Plasmonic enhancement of UV emission from ZnO thin films induced by Al nano-concave arrays

Applied Surface Science 384 (2016) 18–26

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Plasmonic enhancement of UV emission from ZnO thin films induced by Al nano-concave arrays Małgorzata Norek a,∗ , Grzegorz Łuka b , Maksymilian Włodarski c a Department of Advanced Materials and Technologies, Faculty of Advanced Technologies and Chemistry, Military University of Technology, Kaliskiego 2, 00-908 Warsaw, Poland b Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland c Institute of Optoelectronics, Military University of Technology, Str. Kaliskiego 2, 00-908 Warszawa, Poland

a r t i c l e

i n f o

Article history: Received 19 December 2015 Received in revised form 30 April 2016 Accepted 5 May 2016 Available online 9 May 2016 Keywords: ZnO nanostructures Al nano-concaves Plasmonic enhancement Near band edge emission Reflectivity spectra

a b s t r a c t Surface plasmons (SPs) supported by Al nano-concave arrays with increasing interpore distance (Dc ) were used to enhance the ultraviolet light emission from ZnO thin films. Two sets of samples were prepared: in the first set the thin ZnO films were deposited directly on Al nanoconcaves (the Al/ZnO samples) and in the second set a 10 nm − Al2 O3 spacer was placed between the textured Al and the ZnO films (the Al/Al2 O3 -ALD/ZnO samples). In the Al/ZnO samples the enhancement was limited by a nonradiative energy dissipation due to the Ohmic loss in the Al metal. However, for the ZnO layer deposited directly on Al nanopits synthesized at 150 V (Dc = 333 ± 18 nm), the largest 9-fold enhancement was obtained by achieving the best energy fit between the near band-edge (NBE) emission from ZnO and the (0,1) SPP resonance mode. In the Al/Al2 O3 -ALD/ZnO samples the amplification of the UV emission was smaller than in the Al/ZnO samples due to a big energy mismatch between the NBE emission and the (0,1) plasmonic mode. The results obtained in this work indicate that better tuning of the NBE − (0,1) SPP resonance mode coupling is possible through a proper modification of geometrical parameters in the Al/Al2 O3 -ALD/ZnO system such as Al nano-concave spacing and the thickness of the corresponding layer. This approach will reduce the negative influence of the non-radiative plasmonic modes and most likely will lead to further enhancement of the SP-modulated UV emission from ZnO thin films. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Plasmonics have opened up new opportunities for light manipulation via the confinement of the electromagnetic field to regions well below the diffraction limit. Surface plasmons (SPs) generated by a metal can be harnessed to concentrate and intensify incident light if proper conditions are met. Two distinct types of SPs can be distinguished depending on the geometry of the metal: surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs) [1]. The SPPs are the plasmons sustained at a flat metal-dielectric interface. SPP is an evanescent wave and therefore cannot be directly coupled to freely propagating light. A technique is needed to compensate for the momentum mismatch. Efficient coupling of incident light into SPPs can occur on periodically patterned metallic film resulting in a strong electromagnetic field augmentation at the metal surface (surface plasmon resonance, SPR). This field is

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (M. Norek). http://dx.doi.org/10.1016/j.apsusc.2016.05.029 0169-4332/© 2016 Elsevier B.V. All rights reserved.

responsible for very high light transmission efficiencies at specific wavelengths which are strictly linked with the size, shape and periodicity of the pattern [2,3]. In contrast to SPPs, LSPs can be directly excited by propagating light. The LSP resonance exists only over a finite wavelength range, and its spectral position can be tuned by the particle’s size and shape. Metal nanopits or nanoholes can usually support both types of resonances [5–7]. The modification of optical performance in semiconductors by the coupling of light to plasma oscillations in metallic nanostructures residing on or in the proximity of the semiconductor surface has become an active field of research in the last twenty years [8,9]. ZnO is a wide band gap semiconductor with a direct band gap of 3.37 eV [10–12]. Its large exciton binding energy (60 meV) makes it very attractive for potential applications in light emitting devices. Various ZnO nanostructures in addition to the near bandedge emission (NBE) in near UV produce a defect-related emission (DLE), which is positioned in a visible part of the spectrum [13–15]. Different fabrication methods resulted in different nanostructures’ morphologies which in turn induced diverse defect types and concentrations and consequently dissimilar luminescence spectra. The

