Variation of the key morphological, structural, optical and electrical properties of SILAR CdO with alkaline earth Ca2+ ions doping

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Contents lists available atScienceDirect

Ceramics International

journal homepage:www.elsevier.com/locate/ceramint

Variation of the key morphological, structural, optical and electrical

properties of SILAR CdO with alkaline earth Ca

2+

ions doping

Bünyamin

Şahin

a,∗

, Ra

şit Aydın

b

, Hidayet Çetin

c a_{Department of Physics, Faculty of Arts and Sciences, Mustafa Kemal University, Hatay, Turkey} b_{Department of Physics, Faculty of Sciences, Selçuk University, Konya, Turkey}

c_{Department of Physics, Faculty of Arts and Sciences, Bozok University, Yozgat, Turkey}

A R T I C L E I N F O Keywords: Cadmium oxide Calcium Doping Band gap Impedance A B S T R A C T

Pristine and alkaline earth Ca2+_{ions doped (Ca}

xCd1-xO (0≤ x ≤ 0.025)) CdO ﬁlms were fabricated by SILAR

technique on the soda lime glass substrates. The influence of increasing Ca content on the morphological, structural, optical and electrical properties of depositedfilms was analyzed. Metallurgical Microscope (MM), Scanning Electron Microscope (SEM) and Atomic Force Microscopy (AFM) images of the samples exhibited that the morphology was dramatically changed with the addition of Ca to the synthesis solution when compared to the pristine CdOfilm. Energy Dispersive Spectrometry (EDS) analyses confirmed the presence of Ca in the doped films. X-ray diffraction (XRD) analysis of the pristine and Ca-doped CdO films exhibited cubic crystalline structure with preferred orientation of (111) and (200) direction. The existence of chemical bonding was con-firmed by Fourier Transform Infrared Spectroscopy (FTIR) study. Optical studies revealed that the energy band gap were dependent on Ca-doping content in accordance with both Vegard's relation and Tauc's law calculations. The impedance analysis and four-point probe measurement results of CdO thinfilms were studied. Sheet re-sistances of the thinfilms were increased by Ca doping up to doping level of 1.5%. Further doping level causes degradation in sheet resistance.

1. Introduction

In semiconductor optoelectronic device applications, transparent conductingﬁlms of metal oxide materials like indium oxide [1], zinc oxide [2], tin oxide [3] and cadmium oxide [4] play an important role. Among these oxide materials cadmium oxide (CdO), a promising II–VI group material, is one of the preﬀered semiconductors with low elec-trical resistivity due to cadmium interstitials and oxygen vacancies [5,6].

CdO is an n-type degenerate semiconductor oxide material with cubic crystallite lattice structure and low electrical resistivity. Also, CdO has large direct optical band gap energy about 2.2–2.7 eV taining superior optical transmittance and perfect electrical con-ductivity [7,8]. These features make CdO a viable oxide material for technological uses such as solar cells [9], heat mirrors [10], gas sensors [11],ﬂat panel displays [12], photodiodes [13] and phototransistors [14]. Several growth procedures were devoloped to synthesize pristine and doped nanostructured CdO ﬁlms via pulsed laser deposition method [15], sol-gel process [16], spray pyrolysis way [17], sputtering technique [18], chemical bath deposition procedure [19], thermal

evaporation method [20] and successive ionic layer adsorption and reaction (SILAR) [21]. Among these techniques SILAR oﬀers a cheap and easy solution-based synthesis approach in which theﬁlm thickness can easily be controlled. Therefore, SILAR technique was used in this study [22,23].

One of the procedures for the alteration of the physical features of CdOfilms is the doping with different elements such as Cu [24], Zn [25], Li [26] and In [27]. Calcium (Ca), the bivalent alkaline earth metal, is a significant impurity element to control the performances of films for practical optoelectronic applications. In addition, Ca is an abundant, inexpensive and nonhazardous additive element. Since Ca atoms have a greater radius value than Cd atoms they dictate changes in the lattice structure of thefilms. Thus, morphological, structural, op-tical and electrical attributes of Ca doped CdOfilms greatly differ from pristine CdOfilms [28–30]. Due to these important properties, Ca as a doping atom is added into CdO to develop the performance of CdOfilm in this study.

