A surface plasmon (SP)-enhanced nanoporous GaN-based green LED based on top-down processing technology has been successfully fabricated. This SP-enhanced LED consists of nanopores passing through the multiple quantum wells (MQWs) region, with Ag nanorod array filled in the nanopores for SP-MQWs coupling and thin Al2O3 passivation layer for electrical protection. Compared with nanoporous LED without Ag nanorods, the electroluminescence (EL) peak intensity for the SP-enhanced LED was greatly enhanced by 380% and 220% at an injection current density of 1 and 20A/cm2, respectively. Our results show that the increased EL intensity is mainly attributed to the improved internal quantum efficiency of LED due to the SP coupling between Ag nanorods and MQWs.
© 2014 Optical Society of America
GaN-based green light emitting diodes (LEDs) are attractive in display and solid state lighting due to the advantages of long lifetime and energy saving . However, further increase in luminous efficiency of GaN-based green LEDs has being obstructed by the poor crystal  and serious quantum-confined Stark effect (QCSE) . Apart from the improvement of epitaxial growth process, surface plasmon (SP) coupling is another promising technology to increase the efficiency, which can increase the density of states and the spontaneous emission rate in the multiple quantum wells (MQWs) [4,5], and thus enhance the light emission from GaN-based green LED accordingly [6–11]. In general, the key point to fabricate the SP-enhanced GaN-based LED is how to place the energy-matched metal nanoparticals in the vicinity region of the MQWs as close as possible, but without deterioration of the electrical and physical properties of the devices.
Till now, many researchers have already put tremendous efforts to explore the efficient coupling methodology to increase the coupling of SP-LEDs. For example, at the earlier stage, an approach with a thin p-GaN layer was proposed. In this case, the thickness of the p-GaN layer was reduced from conventional 200 to 80 nm, and the metal nanoparticles were then deposited on the surface to enable the coupling with quantum well underneath the p-GaN layer [12–15]. In that approach, the structure of device is simple, but the hole-injection is still limited by the depletion width of the p-side in the p-n junction, the coupling distance cannot be further reduced. In order to decrease the SP coupling distance, the approach with embedded metal has been utilized [12, 16–19], where the metal nanoparticles were embedded above the p-GaN/MQWs or below the n-GaN/MQWs interface. This method can result in a SP coupling distance of 10 nm and a thick enough p-GaN hole-injection layer, but the side effect is that the small size of metal caused by the high temperature regrowth will lead to a low SP scattering efficiency. In addition, the devices also suffer a serious degradation of optical-electrical performance because of the poor crystal quality of epilayers grown on the metal layer and the metal diffusion to MQWs at high temperature. Recently, a more flexible SP coupling based on top-down processing technology has been reported [20–22]. The p-GaN layer with two-dimensional nano-hole array structure has been fabricated using a dry etching mask, followed by the metal deposition into the bottom of hole. It is roughly 30-40 nm above the MQWs for SP coupling. With this method, the metal incorporation is independent of epitaxial growth, which can help to prevent the quality deterioration of the material caused during the growth. Furthermore, the surface plasmon resonance energy and scattering efficiency of metal particles can also be easily manipulated using different mask morphology. However the devices will have a large leakage current because of the short distance between hole-bottom and MQWs, which will not only seriously deteriorate the optical performance, but limit the further decrease of the distance between the hole-bottom and MQWs to increase the SP coupling.
Here, by introducing the Al2O3 passivation layer for electrical protection, we prepared a high performance SP-enhanced nanoporous GaN-based green LED based on top-down processing technology, with the nanopores passing through the MQWs region for the closer SP-MQWs coupling. After deposition of Ag nanorods into nanopores, the electroluminescence (EL) peak intensity of the nanoporous GaN-based LED was enhanced by 220% at 20A/cm2. Under low injection current, a maximum enhancement up to 4 times at 1 A/cm2 was achieved. In addition, the different contribution to EL enhancement has been compared quantitatively, which shows that the increase of EL intensity is mainly due to the improved internal quantum efficiency (IQE) of LED as a result of the lateral SP coupling between Ag nanorods and MQWs.