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Fig. 1. SEM images of ZnO films deposited directly on Al nanoconcaves with increasing interpore distance (the Al/ZnO samples) (a–e) and on the Al nanoconcaves covered by Al2 O3 (the Al/Al2 O3 -ALD/ZnO samples) (f–j) fabricated at 125 (Dc = 273 ± 6 nm) (a, f), 140 (Dc = 286 ± 7 nm) (b, g), 150 (Dc = 333 ± 18 nm) (c, h), 160 (Dc = 362 ± 10 nm) (d, i), and 195 V (Dc = 456 ± 25 nm) (e, j); scale bars = 500 nm.

DLE emission is disadvantageous to produce highly efficient optoelectronic devices operating in UV. Recently, enhancement of the NBE emission from ZnO by metallic nanoparticles has attracted considerable attention. In particular, a number of articles explored the enhancement of photoluminescence (PL) in ZnO/Ag hybrid systems [16,17]. Many factors have impact on the optical response of semiconductors. The enhancement factor was varied between 2 and 10 depending on system configuration and Ag nanoparticle size, shape, and density [18–22]. A thin layer of Ag deposited on the ZnO thin film resulted in an enhancement factor of 15 [23]. When the ZnO film was deposited on the Ag layer, an enhancement ratio as high as 45 was achieved for an Ag layer thickness of ∼120 nm [24]. NBE emission enhancement was also observed in other systems such as in Si nanopillars coated by ZnO ultrathin film [25,26], or in ZnO thin films covered by polystyrene microspheres of different diameters [27]. Although in some cases the NBE emission enhancement was quite large, the effectiveness of Ag nanoparticles in increasing the ZnO luminescence is limited by the intrinsic optical properties of the metal. Optimal plasmonic properties (the strongest and narrowest resonances) are provided by metals with a small imaginary part of the dielectric constant at a given wavelength range [28–30]. Most of the metals, including Ag or Au, possess inter- and intraband transitions in the UV region that increase the imaginary part of their dielectric constants. Therefore, although the metals demonstrate excellent plasmonic properties in the visible and near-infrared regimes [31–34], they are not the best choice to generate SPR in the UV range. Aluminum (Al) has an interband transition near 1.4 eV [28] and therefore its optical spectra show well-defined resonance peaks in the UV or even deep UV regions [35,36]. In addition, the SPRs are very sensitive to the size and shape of nanostructures [37–39]. Although aluminum is easily oxidized, the process is selflimiting, yielding a thin and stable oxide thickness that makes it easy to work with even in atmospheres containing oxygen [40–42]. Aluminum can exhibit strongly enhanced local fields owing to its high electron density (3 electrons per atom as compared to 1 electron per atom in metals such as Au or Ag). Moreover, Al is abundant and cheaper than most other plasmonic metals (e.g. Au, Pt, or Ag). One of first trials using Al nanoparticle arrays for enhancement of the NBE emission from ZnO was completed by Lin et al. [43]. The influence of the shape of Al nanoparticles was demonstrated: the square Al nanoparticles were better (yielding an enhancement factor of 2.6) than the round ones (enhancement factor of 1.6). Lu et al. registered a more than 10-fold enhancement of the sponta-

