Transparent conducting ZnO-CdO mixed oxide thin films grown by the sol-gel method

Accepted Manuscript Transparent conducting ZnO-CdO mixed oxide thin films grown by the sol-gel method Trilok K. Pathak, Jeevitesh K. Rajput, Vinod Kumar, L.P. Purohit, H.C. Swart, R.E. Kroon PII: DOI: Reference:

S0021-9797(16)30829-3 http://dx.doi.org/10.1016/j.jcis.2016.10.062 YJCIS 21695

To appear in:

Journal of Colloid and Interface Science

Received Date: Revised Date: Accepted Date:

25 August 2016 14 October 2016 23 October 2016

Please cite this article as: T.K. Pathak, J.K. Rajput, V. Kumar, L.P. Purohit, H.C. Swart, R.E. Kroon, Transparent conducting ZnO-CdO mixed oxide thin films grown by the sol-gel method, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/10.1016/j.jcis.2016.10.062

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Transparent conducting ZnO-CdO mixed oxide thin films grown by the sol-gel method Trilok K. Pathaka,b#, Jeevitesh K. Rajputb, Vinod Kumara,c, L.P. Purohitb, H.C. Swarta and R.E. Kroona# a

Department of Physics, University of the Free State, Bloemfontein, South Africa

b

Semiconductor Physics Laboratory, Department of Physics, Gurukula Kangri University, Haridwar, India c

Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, New Delhi, India

Abstract Mixed oxides of zinc and cadmium with different proportions were deposited on ordinary glass substrates using the sol-gel spin coating method under optimized deposition conditions using zinc acetate dihydrate and cadmium acetate dihydrate as precursors. X-ray diffraction patterns confirmed the polycrystalline nature of the films. A combination of cubic CdO and hexagonal wurzite ZnO phases was observed. The oxidation states of Zn, Cd and O in the deposited films were determined by X-ray photoelectron spectroscopic studies. Surface morphology was studied by scanning electron microscopy and atomic force microscopy. The compositional analysis of the thin films was studied by secondary ion mass spectroscopy. The transmittance of the thin films was measured in the range 300 nm - 800 nm and the optical bandgap was calculated using Tauc’s plot method. The bandgap decreased from 3.15 eV to 2.15 eV with increasing CdO content. The light emission properties of the ZnO:CdO thin films were studied by photoluminescence spectra recorded at room temperature. The current-voltage characteristics were also assessed and showed ohmic behaviour. The resistance decreased with increasing CdO content. Keywords: Thin film, Sol-gel, X-ray diffraction, Optical properties, Electrical properties. #

corresponding authors: E-mail: (1) [email protected], [email protected] (2) [email protected]

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1. Introduction Transparent conducting oxides (TCOs), which can be grown efficiently as thin films with low cost, are used extensively for a variety of applications, including sensors, solar cells, phototransistors, light transparent electrodes and other optoelectronic devices [1-4]. Among various other TCOs such as tin oxide (SnO2) and indium oxide (In2O3), zinc oxide (ZnO) and cadmium oxide (CdO) are attractive candidates as window materials for many devices such as solar cells, liquid crystal displays, and laser or light-emitting diodes. Both ZnO and CdO are generally n-type semiconductor components [5, 6]. ZnO thin film shows high transparency in the visible region but low electrical conductivity while CdO thin film shows low transparency in the visible region but high electrical conductivity. CdO has a bandgap ranging from 2.15 eV to 2.7 eV with low electrical resistivity while ZnO has a large bandgap of 3.15 eV to 3.4 eV, but exhibits a higher transparency. Hence, the binary system CdO-ZnO may be considered as an important alternative for the preparation of good quality, transparent, conductive materials. Separate ZnO and CdO films have been investigated by a large number of research groups [710]. It is important to study the system of mixed oxides, such as how the bandgap, optical transmittance and electrical resistance are altered when the composition changes. The electrical properties and electronic band structure play crucial roles in order to build functional devices in electronics. Therefore this work aimed to explore the properties of the mixed oxides (ZnO-CdO) and the possibility of applications in electronics and optoelectronic devices. A ZnO-CdO composite may enhance the advantages and reduce the shortcomings of the individual oxide films for a particular application such as optical switching, or buffer and window layers in solar cells. First in 1996, Chit et al. deposited ZnO-CdO mixed oxide thin films and explained the alteration of electrical resistance with increasing Cd concentration [11]. After that a number of studies have appeared on ZnO-Cd thin films, nanowires and nanorods [12-15]. 3