To the best of our knowledge, no scientiﬁc works are available on the physical activities of transparent conductive Ca doped CdOﬁlms obtained by the SILAR method. Hence, in this study we synthesized

https://doi.org/10.1016/j.ceramint.2019.05.210

Received 4 April 2019; Received in revised form 17 May 2019; Accepted 20 May 2019

∗_{Corresponding author.}

E-mail address:bsahin@mku.edu.tr(B.Şahin).

Available online 21 May 2019

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Fig. 1. The counter plot of photographs of CaxCd1-xO (x = 0, 0.005, 0.010, 0.015, 0.020 and 0.025)ﬁlms at 200X magniﬁcation.

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CdOﬁlms with various Ca content (at 0, 0.5, 1.0, 1.5, 2.0 and 2.5 M%) by SILAR procedure on glass slides for theﬁrst time and studied their morphological, structural, optical and electrical properties.

2. Experimental details

Pristine and Ca-doped nanostructured CdO films were coated on soda lime glass substrates by using the SILAR technique. The cationic solution consists 0.1 M Cadmium acetate (Cd(CH3COO)2.H2O) and 0.1 molar ratio Calcium dichloride (CaCl2) for dopant element (0.5,1.0,1.5,2.0 and 2.5 M% calcium ratios). The pre cleaned soda lime glass substrate was immersed into the cationic precursor for 20 s in 85 °C and then immersed in anionic precursor DI water for 30 s in 85 °C. This SILAR cycle was repeated twenty times. Grown films were then annealed at 350 °C for 45 min in order to transform Cd (OH)2to CdO. The surface morphological properties of thefilms were examined by Zeiss Evo Ls 10 scanning electron microscope (SEM) (operated at an acceleration voltage of 20 kV) with an EDX detector, Nikon Eclipse LV 150 metallurgical microscope and TEGRA Solaris Atomic Force Microscope. The present phase components and orientations of the films were investigated by X-ray diffraction (Bruker D8 Advance X-ray diffractometer) with a high-intensity Cu Kα radiation (λ = 0.1506 nm). The optical absorption spectra of the productfilms were acquired using a UV–Vis spectrophotometer (Thermo Scientific Genesys 10S) at the wavelengths from 190 nm to 1100 nm. Four-point probe measurements were obtained with HBE-T11 model probe station in which distance between probe centers of the axis is 1 mm. All current measurements were taken in the range of 1 × 10−6A -1x10−4A with 1 × 10−6A steps for high resolution at room temperature with confirmation from the measurement software regarding the statistical significance.

Impedance of ﬁlms was analyzed via Agilent 4284A LCR meter

through LabVIEW. Real and imaginary impedance and phase angle measurements were in the range of 20 Hz- 1 MHz at 10 mV oscillator voltage level. Phosphor-bronze probes were used for the impedance measurements and the distance between probes was measured as 1.5 mm.

3. Result and discussions

3.1. Metallurgical microscope (MM) images

Film surface photos were taken with 200X magnification and were transformed to counter plots to reveal surface structure and make heights and pits more visible.Fig. 1shows the counter plot of photo-graphs of the pristine and different amount Ca doped CdO films at 200X magnification.Fig. 1-a shows the pristine CdOfilm surface. Although there are regions which have different reflectance, the pristine CdO has relatively homogenous surface. The image of 0.5% Ca doped CdOfilm is shown atFig. 1-b. InFig. 1-b, a significant change can be seen at the contrast of reflected light from the surface when compared to that of the pristine CdO thinfilm. The surface homogeneities changed dramatically to an inhomogeneous structure due to Ca atoms dispersed onto CdO surface and they affected the homogeneity of CdO film.Fig. 1-c shows 1% Ca doped surface and an inhomogeneous surface was again ob-served. 1.5% Ca doped CdO surface is shown inFig. 1-d. Although the Ca doping was increased to 1.5%, a more uniform surface was observed. At 1.5% doping, Ca atoms dispersed to the surface within shorter dis-tances and surface became relatively homogeneous structure with the increasing Ca doping.Fig. 1-e shows 2% Ca doped CdO surface. When Ca doping was increased to 2% level, Ca atoms located to the films more frequently. Thus, an interaction started between each Ca atoms. Accordingly, it has resulted in the formation of new regions on CdO

