Effect of Annealing Temperature and Oxygen Flow ... - Semantic Scholar
Materials 2015, 8, 5289-5297; doi:10.3390/ma8085243
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materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Effect of Annealing Temperature and Oxygen Flow in the Properties of Ion Beam Sputtered SnO2´x Thin Films Chun-Min Wang 1 , Chun-Chieh Huang 2, *, Jui-Chao Kuo 1,: , Dipti Ranjan Sahu 3,: and Jow-Lay Huang 1,4,5,6,: 1
Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan 701, Taiwan; E-Mails: [email protected] (C.-M.W.); [email protected] (J.-C.K.); [email protected] (J.-L.H.) 2 Department of Electrical Engineering, Cheng Shiu University, No. 840, Chengcing Road, Niaosong Township, Kaohsiung 833, Taiwan 3 Amity Institute of Nanotechnology, Amity University, Sector 125, Noida, India; E-Mail: [email protected] 4 Department of Chemical and Materials Engineering, National University of Kaohsiung, No. 700, Kaohsiung University Road, Nan-Tzu District, Kaohsiung 811, Taiwan 5 Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan 6 Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan :
These authors contributed equally to this work.
* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-7-7310606 (ext. 3427); Fax: +886-7-7337390. Academic Editor: Teen-Hang Meen Received: 30 June 2015 / Accepted: 6 August 2015 / Published: 14 August 2015
Abstract: Tin oxide (SnO2´x ) thin films were prepared under various flow ratios of O2 /(O2 + Ar) on unheated glass substrate using the ion beam sputtering (IBS) deposition technique. This work studied the effects of the flow ratio of O2 /(O2 + Ar), chamber pressures and post-annealing treatment on the physical properties of SnO2 thin films. It was found that annealing affects the crystal quality of the films as seen from both X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. In addition, the surface RMS roughness was measured with atomic force microscopy (AFM). Auger electron spectroscopy (AES) analysis was used to obtain the changes of elemental distribution between tin and
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oxygen atomic concentration. The electrical property is discussed with attention to the structure factor. Keywords: SnO2 ; transparent conductive oxide (TCO); oxygen flow ratio; annealing
1. Introduction The most widely-used materials for transparent conductive oxide (TCO) thin films are zinc oxide (ZnO), tin oxide (SnO2 ) and indium oxide (In2 O3 ). SnO2 thin films have been developed and applied in different fields, such as solar cells, gas sensors and light-emitting diodes (LEDs) [1–5] because they have the advantages of their low electrical resistance and high optical transparency in the visible range of the electromagnetic spectrum. Furthermore, they are inexpensive and also stable against thermal and chemical attacks at high temperature. A number of studies have been reported on the effect of the oxygen gas ratio [6], substrate temperature and type, annealing temperature [7–9] and doping elements [10–12] on improving the structural, electrical and optical properties of the SnO2 thin film in the past few decades. The thin films of pure or doped SnO2 can be prepared by different deposition techniques, including sol-gel-dip coatings [13], metal-organic chemical vapor deposition (MOCVD) [14,15], chemical bath deposition (CBD) [16], spray pyrolysis [17], electron beam evaporation [18] and sputtering [19–22]. However, it is necessary to carry out either a heat treatment of substrates during the deposition process or the post-annealing procedure to obtain low resistivity and to retain high transmittance. Here, the ion beam sputtering (IBS) technique was applied to achieve high quality films, because of its high energy of incoming ions and the low deposition rate. In general, SnO2 thin films have defects, such as oxygen vacancies [23], which act as donors in the SnO2 matrix and increase the electron density in the conduction band. This is called the n-type conduction. The formation of excess oxygen vacancies results in decreasing film quality. Thus, increasing the conductivity of SnO2´x lies in a narrow range of oxygen pressure [24]. The post-heat treatment reduces residual stress and the lattice mismatch to obtain good electrical conductivity [25]. In the present work, we investigated the effects of the flow ratio of O2 /(O2 + Ar), chamber pressures and post-annealing treatment on the physical properties of SnO2 thin films. 2. Results and Discussion 2.1. Structural and Morphological Properties Figure 1 shows typical XRD patterns of the as-deposited and annealed SnO2´x films where they were prepared at a flow ratio of 0.5 of O2 /(O2 + Ar) at 4.7 ˆ 10´2 Pa. The structures of the as-deposited and the annealed SnO2´x films deposited at 4 ˆ 10´2 Pa were almost the same as that prepared. It is observed that the films as-deposited and annealed at 350 ˝ C were the typical amorphous structure, as indicated in Figure 1a,b. However, SnO2´x films annealed at 360 ˝ C have crystallization without preferred orientations, as shown in Figure 1c. A similar result was reported by Wulff et al. [26] for ITO thin
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ITO thin films. films. Further, Further, the the {101} peak peak of of SnO SnO phase phase appeared appeared at 400 400 °C, °C, as as shown shown in in Figure 1f, 1f, ITO films.thin Further, the {101} peak{101} of SnO phase appeared at 400 ˝ C, as at shown in Figure 1f, whichFigure was also which was was also also reported reported by by Choe et al. [9]. which reported by Choe et al. [9]. Choe et al. [9].
Figure 1. XRD patterns patterns of of SnO SnO2−x thin films films (a) (a) as-deposited as-deposited and annealed for 3 h at 2´x thin 1. XRD XRD Figure ˝1. patterns of SnO 2−x thin films (a) as-deposited and annealed for 3 h at ˝ ˝ ˝ ˝ ´2 −2 (b) 350 °C; C; (c) 360 °C; C; (d) 370 °C; C; (e) C and C at Pa. (e) 380 380 °C °C and (f) (f) 400 400 °C °C at 4.7 4.7 ˆ 10−2 Pa. (b) 350 °C; (c) 360 °C; (d) 370 °C; (e) 380 and (f) 400 at 4.7 ×× 10 Pa.
prepared with low ratios Figure 22 shows shows the the surface surface morphology morphology of of SnO SnO2´x 2−x films Figure films prepared prepared with with low low and and high high flow flow ratios 2−x films −2 ´2 Pa. It is observed that the as-deposited films have similar and uniform of 0.33 0.33 and and 0.71 0.71 at at 444 ˆ 10−2 Pa. ItItisisobserved observedthat that the the as-deposited as-deposited films films have have similar and uniform of ×× 10 Pa. surface morphologies. However, cracks are found at a high O /(O + Ar) flow ratio of of 0.71 0.71 after after 2 2 surface morphologies. However, However, cracks cracks are are found found at at aa high high O O22/(O /(O22 ++ Ar) Ar) flow flow ratio ˝°C annealing. 380 °C 380 C annealing.
