Canadian Journal on Chemical Engineering & Technology Vol. 3 No. 4, December 2012
Optical properties of thermally evaporated Cr2O3 thin films M. Julkarnain, J. Hossain, K. S. Sharif, and K. A. Khan
Abstract — Chromium Oxide (Cr2O3) thin films have been prepared by thermal evaporation onto glass substrate from Cr powder at a pressure of about 6×10-4 Pa. The thickness of the film ranges from 70 to 300 nm. The deposition rate of Cr 2O3 thin films is about 8.70nm/min. X-ray diffraction (XRD) study reveals that Cr2O3 thin films are amorphous in nature. SEM studies of Cr2O3 thin films exhibit a smooth and uniform growth on the entire surface. Elemental compositions have been estimated by Energy Dispersive Analysis of X-ray (EDAX) method. Optical study of Cr2O3 thin films in the wavelength range 0.3≤λ≤1.1 μm shows a transition of indirect type with a band gap of ≈3.30 eV which is well agreed with reported values. Integrated values of luminous and solar transmittance as well as of reflectance of Cr2O3 thin films have been calculated from the optical data. These studies may be of importance for the uses of this material in selective surface applications. Key Words — Thermal evaporation, Cr2O3, thin films, optical band gap, selective surface studies.
from Goodfellow Chemical Company, England, onto glass substrates. The materials have been evaporated in an EDWARDS E 306A vacuum coating unit. Before deposition, the deposition chamber has been thoroughly cleaned with emery paper and cotton wool by wetting acetone and then dried with a dryer. A small quantity of source materials have been placed on the clean Tungsten boat shaped source. The glass substrates have been first cleaned in chromic acid solution and then washed in distilled water. After washing and drying in hot air, substrates have been then again cleaned in acetone and dried in hot air, and then used for deposition. All the films have been deposited at room temperature and at an ambient pressure of about 6x10-4 Pa. The Cr film may be oxidized to form Cr2O3 in coating unit or in open air after the film has been drawn from the chamber. To make oxidization stable, the as-deposited Cr2O3 thin films have been annealed in open air for about 3 hours at a temperature of 570 K. The thickness of the film has been measured by the Tolanasky interference method with an accuracy of ±5 nm [15].
I. INTRODUCTION
III. RESULTS AND DISCUSSION
This Research and development on thin films have led to the conclusion that different classes of materials are of particular interest for different applications. The Cr2O3 thin films are of great interest due to their wide variety of technological applications. It has been studied for optical and electronic uses such as selectively absorbing films for solar energy conversion [1], solar energy shielding films for windows [2], electrode material for electrochromic windows [3] and cathode material for lithium batteries [4]. Cr2O3 thin films exhibit high order of hardness with high wear and corrosion resistance which are important properties for protective coating [5]. Currently, lowreflective Cr2O3/Cr films are widely used as black matrix films in liquid crystal displays [6]. Literature reports indicate that Cr2O3 thin films have been produced by a number of techniques by a number of researchers. These include the vacuum evaporation [7], sputtering [5], [8], Chemical Vapor Deposition (CVD) [6], [9]-[12], Spray pyrolysis [13] and reactive pulsed laser ablation techniques [14].
A. Structural and compositional studies Structural analysis of the as-deposited and annealed Cr2O3 thin films of different thicknesses (140-300nm), respectively, has been carried out in a PHILIPS PW-3040 X’ Pert PRO XRD System using the monochromatic CuKα radiation. Peak intensities have been recorded corresponding to 2θ values. Fig. 1 shows the X-ray diffractograms of the annealed Cr2O3 thin films of different thickness. It is seen that there is only one peak in all the spectra, besides this there is no noticeable peak in the entire range. It indicates that the films are amorphous in nature. Study of Scanning Electron Microscopy (SEM) and Energy Dispersive Analysis of X-ray (EDAX) has been carried out in a HITACHI S-3400N (SEM) System. SEM micrographs of the annealed Cr2O3 thin films of thickness 140 nm is shown in Fig. 2 and it exhibits almost smooth and uniform surface. The elemental compositions for the Cr2O3 thin films of various thicknesses have been estimated by using the EDAX method. The study shows a non-stoichiometric with oxygen deficient Cr2O3 thin film. The elemental compositions of Cr2O3 thin films are shown in Table I.
Department of Applied Physics & Electronic Engineering, University of Rajshahi, Rajshahi-6205, Bangladesh, (Corresponding author:
[email protected]). II. EXPERIMENTAL DETAILS
Chromium Oxide (Cr2O3) thin films have been prepared by thermal evaporation of Cr powder (purity 99.999%) obtained 81
Canadian Journal on Chemical Engineering & Technology Vol. 3 No. 4, December 2012 60 (a) 50
T / R (%)
40 30 20
O 70 nm x 140 nm 240 nm
10 0 200
400
600 800 1000 Wavelength, (nm)
1200
70
(b)
60
Fig. 1. X-ray diffractograms of the annealed Cr 2O3 thin films, (a) for 140 nm, (b) for 240 nm and (c) for 300 nm. T / R (%)
50 40 30 20
O 70 nm x 140 nm 240 nm
10 0 200
400
600
800
1000
1200
Wavelength, (nm)
Fig. 3. Variation of transmittance (solid line) and reflectance (dashed line) with wavelength for Cr2O3 thin films of different thicknesses; (a) for as-deposited and (b) for annealed films.
