Characterization of oxygen impurities in thermally evaporated LaF3 thin films suitable for oxygen sensor M. Vijayakumara, S. Selvasekarapandiana,*, T. Gnanasekaranb, Shinobu Fujiharac, Shinnosuke Kojic aSolid State and Radiation Physics Laboratory, Department of Physics, Bharathiar University, Coimbatore 641046, India bMaterials Chemistry Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India cDepartment of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-Ku, Yokohama 223-8522, Japan Received 12 June 2003; received in revised form 12 June 2003; accepted 4 August 2003 Abstract The lanthanum fluoride thin films has been prepared by means of thermal evaporation method. The XRD analysis shows the formation of polycrystalline hexagonal LaF3. The depth profile X-ray photoelectron spectroscopy (XPS) analysis shows the presence of oxide ions throughout the films. The formation of lanthanum oxyfluoride (LaOF) has been identified. The [O]/[F] ratio has been found to be 0.35 which is higher than the previously reported values of LaF3 film applied for the oxygen sensor. # 2003 Elsevier B.V. All rights reserved. Keywords: LaF3; Oxygen sensor; XRD analysis; XPS analysis 1. Introduction Lanthanum fluoride based chemical sensors has potential application in sensing the fluorine, oxygen, and carbon monoxide because of its high chemical stability and ionic conductivity [1–4]. The dissolved oxygen sensing property of the LaF3 material has been utilized to construct the biosensors with suitable enzymes as auxiliary electrode [5]. The fast response and good sensitivity of these sensors rely on the existence of metastable oxygen species, specifically superoxide (O2 �) and peroxide ions (HO2 � or O2 2�) [6]. The need for peroxide and/or superoxide ions for low temperature operation is supported by the absence of these ions in CaF2, which does not work as a low temperature oxygen sensor as does LaF3. Miura et al. [7] reported that the water vapor treatment to the above sensor provides the superoxide and peroxide ions to LaF3 and improves the response time of the sensor. However, the water vapor treatment needs extensive optimization, since excessive water vapor may lead to the decomposing of LaF3 material. Further, the low stability of the superoxide and peroxide ions causes the drift in the sensor voltage. Lukaszewicz et al. [8] used the metal pthalocyanines (MePc) as auxiliary electrodes to stabilize the desired ions. Hence, research activities have been spurred to develop the LaF3 thin films having desired ions without water vapor treatment and external auxiliary electrodes. Applied Surface Science 222 (2004) 125–130 * Corresponding author. Tel.: þ91-422-2422222; fax: þ91-422-2422387. E-mail address:
[email protected] (S. Selvasekarapandian). 0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.08.002 In the present study, LaF3 thin film, prepared by thermal evaporation having stable superoxide and peroxide ions, has been reported. 2. Experimental The high purity lanthanum fluoride (99.99%), has been used as a starting material. ‘‘Tungsten helical source’’ has been used for thermal evaporation, and glass slides as substrate material which is kept at 200 8C during coating. The film is coated under a vacuum pressure of around 2� 10�5 Torr using HINDHIVAC 12 AD coating unit. Crystalline phase of the film has been identified with a glancing-angle X-ray diffractometer (Rigaku) using Cu Ka radiation (40 kV; 150 mA; scan speed 28/min). Chemical bond- ing states of the constituent elements in the film were examined by X-ray photoelectron spectroscopy (XPS) (model JPS9000, JEOL) using Mg Ka radiation (10 kV; 10 mA). Peak positions were calibrated by C 1s position at 284 eV. Compositional analysis of the LaF3 film has been done by calculating the ratio of peak heights in the XPS spectra using relative sensi- tivity coefficients. The commercial software PEAK- FIT has been utilized to analyze the XPS spectrum. 3. Results and discussion Fig. 1 shows the XRD pattern of the lanthanum fluoride thin film at room temperature. The peaks are matched with previously reported values of LaF3 film and found to be in hexagonal phase [9]. The particle size of the film has been determined to be 43 nm, using the Debye Scherrer’s equation D ¼ 0:94l b2y cos y (1) where l is the wavelength of the X-ray and b2y the full width at half maximum of the corresponding peak of the XRD pattern. X-ray photoelectron spectra for the surface layer of lanthanum fluoride film have been shown in Fig. 2. The spectra recorded in the binding energy (BE) range of 100–850 eV show three intense peaks correspond- ing to lanthanum (La 3d5/2), fluorine (F 1s1/2) and oxygen (O 1s) at binding energies around 836, 684, and 531 eV, respectively. However, the impurity peaks like silicon and carbon are also traced out. The che- mical state of lanthanum in LaF3 films can be char- acterized by analyzing the energy position, chemical shift, and FWHM of the two core level binding energy peaks of lanthanum (La 3d5/2), fluorine (F 1s1/2), and 20 30 40 50 60 0 1000 In te n si ty (a. u ) 2θ (in degrees) Fig. 1. XRD pattern of LaF3 thin film at room temperature. 