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Properties of amorphous SnO2 films prepared by thermal evaporation

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  • Save International Journal of M aterials and Chemistry 2012, 2(4): 173-177 DOI: 10.5923/j.ijmc.20120204.10 Properties of Amorphous SnO2 Thin Films, Prepared by Thermal Evaporation Shadia J. Ikhmayies Al Isra University, Faculty of Information Technology, Department of Basic Sciences-Physics, Amman, 16197, Jordan Abstract Tin o xide (SnO2) thin films of thickness in the range 100-600 n m are prepared on glass substrates by thermal evaporation at ambient temperatures. The films are characterized by recording their transmittance measurements, X-ray diffraction (XRD) patterns, scanning electron microscope (SEM ) images and energy dispersion X-ray analysis (EDAX). It is found that the films have high t ransmittance and non-sharp absorption edge. XRD d iffractograms showed that the films are amorphous and the SEM micrographs depicted that the surfaces are smooth, uniform and well covered with the material. The EDAX analysis showed that the films are deficient in o xygen. Indirect optical bandgap energy is determined and Urbach tailing in the bandgap is observed and the width of the tail which is related with disorder and localized states is estimated. Keywords Transparent Conducting Oxides (TCOS), A morphous Tin Oxide Thin Films, Optical Properties, Urbach Tail, Gas Sensors 1. Introduction SnO2is an n-type wide bandgap semiconducting material with d irect bandgap energy of 4 eV and indirect bandgap of 2.6 eV[1], where inherent oxygen vacancies act as an n-type dopant[2]. Tin o xide thin films have been used for transparent electrodes in photoelectric conversion devices name ly a morphous silicon solar ce lls, liquid c rystal display, gas discharge display, etc.[3]. Perfect ly amorphous thin SnO2 films show good electrical response to reducing gases in air and could hence be applied to construct semiconductor gas sensors where the metal o xide films function as a monograin-eq ivalent active layer[4]. A morphous SnO2 can also be used in extended gate field-effect transistors (EGFET) as pH sensor, where the sensitive part is made of SnO2/Al structure. This device can be used to detect and to quantify any kind of substances that can produce or consume protons like an enzy mat ic reaction, therefo re showing a large range of applications as biosensors[5]. There are different methods to prepare SnO2 films such as, ato mic lay er depos it io n (A LD)[6], react ive magn et ron sputtering [7], ch emical vapo r d epos it ion ([8],[9]), d ip co at in g [10], s p ray p y ro lys is (SP) ([3],[ 11- 14]) , an d evaporation[8]. Thermal evaporation method was choose to produce SnO2 thin films in th is work because the deposition by t h is techn iqu e does not present any co mp os it ion al prob lems , b eside th e fact th at it is a lo w t emperatu re * Corresponding author: (Shadia J. Ikhmayies) Published online at Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved producing amorphous films. To increase this probability the technique, which increases the probability of films were deposited at ambient temperature. In this work the properties of amorphous SnO2 thin films are investigated by studying their structure, morphology, composition and transmittance at room temperature. The films were characterized by X-ray d iffraction (XRD), scanning electron microscope observations, energy dispersive analysis by X-ray (EDAX), and transmittance measurements. The indirect bandgap of the films and Urbach tailing were estimated and discussed. 2. Experimental Part Undoped SnO2 thin films are deposited by thermal evaporation at amb ient temperature on glass substrates of dimensions (6×2.6×0.1cm3) in a h igh vacuum system (~10-5 mbar) provided with a Turboe pump. The substrates are cleaned in methanol, followed by d istilled water. The evaporation rate is about 10 Å/s and it is measured by a cooled quartz crystal monitor. The distance between the source and the substrate is about 30 cm. Films of thickness 50-600 n m are produced through different indiv idual evaporations. The films showed light brown colour that becomes darker with film thickness. The transmittance of the films is measured by using a double beam Shimad zu UV 1601 (PC) spectrophotometer with respect to a piece of glass simila r to the substrates in the wavelength range 300-1100 n m. The thickness of the films is estimated fro m the interference maxima and min ima in the transmittance by using the method of Alvin [15]. X-ray 174 Shadia J. Ikhmayies: Properties of Amorphous SnO2 Thin Films, Prepared by Thermal Evaporation measurements are made with a Philips PW1840 Co mpact X-ray diffracto meter system with Cu Kα (λ = 1.5405 Å). The SEM images are taken by a FEI scanning electron microscope (Inspect F 50), wh ich is supplied by energy dispersive analysis by X-rays (EDAX), so the compositional analysis of the films is performed by the same system. 