- Email: [email protected]
Investigation of optical properties of ternary Zn-Ti-O thin films prepared by magnetron reactive co-sputtering
Accepted Manuscript Title: Investigation of optical properties of ternary Zn-Ti-O thin films prepared by magnetron reactive co-sputtering ˇ Authors: Marie Netrvalov´a, Petr Nov´ak, Pavol Sutta, Rostislav Medl´ın PII: DOI: Reference:
S0169-4332(17)30784-5 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.125 APSUSC 35501
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-7-2016 12-3-2017 13-3-2017
ˇ Please cite this article as: Marie Netrvalov´a, Petr Nov´ak, Pavol Sutta, Rostislav Medl´ın, Investigation of optical properties of ternary Zn-Ti-O thin films prepared by magnetron reactive co-sputtering, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.03.125 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of optical properties of ternary Zn-Ti-O thin films prepared by magnetron reactive co-sputtering Marie Netrvalová1, Petr Novák1, Pavol Šutta1, Rostislav Medlín1 1. Materials and Technology, New Technologies – Research Centre, University of West Bohemia, Univerzitní 8, 306 14 Pilsen, Czech Republic, [email protected] Graphcal abstract
Highlights Zn-Ti-O films were made by magnetron co-sputtering from metal targets in Ar/O2. Optical properties were obtained by numerical fit of ψ(λ), Δ(λ) and T(λ) withal. Spectral refractive indices and extinction coefficients exhibit two types of shapes. Optical band gap energy grows with Ti content from 2.97 up to 3.78 eV. Optical properties indicate transition from ZnO to TiO2 amorphous structure. Abstract: Zn-Ti-O thin films with different concentrations of titanium were deposited by reactive magnetron co-sputtering in a reactive Ar/O2 atmosphere from zinc and titanium targets. It was found that with increasing Ti content the structure of the films gradually changes from a fully crystalline pure ZnO wurtzite structure with a strongly preferred columnar orientation to an amorphous Zn-Ti-O material with 12.5 at% Ti. The optical parameters (spectral refractive index and extinction coefficient, optical band gap) and thickness of the films were analysed by the combined evaluation of ellipsometric measurements and measurements of transmittance on a UV-Vis spectrophotometer. For evaluation of optical parameters was used Cody-Lorentz dispersion model. Keywords: Zn-Ti-O thin film; magnetron co-sputtering; optical properties; spectroscopic ellipsometry; Cody-Lorentz model; structural properties 1. Introduction ZnO is a semiconductor with a wide direct band gap, the ordinary refractive index for monocrystal being 2.0036 [1], high transparency and high resistivity. In recent years, there has been much interest in the study of impurity-doped zinc oxide with improved conductivity for applications in the field of transparent electrodes in thin-film solar cells or in liquid crystal displays [2, 3, 4] and insulating or ferroelectric layers in opto-electronic devices [5].
While aluminium-doped ZnO is the most investigated dopant system, titanium has attracted attention in the last several years as a dopant due to its potential for greater charges per dopant atom [6], because Ti4+ ions (with ionic radius 0.068 nm) can replace Zn2+ ions (with ionic radius 0.074 nm) at substitutional sites, thanks to its smaller ionic radius [7]. TiO2 is also a semiconductor with a wide direct band gap and a high ordinary refractive index, with table values of 2.612 and 2.562 for Rutile and Anatase [1], respectively. Zn-Ti-O thin films can be fabricated using several deposition techniques such as atomic layer deposition [8], RF and DC magnetron sputtering [9], evaporation [5], pulsed laser deposition [10] or the spray pyrolisis technique [7]. This study investigates the structural evolution and optical properties of ternary Zn-Ti-O thin films with a Ti content up to 12.5 at%. The films were prepared by reactive magnetron co-sputtering in an Ar/O2 atmosphere from zinc and titanium targets. The structure of the prepared films gradually changed from a fully crystalline columnar pure ZnO wurtzite structure with a strong preferred orientation of crystallites in [001] direction perpendicular to the sample surface, through crystallites with decreased crystallite sizes with a structure with a preferred orientation, to amorphous Zn-Ti-O material. All prepared films were fully transparent in the visible spectrum. The extinction coefficient, refractive index and optical band gap were fitted concurrently by a combination of data taken from two independent measurements (spectroscopic ellipsometry and UV-Vis spectrophotometry). Results from Cody-Lorentz dispersion model for evaluating optical parameters are presented. 