Influence of background gas atmosphere on formation of Cr2O3 thin

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Cr2O3 thin films deposited on Si (100) and glass substrates by pulsed laser deposition using. Cr3C2 target were investigated. The films were deposited at base ...
Influence of background gas atmosphere on formation of Cr2O3 thin films prepared by pulsed laser deposition G. Balakrishnan1, P. Kuppusami*2, T. N. Sairam3, R. V. Subba Rao4, E. Mohandas2 and D. Sastikumar1 Cr2O3 thin films deposited on Si (100) and glass substrates by pulsed laser deposition using Cr3C2 target were investigated. The films were deposited at base pressure (461025 mbar) and at different gas atmospheres, such as argon and methane. X-ray diffraction and X-ray photoelectron spectroscopy studies of the films showed only Cr2O3 phase formation irrespective of the background gas environment during deposition. The films deposited on Si (100) substrates contain nanocrystallites with the size varying in the range 24–72 nm, while that on glass substrates contain crystallites of size varying from 19–56 nm. The ultraviolet visible spectroscopy studies showed that the films are transparent in the visible region. The band gap of the deposited films were calculated using Tauc plot and it was found to exhibit direct band gap in the range 3?10–3?60 eV. The band gap was minimum (3?10 eV) with the argon gas atmosphere, while it is 3?60 eV in methane atmosphere. Keywords: Chromium oxide, Thin films, Pulsed laser deposition, X-ray diffraction, Band gap

Introduction Chromium oxide (Cr2O3) exhibits high hardness, chemical inertness, mechanical strength and stability. Among the various chromium oxides (CrO2, CrO3, Cr2O3, etc.), Cr2O3 is antiferromagnetic and is the most stable one under ambient conditions.1 Cr2O3 thin films have good wear, corrosion and oxidation resistance2 and are used as electro chromic coatings,3 infrared (IR) transmitting coatings,4 and optically selective surfaces of solar collectors.5 Thus, Cr2O3 films have important optical applications in the ultraviolet (UV), visible (VIS) and IR regions of the electromagnetic spectrum. Cr2O3 thin films have been deposited by sputtering,2,3 chemical vapour deposition,1,6 e-beam evaporation4,7 and spray pyrolysis.8 However, no reports are available on the preparation of phase pure Cr2O3 thin films by pulsed laser deposition (PLD) using Cr3C2 target. Guinneton et al.9 have used PLD to deposit thin films of mixed chromium oxide phases on sapphire substrates from CrO3 and Cr8O21 targets and discussed the influence of the target composition and of the deposition conditions upon the properties of the films. Pulsed laser deposition

1

National Institute of Technology, Tiruchirapalli-620015, Tamilnadu, India Physical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, Tamilnadu, India 3 Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, Tamilnadu, India 4 Corrosion Science and Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, Tamilnadu, India 2

*Corresponding author, email [email protected]

ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 23 June 2008; accepted 1 August 2008 DOI 10.1179/026708408X356812

technique has become a promising one owing to congruent transfer of multielemental materials on a desired substrate and due to its ability to work with different background gases at different pressures.10,11 However, the properties of the films were found to depend sensitively on laser parameters such as laser energy density (fluence), pulse duration, wavelength and repetition rate. The formation of Cr–O could be easily facilitated with oxygen as the back ground atmosphere.12 Therefore the present work primarily investigates the influence of different background gases such as argon, methane as well as base pressure (461025 mbar) on microstructural, optical and chemical composition of Cr2O3 thin films deposited on glass and (100) oriented Si substrates using Cr3C2 target.

Experimental Commercially available Cr3C2 (99?99% purity) powder was compacted into a pellet of 25 mm diameter and 3 mm thickness at a pressure of 10 MPa using a uniaxial press. These pellets were sintered at 1673 K for 4 h under vacuum. The sintered pellet was found to be phase ˚, pure with an orthorhombic structure (a55?538 A ˚ , c52?834 A ˚ ) in agreement with JCPDS b511?497 A File No35-0804. This sintered pellet was used as a target for PLD. Si (100) and glass substrates were cleaned thoroughly in distilled water, soap solution and finally by ultrasonic cleaning in acetone. The substrates were mounted onto the sample holder of the deposition chamber using silver paste and the chamber was evacuated to a vacuum of 461025 mbar (base pressure)

