shots has been presented in Table 2. It can be observed that the nanocrystal grains on the surfaces grow higher with increasing the number of focus shots. This coincides with the conclusion drawn from the inspection of

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Fig. 5. SEM micrographs of ZnO thin films deposited with (a) 5 shots, (b) 10 shots and (c) 20 shots and (d) cross-sectional SEM view of the thin film deposited with 20 shots.

Table 1 The average thickness of deposited samples with different number of focus shots. Number of shots

Average thickness (nm)

5 10 20

130 230 350

the SEM images and the reasons have been explained following the grain growth in SEM analysis. An increase of the active surface is needed for fabrication of many devices such as gas and light sensors. The active surface is proportional to parameters of surface roughness such as roughness exponent, standard deviation (which indicates vertical grain size) and correlation length (which indicates lateral grain size) [40]. Average and root mean square (rms) roughness of all samples are presented in Table 2. A rough surface with randomly variations can be mathematically characterized by a discretized matrix of positions and heights. As it was already mentioned, all AFM images were digitized into 512 pixels  512 pixels. For each pixel located in (i,j), h(i,j) denotes the height of the surface measured by AFM. It is

important to note that sometimes, instead of h(i,j), the notation h(r) is used where r denotes the position (i,j). Also, the arithmetic average of surface heights is the average surface height. Analytically it can be demonstrated, for a digitized surface, as: hhiN ¼

N 1X hði; jÞ Ni¼1

ð2Þ

where N is the number of data points per line and row (N ¼ 512). For characterizing a rough surface one of the most important parameters is the rms roughness. Analytically, it can be calculated as  X 1=2 1 N 2 hwiN ¼ ðhði; jÞ  hhiN Þ ð3Þ i¼1 N It is known that the average roughness is determined as the average absolute deviation of the roughness irregularities from the mean line over one sampling length. The rms roughness illustrates the standard deviation of the surface heights distribution. This parameter is more sensitive than average roughness. So average and rms roughness results presented in Table 2 indicate that with the increasing of number of focus shots, the height variation from the mean line increases which means the surface becomes rougher.

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non-dimensional factor used to indicate the shape of the distribution, defined as Eq. (4). A Gaussian height distribution has a kurtosis (K) of 3. If K o 3, it means a relatively flat surface and if K 43, the distribution has many high peaks and low valleys, which means a relatively sharp surface [41]. Skewness measures the symmetry of surface data, defined as Eq. (5). A surface with zero skewness has as many peaks as valleys of the same height and depth. Profiles with deep pits have negative skewness while profiles with sharp spikes have positive skewness, [41]. Surface kurtosis is a dimensionless quantity and skewness is dimensional. These parameters illustrate the shape of the surface height distribution and can be calculated as [42] ! 1 XN 4 Rku ¼ ðhði; jÞÞ  hhiN Þ ð4Þ i;j ¼ 1 N hhi4N 

1 XN Rsk ¼ ðhði; jÞÞ  hhiN Þ3 i;j ¼ 1 N hwi3N

Fig. 6. 2D and 3D AFM micrographs of the thin films deposited with (a) 5 shots, (b) 10 shots and (c) 20 shots. Table 2 Average and rms surface roughness, average height and the values of the skewness and kurtosis of thin films deposited with different number of focus shots. Number of shots

RMS roughness (nm)

Ave. roughness (nm)

Ave. height (nm)

Surface skewness

Surface kurtosis

5 10 20

14.1 20.4 23.3

11.4 16.1 18.1

52.1 61.5 87.6

0.02 0.41 0.23

2.59 3.56 3.61

A complete characterization of random variations in surface roughness is provided by the statistical distribution of the random variable h(r). Although samples with different surface roughness may have different height distributions, the most generally used height distribution is the Gaussian height distribution. The statistical analysis of obtained results from AFM data is done using the height distribution histograms. The asymmetry of height can be explained by the statistical parameters such as surface kurtosis and skewness. Graphically, kurtosis suggests the flatness of the surface. This parameter is a