Wu et al. deposited ZnO-CdO thin films by using the radio frequency sputtering method and discussed the structural, optical and electrical properties of the thin films [16]. Ziabari et al. deposited similar ZnO-CdO thin films using dip coating and investigated their electrical and optical properties [17]. ZnO-CdO composite thin films are generally prepared by various physical and chemical deposition techniques like electrodeposition [18], radio frequency magnetron sputtering [19], molecular beam epitaxy [20], sol–gel process [21], chemical vapour deposition [22] and spray pyrolysis [23]. Among these techniques, the sol–gel method is simple and suitable for large area deposition of almost any binary and ternary TCOs. The spin coating technique is simple because there is no need of vacuum or high-temperature for deposition of films, and is feasible for coating large and complex-shaped areas [24]. In the present work the we study influence of various concentration ratios of Zn:Cd on the structural, optical and electrical properties of ZnO:CdO thin films prepared by spin coating technique are reported. 2. Experimental details ZnO:CdO thin films were deposited on glass substrates by the sol–gel spin coating method. All the chemicals used in the present study were of analytical reagent grade. Zinc acetate dihydrate [Zn(COOCH3)2·2H2O, Sigma Aldrich] and cadmium acetate dihydrate [Cd(COOCH3)2·2H2O, Sigma Aldrich] were used as sources for Zn and Cd respectively, while 2-methoxyethanol (C3H8O2, Alfa Asser) and mono ethanolamine (C2H7NO, MEA, Alfa Asser) were used as the solvent and stabilizer, respectively. 2.1 Preparation of ZnO and CdO solutions To prepare a 0.5M ZnO precursor solution, zinc acetate dihydrate of the required amount was dissolved in 50 ml of 2-methoxyethanol by magnetic stirring at 60 °C. Then ethanolamine (molar ratio 1:1) was added drop-wise to this solution as a sol stabilizer. The resulting mixture was

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stirred at room temperature 60 °C for 1 h to yield a colourless and transparent solution. During preparation the reaction occurred. Zn(CH3COO)2・2H2O → ZnO↓ + 2CH3COOH+ H2O The CdO solution was prepared in the same way by exchanging the zinc acetate dihydrate with cadmium acetate dihydrate. 2.2 Preparation of ZnO:CdO thin films After preparation of the ZnO and CdO solutions, these sols were mixed with different volume ratios and coded as shown in Table 1. Table 1 After mixing the sols, they were again stirred at a temperature of 60 °C for 1 h for proper mixing. Then these sols were used for spin coating to deposit the thin films. Before spin-coating, the glass substrates were first rinsed thoroughly by deionized water and then in acetone. In order to remove microscopic contaminations, the substrates were cleaned ultrasonically in distilled water for 15 min. The films were deposited on cleaned substrates with a spin coater using a rotation speed of 2500 rpm for 60 s. After each coating, the as deposited films were dried at 240 °C in air for 10 min to evaporate the organic precursors. The procedure from coating to drying was repeated 10 times to increase the total thickness to about 200 nm. Finally, the films were inserted into a microprocessor controlled furnace and heated at 450 °C in air for 1 h. Fig. 1 shows a flow chart of the preparation process for ZnO:CdO thin films deposited by the sol-gel spin coating method.

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2.3 Characterization of thin films After the furnace returned to room temperature, the films were removed and characterised. X-ray diffraction (XRD) measurements were made using a Bruker D8 Advance diffractometer with Cu Kα radiation (wavelength 0.154 nm) to investigate the crystal structure. The XRD results measured in coupled theta scan type, continuous PSD scan mode with increment 0.0198613 degree and 3258 steps. A PHI5000 Versaprobe was used for X-ray photoelectron spectroscopy (XPS) analysis. The surface morphology of films was observed by scanning electronic microscopy (Jeol JSM-7800F with field emission gun) with a working distance of 7.9 mm and an applied voltage of 5 kV. The topography and roughness of the films were observed using atomic force microscopy (SPM-9600). Elemental mapping and depth profiling were performed using secondary ion mass spectroscopy (TOF SIMS instrument from ION TOF). The optical transmittance spectra were collected using a UV–vis spectrophotometer (PerkinElmer, Lambda 950). The photoluminescence (PL) data was recorded using a 325 nm He-Cd laser based system. The current-voltage characteristics were measured with a Keithley 4200 semiconductor characterization system. Fig. 1 3. Results and Discussions 3.1 X-ray diffraction analysis The material phases and crystallite properties of the ZnO, CdO and ZnO-CdO thin films were assessed by means of XRD measurements in the scan range of 30˚ to 60˚ which are shown in fig.2. The pure ZnO thin film exhibited the hexagonal wurtzite structure (JCPDS: 01-075-0576) while CdO thin film had the cubic structure (JCPDS: 05-0640). For the sample ZC-1 containing predominantly ZnO, the pattern matched with the hexagonal structure, while for the sample ZC-5

