Resistivity Reduction of Nanostructured Undoped Zinc Oxide thin Films for Ag / ZnO Bilayers Using APCVD and Sputtering Techniques

Nanostructured undoped zinc oxide (ZnO) thin films were deposited using atmospheric pressure chemical vapor deposition (APCVD) on glass substrates using zinc acetate dehydrate [C4H6O4Zn·2H2O, ZnAc] in less than 2 minutes for each sample. In order to reduce the resistivity of ZnO films, a very thin layer of Ag was deposited on top of the films via the sputtering method to reduce resistivity from 2.89 to 0.31 Ω.cm, using only a 30Å silver coating. Structural, electrical and optical properties of the resulting bilayers were also investigated. The results show a polycrystalline structure in higher temperatures compared to rather amorphous ones in lower temperatures such as 325°C. The XRD patterns of the optimum polycrystalline films were identified as a hexagonal wurtzite structure of ZnO with the (002) preferred orientation. Also, sheet resistance decreased from 17.8 MΩ/⧠ to 28.9 KΩ/⧠ for the temperatures of 325°C to 450°C, respectively. Based on the physical properties of undoped ZnO, substrate temperature is an important factor which affects the crystallite size and modifies electrical parameters. UV-vis measurements revealed a reduction in the transparency of the layers with increasing substrate temperature. A sharp cut-off was observed in ultraviolet regions at around 380 nm.


Introduction
Zinc oxide (ZnO) is a wide-band-gap (3.37 eV) semiconductor with a large (60 meV) exciton binding energy.It is fairly stable both chemically and physically.Its piezoelectricity and high room temperature electron mobility of 155 cm 2 V -1 s -1 causes lasing action based on exciton recombination above the room temperature.ZnO has received considerable attention because of its effect in fundamental studies and diverse applications in ultraviolet and blue light-emitting diodes, laser diodes, ferroelectrics, piezoelectrics, gas sensors, field-effect transistors, acoustic wave devices, solars, and so on [1][2][3][4][5][6] .ZnO has a wurtzite structure, which is, in fact, a hexagonal crystal structure (lattice parameters: a=b=0.32427nm, c=0.51948 nm), fitting to the space group P6 3 mc, and is identified by two interconnecting sublattices of O 2-and Zn 2+ , in a way that Zn ions are surrounded by O ions, and vice versa 7 .
Manufacturing as low as possible sheet resistance while maintaining the visible transparency is the main objective of many researches carried out on TCOs.According to Chen et.al 24 , there is a lower limit for resistivity (or upper limit for charge carriers' mobility) due to scattering mechanisms.Bellingham et.al 25 suggested 4 х 10 -7 Ω.m as the absolute TCO resistivity lower limit.The thickness of TCO is very crucial to many applications.Therefore, adding a few nanometers of a second layer of a good conductor (such as a metal) might be a reasonable way to keep the resistivity low without increasing the TCO thickness or losing high percentage of optical transparency.Silver is the best candidate because of having the lowest resistivity among metals 26 .
The CVD of films and coatings contains the chemical reactions of reactants in a gaseous phase on or in close vicinity of an already heated substrate surface.This atomistic deposition process can provide highly pure materials with a good structural control at a nanometer or atomic scale level 27 .Atmospheric pressure CVD is an economic method, but has certain difficulties in the process control and most research considerations have been dedicated to low pressure CVD (LPCVD) and sputtering deposition which are, low pressure techniques for deposition.However, because of its potential for mass production, the APCVD of ZnO has been considered 28 .
ZnO films manufactured by APCVD with a zinc source such as zinc acetylacetonate (Zn(C 5 H 7 O 2 ) 2 ) or zinc acetate are chemically stable in air atmospheric pressure and are used as a precursor instead of diethyl zinc and dimethyl zinc 29 .Atmospheric CVD is advanced by the production of active precursor vapors via gas-phase reactions.They diffuse through a thin hydrodynamic boundary film above the substrate.Because of the low reactivity of oxygen or metal precursors, temperatures above 300℃ for the substrate are needed for surface reactions, activating the gas-phase, and having high rates of film growth 30 .a Department of Physics, University of Guilan, Rasht, Iran