Figure 2. FE-SEM micrographs of the SnO2´x thin films prepared at various flow ratios Figure 2. 2. FE-SEM FE-SEM micrographs micrographs of of´2the the SnO SnO2−x 2−x thin films prepared at various flow ratios Figure thin films prepared at various flow ratios and a working pressure of 4 ˆ 10 −2 Pa. The flow ratio of 0.33 of O2 /(O2 + Ar) (a) for and a working pressure of 4 × 10 −2 Pa. The flow ratio of 0.33 of O2/(O2 + Ar) (a) for Thethe flow ratio and a working pressure of ˝4C×annealing 10 Pa. and 2 + Ar) (a) for as-deposited films; (b) 380 flow ratioofof0.33 0.71ofofOO2/(O 2 /(O2 + Ar) (c) for as-deposited films; (b) 380 °C annealing and the flow ratio of 0.71 of O2/(O2 + Ar) as-deposited films;(d) (b)380 380 ˝ °C annealing and the flow ratio of 0.71 of O2/(O2 + Ar) as-deposited films; C annealing. (c) for for as-deposited as-deposited films; films; (d) (d) 380 380 °C °C annealing. annealing. (c)
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In to SEM SEM analysis, analysis, TEM TEM was was employed employed to to characterize characterize the the cross-section cross-section of of SnO SnO2´x 2−x 2−x films. In addition addition to films. −2 −2 −2 −2 Pa The as-deposited films films with with aa high high flow flowratio ratioofof0.71 0.71ofofOO Ar)atat4 4ˆ×10 10´2 and and 4.7 4.7ˆ× 10 10´2 22/(O22++Ar) The as-deposited Pa 2 /(O 2 reveal very high adherence and a smooth and uniform surface, as shown in Figure 3a,c. After reveal very high adherence and a smooth and uniform surface, as shown in Figure 3a,c. After annealing annealing 3 h,show the films show looser, thinner than interfaces than as-deposited films,they because at 370 ˝ C at for370 3 h,°C theforfilms looser, thinner interfaces as-deposited films, because were they were cracked and have large enhanced large spacesgrains between high temperature Ar annealing, cracked and have enhanced spaces between aftergrains high after temperature Ar annealing, as shown as shown in Figure 3b,d. in Figure 3b,d.
Figure 3. The cross-section images of bright field high-resolution TEM analysis at the same Figure 3. The cross-section images of bright field high-resolution TEM analysis at the flow ratio of 0.71 of O2 /(O2 + Ar) and a working pressure of 4 ˆ 10´2 Pa (a) for−2 as-deposited same flow ratio of 0.71 of O22/(O22 + Ar) and a working pressure of 4 × 10 −2 Pa (a) for ˝ ´2 films; (b) 370 C annealing and a working pressure of 4.7 ˆ 10 Pa (c) for as-deposited −2 Pa (c) for as-deposited films; (b) 370 °C annealing and a working pressure of 4.7 × 10−2 films; (d) 370 ˝ C annealing. as-deposited films; (d) 370 °C annealing.
Figure 4. 3D AFM images with a scan area of 2.5 µm ˆ 2.5 µm: (a) as-deposited; Figure 4. 3D AFM images with a scan area of 2.5 μm × 2.5 μm: (a) as-deposited; (b) 380 °C (b) 380 ˝ C annealing SnO2´x thin films at the flow ratio of 0.5 of O2 /(O2 + Ar) and a working annealing SnO2−x films at the flow ratio of 0.5 of˝ O22/(O22 + Ar) and a working pressure 2−x thin ´2 pressure of 4 ˆ 10 Pa. (c) As-deposited; (d) 380 C annealing SnO2´x thin films at the −2 of 4 × 10 −2 Pa. (c) As-deposited; (d) 380 °C annealing SnO2−x 2−x thin films at the flow ratio of flow ratio of 0.5 of O2 /(O2 + Ar) and a working pressure of 4.7 ˆ 10´2 Pa. −2 −2 0.5 of O22/(O22 + Ar) and a working pressure of 4.7 × 10 Pa.
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The AFM images that illustrate the surface topology and the root mean square (RMS) surface The AFM images films that illustrate surface topology andarethe root inmean square (RMS) surface roughness of SnO before andthe after 380 ˝ C annealing shown Figure 4. It is clear that the 2´x roughness of SnO2−x films before and after 380 °C annealing are shown in Figure 4. It is clear that the ˝ surface morphology of the as-deposited film and 380 C annealing at the flow ratio of 0.5 of O2 /(O2 + Ar) surface morphology of the as-deposited film and 380 °C´2annealing at the flow ratio of 0.5 of do not significantly change at a working pressure of 4.7 ˆ 10 Pa, as shown in Figure 4c,d. However, O2/(O2 + Ar) do not significantly change at a working pressure of 4.7 × 10−2 Pa, as shown in the surface morphology of the film changes dramatically from Figure 4a to Figure 4b. The surface Figure 4c,d. However, the surface morphology of the film changes dramatically from Figure 4a to 4b. roughness of the as-deposited film at 4 ˆ 10´2 Pa is 0.20 nm. The film annealed at 380 ˝ C shows an −2 The surface roughness of the as-deposited film at 4 × 10 Pa is 0.20 nm. The film annealed at 380 °C increase of surface roughness from 0.20 nm to 2.76 nm. The increase in film roughness at a working shows an increase ´2 of surface roughness from 0.20 nm to 2.76 nm. The increase in film roughness at a pressure of 4 ˆ 10 Pa can−2be explained with surface melt after 380 ˝ C annealing. However, it does not working pressure of 4 × 10 Pa can be explained with surface melt after 380 °C annealing. However, have an obvious structure transformation at a working pressure of 4.7 ˆ 10´2 Pa. it does not have an obvious structure transformation at a working pressure of 4.7 × 10−2 Pa. 2.2. Electrical Properties 2.2. Electrical Properties Figure 5 shows the variation of electrical resistivity of SnO2 thin films with various flow ratios Figure 5 shows the variation of electrical resistivity of SnO 2 thin films with various flow ratios of of O2 /(O2 + Ar) at the annealing temperature from 350 ˝ C to 380 ˝ C and a working pressure of O2/(O2 +´2 Ar) at the annealing temperature from 350 to 380 °C and a working pressure of 4.7 × 10−2 Pa. 4.7 ˆ 10 Pa.
