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Optical band engineering of metal-oxynitride based on tantalum oxide thin film fabricated via reactive gas-timing RF magnetron sputtering
Optical Band Engineering of Metal-Oxynitride based on Tantalum Oxide Thin Film Fabricated via Reactive Gas-Timing RF Magnetron Sputtering N. Khemasiri, S. Jessadaluk, C. Chananonnawathorn, S. Vuttivong, T. Lertvanithphol, M. Horprathum, P. Eiamchai, V. Patthanasettakul, A. Klamchuen, A. Pankiew, S. Pornthreeraphat, J. Nukeaw PII: DOI: Reference:
S0257-8972(16)30693-4 doi: 10.1016/j.surfcoat.2016.08.002 SCT 21420
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
26 December 2015 15 July 2016 1 August 2016
Please cite this article as: N. Khemasiri, S. Jessadaluk, C. Chananonnawathorn, S. Vuttivong, T. Lertvanithphol, M. Horprathum, P. Eiamchai, V. Patthanasettakul, A. Klamchuen, A. Pankiew, S. Pornthreeraphat, J. Nukeaw, Optical Band Engineering of Metal-Oxynitride based on Tantalum Oxide Thin Film Fabricated via Reactive Gas-Timing RF Magnetron Sputtering, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.08.002
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.
ACCEPTED MANUSCRIPT Optical Band Engineering of Metal-Oxynitride based on Tantalum Oxide Thin Film Fabricated via Reactive Gas-Timing RF Magnetron Sputtering
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N. Khemasiria,d, S. Jessadaluka,e, C. Chananonnawathornb, S. Vuttivongb, T. Lertvanithpholb, M. Horprathumb, P. Eiamchaib, V. Patthanasettakulb, A. Klamchuenc, A. Pankiewd,
College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang,
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S. Pornthreeraphatb,e,*and J. Nukeawa,e,d
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Ladkrabang, Bangkok 10520, Thailand National Electronic and Computer Technology Center (NECTEC), Klong Luang, Pathumthani
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National Nanotechnology Center (NANOTEC), Klong Luang, Pathumthani 12120, Thailand
Thai Microelectronics Center, 51/4 Moo 1, Wangtakien, Amphur Muang, Chachoengsao 24000,
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12120, Thailand
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Nanotec-KMITL Center of Excellence on Nanoelectronic Devices, Ladkrabang, Bangkok 10520, Thailand
Thailand Center of Excellence in Physics, Commission on Higher Education, Ministry of Education, Bangkok 10400, Thailand
* Corresponding author: Email: [email protected] ; Tel: +66-2564-6900 Ext. 2102; Fax: +66-2564-6771; Postal address: Photonics Technology Laboratory, National Electronics and Computer Technology Center, National Science and Technology Development Agency, 112 Thailand Science Park, Phahonyothin Rd., Klong 1, KlongLuang, Pathumthani 12120 Thailand
ACCEPTED MANUSCRIPT Abstract In this paper, we demonstrate a novel technique, as called reactive gas-timing (RGT) rf
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magnetron sputtering, to control and design an optical band engineering of TaON thin films
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without an external heating substrate temperature and post annealing treatment process. The influence of the oxygen intervals ranged from 5 to 60 s on deposition rate, chemical composition
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and optical properties of TaON thin films were investigated. The chemical composition was
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characterized by auger electron spectroscopy (AES). The optical properties were determined by UV-Vis spectrophotometer and spectroscopic ellipsometry. The nitrogen atomic concentration of
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the TaON thin films deposited by RGT decreased when the oxygen gas-timing intervals
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engineering from 1.90 to 2.15 eV.
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increased. In addition, the RGT sputtered TaON films could be demonstrated band gaps
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Keyword: Tantalum oxynitride thin film, reactive gas-timing technique (RGT technique), rf magnetron sputtering system, optical band gap, optical material.
Abstract code: BO04
1. Introduction Optical band engineering based on wide band optical materials has been a subject of interests and has essential for many scientific applications, i.e., filters, photo-activated materials, and optoelectronic devices. Among metal oxynitride compounds, which demonstrates a conspicuous property as wide band optical materials, tantalum oxynitride (TaON) is one of the most popular because of excellence in optical constants and dielectric functions, high hardness values, and
ACCEPTED MANUSCRIPT chemical stability. In addition, TaON allows engineering of the optical band gaps, essentially based on fraction of nitrogen 2p orbital and oxygen 2p orbital in top valence band position[1-5].
