Effect of Ga Implantation and Hole Geometry on Light Transmission

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C: Plasmonics, Optical Materials, and Hard Matter

Effect of Ga Implantation and Hole Geometry on Light Transmission through Nanohole Arrays in Al and Mg Jieying Mao, Yunshan Wang, Kanagasundar Appusamy, Sivaraman Guruswamy, and Steve Blair J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02310 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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The Journal of Physical Chemistry

Effect of Ga Implantation and Hole Geometry on Light Transmission through Nanohole Arrays in Al and Mg Jieying Mao,† Yunshan Wang,‡ Kanagasundar Appusamy,¶ Sivaraman Guruswamy,¶ and Steve Blair∗,†,‡ †Department of Physics and Astronomy, University of Utah, Salt Lake City, Utah 84112, United States ‡Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah 84112, United States ¶Department of Metallurgical Engineering, University of Utah, Salt Lake City, Utah 84112, United States E-mail: [email protected] Abstract The study of plasmonic nanostructures in the ultraviolet (UV) is a relatively uncharted field due to challenges in both engineering and materials science. In this work, two-dimensional periodic nanohole arrays in aluminium (Al) and magnesium (Mg) films were fabricated using gallium (Ga) focused ion beam (FIB). Optical transmission through the arrays was obtained in the UV-visible range, for varying array periodicity. Transmission results show strong resonance transmission enhancement and suppression with corresponding red-shift as the period increases, and the presence of stationary waveguide modes. Comparing Al and Mg, Al hole arrays enable greater

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transmission than those of Mg. Numerical analysis was carried out through the finitedifference-time-domain (FDTD) method, which showed far-field transmission consistent with experiments. The simulation model was constructed based on transmission electron microscopy (TEM) of cross-sectioned samples to take into account tapered sidewall geometry and undercut into the substrate. Energy-dispersive X-ray spectroscopy (EDS) was used to assess elemental composition, including oxidation and alloying. The effect of Ga implantation was qualitatively studied, which indicates that Ga implants inside the hole, with greater implantation concentration within Mg than within Al, and affects the transmission through both Al and Mg arrays.

1

Introduction

In recent years, enthusiasm for UV plasmonics has grown because of applications in bioscience and material sciences. 1 For instance, many biomolecules contain intrinsic fluorophores such as phenylalanine and tryptophan that naturally fluoresce at UV wavelengths. 2,3 Enhancement of intrinsic emission of proteins and other biomolecules can be achieved through metal enhanced fluorescence phenomenon in the UV region. 4 Other applications such as UV spectroscopy and energy harvesting are also quite promising. 5 At this point, in spite of growing interests in UV plasmonics, significant challenges remain in this field due to scarcity of materials that support large plasmonic resonance at UV range. Therefore, new materials need to be carefully analyzed and selected to minimize loss in the UV region. 6–9 As a common choice for a UV plasmonic material, Al possesses a high plasma frequency and a large negative real permittivity component with a small positive imaginary component. Such properties enable strong Surface Plasmon Polariton (SPP) modes across the UV and visible region. 10–13 Other advantages, including low cost, high natural abundance, ease of processing and durability enabled by a stable surface oxide, also make it the most promising material in UV plasmonics applications so far. 14 Investigation of UV excited plasmons are rapidly evolving with different Al nanostructure like ellipsoids, 10 V-grooves, 6 triangular 2


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nanoparticle arrays, 15 nanodisk arrays, 14,16 nanowire arrays, 17 and nanohole arrays. 18,19 Confirmation of SPP excitation can be made via extraordinary optical transmission (EOT), 20 where dips and peaks correlate with excited SPP modes. Similarly, Mg is another suitable metal for active SPPs at short wavelengths, 21 and can be deposited in thin film form with refined grain source. 22 The more promising potential for Al and Mg UV SPP excitation and propagation compared with other metals such as gold and silver can be directly seen from calculation of figure of merit (FOM) in Equation (1): 23

F OMSPP = β 0 /β 00 ,

(1)

where β 0 and β 00 refer to the real and imaginary parts of the propagation constant for SPPs traveling along metal and dielectric planar interface. Higher FOM indicates lower-loss SPP propagation. Comparison of the FOM for UV materials proposed in this paper, along with the FOM for Au and Ag, is shown in Fig. 1. It is obvious that the plasmon tuning range of Al and Mg extend deeper into the UV region compared to Au and Ag. In this work, we fabricated Al and Mg nanohole arrays using FIB and measured their zeroth-order transmission spectra across the UV and visible spectral region. Numerical analysis was then carried out through FDTD simulations, using a simplified cylindrical hole model at first in order to identify the impact both SPP and waveguide mode coupling on the transmission spectrum. The simulation model was optimized afterwards based upon STEM and EDS analysis of the sample cross-section, which achieved good match with experimental transmission spectra. Influences from FIB lithography while preparing nanohole arrays, such as Ga implantation, were also evaluated.