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containing predominantly CdO the pattern matched well to its cubic structure. It is interesting to note that the experimental peaks of sample ZC-1 occur at angles slightly less than the standard, while the experimental peaks of sample ZC-5 occur at angles slightly greater than the standard. This can be explained by the larger ionic radius of Cd compared to Zn: the small amount of CdO in the ZnO making up sample ZC-1 caused a lattice expansion and hence a shift to smaller 2θ angles, while the small amount of ZnO in the CdO making up sample ZC-5 produced the opposite effect. The clear presence of mixed crystallite phases of both ZnO and CdO compounds with hexagonal and cubic structures, respectively was reported by Tabet-Derrazn et al. [25] and a similar result has also been obtained by Vigil et al. [26] for ZnO:CdO thin films deposited by spray pyrolysis. However, in this work no standard structure pattern was obtained for the ZC-2, ZC-3 and ZC-4 thin films, but some intermediate phase with peaks denoted by (*) in the XRD pattern, as the crystal structure of CdO (cubic, (hexagonal,

= 0.324 nm and

= 0.424 nm) is quite different from that of ZnO

= 0.520 nm). Based on the Clausius-Clapeyron equation, the

vapour pressure of Cd at 450 ˚C is about 13 times higher than the vapour pressure of Zn [27]. Therefore evaporation of Cd from the substrate could occur at a higher rate than Zn and some intermediate phase may be obtained. Nevertheless, the sample ZC-5 consisting mostly of CdO produced the pattern expected from this oxide. Fig. 2 The crystallite size was estimated for the ZC-1 and ZC-5 samples using Scherrer’s equation [28];

D

K  cos 

(1)

where K is a constant (taken as 0.94), λ is the wavelength (λ = 0.154 nm for Cu K X-rays) and β is the full width at half maximum (FWHM). The crystallite size for the ZC-1 sample determined 7

from the (002) peak was 20 nm, while a larger value of up to 52 nm was obtained for sample ZC5 with higher CdO content from the (111) peak. During the deposition process, ZnO and CdO were mixed and deposited; different crystal structures and different surface energies would induce lattice distortion due to which strain is generated [29]. The broadening of the XRD peaks which have been observed may also be due to a lattice mismatch developed at the interface of the ZnO and CdO, so the crystallite sizes can only be interpreted as lower limits. This microstrain change can be related to the crystallization process in the thin films. A similar result of strain was also obtained by Velusamy et al. [30] for CdO thin films prepared by the spray pyrolysis technique. The XRD studies showed that the crystallinity of the thin films changed with respect to the Zn:Cd ratio. 3.2 X-ray photoelectron spectroscopy XPS analysis was carried out on the ZC-1 and ZC-5 samples to evaluate the oxidation states of Zn, Cd and O. The different survey spectra obtained using Al Kα X-rays with energy 1486.6 eV are compared in fig. 3. Zn, O, Cd and C peaks were detected. The detected carbon is related to the carbon adsorbed on the surface during the exposure of the sample to the ambient atmosphere. All binding energies were corrected for the charge shift using the C 1s peak of graphitic carbon (284.6 eV) as a reference [31]. Fig. 3 The high resolution XPS spectra of the Zn 2p binding energy region are shown in fig. 4. The Zn 2p spectrum shows a doublet with binding energies 1021.2 eV and 1044.3 eV, which can be as ascribed to the 2p3/2 and 2p1/2 transitions. The binding energy difference is 23.1 eV, which is close to the standard reference value of ZnO [31]. The values of the binding energy and binding energy difference, measured from the XPS study, show that the Zn atoms were in the Zn2+

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oxidation state [32]. Although XRD peaks for ZnO were not observed for the sample ZC-5, the XPS data showed that Zn occurred on the surface of this sample. There is no significant difference of the Zn peaks between the samples ZC-1 and ZC-5, suggesting the Zn occurs as ZnO in both cases. Fig. 4 Very little Cd occurred on the surface of sample ZC-1, as shown by fig. 5(a), even after sputtering, which is not surprising since this sample had the lowest proportion of Cd. The concentration was much larger for the sample ZC-5 (fig. 5(b)) and the binding energy corresponding to the Cd 3d5/2 and 3d3/2 peaks are 403.8 eV and 410.3 eV, respectively. These values are in good agreement with the reported values given in the literature [33, 34]. The binding energy of Cd 3d5/2 indicates the Cd2+ states; this is in good agreement with the literature [33]. Fig. 5 The high resolution oxygen O1s peaks for ZC-1 and ZC-5 samples are shown in fig. 6. Kumar et al. [34, 35] reported that the O1s peak of ZnO may be de-convoluted into three peaks, namely O1, O2 and O3. They attributed the O1 peak at 530.1±0.3 eV to O2- ions in the wurtzite structure of a hexagonal Zn2+ ion array, surrounded by Zn atoms with their full complement of nearest neighbour O2- ions [35-37]. The O2 peak at 531.1±0.3 eV is associated with O2- ions that are in oxygen deficient regions within the matrix of ZnO (such as ZnOH) [34]. The O3 peak at 532.3±0.3 eV is usually attributed to chemisorbed species (such as CO3, adsorbed H2O or O2) on the surface of the ZnO [37]. The intensity of the O3 peak has decreased after sputter cleaning due to the removal of surface contaminants. The extra peak at lower binding energy side (529.2±0.3 eV) is attributed to oxygen correlated to CdO [38]. XPS results confirmed that the defect

