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Properties of V–Ce mixed-oxide thin films deposited by RF magnetron sputtering
Materials Science in Semiconductor Processing 19 (2014) 40–49
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Properties of V–Ce mixed-oxide thin films deposited by RF magnetron sputtering D. Rachel Malini a, C Sanjeeviraja b,n a b
Department of Physics, The American College, Madurai 625 002, India Department of Physics, Alagappa Chettiar College of Engineering & Technology, Karaikudi 630 004, India
a r t i c l e in f o
Keywords: V–Ce mixed-oxide thin films RF magnetron sputtering Photoacoustics
abstract Thin films of vanadium cerium mixed oxides are good counter-electrodes for electrochromic devices because of their passive optical behavior and very good charge capacity. We deposited thin films of V–Ce mixed oxides on glass substrates by RF magnetron sputtering under argon at room temperature using different power settings. The targets were pressed into pellets of a powder mixture of V2O5 and CeO2 at molar ratios of 2:1, 1:1, and 1:2. For a molar ratio of 2:1, the resulting crystalline film comprised an orthorhombic CeVO3 phase and the average grain size was 89 nm. For molar ratios of 1:1 and 1:2, the resulting films were completely amorphous in nature. Scanning electron microscopy images and energy-dispersive X-ray spectroscopy data confirmed these results. The optical properties of the films were studied using UV-Vis-NIR spectrophotometry. The transmittance and indirect allowed bandgap for the films increased with the RF power, corresponding to a blue shift of the UV cutoff. The average transmittance increased from 60.9% to 85.3% as the amount of CeO2 in the target material increased. The optical bandgap also increased from 1.94 to 2.34 eV with increasing CeO2 content for films prepared at 200 W. Photoacoustic amplitude (PA) spectra were recorded in the range 300–1000 nm. The optical bandgap was calculated from wavelengthdependent normalized PA data and values were in good agreement with those obtained from UV-Vis-NIR data. The thermal diffusivity calculated for the films increased with deposition power. For thin films deposited at 200 W, values of 53.556 10 8, 1.069 10 8, and 0.2198 10 8 m2/s were obtained for 2:1, 1:1, and 1:2 V2O5/CeO2, respectively. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Translation of optical energy into mechanical energy by a sample is called the photoacoustic or optoacoustic effect. Photoacoustic spectroscopy can be used to study the optical and thermal properties of a material and involves exciting the electrons of an analyte by illuminating it with modulated electromagnetic waves. Kinetic energy or heat is produced during the de-excitation process. Local warming of the sample matrix by this non-radiative relaxation process generates pressure fluctuations as a result of thermal
n
Corresponding author. Tel.: þ91 9487037305. E-mail address: [email protected] (C. Sanjeeviraja).
expansion within the sample. Owing to its multivalency, layered structure, and large optical bandgap, vanadium pentoxide (V2O5) has been widely studied as a transition metal oxide semiconductor [1]. It exhibits good charge capacity as a counter-electrode in electrochromic devices (ECDs) but shows an undesirable brownish gray color during lithium intercalation [2]. Because of high transparency in both oxidized and reduced states, CeO2 can be used as a passive counter-electrode [3–5] but it exhibits slow reaction kinetics and low charge capacity [6,7]. To obtain high-quality counter-electrodes for ECDs, mixed oxides of these materials have been prepared [8–10]. Good thermal parameters such as thermal diffusivity are essential for semiconductors used in fabricating such technological devices [11]. Therefore, we measured the thermal properties of V–Ce mixed-oxide thin
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D. Rachel Malini, C Sanjeeviraja / Materials Science in Semiconductor Processing 19 (2014) 40–49
films by recording photoacoustic (PA) spectra. In this technique, a sample in an enclosed chamber is illuminated by an intensity-modulated light beam to generate acoustic waves from the sample [12]. This non-destructive and non-contact technique is used to study non-radiative processes and is suitable for samples with rough surfaces [13–15]. The optical bandgap of V–Ce mixed-oxide thin films was calculated from the PA data and was compared to results obtained from UV-Vis-NIR data. To determine the quality of the thin films, structural and morphological properties were measured using X-ray diffraction (XRD), atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDS). 2. Experimental V–Ce mixed-oxide thin films were deposited on clean Corning glass substrates (8 cm 6 cm) by RF magnetron sputtering under pure argon. V2O5 and CeO2 powders were mixed at different molar ratios (V2O5:CeO2 ¼2:1, 1:1; 1:2) and sintered at 600 1C for 5 h. XRD patterns revealed that the 2:1 product contained 49% CeVO4, 18% CeVO3, and 33% V2O5, the 1:1 product contained 76% CeVO4 and 23% V2O5, and the 1:2 product contained 71% CeVO4 and 29% V2O5. In all cases, products comprising pure CeVO4 phase were obtained by repeated sintering. After five repeats, products of a pure wakefieldite CeVO4 phase were obtained. The products were pressed into target pellets of 6 mm in thickness and 50 mm in diameter and were sintered again at 300 1C for 2 h. Before deposition, a pre-sputtering vacuum
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better than 10 5 mbar was obtained in the chamber. Using these targets, three sets of V–Ce mixed–oxide thin films were deposited at room temperature using RF power from 100 to 200 W in steps of 50 W under argon. In all cases, the deposition time was 45 min, the Ar pressure inside the chamber was 1.33 10 2 mbar, and the target–substrate spacing was 6 cm. XRD patterns were recorded on an XPERT-PRO PANalytical diffractometer with Cu Kα radiation (λ¼1.5406 Å) to determine the structural properties of the samples. The morphology of the films was investigated using AFM (Veeco diCP-II instrument), FESEM, and EDS (acceleration voltage 20 keV). Transmittance and absorption spectra were recorded using a UV-Vis-NIR spectrophotometer (Ocean Optics HR-2000). PA spectra were recorded in the range 300–1000 nm using a standard PA spectrometer with a 450 W Xe lamp (Horiba Jobin Yvon) as the source and a mechanical chopper (C-995, Tetrahertz Technologies) for modulation of the pump beam at the desired frequency. A lock-in amplifier (SR-830 DSP, Stanford Research) was used to amplify the microphone output signal. The light falling on the sample was first passed through a monochromator (Triax 180, Horiba Jobin Yvon). 3. Results and discussion 3.1. Structure XRD patterns for the 2:1 V–Ce mixed-oxide target and resulting thin films are shown in Fig. 1a. The target comprises pure CeVO4 phase according to the 11 peaks
Fig. 1. (a) XRD patterns for CeVO4 targets and V–Ce mixed-oxide thin films deposited at different RF powers on glass substrates for V2O5:CeO2 molar ratios of (a) 2:1; (b) 1:1, and (c) 1:2.
