Abstract
ZnO is promising material for the electronic and optoelectronic devices. In present work we have fabricated the ZnO film by DC reactive magnetron sputtering. The variations of reactive and sputtering gases affect the crystallite size and band gap of ZnO film. In present work the ZnO film is prepared at 50 watt power by DC reactive spurting method. The fuzzy simulation has been performed to estimate the best argon oxygen gas ratio which gives the better crystallinity and band-gap. The structural analysis shows that the ZnO film has hexagonal wurtzite structure. The UV-vis spectroscopy has been employed to find the band gap.the measured band gap value of ZnO is 3.21 eV. The fuzzy rule based system and characterization results are in accordance with each other with a minimal difference of less than 1%.
Introduction
A number of transparent conducting oxides have been introduced by the research community in last three to four decades like indium tin oxide (ITO), SnO2, CuO, ZnS, ZnO, CdO etc. Among all ZnO gain much attention by the researchers due to its low cost, good electrical conductivity, high transparency for visible light [1–4], high electron mobility, and wide band gap [5, 6]. These remarkable and potential applications increase the use of ZnO in semiconductor industry to make the photodetectors [7], solar cell [8, 9], thin film transistor [10], light emitting diode [11], gas sensor [12] piezoelectric transducer and surface acoustic wave device [13] etc. Moreover, ZnO is much compatible with polymeric substrates to make flexible, and stretchable electronic devices [14].
Despite all these unique properties there is a need to fabricate the ZnO thin film with superior properties in order to fulfil the demands of emerging technology with demanded properties. To achieve the superior properties according to the demand of industry number of ways were adopted in past to fabricate the ZnO thin film like pulsed laser deposition [15], RF/DC magnetron sputtering [6, 16], sol-gel processing [17], molecular beam epitaxy (MBE) [18], chemical vapour deposition [19], chemical spray pyrolysis [20] etc. Among these, RF/DC magnetron sputtering is used to get film of better quality, good uniformity and good adhesion to substrate. A lot of literature is available on the ZnO thin film fabrication of by RF magnetron sputtering technique but DC reactive sputtering technique is rarely used in literature to fabricate the ZnO thin film on different substrates especially on PET to get good quality film with high transparency and conductivity [21]. A number of parameters control the quality conductivity and optical properties of film in DC sputtering technique like substrate temperature, thickness of film, argon oxygen gas ratio [6]. Bao Ma et al. [21] presented the effects of Ar:O2 ratio on the optical and electrical properties of ZnO:Ga thin film they got the film with lowest resistivity at Ar:O2 = 15:1. Shtepliuk et al [22] studied the effects of concentrations of sputtering gas (Ar) and reactive gas oxygen (O2) and sputtering power on different properties of Zn0.9Cd0.1O film. The lowest band gap was achieved at Ar:O2 = 4:1 and 150 Adrian et al. [23] presented the formation of ZnO film by DC reactive magnetron sputtering technique under deficient and rich oxygen environment. They found that oxygen vacancies increase by increasing the oxygen percentage.
Nowadays fabrication of flexible electronics devices become a hot field for researchers. The much popular polymeric substrate used in flexible electronic devices are mainly polytetrafluoroethylene (Teflon), terephthalate (PET), polyethylene naphthalate (PEN), and polyethylene polyimide (PI) [23–25]. Light weight, flexibility, stretchability, and low cost, are the unique properties these polymers which increase their use in flexible electronics.
The first objective of this work is to produce ZnO film on Poly vinyl chloride (PVC) substrate by DC reactive sputtering technique which is difficult due to heat sensitive issues of polymer PVC.
The second objective is to perform the fuzzy simulation to visualize the effect of argon/oxygen gas ration on various structural and optical parameters o.
Methodology
ZnO thin film has been deposited on the Polyvinyl chloride (PVC) substrate by the DC reactive magnetron sputtering by using pure zinc target. PVC substrates were cut into 2x2 cm2 size and washed with DI water followed by cleaning with PVA poly vinyl alcohol. Finally the PVC substrates were dry with nitrogen gas. The cleaned substrates were mounted in magnetron sputtering chamber where pure zinc target was fitted for the sputtering of pure zinc. Initially the sputtering chamber was evacuated to a base pressure of 10-5 mbar. The sputtering process was performed in the presence of pure oxygen gas whereas argon gas was used as sputtering gas. The working pressure was maintained at 10-3 mbar.
When DC voltage of 300 V is applied the argon gas plasma is generated and positive ion of argon gas sputtered the zinc from negatively biased zinc target.The sputtered zinc particle move towards the PVC substrate and react with oxygen to form ZnO on their way towards the PVC substrate. Compact and smooth films of thickness about 250 nm were obtained after sputtering of zinc target for 65 minutes.the sputtering parameters are listed in Table 1.
