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Abstract
Spectrally selective coatings are multilayer structures that are deposited on glass. These structures consist of a metal/dielectric bi-layer, in which the metal is silver and the dielectric is a wide-bandgap semiconductor with a high refractive index. These layers are typically very thin (< 30 nm), and thus may be etched away if deposited by a sputtering process. This work has two objectives. First, a new design is fabricated that can sustain the sputtering process. Second, a high-refractive-index material (i.e., bismuth oxide) is selected to serve as the dielectric. Bismuth oxide is deposited by reactive direct-current sputtering, and the deposition parameters are optimized to yield films that are suitable for use in spectrally selective coatings. When used in a silver-based multilayer structure, the resulting peak visible transmittance was 68%, and the maximum infrared reflectance was 85%.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Bismuth oxide (Bi2O3) is a wide-bandgap semiconductor that is characterized by a high refractive index, remarkable optical non-linearity, and significant photoconductivity [1–4]. It can exist in six polymorphic forms identified as $\alpha $, $\beta $, $\gamma $, $\delta $, $\epsilon $, and $\omega $ phases, with the monoclinic $\alpha $ phase being a thermodynamically stable structure existing at room temperature up to 720 °C [4,5]. Bi2O3 has found potential applications in diverse technological fields, including optical coatings, solid-oxide fuel cells, gas sensing, ionic conduction, heterogeneous catalysis, and photocatalysis [1–3,6]. Thin films of bismuth oxide have been prepared by a wide range of deposition techniques including thermal oxidation of bismuth [7,8], sol-gel [9], as well as chemical bath [10], chemical vapor [11], atomic layer [2,5], and pulsed laser [12,13] deposition. Magnetron sputtering has emerged as a major technique for synthesizing Bi2O3 thin films. This is because the sputtering process offers several advantages, such as operation far from equilibrium conditions [14], high deposition rates [4,15], dense and adherent coatings [4,15], and adaptability for large-scale industrial production [4,16]. Radio-frequency sputtering (RFS) has been widely used for the deposition of bismuth oxide thin films [1,4,14–21]. However, despite its relative simplicity as compared to RFS, direct-current sputtering (DCS) has been less widely employed in preparing Bi2O3 thin films. DCS has been accomplished in two modes: (i) reactive deposition of metallic bismuth in an oxygen atmosphere [22–24], or (ii) post-deposition laser-irradiation of DC-sputtered Bi thin films [3,6].
The solar spectrum corresponds to electromagnetic waves that extend over a wide wavelength ($\lambda $) range from 400 to 3000 nm. To reduce cooling demands in buildings and vehicles, energy-saving coatings are deposited on commercial glass windows to transmit light ($\lambda $ = 400–700 nm) while simultaneously reflecting infrared heat ($\lambda $ > 700 nm). The functionality of these spectrally selective coatings, which are also known as transparent heat mirrors (THMs), can be achieved through a three-layer (dielectric/metal/dielectric/glass) structure. However, in this study, a bi-layer was adopted (without the top dielectric) for two reasons: economic savings with respect to the required materials and the inability of the top dielectric layer to sustain ion etching in a sputtering process. Typically, silver (Ag) is chosen as the metal for this application because of two of its properties. First, the free-electron character of silver results in a plasmon wavelength (due to $d - d$ transitions) in the ultraviolet range, which enhances visible transparency and infrared reflectivity [25,26]. Second, its low refractive index ($n$ = 0.053) and high extinction coefficient ($k$ = 3.485) at $\lambda $ = 550 nm [27] confirm low absorption in the visible range [26,28]. The dielectric layer serves three purposes: (i) antireflection for the Ag layer, (ii) wavelength tunability, and (iii) protection of the Ag layer against environmental degradation such as corrosion and abrasion [29–31]. To realize these requirements, the dielectric layer should have the following characteristics. First, its refractive index should match the extinction coefficient of Ag in the visible range [32]. Second, the thickness of the dielectric should be greater than ${\lambda _m}/8{n_D}$, where ${n_D}$ is the refractive index of the dielectric at the desired wavelength (${\lambda _m}$) for maximum transmittance [33]. Bi2O3 has a wide bandgap and is a high-refractive-index material. Therefore, it is a potential candidate for use as a dielectric in THMs. Its potential for this application was first realized by George et al. [34] in gold-based THMs. One of the major technical challenges in the design of THMs is the stringent control over the thicknesses of the layers, because these layers are relatively very thin (thickness < 30 nm). Another challenge is the etching of the layers due to ion bombardment when the coatings are deposited by sputtering, as a given layer could be completely removed when the subsequent layer is deposited.