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located at 2θ¼18.271, 20.391, 21.821, 24.151, 26.261, 28.651, 32.531, 39.151, 47.591, 47.981 and 49.331 attributed to the (0 1 1), (–1 1 1), (1 0 1), (0 2 0), (2 0 0), (0 2 1), (1 2 1), (1 3 0), (1 0 3), (–1 2 3), and (0 2 3) planes, respectively, of CeVO4 (JCPDS 79–1066). This confirms that all of the V2O5 reacted with cerium atoms to form CeVO4 according to the reaction V2O5 þ2CeO2-2CeVO4 þ1/2 O2. XRD patterns for thin films deposited at 100 and 150 W are completely amorphous, whereas the film deposited at 200 W shows a peak at 2θ¼19.721 with a height of 316 cts corresponding to the (1 0 1) plane of the orthorhombic CeVO3 phase (JCPDS 78–2306), which has Pbnm symmetry [16]. Trapped electrons near CeVO4, VO, and CeO2 oxides are responsible for the reduction of CeVO4 to CeVO3 [17]. The following microstructural parameters were calculated for V–Ce mixed-oxide thin films deposited at 200 W: d spacing, 4.4975 Å; crystallite size, 9 nm; dislocation density, 109.072 1014 lines/m2; microstrain, 0.5636 10–2; and number of crystallites per unit volume, 1219 1015 m 3 respectively. The number of crystallites per unit volume is
very high, which confirms that CeVO3 particles in the thin films are nanostructure. The results indicate that an increase in RF power during deposition enhances thin film growth. XRD patterns for the 1:1 V–Ce mixed-oxide target and the resulting thin films are shown in Fig. 1b. The target comprises pure CeVO4 phase according to the six peaks located at 2θ¼18.191, 24.011, 32.451, 34.291, 39.011, and 49.261, which are attributed to the (1 0 1), (2 0 0), (1 1 2), (2 2 0), (3 0 1), and (3 1 2 ) plans of CeVO4 (JCPDS 12–0757). It is evident that all the thin films deposited at RF power of 125–200 W are completely amorphous. This indicates that even high RF power of 200 W during deposition does not assist the formation of crystalline CeVO4 because of the existence of V2O5 on the film surface in a highly dispersed or amorphous state. XRD patterns for the 1:2 V–Ce target and the resulting thin films are shown in Fig. 1 c. The target material comprises pure CeVO4 phase according to the 16 peaks located at 2θ ¼18.2021, 24.0571, 30.1941, 32.4281, 34.241,36.7561, 38.991, 43.431, and 46.341, which are attributed to the (1 0 1), (2 0 0), (2 1 1), (1 1 2), (2 2 0), (2 0 2), (3 0 1), (1 0 3), and (3 2 1) planes, respectively, of CeVO4 (JCPDS No: 82–1969). In this case, all of the
Fig. 2. AFM images of V–Ce mixed-oxide thin films grown at 200 W for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
D. Rachel Malini, C Sanjeeviraja / Materials Science in Semiconductor Processing 19 (2014) 40–49
thin films are also amorphous in nature, indicating that 200 W is not sufficient to grow crystalline particles due to a lack of oxygen during deposition [18]. 3.2. Morphological studies 3.2.1. AFM analysis AFM images of V–Ce mixed-oxide thin films deposited at low RF power do not reveal any characteristic features and they are amorphous. Two-dimensional AFM images of V–Ce mixed-oxide thin films prepared at 200 W are shown in Fig. 2. Film prepared from a 2:1 V–Ce target shows a smooth surface with small grains oriented in an elongated and orderly manner, revealing uniform growth (Fig. 2a). The average grain size is approximately 18 nm, which confirms that the film is nanostructured. In this case, high RF power led to a crystalline thin film but of CeVO3 instead of CeVO4 due to insufficient oxygen during deposition. Film prepared from a 1:1 V–Ce target shows uniform growth with elongated spherical particles oriented in an orderly fashion on the surface with an average grain size of 63 nm (Fig. 2b). Film prepared from a 1:2 V–Ce target exhibits a flat surface with uniform growth and an average grain size of 55 nm (Fig. 2c). In the latter two cases, high RF power during deposition enhanced film growth but was
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not sufficient for a crystalline thin film owing to deposition in an Ar atmosphere. 3.2.2. FESEM and EDS studies The surface morphology of the thin films was investigated by FESEM. Images for thin films deposited at low RF power exhibit almost flat and smooth surfaces (data not shown), indicating amorphous films with a dense structure. The FESEM image of a thin film prepared at 200 W from a 2:1 V–Ce target in Fig. 3a shows irregular shaped grains of 160–511 nm in size. XRD data revealed that these grains correspond to CeVO3. The average grain size is 385 nm. The FESEM image of a thin film prepared at 200 W from a 1:1 V–Ce target reveals an almost flat, dense and smooth surface, indicating the film is amorphous (Fig. 3b). Deposition in an Ar atmosphere results in thin films with a high density and small grain size. The FESEM image of a thin film prepared at 200 W from a 1:2 V–Ce target reveals a smooth and compact surface with highdensity packing of nanoparticles ofr10 nm in size (Fig. 3c). The results indicate that an increase in RF power during deposition improves the growth of these thin films. Fig. 4a shows the EDS spectrum for a thin film prepared at 200 W from a 2:1 V–Ce target. The line at 0.54 keV (33.36 wt.%) is due to 2p oxygen and the line at 4.94 keV
Fig. 3. FESEM images of V–Ce mixed-oxide thin films grown at 200 W for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
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Fig. 4. EDS results for V–Ce mixed-oxide thin films grown at 200 W for V2O5:CeO2 RF power in the molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
(8.7 wt.%) is the Kα line for vanadium. Peaks corresponding to cerium ions are observed at 0.87, 4.87, 5.28, and 5.71 keV (19.56 wt.%). The remaining EDS peaks are due to components of the glass substrate. Thus, the EDS data confirm the presence of vanadium, cerium, and oxygen ions in this film and its purity. Fig. 4b shows the EDS spectrum for a thin film prepared at 200 W from a 1:1 V–Ce target. The Kα line for vanadium is evident at 4.95 keV (7.66 wt.%) and a line due to 2p oxygen is observed at 0.55 keV (29.68 wt.%). The peaks at 0.88, 4.87, 5.29, and 5.7 keV correspond to cerium ions (21.45 wt.%). The EDS spectrum for a thin film prepared at 200 W from a 1:2 V–Ce target is shown in Fig. 4 c. The line at 0.54 keV is due to 2p oxygen (22.32 wt.%) and that at 4.95 keV is the Kα line for vanadium (5.57 wt.%). Peaks corresponding to cerium ions are observed at 0.86, 4.86, 5.29, and 5.71 keV (28.58 wt.%). 3.3. Optical studies Transmission spectra for the V–Ce mixed-oxide thin films over the wavelength range 300–900 nm are shown in Fig. 5. For films prepared from a 2:1 V–Ce target, the average transmittance in the visible region was 51%, 63.9%, and 67.7% for RF power of 100, 150, and 200 W, respectively (Fig. 5a). The increase in transmission with RF power reflects an improvement in grain growth on the film surface. The transmittance for films prepared from a 1:1 V–Ce target was 75%, 93%, 97.5%, and 98% for RF power of 125, 150, 175, and 200 W, respectively (Fig. 5b). These