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2 2.1

Methods Sample Structure and Fabrication

Al films of 100 nm thickness were sputtered (Denton Discovery 18) onto a 1”-diameter round UV fused silica coverslip (Esco Optics, Inc) of thickness 160 µm. Mg films of 100 nm thickness were deposited at deposition rates 3.7 Å/s with a 10 nm Al seed layer. 22 Optical permittivities of Al and Mg films were measured using a Woollam Variable Angle Spectroscopic Ellipsometry (VASE), the results of which can be found in Supporting Information. Periodic round nanohole arrays were then milled into Al and Mg films by FIB 24 under iodine gas injection, with uniform hole size and predetermined periods in both the x and y directions (see Fig. 2). Acceleration voltage for nanohole array FIB milling was 30 kV and beam current was 0.23 nA. The Mg samples were milled with a hole design radius of 50 nm, while the Al samples were milled with a hole design radius of 65 nm. As it will be discussed in the latter part of this paper, the actual hole size determined from STEM/EDS analysis and simulation results was larger than the designed radius in FIB milling. FIB resulted in a tapered hole instead of a perfectly straight hole profile with undercut into substrate. Moreover, Mg has consumed longer milling time to reach the designed depth 100 nm, which led to tapered hole with wider opening compared with that in Al.

2.2

Optical Transmission Measurement

Transmission measurements were performed for Al/Mg nanohole arrays with different periods. A high brightness fiber coupled broadband light source (LDLST M Laser-Driven Light Source Model EQ-99-FC) was used to produce light from mid-UV (MUV) wavelengths through visible. Light was linearly polarized using a Rochon prism, with the polarization aligned with a principal axis of the hole array (HA); an adjustable iris was used to select one of the polarized outputs. A biconvex lens was used to image the light from the fiber output onto the 30 µm×30 µm hole array pattern using the 4f imaging condition. In this case, the 4


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beam size at the pattern should be comparable to the beam size at the 100 µm diameter source fiber. After the pattern, transmitted light was collected by a lens-coupled optical fiber connected to either a Maya2000 Pro (200 to 400 nm) or AvaSpec-USB2-DT spectrometer (350 to 800 nm) to obtain transmission in the UV or visible region, respectively. The intensity spectrum measured without a sample was used as the reference signal. A schematic illustration is shown in Fig. 3(a). The spot size of the beam striking the array pattern was larger than the pattern size of 30 µm×30 µm, requiring a fill-fraction correction to the transmission data. Therefore, we performed a laser beam waist measurement using maximized light transmission through holes of sizes 50 µm, 75 µm, and 100 µm, and then determined a focal beam waist w0 ≈ 68 µm. The area correction factor for transmission measurements was thereby 30 µm×30 µm/(πw02 ) = 0.062.

2.3

FDTD Simulation Analysis

Numerical calculations were carried out through the finite-difference time-domain (FDTD) method using Lumerical FDTD Solutions. Mg and Al permittivities were measured through ellipsometry and were imported into Lumerical directly as the film optical data. As shown in Fig. 2(b), circular hole arrays were formed in the metal film which was placed on top of a silica substrate (Palik silica data). Air covered the film and nanohole interior. An incident plane wave along the −z direction with transverse magnetic (TM) polarization (magnetic field along the y axis) was used to illuminate the hole array as a Total-Field-Scattered-Field (TFSF) source. For simulation of a periodic hole array, the unit cell containing a single nanohole was used with periodic boundary conditions on the sides. Reflection symmetries were used to reduce calculation time. Perfectly Matched Layer (PML) boundaries were used in the ±z direction, with separations from the film interfaces larger than the single simulation wavelength in order to allow the field to decay towards the edge of the grid. Mesh size was selected as a compromise between accuracy and computational expense, and was set at 5



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24.3 nm. Simulation mesh type was set as “Auto non-uniform,” which can accurately resolve interfaces and require less memory and less computation time simultaneously. A second finer mesh with resolution of 2 nm encompassed the single hole structure for improved sampling of the field inside the nanohole. A z-normal frequency-domain field and power (DFT) monitor sheet was set below hole bottom and placed outside of the source boundary to avoid incident light collection. The far-field transmission spectrum was obtained through the power passing through the DFT monitor as a function of wavelength.

3 3.1

Results Optical Transmission Results

As was shown in Fig. 3(b) and Fig. 3(c), EOT was observed through periodic nanohole arrays in Al and Mg films, with up to 11% of incident light transmitted through nanohole arrays in Al and more than 7% through Mg. A parametric study of the effect of hole period on transmission was carried out via periods of 260 nm, 280 nm and 300 nm for Al samples and 240 nm, 260 nm, 280 nm and 300 nm for Mg samples. Coupling to SPP modes at the metal interfaces with air and silica can be estimated using the following Equation (2): 18,25

λSPPs = p

P i2 + j 2

r

d m , d + m

(2)

where P is hole array period, i, j refers to the coupling order indexed by i = 0, 1, ... and j = 0, 1, ..., and m and d are the material permittivities of the metal film and the adjacent dielectric medium. The first and second order coupling wavelengths for each hole pitch of Al and Mg HA patterns are indicated in Fig. 3(b) and Fig. 3(c). These SPP modes closely correlate with the antiresonance transmission dip followed by the resonance peak of the Fano-type resonance profile in the transmission spectra, 26 and redshift with increasing hole pitch as predicted by Equation (2).

