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Optical response of bismuth based thin films synthesized via unbalanced magnetron DC sputtering technique
Accepted Manuscript Optical response of bismuth based thin films synthesized via unbalanced magnetron DC sputtering technique
G. Orozco-Hernández, J.J. Olaya, J.E. Alfonso, C.A. PinedaVargas, C. Mtshali PII: DOI: Reference:
S0040-6090(17)30193-1 doi: 10.1016/j.tsf.2017.03.018 TSF 35866
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
21 June 2016 8 March 2017 9 March 2017
Please cite this article as: G. Orozco-Hernández, J.J. Olaya, J.E. Alfonso, C.A. PinedaVargas, C. Mtshali , Optical response of bismuth based thin films synthesized via unbalanced magnetron DC sputtering technique. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2016), doi: 10.1016/j.tsf.2017.03.018
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ACCEPTED MANUSCRIPT OPTICAL RESPONSE OF BISMUTH BASED THIN FILMS SYNTHESIZED VIA UNBALANCED MAGNETRON DC SPUTTERING TECHNIQUE G. Orozco-Hernándeza,b, a
Departamento de Ingeniería Mecánica y Mecatrónica, Facultad de Ingeniería, Universidad Nacional de Colombia sede Bogotá, AA 111321, Bogotá – Colombia, email: [email protected] Universidad ECCI, Cra 19 No. 49-20, Bogotá – Colombia.
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J.J. Olayaa a
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Departamento de Ingeniería Mecánica y Mecatrónica, Facultad de Ingeniería, Universidad Nacional de Colombia sede Bogotá, AA 111321, Bogotá – Colombia, email: [email protected]
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J.E. Alfonsoc,* c
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Departamento de Física, Facultad de Ciencias, Universidad Nacional de Colombia sede Bogotá, AA 111321, Bogotá – Colombia, email: [email protected] C.A. Pineda-Vargasd d
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iThemba LABS, National Research Foundation, PO Box 722, Somerset west 7129, Cape Town – South Africa, email: [email protected]
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C. Mtshalid,e d
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iThemba LABS, National Research Foundation, PO Box 722, Somerset west 7129, Cape Town – South Africa e
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Faculty of Health & Wellness Sciences, CPUT, Bellville, South Africa, email: [email protected] Corresponding author: email: [email protected], mobile phone: +573115060647
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ABSTRACT
Bismuth-based thin films were deposited on AISI 316L stainless steel and glass substrates via the unbalanced magnetron DC sputtering technique. Three different configurations of titanium pieces along the bismuth target race track were used in order to evaluate the influence of Ti content on the optical and electrical properties of the films. We used a reactive ambient mixture of Ar:O (80%:20%) and a constant flux of 9 sccm, along with a power of 40 W. Results showed thin films with high homogeneity and smooth surfaces, with several bismuth droplets, in all the different configurations. It was also found that the optical and electrical properties of the thin films are strongly dependent on the preparation conditions, especially the Ti content. Titanium quantities in the parts-per-million range were found only through the micro-proton induced X-Ray emission technique. This small
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ACCEPTED MANUSCRIPT quantity of Ti, as well as the thickness of the thin films, affected the electrical and optical response of the bismuth-based thin films. KEYWORDS Bismuth, Optical properties, Electrical properties, Unbalanced Magnetron, Proton Induced X-Ray Emission 1. INTRODUCTION
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Bismuth (Bi) and its compounds have been the subject of much interest in research communities due to their interesting properties. This is a semi-metallic group V element on the periodic table with a rhombohedral crystal structure and is generally indexed as a hexagonal lattice (a = 4.574A, c = 11.80A) [1]. It is commonly known to exhibit several interesting properties, such as a highly anisotropic Fermi surface, low carrier density, small carrier effective mass, large Fermi wavelength, and long mean free path [2]. It also has low toxicity and low thermal conductivity [3]. These physical properties have drawn considerable attention because of their usefulness in various applications in industry. For example, ternary compounds based on bismuth, such as BixMyOz (M= Ti, Si, Al, Ga), have the potential to be used as electrolytes for solid oxide fuel cells and sensors [4], and this is due to the properties of photoluminescence [5–8] or oxygen ionic conductivity [9] that they possess. Moreover, depending on the structural formation, some reports suggest that some Bi binary compounds such as Bi2Te3 can be used as thermoelectric materials [10].
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Recently, it has been reported that nanoparticles of Bi have been useful in biological science, such as bio-imaging and bio-sensing [11]. The ability to tune the bandgap of Bi compound material enables a wide range of optical applications. Bi oxide possesses this bandgap tuning capability and therefore qualifies as a promising material for various modern solid-state technologies such as optical coatings, gas sensors, components in optoelectronics, Schottky barrier solar cells, and transparent ceramic glass manufacturing [12, 13]. For instance, the Bi oxide bandgap can be tuned from 2.0 to 3.96 eV [14, 15] and depends on structural formation. On the other hand, the polymorphs of crystalline Bi2O3, such as -Bi2O3 (monoclinic phase), -Bi2O3 (tetragonal phase), and -Bi2O3 (bodycentered cubic phase), have different characteristic values for properties such as the energy bandgap, electrical conductivity, refractive index, permittivity, etc. [16]. Several methods have been employed to achieve various desired structures of Bi oxides and compounds and to alter their properties. These methods include sputtering, pulsed laser deposition, sol-gel (spin coating), and chemical vapor deposition, among others [15–18]. All these methods require variation of different parameters in order to achieve the desired properties. For instance, varying the substrate temperature can favor a particular physical structure while confining optical properties to a particular limit, e.g. the monoclinic αphase, stable up to ~730° C, and after that temperature the cubic high-temperature δ-phase, which is quite attractive in terms of ionic conductivity [19, 20]. On cooling from the δphase, two metastable phases have been reported: the β-phase, with a tetragonal structure, which occurs at 650° C, and the γ-phase, which occurs at 640° C. 2
ACCEPTED MANUSCRIPT It has been reported in the literature that bismuth titanate exists in different phases, such as the Aurivillius, and these phases have shown excellent optical properties due to their photorefractive properties. However, due to its physical characteristic of high-energy bandgap values (~ 4 eV), it has not been used in electric applications.
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In the present paper, we mainly report on some improved structural and optical properties (transmission, reflection and absorption spectra, dispersion of refractive index, and absorption index) of bismuth-based thin films obtained by varying the Ti content during deposition using unbalanced magnetron sputtering. Some correlations between the Ti content and the optical and electrical properties are discussed. 2. EXPERIMENTAL SETUP
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Bismuth-titanium-oxygen thin films were deposited at room temperature on glass and AISI 316 L stainless steel substrates using the variable unbalanced magnetron (GENCOA model PP100) DC sputtering technique, which permits variation of the magnetic field by varying the vertical distance of the central magnet. This technique also allows high deposition rates and high plasma density close to the substrates. The unbalance level of the magnetron can be estimated using the coefficient of geometrical unbalance equation KG, as has been reported [21, 22]: 𝑍0
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𝐾𝐺 =
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where R is the average radius of the erosion zone and Z0 is the distance from the target surface to the point on the axis of the magnetron where the normal component of the magnetic field (Bz) has a value of zero [21, 22]. For the present project, a KG value of 0.88, which represents the highest configuration of the unbalanced magnetron, was used.