Sputtering parameters of ZnO thin film
Sputtering parameters of ZnO thin film
The as deposited films were characterized by x-ray diffractometer for the structural analysis and FESEM for surface morphology. The optical band gap of ZnO film was estimated by using UV-vis spectroscopy.
To analyze the effect of argon and oxygen gas ratio on the crystallite size and band gap, fuzzy rule based system is used. Mamdani model is used as it provides an exact crisp value for comparison with experimental findings. Oxygen and Argon are taken as input and Crystallite size and band-gap is taken as output as shown in FIS figure in Fig. 1.
FIS diagram of the fuzzy rule based system.
The membership functions and ranges for the input and output are then selected based on the experimental parameters. For oxygen/argon ratio, 0/6 and 3/10 are taken as input parameters range. As input parameters are ratio so it does not have any units. Range for grain size is taken from 10-30 nm and for band-gap is taken from 3.18 to 3.22 eV. 9 rules (3∧no. of membership functions) are defined based on the real time data.
The membership function for input oxygen and argon are shown in Fig. 2.
Membership function for input (a) Oxygen (b) Argon.
Membership function for output grain size and band-gap is shown in Fig. 3.
Membership function for output (a) Grain Size (b) Band-Gap.
On the basis of the rules, the 3D graphs are plotted which shows that with increase in Argon concentration grain size as well as band gap increases. Similarly, with increase in oxygen concentration, the grain size and band-gap decrease. Figure 4 (a) shows the 3D graph of the input and its effect on grain size. Figure 4 (b) shows the 3D graph of the input and its effect on band-gap.
3D Graph between input and its effect on (a) grain size (b) Band-gap.
The crisp value in the rule viewer gives a value of grain size as 20 nm and band-gap as 3.22 eV when the ratio for oxygen and argon is taken as 1:8. Figure 5 shows the rule viewer with the crisp values of input and output.
Rule Viewer for the input and output.
XRD analysis
The XRD pattern of DC magnetron sputtered ZnO film is depicted in Fig. 5. It can be observed from the graph the dominant peak is along (002) plane. No other peaks were observed which shows that the formation of c-axis oriented ZnO film with hexagonal wurtzite structure. The formation of c-axis orientation is mainly due to the low surface energy low stresses and high atomic density of (002) plane. As (002) is the only dominant plane.
Therefore Scherer formula is applied to calculate the crystallite size as shown in Equation 1;
Here β is full width at half maximum which is to measure from the XRD graph, λ is the wavelength of the x-rays used and θ is the peak position.
The value of inter planner spacing estimated by above equation is 2.61 Å. By using the value of d in c = d×l the lattice parameter c is obtained to be 5.22 Å which is in good agreement with the reported value of literature [27].
The FESEM image is shown in Fig. 6. The micrographs of ZnO film shows that the uniform and homogeneous growth of ZnO with nearly spherical morphology.it can be observed from the FESEM grasp the grans of film are compact and regular. The gran size was calculated from the FESEM bar scale.The bar scale contain nearly ten grains. The size of single grain is estimated about 20 nm.
The XRD pattern of ZnO thin film on PVC.
The optical properties of ZnO are very interesting. UV-vis spectroscopy has been employed to calculate the band gap of ZnO. The absorbance spectrum of ZnO was obtained by UV -vis spectrometer. The absorbance spectrum of ZnO is shown in Fig. 7. It can be observed from the graph the absorbance reduces near the wavelength 387 nm the band gap of ZnO film is estimated by the absorbance edge by using the formula
The FESEM image of ZnO thin film on PVC.
The absorbance spectrum of ZnO thin film.
Experimental and fuzzy analysis findings and its comparison is shown in Table 2.
Comparison between simulated and experimental values
Comparison between simulated and experimental values
The comparison between the simulated and calculated value shows a minimal error of 1% which shows the simulated and experimental data is in accordance with each other
ZnO is considered as a promising material in opto-electronics and energy harvesting field owning to its excellent band-gap and particle size. Various chemical and physical methods are used to fabricate and synthesize the thin film of ZnO for use in opto-electronics and sensing applications. Physical vapor deposition is considered as an excellent method to deposit thin film of ZnO with great accuracy however, altering the process parameters during the process can help change the properties of deposited ZnO. In this work the process parameter of gas ratio is varied to study its impact on the characteristics of the ZnO film. Fuzzy rule based system is used to analyze the effect of change is oxygen and argon gas ratio on grain size and band-gap. An increase in band-gap and grain size is observed due to increase in oxygen and decrease in observed for increase in argon gas pressure. The fuzzy results are in accordance with the experimental result which shows the accuracy of the simulated and calculated results.