Procedia Materials Science 10 ( 2015 ) 285 – 291
2211-8128 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014) doi: 10.1016/j.mspro.2015.06.052
ScienceDirect
2nd International Conference on Nanomaterials and Technologies (CNT 2014)
Investigations on Physical Properties of Nanostructured Cr Doped
CdO Thin Films for Optoelectronic Applications
B. Hymavathi
a*, B. Rajesh Kumar
b, T. Subba Rao
ca,cMaterials Research Lab, Department of Physics, Sri Krishnadevaraya University, Anantapuram – 515 003, A.P, India bDepartment of Physics, GITAM Institute of Technology, GITAM University, Visakhapatnam – 530 045, A.P, India
Abstract
Cr doped CdO thin films were deposited on glass substrates by DC reactive magnetron sputtering method by varying oxygen flow rates from 1 to 4 sccm. XRD spectra shows that the films exhibit a simple cubic crystal structure with preferred orientation along (111) direction. FESEM images of Cr doped CdO thin films exhibits columnar structure and the grain width decreases with the increase of oxygen flow rate. Atomic force microscopy was used to examine the surface topography of the films. The average roughness and rms roughness for Cr doped CdO thin film deposited at oxygen flow rate of 2 sccm are 3.28 nm and 5.47 nm. The four-probe method was employed to determine the electrical resistivity of the films. The minimum resistivity of 4.47 x 10-4
Ω.cm, high optical transmittance of 90 % and carrier concentration of 1.36 x 1020 cm-3 are obtained for Cr doped CdO thin film
deposited at oxygen flow rate of 2 sccm. © 2015 The Authors. Published by Elsevier Ltd.
Peer-review under responsibility of the International Conference on Nanomaterials and Technologies (CNT 2014).
Keywords: Thin films; Magnetron sputtering; X-ray diffraction; Electrical properties; Optical properties
* Corresponding author. Tel.: +0-851-827-3331; fax: +0-891-279-0399. E-mail address:brkhyma@gmail.com
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.Introduction
Cadmium oxide (CdO) is one of the promising II-IV compound semiconductors that have great potential for optoelectronic devices (Gulino and Tabbi (2005)). CdO is an n-type degenerate semiconductor with a simple cubic structure having direct band gap of 2.2-2.7 eV and two indirect band gaps of 1.18-1.20 eV and 0.8-1.12 eV (Azizar Rahman and Khan (2014)). The high optical transmittance, carrier mobility and conductivity makes the CdO thin films useful for various applications like solar cells, gas sensors, photo-transistors, photodiodes, transparent electrodes, liquid crystal displays and anti reflection coatings (Saha et al. (2007)). The optoelectronic properties of CdO thin films could be controlled by doping with different metallic ions. It was found that dopant ions of smaller radius than that of Cd2+(0.097 nm) could improve its electrical conductivity and increase its optical band gap
(Dakhel (2009), Hymavathi et al. (2014)). However, doping of CdO with ions of much smaller radius like chromium (0.069 nm) has not yet been investigated. CdO thin films have been prepared by various physical and chemical methods such as thermal evaporation, sputtering, pulsed laser deposition, sol-gel, chemical bath deposition, successive ionic layer adsorption and reaction, and spray pyrolysis (Yakuphanoglu (2010)). Among these DC reactive magnetron sputtering offers good control of the films composition because of high deposition rates. High deposition rates minimize the target poisoning effects during reactive sputtering. In this investigation, Cr doped CdO thin films were grown by DC reactive magnetron sputtering method at various oxygen flow rates and we studied the influence of oxygen flow rate on the structural, electrical and optical properties.
2.Experimental details
Cr doped CdO thin films were prepared by DC reactive magnetron sputtering technique at different oxygen flow rates. High purity of Cd (99.99%) and Cr (99.99%) targets with 2 inch diameter and 4 mm thickness are used for deposition on glass substrates. The base pressure in chamber was 4.2 x 10-6 Torr and the distance between target and
substrate were set at 60 mm. The glass substrates were ultrasonically cleaned in acetone and ethanol, rinsed in an ultrasonic bath in deionized water for 15 min, with subsequent drying in an oven before deposition. High purity (99.99%) Ar and O2 gas was introduced into the chamber and was metered by mass flow controllers (Model GFC 17,
Aalborg, Germany) for a flow rate fixed at 30 sccm (standard cubic centimeter) for Ar and 1-4 sccm for O2.