 ð5Þ

We utilized central limit theorem for comparison of these parameters for all deposited thin films. This theorem mean of a sufficiently large number of independent random variables, each with finite mean and variance, will approximately have a normal distribution [43]. Table 2 indicates the values of surface kurtosis and skewness parameters for the produced thin films which are calculated by Eqs. (4) and (5). Presented results in Table 2 show that for thin film deposited with 5 focus shots, the skewness value is very close to zero, hence surface of this sample has as many peaks as valleys of the same height and depth. For thin films deposited with 10 and 20 focus shots surface skewness values are positive and larger than skewness value of sample deposited with 5 focus shots. Hence these samples contain surfaces with sharp spikes. Also, results obtained from kurtosis measurements, show that surface kurtosis for sample deposited by 5 focus shots is smaller than 3, while for samples deposited by 10 and 20 focus shots surface kurtosis are larger than 3. Hence for sample deposited with 5 focus shots, there is a relatively flat surface, while for samples deposited with 10 and 20 focus shots, the distributions have many high peaks and low valleys and therefore, these thin films have relatively sharp surfaces. Moreover, it can be observed that kurtosis variations tend to increase for samples deposited with more focus shots. This result indicates that with an increased number of focus shots, more peaks and valleys appear on the surface of the deposited sample. Mathematically, this means a larger variance for the random heights of points on the surface, hence increased surface roughness. The height distribution histograms for deposited ZnO thin films obtained from AFM analysis are presented in Fig. 7. From Fig. 7 it is observable that when the number of focus shots increases from 5 to 10 and then 20 shots, the patterns given in Fig. 7 demonstrate an increase in the size of most of the grains. This phenomenon can be another reason that when number of shots is increasing the surface roughness increases.

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The electrical properties of ZnO deposited thin films are examined through measurements of the electrical resistivity (ρ) and Hall effect (carrier mobility (μ) and concentration (N)). Fig. 8 presents the electrical properties of samples deposited with different number of focus shots. An unequivocal and clear reduction of ρ for samples deposited with more focus shots is observable from Fig. 8. Indeed, the resistivity falls significantly, in a small interval of a few tens of nm. The resistivity closely relate to the concentration and carrier mobility by the relation presented in below: ρ ¼ 1=μeN

ð6Þ

where e is the electron charge. As it is observable from Fig. 8, μ increases quickly and clearly when the number of focus shots increases from 5 to 20 shots. Also results presented in Fig. 8 indicate that for samples deposited with different number of shots, the variations of the carrier concentration (N), are not significant. Following these results, N seems to be almost independent of focus shots and also film thickness. The results obtained from electrical parameters measurements (ρ, μ and N)

Fig. 7. The height distribution histograms for deposited ZnO thin films deposited with different number of focus shots.

indicate the variations of resistivity are mainly due to the alteration of mobility. In polycrystalline materials like deposited thin films in this research, the variations of mobility are significantly controlled by their crystallinity and microstructure. The increase in crystallinity and grain size lead to an increase in μ. In this work, the XRD results show that with increasing the film thickness, the grain size increases, leading to the reduction of grain boundaries density and consequently decreasing the carrier diffusion by the grain boundaries. This results in an increase in μ and a decrease in ρ; particularly when no considerable decrease in the carrier concentration occurs like the case here. Fig. 9 presents optical transmittance of the substrate (glass) and ZnO samples deposited with different number of focus shots in the UV–vis wavelength (200–1000 nm) range. From patterns presented in Fig. 9 it is observable that larger optical transmittance is achieved for the samples deposited with less focus shots. This phenomenon is attributed to the enhancement in the film thickness with increasing the number of focus shots, which subsequently increases absorption. The thinner deposited layers have higher transmittances and sharper absorption edges. The onset of the absorption edge in the thicker films is less sharp, because bigger crystalline sizes are deposited; and the scattered radiation become remarkable due to surface roughness [44]. Therefore we expect to decrease the transmittance for samples deposited with more focus shots. The band gap energy (Eg) is a significant material property, in the context of semiconductor technology and applications. This feature allows metallurgists to design semiconductor materials with desirable electrical and optical properties. In this study, we calculated band gap energy for all deposited samples from their transmittance spectra. For calculation of band gap energy, we first estimated the absorption coefficient, given by [45]   α ¼ 1=d ln T o =T ð7Þ where in this relationship α is absorption coefficient and d is the thickness of deposited thin film. Also T and To are the transmittance of the deposited ZnO sample and the substrate (glass), respectively. Then the band gap energy is calculated

Fig. 8. The variations of the electrical resistivity, carrier mobility and concentration of the ZnO thin films as a function of focus shots.

