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Enhancement in some physical properties of spray deposited CdO:Mn thin films through Zn doping towards optoelectronic applications
Accepted Manuscript Title: Enhancement in some physical properties of spray deposited CdO:Mn thin films through Zn doping towards optoelectronic applications Author: N. Manjula A.R. Balu K. Usharani N. Raja V.S. Nagarethinam PII: DOI: Reference:
S0030-4026(16)30409-0 http://dx.doi.org/doi:10.1016/j.ijleo.2016.04.129 IJLEO 57612
To appear in: Received date: Accepted date:
4-3-2016 25-4-2016
Please cite this article as: N.Manjula, A.R.Balu, K.Usharani, N.Raja, V.S.Nagarethinam, Enhancement in some physical properties of spray deposited CdO:Mn thin films through Zn doping towards optoelectronic applications, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.04.129 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement in some physical properties of spray deposited CdO:Mn thin films through Zn doping towards optoelectronic applications N. Manjula, A.R. Balu*, K. Usharani, N. Raja, V.S. Nagarethinam PG and Research Department of Physics, AVVM Sri Pushpam College, Poondi – 613 503, Tamilnadu, India
*Corresponding author Dr. A.R. BALU 757 MIG Colony, New Housing Unit, Thanjavur – 613 005. Ph: +91 9442846351 Email: [email protected]
1
Abstract This paper reports the enhancement in the structural, morphological and opto-electrical properties of spray deposited CdO:Mn thin films through Zn doping. Mn concentration is maintained as 1 at.% for all the films whereas Zn concentration in CdO:Mn film is varied as 1, 2 and 3 wt.%. XRD studies showed that the crystalline quality of pure CdO deteriorates with Mn doping which got enhanced with Zn doping. The entire doped films exhibit a (1 1 1) preferential orientation similar to that of the undoped film. Cauliflower shaped nanostructures are evinced for all the films from the FESEM images. The decreased transparency of pure CdO with Mn doping increases with Zn doping and a maximum transmittance of 89.8 % is obtained for the CdO:Mn film coated with 2 wt.% Zn concentration. Optical band gap of pure CdO which exhibit a red shift with Mn doping got blue shifted with Zn doping. Electrical studies showed that the CdO:Mn film is more resistive than the undoped film which experience a decrement with Zn doping. The obtained results confirm that CdO:Mn film when doped with zinc finds application in optoelectronic devices due to their enhanced optoelectrical properties. Keywords Grain boundaries; crystal structure; red shift; blue shift; carrier concentration
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1.
Introduction Cadmium oxide (CdO), an n-type II – VI non-stoichiometric degenerate semiconductor
find applications in optoelectronics like transparent conducting oxides (TCOs), solar cells, smart windows, optical communications, flat panel displays and photo-transistors [1].Thin films of CdO exhibit better transparency in the visible and NIR spectral regions. CdO have a direct band gap energy of 2.2 – 2.7 eV and a low electrical resistivity of 10-2-10-4 Ω-cm [2]. It shows n-type conductivity due to native defects such as oxygen vacancies and cadmium interstitials. The conductivity of pure CdO can be improved by controlling these defects, which can be achieved through doping. Concerning specific applications, dopants like Ti [3], Zn [4], Mg [5], Cl [6], Al [7], and Bi [8] have been proposed, studied and employed in CdO thin films. It has been reported earlier that oxalate materials from 3d series such as Mn, Co, Fe, Ni, Zn, etc act as excellent precursors for the synthesis of pure and homogeneous oxide nanoparticles with high surface area because of its low decomposition temperature. Also due to their isomorphous nature, these oxalates enable the modification of the physical properties of oxide semiconductors by doping the precursor in the course of its synthesis [9]. In our previous work we observed that Mn when added to CdO in low concentration (< 2 at.%) improved its optical and electrical properties [10]. Zinc is an important oxalate material and Zn based TCO thin film appears to be promising material in solar cells as window and buffer layers due to its high transparency in the visible region. ZnO thin films have high resistivity with a band gap around 3.3 eV [11]. So it is expected that the homogeneous mixing of both CdO and ZnO allow intermediate optical and electrical properties between them, making CdO attractive for various applications especially as buffer layer in solar cells [12]. Hence in the present study, CdO thin films co-doped with two oxalate precursors Mn and Zn were deposited by spray pyrolysis technique using perfume atomizer. Mn concentration was kept fixed as 1 at.% because, this concentration has been found to be the best to produce CdO thin
3
films with better physical properties [10]. Zn concentration was varied as 1, 2, and 3 wt.% and the effect of Zn doping on the physical properties of CdO:Mn thin films were investigated and the results are reported here. To the best of our knowledge there is no earlier report on the properties of CdO thin films co-doped with two oxalate precursors. 2.