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A Bi target (99.9999% pure) of 4 inches was used. 3 mm thick pieces of pure Ti (99.99%) were thoroughly cleansed with isopropanol followed by acetone in an ultrasonic cleaner and then dried with pressurized air. They were then placed on top of the Bi target surface inside the chamber along the race track in three different configurations, 1, 9, and 17 pieces (7 x 7 mm), as shown in Fig. 1. The glass substrates were cleaned with tap water, and then were immersed in sulfo-chromic mixture (H2CrO4 + H2SO4) for one week, as reported in references [23–25] in order to eliminate insoluble organic residues. They were then cleaned with doubly-distilled deionized water, followed by isopropanol and finally acetone, using the ultrasonic bath, for ten minutes. The substrates were then placed in sample holders inside the chamber, and the chamber was properly closed for vacuum preparation. A vacuum base pressure of 9 x 10-4 Pa was achieved before introducing the gases. A mixture of 80% argon and 20% oxygen gas was then introduced into the chamber for the reactive sputtering process, and a 3.5 x 10-3 Pa vacuum work pressure was reached. Target to substrate distances were maintained at 50 mm. The thin films were grown at room temperature by bombarding the Bi target with Ti pieces (as described above). Time of deposit was kept at 4 minutes for all samples. Sputtering yields for Bi and Ti were 8.946
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and 2.025, respectively. Films with thickness 400 nm for 1 Ti piece, 330nm for 9 Ti pieces, and 260 nm for 17 Ti pieces were obtained.
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Fig. 1: Schematic representation of bismuth target with titanium pieces positioned along the race track within the sputtering system
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The morphological investigation was carried out by employing a FEI Quanta 200 scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) probe for elemental analysis. Surface topographies and roughness were determined by means of a confocal microscope (Carl Zeiss LSM 700) in areas of 250 µm x 250 µm. An X-ray diffraction (XRD) system (Phillips X-Pert Pro Panalytical) in the conventional BraggBrentano (ϴ-2ϴ) geometry and CuK⍺ radiation with λ=1.540998 Å was used for the structural investigation. The X-ray profiles (XRD) were taken for a 2θ range between 10 and 90 degrees in steps of Δ2θ=0.02. Ti distribution in the Bi-based films was mapped via particle-induced X-ray emission within the nuclear microprobe at the Materials Research Department, iThemba LABS, South Africa. A 3 MeV focused proton beam of about 3.5 µm x 3.5 µm lateral resolution was raster scanned on the sample surface (82 µm x 82 µm) using an electrostatic scanning coil. Beam current was kept at about 150 pA with minimum instability in order to avoid damage. Scanned areas were typically analyzed in a square pattern of up to a maximum of 128 x 128 pixels, with a dwell time of 10 ms/pixel. PIXE and proton backscattering spectra were acquired simultaneously in an event-by-event mode, using a Si (Li) X-ray detector positioned at a take-off angle of 135° and shielded with a 125 µm Beryllium (Be) filter. The PIXE count rate was kept below 1000 counts/second to avoid pulse pile-up and to achieve satisfactory counting statistics. The accumulated PIXE spectra were analyzed using GeoPIXE II software [26]. 4
ACCEPTED MANUSCRIPT A Cary Varian 5000 UV-VIS-NIR spectrophotometer was used to investigate the optical properties of the films, and the measurements were conducted in the 200 nm to 2500 nm wavelength range in both the reflectance and the absorbance mode. The extinction coefficient, k, was obtained from Equation 2 [17]:
𝑘=
𝛼∙𝜆
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𝑛=
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where λ denotes the wavelength in the investigated spectral range and α the absorption coefficient. The refractive index of the films was calculated from reflectance values R using Equation 3 [17, 27]:
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𝛼ℎ𝜐 = 𝐴(ℎ𝜐 − 𝐸𝑔 )
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Equation 4 was used to calculate the optical band gap by taking into consideration the relationship between the absorption coefficient and the photon energy. (4)
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where hʋ is the photon energy, Eg is the energy of the optical bandgap, A is the characteristic parameter for respective transitions, and r is an exponent which may take the values r=1/2 for allowed direct band-to-band transitions and r=2 for indirect allowed transitions. The usual method for determining the value of the band gap involves plotting a graph of (αhʋ )r versus photon energy in accordance with Equation 4. The value of the direct Eg was determined by extrapolating the linear portion of the plot to (αhʋ )2=0 [17, 28, 36].
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For the electrical properties, a classic standard four-point probe was used with a Lucas Labs 302 setup with a Keithley 2400 source meter. Resistivity values of the films were calculated by taking into account the corresponding correction factors that strongly depend on the sample geometry [29]. 3. RESULTS AND DISCUSSION
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3.1 Scanning electron microscope (SEM) and confocal microscope To better understand the influence of the quantity of Titanium (Ti) pieces (see Fig. 1) on the morphology of the films, they were studied by means of the SEM technique. The resulting images for samples grown with the three different configurations of titanium pieces over the bismuth target are reported in Fig. 2. As can be seen, all the samples are generally homogeneous, but there are some droplet structures (circled in order to highlight a few) that formed due to agglomeration. Later analysis with energy dispersive X-Ray spectroscopy (EDX) established that these droplets are metallic bismuth agglomerates that formed because of the high energy of the gas ions. Although these images show a generally homogeneous surface, there is a clear distinction between them. For example, Fig. 2a shows more droplets, Fig. 2b shows fewer droplets together with a fairly smooth surface, and Fig. 2c shows a smooth and homogeneous surface with only one droplet (circled). 5
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Although EDX could not detect the presence of Ti in the sample due to its detection limit (1000 – 3000 ppm; >10% wt %), it is evident that this evolution is associated with the number of Ti pieces. Increasing Ti pieces on the target reduces the number of droplets on the films surface and consequently make them more homogeneous. Moreover, Fig. 2a and 2b show some patches (pointed to by arrows) signaling inhomogeneity, while in Fig. 2c there was a smooth surface with only a patch area surrounding the droplet. Due to the limitation of EDX, the particle-induced X-ray emission (PIXE) technique was employed in order to investigate the amount of Ti in the samples, and the results are shown and discussed in the latter part of this paper.
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Fig. 2: Scanning electron microscope (SEM) images of the bismuth-based thin films deposited on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: (a) 1 titanium piece, (b) 9 titanium pieces, and (c) 17 titanium pieces
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A confocal microscope was used as a complementary technique, besides SEM, in order to stimulate a parallel discussion and draw conclusions. Fig. 2 shows images of the bismuthbased thin films with three different configurations of titanium pieces over the bismuth target, as labelled. Images were taken in an area of 250µm x 250µm, and the roughness of these films was also analyzed, which in all the samples was quite similar, at around 8 nm.
Fig. 3: Confocal microscope images of the bismuth-based thin films on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: (a) 1 titanium piece, (b) 9 titanium pieces, and (c) 17 titanium pieces. 6
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3.2. X-ray diffraction (XRD)
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Fig. 4 shows the XRD spectra of three samples in the three configurations mentioned under the subtitle Experimental Setup. For all the configurations, these spectra only revealed the presence of rhombohedral phase crystalline Bi, with no traces of Ti. Also, there were no Bibased compound phase structures observed maybe because of the highly oriented Bi peaks for which relative intensities are very large and cannot allow to see them. The absence of Ti and Bi compound peaks in the XRD results does not necessarily mean that Ti is not present in the film, but it could mean that quantity of Ti is very low. According to the ICDD reference pattern 00-044-1246 [30], peaks appearing at 27.1 and 55.9 2θ degrees oriented in (0 1 2) and (0 2 4) are a signature of the rhombohedral Bi phase, as we have observed in Fig. 4. Additional peaks in Fig. 4 correspond to the AISI 316L stainless steel (SS) substrate.
