Polycrystalline SnO2 films grown by chemical vapor deposition on quartz glass

Polycrystalline SnO2 films grown by chemical vapor deposition on quartz glass

Vacuum xxx (2015) 1e6 Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum Polycrystalline SnO2 films g...

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Vacuum xxx (2015) 1e6

Contents lists available at ScienceDirect

Vacuum journal homepage: www.elsevier.com/locate/vacuum

Polycrystalline SnO2 films grown by chemical vapor deposition on quartz glass Y.M. Lu a, b, J. Jiang a, M. Becker a, B. Kramm a, L. Chen a, A. Polity a, Y.B. He b, *, P.J. Klar a, B.K. Meyer a a

I. Physics Institute, Justus-Liebig University of Giessen, Heinrich-Buff-Ring 16, D-35392, Giessen, Germany Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Ministry-of-Education Key Laboratory of Green Preparation and Application for Functional Materials, Faculty of Materials Science & Engineering, Hubei University, Wuhan, 430062, China


a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 October 2014 Received in revised form 4 March 2015 Accepted 19 March 2015 Available online xxx

Tin dioxide (SnO2) thin films were deposited on quartz glass substrates by chemical vapor deposition using SnI2 and O2 as reactants. The growth experiments were carried out in the substrate temperature range of 300e900  C. X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectroscopy, spectrophotometry and Raman spectroscopy were used to characterize the films. The films were polycrystalline with their crystallites having a preferred orientation, which was dependent on the film thickness. The average grain size increased with increasing thickness of the films. The binding energies of Sn 3d5/2 and O 1s for all samples showed the Sn4þ and O-Sn4þ bonding state from SnO2. The absolute average transmittance of SnO2 films exceeded 90% in the visible and infrared range. The obtained SnO2 films had optical band gaps between 3.78 and 3.92 eV. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Tin oxide Thin films Chemical vapor deposition Thickness dependence

1. Introduction Tin oxide (SnO2) with intrinsic n-type conductivity caused by oxygen vacancies and tin interstitials has been widely studied due to its wide band-gap (Eg ¼ 3.6 eV), high optical transparency in the visible light range, and good chemical stability, which lead to many practical applications [1]. SnO2 also has advantages such as low costs, abundance of the elements Sn and O and low toxicity of the compound. It is widely employed in gas sensors [2e4], solar cells [5,6], transparent electrodes for flat panel displays [7], architectural window coatings [8], etc. There are several physical and chemical deposition methods to produce SnO2 thin films, including atomic layer deposition [9], spray pyrolysis [10,11], pulsed laser deposition (PLD) [12,13], plasma electrolytic oxidation [14], molecular beam epitaxy (MBE) [15], ion beam sputtering [16], chemical vapor deposition (CVD) [17e19], etc. Many of these techniques suffer from one or more drawbacks such as low deposition rate, prolonged post processing (e.g. annealing) time, and expensive targets, precursors and apparatus. Chemical vapor deposition is an

* Corresponding author. Tel./fax: þ86 27 88661803. E-mail addresses: [email protected], [email protected] (Y.B. He).

inexpensive and versatile process capable of producing films of high quality, therefore suitable for commercial exploitation. Several studies have used the CVD process to produce SnO2 thin films using SnI4 or SnCl4 (Sn4þ) as the Sn precursor [19,20]. In this work we have attempted to use SnI2 (Sn2þ) as the Sn precursor, aiming at achieving SnO in addition to SnO2 films by CVD. This paper reports on the structure, surface morphology, stoichiometry and optical properties of the SnO2 thin films fabricated on quartz glass substrates. 2. Experimental details A series of SnO2 thin films were grown by CVD on quartz glass. The quartz glass substrates were cleaned with acetone, methanol and pure water consecutively in an ultrasonic bath, each for 15 min, and then blown dry with nitrogen gas (99.999%). A specially designed vertical CVD system was used to deposit the films [21]. It consisted of a five temperature zones furnace which was completely equipped with quartz tubes. Prior to film deposition, the chamber was evacuated to 0.1 mbar. Powder tin (II) iodide (SnI2, 99 þ %, from Alfa Aesar Company) was used as tin precursor for the growth of SnO2. SnI2 evaporation rate was controlled in the range of 0.005e0.7 g/h through regulating the temperature of the SnI2 reservoir in the range of 270e410  C. The SnI2 vapor was