Experimental details CdO thin films co-doped with two oxalate dopants (Mn and Zn) were prepared on glass
substrates by spray technique using perfume atomizer. Mn concentration is kept fixed as 1 at.% in pure CdO to get CdO:Mn thin film and to this film Zn doping is performed with 1, 2 and 3 wt.% Zn concentrations, respectively. Undoped CdO is prepared by spraying aqueous solution (50 ml in volume) containing 0.05 M cadmium acetate. To prepare CdO:Mn film, manganese acetate is used as the precursor and for Zn doping, zinc acetate is used. The thicknesses of the films were measured using a stylus type profilometer. The crystal structure was determined using X-ray diffractometer (PANalytical- PW 340/60 X’ pert PRO) with CuKα radiation (λ = 1.5406 Å). Morphology of the films was examined using a HITACHI SU6600 variable pressure field emission scanning electron microscope (FESEM). The optical band gap of the films was determined from the transmittance spectra using a Perkin Elmer double beam UV-Vis-NIR double beam spectrophotometer. Electrical resistivity values were measured using a two point probe setup. 3.
Results and discussion
3.1
XRD studies Fig. 1(a-e) shows the XRD patterns of undoped CdO, Mn-doped CdO (CdO:Mn) and
Zn-doped CdO:Mn thin films (Zn doping levels: 1, 2 and 3 wt.%), respectively. The presence of diffraction peaks at 2θ values approximately equal to 32.944º, 38.27º, 55.293º, 65.891º and 69.227º indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of cubic crystal structure of pure CdO (JCPDS Card No.73-2245) confirmed their polycrystalline nature. It is observed
4
for the XRD patterns that the CdO:Mn (Fig. 1(b)) and Zn doped CdO:Mn thin films (Fig. 1(ce)) exhibit a strong preferential orientation along the (1 1 1) plane similar to that of the undoped film. The preferential orientation along the (1 1 1) plane observed here exactly matches with the results reported by Manjula et al. [13] and Noornisha et al. [14] for undoped and Zn-doped CdO thin films, respectively prepared by spray technique using perfume atomizer. The preferential orientation factor f(1 1 1) values of the films calculated by the way adopted by Suganya et al. [15] are given in Table 1. It is observed that f(1 1 1) of pure CdO decreases with Mn doping and for the Zn-doped CdO:Mn films it slightly increases which supports the fact that the crystalline quality of pure CdO which deteriorates with Mn doping got enhanced with Zn doping. Overall when compared with that of the undoped film, the f(1 1 1) for all the doped films have lesser values confirming their decreased crystalline quality. The deterioration of the crystalline quality of the doped films might have been caused by the following reasons: i) stress induced due to the ionic size differences between the host and dopant ions and ii) segregation of dopants on the grain boundaries as reported by Xu et al. [16] for sol-gel derived Al-doped ZnO thin films. A close examination of the XRD peaks showed that the 2θ values of the (1 1 1) peak of the doped films shift towards higher Bragg angles confirming a contraction in their lattice parameter values (Table 1). The lattice parameter values obtained for the films are compiled in Table 1a. The average crystallite sizes (D) of the films calculated by using the Scherrer formula, (where β is the FWHM of the strongest peak ((1 1 1) in this case), λ is the
wavelength of the X-ray used (1.5406 Å) and θ is the Bragg angle) are compiled in Table 1. It is observed that crystallite size decreased from 33.91 nm obtained for the undoped CdO to 30.48 nm for the CdO:Mn film which got increased with Zn-doping. The strain (ε) and dislocation density (δ) values of the films calculated using the formulae [17]: 5
(1)
are compiled in Table 1b. 3.2
Surface morphology and elemental analysis Fig. 2(a-e) shows the FESEM images of undoped CdO, CdO:Mn and Zn-doped
CdO:Mn thin films. Cauliflower shaped nanostructures are evinced from the FESEM images of all the films. The film surface of the undoped CdO is composed of equally sized cauliflower shaped nanostructures with few pin holes (Fig. 2(a)). For the CdO:Mn thin film, the surface gets modified with unequally sized cauliflower structures interconnected with each other (Fig. 2(b)). Film surface appeared to be smooth and uniform with tightly packed equally sized cauliflower structures for the Zn-doped CdO:Mn (Zn doping levels: 1 and 2 wt.%) thin films (Fig. 2(c, d)). With further increase in Zn doping concentration to 3 wt.%, the surface gets modified with unequal sized cauliflower structures (Fig. 2(e)). Plenty of pin holes are also visible. These results infer that with Mn-doping the surface morphology of pure CdO slightly deterioraties which then got modified with Zn doping which very well support the results obtained in the XRD analysis. The EDX spectra (Fig. 3(a-e)) confirm the presence of Mn in the CdO:Mn thin film, Mn and Zn in the Zn-doped CdO:Mn thin films. The elemental compositions of the films are compiled in Table 2c. 3.3
Optical properties Fig. 4 displays the transmittance spectra of undoped, CdO:Mn and Zn-doped CdO:Mn
thin films. Undoped CdO film has a maximum transparency of nearly equal to 80.63 % which decreased to 77.84 % with Mn doping. However, the transparency got improved with Zn doping and the CdO:Mn film coated with 2 wt.% Zn concentration exhibited a maximum transparency of 89.8 %. The increased transparency observed for the Zn-doped CdO:Mn 6