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Fig. 4. X-ray diffraction spectra of bismuth-based thin films deposited on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: 1 titanium piece, 9 titanium pieces and 17 titanium pieces. 3.3. Particle-induced X-ray emission (PIXE) and electrical measurements Two-dimensional elemental mapping by micro-PIXE revealed the presence of small quantities of Ti within the films, as shown in Fig. 5. Based on this characterization, there are two aspects to be considered in the discussion of the Ti 2D-distribution maps: First, increasing the Ti pieces in the sputtering process resulted in an increase of the Ti content in the films from an average of about 0.1319 wt% (for 1 Ti piece) to a maximum of about 7
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0.4434 wt% (for 17 pieces). It is of importance to note the fact that these values are average values within the areas where the Ti content was enriched in the 2-dimensional elemental map for Ti (highlighted by the red contours in Fig 5). Here it is also important to emphasize that the maximum values shown on the side color-coded scale bars on each map (2, 3, and 8 wt%) represent the maximum of the vertical concentration scale for the 2-dimensional elemental map. It is, however, essential to point out here that in the small regions where Ti is enriched (circled by red lines) there are pixels where the concentration of Ti may reach values that are higher than the average quoted in Table 1. For example, the map in Fig 5.c for the 17 pieces of Ti shows a high-intensity sub-region in the bottom right corner. In this sub-region it can be observed that a yellow color appears in some pixels. That means that in these pixels the “real” concentration of Ti is on the order of 6 wt%. In a similar fashion, we could infer the same analogy for the other enriched sub-regions for maps 5.a and 5.b. Second, the distribution of Ti in the whole scanned area, 82 µm x 82 µm, for the 17-piece map was fairly homogeneous except of course for the areas encircled in the red line, which contained higher amounts of Ti content, as shown by the scale bar.
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Fig. 5: Particle-induced X-ray emission maps of Ti on Bi-based thin films on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: (a) 1 titanium piece, (b) 9 titanium pieces, and (c) 17 titanium pieces. Maps collected from areas: (a) 82µm x48µm; (b) and (c) 82µm x 82µm. Regarding the electrical measurements, a four-point probe was applied to the samples grown on glass substrates. Applying current values on the order of µA, we registered voltage on the order of the mV, and with this data we found the electrical resistance. Applying the geometric correction factors, we calculated the material resistivity, and the results are summarized in Table 1 for the different configurations of titanium pieces over the target. This behavior is in agreement with those reported in the literature for bismuth films’ electrical behavior [31, 32] at room temperature. In the configuration of 17 titanium pieces, an increase in resistivity can clearly be seen, and this could be attributed to the higher presence of titanium as well as the low thickness compared to the other samples. In this sample with 17 titanium pieces, titanium can be considered as an impurity due to the 8
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low concentration found by PIXE measurements; these titanium “impurities” reduce the mean free path of the electrons inside the film. Also the thickness of this sample is the lowest of the set and it plays an important role in the resistivity behavior. As considered by Fuchs-Sondheimer [33, 34] theory in which they talk about the size effect in the mean free path of conduction electrons in thin films, the mean free path is shortened in thin films due to the diffusive scattering and reflection of the electrons at the film surfaces, this fact increases the electrical resistivity. Both phenomena (considering titanium as impurity and the size effect) contribute to the increase of the electrical resistivity in the film with 17 titanium pieces compared with the other films. Applying Fuchs-Sondheimer theory we calculated the electrical conductivity ratio (𝜎0 ⁄𝜎) between bulk conductivity 𝜎0 and film conductivity 𝜎, this is the same as the ratio between film resistivity and bulk resistivity. The results are summarized in Table 1 and show that in fact there is an increase in film resistivity while thickness is reduced because the ratio increases. The classic model of electric conductivity establishes that the resistivity of a material depends inversely on the number of carries per volume unit and on the time between collisions. In a thicker film, these numbers increase, and therefore resistivity decreases.
Table 1. Properties of the films according to the number of pieces of titanium over the target.
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Fuchs-Sondheimer relation of 𝝈𝟎 ⁄𝝈
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Titanium concentration (ppm(wt)) +/- uncertainty and detection limit (0.1319% +/- 0.0374%) 0.0124% (0.2465% +/- 0.0438%) 0.0116% (0.4434% +/- 0.0588%) 0.0147%
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3.4 Optical measurements Optical measurements were carried out on the thin films deposited on glass substrates. Fig. 6 displays UV-VIS spectroscopy spectra collected in the spectroscopy reflectance mode in the 300 nm to 2100 nm wavelength region. There are some vital aspects of these results that should be pointed out here. At about 300 nm, about 15% reflectance was observed for the 1-Ti piece film, while for 9 pieces and 17 pieces the reflectance values were about 10% and almost 0%, respectively. At about 500 nm and 15% reflectance, there was a crossover (circled) whereby the reflectance of 9 Ti pieces went below that of 17 Ti pieces, and that of 1 Ti piece was flat up to 580 nm, where it started to rise again. At 900 nm and 30% reflectance, another crossover (also circled) was observed, where reflectance of the 17 Ti 9
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pieces went below the reflectance of 1 piece at 50% to about 40%, as indicated in Fig. 6 by the black dotted line. These results clearly suggest that Ti enhanced the reflectance of the Bi-based film in the visible region of the electromagnetic spectrum. It has been reported in the literature that the presence of nanoparticles in the film introduces a dispersion process that changes the optical response of the films. For example, N. Chander et al. reported that the inclusion of gold nanoparticles enhanced the photovoltaic response of the films [35], while Ti nanoparticles enhanced reflectance at lower energies (about 1 eV) to about 80%, as reported in reference [36]. Although the reflectance we measured was not enhanced to the level reported in reference [36], our results agreed with published investigations, especially around the 850 nm wavelength.
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Fig. 6: UV-VIS spectroscopy in reflectance mode spectra of Bi-based thin films on glass substrate for three different configurations of titanium pieces over a bismuth target: 1 titanium piece, 9 titanium pieces and 17 titanium pieces The optical parameters extinction coefficient k(λ) and refractive index n(λ) of the films were calculated from the reflection spectra by taking into account Equations (2) and (3). Fig 7 shows the results. From them we can say that the extinction coefficient exhibits a lineal behavior as a function of wavelength and that a thicker film has a higher k and also a higher absorption. Refractive index exhibits a behavior that notably depends on the titanium content and is similar to the reflectance spectra. In the film with less Ti content, the refractive index grows linearly from 200 nm to 1300 nm and after that reaches a constant value (6). In the film with an intermediate value of Ti, it grows linearly from 200 nm to 700 nm, where it reaches a value of stabilization of 3; and finally in the film with a 10
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higher Ti content it grows linearly from 200 nm to 1000 nm and then reaches a stabilization of 4. This behavior probably can be explained by the presence of the titanium in the bismuth lattice, since in the literature it is shown that metallic bismuth films exhibit a behavior similar to the film with less Ti content [37]. For instance, metallic bismuth at 600 nm has a refractive index value of 2.1832, the film with less Ti content at this wavelength has a refractive index value of 2.531, and the film with higher Ti content at the same wavelength has a value of 2.98, values that indicate relative errors of 15.9% and 27.7%, respectively.
