ISSN 0030-400X, Optics and Spectroscopy, 2015, Vol. 119, No. 3, pp. 343–355. © Pleiades Publishing, Ltd., 2015. Original Russian Text © K.V. Baryshnikova, A.S. Kadochkin, A.S. Shalin, 2015, published in Optika i Spektroskopiya, 2015, Vol. 119, No. 3, pp. 367–380.
THE INTERNATIONAL YEAR OF LIGHT 2015
Nanostructural Antireflecting Coatings: Classification Analysis (A Review) K. V. Baryshnikovaa, A. S. Kadochkinb, and A. S. Shalina, b, c a
ITMO University, St. Petersburg, 197101 Russia Ulyanovsk State University, Ulyanovsk, 432017 Russia c Kotel’nikov Institute of Radio Engineering and Electronics, Ulyanovsk Branch, Russian Academy of Sciences, Ulyanovsk, 432011 Russia e-mail:
[email protected] b
Received February 25, 2015
Abstract—Many modern optical instruments require the use of high-quality antireflecting coatings. Singleand multilayer homogeneous films are mainly used for this purpose. However, an alternative line is rapidly developed at present, which is devoted to the design and use of nanostructural systems for increasing the transparency of different media. Despite the unified principle of operation of these coatings, which is based on the destructive interference of waves in the direction of reflection of light, approaches to their implementation may differ significantly. Different types of nanostructural coatings are considered in detail and classified, their optical properties are compared, and special attention is paid to methods of their manufacture. It is shown that different antireflecting coatings should be used for different purposes, and that coatings that combine properties of several classes often have better antireflecting capabilities. DOI: 10.1134/S0030400X15090040
INTRODUCTION Materials and media with light transmittance close to 100% in a wide spectral range are required in modern optics in increasing frequency. To increase the light transmittance, single- or multilayer thin-film antireflecting coatings are commonly used [1]. These films increase the transparency due to the destructive interference in the direction of reflection of light. For this purpose, the phase shift of waves reflected from the upper and lower boundaries of the antireflecting coating should be equal to π + 2π m, m ∈ N. The refractive index of an antireflecting layer should be n1 = n0nsub , where n0 is the refractive index of the ambient medium (usually, air) and nsub is the refractive index of the medium with the transparency to be increased [1, 2]. The optical thickness of an antireflecting film is usually equal to a quarter of the wavelength at which the cancellation of reflection should be maximal; this corresponds to a phase shift equal to π. The light transmittance turns out to be much lower at other wavelengths. Coatings that act according to this principle were historically first, being studied for the first time by Rayleigh in the 1870s–1880s [3]. In 1904, Harold Taylor patented the chemical method for their manufacturing [4]. Use of multilayer coatings increases the transparency in a quite wide spectral range. However, this approach has a series of significant restrictions, caused by the need to apply films of
strictly specified thicknesses [5, 6]. The mechanical strength of these coatings is often low due to a limiting scope of materials with required optical properties [7]. The majority of antireflecting coatings that are widespread at present ensure a relatively low transmittance (of about 99% for coated glass versus 92–93% for uncoated glass) [8]. Many semiconductors used in photovoltaics have quite high transmittance [9], which hampers the creation of antireflecting coatings for them. Thus, the search for alternative approaches to increase the transparency is quite urgent at present. In this review, we consider antireflecting coatings of different types (and techniques for their manufacturing), which can compete in their parameters with homogeneous antireflecting coatings. We compare their properties, and discuss their applicability in different optical systems. Apart from realized optical effects, dimensions of nanoobjects and their spatial distribution will be basic parameters with respect to which antireflecting coatings will be grouped. We note that this grouping is conventional, since many structures discussed have properties of several groups simultaneously or are in the boundary range with respect to their parameters. FLAT COMPOSITE FILMS Composite antireflecting coatings make up a wide class of coatings, layers of which contain a large num-
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Fig. 1. SEM image of a four-layer antireflecting coating on a silicon substrate; layer materials, thicknesses, and effective refractive indices at a wavelength of 485 nm are given [12].