The purpose of this study is two-fold. First, bismuth oxide thin films are deposited by reactive DC sputtering. The properties of the films under various deposition parameters are varied to determine the best conditions for the application of the films in Ag-based THMs. Second, to overcome the challenges of THM fabrication by sputtering, a new reverse design is investigated, in which the dielectric layer is deposited by DC sputtering followed by deposition of the Ag layer by thermal evaporation. Here, the energy and momentum of the evaporated species are so minimal as to cause no surface damage or layer removal. This requires back-surface illumination of the glass substrate to exploit the antireflection effect of the dielectric layer. Finally, to protect the top surface of the Ag layer, a magnesium fluoride layer (MgF2) is deposited by thermal evaporation on top of the Ag layer. MgF2 is selected because of its wide usage in optical coatings as a low-index hard and durable transparent protective coating. The schematic of the multilayer structure is shown in Fig. 1.
Fig. 1. Multilayer design of the transparent heat mirror. The Bi2O3 layer is deposited by DC sputtering, whereas the silver and magnesium fluoride layers are deposited by thermal evaporation. The back illumination of the substrate is also shown.
2. Experimental details
Bismuth oxide thin films were prepared by reactive DC magnetron sputtering in an Oerlikon Univex 350 system using a 3-inch-diameter bismuth target having a purity of 99.99%. The films were deposited on fused silica substrates and single-crystal (100) silicon wafers as well as on molybdenum substrates. Prior to deposition, the substrates were cleaned in methanol and isopropanol in a sonicator for 5 min. They were then loaded into the sputtering system, which was pumped to a base pressure of 5 × 10−6 mbar. Next, argon (Ar) was bled into the system, and DC power was applied to initiate the sputtering plasma. Four sets of films were deposited at Ar flow rates of 10 and 15 sccm and DC sputtering powers (${P_{DC}}$) of 40 and 60 W. Reactive deposition was achieved using an oxygen flow rate of 20% of the Ar flow rate. The deposition time was 1 h. The film thicknesses were calculated from the interference patterns in the transmittance spectra (Fig. 5(a)) and were verified using a surface profilometer (KLA Tencor D-500). The values of the different thicknesses are listed in Table 1. Optical characterization of the bismuth oxide thin films (Section 3.2) indicated that the films best suited for THM applications were those deposited at an Ar flow rate of 10 sccm using a DC power of 40 W. The three-layer THM was fabricated by first depositing a DC-sputtered bismuth oxide layer. This was followed by depositing a silver layer and then a 25-nm top MgF2 layer, both of which were prepared by thermal evaporation in a Leybold L560 box coater at a rate of 4 Å/s using granules of 99.99% purity evaporated from molybdenum boats. In the THMs, the thicknesses of the bismuth oxide and silver layers were varied as described in Section 3.3. The structural properties of the films were analyzed by x-ray diffraction (XRD) of the films deposited on the silicon wafers using a Bruker Phaser D2 x-ray diffractometer that employed a Cu Kα source at a 2θ scanning step size of 0.01°. The surface morphologies of the films were examined using tapping mode atomic force microscopy (AFM, Bruker diInnova SPM). The scan areas of the films were 1 × 1 µm2, and the scan rate of the tip was 0.8 Hz. The chemical properties of the films were investigated using x-ray photoelectron spectroscopy (XPS) through a Thermo Scientific Escalab 250Xi spectrometer equipped with a monochromatic Al Kα x-ray source with an instrumental energy resolution of 0.5 eV. The films deposited on molybdenum substrates were used for the XPS analysis to avoid charging the non-conducting samples. Elemental depth profiles of the THMs were obtained by a sequence of ion etching, followed by XPS measurement. Etching was performed using a 2-keV Ar+ ion beam on a 1-mm2 area of the surface. The transmittance and reflectance spectra of the films deposited on fused silica were measured using a JASCO V-570 double-beam spectrophotometer. Unless otherwise stated, all optical spectra were measured at normal incidence. The resistivity of the bismuth oxide thin films was measured using a two-probe method. Two parallel rectangular silver electrodes (length: 2.54 cm, width: 5 mm, spacing: 0.5 mm, and thickness: 100 nm) were deposited on the films by thermal evaporation. The resistance was measured by a Keithley 6517B electrometer and was found to be in the 1012 Ω range, indicating highly resistive films. The heat-shielding capability of the fabricated THMs was investigated by measuring the surface temperature of the substrate. To that end, the THM was placed at a distance of 2 cm in front of a 50-W tungsten halogen lamp, which simulates solar radiation and can raise the temperature to values comparable to those encountered in hot climates.