The value of the refractive index has been calculated from the transmittance data by using the Murmann’s equation [16]. The variation of refractive index with wavelength for the asdeposited and annealed Cr2O3 thin films is shown in Fig. 4. It is evident from the figures that the value of refractive index lies within 1.3 to 2.1 in the investigated spectral range which is well agreed with reported values [17].
Fig. 2. SEM micrograph of the annealed Cr 2O3 thin film of thickness 140 nm. TABLE I ELEMENTAL COMPOSITION OF CR2O3 THIN FILMS OF VARIABLE THICKNESS
As-deposited Annealed
Thickness (nm)
Cr (wt %)
O (wt %)
300
89.56
10.44
140
73.48
26.52
300
79.96
20.04
2.2
Refractive index, n
Status
B. Optical studies Spectral transmittance T(λ) and reflectance R(λ) of the asdeposited and annealed Cr2O3 thin films of thicknesses 70, 140, 240 and 300 nm, respectively, have been measured at wavelength range 0.3≤λ≤1.1 μm using a SHIMADZU UVdouble beam spectrophotometer. Fig. 3 shows the spectral transmittance and reflectance vs. wavelength spectra for Cr2O3 thin films of thicknesses 70, 140 and 240 nm. By comparing the magnitudes of the transmittance spectra for both the asdeposited and annealed films, it is found that the annealed films are more transparent than the as-deposited films in the visible and near infrared range which is also reported by other worker [12].
70 nm 140 nm 240 nm
2
(a)
1.8 1.6 1.4 200
400 600 800 1000 Wavelength, (nm)
1200
Refractive index, n
2.2 70 nm 140 nm 240 nm
2
(b)
1.8 1.6 1.4 1.2 200
400
600 800 1000 Wavelength, (nm)
1200
Fig. 4. Variation of refractive index with wavelength for Cr 2O3 thin films of different thicknesses, (a) for as-deposited and (b) for annealed films.
In order to determine the value of optical band gap, (αhυ) 1/n 82
Canadian Journal on Chemical Engineering & Technology Vol. 3 No. 4, December 2012 vs. (hν) curves have been plotted, where α indicates the optical absorption co-efficient, hυ photon energy and n represents the nature of transition. Fig. 5 shows the variation of (αhυ)1/2 with photon energy, (hν) for the as-deposited and annealed Cr2O3 thin film of thickness 140 nm. The plot exhibits the indirect band gap of ≈ 3.30 eV, which is well agreed with the reported values [14], [17].
equation [19]
where ν is a light frequency, h is the Plank constant,
1/2 -1
7
1/2
3
(h) x10 (m .eV)
As-deposited Annealed
9 8
E gi is the
I (h ) 2 (h Egd )1 / 2 exp[ h Egd / kBT ]
5 4 1
1.5
2
2.5
3
3.5
4
(2)
Attempts have been taken to calculate both the indirect and direct band gap using Eq. (1) and (2) but the best fitted curve has been observed for indirect band gap. The band calculation according to Eq. (1) is shown in Fig. 7 for the annealed Cr2O3 thin films of thickness 140 nm. It is seen from the figure that the indirect band gap of Cr2O3 thin film is ≈ 3.33 eV, which is well agreed with that observed in author’s result of optical study.
6
3 4.5
Photon Energy, h (eV)
Fig. 5. Variations of (αhν) ½ with photon energy (hν) for Cr2O3 thin film of thickness 140 nm.
C. Photoluminescence study The Photoluminescence (PL) spectra of Cr2O3 thin films have been measured using SHIMADZU RF-5301 Spectro Fluorophotometer. The excitation wavelength for this measurement has been taken as 260 nm whose corresponding energy is well above the band gap of the samples. Fig. 6 shows the PL spectrum of annealed Cr2O3 thin films of different thicknesses. In the PL spectrum as shown in Fig. 6, consists of many bands with varying intensities. At room temperature, these bands may be marked by letters A, B, C and D. The bands A, B and C may be caused due to the radiative transitions [18]. The band D which is the most intensive at room temperature from all the bands may corresponds to the band-to-band transitions.
140 nm
PL intensity (arb. unit)
PL spectra Fitted curve
3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 Photon energy, h (eV)
Fig. 7. The band gap calculation of the annealed Cr 2O3 thin film of thickness 140nm
D. Selective surface study For assessing the performance of coating in selective surface applications, it is convenient to introduce certain integrated optical quantities. These are especially the luminous (lum) and solar (sol) properties obtained from
D
PL intensity (arb. unit)
(1)
indirect band gap of Cr2O3 thin films, kB is the Boltzmann constant and T is the absolute temperature. For direct transitions, the spectrum of intrinsic radiation has a form [19]
11 10
I(hν) ν 2(hν Egi )2 exp [ hν Egi / kBT ]
140 nm 240 nm 300 nm C
Xp
A B
d X / d p
p
(3)
where X denotes transmittance or reflectance. The luminous quantities are obtained with Φp = Φlum, i.e., the standard luminous efficiency function for photopic vision [20] as specified CIE (Commission International de I’ É clairge). For solar quantities, one can use Φp = Φsol, according to the tabulated AM2 irradiance spectrum [21]. Table 2 shows integrated optical quantities, evaluated from the optical data. Appreciable order of integrated luminous and solar transmittance as well as of reflectance suggests that this
3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 Photon energy, h (eV) Fig. 6. Photoluminescence (PL) spectra of the annealed Cr 2O3 thin films of thicknesses 140, 240 and 300nm.