ber of randomly distributed nanoparticles or nanopores with dimensions much smaller than the wavelength of the visible range. Optical properties of coating films are directly governed by the concentration of the nanoinclusions [10, 11]. If the inclusion concentration is controlled, the refractive index of composite structures can be controlled in quite wide limits. Such coatings have better mechanical properties than do homogeneous coatings. Composite coatings containing nanopores are usually manufactured by chemical methods (etching, caustic corrosion, etc.) [11–13]. The shape of nanopores can vary significantly in different coatings; it is determined by the crystal lattice of the material, the pore-formation method, etc. Figure 1 shows an SEM image of a section of a composite multiplayer nanoporous silicon-based coating [12]. Each layer has its own refractive index, which is different from indices of other layers due to the use of different materials (silicon oxide, titanium oxide) and because of a varying degree of porosity. This coating may also be referred to gradient coatings (see the next section), since the refractive index of its layers increases as the layer number decreases. Such a coating makes it possible to ensure, on the average, a 5% reflection in the optical range (compared to ~55% for the clean silicon surface); however, this value, as in the case of homogeneous coatings, is different for different wavelengths and depends on the angle of incidence of light, [12, 13]. In [14, 15], antireflecting coatings with nanoparticles obtained by the sol–gel technology were pro-
posed. These coatings have quite good parameters (for coated glass, the reflectivity averaged over the optical range is 3.3% [15]). The sol–gel technology is one of the most widespread; it is used to manufacture both homogeneous and composite coatings. However, it is rather complicated and requires the use of expensive and labor-consuming methods. The possibility of manufacturing coatings with the use of spin coating technique is studied in [10]. This technique is much simpler, uses readily available colloidal sols, and allows manufacturing thin homogeneous coatings with precisely controlled geometrical and optical parameters. Thus, a coating with the refractive index that is controllable in the 1.46–1.54 range when using silicon oxide and in the 1.54–1.95 range when using ceric oxide is presented in [10]. Figure 2 shows experimental results from [10] on the reflection in the visible range from an acrylic substrate with a composite coating with ceric oxide particles. The reflectivity does not exceed a few basic points. The position of the reflection minimum can be controlled by variations in the film thickness and refractive index. The transmittance integrated over the optical range is 97.6% (90% for clean acrylic surface). A coating that is a layer of silicon oxide particles is suggested in [16]. The manufacturing procedure of this coating consists of the application of colloidal particles from a spray or colloidal solution with subsequent drying, and heat treatment. This method is maximally simple and inexpensive. Different values of the filling factor of the coating and, hence, different optical properties can be attained by controlling the
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Fig. 2. Light reflectivity as a function of the wavelength in the optical ranges for two composite coatings with different concentrations of ceric oxide particles [10].
particle concentration. The refractive index of this film varies from 1.3 to 1.47. For 20-nm particles, this coating yielded a decrease in the reflection from the glass surface from 8 to 3.1% [16]. A composite antireflecting coating was manufactured with the use a colloidal solution of a mixture of 7-nm titanium oxide particles and 22-nm silicon oxide particles in [17] in a similar way, but using layer-bylayer treatment of the surface. Figure 3 shows transmission spectra of uncoated glass and glass coated with different numbers of layers (a glass plate is coated from both sides). Applying the same technology to a colloidal solution of 30-nm mesoporous nanoparticles of silicon oxide, a coating was manufactured in [18], which, being applied to the two sides of a flat glass plate, ensured a reflection of no higher than 2% throughout the visible range. Metal particles may also be used as nanoinclusions in composite coatings; however, they themselves strongly absorb the light. Correspondingly, the transmittance of a medium to be coated can remain invariable or even decrease with a decrease in the reflectivity. Therefore, these coatings have a quite narrow range of possible application, limited mainly to photovoltaic applications [19]. Work [20] describes a complex composite coating on silicon, which combines silicon nanowhiskers and silver nanoparticles in a SiO2 shell. This coating provides for the reflectivity below 0.3% in a wide wavelength range 600–1950 nm (55% for uncoated silicon). GRADIENT ANTIREFLECTING COATINGS Another method to reduce the reflection consists of a gradient change in the refractive index of an antireflecting film from 1 (the refractive index of air) to the refractive index of a medium with the transparence to be increased. The idea of such a coating was sugOPTICS AND SPECTROSCOPY
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Fig. 3. Transmission spectra of five- and six-layer composite coatings of 7-nm titanium oxide particles and 22-nm silicon oxide particles on a glass substrate prior to and after the calcination (thin and thick curves, respectively); the bottom curve refers to the substrate [17].
