Article pubs.acs.org/JPCC
Optical Resonances in Short-Range Ordered Nanoholes in Ultrathin Aluminum/Aluminum Nitride Multilayers Yuichiro Ikenoya,† Masahiro Susa,† Ji Shi,† Yoshio Nakamura,† Andreas B. Dahlin,‡ and Takumi Sannomiya*,† †
Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, Tokyo, Japan Department of Applied Physics, Chalmers University of Technology, Göteborg, Sweden
‡
S Supporting Information *
ABSTRACT: Nanoholes with short-range ordering were fabricated in ultrathin aluminum/aluminum nitride multilayer films where each layer is as thin as a few nanometers. Optical resonances of the trilayer system with a single metallic layer and five-layer system with two metallic layers were successfully tuned in the visible−nearinfrared (vis/NIR) range. The resonance wavelength as well as the width can be predicted and designed by solving the dispersion relation and comparing with the lateral dimension of the short-range ordering. To solve the dispersion relation, we developed a general formulation for multilayer systems. The thermal stability of the fabricated nanoholes in ultrathin multilayers was also tested by vacuum annealing the samples up to 400 °C. While no structural change of the nanohole or the multilayer surface has been observed, the optical property showed almost no change in the resonance confirming no structural change but emergence of the interband transition around the wavelength of 900 nm. It means crystallinity improvement without grain growth by thermal annealing, which is consistent with the previous crystallographic studies on the same multilayer systems. The fabricated sensor revealed comparable refractive index sensitivity to the gold based sensors even with the top protective AlN layer. The chemical sensing test using the nanohole sensor with a bare aluminum top surface confirmed the applicability to the monitoring of aluminum surface reactions. difference in aqueous or humid condition.12−14 Another yet more generic difficulty common for any material is to maintain the interface smoothness of the very thin layer which is usually polycrystalline. Very thin polycrystalline films easily recrystallize even at room temperature or under humid conditions, and induce faceting associated with interface roughening. Rough interfaces are not favorable for the SP and guided waves to propagate nicely along the layer because such waves are scattered or damped by rough interfaces.15 All these interface problems become more serious when fabricating metal/ dielectric multilayers: adhesion layers, if necessary, should be introduced at each interface, and interface roughening would be accumulated as the layer number increases.16 In this study, we attempt to fabricate nanoholes in ultrathin metal/dielectric multilayers with its optical resonance adjusted in the visible−near-infrared (vis/NIR) range for sensing applications. We adopt the face-centered-cubic (fcc) metal/ AlN multilayer fabricated by sputtering deposition technique, recently established in our group.17−19 With this method it is possible to deposit metallic and dielectric films as thin as a few nanometers with atomically smooth interfaces. Such metal/AlN
1. INTRODUCTION Metallic nanohole arrays have been widely investigated due to their potential application to chemical- and biosensing, metamaterials, photovoltaic cells, as well as simply for scientific interests in their unique optical responses.1−4 The useful and also intriguing optical characteristic of the metallic nanohole array is the asymmetric resonance originating from the interhole coupling through supported bonding-mode surface plasmons (SPs).5−8 The film SP is sensitive to the topmost surface when the metallic film is relatively thin compared to the field decay inside the metallic film, which enables refractive index (RI) based bio- and chemical sensing. Thinner metallic films made of a material with a low imaginary part of the dielectric constant would produce coupled SPs from top and bottom interfaces, which improve the sensitivity for the bound SP mode. However, in most research the nanoholes have been fabricated in a metallic layer more than 10 nm thick.1,7,9 This is partially due to the poor adhesion and wettability of the commonly used noble metals, such as gold and silver, to dielectric substrates (mostly glass or oxide), which results in noncontinuous film when the layer is too thin.10,11 An adhesion material, such as titanium, nickel, or chromium, is normally introduced, which can be problematic considering the lossy optical property, recrystallization, alloying at high temperature, or corrosion and surface diffusion due to contact potential © 2013 American Chemical Society
Received: November 30, 2012 Revised: February 23, 2013 Published: February 27, 2013 6373
dx.doi.org/10.1021/jp401372k | J. Phys. Chem. C 2013, 117, 6373−6382
The Journal of Physical Chemistry C
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multilayer films have very high preferential crystalline orientation with the close-packed plane exposed to the interface already in the as-deposited condition, which further improves by thermal annealing without roughening the interfaces.18 We chose aluminum as the metallic layer as it is a low-cost and abundant material with moderately good plasmonic properties,20 and enables the metal/dielectric multilayer deposition from a single aluminum sputtering target, which drastically simplifies the deposition system.21 Since aluminum is ubiquitous with a huge variety of applications ranging from infrastructural materials, such as vehicles or energy storage, to nanometer-scale electronic devices, monitoring the surface state of this relatively reactive metal is of great interest.22 Plasmonic sensor devices using aluminum can offer cheap, disposable, and portable sensing opportunities.23−25 Inexpensive disposable sensor chips using abundant material are also beneficial in terms of hygiene and handling easiness without cleaning process. However, aluminum is known to exhibit its nanoplasmonic resonances in the UV wavelength region when the nanostructure is prepared in the same dimension as conventionally used gold or silver.26 So one task is to tune the nanohole resonance in the vis/NIR range without having the Rayleigh−Wood anomalies in air or substrate medium7,8 so that the optical measurement or future sensing application can be easily and effectively performed. The parameters we mainly optimize here are the layer thickness and the number of layers. We note that this ultrathin metal/AlN multilayer system can be readily applied for different noble metals.19,21 In this paper, we first introduce the general expression of the dispersion relation (DR), i.e., the function to describe the guided wavelength at a given vacuum wavelength, for a multilayer system. With this generic method, it is possible to solve all the waveguide modes for any number of layers in the same manner. The colloidal lithography method is applied to fabricate short-range ordered (SRO) nanoholes, which allows large-area and low-cost nanoscale fabrication.27 We fabricated trilayer systems where a single metallic aluminum layer of 5 or 10 nm is sandwiched by two 4.6 nm dielectric layers and fivelayer systems with two metallic (2.5 or 5 nm) and three inter dielectric layers (2.3 nm) as schematically shown in Figure.1.
