Annealing studies on the structural and optical

Materials Chemistry and Physics 130 (2011) 1366–1371

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Annealing studies on the structural and optical properties of electrodeposited CdO thin films Trilok Singh ∗ , D.K. Pandya, R. Singh Department of Physics, Thin Film Laboratory, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India

a r t i c l e

i n f o

Article history: Received 15 December 2010 Received in revised form 6 September 2011 Accepted 12 September 2011 Keywords: CdO Electrodeposition Annealing Bandgap

a b s t r a c t Cadmium oxide (CdO) thin films were electrodeposited on indium doped tin oxide glass substrates from aqueous solution and subsequently annealed at higher temperatures up to 600 ◦ C in air, oxygen and inert gas ambients, respectively. The effect of annealing on the morphology, crystalline quality and optical properties of CdO films was investigated. The CdO films were polycrystalline in nature and the grain size strongly depended on the annealing temperature as well as annealing ambient. CdO films were highly transparent in the visible region of the spectrum. The transmittance of the films decreased with increasing annealing temperatures in all the three ambients. Optical bandgap of the nanocrystalline CdO films varied in the range of 2.20–2.54 eV. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Transparent metal oxide semiconductor materials such as zinc oxide, indium oxide and cadmium oxide have attracted much attention owing to their potential applications in electronic and optical devices. Among these metal oxide semiconductors, CdO is an n-type semiconductor with non-stoichiometric composition due to the presence of either cadmium interstitials [1] or oxygen vacancies [2], which acts as doubly charged donors [3]. Its high electrical conductivity and high optical transmittance [4] in the visible region of the solar spectrum along with a moderate refractive index makes it useful for various applications such as photovoltaic solar cells, transparent electrodes, gas sensors, liquid crystal display, photodiodes, phototransistors and optoelectronic devices [5–7]. CdO thin films had been prepared by various physical and chemical deposition techniques such as pulsed laser deposition [7,8], magnetron sputtering [9,10], RF sputtering [11], sol–gel [12,13], spray pyrolysis [14,15], chemical bath deposition [16], metalorganic chemical vapor deposition (MOCVD) [17] and electrodeposition [18–22]. Electrodeposition is an important method that is used for the growth of metal oxide films from aqueous or non-aqueous solutions and the advantage of this method compared with other techniques includes low process temperature, inexpensiveness and capability of controlling morphology of the films. The earlier reported results revealed that the characteristics of CdO

∗ Corresponding author. Tel.: +91 011 26596658. E-mail address: [email protected] (T. Singh). 0254-0584/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2011.09.035

films were strongly dependent on the deposition techniques and deposition conditions. However, there are only a few reports published on the effect of thermal annealing on the characteristics of CdO films [23–25]. In this work, CdO films have been deposited by electrodeposition on ITO substrates at room temperature. We carried out various measurements to evaluate the effect of annealing in the presence of different ambients on the morphological, structural and optical properties of CdO films. 2. Experimental The electrodeposition was carried out using a CHI potentiostat/galvanostat (CHI Electrochemical Analyzer) in a specially designed closed glass cell. The indium doped tin oxide (ITO) transparent glass substrate (sheet resistance ∼20 /) was used as a working electrode while platinum sheet and saturated calomel electrode (SCE) were used for counter and reference electrodes, respectively. Prior to electrodeposition, the ITO glass substrates were rinsed with acetone, toluene, de-ionized (DI) water and then ultasonicated in distilled water for 20 min. After that the ITO substrate were dipped in nitric acid (10 wt%) for acid activation and followed by rinsing with DI water. The electrolyte (bath) temperature was kept at 30 ◦ C. All chemicals were procured from commercial sources and were of the highest purity available. They were used without further purification. Electrochemical deposition of CdO thin films were carried out from the 1 molar aqueous solution of cadmium nitrate [Cd(NO)3 ·4H2 O] at the deposition potential of −0.7 V (vs. SCE) for 15 min. After deposition the samples were removed from electrolyte and rinsed in DI water. Subsequently grown samples were annealed in a tubular furnace having temperature accuracy of ±1 ◦ C. The samples were annealed in different atmospheres such as air, oxygen and nitrogen for 1 h at 300, 500 and 600 ◦ C, respectively. The air, oxygen and nitrogen annealed samples are represented by notation CA, CO and CN respectively and temperature range 300, 500 and 600 ◦ C are given as 3, 5 and 6. For the structural studies, X-ray diffrac˚ radiation in 2 range tometer (Model Philips Xpert Pro) using CuK␣ ( = 1.5405 A) 20–80◦ was used. Scanning electron microscopy images were obtained using Zeiss (Model EVO-50). Energy dispersive X-ray spectroscopy was obtained from BrukerASX (Model QuanTax 200). The transmission spectra of grown films were measured using UV–vis spectrophotometer from Perkin Elmer (Model Lambda 650).

T. Singh et al. / Materials Chemistry and Physics 130 (2011) 1366–1371

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Fig. 1. (a) Cyclic voltammograms and (b) chronoamperometry.