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Furthermore, the contribution of these orbitals could be controlled from nitrogen and oxygen
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contents, which is directly related with the other properties of TaON. Most studies of TaON thin film have been utilized an RF magnetron sputtering due to several advantages, i.e., large area
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coating, repeatability, and no chemical process. However, precise control of the element
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components between tantalum, oxygen, and nitrogen within the sputtered films is known to be a major drawback owing to theirs reactivity limitation [6, 7]. Many studies have taken approaches
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by adjustment of reactive gas fraction (O2/N2) ratios during the sputtering process, substrate bias, substrate heating and post annealing treatment to obtain TaON thin film[1, 3, 7-15].
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Since 2002, a novel deposition method, i.e., a reactive gas-timing (RGT) technique has been reported by J. Nukeaw et al. The RGT technique utilizes the alternate supplies of different
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reactive gases during thin film deposition. The reactive gases are separately and independently supplied into the sputtered chamber until the thickness of thin film is obtained. Regarding to this technique, the researchers successfully prepared InN, InON, AlN, AlON, SiN, ZnO, ITO thin films with different chemical compositions[16-25]. In this research, we focus on the fabrication of the TaON thin film by the RF magnetron sputtering with the RGT technique at room temperature without any post annealing process. The chemical composition, and the optical property of the obtained TaON thin film were investigated and discussed. 2. Experiment TaON thin films have been fabricated by rf magnetron sputtering system (AJA international, Inc; ATC 2000-F) cooperated with the RGT technique at room temperature. High purity
ACCEPTED MANUSCRIPT (99.99%) tantalum (Kurt J. Lesker) of 2-inch diameter was used as a sputtering target. Si (100) wafer and glass slide were used as substrates, which had been sequentially cleaned in ultrasonic
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washer with acetone, isopropanol, and deionized water, and then dried in nitrogen atmosphere.
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With a mechanical (ALCATEL) and a turbomolecular pump (Shimazu, TMP-803-LM), a base
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pressure of the deposition chamber was achieved at 1.010-6 Torr. High purity (99.999 %) argon was supplied as sputtering gas. High purity (99.999 %) oxygen and nitrogen were supplied as
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reactive gases. The argon, oxygen, and nitrogen were controlled and monitored with mass flow meters (MKS) at constant flow rates of 10, 5, and 3 sccm, respectively.
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Prior to the depositions, the Ta target was pre-sputtered in argon (99.999 %) ambient for 10 min to remove surface oxide from target poisoning. During the depositions, a constant RF power
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of 200 W and the continuous flow of argon were supplied in order to deposit the thin films. In this study, all the thin films were prepared and controlled at the thickness of 200 nm, based on
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the expected results from preliminary experiments. With the RGT technique, the film fabrication utilized alternate on-off time sequences of the reactive gases in order to control alternate flows of either oxygen or nitrogen into the vacuum chamber. In this study, the oxygen timing was varied at 5, 15, 30, and 60 sec (s), and the nitrogen timing was fixed at 60 sec. We denoted the RGT parameters of the O2:N2 timing (sec: sec) as 5:60, 15:60, 30:60, and 60:60, respectively. Fig. 1 illustrates the schematic of O2:N2 timing control at 30:60 sec at flow rate of 5:3 sccm, respectively with continuous flow of argon at 10 sccm, during the film deposition. For comparison, another TaON thin film was also prepared by conventional sputtering method, where the reactive gases were both simultaneously introduced to the vacuum chamber at the same flow rates as the RGT conditions.
ACCEPTED MANUSCRIPT The obtained TaON thin films were investigated for deposition rates, film compositions, and optical characteristics. The film thickness was confirmed by a step-profiler (Surftest sv-3000,
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Mitutoyo). The film compositions were determined by Auger electron microscopy (AES),
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(PHI700, ULVAC), in a combination with a depth profile technique based on argon etching
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process. The optical constants were investigated and analyzed by variable-angle spectroscopic ellipsometry (J.A. Woollam) at 70 incident angle in the range 1.0-6.0 eV. Optical transmission
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was characterized by UV-Vis spectrophotometer (T90+, PG Instrument).
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3. Result and discussion
Fig. 2 shows the deposition rate of the TaON thin films prepared with the RGT technique
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which the nitrogen gas-timing interval was fixed at 60 s and the oxygen interval ranged from 5 to 60 s. For comparison, the figure also includes the deposition rate of the thin film prepared with a
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conventional reactive sputtering, where the mixed reactive gases were supplied. The results clearly showed that the RGT technique yielded excellent deposition rate at all oxygen gas-timing intervals. Such high deposition rate during the TaON fabrication with the RGT technique could be described as a following mechanism based on the alternate highs-and-lows of the sputtering pressure. In general sputtering process, the deposition rate strongly corresponds to the sputtering pressure, which is also inversely related to the mean free path of the sputtered particles and the oxidation of the target surface. In this study, the sputtering pressure of the RGT technique was lower than the conventional reactive sputtering. When the film deposition began, argon and nitrogen were introduced into the chamber, leading to high sputtering pressure at approximately 3.2 mTorr within the first 60 s. When the nitrogen supply stopped and the oxygen flow was immediately switched on, the sputtering pressure would drift to a lower pressure state at
ACCEPTED MANUSCRIPT approximately 2.7 mTorr in a short amount of time. During such time, the sputtered atoms, which still maintained long mean free paths, now experienced fewer collisions. They therefore
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maintained relatively high particle energies to reach the substrates and allowed the increased
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deposition rate [26]. When the oxygen flow stopped and the nitrogen was immediately resupplied, the operating pressure increased, originating the decrease deposition rate. The
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alternate on-off sequences of each reactive gas would continually repeat through the whole thin-
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film deposition, which would therefore allow high deposition rates at all the O2:N2 gas-timing conditions.
