Transition metal Mn/Cu co-doped CdO transparent conductive films: Effect on structural, morphological and optical characteristics

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Transition metal Mn/Cu co-doped CdO transparent conductive

ﬁlms:

Effect on structural, morphological and optical characteristics

R. Aydin

a,*

, H. Cavusoglu

a

, B. Sahin

b

a_{Department of Physics, Faculty of Sciences, Selcuk University, Konya, Turkey}

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

a r t i c l e i n f o

Article history:

Received 13 August 2018 Accepted 20 January 2019 Available online 22 January 2019 Keywords:

Mn/Cu co-doped CdOﬁlms Co-doping

SILAR Band gap

a b s t r a c t

The transparent conducting un-doped, Cd0.99 Mn0.01O (Mn: 1.0%) and Mn/Cu co-doped CdO

[(CuxMn0.01Cd0.99-x) (x: 0.005, 0.01 and 0.02 respectively)]ﬁlms were prepared by successive ionic layer

adsorption and reaction (SILAR) technique on soda lime glass substrates. The effect of Mn and Cu-dopant on structural, morphological and optical characteristics of the CdOfilms was analyzed by XRD, SEM, UV eVisible spectrophotometer and FT-IR spectroscopy. The XRD studies showed that the all-SILAR prepared films were polycrystalline and had preferential growth along the (111) directions. SEM analysis revealed that the Mn doping and Mn/Cu co-doping significantly influenced the surface morphologies of the CdO films. The EDX results confirmed that the dopant ions were incorporated properly into the CdO lattices. The optical band gap energy values of the all depositedfilms were determined by extrapolation method and observed to be in the range of 2.08e2.38 eV. Optical analysis results disclosed that doping alters the band gap facilitating the potential of transparent conductive films to be exploited in optoelectronic devices. Furthermore, FT-IR was used to confirm the existence of metal-doped CdO. The investigations showed that co-doping significantly affects the physical properties of SILAR-grown CdO films.

1. Introduction

During the recent years, transparent conductive oxide (TCO) thinfilms have been received a great deal of attention because of their tunable physical and optical characteristics by the addition of transition metal (TM) dopants [1e5]. As it is well known that the physical characteristics of the TCO thinfilms can be adjusted by the addition of the TM dopants due to the exchange between the electrons of host TCO material and TM ions [6]. Among different TCOfilms, CdO is widely used for many applications such as gas sensor devices [7,8], photovoltaic solar cells [9e11], transparent electrodes [12], photodiodes [13], etc., due to its unique features including high electrical conductivity, low resistivity and high op-tical transmittance in the visible region of solar spectrum [14e16]. CdO is an n-type semiconductor that possesses optical band gap energy of about ~2.2 eV and crystallizes in the rock-salt structure (fcc) [17].

Until recently, TM-doped CdO thinﬁlms have been synthesized using a variety of deposition techniques, including thermal

evaporation, spray pyrolysis, chemical bath deposition, chemical vapor deposition, solegel, and successive ionic layer adsorption and reaction (SILAR) [18e22]. Among these fabrication techniques, SILAR has a number of advantages such as simple and low cost experimental setup and the possibility to grow at room tempera-ture and normal pressure [23,24].

In recent years, much attention has been focused on the double doping strategy in which the two elements are incorporated into CdO concurrently because the doubly-doped CdO ﬁlms are sup-posed to exhibit some improvements in physical and optical char-acteristics. In mono-doped CdO, Mn and Cu doping have a variety of advantages concerning optical transmission and band gap [25,26]. From this point of view, it would seem that the effect of Mn/Cu co-doping draw interest due to an improvement in several charac-teristics of the materials.

In the present work, the Mn/Cu co-doped transparent con-ducting CdO films were prepared by SILAR method, which is a relatively simple, cost-efficient and favorable to fabricate thin films. To our knowledge, such a nanocrystalline film Mn/Cu co-doped CdO which were produced by SILAR method has not been previ-ously reported yet in the literature. Due to this reason, this is the first study evaluating the role of Mn/Cu co-doping contents on the

* Corresponding author.