mixed-oxide thin films are more transparent than pure V2O5 thin films prepared under similar conditions, which had transmittance values of 78.4%, 80%, 84.1%, and 92.9% [6]. The overall transmission is higher for these thin films (average 90.9%) than for the films prepared from a 2:1 V–Ce target (60.9%) because of the significant role of CeO2 in the films [19]. Thin films prepared from a 1:2 V–Ce target at RF power of 100, 150, and 200 W had transmittance of 75%, 90%, and 91%, respectively, with an average value of 85.3%. These films showed a blue shift in UV cutoff, starting from 330 nm, with increasing RF power. The presence of defective phases in addition to the desired CeVO4 phase in the films is responsible for the shift in fundamental absorption edge to higher energy. The UV cutoff for films prepared from a 1:1 V–Ce target also exhibited a blue shift from 350 to 300 nm. The same result was observed for films prepared from a 1:2 V–Ce target, with a UV cutoff at 300 nm. Experimental data for thin films prepared from targets with different V–Ce molar ratios gave a better fit for (αhν)1/2 versus hν. This suggests an indirect allowed transition. Fig. 6 shows plots of (αhν)1/2 versus hν for V–Ce mixed-oxide thin films. The indirect allowed transition for thin film prepared from a 2:1 V–Ce target was 1.59, 1.75, and 1.94 eV for RF power of 100, 150, and 200 W, respectively (Fig. 6a). The optical bandgap for the mixed-oxide films is low because of the dominance of V2O5, which has a lower bandgap than CeO2. Eg for thin films prepared from a 1:1 target was 1.64, 1.7, 2.2, and 2.34 eV for RF power of 125, 150, 175, and 200 W, respectively (Fig. 6b). For both V2O5 and CeO2, the
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Fig. 5. Transmittance spectra for V–Ce mixed-oxide thin films deposited at different RF powers on glass substrates for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
fundamental absorption edge is due to an indirect allowed transition between 2p electrons in the valance band of oxygen and the conduction band of the metal. For films prepared at 200 W, Eg widens from 1.94 eV for a 2:1 target to 2.34 eV for a 1:1 target because of the greater amount of CeO2, which has a wide bandgap of 3.2 eV [2]. Eg for thin films prepared from a 1:2 target was 2.7, 2.93, and 3.04 eV for RF power of 100, 150, and 200 W, respectively (Fig. 6c). Eg is greater for V–Ce mixed-oxide thin films than for pure V2O5 thin films because of the Burstein–Moss effect (BME), whereby mixing of V2O5 (an n-type semiconductor) into CeO2 shifts the Fermi level inside the conduction band, which results in widening of the energy bandgap. According to the BME, an increase in the Fermi level in the conduction band results in an increase in carrier concentration [20]. High RF power helps to increase this carrier concentration, which in turn is responsible for the increase in Eg. 3.4. PA studies Fig. 7 shows the wavelength-dependent normalized PA signal amplitude for thin films prepared from a 2:1 V–Ce target at different RF powers. The PA signal amplitude increases with the RF power. Cruz-Orea and MendozaÁlvarez investigated the dependence of the latter on laser
intensity for TiO2 thin films grown on Ti [21]. Resonant absorption via interband transitions occurs when the photon energy is greater than the optical bandgap, and the decrease in PA amplitude with respect to photon wavelength can be used to calculate the optical bandgap. The optical bandgap calculated for the samples was 2.19, 2.29, and 2.32 eV for RF power of 100, 150, and 200 W, respectively, which is in good agreement with the UV-VisNIR results. The thermal diffusivity of a thin film can be determined from the slope of a graph of the PA signal phase versus the square root of the modulation frequency in the thermally thick region [22,23]. According to Rosencwaig–Gersho PA theory, the film thickness is less than the thermal diffusion length in thermally thin regions and thus the PA signal strength depends on the thermal property of the medium next to the thin film. In thermally thick regions, the thermal diffusion length is less than the film thickness, so the PA signal depends only on the thermal property of the region of thin film. In this region, thermal diffusivity can be calculated as α¼fcl2, where fc is the characteristic frequency and l is the thickness of the sample. The characteristic frequency can be determined from the region where the PA signal depends on 1/f. Fig. 8 shows the PA phase signal as a function of the square root of the
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Fig. 6. Plots of (αhν)1/2 versus photon energy for mixed-oxide thin films prepared at different RF powers for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
chopping frequency and the best fit taken over the thermally thick region for calculation of the thermal diffusivity. From these fitting data, the thermal diffusivity was calculated as 1.221 10 8, 1.6911 10 8, and 53.556 10 8 m2/s for RF power of 100, 150, and 200 W, respectively. The increase in thermal diffusivity with RF power shows that phonon conduction improved in the thin films and thus the thermal conductivity increases with the RF power as a result of phonon impacts. The thermal conductivity of a phonon is given by K¼
1 C p vl; 3
where Cp is the specific heat capacity, v is the mean velocity, and l is the mean free path of the phonon [24]. As the mean free path and the number of thermally generated phonons in the lattice plane of the thin films increase with the RF power, the thermal diffusivity of the thin films also increases [25]. PA spectra for thin films prepared from a 1:1 V–Ce target at different RF powers are shown in Fig. 9. These films exhibit a higher PA signal amplitude than films prepared from a 2:1 V–Ce target. This indicates that thermal vibrations induced in the material increase with the concentration of Ce3 þ ions in the target. This may be
Fig. 7. PA spectra for 2:1 V–Ce mixed-oxide thin films grown at different RF powers.
due to the generation of more thermal phonons. The energy bandgap for these thin films was 2.25, 2.3, 2.32, and 2.38 eV for RF power of 125, 150, 175, and 200 W, respectively. The higher Eg for these films compared to
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Fig. 8. PA signal phase versus chopping frequency for 2:1 V–Ce mixed-oxide thin films prepared at (a) 100, (b) 150, and (c) 200 W. The best fit region is shown by the solid line.