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Comparing the experimental results for Al and Mg HA samples, some interesting points arise for discussion. As expected, the dips in transmission through Al nanohole arrays occur at similar spectral positions to those of Mg nanohole arrays with the same hole pitches, and Al HA’s achieve greater light transmission than Mg structure’s do. However, there are several differences between nanohole array transmission spectra between Al and Mg. The three prominent transmission peaks from each Al sample are comparable in magnitude, while the transmission peak in the visible part of the spectrum tended to be higher than the UV peaks for the Mg samples. Unlike Al HA transmission profile in the deep UV region, there is a stationary transmission peak at 300 nm for Mg HA transmission spectra, which masks shifting of transmission dips corresponding with 1st and 2nd order SPP modes along the Mgair interface. This stationary peak corresponds to a localized surface-plasmon (LSP) hole mode, which will be discussed in the next section. The increasing transmission tail towards the deep-UV region for Mg HA can be observed in Fig. 3(c), which is likely to be the next higher order of Fabry-Perot like modes within the plasmonic hole waveguide. 6 This will be discussed in the following section as well.

3.2

Distinct Mode Analysis with Simplified Simulation Model

A simplified cylindrical nanohole structure was first taken into consideration to simulate light transmission, as shown in Fig. 4(a). The uniform diameter of the cylindrical hole was set to be 120 nm. The thickness for both Al and Mg films was set to 120 nm, containing a 5 nm thick native oxide layer covering the top of planar film as well as the hole wall side, with a 4 nm interfacial layer Al2O3 in-between the metal film and substrate. Transmission through hole array with pitches corresponding to real sample was calculated and is plotted in Fig. 4(b) and Fig. 4(c). Single hole transmission with PML boundaries in ±x and ±y directions separated by 1000 nm was also calculated as comparison with the periodic simulations. The periodic structure modulates the single hole transmission spectra with Wood’s anomalies corresponding to the hole pitches. Such anomalies are also known as Fano-type reso7



nances, resulting from interference between excited surface-bound states (e.g. SPP’s) and scattered waves. The same hole array model in PEC (perfect electric conductor) film also displays distinct Wood’s anomalies based upon surface-directed diffracted waves. (See Supporting Information.) Due to the predicted dependence of SPPs on metallic permittivities, Al and Mg HA models are found to exhibit red-shifting and broaden of Wood’s anomalies compared to the PEC HA model. 26,27 As pointed out in the previous section, a stationary peak at short wavelength around 300 nm is present in the Mg HA spectra in Fig. 3(c), but not in the Al results. Similarly, Mg HA simulation also shows a short wavelength resonance at 250 nm that does not shift with hole pitch, while Al HA simulation doesn’t. This stationary resonance lines up with a simulated Mg single hole transmission peak in the mid-UV. Therefore, mode analysis is carried out through single hole simulations in order to specify the stationary mode in Mg HA/SH structures. Electric field intensity was mapped within the Mg single hole in Fig. 5(a) and Fig. 5(b) at transmission peaks of 255 nm and 410 nm respectively. More details on the field profiles and model comparisons are provided in Supporting Information using Al, Mg and PEC dielectric properties. A TE1,1 mode exists in the PEC circular waveguide with diameter D = 120 nm and its cut off frequency can be calculated by λc = πD/1.8412 ≈ 205 nm. As for the plasmonic circular waveguide, a similar TE1,1 like mode presents at the short wavelength resonance of 255 nm for Mg SH simulation. The 3D vector field mapping for both electric and magnetic fields is provided in Supporting Information, which matches with the TE1,1 mode profile. 28,29 The longer wavelength resonance at 410 nm for Mg SH, on the other hand, corresponds with a gapplasmon mode that is exclusive to the plasmonic waveguides. Such fields possess hyperbolic core field variation with central field nonzero. That differs from the field distribution at 255 nm having sinusoidal core-field variation with at least one zero-crossing. 30 In Fig. 4(b), the increasing tail towards deep UV for Al SH transmission spectrum indicates the existence of the TE1,1 like waveguide mode, which undergoes significant blue-shifting compared with

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that for Mg SH. In the periodic hole array configuration, surface-bound states are excited along the top metal-air or metal-substrate interface. According to the Poynting vector field in Fig. 5, only longer wavelength (410 nm for Mg) surface waves are guided into the hole to couple with gap-plasmon mode, with Fano resonance signature. Whereas at the mid-UV (255 nm for Mg), the excited surface waves fail to couple with the waveguide mode inside the hole, so that there appears a stationary peak independent of hole pitch in the hole array transmission spectra. Al HA also has a blue-shifted stationary peak at deep UV indicating its waveguide mode, which is beyond our simulation and measurement limits.

3.3

Sample Dimensional and Elemental Analysis

Despite similar resonant features between the experimental and simulation results, there are qualitative and quantitative differences between the two. All of the spectral peaks and dips are blue-shifted in the simplified model simulation and the magnitude variance between UV and visible peaks is different from the measured EOT. The mismatch between simulations and experiments relies largely on the deviations from ideality produced from the FIB lithography method. Therefore, further effort is made to optimize the simulation model so as to match up with measurements and thereby reveal the impact of FIB lithography on the hole array model in terms of transmission spectra. In order to develop a more accurate model for simulation, we performed FIB sectioning and lift-out of thin (∼100 nm) sample cross-sections for high-resolution imaging. Scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) methods were used to analyze the cross-sections. High angle annular dark-field (HAADF) S/TEM images (Fig. 6(a)) are dependent on atomic z-contrast, and are therefore entirely dependent on relative scattering efficiencies in regions of constant thickness. EDS component mapping of characteristic K-line x-ray transitions of certain dominant atomic elements in the Al hole array was obtained and displayed in Fig. 6(b), Fig. 6(c), Fig. 6(e) and Fig. 6(f). 9



