Two-dimensional micro-cylinder patterns of dodecagonal photonic quasi-crystal (12-PQC) and tetragonal photonic crystal (2-PC) were fabricated on the top surface of a GaN light-emitting diode (LED). The patterns were evaporated with a 10-nm-thick Ag film. Spatially resolved surface emission was recorded and analysed by scanning near-field optical microscopy. Electromagnetic energy was confined and enhanced at the top surface when the surface plasmon (SP) resonated. The enhancement factor for 12-PQC was 1.9 times that of 2-PC, 8.6 times that of non-patterned LED in the near field and 6.7 times in the far field, respectively. Finite-difference time-domain simulations are consistent with the experimental data. The results show that with a patterned structure on the top surface of an LED, the light emission can be greatly enhanced due to SP resonance.
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A GaN-based light-emitting diode (LED) has been used for display, printer and the general lighting with lower power consumption. However, the emitting efficiency from the top of such an LED is limited by the total internal reflection at the interface of GaN/air or GaN/sapphire, which restricted the wider application of GaN LED in industry. Photonic crystal (PC) patterns from sub-wavelength to micron scale fabricated on the LED surface make the photonic control and light output enhancement possible by out-coupling the guided modes (Fan et al., 1997). Photonic quasi-crystal (PQC) defined by various tiling rules with high-order rotational and linear symmetries on the top of an LED gives higher scattering of the emitted light and it is easier to achieved omni-directional band gap than through conventional periodic PCs (Fujita et al., 2005). Patterning PQC cavities on the LED surface has shown emission enhancement in our previous work, but the enhancement factor is 2 ∼ 3 in comparison with the non-patterned devices (Zhang et al., 2006).
Surface plasmon (SP) is a hybrid mode of a light field and a collective electron oscillation resonantly excited at the interface of metal/dielectric layers. SP enhancement effect has been used in optoelectronic integrated circuits (Lezec et al., 2002) and focusing plasmonic devices recently (Fang et al., 2009). Nanometer-scale roughened surface on the InGaN/GaN LED was fabricated to improve the light-emitting efficiency using a Ni nanomask and a laser etching technique (Huang et al., 2005). Field enhancement at the top of a pattered silver rod array was reported for the biomedical applications (Kawata et al., 2008). In this paper, we report the application of SP enhancement on surface patterned GaN LED with dodecagonal photonic quasi-crystal (12-PQC) and tetragonal photonic crystal (2-PC) structures.
Material and methods
The starting sample used in this work is an edge-emitting GaN-based LED covered with 30 μm × 30 μm indium tin oxides (ITO)- and 8 μm × 30 μm GaN-exposed broad stripes. It is prepared on conventional LED epitaxial wafers grown by low-pressure metal-organic chemical vapour deposition (MOCVD) on C-plane of the sapphire substrate. The wavelength of emitting light is around 460 nm.
The patterns are located on the interspacing between the broad stripe electrodes where the p-type metal film was etched off. The 12-PQC and 2-PC patterns with a micro-cylinder shape are fabricated on the top of the LED using electron beam lithography and inductively coupled plasma (ICP) etching as follows: (1) the patterns are defined in an area of about 300 mm × 300 mm on polymethylmethacrylate (PMMA) by electron beam lithography, (2) the samples are developed in a solution of methyl isobutyl ketone and isopropyl alcohol, (3) an Ni layer about 50 nm thick is deposited by sputtering on the samples and is lifted off by acetone, (4) ICP dry etching is performed for 3 min using Cl2 and Ar gas at a pressure of 5 mTorr, (5) the remaining Ni mask is removed in hydrochloric acid solution and (6) a 10-nm-thick Ag layer is fabricated on the surface of the pattern by electron beam lithography. The enhancement factors under different DC injections are recorded using scanning near-field optical microscopy (SNOM).
The schematic of the patterned GaN-based LED is shown in Fig. 1(a). The generated guided modes, propagating in p-GaN layer, interact with the patterned structure. The emitting luminescence is scattered at the top surface and induces SP resonance, which is perpendicular to the guiding mode direction. A 1-μm height step between ITO and GaN areas is observed from the Atomic Force Microscopy (AFM) topography of 12-PQC pattern, as shown in Figs 1(b) and (c). Figure 1(d) is the scanning electron microscopy (SEM) image of the 12-PQC pattern, with a scale bar of 500 nm. The diameter and interval period of the micro-cylinders are 500 nm and 1 μm, respectively.