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Fig. 7. Refractive index, n (a) and extinction coefficient, k (b) of Bi-based thin films on glass substrate for three different configurations of titanium pieces over a bismuth target: 1 titanium piece, 9 titanium pieces and 17 titanium pieces
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From the values of absorbance and according to Equation 4, we calculated the optical energy bandgap values of the films, as can be seen in Fig. 8. These values were obtained by extrapolating the linear portion of the curve, and they show that the film with a higher Ti content has a value of 3.84 eV ± 0.05 eV and the other two films have an energy bandgap of around 4 eV. This results are in agreement with those obtained by Condurache-Bota et al. and Celia L. Gomez et al., who reported values from 2.9 to 4.2 eV [17, 18]. The semiconductor behavior of the films can be explained by taking into account that they are formed by an amorphous bismuth oxide immersed in a metallic bismuth lattice.
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Fig. 8 Typical (αhʋ )2 vs photon energy (Tauc plot) for the studied samples and the calculated Eg
CONCLUSIONS
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Bismuth thin films were deposited on AISI 316L stainless steel and glass substrates via unbalanced DC magnetron sputtering. The effect of the variation of specific parameters during deposition showed the dependence on them of the optical and electrical properties of the Bi systems. The results of the optical response indicate that Bi-based thin films exhibit a semiconductor behavior, and the electrical response indicates a metallic behavior. These results can be explained by taking into consideration that the films have bismuth oxide amorphous compounds immersed in a crystalline bismuth lattice.
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Additionally, these results indicate that by controlling the Ti content it is possible to grow films with pre-determined optical properties in accordance with the application. The amount of Ti in the Bi-based thin films is not significant from a stoichiometric point of view. However, it has here been demonstrated that small amounts of Ti can and may determine the optical and electrical properties of the Bi/substrate systems. It is recommended that in the future more research should be devoted not only to the study of the optical behavior but also to the further exploration of the electrical characteristics and properties of these systems. ACKNOWLEDGMENTS The authors are thankful for the financial support of the Universidad Nacional de Colombia during the course of this research under project number 23656 in the CONVOCATORIA 12
ACCEPTED MANUSCRIPT DEL PROGRAMA NACIONAL DE PROYECTOS PARA EL FORTALECIMIENTO DE LA INVESTIGACIÓN, LA CREACIÓN Y LA INNOVACIÓN EN POSGRADOS DE LA UNIVERSIDAD NACIONAL DE COLOMBIA 2013-2015. The National Research Foundation of South Africa and the operators of the VDG accelerator at iThemba LABS are gratefully acknowledged. Conflict of Interest disclosure
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Funding: This study was funded by the Universidad Nacional de Colombia under project number 23656 of the CONVOCATORIA DEL PROGRAMA NACIONAL DE PROYECTOS PARA EL FORTALECIMIENTO DE LA INVESTIGACION, LA CREACIÓN Y LA INNOVACIÓN EN POSGRADOS DE LA UNIVERSIDAD NACIONAL DE COLOMBIA 2013 – 2015. Conflict of interest: The authors declare that they have no conflict of interest
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[5] G. Blasse, O.B. Ho, On the luminescence of bismuth aluminate (Bi2Al4O9), J. Lumin. 21 (1980) 165–168.
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[6] V.V. Volkov, A.V. Egorysheva, Y.F. Kargin, V.I. Solomonov, S.G. Mikhailov, S.I. Buzmakova, B.V. Shul’gin, V.M. Skorikov, Synthesis and luminescent properties of Bi2Ga4O9 single crystals, Inorg. Mater. 32 (1996) 455–458. [7] V.V. Volkov, A.V. Egorysheva, Photoluminescence in fast-response Bi2Al4O9 and Bi2Ga4O9 oxide scintillators, Opt. Mater. 5 (1996) 273–277. [8] H. Fue, T. Hasegawa, T. Sato, S. Mukaigawa, K. Takaki, T. Fujiwara, Development of self-organized filaments in a Micrograp Atmospheric Barrier Discharge on Bismuth Silicon Oxide dielectrics, IEEE T. Plasma Sci. 39 (2011) 2140 – 2141. [9] I. Bloom, M.C. Hash, J.P. Zebrowski, K.M. Myles, M. Krumpelt, Oxide-ion conductivity of bismuth aluminates, Solid State Ionics 53–56 (1992) 739–747.
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ACCEPTED MANUSCRIPT [10] L.M. Goncalves, C. Couto, P. Alpuim, A.G. Rolo, F. Völklein, J.H. Correia, Optimization of thermoelectric properties on Bi2Te3 thin films deposited by thermal coevaporation, Thin Solid Films 518 (2010) 2816-2821. [11] M. Wenkin, P. Ruiz, B. Delmon, M. Devillers, The role of bismuth as promoter in PdBi catalyst for the selective oxidation of glucose to gluconate, J. Mol. Catal. A-Chem. 180 (2002) 141-159.
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[12] P. Zhou, G.J. You, Y.G. Li, T. Han, J. Li, S.Y. Wang, L.Y. Chen, Y. Liu, S.X. Qian, Linear and ultrafast nonlinear optical response of Ag:Bi2O3 composite films, Appl. Phys. Lett. 83 (2003) 3876-3878.
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[13] T. Takeyama, N. Takahashi, T. Nakamura, S. Itoh, Microstructure characterization of delta-Bi2O3 thin film under atmospheric pressure by means of halide CVD on c-sapphire, J. Cryst. Growth 275 (2005) 460-466.
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[14] T.N. Soitah, Y. Chunhui, Y. Yong, N. Yinghua, S. Liang, Properties of Bi2O3 thin films prepared via a modified Pechini route, Curr. Appl. Phys. 10 (2010) 1372-1377.
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[15] M.M. Ivashchenko, I.P. Buryk, V.M. Latyshev, A.O. Stepanenko, K.S. Levchenko, Influence of substrate temperature on structural and optical properties of bismuth oxide thin films deposited by closespaced vacuum sublimation, Superlattice. Microst. 88 (2015) 600608.
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[16] H.T. Fan, X.M. Teng, S.S. Pan, C. Ye, G.H. Li, L.D. Zhang, Structural and thermal stability of δ-Bi2O3 thin films deposited by reactive sputtering, Appl. Phys. Lett. 87 (2005) 231916.
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[17] S. Condurache-Bota, N. Tigau, A.P. Rambu, G.G. Rusu, G.I. Rusu, Optical and electrical properties of thermally oxidized bismuth thin films, Appl. Surf. Sci. 257 (2011) 10545-10550.
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[18] L. Celia. Gomez, Osmary Depablos-Rivera, Phaedra Silva-Bermudez, Stephen Muhl, Andreas Zeinert, Michael Lejeune, Stephane Charvet, Pierre Barroy, Enrique Camps, Sandra E. Rodil, Opto-electronic properties of bismuth oxide films presenting different crystallographic phases, Thin Solid Films 578 (2015) 103–112. [19] N.M. Sammes, G.A. Tompsett, H. Nafe, F. Aldinger, Bismuth based oxide electrolytes – Structures and Ionic Conductivity, J. Eur. Ceram. Soc. 19 (1999) 1801 – 1826. [20] J. W. Medernach, R.L. Snyder, Powder direction patterns and structure of the bismuth oxides, J. Am. Ceram. Soc. 61 (1978) 494-497. [21] I.V. Svadkovski, D.A. Golosov, S.M. Zavatskiy,Characterization parameters for unbalanced magnetron sputtering systems, Vacuum 68 (2003) 283-290. [22] D. Marulanda, J.J. Olaya, Unbalanced magnetron sputtering system for producing corrosion resistance multilayer coatings, Dyna 79 (2012) 171.