It is not surprising that a considerable amount of Ga has been implanted into the hole bottom of the holes and at the film surfaces since a Ga ion beam was used in both FIB milling of the hole array and to create the lift-out cross-section. 24 According to the unevenly distributed Ga across the Al hole array cross-section, most of the Ga is expected to have been implanted during FIB milling rather than during cross-sectioning, which would produce a relatively evenly distributed Ga dose. EDS mapping of the dominant compounds in the hole array cross-section has also been extracted in Fig. 6(d), which helps create a simulation model with realistic material composition. In addition, we plot elemental line scans in Fig. 6(g) and Fig. 6(h), showing net detector counts for each component along a vertical path across the interstitial film layer and along the hole axis. Combining relative counts with pixel measurement from Fig. 6(a), we can determine reasonable dimensions for each distinct material region in the simulation model design. However, since EDS imaging of atomic concentrations are affected by sample thickness and density, composition information remains qualitative. Nevertheless, for our purposes, a reasonable starting approximation is obtained. A similar analysis was performed for the Mg hole array. HAADF S/TEM images are shown in Fig. 7(a). Fig. 7(d) gives EDS mapping of the dominant compounds, while Fig. 7(b), Fig. 7(c), Fig. 7(e) and Fig. 7(f) show element mapping of K-line x-ray transitions of Mg, Ga, O and Al. Compared with Ga implantation in the Al sample, more Ga was implanted into the Mg film, rather than into the hole bottom, as in the Al sample. However, there was much greater concentration of Ga in the Mg sample than in the Al sample, as estimated by the ratio of Ga maximum counts. The Ga implantation profile is also not the same for Mg as for the Al film. Existence of the Al seed layer can be clearly seen from the EDS imaging. A horizontal line-scan across the hole in the Mg film and vertical line-scan along the hole axis are shown in Fig. 7(g) and Fig. 7(h), respectively, which can be used to determine hole cross section parameters when considered with S/TEM image measurement. From the sample analyses in Fig. 6 and Fig. 7, detailed simulation models of Al and Mg

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samples were generated, as shown in Fig. 8(a) and Fig. 8(d), respectively. A tapered hole shape is used rather than a cylindrical hole, and the hole extends through the metal film into the substrate, taking undercut into consideration. A native oxide layer covers the surfaces of both metals, with a thicker oxide layer for Mg than for Al. During FIB fabrication process, Ga was implanted into the film and the bottom of the holes. Al and Mg alloys with Ga need to be considered within the Ga implanted regions, and there is an Al-Mg alloy in the Mg interfacial oxide layer.

4

Discussion

4.1

FDTD Simulation Model Reconstruction and Optimization

Since the EDS results in the previous section are not quantitative, additional optimizations are performed on the model setup in FDTD simulations. As EOT is partly the result of surface phenomena, the details of the two metal-dielectric interfaces can influence the magnitudes and positions of the transmission peaks and dips. We modified the simplified interface model with more accurate oxide layer thicknesses and with Ga implantation percentage and thickness. The dielectric properties of the alloys were calculated using the Bruggeman model (which was used in 14 for partially oxidized Al) based on two constituent materials

f1

2 − 1 − + f2 =0 1 + 2 2 + 2

(3)

where 1 , 2 refers to two materials comprising the composite, and f1 and f2 are the volume fraction of these two materials respectively. The dielectric function for the composite material is determined by the implicit variable . The hole size and shape also plays a key role in EOT, and we consider the tapered nature of the hole in terms of the top and bottom diameters as well as the hole depth. A paraboloid profile was used in the undercut region with a depth of 55 nm in Mg and 54 nm in Al that

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did not change with pitch, along with Ga implantation beneath the undercut. A parametric study of these major factors was carried out to achieve an optimized model from which simulated far-field transmission matches well with measured transmission. Referring to the Al hole array model in Fig. 8(a), 5 nm of native oxide was used both at the film top surface and along the hole sides. An alloy of 5% Ga and 95% Al was considered within the bulk of the film enclosed by the Al2O3 layer to represent Ga implantation into the bulk. The Ga concentration into the bulk of the Al film depends upon array pitch, with smaller pitch resulting in higher Ga concentration (assuming the hole size, and therefore the Ga dose, are the same). To simplify the parametric study for simulation optimization, rather than changing the Ga concentration with pitch (which requires continually solving for a new ), the Ga-Al alloy volume was allowed to vary, as represented by the yellow region in Fig.8(a), with dimensions Htop and Hside . According to EDS analysis, a 4 nm thick layer of 5% Ga-Al alloy existed between the film and substrate rather than Al oxide as expected, likely the result of surface diffusion of Ga. A 10% Ga and 90% SiO2 composite was used along the hole bottom region, with hside and hbottom referring to the thickness along the substrate surface and depth at the hole bottom, respectively. These dimensions changed with array pitch as well. As for the Mg model shown in Fig. 8(d), 8 nm thickness of MgO was used on top of the film surface with 16 nm thickness along the hole side. An alloy of 20% Ga and 80% Mg was considered within the bulk of the film enclosed by the MgO layer to represent Ga implantation into the bulk. Ga concentration in Mg is much higher and more evenly distributed than in Al. This can be verified through comparison of their phase diagram. Al-Ga phase diagram shows the maximum solubility of Ga in Al is about 9% at room temperature and any Ga present above this concentration level will be present as a pure Ga second phase. 31 Ga in Mg, on the other hand, forms Mg5Ga2 intermetallic in equilibrium with Mg terminal solid solution phase when Ga concentration reaches 28.57%. 32 There is a strong interaction of Ga with Mg in contrast to the behavior observed in the case of Ga and Al.

