Influence of growth temperature on the physico-chemical properties of sprayed cadmium oxide thin films

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Influence of growth temperature on the physicochemical properties of sprayed cadmium oxide thin films S.P. Desai, M.P. Suryawanshi, S.M. Bhosale, Jin Hyeok Kim, A.V. Moholkar

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Cite this article as: S.P. Desai, M.P. Suryawanshi, S.M. Bhosale, Jin Hyeok Kim, A.V. Moholkar, Influence of growth temperature on the physico-chemical properties of sprayed cadmium oxide thin films, Ceramics International, http://dx.doi.org/10.1016/j. ceramint.2014.12.045 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 galley proof before it is published in its final citable 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.

Influence of growth temperature on the physico-chemical properties of sprayed cadmium oxide thin films S. P. Desaia, M. P. Suryawanshia,b, S. M. Bhosalea, Jin Hyeok Kimb, A. V. Moholkara* a

Thin Film Nano-materials Laboratory, Department of Physics, Shivaji University, Kolhapur-416004, (MH) India b

Optoelectronics Convergence Research Center, Department of Materials Science and Engineering, Chonnam National University, 300, Yongbong-Dong, Buk-Gu, Gwangju 500-757, South Korea

*Corresponding Author: [email protected]

Abstract Transparent conducting cadmium oxide (CdO) thin films have been deposited onto the soda-lime glass (SLG) substrates using facile and cost-effective spray pyrolysis (SPT) technique. The films have been deposited at various substrate temperatures ranging from 250 to 400oC in steps of 50oC. The influence of substrate temperature on structural, morphological and electro-optical properties of CdO thin films has been investigated. Thermo-gravimetric analysis (TGA) study indicates the formation of CdO by decomposition of cadmium acetate after 250oC. The X-ray diffraction study reveals that all samples are polycrystalline with major reflex along (111) and (200) plane, manifested with the homogeneous distribution of roughly spherical clusters all over the substrate of varying grain size.The optical study shows band gap ranging between 2.3 - 2.5 eV. The Hall effect measurement indicates that the resistivity decreases from 2.43 x 10-3 to 0.99 x 10-3 Ωcm while carrier concentration increases from 2.61 x 1020 – 5 x 1020 /cm3 and mobility lies in the range of 8.26 – 12.55 cm2/Vs

1

Keywords: Transparent conducting oxide, CdO, spray pyrolysis technique (SPT), carrier concentration, Thermo-gravimetric analysis (TGA)

1. Introduction Transparent conducting oxides (TCO’s) are essential for technologies that required both large-area electrical contact and optical access in the visible portion of the light spectrum. High transparency with useful electrical conductivity is achieved by selecting a wide-bandgap oxides like ZnO, TiO2, IrO2, SnO2, Sn:In2O3, CdO [1-6]. For optoelectronic applications, the transparent conductor must be carefully processed to maximize optical transmitivity in the visible regime, while achieving maximum electrical conductivity. CdO has gained substantial interest due to its interesting electronic and optical properties such as carrier mobility, high conductivity with direct band gap energy making it suitable candidate for electro-optical applications [6]. Various deposition methods have been employed to prepare CdO thin films such as plasma laser deposition (PLD) [6], RF Sputtering [7], chemical bath deposition(CBD) [8], Sol-gel [9], vacuum evaporation [10,11], metalorganic chemical vapor deposition (MOCVD) [12], successive ionic layer adsorption and reaction (SILAR) [13] and spray pyrolysis technique (SPT) [5]. Among these methods, SPT is a most promising and widely used method to prepare variety of thin films such as metal oxides, superconducting materials and nanomaterials. It is simple, cost-effective and non-vacuum method which has several advantages such high purity, easy control of chemical composition [5]. The substrate temperature is one of the key factor in deciding the physical, chemical and other properties of any thin films. In this work, we have focused on the optimization of spraypyrolytic synthesis of nanocrystalline CdO thin films in terms of substrate temperature

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aiming for highly transparent conducting oxide films. The effect of substrate temperature on the various properties such as structural, morphological, electrical and optical properties have reported.