neous and stimulated emission from Al-decorated ZnO microrods, with a slight blue shift of the UV peak [44]. Both the enhanced intensity and the blue shift were attributed to the surface plasma resonance (SPR) induced by Al NPs. The same group observed an 8fold enhancement of NBE emission from Al-decorated ZnO nanorod arrays [45]. Tuning of the SPR by adjusting the thickness of the Al nanoparticle layer sputtered on ZnO microrods allowed a 170-fold enhancement of the NBE emission [46]. It was previously observed that luminescence enhanced by SPs suffers from strong luminescence quenching as the light emitter is placed at a very short distance (a few nanometers) from the surface of the plasmonic nanostructures. It is believed that the luminescence quenching originates from the light coupling with nonradiative high-order LSP modes. To eliminate this effect a spacer with optimized thickness was introduced between the metallic nanostructures and the emitter. An over 100-fold enhancement in photoluminescence was observed in the ZnO ultra thin films grown on the uniformly dispersed nanostructured Pt layer and separated from the ZnO by an Al2 O3 spacer [47]. The high impact of the Al2 O3 spacer on ZnO NBE emission was also confirmed in Al, Au and Ag modified ZnO systems [48]. We have recently demonstrated that the Al nano-concave arrays prepared in the anodization process can support SP resonances in the UV range, whose spectral positions can be easily tuned by changing the interpore distance (Dc ) [49]. The ease of preparation and high controllability of the geometrical parameters of the Al nanostructures make them very attractive for application in SPR induced enhancement of light emission in the UV spectral range. In the research described herein, the plasmonic enhancement of UV light emission from ZnO thin films deposited on Al nano-concave arrays with Dc ranging between ∼270 and ∼450 nm is studied. The effect of an Al2 O3 spacer placed between the Al surface and ZnO on intensification of PL from the ZnO films is also analyzed and discussed. 2. Experimental High-purity aluminum foil (99.9995% Al, Puratronic, Alfa Aesar) with a thickness of about 0.25 mm was cut into coupons (2 cm × 1 cm). Before the anodization process the Al foils were degreased in acetone and ethanol and subsequently electropolished in a 1:4 mixture of 60% HClO4 and ethanol at 0 ◦ C, at a constant voltage of 20 V, for 2 min. Next, the samples were rinsed with ethanol, distilled water and then dried. The Al coupons were

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Fig. 2. STEM-HAADF images along with EDX mappings (at.%) of a cross section of Al(150 V)/ZnO (a) and Al(150 V)/Al2 O3 -ALD/ZnO (b) samples.

insulated at the back and the edges with acid resistant paint and served as the anode. A Pt grid was used as the cathode and the distance between both electrodes was kept constant (ca. 5 cm). The anodizing process involved vigorous stirring (750 rpm) in a large 1 l electrochemical cell. The applied voltage was controlled using an adjustable DC power supply with a voltage range of 0–300 V and a current range of 0–5 A, purchased from NDN (model GEN750 TDK Lambda). The Al nano-concaves were prepared using a hard anodization (HA) method at voltages between 120 and 160 V in a 0.3 M H2 C2 O4 water-based solution or in ethanol- modulated solutions with 4:1 v/v water to EtOH, at 0 ◦ C. The samples were pre-anodized at 40 V for 5–8 min prior to the application of a given voltage. Then the voltage was slowly increased to a target value at a rate ranging from around 0.04 to 0.06 V/s, and the samples were anodized for 2 h. The Al-concave substrates with an interpore distance (Dc ) of ∼450 nm were prepared in 0.1 M H3 PO4 solution with 4:1 v/v water to EtOH at 0 ◦ C. Alumina was chemically removed using a mixture of 6 wt% phosphoric acid and 1.8 wt% chromic acid at 60 ◦ C for 120 min. ZnO and Al2 O3 materials were deposited on the Al nano-concave arrays by atomic layer deposition (ALD). Zinc oxide was grown using diethylzinc (DEZ) and water vapor as zinc and oxygen precursors respectively. Aluminum oxide films were grown using trimethylaluminum (TMA) and water vapor reagents. The films were grown in a Savannah-100 ALD reactor (Cambridge NanoTech) at a growth temperature of 200 ◦ C. The process pressure was ∼10−1 mbar and the N2 purging gas flow rate was 20 sccm. Two sets of samples were prepared. In one set the ZnO was directly deposited on Al nano-concaves (samples Al/ZnO). In the second set