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Again, the Ga-Mg alloy volume was allowed to vary, as represented by the yellow region in Fig.8(d), with dimensions Htop and Hside . Similarly, a 10% Ga and 90% SiO2 composite was set along the hole bottom. The seed layer was divided into three parts with different Al-Mg alloy compositions. Starting from the substrate interface upwards, we used a 3 nm 90% Al-Mg alloy, beneath a 9 nm pure Al layer, followed by a 16 nm 40% Al-Mg alloy with Ga implanted at the ends. The seed layer of Al actually helped increase corrosion resistance of Mg film in the quartz interface, 33 which can be easily seen from EDS compounds mapping in Fig.7(d), so that interfacial oxide was not considered in this case. Detailed parameter settings relevant to separate hole pitches for Al and Mg HA are provided in Supporting Information. Simulation results with the optimized models were scaled in amplitude to match the experimental results, and show very good agreement, as indicated in Fig. 8(b) and Fig. 8(e) for Al 260 nm and Mg 240 nm pitch, respectively. Results shown in Fig. 8(c) and Fig. 8(f) for the different array pitches show the same trends as the experimental results showed in Fig. 3(b) and Fig. 3(c).

4.2

Influence of Gallium Implantation for EOT

From EDS analysis of Mg and Al hole array samples, Ga was implanted into the metal film as well as within the substrate at the hole bottom. This is a side-effect of the Ga FIB milling process. In order to quantify the impact of Ga implantation, we performed further simulations with/without Ga implantation, with results shown in Fig. 9. For both Al and Mg arrays, Ga implantation primarily results in a decrease in the overall transmission magnitude, with a red shift of transmission features in the visible spectrum; nevertheless, the transmission features for Al arrays don’t exhibit a significant qualitative change with Ga implantation. However, in addition to a greater drop in transmission due to higher Ga implantation concentration in Mg arrays, there are qualitative differences as seen by the number of transmission peaks/dips in the mid-UV region, which is especially evident 13



for 280 nm and 300 nm pitches. These results suggest that alternative fabrication methods should be explored for patterning Mg films, such as lift-off, if the material response of Mg is desired without the influence of Ga. The redshifts of visible dip/peak for both Al and Mg sample are relatively easy to understand, as similar amount of Ga was expected to implant into the substrate at the hole bottom and red-shifted 1st order SPPs at the metal-quartz interface. Ga implantation in the hole bottom decreases the total transmission in general. As for the dip/peak in the mid-UV region, which is the result of a superposition of SPPs at both metal-air and metal-quartz interfaces, a blue shift of these SPPs is introduced through Ga implantation in the metal. Shifting of SPPs along the metal-air interface is smaller than the shift of SPPs along the metal-quartz interface. The large resonance space between the 1st order SPP along Mg-air and the 2nd order SPP along Mg-quartz interface for the 300 nm pitch Mg hole array marked in Fig. 3(c) should exhibit a small peak, which is shown in the pure Mg hole array simulation in Fig. 9(g). Using 20% Ga-Mg alloy, however, the spectral separation between these two modes closes, causing the peak in between to disappear. This effect did not make an apparent difference to the Al resonances since a small amount of Ga (about 5% as mentioned in 4.1) was implanted into Al and most of the implanted Ga was accumulated along the hole side, see Fig. 8(a). With the larger hole array period in Mg, Ga implantation depth into the hole side is greatly increased, which is shown in the parametric settings provided in Supporting Information as Hside almost doubled from 240 nm to 300 nm hole pitch. These factors contribute to the greater changes in the transmission spectra of 300 nm hole array between Mg arrays with/without Ga implantation as compared to Al. Using the refined model, we also performed simulations of intensity enhancement within the nanoholes for Al and Mg arrays, comparing the cases with and without Ga implantation (see Supporting Information for complete results). The highest enhancement for all samples occurs near the upper edge of the hole and for excitation wavelengths near 400 nm (Figures S13-S19). For both Al and Mg, the 300 nm hole pitch produces the greatest enhancement

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factors of 10.6 and 9.3, respectively. Interestingly, enhancement in the Al arrays near 400 nm excitation is higher with Ga implantation than without, whereas for Mg, the effect is opposite. At other resonant enhancement wavelengths, enhancement is greater without Ga for both Al and Mg arrays.

