Broadband and wide-angle antireflective characteristics of nanoporous anodic alumina films for silicon-based optoelectronic applications

Abstract

technological interest in most of the above applications [1][2][3][4][5][6]. In these applications, it is very crucial to enhance the light extraction or absorption efficiency of the devices. However, a flat Si surface has high inherent reflectivity of >30 % due to its high refractive index (n Si > 3.4) at visible and near-infrared (NIR) wavelengths, which can degrade the device performance due to the optical losses caused by the Fresnel surface reflection. For high-performance Si-based optoelectronic devices, therefore, efficient antireflection coatings (ARCs) which can suppress the unwanted surface reflection losses are required.Recently, using an oblique angle deposition (OAD) method, ARCs consisting of nanoporous materials with inclined columnar structured films have been reported [7][8][9]. The nanoporous films with specified refractive indices can be realized by adjusting the incident vapor flux angle in the OAD process [10,11]. But this technique needs precise control over the process conditions, which causes some problems in practical applications. As an alternative to the OAD method, a self-assembled nanoporous anodic alumina (NPAA) template, which is prepared by a simple, fast, and cost-effective electrochemical oxidation process of aluminum (Al) films in a particular acidic electrolyte solution, can be employed as an optical thin film. The NPAA template is an aluminum oxide (Al 2 O 3 ) film with self-organized random arrays of uniform parallel nanopores. It has been widely used as a template for fabricating various functional nanostructures [12][13][14][15]. The Al 2 O 3 film has been also utilized as a passivation layer to suppress the degradation of electrical properties in Si-based optoelectronic devices due to its relatively good mechanical/ chemical stability [16][17][18]. However, NPAA films as an ARC on the Si are rarely reported. In addition, because the NPAA films have no absorption in the visible and NIR wavelength regions, it is very suitable for Si-based optical Abstract Nanoporous anodic alumina (NPAA) films are fabricated on silicon (Si) surfaces by the anodization of aluminum (Al). Effect of the thickness of initial Al films on the antireflective characteristics of NPAA films consisting of random pore structures is investigated in the wavelength range of 300-1,100 nm at incident light angles (θ inc ) of 3°-70°, together with theoretical analysis using a rigorous coupled-wave analysis method. The reflectance strongly depends on the thickness and porosity of NPAA films, resulting from anodization parameters such as applied voltage, anodization time, and pore widening time. For the obtained NPAA film at the initial Al thickness of 100 nm and the applied voltage of 30 V, the surface reflectance is reduced over a wide wavelength region of 300-1,100 nm at normal incidence of ~3°, exhibiting the relatively lower average reflectance (R avg ) of ~19 % (i.e., R avg ~ 39 % for the bare Si substrate). The lower angle-dependent reflective properties are observed compared to the bare Si substrate at θ...

Hierarchical Ag/TiO2/Si Forest-Like Nano/Micro-Architectures as Antireflective, Plasmonic Photocatalytic, and Self-Cleaning Coatings

“…In addition, for solar cell applications, we estimated the solar weighted reflectance (R SW ), which is defined as the ratio of the usable photons reflected to the total usable photons, by normalizing the reflectance spectrum integrated with the terrestrial AM1.5 global spectrum in a wavelength range of 300–1000 nm. − For the optimum TiO 2 -NRs/MP-Si sample, the smaller R SW value of ∼3.3% is evaluated compared to the TiO 2 -NRs/flat-Si, TiO 2 seed layer/MP-Si, MP-Si, and bare Si (i.e., R SW = 7.48, 15.6, 23.4, and 39.2%, respectively). Therefore, it is noted that the TiO 2 -NRs/MP-Si can be used as an efficient ARC to enhance the efficiency of Si-based solar cells, considering that the ARCs with lower R SW values can further improve the device performance of solar cells …”

Atomic scale structure of amorphous aluminum oxyhydroxide, oxide and oxycarbide films probed by very high field27Al nuclear magnetic resonance

“…[5][6][7] Also, the optical properties of alumina films show potential for RF transparency, light polarization, and broadband and wide-angle antireflection. [8][9][10] The development of materials with such superior properties is strongly linked to the understanding of the atomic structure dictated by the fabrication process. This has been illustrated for amorphous alumina films concerning, for example, the corrosion resistance measured by electrochemical impedance spectroscopy 5 or their superior mechanical properties (hardness and Young's modulus).…”

Blue photoluminescence of sponge-like highly porous alumina synthesized in hydrofluoric acid based electrolytes

“…Unique physical and chemical properties allow to utilize it as functional medium for forming of composite structures [2], producing of filters [3], thermoconducting coatings [4], active elements of optoelectronics [5] etc.…”