films might be due to structural homogeneity, less scattering effects and improved crystallinity, as reported by Usharani et al. [18] for Zn, Mg incorporated CdO thin films. The decreased transparency observed for the CdO:Mn film might be due to the high thickness value obtained for this film (Table 1). Due to increased film thickness, the absorbance increases which might be due to the lot of defects introduced by Mn dopant and this is noticeable in the lower wavelengths below 500 nm as seen in the absorbance spectra (Fig. 5). The crystal structure, the arrangement and distribution of atoms in the crystal lattice play a vital role on the energy gap values of thin film samples. The optical band gap (Eg) values of the undoped CdO, CdO:Mn and Zn-doped CdO:Mn thin films are determined from the relation between the absorption coefficient (α) and the incident photon energy (hυ) [19]: (
)
(3)
where A is a constant. From the plots of (αhυ)2 vs. hυ (Fig. 6), the band gap values are determined from the extrapolation of the straight line portion to the hυ axis at α = 0 and the calculated values are given in Table 2. Undoped CdO film has a band gap value of 2.42 eV which exactly matches with the value reported by Usharani et al. [20]. The band gap decreased to 2.32 eV for the CdO:Mn film which increased with Zn doping attaining a maximum value of 2.54 eV for the CdO:Mn film coated with 3 wt.% Zn concentration. The red shift in the optical band gap observed for the CdO:Mn film might be due to the existence of Mn impurities in the CdO lattice which induce the formation of new recombination centers with lower emission energy [21]. The blue shift in the band gaps observed for the Zn-doped CdO:Mn films may be explained by several mechanisms such as i) Moss-Burstein effect which originates from the lifting of Fermi level into the conduction band due to the increase in charge carrier concentration; ii) the decrease in shallow-level trap concentration near the conduction band of Zn-doped CdO:Mn resulting from the improved crystallinity as reported by Usharani et al. [6] for CdO:Cl thin films, and iii) reduction of band bending effect at the
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grain boundaries. In nanocrystalline materials band bending effect takes place at grain boundaries due to their increased surface to volume ratio. For smaller grains, the band bending effect is large whereas it becomes flatter for larger grains. Similar results have been reported by Dutta et al. [22] for undoped ZnO thin films. The exponential behaviour of the absorption edge which depends on the incident photon energy (hυ) obeys the empirical Urbach relation as [23]: ( )
where
is a constant, and
is the Urbach energy which is the width of the tails of
localized states associated with the amorphous states of the films in forbidden band. The values are estimated from the slopes of the plots of ln(α) vs. hυ (inset of Fig. 6), and the values are compiled in Table 2. Fig. 7 shows the variation of
and
values of the undoped CdO, CdO:Mn, Zn-doped
CdO:Mn thin films. It is observed that both shift in the
and
values vary in opposite ways. The red
value observed for the CdO:Mn film is well supported with its high
which confirms the increased disorderliness of this film. The low values of
value
observed for
the Zn-doped CdO:Mn films confirm the minimization in the number of defects of CdO:Mn with Zn doping and this might be the reason for the increased
values obtained for these
films. 3.4
Electrical studies The electrical resistivity values of the undoped CdO, Mn-doped CdO (CdO:Mn) and
Zn-doped CdO:Mn thin films are presented in Table 2. Undoped film has a resistivity of 1.82 x 10-3 Ω-cm which slightly increased to 2.01 x 10-3 Ω-cm with Mn doping. The increased resistivity value observed for the CdO:Mn thin film might be due to the occupancy of few Mn2+ ions at the interstitial sites of the CdO lattice besides replacing Cd2+ ions substitutionally, thereby creating structural deformation. Interstitially occupied Mn2+ ions act 8
as trap for free carriers due to the movement of the dopant ions towards the grain boundaries causing a high dispersion of free carriers through the grain boundaries, thus contributing to an increase in its resistivity [24]. Decreased crystallinity observed for this film due to increased number of defects might be another reason for its decreased resistivity. The increased strain and dislocation density values obtained for this film (Table 1) strongly supports the above fact. It is observed that the resistivity of CdO:Mn film decreases with Zn doping attaining values lesser than even the undoped film. The decreased resistivity observed with Zn doping might be due to the following reasons: i)
Successful replacement of Zn2+ ions directly replacing Cd2+ ions or due to the substitution of Zn2+ ions in the Mn2+ sites particularly in the interstitial sites of the CdO lattice,
ii)
Aggregation on CdO grains in form of small Zn-oxide cluster (i.e. crystalliteand grain-boundary) accumulation effect.
4.
Conclusion In this work undoped CdO, Mn-doped CdO (CdO:Mn) and Zn-doped CdO:Mn thin
films were prepared by spray technique using perfume atomizer. Zn-doping was performed on the CdO:Mn film to improve its physical properties to make it suitable for optoelectronic applications. XRD studies confirmed that the crystalline quality of CdO:Mn film improved with Zn doping. Decreased strain and dislocation density values were obtained for the Zndoped CdO:Mn film. Surface morphology of the CdO:Mn film got enhanced with Zn doping. Optical transparency of CdO:Mn film got enhanced with Zn doping and band gap was found to be blue shifted from 2.32 eV to 2.54 eV. Electrical resistivity of CdO:Mn film decreased with Zn doping and a minimum resistivity of 0.71 x 10-3 Ω-cm was obtained for the film coated with 2 wt.% Zn doping concentration. Increased transparency, widened band gap and
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decreased electrical resistivity values obtained through Zn doping make CdO:Mn film suitable for optoelectronic and photovoltaic applications especially in solar cells. 5.
