VACUUM ANNEALING EFFECT ON THE STRUCTURAL AND OPTICAL PROPERTIES OF ANTIMONY TRIOXIDE THIN FILMS

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1 VACUUM ANNEALING EFFECT ON THE STRUCTURAL AND OPTICAL PROPERTIES OF ANTIMONY TRIOXIDE THIN FILMS N. TIGAU 1, S. CONDURACHE-BOTA 1, R. DRASOVEAN 1, J. CRINGANU 1, R. GAVRILA 1 Dunarea de Jos University of Galati, Faculty of Sciences and Environment, 111 Domneasca Street, 80001, Galati, Romania, s: ntigau@ugal.ro, scondurache@ugal.ro, jcringanu@ugal.ro National Institute for Research and Development in Microtechnology (IMT-Bucharest), 16A Erou Iancu Nicolae Street, Bucharest, Romania, raluca.gavrila@imt.ro Received September 4, 016 Abstract. Antimony trioxide (Sb O 3 ) thin films were deposited onto glass substrates held at 5 C using the thermal vacuum evaporation technique. The films were annealed at 50 C in vacuum after evaporation. The morpho-structural properties of the films were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. The optical properties were investigated for both the as-deposited and annealed film in the wavelength range of nm. The optical constants such as the refractive index, the absorption coefficient and the energy bandgap were determined from the transmission spectra, by using Swanepoel s method. Key word: Sb O 3, thin films, annealing effect, structural properties, optical properties. 1. INTRODUCTION Sb O 3 is an important member of semiconducting V VI-type of compounds because of its large band gap located in the near ultraviolet region. The optical band gap of senarmontite Sb O 3 it was found to be 3.6 ± 0.1 ev as deduced from spectroscopic measurements [1]. Due to all the properties presented above, antimony trioxide has wide applications as transparent conductive material, as effective catalyst, flame-retardant synergist in plastics, paints, adhesives and textile back coatings or as functional filter, as component of optical materials and for visible light photodetectors [ 4]. The authors of the present paper have already analyzed as-deposited and open-air annealed antimony trioxide thin films deposited on glass substrates through the thermal vacuum evaporation technique [5 9]. Instead, this paper deals with the vacuum annealing of such films and with the influence of this type of thermal treatment on the morpho-structural and optical properties of the films. Comparisons are made not only between the annealed and as-deposited films, but also between those annealed in vacuum, as done for this paper and those annealed in open-air, as studied elsewhere [5]. Romanian Journal of Physics 6, 604 (017)

2 Article no. 604 N. Tigau et al.. EXPRIMENTAL The thickness of the films was inferred through the multiple-beam Fizeau fringe method by means of a Linnik interferometric microscope operated with white light [10]. The films resulted to be 54 nm thick. The as-deposited antimony trioxide thin films were annealed in vacuum as follows: 1 st step consisted in a gradual heating with 4.5 deg./minute up till 50 C, the nd step: 50 C held for one hour and 3 rd step: cooling the films back till room temperature (5 C) with the same rate as for heating. The crystallinity of both the as-deposited and the annealed films was examined by means of X-ray diffraction (XRD) with CuK radiation (λ = Å) in the scanning range θ = The surface morphology of Sb O 3 thin films was studied by atomic force microscopy (AFM). The optical transmission spectra of the films were recorded by using a Perkin Elmer Lambda 35 UV-Vis double beam spectrophotometer, operated in air, at normal incidence, for the nm spectral range. 3. RESULTS AND DISCUSSION Figure 1 presents the XRD spectra of both the as-deposited and annealed Sb O 3 thin films. The presence of individual maxima in the XRD patterns proves that the films are polycrystalline. Fig. 1 XRD patterns of the Sb O 3 thin films: (a) as-deposited at 5 C, (b) annealed in vacuum at 50 C. Following the indexation made according to the JCPDS database, it results that the films contain only the cubic phase of Sb O 3. It can be seen that both types of films contain six families of crystalline planes, namely those with the following Miller indices: (), (400), (331), (440), (6) and (551). Since the peak

