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Abstract
Localized surface plasmon resonance (LSPR) with Ga nanoparticles (NPs) was achieved and tuned over the entire deep-ultraviolet (DUV) wavelength range. Ga NPs with nano hemisphere structures were fabricated by combining vapor deposition and thermal annealing without top-down nanofabrication technology. We successfully fabricated Ga2O3 NPs by thermally annealing Ga NPs at high temperatures. The coating of Ga NPs with Al2O3 thin films prevented oxidation and improved the robustness of Ga NPs, which have a low melting point and are unstable at room temperature, enabling device applications. Furthermore, we fabricated a new NP structure with Ga or Ga2O3 located on Al mirror substrates, which can be applied to LSPR-enhanced light-emitting materials and devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Light-emitting diodes (LEDs) in the deep-ultraviolet (DUV) wavelength region have a wide range of applications, including sterilization, processing technology, and optical memory. DUV-LEDs have great advantages in terms of saving energy, whereas mercury lamps, which are the main source of deep-ultraviolet light, have a high environmental impact. DUV-LEDs are expected to serve as an alternative light source; however, the associated external quantum efficiencies are still low, and hence, DUV-LEDs are far from practical applications [1–3]. One promising method for solving this problem is the use of surface plasmon resonance (SPR). SPR has recently attracted considerable attention owing to its wide range of applications, including sensing [4–8], imaging [9], and light-emitting devices [10–12]. Among the excellent effects of SPR, we have focused on improving the emission efficiency of LEDs [13]. In 2010, for the first time, we succeeded in enhancing deep UV emissions from AlGaN/AlN quantum wells by using propagating SPR with Al thin films [14,15]. However, in propagating SPR, the resonance wavelength is fixed by the metallic material; consequently, the emission enhancement effect is rapidly weakened when the resonance wavelength is far from the emission wavelength [13]. However, in localized surface plasmon resonance (LSPR) generated by metal nanostructures, the resonance wavelength can be tuned by changing the size of the nanostructure. Therefore, we have been working on controlling LSPR to extend our proposed method of light emission enhancement to all UV wavelengths. In most previous studies, aluminum nanostructures that produce LSPR in the UV region have been fabricated using top-down technology [16–19]. However, this top-down nanofabrication process is costly and time consuming, making it unsuitable for engineering applications. In our previous studies, we fabricated nanoparticles (NPs) of the UV plasmonic metals Al and Ga by annealing thin films using a simple method. As a result, we found that Ga nanoparticles can be fabricated with a better shape and higher density than Al nanoparticles because of the difference in surface wettability [20]. In this study, we further investigated the properties of Ga NPs using calculations and experiments. We succeeded in experimentally producing intense deep-ultraviolet LSPR and tuning its wavelength. Moreover, we overcame the weak point of Ga, which is that it is easily oxidized by heat processes, by coating it with Al2O3 protective films. In a previous study, we confirmed the peak splitting of LSPR by fabricating a nano hemisphere on a mirror (NHoM) structure [20–22]. Based on these results, in this study, we fabricated NHoM structures using Ga NPs, successfully split the resonance peak, and experimentally extended the range of LSPR to the UVC region. Finally, we designed and fabricated a new structure combining gallium oxide nanoparticles and an Al mirror and succeeded in producing a sharp LSPR in the UVC region.
