Investigation of Antireflection Nb2o5 Thin Films by the Sputtering Method

micromachines

Article Investigation of Antireﬂection Nb2O5 Thin Films by the Sputtering Method under Different Deposition Parameters

Kun-Neng Chen 1, Chao-Ming Hsu 2, Jing Liu 3, Yu-Chen Liou 4 and Cheng-Fu Yang 4,* 1 Department of Electrical Engineering, Kun-Shan University, Tainan 710, Taiwan; [email protected] 2 Department of Mechanical Engineering, National Kaohsiung University of Applied Science, Kaohsiung 807, Taiwan; [email protected] 3 School of Information Engineering, Jimei University, Xiamen 361021, China; [email protected] 4 Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 811, Taiwan; [email protected] * Correspondence: [email protected]; Tel.: +886-7-591-9283; Fax: +886-7-591-9277

Academic Editors: Teen-Hang Meen, Shoou-Jinn Chang, Stephen D. Prior and Artde Donald Kin-Tak Lam Received: 11 July 2016; Accepted: 24 August 2016; Published: 1 September 2016

Abstract: In this study, Nb2O5 ceramic was used as the target to deposit the Nb2O5 thin films on glass substrates with the radio frequency (RF) magnetron sputtering method. Different deposition temperatures and O2 ratios were used as parameters to investigate the optical properties of Nb2O5 thin films. The deposition parameters were a pressure of 5 × 10−3 Torr, a deposition power of 100 W, a deposition time of 30 min, an O2 ratio (O2/(O2 + Ar), in sccm) of 10% and 20%, and deposition temperatures of room temperature (RT), 200, 300 and 400 ◦C, respectively. We found that even if ◦ the deposition temperature was 400 C, the deposited Nb2O5 thin films revealed an amorphous phase and no crystallization phase was observed. The optical properties of transmittance of Nb2O5 thin films deposited on glass substrates were determined by using a ultraviolet-visible (UV-vis) spectrophotometer (transmittance) and reflectance spectra transmittance (reflectance, refractive index, and extinction coefficient) in the light wavelength range of 250–1000 nm. When the O2 ratio was 10% and the deposition temperature increased from RT to 200 ◦C, the red-shift was clearly observed in the transmittance curve and the transmission ratio had no apparent change with the increasing deposition temperature. When the O2 ratio was 20%, the red-shift was not observed in the transmittance curve and the transmission ratio apparently decreased with the increasing deposition temperature. The variations in the optical band gap (Eg) values of Nb2O5 thin films were evaluated from the Tauc plot by using the quantity hν (the photon energy) on the abscissa and the quantity (αhν)r on the ordinate, where α is the optical absorption coefficient, c is the constant for direct transition, h is Planck’s constant, ν is the frequency of the incident photon, and the exponent r denotes the nature of the transition. As the O2 ratio of 10% or 20% was used as the deposition atmosphere, the measured Eg values decreased with the increase of the deposition temperature. The reflectance ratio, extinction coefficient, and refractive index curves of Nb2O5 thin films were also investigated in this study. We would show that those results were influenced by the deposition temperature and O2 ratio.

Keywords: Nb2O5 thin ﬁlms; anti-reﬂection; deposition temperature; transmittance; optical band gap

1. Introduction Different metal-oxide thin ﬁlms are increasingly applied for a wide range of optical and microelectronic applications. In those materials, the most stable form of niobium oxides is Nb2O5 and it is very attractive for electronic and optical applications. Nb2O5, along with other oxides of metals

Micromachines 2016, 7, 151; doi:10.3390/mi7090151 www.mdpi.com/journal/micromachines Micromachines 2016, 7, 151 2 of 12 from the Vth group, have also been investigated as possible candidates for thin film catalysts and corrosion barrier coatings [1,2]. In the case of optical applications, Nb2O5-based thin films are often used as high index and low loss materials; for example, they can be used as optical waveguides [3,4], interference filters, antireflective or antireflection (AR) coatings or electroluminescent devices [5–7]. An AR coating is a type of optical coating applied to the surface of lenses and other optical elements to reduce reflection. For example, bare silicon–based solar cells have a high surface reflection of over 30%. AR coatings are applied to reduce surface reflection and maximize the devices’ efficiency for the solar cells manufactured on glass and silicon substrates [8]. The simplest interference antireflection (AR) coating can be achieved by using a single quarter-wave layer of oxide thin films whose refractive index is chosen as the square root of the substrate’s refractive index. The single-layer AR coating consists of a dielectric thin film. As the thin films have a specially chosen thickness, the interference effects in the AR coating will cause the wave to be reflected from the top surface of the coating layers to be out of phase with the wave reflected from the surfaces of the semiconductor [9]. For that, the thin films can have an effect of zero reflectance at the center light wavelength which can decrease reflectance for light wavelengths in a broad band around the center. As the multi-layer AR coating thin films are used, the alternating layers of a low-index material (such as SiO2) and a higher-index material (such as ZrO2) are possible to obtain reflectivity as low as 0.1% at a special or single wavelength [10,11]. Nb2O5 is one of the useful optical thin-film materials because of its desirable properties, including stability in air and water and resistance to acids and bases. Also, the Nb2O5 thin films have the properties of high refractive index, low extinction coefficient, and high transparent ratio in the UV-vis-NIR (ultraviolet-visible near-infrared) region [12,13]. The properties of Nb2O5 thin films closely rely on the sputtered materials, deposition techniques, deposition parameters, and the thicknesses of thin films. For that, many different methods are investigated to deposit Nb2O5 thin films. For example, Nb2O5 thin films can be deposited on variety of substrates using the sol-gel dip coating technique [14], and the deposited Nb2O5 thin films revealed an amorphous phase [1,15,16]. Agarwal and Reddy ◦ found that after annealing Nb2O5 thin films under controlled ambience at 600 C for 5 h, the thin films deposited on NaCl substrates crystallized into a stable monoclinic phase and those deposited on single-crystal Si substrates crystallized into a hexagonal phase [13]. The optical band gap of Nb2O5 thin films increased from 4.35 eV when the films were in an amorphous state to 4.87 eV on crystallization [13]. Lazarova et al. obtained Nb2O5 thin films by mixing NbCl5 with ethanol to form the spin-coating precursor. The mixing material was spin-coated on glass and silicon substrates and then they were annealed at different temperatures to obtain the Nb2O5 thin films [17]. Nb2O5 thin films could also be formed by annealing the sputter-deposited Nb thin films under a pure Ar flow plasma and annealing them in a quartz tube at 850 ◦C. Jose et al. observed vibrational modes including longitudinal optical, transverse optical, and triply degenerate modes, and found the direct optical band gap to be ~3.49 eV [18]. Sputtering is a simple method to deposit Nb2O5 thin films with a controllable thickness [19]. In the sputtering method, the properties of the Nb2O5 thin films depend on the deposition parameters, such as the deposition temperature [20], reactive gas flow or pressure. In this paper, Nb2O5 thin films were deposited on glass substrates by using the sputtering method. The effects of the deposition temperature and oxygen concentration (10% and 20%) in the deposition atmosphere on the properties of Nb2O5 thin films were investigated by X-ray diffraction (XRD) patterns and field emission scanning electron microscopy (FESEM), respectively. In this paper we found two important novelties. The first novelty in this paper is that the Nb2O5 thin films deposited in pure argon atmosphere revealing an un-transparent result. The second novelty is that the O2 flow rate has a large effect on the properties of Nb2O5 thin films; only a few studies focused on the effect of the O2 flow rate on that topic. In the past, the Nb2O5 thin films had been investigated by different methods, for example by the microwave-assisted reactive magnetron sputtering process [6], by the glancing angle deposition process [12], by the sol-gel dip coating technique [13], by the sol-gel and evaporation-induced self-assembly method [14], by radio frequency (RF) magnetron sputtering [16], and by a low frequency reactive magnetron sputtering Micromachines 2016, 7, 151 3 of 12