3 3 Vacuum annealing effect on the properties of Sb O 3 films Article no. 604 appearing at = 7.68, given by the () planes is much higher than the other XRD peaks, this proves that the films are texturized, having the () direction as preferential orientation. After the thermal treatment, all the XRD peaks get higher, especially the one given by the () planes. This fact indicates crystallinity and texturizing improvement following the annealing process [10]. The lattice parameter, a, for cubic phase of Sb O 3 thin films was calculated according to: 1/ a ( h k ) (1) sin where λ is the wavelength of the XRD radiation, h, k and l are the Miller indices and is the diffraction angle corresponding to a certain crystalline plane. It was found that the lattice parameter decreased from Å to Å after annealing. The obtained values of the lattice parameter are consistent with the standard value for the Sb O 3 cubic phase (a = Å according to the JCPDS ). The grain size of the Sb O 3 crystallites was determined from the XRD patterns by using the Debye-Scherrer formula [11]: 0.9 D, () cos where D is the average diameter of the crystallites within the film, λ is the wavelength of the XRD radiation, β is the peak width at half maximum (FWHM), expressed in radians and θ is the Bragg diffraction angle. The calculated grain size for the as-deposited film is approximately nm, which increases to nm after the 50 C annealing. This is a similar behavior with that of the same type of films annealed in open atmosphere [5]. This was not the case when open-air annealing was performed to similar Sb O 3 thin films, when grains grew bigger, but their uniformity didn t improve visibly after the thermal treatment [5]. The increase of grain size after the annealing treatment could be attributed to the improvement in the mobility of the surface ad-atoms and to an increase in the cluster formation leading to the agglomeration of small grains, followed by their coalesce, leading to the formation of larger grains with improved crystallinity [1]. Figures (a) and (b) show the 5 µm 5 µm AFM images of the antimony trioxide films as-deposited, while Figures (c) and (d) exhibit the AFM images of the same films after their vacuum annealing. The surface of the as-deposited film is characterized by high-density columnar morphology. The as-deposited films are continuous, without any voids or cracks on the surface. The post-deposition thermal treatment clearly changes the film morphology, as proved by the AFM images. Thus, Figs. (c) and (d) demonstrate that following the annealing, the larger crystallites get even bigger,

4 Article no. 604 N. Tigau et al. 4 while the smaller crystallites shrink. This fact is in agreement with the growth mechanism of polycrystalline semiconducting film [10]. Since the Sb O 3 thin films under study exhibit a texturized surface, this recommends them for solar cell applications, where light trapping may be induced and thus, the reflection of photons incident on the film surface may be significantly inhibited [13]. Fig. D and 3D-AFM images of the Sb O 3 thin films: (a) and (b) as-deposited at 5 C; (c) and (d) annealed in vacuum at 50 C. After annealing, the average roughness R a increases from 4.13 to 17.8 and the root mean square roughness R rms increases from 6.34 to.1 nm (Table 1). This is a significantly different behavior of the thermal treatment in vacuum of antimony trioxide thin films as compared to open-air annealing, since in the latter case, a significant decrease of the root mean square roughness happened [5]. The increase of surface roughness following the vacuum annealing could be due to the increase in grain size and to their more closer packing, as it was proved by XRD analysis, which induced a reduction of grain boundaries [14]. Thus, the AFM analysis proves that vacuum annealing changed significantly the morphology of the films.