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ACCEPTED MANUSCRIPT [23] J.V. de Melo, A.P. Soldaktin, C. Martelet, N. Jaffrezic-Renault, S. Cosnier, Use of competitive inhibition for driving sensitivity and dynamic range of urea ENFETs, Biosens. Bioelectron. 18 (2003) 345-351. [24]. A. Pardo, J. Torres, Substrate and annealing temperature effects on the crystallographic and optical properties of MoO3 thin films prepared by laser assisted evaporation. Thin Solid Films 520 (2012) 1709-1717.
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[25] S. Daoud-Mahammed, C. Ringard-Lefebvre, N. Razzouq, et. al., Spontaneous association of hydrophobized dextran and poly-beta-cyclodextrin into nanoassemblies. Formation and interaction with a hydrophobic drug, J. Colloid Interface Sci. 307 (2007) 8393.
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[26] C.G. Ryan, Quantitative trace element imaging using PIXE and the nuclear microprobe, J. Imaging Syst. Technol 11 (2000) 219-230.
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[27] L. Leontie, M. Caraman, M. Delibas, G.I. Rusu, Optical properties of bismuth trioxide thin films, Mater. Res. Bull. 36 (2001) 1629.
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[28] V. Dolocan, Transmission spectra of bismuth trioxide thin films, Appl. Phys. 16 (1978) 405–407.
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[29] M. E. Girotto, A.S. Ivair, Medidas de resistividade eléctrica DC em sólidos: como efetuá-las corretamente, Quim. Nova, 25, 4 (2002) 639-647.
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[30] R. Sailer, G. McCarthy, Philips X’Pert Software. North Dakota State University, Fargo, N D, USA. (1992) ICDD Grant-in-Aid.
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[31] Christopher Grant Padwick, Resistivity measurements of thin films of bismuth: applications for building bolometric detectors. Thesis for Masters of Science, The University of British Columbia, The Faculty of Graduate Studies, Department of Physics and Astronomy (1997).
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[32] A.K. Shigeru Baba, H. Sugawara, Electrical resistivity of thin bismuth films, Thin Solid Films, 31 (1976) 329-335.
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[33] K. Fuchs, The conductivity of thin metallic films according to the electron theory of metals, Math. Proc. Cambridge Philos. Soc. 34, 1 (1938) 100-108. [34] E.H. Sondheimer, The mean free path of electrons in metals, Adv. Phys. 1,1 (1952) 142. [35] N. Chander, A.F. Khan, E. Thouti, S.K. Sardana, P.S. Chandrasekhar, V. Dutta, V.K. Komarala, Size and concentration effects of gold nanoparticles on optical and electrical properties of plasmonic dye sensitized solar cells, Sol. Energy 109 (2014) 11-23. [36] William Edgar Wall, Optical reflectivity and Auger Spectroscopy of Titanium and Titanium-Oxygen surfaces. Dissertation, Georgia Institute of Technology, (1978).
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ACCEPTED MANUSCRIPT [37] H.J. Hagemann, W. Gudat, C. Kunz, Optical constants from the far infrared to the xray region: Mg, Al, Cu, Ag, Au, Bi, C, and Al2O3, J. Opt. Soc. Am. 65 (1975) 742-744.
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ACCEPTED MANUSCRIPT HIGHLIGHTS
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Unbalanced magnetron sputtering of films from Bi targets decorated with Ti coupons. While being low, Ti content affects the surface morphology of the Bi films. Optical properties showed films’ semiconductor behavior. Fuchs-Sondheimer theory used to explain the electrical properties of the films. Ti content shown to strongly influence the films’ optical and electrical behavior.
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G. Orozco-Hernández, J.J. Olaya, J.E. Alfonso, C.A. PinedaVargas, C. Mtshali PII: DOI: Reference:
S0040-6090(17)30193-1 doi: 10.1016/j.tsf.2017.03.018 TSF 35866
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
21 June 2016 8 March 2017 9 March 2017
Please cite this article as: G. Orozco-Hernández, J.J. Olaya, J.E. Alfonso, C.A. PinedaVargas, C. Mtshali , Optical response of bismuth based thin films synthesized via unbalanced magnetron DC sputtering technique. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2016), doi: 10.1016/j.tsf.2017.03.018
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ACCEPTED MANUSCRIPT OPTICAL RESPONSE OF BISMUTH BASED THIN FILMS SYNTHESIZED VIA UNBALANCED MAGNETRON DC SPUTTERING TECHNIQUE G. Orozco-Hernándeza,b, a
Departamento de Ingeniería Mecánica y Mecatrónica, Facultad de Ingeniería, Universidad Nacional de Colombia sede Bogotá, AA 111321, Bogotá – Colombia, email: [email protected] Universidad ECCI, Cra 19 No. 49-20, Bogotá – Colombia.
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Departamento de Ingeniería Mecánica y Mecatrónica, Facultad de Ingeniería, Universidad Nacional de Colombia sede Bogotá, AA 111321, Bogotá – Colombia, email: [email protected]
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J.E. Alfonsoc,* c
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Departamento de Física, Facultad de Ciencias, Universidad Nacional de Colombia sede Bogotá, AA 111321, Bogotá – Colombia, email: [email protected] C.A. Pineda-Vargasd d
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iThemba LABS, National Research Foundation, PO Box 722, Somerset west 7129, Cape Town – South Africa, email: [email protected]
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iThemba LABS, National Research Foundation, PO Box 722, Somerset west 7129, Cape Town – South Africa e
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Faculty of Health & Wellness Sciences, CPUT, Bellville, South Africa, email: [email protected] Corresponding author: email: [email protected], mobile phone: +573115060647
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ABSTRACT
Bismuth-based thin films were deposited on AISI 316L stainless steel and glass substrates via the unbalanced magnetron DC sputtering technique. Three different configurations of titanium pieces along the bismuth target race track were used in order to evaluate the influence of Ti content on the optical and electrical properties of the films. We used a reactive ambient mixture of Ar:O (80%:20%) and a constant flux of 9 sccm, along with a power of 40 W. Results showed thin films with high homogeneity and smooth surfaces, with several bismuth droplets, in all the different configurations. It was also found that the optical and electrical properties of the thin films are strongly dependent on the preparation conditions, especially the Ti content. Titanium quantities in the parts-per-million range were found only through the micro-proton induced X-Ray emission technique. This small
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ACCEPTED MANUSCRIPT quantity of Ti, as well as the thickness of the thin films, affected the electrical and optical response of the bismuth-based thin films. KEYWORDS Bismuth, Optical properties, Electrical properties, Unbalanced Magnetron, Proton Induced X-Ray Emission 1. INTRODUCTION
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Bismuth (Bi) and its compounds have been the subject of much interest in research communities due to their interesting properties. This is a semi-metallic group V element on the periodic table with a rhombohedral crystal structure and is generally indexed as a hexagonal lattice (a = 4.574A, c = 11.80A) [1]. It is commonly known to exhibit several interesting properties, such as a highly anisotropic Fermi surface, low carrier density, small carrier effective mass, large Fermi wavelength, and long mean free path [2]. It also has low toxicity and low thermal conductivity [3]. These physical properties have drawn considerable attention because of their usefulness in various applications in industry. For example, ternary compounds based on bismuth, such as BixMyOz (M= Ti, Si, Al, Ga), have the potential to be used as electrolytes for solid oxide fuel cells and sensors [4], and this is due to the properties of photoluminescence [5–8] or oxygen ionic conductivity [9] that they possess. Moreover, depending on the structural formation, some reports suggest that some Bi binary compounds such as Bi2Te3 can be used as thermoelectric materials [10].