Fig. 9. PA spectra for 1:1 V–Ce mixed-oxide thin films grown at different RF powers.
those prepared from a 2:1 V–Ce target is due to the higher CeO2 content in the target material, since CeO2 has a high optical bandgap of 3.2 eV. The Eg values calculated are in
good agreement with the results obtained from UV-VisNIR data. Fig. 10 shows the PA phase signal as a function of the square root of the chopping frequency for thin films prepared from a 1:1 V–Ce target at different RF powers. From the best fit taken over the thermally thick region, the thermal diffusivity calculated for the thin films was 0.2355 10–8, 0.2694 x10–8, 0.4118 10–8, and 1.069 10 8 m2/s for RF power of 125, 150, 175, and 200 W, respectively. The increase in thermal diffusivity with RF power for the thin films is due to increases in the mean free path and thermally generated phonons in the lattice structure on mixing of CeO2 with V2O5. Fig. 11 shows PA spectra for thin films prepared from a 1:2 V–Ce target at different RF powers. The energy bandgap was 2.33, 2.36, and 2.39 eV for RF power of 100, 150, and 200 W, respectively, and these values are in good agreement with the results obtained from UV-Vis-NIR data. The higher Eg compared to films prepared from 2:1 and 1:1 V–Ce targets is due to higher CeO2 content in the target material, since CeO2 has a high optical bandgap of 3.2 eV. Fig. 12 shows the PA phase signal as a function of the square root of the chopping frequency for films deposited at different RF powers. From the best fit taken over the thermally thick
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Fig. 10. PA signal phase versus chopping frequency for 1:1 V–Ce mixed-oxide thin films prepared at (a) 125, (b) 150, (c) 175, and (d) 200 W. The best fit region is shown by the solid line.
in the mean free path and thermally generated phonons in the lattice plane of the thin films with increasing RF power are responsible for the increase in thermal diffusivity. 4. Conclusions
Fig. 11. PA spectra for 1:2 V–Ce mixed-oxide thin films grown at different RF powers.
region, the thermal diffusivity calculated for the thin films was 0.2083 10 8, 0.2121 10 8, and 0.2198 10 8 m2/s for RF power of 100, 150, and 200 W, respectively. Increases
We deposited V–Ce mixed-oxide thin films by RF magnetron sputtering. XRD patterns for films prepared at high RF power from 2:1 V–Ce targets showed peaks corresponding to CeVO3 whereas those prepared from 1:1 and 1:2 V–Ce targets were amorphous in nature. AFM and FESEM images and EDS data confirmed these results. The optical bandgap and transmittance increased with the RF power. The fundamental absorption edge shifted to higher energy due to the presence of defective phases in addition to the desired CeVO4 phase in the thin films. CeO2 has higher transmittance and optical bandgap than V2O5 does, so a higher CeO2 content in the target material increases the transmittance and bandgap of the resulting thin films. The PA signal amplitude increased with the RF power because of an increase in Ce3 þ concentration in the films, which induced thermal vibrations in the material by generating more thermal phonons. The optical bandgap calculated using the normalized PA
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Fig. 12. PA signal phase versus chopping frequency for 1:2 V–Ce mixed-oxide thin films prepared at (a) 100, (b) 150, and (c) 200 W. The best fit region is shown by the solid line.
signal amplitude is in good agreement with results obtained from UV-Vis-NIR data. The thermal diffusivity of the films increased with the RF power due to increases in the mean free path and thermally generated phonons in the lattice plane of the thin films. However, it decreased with increasing CeO2 content in the target. References [1] R.T. Rajendra Kumar, B. Karunagaran, V. Senthil Kumar, Y.L. Jeyachandran, D. Mangalraj, S.K. Narayandass, Mater. Sci. Semicond. Process. 6 (2003) 543–546. [2] E. Masetti, F. Varsano, F. Decker, A. Krasilnikova, Electrochim. Acta 46 (2001) 2085–2090. [3] S.-Y. Zheng, A.M. Andersson-Faldt, B. Stjerna, C.G. Granqvist, Appl. Opt. 32 (1993) 6303–6309. [4] Z. Crnjak Orel, B. Orel, Proc. SPIE, 2255, , 1994, 285. [5] U. Lavrencic Stangar, B. Orel, I. Grabec, B. Ogorevc, K. Kalcher, Sol. Energy Mater. Sol. Cells 31 (1993) 171–185. [6] E. Masetti, F. Varsano, F. Decker, Electrochim. Acta 44 (1999) 3117–3119. [7] F. Varsano, F. Decker, E. Masetti, Ionics 5 (1999) 80–85. [8] U. Opara Krasovec, B. Orel, A. Surca, N. Bukovec, R. Reisfeld, Solid State Ionics 118 (1999) 195–214. [9] Z. Crnjak Orel, I. Musevic, Nanostruct. Mater. 12 (1999) 399–404.