Table 1. Properties of the bismuth oxide thin films
3. Results and discussion
3.1 Structural and chemical characterization of bismuth oxide thin films
The deposition rate was increased as the DC power increased. This was due to the higher kinetic energy of the ions at the higher sputtering power, which resulted in more deposited material impinging on the substrates. However, the deposition rate decreased with the higher Ar flow rate, which may be attributed to an increased number of collisions between the ions and the surface of the growing film [35]. The XRD patterns of the films are shown in Fig. 2(a) and exhibit a single peak centered at 2$\theta $ = 28.8°, which corresponds to the (012) orientation of the monoclinic $\alpha $-phase [2,14]. In an attempt to improve the crystallinity of the films, one set of films was deposited at an Ar flow rate of 10 sccm at a DC power of 40 W on substrates heated to 300 °C. As shown from the intensity of the XRD pattern in Fig. 2(b), the crystallinity was substantially enhanced, resulting in nearly an epitaxial growth along the (012) direction. However, these films possessed severely inhomogeneous and rough surfaces, as shown by the optical microscope image shown in the inset of Fig. 2(b). This effect could be attributed to the low melting point (273 °C [23]) of bismuth that resulted in the deposits being transported like a fluid onto the heated substrate. These films are unacceptable for the desired optical applications, which require highly transparent films with very smooth surfaces. Therefore, all of the films reported in this study were deposited on unheated substrates. The surface morphologies of the films were demonstrated by the two-dimensional AFM images shown in Fig. 3. Statistical analysis of the images yielded the root-mean-square surface roughness (${R_{rms}}$) values listed in Table 1, which indicate very smooth surfaces that are best suited for optical applications, as they result in the least scattering of light. The surface roughness increased as the DC power increased, which was expected for the higher kinetic energy of the ions. However, the surface roughness decreased for the higher argon flow rate, as would be expected for the lower growth rate.
Fig. 2. XRD patterns of the bismuth oxide films: (a) deposited on unheated silicon substrates at the indicated Ar flow rates and sputtering powers, and (b) deposited on a silicon substrate heated to 300 °C at an Ar flow rate of 10 sccm and a DC power of 40 W. The inset in (b) shows an optical-microscope image (1 × 1 in.) of the film deposited on a heated fused silica substrate (300 °C) at 10 sccm of Ar and 40-W DC power.
Fig. 3. Two-dimensional AFM images of the bismuth oxide thin films deposited on fused silica substrates: (a) ${P_{DC}}$ = 40 W, Ar = 10 sccm; (b) ${P_{DC}}$ = 40 W, Ar = 15 sccm; (c) ${P_{DC}}$ = 60 W, Ar = 10 sccm; and (d) ${P_{DC}}$ = 60 W, Ar = 15 sccm.
The chemical state of the films was investigated using XPS. The bismuth XPS 4f core level spectrum consisted of two sublevels (4f5/2 and 4f7/2) due to spin-orbit splitting that are different in terms of binding energy because of an energy shift $\delta $. A typical XPS wide-scan survey spectrum for the surface of a bismuth oxide thin film is shown in Fig. 4(a), which indicates that the films contained only the constituent elements (Bi and O). In addition, high-resolution spectra in the Bi 4f and O 1s core level regions are shown in Figs. 4(b) and 4(c), respectively. To determine the binding energies of the Bi 4f7/2, Bi 4f5/2, and O 1s levels, the spectra were deconvoluted using a non-linear least squares algorithm with a Shirley background and a Gaussian–Lorentzian mixed line shape. The binding energies of the Bi 4f7/2, Bi 4f5/2, and O 1s levels were 158.4 ± 0.1, 163.7 ± 0.1, and 529.6 ± 0.3 eV, respectively. These values match those reported in the literature [36] for Bi in the Bi3+ oxidation state, indicating that our films had the stoichiometric Bi2O3 composition. The purity of the films was also confirmed by the single-component O 1s spectrum, which reflects the absence of adsorbed oxygen species, such as those absorbed due to exposure to the atmosphere.
Fig. 4. X-ray photoelectron spectra of the bismuth oxide film deposited on a molybdenum substrate with a DC power of 40 W under an Ar flow rate of 10 sccm: (a) wide survey scan showing the constituents of the film; (b) and (c) detailed high-resolution core-level spectra in the Bi 4f and O 1s regions, respectively.