The Experiment
In this work, we used a homemade CVD apparatus to deposit ZnO films in atmospheric pressure (See Fig. 1) 31 .
(3) (4) Lin et al. considered the thermal process of zinc acetate dihydrate as the dehydration, vaporization/decomposition, and formation of ZnO 33 .The gaseous by-products desorb from the surface, starting to diffuse into the stream and transported by the carrier gas 34 .Then, they must be ventilated by a chemical fume hood 35 in order to be safe for the environment.
In Set 2: a very thin Ag layer of a few angstrom as the second layer was sputtered in the vacuum condition using the DSR sputter vacuum coating sputtering system which employed a 2-inch-diameter magnetron silver target.The sputtering system had a film thickness monitor using a quartz crystal microbalance which was included a 6 MHz quartz crystal.
The transmittance spectra were recorded using a UV-vis spectrophotometer (Perkin-Elmer Lambda 25 UV/vis) in the range of 350-1100 nm.The electrical properties of thin films were measured by the Hall effect and Van der Pauw setup (RH 2010 PhysTech System).X-ray diffraction (XRD) was applied to determine the crystalline quality of the samples using Cu K α radiation (model Philips PW-1800).Field emission scanning electron microscopy (FESEM) was carried out by a Mira 3-XMU equipped with an energy dispersive X-ray (EDX) system for the morphological analysis of the thin films.The Swanepoel method 36 was employed for thickness measurements of the samples with interference fringes and FESEM cross-section for interference-free thin films.

Structural properties
The XRD pattern on the ZnO thin films for different temperatures is demonstrated in Fig. 2. The peaks correlated with the theoretical ZnO patterns of Joint Committee on Powder Diffraction Standards (JCPDS), reference code 01-075-0576, indexed as a hexagonal wurtzite structure with no impurities.In the wurtzite structure of ZnO, the a-and c-axes of the orientation corresponded to (100) and (002) peaks.
At low temperatures, the atomic mobility is low which restricts the crystal growth 37 and a weak peak of (100) is dominated.The lowest sheet resistance is at 450℃ in which we have the highest carrier mobility.As Ardyanian et al. 38 suggested, this temperature in the best substrate temperature for ZnO deposition, because atoms have optimum energy for chemical reactions.By increasing the temperature up to 500℃, adatoms receive enough thermal energy and mobility increase which causes the growth of (002) preferred orientation 37 .Hyun et al. 39 stated that the main effect of a change in composition All of the samples were deposited on glass substrates (75 х 25 х 1 mm 3 ) by the CVD technique in a tube through the oxidation of zinc acetate dihydrate [C 4 H 6 O 4 Zn•2H 2 O, ZnAc] as the precursor.The initial mass of zinc acetate powder was 0.5 g for all the samples and was directly introduced in the reactor.The system contained a horizontal tabular furnace with the diameter of 80 mm and the length of approximately 1 m.
All substrates were put in a detergent solution and then rinsed with deionized water.Next, these glass substrates were ultrasonically cleaned with a mixture of deionized water and HCl acid for 20 minutes.The substrates were rinsed in deionized water again.To prevent the cleaned substrates from oxidization before the deposition, the substrates were heated, while a nitrogen flow was applied to clean the surface of the furnace tube before resuming the deposition process.The deposition duration for each sample was less than 2 minutes depending on the temperature.The less the temperature, the more time it takes to vaporize the fixed amount of 0.5 g as the precursor material.In general, source material, system geometry, and quantity all affect the local vapor pressure of the reactants present at the substrate and, therefore, the properties and morphology of the obtained films 32 .
Two sets of experiments were conducted.In Set 1: the temperature of the source material was varied from 325℃ to 550℃ in steps of 25℃.Air was utilized as the source of oxygen.The zinc acetate dihydrate powder vapor entered the substrate at the atmospheric pressure, and undoped ZnO films were deposited on the substrate.By increasing the temperature from the room temperature, water (H 2 O) was vaporised from 50℃, as shown in the reaction Eq. ( 1).Other products in the gas phase such as acetone, carbon dioxide, and acetic acid started to appear when the temperature reached around 200℃.These materials reached their highest concentration at around 270℃, as shown in Equations ( 2)-( 4).As the temperature rose, ZnO thin films were formed by the chemical reactions below: (1) (2) from an oxygen-deficient material to a more stoichiometric zinc oxide is the (002) preferred orientation.This causes the sheet resistance to increase for temperatures above 450℃.
The growth rate of each crystallographic plane differs based on crystal orientation and surface energy.The (002) to (100) polar planes ratio of intensity (I 002 / I 100 ) can affect the photocatalytic activity due to the ZnO polar planes' contribution in the formation of oxygen vacancies 40 .The full width at half maximum (FWHM), crystallite sizes, peak positions, and d-spacing are listed in Table 1 (calculated by the PANalytical X'pert Highscore Plus software).Based on the XRD patterns, the dominant orientation of (002) was at 2θ ≈ 34º which is in agreement with a few other works 14,41- 44 .The other observed peaks were (100), (101), and (110).Hence, the crystallites were highly oriented with their c-axes which are perpendicular to the substrate plane 45 .
Average crystallite size was calculated by the Scherrer's equation ( 5), (5)   where λ=1.542Å is the X-ray wavelength, β is the FWHM of the diffraction in radians, and Ɵ is the Bragg diffraction angle 46,47 .
Inter-planar spacing (d) was measured using the following equation 48 (6)