Figure 5. The variation of the electrical resistivity of SnO2´x thin films with various flow thin films with various flow Figure 5. The variation of the electrical resistivity of SnO2−x ratios of O2 /(O2 + Ar) at the annealing temperature from 350 ˝ C to 380 ˝ C and a working ratios of O2/(O2 + Ar) at the annealing temperature from 350 to 380 °C and a working ´2 pressure of 4.7 ˆ 10−2 Pa. pressure of 4.7 × 10 Pa. ´2 The as-deposited films show Ω cm cm at at the the flow flow ratio of 0.5. show the the lowest lowest resistivity resistivity of of 1.05 1.05 ˆ × 10−2 Ω A strong dependence of resistivity on the oxygen oxygen flow flow ratio ratio was was observed. observed. It is observed that the ˝ resistivity increased increased about abouttwo twoorders ordersofofhigher higher magnitude at 360 C than thattheofas-deposited the as-deposited magnitude at 360 °C than that of films, films, except at the flow ratio of 0.33. This is due to the change in the structure from amorphous except at the flow ratio of 0.33. This is due to the change in the structure from amorphous to of of carrier concentration withwith a serious crackcrack formation at a higher crystallization and and mainly mainlythe thedecrease decrease carrier concentration a serious formation at a flow ratio, depicted by SEM and TEM and analyses. et Shanthi al. [27] reported thatreported the chemisorbed higher flowasratio, as depicted by SEM TEM Shanthi analyses. et al. [27] that the oxygen removes oxygenremoves vacanciesoxygen at high vacancies temperatureatannealing and increases chemisorbed at chemisorbed oxygen high temperature annealing and oxygen increases the surfaces and grainatboundaries, resultsboundaries, from further lowering of from free electrons. The degree of chemisorbed oxygen the surfaceswhich and grain which results further lowering of free reduction in carrier concentration high for SnO2 deposited a high flow2ratio. At theatoxygen electrons. The degree of reductionwas in carrier concentration was at high for SnO deposited a high flow ˝ ˝ ratio of initialflow decrease at the decrease annealingintemperature C to 370 C is due ratio. At0.3, thethe oxygen ratio inofresistivity 0.3, the initial resistivity from at the360 annealing temperature to the360 number of oxygen vacancies and excess metal ions arising and fromexcess the non-stoichiometry, from to 370 °C is due to the number of oxygen vacancies metal ions arisingwhich fromwas the also reported by Ku which et al. [28]. ratio by of 0.6 the[28]. highest dueofto0.6 its lowest non-stoichiometry, was The also flow reported Ku has et al. Theresistivity flow ratio has themobility highest resistivity due to its lowest mobility or carrier concentration at high and low working pressure after
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or carrier concentration at high and low working pressure after annealing. Williams and Ho [29,30] Materials 2015, 8 5307 reported the conductivity performance of SnO2 sensors, which were very a sensitive probe of changes in surface chemistry due and to electrons transfer, suchtheas conductivity the parameters of oxygenofpartial annealing. Williams Ho [29,30] reported performance SnO2 pressure sensors, and whichsize, wereetc. very a sensitive probe of changes in surface chemistry due to electrons transfer, such as the crystallite parameters of oxygen partial pressure size, and etc. 380 ˝ C annealing SnO2´x thin films at the Figure 6 shows the AES depth profileand of crystallite as-deposited Figure 6 shows the AES depth profile of as-deposited and 380 °C annealing SnOdepth 2−x thin films at the ´2 flow ratio of 0.5 of O2 /(O Pa. The profile shows the 2 + Ar) and a working pressure of 4.7 ˆ 10 flow ratio of 0.5 of O2/(O2 + Ar) and a working pressure of 4.7 × 10−2 Pa. The depth profile shows the change of atomic percent at the sputtering time from 0 to 120 s. The change of composition after the change of atomic percent at the sputtering time from 0 to 120 s. The change of composition after the annealing temperature of 380 ˝ C indicates the decrease of oxygen atoms and the increase of tin atoms annealing temperature of 380 °C indicates the decrease of oxygen atoms and the increase of tin atoms in theinthin surface. TheThe AES profiles usedfor forchemical chemicalcomposition composition analysis. We used the film thin film surface. AES profilesare areusually usually used analysis. We used this technique to confirm thethe correction and atomic atomicpercentage percentage with TEM. this technique to confirm correctionfor forthe thethickness thickness and with TEM.
˝ Figure 6. Atomic depth profile and380 380°C Cannealing annealing SnO 2´x thin films Figure 6. Atomic depth profileofofthe theas-deposited as-deposited and SnO 2−x thin films ´2 at theatflow ratioratio of 0.5 of of O2O /(O Ar) and a working pressure of 4.7 ˆ −210 Pa. 2 + the flow of 0.5 2/(O 2 + Ar) and a working pressure of 4.7 × 10 Pa.