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For further validations from the results in Fig. 2, we have explored the cross-sectional micrographs based on FE-SEM analyses for confirmation of the film thickness. The results
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corresponded to those from the depth profiler. The authors noted that, there were no multilayer microstructures presented from all the fabricated TaON thin films. In essence, during the film
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deposition with the argon constantly supplied as the sputtering gas, the nitrogen and oxygen were alternatively supplied as the reactive gas. Although each gas were supplied into the vacuum chamber at the alternate cycles, both gases were supposed to mix well with the argon gas within the deposition process. When the nitrogen was stopped and the oxygen was supplied, the nitrogen atomic energy of was increased, and vice versa. Therefore, the RGT technique actually exerted more atomic energy of each reactive gas into the film stoichiometry. This assumption could help predict that the conventional sputtering technique would yield slightly inhomogeneous films with included porosity, while the RGT technique would yield homogeneous dense films, without causing the multilayer structures. The predictions were to be confirmed by the AES depth profiles from the argon etching process, to be discussed in the next paragraph.
ACCEPTED MANUSCRIPT Fig. 3 shows the compositions of the TaON thin films prepared by the RGT technique in comparison with the conventional technique, as analyzed by the AES analyses. The results
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showed that both the nitrogen and tantalum atomic concentrations of the TaON film prepared by
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conventional reactive sputtering were lower than those prepared by the RGT technique. Generally, tantalum possesses a stronger affinity to oxygen than nitrogen. When the film from
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the conventional sputtering showed lower nitrogen concentration corresponded to higher particle
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energies caused by low sputtering pressure. As the oxygen gas-timing intervals were increased, the oxygen concentration in the films was progressively increased, while the nitrogen content
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was decreased. The concentration of tantalum slightly decreased from 70 to 60 at.%. With the spectroscopic ellipsometer (SE) measurements and analyses, the optical constants of
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the fabricated films could be obtained. In this work, the SE physical models included the silicon substrate, the native oxide layer, the TaON layer, and the surface roughness. The native oxide of
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2 nm was included because the silicon substrates had not been etched by HF before the film deposition. The SE optical models of TaON utilized a single Tauc-Lorentz oscillator. The refractive index and the extinction coefficient of the thin films, obtained from the SE model fits, were plotted as shown in Fig. 4. The results showed that the TaON films from the RGT technique yielded higher optical constants (n and k at 550 nm) than the film from the convention sputtering, because of their density variations depended on the nitrogen concentrations. At the smallest oxygen gas-timing interval from the O2:N2 RGT parameter of 5s:60s, both optical constants were very high, with the reactive index at approximately 3.0. At this parameter, only small amount of oxygen was incorporated among the nitrogen concentration within the film structure. When the oxygen gas-timing intervals were increased to 60 s, the refractive index of the TaON films was rapidly decreased because of the higher amount of oxygen incorporations
ACCEPTED MANUSCRIPT contributed to the decrease in the nitrogen concentrations and the film density. Such results also corresponded to other publications, whose bulk density materials yielded higher optical index [8,
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10, 27]. The obtained values of the refractive index by the SE analyses corresponded well with
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the nitrogen concentrations within the thin films previously analyzed by AES. Fig. 5 shows the UV-Vis transmission spectra in the wavelength range 200-850 nm for the
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fabricated TaON thin films on the glass substrates. In this study, a blank glass was used as a
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reference in the double-beam measurements, whose results allowed relative optical transmission of the thin films. From the results, the TaON film prepared by the conventional mixed reactive
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gases was highly transparent with an average transmittance of ~ 90% in the visible region. However, the TaON films obtained with the RGT technique, with the 5 s O2 timing, revealed
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lower transmittance because of high nitrogen content incorporated within the film. With the increased oxygen gas-timing intervals, the TaON thin films became less absorbing and the
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absorption edge significantly shifted toward shorter wavelengths. The transmission spectra were then used to determine the optical band gaps of the TaON thin films. For wavelengths close to values where loss scattering are dominated by the fundamental absorption of light, the absorption coefficient can be calculated using the expression = d-1ln(1/T)
(1)
Where d is the thickness of the film and T is the optical transmittance. In the vicinity of a fundamental absorption, the indirect allowed transition dominates over the optical absorption, according to the relation [1]. (h)1/2 = A(h-Eg)