2. Experimental details:The CdO films were synthesized by using cadmium acetate [(Cd(CH3COO)2] aqueous solution as a precursor. By using double distilled water, 0.1 M Cadmium acetate [A. R. grade (99 % pure), Loba Chemie Pvt. Ltd. Mumbai] was prepared and sprayed onto the preheated ultrasonically cleaned glass substrates. The deposition parameters like solution concentration (0.1 M), spray rate (4 ml/min), nozzle to substrate distance (33 cm), pressure of carrier gas (75 kg/cm2) and quantity of spraying solution (20 ml) were kept constant. The optimized values are indicated in bracket. The substrate temperature was varied from 250oC to 400oC in steps of 50oC using electronic temperature controller with an accuracy of ± 5oC. The films deposited at 250oC, 300oC, 350oC and 400oC were allowed to cool naturally at room temperature and were further used for their different characterization. These films are denoted by T250, T300, T350 and T400, respectively. All the as deposited films were faint yellowish-brown in color, transparent, uniform, well adherent to the substrates and pinhole free. The deposited films were characterized by means of their chemical, structural, morphological, optical and electrical properties with the help of different characterization technique. To select the range of substrate temperature for deposition, thermo-gravimetric analysis (TGA) of cadmium acetate was carried out using TA instrument SDT Q600 V20.9 Build 20. The structural properties of sprayed films were studied by X-ray powder diffractometer Bruker AXS (D2-Phaser) using Cu Kα (λ = 1.5418 Å) operated at 25 kV, 20

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mA. The microstructural study was carried out using JEOL JSM 6360, (Japan) scanning electron microscope. The 3 dimensional surface morphology of sample was studied using AFM images obtained from INNOVA IB3BE model (Bruker, USA) in contact mode. The thickness of the samples measured using surface profiler make Ambios Technology model XP-1 (USA). The optical absorption spectrum was studied at room temperature within 200800 nm wavelength range using spectrophotometer UV-1800 SHIMADZU. The room temperature electrical measurements were carried out with Hall effect set up in Van-der Pauw configuration. .

3. Results and discussion 3.1 Thermogravimetric analysis It is very important to determine the possible temperature range in which the metal oxide films are formed, hence thermo-gravimetric (TGA) analysis of cadmium acetate precursor powder, taken in the appropriate proportion has been carried out. The typical thermogram obtained for 4.029 mg cadmium acetate powder is shown in Fig.1. The thermal evolution in air takes place in five consecutive stages with weight losses for which inflection point coincide with the temperature with the temperature corresponding to exothermic and endothermic peaks in DTA trace. It is clearly seen that the loss of water from the precursor take place at various temperatures in the range of 25-140oC, corresponding to which endothermic peaks are observed. The total weight loss corresponding to removal of both the physisorbed and chemisorbed water is calculated is about 13.97%. The rapid weight loss commences at about 250oC, which is indication of onset of the thermal decomposition of the precursor. This regular weight loss continues upto 500oC as shown in Fig.1. The weight loss during this temperature is mainly due to removal of acetate group from the precursor, which

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leads to the formation of cadmium oxide. Thus, this result indicates the formation of CdO by decomposition of cadmium acetate after 250oC. The similar result has been reported by Deokate et al. [15].

3.2 X-ray diffraction studies Fig. 2 show the XRD patterns of all the CdO thin films deposited at different substrate temperatures. It is found that all the films are polycrystalline in nature with face centered cubic (FCC) structure having major reflex along (111) and (200) planes. The presence of other planes corresponding to (220), (311) and (222) has been observed. The observed ‘d’ values of the films are in well agreement with those reported for CdO [14] (JCPDS card no.01-075-0592). It is manifested that the intensity corresponding to major (111) and (200) planes gets enhanced with substrate temperature, which indicates that the films sprayed at higher substrate temperature have better crystallinity. The enhancement in crystallinity of films is due to increase in thermal energy required for crystallization, recrystallization and growth of the grains with temperature. The crystallite size of different planes is estimated using the Scherrer’s formula [16]
=

0.9  

where, D is the crystallite size, λ is the X-ray wavelength, θ is the Bragg’s angle and β is the full-width of the diffraction line at half of its maximum intensity (FWHM). The calculated average crystallite size (D) for all samples is given in Table 1. It is found that the grain size increases with increase in substrate temperature as 34.94 nm,47.02 nm, 52.81 nm and 56.37 nm for T250, T300, T350 and T400 samples respectively. This may be due to the fact that the smaller crystallites with sharper convexity provide larger area of contact between adjacent crystallites, facilitating coalescence process to form larger crystallites.