[10] G. Picardi, F. Varsano, F. Decker, U. Opara-Krasovec, A. Surca, B. Orel, Electrochim. Acta 44 (1999) 3157–3164. [11] T.A. El-Brolossy, S.S. Ibrahim, Thermochim. Acta 509 (2010) 46–49. [12] A. Rosencwaig, Phys. Today 28 (1975) 23–30. [13] T. Terasako, T. Yamanaka, S. Yura, M. Yagi, S. Shirakata, Thin Solid Films 519 (2010) 1546–1551. [14] A. Rosencwaig, Photoacoustic and Photoacoustic Spectroscopy, Wiley Interscience, New York, 1980. [15] C.W. Van Neste, L.R. Senesac, T. Thundat, Appl. Phys. Lett. 92 (2008) 234102–234104. [16] D.R. Malini, C. Sanjeeviraja, Preparation and characterization of RF sputtered Ce-V mixed oxide thin films, AIP Conf. Proc 1447 (2012) 633–634. [17] J. Matta, E. Abi-Aad, D. Courcot, A. Aboukais, in: J.P. Fraissard, O. Lapina (Eds.), Magnetic Resonance in Colloid and Interface Science, Kluwer Academic, Dordrecht, 2002, pp. 365–374. [18] D.R. Malini, C. Sanjeeviraja, Electrochim. Acta 104 (2013) 162–169. [19] Z.Crnjak Orel, Solid State Ionics 116 (1999) 105–116. [20] J.-W. Kim, H.-B. Kim, D.K. Kim, J. Kor, Phys, Soc 59 (2011) 2349–2353. [21] A. Cruz-Orea, J.G. Mendoza-Álvarez, Superficies Vacío 16 (2003) 34–37. [22] P. Raji, C. Sanjeeviraja, K. Ramachandran, Bull. Mater. Sci. 28 (2005) 233–238. [23] K. Jeyadheepan, P. Palanichamy, P. Kalyanasundaram, M. Jayaprakasam, C. Sanjeeviraja, K. Ramachandran, Measurement 43 (2010) 1336–1344. [24] R. Mévrel, J.-C. Laizet, A. Azzopardi, B. Leclercq, M. Poulain, O. Lavigne, D. Demange, J. Eur. Ceram. Soc. 24 (2004) 3081–3089. [25] K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969.
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Properties of V–Ce mixed-oxide thin films deposited by RF magnetron sputtering D. Rachel Malini a, C Sanjeeviraja b,n a b
Department of Physics, The American College, Madurai 625 002, India Department of Physics, Alagappa Chettiar College of Engineering & Technology, Karaikudi 630 004, India
a r t i c l e in f o
Keywords: V–Ce mixed-oxide thin films RF magnetron sputtering Photoacoustics
abstract Thin films of vanadium cerium mixed oxides are good counter-electrodes for electrochromic devices because of their passive optical behavior and very good charge capacity. We deposited thin films of V–Ce mixed oxides on glass substrates by RF magnetron sputtering under argon at room temperature using different power settings. The targets were pressed into pellets of a powder mixture of V2O5 and CeO2 at molar ratios of 2:1, 1:1, and 1:2. For a molar ratio of 2:1, the resulting crystalline film comprised an orthorhombic CeVO3 phase and the average grain size was 89 nm. For molar ratios of 1:1 and 1:2, the resulting films were completely amorphous in nature. Scanning electron microscopy images and energy-dispersive X-ray spectroscopy data confirmed these results. The optical properties of the films were studied using UV-Vis-NIR spectrophotometry. The transmittance and indirect allowed bandgap for the films increased with the RF power, corresponding to a blue shift of the UV cutoff. The average transmittance increased from 60.9% to 85.3% as the amount of CeO2 in the target material increased. The optical bandgap also increased from 1.94 to 2.34 eV with increasing CeO2 content for films prepared at 200 W. Photoacoustic amplitude (PA) spectra were recorded in the range 300–1000 nm. The optical bandgap was calculated from wavelengthdependent normalized PA data and values were in good agreement with those obtained from UV-Vis-NIR data. The thermal diffusivity calculated for the films increased with deposition power. For thin films deposited at 200 W, values of 53.556 10 8, 1.069 10 8, and 0.2198 10 8 m2/s were obtained for 2:1, 1:1, and 1:2 V2O5/CeO2, respectively. Crown Copyright & 2013 Published by Elsevier Ltd. All rights reserved.
1. Introduction Translation of optical energy into mechanical energy by a sample is called the photoacoustic or optoacoustic effect. Photoacoustic spectroscopy can be used to study the optical and thermal properties of a material and involves exciting the electrons of an analyte by illuminating it with modulated electromagnetic waves. Kinetic energy or heat is produced during the de-excitation process. Local warming of the sample matrix by this non-radiative relaxation process generates pressure fluctuations as a result of thermal
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Corresponding author. Tel.: þ91 9487037305. E-mail address: [email protected] (C. Sanjeeviraja).
expansion within the sample. Owing to its multivalency, layered structure, and large optical bandgap, vanadium pentoxide (V2O5) has been widely studied as a transition metal oxide semiconductor [1]. It exhibits good charge capacity as a counter-electrode in electrochromic devices (ECDs) but shows an undesirable brownish gray color during lithium intercalation [2]. Because of high transparency in both oxidized and reduced states, CeO2 can be used as a passive counter-electrode [3–5] but it exhibits slow reaction kinetics and low charge capacity [6,7]. To obtain high-quality counter-electrodes for ECDs, mixed oxides of these materials have been prepared [8–10]. Good thermal parameters such as thermal diffusivity are essential for semiconductors used in fabricating such technological devices [11]. Therefore, we measured the thermal properties of V–Ce mixed-oxide thin
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films by recording photoacoustic (PA) spectra. In this technique, a sample in an enclosed chamber is illuminated by an intensity-modulated light beam to generate acoustic waves from the sample [12]. This non-destructive and non-contact technique is used to study non-radiative processes and is suitable for samples with rough surfaces [13–15]. The optical bandgap of V–Ce mixed-oxide thin films was calculated from the PA data and was compared to results obtained from UV-Vis-NIR data. To determine the quality of the thin films, structural and morphological properties were measured using X-ray diffraction (XRD), atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDS). 2. Experimental V–Ce mixed-oxide thin films were deposited on clean Corning glass substrates (8 cm 6 cm) by RF magnetron sputtering under pure argon. V2O5 and CeO2 powders were mixed at different molar ratios (V2O5:CeO2 ¼2:1, 1:1; 1:2) and sintered at 600 1C for 5 h. XRD patterns revealed that the 2:1 product contained 49% CeVO4, 18% CeVO3, and 33% V2O5, the 1:1 product contained 76% CeVO4 and 23% V2O5, and the 1:2 product contained 71% CeVO4 and 29% V2O5. In all cases, products comprising pure CeVO4 phase were obtained by repeated sintering. After five repeats, products of a pure wakefieldite CeVO4 phase were obtained. The products were pressed into target pellets of 6 mm in thickness and 50 mm in diameter and were sintered again at 300 1C for 2 h. Before deposition, a pre-sputtering vacuum
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better than 10 5 mbar was obtained in the chamber. Using these targets, three sets of V–Ce mixed–oxide thin films were deposited at room temperature using RF power from 100 to 200 W in steps of 50 W under argon. In all cases, the deposition time was 45 min, the Ar pressure inside the chamber was 1.33 10 2 mbar, and the target–substrate spacing was 6 cm. XRD patterns were recorded on an XPERT-PRO PANalytical diffractometer with Cu Kα radiation (λ¼1.5406 Å) to determine the structural properties of the samples. The morphology of the films was investigated using AFM (Veeco diCP-II instrument), FESEM, and EDS (acceleration voltage 20 keV). Transmittance and absorption spectra were recorded using a UV-Vis-NIR spectrophotometer (Ocean Optics HR-2000). PA spectra were recorded in the range 300–1000 nm using a standard PA spectrometer with a 450 W Xe lamp (Horiba Jobin Yvon) as the source and a mechanical chopper (C-995, Tetrahertz Technologies) for modulation of the pump beam at the desired frequency. A lock-in amplifier (SR-830 DSP, Stanford Research) was used to amplify the microphone output signal. The light falling on the sample was first passed through a monochromator (Triax 180, Horiba Jobin Yvon). 3. Results and discussion 3.1. Structure XRD patterns for the 2:1 V–Ce mixed-oxide target and resulting thin films are shown in Fig. 1a. The target comprises pure CeVO4 phase according to the 11 peaks
Fig. 1. (a) XRD patterns for CeVO4 targets and V–Ce mixed-oxide thin films deposited at different RF powers on glass substrates for V2O5:CeO2 molar ratios of (a) 2:1; (b) 1:1, and (c) 1:2.