126 M. Vijayakumar et al. / Applied Surface Science 222 (2004) 125–130 oxygen (O 1s). The presence of oxygen peak in the surface of the film is due to the adsorbed oxygen. This may be due to the presence of oxygen during coating procedure. The vacuum level during the evaporation is around 8� 10�5 Torr. Hence, there will be possible oxygen inside the chamber which reacts with LaF3 vapor, resulting the oxygen impurities in the films. Fig. 3a–c, shows depth profile XPS spectra of lanthanum, fluorine, and oxygen, respectively, along the direction of the film thickness. The oxygen peak in the depth profile XPS spectra (shown in Fig. 3c) reveals the presence of oxygen throughout the LaF3 film. The oxygen spectra for surface layer of the LaF3 show a single oxygen peak at 531.5 eV, which is due to the adsorbed oxygen [10]. The lanthanum fluoride films were etched by Arþ ion up to 2000 s to analyze compositions in direction of film thickness. Interest- ingly, two peaks at 531.5 and 528.7 eV have been observed for oxygen spectra in the inner layers of the film, which reveals that the oxygen has two binding state in LaF3 film. The presence of additional peak at the lower energy side may be due to the presence of oxide impurities such as superoxide and peroxide ions [11]. Presence of these ions leads to the formation of lanthanum oxyfluoride (LaOF) in the lanthanum fluor- ide films. This has been confirmed from the chemical shift of the fluorine peak (F 1s) towards higher binding energy in the XPS spectra (shown in Fig. 3b) with increase in oxygen peak intensity in the inner layer of the film. It has been reported that the LaF3 chemically react with oxide ions to form lanthanum oxyfluoride with binding energy of F 1s will move toward higher value [12]. Further, the reaction of rare earth fluoride with oxide ions and formation of oxyflouride has already been reported [13]. The formation of the lanthanum oxyfluorides in the sol–gel prepared lantha- num fluoride has also been reported elsewhere [10]. Due to the similarity in size between oxide ion and fluoride ion, oxide ion must substitute or incorporate for fluoride ion or vacant site in the lanthanum fluor- ide. Hence, the formation of lanthanum oxyfluoride causes the F� vacancies and free fluoride ions, which gives rise to the F� ionic conductivity and hence, high response rate of the sensor. This is represented in Kroger–Vink notation as: O2 � þ FFx ¼ OF0 x þ F� (2) OH� þ FFx ¼ OHFx þ F� (3) Miura et al. [14] reported the presence of oxide impurities in the sputtered lanthanum fluoride films with [O]/[F] ratio of 0.22, which gives the response 0 100 200 300 400 500 600 700 800 900 0 5000 10000 15000 20000 Si 1S C 1S O 1S F 1S1/2 La 3d5/2 In te n si ty (a. u) Binding Energy (eV) Fig. 2. X-ray photoelectron spectra of LaF3 thin film. M. Vijayakumar et al. / Applied Surface Science 222 (2004) 125–130 127 820825830835840845850 1 2 3 4 5 6 La 3d5/2 In te n si ty (a. u ) Ar + E tc hin g S tep s Binding Energy (eV) 680685690695700705710 1 2 3 4 5 6 F 1S1/2 In te ns ity (a. u) Ar + E tc hi ng S tep s Binding Energy (eV) (a) (b) Fig. 3. (a) Depth profile XPS spectrum of the LaF3 film in the region of La 3d5/2, (b) depth profile XPS spectrum of the LaF3 film in the region of F 1s1/2, and (c) depth profile XPS spectrum of the LaF3 film in the region of O 1s, where (1) before Ar þ etching; (2) after 200 s Arþ etching; (3) after 400 s Arþ etching; (4) after 600 s Arþ etching; and (5) after 800 s Arþ etching. 128 M. Vijayakumar et al. / Applied Surface Science 222 (2004) 125–130 rate as 5 min. The [O]/[F] ratio has been increased to 0.31 by the water vapor treatment at 90 8C for 1 h of sputtered films which shows high response rate of 0.5 min. In the present work, the average [O]/[F] ratio has been observed as 0.35 for the thermally evaporated LaF3 film even without water vapor treatment. This high oxygen content of the as deposited LaF3 film will allow a quick response to oxygen, rather than the water vapor treated samples reported previously. The lanthanum content of the film along the depth of film thickness has been shown in Fig. 3a. The La 3d peak at the surface layer has been observed at 836.6 eV. The shape and peak positive are similar to those of LaF3 powder data, evidencing the presence of La–F bonding in the film [15]. The lanthanum peak has been found to split into two peaks in the inner layer of the film. This confirms the presence of two different chemical bonding nature of the La atom. As explained previously, this may be due to the formation of lanthanum oxyfluoride where La has bonding with oxygen and fluorine. This is further evidence for the formation of lanthanum oxyfluoride. The atomic ratio of lanthanum is almost constant throughout the film, while those of fluorine and oxy- gen slightly change along the depth direction. In the surface, the amount of fluorine is larger and that of oxygen is smaller as compared to those detected in the inner area. About 30 at.% oxygen has been detected throughout the film. The LaF3 stoichiometric has been calculated from the ratio of peak areas between F and La when corrected with atomic sensitivity factors, and it is found to be 1:2.84 at the surface of the film. The presence of lanthanum oxyfluoride and other oxide impurities are below the detectable limit of X-ray diffraction analysis and hence, it is not observed in the XRD pattern of LaF3 thin film as shown in Fig. 1. 4. Conclusion The lanthanum fluoride thin film has been prepared by thermal evaporation method. The XRD diffraction pattern reveals the polycrystalline nature of the film in the hexagonal phase. The depth profile XPS ana- lysis reveals the presence of oxide impurities such as superoxide and peroxide ions, which leads to the formation of lanthanum oxyfluoride. Hence, 530535540545550 1 2 3 4 5 O 1s In te ns ity (a. u) Ar + E tc hi ng S te ps Binding Energy (eV)(c) Fig. 3. (Continued ). M. 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