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Fig. 3. Elemental composition of pristine CdO and Ca0.005Cd0.995Oﬁlms.

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surface. In this case, the surface homogeneity deteriorates. Fig. 1-f shows CdO surface with 2.5% Ca doping. When the amount of the Ca doping was increased to 2.5%, an inhomogeneous surface was obtained due to the closer Ca regions and extensive interactions between them.

3.2. SEM and EDS analysis

Micro morphological structure of these films were examined by SEM. Fig. 2 indicates the SEM images of the surface structures of CaxCd1-xO (x = 0, 0.005, 0.010, 0.015, 0.020 and 0.025)films. As can be seen fromFig. 2, nanosized particles were successfully obtained and all of the glass slides were coated by CdO and Ca nanoparticles. The surface structures of the Ca dopedfilms were significantly altered with the addition of Ca to the synthesis solution when compared to the pristine CdOfilm. These differences in the surface of the CdO samples could be caused by the change in the lattice matrix of CdOfilm by the doping of Ca2+[31,32]. The SEM surface pictures also demonstrate that the nanostructured CdO films have a porous and agglomerated structure with Ca doping. As seen from Fig. 2 (b, d) the CaxCd1-xO samples with varied Ca2+contents exhibit similar surface morpholo-gical structure and grain size. In other words, Ca2+doping concentra-tion has obvious impact on the surface morphology and grain size of CdOfilms [33–35].

To examine the elementary analysis of pristine CdO and Ca0.005Cd0.995Oﬁlms EDS analysis was utilized. The chemical elemental analysis of theseﬁlms is shown inFig. 3and the chemical elemental composition is presented in Table 1. The expected elements are ob-served in the EDS spectrum which reveals the entity of Cd, O and Ca without any other impurity element. As can be seen fromTable 1the atomic composition percentages of Cd, O and Ca contents were found as 26.37%, 72.82% and 0.80%, respectively.

3.3. AFM images

The surface morphologies of the pristine and Ca-doped CdOﬁlms were also examined by AFM. AFM is a convenient technique to check the surface topography of the nanostructured thinﬁlm materials. Three surface roughness (SR) parameters (averages of surface roughness, root-mean square (RMS) and height result) were measured by using TEGRA Solaris Atomic Force Microscope in the contact imaging mode. All measurements were taken from 3 × 3μm2_{areas. The results were also}

presented as 2D and 3D images. As presented in Fig. 4, all samples exhibited well adheredfilms that covered the substrates. The impacts of Ca content in the growth solution on the SR properties of nanos-tructured CdO samples were examined.Table 2shows SR parameters for pristine and different concentration of Ca doped CdO films. The SR parameters of CdOfilms were firstly decreased and then increased with the increasing amount of Ca doping as shown inTable 2. In addition, Ca0.020Cd0.980Ofilm resulted in minimum SR values.