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concentration in the films, as indicated by the O2 peak, increased with an increase in the CdO concentration. Fig. 6 3.3 Surface morphology The morphology of the ZnO, CdO and ZnO-CdO thin films was evaluated using SEM images as shown in fig.7 and fig.8. Fig.7 A wrinkled structure was obtained for the ZnO thin film while a flower-like structure was obtained for the CdO thin film. The ZC-1 film showed that some small grains were located between or on some bigger grains. The wrinkle structure may be due to high amount of ZnO. In ZnO thin films produced by sol–gel, compressive membrane forces due to the difference in thermal expansion coefficients between the ZnO films and substrate is generated during the drying process, which leads to bending of the gelated thin films and generates disordered wrinkles [39]. The morphology of the thin films continuously changed as the ratio of the Zn:Cd changed. The agglomerated grains are obtained for ZC-2 and ZC-4 thin films, respectively, which were uniformly distributed on the surface. A flake-like morphology was obtained for the ZC-3 thin film in which ZnO and CdO are in equal proportion, while a cauliflower like morphology was obtained for the CdO rich ZC-5 thin film. Such a cauliflower structure has a great importance due to its high specific surface area and potential applications in various fields. Similar results were obtained by Ren et al. [40] with different Zn concentration in CdO. The energy dispersive X-ray spectroscopy (EDS) analysis showed the homogeneous distribution of both Zn and Cd elements in the ZC-3 thin film. Carbon and Si peaks were also observed due to contamination and the substrate, respectively.

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Fig. 8 3.4 Atomic force microscopy (AFM) AFM measurements were performed to study the differences on the surface microstructure and roughness of the ZnO:CdO thin films. The AFM images (fig. 9) were taken in a 5×5 μm2 area and show that all the films consisted of nanoparticles while the particle size continuously increased with increasing Cd content. The surface root-mean-square roughness values of different films were found to be 12, 23, 17, 14 and 29 nm for ZC-1, ZC-2, ZC-3, ZC-4 and ZC-5, respectively. The increase in surface roughness with increasing CdO content is associated with the Zn/Cd ratio for different sample. The average grain sizes were found to be almost in the same range of 100 nm for all of the samples. Similar results were obtained by Gupta et al. [41] in CdZnO thin films on glass substrates by sol-gel method. It was observed that the surface characteristics of all samples were the same, only the grain size and roughness changed with relative oxide proportion. Fig. 9

3.5 Secondary ion mass spectrometry (SIMS) SIMS is a sensitive surface analysis technique to determine the elemental, isotopic or molecular composition of the surface of thin films. In this work, TOF SIMS was performed on the ZC-1 and ZC-5 thin films to record the compositional analysis as a function of depth. Some isotopes of Zn (66Zn+,

68

Zn+,

70

Zn+) and Cd (108Cd+,

110

Cd+,

112

Cd+) as well as hydrocarbons (e.g. C3H5+)

were observed in the SIMS analysis. Positive ion spectroscopy was performed using an oxygen sputtering gun at 1 kV, 250 nA for 10 s intervals on the surface area 300×300 μm2 and base pressure of 1.2 × 10-8 mbar. The element analysis is shown in fig. 10. The position of the

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interface was estimated as the sputtering time for which the Si+ signal from the substrate reached half its maximum value and is indicated by vertical dotted lines in the figure. As expected, the ZC-5 thin film contains much more Cd as compared to the ZC-1 film, and it can be observed that the Cd of the thin films segregated to the substrate interface (as well as to the thin film surface) during annealing at 450 °C in the preparation process. Kumar et al. have reported a similar analysis on ZnO nanofibre thin films grown by spray pyrolysis and observed than due to annealing the source atoms were embedded into the substrate surface [42]. Fig. 10 3.6 Optical study The optical properties of the thin films were studied for ZnO, CdO and different proportion of Zn:Cd. fig.11 shows the transmittance spectra in the wavelength range 300-800 nm respectively. The films ZC-1 to ZC-5 were roughly 60% - 80% transparent in the visible region 400 nm - 800 nm. The transparency decreased as the CdO concentration increased in the films. Fig. 11 The decrease in the transmittance with an increasing CdO concentration in the doped samples might be due to band-to-band absorption of the CdO with its bandgap smaller than that of ZnO or due to the increased optical scattering of incident light on the film surface. Similar results were obtained by Acharya et al. [43] in Cd doped ZnO thin films prepared by spray pyrolysis. They concluded that all undoped and Cd doped ZnO thin films have an 80% transmittance in the visible region and obtained a sharp absorption edge in the ultraviolet region. The dependence of the absorption coefficient with the photon energies fitted to the relationship for the allowed direct transition [44];