2. Experimental details Films were prepared by co-sputtering in a BOC Edwards TF600 sputtering deposition system. A zinc target and a titanium target were placed on a magnetron connected with RF and DC power supplies, respectively. Titanium concentration up to 12.5 at% was controlled by the power delivered to the Ti and Zn targets. The films, with thickness in the range of 170 – 320 nm, were sputtered in an Ar + O2 atmosphere at a total pressure of 1.4 Pa. Both the argon and oxygen flows were set to 5 sccm. The depositions were performed in reactive mode; thus the Zn and Ti targets were poisoned. A substrate holder with corning glass and an Si [100] oriented wafer was heated to 200°C. The substrates were at a floating potential during deposition and the substrate holder rotated to ensure homogeneity of the films. The Ti/Zn ratio was determined from SEM EDS measurements of the films deposited on Si substrates. Oxygen signals from the thin films were artificially increased by the fluorescence effect of Si characteristic X-rays from the substrate and therefore only the Ti/Zn value could be measured. Titanium atomic volumes in the oxide thin films were then calculated from stoichiometry of Zn and Ti oxides. Structural properties of the films were analysed by X-ray diffraction (XRD) using automatic powder diffractometer X´Pert Pro with CuKα radiation. Two goniometer setups (symmetric and asymmetric 2, where = 0.5 degree) were used. XRD patterns were collected from 25 to 65 degrees () and from 25 to 75 degrees (2) in 2 scales. In order to ascertain the basic microstructure parameters (crystallite sizes and micro-strains) of the films, a line profile analysis of the strongest lines was performed [11, 12]. High-resolution transmission electron microscopy (HRTEM) was carried out on transmission electron microscope JEOL JEM 2200FS operated at 200 kV (autoemission Shottky gun, point resolution 0.19 nm) with: an in-column energy Ω-filter for EELS/EFTEM analyses, a STEM unit, and an Energy Dispersive X-ray (EDX) SDD detector Oxford Instruments X-Max attached. Images were recorded on a Gatan CCD camera with resolution 2048×2048 pixels using the Digital Micrograph software package.
For the optical studies, spectroscopic ellipsometry measurements and UV-Vis spectrophotometry measurements of films were made using spectroscopic ellipsometer SE850 (Sentech GmbH) and UV-Vis spectrophotometer Specord 210 (AnalyticJena AG). In this work, the ellipsometric measurements of the relative amplitude change ψ(λ) and the relative phase change Δ(λ) were acquired in reflection mode on all samples over the spectral range of 240 – 2500 nm in air at room temperature. Angles of incidence of the light beam on the sample surfaces were chosen to be 60°, 65° and 70°. Ellipsometric measurements were obtained from co-sputtered Zn-Ti-O films on corning glass substrates. In addition to the ellipsometric parameter, normal transmission (T) measurements were carried out using a double beam UV-Vis spectrophotometer Specord 210 over the spectral range 190 – 1100 nm. Normal transmission data were obtained from films deposited on the corning glass substrates. Calculations were performed by the Sentech authorized program SpectraRay/3. In that program were combined data from ellipsometry and normal transmission data and were fitted concurrently by using the Cody-Lorentz model. The Cody-Lorentz model consists of seven fitted parameters. A (amplitude), E0 (resonance energy) and C (oscillator width) are the parameters of the Lorentz-oscillator. Eg (band gap energy) and Ep (transition energy) are the parameters of the Cody-formula (Eg + Ep separates the absorption onset behaviour from the Lorentz-oscillator behaviour. Et (transition energy between Cody-Lorentz and Urbach tail) and Eu (Urbach energy) are the parameters of the Urbach absorption tail which describes the weak exponentially increasing absorption with increasing energy below the band gap. [13] 3. Results and discussion Results are divided into two main parts: structural and surface properties and investigation of optical properties. 3.1. Structural and surface properties XRD analysis indicated that almost all films are polycrystalline (see Fig. 1 a) – d)) except for the film with the highest Ti concentration, which is amorphous. Average dimensions of crystallites (coherently diffracting domains) decreased from 85 nm for pure ZnO to 19 nm for ZnO with 8.7 at% Ti. The most significant feature of all investigated films is the preferred orientation of their crystallites in a certain direction against the sample surface (Fig. 1a) and c)). The other important feature is that the films change their phase composition step by step from ZnO to ZnxTi1-xOy, and also from a more crystalline to a less crystalline structure (Fig. 1b) and d)). This can be clearly seen from the asymmetry of the individual diffraction line profiles analysed. Samples with 2.4, 6.7 and 8.7 at% Ti were prepared for TEM measurements as X-TEM samples. TEM pictures of cross-sections of the thin films are in Fig. 2. The columnar structure of the ZnO thin film with increased Ti content changes and the single column starts to divide into more disoriented crystallite structures, as can be clearly seen from HR-TEM. Acquired crystallite sizes correspond with the sizes of coherently diffracted domains obtained from XRD analysis. Surface pictures from SEM measurements (see Fig. 3) correspond with TEM and show that increasing the Ti content has moderately increased the size of the structures, which are subsequently divided into smaller domains (but still oriented in a highly preferential way). Inter-planar distances observed by HR-TEM increase with the increase of the Ti content (Fig. 4), conforming to XRD measurements. 3.2. Optical properties