the Al nano-concaves were first covered by an Al2 O3 layer and next the ZnO layer was deposited (samples Al/Al2 O3 /ZnO). The morphology of the samples was studied using a fieldemission scanning electron microscope FE-SEM (FEI, Quanta) equipped with an energy dispersive X-ray spectrometer (EDS). To obtain the geometrical parameters of the fabricated Al nanoconcaves, Fast Fourier transforms (FFTs) were generated based on three SEM images taken at the same magnification for every anodizing voltage, and were further used in calculations with WSxM software [50]. The average interpore distance (Dc ) was estimated as an inverse of the FFT’s radial average abscissa from three FE-SEM images for each sample. The microstructure of the samples was analyzed with a high-resolution transmission electron microscope (HR-TEM). The analyses were performed in a TITAN CUBED 80–300 microscope with aberration correction of objective lens, operating at 300 kV. Scanning transmission electron microscopy (STEM) was done with the use of a high angle annular dark field detector (HAADF). Elemental mapping was performed using an energy-dispersive X-ray (EDX) in scanning TEM mode. Photoluminescence spectra were measured in the 350–700 nm spectral range using a CM2203 spectrofluorimeter with a xenon lamp. The excitation wavelength was 300 nm. Reflectivity measurement was performed using CCD spectrometer with a fiber-optics reflection probe (Avantes) and a deuterium/halogen light source (Ocean Optics Inc.). The probe was set up at a normal angle to measured sample. All reflectance spectra were collected in the 200–1100 nm wavelength range. An electropolished Al coupon was used as a reference sample with 100% reflection.

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Fig. 3. HR TEM images of a cross section (individual crown) of Al(150 V)/ZnO (a, b) and Al(150 V)/Al2 O3 -ALD/ZnO (c, d) samples. (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

3. Results and discussion The Al textured substrates were fabricated in hard anodization processes at 125, 140, 150, and 160 V. The distance between concaves obtained during these processes was on average 273 ± 6, 286 ± 7, 333 ± 18, 362 ± 10 nm, respectively (Fig. 1). For the sample prepared in mild anodization at 195 V the distance between the pits was on average 456 ± 25 nm. The ALD technique was used to deposit ZnO layers directly onto the Al nano-concaves with different pore intervals (the samples Al/ZnO, Fig. 1a–e) and on the Al nano-pits coated with Al2 O3 layer before (samples Al/Al2 O3 ALD/ZnO, Figs. 1f–j). The concave diameters are visibly smaller in the Al/Al2 O3 -ALD/ZnO samples than in the Al/ZnO ones suggesting that the film formed on Al concaves in the former samples are thicker (Fig. 1). In Fig. 2 STEM HAADF images of cross sections of the Al/ZnO (Fig. 2a) and Al/Al2 O3 -ALD/ZnO (Fig. 2b) samples anodized at 150 V are provided, along with chemical analysis. The distribution maps of Al, O, Zn obtained by EDX demonstrate a completely uniform covering of Al concaves by the ZnO layer. The Al2 O3 layer fabricated in the ALD process is also confirmed by EDX microanalysis as illustrated in a more detailed picture of the Al/Al2 O3 -ALD/ZnO sample in Fig. 2b. In the ALD process the thickness of the deposited layer is a function of the number of cycles. However, despite the same number of cycles applied during ZnO deposition, the layer

grown directly on Al nano-pits is almost two times thinner than the one grown on the Al2 O3 layer produced in the ALD process (Al2 O3 ALD layer), as demonstrated by HR TEM images (Fig. 3a and c). Apart from the Al2 O3 -ALD layer there is also native aluminum oxide formed within a few hours on aluminum after exposure to air (passivation layer). This native oxide layer is also visible in Fig. 3b and its thickness of around 3 nm is in agreement with the data presented by Langhammer et al. [41]. The total thickness of the amorphous layer in the Al/Al2 O3 -ALD/ZnO sample is 12.9 nm (Fig. 3d). Therefore, the Al2 O3 -ALD layer thickness is around 10 nm. Although both aluminum oxides are amorphous, the microstructure of the Al2 O3 ALD layer may be different than that of the native Al2 O3 . This resulted in apparently easier nucleation of ZnO on the Al2 O3 -ALD layer than on native Al2 O3 film, manifested in the approximately two times thicker ZnO layer in the Al/Al2 O3 -ALD/ZnO samples. This indicates that there is a big mismatch between the ZnO lattice parameters and the arrangement of atoms in the native Al2 O3 film formed on Al in air that delays the ZnO layer formation at the beginning of the process. Larger magnifications of the interface between the selected Al crown and the ZnO layer demonstrate that the ZnO layer consists of small crystallites with random crystallographic orientations (Fig. 3b and c). The ZnO crystallites in both samples overlap. However, in the Al/Al2 O3 -ALD/ZnO sample (Fig. 3c) the ZnO crystals seem to be larger and of more regular shapes than the ZnO crystals in the Al/ZnO sample (Fig. 3a). Moreover, in contrast