5 5 Vacuum annealing effect on the properties of Sb O 3 films Article no. 604 Table 1 Morpho-structural and optical parameters of the Sb O 3 thin films Sb O 3 thin films t [ o C] D [nm] R a [nm] R rms [nm] S 0 [10 1 m - ] λ 0 [nm] as-deposited annealed in vacuum The optical studies were carried out on both for the as-deposited and the annealed Sb O 3 thin films, by using transmittance and reflectance versus wavelength measurements. The resulting transmission and reflection spectra are shown in Figure 3. Fig. 3 Transmission and reflection spectra of the as-deposited and annealed in vacuum Sb O 3 thin films. The presence of the interference fringes indicates that the films are uniform, with smooth surfaces. As a consequence of the thermal treatment, it can be seen from Fig. 3 that the optical transmittance increases, reaching as high as 75 % in the near-infrared region. The higher transmittance observed for the annealed films as compared to the as-deposited films can be attributed to the crystallinity improvement and subsequent less scattering effects, as well as to lower defect density near the band edge [10, 15]. A fast decrease in the transmittance at lower wavelengths results from the excitation of charge carriers across the optical band gap [16]. Swanepoel s algorithm [17, 18] was applied in order to infer the most important optical constants of the films, namely the refraction index, n, the extinction coefficient, k, and the absorption coefficient, α.

6 Article no. 604 N. Tigau et al. 6 In the region of weak and medium absorption, the transmittance decreases mainly due to the effect of absorption coefficient. In this case, the refractive index of the films can be calculated by using the next equation [17, 18]: where n s n N N, (3) ns 1 ns ( TM T N m), (4) TM Tm where n s is the refractive index of the glass substrate at a certain wavelength, while T M and T m are the transmission maximum and minimum of envelope functions, respectively, at the same wavelength. The refractive index of glass substrate, n s was found to be nearly frequency independent and equal to In the strong absorption region, where the interference fringes disappear, the dependence of refractive index, n as a function of wavelength, λ, can be fitted with Sellmeier s dispersion equation [19 1]: S n, (5) [ ] where S 0 is the average oscillator strength, λ 0 is the average oscillator wavelength and λ is the wavelength of incident radiation. In order to apply this model, we assumed that the material is composed of individual dipole oscillators, which are set to forced vibrations by the incident photons [19]. The resulting values of the dispersion parameters (S 0 and λ 0 ) are presented in the Table 1 given above. The spectral dependences of the refractive index of the as-deposited and annealed Sb O 3 thin films are shown in Figure 4. The values of the refractive index are decreasing with increasing wavelength, proving a normal dispersion behavior of the studied films. Since there is a good fit between Sellmeier s model and the experimental data, it results that the single-oscillator model is adequate to describe the dispersion behavior of this material in the investigated spectral range. It can be noticed from Fig. 4 that the refractive index of the analyzed films increases after annealing, which may be attributed to the increase in surface roughness []. The extinction coefficient, k can be calculated by using the next formula [17, 18]: 0 1 k ln, (6) 4 d x where d is the thickness of the film and x is the absorbance.

7 7 Vacuum annealing effect on the properties of Sb O 3 films Article no. 604 In what the extinction coefficient is concerned it was found that its average value in the visible and near infrared region is found to be approximately Such low values represent a qualitative indication of the excellent surface smoothness of the analyzed films [19]. The fact that the extinction coefficient decreases after annealing proves that the fraction of light lost due to scattering and to absorbance processes decreases after annealing, as related to crystallinity improvement [3]. Fig. 4 The dispersion curves for the refractive index of both the as-deposited and the annealed in vacuum Sb O 3 thin films. In the region of weak and medium absorption the absorbance, x, can be calculated by [17, 18]: where [17, 5]: 3 4 1/ EM [ EM ( n 1) ( n ns )] x (7) 3 ( n 1) ( n ns ) 8n n E s M ( n 1)( n ns ) (8) TM In the strong absorption region, where the interference maxima and minima converge to a single curve T 0, the absorbance, x, is given by [17]: 3 ( n 1) ( n ns ) 16nns x T. (9) The absorption coefficient, α, was calculated by means of [17, 18]: 0

