1. Introduction

Recently, near-ultraviolet (NUV) light-emitting diodes (LEDs) have been used in many applications such as light sources for photometric detecting systems, biochemical sensors, medical equipment, counterfeit bill detection, and pumping sources of white LEDs [1–5]. However, the internal quantum efficiency (IQE) of NUV-LEDs is much lower than those of blue and green LEDs [1, 6], because the NUV-LEDs have a reduced potential confinement of carriers due to the low indium (In) composition in InGaN quantum wells (QWs) compared with blue and green LEDs. Much research has been performed to improve the IQE of NUV-LEDs by improving crystal quality [7–10]. However, it is still necessary to investigate means to enhance the IQE for high efficiency NUV-LEDs.

It was reported that the efficiency of GaN-based blue and green LEDs can be enhanced by using localized surface plasmon (LSP) resonance in the visible region [11, 12]. Surface plasmons (SPs) are collective oscillations of free electrons in metals at the interfaces between metals and dielectric materials [13, 14]. In particular, the collective oscillations of electrons in metal nanoparticles (NPs) embedded in a dielectric material are LSPs. In LEDs with NPs, excitons in the QW can be affected by a resonant coupling to SP modes within the penetration depth of the SP fringing field. The spontaneous emission rate in QW can be enhanced by the influence of the local electric field from the SP mode [15]. Silver (Ag) and platinum (Pt) NPs have been reported to have SP characteristic wavelengths for blue and ultraviolet emission by photoluminescence (PL) and absorption measurements [11, 16]. However, investigations on the coupling between NUV emission of NUV-LEDs and LSPs in metal NPs have not been reported even though these are expected to improve the IQE of NUV-LEDs. In this study, we successfully demonstrated the LSP-enhanced NUV-LEDs using Ag and Pt NPs embedded in p-GaN layer. The NUV-LEDs with p-GaN containing either Ag or Pt showed good electrical properties and the increased optical output power. These results were attributed to an increase in IQE by LSPs resonance coupling between excitons in the multiple quantum wells (MQWs) and metal NPs in the p-GaN layer. Pt NPs showed more advantages over Ag NPs with respect to the particle density and the SP characteristic peak. The high density of Pt NPs, which can increase the penetration depth, could be easily achieved and a closer match between the SP characteristic wavelength of Pt NPs and an emission wavelength of NUV-LEDs was highly desirable for high efficiency NUV-LEDs.

2. Experiments

The NUV-LEDs were grown on a c-plane (0001) sapphire by metal-organic chemical vapor deposition (MOCVD). Figure 1(a) shows a schematic diagram of LSPs-enhanced NUV-LEDs with metal NPs. 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 Si-doped n-GaN layer were grown at 1020 °C. Then, five periods of InGaN/GaN NUV-MQWs consisting of 3 nm-undoped InGaN wells and 12 nm-Si doped GaN barriers were grown at 770 °C, followed by a 20 nm-thick p-GaN layer grown at 950 °C as a spacer layer between MQWs and metal NPs. To estimate the effective thickness of p-GaN spacer layer between MQWs and metal NPs layer, the penetration depth of the SP fringing field was first calculated from Z = (λ/2π)[(ε´_GaN - ε´_metal)/ε´_metal²]^1/2, where ε´_GaN and ε´_metal are the real part of the dielectric constants for GaN and metal, and λ is the wavelength of the SP resonance [13]. The penetration depth of SP fringing field for Ag and Pt was calculated as 41.0 and 38.9 nm for SP-QW coupling, respectively, at an NUV emission wavelength of 406 nm when the real parts of the dielectric constants for GaN and metal were used [17, 18]. However, Jang et al. showed that the penetration depth of Ag NPs surrounded by dielectric materials is estimated to be in the range of 8-18 nm with increasing the number of Ag NPs from single NP to 7 NPs [19]. In this study, the NUV-LEDs with metal NPs and a p-GaN spacer layer with a thickness of less than 10 nm showed a poor electrical performance presumably due to the rapid thermal annealing (RTA) process for metal NP formation and the subsequent overgrowth of 200 nm-thick p-GaN on metal NPs. To study the resonance coupling effect of LSPs and MQWs of NUV-LEDs with stable operation, a 20 nm-thick p-GaN was used as a spacer layer in this study. The Ag or Pt layer was deposited on the p-GaN spacer layer by e-beam evaporation and metal NPs were formed by RTA process. A p-GaN layer was overgrown on the metal NPs and metal NPs were embedded in the 200 nm-thick p-GaN layer. To fabricate NUV-LEDs, a dry etching process was performed until the n-GaN layer was exposed for n-type ohmic contact by an inductively-coupled plasma etching process using Cl₂/CH₄/H₂/Ar source gases. Then, the NUV-LEDs, each of size 300 × 300 µm², were fabricated by using 150 nm-thick indium tin oxide as a transparent current spreading layer and Cr (50 nm)/Au (150 nm) layers were deposited as n- and p-contact by e-beam evaporation. Figure 1(b) and 1(c) show scanning electron microscopy (SEM) images of metal NPs on the p-GaN spacer layer after the RTA process, performed at 600 °C for 5 min under an N₂ ambient gas prior to the over-growth of the p-GaN layer on the metal NPs. The thin metal layer was transformed into NPs via the Ostwald ripening process. The diameter and density of Ag NPs are in the range of 35-75 nm and 2 × 10⁹ cm⁻², respectively, whereas those of Pt NPs are in the range of 10-40 nm and 4 × 10¹⁰ cm⁻², respectively. The density of Pt NPs was always higher by ~20 times than Ag NPs on p-GaN layer under the same deposition and annealing conditions.