The data from UV-Vis spectrophotometry in Fig. 5 shows a high transmission in the entire visible range. The inset shows the onset of absorption, which shifts towards lower wavelengths with the increase in the Ti content, due to an increase in the band gap. The optical parameters, refractive index (n) and extinction coefficient (k), of the Zn-Ti-O films were derived from numerical fitting of the experimental data (ψ(λ), Δ(λ) and T(λ)) of the optical model structure: air / surface roughness / Zn-Ti-O film / corning glass. Film thickness was also obtained as a by-product of fitting process. The optical constants of the corning glass were fitted on pure materials without a deposited layer and were not allowed to vary during the fitting process. Measured ellipsometric data (see Fig. 6a) and transmittance data (see Fig. 6b) of Zn-Ti-O film sputtered onto a corning glass substrate over the spectral range of 240 – 1100 nm and results from fitting Zn-Ti-O film Cody-Lorentz (CL) dispersion model [13, 14, 15] are presented. The refractive index values obtained with Cody-Lorentz model are shown in Fig. 7. It is clear from the result that the refractive index at 632.8 nm has values in the range of 1.94 to 2.11. The variation of spectral refractive indices (n) and extinction coefficients (k) are presented in Fig. 8 a) and b), respectively. The shift of anomalous dispersion to lower wavelengths and higher values of refractive indices for samples with a Ti content above 5.2 at% Ti indicates that there is a phase transformation from the mostly two-component to the three-component material, as well as from the more ordered to the less ordered structure of the material. This has an influence on the velocity of the photons´ motion passing through the material. The slightly lower value of the refractive index for the film having an amorphous structure (with 12.5 at% Ti) could be explained by lower optical density and possibly by the higher content of oxygen in the interstitial position. In contrast, films with lower than 5.2 at% Ti have, from the optical point of view, an almost bulk crystalline ZnO-like structure, while a higher Ti content smoothly changes to TiO2–like properties. Dependency of optical band gap energy Eg on Ti content is presented in figure 9. As can be seen, with growing Ti content optical band gap grows from value 2.97 eV up to 3.78 eV.
4. Conclusion Transparent Zn-Ti-O films were prepared by magnetron co-sputtering from zinc and titanium targets in a reactive Ar + O2 atmosphere. The titanium content was controlled by power delivered to the Ti target and Zn target and observed by EDS analysis. The structure of the prepared films gradually changes from a fully crystalline columnar structure with a strongly preferred orientation of a pure ZnO wurtzite structure in [001] direction perpendicular to the substrate surface, through the intermediate preferably-oriented nanocrystalline structure, and finally to the Zn-Ti-O film with a Ti content of 12.5 at%, which is completely amorphous. Optical properties obtained by combined numerical fitting of ψ(λ), Δ(λ) and T(λ) by CodyLorentz dispersion model are presented. Spectral refractive indices and extinction coefficients exhibit two types of shapes, which indicates that they change from a predominantly ZnO structure to a TiO2 amorphous structure (in the state as deposited). Optical band gap energy continuously grows with growing Ti content. Furthermore, an important consequence of this experiment is that there is a possibility to prepare Zn-Ti-O films having the optical properties mentioned above with a simple deposition technology at relatively low temperatures (200°C) without a post-deposition thermal treatment.