3.4. XRD analysis

The XRD method is an effective tool for phase identification of crystalline CdOfilms. The crystalline structures of the SILAR deposited CdOfilms as a function of the Ca content were examined by XRD. The XRD plots of the pristine and Ca doped CdOfilms prepared with dif-ferent Ca contents are given inFig. 5. According to the value of JCPDS card no 05–0640 all the deposited samples are the cubic crystalline

structure of CdO. As shown inFig. 5the two pre-potent diﬀraction lines can be indexed to (111) and (200) planes. The two peaks corresponding to the (220) and (311) orientations also displayed thin peak intensity. The (111) and (200) peak intensities raised with Ca dopant con-centration up to 2.0 M% and then reduced (Ca: 2.5 M%). These changes in peak intensities are due to the various ionic radii of the Ca2+_and Cd2+ _{ions (Cd}2+_{: 0.95}_{Å, Ca}2+_{: 1.00 Å; for 6-coordination number)} [36,37]. The peak intensity and preferred orientation values obtained from the XRD analysis were tabulated inTable 3.

The preferential growth orientation of the pristine and Ca doped CdOﬁlms can be examined by estimating the texture coeﬃcient (TC (hkl)) for hegemonic (111) and (200) planes using [38]:

= ∑ − TC I I N I I / / hkl hkl hkl N hkl hkl ( ) ( ) 0( ) 1 ( ) 0( ) (1)

where I(hkl)represents measured relative intensity, I0(hkl)standard re-ference intensity (taken from data card; JCPDS card No. 05–0640) and N is the total diffraction numbers. The calculated TC(hkl)values of all synthesized CdOfilms were shown inTable 3. These calculated TC(hkl) values of allfilms are higher than one for the preponderant (111) and (200) planes. FromTable 3, it is obvious that the biggest TC(111)values for the (111) preferred orientations are calculated to be 1.52, 1.54, 1.54, 1.51, 1.52 and 1.52 for the pristine and Ca doped CdO films (Ca:0.5, 1.0, 1.5, 2.0 and 2.5 M%), respectively. TC(200)plane denotes a same tendency like that of TC(111)plane but it has smaller values than that of TC(111).

The structural factors of the pristine and Ca doped CdOﬁlms like crystallite size (D) and microstrain (ε) were calculated using [39]:

= D λ β θ 0.94 cos (2) = ε βcosθ 4 (3)

hereλ symbolizes the X-ray wavelength, θ the diffraction angle, β the FWHM value of the diffraction lines. The calculated mean D and ε va-lues were listed inTable 4. It is found that the D andε of pristine CdO film have 17.90 nm and 2.02 × 10−3

, respectively. The estimated mean D value increased with Ca doping but started to degrade with heavy doping of Ca. The trend inε was similar to the D value but in the reverse direction. This opposite relationship between ε and D is shown in Table 4. The change of the D andε can be attributed to the replacement of Cd2+_{ions by Ca}2+_{ions in CdO matrix structure due to the ionic} radius diﬀerences between Ca2+

and Cd2+ions [40,41].

3.5. FTIR analysis

The bond structure and chemical nature of the grownfilms were studied via FTIR spectroscopy. Fig. 6 shows the FTIR spectrum of pristine and different concentrations of Ca-doped CdO nanostructures recorded at the range 4000–400 cm−1_{. It exhibits that the group of} metal-hydroxide (M-OH) observed near the 865 cm−1 and the band observed at 713 cm−1 are related to the vibration of the calcium-oxygen bond [42] The absorption peak at 800- 1400 cm−1can be as-signed to the CdO nanostructures [43]. The peak in the region of 1381–1636 cm−1_{can be imputed to the stretching vibration of O–C–O} bonding [44]. FTIR spectrum exhibits the entity of O–H bonds, Ca–O bond and Cd–O bonds in the grown films.

3.6. Optical analysis

The energy band gaps of CdO: Caﬁlms grown on glass substrates using diﬀerent Ca-dopant concentrations are calculated from the plots of (αhν)2 _{versus photon energy h}_{ν and by extrapolating the energy} linear region to (αhν)2

= 0 (using Tauc method) [45].

Table 1

Elemental composition of pristine CdO and Ca0.005Cd0.995Oﬁlms.

Sample Name Cd(at%) O(at%) Ca(at%)

CdO 29.21 70.79 –

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Fig. 4. 2D and 3D AFM images of CaxCd1-xO (x = 0, 0.005, 0.010, 0.015, 0.020 and 0.025)ﬁlms.

