Enhanced optical output and reduction of the quantum-confined Stark effect in surface plasmon-enhanced green light-emitting diodes with gold nanoparticles

We report the optical properties of localized surface plasmon (LSP)-enhanced green light-emitting diodes (LEDs) containing gold (Au) nanoparticles embedded in a p-GaN layer. The photoluminescence (PL) and electroluminescence (EL) intensities of a green LED with Au nanoparticles were enhanced by the coupling between excitons and LSPs. Excitation power-dependent PL and injection current-dependent EL measurements revealed that the blue-shift of PL and EL peaks with increasing carrier density was smaller for the LSP-enhanced LED compared with that for a conventional LED. The increased optical output power and decrease in blue-shift of the LED with Au nanoparticles were attributed to the increased radiative recombination efficiency of carriers induced by the LSP-coupling process and the compensation of the polarization-induced electric fields with LSP-enhanced local fields, both of which suppressed the quantumconfined Stark effect. ©2016 Optical Society of America OCIS codes: (230.0230) Optical devices; (230.3670) Light-emitting diodes; (240.6680) Surface plasmons; (350.4238) Nanophotonics and photonic crystals. References and links 1. S. Nakamura, M. Senoh, N. Iwasa, and S. Nagahama, “High-Brightness InGaN Blue, Green and Yellow LightEmitting Diodes with Quantum Well Structures,” Jpn. J. Appl. Phys. 34(7), L797–L799 (1995). 2. H. K. Cho, J. Y. Lee, C. S. Kim, and G. M. Yang, “Influence of strain relaxation on structural and optical characteristics of InGaN/GaN multiple quantum wells with high indium composition,” J. Appl. 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Introduction
Recently, tremendous developments of GaN-based light-emitting diodes (LEDs) have been achieved in terms of epitaxial growth, device fabrication, and optoelectronic device physics.However, the quantum efficiency and output power of green LEDs still need to be increased.The main challenge to fabricate highly efficient green LEDs is the fundamental difficulty associated with the growth of InGaN/GaN multiple quantum wells (MQWs) with high indium (In) composition.In practical use, the emission wavelength range of LEDs has been limited because of the severe drop of internal quantum efficiency (IQE) at green/yellow wavelengths, which is referred to as the "green gap" region [1].The IQE degradation of green LEDs is mainly caused by the low crystal quality of InGaN layers with high In composition [2] and the strong polarization-induced electric fields in highly strained InGaN-based MQWs [3,4].Crystal degradation of In-rich MQWs mainly originates from the considerable lattice mismatch and the thermal instability of the InGaN well layer [5,6].Furthermore, the polarization-induced electric fields cause energy band bending and force the electrons and holes to opposite ends of the quantum well (QW).Consequently, the electron-hole wave function overlap is decreased, resulting in a red-shift of the radiative recombination wavelength and a reduction in the radiative recombination rate.This phenomenon is called the quantum-confined Stark effect (QCSE) [3,7].The QCSE makes it difficult to stabilize the peak wavelength of LEDs under various operating conditions.Moreover, the QCSE increases with the In composition of InGaN, thus leading to relatively poor emission in the green spectral region compared with that in the blue.To overcome these problems, various approaches have been pursued, including the use of a nonpolar or semipolar substrate [8][9][10][11], prestrained MQWs [12], screening by doping [13], quantum dot structure [14], and design of QW structure by band gap engineering [15].
Recently, it was reported that the optical output power of green LEDs can be enhanced by localized surface plasmons (LSPs) of metal nanoparticles embedded in the p-GaN layer of LEDs [16].Surface plasmons (SPs) are the collective oscillations of free electrons in metals at the interfaces between metals and dielectrics.Specifically, the collective oscillations of electrons in noble metal nanoparticles embedded in a dielectric matrix are LSPs.A SPenhanced LED was designed to improve both radiative recombination and IQE, because effective energy-transfer from the light emitter to the SP generates a fast relaxation channel, thus increasing the spontaneous emission rate [17][18][19].Recently, some groups reported the physical mechanisms for the relationship between SP-coupling and QCSE [20,21].However, the direct experimental evidences, which can explain the reduction of QCSE by SP-coupling effect in the InGaN-based green LED, have not been reported yet, even though a detailed understanding of the relationship between SP-coupling and QCSE is highly desirable to further improve the performance of green LEDs.
In this paper, we present the new results obtained from a comparative study of the optical and electrical properties of InGaN-based green LEDs with and without LSP-coupling effect.To observe the LSP-coupling effect, gold (Au) nanoparticles are embedded in the p-GaN layer of green LEDs.Particularly, excitation power-dependent photoluminescence (PL) and injection current-dependent electroluminescence (EL) measurements are used to clearly examine the influence of LSP-coupling on the QCSE of green LEDs.The effect of LSPcoupling on the IQE and optical properties of MQWs are also studied by temperaturedependent PL measurement.