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transported towards the reaction zone by argon carrier gas (99.999%) flow in a separate quartz tube. The substrates were placed on a sample holder made by quartz glass which is approximately 5 cm apart from the orifice of that separate tube. The sample holder was rotating at about 15 rpm during the deposition to improve the film homogeneity. Molecular oxygen gas (99.999%) was used as the oxygen precursor, and the O2 gas flow was varied from 10 to 200 sccm for the films deposition. Both Ar carrier gas flow and O2 reactive gas flow were controlled by mass flow controllers. The pressure in the chamber was kept at 40 mbar during the deposition. In order to achieve a constant temperature of the deposition zone, the whole chamber was heated for about one hour before deposition. The films were deposited at a substrate temperature between 300 and 900  C for 0.5e7.2 h with thicknesses of 100 nm e 8 mm. The crystalline phases and the preferred orientation of the films were studied by X-ray diffraction (XRD) using a Siemens D5000 diffractometer with Cu Ka (l ¼ 0.15418 nm) radiation. The surface morphology and cross-sectional structure of the films were investigated with field-emission scanning electron microscopy (FESEM JSM 7001F, JEOL) at 5 kV. X-ray photoelectron spectroscopy (XPS) was used to determine the composition stoichiometry of the films with a SPECS PHOIBOS 150 system at photon energy of 1486.6 eV (Al Ka radiation). Optical transmittance was measured at room temperature by a PE Lambda 900 spectrometer (Perkin Elmer) in the wavelength range of 200e3000 nm. The film thickness was estimated based on interference oscillations of the optical transmittance spectrum. Raman spectra were recorded in the backscattering geometry at room temperature with a Renishaw in Via micro-Raman microscope. A linearly polarized laser with a wavelength of 532 nm was used for excitation and focused onto the sample surface with a 50 objective.

Harsta et al. They used SnI4 as Sn precursor and attributed the last declined deposition rate to a depletion of the amount of tincontaining species reaching the reaction zone [22]. In fact, in addition to the substrate temperature, there are many experimental factors which may affect the growth rate of SnO2 films in our study, for instance, the oxygen gas flow rate, the evaporation rate of SnI2 (Tsai et al. studied in detail the relationship between variation of the growth rate and the evaporation rate of the Sn precursors [23]), the distance from the SnI2 tube orifice to the substrate, the position of the substrate on the substrate holder, etc. When the substrate temperature was lower than 350  C, no SnO2 films could be obtained. In other words, the reaction SnI2 þ O2 / SnO2 þ I2 was suppressed at temperatures below 350  C. When the oxygen gas flow was low, the reaction to form SnO2 was slow, but no SnO could be observed in this study. The Gibbs free energy of formation was calculated for different experimental conditions (not shown). The formation energy of SnO2 was clearly lower than that of SnO, so all films produced in this study showed a pure SnO2 phase without any SnO precipitates. Using other O-precursor that works via a ligand-exchange mechanism instead of involving oxidation reactions or applying a non-equilibrium growth technique might be solutions to grow SnO. Actually, typical nonequilibrium physical vapor deposition techniques such as sputtering and PLD had already been used to prepare SnO films successfully [24e26]. To produce SnO2 films with a relatively high quality at a sufficient growth rate, the substrate temperature was set between 500 and 700  C while keeping the SnI2 evaporation rate of 0.2 g/h in the following study. Almost all deposited films were colorless and transparent, while only films thicker than 1.5 mm showed a whitish milky appearance caused by light scattering from the rough surface. 3.2. Structure and morphology of SnO2 films

3. Results and discussion 3.1. Deposition rate Fig. 1 shows the relationship between the growth rate of SnO2 films and the substrate temperature. All these films were deposited under the same conditions (oxygen gas flow of 20 sccm and SnI2 evaporation rate of 0.02 g/h) but different substrate temperatures. As can be seen in Fig. 1, the growth rate increased almost linearly from 24 nm/h at 450  C to 243 nm/h at 650  C. But the growth rate decreased to 207 nm/h at 700  C, which was also observed by

Fig. 1. Dependence of the growth rate on the substrate temperature.