Acknowledgements The result was developed within the CENTEM project, reg. no. CZ.1.05/2.1.00/03.0088, cofunded by the ERDF as part of the Ministry of Education, Youth and Sports OP RDI programme and, in the follow-up sustainability stage, supported through CENTEM PLUS (LO1402) by financial means from the Ministry of Education, Youth and Sports under the National Sustainability Programme I. Special thanks to A. Blümich from SENTECH Instruments GmbH, Berlin, Germany, for help with evaluation of measurements of spectroscopic ellipsometry and Jarmila Savková for SEM analyses. References [1] Lide, D. R.: Handbook of Chemistry and Physics, 90th edition, 2009, ISBN 978-1-42009084-0 [2] J. I. Nomoto, J.I. Oda, T. Miyata, T. Minami, Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering, Thin Solid Films 519 (2010) 1587-1593, doi:10.1016/j.tsf.2010.08.093 [3] Jagadish, C., Pearton, S.: Zinc Oxide Bulk, Thin Films and Nanostructures – Processing, Properties and Applications, Oxford (2006), ISBN 978-0-08-044722-3 [4] K. Ellmer, A. Klein, B. Rech, Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells, Springer, Berlin, 2008, ISBN 978-3-54073611-0 [5] A. A. Dakhel, Structural and optical properties of evaporated Zn oxide, Ti oxide and Zn-Ti oxide films, Applied Physics A 77 (2003) 677-682, doi: 10.1007/s00339-002-1763-3 [6] K. Bergum, P.-A. Hansen, H. Fjellvag, O. Nilsen, Structural, electrical and optical characterization of Ti-doped ZnO films grown by atomic layer deposition, Journal of Alloys and Compounds 616 (2014) 618-624, doi: 10.1016/j.jallcom.2014.07.177 [7] R. Sridhar, C. Manoharan, S. Ramalingam, S. Dhanapandian, M. Bououdina, Spectroscopit cstudy and optical and electrical properties of Ti-doped ZnO thin films by spray pyrolysis, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 297-303, doi: 10.1016/j.saa.2013.09.149 [8] Z. Wan, W.S. Kwack, W.J. Lee, S.I. Jang, H.R. Kim, J.W. Kim, K.W. Jung, W.J. Min, K.S. Yu, S.H. Park, E.Y. Yun, J.H. Kim, S.H. Kwon, Electrical and optical properties of Ti doped ZnO films grown on glass substrate by atomic layer deposition, Materials Research Bulletin 57 (2014) 23-28, doi: 10.1016/j.materresbull.2014.04.070 [9] S.S. Lin, J.L. Huang, P. Šajgalik, The properties of Ti-doped ZnO films deposited by simultaneous RF and DC magnetron sputtering, Surface & Coatings Technology 191 (2005) 286-292, doi: 10.1016/j.surfcoat.2004.03.021 [10] W. Zhao, Q. Zhou, X. Zhang, X. Wu, A study on Ti-doped ZnO transparent conducting thin films fabricated by pulsed laser deposition, Applied Surface Science 305 (2014) 481-486, doi: 10.1016/j.apsusc.2014.03.119 [11] R. Siddeswaran, P. Šutta, P. Novak, A. Hendrych, O. Zivotsky, In-situ X-ray diffraction studies and magneto-optic Kerr effect on RF sputtered thin films of BaTiO3 and Co, Nb co-doped BaTiO3, Ceramics International 42, (2016) 3882-3887, doi:10.1016/j.ceramint.2015.11.054 [12] M. Zeman, G. van Elzakker, F.D. Tichelaar, P. Sutta, Structural properties of amorphous silicon prepared from hydrogen-diluted silane, Philosophical Magazine 89 (2009) 2435-2448, doi: 10.1080/14786430902960137 [13] A. S. Ferluato, G. M. Ferreira, J. M. Pearce, C. R. Wronski, R. W. Collins, X. Deng, G. Ganguly, Analytical model for the optical functions of amorphous semiconductors
from the near-infrared to ultraviolet: Applications in thin film photovoltaics, J. Appl. Phys. 92 (2002) 2424, doi: 10.1063/1.1497462 [14] J. Sancho-Parramon, M. Modreanu, S. Bosch, M. Stchakovsky, Optical characterization of HfO2 by spectroscopic ellipsometry: Dispersion models and direct data inversion, Thin Solid Films 516 (2008) 7990-7995. doi: 10.1016/j.tsf.2008.04.07 [15] G.E. Jellison, Jr., The calculation of thin film parameters from spectroscopic ellipsometry data, Thin Solid Films 290-291 (1996) 40-46, doi: 10.1016/S0040-6090(96)09009-8
List of Figures: Fig. 1: XRD patterns of films with a crystalline structure at symmetric geometry: a) in full range of measured 2 angle, b) in detail to diffraction lines around position ZnO line [002] and at asymmetric 2 geometry with fixed angle = 0.5 degree c) in full range of measured 2 angle, d) in detail to diffraction lines around position [103] ZnO line. Fig. 2: X-TEM of thin films with 2.4 (left), 6.7 (middle) and 8.7 (right) at% Ti. Fig. 3: SEM surface morphology of Zn-Ti-O layers with Ti content up to 12.5 at% Ti. Fig. 4: HR-TEM with measurements of inter-planar distances of films with 2.4 (left), 6.7 (middle) and 8.7 (right) at% Ti. Fig. 5: Spectral transmittance of samples up to 12.5 at% Ti. The inset represents the shift of the absorption edge towards a lower wavelength with increasing Ti content. Fig. 6: Example of measured spectra and spectra calculated by Cody-Lorentz model: a) ψ(λ) and Δ(λ) at 70° angle of incidence and b) normal transmittance T(λ) as a function of wavelength for a sample with 6.7 at% Ti with layer thickness 235 nm. Values of particular parameters in CL model are Eg = 3.49 eV, A = 76.17 eV, E0 = 11.43 eV, C = 31.79 eV, Ep = 0.65 eV, Et = 3.69 eV, Eu = 0.23 eV. Fig. 7: Values of refractive indices at wavelength 632.8 nm for Zn-Ti-O films up to 12.5 at% Ti. Fig. 8: a) Spectral refractive indices and b) spectral extinction coefficients of Zn-Ti-O films. Fig. 9: Optical band gaps for Zn-Ti-O films with different Ti content determined by using Cody-Lorentz dispersion model.