Experimental details
Figure 1(a) shows a schematic diagram of the LSP-enhanced green LED with Au nanoparticles embedded in the p-GaN layer.The LED exhibiting green emission at 540 nm was grown on a c-plane (0001) sapphire substrate by metal-organic chemical vapor deposition (MOCVD).After the growth of a 25 nm-thick GaN nucleation layer at 550 °C, a 2 μm-thick undoped GaN layer and a 2 μm-thick n-GaN layer were grown at 1030 °C.Then, five periods MQWs consisting of undoped InGaN wells (3 nm) and GaN barriers (12 nm) were grown at 730 °C, followed by a 20-nm-thick p-GaN spacer layer at 980 °C.To deposit the Au nanoparticles on the p-GaN spacer layer, the samples were taken out of the MOCVD growth chamber and a 0.2 nm-thick Au layer was deposited on the p-GaN spacer-layer by electronbeam evaporation.Figure 1(b) shows an atomic force microscopy (AFM) image of the Au nanoparticles deposited on the p-GaN spacer-layer after thermal annealing.The as-deposited Au layer with a smooth surface is transformed into nanoparticles and their size is increased by thermal annealing via the Ostwald ripening process after thermal annealing [16,22].Annealing was performed at 850 °C for 3 min in the MOCVD chamber prior to the regrowth of a p-GaN capping layer.The average diameter and height of the Au nanoparticles on the p-GaN spacer-layer were 65 ± 40 and 15 ± 5 nm, respectively.Even though the size distribution was different from the previous result [16], the PL enhancement spectra were very similar presumably due to the broadening of extinction spectra because of the large size distribution of Au nanoparticles on the p-GaN surface.After thermal annealing of the Au layer in the MOCVD chamber, a 30 nm-thick p-GaN capping layer was deposited on the Au nanoparticles at 850 °C for 1.5 min and a 150 nm-thick p-GaN layer was grown on the p-GaN capping layer at 980 °C.To fabricate the LEDs, the p-GaN layer was etched by an inductively coupled plasma etching process using Cl2/CH4/H2/Ar gases until the n-GaN layer was exposed for ntype ohmic contact formation.Next, LEDs with a dimensions of 300 × 300 μm2 were fabricated using indium tin oxide with a thickness of 150 nm as a transparent current spreading layer and Cr/Au as the n-and p-pad electrodes by electron-beam evaporation.The fabrication of LSP-enhanced LEDs with metal nanoparticles was reported in detail elsewhere [16].