Deposition was carried out at a working pressure of 2 mTorr after pre-sputtering with argon for 10 min. The DC sputtering power maintained at the time of deposition for Cd target is 100 W and 40 W for Cr target. Film thickness was measured by Talysurf thickness profilometer. The resulting thickness of the films is found to be ~ 300-350 nm. X-ray diffraction (XRD) patterns of the films were recorded with the help of Philips (PW 1830) X-ray diffractometer
using CuKα radiation. The tube was operated at 30 KV, 20 mA with the scanning speed of 0.03(2θ)/sec. Surface
morphology of the samples has been studied using HITACHI S-3400 Field Emission Scanning Electron Microscope (FESEM) with Energy Dispersive Spectrum (EDS). EDS is carried out for the elemental analysis of prepared thin film samples. Surface topography of the films have been studied using AFM (Park XE-100: Atomic Force
Microscopy).The electrical resistivity of the films (ρ) was measured using the four-point probe method. Optical
transmittance of the films was recorded as a function of wavelength in the range of 300 – 1200 nm using JASCO Model V-670 UV-Vis-NIR spectrophotometer (Japan).
3.Results and discussion
3.1.Structural and morphological properties
XRD patterns of Cr doped CdO thin films deposited by varying oxygen flow rates from 1 to 4 sccm are shown in Fig. 1. It revealed that the films are polycrystalline in nature with cubic structure. The films exhibit predominantly a (1 1 1) orientation which is in accordance with the reports on activated reactive evaporation and DC sputtering (Armstrong and Karceff (1999), Calnon and Tiwari (2000)). It also exhibits the peaks with (2 0 0) and (2 2 0) orientations with small intensities. The intensity of the peak (2 0 0) decreases with the increase of oxygen flow rate from 1 to 4 sccm. A small shift in the diffraction peaks towards the lower angles is observed due to the stress developed in the films. It is also observed that the intensity of the peak (1 1 1) increases, while the peak width
decreases with the increase of the oxygen flow rate. The average crystallite size (D) of Cr doped CdO thin films is estimated from Scherrer’s formula (Cullity (1978)). The crystallite size of Cr doped CdO thin films decreased from 48 to 32 nm with the increase of oxygen flow rate from 1 to 4 sccm. The results obtained are in good agreement with the previous reported literature (Young-Soon Kim et al. (2003)).
Fig. 1. XRD spectra of Cr doped CdO thin films deposited at different oxygen flow rates.
The lattice parameter (a) of Cr doped CdO thin films was evaluated from the XRD data, which increases from 0.4694 to 0.4740 nm with the increase of oxygen flow rate. The origin of microstrain is related to the lattice misfit which in turn depends upon the deposition conditions. A dislocation is an imperfection in a crystal associated with the misregistry of the lattice in one part of the crystal with that in another part. In fact, the growth mechanism involving dislocation is a matter of importance. Unlike vacancies and interstitial atoms, dislocations are not equilibrium imperfections, i.e. thermodynamic considerations are insufficient to account for their existence in the observed densities. In fact, the growth mechanism involving dislocation is a matter of importance. In the present study, the microstrain (ε) and dislocation density (δ) are calculated from the relations (Dhivya et al. (2014))
ε = β cosθ
4 (1)
where β is the full width at half maximum of (1 1 1) peak, θ is the diffracted angle and
δ = n2
D (2) where n is a factor, which is equal to unity gives the minimum dislocation density and D is the average crystallite size. The microstrain values increases from 0.193 x 10-3 to 0.228 x 10-3 lin-2.m-4 with the increase of oxygen flow
rate from 1 to 4 sccm. The dislocation density (δ) values are found to be varying from 0.43 x 1015 to 0.97 x 1015
lin.m-2 with the increase of oxygen flow rate from 1 to 4 sccm. The microstructural parameters of Cr doped CdO thin films are given in Table 1.
Table 1. Microstructural parameters of Cr doped CdO films deposited at different oxygen flow rates Oxygen
flow rate (sccm)
FWHM, β
(degrees) d-spacing (nm) Lattice constant, a (nm) Crystallite size, D (nm) Micro strain, ε x 10-3 (lin-2. m-4) Dislocation density, δ x 1015 (lin.m-2) 1 0.809 0.2710 0.4694 48 0.193 0.43 2 0.819 0.2718 0.4708 43 0.197 0.54 3 0.880 0.2727 0.4724 38 0.215 0.70 4 0.926 0.2736 0.4740 32 0.228 0.97
FESEM images of Cr doped CdO thin films deposited at different oxygen flow rates are shown in Fig. 2. All films have columnar structure and the grain width decreases as the oxygen flow rate increases from 1 to 4 sccm. All the films grow mainly with columnar grains perpendicular to the substrate and some granular grains also exist in the films. The compositional analysis of the films was determined by using energy dispersive spectrum of X-rays. The relative compositions for Cr doped CdO thin films deposited at oxygen flow rates of 1, 2, 3 and 4 sccm are in an atomic ratio of Cd/O/Cr are 59.52/37.46/3.02%, 58.86/38.18/2.96%, 54.18/42.78/3.04% and 52.34/44.64 /3.02 %.