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h 2  B h  E g 

(2)

where B is a constant. The optical bandgap Eg values were obtained by extrapolating the linear portion of the plots of (αhν)2 vs. hν shown in fig. 12. With the increase in Cd concentration the optical bandgap decreased from 3.15 eV to 2.15 eV. The optical absorption edge exhibited a red shift with the increase of Cd concentration from ZnO to CdO. The change in the bandgap observed because CdO has smaller bandgap than ZnO [45]. Fig. 12 The PL spectra of the ZnO-CdO composite films are shown in fig. 13. All the films showed ultraviolet emissions centred at 389 nm, which correspond to the near band edge emission due to the free excitonic emission of ZnO. Except in sample ZC-4 the position of this peak remained the same, suggesting it originated from ZnO. However, its intensity did not correlate to the proportion of ZnO in the composite. Some emission in the visible region 500 nm - 800 nm as one prominent at 730 nm is also present, which is attributed to defects such as zinc interstitials and oxygen vacancies. As the CdO content increased the defect peak intensity increased due to the replacement of Zn atoms by Cd atoms [46]. Further increasing the Cd concentration (ZC-5) led to a broad low intensity peak at 520 nm, which may be due to different oxygen defects. Fig. 13 4.

Electrical Properties

The electrical characterization of the ZnO, CdO and ZnO-CdO thin films was carried out by using the two probe method. After making contacts with silver paste the samples were annealed at 80 ˚C for 1 h to make the contacts ohmic. The variation of current with voltage was noted between -10 and 10 V at room temperature and room lighting for all samples and is shown in fig. 13

14. The ohmic conduction mechanism was observed for rich CdO content thin films (ZC-4, ZC5, CdO) and they have a low resistivity compared to the other samples, for which there was a deviation from ohmic behaviour, together with high resistivity. It was observed that the ZnO rich thin film (ZnO, ZC-1 and ZC-2) has a high resistivity, which may be due to grain boundary effects and also, due to the semiconducting nature of the ZnO which will create a potential barrier that affects the electrical transport causing a reduction in conductivity [47]. With increasing Cd content the resistivity has decreased as observed for the ZC-5 thin film. This interpretation is consistent with the optical measurements where a decrease in bandgap with an increase in Cd concentration occurred. The electrical current varies from 4×10-9 A to 6×10-5 A at 10 V with samples ZC-1 to ZC-5 respectively. Similar electrical results were obtained by Flores et al. [48] for spray deposited (ZnO) X (CdO)1−X thin films. The high value of the conductivity in the CdO rich films is characteristic for them, and it has been attributed to the relatively large concentration of ionized atomic defects, such as oxygen vacancies or cadmium interstitials. It was found that ZC-1 thin films showed high resistivity due to more ZnO content but a dramatic fall in resistivity was found when the CdO content was increased. The bend in the ZC-3 thin films at low voltage may be due to a potential barrier at the interface of the ZnO-CdO. Further increasing of the CdO concentration has led to the continuous decrease in resistivity. The Cd proportion therefore plays an important role in the electrical characteristics. Fig. 14 Conclusion Mixed oxides with different ratios of Zn:Cd thin films were successfully deposited on ordinary glass substrates using the sol-gel spin coating method. The XRD diffraction results showed that

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the polycrystalline nature of the thin films changed from the hexagonal structure characteristic of ZnO to a cubic structure characteristic of CdO for the thin films containing a high concentration of ZnO to the films containing a high concentration of CdO, with no smooth transition. XPS results confirmed that defects in the films increased with an increase in the CdO concentration. A spheroidal agglomerated morphology was obtained for all thin films. The roughness of the surface increased from 12 nm to 29 nm for the highest concentration CdO thin films. The depth profile obtained by SIMS showed that Cd had segregated to the substrate interface and the film surface for the annealed films. The transmittance and bandgap of the sample decreased with an increasing CdO concentration. The resistivity of the mixed oxide thin films decreased with increasing CdO proportion. These highly conducting and low optical bandgap thin films may be suitable for applications in solar cells and optoelectronic devices. Acknowledgement The authors acknowledge the DST, Govt. of India for supporting under FIST programme to Department of Physics, Gurukula Kangri Vishwavidyalaya, Haridwar, India. One of the authors (VK) is thankful to DST, New Delhi, India for support through DST-Inspire faculty award [DST/INSPIRE/04/2015/001497]. The PL system used in this study is supported both technically and financially by rental pool programme of the national laser centre (NCL) (Grant No. NLCLREGM00-CON-001). This work is supported both by the South African Research Chairs Initiative of the Department of Science and Technology, the National Research Foundation of South Africa (84415). Dr. Mart-Mari Duvenhage and Dr. Liza Coetsee-Hugo are acknowledged for SIMS and XPS measurements, respectively. The University of the Free State is acknowledged for financial support.