Acknowledgements The authors thank Mr. Vincent, St. Joseph’s College, Trichy for the UV-vis-NIR
studies. Figure captions Fig. 1
XRD patterns of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films
Fig. 2 FESEM images of a) undoped CdO, b) CdO:Mn, (c-e) Zn-doped CdO:Mn thin films Fig. 3 EDX spectra of a) undoped CdO, b) CdO:Mn, (c-e) Zn-doped CdO:Mn thin films Fig. 4 Transmittance spectra of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films Fig. 5 Absorption spectra of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films Fig. 6 Plots of (αhυ)2 vs. hυ. Inset Fig. Plots of ln(α) vs. hυ of undoped CdO, CdO:Mn, Zndoped CdO:Mn thin films Fig. 7 Variation of optical band gap energy and Urbach energy of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films
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K. Usharani, N. Manjula, A.R. Balu, V.S. Nagarethinam, Characteristic analysis of nanostructured Cl-doped CdO thin films – doping effect, Mater. Res. Innov. (2015) (Article in Press) DOI:10.1179/1433075X15Y.0000000045.
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[10] N. Manjula, M. Pugalenthi, V.S. Nagarethinam, K. Usharani, A.R. Balu, Effect of doping concentration on the structural, morphological, optical and electrical properties of Mn-doped CdO thin films, Mater. Sci. – Poland 33 (2015) 774–781. [11] S. Tewari, A. Bhattacharjee, Pramana, Structural, electrical and optical studies on spray-deposited aluminium-doped ZnO thin films, J. Phys .76 (2011) 153–156. [12] A.A. Ziabari, F.E. Ghodsi, Optoelectronic studies of sol-gel derived nanostructured CdO-ZnO composite films, J. Alloys Compnds. 509 (2011) 8748–8755. [13] N. Manjula, A.R. Balu, CdO thin films fabricated by a simplified spray technique using perfume atomizer with different molar concentrations of cadmium acetate for optoelectronic applications, Int. J. Chem. Phys. Sci. 3 (2014) 54–62.
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[14] T. Noorunisha, V.S. Nagarethinam, M. Suganya, D. Praba, S. Ilangovan, K. Usharani, A.R. Balu, Doping concentration and annealing temperature effects on the properties of nanostructured ternary CdZnO thin films towards optoelectronic applications, Optik 127 (2016) 2822–2829. [15] M. Suganya, A.R. Balu, K. Usharani, Role of substrate temperature on the growth mechanism and physical properties of spray deposited lead oxide thin films, Mat. Sci.– Poland, 32 (2014) 448–456. [16] Z.Q. Xu, H. Deng, Y. Li, Q.H. Guo, Y.R. Li, Characteristic of Al-doped c-axis orientation ZnO thin films prepared by the sol-gel method, Mater.Res.Bull. 41 (2006) 354–358. [17] K. Usharani, A. R. Balu, G. Shanmugavel, M. Suganya, V. S. Nagarethinam, Transparent conducting CdO thin films fabricated by low cost simplified spray technique using perfume atomizer, Int. J. Sci. Res. Rev. 2 (2013) 53–68. [18] K. Usharani, A.R. Balu, Properties of spray deposited Zn, Mg incorporated CdO thin films, J. Mater. Sci.– Mater. Electron. (2015) (Article in Press) DOI 10.1007/s10854015-3993-0. [19] C. Rajashree, A.R. Balu, V.S. Nagarethinam, Properties of Cd doped PbS thin films: Doping concentration effect, Sur. Eng. 31 (2015) 316–321. [20] K. Usharani, N. Raja, N. Manjula, V.S. Nagarethinam, A.R. Balu, Characteristic analysis on the suitability of CdO thin films towards optical device applications – Substrate temperature effect, Int. J. Thin Fil. Sci. Tec. 4 (2015) 89–96. [21] 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) 732–743. [22] S. Dutta, S. Chattopadhyay, M. Sutradhar, A. Sarkar, M. Chakrabarti, D. Sanyal, D. Jana, Defects and the optical absorption in nanocrystalline ZnO, J. Phy.cond. Matter.19 (2007) 236218–236218. [23] F. Urbach, The long wavelength edge of photographic sensitivity and electronic absorption of solids, Phys., Rev. 92 (1953) 1324–1324. [24] R. Kumaravel, K. Ramamurthi, V. Krishnakumar, Effect of indium doping in CdO thin films prepared by spray pyrolysis technique, J. Phys. Chem. Sol. 71 (2010) 1545–1549.
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Table 1 Thickness and structural parameters of CdO, CdO:Mn and Zn-doped CdO:Mn thin films Sample
t 2θ(1 1 1) (nm)
f(1 1 1)
Undoped CdO CdO:Mn 1 wt.% Zn 2 wt.% Zn 3 wt.% Zn
463 493 486 472 476
0.4982 0.4464 0.4637 0.4865 0.4723
a
32.944º 32.988º 33.061º 33.109º 33.108º
Lattice parameter ‘a’ (Å) 4.709 4.703 4.693 4.686 4.687
D (nm)
Strain ε x 10-3
δ x 1015 lines/m2
33.91 30.48 32.49 31.94 31.65
1.022 1.137 1.067 1.085 1.095
0.8696 1.0764 0.9473 0.9802 0.9983
It is observed that the lattice parameter values decreased with Mn and Zn doping which
might be due to the smaller ionic radii of Mn2+ (0.67 Å) and Zn2+ (0.74 Å) when compared to that of Cd2+ (0.97 Å).
b
It is observed that both the strain and dislocation density values increased with Mn doping
got reduced with Zn doping indirectly supporting the fact that the amount of defects introduced in the CdO lattice through Mn doping got reduced with Zn doping.