Fig. 2. FESEM images of Cr doped CdO thin films deposited at different oxygen flow rates.
Atomic force microscopy (AFM) was used to examine the surface topography of the thin films. The AFM images (2D and 3D) of Cr doped CdO thin film deposited at oxygen flow rate of 2 sccm is shown in Fig. 3(a) and
3(b). The images show the growth of columnar grains perpendicular to the substrate. Also, flat and dense surface topography with high grain size was observed for the film prepared at oxygen flow rate of 2 sccm. The average roughness and rms roughness for Cr doped CdO thin film deposited at oxygen flow rate of 2 sccm are 3.28 nm and 5.47 nm.
Fig. 3. (a) 2D and (b) 3D topographic AFM images of Cr doped CdO thin film deposited at oxygen flow rate of 2 sccm. 3.2.Electrical studies
Four probe method was employed to determine the electrical resistivity of the films. The resistivity increases from 3.05 x 10-4 to 6.26 x 10-4Ω.cm with the increase of oxygen flow rate from 1 to 4 sccm as shown in Fig. 4. The
increase of resistivity is due to the decrease of interstitial cadmium atoms and/or oxygen vacancies. It is known that the electrical resistivity of Cr doped CdO thin films is related to the variation in the carrier concentration and mobility of the films, which in turn is related to the variation of oxygen vacancies and interstitials cadmium.
Fig. 4. Variation of electrical resistivity of Cr doped CdO films as a function of oxygen flow rate.
The carrier concentration and mobility of the films are determined from the equations as reported in the earlier literature (Vijayan et al.(2009)). The carrier concentration of the films decreased from 1.41 x 1020 to 1.23 x 1020 cm-3
with the increase of oxygen flow rate from 1 to 4 sccm. The higher concentration in the films at low oxygen flow rate is due to the present of cadmium interstitials in addition to the oxygen vacancies. The carrier mobility of the films decreases from 42 to 22 cm2 V-1 s-1 with the increase of oxygen flow rate from 1 to 4 sccm. The decrease of
carrier mobility with the increase of oxygen flow rate indicates that additional scattering centers must be introduced for the films deposited at higher oxygen flow rate, such as neutralized donors (Lamb and Irvine (2011)).
3.3.Optical properties
The optical transmittance spectra of Cr doped CdO thin films deposited at different oxygen flow rates is shown in Fig. 5. An average transmittance of 65 to 92 % was obtained for all the films in the wavelength ranging from 500 to 800 nm. The transmittance of the films increases as the oxygen flow rate increases which is a result of the improved stoichiometry of the films or lowered intrinsic defects density deposited at higher oxygen flow rate (Kim et al. (2010)). The film thickness decreased with the increase of oxygen flow rate which was due to the oxidation of the target material and the resultant low sputtering yield of the metal oxide (Subramanyam et al. (1998)).
The analysis of the dependence of absorption coefficient (α) on photon energy (hν) is performed to obtain the
detailed information about the energy band gaps of the films. The optical band gap is calculated from the relationship between absorption coefficient and photon energy (Rajesh Kumar and Subba Rao (2012))
(αhν) = B (hν - E )g 1/2 (3) where α is the optical absorption coefficient, hν is the photon energy, Eg is the optical band gap, and B is a
constant. We have calculated the direct optical band gap from (αhν)2vs hν plot by extrapolating the linear portion of
the graph to hν axis (shown in Fig. 6). The intercept on the hν axis determines the direct optical band gap. The
optical band gap increases from 2.76 to 2.83 eV with the increase of oxygen flow rate from 1 to 4 sccm. A blue shift of the band edge has been observed for the Cr doped CdO films with the increase of oxygen flow rate. This blue shift is due to Burstein-Moss effect (Burstein (1954)), which arises due to increase in free carrier concentration.
Fig. 5. Optical transmittance of Cr doped CdO films as a function of oxygen flow rate.
The optical constants such as absorption coefficient (α), extinction coefficient (k) and refractive index (n) are
determined from the equations as reported in the previous literature (Rajesh Kumar and Subba Rao (2013)). The
absorption coefficient (α) values for Cr doped CdO films are found to be increase from 2.46 x 104 to 7.63 x 104 cm-1
with the increase of oxygen flow rate from 1 to 4 sccm. The extinction coefficient (k) values also increases from 0.018 to 0.041 with the increase of oxygen flow rate. The refractive index (n) increases from 1.71 to 2.15 with the increase of oxygen flow rates from 1 to 4 sccm. The increase in the refractive index of the films is due to increase of density of the films.