 

Fig. 1 (a) Schematic diagram of the LSP-enhanced NUV-LEDs with metal NPs in p-GaN layer. SEM images of (b) Ag NPs and (c) Pt NPs on the 20 nm-thick p-GaN spacer layer after a rapid thermal annealing process.

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3. Results and discussion

Figure 2(a) , 2(b), and 2(c) show the PL spectra at 10 K and 300 K of NUV-LEDs with and without metal NPs, respectively. The IQE of NUV-LEDs was estimated by comparing the integrated PL intensities, assuming that the IQE is 100% at 10 K [13]. The IQEs of NUV-LEDs with Ag, Pt, and without metal NPs are calculated to be 52.2%, 60.5% and 41.8%, respectively, and the IQEs of NUV-LEDs with Ag and Pt NPs are increased by 24.9% and 44.7%, respectively, compared with that of NUV-LEDs without metal NPs. It has been reported that the probability of coupling between the excitons in MQWs and LSPs increases with increasing density of metal NPs in the p-GaN layer [20]. The fact that the enhancement in IQE of NUV-LEDs with Pt NPs is higher than that of NUV-LEDs with Ag NPs is attributed partly to the particle density of Pt which is higher than that of Ag NPs. Figure 2(d) shows the PL enhancement ratio of the PL intensity of NUV-LEDs with metal NPs to that of NUV-LEDs without metal NPs. The inset of Fig. 2(d) shows the absorbance of metal NPs as a function of wavelength. As shown in the inset of Fig. 2(d), the peak position of absorbance of Pt NPs is closer to a 406 nm emission wavelength of NUV-LEDs compared with that of Ag NPs. The absorbance spectra of Pt NPs is much broader than that of Ag NPs due to the different nature of SPs in Pt and Ag nanostructures [21] and high density of Pt NPs. The high density of Pt NPs can also cause a large penetration depth in MQWs, resulting in a strong SP resonance with excitons in MQWs. For this reason, the PL enhancement ratio of NUV-LEDs with Pt NPs is higher than that of NUV-LEDs with Ag NPs as shown in Fig. 2(d) because the peak position of absorbance of Pt NPs, which represents the SP characteristic wavelength of Pt NPs is closer to the 406 nm emission wavelength of NUV-LEDs and the density of Pt NPs is higher than that of Ag NPs.

 

Fig. 2 PL spectra at 10 K and 300 K of the NUV-LEDs; (a) without metal NPs, (b) with Ag NPs, and (c) with Pt NPs. (d) PL enhancement ratio of PL intensity of the NUV-LEDs with Ag and Pt NPs to that of the NUV-LED without metal NPs. The inset shows the absorbance of Ag and Pt NPs as a function of wavelength.