3.2 Optical characterization of bismuth oxide thin films
The transmittance ($T$) and reflectance ($R$) spectra of the bismuth oxide thin films are shown in Fig. 5. The films were transparent in the visible region and exhibited interference extrema due to multiple interference of the light beams reflected from the various interfaces, indicating the excellent optical quality of the films. In the transparency region of the films (λ ≥ 500 nm), the transmittance spectra were fitted using the equations for the transmittance of a thin film on a transparent substrate [37]. The refractive index was modeled using a Cauchy dispersion equation:
where ${A_o}$, ${A_2}$, and ${A_4}$ are constants. The extinction coefficient was considered to be a constant (${k_o}$). The best-fit parameters (${A_o}$, ${A_2}$, ${A_4}$, and ${k_o}$) are listed in Table 1, and the resulting dispersion curves are shown in Fig. 6. Our results for the refractive index were close to those reported in previous studies for bismuth oxide thin films [4,19,24]. For a given sputtering power, a significant reduction was observed in the refractive index as the Ar flow rate increased. These results were consistent with the XRD and surface roughness results. At the desired design wavelength for a THM (${\lambda _m}$ = 550 nm), the variation in the refractive index for a given sputtering power was 9.5% for ${P_{DC}}$ = 40 W and 10.7% for ${P_{DC}}$ = 60 W. However, for a given Ar flow rate, the variation in the refractive index as ${P_{DC}}$ changed was 2.7% for 10 sccm of Ar and 3.9% for 15 sccm of Ar.Fig. 5. Normal-incidence optical spectra of the bismuth oxide thin films deposited on fused silica substrates: (a) transmittance and (b) reflectance.
Fig. 6. Dispersion curves of the refractive index of bismuth oxide thin films deposited on fused silica substrates.
In the fundamental absorption region, the absorption coefficient ($\alpha $) (not to be confused with the monoclinic phase) is related to the transmittance and reflectance of the film as [38]:
Moreover, bismuth oxide was reported to have a direct bandgap [5,7,10,18], for which the dependence of the absorption coefficient on the incident photon energy ($E$) is given by: where C is a constant, and ${E_g}$ is the bandgap. Plots of ${({\alpha E} )^2}$ versus E (Tauc plots) are shown in Fig. 7, and the linear portions of the curves were extrapolated to the energy axis, where the intercept gives the bandgap. The bandgap values are listed in Table 1, and they show little variation (2.94 ± 0.02 eV) as a function of the deposition parameters, which is well within the uncertainty of the Tauc method. These values are close to those reported in the literature (2.88–3.2 eV) for the bandgap of Bi2O3 [5,7,10,18].Fig. 7. Tauc plots for the determination of the bandgaps of the films deposited on fused silica substrates. The numbers given in the inset represent the maximum of the vertical scale for each type of film.
3.3 Transparent heat mirrors
The films with the highest refractive index (2.48) at the design wavelength (${\lambda _m}$ = 550 nm) were those deposited at an Ar flow rate of 10 sccm with ${P_{DC}}$ = 40 W. In addition, these films had a wide bandgap (2.95 eV), which is crucial for visible transparency. These are the conditions that were used to deposit the bismuth oxide layer in the THM. Consequently, as discussed in Section 1, the minimum thickness of the bismuth oxide layer should be ${\lambda _m}/8{n_D} \approx $ 28 nm. To investigate the influence of the thickness of the bismuth oxide film, two layers were deposited (28 and 38 nm) that were each over-coated with a 20-nm-thick silver layer. The measured transmittance spectra of these bi-layers are shown in Fig. 8. The thicker bismuth oxide layer shifted the peak wavelength favorably into the visible range but at the expense of a 10% reduction in transmittance. This confirms the optimal choice of a 28-nm-thick bismuth oxide layer in the THMs, as theory suggests. Thus, this value was used in all THMs fabricated in this study. Next, the influence of the silver layer was investigated, where silver layers of different thickness (i.e., 10, 15, 20, and 25 nm) were deposited. The resulting optical spectra of these three-layer structures (MgF2/Ag/Bi2O3/substrate) are shown in Fig. 9. An interplay between transmittance and reflectance was evident. As the Ag thickness increased, the density of free-charge carriers increased, resulting in enhanced reflectance and reduced transmittance. No Ag thickness lower than 10 nm was investigated, because such film thicknesses correspond to discontinuous films that consist of island structures [26,33]. The maximum transmittance (68% at $\lambda = $ 510 nm) was obtained with an Ag film having a thickness of 10 nm. The influence of the angle of incidence (${\vartheta _i}$) on reflectance is illustrated in Fig. 10, where ${\vartheta _i}$ was varied from 0 to 60° for the THM with a 15-nm silver layer. As the figure shows, in the near infrared range (where reflectance is to be maximized), the change in reflectance was insignificant (< 3% relative to that at normal incidence) for ${\vartheta _i}$ up to 45°. However, for larger angles (for example, ${\vartheta _i}$ = 60°), considerable change and thus deterioration of infrared reflectance was observed.