E-mail address:raydin@selcuk.edu.tr(R. Aydin).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u rn a l h o m e p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m

https://doi.org/10.1016/j.jallcom.2019.01.221

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structural, morphological and optical characteristics of CdOﬁlms synthesized by SILAR method.

2. Experimental details 2.1. Materials

Cadmium acetate dihydrate [Cd(CH3COO)2$H2O, 98%, Alfa

Aesar], Manganese (II) chloride dihydrate (Cl2H4MnO2, Merck),

Copper (II) chloride dihydrate (CuCl2$2H2O, 99.99%, Merck),

aqueous ammonia solution (25%, Merck) and soda lime glass sub-strates were purchased from different commercial suppliers and they were used as received. All the chemicals and solvents used in the synthesis were of analytical grade and were used without further puriﬁcation.

2.2. Fabrication of nanocrystalline CdOﬁlms

In the present work, Cd (CH3COO) 2$H2O was dissolved in

100 mL of ultra-pure water to prepare 0.1 M cadmium acetate so-lution for the deposition of un-doped CdOfilms. Then, CdO films with fixed Mn doping ratio (1%) and various Cu doping levels (0.05e2%) were deposited on glass substrates by the cost-effective and simple SILAR method. By adding aqueous ammonia solution to the precursor solution, the value of the pH was modulated to ~12.0. Before proceeding to the deposition process, soda lime glass sub-strates were well cleaned with acetone in an ultrasonic bath. After the cleaning process, cadmium acetate solution and ultra-pure water which were used in the deposition process were heated to 85C. Then, the substrate was perpendicularly submerged into a prepared cadmium acetate solution for 20 s. After that, it was perpendicularly submerged into the heated ultra-pure water for 20 s. This SILAR cycle was iterated for 10 times. In order to obtain CdO, the films were sintered at 350_{C for 45 min in the air}

atmosphere.

2.3. Characterization of nanocrystalline CdOﬁlms

The crystalline phase and microstructure of the ﬁlms were evaluated by X-ray diffraction (XRD, Bruker D8 with Cu K

a

radia-tion, 1.5406 Å) in scanning range between 30 and 70 (2

q

). The surface morphologies and the elemental composition of the all deposited _{films were examined by a Zeiss EVO LS-10 scanning} electron microscope (SEM) equipped with energy-dispersive X-ray spectroscopy (EDX) system. Optical characterizations of thefilms were recorded in the wavelength range of 190e1100 nm by UltravioleteVisible (UVeVis) spectrophotometer (Thermo Scienti-fic Genesys 10S) at room temperature. The Fourier transform infrared (FT-IR) spectra of all thefilms were recorded in the range of 400e4000 cm1_{on a Bruker Vertex 70 (Rheinstetten, Germany)}

spectrophotometer with a resolution of 4 cm1. 3. Results and discussion

3.1. XRD analysis

The structural properties of the as-prepared CdO ﬁlms were examined by using XRD analysis. The diffraction angles were recorded in the range 30e70_._{Fig. 1}_{shows the XRD patterns of the}

un-doped, Mn-doped, and Mn/Cu co-doped CdO films. The XRD patterns of all the films indicated the polycrystalline CdO phase [JCPDS Card no: 05-0640]. There were no extra peaks observed corresponding to metallic Mn and Cu or the related phases of these metals in the doped CdOfilms. This means that Mn and Cu atoms have been successfully incorporated into the Cd sites. As can be

apparently seen, these _{ﬁlms exhibited a preferred (111) crystal} orientation. The other planes were (200), (220), and (311). It was explicitly seen from the patterns that both the peak intensities and the preferred orientations of the CdOﬁlms were notably changed by the Mn doping and Mn/Cu co-doping.

Fig. 1. XRD patterns of different compositions of un-doped, Mn-doped and Mn/Cu co-doped CdOﬁlms.

Fig. 2. The relative peak intensity values for (111), (200), (220) and (311) planes of CdO ﬁlms for the un-doped, Mn-doped and Mn/Cu co-doped CdO ﬁlms.