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ACCEPTED MANUSCRIPT Where h is the photon energy, A is parameter independent of photon energy for respective transition, and Eg is the optical band gap. Hence, at photon energies h>Eg, the TaON material
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can absorb photons, while at h
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The optical band gaps could be obtained by extrapolating the linear part the curves of 1/2
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versus h. As shown in Fig. 6, the plots represented the curves to be determined for the band gaps of the TaON thin films in this study. From the results, the deposition with conventional
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mixed reactive gases yielded the thin film with very high bang gap of 4.21 eV. The depositions
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with the RGT technique however yielded the thin films with very small band gaps in the range of 1.90 – 2.15 eV. The variations of the RGT parameters of the O2:N2 timing intervals could alter
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the band gaps around 2 eV. This behavior of band gap engineering from the reactive gas-timing
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technique was because of different nitrogen contents being supplied during the deposition process. The results of the optical band gaps were finally plotted with respect to the composition
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ratio of (nitrogen + oxygen)/tantalum as a zone diagram, as shown in Fig. 7. The diagram also compared the thin film results in this work with other notable publications. From the Fig. 7, the TaON films fabricated by most publications yielded the (N+O)/Ta ratio higher than 1.5. From the J. H. Hsieh's works [1, 8], while the TaON thin films allowed wide variations of the band gap engineering at high atomic ratio, their deposition processes required the substrate bias and the annealing treatments. On the other hand, the publication by H. Le Dr'eo, et al. [28] experimented on oxygen pulses and constant nitrogen flow during the film depositions. Their work also reported a wide range of band gaps from the thin films with the highest atomic ratio up to 3.25. In our work, the fabricated thin films however yielded the (N+O)/Ta atomic ratio at approximately 0.5 for all reactive depositions with a moderate range of the optical band gap variations. The results proved that the sputtering deposition with the
ACCEPTED MANUSCRIPT proposed reactive gas-timing technique without the annealing treatments allowed the smallest (N+O)/Ta atomic ratio. Since this work well demonstrated the band gap engineering from the
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film fabrications with the RGT technique, further studies of the TaON thin films with alternate
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atomic compositions could be explored for many potential applications.
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4. Summary
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The TaON thin films have been deposited using rf reactive magnetron sputtering with RGT technique onto glass slide and c-Si substrate at room temperature. The analysis indicates that the
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compositions of TaON thin films were related to the O2:N2 gas-timing parameters. The AES results demonstrated all the RGT sputtered TaON were chemical compositions ratio ((N+O)/Ta)
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at approximately 0.5. Spectroscopic ellipsometry results indicate that the values of refractive index for the RGT sputtered TaON thin films were higher than conventional reactive sputtered
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TaON thin films. In particular, the refractive index of the RGT sputtered TaON thin films decrease with increasing oxygen interval time. The obtained optical band gap of the RGT sputtered TaON thin film shows widely range from 1.90 – 2.15 eV. In addition, the RGT was an alternative technique to fabricated metal oxynitride films with large range of chemical compositions and band gap engineering for optical coating applications.
Acknowledgments We would like to appreciate to the National Electronic and Computer Technique Center (NECTEC), NSTDA, Ministry of Science and Technology, Thailand, under the R&D project as “white light emitting diode based on zinc oxide optoelectronics material; phase-1: method of ZnO-wafer fabrication” for instrument supporting and National Nanotechnology Center
ACCEPTED MANUSCRIPT (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network.
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Reference
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[28] H. Le Dréo, O. Banakh, H. Keppner, P.A. Steinmann, D. Briand, N.F. de Rooij, Thin Solid Films 515 (2006) 952.
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Figure Caption
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Fig. 1 Illustrates the schematic of O2:N2 timing control at 30:60 sec.
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Fig. 2 Variation of deposition rate for the TaON thin films as a function of oxygen gas-timing
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intervals.
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Fig. 3 AES results of the TaON thin films deposited by conventional reactive sputtering and
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RGT technique at different oxygen gas-timing intervals.
Fig. 4 Refractive index and extinction coefficient of the TaON thin films deposited by conventional reactive sputtering and RGT technique at different oxygen gas-timing intervals.
Fig. 5 Transmittance of the TaON thin films deposited by conventional reactive sputtering and RGT technique at different oxygen gas-timing intervals.
Fig. 6 (h)1/2 as a function photon energy for the TaON thin films deposited by conventional reactive sputtering and RGT technique at different oxygen gas-timing intervals.