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Fig. 4. HR TEM image of a cross section (a single crown) of Al(150 V)/Al2 O3 -ALD/ZnO representing a single ZnO crystal.

to ZnO crystallites in the Al/ZnO sample, the ZnO crystals in the Al/Al2 O3 -ALD/ZnO sample demonstrate a tendency for columnar growth. The base of the columns is anchored in the Al2 O3 -ALD layer (Fig. 3d), therefore the ZnO film looks a bit like a brush surrounding Al crowns (Fig. 3c). One of the elongated ZnO monocrystals is shown in Fig. 4. Its length corresponds with the ZnO film thickness grown on Al2 O3 -ALD covered Al nano-pits. The diameter of the single ZnO crystal is ∼10 nm. The clear evidence of the columnar nature of ZnO growth in the Al/Al2 O3 -ALD/ZnO samples and more random arrangement of ZnO crystals in the Al/ZnO samples may also support the assumption that the nucleation process of ZnO on the native amorphous Al2 O3 layer is somewhat hindered. The SAED (Selected Area Electron Diffraction) image of individual Al/Al2 O3 -ALD/ZnO crowns for the sample prepared at 150 V along with the relevant HR TEM image are shown in Fig. 5. The SAED image proves the polycrystalline nature of the sample. A simulated Gaussian blurring of diffraction peaks indicated by the white profile is characteristic for small crystals. The large circles correspond to the theoretical position of the dhkl rings. The diameter and intensity of the powder diffraction red rings correspond exactly to the model of the nanocrystalline ZnO. Bragg spots corresponding to the polycrystalline Al can be also distinguished on the SAED pattern. The spots are indexed and shown in small green circles. The first necessary condition to achieve a meaningful plasmonic enhancement of light emission from a semiconductor is the energy adjustment between the near band-edge emission (NBE) and the electron oscillation of surface plasmons at the metal/semiconductor surface (the NBE-SPP coupling) [8]. Typical experiments proceed by monitoring a dip in reflectance (R), when an evanescent light field travels through a metal thin film and excites SPPs at the metal–dielectric interface. As a result, the normal reflectivity of the metal surface is greatly reduced on resonance due to optical absorption by the metal [4]. In Fig. 6a the reflectivity spectra of Al nano-concave arrays with different period and that of electropolished aluminum surface measured at normal incident are shown. Apart from a significant drop in the reflectivity’s intensity, characteristic dips are observed in reflectance curves registered for patterned Al substrates as compared to unstructured Al. The minima were previously classified as a signature of SPPs excitations, which means that the incoming light has coupled to surface plasmons (SPs) at Al nano-concave arrays [49,51]. The dependence of

Fig. 5. SAED analysis of individual crown in the Al(150 V)/Al2 O3 -ALD/ZnO sample (the inset: HR TEM image of the analyzed area). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

the reflectance dip position on the interpore intervals (anodization potentials) and its sensitivity to the refractive index of a medium in contact with the surface of Al nano-pits clearly indicate that the Al nano-pits sustain propagating plasmons (SPPs). The surface plasmon resonance (SPR) is directly associated with the periodicity of the nanoconcave arrays and to the optical constants of Al. The nanoconcave arrays in Al substrate provide the additional momentum G necessary to fulfill the resonance conditions, giving rise to the dips in reflectivity spectra [3]. At normal incidence, the coupling of photons of a given energy with a 2-dimensional hexagonal periodic array yields SP resonances (SPRs) and always occurs at specific wavelengths calculated as follows (Eq. (1)) [3,52]: =

  4 3

a0 i2 + j2 + ij





εm εd εm + εd

(1)

where a0 is the pitch size (interpore distance, Dc ), εd is the frequency-dependent permittivity of the dielectric material and εm is the real part of the frequency dependent permittivity of the metal. The i, j integers specify the orders of resonances. The (i, j) = (1,0) or (0,1) resonance order is the most effective resonance mode ((1,0) ) that couples easily to the outside radiating modes [53]. In Fig. 6a, b and d the dips associated with higher order SPP resonance mode ((1,1) ) are also visible, and are always located at the lower wavelengths part of the spectrum. Energy is highly concentrated in the higher order modes, however the modes are difficult to couple to the external radiating modes, and are thus known as non-radiative modes [53]. The SPPs, as electromagnetic waves propagating at a metal/dielectric interface, are very sensitive to any change in the near-surface dielectric constant (index of refraction, n). The resonances of SPPs shift to the red part of the spectrum when n is increased as compared to the refractive index of air (n ∼ 1). The n of crystalline ZnO film at  = 589 nm is around 2 [54]. The refractive index of amorphous alumina may vary considerably depending on the deposition technique and layer thickness [55,56]. A value of n