3. Experimental Section 3. Experimental Section of SnO2 were sputtered from a diameter of 4-inch using a high purity of 99.99 wt% SnO2 Thin Thin filmsfilms of SnO 2 were sputtered from a diameter of 4-inch using a high purity of 99.99 wt% ceramic target (Ultimate Materials Technology Co., Hsinchu, Taiwan) in an atmosphere of argon and SnO2 ceramic target (Ultimate Materials Technology Co., Hsinchu, Taiwan) in an atmosphere of argon oxygen gases having 99.999% purity. SnO2 layers of about 60 nm in thickness were deposited onto the and oxygen gases having 99.999% purity. SnO2 layers of about 60 nm in thickness were deposited glass substrates (Corning EAGLE 2000 AMLCD glass, Taichung, Taiwan) using an ion beam onto sputtering the glass deposition substrates system, (Corning 2000 AMLCD using an ion the EAGLE Commonwealth Scientific glass, IBS250Taichung, Kaufman Taiwan) ion source, at the beamconditions sputtering Scientific Kaufman source, of deposition 600 V and 20system, A, wherethe theCommonwealth glass substrates were heated atIBS250 373 K. The vacuumion of the ion at −4 the conditions of 600 V andwas 20 maintained A, where atthe3 ×glass were heated at 373 K. ion Thebeam vacuum Pa before deposition. In the case of beam sputtering chamber 10 substrates ´4 of SnO the flow ratio mixture gasatcomposition was controlled 2 films, chamber 2/(O 2 + Ar)deposition. of thesputtering ion beam sputtering wasofmaintained 3 ˆ 10 OPa before Infrom the case −2 −2 0.33 to 0.71 at a working pressure of 4.7 × 10 and 4.0 × 10 Pa during deposition. The as-deposited of ion beam sputtering of SnO2 films, the flow ratio of mixture gas composition O2 /(O2 + Ar) was SnO2 films annealed in Ar atmosphere to 4.0 400 ˆ °C10 for ´23 h. The phase and controlled from were 0.33 subsequently to 0.71 at a working pressure of 4.7 ˆfrom 10´2350 and Pa during deposition. lattice structure of the films were analyzed using grazing angle X-ray diffraction (XRD) ˝(D/Max2500 The as-deposited SnO2 films were subsequently annealed in Ar atmosphere from 350 C to 400 ˝ C for Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 100 mA. The field-emission 3 h. The phase and lattice structure of the films were analyzed using grazing angle X-ray diffraction scanning electron microscope (FE-SEM) (S4800-I Hitachi, Tokyo, Japan) was used to observe the (XRD) (D/Max2500 Rigaku, Tokyo, Japan) Cu Kα radiation = 1.5406 Å) at morphologies 40 kV and 100 surface topography of the films before and with after annealing at 15 kV.(λThe cross-section of mA. The field-emission scanning electronfilms microscope (FE-SEM)by (S4800-I Hitachi, Tokyo, transmission Japan) was used as-deposited and post-annealing were investigated an ultrahigh resolution to observe themicroscopy surface topography the films before after annealing kV.kV, Thewhich cross-section electron (HR-TEM) of (JEM-2100F, JEOL,and Peabody, MA, USA)atat15200 was equipped with an energy dispersive spectrometer (EDS) Oxford, for chemical morphologies of as-deposited and post-annealing films INCA were x-stream-2 investigated by anU.K., ultrahigh resolution transmission electron microscopy (HR-TEM) (JEM-2100F, JEOL, Peabody, MA, USA) at 200 kV, which was equipped with an energy dispersive spectrometer (EDS) INCA x-stream-2 Oxford, U.K.,
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for chemical elemental analysis. For EDS measurements, a silicon drift detector of 80 mm2 was used. TEM samples were prepared using focus ion beams SMI3050SE (SEIKO, Chiba, Japan) with the first milling at 30 kV and 100 pA and the final milling at 5 kV and 40 pA. After that, Pt was coated on the SnO2 surface to protect thin films from damage. The Hall effect measurement setup AHM-800A with Advance Design Technology in van der Pauw configuration was used to study the electrical properties. Atomic force microscopy (AFM) (Force Precision Instrument Co., Taipei, Taiwan) was used to evaluate the surface roughness of the films by tapping mode. The sample surface was probed with a silicon tip of 10 nm in radius oscillating at its resonant frequency between 200 and 400 kHz. The scan area was 2.5 µm ˆ 2.5 µm, and the scan rate was 0.3 Hz. The compositions and depth profile of the films were determined by the Auger Electron Spectroscopy (AES) (Microlab 350, Thermo Fisher Scientific, Warrangton, UK). 4. Conclusions As-deposited SnO2 thin films show an amorphous structure, and crystallization occurs at 360 ˝ C. The resistivity of the film depends strongly on the oxygen flow ratio of 0.5 and above, due to the decrease in carrier concentration. In the case of an oxygen flow ratio of 0.3, metallic ions are dominated, and stable conductivity after annealing is observed due to the change in the numbers of oxygen vacancies and excess metal ions. Therefore, the dependence of resistivity on the oxygen partial pressure could be interesting in view of the use of these materials as oxygen sensors. Acknowledgments The authors acknowledge Sean Wu for the assistance in the measurement apparatus of the Hall effect at the College of Applied Design, TungFang Design Institute. The authors wish to thank H.C. Liu of the Department of Materials Science and Engineering, National Cheng Kung University, for giving great help with the AFM instrument in this work. This project was financially supported by the National Science Council of Taiwan with Grant No. 101-2221-E-006-124-MY3. Author Contributions All authors made contributions to this manuscript. Chun-Min Wang performed the experimental works, analyzed the results and wrote the manuscript. Chun-Chieh Huang and Dipti Ranjan Sahu greatly contributed to the concept and the design of the experimental parameters. Jui-Chao Kuo strongly contributed to the discussion of all of the results and revised the manuscript. Jow-Lay Huang provided overall guidance. Conflicts of Interest The authors declare no conflict of interest. References 1. Snaith, H.J.; Ducati, C. SnO2 -based dye-sensitized hybrid solar cells exhibiting near unity absorbed photon-to-electron conversion efficiency. Nano Lett. 2010, 10, 1259–1265. [CrossRef] [PubMed]
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17. Agashe, C.; Mahamuni, S. Competitive effects of film thickness and growth rate in spray pyrolytically deposited fluorine-doped tin dioxide films. Thin Solid Films 2010, 518, 4868–4873. [CrossRef] 18. Punitha, K.; Sivakumar, R.; Sanjeeviraja, C. Structural and surface morphological studies of magnesium tin oxide thin films. Energy Procedia 2012, 15, 312–317. [CrossRef] 19. Jäger, S.; Szyszka, B.; Szczyrbowski, J.; Bräuer, G. Comparison of transparent conductive oxide thin films prepared by a.c. and d.c. reactive magnetron sputtering. Surf. Coat. Technol. 1998, 98, 1304–1314. [CrossRef] 20. Ding, X.; Fang, F.; Jiang, J. Electrical and optical properties of N-doped SnO2 thin films prepared by magnetron sputtering. Surf. Coat. Technol. 2013, 231, 67–70. [CrossRef] 21. Becker, M.; Hamann, R.; Polity, A.; Meyer, B.K. Stannic oxide thin film growth via ion-beam-sputtering. Thin Solid Films 2014, 553, 26–29. [CrossRef] 22. Tosun, B.S.; Feist, R.K.; Gunawan, A.; Mkhoyan, K.A.; Campbell, S.A.; Aydil, E.S. Sputter deposition of semicrystalline tin dioxide films. Thin Solid Films 2012, 520, 2554–2561. [CrossRef] 23. Jarzebski, Z.M.; Marton, J.P. Physical properties of SnO2 materials: II. Electrical properties. J. Electrochem. Soc. 1976, 123, 299C–310C. [CrossRef] 24. Wang, C.-M.; Huang, C.-C.; Kuo, J.-C.; Lin, H.-T.; Huang, J.-L. Preparing transparent conductive Sn/SbO2´x thin films by annealing bi-layer Sb/SnO2 with pulse UV laser. Mater. Lett. 2012, 72, 29–31. [CrossRef] 25. Ordaz-Flores, A.; Bartolo-Pérez, P.; Castro-Rodríguez, R.; Oliva, A. Annealing effects on the mass diffusion of the CdS/ITO interface deposited by chemical bath deposition. Rev. Mex. Fís. 2006, 52, 15–18. 26. Wulff, H.; Quaas, M.; Steffen, H.; Hippler, R. In situ studies of diffusion and crystal growth in plasma deposited thin ITO films. Thin Solid Films 2000, 377–378, 418–424. [CrossRef] 27. Shanthi, E.; Banerjee, A.; Dutta, V.; Chopra, K.L. Annealing characteristics of tin oxide films prepared by spray pyrolysis. Thin Solid Films 1980, 71, 237–244. [CrossRef] 28. Ku, D.Y.; Kim, I.H.; Lee, I.; Lee, K.S.; Lee, T.S.; Jeong, J.H.; Cheong, B.; Baik, Y.J.; Kim, W.M. Structural and electrical properties of sputtered indium–zinc oxide thin films. Thin Solid Films 2006, 515, 1364–1369. [CrossRef] 29. Williams, D.E. Oxide Gas Sensors. In Encyclopedia of Materials: Science and Technology, 2nd ed.; Elsevier: Oxford, UK, 2001; pp. 6609–6613. 30. Ho, G.W. Gas Sensor with Nanostructured Oxide Semiconductor Materials. Sci. Adv. Mater. 2011, 3, 150–168. [CrossRef] © 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).