gested by Rayleigh in 1879 [3]. The gradient profile of the effective refractive index may be obtained by two basic methods: doping of a homogeneous material [21, 22] and surface texturing [23–25]. In the first case, a minimal refractive index is created by the material of an antireflecting film and cannot be unity for perfect matching with air; in the second case, in which there are nanosize structures on the surface of the medium, which provide for a smooth variation in the effective refractive index, the theoretical minimal refractive index may be equal to unity. The profile of the effective refractive index gradient may be different (linear, parabolic, cubic, etc.) [26]. The question on the optimal gradient is theoretically considered in [26]; a conclusion is drawn that a profile described by a quintic polynomial provides for a minimal reflection from a structure in the case in which the refractive indices at the coating boundaries are perfectly matched. A similar situation was also studied in [27] numerically, in which the gradient profile was simulated by 1000 layers of the same thickness. The average reflection from coated aluminum nitride, the profile of effective refractive index of which was described by a quintic polynomial, was calculated to be 0.1–0.5%. If the condition for the perfect matching at boundaries is not satisfied, another gradient profile can be optimal. Nevertheless, there is also work [28], where an array of surface pyramid-like gallium nitride particles with different profiles that are deposited on the surface of gallium nitride is considered. Although the refractive indices are matched at both boundaries, the numerical simulation has shown that a coating with a linear profile is the best. Note that, technologi-
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(i) Index profile (i) Air
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Fig. 4. SEM image and calculated effective refractive index of an array of (i) bullet-shaped particles and (ii) cone-shaped particles. The array period is 510 nm [23].
cally, it is easier to attain linear or parabolic profile rather than a quintic profile. Work [21] is a good example of implementation of a quasi-gradient profile of the refractive index by means of formation of 46 a-Si, SiNx, SiNxOy, and SiO2 layers, where x initially increases and then decreases with an increase in coordinate y. This coating on silicon, which was manufactured using the magnetron sputtering and electron-beam vapor deposition pro-
750 nm Fig. 5. Electron micrograph of a subwavelength structure on a silicon surface [25].
vides for an average reflection of about 1.7% for normal incidence (~55% for uncoated Si) in the optical and IR ranges. In [23], the gradient profile of the refractive index was formed by an ordered structure of (i) narrow bullet-shaped and (ii) wide cone-shaped submicron germanium particles on a germanium substrate manufactured with the laser-lithography method (Fig. 4). Profiles of the refractive indices are shown on the left from the SEM images of the structures. These structures allowed an increase in the reflection from the substrate by factors of about two and four for cases (i) and (ii), respectively. A similar structure was considered in [24], where a coating consisted of ZnS cones (radius of 1 μm, height of up to 4 μm) and was used to increase the transparency of the ZnS layer. The reflection was different for cones of different heights and attained 8% (R = 25% for a flat ZnS surface); lower reflection was observed for longer cones located at both ends of the sample, and the antireflecting properties deteriorated with an increase in the angle of incidence. Subwavelength gradient structures are usually manufactured using different lithography techniques. Thus, a two-dimensional subwavelength structure produced on a crystalline silicone surface with the use of electron-beam lithography and subsequent surface treatment with an SF6 flow is presented in [25]. The structure elements are cone-shaped, the structure period is 150 nm, and the depth is 350 nm (Fig. 5). The
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Fig. 6. Reflection spectrum from uncoated silicon (dashed curve) and silicon with structured surface (solid curve) [25].