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Fig. 6. Reflectivity spectra of pure Al nanoconcaves with increasing Dc (increasing anodizing voltage) (a), and covered with Al2 O3 (the Al/Al2 O3 -ALD samples) (b) along with the reflectivity registered for flat, electropolished Al; Al nanoconcaves covered with ZnO (the Al/ZnO samples) (c), and Al nanoconcaves covered with Al2 O3 and ZnO (the Al/Al2 O3 -ALD/ZnO samples) (d); the vertical, dotted lines in a larger magnification of Fig. 6c and d demonstrate a position of the near band-edge emission (NBE) in ZnO; red circle in a larger magnification of Fig. 6c signifies the best NBE − (0,1) SPP resonance mode energy adjustment and thus the best plasmonic enhancement achieved in the Al/ZnO system. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

as low as 1.3 was observed for amorphous alumina deposited using the HV-CVD technique at 493 ◦ C [57]. In addition, the refractive index may also be sensitive to layer integrity: less dense layers with significant amounts of voids may result in a decrease in the refractive index. It can be expected that textured aluminum promotes the formation of a non-continuous Al2 O3 -ALD layer thus further reducing its index of refraction. In accordance with the SPP phenomena the reflectivity dips for the relevant Al nano-pits (Fig. 6a) are red-shifted upon changing the material in direct contact with their surface (Fig. 6). The reflectivity dips for Al/Al2 O3 -ALD samples are only slightly shifted in comparison to uncovered Al nanoconcaves (Fig. 6b). The largest shift is observed for the Al/Al2 O3 -ALD/ZnO samples (Fig. 3d). Moreover, for the latter samples the reflectivity dips characteristic for the higher order of SPP resonances ((1,1) ) have become more pronounced. For the ZnO layers deposited directly on Al textured surfaces prepared in HA processes at 125, 140, 150, and 160 V (samples Al(125 V)/ZnO, Al(140 V)/ZnO, Al(150 V)/ZnO, Al(160 V)/ZnO, respectively), apart from a large reflectance dip shift, a significant blurring of the (1,0) dips occurs (Fig. 3c). At the same time, the strongly blurred (1,0) dips are the closest to the NBE wavelength emitted by the ZnO material ( ∼ 380 nm), as indicated by the vertical dotted line in Fig. 3c. Moreover, the dips connected with the (1,1) SPP resonance mode were no longer discernible. The observed reflectivity curves may indicate a significant suppression of the SPP resonances when the ZnO layers are in direct contact with Al nanoconcaves. The quenching of SPP resonance dips may be associated with the coupling of light with non-radiative high-order plasmonic modes when the light emitter is placed at a very short distance (a few nanometers) from the surface of plasmonic nanostructures [47]. The absence of the dips in reflectivity spectra acquired for the Al/ZnO samples associated with the (1,1) SPP resonance mode and the blurring of the dips pertinent for the (1,0) mode may be due to the efficient coupling of light to the non-radiative modes. The quenching of SPPs can be understood by taking into account the imaginary part of the dielectric constant of a metal. Sun et al. proved that when the emitter is in close vicinity of the corrugated metal surface the radiative process competes with a nonradiative dissipation due to the Ohmic loss determined by the imaginary part of the SPP propagation constant, even though the energy is very efficiently transferred from the emitter to the high density SPP

