Nanophotonics 2021; 10(1): 627–637 Research article Chang-Wei Cheng, Soniya S. Raja, Ching-Wen Chang, Xin-Quan Zhang, Po-Yen Liu, Yi-Hsien Lee, Chih-Kang Shih and Shangjr Gwo* Epitaxial aluminum plasmonics covering full visible spectrum https://doi.org/10.1515/nanoph-2020-0402 spectroscopy and high-quality-factor plasmonic surface Received July 19, 2020; accepted September 18, 2020; lattices based on standing localized surface plasmons and published online November 25, 2020 propagating surface plasmon polaritons, respectively, in the entire visible spectrum (400–700 nm). Abstract: Aluminum has attracted a great deal of attention as an alternative plasmonic material to silver and gold Keywords: aluminum epitaxial film; molecular-beam because of its natural abundance on Earth, material sta- epitaxy; monolayer transition metal dichalcogenide; bility, unique spectral capability in the ultraviolet spectral plasmonic surface lattice; surface-enhanced Raman spec- region, and complementary metal-oxide-semiconductor troscopy; surface plasmon interferometry. compatibility. Surprisingly, in some recent studies, aluminum has been reported to outperform silver in the visible range due to its superior surface and interface 1 Introduction properties. Here, we demonstrate excellent structural and optical properties measured for aluminum epitaxial films Plasmonics is a rapidly evolving field that takes advantage grown on sapphire substrates by molecular-beam epitaxy of strong light confinement and drastically enhanced under ultrahigh vacuum growth conditions. Using the light–matter interactions beyond the diffraction limit near epitaxial growth technique, distinct advantages can be the surfaces and interfaces of metal nanostructures. In the achieved for plasmonic applications, including high- past few decades, remarkable advances based on plas- fidelity nanofabrication and wafer-scale system integra- monic nanostructures, metamaterials, and metasurfaces tion. Moreover, the aluminum film thickness is controllable have been made for surface-enhanced spectroscopies, down to a few atomic monolayers, allowing for plasmonic sensors, photovoltaics, super-resolution microscopy and ultrathin layer devices. Two kinds of aluminum plasmonic lithography, metalenses, biomedical therapeutics, applications are reported here, including precisely engi- nonlinear optics, and integrated nanophotonics [1–3]. neered plasmonic substrates for surface-enhanced Raman However, there are still significant material issues to be resolved such that plasmonics can be elevated to a trans- formative technology for general applications. *Corresponding author: Shangjr Gwo, Department of Physics, National One critical issue of plasmonics is related to the Tsing-Hua University, Hsinchu 30013, Taiwan; Institute of intrinsic properties of available plasmonic materials. NanoEngineering and Microsystems, National Tsing-Hua University, Hsinchu 30013, Taiwan; and Research Center for Applied Sciences, Nearly all of the research results adopt noble metals (silver Academia Sinica, Nankang, Taipei 11529, Taiwan, [Ag] and gold [Au] are the most popular choices) as the E-mail: [email protected]. https://orcid.org/0000-0002-3013-0477 plasmonic materials because they exhibit negative real and Chang-Wei Cheng, Department of Physics, National Tsing-Hua small imaginary parts of dielectric function in the visible University, Hsinchu 30013, Taiwan. https://orcid.org/0000-0001- and near-infrared spectral regions. However, noble metals 8937-5084 Soniya S. Raja, Institute of NanoEngineering and Microsystems, suffer from high material cost (Au), low material stability National Tsing-Hua University, Hsinchu 30013, Taiwan (Ag), and incompatibility with existing semiconducting Ching-Wen Chang, Research Center for Applied Sciences, Academia technology (Ag, Au). Furthermore, owing to the interband Sinica, Nankang, Taipei 11529, Taiwan transitions in noble metals, spectral responses of plas- Xin-Quan Zhang, Po-Yen Liu and Yi-Hsien Lee, Department of monic devices are limited in some specific ranges. To Materials Science and Engineering, National Tsing-Hua University, overcome these difficulties, alternative plasmonic mate- Hsinchu 30013, Taiwan Chih-Kang Shih, Department of Physics, The University of Texas at rials, such as aluminum (Al), copper (Cu), transition metal Austin, Austin, TX 78712, USA nitrides, conducting metal oxides, and graphene have Open Access. © 2020 Chang-Wei Cheng et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License. 628 C.