ACCEPTED MANUSCRIPT Fig. 7 The zone diagrams of the optical band gap and the composition ratio of (oxygen +
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ACCEPTED MANUSCRIPT Highlights An innovated technique to fabricate TaON thin film at room temperature.
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The chemical composition is precisely controlled.
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The optical band gap of TaON thin film varies from 1.90 to 4.21eV.
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The reactive gas-timing technique is suitable for low melting point material.
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S0257-8972(16)30693-4 doi: 10.1016/j.surfcoat.2016.08.002 SCT 21420
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
26 December 2015 15 July 2016 1 August 2016
Please cite this article as: N. Khemasiri, S. Jessadaluk, C. Chananonnawathorn, S. Vuttivong, T. Lertvanithphol, M. Horprathum, P. Eiamchai, V. Patthanasettakul, A. Klamchuen, A. Pankiew, S. Pornthreeraphat, J. Nukeaw, Optical Band Engineering of Metal-Oxynitride based on Tantalum Oxide Thin Film Fabricated via Reactive Gas-Timing RF Magnetron Sputtering, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.08.002
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.
ACCEPTED MANUSCRIPT Optical Band Engineering of Metal-Oxynitride based on Tantalum Oxide Thin Film Fabricated via Reactive Gas-Timing RF Magnetron Sputtering
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N. Khemasiria,d, S. Jessadaluka,e, C. Chananonnawathornb, S. Vuttivongb, T. Lertvanithpholb, M. Horprathumb, P. Eiamchaib, V. Patthanasettakulb, A. Klamchuenc, A. Pankiewd,
College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang,
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a
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S. Pornthreeraphatb,e,*and J. Nukeawa,e,d
b
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Ladkrabang, Bangkok 10520, Thailand National Electronic and Computer Technology Center (NECTEC), Klong Luang, Pathumthani
c
National Nanotechnology Center (NANOTEC), Klong Luang, Pathumthani 12120, Thailand
Thai Microelectronics Center, 51/4 Moo 1, Wangtakien, Amphur Muang, Chachoengsao 24000,
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d
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12120, Thailand
e
Thailand
d
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Nanotec-KMITL Center of Excellence on Nanoelectronic Devices, Ladkrabang, Bangkok 10520, Thailand
Thailand Center of Excellence in Physics, Commission on Higher Education, Ministry of Education, Bangkok 10400, Thailand
* Corresponding author: Email: [email protected] ; Tel: +66-2564-6900 Ext. 2102; Fax: +66-2564-6771; Postal address: Photonics Technology Laboratory, National Electronics and Computer Technology Center, National Science and Technology Development Agency, 112 Thailand Science Park, Phahonyothin Rd., Klong 1, KlongLuang, Pathumthani 12120 Thailand
ACCEPTED MANUSCRIPT Abstract In this paper, we demonstrate a novel technique, as called reactive gas-timing (RGT) rf
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magnetron sputtering, to control and design an optical band engineering of TaON thin films
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without an external heating substrate temperature and post annealing treatment process. The influence of the oxygen intervals ranged from 5 to 60 s on deposition rate, chemical composition
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and optical properties of TaON thin films were investigated. The chemical composition was
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characterized by auger electron spectroscopy (AES). The optical properties were determined by UV-Vis spectrophotometer and spectroscopic ellipsometry. The nitrogen atomic concentration of
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the TaON thin films deposited by RGT decreased when the oxygen gas-timing intervals
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engineering from 1.90 to 2.15 eV.
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increased. In addition, the RGT sputtered TaON films could be demonstrated band gaps
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Keyword: Tantalum oxynitride thin film, reactive gas-timing technique (RGT technique), rf magnetron sputtering system, optical band gap, optical material.
Abstract code: BO04
1. Introduction Optical band engineering based on wide band optical materials has been a subject of interests and has essential for many scientific applications, i.e., filters, photo-activated materials, and optoelectronic devices. Among metal oxynitride compounds, which demonstrates a conspicuous property as wide band optical materials, tantalum oxynitride (TaON) is one of the most popular because of excellence in optical constants and dielectric functions, high hardness values, and
ACCEPTED MANUSCRIPT chemical stability. In addition, TaON allows engineering of the optical band gaps, essentially based on fraction of nitrogen 2p orbital and oxygen 2p orbital in top valence band position[1-5].
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Furthermore, the contribution of these orbitals could be controlled from nitrogen and oxygen
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contents, which is directly related with the other properties of TaON. Most studies of TaON thin film have been utilized an RF magnetron sputtering due to several advantages, i.e., large area
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coating, repeatability, and no chemical process. However, precise control of the element
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components between tantalum, oxygen, and nitrogen within the sputtered films is known to be a major drawback owing to theirs reactivity limitation [6, 7]. Many studies have taken approaches
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by adjustment of reactive gas fraction (O2/N2) ratios during the sputtering process, substrate bias, substrate heating and post annealing treatment to obtain TaON thin film[1, 3, 7-15].