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The relative peak intensities of the CdOﬁlms were shown in

Fig. 2and tabulated inTable 1. It can be clearly observed that the changes in peak intensities obviously represent a change in crys-tallite size. This comes in sight from the differences between the ionic radii of the dopants and host metals [1,27].

The texture coefﬁcient has been used to specify the preferential crystal orientation. From this point of view, the texture coefﬁcient (TC(hkl)) has been determined from the XRD data by using the

following equation [28]; TC_ðhklÞ¼ IðhklÞ . I_0ðhklÞ 1 N P I_ðhklÞ.I_0ðhklÞ (1)

where I(hkl)is the measured intensity of the (hkl) plane, I0 (hkl)is the

standard intensity of the (hkl) plane, N is the re_{ﬂection number of} diffraction peaks. The TC(hkl)values of the (111), (200), (220) and

(311) crystal planes were listed inTable 1. If the TC(hkl)values of the

corresponding plane greater than one, it is accepted as a prefer-entially grown facet [29]. From the examination of TC(hkl)values, it

was easy to see that (111) crystal plane were the preferential ones. Comparable changes of the TC (hkl) values for the planes as

mentioned above also exhibited inFig. 3.

The average crystallite size (D) of the CdOﬁlms has been ob-tained from the full-width half-maximum (FWHM), by applying the well-known Scherrer equation [30];

D¼

_b

0:94:_:cos

_q

l

b

(2)

where

l

is the wavelength of the X-Ray source which was used in XRD,

b

is the peak width of the diffraction peak proﬁle at half maximum height in radians and

q

is the Bragg angle. The obtained average crystallite sizes of the CdOﬁlms were listed inTable 2. From the obtained XRD data and the Scherrer equation, the crystallite size of CdOﬁlms was found to be in the range 19.32e20.64 nm as seen inTable 2.

Many researchers have focused on the investigation of the structural parameters such as micro-strain and dislocation density in metal oxide materials. In this regard, the micro-strain (ε) and dislocation density (

r

) values for the CdO ﬁlms were calculated using the standard relations [31,32]:

ðεÞ ¼

b

:cos₄

q

(3)

and

ð

r

Þ ¼15:ε_a:D (4)

where a is the lattice parameter. The variations of these structural parameters as a function of un-doped, Mn-doped, Mn/Cu co-doped CdOﬁlms were given inTable 2. It was observed that both the micro-strain and dislocation density values of theﬁlms decreased as crystallite size increased with Mn doping and Cu0.005Mn0.01Cd0.985. On the other hand, the values of both

struc-tural parameters of the ﬁlms increased as the crystallite size decreased for Mn/Cu co-doped CdO_{ﬁlms (Cu}0.01Mn0.01Cd0.98and

Cu0.02Mn0.01Cd0.97). This inverse relation among the crystallite size,

micro-strain, and dislocation density was demonstrated inFig. 4. The opposite relation among the structural parameters of the CdO ﬁlms originated from the change in crystallite sizes [33]. Similar results were obtained by researchers [1,34].

3.2. SEM analysis

Fig. 5(A-E) shows the SEM images of un-doped, Cd0.99Mn0.01O

(Mn: 1.0%) and Mn/Cu co-doped CdO [(CuxMn0.01Cd0.99-x) (x: 0.005,

0.01 and 0.02 respectively)]films. All the obtained CdO films sur-faces seem tofirmly packed with particles of different sizes. From

Table 1

Relative peak intensity and TC(hkl)values of the CdOﬁlms.

Sample Name Relative Peak

Intensity (cps) TC(hkl) (111) (200) (111) (200) (220) (311) CdO 3068 1976 1.94 1.25 0.49 0.31 Mn0.01Cd0.99O 1852 762 2.24 0.92 0.50 0.33 Cu0.005Mn0.01Cd0.985O 1288 1044 1.59 1.29 0.67 0.45 Cu0.01Mn0.01Cd0.98O 2538 848 2.56 0.86 0.33 0.25 Cu0.02Mn0.01Cd0.97O 5712 1414 2.79 0.69 0.33 0.20

Fig. 3. The texture coefﬁcient for (111), (200), (220) and (311) planes of un-doped, Mn-doped and Mn/Cu co-Mn-doped CdOﬁlms.