2. Methods
FDTD simulations were performed using a commercially available software (Poynting for Optics, Fujitsu, Japan). Periodic and absorption boundary conditions were applied in the X and Y directions and in the Z direction, respectively. Pulsed light consisting of a differential Gaussian function with a pulse width of 0.5 fs and an electric field of 1 V/m was irradiated under an X-polarized source. The peak position of the excitation pulse spectrum was approximately 600 THz (wavelength: 500 nm). The dielectric functions of Al and Ga were optimized by the Drude equation using the values reported values [23,24], and the refractive index (n) of Al2O3 was set to 1.7 without dispersion. A nonuniform mesh of 0.5–5 nm grid size was used for the simulations. Ga nanoparticles were deposited by resistive evaporation (Sanyu Electron, SVC-700TM) and then heated in an electric furnace (Yamato Scientific, FO100) at 300 °C for 10 min in a nitrogen atmosphere. The chamber size was 100×150×105 mm and the N2 gas flow rate was 1-2 ml/min under atmospheric pressure. The Al2O3 protective film was deposited at a thickness of 5 nm by atomic layer deposition (Suga Co., Ltd., SAL1000). In the NHoM structure, 50 nm of Al was deposited as a mirror layer via resistively heated evaporation. In addition, as the spacer layer, SiO2 was used as a high-vacuum RF sputter coater (Sanyu Electron, SVC-700RF). The Ga2O3 nanostructure was formed by the resistive evaporation of metallic Ga and heating at 500 °C for 20 min in air in an electric furnace. The surface morphologies of the samples were observed using atomic force microscopy (AFM) (Bruker, JPK Instruments AG, NanoWizard). The transmission and reflection spectra were measured using a UV-visible spectrophotometer at an incidence angle of 5° (Shimadzu, UV-1800) and converted to extinction spectra.
3. Results and discussion
Figure 1 shows a schematic of the hemisphere on a sapphire (NHoS) substrate structure of Ga (Fig. 1(a)), the extinction spectrum (Fig. 1(b)), and the electric field distribution at the peak wavelength (Fig. 1(c)) obtained by the FDTD simulations. Figure 1(b) shows that the extinction spectra include two peaks in all the spectra, large long-wavelength peaks, and weak short-wavelength peaks. As the diameter of the hemisphere increased, the peak wavelength red-shifted, and the extinction became stronger. Figure 1(c) shows the electric field distribution around the NHoS structure for the long- and short-wavelength peaks, which shows that they are caused by dipole and quadrupole oscillation modes, respectively.
Fig. 1. (a) Schematic of the NHoS structure with Ga. (b) Extinction spectra of NHoS structures for nano hemisphere diameters of 30, 60, and 80 nm calculated by the FDTD method. (c) Spatial distribution of the electric field at the 282 nm and 139 nm peaks around the NHoS structure
Figure 2(a) shows the extinction spectra of the NHoS structure fabricated before and after annealing. The LSPR peak was blue-shifted and sharpened after annealing compared with that before annealing. Figure 2(b) shows the surface morphology measured via AFM, and Fig. 2(c) shows the particle size distribution of the NHoS structure obtained from the analysis of the results shown in Fig. 2(b). Most of the film layers became fine particles after annealing. Figure 2(c) shows that the particle size distribution has two peaks: one is the peak of small particles (20–30 nm) in the film layer and the other is the peak of formed medium particles (40–70 nm). It was found that the number of small particles was reduced, whereas the number of medium-sized particles increased with annealing. From these results, the difference in the spectra before and after annealing, as shown in Fig. 2(a), can be explained as follows. Before annealing, the propagating SP resonance due to the film layer and LSPR due to the fine particles were mixed and originated from the broad peaks. However, after annealing, most of the film layer became particulate, the small particles in the film layer became larger, and the average size of the medium-sized particles decreased, resulting in sharper peaks and a blue shift
Fig. 2. (a) Extinction spectra, (b) surface morphology of the AFM image, and (c) histogram of diameters for Ga nanoparticles of NHoS structure before and after annealing.
Figure 3(a) shows the extinction spectrum of the Ga NPs formed by Ga thin films with various initial thicknesses. The LSPR peak wavelengths in the deep UV regions were red-shifted with increasing initial thickness. Figures 3(b) and 3(c) show the AFM images and their size histograms of formed Ga NPs, indicating that the average particle size can be adjusted by changing the initial film thickness. Figure 4 shows a comparison of the experimental and calculated results for the relationship between particle size and resonance peak wavelength. The agreement between the two is very good, indicating that Ga can form a well- hemispherical structure.
Fig. 3. (a) Extinction spectra of NHoS structures fabricated at initial Ga film thicknesses of 5, 10, and 15 nm. (b) AFM images and (c) histograms of diameters of the nanoparticles in NHoS structures with initial film thicknesses of 5, 10, and 15 nm.