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Since 2002, a novel deposition method, i.e., a reactive gas-timing (RGT) technique has been reported by J. Nukeaw et al. The RGT technique utilizes the alternate supplies of different
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reactive gases during thin film deposition. The reactive gases are separately and independently supplied into the sputtered chamber until the thickness of thin film is obtained. Regarding to this technique, the researchers successfully prepared InN, InON, AlN, AlON, SiN, ZnO, ITO thin films with different chemical compositions[16-25]. In this research, we focus on the fabrication of the TaON thin film by the RF magnetron sputtering with the RGT technique at room temperature without any post annealing process. The chemical composition, and the optical property of the obtained TaON thin film were investigated and discussed. 2. Experiment TaON thin films have been fabricated by rf magnetron sputtering system (AJA international, Inc; ATC 2000-F) cooperated with the RGT technique at room temperature. High purity
ACCEPTED MANUSCRIPT (99.99%) tantalum (Kurt J. Lesker) of 2-inch diameter was used as a sputtering target. Si (100) wafer and glass slide were used as substrates, which had been sequentially cleaned in ultrasonic
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washer with acetone, isopropanol, and deionized water, and then dried in nitrogen atmosphere.
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With a mechanical (ALCATEL) and a turbomolecular pump (Shimazu, TMP-803-LM), a base
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pressure of the deposition chamber was achieved at 1.010-6 Torr. High purity (99.999 %) argon was supplied as sputtering gas. High purity (99.999 %) oxygen and nitrogen were supplied as
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reactive gases. The argon, oxygen, and nitrogen were controlled and monitored with mass flow meters (MKS) at constant flow rates of 10, 5, and 3 sccm, respectively.
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Prior to the depositions, the Ta target was pre-sputtered in argon (99.999 %) ambient for 10 min to remove surface oxide from target poisoning. During the depositions, a constant RF power
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of 200 W and the continuous flow of argon were supplied in order to deposit the thin films. In this study, all the thin films were prepared and controlled at the thickness of 200 nm, based on
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the expected results from preliminary experiments. With the RGT technique, the film fabrication utilized alternate on-off time sequences of the reactive gases in order to control alternate flows of either oxygen or nitrogen into the vacuum chamber. In this study, the oxygen timing was varied at 5, 15, 30, and 60 sec (s), and the nitrogen timing was fixed at 60 sec. We denoted the RGT parameters of the O2:N2 timing (sec: sec) as 5:60, 15:60, 30:60, and 60:60, respectively. Fig. 1 illustrates the schematic of O2:N2 timing control at 30:60 sec at flow rate of 5:3 sccm, respectively with continuous flow of argon at 10 sccm, during the film deposition. For comparison, another TaON thin film was also prepared by conventional sputtering method, where the reactive gases were both simultaneously introduced to the vacuum chamber at the same flow rates as the RGT conditions.
ACCEPTED MANUSCRIPT The obtained TaON thin films were investigated for deposition rates, film compositions, and optical characteristics. The film thickness was confirmed by a step-profiler (Surftest sv-3000,
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Mitutoyo). The film compositions were determined by Auger electron microscopy (AES),
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(PHI700, ULVAC), in a combination with a depth profile technique based on argon etching
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process. The optical constants were investigated and analyzed by variable-angle spectroscopic ellipsometry (J.A. Woollam) at 70 incident angle in the range 1.0-6.0 eV. Optical transmission
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was characterized by UV-Vis spectrophotometer (T90+, PG Instrument).
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3. Result and discussion
Fig. 2 shows the deposition rate of the TaON thin films prepared with the RGT technique
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which the nitrogen gas-timing interval was fixed at 60 s and the oxygen interval ranged from 5 to 60 s. For comparison, the figure also includes the deposition rate of the thin film prepared with a
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conventional reactive sputtering, where the mixed reactive gases were supplied. The results clearly showed that the RGT technique yielded excellent deposition rate at all oxygen gas-timing intervals. Such high deposition rate during the TaON fabrication with the RGT technique could be described as a following mechanism based on the alternate highs-and-lows of the sputtering pressure. In general sputtering process, the deposition rate strongly corresponds to the sputtering pressure, which is also inversely related to the mean free path of the sputtered particles and the oxidation of the target surface. In this study, the sputtering pressure of the RGT technique was lower than the conventional reactive sputtering. When the film deposition began, argon and nitrogen were introduced into the chamber, leading to high sputtering pressure at approximately 3.2 mTorr within the first 60 s. When the nitrogen supply stopped and the oxygen flow was immediately switched on, the sputtering pressure would drift to a lower pressure state at
ACCEPTED MANUSCRIPT approximately 2.7 mTorr in a short amount of time. During such time, the sputtered atoms, which still maintained long mean free paths, now experienced fewer collisions. They therefore
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maintained relatively high particle energies to reach the substrates and allowed the increased
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deposition rate [26]. When the oxygen flow stopped and the nitrogen was immediately resupplied, the operating pressure increased, originating the decrease deposition rate. The
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alternate on-off sequences of each reactive gas would continually repeat through the whole thin-
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film deposition, which would therefore allow high deposition rates at all the O2:N2 gas-timing conditions.