system [19], respectively. In the past, as the physical plasma method was used to deposit the Nb2O5 thin films, no reports were focused on the effect of the O2 flow rate in the (O2 + Ar) mixing atmosphere on the properties’ extinction coefficient, optical band gap (Eg) values, and refractive index. Coskuna et al. used the RF magnetron sputtering method to deposit the Nb2O5 thin films [16], and they investigated the effect of the deposition temperature but they did not investigate the effect of the O2 flow rate on the properties of Nb2O5 thin films. Mazur et al. used the microwave-assisted reactive magnetron sputtering process [6] and Lai et al. used the low frequency reactive magnetron sputtering system [19] to deposit the Nb2O5 thin films by using a metallic Nb target in the O2 + Ar mixing atmosphere, and they also did not investigate the effect of the O2 flow rate on the properties of Nb2O5 thin films. RF magnetron sputtering is a well-established high-rate vacuum coating method for the deposition of industrial thin film materials and the thickness of deposited thin films can be well controlled by controlling the deposition parameters. For that, we used RF magnetron sputtering and the Nb2O5 ceramic target to deposit the Nb2O5 thin films. We found that the O2 flow rate had a large effect on the transmittance and reflectance of Nb2O5 thin films. Additionally, optical properties of Nb2O5 thin films, such as transmittance, reflectance, and reactance, were determined by using a spectrophotometer and elliposmeter. The second novelty is that we discussed different O2 flow rates in the (O2 + Ar) mixing atmosphere to investigate its effect on the properties of Nb2O5 thin films, especially on the extinction coefficient, optical band gap (Eg) values, and refractive index. For example, the refractive index of Nb2O5 thin films increased with the increase of the O2 flow rate in the (O2 + Ar) mixing atmosphere. The results in this study show that the Nb2O5 antireflective layer with protective properties can be proposed for possible use in silicon solar cells.

2. Experimental Section

At the first, Nb2O5 powder was mixed with acetone, dried, and ground, and mixed with polyvinyl ◦ alcohol (PVA) as binder. After being debindered, Nb2O5 ceramic was sintered at 1400 C for 2 h in air. The crystalline phases were analyzed by using X-ray diffraction (XRD) patterns to find the crystallization of Nb2O5 ceramic, and only the Nb2O5 phase was observed in the sintered ceramic. Glass substrates (Corning 1737, Coring, New York, NY, USA) with an area of 2 × 2 cm2 were cleaned ultrasonically with isopropyl alcohol (IPA) and deionized (DI) water and then dried under a blown nitrogen gas. Deposition power of Nb2O5 thin films was 100 W, deposition time was 30 min, and the deposition temperatures were room temperature (RT), 200, 300 and 400 ◦C, respectively. In this study, the radio frequency (RF) magnetron sputtering (SYSKEY, Hsinchu, Taiwan) was used as equipment −6 to deposit the Nb2O5 thin films. The base pressure of sputtering chamber was below 5 × 10 Torr, the deposition time and power were 30 min and 100 W, the working pressure was maintained at −3 3 × 10 Torr in O2 5 sccm-Ar 45 sccm (O2 ratio of 10%) and in O2 10 sccm-Ar 40 sccm (O2 ratio of 20%) ambient. Thickness and surface morphology of Nb2O5 thin films were measured using a field emission scanning electron microscopy (FESEM), the roughness (or flatness) was measured using atomic force microscopy (AFM), and their crystalline structures were measured using X-ray diffraction (XRD) patterns with Cu Kα radiation (λ = 1.5418 Å). The optical transmission spectrum was recorded using a Hitachi U-3300 ultraviolet–visible (UV-vis) spectrophotometer in the 250–1000 nm light wavelength range and the reflectance spectra of the films measured at normal light incidence by UV-vis-NIR spectrophotometer Cary 05E (Varian Australia Pty. Ltd., Mulgrave, Australia) using non-linear curve fitting method, respectively. Refractive index (RI) is a complex number comprising a real refractive index and an imaginary part: the absorption (or extinction) coefficient. Therefore, the well-established technique of ellipsometry can determine both the real refractive index (n) and extinction coefficient (k). The optical properties (n and k) along with the thickness of Nb2O5 thin films were determined from n&k Analyzer 1280 (n&k Technology, San Jose, CA, USA). At the first, the baseline sample was put on the n&k Analyzer 1280, after that the measured samples were put on the n&k Analyzer 1280 and the software would calculate the n and k value. Micromachines 2016, 7, 151 4 of 12 measured samples were put on the n&k Analyzer 1280 and the software would calculate the n and k Micromachines 2016, 7, 151 4 of 12 value.

3. Results Results and and Discussion Discussion

Figure 11 showsshows thethe surfacesurface andand cross-sectioncross-section observationsobservations ofof Nb Nb22O5 thinthin films films with various deposition temperatures and and the O 2 ratioratio was was 10%. 10%. As As the the results results in in Figure Figure 11 show,show, highhigh resolutionresolution SEM revealsreveals that that when when the depositionthe deposition temperature temperature was RT, was there RT, was there a densified was a surface densified morphology, surface morphology,consisting of nanocrystallineconsisting of nanocrystalline particles growing particles in a randomgrowing orientation.in a random Further orientation. increasing Further the ◦ increasingdeposition the temperature deposition to temperature 300 and 400 toC 300 caused and the400 Nb °C2 Ocaused5 thin the films Nb to2O have5 thin larger films nanocrystalline to have larger nanocrystallinegrains. Even when grains. the OEven2 ratio when was the changed O2 ratio to 20%, was thechanged variations to 20%, in the the surface variations morphology in the surface of the morphologyNb2O5 thin films of the had Nb similar2O5 thin results, films andhad thesimilar sizes results, of the nanocrystallineand the sizes of grains the nanocrystalline increased (not showngrains increasedhere). The (not root shown mean squarehere). The (RMS) root surface mean square roughness (RMS) of thesurface Nb2 roughnessO5 thin films of the was Nb measured2O5 thin films to be was0.30 nmmeasured by AFM. to Whenbe 0.30 the nm deposition by AFM. temperatureWhen the deposition was RT, 200, temperature 300 and 400 was◦C, RT, the 200, measured 300 and values 400 were°C, the 2.56, measured 3.46, 4.39 values and 5.25 were nm 2.56, when 3.46, the O4.392 ratio and was 5.25 10% nm and when the measuredthe O2 ratio values was were 10% 2.44, and 3.32,the measured4.24 and 5.08 values nm when were the 2.44, O2 ratio3.32, was4.24 20%, and respectively.5.08 nm when Apparently, the O2 theratio roughness was 20%, increased respectively. as the Apparently,deposition temperature the roughness was raised.increased Apparently, as the deposition the results te showmperature that the was roughness raised. ofApparently, the Nb2O5 thinthe resultsfilms increased show that with the the roughness increasing of deposition the Nb2O temperature.5 thin films increased The results with in Figure the increasing1 have also deposition revealed temperature.an important result:The results that thein Figure nanocrystalline 1 have also grains revealed had uniforman important particle result: sizes. that The the average nanocrystalline crystallite grainssizes of had Nb 2uniformO5 thin filmsparticle can sizes. be calculated The average using crystallite the following sizes of equation: Nb2O5 thin films can be calculated using the following equation: G = −2.9542 + 1.4427ln(N) (1) G = −2.9542 + 1.4427ln(N) (1) where G isis thethe numbernumber ofof grainsgrains perper unitunit areaarea atat aa particularparticular magnification,magnification, N is the number of grains/mm22. .The averageaverage crystallite crystallite sizes sizes were were about about 12.3, 16.5,12.3, 22.716.5, and 22.7 34.5 and nm when34.5 nm the depositionwhen the depositiontemperatures temperatures were RT (Figure were1 a),RT 200 (Figure◦C (not 1a), shown 200 °C here), (not shown 300 ◦C (Figurehere), 3001b), °C and (Figure 400 ◦ C1b), (Figure and 1400c), °Crespectively. (Figure 1c), When respectively. the O2 ratio When was the 20%, O2 ratio the average was 20%, crystallite the average sizes crystallite were about sizes 11.2, were 15.1, about 21.4 11.2, and 15.1,32.5 nm21.4 when and 32.5 the depositionnm when temperaturesthe deposition were temperatures RT, 200 ◦C, were 300 ◦RT,C, and200 400°C, ◦300C, respectively°C, and 400 (not°C, respectivelyshown here). (not shown here).

