1. Introduction

Surface plasmons (SPs) are electromagnetic waves propagating along a metal interface. They generally arise from interactions between photons and electrons at the interface and exhibit various unique characteristics, including localization of electric fields at the interface and drastically increased density of modes near the resonance condition [1–4]. Such properties provide the possibility of guiding light in subwavelength metallic structures and enhancing the light emission and absorption [1–8]. Thus, the SPs have attracted considerable research attention due to scientific interest and applications in optoelectronic devices.

ZnO, with a direct bandgap of 3.37 eV, has been widely used as a transparent conducting electrode and is a strong candidate for ultraviolet light-emitting devices and photodetectors [9–11]. It has been reported that SPs at the ZnO/metal interface can enhance the spontaneous electron–hole recombination rate and subsequent photoluminescence (PL) intensity [5–8]. The SP dispersion relation at a metal–semiconductor interface is determined by the dielectric functions of both materials [1–4]. The SP density of modes (proportional to the inverse of $d\omega /dk$), and thus the emission rate, has peaks near the resonance energy [2]. In addition to the near band-edge UV emission, defect states in ZnO thin films usually result in broad visible-range luminescence [12]. In many studies of ZnO/metal structures without artificial periodicity, near-UV emission was enhanced while visible emission was suppressed, because the SP resonance was close to the bandgap energy of ZnO [5–8]. In contrast, periodic structures can allow multi-mode SP excitation in which the energy can be controlled by the periodicity and the properties of the constituent materials [4,9]. Influences of such multiple SP resonances on the optical properties of ZnO have not been explicitly investigated. Careful optical characterization of the grating structures will be very helpful for understanding the interplay between photons, excitons, and SPs in ZnO/metal structures.

In this article, we report the fabrication and optical characterization of ZnO/Ag nanograting structures. The experimental data clearly show that the optical properties of the grating structures were very different compared with the planar thin films. In particular, multiple PL peaks and notable enhancement of intensity were observed. The PL peaks were indicative of the resonant SP energies, which was supported by the reflectance spectra and the finite-difference time-domain simulation results.

2. Sample fabrication and characterization

We deposited ZnO and Ag thin films at room temperature using a radio-frequency (RF) magnetron sputtering technique. The base pressure of the sputtering system was 1 × 10⁻⁶ Torr, and the working pressure of pure Ar was 7.5 × 10⁻³ Torr during thin-film growth. The applied RF power was 50 W for both films. For the gratings, 100-nm-thick ZnO/100-nm-thick Ag layers were grown on polymer patterns fabricated by nanoimprint lithography [13]. Polydimethylsiloxane (PDMS) with nanoscale features was used as a stamp. Patterns were transferred to the UV-curable agent, trimethylolpropane propoxylate triacrylate (TPT), with a photoinitiator of 2-hydroxy-2-methylpropiophrnone. The patterns were cured under UV light (wavelength: 365 nm) for 30 minutes. The area of the pattern was 3 × 3 cm², which was large enough for conventional optical measurements. For comparison, ZnO/Ag thin films were also prepared on planar Si substrates.

The morphology of the films and the fabricated patterns were examined using an atomic force microscopy (AFM) system (Digital Instruments, Dimension 3100) and a field-emission scanning electron microscope (SEM) (JOEL, JSM-6700F). For PL measurements, samples were excited by a HeCd laser (wavelength: 325 nm), and the luminescence spectra were obtained by a nitrogen-cooled charge-coupled device (CCD) detector that was attached to a monochromator with a spectral resolution of 1 nm. The optical reflectance of the films was studied using a UV-visible spectrophotometer (PerkinElmer, 750).