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The dislocation density is then evaluated from the crystallite size by the relation [17]



=  where, n is a factor which equals unity giving minimum dislocation density. The calculated δ values (Table 1) indicate that the dislocations per unit volume of unit cell decreases with increase in temperature, it means with increase in temperature the defects decreases and improves the crystalline quality. The preferential orientation of the film can be studied by calculating the texture coefficient TC(hkl) for all the planes using the following expression [5] 

 =

 

 

∑



where, TC(hkl) is the texture coefficient of (hkl) plane, I (hkl) is the measured intensity of peak, I0 is the standard intensity of (hkl) plane from JCPDS data (JCPDS card no.01-075-0592) and N is the number of reflection observed. From TC measured for various planes for films deposited at different temperatures, it is clear that all films have preferential orientation along (111) and (200) planes. The exact values of the lattice parameter have been calculated by plotting the graph of the lattice parameter (a) versus Nelson-Riley Function (NRF) for each plane and are cited in Table 1. It is seen that in Fig.3 that all the points do not lie on a linear fitted straight line due to strain present in polycrystalline material. Therefore, it is necessary to determine the strain generated in the deposited films. It is well known that depending on the method used to deposit the thin films, inherent strain and stress are get generated in the films. The other

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possible reasons to develop the strain in film are such as the interlayer lattice mismatch during the growth of the film, deficiencies and defect generated in film like edge dislocation, screw dislocation and void spaces etc. Because of one or more reasons above mentioned the unit cell of crystal structure gets deformed resulting in developing strain. Consequently, the observed lattice strain depends on material properties like viscosity and elastic compliance as well as the applied macroscopic stress and the stress-strain distribution among various grains [18]. The strain in the sample can be determined by using the equation [19]

 =

!"#$

− &'()

where, β is the FWHM, λ is the wavelength of X-Ray, D is the crystallite size, ε is the strain in the sample and θ is the Bragg’s angle. Here by plotting the graph of (βcosθ)/λ Vs sinθ/λ and taking slope of it the average strain is calculated. For T350 it is as shown in Fig.4 which is 1.52 x 10-3. The average strain for T250, T300 and T400 are found to be 8.9 x 10-3, 6.51 x 10-3, and 5.85 x 10-3 respectively. The initial decrease in the strain may be due to change in crystallite size with respect to temperature. The possible explanation for the decrease in stain is that as the temperature increases crystallite size increase which influences grain size consequently the grain boundary decreases. With rise in temperature, the growth rate of film formation increases, increasing the crystallinity and grain size, minimizing the strain. This affects the lattice constants. The increase in strain after T350 is due to deterioration of the film quality. Further increase in temperature increases the strain it may be due stress gets relaxed due to weak adhesion of film with substrate or plastic deformation of films [18].

3.3 Surface morphology studies

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a) SEM studies: Fig.5 (a-d) shows SEM images of T250, T300, T350 and T400 samples respectively. It is found that sample T250 has more rough surface covered with spheres of micron sized grains, which renders higher room temperature electrical resistivity into the sample. Sample T300 has more uniform surface than T250, which is probably responsible for its slightly lower value of electrical resistivity. Upon further increase in the substrate temperature, the surface of sample T350 became highly smooth with more uniformity and void-free. It consists of uniform distribution of spherical grains with relatively higher density, thereby minimizing the grain boundary scattering. This sample has lowest electrical resistivity amongst all samples. Thus, it is noteworthy that the substrate temperature of 3500C provides sufficient thermal energy to grow thin film of CdO well adherent, more uniform with fine grain structure and subsequently become more conductive. Sample T400 exhibits the dissemination of spherical grains into each other, which leads to the formation of interconnected grain structure upon further increase in substrate temperature. This sample has better crystallinity among all samples but exhibits relatively higher resistivity. This could be due to the disordering into the structure of sample at such a higher substrate temperature. The average grain size estimated for T250, T300, T350 and T400 are 200 nm, 250 nm, 320 nm and 890 nm respectively. b) AFM Analysis Fig. 6 (a-d) shows AFM images of T250, T300, T350 and T400 samples respectively. It is found that all the films cover entire surface area with small spherical grains which are randomly distributed. Deposition temperature, shadowing effect and energy acquired by depositing atom are responsible for improvement in microstructure of CdO films. Due to mass diffusion and increase in thermal energy along with rise in temperature voids fill with lattice sites. RMS roughness of thin films has important role in development of optical