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Influence of background gas atmosphere on formation of Cr 2 O 3 thin films

a base pressure (461025 mbar); b argon at 461023 mbar; c methane at 261022 mbar; d energy dispersive X-ray spectroscopy of sample c 1 Scanning electron microscopy photographs of films prepared at 873 K with different gas atmospheres

by a turbo molecular pump backed with a rotary pump. The target was rotated and translated along the target with the help of an electric motor to reduce pitting on the target. Pulsed laser deposition experiments were performed with KrF excimer laser (l5248 nm) with pulse energy of 200–300 mJ. The films were deposited for a constant period of 60 min. The thickness of the films was measured by the Dektak profilometer (DEKTAK 6M-stylus profiler, USA). The structure of the films deposited on Si (100) was studied using INEL XRG-3000 X-ray diffractometer using Cu Ka1 radiation. For the optical studies, films deposited on glass substrates were analysed using the UV-VIS-NIR (Shimadzu UV3101PC) spectrophotometer in the range of 190–890 nm and optical band gap energy was determined. X-ray photoelectron spectroscopy (XPS) studies of the films were carried out in an XPS instrument (SPECS) fitted with PHOIBOS 150-9 MCD analyser with an energy resolution of 0?45 eV using Al Ka (486?7 eV) source.

a 300 mJ/973 K/CH4/261021 mbar; b 300 mJ/873 K/ CH4/261021 mbar; c 200 mJ/873 K/CH4/261022 mbar; d 200 mJ/873 K/Ar/461023 mbar; e 200 mJ/873 K/ 461025 mbar 2 X-ray diffraction patterns of films grown on Si (100) substrates at different ambient gases

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Results and discussion Microstructural characterisation Figure 1 shows the microstructure of the films deposited at 873 K on Si (100) under different ambient gases. Scanning electron microscopy revealed structureless morphology with droplets. The droplets were small and large in number density when the films were prepared in the base pressure (461025 mbar), while they were large in size and few in number when formed in argon and methane atmosphere. These droplets are known to be formed due to subsurface boiling, splashing and exfoliation.13 When the deposition takes place at higher partial pressure, these droplets grew in size because of the increased interaction with the gas atmosphere. Similar observation was also noticed for the pulsed laser deposited titania films as a function of oxygen partial pressure.14 X-ray diffraction (XRD) pattern of the films deposited on Si (100) is shown in the Fig. 2. All the films showed peaks, which coincide with the reflections of the Cr2O3 structure given in the JCPDS file no. 38-1479. The films prepared with the base pressure showed only two peaks at an angle (2h) of 33?37 and 54?49u corresponding to (104) and (116) reflections of the hexagonal structure of Cr2O3. The films prepared in argon and methane atmosphere showed several peaks compared to the films prepared in the base pressure of 461025 mbar. At a higher methane partial pressure (261021 mbar), the film showed several reflections of Cr2O3 confirming the formation of Cr2O3 films with hexagonal crystal structure.12,15,16 These results illustrate that higher partial pressures of the background gas facilitate the growth of the crystalline films. The size of the crystallites was determined from the XRD data using Scherrer formula D~Kl=(bcosh) with b~(B2 {b2 )1=2 where b is the full width at half maximum of the coherently diffracting domain (crystallite), B is the experimental full width at half maximum of the (104) reflection and b is the instrumental broadening for the

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standard Si powder, K is Scherrer constant, equal to 0?9, l is the wavelength of the X-rays, D is the crystallite size and h is the diffraction angle. However, the strain in the films is not taken into consideration in the determination of the crystallite size. The deposition conditions, thickness and crystallite size (D) of the films are shown in Table 1. It is seen from Table 1 that the crystallite sizes are in the nanometre range of 24–72 nm for the films deposited on Si substrate and 19–56 nm for the films deposited on glass substrates. A slight decrease in crystallite size in the films formed on glass could be related to the insulating characteristics of the substrates in the vicinity of the plasma species. A similar observation was reported in the growth of CrN.17 It is believed that the electrical conductivity of the substrates plays a major role in the control of surface morphology and silicon being a semiconductor, could attract more charged species than that of glass substrates. Also, there is a significant increase in the crystallite size at 973 K compared to those of the films prepared at 873 K due to increased mobility of the sputtered species on the substrate surface. In contrast to the films prepared at the laser energy of 200 mJ/pulse at 10 Hz, the films prepared at 300 mJ/pulse and at 20 Hz showed an increased film thickness due to higher deposition rates. The increase in the repetition rate and energy/pulse, causes higher deposition rate due to rapid ablation promoted by higher energy. At higher deposition rate and pulse energy, it is possible for the ablated particles to coarsen further to yield a slight increase in the crystallite size. The XPS spectra of the films deposited in various atmospheres are shown in Fig. 3. It shows almost the same peak positions for the films prepared in various conditions. The major peaks observed at the binding energies of 531?8, 699?9, 587?9 and 578 eV are due to oxygen and chromium corresponding to O1s and Cr2s, Cr2p1/2 and Cr2p3/2 respectively.12,18 The corresponding Auger peaks were also noticed in the region around 1 keV. From the binding energy shifts of the photoelectron peaks and the relative intensities of Auger peaks, it is observed that CrO3 and Cr2O3 phases19–21 are significantly present at the surface of the films deposited in the base pressure (461025 mbar) whereas the component of CrO3 phase is reduced and the Cr2O3 is pronounced in those prepared in argon and methane atmospheres. There are also peaks at ,415 and 400 eV in the films deposited in argon and methane atmosphere and these peaks are very small and possibly arise due to contamination. The peaks due to contamination disappeared after sputtering for a few minutes. Figure 3d shows XPS spectrum of film developed in methane atmosphere after sputtering for 4 min. The intensity of the carbon peak is reduced substantially whereas the Cr2O3 phase is unchanged. This clearly indicates that the film did not contain carbon to promote chromium