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materials ISSN 1996-1944 www.mdpi.com/journal/materials Article
Effect of Annealing Temperature and Oxygen Flow in the Properties of Ion Beam Sputtered SnO2´x Thin Films Chun-Min Wang 1 , Chun-Chieh Huang 2, *, Jui-Chao Kuo 1,: , Dipti Ranjan Sahu 3,: and Jow-Lay Huang 1,4,5,6,: 1
Department of Materials Science and Engineering, National Cheng Kung University, No. 1, University Road, Tainan 701, Taiwan; E-Mails: [email protected] (C.-M.W.); [email protected] (J.-C.K.); [email protected] (J.-L.H.) 2 Department of Electrical Engineering, Cheng Shiu University, No. 840, Chengcing Road, Niaosong Township, Kaohsiung 833, Taiwan 3 Amity Institute of Nanotechnology, Amity University, Sector 125, Noida, India; E-Mail: [email protected] 4 Department of Chemical and Materials Engineering, National University of Kaohsiung, No. 700, Kaohsiung University Road, Nan-Tzu District, Kaohsiung 811, Taiwan 5 Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan 701, Taiwan 6 Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan :
These authors contributed equally to this work.
* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-7-7310606 (ext. 3427); Fax: +886-7-7337390. Academic Editor: Teen-Hang Meen Received: 30 June 2015 / Accepted: 6 August 2015 / Published: 14 August 2015
Abstract: Tin oxide (SnO2´x ) thin films were prepared under various flow ratios of O2 /(O2 + Ar) on unheated glass substrate using the ion beam sputtering (IBS) deposition technique. This work studied the effects of the flow ratio of O2 /(O2 + Ar), chamber pressures and post-annealing treatment on the physical properties of SnO2 thin films. It was found that annealing affects the crystal quality of the films as seen from both X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis. In addition, the surface RMS roughness was measured with atomic force microscopy (AFM). Auger electron spectroscopy (AES) analysis was used to obtain the changes of elemental distribution between tin and
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oxygen atomic concentration. The electrical property is discussed with attention to the structure factor. Keywords: SnO2 ; transparent conductive oxide (TCO); oxygen flow ratio; annealing
1. Introduction The most widely-used materials for transparent conductive oxide (TCO) thin films are zinc oxide (ZnO), tin oxide (SnO2 ) and indium oxide (In2 O3 ). SnO2 thin films have been developed and applied in different fields, such as solar cells, gas sensors and light-emitting diodes (LEDs) [1–5] because they have the advantages of their low electrical resistance and high optical transparency in the visible range of the electromagnetic spectrum. Furthermore, they are inexpensive and also stable against thermal and chemical attacks at high temperature. A number of studies have been reported on the effect of the oxygen gas ratio [6], substrate temperature and type, annealing temperature [7–9] and doping elements [10–12] on improving the structural, electrical and optical properties of the SnO2 thin film in the past few decades. The thin films of pure or doped SnO2 can be prepared by different deposition techniques, including sol-gel-dip coatings [13], metal-organic chemical vapor deposition (MOCVD) [14,15], chemical bath deposition (CBD) [16], spray pyrolysis [17], electron beam evaporation [18] and sputtering [19–22]. However, it is necessary to carry out either a heat treatment of substrates during the deposition process or the post-annealing procedure to obtain low resistivity and to retain high transmittance. Here, the ion beam sputtering (IBS) technique was applied to achieve high quality films, because of its high energy of incoming ions and the low deposition rate. In general, SnO2 thin films have defects, such as oxygen vacancies [23], which act as donors in the SnO2 matrix and increase the electron density in the conduction band. This is called the n-type conduction. The formation of excess oxygen vacancies results in decreasing film quality. Thus, increasing the conductivity of SnO2´x lies in a narrow range of oxygen pressure [24]. The post-heat treatment reduces residual stress and the lattice mismatch to obtain good electrical conductivity [25]. In the present work, we investigated the effects of the flow ratio of O2 /(O2 + Ar), chamber pressures and post-annealing treatment on the physical properties of SnO2 thin films. 2. Results and Discussion 2.1. Structural and Morphological Properties Figure 1 shows typical XRD patterns of the as-deposited and annealed SnO2´x films where they were prepared at a flow ratio of 0.5 of O2 /(O2 + Ar) at 4.7 ˆ 10´2 Pa. The structures of the as-deposited and the annealed SnO2´x films deposited at 4 ˆ 10´2 Pa were almost the same as that prepared. It is observed that the films as-deposited and annealed at 350 ˝ C were the typical amorphous structure, as indicated in Figure 1a,b. However, SnO2´x films annealed at 360 ˝ C have crystallization without preferred orientations, as shown in Figure 1c. A similar result was reported by Wulff et al. [26] for ITO thin
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ITO thin films. films. Further, Further, the the {101} peak peak of of SnO SnO phase phase appeared appeared at 400 400 °C, °C, as as shown shown in in Figure 1f, 1f, ITO films.thin Further, the {101} peak{101} of SnO phase appeared at 400 ˝ C, as at shown in Figure 1f, whichFigure was also which was was also also reported reported by by Choe et al. [9]. which reported by Choe et al. [9]. Choe et al. [9].