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located at 2θ¼18.271, 20.391, 21.821, 24.151, 26.261, 28.651, 32.531, 39.151, 47.591, 47.981 and 49.331 attributed to the (0 1 1), (–1 1 1), (1 0 1), (0 2 0), (2 0 0), (0 2 1), (1 2 1), (1 3 0), (1 0 3), (–1 2 3), and (0 2 3) planes, respectively, of CeVO4 (JCPDS 79–1066). This confirms that all of the V2O5 reacted with cerium atoms to form CeVO4 according to the reaction V2O5 þ2CeO2-2CeVO4 þ1/2 O2. XRD patterns for thin films deposited at 100 and 150 W are completely amorphous, whereas the film deposited at 200 W shows a peak at 2θ¼19.721 with a height of 316 cts corresponding to the (1 0 1) plane of the orthorhombic CeVO3 phase (JCPDS 78–2306), which has Pbnm symmetry [16]. Trapped electrons near CeVO4, VO, and CeO2 oxides are responsible for the reduction of CeVO4 to CeVO3 [17]. The following microstructural parameters were calculated for V–Ce mixed-oxide thin films deposited at 200 W: d spacing, 4.4975 Å; crystallite size, 9 nm; dislocation density, 109.072 1014 lines/m2; microstrain, 0.5636 10–2; and number of crystallites per unit volume, 1219 1015 m 3 respectively. The number of crystallites per unit volume is
very high, which confirms that CeVO3 particles in the thin films are nanostructure. The results indicate that an increase in RF power during deposition enhances thin film growth. XRD patterns for the 1:1 V–Ce mixed-oxide target and the resulting thin films are shown in Fig. 1b. The target comprises pure CeVO4 phase according to the six peaks located at 2θ¼18.191, 24.011, 32.451, 34.291, 39.011, and 49.261, which are attributed to the (1 0 1), (2 0 0), (1 1 2), (2 2 0), (3 0 1), and (3 1 2 ) plans of CeVO4 (JCPDS 12–0757). It is evident that all the thin films deposited at RF power of 125–200 W are completely amorphous. This indicates that even high RF power of 200 W during deposition does not assist the formation of crystalline CeVO4 because of the existence of V2O5 on the film surface in a highly dispersed or amorphous state. XRD patterns for the 1:2 V–Ce target and the resulting thin films are shown in Fig. 1 c. The target material comprises pure CeVO4 phase according to the 16 peaks located at 2θ ¼18.2021, 24.0571, 30.1941, 32.4281, 34.241,36.7561, 38.991, 43.431, and 46.341, which are attributed to the (1 0 1), (2 0 0), (2 1 1), (1 1 2), (2 2 0), (2 0 2), (3 0 1), (1 0 3), and (3 2 1) planes, respectively, of CeVO4 (JCPDS No: 82–1969). In this case, all of the
Fig. 2. AFM images of V–Ce mixed-oxide thin films grown at 200 W for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
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thin films are also amorphous in nature, indicating that 200 W is not sufficient to grow crystalline particles due to a lack of oxygen during deposition [18]. 3.2. Morphological studies 3.2.1. AFM analysis AFM images of V–Ce mixed-oxide thin films deposited at low RF power do not reveal any characteristic features and they are amorphous. Two-dimensional AFM images of V–Ce mixed-oxide thin films prepared at 200 W are shown in Fig. 2. Film prepared from a 2:1 V–Ce target shows a smooth surface with small grains oriented in an elongated and orderly manner, revealing uniform growth (Fig. 2a). The average grain size is approximately 18 nm, which confirms that the film is nanostructured. In this case, high RF power led to a crystalline thin film but of CeVO3 instead of CeVO4 due to insufficient oxygen during deposition. Film prepared from a 1:1 V–Ce target shows uniform growth with elongated spherical particles oriented in an orderly fashion on the surface with an average grain size of 63 nm (Fig. 2b). Film prepared from a 1:2 V–Ce target exhibits a flat surface with uniform growth and an average grain size of 55 nm (Fig. 2c). In the latter two cases, high RF power during deposition enhanced film growth but was
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not sufficient for a crystalline thin film owing to deposition in an Ar atmosphere. 3.2.2. FESEM and EDS studies The surface morphology of the thin films was investigated by FESEM. Images for thin films deposited at low RF power exhibit almost flat and smooth surfaces (data not shown), indicating amorphous films with a dense structure. The FESEM image of a thin film prepared at 200 W from a 2:1 V–Ce target in Fig. 3a shows irregular shaped grains of 160–511 nm in size. XRD data revealed that these grains correspond to CeVO3. The average grain size is 385 nm. The FESEM image of a thin film prepared at 200 W from a 1:1 V–Ce target reveals an almost flat, dense and smooth surface, indicating the film is amorphous (Fig. 3b). Deposition in an Ar atmosphere results in thin films with a high density and small grain size. The FESEM image of a thin film prepared at 200 W from a 1:2 V–Ce target reveals a smooth and compact surface with highdensity packing of nanoparticles ofr10 nm in size (Fig. 3c). The results indicate that an increase in RF power during deposition improves the growth of these thin films. Fig. 4a shows the EDS spectrum for a thin film prepared at 200 W from a 2:1 V–Ce target. The line at 0.54 keV (33.36 wt.%) is due to 2p oxygen and the line at 4.94 keV
Fig. 3. FESEM images of V–Ce mixed-oxide thin films grown at 200 W for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
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Fig. 4. EDS results for V–Ce mixed-oxide thin films grown at 200 W for V2O5:CeO2 RF power in the molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