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Recently, it has been reported that nanoparticles of Bi have been useful in biological science, such as bio-imaging and bio-sensing [11]. The ability to tune the bandgap of Bi compound material enables a wide range of optical applications. Bi oxide possesses this bandgap tuning capability and therefore qualifies as a promising material for various modern solid-state technologies such as optical coatings, gas sensors, components in optoelectronics, Schottky barrier solar cells, and transparent ceramic glass manufacturing [12, 13]. For instance, the Bi oxide bandgap can be tuned from 2.0 to 3.96 eV [14, 15] and depends on structural formation. On the other hand, the polymorphs of crystalline Bi2O3, such as -Bi2O3 (monoclinic phase), -Bi2O3 (tetragonal phase), and -Bi2O3 (bodycentered cubic phase), have different characteristic values for properties such as the energy bandgap, electrical conductivity, refractive index, permittivity, etc. [16]. Several methods have been employed to achieve various desired structures of Bi oxides and compounds and to alter their properties. These methods include sputtering, pulsed laser deposition, sol-gel (spin coating), and chemical vapor deposition, among others [15–18]. All these methods require variation of different parameters in order to achieve the desired properties. For instance, varying the substrate temperature can favor a particular physical structure while confining optical properties to a particular limit, e.g. the monoclinic αphase, stable up to ~730° C, and after that temperature the cubic high-temperature δ-phase, which is quite attractive in terms of ionic conductivity [19, 20]. On cooling from the δphase, two metastable phases have been reported: the β-phase, with a tetragonal structure, which occurs at 650° C, and the γ-phase, which occurs at 640° C. 2
ACCEPTED MANUSCRIPT It has been reported in the literature that bismuth titanate exists in different phases, such as the Aurivillius, and these phases have shown excellent optical properties due to their photorefractive properties. However, due to its physical characteristic of high-energy bandgap values (~ 4 eV), it has not been used in electric applications.
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In the present paper, we mainly report on some improved structural and optical properties (transmission, reflection and absorption spectra, dispersion of refractive index, and absorption index) of bismuth-based thin films obtained by varying the Ti content during deposition using unbalanced magnetron sputtering. Some correlations between the Ti content and the optical and electrical properties are discussed. 2. EXPERIMENTAL SETUP
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Bismuth-titanium-oxygen thin films were deposited at room temperature on glass and AISI 316 L stainless steel substrates using the variable unbalanced magnetron (GENCOA model PP100) DC sputtering technique, which permits variation of the magnetic field by varying the vertical distance of the central magnet. This technique also allows high deposition rates and high plasma density close to the substrates. The unbalance level of the magnetron can be estimated using the coefficient of geometrical unbalance equation KG, as has been reported [21, 22]: 𝑍0
2𝑅
(1)
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𝐾𝐺 =
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where R is the average radius of the erosion zone and Z0 is the distance from the target surface to the point on the axis of the magnetron where the normal component of the magnetic field (Bz) has a value of zero [21, 22]. For the present project, a KG value of 0.88, which represents the highest configuration of the unbalanced magnetron, was used.
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A Bi target (99.9999% pure) of 4 inches was used. 3 mm thick pieces of pure Ti (99.99%) were thoroughly cleansed with isopropanol followed by acetone in an ultrasonic cleaner and then dried with pressurized air. They were then placed on top of the Bi target surface inside the chamber along the race track in three different configurations, 1, 9, and 17 pieces (7 x 7 mm), as shown in Fig. 1. The glass substrates were cleaned with tap water, and then were immersed in sulfo-chromic mixture (H2CrO4 + H2SO4) for one week, as reported in references [23–25] in order to eliminate insoluble organic residues. They were then cleaned with doubly-distilled deionized water, followed by isopropanol and finally acetone, using the ultrasonic bath, for ten minutes. The substrates were then placed in sample holders inside the chamber, and the chamber was properly closed for vacuum preparation. A vacuum base pressure of 9 x 10-4 Pa was achieved before introducing the gases. A mixture of 80% argon and 20% oxygen gas was then introduced into the chamber for the reactive sputtering process, and a 3.5 x 10-3 Pa vacuum work pressure was reached. Target to substrate distances were maintained at 50 mm. The thin films were grown at room temperature by bombarding the Bi target with Ti pieces (as described above). Time of deposit was kept at 4 minutes for all samples. Sputtering yields for Bi and Ti were 8.946
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and 2.025, respectively. Films with thickness 400 nm for 1 Ti piece, 330nm for 9 Ti pieces, and 260 nm for 17 Ti pieces were obtained.
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Fig. 1: Schematic representation of bismuth target with titanium pieces positioned along the race track within the sputtering system
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The morphological investigation was carried out by employing a FEI Quanta 200 scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) probe for elemental analysis. Surface topographies and roughness were determined by means of a confocal microscope (Carl Zeiss LSM 700) in areas of 250 µm x 250 µm. An X-ray diffraction (XRD) system (Phillips X-Pert Pro Panalytical) in the conventional BraggBrentano (ϴ-2ϴ) geometry and CuK⍺ radiation with λ=1.540998 Å was used for the structural investigation. The X-ray profiles (XRD) were taken for a 2θ range between 10 and 90 degrees in steps of Δ2θ=0.02. Ti distribution in the Bi-based films was mapped via particle-induced X-ray emission within the nuclear microprobe at the Materials Research Department, iThemba LABS, South Africa. A 3 MeV focused proton beam of about 3.5 µm x 3.5 µm lateral resolution was raster scanned on the sample surface (82 µm x 82 µm) using an electrostatic scanning coil. Beam current was kept at about 150 pA with minimum instability in order to avoid damage. Scanned areas were typically analyzed in a square pattern of up to a maximum of 128 x 128 pixels, with a dwell time of 10 ms/pixel. PIXE and proton backscattering spectra were acquired simultaneously in an event-by-event mode, using a Si (Li) X-ray detector positioned at a take-off angle of 135° and shielded with a 125 µm Beryllium (Be) filter. The PIXE count rate was kept below 1000 counts/second to avoid pulse pile-up and to achieve satisfactory counting statistics. The accumulated PIXE spectra were analyzed using GeoPIXE II software [26]. 4
ACCEPTED MANUSCRIPT A Cary Varian 5000 UV-VIS-NIR spectrophotometer was used to investigate the optical properties of the films, and the measurements were conducted in the 200 nm to 2500 nm wavelength range in both the reflectance and the absorbance mode. The extinction coefficient, k, was obtained from Equation 2 [17]:
𝑘=
𝛼∙𝜆
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1+𝑅+√(1+𝑅)2 −(1−𝑅)2 (1−𝑘 2 ) 1−𝑅
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where λ denotes the wavelength in the investigated spectral range and α the absorption coefficient. The refractive index of the films was calculated from reflectance values R using Equation 3 [17, 27]:
𝑟
𝛼ℎ𝜐 = 𝐴(ℎ𝜐 − 𝐸𝑔 )
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Equation 4 was used to calculate the optical band gap by taking into consideration the relationship between the absorption coefficient and the photon energy. (4)
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where hʋ is the photon energy, Eg is the energy of the optical bandgap, A is the characteristic parameter for respective transitions, and r is an exponent which may take the values r=1/2 for allowed direct band-to-band transitions and r=2 for indirect allowed transitions. The usual method for determining the value of the band gap involves plotting a graph of (αhʋ )r versus photon energy in accordance with Equation 4. The value of the direct Eg was determined by extrapolating the linear portion of the plot to (αhʋ )2=0 [17, 28, 36].