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Figure 3 shows time-resolved PL (TR-PL) spectra of NUV-LEDs with and without metal NPs. The light source was a Ti:sapphire laser (MaiTai, Spectra Physics) with 100 fs pulse width, 710 nm wavelength, and repetition rate of 80 MHz, which was further reduced to 4.44 MHz using a pulse picker to obtain a temporal window as long as 200 ns. The beam was frequency doubled to 355 nm by using a β-BaB₂O₄ (BBO) crystal. The collected PL was dispersed by a 30-cm monochromator and detected by a streak scope (C10627, Hamamatsu K.K.) to obtain PL decay curves. The TR-PL measurement was performed at 10 K to remove non-radiative recombination process that competes with the LSPs coupling. The decay lifetime τ is defined such that the ideal radiative decay curve can be shown as I_(t) = Ae^-t/τ, where A is the normalization constant. The fast part of each decay curve is fitted with a monoexponential decay model to investigate the resonance coupling between excitons in the MQWs and LSPs of the metal NPs. The decay time of NUV-LEDs with Ag and Pt NPs is 3.77 ns and 3.68 ns, respectively, whereas that of NUV-LEDs without metal NPs is 4.26 ns. The faster decay time of NUV-LEDs with metal NPs can be attributed to resonance coupling between MQWs and LSPs. When the energy of the excitons in QW is close to the electron vibrational energy of the LSP in the metal NPs, the recombination rate of excitons can be increased via fast recombination path due to the energy transfer from excitons to LSPs [13, 14].

 

Fig. 3 Time resolved-PL spectra of the NUV-LEDs with and without metal NPs at 10 K. (a) without metal NPs, (b) with Ag NPs, and (c) with Pt NPs.Sample figure.

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Figure 4(a) shows the current-voltage (I-V) curves of NUV-LEDs with and without metal NPs. The forward voltages of NUV-LEDs with and without metal NPs are nearly the same as 3.9 V at 20 mA. Additionally, the series resistance of NUV-LEDs with Ag, Pt NPs, and without metal NPs is 16.1 Ω, 16.0 Ω, and 16.1 Ω, respectively. These results indicate that the electrical properties of NUV-LEDs with metal NPs are not degraded by the metal NPs embedded in p-GaN.

 

Fig. 4 (a) I-V curves of the NUV-LEDs with and without metal NPs. (b) Optical output power of the NUV-LEDs with and without metal NPs as a function of injection current.

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Figure 4(b) shows the optical output power of the NUV-LEDs with and without metal NPs as a function of injection current. The optical output power of the NUV-LEDs was measured at a distance of 5 cm from the top of the NUV-LEDs by Si photodiode. As shown in Fig. 4(b), the optical output power of the NUV-LEDs with metal NPs is higher than that of the NUV-LEDs without metal NPs. Compared with NUV-LEDs without metal NPs, the optical output power of NUV-LEDs with Ag and Pt NPs is increased by 20.1% and 57.9% at an injection current of 20 mA, respectively. The measurements of IQE and TR-PL of the NUV-LEDs show that the enhancement of optical output power is attributed to the increase in the spontaneous emission rate by resonance coupling between excitons in the MQWs and LSPs in the metal NPs.

4. Summary

In summary, we investigated the LSP-enhanced NUV-LEDs by using Ag or Pt NPs embedded in p-GaN layer. The optical output power of NUV-LEDs with metal NPs is increased by 20.1% for NUV-LEDs with Ag NPs and 57.9% for NUV-LEDs with Pt NPs at 20 mA compared with that of NUV-LEDs without metal NPs. The IQE of NUV-LEDs with metal NPs is increased by 24.9% for NUV-LEDs with Ag NPs and 44.7% for NUV-LEDs with Pt NPs compared with that of NUV-LEDs without metal NPs. The TR-PL measurement shows the PL decay time is decreased in NUV-LEDs with metal NPs embedded in p-GaN layer. These results indicate that the increase in optical output power of the NUV-LEDs with metal NPs is attributed to an increase in IQE by LSPs resonance coupling between excitons in the MQWs and metal NPs in the p-GaN layer. In addition, Pt NPs are found to be more suitable for LSP-enhanced NUV-LEDs due to the easy formation of high density NPs and a closer match between the SP characteristic wavelength of Pt NPs and an emission wavelength (406 nm) of NUV-LEDs, compared to Ag NPs.