8 Article no. 604 N. Tigau et al. 8 4 k. (10) Another consequence of the vacuum annealing is the shift towards shorter wavelengths of the position of the fundamental absorption edge, corresponding to the electron excitation from the valence band to the conduction band, which can be used to determine the nature and the value of the optical band gap. In order to study the type of optical transitions happening within the films, as well as to assess the value of optical energy band gap, Tauc s equation was used [4]: r ( g h ) A( h E ) (11) where A is a parameter that depends on the transition probability, E g is the energy band gap of the material, while the exponent r depends on the type of electronic transition. Thus, r = 1/ for indirect allowed transitions, while r = for direct allowed transitions. A good fit between the experimental data and ( h ) versus h plots, shown in Fig. 5, indicates that the direct allowed transitions represent the mechanism responsible for the optical absorption in the Sb O 3 thin films under study. The direct optical band gap energy can be determined from the intercept of the extrapolated linear part of ( h ) curve with the h axis [5]. Fig. 5 Plots of (αhυ) vs. hυ of the as-deposited and the annealed in vacuum Sb O 3 thin films. It can be easily seen from Fig. 5 that annealing increases the optical band gap energy of the studied films from 3.58 ev to 3.73 ev. The values of the energy band gap for both the as-deposited and vacuum-annealed antimony trioxide thin films are in good agreement with those found in the literature [1, 6] and correspond to

9 9 Vacuum annealing effect on the properties of Sb O 3 films Article no. 604 the fact that the optical band gap energy of a certain thin film is affected by its crystallinity changes, following the thermal treatment [1]. 4. CONCLUSIONS Antimony trioxide (Sb O 3 ) thin films were deposited through vacuum thermal evaporation on glass substrates kept at room temperature (5 C) and then they were annealed in vacuum at 50 C, for one hour. Morpho-structural and optical analysis of both the as-deposited and vacuum-annealed films were performed and relevant parameters were inferred and compared between the two types of studied films, but also with similar films either annealed in open air or deposited at higher substrate temperatures. The X-ray diffraction analysis showed that both the as-deposited and the annealed Sb O 3 thin films are polycrystalline and have a cubic structure. The grain size of the as-deposited films, calculated from XRD patterns, increases from nm to nm after annealing. The AFM micrographs revealed that the surfaces of the films are smooth and pinhole free. The annealing in vacuum improved the crystallinity and increases the surface roughness of films, as well as the optical transmittance in the visible and near infrared regions. It was found that the dispersion of the refractive index obeys the single oscillator model. The optical band gap energy, determined from the variation of the absorption coefficient with wavelength, increases from 3.58 ev for as-deposited films to 3.73 ev for the vacuum-annealed Sb O 3 thin films. REFERENCES 1. M. Yasir, M. Kuzmin, M.P.J. Punkkinen, J. Makela, M. Tuominen, J. Dahl, P. Laukkanen, K. Kokko, Applied Surface Science 349, (015).. Y. Hu, H. Zhang, H. Yang, J. Alloys and Compounds 48, (007). 3. L. Li, Y. X. Zhang, X. S. Fang, T. Y. Zhai, M. Y. Liao, H. Q. Wang, G. H. Li, Y. Koide, Y. Bando, D. Golberg, Nanotechnology (16), (011). 4. Y. Zhang, G. Li, J. Zhang, L. Zhang, Nanotechnology 15(7), 76 (004). 5. N. Tigau, V. Ciupina, G. Prodan, G. I. Rusu, C. Gheorghies, E. Vasile, J. Optoelectron. Adv. Mater., 5(4), (003). 6. N. Tigau, V. Ciupina, G. Prodan, G.I.Rusu, E. Vasile, J. Cryst. Growth 69, (004). 7. N. Tigau, V. Ciupina, G. Prodan, J. Cryst. Growth 7, (005). 8. N. Tigau, Cryst. Res. Technol. 41(11), (006). 9. N. Tigau, V. Ciupina, G. Prodan, J. Optoelectron. Adv. Mater. 8(1), 37 4 (006). 10. K. L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, USA (1969). 11. E. Lifshin, X-ray Characterization of Materials, Wiley, New York, USA, (1999). 1. P. Prathap, G. Gowri Devi, Y.P.V. Subbaiah, K.T.Ramakrishna Reddy, V. Ganesan, Current Applied Physics 8(), (008).

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