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= −

αhν A hν( Eg)n (4)

In the above equation, A, hν, α, and Eg symbolize an energy-in-dependent constant, incident photon energy, absorption coeﬃcient and the optical bandgap, respectively. n is dependent upon the transition of carriers and it is theoretically equal to 0.5 for direct allowed transitions. Optical band gap of CdOﬁlms increases from 2.13 to 2.35 eV with the increase of Ca content (Fig. 7) as listed inTable 4. Due to a broad band gap of CaO (about 7.1 eV), it was suggested that the incorporation of Ca in CdO host lattice to realize its optical bandgap engineering [46–48].

Alteration of electronic structure by Ca-doping; the incorporation of Ca can lead to the swap of electrons in CdO orbitals. These phenomena achieve new electronic states in CdO bandgap. Also, the electron lo-calization of Cd changed with Ca doping in CdO, doping provokes the band edge bending [49]. It is marked that Ca atoms substituted for Cd and O ions on neighboring positive ion sites. This is consolidated via SEM, XRD and FTIR results.

Table 2

Various surface roughness parameters of pristine and CaxCd1-xO

(0≤ x ≤ 0.025) ﬁlms.

Sample Name Average Surface Roughness (nm) RMS (nm) Height (nm)

CdO 41,97 54,38 382,20 Ca0.005Cd0.995O 27,10 36,33 295,86 Ca0.010Cd0.990O 24,58 32,09 268,44 Ca0.015Cd0.985O 25,49 32,20 220,02 Ca0.020Cd0.980O 17,04 22,36 198,35 Ca0.025Cd0.975O 40,54 50,45 424,55

Fig. 5. XRD patterns of CaxCd1-xO (x = 0, 0.005, 0.010, 0.015, 0.020 and 0.025)ﬁlms.

Table 3

Relative peak intensity and Tc (hkl) values of pristine and CaxCd1-xO

(0≤ x ≤ 0.025) ﬁlms.

Sample Name Relative Peak Intensity (cps) Tc(hkl) (111) (200) (111) (200) (220) (311) CdO 5373 4643 1.52 1.32 0.72 0.44 Ca0.005Cd0.995O 3575 3082 1.54 1.33 0.71 0.42 Ca0.010Cd0.990O 4459 3736 1.54 1.29 0.72 0.44 Ca0.015Cd0.985O 4522 3936 1.51 1.32 0.72 0.46 Ca0.020Cd0.980O 5667 4897 1.52 1.32 0.71 0.46 Ca0.025Cd0.975O 5380 4694 1.52 1.33 0.72 0.44

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Additionally, the relation between optical bandgap of CdO and dopant (Ca) content was supported by using a theoretical relation en-titled Vegard's law [50], as given below

= CaO + −

Eg, (x) x Eg, (1 x)Eg, CdO (5)

where Eg(x) is the optical bandgap energy of the Cd1-xCaxO. In ac-cordance with this relation, increase in Ca content outcomes in increase in the optical bandgap value of CdO, as found in Tauc's equation. (Fig. 8) shows that crosscheck the optical bandgap value obtained from Tauc's equation and Vegard's relation.

3.7. Electrical measurements

InFig. 9, Nyquist plots of the CdOﬁlms and equivalent circuit are shown. The Cole-Cole plot of pristine CdO which is indicated by red circles is shown at the left corner of x-y axis as a small point due to