XRD measurements revealed that all films obtained in this work consisted of pure rutile SnO2 phase without any secondary phases such as SnO and Sn3O4. In Fig. 2, XRD spectra of two typical samples are presented, together with the standard powder diffraction spectrum of SnO2 (ICDD PDF #41e1445) for comparison. The film in Fig. 2(a) was deposited at a substrate temperature of 650  C for 2 h with a thickness of 410 nm. As can be seen, all diffraction peaks of the film can be identified as reflections from crystal planes of SnO2 with a tetragonal rutile structure, and the (110) and (101) reflections are dominant in the spectrum, well according to the standard diffraction data of SnO2 powder. This observation is somewhat expected as the (110) plane represents the lowest energy face in SnO2 single crystal [27]. As reported in ref. [27], the {110} faces make up the largest fraction of the surface area followed by the {101} faces and small {100} faces in the single-crystal SnO2. For films thicker than 1 mm, the XRD spectra changed significantly. As shown in Fig. 2(b), the film deposited for 6.5 h with a thickness of 1780 nm exhibited the strongest reflection from (211) instead of (110), indicating a (211) preferred orientation of the film. Other researchers also noticed variation of the preferred orientation of SnO2 films prepared on glass substrates by XRD [17]. They attributed the orientation changes mainly to an effect of the substrate temperature. In this work, we found that the film thickness actually played a key role in the orientation change of the resulting films. At the beginning of growth (the film was thin) the film surface preferred to assume the lowest energy face of (110). As the growth continued, the crystallite size increased. When the film thickness exceeded a critical value, the (110) orientation was no longer preferred, but rather other orientations of (211) and (301) became more pronounced, as revealed by the XRD. The work of Viirola et al. demonstrated in detail a similar orientation evolution of SnO2 films

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Fig. 2. XRD spectra of SnO2 films deposited at a substrate temperature of 650  C for (a) 2 h with thickness of 410 nm, and (b) 6.5 h with thickness of 1780 nm. The red lines correspond to standard powder diffraction spectrum of SnO2 (ICDD PDF #41e1445). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

on glass from (110)- to (211)-dominated upon increasing of the films thickness [28]. Similar observations were reported as well by Kim and Utriainen et al. [18,29]. Fig. 3 shows the plan-view (a, c) and cross-sectional (b, d) SEM images of two samples, demonstrating the typical surface morphology and microstructure of SnO2 films deposited in this


study. The first sample corresponding to Fig. 3(a,b) was grown at a substrate temperature of 550  C for 3 h with a thickness of 870 nm. The stone-shaped grains had grain sizes in the range of 100e500 nm. The second sample corresponding to Fig. 3(c,d) was deposited at a substrate temperature of 650  C for 6.5 h yielding a thickness of 1780 nm. It can be seen that most grains in the second sample had an arrow shape with a length over 500 nm Fig. 3(c). The arrow-shaped grains cracked further on the surface yielding a subtle grain structure. The “cracks” in the grains are most likely a result of twinning, i.e., formation of stacking faults in the crystal lattice of the grains. Both cross-sectional images show columnar grains of SnO2 grown directly on the quartz glass substrates. The grains at the bottom were well compacted in both samples. However, the grains at the top became bigger and there were gaps between grains in Fig. 3(d). These significant changes in grain shape and grain size could be attributed to the enhanced atomic diffusion and migration during the prolonged deposition at elevated temperature, in which crystalline grains grow along low stress directions, resulting in a faceted surface with larger roughness [30]. After comparing and analyzing about 30 samples, we found that the morphology of the SnO2 films was strongly influenced by the film thickness and the growth temperature. The films thinner than 1 mm had smaller grain sizes and showed a uniform and smooth surface and a dense structure (cf. Fig. 3a,b). When the films became thicker, the grain sizes increased and the three-dimensional growth of the rutile SnO2 grains led to a rough surface and loose structure in the films (Fig. 3c,d). 3.3. Composition analysis XPS measurements were carried out to analyze the chemical state of Sn, and determine the atomic ratio of Sn to O. Because the SnO2 films had been exposed to ambient air for an extended period of time before the XPS measurements, the film surfaces were very likely covered by carbon-containing adsorbates and other contaminations. To obtain the actual chemical composition of the SnO2 films, we used high doses of argon ions (ion energy of 1.5 keV) to