3. Results and Discussion Fig.1 d isplays the X-ray diffractograms for two Sn O2 thin films of different thickness 200 and 600 n m, deposited on glass substrates at ambient temperature. Both diffractograms showed a broad hump including the positions of the (1 1 0) and (1 0 1) lines, indicating that the films have amorphous structure. It is noticed that the peak of the hump for the thicker film is centred at about 2θ = 29°, while that of the thinner one is centred at about 2θ = 28°. Beside this the width (a) of the hump of the thinner film is larger than that of the thicker one, and hence the full widths at half maximu m FWHM are 4.5 and 5.0° for the thicker and thinner films respectively. Fro m these results we conclude that the thicker film is closer to the transformation fro m the amo rphous phase to the polycrystalline phase than the thinner one. (b) Fi gure 1. X-ray diffract ograms of t wo thermally evaporat ed SnO2 thin films with thickness a) 200 nm. b) 600 nm Fig.2 shows the SEM micrographs at two magnification values (at 80000X and 160000X) of two films of different thickness 300 n m (Fig.2a and b) and 400 n m (Fig.2c and d). The SEM images show smooth, featureless surfaces in agreement with the amorphous structures, observed by XRD patterns. No evidence of granularity as would be seen for a crystalline film. (c) International Journal of M aterials and Chemistry 2012, 2(4): 173-177 175 for respective transitions, hν denotes photon's energy, E opt g is the optical energy gap and m is the number which characterizes the transition process, where m = 2 for most amorphous semiconductors (indirect transition) and m = 1/ 2 for most of crystalline semiconductors (direct transition). (d) Figure 2. SEM micrographs at two magnifications of two thermally evaporated SnO2 thin films of thickness 300 nm (a) and (b); and 400 nm (c) and (d) Fig.3 depicts the EDAX spectra for the two films of thickness 300 and 400 n m with the SEM photos to show the positions where the co mpositional analysis was performed. The elemental concentrations of tin and oxygen are summarized in table 1. Table 1. Results of the EDAX compositional analysis obtained for two thermally evaporated SnO2 thin films with different thickness t (nm) Concentration of O Concentration of Sn wt .% wt .% 300 62.96±22.2 37.04±14.1 (a) 400 55.81±18.5 44.19±15.1 As seen from the data presented in Table 1, the films are deficient in o xygen, where the rat io of o xygen to tin is 1.7 and 1.26 in the thinner and thicker films respectively, wh ile the stoichiometric ratio is 2. This means that there are inherent oxygen vacancies and hence the films are n-type . Fig.4 shows the transmittance of the films with different thickness measured at room temperature. The transmittance is high in the visible and infra-red regions and reaches about 100% there for most of the films in the set. Interference fringes which are indication on the uniformity o f the films are used to estimate film th ickness as mentioned before. The observed broadening of the absorption edge towards longer wavelengths is due to the disorder effect, wh ich is always seen in amo rphous films. It is known that, in the vicinity of the fundamental absorption edge, for allowed direct band-to-band transitions, neglecting exciton effects, the absorption coefficient is described by[16]; where α α (hν ) = K (hν − E opt g ) m hν is the absorption coefficient, (1) h is Planck's constant, ν is the frequency of the radiation, K is the characteristic parameter (independent of photon's energy) (b) Figure 3. EDAX patterns for amorphous SnO2 thin films and accompanied SEM images to show the point at which the compositional analysis is performed. a) Film thickness =300 nm. b) Film thickness =400 nm 176 Shadia J. Ikhmayies: Properties of Amorphous SnO2 Thin Films, Prepared by Thermal Evaporation 100 80 T% 60 t = 100 nm 40 t = 200 nm t = 300 nm t = 400 nm 20 t = 500 nm t = 600 nm 0 400 600 800 1000 hν(eV) Figure 4. Transmittance of thermally evaporated SnO2 thin films of different thickness against photon's energy α = α0 exp(hν / Ee ) (2) where α 0 is a constant and Ee is the width of the Urbach tail. The exponential tail appears because disordered and amorphous materia ls produce localized states extended in the bandgap. The absorption edge of these materials at α less than 104 cm-1, emp irically fo llo ws the Urbach law, and this is interpreted as evidence for the existence of localized states. But Urbach law can be applicable to α larger than 104 cm-1[20]. To find the width of Urbach tail Ee a plot of ln(α ) against the photon's energy is shown in Fig.6. Linear fits were performed and the values of Ee and α 0 for the whole set of curves were estimated and inserted in table 2. 