S0169-4332(17)30784-5 http://dx.doi.org/doi:10.1016/j.apsusc.2017.03.125 APSUSC 35501
To appear in:
APSUSC
Received date: Revised date: Accepted date:
31-7-2016 12-3-2017 13-3-2017
ˇ Please cite this article as: Marie Netrvalov´a, Petr Nov´ak, Pavol Sutta, Rostislav Medl´ın, Investigation of optical properties of ternary Zn-Ti-O thin films prepared by magnetron reactive co-sputtering, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.03.125 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation of optical properties of ternary Zn-Ti-O thin films prepared by magnetron reactive co-sputtering Marie Netrvalová1, Petr Novák1, Pavol Šutta1, Rostislav Medlín1 1. Materials and Technology, New Technologies – Research Centre, University of West Bohemia, Univerzitní 8, 306 14 Pilsen, Czech Republic, [email protected] Graphcal abstract
Highlights Zn-Ti-O films were made by magnetron co-sputtering from metal targets in Ar/O2. Optical properties were obtained by numerical fit of ψ(λ), Δ(λ) and T(λ) withal. Spectral refractive indices and extinction coefficients exhibit two types of shapes. Optical band gap energy grows with Ti content from 2.97 up to 3.78 eV. Optical properties indicate transition from ZnO to TiO2 amorphous structure. Abstract: Zn-Ti-O thin films with different concentrations of titanium were deposited by reactive magnetron co-sputtering in a reactive Ar/O2 atmosphere from zinc and titanium targets. It was found that with increasing Ti content the structure of the films gradually changes from a fully crystalline pure ZnO wurtzite structure with a strongly preferred columnar orientation to an amorphous Zn-Ti-O material with 12.5 at% Ti. The optical parameters (spectral refractive index and extinction coefficient, optical band gap) and thickness of the films were analysed by the combined evaluation of ellipsometric measurements and measurements of transmittance on a UV-Vis spectrophotometer. For evaluation of optical parameters was used Cody-Lorentz dispersion model. Keywords: Zn-Ti-O thin film; magnetron co-sputtering; optical properties; spectroscopic ellipsometry; Cody-Lorentz model; structural properties 1. Introduction ZnO is a semiconductor with a wide direct band gap, the ordinary refractive index for monocrystal being 2.0036 [1], high transparency and high resistivity. In recent years, there has been much interest in the study of impurity-doped zinc oxide with improved conductivity for applications in the field of transparent electrodes in thin-film solar cells or in liquid crystal displays [2, 3, 4] and insulating or ferroelectric layers in opto-electronic devices [5].
While aluminium-doped ZnO is the most investigated dopant system, titanium has attracted attention in the last several years as a dopant due to its potential for greater charges per dopant atom [6], because Ti4+ ions (with ionic radius 0.068 nm) can replace Zn2+ ions (with ionic radius 0.074 nm) at substitutional sites, thanks to its smaller ionic radius [7]. TiO2 is also a semiconductor with a wide direct band gap and a high ordinary refractive index, with table values of 2.612 and 2.562 for Rutile and Anatase [1], respectively. Zn-Ti-O thin films can be fabricated using several deposition techniques such as atomic layer deposition [8], RF and DC magnetron sputtering [9], evaporation [5], pulsed laser deposition [10] or the spray pyrolisis technique [7]. This study investigates the structural evolution and optical properties of ternary Zn-Ti-O thin films with a Ti content up to 12.5 at%. The films were prepared by reactive magnetron co-sputtering in an Ar/O2 atmosphere from zinc and titanium targets. The structure of the prepared films gradually changed from a fully crystalline columnar pure ZnO wurtzite structure with a strong preferred orientation of crystallites in [001] direction perpendicular to the sample surface, through crystallites with decreased crystallite sizes with a structure with a preferred orientation, to amorphous Zn-Ti-O material. All prepared films were fully transparent in the visible spectrum. The extinction coefficient, refractive index and optical band gap were fitted concurrently by a combination of data taken from two independent measurements (spectroscopic ellipsometry and UV-Vis spectrophotometry). Results from Cody-Lorentz dispersion model for evaluating optical parameters are presented. 2. Experimental details Films were prepared by co-sputtering in a BOC Edwards TF600 sputtering deposition system. A zinc target and a titanium target were placed on a magnetron connected with RF and DC power supplies, respectively. Titanium concentration up to 12.5 at% was controlled by the power delivered to the Ti and Zn targets. The films, with thickness in the range of 170 – 320 nm, were sputtered in an Ar + O2 atmosphere at a total pressure of 1.4 Pa. Both the argon and oxygen flows were set to 5 sccm. The depositions were performed in reactive mode; thus the Zn and Ti targets were poisoned. A substrate holder with corning glass and an Si [100] oriented wafer was heated to 200°C. The substrates were at a floating potential during deposition and the substrate holder rotated to ensure homogeneity of the films. The Ti/Zn ratio was determined from SEM EDS measurements of the films deposited on Si substrates. Oxygen signals from the thin films were artificially increased by the fluorescence effect of Si characteristic X-rays from the substrate and therefore only the Ti/Zn value could be measured. Titanium atomic volumes in the oxide thin films were then calculated from stoichiometry of Zn and Ti oxides. Structural properties of the films were analysed by X-ray diffraction (XRD) using automatic powder diffractometer X´Pert Pro with CuKα radiation. Two goniometer setups (symmetric and asymmetric 2, where = 0.5 degree) were used. XRD patterns were collected from 25 to 65 degrees () and from 25 to 75 degrees (2) in 2 scales. In order to ascertain the basic microstructure parameters (crystallite sizes and micro-strains) of the films, a line profile analysis of the strongest lines was performed [11, 12]. High-resolution transmission electron microscopy (HRTEM) was carried out on transmission electron microscope JEOL JEM 2200FS operated at 200 kV (autoemission Shottky gun, point resolution 0.19 nm) with: an in-column energy Ω-filter for EELS/EFTEM analyses, a STEM unit, and an Energy Dispersive X-ray (EDX) SDD detector Oxford Instruments X-Max attached. Images were recorded on a Gatan CCD camera with resolution 2048×2048 pixels using the Digital Micrograph software package.