2. Device fabrication and working principle
The green LED epilayer was grown on a 2 inch (0001) sapphire substrate with a low temperature GaN buffer layer using metal-organic chemical vapor deposition (MOCVD). The structure consisted of a 2µm undoped GaN layer, a 2µm n-GaN layer, a three-period InGaN/GaN MQWs and a 200 nm Mg doped p-GaN layer. Afterwards, the LED devices were processed as follows: firstly, the surface region of the p-GaN layer was partially etched by an inductively coupled plasma-etching process (ICP) using Cl2/BCl3/Ar gases to expose the n-GaN layer, then a 150 nm indium tin oxide (ITO) layer was deposited as a transparent current-spreading layer on the p-GaN layer followed by the evaporation of Cr/Pt/Au (30/100/1500 nm) on both the n-GaN layer and the ITO current-spreading layer as electrode by e-beam evaporation. After that, a 300 nm thick AAO (Anodic Aluminum Oxide) membrane with a period of 400 nm was transferred to the LED chip wafer. The details of the transfer method have been reported elsewhere . Using the AAO membrane as a mask, the LEDs were then etched into the nanoporous structure by low damage ICP etching using Cl2/BCl3/Ar gas and all the pores completely pass through the ITO current spreading layer, p-GaN and MQWs and extend to the n-GaN layer. The residues of AAO template were removed by dilute NaOH solution prior to the deposition of a 15 nm Al2O3 dielectric layer by atomic layer deposition (ALD). By introducing this Al2O3 passivation, it can help to minimize the leakage current of the GaN-based SP-enhanced LEDs based on top-down processing technology, meanwhile a closer SP-MQWs coupling can be obtained by extending the nanopores through the MQWs region. Then a standard photolithographic process and ICP etching procedure were followed to expose the p/n pad electrodes, and the resulted “nanoporous LED” serves as the reference sample. As for the surface plasmon GaN LEDs, a 500 nm thick Ag nanorod array was filled into the nanopores of the nanoporous LED using standard photolithographic process and e-beam evaporation. EL measurements were carried out from the bottom sapphire side of the LEDs using a calibrated Si-photodiode connected to an optical power meter.
Figure 1 shows the schematic diagram of the nanoporous GaN-based green LED with lateral surface plasmon coupling methodology. Except for the conventional p-n electrodes and ITO current spreading layer, this SP-enhanced LEDs consist of nanopores passing through the MQWs with a period of 400 nm, with 15 nm thick Al2O3 passivation layer and 200 nm Ag nanorods array filled in the nanopores. The Al2O3 passivation layer not only serves as an electrical isolation layer, its thickness also determines the lateral SP coupling distance between Ag nanorods and MQWs. In this work, the lateral distance is 15 nm, far less than the SP penetration depth, which can allow the SP field of Ag nanorods penetrate through the thin Al2O3 and extend to the MQWs region for SP-MQWs coupling. To confirm the lateral penetration depth into the MQWs, the SP field distribution of Ag nanorods was simulated using three-dimensional (3D) finite-difference time-domain (FDTD), which is shown in Fig. 1(b). Since the environment of each nanorod is equal, the SP field distribution of each Ag nanorod in the actual LED would be the same. To simplify the simulation, seven 200 nm Ag nanorods with 400 nm periods were used. The nanorods were coated with 15 nm thick Al2O3 layer and embedded in GaN materials, which is similar to our studied LED. Meanwhile, two dipoles were placed next to the middle Ag nanorod to stimulate its SP mode. Numerical simulation results show that the SP field of Ag nanorods can laterally penetrate in to the MQWs region with a depth of about 70 nm. In this region, the electron-hole pairs can be strongly coupled with SP, which will lead to a greatly increase of the radiative recombination rate caused by the extremely high density of states of the SP, and the enhancement of the IQE.