Table 2

Crystallite size, micro-strain, dislocation density and band gap values of the CdOﬁlms.

Sample Name Crystallite Size (nm) Micro-strain (3) x 103 Dislocation Density (r) x 1011_(cm2₎ _{Optical Band Gap (eV)}

CdO 19.32 1.90 3.23 2.08

Mn0.01Cd0.99O 19.72 1.84 3.00 2.38

Cu0.005Mn0.01Cd0.985O 20.64 1.78 2.83 2.31

Cu0.01Mn0.01Cd0.98O 20.34 1.79 2.83 2.26

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Fig. 5, it is obviously seen that the surface morphologies of theﬁlms change depending on the Mn doping and Mn/Cu co-doping. From SEM photos theﬁlm surfaces appear to be altered with micro-sized

particles by adding Mn and Cu to the aqueous solution. Namely, it has been observed that the Mn and Cu elements play an important role in shaping the surface structure of theﬁlms. Similar results

Fig. 4. Variation in crystallite size, micro-strain and dislocation density of un-doped, Mn-doped and Mn/Cu co-doped CdOﬁlms.

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have been notiﬁed by several researchers [35,36]. This change in the surface morphology and particle size ofﬁlms can be due to the ionic radius differences between the host Cd2þ ion and dopant Mn2þand Cu2þions (Cd2þ: 0.87Å; Mn2þ: 0.75Å; Cu2þ: 0.65 Å; for 5-coordination number). So the Mn2þand Cu2þions can occupate the location of Cd2þin lattice readily, leading to a deformation of the CdO lattice [37,38].

The chemical elemental analysis of the un-doped Mn-doped and Mn/Cu co-doped CdOﬁlms were performed using EDX. The EDX images of these deposited nanostructures are displayed in

Fig. 6(aec). Fig. 6(a) shows that the Cd (23.59%) and O (76.41%) elements are present in undoped CdO,Fig. 6(b) shows that the Cd (31.58%), O (67.68%) and Mn (0.74%) elements are contained in Mn-doped Cd0.99Mn0.01O andFig. 6(c) shows Cd (17.60%), O (81.42%),

Mn (0.40%) and Cu (0.57%) elements are included in Mn/Cu co-doped Cu0.005Mn0.01Cd0.985O. The values of chemical composition

of un-doped, Cd0.99 Mn0.01O (Mn: 1%) and Mn/Cu co-doped CdO

[(CuxMn0.01Cd0.99-x) (x: 0.005, 0.01 and 0.02 respectively)] are

shown inTable 3. FromTable 3, it is obvious that Cu content has increased as the concentration ratio (0.5%, 1.0% and 2.0%) increase of Cu is increased.

3.3. UVeVis analysis

The optical energy band gap values of all theﬁlms were esti-mated by using the relationship between the absorption coefﬁcient (

a

) and the photon energy (h

y

) given by the following equation [39];

a

h

n

¼ Ah

n

Egm (5)

where A is an energy-independent constant. It is well known that the exponent m is equal to 1/2, 2, 3/2 and 3 for the allowed direct, allowed indirect, forbidden direct and forbidden indirect transi-tions, respectively [40]. The allowed transitions occurred in CdO, which corresponds to 1/2. Thus, in order to observe the effect of Mn doping and Mn/Cu co-doping on the optical band gap energies,

Fig. 7was plotted with (

a

h

y

)2versus h

y

. The obtained Egvalues of

all theﬁlms were presented inTable 2.