3. Results and discussion The electrochemical deposition takes place on the surface of ITO and the possible formation mechanism of CdO is suggested as follows [21] NO3 − + H2 O + 2e− → NO2 − + 2OH− 2+

Cd

(1)

−

+ 2OH → Cd(OH)2

(2)

As-grown sample of Cd(OH)2 was annealed at temperatures 300, 500 and 600 ◦ C in different gas ambient. The conversion of Cd(OH)2 to CdO takes place above 280 ◦ C by the following reactions [23]. Cd(OH)2 → CdO + H2 O

(3)

Fig. 1(a) shows the typical cyclic voltammograms recorded with a ITO working electrode in 1molar aqueous solution of Cd(NO)3 ·4H2 O at room temperature. During the positive going scan the cathodic current onset at approximately −1.5 V and an anodic peak appeared at −0.4 V. Fig. 1(b) shows the chronoamperometry of Cd(OH)2 thin films at a constant deposition potential of −0.7 V for 15 min and the average deposition current was about 4.7 mA. The surface morphology of the as-grown and annealed CdO films was studied by using SEM. The surface morphology of the as-grown sample exhibited entangled nanostructures as shown in Fig. 2(a). Fig. 2(b)–(h) shows the SEM micrographs of CdO films annealed at 300, 500 and 600 ◦ C air, oxygen and nitrogen gas ambient, respectively. It can be clearly seen from SEM micrographs that the films do not have a smooth and homogeneous surface morphology in case of as-grown thin films. However, on annealing in air and oxygen the films smoothness and homogeneity increased. The surface properties of the CdO films appear to change significantly as a function of annealing temperature. The CdO film becomes quite smooth at 600 ◦ C. Fig. 1(b)–(g) shows that the CdO films quality and roughness improved with increasing of annealing temperature. It is seen

(Fig. 1(h)) that on annealing in nitrogen ambient there are lot of nanostructures formed on the surface of CdO film and it has been observed that in nitrogen annealing from 300 ◦ C to 600 ◦ C there were no significant change in the SEM images. In order to investigate the structural properties of the annealed CdO films in air, oxygen and nitrogen, the XRD measurements were performed and the spectra of as-grown and annealed films are given in Figs. 3 and 4. As-grown films is Cd(OH)2 with the dominant reflection peaks (1 0 0), (0 1 1), (2 0 0), (0 1 2), (1 1 0), (1 1 1), (2 0 1), (1 0 3) and (2 0 2) (JCPDF Card No. 73-0969). In Fig. 4 the XRD patterns reveal that all the films are polycrystalline in nature and exhibit only cubic CdO structure. The lattice constant calculated for all the films are given in Table 1, which are almost identical with the values earlier reported [10]. The variation of lattice parameter as a function of annealing temperature and ambient for CdO thin films deposited on ITO are sown in Table 1. It can be seen that the lattice parameter decreases with annealing temperature for all three ambients. During film deposition and post-deposition treatments, there is always possibility for developing some strain (stress) in the films. Strain affects the mechanical properties of the films such as the stability of microstructure and the adhesion between film and substrate. Strain in the films can be intrinsic, caused by the condition prevailing during deposition. The usually observed (2 0 0) orientation growth in CdO films prepared by different techniques [26,27] was enhanced by postannealing in different ambients. The observed ‘d’ values are in good agreement with standard (d = 0.234 nm) values and diffraction peaks indexed to the cubic phase of CdO (JCPDF card No. 75-0591). Fig. 4 shows the XRD patterns of annealed CdO films. It shows well defined peaks having orientations in the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes. There is a (2 0 0) preferred orientation of all annealed CdO samples and the crystalline quality increases with increase of annealing temperature. From Fig. 4, it can be clearly seen that the preferential orientation peak for

Table 1 The structural and optical parameters of annealed CdO samples in air, oxygen and nitrogen ambient. Film

2 ± 0.001 (◦ )

FWHM ± 0.001 (◦ )

˚ d (A)

˚ a (A)

˚ D (A)

T%

Eg (eV)

Air annealed (CA) 300 ◦ C (CA3) 500 ◦ C (CA5) 600 ◦ C (CA6)

38.296 38.31 38.33

0.321 0.278 0.253

2.348 2.347 2.346

4.695 4.692 4.690

258.95 300.23 331.13

75 65 55

2.32 2.25 2.20

Oxygen annealed (CO) 300 ◦ C (CO3) 500 ◦ C (CO5) 600 ◦ C (CO6)

38.26 38.30 38.33

0.525 0.329 0.295

2.350 2.348 2.347

4.699 4.695 4.691

157.37 252.78 282.93

75 65 50

2.45 2.36 2.21

Nitrogen annealed (CN) 300 ◦ C (CN3) 500 ◦ C (CN5) 600 ◦ C (CN6)