3. Results and discussion
Figure 2(a) shows a top view SEM image of the nanoporous LED before Ag deposition. It has ordered hexagonal pore arrangement, where the average pore-size and inner-pore distance is 200 and 400 nm, respectively. The cross section of the nanoporous LED with Ag nanorods is shown in Fig. 2(b). The interface of ITO/p-GaN, p-GaN/MQWs and GaN/ Al2O3/Ag can be clearly observed. The pores pass through the ITO current spreading layer, p-GaN and MQWs and extend to the n-GaN layer. The 15 nm Al2O3 passivation layer covered the outside of the nanopore and ITO layer entirely. The lateral diameter of Ag nanorods is roughly about 200 nm. With this lateral diameter, Ag nanorods can have more than 80% of SP scattering efficiency [24,25]. Figure 2(c) shows the image of the Ag nanorods array separated from nanoporous LED. The extinction spectrum of Ag nanorods is shown in Fig. 2(d), which is calculated from the reflection spectrum. There are two peaks at 452 and 541 nm, which can be attributed to the quadrupole and dipole component of transverse SP resonance of the silver nanorods, respectively . Compared with nanoporous LED without Ag nanorods, the current density-voltage (J-V) characteristics of nanoporous LED with Ag nanorods exhibit a very low leakage current of less than 10-3A/cm2 at -8 V and similar value at forward operation voltage, as shown in Fig. 2(e), which suggests that the Al2O3 passivation layer can efficiently protect the pore-wall and pore-bottom, and the Ag nanorods were electrically isolated from the nanoporous LED. Due to this good electrical protection of Al2O3 passivation layers and the same nanoporous structure, the nanoporous LEDs with and without Ag nanorods present almost the same electrical properties, such as the same J-V characteristics and current-voltage (I-V) characteristics, which make it reasonable to compare and analyze the luminescence properties of these two LEDs at the same current injection condition.
Figure 3(a) shows the electroluminescence (EL) spectrum of the nanoporous LED with and without Ag nanorods at 1A/cm2. After the filling of Ag nanorods into the nanoporous LED, a 380% enhancement in the EL peak intensity and an obvious blue shift from 567 to 562 nm was observed. To explore the different contributions to EL enhancement, all possible factors, which are regularly associated with SP coupling process as well [6,12,20], need to be considered, such as epitaxial materials properties, device electrical properties, SP coupling effect and light extraction efficiency difference. The optical image of these two LEDs operated under 1A/cm2 are shown in the Fig. 3(b), which were illuminated at an adjacent region where the optical properties of InGaN/GaN epi-layers should be uniform and the difference of EL spectrum due to the wafer non-uniformity can be neglected. In addition, the change of electrical properties caused by the Ag nanorods can be negligible as well since they are electrically isolated from nanoporous LED completely. Furthermore, the Ag was deposited just at room temperature (see experimental Section) and there should be no metal thermal diffusion or self-change in the InGaN/GaN materials. Therefore, the EL intensity enhancement and spectrum shift should not be due to the change of electrical property or any InGaN/GaN materials quality differences. Figure 3(c) shows the time-resolved photoluminescence (TRPL) of nanoporous LED with and without Ag nanorods measured at room temperature. The effective exciton lifetimes were calculated to be 5.9 and 1.3ns respectively. The faster decay time for nanoporous LED with Ag nanorods suggests that the MQWs were strongly coupled with SP in the Ag nanorods .With this structure, the electron-hole pair energy of the MQWs (560 nm) is close to the SP dipole resonance energy (541 nm) In addition, the electron-hole pairs generated in the MQWs near nanopores are located in the SP near field, so there will be a strong lateral coupling between SP and the electron-hole pairs. Meanwhile, due to the very large density of states of SP, the energy of electron-hole pairs will transfer to SP mode with an extremely fast rate, which will increase the radiative recombination rate and PL decay rate, and the exciton lifetime ratio of radiative lifetimes to non-radiative lifetimes accordingly. Moreover, because of a large SP scattering efficiency, the IQE will be significantly enhanced and the peak position will shift to the SP resonance peak position .Therefore, we can conclude that the SP-MQWs coupling caused this great luminescence enhancement and spectrum blue shift.