13 Fig.5 depicts the relation between (αhν )1/ 2 against the photon's energy. A linear fit was performed for each curve and the indirect optical bandgap energy Ein was estimated fro m the intercept with the energy axis with neglecting phonon's energy. The deduced values are inserted in table 2. As the table shows, the indirect optical bandgap energy Ein decreases with film thickness. These values are close to the values that we obtained in a previous work[17] for partially polycrystalline SnO2 thin films prepared by thermal evaporation. On the other hand, these values are smalle r than the values given in our previous work[11] for polycrystalline SnO2:F thin films prepared by the spray pyrolysis technique, the values obtained by Díaz-Flores et. al.[1], the values given by Mohammad and Abdul-Ghafor[18], and those given by Mohammad[19] for spray-deposited SnO2:F thin films. The reason of this difference is that the optical bandgap increases with doping and our films in this work are not doped. 12 ln(α) 11 t= 100 nm t= 200 nm t= 300 nm t= 400 nm 10 t= 500 nm t= 600 nm 9 2.4 2.6 2.8 3.0 3.2 3.4 3.6 hν(eV) Figure 6. The plot of ln(α ) against the photon's energy hν with the linear fits for amorphous, thermally evaporated SnO2 thin films of different thickness (αhν)1/2( 1000 t= 100 nm 800 t= 200 nm t= 300 nm t= 400 nm 600 t= 500 nm t= 600 nm 400 Table 2. The values of the indirect optical bandgap energy Ein, the width of Urbuch tail Ee and the constant α0 deduced from the linear fits in Figs.4 and 5 respect ively beside values of film thickness t (nm) 100 200 300 400 500 600 α0(cm-1) 1.13×103 2.94×102 3.63×102 2.06×102 2.50×102 3.19×102 Ee(meV) 659±2.0 531±1.6 548±1.8 494±0.5 510±1.0 524±1.9 Ein(eV) 2.21±0.008 2.09±0.008 2.02±0.012 2.06±0.015 2.03±0.016 1.98±0.013 200 0 2.0 2.4 2.8 3.2 3.6 hν(eV) Figure 5. The plot of (αhν )1/ 2 against the photon's energy hν wit h the linear fit s for amorphous, t hermally evaporat ed SnO2 thin films of different thickness It is assumed that in the low photon energy range the spectral dependence of the absorption edge follows the emp irical Urbach rule g iven by[20] Co mparing these values with the values that we obtained in ref.[17] for part ially polycrystalline SnO2 thin films prepared by vacuum evaporation, it is found that they are comparable. A lso these values are in good agreement with the values obtained by Melsheimer and Zieg ler[21] for undoped amorphous SnO2 thin films prepared by the spray pyrolysis technique which were prepared at deposition temperatures 340 and 350℃. On the other hand these values are larger than the values that we[11] obtained for SnO2:F thin films prepared by the spray pyrolysis technique and the values obtained by Melsheimer and Zieg ler[21] for undoped SnO2 films wh ich were also prepared by the spray pyrolysis technique at 360-410 ℃ and they are partially International Journal of M aterials and Chemistry 2012, 2(4): 173-177 177 polycrystalline. Fo r these films they[21] got values in the range 350-460 meV. These results confirm that the width of the Urbach tail increases with disorder. 4. Conclusions Amorphous SnO2 thin films of thickness in the range 100-600 n m were prepared by thermal evaporation at amb ient temperature. X-ray diffractograms showed a wide hump which confirms the amorphous structure of the films. SEM micrographs showed smooth surfaces without features in agreement with the amorphous structures observed by XRD patterns. EDA X analysis revealed that the films are deficient in o xygen which means that they are n-type. The transmittance of the films is very high in the visib le and infrared reg ions. The indirect bandgap and Urbach tail were estimated and related to film thickness. The deduced values are co mpared with those obtained by different authors. ACKNOWLEDGEMENTS I would like to thank Sameer Farrash fro m the physics department in the University of Jordan for producing the films by thermal evaporation. Also I want to thank Waddah Fares Mahmoud and Azzam Karadsheh from the department of geology in the University of Jordan fo r taking the SEM micrographs with EDA X analysis and for recording XRD diffractograms, respectively. REFERENCES [1] L. L. Díaz-Flores, R. Ramírez-Bon, A. M endoza-Galván, E. Prokhorov, J. González-Hernández, " Impedance spectroscopy studies on SnO2 films prepared by the sol–gel process", Elsevier, J. Phys. Chem. Solids, Vol.64, pp.1037-1042, 2003. [2] Z. Chen, J. K. L. 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