38.22 38.23 38.33

0.956 0.507 0.326

2.352 2.352 2.346

4.70 4.70 4.69

85.89 162.22 286.53

75 65 65

2.54 2.39 2.20

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Fig. 2. SEM images of (a) as-grown CdO, air annealed CdO at (b) 300 ◦ C, (c) 500 ◦ C, (d) 600 ◦ C, Oxygen annealed CdO at (e) 300 ◦ C, (f) 500 ◦ C, (g) 600 ◦ C and nitrogen annealed CdO at (h) 600 ◦ C.

annealed films became sharper and more intense, especially for 600 ◦ C annealed film. This may be attributed to the crystallinity of the CdO films being improved with annealing temperature. For all CdO films, the grain size (D) perpendicular to [20] direction was estimated by using Scherrer’s relation [28] Dh

kl

=

0.9 ˇh k l cos h k l

(4)

where is the X-ray wavelength, is the Bragg diffraction angle and ˇ is full width at half maximum (FWHM) of the mean peak

in the XRD pattern. The average size of crystallites calculated for all samples are shown in Table 1. In general, by increasing annealing temperature from 300 ◦ C to 600 ◦ C the grain size (D) variation between 8.5 and 33 nm was observed. The transmittance spectra of CdO thin films is expected to depend mainly on three factors (1) oxygen deficiency, (2) surface roughness and (3) impurity centers. The transmission spectra of CdO films were measured to investigate the effect of annealing temperature and ambient. Optical transmittance spectra of annealed CdO films were measured in the range of 400–850 nm at room


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Fig. 3. a XRD pattern of Cd(OH)2 thin films grown at 30 ◦ C and (b) Standard (JCPDS) XRD patterns of Cd(OH)2 and CdO films.

temperature. All films show very similar optical transmissions in the 550–850 nm range, and the difference becomes more pronounced at shorter wavelengths. The optical band edge shifts to higher wavelength with increasing the annealing temperature. The films annealed in air and nitrogen atmospheres show higher transmission than the as deposited film due to the removal of defects, while the film annealed in oxygen shows slightly lower transmission and sharper band edge due to the reduction in the concentration of oxygen vacancies (inset Fig. 5). In the visible region of the spectrum, the transmittance of annealed films was found to vary from about 55% to 75% depending on the annealing temperature and annealing ambient. The cut-off wavelength shifts towards the red when annealing temperature increases and simultaneously a reduction in the transmittance is observed. The transparent film become yellow to reddish on annealing at higher temperature hence, subsequently the transmittance of the film decreases in all the ambients. In order to study the effect of annealing temperature and annealing ambient on optical bandgap of CdO films, the absorption edge was investigated for all the samples annealed at higher temperatures and shown in Fig. 5. The optical bandgap was calculated by the following relationship [29] ˛hv = A(hv − Eg )

m

Fig. 4. XRD patterns of CdO thin films annealed in (a) air, (b) oxygen (c) nitrogen at 300, 500 and 600 ◦ C for 1 h, respectively and (d) standard (JCPDS) XRD patterns of CdO films.

(5)

where A is an energy-independent constant, Eg is the optical bandgap and m is the constant which determine type of optical transition. The optical bandgaps were calculated from this plot and total variation in the range of 2.20–2.54 eV was observed. One obvious result is that the behavior is similar to that observed for other thin film materials like CdS and CdSe annealed at different temperatures and atmosphere [30–33]. The decrease in energy bandgap after annealing can be attributed to improvement in the crystallinity with annealing temperature as supported by XRD studies. As the annealing temperature increased, the crystallite size of the CdO films was found to increase in the range of 8.5–33 nm, resulting in the decrease of the bandgap of the samples potentially due to quantum confinement

effect. Due to the quantum confinement effect, smaller the crystallite size, higher is the bandgap. In the present work, we found that in all the ambients, the bandgap decreased when the annealing temperature increased from 300 ◦ C to 600 ◦ C. Moreover, the crystallite size also changed with increase in annealing temperature from air annealed films to nitrogen annealed films. The crystallite size for the samples CO3, CN3 and CN5 is 15.7, 8.6 and 16.2 nm, respectively. However, we could not find exciton Bohr radius of CdO in the literature but the exciton Bohr radius of ZnCdO increased with higher Cd concentration of the alloys [34]. We expect that the exciton Bohr radius of CdO is larger than the ZnO [35]. Thus for abovementioned samples quantum confinement potentially affects the bandgap value. The other parameters that may potentially affect the

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band effective mass and the reduced effect mass, respectively. The ratio of reduced effective mass to free electron mass (m* vc )/m0 for the pure CdO system is 0.274 [11] and bandgap of pure CdO is 2.20 eV. Thus from Eq. (7) the carrier concentration for example for CN3 sample comes out to be 1.3 × 1020 cm−3 . Hence the carrier concentration is very high which supports the BM shift in CdO thin films. Moreover on annealing at higher temperature the concentration of Cd interstitials and oxygen vacancies decreased and carrier concentration also decreased. Hence the bandgap of CdO thin films decreased with higher annealing temperatures 4. Conclusions We have shown that the electrodeposition is an effective technique to obtain thin film and nanostructures of CdO at room temperature. The thin films were synthesized on ITO by electrodeposition at room temperature (