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References [1] K.T. Ramakrishna Reddy, G.M. Shanthini, D. Johnston, R.W. Miles, Highly transparent and conducting CdO films grown by chemical spray pyrolysis, Thin Solid Films 427 (2003) 397. [2] R.A. Ismail, O.A. Abdulrazaq, A new route for fabricating CdO/Si heterojunction solar cells, Solar Energy Mater, Solar Cells 91 (2007) 903. [3] H. Kim, J.S. Horwitz, G.P. Kushto, S.B. Qadri, Z.H. Kafafi, D.B. Chrisey, Transparent conducting Zr-doped In2O3 thin films for organic light-emitting, Appl. Phys. Lett.78 (2001) 1050. [4] L.M. Su, N. Grote, F. Schmitt, Characterization of sprayed CdO thin films, Electron. Lett. 20 (1984) 716. [5] R. Ferro, J.A. Rodriguez, O. Vigil, A. Morales-Acevedo, G. Contreras-Puente, F-doped CdO thin films deposited by spray pyrolysis, Phys. Stat. Sol. A 177 (2000) 477. [6] S.B. Zhang, S.H. Wei, A. Zunger, Intrinsic n-type versus p-type doping asymmetry and the defect physics of ZnO, Phys. Rev. B 63 (2001) 075205. [7] V. Kumar, V. Kumar, S. Som, A. Yousif, N. Singh, O. M. Ntwaeaborwa, A. Kapoor and H.C. Swart, Effect of annealing on the structural, morphological and photoluminescence properties of ZnO thin film prepared by spin coating. J. Colloid and Interface: 428 (2014) 8. [8] T.K. Pathak, V. Kumar, H.C. Swart, L.P.Purohit, Electrical and optical properties of p-type codoped ZnO thin films prepared by spin coating technique, Physica E77 (2016) 1. [9] C. Sravani, K.T.R. Reddy, P.J. Reddy, Physical behaviour of CdO films prepared by activated reactive evaporation, Semicond. Sci. Technol. 6 (1991) 1036.

16

[10] O. Vigil, F. Cruz, A. Morales-Acevedo, G. Contreras-Puente, L. Vaillant, G. Santana, Structural and optical properties of annealed CdO thin films prepared by spray pyrolysis, Mater. Chem. Phys. 68 (2001) 249. [11] Y.-S. Choi, C.G. Lee, S.M. Cho, Transparent conducting ZnxCd1−xO thin films prepared by the sol-gel process, Thin Solid Films 289 (1996) 153. [12] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Band gap engineering based on MgxZn1−xO and CdyZn1−yO ternary alloy films, Appl. Phys. Lett. 78 (2001)1237. [13] H. Tabet-Derraz, N. Benramdane, D. Nacer, A. Bouzidi, M. Medles, Investigations on ZnxCd1−xO thin films obtained by spray pyrolysis, Sol. Energy Mater. Sol. Cells 73 (2002) 249. [14] K. Sakurai, T. Takagi, T. Kubo, D. Kajita, T. Tanabe, H. Takasu, S. Fujita, S. Fujita, Spatial composition fluctuations in blue-luminescent ZnCdO semiconductor films grown by molecular beam epitaxy, J. Cryst. Growth 237–239 (2002) 514. [15] F. Cruz-Gandarilla, A. Morales-Acevedo, O. Vigil, M. Hesiquio-Garduño, L. Vaillant, G. Contreras-Puente, Micro-structural characterization of annealed cadmium-zinc oxide thin films obtained by spray pyrolysis, Mater. Chem. Phys. 78 (2003) 840. [16] X. Wu, T. J. Couttsand, W. P. Mulligan, Properties of transparent conducting oxides formed from CdO and ZnO alloyed with SnO2 and In2O3, J. Vac. Sci. Technol. A15 (1997) 1057. [17] A.A. Ziabari, F.E. Ghodsi, Optoelectronic studies of sol–gel derived nanostructured CdO– ZnO composite films, Journal of Alloys and Compounds, 509(35) (2011) 8748. [18] M. Tortosa, M. Mollar, B. Mar, Synthesis of ZnCdO thin films by electrodeposition, J Crys. Growth 304(1) (2007) 97.