reflection decreases from 55% (for uncoated Si surface) to 0.5% (minimal value in the optical range) (Fig. 6). It should be noted that the lithography is very expensive method. Attempts are made to simplify the technology. Chemical etching, electron-beam treatment, and other nonselective surface-treatment techniques are less expensive. A technique for producing similar structures using etching without masks is suggested in [29]. In that work, an antireflecting coating with silicon cones and cylinders on the silicon surface are considered; it is shown that the reflection from this structure can be reduced to 1% at submicron sizes of its elements (10% upon averaging over all angles), and this value can be halved when introducing a fine structure at the nanoelement tops. Different types of surface structuring are used. Thus, a coating is suggested in [30], which consists of pyramids, which, it turn, are composed by double layers on a polycrystalline silicon surface. The reflection
is lower than 1% at a structure period of 100 nm and the same depth. Moth eye coatings are very popular in the Western literature (see, e.g., [31–35]. The term is connected with the fact that subwavelength structures have been revealed on the surface of cornea of night insects, which lower the reflection (Fig. 7). The influence of paraboloid aluminum-doped zinc oxide (AZO) nanoparticles located on the plane AZO surface on its reflectivity is studied in detail in [31] Fig. 8. An approach to the optimization of highly transparent antireflecting coatings, which are subwavelength structures on the glass surface, is described in [38]. Transmittance of 99.58% in the whole visible range is attained when using paraboloid subwavelength structures with a period of 200 nm and a height of 200 nm. Figure 9 shows the reflection spectrum calculated for a glass plate with antireflecting nanostructures on both surfaces under normal light incidence. A combined coating composed by classical interference layers and a textured layer is proposed in [39] (Fig. 10). The authors point out that, even a coating composed by a large number of interference layers cannot ensure a decrease in the reflection in a wide range below a certain limit (as was mentioned in the Introduction). The reflection in a wide range is mainly determined by the refractive index of the upper layer, which cannot be lower than a certain value for homogeneous materials due to natural causes. A way out of this situation is apparently to use of a structured coating, which provides for the surface refractive index to become close to unity due to the corresponding choice of the geometry. This approach allowed the authors of that work to reduce the reflection from glass to a value of no higher than 0.3% in the optical range. We should mention separately works [40, 41]. The coating proposed in them can be referred to gradient, even though the profile of its refractive index significantly differs from the profile of common gradient coatings, which usually has no maxima and smoothly connects the refractive indices of two media. The
Fig. 7. Moth eye [36] and its SEM image [37], which shows nanospheroids on its surface [1]. OPTICS AND SPECTROSCOPY
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authors of [40, 41] state that similar coatings on both sides of a nonabsorbing dielectric plate make it invisible. To substantiate theoretically the action of this broadband all-angle antireflecting coating, they use so-called reflectionless potentials, which are usually used in quantum mechanics [42]. Figure 11 shows the profile of the coating refractive index and the reflectivity as a function of the angle of incidence and wavelength.
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Fig. 9. Calculated reflection spectrum of light normally incident on the surface of a glass plate with subwavelength structure applied to its two sides [38].
Light trapping is another way to reduce the surface reflection, in which the light is rereflected many times inside surface traps (the fraction of reflected light decreases with each rereflection) or the incident wave is efficiently transformed into “hot spots” inside the substrate by traps. Coatings that act on this principle are used because of, e.g., the need in producing highly efficient solar cells, and the surface texturing can be less expensive than common antireflecting coatings [43–45]. We note in this case that geometrical parameters of the structure, such as its period and depth,
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Fig. 11. Refractive index as a function of (top) the angle of incidence and (bottom) the wavelength [40]. The profile of the effective refractive index is shown in the insets.