radiation modes (strong NBE-SPP coupling) [58,59]. Thus a substantial fraction of the energy is lost in the metal instead of being temporarily stored and used to excite the SPPs. The (1,0) dip in the spectra obtained for the ZnO layer deposited directly on the Al nanopits anodized at 195 V (the Al(195 V)/ZnO sample) is shifted to the position at around 510 nm, thus much farther than the NBE ZnO emission. There was no significant damping of energy in the Al metal in this sample, most probably owing to the large shift. However, the dip related with the (1,1) SPP resonance mode, being close to the NBE line in ZnO (the vertical line in a magnification of Fig. 3c), has been flattened. It was suggested that reduction of the coupling with nonradiative high-order plasmonic modes can be prevented by placing a spacer layer with an optimal thickness between the emitter and a metal surface [47,48]. As can be seen in Fig. 6d, both (1,0) and (1,1) SPP resonances are well-defined in the Al/Al2 O3 -ALD/ZnO samples. However, the (1,0) dips are shifted far above 380 nm. Therefore, it can be expected that for these samples the plasmonic enhancement of ZnO UV emission will not be the strongest due to a considerable mismatch between the near-band edge emission and the strongest (1,0) plasmonic mode. The geometry of the Al nanoconcaves and resulting efficiencies of the light scattering and absorption by the textured Al is decisive for modifying the optical performance of semiconductors. In Fig. 7, the PL spectra of the Al/ZnO and Al/Al2 O3 -ALD/ZnO samples are presented. First of all, the emission intensity of thin ZnO layers deposited on flat, electropolished Al is very weak, signifying a low radiative recombination rate of free excitons. The PL intensity in the UV region has become significant for the ZnO layers grown on the Al nanoconcaves, indicating enhancement of UV luminescence by SPPs supported by the Al nanoconcaves. The visible broad band emission originated from deep level defects is also observed in the studied samples, although its intensity is considerably smaller than the UV emission. It is difficult to avoid the defect emission in strongly nanostructured samples as those analyzed in this work (Figs. 3 and 4). The strongest, 9-fold enhancement (as compared to the emission from ZnO deposited on a flat Al substrate) is observed in the ZnO layer deposited directly on Al nanopits synthesized at 150 V. In this sample (as indicated by red circle in Fig. 3c), the strongest enhancement factor corresponds to the position of the reflectivity dip closest to the NBE emission from ZnO. The lowest,

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Fig. 7. PL spectra of the respective Al/ZnO (a) and the Al/Al2 O3 -ALD/ZnO samples (b). (For interpretation of the references to color in the text, the reader is referred to the web version of this article.)

5-fold enhancement was obtained from the ZnO film deposited on Al nanopits fabricated at 120 and 140 V. The PL luminescence is also enhanced in the case of ZnO film deposited directly on Al concaves prepared at 195 V, even though the (1,0) dip is far above the NBE wavelength of ZnO. However, the (1,1) mode is not completely suppressed in the Al(195 V)/ZnO sample (Fig. 6c), suggesting low Ohmic loss in the aluminum substrate. Therefore, a fraction of energy can be still stored in this system to excite SPPs that can be coupled to the NBE emission and used for the enhancement of the UV emission. The energy dissipation owing to the high-order plasmonic modes can severely reduce the overall SPP enhancement of radiation efficiency in the ZnO layers deposited directly on the Al nanoconcaves. In order to decrease this effect the Al2 O3 -ALD spacer was placed between the Al and ZnO layers. The SPP electromagnetic waves decay exponentially with increasing distance from the surface, thus they are strongly localized at the metal/dielectric interface. Consequently the enhancement factor of luminescence depends on spacer thickness. There is always an optimal spacer thickness above which the PL intensity drops substantially [60,61]. This is due to the evanescent character of the SPP modes. The penetration depth of the SPP into a semiconductor depends on the wavelength and the dielectric constant of the semiconductor. Usually its value is smaller than 1 ␮m [62]. For the 10 nm Al2 O3 -ALD film thickness a substantial number of electron-hole pairs in the ZnO semiconductor should still be coupled to the SPP modes. In the reflectivity curves shown in Fig. 6d the SPP resonances are well resolved, suggesting that the light coupling to the higher SPP modes was indeed inefficient, resulting in low loss of energy to the Al metal. However, the (0,1) resonance dips in the Al/Al2 O3 -ALD/ZnO samples have shifted far above the NBE emission (vertical line in Fig. 6d). The PL spectra for the Al/Al2 O3 -ALD/ZnO samples are given in Fig. 7b. The enhancement factors for the samples (as compared to the Al2 O3 -ALD/ZnO layer deposited on flat Al) are between 5 and 3.5. The factors are smaller than the respective factors measured for the Al/ZnO samples due to the larger NBE- (0,1) SPP mode energy mismatch. The PL reinforcement is lower despite the ZnO layer being around two times thicker than that in the Al/ZnO samples (Fig. 3), implying strong plasmonic effects in the latter samples. The dips related with the (1,1) SPP mode are much closer to the NBE emission line, which could contribute to the enhancement of UV emission in the Al/Al2 O3 -ALD/ZnO samples owing to a strong electromagnetic field concentration. The strongest enhancement was however achieved with the Al(195 V)/Al2 O3 -ALD/ZnO sample, in which both the (0,1) and the (1,1) modes are far away from the NBE emission (Fig. 6d). To explain this behavior it should be remembered that proximity of the SPP resonant energy to a semiconductor’s emission energy is necessary to achieve an optimal