The SNOM used in this work consists of an NSOM-100 scanner (Nanonics Imaging Ltd, Jerusalem, Israel), an electronic controller system SPM-100 (RHK Technology, MI, U.S.A.) and a sensitive avalanche photon detector (APD), as shown in Fig. 2 (see Fang et al., 2008, for details). The probe is an Al-coated, tapered cantilever optical fiber with a 50-nm apex diameter (Nanonics Co.). The topography and optical images of the sample can be obtained simultaneously by SNOM, which provides a higher resolution (50 nm in our experiment) than the diffraction limit and is a crucial technique to observe the spatially resolved scattering characters on the sample surface and the field enhancement of SP resonance.
Results and discussions
The SP enhancement effect was measured as the following. Figure 3(a) is the topography of 12-PQC pattern obtained by SNOM. The near-field emission intensities of a patterned LED without and with an Ag layer with 10 nm thickness evaporated are compared under 2 mA DC injection, as shown in Figs 3(b) and (c). Figures 3(b) and (c) are taken from the same region as shown in Fig. 3(a). With an Ag layer, the average emission intensity is enhanced and the micro-cylinder profiles can be distinguished in the near-field optical images.
Figures 4(a–c) are the topography images measured by SNOM of 12-PQC, 2-PC and non-patterned LEDs with ITO and GaN areas at the left and right side, respectively. The propagation vector of the guided modes is along the x-direction. Figures 4(a1–a3) are near-field optical images corresponding to the area of Fig. 4(a) under DC injections of 100 μA, 800 μA and 4 mA, respectively. Higher emission in the patterned LEDs is recorded than in the non-patterned one, which shows an emission enhancement induced by the micro-structures on the top surface. In comparison with the 2-PC pattern, as shown in Figs 4(b1–b3), the micro-cylinder profile of the 12-PQC pattern is gradually observed with the increase in the concentration of DC injection, but only the vertical lines of the 2-PC pattern are distinguished from the images. Under the same DC injection, the emission intensity of the 12-PQC patterned LED is larger than that of the 2-PC patterned one in the far field by naked eyes. There is also an intensity jump at the step between ITO and GaN areas, which is considered as a second scattering to enhance the emission efficiency by the guided modes impinging onto the defects or the corners of the step sidewall. For the non-patterned LED, there are few changes for the emitting intensity when the concentration of the DC injection is increased, as shown in Figs 4(c1–c3).
The intensity distributions are analysed to compare the enhancement factor of 12-PQC and 2-PC patterned LEDs. If a function I (x, y) represents the surface intensity of each point (x, y) in the near-field optical image, an averaged light intensity yields
where w is the range of the area in the y-direction. Figure 5(a) shows the emission intensity profiles from the data of 12-PQC and 2-PC patterns. Both intensities first decrease in the ITO area during the propagation but increase rapidly to a peak near the ITO/GaN step and finally decrease exponentially to a plateau with slightly reducing intensity. After the SNOM tip detects the intensity of each point of the sample, the wavelength versus intensity curves are recorded using a spectrometer (SPEX 0.34 (HORIBA Jobin Yvon, Edison, NJ, U.S.A.)) and a photomultiplier tube (PMT), as shown in Fig. 5(b). As a result, under 4 mA DC injection, the integrated light intensity of 12-PQC is about 1.9 times that of 2-PC and 8.6 times in comparison with the non-patterned LED in the near field.
PQC is classified as a dodecagonal structure constructed by combining equilateral triangles and squared, keeping a 12-fold rotational symmetry as a whole, and is isotropic for possessing the lowest orientation order of the system (Zoorob et al., 2000). The photons’ Bragg scattering along a spherically symmetric set of directions leads to a higher enhancement of surface light emission in comparison with the conventional PCs (Man et al., 2005). Thus, the emission intensity of a 12-PQC patterned LED is always higher than the one with a 2-PC pattern, as shown in Fig. 5(a). For a 2-PC pattern, since the wave vectors are parallel to the transverse lines, more scattering light along the vertical direction is expected, as shown in Figs 4(b1–b3). Moreover, the surface defects near the step between ITO and GaN areas result in a second scatter according to the edge effect and can be used to increase the luminescence intensity, shown as the intensity jump in the 12-PQC and 2-PC near-field optical images.