Figure 1. Surface morphology and cross observations of Nb2O5 thin film deposited at (a) room Figure 1. Surface morphology and cross observations of Nb2O5 thin ﬁlm deposited at (a) room temperature (RT), (b) 300 ◦°C, and (c) 400 ◦°C, respectively. (d) Cross-section observation of Nb2O5 thin temperature (RT), (b) 300 C, and (c) 400 C, respectively. (d) Cross-section observation of Nb2O5 thin films, the deposition temperature was 200 ◦°C and the O2 ratio was 10%, respectively. ﬁlms, the deposition temperature was 200 C and the O2 ratio was 10%, respectively.

The average crystallite sizes and the deviation of the crystallite sizes of Nb2O5 thin films are The average crystallite sizes and the deviation of the crystallite sizes of Nb2O5 thin films are shown shown in Table 1. We believe that the higher deposition temperature will cause the Nb2O5 molecules in Table1. We believe that the higher deposition temperature will cause the Nb 2O5 molecules to have a to have a large activation energy and then the Nb2O5 thin films have a larger particle size and larger large activation energy and then the Nb2O5 thin films have a larger particle size and larger roughness. roughness. The thickness of Nb2O5 thin films is also observed and the result is shown in Figure 1d. The thickness of Nb2O5 thin films is also observed and the result is shown in Figure1d. The thickness The thickness was around 75 nm when the deposition temperature◦ was 200 °C and the O2 ratio was was around 75 nm when the deposition temperature was 200 C and the O2 ratio was 10%, respectively. 10%, respectively. We found that as the deposition temperature was changed◦ from RT to 400 °C and We found that as the deposition temperature was changed from RT to 400 C and the O2 ratio was the O2 ratio was 10% and 20%, the average thicknesses of Nb2O5 thin films were in the range of 72.7– 10% and 20%, the average thicknesses of Nb O thin films were in the range of 72.7–77.7 nm and 77.7 nm and 46.4–47.9 nm, respectively. The averag2 5 e thicknesses and the deviation of the thicknesses 46.4–47.9 nm, respectively. The average thicknesses and the deviation of the thicknesses of Nb2O5 thin films are also shown in Table1. Those results suggest that in this study, the O 2 ratio has no effect but Micromachines 2016, 77,, 151 5 of 12 of Nb2O5 thin films are also shown in Table 1. Those results suggest that in this study, the O2 ratio hasthe depositionno effect but temperature the deposition has temperature apparent effect has onapparent the surface effect morphology, on the surface and morphology, the O2 ratio and has the an Oeffect2 ratio but has the depositionan effect but temperature the deposition has no temperat apparenture effect has on no the apparent thickness, effect respectively. on the thickness, respectively.The XRD patterns of Nb2O5 thin films developed as a function of deposition temperatures are shownThe in XRD Figure patterns2a, where of theNb2 oxygenO5 thin concentrationfilms developed was as 10%. a function As the differentof deposition sintering temperatures temperatures are wereshown used, in theFigure differently 2a, where crystalline the oxygen phases wouldconcentration be formed was in the10%. Nb 2AsO5 ceramicthe different targets, sintering and the temperaturesmulti-crystal phaseswere used, were onlythe differently observed incrys thetalline Nb2O phases5 ceramic would targets. be Nbformed2O5 thin in the films Nb deposited2O5 ceramic by targets,the sputtering and the method multi-crystal in the O 2phas/Ares mixed were atmosphere only observed would in revealthe Nb the2O5 amorphous ceramic targets. phase Nb rather2O5 thanthin filmsthe polycrystal deposited phase by the since sputtering no characteristic method peaks in werethe O observed2/Ar mixed in the atmosphere XRD patterns. would The amorphousreveal the amorphousnature of the phase deposited rather thinthan filmsthe polycrystal can result phase from thesince low no temperature characteristic of peaks the sputtering were observed process. in ◦ theEven XRD when patterns. the Nb2 OThe5 thin amorphous films were nature deposited of the at 400depositedC, the depositionthin films temperaturecan result from was notthe highlow temperatureenough to crystallize of the sputtering the Nb2 process.O5 thin films.Even when Figure the2b Nb shows2O5 thin that films when were the deposited oxygen concentration at 400 °C, the ◦ wasdeposition 20% and temperature the deposition was not temperature high enough was to 400 crystallizeC, the XRDthe Nb patterns2O5 thin of films. Nb2O Figure5 thin films2b shows also thatrevealed when the the amorphous oxygen concentration phase. was 20% and the deposition temperature was 400 °C, the XRD patterns of Nb2O5 thin films also revealed the amorphous phase. Table 1. Average crystallite sizes, variation of crystallite sizes, average thicknesses, and variation of Tablethicknesses 1. Average of Nb 2crystalliteO5 thin films. sizes, variation of crystallite sizes, average thicknesses, and variation of

thicknesses of Nb2O5 thin films. Average Variation of Average Variation of Atmosphere TemperatureAverageCrystallite VariationCrystallite of ThicknessesAverage ThicknessesVariation of Atmosphere Temperature CrystalliteSizes (nm)CrystalliteSizes Sizes (nm) Thicknesses(nm) Thicknesses(nm) RTSizes (nm) 12.3(nm) 10.2–13.9 (nm) 72.7 70.5–74.7(nm) RT 200 ◦C12.3 16.5 10.2–13.9 14.3–18.5 75.172.7 73.2–77.5 70.5–74.7 10% O 2 200 °C 300 ◦C 16.5 22.7 14.3–18.5 19.7–25.9 75.875.1 73.8–77.3 73.2–77.5 10% O2 300 °C 400 ◦C 22.7 34.5 19.7–25.9 31.1–38.6 77.775.8 75.2–79.6 73.8–77.3 400 °C RT 34.5 11.2 31.1–38.6 9.5–13.3 47.277.7 45.8–48.7 75.2–79.6 RT 200 ◦C11.2 15.1 9.5–13.3 12.5–18.1 47.547.2 45.7–49.2 45.8–48.7 20% O 2 200 °C 300 ◦C 15.1 21.4 12.5–18.1 19.4–23.8 47.947.5 46.1–49.3 45.7–49.2 20% O2 300 °C 400 ◦C 21.4 32.5 19.4–23.8 29.7–36.9 46.447.9 44.9–47.8 46.1–49.3 400 °C 32.5 29.7–36.9 46.4 44.9–47.8