strongly affect the efficiency of this structure. When talking about the surface texturing and light-trapping coatings, microinhomogeneities or monolayers of microparticles are usually meant. At the same time, many different nanostructural plasmonic [46–48] (with nanoparticles of several tens of nanometers in size) and dielectric [49, 50] coatings are used for light trapping in thin-film solar cells with layers thinner than 500 nm, but their consideration is beyond the scope of this work, since they satisfy only specific photovoltaic problems and their use for antireflection purOPTICS AND SPECTROSCOPY
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poses is often unreasonable in view of, e.g., significant absorption of these coatings. The geometry suggested in [44, 45] is a set of mutually perpendicular faces, which provides for multiple internal reflections due to the arrangement of the face planes at an angle higher than the critical angle to the incident light, which causes the light trapping (Fig. 12). This structure on the silicon surface decreases the reflection by 80–85%. A similar mechanism is implemented in [51], where a structure of pyramids and tetrahedrons is
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Fig. 12. Textured silicon surface. The light trapping scheme is shown below [45].
compared with an anisotropically textured surface. It is shown that the structure of pyramids is preferable in the range where the silicon absorption is high, while tetrahedrons act better at the absorption edge. In any case, the texturing increases in the absorption in the silicon layer, which potentially improves parameters of a solar cell. The texturing of the back surface of solar cells with pyramids [52] or the front surface with inverted pyramids [53] is also used. An increase in the photocurrent by 80% due to pyramidal texturing of the surface of a tandem perovskite-on-silicon solar cell is shown in [54]. An uncommon way of surface texturing in the form of a honeycomb pattern (Fig. 13) is considered in [55,
56]. It allows a decrease in the reflection from the silicon surface to 10% at a wavelength of 600 nm [56]. A coating that combines the gradient profile of the refractive index and light trapping due to the multiple internal rereflection was proposed in 2014 in [57]. The morphology of this structure is shown in Fig. 14: silicon microneedles are coated with a huge number of polyaniline nanoneedles, and silicon needles are connected in a conelike complex hierarchical structure. This coating has shown a record high transmittance in the visible, UV, and near-IR ranges (see table). Examples of texturing metals by a femtosecond pulsed laser (Fig. 15) are given in [58–60]. The reflection from their surfaces is significantly decreased
Antireflecting properties of a coating presented in [57] Plane silicon surface
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nanospheres on the glass surface by means of selforganization. As is seen from Fig. 17, particles of 500 nm in diameter form an ordered hexagonal structure on the glass surface. If particles of other diameters are used (200, 700 nm), a structure with a lower degree of the order is obtained. Nevertheless, from the viewpoint of antireflecting properties, the coating formed by 200-nm particles is the best. It increases the transmittance from 88.8 to 97.2% at a wavelength of 600 nm (Fig. 18). Similar results are presented in [64–66]; the authors of which show that a two-dimensional photonic crystal on the surface of a homogeneous dielectric can act as an antireflecting coating. We also proposed a broadband antireflecting coating, which is a discrete ordered layer of nanoobjects incorporated into the near-surface region of a medium the transparency of which is to be increased [67–69]. In these works, we explained for the first time the above-mentioned phenomenon of broadband antireflection in the absence of a smooth gradient of the effective refractive index from the upper to lower boundaries of the monolayer, received a good agreement between the analytical and exact electrodynamic solutions, and predicted the possibility of a 100% transmittance of the medium in a wide spectral range. The imaginary boundary method has been proposed in [67–69]. It allows one to obtain an analytical solution of the problem of the light interaction with an ordered layer of spherical nanoparticles on the surface of the substrate or in its near-surface region. This solution is accurate as compared to an ab initio numerical solution in the case of small particles. Based on this method, it has been shown that a film of spatially
Fig. 13. SEM image of a honeycomb pattern (honeycomb diameter is about 14 μm) [55].