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Fig. 9. Optical transmittance spectra of samples deposited with different number of focus shots. The inset shows the variation of (αhν)2 against hν for deposited ZnO thin films.

using Tauc's equation [45] ðαhνÞ2 ¼ A ðhν  Eg Þ

ð8Þ

where Eg is band gap energy, hν is the photon energy, and A is a constant. The band gap energy values obtained by extrapolating the linear portion of the plots of (αhν)2 versus (hν) indicated that for ZnO samples deposited with 5, 10 and 20 focus shots, these values change from 3.36 eV to 3.33 eV and then 3.32 eV, respectively (see Fig. 9). The rise in the band gap energy with decreasing of focus shots can be attributed to the decrease in the degree of crystallinity and size of grains. To illustrate the rise in the band gap energy with decreasing the film thickness let us discuss that; this behavior can be attributed to the carrier concentration variations. From Fig. 8 it is observable that N decreases gently when the film thickness increases. The reduction in N with increasing the focus shots can be attributed to the enhancement of the degree of crystallinity and the growth in the size of grains. With increasing the size of grains, the amount of oxygen vacancies which are a significant source of free carriers decreases. This phenomenon leads to a decrease in the free carrier density. With the reduction in the free carrier density, the absorption edge shifts towards the longer wavelength region and the band gap decreases [46,47]. On the other hand, from the AFM results it is observable that the surface roughness increases when the number of focus shots increase from 5 to 20 shots. With the increasing of surface roughness, scattering also enhance. Consequently, the absorption edge shift towards the shorter wavelength region [48] and the band gap increase. 4. Conclusion This research demonstrated successful application of the emitted energetic ions in a radial collapse phase of plasma focus discharge for the highly effective plasma-based nanofabrication. Highly energetic ion species (Zn þ ,O þ , Ar þ and ZnO þ ) were originated from the fully ionized gas and evaporated anode tip material (zinc), and were deposited onto glass substrate without using any additional substrate heating or biasing, two most

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common artefacts in most existing PVD processes. As a result of these energetic ions, ZnO nanostructures have been deposited on glass substrate at room temperature. In these experiments, ZnO thin films were deposited with different number of focus shots (5, 10 and 20 shots) by keeping other experimental parameters such as the distance of the substrate from the anode tip (8 cm), total pressure of gas mixture (1 Torr) and argon:oxygen gas admixture ratio (7:3). Deposited samples were studied and compared for various characteristics using different techniques like XRD, SEM, surface profiler, AFM analyses and UV/VIS spectrophotometer. The XRD results suggested that the degree of crystallinity of the deposited thin films and also the average crystallite sizes of the all ZnO diffraction peaks strongly enhance with increasing the number of focus shots. The SEM images of the all deposited thin films revealed the presence of individual nano-phase grains and the conglomeration of these grains by increasing the number of focus shots, to form the larger grains or particulates. Thickness measurements indicated the expansion of thickness with increasing the number of focus shots which was well adapted with the XRD results. AFM analysis provided a powerful tool to measure roughness parameters conveniently. This analysis revealed that both average and rms roughness tend to be larger with the increasing of focus shots. Furthermore, the kurtosis and skewness values measured in our experiments showed that with an increased number of focus shots, more peaks and valleys appear on the surface of the deposited sample. This means a larger variance for the random heights of points on the surface, hence increased surface roughness. The optical transmission patterns revealed that the transmittance and band gap energy decrease with increasing the film thickness. For the electrical properties, measurements indicated behavior dependent upon number of focus shots. The electrical resistivity decreased and the carrier mobility increased when the number of shots increased from 5 to 20 shots while the carrier concentration seems to be less affected by the thickness of deposited thin film. Nevertheless, a tendency to a decrease in carrier concentration was noticed. Moreover, optical investigations determined that the band gap energy of ZnO thin films decreases with increasing the number of focus shots, due to the changes in grain size and degree of crystallinity. Acknowledgment The first author gratefully acknowledges Dr. Reza Hoseinnezhad at RMIT University, Australia, for his valuable suggestions to improve the manuscript. References [1] M.C. Carotta, A. Cervi, V. diNatale, S. Gherardi, A. Giberti, V. Guidi, et al., ZnO gas sensors: a comparison between nanoparticles and nanotetrapods-based thick films, Sens. Actuators B 137 (2009) 164–169. [2] X. Ma, J. Zhang, J. Lu, Z. Ye, Room temperature growth and properties of ZnO films by pulsed laser deposition, Appl. Surf. Sci. 257 (2010) 1310–1313. [3] J. Li, Q. Sun, C. Jin, J. Li, Comprehensive studies of the hydrothermal growth of ZnO nanocrystals on the surface of bamboo, Ceram. Int. 41 (2015) 921–929.

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