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[19] Z. Zhao, D.L. Morel, C.S. Ferekides, Electrical and optical properties of tin-doped CdO films deposited by atmospheric metalorganic chemical vapor deposition, Thin Solid Films 413 (2002) 203. [20] S. Sadofev, S. Blumstengel, J. Cui, J. Puls, S. Rogaschewski, P. Schafer, Visible band-gap ZnCdO heterostructures grown by molecular beam epitaxy, Appl Phys Lett 89 (2006) 201907. [21] G. Torres-Delgado, C.I. Zuniga-Romero, O. Jimenez-Sandoval, R. Castanedo-Perez, B. Chao, S. Jimenez-Sandoval, Percolation mechanism and characterization of (CdO) y(ZnO)1-y thin films. Adv. Funct. Mater 12 (2002) 129. [22] F. Wu, L. Fang, Y.J. Pan, K. Zhou, Q.L. Huang, C.Y. Kong, Effect of substrate temperature on the structural, electrical and optical properties of ZnO:Ga thin films prepared by RF magnetron sputtering, Physica E 43 (2010) 228. [23] H. Tabet-Derraz, N. Benramdane, D. Nacer, A. Bouzidi, M. Medles, Investigations on ZnxCd1xO thin films obtained by spray pyrolysis, Sol Energ Mater Sol Cells 73 (2002) 249. [24] Vinod Kumar, N. Singh, R.M. Mehra, A. Kapoor, L.P. Purohit, H.C. Swart, Role of film thickness on the properties of ZnO thin films grown by sol-gel methodThin Solid Films 539 (2013) 161–165. [25] H. Tabet-Derrazn, N. Benramdane, D. Nacer, A. Bouzidi, M. Medles, Investigations on ZnxCd1-xO thin films obtained by spray pyrolysis, Sol. Energy Mater. Sol. Cells 73 (2002) 249. [26] O. Vigil, L. Vaillant, F. Cruz, G. Santana, A. Morales-Acevedo, G. Contreras-Puente, Spray pyrolysis deposition of cadmium–zinc oxide thin films, Thin Solid Films 361–362 (2000) 53.

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[27] J.C. Greenbank, B.B. Argent, Vapour pressure of magnesium, zinc and cadmium, Trans. Faraday Soc. 61(1965) 655. [28] R.E. Kroon, Nanoscience and the Scherrer equation versus the Scherrer–Gottingen equation, South African Journal of Science 109(5/6) (2013) Art. #a0019, 2 pages. [29] S. Fujihara, C. Sasaki, T. Kimura, Crystallization behavior and origin of c-axis orientation in sol–gel-derived ZnO:Li thin films on glass substrates, Appl. Surf. Sci. 180 (2001) 341. [30] P. Velusamy, R. Ramesh Babu, K. Ramamurthi, M.S. Dahlem, E. Elangovan, Highly transparent conducting cerium incorporated CdO thin films deposited by a spray pyrolytic technique, RSC Adv. 5 (2015) 102741. [31] J.F. Moulder, W.F. Strickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, ULVAC-PHI Inc, Enzo, Chigasaki, 1995. [32] M.N. Islam, T.B. Ghosh, K.L. Chopra, H.N. Acharya, XPS and X-ray diffraction studies of aluminum-doped zinc oxide transparent conducting films, Thin Solid Films 280 (1996) 20. [33] L.L. Pan, K.K. Meng, G.Y. Li, H.M. Sun, J.S. Lian, Structural, optical and electrical characterization of gadolinium and indium doped cadmium oxide/p-silicon heterojunctions for solar cell applications, RSC Adv. 4 (2014) 52451. [34] C. Zhang, J. Lin, Visible-light induced oxo-bridged ZrIV−O−CeIII redox centre in tetragonal ZrO2–CeO2 solid solution for degradation of organic pollutants, Phys. Chem. Chem. Phys., 13 (2011) 3896. [35] V. Kumar, H.C. Swart, O.M. Ntwaeaborwa, R.E. Kroon, J.J. Terblans, S.K.K. Shaat, A. Yousif, M.M. Duvenhage, Origin of the red emission in zinc oxide nanophosphors, Mater. Lett. 101 (2013) 57.

19

[36] Vinod Kumar, O.M. Ntwaeaborwa, H.C. Swart, Deep level defect correlated emission and Si diffusion in ZnO:Tb3+ thin films prepared by pulsed laser deposition, J. Colloid and Interface science 465 (2016) 295. [37] R. Al-Gaashani, S. Radiman, A.R. Daud, N. Tabet, Y. Al-Douri, XPS and optical studies of different morphologies of ZnO nanostructures prepared by microwave methods, Ceram. Int. 39 (2013) 2283. [38] Hani Khallaf, Chia-Ta Chen, Liann-Be Chang, Oleg Lupan, Aniruddha Duttaa, Helge Heinrich, A. Shenouda, Lee Chow, Investigation of chemical bath deposition of CdO thin films using three different complexing agents, Applied Surface Science 257 (2011) 9237. [39] C. Justin Raj, S.N. Karthick, K.V. Hemalatha, Soo-Kyoung Kim, Byung Chul Kim, KookHyun Yu, Hee-Je Kim, Synthesis of self-light scattering wrinkle structured ZnO photoanode by sol-gel method for dye-sensitized solar cells, Applied Physics A 116, 2 (2014) 811. [40] H.X. Ren, X.J. Huang, O. Yarimaga, Y.K. Choi, N. Gu, A cauliflower-like gold structure for super hydrophobicity, J. Colloid Interface Sci. 334 (2009) 103. [41] R.K. Gupta, M. Cavas, F. Yakuphanoglu, Structural and optical properties of nanostructure CdZnO films, SpectrochimicaActa Part A: Molecular and Biomolecular Spectroscopy 95 (2012) 107. [42] N. Sadananda Kumar, Kasturi V. Bangera, G. K. Shivakumar, Effect of annealing on the properties of zinc oxide nanofiber thin films grown by spray pyrolysis technique, Appl Nanosci 4 (2014) 209. [43] A.D. Acharya, Shweta Moghe, Richa Panda, S.B. Shrivastava, Mohan Gangrade, T. Shripathi, D.M. Phase, V. Ganesan, Effect of Cd dopant on electrical and optical properties of ZnO thin films prepared by spray pyrolysis route, Thin Solid Films 525 (2012) 49. 20