(8.7 wt.%) is the Kα line for vanadium. Peaks corresponding to cerium ions are observed at 0.87, 4.87, 5.28, and 5.71 keV (19.56 wt.%). The remaining EDS peaks are due to components of the glass substrate. Thus, the EDS data confirm the presence of vanadium, cerium, and oxygen ions in this film and its purity. Fig. 4b shows the EDS spectrum for a thin film prepared at 200 W from a 1:1 V–Ce target. The Kα line for vanadium is evident at 4.95 keV (7.66 wt.%) and a line due to 2p oxygen is observed at 0.55 keV (29.68 wt.%). The peaks at 0.88, 4.87, 5.29, and 5.7 keV correspond to cerium ions (21.45 wt.%). The EDS spectrum for a thin film prepared at 200 W from a 1:2 V–Ce target is shown in Fig. 4 c. The line at 0.54 keV is due to 2p oxygen (22.32 wt.%) and that at 4.95 keV is the Kα line for vanadium (5.57 wt.%). Peaks corresponding to cerium ions are observed at 0.86, 4.86, 5.29, and 5.71 keV (28.58 wt.%). 3.3. Optical studies Transmission spectra for the V–Ce mixed-oxide thin films over the wavelength range 300–900 nm are shown in Fig. 5. For films prepared from a 2:1 V–Ce target, the average transmittance in the visible region was 51%, 63.9%, and 67.7% for RF power of 100, 150, and 200 W, respectively (Fig. 5a). The increase in transmission with RF power reflects an improvement in grain growth on the film surface. The transmittance for films prepared from a 1:1 V–Ce target was 75%, 93%, 97.5%, and 98% for RF power of 125, 150, 175, and 200 W, respectively (Fig. 5b). These
mixed-oxide thin films are more transparent than pure V2O5 thin films prepared under similar conditions, which had transmittance values of 78.4%, 80%, 84.1%, and 92.9% [6]. The overall transmission is higher for these thin films (average 90.9%) than for the films prepared from a 2:1 V–Ce target (60.9%) because of the significant role of CeO2 in the films [19]. Thin films prepared from a 1:2 V–Ce target at RF power of 100, 150, and 200 W had transmittance of 75%, 90%, and 91%, respectively, with an average value of 85.3%. These films showed a blue shift in UV cutoff, starting from 330 nm, with increasing RF power. The presence of defective phases in addition to the desired CeVO4 phase in the films is responsible for the shift in fundamental absorption edge to higher energy. The UV cutoff for films prepared from a 1:1 V–Ce target also exhibited a blue shift from 350 to 300 nm. The same result was observed for films prepared from a 1:2 V–Ce target, with a UV cutoff at 300 nm. Experimental data for thin films prepared from targets with different V–Ce molar ratios gave a better fit for (αhν)1/2 versus hν. This suggests an indirect allowed transition. Fig. 6 shows plots of (αhν)1/2 versus hν for V–Ce mixed-oxide thin films. The indirect allowed transition for thin film prepared from a 2:1 V–Ce target was 1.59, 1.75, and 1.94 eV for RF power of 100, 150, and 200 W, respectively (Fig. 6a). The optical bandgap for the mixed-oxide films is low because of the dominance of V2O5, which has a lower bandgap than CeO2. Eg for thin films prepared from a 1:1 target was 1.64, 1.7, 2.2, and 2.34 eV for RF power of 125, 150, 175, and 200 W, respectively (Fig. 6b). For both V2O5 and CeO2, the
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Fig. 5. Transmittance spectra for V–Ce mixed-oxide thin films deposited at different RF powers on glass substrates for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
fundamental absorption edge is due to an indirect allowed transition between 2p electrons in the valance band of oxygen and the conduction band of the metal. For films prepared at 200 W, Eg widens from 1.94 eV for a 2:1 target to 2.34 eV for a 1:1 target because of the greater amount of CeO2, which has a wide bandgap of 3.2 eV [2]. Eg for thin films prepared from a 1:2 target was 2.7, 2.93, and 3.04 eV for RF power of 100, 150, and 200 W, respectively (Fig. 6c). Eg is greater for V–Ce mixed-oxide thin films than for pure V2O5 thin films because of the Burstein–Moss effect (BME), whereby mixing of V2O5 (an n-type semiconductor) into CeO2 shifts the Fermi level inside the conduction band, which results in widening of the energy bandgap. According to the BME, an increase in the Fermi level in the conduction band results in an increase in carrier concentration [20]. High RF power helps to increase this carrier concentration, which in turn is responsible for the increase in Eg. 3.4. PA studies Fig. 7 shows the wavelength-dependent normalized PA signal amplitude for thin films prepared from a 2:1 V–Ce target at different RF powers. The PA signal amplitude increases with the RF power. Cruz-Orea and MendozaÁlvarez investigated the dependence of the latter on laser
intensity for TiO2 thin films grown on Ti [21]. Resonant absorption via interband transitions occurs when the photon energy is greater than the optical bandgap, and the decrease in PA amplitude with respect to photon wavelength can be used to calculate the optical bandgap. The optical bandgap calculated for the samples was 2.19, 2.29, and 2.32 eV for RF power of 100, 150, and 200 W, respectively, which is in good agreement with the UV-VisNIR results. The thermal diffusivity of a thin film can be determined from the slope of a graph of the PA signal phase versus the square root of the modulation frequency in the thermally thick region [22,23]. According to Rosencwaig–Gersho PA theory, the film thickness is less than the thermal diffusion length in thermally thin regions and thus the PA signal strength depends on the thermal property of the medium next to the thin film. In thermally thick regions, the thermal diffusion length is less than the film thickness, so the PA signal depends only on the thermal property of the region of thin film. In this region, thermal diffusivity can be calculated as α¼fcl2, where fc is the characteristic frequency and l is the thickness of the sample. The characteristic frequency can be determined from the region where the PA signal depends on 1/f. Fig. 8 shows the PA phase signal as a function of the square root of the
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Fig. 6. Plots of (αhν)1/2 versus photon energy for mixed-oxide thin films prepared at different RF powers for V2O5:CeO2 molar ratios of (a) 2:1, (b) 1:1, and (c) 1:2.