No adhesion layer was introduced. In comparison with the calculated DR for each multilayer system, the optical resonance is analyzed both in terms of resonance wavelength position and broadening. We show that the resonances of such ultrathin aluminum multilayer systems can be designed to be in the vis/ NIR range and have acceptable peak width on the basis of DR. Such multilayer systems provide more engineering parameters for designing nanohole arrays, namely, layer numbers and the thickness of each layer other than nanohole size and single film thickness. We also tested the RI sensing performance of the fabricated sample as well as the chemical sensing of the surface reaction of aluminum by monitoring the resonance wavelength. The sensing results were analyzed using the multilayer dispersion relation calculation.
2. THEORY: GENERAL EXPRESSION OF DISPERSION RELATION FOR MULTILAYERS In previous works we have shown that the optical resonance of SRO nanohole arrays can be explained by the resonance of the guided propagating waves on the metal surface.8,28,29 Especially the transmission minimum directly correspond to the interference of the guided wave, where the characteristic hole−hole distance matches the propagation wavelength. Therefore, to understand the optical property of the SRO nanoholes, we should solve for the eigen propagation modes of the guided waves, or the so-called dispersion relation (DR). Because we will deal with a multilayer film, we solve DR for n films in general. Here we consider the transverse magnetic (TM) mode for the surface wave on the non-magnetic metallic material (see Supporting Information for TE mode). To describe the propagation mode in a multilayer without “leaking”, we consider two plane waves in each film and single evanescent waves for the top and bottom media with the common propagation number kx. By matching the boundary conditions of the magnetic field at all the plane interfaces, we obtain the following system of n+1 equations for n films with two media on the top and bottom: A top = A1upe−ik1d1 + A1down eik1d1 up
A1up + A1down = A 2upe−ik 2d2 + A 2down eik 2d2 ⋮ A mup + A mdown = A m + 1upe−ikm+1dm+1 + A m + 1down eikm+1dm+1 ⋮ A nup + A ndown = A bottomdown (1)
The coefficient A corresponds to the magnetic field for TM mode. For the boundary condition of the electric field we get another set of n+1 equations:
Figure 1. Schematic illustration of the nanohole in (a) trilayer and (b) five-layer consisting of metallic aluminum and dielectric aluminum nitride. 6374
dx.doi.org/10.1021/jp401372k | J. Phys. Chem. C 2013, 117, 6373−6382
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A top = up
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3. METHODS 3.1. Colloid Deposition. Commonly used colloidal lithography steps were applied to fabricate SRO nanoholes.27 A borosilicate glass substrate (1 × 1 cm2, Matsunami) was rinsed with water and ethanol, and plasma cleaned for 4 min (Harrick Plasma PDC32G, USA). Aluminum chlorohydrate solution with 5 wt % Al (Summit Research Laboratories. USA) was then deposited on the cleaned glass substrate as glue, followed by water rinsing and drying by nitrogen. Sulfatefunctionalized 100 nm polystyrene (PS) colloid particles (Sigma Aldrich, USA) were deposited and affixed by pouring 150 °C ethylene glycol solution. The substrate was finally water-rinsed and dried by nitrogen blow. 3.2. Multilayer Deposition. Multilayers of Al and AlN were sputter-deposited on the substrate with colloid particles, which were subsequently removed by an adhesive tape (tape lift-off). Multilayers always start with AlN film on the glass substrate to obtain good crystallinity of the subsequent metallic Al layers17−19 and capped by AlN on top to avoid possible oxidation except the sample for chemical sensing. We used a single aluminum target to deposit both metallic aluminum and dielectric aluminum nitride by controlling the reactive gas pressure in the sputtering chamber. The optimized conditions to realize smooth continuous ultrathin layers were Ar gas flow of 4 sccm and N2 2 sccm for AlN, and pure Ar gas with 6 sccm for metallic Al, with the total working pressure of 0.5 Pa. The film thickness was calculated from the deposition rate. It is noted that no adhesion interlayer was needed for this multilayer system on glass. Because wettability and adhesion of Al on AlN (or vice versa), as well as of AlN on glass, are high enough for smooth interfaces, no additional gluing layer is required. We name the multilayer samples with material abbreviation and layer thickness in nanometers, such as “N4.6/A10/N4.6” for the multilayer sample of AlN[4.6 nm]/Al[10 nm]/AlN[4.6 nm], where “N” and “A” stand for aluminum nitride and metallic aluminum, respectively, and the numbers correspond to the layer thickness. 3.3. Characterization. The structures of the fabricated samples were analyzed by atomic force microscopy (AFM, Shimadzu SPM-9600, Japan) in tapping mode and scanning electron microscopy (SEM, JEOL JSM-7000F, Japan) in secondary electron imaging mode. The optical properties were characterized by transmission spectra using a spectrophotometer (Hamamatsu C10083MD, Shimadzu UV3100PC, Japan) with halogen lamp illumination and fiber optics. Optical measurements were done in ambient atmosphere except sensing measurements. X-ray diffractometry (XRD, JEOL, Japan) was used to analyze the crystallinity. 3.4. Vacuum Anneal. The annealing for the stability test was performed at the pressure