Fig. 3. Typical SEM images for SnO2 thin films deposited for 3 h with thickness of 870 nm (a, b), and for 6.5 h with thickness of 1780 nm (c, d).

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sputter away the top layer (about 20 nm), in order to remove the hydrocarbon contaminations [30]. Fig. 4(a) presents the survey XP spectrum of a typical SnO2 film after sputter cleaning. The peak C 1s presented in the spectrum of the air-exposed film disappeared after sputtering, and only peaks from Sn and O showed up in the spectrum. Because there was no monochromator equipped with our XPS setup, a satellite peak from Sn 3d due to Al Ka3 was also seen in the spectrum. The survey scan revealed that the film was composed of Sn and O only, and free of any impurities in the bulk. The corelevel spectra of Sn 3d and O 1s of the film are shown in Fig. 4(b). All peak positions were corrected referring to the C 1s peak at 284.5 eV. The Sn 3d5/2, Sn 3d3/2 and O 1s peaks were located at 486.7, 495.1 and 530.4 eV, respectively, in excellent agreement with the report of Liang et al. [30], based on which the chemical state of Sn4þ and bonding of O-Sn4þ can be unambiguously identified in the film. Quantification resulted in the Sn/O atomic ratio of 39.5/60.5, higher than 1/2 as expected for SnO2. This might be attributed to the existence of oxygen vacancies in the film, and the preferential sputtering of oxygen during Arþ sputter cleaning of the surface. All these results confirmed that the films grown by CVD consist of pure SnO2 phase as revealed previously by XRD. 3.4. Optical properties Fig. 5 shows the optical transmittance spectra of samples with different SnO2 thicknesses as a function of the incident light wavelength in the range of 200e2600 nm. The average transmittance in the visible range for all SnO2 samples was over 80%,

Fig. 4. XPS spectra of a typical SnO2 film: (a) Survey scan, (b) O 1s and Sn 3d core level spectra.

Fig. 5. Optical transmittance spectra of SnO2 thin films with different thicknesses.

while it was about 93% for the quartz glass substrate. The absolute average transmittance (i.e. corrected for the substrate contribution) of the SnO2 films was about 90%. The inset in Fig. 5 shows that the optical transmission edge of SnO2 films shifted towards longer wavelengths with increasing of the film thickness. At first glance, this thickness-dependent variation behavior seems to be consistent with the general observation for very thin films that the optical absorption edge shifts toward higher energies/shorter wavelengths with decreasing film thickness due to the quantum size effect [31,32]. However, as presented below (cf. Fig. 6) the absorption edge of our SnO2 films actually did not exhibit such a thicknessdependent shift, due probably to large enough film thickness which led to no quantum size effects and various film orientations which influenced the absorption edges. For allowed direct transition, the energy-dependent absorption coefficient a (hn) can be expressed by the relation a ¼ A (hn e Eg)½, where Eg is the optical band gap, h is the Planck's constant, n is the frequency of the incident photon and A is a material dependent

Fig. 6. Plots of a2 as a function of photon energy (hn) for SnO2 thin films with different thicknesses.