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coatings. The RMS roughness gives the quality of surface and light scattering ability of sample [20]. AFM images of 5µm x 5µm are used to determine surface roughness of films. It was observed that substrate temperature is responsible for alteration of microstructure of the films, as can be seen from Fig.6 (a-d), large number of grains forms the cluster. For T350 sample, compact morphology with low roughness of 44.5 nm was found. The RMS roughness of T250, T300 and T400 was observed as 124 nm, 121 nm and 87.9 nm respectively. The AFM images of all samples show similar morphology as that of SEM images. 3.4 Optical Properties The variation of optical density with wavelength is further analyzed to find out the nature of transition involved and the optical band gap. The nature of the transition involved is determined by using following the relation [21]. (αhν)1/n =A(hν − Eg) where, the symbols have their usual meanings. For allowed direct transition, n = 1/2, for allowed indirect transition n = 2, for forbidden indirect transition n=3/2 and for forbidden indirect transition it is 3. Taking n = 1/2 we have calculated direct optical band gap from (αhν)2 Vs (hν) plot by extrapolating the linear portion of the graph on hν axis at y=0 as shown in Fig. 4. The value of direct band gap lies in the range of 2.3 eV – 2.5 eV which is in well agreement with the others reported [21]. The sample T250 has minimum band gap energy value of 2.3 eV, amongst all other samples, owing to lower carrier concentration. It increases gradually with increase in substrate temperature and attain the maximum value 2.5 eV for sample T350 Thus, the carrier concentration is higher in sample T350. As the carrier concentration is higher, absorption of the light by the carriers also increases, leading to the higher absorption coefficient (α) in the sample T350. As carrier concentration decreases,

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absorption by the carriers also decreases, resulting into lower α values in other samples. For sample T400, the band gap energy value slightly decrease to 2.41 eV. The increase in bandgap of CdO film is attributed to increase in carrier concentration which leads to Burstein-Moss (BM) effect. There are other few factors which are responsible for the change in bandgap like electron-electron interaction, coulomb interaction within the conduction band which leads to shrinkage and renormalization of bandgap of CdO as described by various groups [5,6,11,22]. 3.5 Electrical Properties Fig.8 and 9 show the variation of electrical resistivity (ρ), carrier concentration (n) and mobility (µ) of T250, T300, T350 and T400 samples. The resistivity, carrier concentration and mobility measured at room temperature by four probe Hall method in standard Van der Pauw configuration and are presented in Table 2.The resistivity decreases from 2.43 x 10-3 to 0.99 x 10-3 Ωcm with increase in substrate temperature upto 350oC and further increases at 400oC. Opposite trend is observed in the values of carrier concentration and mobility of samples. The electrical parameters presented in this work are in well agreement with literature results [23]. The decrement in the electrical resistivity of the samples with the substrate temperature is due to increment in grain boundary concentration, which enhances carrier concentration and mobility of charge carriers. It is well known that the nature of defects and grain boundaries of different thin films are strongly dependent on the crystal quality. The defects and grain boundary acts as free electron trap centers, which reduces the charge carrier concentration and become scattering centers, leading to a decrease in electrical mobility [24]. The electrical mobility of conducting oxides is related to the surface morphology of films. The rough surface morphology also reduces the mean free path of free electrons on surface of

10

the film, leading to decrease in the electrical conductivity of films. Therefore, sample T350 shows improved electrical properties compared to that of all other samples. 4. Conclusions The simple and cost-effective spray pyrolysis technique is used to prepare CdO thin films at different substrate temperatures. It is noteworthy that the substrate temperature plays a vital role in changing the properties of CdO thin films. It is observed that the pyrolytic decomposition of cadmium acetate precursor solution occurs at the substrate temperature 350oC. The XRD studies reveal that all the samples are polycrystalline in nature and crystallinity and grain size ameliorates with increase in substrate temperature. The optical direct band gap lies in range between 2.3 and 2.5 eV. The electrical resistivity of all samples lies in the range of 2.43 x 10-3 to 0.99 x 10-3 Ωcm. All the samples show n-type semiconducting behavior. The films sprayed at 3500C exhibits the lowest resistivity of 9.9 x 10-4 Ωcm, carrier concentration of 5 x 1020 /cm3 and mobility of 12.55 cm2/Vs. 5. Acknowledgements: Author is grateful to University Grant Commission (UGC), New Delhi and Department of Science and Technology (DST) New Delhi for the financial assistance.