(a) (b) o 400oC 400 C

o 300oC 300 C

o 200oC 200 C Intensity (a.u) Intensity (a.u) Intensity RT RT

20 30 40 50 60 20 30 40 50 60 θ ( θ ( 2 degree) 2 degree)

Figure 2. X-ray diffraction (XRD) patterns of Nb2O5 thin films as a function of deposition temperature Figure 2. X-ray diffraction (XRD) patterns of Nb2O5 thin ﬁlms as a function of deposition temperature and O2 ratio. (a) O2 ratio 10% and (b) O2 ratio 20%, respectively. and O2 ratio. (a)O2 ratio 10% and (b)O2 ratio 20%, respectively.

The transmittance of Nb2O5 thin films under various deposition temperatures and O2 ratios is The transmittance of Nb2O5 thin films under various deposition temperatures and O2 ratios is depicted in Figure 3. When the O2 ratio was 10%, the average total transmittance of the RT-deposited depicted in Figure3. When the O 2 ratio was 10%, the average total transmittance of the RT-deposited Nb2O5 thin films was 74.42%, whereas the Nb2O5 thin films showed that the average total Nb O thin films was 74.42%, whereas the Nb O thin films showed that the average total transmittance transmittance2 5 was almost unchanged with the2 increa5 sing deposition temperature in the visible-light was almost unchanged with the increasing deposition temperature in the visible-light wavelength wavelength (400–800 nm) region. The average total transmittances of 74.15%, 73.87%, and 75.17% (400–800 nm) region. The average total transmittances of 74.15%, 73.87%, and 75.17% were shown as the were shown as the deposition temperature was 200, 300 and 400 °C, respectively. The maximum deposition temperature was 200, 300 and 400 ◦C, respectively. The maximum transmittance ratio was transmittance ratio was 85.73% at 361 nm, 90.78% at 382 nm, 91.06% at 387 nm, and 91.54% at 402 85.73% at 361 nm, 90.78% at 382 nm, 91.06% at 387 nm, and 91.54% at 402 nm, respectively. However, nm, respectively. However, when the O2 ratio was 20%, the average total transmittances of the when the O2 ratio was 20%, the average total transmittances of the Nb2O5 thin films had different Nb2O5 thin films had different results. When the deposition temperature was RT, 200, 300 and 400 results. When the deposition temperature was RT, 200, 300 and 400 ◦C, the average total transmittance °C, the average total transmittance was 73.82%, 69.90%, 68.42%, and 64.72%, respectively.

Micromachines 2016, 7, 151 6 of 12 Micromachines 2016, 7, 151 6 of 12 wasApparently, 73.82%, 69.90%,the average 68.42%, total and transmittances 64.72%, respectively. of the Nb Apparently,2O5 thin films the decreased average totalwith transmittancesthe increasing depositionof the Nb2 Otemperature5 thin films in decreased the visible-light with the wavelength increasing depositionregion. The temperature results in Figure in the 3b visible-light also show wavelengththat the transmittance region. The of results the Nb in2O Figure5 thin3 filmsb also increased show that with thetransmittance the increasing of measuring the Nb 2O 5frequency.thin films Theincreased red-shift with of the maximum increasing transmittance measuring frequency. in Nb2O5 thin The films red-shift was ofalso the apparently maximum observed transmittance as the depositionin Nb2O5 thin temperature films was alsoincreased. apparently When observed the same as thedeposition deposition temperature temperature was increased. used, in When the transmissionthe same deposition spectra temperatureof the 20% O was2-deposited used, in Nb the2O transmission5 thin films, spectrathe optical of the band 20% edge O2-deposited was also shiftedNb2O5 thinto a films,shorter the light optical wavelength. band edge The was optical also shiftedtransmittance to a shorter properties light wavelength. of Nb2O5 thin The films optical are listedtransmittance in Table properties2. of Nb2O5 thin films are listed in Table2.

100 100 (a) (b)

80 80

60 60

40 40 RT RT o 200oC 200 C ansmittance (%) ansmittance (%) 20 20 o 300oC 300 C Tr Tr o 400oC 400 C 0 0 400 600 800 1000 400 600 800 1000 Wavelength (nm) Wavelength (nm)

Figure 3. Measured transmittance of Nb2O5 thin films as a function of deposition temperature and O2 Figure 3. Measured transmittance of Nb2O5 thin ﬁlms as a function of deposition temperature and O2 ratio. (a) O2 ratio 10% and (b) O2 ratio 20%, respectively. ratio. (a)O2 ratio 10% and (b)O2 ratio 20%, respectively.

Table 2. Average total transmittances, maximum transmittance ratio, and light wavelength of Table 2. Average total transmittances, maximum transmittance ratio, and light wavelength of maximum maximum transmittance ratio of Nb2O5 thin films. transmittance ratio of Nb2O5 thin ﬁlms. Maximum Light Average Total Atmosphere TemperatureAverage Total TransmittanceMaximum WavelengthLight Wavelength Atmosphere Temperature Transmittance Transmittance TransmittanceRatio Ratio (nm) (nm) RTRT 74.42%74.42% 85.73%85.73% 361 361 200 ◦C200 °C 74.15% 74.15% 90.78% 90.78% 382 382 10% O2 10% O2 ◦ 300 C300 °C 73.87% 73.87% 91.06% 91.06% 387 387 ◦ 400 C400 °C 75.17% 75.17% 91.54% 91.54% 402 402 RTRT 73.82%73.82% -- - - ◦ 200 C200 °C 69.90% 69.90% -- - - 20% O2 20% O2 ◦ 300 C300 °C 68.42% 68.42% -- - - ◦ 400 C400 °C 64.72% 64.72% -- - -