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Table 2 Elemental composition, optical parameters and electrical resistivity values of the undoped CdO, CdO:Mn and Zn-doped CdO:Mn thin films Sample
Undoped CdO CdO:Mn 1 wt.% Zn 2 wt.% Zn 3 wt.% Zn
Elemental composition (at.%) Cd
O
Mn
Zn
O/(Cd+ Mn+Zn)
48.46 47.97 45.12 42.14 43.08
47.79 46.21 46.02 45.29 45.94
--2.19 2.19 2.19 2.19
----2.07 3.06 2.87
0.99 0.92 0.93 0.96 0.95
Optical parameters Band Urbach gap, Eg energy, (eV) Eu (µeV) 2.42 2.937 2.32 4.40 2.46 3.40 2.5 3.03 2.54 2.41
Electrical resistivity, ρ x 10-3 (Ω-cm)
1.82 2.01 1.03 0.71 0.84
c
It is observed that the elements Cd and O show a decreasing trend with doping. It is also
observed that for the undoped film, O/(Cd+Mn+Zn) atomic ratio is equal to 0.99 confirming its perfect stoichiometric nature. The films become non-stoichiometric with doping and for the CdO:Mn film, O/(Cd+Mn+Zn) ratio takes a lesser value of 0.92 and this might be the reason for the increased resistivity value obtained for this film. However, with Zn doping, this ratio slightly increases confirming an improvement in the stoichiometry which reflected in their decreased resistivity values.
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S0030-4026(16)30409-0 http://dx.doi.org/doi:10.1016/j.ijleo.2016.04.129 IJLEO 57612
To appear in: Received date: Accepted date:
4-3-2016 25-4-2016
Please cite this article as: N.Manjula, A.R.Balu, K.Usharani, N.Raja, V.S.Nagarethinam, Enhancement in some physical properties of spray deposited CdO:Mn thin films through Zn doping towards optoelectronic applications, Optik - International Journal for Light and Electron Optics http://dx.doi.org/10.1016/j.ijleo.2016.04.129 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhancement in some physical properties of spray deposited CdO:Mn thin films through Zn doping towards optoelectronic applications N. Manjula, A.R. Balu*, K. Usharani, N. Raja, V.S. Nagarethinam PG and Research Department of Physics, AVVM Sri Pushpam College, Poondi – 613 503, Tamilnadu, India
*Corresponding author Dr. A.R. BALU 757 MIG Colony, New Housing Unit, Thanjavur – 613 005. Ph: +91 9442846351 Email: [email protected]
1
Abstract This paper reports the enhancement in the structural, morphological and opto-electrical properties of spray deposited CdO:Mn thin films through Zn doping. Mn concentration is maintained as 1 at.% for all the films whereas Zn concentration in CdO:Mn film is varied as 1, 2 and 3 wt.%. XRD studies showed that the crystalline quality of pure CdO deteriorates with Mn doping which got enhanced with Zn doping. The entire doped films exhibit a (1 1 1) preferential orientation similar to that of the undoped film. Cauliflower shaped nanostructures are evinced for all the films from the FESEM images. The decreased transparency of pure CdO with Mn doping increases with Zn doping and a maximum transmittance of 89.8 % is obtained for the CdO:Mn film coated with 2 wt.% Zn concentration. Optical band gap of pure CdO which exhibit a red shift with Mn doping got blue shifted with Zn doping. Electrical studies showed that the CdO:Mn film is more resistive than the undoped film which experience a decrement with Zn doping. The obtained results confirm that CdO:Mn film when doped with zinc finds application in optoelectronic devices due to their enhanced optoelectrical properties. Keywords Grain boundaries; crystal structure; red shift; blue shift; carrier concentration
2
1.