For the optical studies, spectroscopic ellipsometry measurements and UV-Vis spectrophotometry measurements of films were made using spectroscopic ellipsometer SE850 (Sentech GmbH) and UV-Vis spectrophotometer Specord 210 (AnalyticJena AG). In this work, the ellipsometric measurements of the relative amplitude change ψ(λ) and the relative phase change Δ(λ) were acquired in reflection mode on all samples over the spectral range of 240 – 2500 nm in air at room temperature. Angles of incidence of the light beam on the sample surfaces were chosen to be 60°, 65° and 70°. Ellipsometric measurements were obtained from co-sputtered Zn-Ti-O films on corning glass substrates. In addition to the ellipsometric parameter, normal transmission (T) measurements were carried out using a double beam UV-Vis spectrophotometer Specord 210 over the spectral range 190 – 1100 nm. Normal transmission data were obtained from films deposited on the corning glass substrates. Calculations were performed by the Sentech authorized program SpectraRay/3. In that program were combined data from ellipsometry and normal transmission data and were fitted concurrently by using the Cody-Lorentz model. The Cody-Lorentz model consists of seven fitted parameters. A (amplitude), E0 (resonance energy) and C (oscillator width) are the parameters of the Lorentz-oscillator. Eg (band gap energy) and Ep (transition energy) are the parameters of the Cody-formula (Eg + Ep separates the absorption onset behaviour from the Lorentz-oscillator behaviour. Et (transition energy between Cody-Lorentz and Urbach tail) and Eu (Urbach energy) are the parameters of the Urbach absorption tail which describes the weak exponentially increasing absorption with increasing energy below the band gap. [13] 3. Results and discussion Results are divided into two main parts: structural and surface properties and investigation of optical properties. 3.1. Structural and surface properties XRD analysis indicated that almost all films are polycrystalline (see Fig. 1 a) – d)) except for the film with the highest Ti concentration, which is amorphous. Average dimensions of crystallites (coherently diffracting domains) decreased from 85 nm for pure ZnO to 19 nm for ZnO with 8.7 at% Ti. The most significant feature of all investigated films is the preferred orientation of their crystallites in a certain direction against the sample surface (Fig. 1a) and c)). The other important feature is that the films change their phase composition step by step from ZnO to ZnxTi1-xOy, and also from a more crystalline to a less crystalline structure (Fig. 1b) and d)). This can be clearly seen from the asymmetry of the individual diffraction line profiles analysed. Samples with 2.4, 6.7 and 8.7 at% Ti were prepared for TEM measurements as X-TEM samples. TEM pictures of cross-sections of the thin films are in Fig. 2. The columnar structure of the ZnO thin film with increased Ti content changes and the single column starts to divide into more disoriented crystallite structures, as can be clearly seen from HR-TEM. Acquired crystallite sizes correspond with the sizes of coherently diffracted domains obtained from XRD analysis. Surface pictures from SEM measurements (see Fig. 3) correspond with TEM and show that increasing the Ti content has moderately increased the size of the structures, which are subsequently divided into smaller domains (but still oriented in a highly preferential way). Inter-planar distances observed by HR-TEM increase with the increase of the Ti content (Fig. 4), conforming to XRD measurements. 3.2. Optical properties
The data from UV-Vis spectrophotometry in Fig. 5 shows a high transmission in the entire visible range. The inset shows the onset of absorption, which shifts towards lower wavelengths with the increase in the Ti content, due to an increase in the band gap. The optical parameters, refractive index (n) and extinction coefficient (k), of the Zn-Ti-O films were derived from numerical fitting of the experimental data (ψ(λ), Δ(λ) and T(λ)) of the optical model structure: air / surface roughness / Zn-Ti-O film / corning glass. Film thickness was also obtained as a by-product of fitting process. The optical constants of the corning glass were fitted on pure materials