When the oxygen ratio was 10% and the deposition temperature temperature increased increased from from RT RT to to 400 °C,◦C, the optical band edges in the transmissiontransmission spectra of the NbNb2O5 thinthin films films were were shifted shifted to to a longer light wavelength and a greater sharpness was noticeablenoticeable in the curves of the absorption edges. Those results mean that as thethe depositiondeposition temperaturetemperature increases,increases, the absorptionabsorption edgeedge ofof thethe NbNb22O5 thin filmsfilms is red-shifted. In the past, the determination of the optical band gap (Eg)) was was often necessary to develop thethe electronicelectronic band band structure structure of aof thin a thin film film material. material. Typically, Typically, a Tauc a plot Tauc shows plot the shows quantity the quantityhν (the photon hν (the energy) photon on energy) the abscissa on the and abscissa the quantity and the ( αquantityhν)r on the(αh ordinate,ν)r on the where ordinate,α is where the optical α is theabsorption optical absorption coefficient, ccoefficient,is the constant c is the for constant direct transition, for directh transition,is Planck’s h constant, is Planck’sν is constant, the frequency ν is the of frequencythe incident of photon, the incident and the photon, exponent and rthedenotes exponent the naturer denotes of the the transition nature of[ 21the]. transition When using [21]. the When Tauc usingplot method, the Tauc the plotEg method,values of the thin Eg filmsvalues can of bethin determined films can be from determined the absorption from the edge absorption for the direct edge forinterband the direct transition, interband which transition, can be which calculated can be using calculated the relation using in the Equation relation (1) in [ 21Equation]: (1) [21]:

2 (αhv2) = c(hν − Eg) (2) (αhv) = c(hν − Eg) (2) where α is the optical absorption coefficient, c is the constant for direct transition, h is Planck’s whereconstant,α is and the ν optical is the absorptionfrequency coefﬁcient,of the incidentc is the photon constant [21]. for Figure direct 4 transition,shows theh energyis Planck’s band constant, gap of theand plasma-treatedν is the frequency Nb2O of5 thin the films incident plotted photon against [21 ].light Figure wavelengths4 shows thein the energy region band of 300–1000 gap of thenm. The linear dependence of (αhv)2 on hν indicates that Nb2O5 thin films are a direct transition material

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plasma-treatedMicromachines 2016, Nb 7, 1512O 5 thin films plotted against light wavelengths in the region of 300–1000 nm.7 of The 12 2 linear dependence of (αhv) on hν indicates that Nb2O5 thin films are a direct transition material [21]. The[21]. experimentalThe experimental band-gap band-gap width width of Nb 2ofO 5Nbis2 generallyO5 is generally measured measured in the orderin the oforder 3.3 toof 3.9 3.3 eV to [3.922, 23eV]. [22,23].In accordance In accordance with Equation with Equation (1), the calculated (1), the calculated optical band optical gap band of the gap Nb 2ofO 5thethin Nb films2O5 decreasedthin films decreasedfrom 3.86 tofrom 3.73 3.86 eV to when 3.73 theeV when O2 ratio the wasO2 ratio 10% was and 10% decreased and decreased from 3.91 from to 3.813.91 eVto 3.81 when eV the when O2 theratio O was2 ratio 20%. was As 20%. the resultsAs the in results Figure in4a,b Figure were 4a,b compared, were compared, we found thatwe found as the samethat as deposition the same depositiontemperature temperature was used to was deposit used Nbto deposit2O5 thin Nb films,2O5 thin the Efilms,g value the for Eg usingvalue thefor using O2 ratio the of O 20%2 ratio was of 20%higher was than higher that than for using that for the using O2 ratio the of O 10%.2 ratio of 10%.

12 1.5x1012 1.0x10 RT (Eg=3.91eV) (b) RT (Eg=3.86eV) (a) o o 200 C(Eg=3.87eV)

200 C(Eg=3.76eV) ) o o 2 300 C(Eg=3.84eV)

) 300 C(Eg=3.74eV)

2 o o 12 400 C(Eg=3.81eV) 400 C(Eg=3.73eV) /cm 1.0x10 2 /cm 2

11 eV

5.0x10 ( 2 eV ) ( ν

2 11 ) h 5.0x10 ν α ( h α (

0.0 0.0 3.0 3.5 4.0 3.0 3.5 4.0 Photon Energy, hν (eV) Photon Energy(eV)

Figure 4. The (αhv)2 versus hν − Eg plots of Nb2O5 thin films as a function of deposition temperature Figure 4. The (αhv) versus hν − Eg plots of Nb2O5 thin ﬁlms as a function of deposition temperature and O2 ratio. (a) O2 ratio 10% and (b) O2 ratio 20%, respectively. and O2 ratio. (a)O2 ratio 10% and (b)O2 ratio 20%, respectively.

The optical band gap of amorphous materials depends on their structure and components; thus, The optical band gap of amorphous materials depends on their structure and components; thus, it is closely related to the deposition techniques. For a transparent material or thin film, when the it is closely related to the deposition techniques. For a transparent material or thin film, when the light wavelength is equal to 300 nm, the visible light absorbed by the thin films is due to a quantum light wavelength is equal to 300 nm, the visible light absorbed by the thin films is due to a quantum phenomenon called band edge absorption [24]. In this study, the thicknesses of Nb2O5 thin films are phenomenon called band edge absorption [24]. In this study, the thicknesses of Nb2O5 thin films about 75 nm when the O2 ratio is 10% and about 47 nm when the O2 ratio is 20%. Because the are about 75 nm when the O2 ratio is 10% and about 47 nm when the O2 ratio is 20%. Because the thicknesses of Nb2O5 thin films are smaller than 300 nm, the band edge absorption effect will not thicknesses of Nb O thin films are smaller than 300 nm, the band edge absorption effect will not happen. We believed2 5 that the decrease in the band gap with the increased deposition temperature happen. We believed that the decrease in the band gap with the increased deposition temperature could be attributed to a shift in either the conduction or valence bands leading to narrower energy could be attributed to a shift in either the conduction or valence bands leading to narrower energy band gaps. In the past, the red-shift effect could be explained by the Burstein-Moss shift, a shift of band gaps. In the past, the red-shift effect could be explained by the Burstein-Moss shift, a shift of the Fermi level into the conduction band, which enhances the optical band gap by the energy, as the Fermi level into the conduction band, which enhances the optical band gap by the energy, as follows [25,26]: follows [25,26]: 2 222 22 2 2 } kkk1111 } k EBMΔ==EBM F FF + == F ∆ g g * ∗ (3)(3) 2 2 mmmeehmh 2mvc 2mvc where k stands for the Fermi wave vector, m is the effective mass of electrons in the conduction where kF stands for the Fermi wave vector, me is the effective mass of electrons in the conduction band, and m is the effective mass of holes in the valence band. The Nb O thin films are an insulating band, and mh h is the effective mass of holes in the valence band. 2The5 Nb2O5 thin films are an insulatingmaterial as thematerial oxygen as is the introduced oxygen duringis introduced the deposition during process. the deposition For that, the process. Burstein-Moss For that, shift the is Burstein-Mossnot the reason thatshift causesis not the the reason increase that in causes the value the of increase the optical in the band value gap. of the The optical formal band Nb-oxidation gap. The state of Nb O is +5 and the corresponding electronic configuration of the Nb atoms is [Kr]4d0, i.e., all formal Nb-oxidation2 5 state of Nb2O5 is +5 and the corresponding electronic configuration of the Nb of the d-electrons have been transferred to the O band and the Nb band is empty [23]. Moreover, atoms is [Kr]4d0, i.e., all of the d-electrons have been2p transferred to the4d O2p band and the Nb4d band is emptyBrayner [23]. et al. Moreover, experimentally Brayner observed et al. experimentally that the energy observed of the absorptionthat the energy edge of is the influenced absorption not edge only isby influenced the number not of only interconnected by the number polyhedrals of interconnected (size effect) polyhedrals but also by(size the effect) local but coordination also by the as local the particlecoordination sizes decreaseas the particle from about sizes 40 decrease to 4.5 nm. from They about also found 40 to that 4.5 thenm. transmission They also spectrumfound that shows the transmissiona significant blue spectrum shift of shows the absorption a significant edge blue and anshift increase of the ofabsorption the optical edge band and gap an from increase 3.4 to of 4.2 the eV as the size of the Nb O particles decreases from about 40 to 4.5 nm. They ascribed this modification optical band gap from2 53.4 to 4.2 eV as the size of the Nb2O5 particles decreases from about 40 to 4.5 nm.to a quantum-sizeThey ascribed effect this modification [27]. Figure1 toshows a quantum-size that particle effect sizes of[27]. the Figure Nb 2O 15 showsthin films that increased particle sizes with the increasing deposition temperature and Figure4 shows that the values of the optical band gap of the Nb2O5 thin films increased with the increasing deposition temperature and Figure 4 shows thatdecreased the values with theof the increasing optical depositionband gap decreased temperature. with We the believe increasing that as deposition the deposition temperature. temperature We believeincreases, that the as quantum-size the deposition effect temperature is the reason increase that causess, the the quantum-size decrease in the effect optical is bandthe reason gap values that of Nb O thin films. causes2 the5 decrease in the optical band gap values of Nb2O5 thin films. Reflectance is a ratio of the intensity of the incident optical light or other electromagnetic radiation on the surface of a material or a thin film. As usual, the reflectance spectrum can be plotted