5

Conclusions

In conclusion, optical transmission through two-dimensional periodic hole arrays in Al and Mg has been investigated in the UV and visible range. Transmission results showed strong resonance enhancement and suppression in the UV and visible region, with corresponding red-shift as the period increases. Comparing Al and Mg, the Al hole-array enabled greater transmission. Dips in transmission through Al arrays occur at similar spectral positions to those of Mg arrays with same periods. A waveguide mode that interacts with SPP resonance in the mid-UV in the Mg hole-array was observed in the experiments and analyzed in the simulations. Simulation models took into account the hole geometry and its material compositions based on TEM and EDS of the cross-sectioned samples. Metal oxidation, Ga implantation and hole dimensions were found to have significant influence on EOT. Ga implantation from FIB fabrication was qualitatively analyzed, which indicated Ga implants inside the hole bottom as well as higher implantation concentration into the bulk of Mg than within Al. The effects of implanted Ga on EOT was specifically simulated to show suppression of resonances features in Mg hole arrays in the mid-UV, along with reduction of overall transmission and red-shifting of resonance features in both Mg and Al arrays.

Supporting Information Available Supporting Information. One PDF file containing material dielectric functions, FDTD simulation spectra and vector field plots. AVI files that show field distribution/evolution within cylindrical SH/HA for PEC/Al/Mg, and fields within final optimized truncated tapered hole 15



models for PEC/Al/Mg SH/HA. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement All authors received funding from NSF MRSEC grant DMR-1121252 to support this work. This work was performed in part at the Utah Nanofab sponsored by the College of Engineering, Office of the Vice President for Research, and the Utah Science Technology and Research (USTAR) initiative of the State of Utah. We thank the Surface Analysis and Nano-Scale Imaging Lab at Utah Nanofab to the access of the ellipsometry, FIB and STEM and help from the supporting staff - Brian R. Van Devener, Paulo Perez and Randy C. Polson. The support and resources from the Center for High Performance Computing at the University of Utah are gratefully acknowledged.

References (1) Barrios, C. A.; Canalejas-Tejero, V.; Herranz, S.; Urraca, J.; Moreno-Bondi, M. C.; Avella-Oliver, M.; Maquieira, Á.; Puchades, R. Aluminum nanoholes for optical biosensing. Biosensors 2015, 5, 417. (2) Chowdhury, M. H.; Chakraborty, S.; Lakowicz, J. R.; Ray, K. Feasibility of using bimetallic plasmonic nanostructures to enhance the intrinsic emission of biomolecules. The Journal of Physical Chemistry C 2011, 115, 16879–16891, PMID: 21984954. (3) Jiao, X.; Wang, Y.; Blair, S. UV fluorescence enhancement by Al and Mg nanoapertures. Journal of Physics D: Applied Physics 2015, 48, 184007. (4) Ray, K.; Szmacinski, H.; Lakowicz, J. R. Enhanced fluorescence of proteins and labelfree bioassays using aluminum nanostructures. Analytical Chemistry 2009, 81, 6049– 6054, PMID: 19594133. 16


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Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Maier, S. A.; Atwater, H. A. Plasmonics: Localization and guiding of electromagnetic energy in metal/dielectric structures. Journal of Applied Physics 2005, 98 . (6) Mao, J.; Blair, S. Nanofocusing of UV light in aluminum V-grooves. Journal of Physics D: Applied Physics 2015, 48, 184008. (7) Jiao, X.; Peterson, E. M.; Harris, J. M.; Blair, S. UV fluorescence lifetime modification by aluminum nanoapertures. ACS Photonics 2014, 1, 1270–1277. (8) McMahon, J. M.; Schatz, G. C.; Gray, S. K. Plasmonics in the ultraviolet with the poor metals Al, Ga, In, Sn, Ti, Pb, and Bi. Phys. Chem. Chem. Phys. 2013, 15, 5415–5423. (9) Naik, G. V.; Shalaev, V. M.; Boltasseva, A. Alternative plasmonic materials: Beyond gold and silver. Advanced Materials 2013, 25, 3264–3294. (10) Zeman, E. J.; Schatz, G. C. An accurate electromagnetic theory study of surface enhancement factors for silver, gold, copper, lithium, sodium, aluminum, gallium, indium, zinc, and cadmium. The Journal of Physical Chemistry 1987, 91, 634–643. (11) Dörfer, T.; Schmitt, M.; Popp, J. Deep-UV surface-enhanced Raman scattering. Journal of Raman Spectroscopy 2007, 38, 1379–1382. (12) Maidecchi, G.; Gonella, G.; Zaccaria, R. P.; Moroni, R.; Anghinolfi, L.; Giglia, A.; Nannarone, S.; Mattera, L.; Dai, H.-L.; Canepa, M.; et al, Deep ultraviolet plasmon resonance in aluminum nanoparticle arrays. ACS Nano 2013, 7, 5834–5841, PMID: 23725571. (13) Martin, J.; Proust, J.; Gérard, D.; Plain, J. Localized surface plasmon resonances in the ultraviolet from large scale nanostructured aluminum films. Opt. Mater. Express 2013, 3, 954–959. (14) Knight, M. W.; King, N. S.; Liu, L.; Everitt, H. O.; Nordlander, P.; Halas, N. J. Aluminum for plasmonics. ACS Nano 2014, 8, 834–840, PMID: 24274662. 17