Fig. 4. Comparison between the FDTD calculations and experimental results of the relationship between nanoparticle diameter and SP resonance peak wavelength.
One serious problem with the use of Ga in device applications is its low melting point of 30 °C, which meant that the fabricated Ga NPs were very unstable and easily broken when touched. We therefore aimed to improve the stability and durability of the Ga NPs by applying a thin-film coating. Figure 5 shows the extinction spectrum of the Ga NPs coated with the 5-nm-thick Al2O3 thin film. The LSPR peak disappeared upon heating at 400 °C for 30 min, when the Ga NPs were not protected by the Al2O3 film (red line). Because this spectrum is almost identical to that of gallium oxide in the literature [25], it is assumed that Ga is completely oxidized to gallium oxide. After the Al2O3 coating, the LSPR peak wavelength was slightly shifted to a longer wavelength because the refractive index near the Ga NPs increased (green line). The LSPR peaks of the Ga NPs protected by the Al2O3 coating remained in the deep UV wavelength region after heating, as shown in the spectra (blue line). These results suggest that the oxidation of Ga NPs was prevented by the formation of an Al2O3 coating. These Ga NPs with the Al2O3 coating are very robust and stable despite being easy to fabricate, they should be useful for enhancing deep UV emissions from AlGaN/AlN or other light-emitting materials in various deep UV wavelength regions.
Fig. 5. Extinction spectra of the Ga NHoS structure with and without the 5-nm-thick Al2O3 coating before and after annealing.
Next, we fabricated NHoM structures with Ga NPs to obtain a much stronger and sharper resonance peak, and flexible tunability of the peak wavelength. Figure 6(a) shows the calculated extinction spectrum of the NHoM structure with Ga nano hemispheres of various initial Ga film thicknesses. A schematic of the NHoM structure is shown in the figure. In the NHoM structure, two peaks are observed at approximately 150-200 nm and 400-700 nm. Figure 6(b) shows the electric field distribution around the NHoM structure at each peak wavelength. The dipole and quadrupole oscillation modes were observed near the equator of the metallic hemisphere at each peak wavelength. Additionally, in both peaks, mirror image charges of opposite signs to those of the hemisphere were observed. Therefore, the charge distribution on the hemisphere and mirror forms a coupled mode, which is the origin of the respective peaks. Figure 6(c) shows the extinction spectrum of the experimentally fabricated NHoM structure. Two clear peaks were observed in the UVC and visible regions, similar to the calculated results. We succeeded in obtaining a strong LSPR in the DUV region, even at approximately 200 nm, using the NHoM structure of Ga, and additionally succeeded in flexibly tuning the resonance peak wavelength by changing the initial thickness of the Ga film.
Fig. 6. (a) Extinction spectra of the NHoM structure for Ga nanoparticles with diameters of 40, 60 and 80 nm and 5-nm spacers calculated by the FDTD method. (b) Spatial distribution of the electric field at the 544 and 175 nm peaks of the NHoS structure. (c) Experimentally obtained spectra of NHoM structures at initial Ga thicknesses of 5, 10, and 15 nm and 5-nm Al2O3 spacers.