without a deposited layer and were not allowed to vary during the fitting process. Measured ellipsometric data (see Fig. 6a) and transmittance data (see Fig. 6b) of Zn-Ti-O film sputtered onto a corning glass substrate over the spectral range of 240 – 1100 nm and results from fitting Zn-Ti-O film Cody-Lorentz (CL) dispersion model [13, 14, 15] are presented. The refractive index values obtained with Cody-Lorentz model are shown in Fig. 7. It is clear from the result that the refractive index at 632.8 nm has values in the range of 1.94 to 2.11. The variation of spectral refractive indices (n) and extinction coefficients (k) are presented in Fig. 8 a) and b), respectively. The shift of anomalous dispersion to lower wavelengths and higher values of refractive indices for samples with a Ti content above 5.2 at% Ti indicates that there is a phase transformation from the mostly two-component to the three-component material, as well as from the more ordered to the less ordered structure of the material. This has an influence on the velocity of the photons´ motion passing through the material. The slightly lower value of the refractive index for the film having an amorphous structure (with 12.5 at% Ti) could be explained by lower optical density and possibly by the higher content of oxygen in the interstitial position. In contrast, films with lower than 5.2 at% Ti have, from the optical point of view, an almost bulk crystalline ZnO-like structure, while a higher Ti content smoothly changes to TiO2–like properties. Dependency of optical band gap energy Eg on Ti content is presented in figure 9. As can be seen, with growing Ti content optical band gap grows from value 2.97 eV up to 3.78 eV.
4. Conclusion Transparent Zn-Ti-O films were prepared by magnetron co-sputtering from zinc and titanium targets in a reactive Ar + O2 atmosphere. The titanium content was controlled by power delivered to the Ti target and Zn target and observed by EDS analysis. The structure of the prepared films gradually changes from a fully crystalline columnar structure with a strongly preferred orientation of a pure ZnO wurtzite structure in [001] direction perpendicular to the substrate surface, through the intermediate preferably-oriented nanocrystalline structure, and finally to the Zn-Ti-O film with a Ti content of 12.5 at%, which is completely amorphous. Optical properties obtained by combined numerical fitting of ψ(λ), Δ(λ) and T(λ) by CodyLorentz dispersion model are presented. Spectral refractive indices and extinction coefficients exhibit two types of shapes, which indicates that they change from a predominantly ZnO structure to a TiO2 amorphous structure (in the state as deposited). Optical band gap energy continuously grows with growing Ti content. Furthermore, an important consequence of this experiment is that there is a possibility to prepare Zn-Ti-O films having the optical properties mentioned above with a simple deposition technology at relatively low temperatures (200°C) without a post-deposition thermal treatment.
Acknowledgements The result was developed within the CENTEM project, reg. no. CZ.1.05/2.1.00/03.0088, cofunded by the ERDF as part of the Ministry of Education, Youth and Sports OP RDI programme and, in the follow-up sustainability stage, supported through CENTEM PLUS (LO1402) by financial means from the Ministry of Education, Youth and Sports under the National Sustainability Programme I. Special thanks to A. Blümich from SENTECH Instruments GmbH, Berlin, Germany, for help with evaluation of measurements of spectroscopic ellipsometry and Jarmila Savková for SEM analyses. References [1] Lide, D. R.: Handbook of Chemistry and Physics, 90th edition, 2009, ISBN 978-1-42009084-0 [2] J. I. Nomoto, J.I. Oda, T. Miyata, T. Minami, Effect of inserting a buffer layer on the characteristics of transparent conducting impurity-doped ZnO thin films prepared by dc magnetron sputtering, Thin Solid Films 519 (2010) 1587-1593, doi:10.1016/j.tsf.2010.08.093 [3] Jagadish, C., Pearton, S.: Zinc Oxide Bulk, Thin Films and Nanostructures – Processing, Properties and Applications, Oxford (2006), ISBN 978-0-08-044722-3 [4] K. Ellmer, A. Klein, B. Rech, Transparent Conductive Zinc Oxide: Basics and Applications in Thin Film Solar Cells, Springer, Berlin, 2008, ISBN 978-3-54073611-0 [5] A. A. Dakhel, Structural and optical properties of evaporated Zn oxide, Ti oxide and Zn-Ti oxide films, Applied Physics A 77 (2003) 677-682, doi: 10.1007/s00339-002-1763-3 [6] K. Bergum, P.