The energy band gap of Cr2O3 thin films has been calculated from the PL spectra at room temperature. For indirect transition, the D band intensity can be described by the 83
Canadian Journal on Chemical Engineering & Technology Vol. 3 No. 4, December 2012 [6]
material is a potential candidate for the application in selective surface devices. TABLE II DATA FOR THICKNESS DEPENDENCE OF LUMINOUS AND SOLAR TRANSMITTANCE AS WELL AS REFLECTANCE OF CR2O3 THIN FILMS Solar Luminous Thickness Status Tsol Rsol Tlum Rlum t (nm) (%) (%) (%) (%) 70 40.67 52.28 43.65 52.37 140 36.13 48.53 39.16 48.36 As-deposited 240 31.46 41.01 34.54 40.03 300 27.87 41.61 30.46 41.17 70 57.29 23.13 60.01 19.71 140 47.15 26.14 50.05 22.68 Annealed 240 30.39 30.00 33.89 26.49 300 19.26 16.95 21.71 16.25
[7]
[8]
[9]
[10]
[11]
IV. CONCLUSION Chromium Oxide (Cr2O3) thin films of thickness ranges 70 – 300 nm have been prepared onto glass substrate from Cr powder by thermal evaporation method at a pressure of about 6×10-4 Pa. The deposition rate is about 8.70 nm/min. X-ray diffraction (XRD) study shows that Cr2O3 thin films are amorphous in nature. SEM studies of the samples exhibit a smooth and uniform growth on the entire surface. The EDAX study shows a non-stoichiometric Cr2O3 thin film. Optical study shows a transition of indirect allowed type with a band gap of Eg ≈ 3.30 eV, which is an excellent agreement with the reported value. The value of refractive index lies within 1.4 to 2.1 in the studied spectral range. Appreciable order of integrated optical quantities suggests that this material is a potential candidate for the application in selective surface devices.
[12]
[13]
[14]
[15] [16]
[17]
[18]
ACKNOWLEDGMENT [19]
M. Julkarnain is indebted to Dr. Jibon Poddar, Deptt. of Physics, BUET, Dr. D. K. Saha, Atomic Energy Centre, Dhaka and Mr. F. U. Forhad, BCSIR, Dhaka, Bangladesh for providing the support of the XRD and SEM studies of Cr2O3 thin films.
[20]
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Md. Julkarnain was born in Sirajganj, Bangladesh, on January 1, 1985. He has been completed his B. Sc. (Honour’s) and M. Sc. degree from the Department of Applied Physics and Electronic Engineering, University of Rajshahi, Bangladesh. After completion of his postgraduation he joined as a lecturer in his own department and now he is serving in this position. His interested areas of research are Nanoparticles, Nanowires and Nanorods, Nanophotonics and Solar cells.
Jaker Hossain was born in Tangail, Bangladesh, on October 30, 1984. He received his Bachelors with Honors, and Masters degree in Applied Physics and Electronic Engineering from the University of Rajshahi, Bangladesh. Now he has been working as a lecturer in the department of Applied Physics and Electronic Engineering, University of Rajshahi, Bangladesh. His research interest in nanotechnology, photonics and organic solar cells.
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Canadian Journal on Chemical Engineering & Technology Vol. 3 No. 4, December 2012
Kazi Shaifullah Sharif was born in Kustia, Bangladesh. He received his B. Sc. (Honour’s) and M. Sc. degree from the Department of Applied Physics and Electronic Engineering, University of Rajshahi, Bangladesh. He joined in the Civil Aviation Authority of Bangladesh in 2011. His interested areas of research are Nanoscience and Nanotechnology, Material Science.
Professor Khairul Alam Khan was born in Jessor, Bangladesh, on September 13, 1949. He received his B.Sc. and M.Sc. degree from Rajshahi University. In 1974 he joined as a Lecturer in the Department of Applied Physics & Electronics, University of Rajshahi, Bangladesh. Again he obtained his M.Sc. degree from Regina University, Canada, in 1982. Following that, in 1990 he completed his Ph.D. degree in Experimental Solid State Physics under a sandwitch work done at Chalmers Univ. of Tech. Sweden & Rajshahi University by IPPS support. He has a sorts of paper published in well renouned journal. At present, he is acting as a Vice-Chencellor of Bangabandhu Sheikh Mujibur Rahman University of Science and Technology, Gopalganj, Bangladesh. His field of expertise on thin film selective surface coatings, Photoelectrochemical solar cells, cermet thin films for solar, electronic and optoelectronics applications.
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