small imaginary and real part values. When 0.5% Ca was added to the CdO, the real and imaginary part of impedance were increased. Ca atom is located 2nd_{column in the periodic table and Ca atom has relatively} big atomic radius. Thus, the bigger atoms in the lattice that formed by relatively small atoms behave as scatter centers for the charge carriers [51,52]. When Ca amount was increased to 1.0% and 1.5% values, the semicircles expanded. However, there was a turning point of the trend that starts with 2.0% Ca doping. An interaction starts with increasing Ca atoms into CdO host lattice due to the more added atoms located at the smaller distance. The energy levels of Ca atoms interact with each other and discrete energy levels of Ca atoms split into a band energies. If there are enough Ca atoms, semiconductor will be degenerate [53]. 2.0 and 2.5% Ca doping caused a degenerated semiconductor and in-creasing Ca doping increased the number energy states at the CdO bandgap and conductivity. The experimental data were analyzed using the equivalent circuit shown inFig. 9. For all samples single semicircle ﬁt was obtained, that indicates the single relaxation process. Circuit parameters were calculated byﬁtting the experimental data through LabVIEW. ′ = + + Z R R wC R 1 ( ) s p p p2 (6) and = + Z wC R wC R " 1 ( ) p p p p2 (7)

equations, wherew=2πf, f is frequency,Cp andRpshow parallel

ca-pacitance and resistance,Z′andZ"show real and imaginary part of the impedance, respectively.Rs was close to zero for all samples and was

ignored it in the equivalent circuit. The impedance analysis and

four-Table 4

Crystallite size, micro-strain and band gap values of pristine and CaxCd1-xO

(0≤ x ≤ 0.025) ﬁlms.

Sample Name Crystallite Size (nm)

Micro-strain (ɛ) x 10−3

Energy gap (eV) From Vegard's law From Tauc's plot CdO 17.90 2.02 2.160 2.160 Ca0.005Cd0.995O 16.57 2.19 2.184 2.200 Ca0.010Cd0.990O 18.33 1.97 2.209 2.250 Ca0.015Cd0.985O 18.20 1.99 2.234 2.260 Ca0.020Cd0.980O 17.54 2.06 2.258 2.280 Ca0.025Cd0.975O 17.01 2.13 2.283 2.350

Fig. 6. FTIR spectrum of CaxCd1-xO (x = 0, 0.005, 0.010, 0.015, 0.020 and 0.025)ﬁlms.

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Fig. 7. Tauc's plot of CaxCd1-xO (x = 0, 0.005, 0.010, 0.015, 0.020 and 0.025).

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point probe measurement results of CdO thin ﬁlms are shown in Table 5. The sheet resistances of theﬁlms were increased by Ca doping until the doping level reaches 1.5% inTable 5. The further doping level causes a decrease at sheet resistance. Similar behavior was obtained for parallel resistance, Rp which is represent charge-transfer impedance [54]. On the other hand, the parallel capacitance was increased with increasing Ca doping at whole range. It shows us formation of the charge storage regions with the Ca doping [55]. Furthermore, it was observed that both surface analysis and electrical measurement results support to each other.

4. Conclusions

The pristine and Ca-doped (CaxCd1-xO (0≤ x ≤ 0.025)) CdO films were synthesized onto soda-lime glass slides via SILAR method at room temperature. The impact of Ca content on morphological, structural, optical and electrical features was examined by SEM, AFM, XRD, UV–Vis spectrophotometer, impedance analysis and four-point probe measurement, respectively. The SEM and AFM pictures demonstrated that the morphological structures of thefilms were modified by the Ca doping. EDX graphs verified the existence of Ca in the doped films. XRD patterns depicted that all the obtained films display polycrystalline nature and hegemonic (111) and (200) preferred growth orientation. The optical analyses illustrated that the band gap values of thefilms increased with rising Ca concentration according to both Vegard's re-lation and Tauc's law calcure-lations. According to the four-point probe measurements and impedance analysis, sheet resistance and parallel resistance showed an increase until 1.5% Ca doping. The higher Ca

doping caused a decrease for the resistances. Increased Ca concentra-tion not only increased bandgap, but also resulted the formaconcentra-tion of new energy levels in it. Especially, beyond to the 1.5% Ca doping, the re-duction in sheet resistance and parallel resistance points out the for-mation of a large number of energy levels in the bandgap.

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Table 5

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