Figure 1. XRD patterns patterns of of SnO SnO2−x thin films films (a) (a) as-deposited as-deposited and annealed for 3 h at 2´x thin 1. XRD XRD Figure ˝1. patterns of SnO 2−x thin films (a) as-deposited and annealed for 3 h at ˝ ˝ ˝ ˝ ´2 −2 (b) 350 °C; C; (c) 360 °C; C; (d) 370 °C; C; (e) C and C at Pa. (e) 380 380 °C °C and (f) (f) 400 400 °C °C at 4.7 4.7 ˆ 10−2 Pa. (b) 350 °C; (c) 360 °C; (d) 370 °C; (e) 380 and (f) 400 at 4.7 ×× 10 Pa.
prepared with low ratios Figure 22 shows shows the the surface surface morphology morphology of of SnO SnO2´x 2−x films Figure films prepared prepared with with low low and and high high flow flow ratios 2−x films −2 ´2 Pa. It is observed that the as-deposited films have similar and uniform of 0.33 0.33 and and 0.71 0.71 at at 444 ˆ 10−2 Pa. ItItisisobserved observedthat that the the as-deposited as-deposited films films have have similar and uniform of ×× 10 Pa. surface morphologies. However, cracks are found at a high O /(O + Ar) flow ratio of of 0.71 0.71 after after 2 2 surface morphologies. However, However, cracks cracks are are found found at at aa high high O O22/(O /(O22 ++ Ar) Ar) flow flow ratio ˝°C annealing. 380 °C 380 C annealing.
Figure 2. FE-SEM micrographs of the SnO2´x thin films prepared at various flow ratios Figure 2. 2. FE-SEM FE-SEM micrographs micrographs of of´2the the SnO SnO2−x 2−x thin films prepared at various flow ratios Figure thin films prepared at various flow ratios and a working pressure of 4 ˆ 10 −2 Pa. The flow ratio of 0.33 of O2 /(O2 + Ar) (a) for and a working pressure of 4 × 10 −2 Pa. The flow ratio of 0.33 of O2/(O2 + Ar) (a) for Thethe flow ratio and a working pressure of ˝4C×annealing 10 Pa. and 2 + Ar) (a) for as-deposited films; (b) 380 flow ratioofof0.33 0.71ofofOO2/(O 2 /(O2 + Ar) (c) for as-deposited films; (b) 380 °C annealing and the flow ratio of 0.71 of O2/(O2 + Ar) as-deposited films;(d) (b)380 380 ˝ °C annealing and the flow ratio of 0.71 of O2/(O2 + Ar) as-deposited films; C annealing. (c) for for as-deposited as-deposited films; films; (d) (d) 380 380 °C °C annealing. annealing. (c)
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In to SEM SEM analysis, analysis, TEM TEM was was employed employed to to characterize characterize the the cross-section cross-section of of SnO SnO2´x 2−x 2−x films. In addition addition to films. −2 −2 −2 −2 Pa The as-deposited films films with with aa high high flow flowratio ratioofof0.71 0.71ofofOO Ar)atat4 4ˆ×10 10´2 and and 4.7 4.7ˆ× 10 10´2 22/(O22++Ar) The as-deposited Pa 2 /(O 2 reveal very high adherence and a smooth and uniform surface, as shown in Figure 3a,c. After reveal very high adherence and a smooth and uniform surface, as shown in Figure 3a,c. After annealing annealing 3 h,show the films show looser, thinner than interfaces than as-deposited films,they because at 370 ˝ C at for370 3 h,°C theforfilms looser, thinner interfaces as-deposited films, because were they were cracked and have large enhanced large spacesgrains between high temperature Ar annealing, cracked and have enhanced spaces between aftergrains high after temperature Ar annealing, as shown as shown in Figure 3b,d. in Figure 3b,d.
Figure 3. The cross-section images of bright field high-resolution TEM analysis at the same Figure 3. The cross-section images of bright field high-resolution TEM analysis at the flow ratio of 0.71 of O2 /(O2 + Ar) and a working pressure of 4 ˆ 10´2 Pa (a) for−2 as-deposited same flow ratio of 0.71 of O22/(O22 + Ar) and a working pressure of 4 × 10 −2 Pa (a) for ˝ ´2 films; (b) 370 C annealing and a working pressure of 4.7 ˆ 10 Pa (c) for as-deposited −2 Pa (c) for as-deposited films; (b) 370 °C annealing and a working pressure of 4.7 × 10−2 films; (d) 370 ˝ C annealing. as-deposited films; (d) 370 °C annealing.
Figure 4. 3D AFM images with a scan area of 2.5 µm ˆ 2.5 µm: (a) as-deposited; Figure 4. 3D AFM images with a scan area of 2.5 μm × 2.5 μm: (a) as-deposited; (b) 380 °C (b) 380 ˝ C annealing SnO2´x thin films at the flow ratio of 0.5 of O2 /(O2 + Ar) and a working annealing SnO2−x films at the flow ratio of 0.5 of˝ O22/(O22 + Ar) and a working pressure 2−x thin ´2 pressure of 4 ˆ 10 Pa. (c) As-deposited; (d) 380 C annealing SnO2´x thin films at the −2 of 4 × 10 −2 Pa. (c) As-deposited; (d) 380 °C annealing SnO2−x 2−x thin films at the flow ratio of flow ratio of 0.5 of O2 /(O2 + Ar) and a working pressure of 4.7 ˆ 10´2 Pa. −2 −2 0.5 of O22/(O22 + Ar) and a working pressure of 4.7 × 10 Pa.