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18. L. B. Freund and S. Suresh, Thin film materials: stress, defect formation and surface evoluation, Cambridge University press, United Kingdom, 2008. 19. A.A. Bagade, V.V. Ganbavle, and K.Y. Rajpure, J. Mater. Engg. Perfor. 23 (2014) 2787– 2794. 20. A. A. Ziabari, F.E. Ghodsi, G. Kiriakidis , Surf. Coat. Tech. 213 (2012) 15–20. 21. A.V. Moholkar, G.L. Agawane, Kyu-Ung Sim, Ye-bin Kwon, K.Y. Rajpure, J.H. Kim, App. Surf. Sci. 257 (2010) 93–101. 22. B. Saha, S. Das, K.K. Chattopadhyay, Sol. Energ. Mater. Sol. Cells 91 (2007) 1692– 1697. 23. N. Wongcharoen, T. Gaewdang, and T. Wongcharoen, Energy Proc. 15 (2012)

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Figure Captions: Fig.1 (a) Thermo gravimetric analysis (TGA) and (b) differential thermal analysis (DTA) of the precursor powder of cadmium acetate in the temperature range 0-1000 0C. Fig.2 The XRD patterns of CdO samples, T250, T300, T350 and T400, respectively. Fig.3 Typical Nelson-Riley plot for lattice parameter of T350 sample. Fig.4 The plot of (βcosθ)/λ Vs sinθ/λ for various planes of CdO T350 sample. Fig.5 Scanning electron micrographs (SEMs) of CdO samples deposited at different substrate temperatures, T250, T300, T350 and T400 respectively. Fig.6 Atomic force micrographs (AFMs) of CdO samples T250, T300, T350 and T400 respectively. Fig.7 The variation of (αhν)2 Vs hν for CdO samples T250, T300,T350 and T400 respectively.

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Fig.8 Variation of resistivity of T250, T300, T350, T400 samples respectively. Fig.9 Variation of carrier concentration (n) and mobility (µ) with temperature, of different studied samples.

Table captions: Table 1 Values of texture coefficient (TC), strain (ε) and crystallite size (D) and defect density (δ) of CdO samples sprayed at various substrate temperatures, T250, T300, T350 and T400. Table 2 Various electrical parameters of CdO samples sprayed at various substrate temperatures, T250 , T300 , T350, and T400 .

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Sample

2θ

(hkl)

(degree)

T250

T300

T350

T400

TC

a in A0

(hkl)

(ε) x 10

33.06

(111)

1.45

4.69

38.43

(200)

1.39

4.68

55.41

(220)

65.95

(311)

69.38

(222)

33.10

(111)

38.35

(200)

55.35

(220)

65.97

(311)

69.47

(222)

33.14

(111)

38.38

(200)

55.40

(220)

66.10

(311)

69.49

(222)

33.10

(111)

38.46

(200)

55.43

(220)

66.04

(311)

69.35

(222)

4.68 <1

Strain

D (nm) -3

40.83 41.45 8.9

33.15 29.99

4.68

29.27

1.30

4.68

66.81

1.45

4.69

53.28

<1

6.5

33.15

4.69

52.53

4.68

29.29

1.38

4.68

40.83

1.38

4.69

46.61

4.69 <1

1.5

99.46

4.68

41.99

4.68

35.14

1.43

4.68

81.67

1.19

4.68

93.29

4.68 <1

D δ

( nm)

4.69

4.69

Average

5.8

4.69

39.79

34.94

47.01

0.082

0.045

52.81

0.035

56.37

0.031

42.01

4.69

25.08

Table 1

Sample

Resistivity (ρ )

Carrier concentration

Mobility (µ)

Ωcm

(n) (x 1020) /cm3

cm2/Vs

T250

2.42 x 10-3

2.61

8.26

T300

2.02 x 10-3

3.11

11.79

T350

0.99 x 10-3

5.00

12.55

T400

1.80 x 10-3

3.00

11.49

15

Figure

(222)

(311)

(220)

(200)

Intensity (a.u.)

(111)

Fig.1

T4 T3 T2 T1

20

30

40

50

2q (deg.) Fig.2

60

70

80

lattice constant (a.u.)

4.692

4.688

4.684

4.680 1.0

1.5

2.0

2.5

3.0

3.5

NRF Fig.3

(bcosq)/l, 10

-3

5

4

3

2 0.15

0.20

0.25

0.30

(sinq)/l Fig.4

0.35

0.40 

Fig.5 .

Fig.6

10

x 108 (eV/cm2)

T250 8

T300 6

T350

2

4

( ah u)

T350 2

0 1.0

1.5

2.0

2.5

3.0

3.5

Photon energy (eV)

-3

Resistivity(r),10 (ohm.cm)

Fig.7

2.4

T250

T300

2.0

T400 1.6

1.2

T350

0.8 250

300

350

400

0

Temperature( C)

Fig. 8







5.0 12 4.5 11

2

4.0

Mobility (cm /Vs)

Carrier Concentration, 10

20

-3

(cm )

13

10

3.5 3.0

9

2.5

8 250

300

350

400 o

Substrate Temperature ( C)                                Fig.9