A 10-nm-thick Ag layer is transparent for the emitting luminescence from GaN micro-cylinders to the air, and most of the scattering light impinging on the Ag layer will couple to the SP modes at the GaN/Ag interface. If the wave vectors of the scattering modes fulfilled the SP phase-matching condition (Raether, 1988), SP modes will resonate and will be enhanced. After the enhancement, the SP modes radiate again into the air, as shown in Figs 4(a1–a3) each micro-cylinder functions as an optical emitter, which results in the enhancement of the LED emission. The residue modes, propagating along the incident vector, interact with other structures and are gradually scattered out. Different thicknesses of the Ag film also have been tested to find the best SP coupling efficiency. From the experiments, we find that with the Ag film thickness increasing, the LED emitting intensity will be increased, but with 10 nm as the critical point for the maximum light intensity. There will still be intensity increase when the Ag layer is thicker than 10 nm; however, the enhancement factor begins to decrease. When the thickness is more than 100 nm, most scattering lights are reflected by the Ag layer and no enhancement is expected.
The emitting properties of 12-PQC in the far field were also studied under 4 mA DC injection using an integrated optical microscopy. We find that the average enhancement factor in the far field is lower than in the near field, which may be explained by the exponential decay of non-radiation field that localized on the top surface. The resonance of SP belongs to the non-radiation field; however, most of the SP modes can be transformed to radiation field by pattern scattering. Thus, in the far field, it still has an enhancement factor 6.7 in comparison with the non-patterned LED.
The SP coupling and scattering in this structure are simulated by using the finite-difference time-domain (FDTD) method. We use an eigenmode solver at the front edge boundary. Using the commercial software XFDTD (Remcom Co., State College, PA, U.S.A.), a GaN layer (n= 2.31) (Cavallini et al., 2007, for more details) covered by a 10-nm-thick Ag film (permittivity ɛ=–7.6321 + 0.7306i) (Fang et al., 2008, for details) is modelled in Fig. 6(a). The parameters are h= 5 μm, w= 10 μm and l= 10 μm. According to the experimental data, the 12-PQC and 2-PC patterned micro-cylinders are modelled at the top surface with a 500-nm diameter and a 1-μm interval period. The guided modes propagate in the direction of the arrow, as shown in Fig. 6(a).
Figures 6(b) and (c) are the electrical distributions for 12-PQC and 2-PC patterns in the y-direction detected at 5 nm above the Ag film surface. The average emitting intensity of 12-PQC pattern is stronger than that of 2-PC pattern, and most of the electromagnetic energy is confined and enhanced at the pattern surface when the SP resonance occurs. Only the vertical lines are observed in the simulation for 2-PC pattern, which is consistent with the experimental observations in Figs 4(b1–b3). The average emitting intensities of the simulation of both 12-PQC and 2-PC patterns in the near field are shown in Fig. 6(d). The enhancement factor of 12-PQC pattern is 1.96 times higher than that of 2-PC pattern, which is close to the experimental factor 1.9. The difference may come from the surface defect scattering during the experiment. For the far field, the filed intensity strongly depends on the distance between the detector and the top surface, but it still can find a maximum enhancement factor 6.2 for 12-PQC in comparison with non-patterned LED in our simulations.
In summary, 12-PQC and 2-PC patterned photonic crystal micro-cylinders covered by a 10-nm-thick Ag film are fabricated on the top surface of a GaN-based LED. Surface emission mapping from 12-PQC, 2-PC and non-patterned samples are investigated using SNOM. The enhancement factor of 12-PQC under 4 mA DC injection is 1.96 times higher than that of the 2-PC, 8.6 times in comparison with non-patterned LED in the near field, and 6.7 times in the far field. A second scattering resulted by the edge effect, with the guided modes impinging onto the defects or the corners of the sidewall at the step areas, is observed. FDTD simulations are consistent with the experimental observations. With the combination of the SP enhancement and the 12-PQC patterned structure, the top-surface emission efficiency of the GaN-based LED is effectively improved.
The work is supported by the National Science Foundation of China (Grant No. 10574002 and 60607003) and National Basic Research Program of China (973 Program) (Grant No. 2007CB936800 and 2007CB307004).