4.Conclusions
Cr doped CdO thin films with preferred orientation along (1 1 1) direction have been prepared on glass substrates by DC reactive magnetron sputtering method by varying oxygen flow rates from 1 to 4 sccm. FESEM images of films had a columnar structure and the grain width decreases with the increase of oxygen flow rate. The minimum resistivity of 4.47 x 10-4Ω.cm, high optical transmittance of 90 % and carrier concentration of 1.36 x 1020 cm-3 is
obtained for the film deposited at oxygen flow rate of 2 sccm. These low electrical resistivity and high optical transmittance enable the Cr doped CdO thin films is a promising candidate for future optoelectronic applications.
Acknowledgements
One of the authors Ms. B. Hymavathi would like to express thanks to the authorities of UGC, New Delhi for awarding UGC-BSR fellowship (No. F. 11-23/2008(BSR)) in sciences for meritorious students.
References
Armstrong, N., Karceff, W., 1999. A maximum entropy method for determining the column-length distributions from size-broadened X-ray diffraction profiles. Journal of Applied Crystallography 32, 600-613.
Azizar Rahman, M., Khan, M.K.R., 2014. Effect of annealing temperature on structural, electrical and optical properties of spray pyrolytic nanocrystalline CdO thin films. Materials Science in Semiconductor Processing 24, 26-33.
Bandyopadhyay, G.K., Paul, G.K., Sen, S.K., 2002. Study of optical properties of some sol-gel derived films of ZnO. Solar Energy Materials and Solar cells 71, 103-113.
Burstein, E., 1954. Anomalous optical absorption limit in InSb. Physical Review 93, 632-633.
Calnan, S., Tiwari, A.N., 2010. High mobility transparent conducting oxides for thin film solar cells. Thin Solid Films 518, 1839-1849. Cullity, B.D., 1978. “Elements of X-Ray Diffraction”. Addison- Wesley Pub. Co. Inc. MA, London, pp. 102.
Dakhel, A.A., 2009. Transparent conducting oxides of samarium-doped CdO. Journal of Alloys and Compounds 475, 51-54.
Dhivya, P., Prasad, A.K., Sridharan, M., 2014. Magnetron sputtered nanostructured cadmium oxide films for ammonia sensing. Journal of Solid State Chemistry 214, 24-29.
Gulino, A., Tabbi, G., 2005. CdO thin films: a study of their electronic structure by electron spin resonance spectroscopy. Applied Surface Science 245, 322-327.
Hymavathi, B., Rajesh Kumar, B., Subba Rao, T., 2014. Temperature dependent structural and optical properties of nanostructured Cr doped CdO thin films prepared by DC reactive magnetron sputtering. Procedia Materials Science 6, 1668-1673.
Kim, Y.J., Jin, S.B., Kim, S.I., Choi, Y.S., Choi, I.S., Han, J.G., 2010. Effect of oxygen flow rate on ITO thin films deposited by facing targets sputtering, Thin Solid Films 518, 6241-6244.
Lamb, D.A., Irvine, S.J.C., 2011. A temperature dependant crystal orientation transition of cadmium oxide films deposited by metal organic chemical vapour deposition. Journal of Crystal Growth 332, 17-20.
Rajesh Kumar, B., Subba Rao, T., 2012. Effect of Aluminum concentration on structural and optical properties of DC reactive magnetron sputtered zinc aluminum oxide thin films for transparent electrode applications. Journal of Physics: Conference Series 390, 012032/1-6. Rajesh Kumar, B., Subba Rao, T., 2013. Investigations on opto-electronical properties of DC reactive magnetron sputtered zinc aluminum oxide thin films annealed at different temperatures. Applied Surface Science 265, 169-175.
Saha, B., Das, S., Chattopadhyay, K.K., 2007. Electrical and optical properties of Al doped cadmium oxide thin films deposited by radio frequency magnetron sputtering. Solar Energy Materials and Solar Cells 91, 1692-1697.
Subramanyam, T.K., Uthanna, S., Srinivasulu Naidu, B., 1998. Preparation and characterization of CdO films deposited by dc magnetron reactive sputtering. Materials Letters 35, 214-220.
Vijayan, T.A., Chandramohan, R., Valanarasu, S., Thirumalai, J., Subramanian, S.P., 2008. Comparative investigation on nanocrystal structure, optical, and electrical properties of ZnO and Sr-doped ZnO thin films using chemical bath deposition method. Journal of Materials Science 43, 1776–1782.
Yakuphanoglu, F., 2010. Nanocluster n-CdO thin film by sol-gel for solar cell applications. Applied Surface Science 257, 1413-1419.
Young-Soon Kim, Young-Chul Park, Ansari, S.G., Jeong-Young Lee, Byung -Soo Lee, Hyung-Shik Shin, 2003. Influence of O2 admixture and sputtering pressure on the properties of ITO thin films deposited on PET substrate using RF reactive magnetron sputtering. Surface and Coatings Technology 173, 299-308.