-W. Cheng et al.: Epitaxial aluminum plasmonics covering full visible spectrum been extensively pursued in recent years [4–6]. Among can have some distinct advantages for plasmonic appli- them, aluminum is particularly interesting because it acts cations, including high-fidelity nanofabrication for precise as an ideal Drude metal except a narrow interband tran- control of surface plasmon resonances owing to the single- sition window in the near-infrared (at 800 nm) [7]. In crystalline material structure and large-scale, highly uni- particular, for the ultraviolet (UV) and deep-UV plasmonic form plasmonic structures required for SERS substrates applications, aluminum is the best plasmonic material due and high-quality-factor (high-Q) plasmonic surface lat- to the negative real and relatively small imaginary parts of tices. Moreover, the aluminum film thickness is control- aluminum dielectric function in the UV region. lable down to a few atomic monolayers, allowing for Considering practical applications, aluminum is also a ultrathin metal layer plasmonic applications [43–45]. It is sustainable plasmonic material since it is naturally abun- worth noting that, since aluminum is considered as the dant in the Earth’s crust and has a native oxide protection “silicon” of superconductivity [46], aluminum epitaxial layer (∼3–5nmAl2O3) [8, 9]. However, aluminum was not films can also be used as the building material for quantum previously considered as a good candidate for alternative computers requiring high-performance superconducting plasmonic material [5] before the advent of high-quality qubits. aluminum nanocrystals [10, 11] and epitaxial films [9, 12, 13] with greatly improved material properties. During the past few years, the fast development of aluminum plas- 2 Epitaxial growth and structural monics has attract a great deal attention not only for the expected good performance of aluminum for UV plas- properties monics, such as UV surface-enhanced Raman spectros- copy (UV-SERS) [14, 15], plasmonic lasers [16–22], and Previous works on silver epitaxial films and nanostructures deep-UV resonances [8, 23], but also for its unexpected have demonstrated that crystalline properties and surface excellent performance in the visible region, including morphologies play an important role in plasmonic applica- complementary metal-oxide-semiconductor (CMOS)- tions [47–53]. Especially, uniform and controllable plasmonic compatible color filters [24–28], photocatalysis [29], hot spots can realized by high-fidelity top-down nano- nonlinear optics [30–32], and SERS [15, 33]. Very recently, fabrication on ultrasmooth, single-crystalline Ag colloidal aluminum has even been found to outperform silver in crystals [53, 54]. In this work, aluminum epitaxial growth was some important plasmonic applications [15, 34]. conducted by using a MBE system under ultrahigh vacuum The key to understand these finding is that the per- conditions. Two-inch double-side-polished c-plane sapphire formance of plasmonic materials depends not only on their (0001) wafers were used as the substrates, and the base − intrinsic optical properties but also their material proper- vacuum pressure was kept about 1 × 10 10 Torr during growth. ties, such as crystallinity, surface and interface quality, as Before growth, the c-plane sapphire substrate was thermally well as stability. In the literature, aluminum nano- cleaned at 950 °Cfor2h.Astreakyreflection high-energy structures and metasurface are typically fabricated by electron diffraction (RHEED) pattern of the c-plane sapphire lithographic methods using thermally evaporated surface can be obtained after this cleaning step (Figure 1A). aluminum films. In such cases, amorphous or poly- Then, aluminum was evaporated by using a Kundsen cell. crystalline film growth, as well as residual oxygen in the The deposition rate (about 200 nm/h) was controlled by the growth chamber, will eventually affect the performance of cell temperature, and the substrate temperature was main- aluminum-based plasmonic devices. Recently, epitaxial tained at room temperature (∼300 K) during growth. A streaky aluminum film growth on commercially available sub- RHEED pattern (Figure 1A) of aluminum film indicates a strates (silicon [13, 35, 36], GaAs [12, 37, 38], sapphire [9, 39, smooth film morphology during epitaxial growth. 40]) has been developed by using the molecular-beam The crystal orientation of epitaxial aluminum film was epitaxy (MBE) technique under ultrahigh vacuum condi- measured by X-ray diffraction (XRD) using the copper Kα1 tions. The availability of high-quality aluminum epitaxial line at the wavelength of 0.15406 nm. The XRD pattern films opens the way to explore aluminum plasmonics for (Figure 1B) shows the aluminum film is single-crystalline real-world applications [8, 41, 42]. and grown along the (111) direction. The Al (111) and Here, we report on aluminum epitaxial films grown on c-sapphire (0006) diffraction