Influence of background gas atmosphere on formation of Cr 2 O 3 thin films

a base pressure (461025 mbar); b argon at 461023 mbar; c methane at 261022 mbar; d after sputtering sample c for 4 min 3 X-ray photoelectron spectroscopy spectra of films deposited on Si (100) at different background atmospheres

carbide formation. X-ray diffraction and XPS analyses confirmed that only Cr2O3 is formed due to the high affinity of the chromium with oxygen and its formation is found to be independent of the background atmosphere.22 A few ppm of oxygen impurities contained in the base vacuum and in the gases (,1000 ppm) used in the present study could have promoted the formation of chromium oxide.23–26

Optical studies Optical applications of thin films require detailed knowledge of their optical properties. These include the optical constants such as refractive index n, extinction coefficient k and the optical band gap. For example, absorption characteristics of Cr2O3 thin films prepared on glass substrates are given in Fig. 4. It is found that the fundamental absorption edge falls around 380 nm. The band gap is evaluated from the Tauc plot: (ahu)2 versus hu; where a is the absorption coefficient, h is Planck’s constant and u is the frequency of radiation. The absorption coefficient a is determined using the relation a52?3036(A/d), where ‘A’ is the absorbance and ‘d’ is the thickness of the film. According to the theory of optical transitions (direct or indirect) in solids, near the absorption edge, the absorption coefficient varies with the photon energy, hu and follows the expression ahu5A(hu2Eg)n, where A is a constant and Eg is the optical band gap and n depends on the kind of optical transition. The optical band gaps were calculated for the films deposited under various ambient conditions. Inset of Fig. 4 shows the Tauc plot from which the direct band gap was calculated by extrapolating the linear portion of the intercept on the energy axis. The calculated value is 3?10 eV for the film

Table 1 Deposition condition, thickness, crystallite sizes and calculated band gap for chromia films S. no.

Deposition conditions

1 2 3 4 5

200 200 200 300 300

mJ/873 mJ/873 mJ/873 mJ/873 mJ/973

K/4.7610 25 mbar K/Ar/461023 mbar K/CH4/261022 mbar K/CH4/261021 mbar K/CH4/261021 mbar

Repetition rate, Hz

Thickness of the film, A˚

Crystallite size on Si (100), nm

Crystallite size on glass, nm

Band gap, eV

10 10 10 20 20

745 787 652 714 1488

24 34 33 54 72

19 23 26 35 56

3.25 3.10 3.55 3.55 3.60

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4 Ultraviolet visible absorption spectra of deposited films; inset shows (ahu)2 versus hu plot for film deposited in argon atmosphere

deposited in the argon atmosphere and it is in agreement with the reported values.8,12,18 The films deposited in the base pressure have a band gap value of 3?25 eV. When the substrate temperature is increased from 873 to 973 K in methane atmosphere, band gap energy also increases to a value of 3?6 eV.27,28 Table 1 also lists the band gap as a function of crystallite size obtained on the films prepared under different deposition conditions. It is seen that for films grown in methane atmosphere, the band gap is more or less constant although the particle sizes are different. This is understandable since quantum confinement is not expected to be pronounced for larger particles. It is clear from the XRD patterns that the films are more crystalline at higher substrate temperature and at higher background gas pressure. In agreement with the reported results,12,18 the increased crystallinity of the films might also have caused a slight increase in the band gap for the films prepared at higher substrate temperature and at higher background gas pressure.

Conclusion It is demonstrated that the crystalline chromia films could be prepared by pulsed laser deposition from Cr3C2 target with varying background atmospheres. Xray diffraction and XPS analyses confirmed the presence of only Cr2O3 with hexagonal structure in these films. The crystallites were found to be nanocrystalline and substrate sensitive. The films contained several spherical droplets for the films formed in the base pressure, while the droplets were large in size and few in number for the films prepared in background gases at higher pressures. Optical studies showed a direct band gap of 3?1–3?6 eV and found to increase with increasing crystallite size. It is believed that a few ppm of oxygen present in the base pressure (461025 mbar) and in the background gases (argon and methane) might have helped for the formation of chromia films.