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Micromachines 2016, 7, 151 8 of 12 Reflectance is a ratio of the intensity of the incident optical light or other electromagnetic radiation ason a the function surface of of the a material light wavelength. or a thin film. Figure As usual, 5 shows the the reflectance measured spectrum reflectance can beof plottedNb2O5 thin as a functionfilms as aof function the light of wavelength. the deposition Figure temperature5 shows the and measured the O2 ratio reflectance in the light of Nb wavelength2O5 thin filmsrange as of a 350–1000 function nm.of the As deposition we know, temperaturereflectivity can and be the defined O2 ratio as inthe the square light of wavelength the magnitude range of of the350–1000 Fresnel nm. reflectionAs we coefficientknow, reflectivity [28], which can be is definedthe ratio as of the the square reflected of the electric magnitude field to of incident the Fresnel on a reflection material coefficientor a thin film. [28], Ifwhich the reflection is the ratio occurs of the from reflected a thin electric layer field of mate to incidentrials and on the a material thicker orthe a thinsample film. that If the is used, reflection the effectsoccurs fromof the a thininternal layer ofreflection materials can and cause the thicker the measured the sample reflectance that is used, to thechange effects as of the the internalsurface thicknessreflection canand causereflectivity the measured are the limit reflectance values to of change reflectance. as the As surface the spectrum thickness in and Figure reflectivity 5a shows, are theas thelimit O values2 ratio was of reflectance. 10%, the average As the spectrumreflectances in Figureof 26.0%,5a shows,26.3%, 26.6%, as the Oand2 ratio 25.3% was were 10%, shown the average as the depositionreflectances temperature of 26.0%, 26.3%, was RT, 26.6%, 200, and300 and 25.3% 400 were °C. shown as the deposition temperature was RT, ◦ 200, 300The andreflectance 400 C. first increased as the light wavelength increased and reached a maximum at aroundThe 325 reflectance nm (except first the increased RT-deposited as the thin light film wavelengths), then it increaseddecreased andand reachedreached aa maximumminimum at around 400 325 nm. nm (exceptThe minimum the RT-deposited reflectance thinwas films),8.13% thenin the it 400 decreased °C–deposited and reached Nb2O5 athin minimum films. As at ◦ thearound spectrum 400 nm. in TheFigure minimum 5b shows, reflectance when wasthe 8.13%O2 ratio in thewas 400 20%,C–deposited the average Nb reflectances2O5 thin films. of 26.7%, As the 30.6%,spectrum 32.0%, in Figure and 33.7%5b shows, were when shown the as O the2 ratio depo wassition 20%, temperature the average was reflectances RT, 200, 300 of 26.7%, and 400 30.6%, °C, respectively.32.0%, and 33.7% The werereflectance shown first as the increased deposition as temperaturethe light wavelength was RT, 200, increased 300 and and 400 ◦theC, respectively.reflectance reachedThe reflectance a maximum first increased when the as thelight light wavelength wavelength wa increaseds around and 400 the nm. reflectance When the reached light wavelength a maximum increasedwhen the from light 400 wavelength to 1000 nm, was the around reflectance 400 nm. linearly When decreased. the light wavelengthThe results shown increased in Figure from 4003 and to Figure1000 nm, 5 suggest the reflectance that the linearlyO2 ratio decreased.is an important The resultsfactor that shown affects in Figures the optical3 and properties5 suggest of that Nb the2O5 thinO2 ratio films. is anThe important red-shift factorof the thatminimum affects reflectance the optical in properties the Nb2O of5 Nbthin2O films5 thin was films. also The apparently red-shift observedof the minimum as the deposition reflectance temperature in the Nb2O 5increathin filmssed. The was optical also apparently reflectance observed properties as theof Nb deposition2O5 thin filmstemperature are listed increased. in Table The3. optical reflectance properties of Nb2O5 thin films are listed in Table3.

100 100 RT (a) RT (b) o 200 C 200oC 80 o 80 300 C 300oC o 400 C 400oC 60 60

40 40 Reflectance (%) Reflectance

Reflectance (%) 20 20

0 0 400 600 800 1000 400 600 800 1000 Wavelength (nm) Wavelength (nm)

Figure 5. Measured reflectance of Nb2O5 thin films as a function of deposition temperature and O2 Figure 5. Measured reﬂectance of Nb2O5 thin ﬁlms as a function of deposition temperature and O2 ratio. (a) O2 ratio 10% and (b) O2 ratio 20%, respectively. ratio. (a)O2 ratio 10% and (b)O2 ratio 20%, respectively.

Table 3. Average reflectance, minimum reflectance, and light wavelength to reveal the minimum Table 3. Average reflectance, minimum reflectance, and light wavelength to reveal the minimum reflectance of Nb2O5 thin films. reflectance of Nb2O5 thin films. Average Minimum Light Atmosphere Temperature Average Minimum Light Wavelength Atmosphere Temperature Reflectance Reflectance Wavelength (nm) RT Reflectance26.0% Reflectance8.17% (nm)341 200 °CRT 26.3% 26.0% 8.17%8.12% 341375 10% O2 200 ◦C 26.3% 8.12% 375 10% O 300 °C 26.6% 8.27% 389 2 300 ◦C 26.6% 8.27% 389 400 °C 400 ◦C25.3% 25.3% 8.13% 402402 RT 26.7% 14.5% 300 RT 26.7% 14.5% 300 200200 °C ◦C30.6% 30.6% 13.7% 309309 20%20% O O2 2 300300 °C ◦C32.0% 32.0% 12.9% 311311 ◦ 400400 °C C33.7% 33.7% 9.90% 316316

In optics, the refractive index or index of refraction n of an optical medium is a dimensionless number that describes how light, or any other radiation, propagates through that medium. It is defined as:

Micromachines 2016, 7, 151 9 of 12

In optics, the refractive index or index of refraction n of an optical medium is a dimensionless numberMicromachines that 2016 describes, 7, 151 how light, or any other radiation, propagates through that medium.9 Itof 12 is defined as: nn= = c/vv (4)(4) where c isis the the speed speed of of light light in in a avacuum vacuum and and v isv theisthe phase phase velocity velocity of light of light in the in medium. the medium. The Therefractive refractive index index is not is nota stable a stable value value and and it itwill will vary vary with with the the light light wavelength. wavelength. That That is called dispersion and it causes the splitting of white light into its constituent colors into rainbows and chromatic aberration in lenses. However, However, the the li lightght propagation propagation in in absorbing materials is very complicated and can be described using a complex-valuedcomplex-valued refractive index. We We can measured the refractive index by using n&k Analyzer 1280 and Figure 66 shows shows thethe measuredmeasured refractiverefractive indexindex ofof Nb2OO55 thinthin films films as as a a function function of of the the deposition deposition temperature temperature and and O O22 ratio.ratio. No matter whether the O2 ratio was 10% or 20%,20%, thethe refractiverefractive indexindex ofof thethe NbNb22O5 thin filmsfilms decreased with the increasing light wavelength. For example, as Figure6 6aa showsshows forfor whenwhen OO22 ratio was 10%, the refractive index decreased from 3.14 to around 2.3 when light wavelength was longer than 450 nm and then the refractive index had had a a stable stable value value when when the the light light wavelength wavelength was was longer. longer. Figure6 6bb (when(when OO22 ratio was 20%) also shows that the refractive index at the same light wavelength increased as the deposition temperature was increase increasedd from RT to 200 ◦°CC and slightly increased as the deposition temperature was furtherfurther increased. As Figure 6 6bb shows,shows, thethe variationvariation inin thethe refractiverefractive index of NbNb2O55 thinthin films films was similar to that ofof NbNb22O5 thinthin films films with an OO22 ratio of 10%.10%. As the results in Figure 66 are are compared,compared, wewe havehave foundfound thatthat asas thethe samesame depositiondeposition temperaturetemperature waswas used,used, the refractive index ofof thethe NbNb22O5 thinthin films films deposited with the O 2 ratioratio of of 20% was higher than that deposited with the O2 ratio of 10%. Those Those results results suggest again that the deposition temperature and O2 ratio are two important factors that will affectaffect thethe propertiesproperties ofof NbNb22O5 thin films. films. Those results suggest that asas aa higherhigher substratesubstrate temperaturetemperature isis usedused toto depositdeposit NbNb22O5 thin films, films, the probability of collisions betweenbetween atomsatoms is is high high and and this this results results in in the the sputtered sputtered atoms atoms having having higher higher energies, energies, so they so canthey easily can easily diffuse diffuse and the and thin the films’ thin structurefilms’ structur is moree is compact. more compact. Thus, the Thus, refractive the refractive index is relatively index is high.relatively On thehigh. contrary, On the whencontrary, a lower when substrate a lower temperature substrate temperature is used to depositis used theto deposit Nb2O5 thethin Nb films,2O5 thethin structure films, the becomes structure less becomes densified less and densified the refractive and indexthe refractive would be index expected would to bebe lower; expected therefore, to be thelower; results therefore, of the the experiments results of arethe as experiments expected. Theare refractiveas expected. index The values refractive of the index Nb2 Ovalues5 thin of films the investigatedNb2O5 thin films increased investigated from 2.14 increased to 2.34 from when 2.14 the Oto2 2.34ratio when was 10%the O and2 ratio increased was 10% from and 2.18 increased to 2.44 ◦ whenfrom 2.18 the Oto2 2.44ratio when was 20%. the O When2 ratio the was deposition 20%. When temperature the deposition was RT temperature and 200 C, was the refractiveRT and 200 index °C, valuesthe refractive of the Nbindex2O5 valuesthin films of the were Nb lower2O5 thin than films the were values lower revealed than inthe Reference values revealed [19], which in Reference changed from[19], which 2.23 to changed 2.28. In thefrom past, 2.23 there to 2.28. was In no the research past, there using was both no theresearch deposition using temperatureboth the deposition and O2 ratiotemperature (or they and only O used2 ratio the (or O 2theyratio) only as theused deposition the O2 ratio) parameters. as the deposition We believe parameters. that the high We refractive believe indexthat the values high refractive in this study index are values caused in by this the study use ofare O caused2 during by the the deposition use of O2 during process, the and deposition this will compensateprocess, and forthis the will oxygen compensate defects for in the Nboxygen2O5 thin defects films. in the Nb2O5 thin films.

3.6 3.6 (a) RT (b) RT o 200oC 200 C o 3.2 300oC 3.2 300 C o o 400 C 400 C

2.8 2.8

2.4 2.4 Refractive Index (n) Refractive Index (n) Index Refractive

2.0 2.0 400 600 800 1000 400 600 800 1000 Wavelength (nm) Wavelength (nm)

Figure 6. Measured refractive index of Nb2O5 thin films as a function of deposition temperature and Figure 6. Measured refractive index of Nb2O5 thin ﬁlms as a function of deposition temperature and O2 ratio. (a) O2 ratio 10% and (b) O2 ratio 20%, respectively. O2 ratio. (a)O2 ratio 10% and (b)O2 ratio 20%, respectively.

Forouhi and Bloomer deduced dispersion equations for the refractive index, n, and the extinction coefficient, k [7,29]. The refractive index (n) and extinction coefficient (k) are related to the interaction between a material and incident light, and are associated with refraction and absorption. Those descriptions suggest again that the properties of the extinction coefficient of Nb2O5 thin films are very complicated. Figure 7 shows the extinction coefficient of Nb2O5 thin films as a function of

