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We have fabricated the surface plasmon (SP) coupled GaN-based nanorod LEDs with Ag nanoparticles (Nps), and demonstrate the enhancement of the optical modulation bandwidth by SPs. Compared with the LED without Ag Nps, the optical modulation bandwidth of the LED with Ag Nps increases by a factor of ~2 at 57 A/cm2. The photoluminescence (PL) and electroluminescence (EL) experimental results are consistent with each other, and both suggest the effective coupling between quantum wells (QWs) and SPs. Furthermore, the current dependent modulation frequency characteristics show that the QW-SP coupling can increase the modulation bandwidth, especially for LEDs with high intrinsic internal quantum efficiency (IQE). These findings will help to open a new solution to design the ultrafast LED light source for the application of the visible light communication.
© 2015 Optical Society of America
1. Introduction
High-power GaN-based LEDs offer many advantages over incandescent, fluorescent, and discharge light sources, including longer lifetime, smaller size, higher energy efficiency and faster response. With the increase of the luminous efficacy, LEDs are fast replacing traditional light sources in numerous applications, such as illumination and displays. Besides the advantages for illumination, the use of LEDs also offer the possibilities for high data-rate communication, which is very important for their potential application of visible light communication (VLC) [1–4]. However, the optical modulation bandwidth of conventional commercial LEDs is still quite low [5, 6], which restricts the development and application of VLC.
Normally there are two major factors that influence the optical modulation bandwidth of LEDs: the RC time constant and the carrier recombination time [7, 8]. So far, many methods have been reported to improve the optical modulation bandwidth of LEDs in terms of reducing the RC time constant, such as decreasing the thickness of the active layer [9], using barrier-doped multiple quantum wells (MQWs) structure [7, 9, 10], and decreasing the effective active area [7, 11–13]. However, once the size of the LEDs reaches the limit, the RC time cannot be further decreased and the modulation bandwidth will be mainly restricted by the carrier recombination time. In addition, decreasing the chip size at the same current density will also reduce the optical power, which defeats the purpose of VLC applications. In this case, reducing the carrier recombination time in the active region will be an effective solution to increase the optical modulation bandwidth for LEDs. In order to maintain high illumination efficiency, it is preferable to shorten the recombination lifetime τr, which can be expressed by hole concentration P0, electron concentration N0 in thermal equilibrium, excess carrier concentration Δp (Δn = Δp) and the radiative recombination constant B as follows [14, 15]:
It can be seen that increasing the injected current density to introduce a large carrier concentration in the active region can reduce the carrier lifetime. Therefore, methods such as decreasing the effective active area (μLED) [7, 11–13] and decreasing the junction temperature [16] have been used to realize a larger maximum withstand current density. However, the overdrive will decrease the optical power (the so-called droop effect) [17, 18] and reliability of LEDs. Although the resonant-cavity LED (RCLED) is an alternative solution to realize both high speed and high power [16, 19, 20] by increasing the radiative recombination constant B, fabrication of the GaN-based RCLEDs is quite challenging [21] owing to difficulties in producing micro-cavities in this alloy system.It has been reported that SPs can effectively increase the carrier spontaneous emission rate (the radiative recombination constant B) [22–24], which is attributed to the new energy transition channel of electron-hole pairs in LEDs created by the QW-SP coupling. Since the density of states of SP mode is much large, the QW-SP coupling rate will be very fast, and will increase the spontaneous emission rate and decrease the carrier recombination time accordingly. Over ten-folds reduction of carrier lifetime has been achieved in the InGaN single quantum well [22], the GaAsP PIN double heterostructure [25] and the core-shell nanowire LED [26]. Recently, fluorescence lifetime measurements on emitters of plasmonic nano-antennas reveal a spontaneous emission rate enhancement exceeding even 1000 times [27], which cannot be achieved by only increasing the injected current density. In addition, if the scattering efficiency of SPs is high enough compared to the IQE of MQWs [24, 28], the optical intensity can also be increased simultaneously. Therefore, the QW-SP coupled LED is an attractive alternative solution for VLC, which will increase the optical modulation bandwidth without greatly increasing current density. Nevertheless, most of the results are based on the optical pumping and simple quantum well structure. Since the SP is an evanescent wave that exponentially decays with increasing the distance from the metal surface, the nanostructure will be beneficial for decreasing the SP coupling distance in LEDs with electrical injection. Whereas all of them are focused on increasing the luminous efficacy for GaN-based LEDs [29–32], and there has been no report on the optical modulation bandwidth of electrically injection LEDs based on the QW-SP coupling.