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For the electrical properties, a classic standard four-point probe was used with a Lucas Labs 302 setup with a Keithley 2400 source meter. Resistivity values of the films were calculated by taking into account the corresponding correction factors that strongly depend on the sample geometry [29]. 3. RESULTS AND DISCUSSION
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3.1 Scanning electron microscope (SEM) and confocal microscope To better understand the influence of the quantity of Titanium (Ti) pieces (see Fig. 1) on the morphology of the films, they were studied by means of the SEM technique. The resulting images for samples grown with the three different configurations of titanium pieces over the bismuth target are reported in Fig. 2. As can be seen, all the samples are generally homogeneous, but there are some droplet structures (circled in order to highlight a few) that formed due to agglomeration. Later analysis with energy dispersive X-Ray spectroscopy (EDX) established that these droplets are metallic bismuth agglomerates that formed because of the high energy of the gas ions. Although these images show a generally homogeneous surface, there is a clear distinction between them. For example, Fig. 2a shows more droplets, Fig. 2b shows fewer droplets together with a fairly smooth surface, and Fig. 2c shows a smooth and homogeneous surface with only one droplet (circled). 5
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Although EDX could not detect the presence of Ti in the sample due to its detection limit (1000 – 3000 ppm; >10% wt %), it is evident that this evolution is associated with the number of Ti pieces. Increasing Ti pieces on the target reduces the number of droplets on the films surface and consequently make them more homogeneous. Moreover, Fig. 2a and 2b show some patches (pointed to by arrows) signaling inhomogeneity, while in Fig. 2c there was a smooth surface with only a patch area surrounding the droplet. Due to the limitation of EDX, the particle-induced X-ray emission (PIXE) technique was employed in order to investigate the amount of Ti in the samples, and the results are shown and discussed in the latter part of this paper.
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Fig. 2: Scanning electron microscope (SEM) images of the bismuth-based thin films deposited on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: (a) 1 titanium piece, (b) 9 titanium pieces, and (c) 17 titanium pieces
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A confocal microscope was used as a complementary technique, besides SEM, in order to stimulate a parallel discussion and draw conclusions. Fig. 2 shows images of the bismuthbased thin films with three different configurations of titanium pieces over the bismuth target, as labelled. Images were taken in an area of 250µm x 250µm, and the roughness of these films was also analyzed, which in all the samples was quite similar, at around 8 nm.
Fig. 3: Confocal microscope images of the bismuth-based thin films on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: (a) 1 titanium piece, (b) 9 titanium pieces, and (c) 17 titanium pieces. 6
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3.2. X-ray diffraction (XRD)
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Fig. 4 shows the XRD spectra of three samples in the three configurations mentioned under the subtitle Experimental Setup. For all the configurations, these spectra only revealed the presence of rhombohedral phase crystalline Bi, with no traces of Ti. Also, there were no Bibased compound phase structures observed maybe because of the highly oriented Bi peaks for which relative intensities are very large and cannot allow to see them. The absence of Ti and Bi compound peaks in the XRD results does not necessarily mean that Ti is not present in the film, but it could mean that quantity of Ti is very low. According to the ICDD reference pattern 00-044-1246 [30], peaks appearing at 27.1 and 55.9 2θ degrees oriented in (0 1 2) and (0 2 4) are a signature of the rhombohedral Bi phase, as we have observed in Fig. 4. Additional peaks in Fig. 4 correspond to the AISI 316L stainless steel (SS) substrate.
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Fig. 4. X-ray diffraction spectra of bismuth-based thin films deposited on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: 1 titanium piece, 9 titanium pieces and 17 titanium pieces. 3.3. Particle-induced X-ray emission (PIXE) and electrical measurements Two-dimensional elemental mapping by micro-PIXE revealed the presence of small quantities of Ti within the films, as shown in Fig. 5. Based on this characterization, there are two aspects to be considered in the discussion of the Ti 2D-distribution maps: First, increasing the Ti pieces in the sputtering process resulted in an increase of the Ti content in the films from an average of about 0.1319 wt% (for 1 Ti piece) to a maximum of about 7
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0.4434 wt% (for 17 pieces). It is of importance to note the fact that these values are average values within the areas where the Ti content was enriched in the 2-dimensional elemental map for Ti (highlighted by the red contours in Fig 5). Here it is also important to emphasize that the maximum values shown on the side color-coded scale bars on each map (2, 3, and 8 wt%) represent the maximum of the vertical concentration scale for the 2-dimensional elemental map. It is, however, essential to point out here that in the small regions where Ti is enriched (circled by red lines) there are pixels where the concentration of Ti may reach values that are higher than the average quoted in Table 1. For example, the map in Fig 5.c for the 17 pieces of Ti shows a high-intensity sub-region in the bottom right corner. In this sub-region it can be observed that a yellow color appears in some pixels. That means that in these pixels the “real” concentration of Ti is on the order of 6 wt%. In a similar fashion, we could infer the same analogy for the other enriched sub-regions for maps 5.a and 5.b. Second, the distribution of Ti in the whole scanned area, 82 µm x 82 µm, for the 17-piece map was fairly homogeneous except of course for the areas encircled in the red line, which contained higher amounts of Ti content, as shown by the scale bar.
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Fig. 5: Particle-induced X-ray emission maps of Ti on Bi-based thin films on AISI 316L stainless steel (SS) substrate for three different configurations of titanium pieces over a bismuth target: (a) 1 titanium piece, (b) 9 titanium pieces, and (c) 17 titanium pieces. Maps collected from areas: (a) 82µm x48µm; (b) and (c) 82µm x 82µm. Regarding the electrical measurements, a four-point probe was applied to the samples grown on glass substrates. Applying current values on the order of µA, we registered voltage on the order of the mV, and with this data we found the electrical resistance. Applying the geometric correction factors, we calculated the material resistivity, and the results are summarized in Table 1 for the different configurations of titanium pieces over the target. This behavior is in agreement with those reported in the literature for bismuth films’ electrical behavior [31, 32] at room temperature. In the configuration of 17 titanium pieces, an increase in resistivity can clearly be seen, and this could be attributed to the higher presence of titanium as well as the low thickness compared to the other samples. In this sample with 17 titanium pieces, titanium can be considered as an impurity due to the 8
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low concentration found by PIXE measurements; these titanium “impurities” reduce the mean free path of the electrons inside the film. Also the thickness of this sample is the lowest of the set and it plays an important role in the resistivity behavior. As considered by Fuchs-Sondheimer [33, 34] theory in which they talk about the size effect in the mean free path of conduction electrons in thin films, the mean free path is shortened in thin films due to the diffusive scattering and reflection of the electrons at the film surfaces, this fact increases the electrical resistivity. Both phenomena (considering titanium as impurity and the size effect) contribute to the increase of the electrical resistivity in the film with 17 titanium pieces compared with the other films. Applying Fuchs-Sondheimer theory we calculated the electrical conductivity ratio (𝜎0 ⁄𝜎) between bulk conductivity 𝜎0 and film conductivity 𝜎, this is the same as the ratio between film resistivity and bulk resistivity. The results are summarized in Table 1 and show that in fact there is an increase in film resistivity while thickness is reduced because the ratio increases. The classic model of electric conductivity establishes that the resistivity of a material depends inversely on the number of carries per volume unit and on the time between collisions. In a thicker film, these numbers increase, and therefore resistivity decreases.