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Acknowledgements The authors thank Dr R. K. Dayal, Head, CSTD for extending XPS facility and C. Sudha for the help in XPS analysis. The authors also thank Dr M. Vijayalakshmi, Head, PMD, Dr K. B. S. Rao, AD, MDCG, Dr P. R. Vasudeva Rao, Director, MMG, Dr Baldev Raj, Director, IGCAR and Dr M. Chidambaram, Director, National Institute of Technology, Tiruchirappalli, for the encouragement and support.

References 1. V. M. Bermudez and W. J. Desisto: J. Vac. Sci. Technol. A, 2001, 19A, 576–583. 2. F. D. Lai, C. Y. Huang, C. M. Chang, L. A. Wang and W. A. Cheng: Microelectron. Eng., 2003, 17, 67–68. 3. A. Azens, G. Vaivars, L. Kullman and C. G. Granqvist: Electrochim. Acta, 1999, 44, 3059– 3061. 4. J. D. Kruschwitz and W. T. Pawlewicz: Appl. Opt., 1997, 36, 2157– 2159. 5. C. G. Garnqvist: Appl. Phys. A Matr., 1991, 52A, 83–93. 6. J. C. Nable, S. L. Suib and F. S. Galasso: Surf. Coat. Technol., 2004, 186, 423–430. 7. T. Seike and J. Nagai: Sol. Energy Mater., 1991, 22, 107–117. 8. R. H. Misho, W. A. Murad and G. H. Fattahallah: Thin Solid Films, 1989, 169, 235–239. 9. F. Guinneton, O. Monnereau, L. Argeme, D. Stanoi, G. Socol, I. N. Mihailescu, T. Zhang, C. Grigorescu, H. J. Trodahl and L. Tortet: Appl. Surf. Sci., 2005, 247, 139–144. 10. E. W. Kreutz: Appl. Surf. Sci., 1998, 606, 127–129. 11. P. Kuppusami and V. S. Raghunathan: Surf. Eng., 2006, 22, 81– 83. 12. S. Hong, E. Kim, D. W. Kim, T. H. Sung and K. No: J. Non-Cryst. Solids, 1997, 221, 245–254. 13. D. B. Chrisey and G. K. Hubler: ‘Pulsed laser deposition of thin films’; 1994, New York, John Wiley & Sons. 14. S. Murugesan, P. Kuppusami, N. Parvathavarthini and E. Mohandas: Surf. Coat. Technol., 2007, 201, 7713–7719. 15. D.-Y. Wang, J.-H. Lin and W.-Y. Ho: Thin Solid Films, 1998, 332, 295–299. 16. L. I. Maissel and R. Glang: ‘Handbook of thin film technology’; 1980, New York, McGraw-Hill. 17. A. DasGupta, P. Kuppusami, F. Lawrence, V. S. Raghunathan, P. A. Premkumar and K. S. Nagaraja: Mater. Sci. Eng. A, 2004, A374, 362–368.

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18. M. F. Al-Kuhaili and S. M. A. Durrani: Opt. Mater., 2007, 29, 709–713. 19. J. Chastain: ‘Handbook of X-ray photoelectron spectroscopy’; 1992, Eden Prairie, MN, Perkin-Elmer. 20. G. Hollinger, R. Skheyta-Kabbani and M. Gendry: Phys. Rev. B, 1994, 49B, 11159–11167. 21. L. I. Yin, T. Tsang, G. J. Coyle, W. Yin and I. Adler: Phys. Rev. B, 1982, 26B, 1093–1098. 22. M. Detroye, F. Reniers, C. Buess-Herman and J. Vereecken: Appl. Surf. Sci., 1997, 120, 85–93.

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23. I. Barin: ‘Thermochemical data of pure substances’; 1993, Weinheim, VCH. 24. W. F. Chu and A. Rahmel: Oxid. Met., 1981, 15, 331–344. 25. M. J. Ledoux, C. Pham-Huu, J. Guille, H. Dunlop, S. Hantzer, S. Martin and M. Weibel: Catal. Today, 1992, 15, 263–284. 26. J. Lemaitre, B. Vidick and B. Delmon: J. Cataly., 1986, 99, 415–427. 27. M. Tabbal, S. Kahwaji, T. C. Christidis, B. Nsouli and K. Zahraman: Thin Solid Films, 2006, 515, 1976–1984. 28. P. Hones, M. Diserens and F. Le´vy: Surf. Coat. Technol., 1999, 120, 277–283.

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