In this study, we fabricated QW-SP coupled LEDs based on the nanorod structure, with Ag Nps laterally proximity to the MQWs region. In order to guarantee a larger proportion of MQWs region coupled to SPs, the radius of the nanorods was chosen to be around 70 nm which is comparable to the penetration depth of the SP [24, 32]. Compared to the LED without SP coupling, the optical modulation bandwidth of the LED with Ag Nps increased by a factor of ~2 at 57 A/cm2, which is mainly due to the QW-SP effective coupling. In addition, for LEDs with a higher IQE, the enhancement of the bandwidth is more significant. Our findings will pave an alternative solution for high speed LEDs.
2. Experiment
GaN-based LED epilayers were grown on a c-axial sapphire (0001) substrate using metal organic chemical vapor deposition (MOCVD). The epitaxial structure consists of 18 pairs of InGaN/GaN (3nm/12nm) MQWs sandwiched in a 220 nm thick Mg-doped p-type GaN and a 3 μm thick layer of Si-doped n-type GaN. Figure 1 shows the schematic procedures for fabricating the GaN-based nanorod LEDs with Ag Nps. For the nanorod array structure fabrication, a 35 nm thick Al2O3 layer was firstly deposited on top of the p-GaN using plasma-enhanced CVD (PECVD) followed by the deposition of 10 nm thick Ni layer using an e-beam evaporator. Subsequently, a rapid thermal annealing (RTA) process was implemented to form self-assembled Ni metal clusters as etching masks, then the Al2O3 layer, p-GaN and active layer were etched by inductively coupled plasma (ICP), where the etched depth of the active layer was ~25 nm. After that, a 10 nm thick HfO2 layer was used to cover the nanorod and active layer before coating the Ag film (9 nm). In this case, the Ag can be separated from the p-GaN and the MQWs. After a RTA under N2 at 550 °C, the Ag Nps in Fig. 1(c) were obtained. In order to fill up the space between the nanorods, a 500 nm thick spin-on-glass (SOG) was spun onto the structure. The SOG was firstly baked with a hot plate at 110 °C for 1 min and then in a furnace at 550 °C for 30 min. The cured SOG and the remaining Ag Nps on the top of the devices were removed to expose the p-GaN. After ITO deposition, ICP etching of a mesa and the metal electrode deposition, the nanorod LEDs with Ag Nps were finally obtained, as illustrated by Fig. 1(f). The preparation of nanorod LEDs without Ag Nps is similar, but without the Ag deposition and the following RTA process.
The PL measurements were carried out on the LED wafers with a configuration of top excitation and top detection at room temperature and 10 K, respectively. A 405 nm laser diode with a power of 10 mW was employed as the excitation source. The EL of the encapsulated chips was measured in an integrating sphere in the continuous-wave current mode at room temperature.
3. Results and discussions
Figure 2 shows the SEM images of the nanorod LED with Ag Nps during the fabrication. In Fig. 2(a), 35 nm thick Al2O3 is covered on the top of the 245 nm nanorods. The etched depth of MQWs is roughly about 25 nm. This distance can guarantee the effective lateral energy coupling between the QWs and the SPs [24, 33]. The nanorods covered with HfO2 are shown in Fig. 2(b). The 10 nm thick HfO2 layer coats the nanorods uniformly, and makes an effective isolation between Ag Nps and nanorods. This thickness meanwhile determines the coupling distance between SPs and MQWs. The filling factor of nanorods is estimated as ~40% and the active area is calculated as 8800 μm2. Figure 2(c) shows the morphology of the annealed Ag Nps, where Ag Nps at the bottom are different from those on the top of the nanorods. The size at the bottom is slightly smaller with the diameter range between 25 nm and 45 nm, which ensures both the energy match and a large SP field enhancement. The Ag Nps at the top were then removed to expose the GaN layer. Figure 2(d) shows the nanorod array covered by the etched SOG. The filling SOG acts as an efficient electric insulating layer between MQWs and the following ITO.
Fig. 2 SEM images of (a) the cross section of the nanorod array, top view of (b) the nanorod array covered by HfO2 and (c) Ag nanoparticles, and (d) tiled view of the nanorod array covered by the etched SOG.