(15) Chan, G. H.; Zhao, J.; Schatz, G. C.; Duyne, R. P. V. Localized surface plasmon resonance spectroscopy of triangular aluminum nanoparticles. The Journal of Physical Chemistry C 2008, 112, 13958–13963. (16) Langhammer, C.; Schwind, M.; Kasemo, B.; Zorić, I. Localized surface plasmon resonances in aluminum nanodisks. Nano Letters 2008, 8, 1461–1471, PMID: 18393471. (17) Christ, A.; Ekinci, Y.; Solak, H. H.; Gippius, N. A.; Tikhodeev, S. G.; Martin, O. J. F. Controlling the Fano interference in a plasmonic lattice. Phys. Rev. B 2007, 76, 201405. (18) Ebbesen, T. W.; Lezec, H. J.; Ghaemi, H. F.; Thio, T.; Wolff, P. A. Extraordinary optical transmission through sub-wavelength hole arrays. Nature 1998, 391, 667–669. (19) Braun, J.; Gompf, B.; Weiss, T.; Giessen, H.; Dressel, M.; Hübner, U. Optical transmission through subwavelength hole arrays in ultrathin metal films. Phys. Rev. B 2011, 84, 155419. (20) Gan, Q.; Zhou, L.; Dierolf, V.; Bartoli, F. J. UV plasmonic structures: Direct observations of UV extraordinary optical transmission and localized field enhancement through nanoslits. IEEE Photonics Journal 2009, 1, 245–253. (21) Sterl, F.; Strohfeldt, N.; Walter, R.; Griessen, R.; Tittl, A.; Giessen, H. Magnesium as novel material for active plasmonics in the visible wavelength range. Nano Letters 2015, 15, 7949–7955, PMID: 26312401. (22) Appusamy, K.; Jiao, X.; Blair, S.; Nahata, A.; Guruswamy, S. Mg thin films with Al seed layers for UV plasmonics. Journal of Physics D: Applied Physics 2015, 48, 184009. (23) West, P.; Ishii, S.; Naik, G.; Emani, N.; Shalaev, V.; Boltasseva, A. Searching for better plasmonic materials. Laser and Photonics Reviews 2010, 4, 795–808.

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Page 18 of 29

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(24) Langford, R. M.; Petford-Long, A. K. Preparation of transmission electron microscopy cross-section specimens using focused ion beam milling. Journal of Vacuum Science and Technology A 2001, 19, 2186–2193. (25) Ghaemi, H. F.; Thio, T.; Grupp, D. E.; Ebbesen, T. W.; Lezec, H. J. Surface plasmons enhance optical transmission through subwavelength holes. Phys. Rev. B 1998, 58, 6779–6782. (26) Miroshnichenko, A. E.; Flach, S.; Kivshar, Y. S. Fano resonances in nanoscale structures. Rev. Mod. Phys. 2010, 82, 2257–2298. (27) Lutolf, F.; Martin, O. J. F.; Gallinet, B. Fano-resonant aluminum and gold nanostructures created with a tunable, up-scalable process. Nanoscale 2015, 7, 18179–18187. (28) Shin, H.; Catrysse, P. B.; Fan, S. Effect of the plasmonic dispersion relation on the transmission properties of subwavelength cylindrical holes. Phys. Rev. B 2005, 72, 085436. (29) Webb, K. J.; Li, J. Analysis of transmission through small apertures in conducting films. Phys. Rev. B 2006, 73, 033401. (30) Kekatpure, R. D.; Hryciw, A. C.; Barnard, E. S.; Brongersma, M. L. Solving dielectric and plasmonic waveguide dispersion relations on a pocket calculator. Opt. Express 2009, 17, 24112–24129. (31) Murray, J. L. The Al-Ga (Aluminum-Gallium) system. Bulletin of Alloy Phase Diagrams 1984, 5, 21–21. (32) Nayeb-Hashemi, A.; Clark, J. Phase diagrams of binary magnesium alloys. Binary Alloy Phase Diagrams (American Society for Metals) 1986, 2, 1138–1144. (33) Chiu, L.-H.; Chen, C.-C.; Yang, C.-F. Improvement of corrosion properties in an

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aluminum-sprayed AZ31 magnesium alloy by a post-hot pressing and anodizing treatment. Surface and Coatings Technology 2005, 191, 181–187.

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Figures

Figure 1: Surface plasmon polariton (SPP) figure of merit (FOM) versus wavelength, calculated for a single metal-air interface. Solid red line and dash-dot green line indicate FOM of Al and Mg, while dotted blue line and dashed black line represent FOM of the more common metals Au and Ag. Optical permittivities of UV materials Al and Mg are measured and provided in Supporting Information, while Au and Ag are Palik Handbook materials.

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(a)

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(b)

Figure 2: Nanohole array in Al/Mg film. (a) SEM image of 300 nm pitch nanohole array in Mg with ∼ 120 nm hole diameter, while (b) shows partial schematic of the 30 µm×30 µm nanohole array pattern that was milled into ∼ 100 nm thick Al/Mg film deposited on top of silica. A 100 nm scale bar is labeled in (a). Al and Mg samples with different periods were prepared for transmission measurements.

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(a) Measurement setup

(b) Al nanohole array transmission spectrum

(c) Mg nanohole array transmission spectrum

Figure 3: Illustration of the measurement setup (a) and transmission measurements of (b) Al and (c) Mg nanohole arrays. Al patterns had hole periods of 260 nm, 280 nm, and 300 nm and Mg patterns had periods of 240 nm, 260 nm, 280 nm, and 300 nm. The 1st and 2nd order of SPP modes along metal-air interface are marked with “•” and “”, while these along the metal-quartz interface are marked with “◦” and “♦” for each hole pitch pattern. Fano-type resonance profiles with transmission dips followed by peaks were observed in both Al and Mg HA transmission spectra, which red-shifted with increasing hole pitch. For Mg HA transmission, a stationary transmission peak around 300 nm was found that doesn’t change with hole pitch.