Results and discussion
Figure 2 presents the room-temperature PL spectra of InGaN/GaN MQWs with and without Au nanoparticles in the p-GaN layer.The PL intensity of the MQWs with Au nanoparticles is much stronger than that of the MQWs without Au nanoparticles.The integrated PL intensity of the MQWs with Au nanoparticles is about three times higher than that of the MQWs without Au nanoparticles.This remarkable enhancement of PL intensity is believed to be caused by the fast spontaneous recombination rate of the excitons in the MQWs induced by QW-LSP coupling [17][18][19].Furthermore, the blue-shift of the PL peak of MQWs with Au nanoparticles in Fig. 2 also indicates the existence of a QW-LSP coupling process.The PL intensity of MQWs with Au nanoparticles is dominantly increased at 517 nm as shown in Fig. 2 because of the LSP mode in the Au nanoparticles at 517 nm [16].Therefore, the enhanced intensity and observed blue-shift of the PL spectra are attributed to the QW-LSP coupling.To further confirm that the improvement in the IQE of the InGaN/GaN MQWs with Au nanoparticles in the p-GaN layer is caused by an increase in the spontaneous recombination rate through a QW-LSP coupling process, temperature-dependent PL measurements were conducted over the temperature range from 10 to 300 K.The inset of Fig. 2 shows Arrhenius plots of the integrated PL intensities of MQWs with and without Au nanoparticles.The IQE of the MQWs can be estimated by comparing the integrated PL intensities measured at two different temperatures by assuming that the IQE is 100% at a low temperature of 10 K [23].Here, the IQE was obtained by taking the ratio of the integrated PL intensity at 10 K and 300 K.The IQE of the MQWs with and without Au nanoparticles is 23.1% and 11.7%, respectively.These observed enhancements of PL intensity and IQE in the LEDs with Au nanoparticles are ascribed to resonant coupling between excitons in the MQWs and LSPs of the Au nanoparticles.Because of the Purcell effect resulting from the increased photon density of states near the SP frequency, the QW-LSP coupling rate is very fast, and the resulting new recombination path can increase the spontaneous emission rate [17].To investigate the effect of coupling with LSP on the QCSE in MQWs, excitation powerdependent PL measurements were performed.Figure 3 shows the excitation power-dependent PL spectra of InGaN/GaN MQWs with and without Au nanoparticles in the p-GaN layer measured at room-temperature.The MQWs were excited at 325 nm by a continuous He-Cd laser and the excitation power was varied from 2.5 to 25 mW using neutral density filters.The PL peak positions of both LEDs with and without Au nanoparticles are blue-shifted with increasing excitation power, as shown in Figs.3(a) and 3(b).This observation can be attributed to the compensation of the polarization-induced electric field by the increased free carrier screening effect with increasing carrier density [24,25].In particular, the PL peak energy of the MQWs without Au nanoparticles exhibits a large blue-shift of 43.4 meV with increasing excitation power.Conversely, the PL peak energy of the MQWs with Au nanoparticles exhibits a blue-shift of only 9.5 meV in the same range of excitation power, indicating that the QCSE in MQWs with Au nanoparticles is decreased by LSP-coupling.These results clearly indicate that MQWs with Au nanoparticles have a relatively weaker polarization-induced electric field in MQWs compared with that in conventional MQWs without Au nanoparticles because of the compensation of the polarization-induced electric field with LSP-enhanced local fields by Au nanoparticles and this led to the suppression of the QCSE in MQWs [20].To better understand the effect of LSP-coupling on the QCSE, the EL from two green LEDs was measured.Figure 4 displays EL spectra of green LEDs with and without Au nanoparticles at injection currents from 10 to 150 mA.As shown in Fig. 4(a), the EL intensities of the green LED with Au nanoparticles are much higher than those of the LED without Au nanoparticles.Moreover, the EL emission peaks of both green LEDs are blueshifted with increasing injection current because of a screening effect of the polarizationinduced electric field by carriers and the band-filling effect of the localized energy states of potential fluctuation in MQWs [26].In particular, the current-induced blue-shift of the green LED with Au nanoparticles is much smaller than that of the conventional green LED without Au nanoparticles.As illustrated in Fig. 4 eV at 150 mA.The EL emission peak of the LSP-enhanced green LED with Au nanoparticles exhibits a blue-shift of 70 meV in the same range of injection current.These results are similar to those obtained for nonpolar and semipolar LEDs [9,10], indicating that the decrease in the blue-shift of EL emission energy in the LED with Au nanoparticles can be attributed to the compensation of the polarization-induced electric fields in MQWs with the LSP-enhanced local fields of metal nanoparticles [20].

Conclusion
In conclusion, we analyzed the contribution of LSP-coupling to the QCSE in LEDs by excitation power-dependent PL and current-dependent EL measurements.The blue-shifts of PL and EL of the green LED with Au nanoparticles were much smaller than those of the conventional green LED without Au nanoparticles due to the compensation of the polarization-induced electric field with the LSP-enhanced local fields of Au nanoparticles.These results indicate that such an LSP-coupling effect contributes substantially to the enhancement of optical output power and decreases the blue-shift of EL peaks because of the reduced QCSE in MQWs.This study shows that Au nanoparticles can significantly improve the optical properties of InGaN-based LEDs with "green gap" emission wavelength.

Fig. 1 .
Fig. 1.(a) Schematic diagram of the structure of LSP-enhanced green LEDs with Au nanoparticles embedded in the p-GaN layer.(b) AFM image of Au nanoparticles deposited on a p-GaN spacer-layer after thermal annealing.

Fig. 2 .
Fig. 2. Room-temperature PL spectra of InGaN/GaN MQWs with and without Au nanoparticles.The inset shows the temperature-dependent integrated PL intensity of the MQWs with and without Au nanoparticles.

Fig. 4 .
Fig. 4. (a) EL spectra of LSP-enhanced green LEDs with and without Au nanoparticles measured with increasing injection current at room-temperature.(b) EL emission wavelength of LSP-enhanced green LEDs with and without Au nanoparticles as a function of injection current.
(b), the EL emission peak of the conventional green LED without Au nanoparticles is blue-shifted by 109 meV from 2.230 eV at 10 mA to 2.339