Figure 3 shows the PL spectra of the nanorod LED with and without Ag Nps at room temperature and 10 K. Assuming the IQE at low temperature (10 K) is equal to 100% [34], by calculating the integrated PL intensity at both room temperature and 10 K, the IQE of nanorod LED with and without Ag Nps at room temperature is 61% and 51%, respectively. Compared to nanorod LED without Ag Nps, the PL intensity of LED with Ag Nps was suppressed by 4.6 times with a red shift of 4 nm from 532 nm to 536 nm. Both the enhancement of IQE and the changes of the PL spectra with Ag Nps suggest the effective QW-SP coupling, where the energy of excitons in MQWs is coupled and transferred to SPs. The suppression of the PL intensity is due to the high IQE of the MQWs region and the low energy scattering efficiency of Ag Nps. Since the size of Ag Nps here is small, most of the SP energy was dissipated instead of scattered [35–37]. These Ag Nps with low scattering efficiency have difficulties increasing the PL intensity of MQWs with the high intrinsic IQE (51%) [24, 28]. Although the luminous efficacy is very low, the QW-SP coupling can create a new energy conversion channel for the excitons in QWs, and increase spontaneous emission rate to realize a high speed LED. In addition, the small size Ag Nps have a small effective mode volume and a high quality factor, which will lead to a higher Purcell factor compared with bigger ones [38–40], and thus can effectively enhance the spontaneous emission rate [41]. This is also the reason that the IQE for the nanorod LED increases from 51% to 61% after Ag incorporation. The PL emission ratio spectrum is shown in Fig. 3(b), which uses the corresponding nanorod LED for normalization. The ratio reaches the minimum value at 526 nm. For comparison, the inset shows the transmission spectra of Ag Nps on glass, which have the similar size to the Ag Nps in the nanorod LED. Consistent with the emission ratio spectrum, the resonant absorption peak at 516 nm of the transmission spectra also locates at the high energy side of the PL emission peak. Therefore the large absorption and weak scattering at high energy region caused by QW-SP coupling lead to the 4 nm red shift [23].
Fig. 3 (a) PL spectra of the nanorod LED without Ag Nps and nanorod LED with Ag Nps at room temperature (dashed lines) and 10 K (solid lines). (b) PL intensity ratio spectra of nanorod LED with Ag Nps to that without Ag Nps. The inset shows the transmission spectrum of similar Ag Nps on glass.
The J-V characteristics of the two devices are shown in Fig. 4(a). The two curves coincide with each other very well. Because of the large equivalent series resistance caused by the tiny active area, the turn-on voltages for both devices are around 4 V. While at negative voltage −5 V, the reverse current is roughly about 5 μA, which indicates that there is not much etching damage during the fabrication of nanorods. The EL spectra of these two devices operated at current density 57 A/cm2 is shown in Fig. 4(b). The change of the EL spectra is quite similar to that of the PL spectra. Compared to the nanorod LED without Ag Nps, the EL intensity of nanorod LED after Ag incorporation have been decreased with a red shift of 1 nm from 524 nm to 525 nm. The suppression of EL intensity can also be attributed to the high IQE and the low scattering efficiency. While the difference between the PL and EL results is mainly due to the discrepancy of these two measurement methods [38, 39]: The PL measurement consists of two stages: one is the optical excitation stage and the other is the light emission. The absorption of the pumping light by the Ag Nps during the optical excitation does not exist in the EL measurement. So the suppression and the red-shift of PL are larger than that of EL. The EL emission ratio at various injection current as a function of the wavelength is illustrated in Fig. 4(c). A minimum EL emission ratio of 0.49 is observed at 68 A/cm2, and the peak locates just between the emission peak and the SP resonance peak, which is also comparable with the PL emission ratio in Fig. 3(b). In order to further understand how the injection current influences the EL emission ratio, current dependent EL measurements for these two devices were measured, as shown in Fig. 4(d). Integrated EL intensity of the LED with Ag Nps is always smaller than that of the nanorod LEDs at different current density. With increasing the injection current density, the EL intensity ratio of nanorod LED with Ag Nps compared to LED without Ag Nps decreases from 0.67 to 0.56, as shown in the inset of Fig. 4(d). This is mainly due to the increased IQE, where the IQE is directly related to the external quantum efficiency (EQE) and the light extraction efficiency (LEE), as IQE = EQE/LEE. The EQE of both LEDs increases within the injection current density from 0 to 68 A/cm2, which can be extracted from Fig. 4(d). Since LEE is a constant at different current density, we can conclude that the IQE increases with current density increasing in this region. In addition, as discussed above, it will become more difficult to enhance the EL intensity by the Ag Nps incorporation with current density increasing, or in another word, the EL intensity ratio for the nanorod LED with Ag Nps compared to the LED without Ag Nps will be reduced more obviously. More details about the current dependent EL by SPs will be further investigated in the future.
Fig. 4 (a) Current density vs. voltage characteristics for the nanorod LED without Ag Nps (black dashed line) and nanorod LED with Ag Nps (red solid line) (b) EL spectra for the nanorod LED without Ag Nps and with Ag Nps at 57 A/cm2. (c) EL emission ratio spectra of nanorod LED with Ag Nps compared with that without Ag Nps at various injection current densities. (d) Integrated EL intensity in the visible range as a function of the injected current density. The inset shows the integrated EL intensity ratio of nanorod LED with Ag Nps to that without Ag Nps as a function of current density.