enhancement of light emission, but it is not a sufficient condition. The second important factor responsible for the emission efficiency is probability of photon extraction from the SPs energy [62]. This factor in turn depends on roughness and the structure of the metal surface. The 5-fold enhancement of UV luminescence measured for the Al(195 V)/Al2 O3 -ALD/ZnO sample was most probably due to some favorable geometrical features (e.g. induced by the largest Dc ), which increased the probability of light extraction. Apart from the plasmonic effects discussed above, the lower enhancement in the Al/Al2 O3 -ALD/ZnO samples as compared to the Al/ZnO ones may also be caused by simple absorption and scattering effects happening at the additional Al/Al2 O3 and Al2 O3 /ZnO interfaces. Based on the above analysis one can conclude that there is room for further improvement of UV emissions from ZnO layers induced by Al nanoconcave arrays. Direct deposition of ZnO on Al nanopits gives a rather moderate UV emission improvement due to metal losses at the metal surface. As proved in previous research [47,48], it seems that the application of the additional Al2 O3 layer between a textured Al surface and a ZnO layer is necessary in order to take full advantage of the plasmonic properties of Al in the UV region. However, the NBE-SPP energy coupling in the Al/Al2 O3 -ALD/ZnO samples should be corrected through better tailoring of the geometrical parameters of the system including Dc and the thickness of the ZnO and Al2 O3 layers. Moreover, the SPR dip position and the dip broadening are influenced by the regularity of hexagonal pore arrangement and the pitch (Dc ) size distribution [49], which vary with the applied potential [63]. This issue should also be taken into account when optimizing the optical performance of the studied system. 4. Conclusions Plasmonic enhancement of ultraviolet light emission from ZnO thin films was analyzed. The SPPs were generated in Al nanoconcave arrays with increasing interpore distance (Dc ). The ZnO layers were deposited directly on Al nanoconcaves (the Al/ZnO samples) and in some samples a 10 nm Al2 O3 spacer was placed between the textured Al and ZnO films (in the Al/Al2 O3 -ALD/ZnO samples). In the Al/ZnO samples the enhancement was limited by a nonradiative energy dissipation due to Ohmic loss in the Al metal. In the Al/Al2 O3 -ALD/ZnO samples, UV light amplification was hindered by a big energy mismatch between the NBE emission from ZnO and the (0,1) plasmonic mode. The largest enhancement (around 9fold) was achieved in samples where the ZnO layer was deposited directly on Al nanopits synthesized at 150 V (Dc = 333 ± 18 nm). The results obtained in this work suggest that further improvement of UV emission from ZnO thin films in the Al/Al2 O3 -ALD/ZnO samples is possible after better adjustment of geometrical parameters of

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the system such as distance between the Al nano-concave centers and/or the thickness of the corresponding layers. The modification will most probably enable better tuning of the NBE − (0,1) SPP resonance mode energy coupling and at the same time will reduce energy dissipation due to Ohmic loss. The optimal material can be interesting for application in various SP-based sensors, solar cells or in light emission devices.

Acknowledgments The research was financed by Polish National Science Centre (Decision number: DEC-2012/07/D/ST8/02718). The work has been financially supported by the Polish Ministry of Science and Higher Education, Project: LAPROMAW (POIG.02.01.00-14-071/08/00).

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