(from 70 to 10% for tungsten [18]), and the absorption in a metal is increased. NONGRADIENT STRUCTURED COATINGS There is a series of structured coatings without gradient or light trapping properties. Thus, a coating of silicon oxide microspheres (Fig. 16) is considered in [61, 62]; it decreases the reflection to 18% for the silicon surface and to 6% for the glass surface [61]. A simple and inexpensive method for obtaining such antireflecting coatings on glass is suggested in [63]. The method consists of the deposition of quartz (a)
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Fig. 14. (a) SEM image of a microneedle structure and a self-organizing hierarchical structure. (a) Plane view; the inset shows side view (cross-section). (b) Plane view, magnified. (c and d) Magnified 3D images of the hierarchical structure [57]. OPTICS AND SPECTROSCOPY
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(b)
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Fig. 16. SEM images of a monolayer of silicon oxide spheres of 2 μm in diameter on glass: (a) plane view and (b) side view [62].
ordered nanospheres has an effective optical depth, different for different wavelengths of the incident radiation (even in the absence of frequency dispersion of the material of the objects), and could serve as a quarter-wavelength antireflecting coating in a certain spectral range, rather than at a single wavelength, as is the case with homogeneous films [67]. We have shown in [70, 71] that the use of nanoinclusions in substrates from materials the refractive index of which is lower than the refractive index of the medium itself (cavities or pores in the simplest case) is the most promising for problems of medium transparency increase. Such antireflecting coatings are universal [71]; i.e., they can be adjusted to a particular medium by varying the cavity sizes. In addition, no materials other than the substrate material are used; hence, there are no problems of physicochemical compatibility of materials typical for common multilayer structures, lamination, fluctuations of film thickness, and so on.
Sum Fig. 17. SEM image of glass surfaces with 500-nm particles applied by means of self-organization [63].
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modified by cavities of different shapes, which have been calculated using the commercial Comsol Multiphysics software. Thus, the use of the effect of frequency dependent optical depth of an ordered layer of nanopores allows a decrease in the reflection from glass to about 0.01%.
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Fig. 19. Reflection spectra of glass modified by layers of spherical (1) and (2) cylindrical nanocavities and frustoconical pores with apex angles of (3) 8° and (4) 15°. The radius of spherical cavities is 50 nm, the radius of the cylinder and cone bases is 60 nm and their depth is 140 nm. Square closely packed lattice.
It should be noted that not only spherical, but also cylindrical or frusto-conical spatially ordered pores can also be used to increase the medium transparency. Such pores are the simplest for experimental implementation by nanolithography methods. Figure 19 shows glass reflection spectra (the refractive index has been taken equal to 1.5 throughout the optical range) OPTICS AND SPECTROSCOPY
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CONCLUSIONS In this review, we have described different types of antireflecting structures and main directions in the development of this field. The main attention has been paid to the principles of action of coatings, manufacturing techniques, and (most importantly) their antireflecting properties, which has allowed classification of coatings. Despite the unified principle of the action of coatings, based on the destructive wave interference in the direction of the light reflection (except for lighttrapping coatings), they can be implemented in different ways. Thus, doping a layer or incorporating nanoparticles into it, materials with a required unnatural refractive index can be manufactured. The surface structuring allows its reflectivity to be reduced and the refractive index gradient profile to be produced for better material matching. There are coatings that can be described as a film with an effective optical thickness that can be varied by varying the radiation wavelength, which provides for conditions for an increase in the medium transparency in a wide spectral range. Antireflecting coatings are widely and differently used. These coatings are required for correct operation of photo cameras, telescopes, glasses, and many other optical instruments. Antireflecting properties of a coating determine the quality of monitors (anti-glare effect) and glasses. A decrease in the reflection from the solar cell surface increases its efficiency. Antireflecting coatings of different types have different application domains. Thus, multilayer interference coatings are used to increases the glass transparency in optical instruments; the layers are often composite with nanoinclusions, and/or implement the gradient of refractive index profile. Structured coatings with the gradient of the effective refractive index profile and light-trapping coatings are used to increase the transparency of surfaces of materials with a high refractive index. There are coatings that combine features of several types; such coatings often have the best antireflecting properties. ACKNOWLEDGMENTS The work was supported by the Russian Foundation for Basic Research (projects nos. 14-08-31730 mol-a and 14-02-31765 mol-a), by a grant of the President of the Russian Federation, by the Government of the Russian Federation (grant no. 074-U01), and by the Ministry of the Education and Science of the Russian Federation (State Target 2014/190, project no. 14.Z50.31.0015).
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Translated by O. Ponomareva