Introduction Cadmium oxide (CdO), an n-type II – VI non-stoichiometric degenerate semiconductor
find applications in optoelectronics like transparent conducting oxides (TCOs), solar cells, smart windows, optical communications, flat panel displays and photo-transistors [1].Thin films of CdO exhibit better transparency in the visible and NIR spectral regions. CdO have a direct band gap energy of 2.2 – 2.7 eV and a low electrical resistivity of 10-2-10-4 Ω-cm [2]. It shows n-type conductivity due to native defects such as oxygen vacancies and cadmium interstitials. The conductivity of pure CdO can be improved by controlling these defects, which can be achieved through doping. Concerning specific applications, dopants like Ti [3], Zn [4], Mg [5], Cl [6], Al [7], and Bi [8] have been proposed, studied and employed in CdO thin films. It has been reported earlier that oxalate materials from 3d series such as Mn, Co, Fe, Ni, Zn, etc act as excellent precursors for the synthesis of pure and homogeneous oxide nanoparticles with high surface area because of its low decomposition temperature. Also due to their isomorphous nature, these oxalates enable the modification of the physical properties of oxide semiconductors by doping the precursor in the course of its synthesis [9]. In our previous work we observed that Mn when added to CdO in low concentration (< 2 at.%) improved its optical and electrical properties [10]. Zinc is an important oxalate material and Zn based TCO thin film appears to be promising material in solar cells as window and buffer layers due to its high transparency in the visible region. ZnO thin films have high resistivity with a band gap around 3.3 eV [11]. So it is expected that the homogeneous mixing of both CdO and ZnO allow intermediate optical and electrical properties between them, making CdO attractive for various applications especially as buffer layer in solar cells [12]. Hence in the present study, CdO thin films co-doped with two oxalate precursors Mn and Zn were deposited by spray pyrolysis technique using perfume atomizer. Mn concentration was kept fixed as 1 at.% because, this concentration has been found to be the best to produce CdO thin
3
films with better physical properties [10]. Zn concentration was varied as 1, 2, and 3 wt.% and the effect of Zn doping on the physical properties of CdO:Mn thin films were investigated and the results are reported here. To the best of our knowledge there is no earlier report on the properties of CdO thin films co-doped with two oxalate precursors. 2.
Experimental details CdO thin films co-doped with two oxalate dopants (Mn and Zn) were prepared on glass
substrates by spray technique using perfume atomizer. Mn concentration is kept fixed as 1 at.% in pure CdO to get CdO:Mn thin film and to this film Zn doping is performed with 1, 2 and 3 wt.% Zn concentrations, respectively. Undoped CdO is prepared by spraying aqueous solution (50 ml in volume) containing 0.05 M cadmium acetate. To prepare CdO:Mn film, manganese acetate is used as the precursor and for Zn doping, zinc acetate is used. The thicknesses of the films were measured using a stylus type profilometer. The crystal structure was determined using X-ray diffractometer (PANalytical- PW 340/60 X’ pert PRO) with CuKα radiation (λ = 1.5406 Å). Morphology of the films was examined using a HITACHI SU6600 variable pressure field emission scanning electron microscope (FESEM). The optical band gap of the films was determined from the transmittance spectra using a Perkin Elmer double beam UV-Vis-NIR double beam spectrophotometer. Electrical resistivity values were measured using a two point probe setup. 3.
Results and discussion
3.1
XRD studies Fig. 1(a-e) shows the XRD patterns of undoped CdO, Mn-doped CdO (CdO:Mn) and
Zn-doped CdO:Mn thin films (Zn doping levels: 1, 2 and 3 wt.%), respectively. The presence of diffraction peaks at 2θ values approximately equal to 32.944º, 38.27º, 55.293º, 65.891º and 69.227º indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of cubic crystal structure of pure CdO (JCPDS Card No.73-2245) confirmed their polycrystalline nature. It is observed
4
for the XRD patterns that the CdO:Mn (Fig. 1(b)) and Zn doped CdO:Mn thin films (Fig. 1(ce)) exhibit a strong preferential orientation along the (1 1 1) plane similar to that of the undoped film. The preferential orientation along the (1 1 1) plane observed here exactly matches with the results reported by Manjula et al. [13] and Noornisha et al. [14] for undoped and Zn-doped CdO thin films, respectively prepared by spray technique using perfume atomizer. The preferential orientation factor f(1 1 1) values of the films calculated by the way adopted by Suganya et al. [15] are given in Table 1. It is observed that f(1 1 1) of pure CdO decreases with Mn doping and for the Zn-doped CdO:Mn films it slightly increases which supports the fact that the crystalline quality of pure CdO which deteriorates with Mn doping got enhanced with Zn doping. Overall when compared with that of the undoped film, the f(1 1 1) for all the doped films have lesser values confirming their decreased crystalline quality. The deterioration of the crystalline quality of the doped films might have been caused by the following reasons: i) stress induced due to the ionic size differences between the host and dopant ions and ii) segregation of dopants on the grain boundaries as reported by Xu et al. [16] for sol-gel derived Al-doped ZnO thin films. A close examination of the XRD peaks showed that the 2θ values of the (1 1 1) peak of the doped films shift towards higher Bragg angles confirming a contraction in their lattice parameter values (Table 1). The lattice parameter values obtained for the films are compiled in Table 1a. The average crystallite sizes (D) of the films calculated by using the Scherrer formula, (where β is the FWHM of the strongest peak ((1 1 1) in this case), λ is the
wavelength of the X-ray used (1.5406 Å) and θ is the Bragg angle) are compiled in Table 1. It is observed that crystallite size decreased from 33.91 nm obtained for the undoped CdO to 30.48 nm for the CdO:Mn film which got increased with Zn-doping. The strain (ε) and dislocation density (δ) values of the films calculated using the formulae [17]: 5
(1)
are compiled in Table 1b. 3.2
Surface morphology and elemental analysis Fig. 2(a-e) shows the FESEM images of undoped CdO, CdO:Mn and Zn-doped