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The AFM images that illustrate the surface topology and the root mean square (RMS) surface The AFM images films that illustrate surface topology andarethe root inmean square (RMS) surface roughness of SnO before andthe after 380 ˝ C annealing shown Figure 4. It is clear that the 2´x roughness of SnO2−x films before and after 380 °C annealing are shown in Figure 4. It is clear that the ˝ surface morphology of the as-deposited film and 380 C annealing at the flow ratio of 0.5 of O2 /(O2 + Ar) surface morphology of the as-deposited film and 380 °C´2annealing at the flow ratio of 0.5 of do not significantly change at a working pressure of 4.7 ˆ 10 Pa, as shown in Figure 4c,d. However, O2/(O2 + Ar) do not significantly change at a working pressure of 4.7 × 10−2 Pa, as shown in the surface morphology of the film changes dramatically from Figure 4a to Figure 4b. The surface Figure 4c,d. However, the surface morphology of the film changes dramatically from Figure 4a to 4b. roughness of the as-deposited film at 4 ˆ 10´2 Pa is 0.20 nm. The film annealed at 380 ˝ C shows an −2 The surface roughness of the as-deposited film at 4 × 10 Pa is 0.20 nm. The film annealed at 380 °C increase of surface roughness from 0.20 nm to 2.76 nm. The increase in film roughness at a working shows an increase ´2 of surface roughness from 0.20 nm to 2.76 nm. The increase in film roughness at a pressure of 4 ˆ 10 Pa can−2be explained with surface melt after 380 ˝ C annealing. However, it does not working pressure of 4 × 10 Pa can be explained with surface melt after 380 °C annealing. However, have an obvious structure transformation at a working pressure of 4.7 ˆ 10´2 Pa. it does not have an obvious structure transformation at a working pressure of 4.7 × 10−2 Pa. 2.2. Electrical Properties 2.2. Electrical Properties Figure 5 shows the variation of electrical resistivity of SnO2 thin films with various flow ratios Figure 5 shows the variation of electrical resistivity of SnO 2 thin films with various flow ratios of of O2 /(O2 + Ar) at the annealing temperature from 350 ˝ C to 380 ˝ C and a working pressure of O2/(O2 +´2 Ar) at the annealing temperature from 350 to 380 °C and a working pressure of 4.7 × 10−2 Pa. 4.7 ˆ 10 Pa.
Figure 5. The variation of the electrical resistivity of SnO2´x thin films with various flow thin films with various flow Figure 5. The variation of the electrical resistivity of SnO2−x ratios of O2 /(O2 + Ar) at the annealing temperature from 350 ˝ C to 380 ˝ C and a working ratios of O2/(O2 + Ar) at the annealing temperature from 350 to 380 °C and a working ´2 pressure of 4.7 ˆ 10−2 Pa. pressure of 4.7 × 10 Pa. ´2 The as-deposited films show Ω cm cm at at the the flow flow ratio of 0.5. show the the lowest lowest resistivity resistivity of of 1.05 1.05 ˆ × 10−2 Ω A strong dependence of resistivity on the oxygen oxygen flow flow ratio ratio was was observed. observed. It is observed that the ˝ resistivity increased increased about abouttwo twoorders ordersofofhigher higher magnitude at 360 C than thattheofas-deposited the as-deposited magnitude at 360 °C than that of films, films, except at the flow ratio of 0.33. This is due to the change in the structure from amorphous except at the flow ratio of 0.33. This is due to the change in the structure from amorphous to of of carrier concentration withwith a serious crackcrack formation at a higher crystallization and and mainly mainlythe thedecrease decrease carrier concentration a serious formation at a flow ratio, depicted by SEM and TEM and analyses. et Shanthi al. [27] reported thatreported the chemisorbed higher flowasratio, as depicted by SEM TEM Shanthi analyses. et al. [27] that the oxygen removes oxygenremoves vacanciesoxygen at high vacancies temperatureatannealing and increases chemisorbed at chemisorbed oxygen high temperature annealing and oxygen increases the surfaces and grainatboundaries, resultsboundaries, from further lowering of from free electrons. The degree of chemisorbed oxygen the surfaceswhich and grain which results further lowering of free reduction in carrier concentration high for SnO2 deposited a high flow2ratio. At theatoxygen electrons. The degree of reductionwas in carrier concentration was at high for SnO deposited a high flow ˝ ˝ ratio of initialflow decrease at the decrease annealingintemperature C to 370 C is due ratio. At0.3, thethe oxygen ratio inofresistivity 0.3, the initial resistivity from at the360 annealing temperature to the360 number of oxygen vacancies and excess metal ions arising and fromexcess the non-stoichiometry, from to 370 °C is due to the number of oxygen vacancies metal ions arisingwhich fromwas the also reported by Ku which et al. [28]. ratio by of 0.6 the[28]. highest dueofto0.6 its lowest non-stoichiometry, was The also flow reported Ku has et al. Theresistivity flow ratio has themobility highest resistivity due to its lowest mobility or carrier concentration at high and low working pressure after
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or carrier concentration at high and low working pressure after annealing. Williams and Ho [29,30] Materials 2015, 8 5307 reported the conductivity performance of SnO2 sensors, which were very a sensitive probe of changes in surface chemistry due and to electrons transfer, suchtheas conductivity the parameters of oxygenofpartial annealing. Williams Ho [29,30] reported performance SnO2 pressure sensors, and whichsize, wereetc. very a sensitive probe of changes in surface chemistry due to electrons transfer, such as the crystallite parameters of oxygen partial pressure size, and etc. 380 ˝ C annealing SnO2´x thin films at the Figure 6 shows the AES depth profileand of crystallite as-deposited Figure 6 shows the AES depth profile of as-deposited and 380 °C annealing SnOdepth 2−x thin films at the ´2 flow ratio of 0.5 of O2 /(O Pa. The profile shows the 2 + Ar) and a working pressure of 4.7 ˆ 10 flow ratio of 0.5 of O2/(O2 + Ar) and a working pressure of 4.7 × 10−2 Pa. The depth profile shows the change of atomic percent at the sputtering time from 0 to 120 s. The change of composition after the change of atomic percent at the sputtering time from 0 to 120 s. The change of composition after the annealing temperature of 380 ˝ C indicates the decrease of oxygen atoms and the increase of tin atoms annealing temperature of 380 °C indicates the decrease of oxygen atoms and the increase of tin atoms in theinthin surface. TheThe AES profiles usedfor forchemical chemicalcomposition composition analysis. We used the film thin film surface. AES profilesare areusually usually used analysis. We used this technique to confirm thethe correction and atomic atomicpercentage percentage with TEM. this technique to confirm correctionfor forthe thethickness thickness and with TEM.
˝ Figure 6. Atomic depth profile and380 380°C Cannealing annealing SnO 2´x thin films Figure 6. Atomic depth profileofofthe theas-deposited as-deposited and SnO 2−x thin films ´2 at theatflow ratioratio of 0.5 of of O2O /(O Ar) and a working pressure of 4.7 ˆ −210 Pa. 2 + the flow of 0.5 2/(O 2 + Ar) and a working pressure of 4.7 × 10 Pa.