Morphological properties
The surface morphology of the thin films was observed using a field emission scanning electron microscope at room temperature.Fig. 3 illustrates FESEM micrographs of ZnO structures deposited on a glass substrate at four different temperatures.By comparing the images, the role of the deposition temperature is vivid.The surface morphology of ZnO thin films grown at a low substrate temperature (325℃) showed small crystallites with a high density.By increasing the temperature to 525℃, the crystallinity of the thin films was enhanced and we obtained the largest crystallites.
The morphology of grown ZnO with different thicknesses of Ag nanoparticle coating is depicted in Fig. 4. Slight changes were observed in the microstructure that can be attributed to the role of Ag thickness.As can be seen, the distribution of Ag nanoparticles over the ZnO layer was found to be relatively uniform.
EDX was implemented in different areas of the samples which confirmed the chemical composition of ZnO, as can be seen in Fig. 5. Table 2 shows the quantitative EDX results for undoped zinc oxide thin films prepared at optimum temperature 450℃.Fig. 6 demonstrates the silver appearance in ZnO/Ag bilayers in the EDX spectrum.Table 3 shows the quantitative EDX results for ZnO/Ag bilayer thin films.

Electrical properties
If close to stoichiometry, ZnO films, show very high sheet resistance, while nonstochiometric ZnO films exhibit low sheet resistance.The electrical characteristics of ZnO thin films are mainly dominated by electrons which are generated from Zn interstitial atoms and oxygen vacancies 49 .As in many other semiconductors, the conductivity of ZnO increases with increasing the temperature 50 .
Hall effect measurements were conducted using van der Pauw method with the magnetic field of 0.56 T. The samples were cut into square shapes (5 х 5 mm 2 ).Silver dots were formed as electrodes at the surfaces corners of each sample.
Variations in sheet resistance, resistivity, Hall Co., mobility, and carrier concentration at different substrate temperatures are listed in Table 4.The lowest sheet resistance was achieved at 450℃.
According to Table 5, by taking 450℃ as the optimum temperature, films revealed a significant drop in sheet resistance by sputtering a second Ag thin layer on top of the first undoped ZnO layers.As a metal, Ag is a very good conductor but it has a high reflectance.Therefore, we should be very careful about the thickness of the second silver layer because we lose the optical transparency of the films.For this reason, the thickness of the bilayers was limited to 30Å by our sputtering system.