Table 1. Properties of the films according to the number of pieces of titanium over the target.
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Thickness (nm)
Fuchs-Sondheimer relation of 𝝈𝟎 ⁄𝝈
1.2849*10
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4.1422
1.6272*10
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330
4.4848
6.9033*10
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260
5.0271
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Titanium concentration (ppm(wt)) +/- uncertainty and detection limit (0.1319% +/- 0.0374%) 0.0124% (0.2465% +/- 0.0438%) 0.0116% (0.4434% +/- 0.0588%) 0.0147%
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3.4 Optical measurements Optical measurements were carried out on the thin films deposited on glass substrates. Fig. 6 displays UV-VIS spectroscopy spectra collected in the spectroscopy reflectance mode in the 300 nm to 2100 nm wavelength region. There are some vital aspects of these results that should be pointed out here. At about 300 nm, about 15% reflectance was observed for the 1-Ti piece film, while for 9 pieces and 17 pieces the reflectance values were about 10% and almost 0%, respectively. At about 500 nm and 15% reflectance, there was a crossover (circled) whereby the reflectance of 9 Ti pieces went below that of 17 Ti pieces, and that of 1 Ti piece was flat up to 580 nm, where it started to rise again. At 900 nm and 30% reflectance, another crossover (also circled) was observed, where reflectance of the 17 Ti 9
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pieces went below the reflectance of 1 piece at 50% to about 40%, as indicated in Fig. 6 by the black dotted line. These results clearly suggest that Ti enhanced the reflectance of the Bi-based film in the visible region of the electromagnetic spectrum. It has been reported in the literature that the presence of nanoparticles in the film introduces a dispersion process that changes the optical response of the films. For example, N. Chander et al. reported that the inclusion of gold nanoparticles enhanced the photovoltaic response of the films [35], while Ti nanoparticles enhanced reflectance at lower energies (about 1 eV) to about 80%, as reported in reference [36]. Although the reflectance we measured was not enhanced to the level reported in reference [36], our results agreed with published investigations, especially around the 850 nm wavelength.
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Fig. 6: UV-VIS spectroscopy in reflectance mode spectra of Bi-based thin films on glass substrate for three different configurations of titanium pieces over a bismuth target: 1 titanium piece, 9 titanium pieces and 17 titanium pieces The optical parameters extinction coefficient k(λ) and refractive index n(λ) of the films were calculated from the reflection spectra by taking into account Equations (2) and (3). Fig 7 shows the results. From them we can say that the extinction coefficient exhibits a lineal behavior as a function of wavelength and that a thicker film has a higher k and also a higher absorption. Refractive index exhibits a behavior that notably depends on the titanium content and is similar to the reflectance spectra. In the film with less Ti content, the refractive index grows linearly from 200 nm to 1300 nm and after that reaches a constant value (6). In the film with an intermediate value of Ti, it grows linearly from 200 nm to 700 nm, where it reaches a value of stabilization of 3; and finally in the film with a 10
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higher Ti content it grows linearly from 200 nm to 1000 nm and then reaches a stabilization of 4. This behavior probably can be explained by the presence of the titanium in the bismuth lattice, since in the literature it is shown that metallic bismuth films exhibit a behavior similar to the film with less Ti content [37]. For instance, metallic bismuth at 600 nm has a refractive index value of 2.1832, the film with less Ti content at this wavelength has a refractive index value of 2.531, and the film with higher Ti content at the same wavelength has a value of 2.98, values that indicate relative errors of 15.9% and 27.7%, respectively.
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Fig. 7. Refractive index, n (a) and extinction coefficient, k (b) of Bi-based thin films on glass substrate for three different configurations of titanium pieces over a bismuth target: 1 titanium piece, 9 titanium pieces and 17 titanium pieces
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From the values of absorbance and according to Equation 4, we calculated the optical energy bandgap values of the films, as can be seen in Fig. 8. These values were obtained by extrapolating the linear portion of the curve, and they show that the film with a higher Ti content has a value of 3.84 eV ± 0.05 eV and the other two films have an energy bandgap of around 4 eV. This results are in agreement with those obtained by Condurache-Bota et al. and Celia L. Gomez et al., who reported values from 2.9 to 4.2 eV [17, 18]. The semiconductor behavior of the films can be explained by taking into account that they are formed by an amorphous bismuth oxide immersed in a metallic bismuth lattice.
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Bismuth thin films were deposited on AISI 316L stainless steel and glass substrates via unbalanced DC magnetron sputtering. The effect of the variation of specific parameters during deposition showed the dependence on them of the optical and electrical properties of the Bi systems. The results of the optical response indicate that Bi-based thin films exhibit a semiconductor behavior, and the electrical response indicates a metallic behavior. These results can be explained by taking into consideration that the films have bismuth oxide amorphous compounds immersed in a crystalline bismuth lattice.
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Additionally, these results indicate that by controlling the Ti content it is possible to grow films with pre-determined optical properties in accordance with the application. The amount of Ti in the Bi-based thin films is not significant from a stoichiometric point of view. However, it has here been demonstrated that small amounts of Ti can and may determine the optical and electrical properties of the Bi/substrate systems. It is recommended that in the future more research should be devoted not only to the study of the optical behavior but also to the further exploration of the electrical characteristics and properties of these systems. ACKNOWLEDGMENTS The authors are thankful for the financial support of the Universidad Nacional de Colombia during the course of this research under project number 23656 in the CONVOCATORIA 12
ACCEPTED MANUSCRIPT DEL PROGRAMA NACIONAL DE PROYECTOS PARA EL FORTALECIMIENTO DE LA INVESTIGACIÓN, LA CREACIÓN Y LA INNOVACIÓN EN POSGRADOS DE LA UNIVERSIDAD NACIONAL DE COLOMBIA 2013-2015. The National Research Foundation of South Africa and the operators of the VDG accelerator at iThemba LABS are gratefully acknowledged. Conflict of Interest disclosure
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Funding: This study was funded by the Universidad Nacional de Colombia under project number 23656 of the CONVOCATORIA DEL PROGRAMA NACIONAL DE PROYECTOS PARA EL FORTALECIMIENTO DE LA INVESTIGACION, LA CREACIÓN Y LA INNOVACIÓN EN POSGRADOS DE LA UNIVERSIDAD NACIONAL DE COLOMBIA 2013 – 2015. Conflict of interest: The authors declare that they have no conflict of interest
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Unbalanced magnetron sputtering of films from Bi targets decorated with Ti coupons. While being low, Ti content affects the surface morphology of the Bi films. Optical properties showed films’ semiconductor behavior. Fuchs-Sondheimer theory used to explain the electrical properties of the films. Ti content shown to strongly influence the films’ optical and electrical behavior.
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