Except for the IQE enhancement induced by SP-MQWs coupling, the metal reflection would be another reason for the increased EL intensity because of the improvement of light extraction efficiency after Ag coating. In order to distinguish the influence of the light extraction from the SP coupling, we also prepared a series of corresponding nanoporous control LEDs, which have the same surface structure of pore morphology, passivation layer thickness and Ag nanorod morphology to above mentioned nanoporous SP-enhanced LED, but thicker p-GaN layer with 360 nm. In this case, Ag nanorods are located at 100 nm above the MQWs, and no SP coupling would be expected because of the large coupling distance. The EL enhancement spectrum of the nanoporous control LEDs is shown in the Fig. 3(d), which is almost flat with the value of about 1.4 in the range of wavelength from 500 to 600 nm, indicating that no SP coupling was achieved , and this EL intensity enhancement is only attributed to the light extraction enhancement caused by the metal reflection. It should be noted that because of the same surface structure, the EL enhancement spectrum of the nanoporous control LEDs may also be correlated to the light extraction enhancement of the SP-enhanced nanoporous LEDs. The EL enhancement spectrum of the SP-enhanced nanoporous LEDs is also shown in Fig. 3(d), which corresponds to the total enhancement factor of SP coupling and light extraction. A peak enhancement factor of 4.8 was observed at 553 nm. Extracting the EL enhancement spectrum of the corresponding control LEDs, the IQE enhancement spectrum of SP-enhanced nanoporous LED can be achieved. A maximum IQE enhancement factor of 3.6 was observed at 550 nm, which is very close to that of the dipole resonance of the silver nanorods. This indicates that the IQE enhancement is attributed to the coupling between SP and MQWs .Therefore, the significant EL intensity enhancement of nanoporous LED after Ag nanorods filling is mainly due to the SP-MQWs coupling, which will increase the IQE of LED, and partially the effect caused by the light extraction efficiency enhancement.
Figure 4(a) and 4(b) shows the electroluminescence of nanoporous LED with and without Ag nanorods were measured at different current density. The calculated EL enhancement factor as a function of injection current density is shown in Fig. 4(c). A maximum EL intensity enhancement factor of 4.8 was observed at 553 nm with the current density of 1A/cm2, which is attributed to the coupling between MQWs and dipole resonance of the silver nanorods. However, in short wavelength, the EL is suppressed and the enhancement factor is less than 1. This is caused by the coupling between MQWs and quadrupole resonance of the silver nanorods, the quadrupole has a low scattering efficiency and most energy transferred from electron-hole pairs to quadrupole is absorbed .The maximum EL enhancement was rapidly decreased until leveling off at approximately 3 as the current density approached 20 A/cm2. This result can be explained by the screening effect of excess charges in MQWs with increasing current, resulting in a decrease of exciton dipole coupling with SP . Figure 4(d) depicts the optical output power of nanoporous LED with and without Ag nanorods, as a function of injection current. The optical output power of nanoporous LED was enhanced by 250% and 120% after Ag nanorods filling at an input current density of 1 and 20A/cm2, respectively. The improvement in optical output power can mainly be attributed to the SP coupling with MQWs.
We have successfully fabricated a SP-enhanced LED with nanopores passing through the InGaN/GaN MQWs region, silver nanorods array filled in pores separated from MQWs by thin Al2O3 passivation layer. Benefit from the strong lateral SP coupling between Ag nanorods and MQWs and good electrical protection by the passivation layer, the EL intensity of SP-enhanced nanoporous LED was significantly enhanced. By introducing a series of corresponding nanoporous LED with thicker p-GaN layer, we confirm that this significant EL enhancement is mainly attributed to SP coupling between Ag nanorods and MQWs. This SP coupling method can also be applied to the similar structures such as top-down or bottom-up nanorods LED, which can pave a way to apply the SP coupling technology to increase the luminous efficacy of GaN based LEDs.
This work was financially supported by National Natural Science Foundation of China (grants No. 61334009) and International Science and Technology Cooperation Program of China (grants No. 2014DFG62280).
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