We also propose a new method to control the LSPR spectra using Ga2O3 nanostructures formed by the oxidation of Ga NPs. Figure 7(a) shows the extinction spectrum of the NHoM structure with 75-nm-diameter Ga2O3 NPs when the spacer layer is 0, 5, and 10 nm calculated via FDTD simulations. Very sharp peaks were observed in the deep-ultraviolet wavelength region. The linewidths of these peaks are much sharper than those of the NHoM of Ga, as shown in Fig. 7(a). The peak wavelength was red-shifted with increasing spacer thickness. Figure 7(b) shows the electric field distribution at the peak wavelength of the NHoM structure with 5-nm spacers after 1.5 fs and 3.5 fs from pulse irradiation. After 1.5 fs, quadrupole oscillations of the optical near-field were observed around the dielectric hemisphere, and weak mirror image oscillations were observed on the metal substrate. After 3.5 fs, the oscillation of the optical near-field around the dielectric hemisphere disappeared, but the oscillations on the metal substrate remained plasmonic. This result suggests that the optical near-field mode of Ga2O3 induced a strong plasmonic mode on the Al substrate. Therefore, the sharp peak in Fig. 7(a) is considered to be the LSPR mode generated at the Al interface. Figure 7(c) shows the extinction spectrum of the experimentally fabricated NHoM structure with Ga2O3 with spacer thicknesses of 0 and 5 nm. In the metallic Ga NHoS structure before thermal oxidation, the LSPR peak was observed at approximately 300 nm, whereas in the Ga2O3 NHoS structure after thermal oxidation, band-edge absorption of the Ga2O3 was observed at wavelengths shorter than 260 nm. For the NHoM structure, other new peaks were clearly generated at approximately the same wavelength as the calculation results for both the 0 nm and 5 nm spacers. The experimentally obtained spectra were much broader than the calculated spectra, probably owing to the inhomogeneity of the size and shape of the fabricated Ga2O3 nanostructures; however, the peak intensities and wavelengths were almost similar.
Fig. 7. (a) Extinction spectra of the NHoM structure with a 75-nm-diameter Ga2O3 nanostructure when the spacer layer is 0, 5, and 10 nm calculated by the FDTD method. (b) Spatial distribution of the electric field at the peak of the NHoM structure after 1.5 and 3.5 fs from pulse irradiation. (c) Experimentally obtained extinction spectra of NHoM structures with spacer thicknesses of 0 and 5 nm, Ga NHoS, and Ga2O3 NHoS structures.
Although their trends were similar, there are some differences between the numerical and experimental spectra shown in Fig. 7. First, approximately 5 nm from the surface of the Al substrate would be naturally oxidized by thermal annealing in air to Al2O3 [20]. In addition, only the surface of the Ga NPs may be oxidized, partially leaving Ga in the inner core. Figure 8 shows the calculated extinction spectra of 75-nm-diameter Ga2O3 NPs with a 0-, 10-, 20-, or 30-nm Ga core inside on Al substrate with a 10-nm-thick Al2O3 spacer layer via the FDTD method. The broad peaks appeared around 300-400 nm due to the presence of the Ga core, similar to the experimental results. This suggests that the interior of the Ga NPs may not be sufficiently oxidized. Some details still need to be further clarified, nevertheless, we conclude that the Ga2O3 nano hemisphere on the Al substrate is a useful structure for DUV plasmonics.
Fig. 8. Extinction spectra of Ga2O3 NPs of 75 nm diameter with 0, 10, 20, or 30 nm Ga core inside on Al substrate with 10-nm-thick Al2O3 spacer layer calculated by the FDTD method.
4. Conclusions
We succeeded in both theoretically and experimentally tuning LSPR over the entire deep-ultraviolet wavelength range, using NHoS and NHoM structures with Ga and Ga2O3. These structures, based on random nanostructures, can be fabricated easily, inexpensively, and over a large area using a bottom-up approach. Although it was difficult to fabricate NHoS and NHoM structures in Al using this method, exquisite hemispherical structures could be fabricated in Ga, which could be oxidized directly into Ga2O3 hemispherical structures. The LSPR peaks were enhanced, dramatically sharpened, and flexibly tuned from the UVC wavelength regions by optimizing the thickness of the Ga film before annealing or the spacer layer. The sharpest and strongest LSPR peaks were obtained for the NHoM structure with Ga2O3 on the Al substrate because of the localized mode of the Al substrate. Our proposed structures have the potential to dramatically advance various conventional optical technologies, including ultra-high-efficiency energy devices, such as high-efficiency deep-UV LEDs, threshold-free nano-lasers, ultra-high-resolution imaging, and ultra-sensitive sensing.
Funding
Japan Society for the Promotion of Science (JP19H05627, JP20H05622, JP21K19218).
Acknowledgments
The authors wish to thank Prof. K. Tamada of Kyushu University and Prof. Y. Kawakami, Prof. M. Funato, and Prof. Terazima of Kyoto University for their valuable discussions and support.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data supporting the findings of this study are available from the corresponding author upon reasonable request.
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