Fig. 8. Transmittance spectra of transparent heat mirrors deposited on fused silica substrates using a 20-nm silver layer deposited on top of bismuth oxide films of thicknesses 28 and 38 nm.
Fig. 9. Normal-incidence transmittance and reflectance spectra of transparent heat mirrors based on the structure MgF2/Ag/Bi2O3/fused silica substrate, where the thicknesses of Bi2O3 and MgF2 were 28 and 25 nm, respectively. The thickness of the silver layer was varied as indicated in the figure.
Fig. 10. Variation of the reflectance of the THM deposited on a fused silica substrate with a 15-nm silver layer as a function of the angle of incidence.
Ideally, the transmittance of a THM should be confined to the visible range. This necessitates the presence of sharp interfaces between the layers [39]. XPS depth profiling was used to investigate the inter-diffusion among the layers. Figure 11 shows typical elemental depth profiles of the THM with a 15-nm-thick silver layer, which demonstrates the spatial distribution of the elements along the thickness of the THM. These profiles were calculated from the areas of the Ag 3d, Bi 4f, F 1s, Mg 1s, and O 1s peaks. The figure clearly shows that the interfaces between the layers were not sharp, and thus diffusion among the layers occurred.
Fig. 11. XPS depth profile spectra showing the variation of the relative atomic concentration of the various elements as a function of etching time for the three-layer structure whose optical spectra are shown in Fig. 9 with a silver layer thickness of 15 nm.
The performance of a heat mirror is typically evaluated in terms of the integrated visible transmittance (${T_{vis}}$) and infrared reflectance (${R_{IR}}$) defined as [26,33,40]:
Fig. 12. Performance curves of the transparent heat mirrors (whose spectra are given in Fig. 9) showing the variation of the integrated visible transmittance and infrared reflectance as functions of the thickness of the silver layer.
Fig. 13. Surface temperature of an uncoated fused silica substrate as well as a fused silica substrate coated with a THM having a 15-nm silver layer. Heating is achieved by radiation from a tungsten halogen lamp.
4. Conclusion
A transparent heat mirror is an energy-saving coating that transmits light and reflects infrared heat. This type of coating can be used in the heat insulation of commercial residential and automotive glass windows to reduce cooling demands in warm climates. This coating consists of a multilayer metal/dielectric structure, in which the metal is typically silver. The layers are very thin and may not withstand dry etching that occurs when the layers are deposited by sputtering due to ion bombardment. In this study, a new design was fabricated to overcome this challenge by (i) depositing the dielectric layer by sputtering, (ii) depositing the silver layer by evaporation, (iii) depositing a top protective magnesium fluoride layer, and (iv) back-illuminating the glass substrate. Bismuth oxide was investigated as the dielectric, where Bi2O3 thin films were deposited by reactive DC sputtering of a bismuth target in an oxygen atmosphere. Two deposition parameters were investigated: the sputtering power (40 and 60 W) and the argon flow rate (10 and 15 sccm). Structural, chemical, and optical characterizations indicated that the best films for use in energy-saving coatings were those deposited at a DC power of 40 W under an argon flow rate of 10 sccm. These films exhibited the desirable properties of a wide bandgap (2.95 eV) and a high refractive index (2.48 at 550 nm). This deposition process was superbly adapted for this application because it is a low-temperature process that required no further annealing, and the resulting films were transparent with very low surface roughness. A transparent heat mirror was then deposited encompassing a bismuth oxide layer with an optimal thickness of 28 nm. This layer was over-coated with silver layers having thicknesses varying from 10 to 25 nm, on top of which was a 25-nm MgF2 layer for environmental protection. The best performance was obtained with a 10-nm silver layer, where the maximum visible transmittance and maximum infrared reflectance were 68% and 85%, respectively. These coatings were found to reduce the surface temperature of glass by at least 16 °C.
Acknowledgment
This work was supported by the Physics Department of King Fahd University of Petroleum and Minerals.
Disclosures
The authors declare no conflicts of interest.
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