CdO:Mn thin films. Cauliflower shaped nanostructures are evinced from the FESEM images of all the films. The film surface of the undoped CdO is composed of equally sized cauliflower shaped nanostructures with few pin holes (Fig. 2(a)). For the CdO:Mn thin film, the surface gets modified with unequally sized cauliflower structures interconnected with each other (Fig. 2(b)). Film surface appeared to be smooth and uniform with tightly packed equally sized cauliflower structures for the Zn-doped CdO:Mn (Zn doping levels: 1 and 2 wt.%) thin films (Fig. 2(c, d)). With further increase in Zn doping concentration to 3 wt.%, the surface gets modified with unequal sized cauliflower structures (Fig. 2(e)). Plenty of pin holes are also visible. These results infer that with Mn-doping the surface morphology of pure CdO slightly deterioraties which then got modified with Zn doping which very well support the results obtained in the XRD analysis. The EDX spectra (Fig. 3(a-e)) confirm the presence of Mn in the CdO:Mn thin film, Mn and Zn in the Zn-doped CdO:Mn thin films. The elemental compositions of the films are compiled in Table 2c. 3.3
Optical properties Fig. 4 displays the transmittance spectra of undoped, CdO:Mn and Zn-doped CdO:Mn
thin films. Undoped CdO film has a maximum transparency of nearly equal to 80.63 % which decreased to 77.84 % with Mn doping. However, the transparency got improved with Zn doping and the CdO:Mn film coated with 2 wt.% Zn concentration exhibited a maximum transparency of 89.8 %. The increased transparency observed for the Zn-doped CdO:Mn 6
films might be due to structural homogeneity, less scattering effects and improved crystallinity, as reported by Usharani et al. [18] for Zn, Mg incorporated CdO thin films. The decreased transparency observed for the CdO:Mn film might be due to the high thickness value obtained for this film (Table 1). Due to increased film thickness, the absorbance increases which might be due to the lot of defects introduced by Mn dopant and this is noticeable in the lower wavelengths below 500 nm as seen in the absorbance spectra (Fig. 5). The crystal structure, the arrangement and distribution of atoms in the crystal lattice play a vital role on the energy gap values of thin film samples. The optical band gap (Eg) values of the undoped CdO, CdO:Mn and Zn-doped CdO:Mn thin films are determined from the relation between the absorption coefficient (α) and the incident photon energy (hυ) [19]: (
)
(3)
where A is a constant. From the plots of (αhυ)2 vs. hυ (Fig. 6), the band gap values are determined from the extrapolation of the straight line portion to the hυ axis at α = 0 and the calculated values are given in Table 2. Undoped CdO film has a band gap value of 2.42 eV which exactly matches with the value reported by Usharani et al. [20]. The band gap decreased to 2.32 eV for the CdO:Mn film which increased with Zn doping attaining a maximum value of 2.54 eV for the CdO:Mn film coated with 3 wt.% Zn concentration. The red shift in the optical band gap observed for the CdO:Mn film might be due to the existence of Mn impurities in the CdO lattice which induce the formation of new recombination centers with lower emission energy [21]. The blue shift in the band gaps observed for the Zn-doped CdO:Mn films may be explained by several mechanisms such as i) Moss-Burstein effect which originates from the lifting of Fermi level into the conduction band due to the increase in charge carrier concentration; ii) the decrease in shallow-level trap concentration near the conduction band of Zn-doped CdO:Mn resulting from the improved crystallinity as reported by Usharani et al. [6] for CdO:Cl thin films, and iii) reduction of band bending effect at the
7
grain boundaries. In nanocrystalline materials band bending effect takes place at grain boundaries due to their increased surface to volume ratio. For smaller grains, the band bending effect is large whereas it becomes flatter for larger grains. Similar results have been reported by Dutta et al. [22] for undoped ZnO thin films. The exponential behaviour of the absorption edge which depends on the incident photon energy (hυ) obeys the empirical Urbach relation as [23]: ( )
where
is a constant, and
is the Urbach energy which is the width of the tails of
localized states associated with the amorphous states of the films in forbidden band. The values are estimated from the slopes of the plots of ln(α) vs. hυ (inset of Fig. 6), and the values are compiled in Table 2. Fig. 7 shows the variation of
and
values of the undoped CdO, CdO:Mn, Zn-doped
CdO:Mn thin films. It is observed that both shift in the
and
values vary in opposite ways. The red
value observed for the CdO:Mn film is well supported with its high
which confirms the increased disorderliness of this film. The low values of
value
observed for
the Zn-doped CdO:Mn films confirm the minimization in the number of defects of CdO:Mn with Zn doping and this might be the reason for the increased
values obtained for these
films. 3.4
Electrical studies The electrical resistivity values of the undoped CdO, Mn-doped CdO (CdO:Mn) and
Zn-doped CdO:Mn thin films are presented in Table 2. Undoped film has a resistivity of 1.82 x 10-3 Ω-cm which slightly increased to 2.01 x 10-3 Ω-cm with Mn doping. The increased resistivity value observed for the CdO:Mn thin film might be due to the occupancy of few Mn2+ ions at the interstitial sites of the CdO lattice besides replacing Cd2+ ions substitutionally, thereby creating structural deformation. Interstitially occupied Mn2+ ions act 8
as trap for free carriers due to the movement of the dopant ions towards the grain boundaries causing a high dispersion of free carriers through the grain boundaries, thus contributing to an increase in its resistivity [24]. Decreased crystallinity observed for this film due to increased number of defects might be another reason for its decreased resistivity. The increased strain and dislocation density values obtained for this film (Table 1) strongly supports the above fact. It is observed that the resistivity of CdO:Mn film decreases with Zn doping attaining values lesser than even the undoped film. The decreased resistivity observed with Zn doping might be due to the following reasons: i)
Successful replacement of Zn2+ ions directly replacing Cd2+ ions or due to the substitution of Zn2+ ions in the Mn2+ sites particularly in the interstitial sites of the CdO lattice,
ii)
Aggregation on CdO grains in form of small Zn-oxide cluster (i.e. crystalliteand grain-boundary) accumulation effect.