Figure 1(a) shows a schematic diagram of the ZnO/Ag grating with line-and-space structures such as one-dimensional square wave shapes. Figure 1(b) shows a typical AFM image of the polymer pattern. The period and amplitude of the grating were estimated to be 1000 and 500 nm, respectively. SEM measurements of the polymer patterns showed that the widths of the line and the space were nearly the same. After growth of the ZnO/Ag layers, the space-width was reduced, and the line-to-space ratio was 3:2, indicating that the sputtering method allowed very conformal growth of the layers on the polymer pattern. The root-mean-square (rms) roughnesses of ZnO and Ag thin films on flat Si substrates were estimated to be 3.2 and 1.9 nm, respectively, based on the AFM measurements. The surface roughness of both of the films was negligible compared with the peak-to-valley height of the grating.

 

Fig. 1 (a) Schematic diagram of a ZnO/Ag grating structure and (b) AFM morphology of a polymer pattern with a period of 1 μm and a line-to-space ratio of 1:1.

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3. Surface plasmon dispersion relation

SP resonance energies at the ZnO/Ag interface can be determined based on the dielectric functions of ZnO and Ag [2]. The dispersion relation of SPs at a metal/dielectric interface is given by the following expression:

 

Fig. 2 (a) Real-part dielectric constants of Ag, ZnO, and air, and expected SPR and LSPR energies. (b) SP dispersion relation of a ZnO/Ag grating structure with a period of 1 μm. Blue dashed lines and dots indicate the SP (photon) energy that can couple photons (SPs).

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In a periodic structure, SPs can be excited by incident photons, while the momentum mismatch in multiples of the reciprocal lattice vector are compensated according to the relationship

4. Photoluminescence and reflectance spectra

Figure 3 shows the PL spectra of the ZnO/Ag films grown on a flat Si substrate (gray line) and on the nanograting structure (black line). For the planar sample, the PL spectra consisted of a peak at ~3.2 eV, originating from the band-to-band emission, and a broad peak in the visible range due to defect electronic states inside the bandgap region [12]. The PL spectra of the grating sample were drastically different from those of the flat sample: (1) the intensity increased significantly, and (2) wide-range (nearly white light) emission was observed.

 

Fig. 3 PL spectra of a ZnO/Ag planar thin film and a ZnO/Ag grating structure. Enhancement indicates the ratio of the PL intensity of the grating structure to that of the planar thin film.

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Photons with an energy exceeding the bandgap of ZnO will generate electron–hole pairs (EHPs) in ZnO. The band electrons will undergo one of two types of decay: (1) radiative recombination that generates near-UV emission or (2) relaxation to defect states. The electrons at defect states can recombine radiatively and emit luminescence in the visible range. The SPs can modify the radiative recombination rate for both excitons and electrons at defect states [7]. The SPs are the waves bound to the interface; thus, they are inherently nonradiative [2]. For a flat sample, it would be difficult to extract light from the SP mode because of the large momentum mismatch between the light and the SP. Interfacial roughness and impurities enabled the SP energies to be transferred to photons [6]. The ZnO films showed both near-UV and visible emissions (Fig. 3), suggesting that ZnO is a good material for studying the coupling of SPs with photons over a wide spectral range. In the flat sample, the SPs at low energies (in the visible range) could not readily couple with photons, and efficient release of the SP energy (i.e., visible light emission) did not occur. As shown in Fig. 2(b), the gratings facilitated the coupling of SPs with light at various energies, and vice versa, by solving the in-plane momentum problem. As a result, the excited SPs emitted light according to the dispersion relation depicted in Fig. 2(b). Thus, the broad PL spectra of the grating indicated that the SPs promoted emissions due to band-to-band transitions and also due to defect state transitions.

Figure 3 shows the ratio of the PL intensity for the grating to that of the flat sample, which reached 10² at 2.8 eV. The PL enhancement ratio has been reported to be less than several tens for semiconductors with metal thin films and nanoparticles [3–8]. The electronic density of states in ZnO for the grating should have been similar to those for the flat samples. Thus, the grating structures likely played key roles in the modification of the spectral dependence and the enhancement of the PL intensity. First, the grating enabled SPs to exchange energy with photons efficiently, as mentioned previously. Diffraction can also occur in the grating; thus, the optical path can be increased [9]. When the photons remain in the ZnO layers longer, more electron–hole pairs can be generated, resulting in emission enhancement. The electric field distributions in the grating should also be considered, and details will be discussed in Sec. 5.