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For further validations from the results in Fig. 2, we have explored the cross-sectional micrographs based on FE-SEM analyses for confirmation of the film thickness. The results
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corresponded to those from the depth profiler. The authors noted that, there were no multilayer microstructures presented from all the fabricated TaON thin films. In essence, during the film
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deposition with the argon constantly supplied as the sputtering gas, the nitrogen and oxygen were alternatively supplied as the reactive gas. Although each gas were supplied into the vacuum chamber at the alternate cycles, both gases were supposed to mix well with the argon gas within the deposition process. When the nitrogen was stopped and the oxygen was supplied, the nitrogen atomic energy of was increased, and vice versa. Therefore, the RGT technique actually exerted more atomic energy of each reactive gas into the film stoichiometry. This assumption could help predict that the conventional sputtering technique would yield slightly inhomogeneous films with included porosity, while the RGT technique would yield homogeneous dense films, without causing the multilayer structures. The predictions were to be confirmed by the AES depth profiles from the argon etching process, to be discussed in the next paragraph.
ACCEPTED MANUSCRIPT Fig. 3 shows the compositions of the TaON thin films prepared by the RGT technique in comparison with the conventional technique, as analyzed by the AES analyses. The results
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showed that both the nitrogen and tantalum atomic concentrations of the TaON film prepared by
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conventional reactive sputtering were lower than those prepared by the RGT technique. Generally, tantalum possesses a stronger affinity to oxygen than nitrogen. When the film from
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the conventional sputtering showed lower nitrogen concentration corresponded to higher particle
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energies caused by low sputtering pressure. As the oxygen gas-timing intervals were increased, the oxygen concentration in the films was progressively increased, while the nitrogen content
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was decreased. The concentration of tantalum slightly decreased from 70 to 60 at.%. With the spectroscopic ellipsometer (SE) measurements and analyses, the optical constants of
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the fabricated films could be obtained. In this work, the SE physical models included the silicon substrate, the native oxide layer, the TaON layer, and the surface roughness. The native oxide of
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2 nm was included because the silicon substrates had not been etched by HF before the film deposition. The SE optical models of TaON utilized a single Tauc-Lorentz oscillator. The refractive index and the extinction coefficient of the thin films, obtained from the SE model fits, were plotted as shown in Fig. 4. The results showed that the TaON films from the RGT technique yielded higher optical constants (n and k at 550 nm) than the film from the convention sputtering, because of their density variations depended on the nitrogen concentrations. At the smallest oxygen gas-timing interval from the O2:N2 RGT parameter of 5s:60s, both optical constants were very high, with the reactive index at approximately 3.0. At this parameter, only small amount of oxygen was incorporated among the nitrogen concentration within the film structure. When the oxygen gas-timing intervals were increased to 60 s, the refractive index of the TaON films was rapidly decreased because of the higher amount of oxygen incorporations
ACCEPTED MANUSCRIPT contributed to the decrease in the nitrogen concentrations and the film density. Such results also corresponded to other publications, whose bulk density materials yielded higher optical index [8,
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10, 27]. The obtained values of the refractive index by the SE analyses corresponded well with
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the nitrogen concentrations within the thin films previously analyzed by AES. Fig. 5 shows the UV-Vis transmission spectra in the wavelength range 200-850 nm for the
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fabricated TaON thin films on the glass substrates. In this study, a blank glass was used as a
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reference in the double-beam measurements, whose results allowed relative optical transmission of the thin films. From the results, the TaON film prepared by the conventional mixed reactive
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gases was highly transparent with an average transmittance of ~ 90% in the visible region. However, the TaON films obtained with the RGT technique, with the 5 s O2 timing, revealed
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lower transmittance because of high nitrogen content incorporated within the film. With the increased oxygen gas-timing intervals, the TaON thin films became less absorbing and the
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absorption edge significantly shifted toward shorter wavelengths. The transmission spectra were then used to determine the optical band gaps of the TaON thin films. For wavelengths close to values where loss scattering are dominated by the fundamental absorption of light, the absorption coefficient can be calculated using the expression = d-1ln(1/T)
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Where d is the thickness of the film and T is the optical transmittance. In the vicinity of a fundamental absorption, the indirect allowed transition dominates over the optical absorption, according to the relation [1]. (h)1/2 = A(h-Eg)
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ACCEPTED MANUSCRIPT Where h is the photon energy, A is parameter independent of photon energy for respective transition, and Eg is the optical band gap. Hence, at photon energies h>Eg, the TaON material