In order to further investigate the influence of the QW-SP coupling on the spontaneous emission rate, current dependent optical modulation frequency responses from 500 kHz to 40 MHz were also measured using a network analyzer (Agilent E5061B). The experimental details were reported elsewhere [15]. At 57 A/cm2, the optical 3-dB modulation bandwidth (f3dB) increased from 15.1 MHz for the nanorod LED without Ag Nps to 29.8 MHz for the LED with Ag Nps as shown in Fig. 5(a). Since the active area of two devices is 8800 μm2, the capacitances of both LEDs are small, measured as ~3 pF at 57 A/cm2. The series resistances of the nanorod LEDs without and with Ag Nps can be obtained from Fig. 4(a), as ~89 Ω and ~67 Ω, respectively. In this case, the RC-limited bandwidths of nanorod LED with and without Ag Nps are about 790 MHz and 590 MHz, respectively. Both of them are much larger than the measured modulation bandwidth ~30 MHz. Therefore, the modulation bandwidths of both devices are mainly limited by the carrier recombination lifetime. This ~2 times enhancement of the optical 3-dB modulation bandwidth of the nanorod LED with Ag Nps indicates that the QW-SP coupling can effectively raise the spontaneous emission rate, and shorten carrier spontaneous recombination lifetime. The modulation bandwidths for the two devices at different injection currents are also shown in Fig. 5(b), where the bandwidth for both devices increases with increasing the injected current. It is because that the carrier lifetime can be affected by the carrier density or injected current density [8, 11]. Assuming a bi-molecular recombination mechanism, the modulation bandwidth can be expressed as below [14, 42]:
where τeff is the effective carrier lifetime, B is the radiative recombination constant, J is the injected current density, e is the elementary charge and d is the thickness of the active layers. Therefore, the increase of the modulation bandwidth will become slower as the current density increases. Similar results (relationship between bandwidth and injected current) were also reported [11]. The modulation bandwidth ratio of the LED with Ag Nps to the LED without Ag Nps is also shown in Fig. 5(b), which increases from 1.1 at 11 A/cm2 to 1.9 at 57 A/cm2. The increase of the modulation bandwidth ratio with increasing the injection current density may be attributed to the increase of IQE ηint, which is mainly determined by τr and nonradiative recombination lifetime τnr as follows:Considering the QW-SP coupling, the effective carrier lifetime can be written as below:where F is the Purcell factor. F = 1 represents the nanorod LED without Ag Nps. From (2)-(4), the modulation bandwidth ratio M can be obtained:It can be seen that the modulation bandwidth ratio M increases with increasing the IQE. For LEDs with a higher IQE, radiative recombination will contribute more significantly to the optical modulation bandwidth. In this case, QW-SP coupling will be more effective to realize high speed LEDs.Fig. 5 (a) Optical response of the nanorod LED without Ag Nps (black solid line) and nanorod LED with Ag Nps (red solid line) at 57 A/cm2. (b) The optical 3-dB bandwidth modulation as a function of injection current density for the nanorod LED without Ag Nps (black dashed line) and nanorod LED with Ag Nps (red solid line). The error bars come from the experimental measurements. The right axis gives the bandwidth ratio of nanorod LED with Ag Nps compared to nanorod without Ag Nps.
4. Conclusion
The QW-SP coupled nanorod LEDs were successfully fabricated and the optical characteristics were studied using both photonic and electrical pumping. Compared to the nanorod LED without Ag Nps, both the PL and EL spectra showed a red shift for the nanorod LED with Ag Nps, which indicated the existence of the effective coupling between QWs and SPs. In addition, the current dependent optical modulation frequency responses of the QW-SP coupled LED were also measured. Compared with the nanorod LED without Ag Nps, the enhancement of the optical 3-dB modulation bandwidth of the LED with Ag Nps increased by ~2 times to 29.8 MHz with the injection current density of 57 A/cm2. Our results demonstrated that the QW-SP coupling can effectively enhance the carrier spontaneous emission rate to increase the modulation bandwidth for LEDs, especially for LEDs with high intrinsic IQE. Our findings could pave a way to design the ultrafast LED light source for the application of the visible light communication.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grants No. 61334009), the China International Science and Technology Cooperation Program (Grants No. 2014DFG62280), the National High Technology Program of China (2015AA03A101) and “Import Outstanding Technical Talent Plan” of Chinese Academy of Sciences.
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