chopping frequency and the best fit taken over the thermally thick region for calculation of the thermal diffusivity. From these fitting data, the thermal diffusivity was calculated as 1.221 10 8, 1.6911 10 8, and 53.556 10 8 m2/s for RF power of 100, 150, and 200 W, respectively. The increase in thermal diffusivity with RF power shows that phonon conduction improved in the thin films and thus the thermal conductivity increases with the RF power as a result of phonon impacts. The thermal conductivity of a phonon is given by K¼
1 C p vl; 3
where Cp is the specific heat capacity, v is the mean velocity, and l is the mean free path of the phonon [24]. As the mean free path and the number of thermally generated phonons in the lattice plane of the thin films increase with the RF power, the thermal diffusivity of the thin films also increases [25]. PA spectra for thin films prepared from a 1:1 V–Ce target at different RF powers are shown in Fig. 9. These films exhibit a higher PA signal amplitude than films prepared from a 2:1 V–Ce target. This indicates that thermal vibrations induced in the material increase with the concentration of Ce3 þ ions in the target. This may be
Fig. 7. PA spectra for 2:1 V–Ce mixed-oxide thin films grown at different RF powers.
due to the generation of more thermal phonons. The energy bandgap for these thin films was 2.25, 2.3, 2.32, and 2.38 eV for RF power of 125, 150, 175, and 200 W, respectively. The higher Eg for these films compared to
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Fig. 8. PA signal phase versus chopping frequency for 2:1 V–Ce mixed-oxide thin films prepared at (a) 100, (b) 150, and (c) 200 W. The best fit region is shown by the solid line.
Fig. 9. PA spectra for 1:1 V–Ce mixed-oxide thin films grown at different RF powers.
those prepared from a 2:1 V–Ce target is due to the higher CeO2 content in the target material, since CeO2 has a high optical bandgap of 3.2 eV. The Eg values calculated are in
good agreement with the results obtained from UV-VisNIR data. Fig. 10 shows the PA phase signal as a function of the square root of the chopping frequency for thin films prepared from a 1:1 V–Ce target at different RF powers. From the best fit taken over the thermally thick region, the thermal diffusivity calculated for the thin films was 0.2355 10–8, 0.2694 x10–8, 0.4118 10–8, and 1.069 10 8 m2/s for RF power of 125, 150, 175, and 200 W, respectively. The increase in thermal diffusivity with RF power for the thin films is due to increases in the mean free path and thermally generated phonons in the lattice structure on mixing of CeO2 with V2O5. Fig. 11 shows PA spectra for thin films prepared from a 1:2 V–Ce target at different RF powers. The energy bandgap was 2.33, 2.36, and 2.39 eV for RF power of 100, 150, and 200 W, respectively, and these values are in good agreement with the results obtained from UV-Vis-NIR data. The higher Eg compared to films prepared from 2:1 and 1:1 V–Ce targets is due to higher CeO2 content in the target material, since CeO2 has a high optical bandgap of 3.2 eV. Fig. 12 shows the PA phase signal as a function of the square root of the chopping frequency for films deposited at different RF powers. From the best fit taken over the thermally thick
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Fig. 10. PA signal phase versus chopping frequency for 1:1 V–Ce mixed-oxide thin films prepared at (a) 125, (b) 150, (c) 175, and (d) 200 W. The best fit region is shown by the solid line.
in the mean free path and thermally generated phonons in the lattice plane of the thin films with increasing RF power are responsible for the increase in thermal diffusivity. 4. Conclusions
Fig. 11. PA spectra for 1:2 V–Ce mixed-oxide thin films grown at different RF powers.
region, the thermal diffusivity calculated for the thin films was 0.2083 10 8, 0.2121 10 8, and 0.2198 10 8 m2/s for RF power of 100, 150, and 200 W, respectively. Increases
We deposited V–Ce mixed-oxide thin films by RF magnetron sputtering. XRD patterns for films prepared at high RF power from 2:1 V–Ce targets showed peaks corresponding to CeVO3 whereas those prepared from 1:1 and 1:2 V–Ce targets were amorphous in nature. AFM and FESEM images and EDS data confirmed these results. The optical bandgap and transmittance increased with the RF power. The fundamental absorption edge shifted to higher energy due to the presence of defective phases in addition to the desired CeVO4 phase in the thin films. CeO2 has higher transmittance and optical bandgap than V2O5 does, so a higher CeO2 content in the target material increases the transmittance and bandgap of the resulting thin films. The PA signal amplitude increased with the RF power because of an increase in Ce3 þ concentration in the films, which induced thermal vibrations in the material by generating more thermal phonons. The optical bandgap calculated using the normalized PA
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Fig. 12. PA signal phase versus chopping frequency for 1:2 V–Ce mixed-oxide thin films prepared at (a) 100, (b) 150, and (c) 200 W. The best fit region is shown by the solid line.
signal amplitude is in good agreement with results obtained from UV-Vis-NIR data. The thermal diffusivity of the films increased with the RF power due to increases in the mean free path and thermally generated phonons in the lattice plane of the thin films. However, it decreased with increasing CeO2 content in the target. References [1] R.T. Rajendra Kumar, B. Karunagaran, V. Senthil Kumar, Y.L. Jeyachandran, D. Mangalraj, S.K. Narayandass, Mater. Sci. Semicond. Process. 6 (2003) 543–546. [2] E. Masetti, F. Varsano, F. Decker, A. Krasilnikova, Electrochim. Acta 46 (2001) 2085–2090. [3] S.-Y. Zheng, A.M. Andersson-Faldt, B. Stjerna, C.G. Granqvist, Appl. Opt. 32 (1993) 6303–6309. [4] Z. Crnjak Orel, B. Orel, Proc. SPIE, 2255, , 1994, 285. [5] U. Lavrencic Stangar, B. Orel, I. Grabec, B. Ogorevc, K. Kalcher, Sol. Energy Mater. Sol. Cells 31 (1993) 171–185. [6] E. Masetti, F. Varsano, F. Decker, Electrochim. Acta 44 (1999) 3117–3119. [7] F. Varsano, F. Decker, E. Masetti, Ionics 5 (1999) 80–85. [8] U. Opara Krasovec, B. Orel, A. Surca, N. Bukovec, R. Reisfeld, Solid State Ionics 118 (1999) 195–214. [9] Z. Crnjak Orel, I. Musevic, Nanostruct. Mater. 12 (1999) 399–404.
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