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constant [33]. The plots of a2 as a function of photon energy hn for films with different thicknesses are shown in Fig. 6. In all cases absorption occurred already at energies below the band-gap, indicating considerable tail-states associated with defects existed in the band-gap of the SnO2 films. It appears that the tail-states/ defects density increased with decreasing film thickness except the film with 291 nm thickness. Further study is, however, needed to assure and rationalize this observation. Extrapolating the linear portions of a2 e hn plots to zero absorption yielded band-dge values between 3.78 and 3.92 eV for the SnO2 films. It was well documented in literature that SnO2 single crystals show anisotropic absorption edges of 3.93 and 3.57 eV for radiation polarized parallel and perpendicular to the c axis of the crystal, respectively [34,35]. As SnO2 films produced in this study are polycrystalline showing no c-axis preferential orientation, they exhibit optical absorption edges within the anisotropic absorption edges of SnO2 single crystals. In a recent theoretical report, Schleife et al. [36] explained the difference in the experimentally observed gaps for E perpendicular and parallel to c as a result of the anisotropy in the dipoleallowed direct transitions in the vicinity of the valence band maximum (VBM) at G rather than due to transitions from lowerlying VBs.

Raman lines at 497 cm-1, 541 cm-1 and 695 cm-1 in Fig. 7, which had not been detected in bulk SnO2. The weak Raman bands of 497 cm-1 and 695 cm-1 were also observed in the work of Peng [40]. Abello et al. proposed that the relaxation of the k ¼ 0 selection rule is progressive when the rate of disorder increased or size decreased, and IR modes could become weakly active when the structure changes induced by disorder and size effects took place [41]. Therefore, the weak Raman bands of 497 cm-1 and 695 cm-1 might correspond to the IR-active modes of transverse optical phonons (TO), and the longitudinal optical phonons (LO) of A2u modes, respectively [41]. The weak Raman band of 541 cm-1 was also observed by Dieguez et al. in the Raman spectrum of nanometric SnO2 particles [42]. They proposed that appearance of the band at 541 cm-1 was a consequence of the disorder activation. The reasons for the appearance of these “Raman-forbidden” modes could be manifold. Another possible reason might be that the oxygen vacancies induced the Raman activity. A somewhat similar effect was recently reported for CuO2 [43]. Further work is required to clarify this point and to correlate Raman signals with the defect states in SnO2 films with different thicknesses.

3.5. Raman analysis

Polycrystalline SnO2 thin films were fabricated on quartz glass substrates using CVD. X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy showed that pure-phase SnO2 films were formed and precipitates of other SnxOy phases (such as SnO and Sn3O4) did not occur. Detailed analysis using XRD and SEM techniques found that the microstructures in terms of grain orientation, size and shape, depended on the thickness of the films. The (110) reflection was the most intense in films of thickness up to about 400 ~ 500 nm, but for thicker films the (211) reflection became dominant. The optical band-gaps of the films were in the range of 3.78e3.92 eV, higher than the reported value for SnO2 single crystal.

With a rutile structure, the unit cell of SnO2 contains two Sn and four O ions and belongs to the space group D14 (P42/mnm). The 4h symmetry of the normal lattice vibration at the G point of the Brillouin zone may be derived by group theory:

Grutile ¼ A1g þ A2g þ A2u þ B1g þ B2g þ 2Bu þ Eg þ 3Eu ; where the modes of A1g, B1g, B2g and Eg symmetry are Raman active. Fig. 7 shows the Raman scattering spectrum of a typical SnO2 thin film. Raman lines were observed at 474 cm-1, 633 cm-1 and 775 cm-1, corresponding to Eg, A1g and B2g modes, respectively, in good agreement with results of Scott and co-workers [37]. The calculated value of the mode B1g at k ¼ 0 is 121 cm-1 [38]. A careful search of this region using different excitation wavelengths led to the conclusion that this band must have an intensity less than 0.001 of the intensity of the A1g mode [39]. The mode B1g was not detected in our SnO2 films. However, there existed three weak

4. Conclusions

Acknowledgments Y. B. He thanks the National Natural Science Foundation of China (Grant No. 61274010) and Ministry of Education of China (Program for New Century Excellent Talents in University, NCET-09-0135) for final support. References

Fig. 7. Raman scattering spectrum of a typical SnO2 thin film.

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