3. Experimental Section 3. Experimental Section of SnO2 were sputtered from a diameter of 4-inch using a high purity of 99.99 wt% SnO2 Thin Thin filmsfilms of SnO 2 were sputtered from a diameter of 4-inch using a high purity of 99.99 wt% ceramic target (Ultimate Materials Technology Co., Hsinchu, Taiwan) in an atmosphere of argon and SnO2 ceramic target (Ultimate Materials Technology Co., Hsinchu, Taiwan) in an atmosphere of argon oxygen gases having 99.999% purity. SnO2 layers of about 60 nm in thickness were deposited onto the and oxygen gases having 99.999% purity. SnO2 layers of about 60 nm in thickness were deposited glass substrates (Corning EAGLE 2000 AMLCD glass, Taichung, Taiwan) using an ion beam onto sputtering the glass deposition substrates system, (Corning 2000 AMLCD using an ion the EAGLE Commonwealth Scientific glass, IBS250Taichung, Kaufman Taiwan) ion source, at the beamconditions sputtering Scientific Kaufman source, of deposition 600 V and 20system, A, wherethe theCommonwealth glass substrates were heated atIBS250 373 K. The vacuumion of the ion at −4 the conditions of 600 V andwas 20 maintained A, where atthe3 ×glass were heated at 373 K. ion Thebeam vacuum Pa before deposition. In the case of beam sputtering chamber 10 substrates ´4 of SnO the flow ratio mixture gasatcomposition was controlled 2 films, chamber 2/(O 2 + Ar)deposition. of thesputtering ion beam sputtering wasofmaintained 3 ˆ 10 OPa before Infrom the case −2 −2 0.33 to 0.71 at a working pressure of 4.7 × 10 and 4.0 × 10 Pa during deposition. The as-deposited of ion beam sputtering of SnO2 films, the flow ratio of mixture gas composition O2 /(O2 + Ar) was SnO2 films annealed in Ar atmosphere to 4.0 400 ˆ °C10 for ´23 h. The phase and controlled from were 0.33 subsequently to 0.71 at a working pressure of 4.7 ˆfrom 10´2350 and Pa during deposition. lattice structure of the films were analyzed using grazing angle X-ray diffraction (XRD) ˝(D/Max2500 The as-deposited SnO2 films were subsequently annealed in Ar atmosphere from 350 C to 400 ˝ C for Rigaku, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å) at 40 kV and 100 mA. The field-emission 3 h. The phase and lattice structure of the films were analyzed using grazing angle X-ray diffraction scanning electron microscope (FE-SEM) (S4800-I Hitachi, Tokyo, Japan) was used to observe the (XRD) (D/Max2500 Rigaku, Tokyo, Japan) Cu Kα radiation = 1.5406 Å) at morphologies 40 kV and 100 surface topography of the films before and with after annealing at 15 kV.(λThe cross-section of mA. The field-emission scanning electronfilms microscope (FE-SEM)by (S4800-I Hitachi, Tokyo, transmission Japan) was used as-deposited and post-annealing were investigated an ultrahigh resolution to observe themicroscopy surface topography the films before after annealing kV.kV, Thewhich cross-section electron (HR-TEM) of (JEM-2100F, JEOL,and Peabody, MA, USA)atat15200 was equipped with an energy dispersive spectrometer (EDS) Oxford, for chemical morphologies of as-deposited and post-annealing films INCA were x-stream-2 investigated by anU.K., ultrahigh resolution transmission electron microscopy (HR-TEM) (JEM-2100F, JEOL, Peabody, MA, USA) at 200 kV, which was equipped with an energy dispersive spectrometer (EDS) INCA x-stream-2 Oxford, U.K.,
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for chemical elemental analysis. For EDS measurements, a silicon drift detector of 80 mm2 was used. TEM samples were prepared using focus ion beams SMI3050SE (SEIKO, Chiba, Japan) with the first milling at 30 kV and 100 pA and the final milling at 5 kV and 40 pA. After that, Pt was coated on the SnO2 surface to protect thin films from damage. The Hall effect measurement setup AHM-800A with Advance Design Technology in van der Pauw configuration was used to study the electrical properties. Atomic force microscopy (AFM) (Force Precision Instrument Co., Taipei, Taiwan) was used to evaluate the surface roughness of the films by tapping mode. The sample surface was probed with a silicon tip of 10 nm in radius oscillating at its resonant frequency between 200 and 400 kHz. The scan area was 2.5 µm ˆ 2.5 µm, and the scan rate was 0.3 Hz. The compositions and depth profile of the films were determined by the Auger Electron Spectroscopy (AES) (Microlab 350, Thermo Fisher Scientific, Warrangton, UK). 4. Conclusions As-deposited SnO2 thin films show an amorphous structure, and crystallization occurs at 360 ˝ C. The resistivity of the film depends strongly on the oxygen flow ratio of 0.5 and above, due to the decrease in carrier concentration. In the case of an oxygen flow ratio of 0.3, metallic ions are dominated, and stable conductivity after annealing is observed due to the change in the numbers of oxygen vacancies and excess metal ions. Therefore, the dependence of resistivity on the oxygen partial pressure could be interesting in view of the use of these materials as oxygen sensors. Acknowledgments The authors acknowledge Sean Wu for the assistance in the measurement apparatus of the Hall effect at the College of Applied Design, TungFang Design Institute. The authors wish to thank H.C. Liu of the Department of Materials Science and Engineering, National Cheng Kung University, for giving great help with the AFM instrument in this work. This project was financially supported by the National Science Council of Taiwan with Grant No. 101-2221-E-006-124-MY3. Author Contributions All authors made contributions to this manuscript. Chun-Min Wang performed the experimental works, analyzed the results and wrote the manuscript. Chun-Chieh Huang and Dipti Ranjan Sahu greatly contributed to the concept and the design of the experimental parameters. Jui-Chao Kuo strongly contributed to the discussion of all of the results and revised the manuscript. Jow-Lay Huang provided overall guidance. Conflicts of Interest The authors declare no conflict of interest. References 1. Snaith, H.J.; Ducati, C. SnO2 -based dye-sensitized hybrid solar cells exhibiting near unity absorbed photon-to-electron conversion efficiency. Nano Lett. 2010, 10, 1259–1265. [CrossRef] [PubMed]
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