4.
Conclusion In this work undoped CdO, Mn-doped CdO (CdO:Mn) and Zn-doped CdO:Mn thin
films were prepared by spray technique using perfume atomizer. Zn-doping was performed on the CdO:Mn film to improve its physical properties to make it suitable for optoelectronic applications. XRD studies confirmed that the crystalline quality of CdO:Mn film improved with Zn doping. Decreased strain and dislocation density values were obtained for the Zndoped CdO:Mn film. Surface morphology of the CdO:Mn film got enhanced with Zn doping. Optical transparency of CdO:Mn film got enhanced with Zn doping and band gap was found to be blue shifted from 2.32 eV to 2.54 eV. Electrical resistivity of CdO:Mn film decreased with Zn doping and a minimum resistivity of 0.71 x 10-3 Ω-cm was obtained for the film coated with 2 wt.% Zn doping concentration. Increased transparency, widened band gap and
9
decreased electrical resistivity values obtained through Zn doping make CdO:Mn film suitable for optoelectronic and photovoltaic applications especially in solar cells. 5.
Acknowledgements The authors thank Mr. Vincent, St. Joseph’s College, Trichy for the UV-vis-NIR
studies. Figure captions Fig. 1
XRD patterns of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films
Fig. 2 FESEM images of a) undoped CdO, b) CdO:Mn, (c-e) Zn-doped CdO:Mn thin films Fig. 3 EDX spectra of a) undoped CdO, b) CdO:Mn, (c-e) Zn-doped CdO:Mn thin films Fig. 4 Transmittance spectra of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films Fig. 5 Absorption spectra of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films Fig. 6 Plots of (αhυ)2 vs. hυ. Inset Fig. Plots of ln(α) vs. hυ of undoped CdO, CdO:Mn, Zndoped CdO:Mn thin films Fig. 7 Variation of optical band gap energy and Urbach energy of undoped CdO, CdO:Mn, Zn-doped CdO:Mn thin films
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Table 1 Thickness and structural parameters of CdO, CdO:Mn and Zn-doped CdO:Mn thin films Sample
t 2θ(1 1 1) (nm)
f(1 1 1)
Undoped CdO CdO:Mn 1 wt.% Zn 2 wt.% Zn 3 wt.% Zn
463 493 486 472 476
0.4982 0.4464 0.4637 0.4865 0.4723
a
32.944º 32.988º 33.061º 33.109º 33.108º
Lattice parameter ‘a’ (Å) 4.709 4.703 4.693 4.686 4.687
D (nm)
Strain ε x 10-3
δ x 1015 lines/m2
33.91 30.48 32.49 31.94 31.65
1.022 1.137 1.067 1.085 1.095
0.8696 1.0764 0.9473 0.9802 0.9983
It is observed that the lattice parameter values decreased with Mn and Zn doping which
might be due to the smaller ionic radii of Mn2+ (0.67 Å) and Zn2+ (0.74 Å) when compared to that of Cd2+ (0.97 Å).
b
It is observed that both the strain and dislocation density values increased with Mn doping
got reduced with Zn doping indirectly supporting the fact that the amount of defects introduced in the CdO lattice through Mn doping got reduced with Zn doping.
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Table 2 Elemental composition, optical parameters and electrical resistivity values of the undoped CdO, CdO:Mn and Zn-doped CdO:Mn thin films Sample
Undoped CdO CdO:Mn 1 wt.% Zn 2 wt.% Zn 3 wt.% Zn
Elemental composition (at.%) Cd
O
Mn
Zn
O/(Cd+ Mn+Zn)
48.46 47.97 45.12 42.14 43.08
47.79 46.21 46.02 45.29 45.94
--2.19 2.19 2.19 2.19
----2.07 3.06 2.87
0.99 0.92 0.93 0.96 0.95
Optical parameters Band Urbach gap, Eg energy, (eV) Eu (µeV) 2.42 2.937 2.32 4.40 2.46 3.40 2.5 3.03 2.54 2.41
Electrical resistivity, ρ x 10-3 (Ω-cm)
1.82 2.01 1.03 0.71 0.84
c
It is observed that the elements Cd and O show a decreasing trend with doping. It is also
observed that for the undoped film, O/(Cd+Mn+Zn) atomic ratio is equal to 0.99 confirming its perfect stoichiometric nature. The films become non-stoichiometric with doping and for the CdO:Mn film, O/(Cd+Mn+Zn) ratio takes a lesser value of 0.92 and this might be the reason for the increased resistivity value obtained for this film. However, with Zn doping, this ratio slightly increases confirming an improvement in the stoichiometry which reflected in their decreased resistivity values.
14