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(a) Simplified model setup

simulation

(b) AlHA transmission simulation

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(c) MgHA transmission simulation

Figure 4: FDTD simulation with simplified hole array model. (a) Simulation model crosssection of cylindrical hole cut in Al or Mg film. Hole diameter is 120 nm and film thickness is 120 nm, including a 5 nm thick oxide layer on the top surface and a 4 nm interfacial oxide layer between the film and substrate. Far-field transmission results from the simplified model for Al (b) and Mg (c) hole arrays with different hole pitch. Single hole simulation results (green line) were also plotted and scaled for comparison.

(a) Mg SH |E|2 at 255 nm

(b) Mg SH |E|2 at 410 nm

Figure 5: Cross-section electric field intensity |E|2 (color map) and Poynting vector S (arrow plot) distributions through Mg single hole model at transmission peaks (a) 255 nm and (b) 410 nm. Simulation geometry was the same as Mg periodic model with 300 nm pitch. Color scale for |E|2 is in logarithm scale. Arrow length is proportional to the Poynting vector magnitude on a linear scale. The cross-section plane x-z is also the plane of incidence. TM incident light was considered with linear polarization in x.

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(a) STEM DF - Al array

(d) EDS composite

(b) EDS mapping: Al

(c) EDS mapping: Ga

(e) EDS mapping: O

(f) EDS mapping: Si

(g) Vertical linescan through film

(h) Vertical linescan through hole

Figure 6: (a) HAADF S/TEM image of 100 nm thick, 120 nm diameter designed Al hole array, which is sensitive to z-contrast differences. The higher the element density, the darker it appears. EDS mapping was performed to determine distribution of certain elements within the cross-section: (b) Al, (c) Ga, (e) O, and (f) Si. (d) Composite element mapping image. In these images, the color intensity corresponds to the element net detector count. Net counts along vertical lines-scan from the substrate into the Pt overcoat layer, going across the unpatterned film (g) and across the hole bottom (h).

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(a) STEM DF - Mg array

(d) EDS composite

(b) EDS mapping: Mg

(e) EDS mapping: O

(g) Horizontal linescan through film

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(c) EDS mapping: Ga

(f) EDS mapping: Al

(h) Vertical linescan through hole

Figure 7: (a) HAADF S/TEM image of 100 nm thick, 120 nm diameter designed Mg hole array. EDS mapping was performed to determine distribution of certain elements within the cross-section: (b) Mg, (c) Ga, (e) O, and (f) Al. (d) Composite element mapping image. Net counts along vertical lines-scan from the substrate into the Pt overcoat layer, going across the unpatterned film (g) and across the hole bottom (h).

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(a) Al model

(b) Exp vs. FDTD: Al

(c) Optimized simulation: Al

(d) Mg model

(e) Exp vs. FDTD: Mg

(f) Optimized simulation: Mg

Figure 8: Simulation optimization results for Al and Mg samples. Optimized model setup is shown in (a) for Al and (d) for Mg. Alloy (Al-Ga) refers to 5% Ga and 95% Al composite and Alloy (Mg-Ga) assumes 20% Ga implanted in Mg. Ga-SiO2 in both samples considered consists of 10% Ga and 90% SiO2 composite. The Al seed layer for Mg films was divided into three different composite layers, one of which was 40% Al-Mg alloy sandwiched by pure Mg layer on top and pure Al seed layer below. The last layer beneath pure Al seed layer was 90% Al-Mg alloy. Simulation using simplified and optimized hole model are compared with experimental EOT results for Al HA of 260 nm pitch and Mg HA of 240 nm pitch in (b) and (e), respectively. All spectra were scaled in magnitude to identify discrepancies, where good agreement between simulation and experiments has been achieved. (c) and (f) display final simulation results for all hole array periods, which match experimental spectra in Fig. 3(b) and Fig. 3(c) well.

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(a) Al P260

(d) Mg P240

(b) Al P280

(e) Mg P260

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(c) Al P300

(f) Mg P280

(g) Mg P300

Figure 9: Impact of Ga implantation on EOT spectra of hole arrays in Al and Mg. Comparison of transmission between experimental data and the simulation model with/without Ga implantation in Al arrays with (a) 260 nm hole pitch, (b) 280 nm hole pitch, and (c) 300 nm hole pitch; transmission for Mg arrays with (d) 240 nm hole, (e) 260 nm hole pitch, (f) 280 nm hole pitch, and (g) for 300 nm hole pitch. Solid lines refer to measured transmission spectra from experiments and dashed lines refer to simulated spectra using previous simulation models with Ga implantation, while dotted lines indicate simulation results without considering Ga. For simulation spectrum with Ga, 1st order SPP modes along the metal-air interface are marked with “” and 2nd along the metal-quartz interface are marked with “♦” for the Mg hole array with 300 nm pitch. For simulation spectrum without Ga, 1st order SPP modes along the metal-air interface are marked with “•” and 2nd along the metal-quartz interface are marked with “◦” for Mg hole array 300 nm pitch.

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TOC Graphic

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Effect of Ga Implantation and Hole Geometry on Light Transmission

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