-A. Hansen, H. Fjellvag, O. Nilsen, Structural, electrical and optical characterization of Ti-doped ZnO films grown by atomic layer deposition, Journal of Alloys and Compounds 616 (2014) 618-624, doi: 10.1016/j.jallcom.2014.07.177 [7] R. Sridhar, C. Manoharan, S. Ramalingam, S. Dhanapandian, M. Bououdina, Spectroscopit cstudy and optical and electrical properties of Ti-doped ZnO thin films by spray pyrolysis, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 297-303, doi: 10.1016/j.saa.2013.09.149 [8] Z. Wan, W.S. Kwack, W.J. Lee, S.I. Jang, H.R. Kim, J.W. Kim, K.W. Jung, W.J. Min, K.S. Yu, S.H. Park, E.Y. Yun, J.H. Kim, S.H. Kwon, Electrical and optical properties of Ti doped ZnO films grown on glass substrate by atomic layer deposition, Materials Research Bulletin 57 (2014) 23-28, doi: 10.1016/j.materresbull.2014.04.070 [9] S.S. Lin, J.L. Huang, P. Šajgalik, The properties of Ti-doped ZnO films deposited by simultaneous RF and DC magnetron sputtering, Surface & Coatings Technology 191 (2005) 286-292, doi: 10.1016/j.surfcoat.2004.03.021 [10] W. Zhao, Q. Zhou, X. Zhang, X. Wu, A study on Ti-doped ZnO transparent conducting thin films fabricated by pulsed laser deposition, Applied Surface Science 305 (2014) 481-486, doi: 10.1016/j.apsusc.2014.03.119 [11] R. Siddeswaran, P. Šutta, P. Novak, A. Hendrych, O. Zivotsky, In-situ X-ray diffraction studies and magneto-optic Kerr effect on RF sputtered thin films of BaTiO3 and Co, Nb co-doped BaTiO3, Ceramics International 42, (2016) 3882-3887, doi:10.1016/j.ceramint.2015.11.054 [12] M. Zeman, G. van Elzakker, F.D. Tichelaar, P. Sutta, Structural properties of amorphous silicon prepared from hydrogen-diluted silane, Philosophical Magazine 89 (2009) 2435-2448, doi: 10.1080/14786430902960137 [13] A. S. Ferluato, G. M. Ferreira, J. M. Pearce, C. R. Wronski, R. W. Collins, X. Deng, G. Ganguly, Analytical model for the optical functions of amorphous semiconductors
from the near-infrared to ultraviolet: Applications in thin film photovoltaics, J. Appl. Phys. 92 (2002) 2424, doi: 10.1063/1.1497462 [14] J. Sancho-Parramon, M. Modreanu, S. Bosch, M. Stchakovsky, Optical characterization of HfO2 by spectroscopic ellipsometry: Dispersion models and direct data inversion, Thin Solid Films 516 (2008) 7990-7995. doi: 10.1016/j.tsf.2008.04.07 [15] G.E. Jellison, Jr., The calculation of thin film parameters from spectroscopic ellipsometry data, Thin Solid Films 290-291 (1996) 40-46, doi: 10.1016/S0040-6090(96)09009-8
List of Figures: Fig. 1: XRD patterns of films with a crystalline structure at symmetric geometry: a) in full range of measured 2 angle, b) in detail to diffraction lines around position ZnO line [002] and at asymmetric 2 geometry with fixed angle = 0.5 degree c) in full range of measured 2 angle, d) in detail to diffraction lines around position [103] ZnO line. Fig. 2: X-TEM of thin films with 2.4 (left), 6.7 (middle) and 8.7 (right) at% Ti. Fig. 3: SEM surface morphology of Zn-Ti-O layers with Ti content up to 12.5 at% Ti. Fig. 4: HR-TEM with measurements of inter-planar distances of films with 2.4 (left), 6.7 (middle) and 8.7 (right) at% Ti. Fig. 5: Spectral transmittance of samples up to 12.5 at% Ti. The inset represents the shift of the absorption edge towards a lower wavelength with increasing Ti content. Fig. 6: Example of measured spectra and spectra calculated by Cody-Lorentz model: a) ψ(λ) and Δ(λ) at 70° angle of incidence and b) normal transmittance T(λ) as a function of wavelength for a sample with 6.7 at% Ti with layer thickness 235 nm. Values of particular parameters in CL model are Eg = 3.49 eV, A = 76.17 eV, E0 = 11.43 eV, C = 31.79 eV, Ep = 0.65 eV, Et = 3.69 eV, Eu = 0.23 eV. Fig. 7: Values of refractive indices at wavelength 632.8 nm for Zn-Ti-O films up to 12.5 at% Ti. Fig. 8: a) Spectral refractive indices and b) spectral extinction coefficients of Zn-Ti-O films. Fig. 9: Optical band gaps for Zn-Ti-O films with different Ti content determined by using Cody-Lorentz dispersion model.