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can absorb photons, while at h
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The optical band gaps could be obtained by extrapolating the linear part the curves of 1/2
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versus h. As shown in Fig. 6, the plots represented the curves to be determined for the band gaps of the TaON thin films in this study. From the results, the deposition with conventional
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mixed reactive gases yielded the thin film with very high bang gap of 4.21 eV. The depositions
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with the RGT technique however yielded the thin films with very small band gaps in the range of 1.90 – 2.15 eV. The variations of the RGT parameters of the O2:N2 timing intervals could alter
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the band gaps around 2 eV. This behavior of band gap engineering from the reactive gas-timing
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technique was because of different nitrogen contents being supplied during the deposition process. The results of the optical band gaps were finally plotted with respect to the composition
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ratio of (nitrogen + oxygen)/tantalum as a zone diagram, as shown in Fig. 7. The diagram also compared the thin film results in this work with other notable publications. From the Fig. 7, the TaON films fabricated by most publications yielded the (N+O)/Ta ratio higher than 1.5. From the J. H. Hsieh's works [1, 8], while the TaON thin films allowed wide variations of the band gap engineering at high atomic ratio, their deposition processes required the substrate bias and the annealing treatments. On the other hand, the publication by H. Le Dr'eo, et al. [28] experimented on oxygen pulses and constant nitrogen flow during the film depositions. Their work also reported a wide range of band gaps from the thin films with the highest atomic ratio up to 3.25. In our work, the fabricated thin films however yielded the (N+O)/Ta atomic ratio at approximately 0.5 for all reactive depositions with a moderate range of the optical band gap variations. The results proved that the sputtering deposition with the
ACCEPTED MANUSCRIPT proposed reactive gas-timing technique without the annealing treatments allowed the smallest (N+O)/Ta atomic ratio. Since this work well demonstrated the band gap engineering from the
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film fabrications with the RGT technique, further studies of the TaON thin films with alternate
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atomic compositions could be explored for many potential applications.
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4. Summary
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The TaON thin films have been deposited using rf reactive magnetron sputtering with RGT technique onto glass slide and c-Si substrate at room temperature. The analysis indicates that the
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compositions of TaON thin films were related to the O2:N2 gas-timing parameters. The AES results demonstrated all the RGT sputtered TaON were chemical compositions ratio ((N+O)/Ta)
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at approximately 0.5. Spectroscopic ellipsometry results indicate that the values of refractive index for the RGT sputtered TaON thin films were higher than conventional reactive sputtered
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TaON thin films. In particular, the refractive index of the RGT sputtered TaON thin films decrease with increasing oxygen interval time. The obtained optical band gap of the RGT sputtered TaON thin film shows widely range from 1.90 – 2.15 eV. In addition, the RGT was an alternative technique to fabricated metal oxynitride films with large range of chemical compositions and band gap engineering for optical coating applications.
Acknowledgments We would like to appreciate to the National Electronic and Computer Technique Center (NECTEC), NSTDA, Ministry of Science and Technology, Thailand, under the R&D project as “white light emitting diode based on zinc oxide optoelectronics material; phase-1: method of ZnO-wafer fabrication” for instrument supporting and National Nanotechnology Center
ACCEPTED MANUSCRIPT (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network.
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Figure Caption
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Fig. 1 Illustrates the schematic of O2:N2 timing control at 30:60 sec.
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Fig. 2 Variation of deposition rate for the TaON thin films as a function of oxygen gas-timing
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intervals.
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Fig. 3 AES results of the TaON thin films deposited by conventional reactive sputtering and
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RGT technique at different oxygen gas-timing intervals.
Fig. 4 Refractive index and extinction coefficient of the TaON thin films deposited by conventional reactive sputtering and RGT technique at different oxygen gas-timing intervals.
Fig. 5 Transmittance of the TaON thin films deposited by conventional reactive sputtering and RGT technique at different oxygen gas-timing intervals.
Fig. 6 (h)1/2 as a function photon energy for the TaON thin films deposited by conventional reactive sputtering and RGT technique at different oxygen gas-timing intervals.
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nitrogen)/tantalum, in comparison with notable published results.
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ACCEPTED MANUSCRIPT Highlights An innovated technique to fabricate TaON thin film at room temperature.
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The chemical composition is precisely controlled.
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The optical band gap of TaON thin film varies from 1.90 to 4.21eV.
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The reactive gas-timing technique is suitable for low melting point material.
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