The Egvalue of un-doped CdO value was obtained at 2.08 eV. It

was clear that the optical band gap energy for Mn-doped CdOfilms (2.38 eV) has increased when compared to un-doped CdO (2.08 eV) and similar results have also been reported [41]. The reason for this that the band gap energy of MnO (about 4.20 eV) is more than that of CdO (2.08 for the present work), the band gap energy of Mn-doped CdO should be bigger than the band gap energy of un-doped CdO. In Mn/Cu co-un-doped CdOfilms, the band gap energy values were varied (2.31e2.20) due to the variation of Cu concen-trations. For the co-doped CdO_{films, the band gap energy values} significantly varied with the addition of doping materials. These variations in the band gap energy of CdOfilms after Mn doping and Mn/Cu co-doping originates from the structural variation of CdO films during the growth process. The crystal structure alteration in the CdO films could be due to the replacement of Cd2þ_{ions by}

dopant Mn2þand Cu2þ ions. The change between Eg with

Mn-doped and Mn/Cu co-Mn-doped films may be attributed to the BursteineMoss shift and density of carrier concentration [42]. The variation in band gap values as a function of un-doped, Mn-doped, Mn/Cu co-doped CdOfilms were also plotted inFig. 8. It is seen that with the addition of doping materials (Mn and Cu) leads to sig-nificant changes in the Egvalues of thefilms which mean that Mn

and Cu as a dopant can be used as a regulator of the band gap of a semiconductor.

Table 3

Elemental composition of un-doped, mono doped and co-doped CdOﬁlms.

Sample Name Cd (at%) O (at%) Mn (at%) Cu (at%)

CdO 23.59 76.41 e e

Mn0.01Cd0.99O 31.58 67.68 0.74 e

Cu0.005Mn0.01Cd0.985O 17.60 81.42 0.40 0.57

Cu0.01Mn0.01Cd0.98O 13.01 86.01 0.32 0.66

Cu0.02Mn0.01Cd0.97O 14.16 84.66 0.40 0.78

Fig. 7. Band gap energy of un-doped, Mn-doped and Mn/Cu co-doped CdOﬁlms.

Fig. 8. The effect of doping on the band gap of the un-doped, Mn-doped and Mn/Cu co-doped CdOﬁlms.

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3.4. FT-IR analysis

In order to identify the presence (conjugation) of various chemical functional groups to the specified material, the Fourier transform infrared (FT-IR) spectroscopy can be widely used by the researchers. In this context, FT-IR analysis was performed in the range of 400e4000 cm1 in order to identify the presence of intended elements.Fig. 9(a and b) shows the FT-IR spectra of the product (un-doped, Mn-doped and Mn/Cu co-doped CdO films) after annealing at 350C for 45 min. As a result of the annealing process, the organic section of the precursor solution was removed and the specified sharp and strong band at 829 and 1381 cm1

belongs to the CdeO vibration and conﬁrms the formation of CdO [43,44]. The spectra obtained for Mn-doped and Mn/Cu co-doped CdOﬁlms were more or less similar. In Fig. 9 (b), many peaks were observed attached to CdeO which assigned as CueO stretching mode below 700 cm1. This is because of the presence of Mn and Cu ions in the doped sample. Various shifts which were observed in the spectra might be due to the incorporation of Mn and Cu elements [45].

4. Conclusions

In summary, the CdO:(Mn/Cu) TCO films were successfully synthesized through a simple SILAR method. The influence of Mn doping and Mn/Cu co-doping on the structural, morphological and optoelectronic characteristics of thefilms was scrutinized. Our XRD results illustrated that all thefilms were of polycrystalline nature and no characteristic peaks of any impurities were detected. Morphological analysis results demonstrate that all of the samples were well crystallized. The EDX results display that Mn and Cu dopants were successfully incorporated with CdO_{films. The optical} band gap of the films was observed to be in the range of 2.08e2.38 eV. According to the optical analysis results, a clear increment and decrement of the optical band gap from 2.08 to 2.38 eV and from 2.38 to 2.20 eV for the Mn and Cu dopants, respectively. To the authors' knowledge, this is thefirst study on the benefit of Mn/Cu co-doping for improving the physical and optical characteristics of SILAR deposited CdOfilms. Such thin films are potentially suitable alternatives for pharmaceutical, spintronic and optoelectronic applications.

Acknowledgements

The authors wish to acknowledgeﬁnancial support from the Scientiﬁc Research Project Commission of Selcuk University (Grant No. BAP-17401102).

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