Figure 4 shows the PL and total reflectance spectra of a ZnO/Ag grating structure. The reflectance data were obtained with an integrating sphere to minimize losses due to interference and diffraction. The incident angles of light were 0° and 7° from the surface normal for the PL and the reflectance experiments, respectively. Dashed blue lines indicate the excited SP modes expected from Fig. 2(b). According to Eq. (2), the term sin(7°) = 0.12 can cause a small difference in the SP energy involved in the PL and the reflectance data. The blue dashed lines agreed well with the PL peaks and the reflectance dips, suggesting that the coupling between the photons and SPs enhanced the PL intensity and modified the PL spectra, as discussed above. However, the reflectance dips and the corresponding PL peaks were located at somewhat higher energies than the blue lines (note the differences at 1.85 and 1.68 eV). Haug et al. reported similar reflectance results for their ZnO/Ag grating structures: the SP resonance at the Ag interface was not sensitive to the presence of the ZnO layer at low energies, and a transition between the relationships of air/Ag and ZnO/Ag was observed at photon energies above 2 eV [9]. At low energies (i.e., long wavelengths), the propagation length in the ZnO layer can be less than one third of the effective wavelength, ${\lambda }_{eff}={\lambda }_0/\sqrt{\epsilon }\approx 330\text{ nm}$. For high-energy photons, the 100-nm-thick ZnO layer acted like a bulk material, resulting in the ZnO/Ag SP dispersion relation depicted in Fig. 2(b). As a result, the PL peaks and the reflectance dips were in good agreement with the dashed lines.

 

Fig. 4 PL spectra and reflectance of a ZnO/Ag grating structure. Dashed blue lines correspond to the SP mode energies expected from the dispersion relation.

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5. Finite-difference time-domain simulation

To describe the wave propagation within the nanograting structure, Maxwell’s equations had to be solved. Real-space electric field distributions can be obtained by finite-difference time-domain (FDTD) simulations, which numerically solve the differential equations. In this work, the nanograting structure was modeled in FDTD (Lumerical FDTD Solutions 7.0.1) as a unit cell (Fig. 5 ) embedded in air with a perfectly matched layer (PML) and periodic boundary conditions. Figure 5 shows the electric field intensity,${\left|\overrightarrow{E}\right|}^2$, for four different photon energies (hν) for a p-polarized plane wave at normal incidence. For hν = 1.0 eV, no SPP excitation was expected from Fig. 2(b), and no large field formed in the ZnO layer, as shown in Fig. 5(a). Figures 5(b), 5(c), and 5(d) show that the field intensity built up in the ZnO layers, especially at the ZnO/Ag interface, for hν = 2.1, 2.7, and 3.0 eV, respectively. For these energies, SPP excitation was possible according to the dispersion relation depicted in Fig. 2(b). At hν = 2.7 eV, a large field was observed at the top as well as the side of the ZnO/Ag interface. Notable field intensity enhancement and a large number of excited SP modes at hν = 2.7 eV accounted for the PL intensity enhancement, as shown in Fig. 3.

 

Fig. 5 Electric field intensity distribution of a ZnO/Ag grating structure at various photon energies obtained by FDTD simulations. All simulations were carried out for p-polarized plane waves.

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6. Conclusion

We demonstrated that the optical properties of ZnO/Ag grating structures were significantly different from those of ZnO/Ag planar thin films. PL spectra for the grating showed multiple peaks and nearly white light emission. The PL intensity of the grating was larger than that of the planar sample by two orders of magnitude. The PL peak positions and the reflectance dips were well matched, which was understood based on the SP dispersion relation. The spatial electric field distribution was simulated, revealing that the SPs influenced the optical characteristics significantly.