[44] T.K. Pathak, V. Kumar, H.C. Swart, L.P. Purohit,

P-type conductivity in doped and

codoped ZnO thin films synthesized by RF magnetron sputtering, Journal of Modern Optics, 62, 17 (2015) 1368. [45] F. Yakuphanoglu, S. Ilican, M. Caglar, Y. Caglar, Microstructure and electro-optical properties of sol–gel derived Cd-doped ZnO films, Superlattices Microstruct. 47 (2010) 732. [46] R. Hong, J. Shao, H. He, Z. Fan, Enhancement of near band edge photoluminescence of ZnO thin films in sandwich configuration at room temperature, J. Appl. Phys. 99 (2006) 093520. [47] B. Joseph, K.G. Gopchandran, P.K. Manoj, P. Koshy, V.K. Vaidyan, Optical and electrical properties of zinc oxide films prepared by spray pyrolysis, Bull Mater Sci 22 (1999) 921. [48] G. Alarcon-Flores, B. Vasquez-Perez, A. Pelaez-Rodrıguez, M. Villa-Garcia, S. CarmonaTellez, J. A. Luna-Guzman, C. Falcony, M. Aguilar-Frutis, Electrical and structural characteristics of spray deposited (ZnO) X-(CdO)1−X thin films, Revista Mexicana de Fisica 59 (2013) 403.

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Fig. 1 Flow chart for the deposition of the ZnO:CdO thin films by the sol-gel spin coating method.

22

Fig. 2 XRD patterns of the different ZnO:CdO thin films with JCPDS: 01-075-0576 and JCPDS: 05-0640.

Fig. 3 XPS survey spectra of (a) ZC-1 (b) ZC-5 samples, before and after 30 s of Ar+ ion sputtering.

23

Fig. 4 High-resolution XPS spectra of Zn peaks for (a) ZC-1 (b) ZC-5 samples, before and after

395

400

405

(b)

410

415

420

425

430

Before Sputtering After Sputtering

395

400

Cd 3d5

Cd 3d3

Before Sputtering After Sputtering

Intensity (a.u.)

Cd 3d5

Intensity (a.u.)

(a)

Cd 3d3

30 s of Ar+ sputtering.

405

410

415

420

425

430

Binding Energy (eV)

Binding Energy (eV)

Fig. 5 High-resolution XPS spectra of Cd peaks for (a) ZC-1 (b) ZC-5 samples, before and after 30 s of Ar+ sputtering.

24

(a) Intinsity (a.u.)

O1 O2 O3 CdO

536

534

532

530

528

526

Binding Energy (eV)

Intensity (a.u.)

(b)

524 536

532

530

528

526

524

Binding Energy (eV)

(c)

(d) Intensity (a.u.)

Intensity (a.u.) 536

534

534

532

530

528

526

Binding Energy (eV)

524 536

534

532

530

528

526

Binding Energy (eV)

524

Fig. 6 High-resolution XPS spectra of O peaks for (a) ZC-1 before sputtering (b) ZC-1 after sputtering (c) ZC-5 before sputtering (d) ZC-5 after sputtering.

Fig. 7 FE-SEM image of ZnO, CdO thin films. 25

Fig. 8 FE-SEM and EDS of different content ZnO-CdO thin films.

26

Fig. 9 AFM topographic images (5 μm x 5 μm) of the different ZnO:CdO thin films.

Fig. 10 TOF SIMS depth profiles for (a) ZC-1 thin film (b) ZC-5 thin film (Please note: Si+ intensity in the depth profile has been divided by 5 for clarity.)

27

Fig. 11 Transmittance spectra of the different ZnO-CdO thin films.

28

Fig. 12 Tauc plots of the different ZnO-CdO thin films.

Fig. 13 Photoluminescence of ZnO-CdO thin films. 29

Fig. 14 Current-Voltage characteristics of ZnO-CdO thin films.

30

Table 1 Composition and abbreviation of ZnO-CdO mixed oxides thin films. Sample Composition (ZnO:CdO) 3:1

Abbreviation ZC-1

3:2

ZC-2

1:1

ZC-3

2:3

ZC-4

1:3

ZC-5

31

Graphical Abstract

32