Optical properties
Fig. 8 illustrates the optical transmittance spectra obtained from ZnO thin films, at the wavelengths ranging from 300-1100 nm, as a function of substrate temperature which plays an important role in film formation.We had a transmittance of about 80% in the visible section of the spectrum from 325℃ to the optimum temperature, i.e. 450℃.At higher temperatures, transmittance was reduced and we lost most interference fringes.A sharp cut-off was observed in ultraviolet region at around 380 nm that is due to band edge absorption of ZnO in this area, the monodispersed nature 51 , and an almost uniform size of nanoparticles 52 which is also observed by Look 53 and Zhang et al. 54 at 373 nm and 369 nm, respectively.Fig. 9 demonstrates the optical transmittance spectra resulting from ZnO thin films covered by a few angstroms of Ag with different thicknesses.Ag has a very high reflectance.Therefore, for ZnO/Ag bilayers, as the thickness of Ag is increased, transmittance is reduced.
Optical constants such as refractive index, extinction coefficient, and absorption coefficient were calculated using the average transmittance in the visible range.The transmittance of the glass substrate was only T s = 91.33% in the visible range.Afterwards, the refractive index of the substrate, s=1.540754, was obtained using (7)   According to the Swanepoel method 36 which is based on the study by Manifacier et al. 55 on making upper and lower envelopes of the transmittance spectrum (see Fig. 10), the refractive index of the film n in the transparent, weak, and medium absorption regions can be calculated using (8)   where for the transparent region and (9)   for medium and weak absorption regions.T m and T M are thecorresponding minimum and maximum transmittances at a certain wavelength, respectively.The thickness of the film was given by: (10)   where n(λ 1 ) and n(λ 2 ) are the refractive indices at two adjacent minimums (or maximums) at λ 1 and λ 2 , respectively.The fundamental equation for interference fringes is 2nd=mλ where m is an integer for the maximum and half integer for the minimum.
The calculations of Table 6 correspond to the undoped ZnO sample deposited at 375℃ using Eq. ( 10) and the average thickness of 1378 nm was obtained for this sample.By increasing the deposition temperature, the thickness of the ZnO layers decreases which causes a decrease of grow rate 56 .The thickness of the film influences the optical transmission in which the number of interference fringes and the depth of these fringes depend on the film thickness 57 .Therefore, we performed the Manifacier calculations for the sample with fringes (deposition temperature less than 475℃), resulting an average of 1300nm.For layers with no or not enough interference fringes in the visible region of the transmission spectra, SEM crosssections were utilized to measure the thickness of the layers.The average thickness for ZnO layers prepared at 475℃ and higher temperatures using SEM cross sections was 721 nm.For these films, the absorption coefficient can be calculated using Since the Swanepoel method is not valid in the strong absorption region, to determine the absorption coefficient of the film in the zone, we used Lambert's equation

64
. Following the Swanepoel method, the absorption coefficient, α, and extinction coefficient, κ, can be obtained from T M and T m , s and thickness by the following expressions, respectively 49 .13)   where B is an energy-independent constant, α is the material's linear absorption coefficient, E g is the optical band gap and m is a constant which regulates the type of optical transitions.For example, in an indirect allowed transition, m=2, for an indirect forbidden transition, m=3, for a direct allowed transition, m=1/2, and for a direct forbidden transition, m=3/2

Conclusion
In this work, we presented the synthesis of ZnO nanostructures prepared by atmospheric pressure CVD on glass substrates at different substrate temperatures in order to achieve the highest optical transparency and electrical conductivity.The optimum set of ZnO films were then used as a substrate for the second Ag layer to be sputtered on top of the first layer.Also, sheet resistance decreased from 17.8 MΩ/⧠ to 28.9 KΩ/⧠ for the temperatures of 325℃ to 450℃, respectively.Based on the physical properties of undoped ZnO, substrate temperature is an important factor which affects the crystallite size and modifies electrical parameters.UV-vis measurements revealed a reduction in the transparency of the layers with increasing substrate temperature.A sharp cut-off was observed in ultraviolet regions at around 380 nm.

Figure 2 .Figure 3 .
Figure 2. The XRD pattern of undoped zinc oxide thin films deposited at different substrate temperatures

Figure 5 .
Figure 5. EDX spectrum measured for undoped zinc oxide thin films prepared at optimum temperature 450℃

Figure 7 .
Figure 7. Variations of resistivity, mobility and carrier concentration versus deposition temperature

Figure 8 .
Figure 8. Optical transmittance spectra for ZnO films deposited with different substrate temperatures

Figure 9 .
Figure 9. Optical transmittance spectra for undoped zinc oxide compared to ZnO/Ag bilayers deposited with different Ag thicknesses

Figure 10 .
Figure 10."Envelope method" graph including extremum values for thickness measurement equations, where d is the film thickness, and T and T 0 are the transmittance of the ZnO thin film samples and the substrate (glass) without coating, respectively.

Figs. 11
Figs. 11 and 12 depict the absorption and extinction coefficients for samples prepared at different temperatures.

Figure 11 .Figure 12 .
Figure 11.Variations in absorption coefficient, α, as a function of wavelength for ZnO films at different temperatures

Table 1 .
Structural properties of ZnO thin films prepared at different substrate temperatures

Table 2 .
Quantitative EDX results for undoped zinc oxide thin films prepared at optimum temperature 450°C.

Table 3 .
Quantitative EDX results for ZnO/Ag bilayer thin films.

Table 4 .
Electrical properties of ZnO thin films prepared at different substrate temperatures.

Table 5 .
Electrical properties of ZnO/Ag bilayers prepared at 450 °C with different Ag thicknesses.

Table 6 .
The Manifacier's calculations corresponding to the undoped ZnO sample deposited at 375°C using Eq.(10)