Micromachines 2016, 7, 151 10 of 12

Forouhi and Bloomer deduced dispersion equations for the refractive index, n, and the extinction coefficient, k [7,29]. The refractive index (n) and extinction coefficient (k) are related to the interaction Micromachinesbetween a material2016, 7, 151 and incident light, and are associated with refraction and absorption.10 Those of 12 descriptions suggest again that the properties of the extinction coefficient of Nb2O5 thin films are thevery deposition complicated. temperature Figure7 shows and the the extinctionO2 ratio in coefficient a spectrum of Nbrange2O 5ofthin 350–1000 films as nm. a function As the oflight the wavelengthdeposition temperature changed, the and extinction the O2 ratio coefficient in a spectrum linearly range decreased of 350–1000 from nm. As~0.35 the lightwhen wavelength the light wavelengthchanged, the was extinction 350 nm coefficientto smaller linearlythan 10− decreased4 when light from wavelength ~0.35 when was the longer light than wavelength the 361 wasnm. −4 When350 nm theto smallerO2 ratio than was 10 10%when and lightthe deposition wavelength temperature was longer thanwas theRT, 361200, nm. 300 When and 400 the O°C,2 ratio the extinctionwas 10% and coefficient the deposition was 4.05 temperature × 10−5, 9.40 was × RT, 10 200,−5, 4.82 300 and× 10 400−5, and◦C, the2.75 extinction × 10−5 when coefficient the light was wavelength4.05 × 10−5, was 9.40 361,× 10 360,−5, 4.82358 and× 10 357−5, nm, and and 2.75 the× 10extinction−5 when coefficient the light wavelength was zero when was the 361, light 360, wavelength358 and 357 nm,was andlonger the extinctionthan 362, coefficient361, 358 and was 357 zero nm; when when the light the wavelengthO2 ratio was was 20% longer andthan the ◦−5 deposition362, 361, 358 temperature and 357 nm; was when RT, the 200 O °C,2 ratio 300 was°C, and 20% 400 and °C, the the deposition extinction temperature coefficient was RT,2.75 200 × 10C,, 9.81300 ◦ ×C, 10 and−5, 1.68 400 ◦×C, 10 the−5 and extinction 6.89 × 10 coefficient−5 when wasthe light 2.75 ×wavelength10−5, 9.81 ×was10 −370,5, 1.68 367,× 36110− and5 and 345 6.89 nm,× 10and−5 thewhen extinction the light coefficient wavelength was was zero 370, when 367, 361the andligh 345t wavelength nm, and the was extinction longer than coefficient 370, 368, was 361 zero and when 346 nm,the lightrespectively. wavelength The was extinction longer than coefficient 370, 368, appa 361 andrently 346 decreases nm, respectively. as the Thedeposition extinction temperature coefficient increasesapparently and decreases it has no as apparent the deposition trend as temperature the O2 ratio increases is changed. and The it has results no apparent in Figure trend 5 suggest as the that O2 Nbratio2O is5 thin changed. films Theare resultsstable dielectric in Figure5 films suggest with that a low Nb 2 extinctionO5 thin films coefficient are stable in dielectric the range films of visible with a light.low extinction coefficient in the range of visible light.

) 0.4 0.4 κ RT ) (b) RT (a) κ o 200oC 200 C o 0.3 300oC 0.3 300 C o 400oC 400 C

0.2 0.2

0.1 0.1 Extinction coefficient (

0.0 ( Extinction coefficient 0.0 400 600 800 1000 400 600 800 1000 Wavelength(nm) Wavelength (nm)

Figure 7. Measured extinction coefficient of Nb2O5 thin films as a function of deposition temperature Figure 7. Measured extinction coefﬁcient of Nb2O5 thin ﬁlms as a function of deposition temperature and O2 ratio. (a) O2 ratio 10% and (b) O2 ratio 20%, respectively. and O2 ratio. (a)O2 ratio 10% and (b)O2 ratio 20%, respectively.

4. Conclusions Conclusions

When the O2 ratio was 10% or 20%, the average crystallite sizes increased with the increasing When the O2 ratio was 10% or 20%, the average crystallite sizes increased with the increasing deposition temperature. When the deposition temperature was increased from RT to 400◦ °C and the deposition temperature. When the deposition temperature was increased from RT to 400 C and the O2 Oratio2 ratio was 10%,was the10%, average the average total transmittance total transmittance had no apparent had no change, apparent the maximumchange, the transmittance maximum transmittanceratio increased, ratio and increased, the light wavelength and the light to revealwavele thength maximum to reveal transmittance the maximum ratio transmittance was shifted ratio to a was shifted to a higher value (red-shift). When the O2 ratio was 20%, the average total transmittance higher value (red-shift). When the O2 ratio was 20%, the average total transmittance decreased with decreased with the increasing deposition temperature. The values of the optical band gap of the the increasing deposition temperature. The values of the optical band gap of the Nb2O5 thin films Nbdecreased2O5 thin withfilms the decreased increasing with deposition the increasing temperature, deposition and temperature, the quantum-size and the effect quantum-size was the reason effect was the reason causing those results. When the O2 ratio was 10% (20%), the refractive index of the causing those results. When the O2 ratio was 10% (20%), the refractive index of the Nb2O5 thin films Nbdecreased2O5 thinfrom films 3.14 decreased (3.48) to from around 3.14 2.13(3.48) (2.16) to around when light2.13 (2.16) wavelength when light was longerwavelength than 450was nm longer and thanthen 450 the nm refractive and then index the hadrefractive a stable index value had when a stable the value light wavelengthwhen the light was wavelength longer. The was refractive longer. The refractive index values of the Nb2O5 thin films at 550 nm increased from 2.14 (2.18) to 2.34 (2.44) index values of the Nb2O5 thin films at 550 nm increased from 2.14 (2.18) to 2.34 (2.44) when the O2 whenratio was the 10%O2 ratio (20%) was and 10% the (20%) refractive and the index refractive values also index increased values also with increased the increase with of the the increase deposition of thetemperature. deposition We temperature. believe that We the believe high refractive that the index high valuesrefractive in thisindex study values are causedin this bystudy the useare caused by the use of O2 during the deposition process, and this will compensate for the oxygen of O2 during the deposition process, and this will compensate for the oxygen defects in the Nb2O5 defectsthin films. in the The Nb refractive2O5 thin index films. at The the refractive same light index wavelength at the same increased light as wavelength the deposition increased temperature as the depositionwas increased temperature from RT towas 200 increased◦C and slightly from RT increased to 200 °C as and the depositionslightly increased temperature as the was deposition further temperature was further increased. When the O2 ratio was 10% (20%) and the deposition increased. When the O2 ratio was 10% (20%) and the deposition temperature was RT, 200, 300 and temperature was RT, 200, 300 and 400 °C, the extinction coefficient was 4.05 × 10−5 (2.75 × 10−5), 9.40 × 10−5 (9.81 × 10−5), 4.82 × 10−5 (1.68 × 10−5), and 2.75 × 10−5 (6.89 × 10−5) when the light wavelength was 361 nm (370 nm), 360 nm (368 nm), 358 nm (361 nm), and 357 nm (346 nm), and the extinction coefficient was zero when the light wavelength was longer than 362 nm (370 nm), 361 nm (367 nm), 358 nm (361 nm), and 357 nm (345 nm), respectively. The extinction coefficient apparently decreases as the deposition temperature increases and it has no apparent trend as the O2 ratio is changed.

Micromachines 2016, 7, 151 11 of 12

400 ◦C, the extinction coefficient was 4.05 × 10−5 (2.75 × 10−5), 9.40 × 10−5 (9.81 × 10−5), 4.82 × 10−5 (1.68 × 10−5), and 2.75 × 10−5 (6.89 × 10−5) when the light wavelength was 361 nm (370 nm), 360 nm (368 nm), 358 nm (361 nm), and 357 nm (346 nm), and the extinction coefficient was zero when the light wavelength was longer than 362 nm (370 nm), 361 nm (367 nm), 358 nm (361 nm), and 357 nm (345 nm), respectively. The extinction coefficient apparently decreases as the deposition temperature increases and it has no apparent trend as the O2 ratio is changed.

Acknowledgments: The authors acknowledge the financial supports of SUMMIT-TECH Resource Corp, and Ministry of Science and Technology, R.O.C. with the contract Nos. 104-2221-E-390-013-MY2 and 104-2622-E-390-004-CC3. Author Contributions: K.-N.C. and C.-M.H. helped with the experimental processes, measurements, and data analysis; C.-F.Y. organized the paper and encouraged the paper-writing; Y.-C.L. and J.L. helped with the experimental processes and measurements. Conflicts of Interest: The authors declare no conflict of interest.

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