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Abstract
The efficiency enhancement of light color conversion from blue quantum well (QW) emission into red quantum dot (QD) emission through surface plasmon (SP) coupling by coating CdSe/ZnS QDs on the top of an InGaN/GaN QW light-emitting diode (LED) is demonstrated. Ag nanoparticles (NPs) are fabricated within a transparent conductive Ga-doped ZnO interlayer to induce localized surface plasmon (LSP) resonance for simultaneously coupling with the QWs and QDs. Such a coupling process generates three enhancement effects, including QW emission, QD absorption at the QW emission wavelength, and QD emission, leading to an overall enhancement effect of QD emission intensity. An Ag NP geometry for inducing an LSP resonance peak around the middle between the QW and QD emission wavelengths results in the optimized condition for maximizing QD emission enhancement. Internal quantum efficiency and photoluminescence (PL) decay time measurements are performed to show consistent results with LED performance characterizations, even though the QD absorption of PL excitation laser may mix with the SP-induced QD absorption enhancement effect in PL measurement.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Light color conversion is an important process for white-light generation and color display. For white-light generation application, various phosphors are used to absorb shorter-wavelength photons (typically blue light) and emit lights of longer wavelengths, such as green, yellow, or red light, to mix into white light [1–3]. For color display application with a monolithic structure, color conversions from a common blue-emitting base through certain quantum dot (QD) phosphors can provide us with desired colors in different pixels [4–8]. Because of its brightness and long-lifetime of GaN-based light-emitting diode (LED), the use of a micro-LED array for color display has become an attractive technique. In implementing such a color micro-LED array, one promising approach is to coat green- and red-emitting QDs on blue-emitting LEDs in the pixels of designated green and red emissions, respectively [9–12]. In this approach, the color conversion efficiency from shorter-wavelength light into longer-wavelength emission is a crucial factor.
The conversion from a shorter-wavelength photon into a longer-wavelength photon involves three processes, including the emission of a shorter-wavelength emitter, the absorption at the shorter wavelength by a longer-wavelength emitter, and the emission of the longer-wavelength emitter. The efficiency enhancement of either process can help in increasing the overall light color conversion efficiency. Surface plasmon (SP) coupling has been used for improving the performance of an LED, including the enhancements of internal quantum efficiency (IQE) and electroluminescence (EL) intensity [13–18], the suppression of the efficiency droop effect [19–22], the increase of LED modulation bandwidth [20–22], and the generation of polarized LED output [23,24]. SP coupling can also be used to enhance material absorption for sunlight harvest in a solar cell [25,26]. In other words, when SP couples with a light absorber, such as a QD, its absorption efficiency can be enhanced. Therefore, if the spectrum of an SP resonance feature based on a certain metal nanostructure, such as surface Ag NPs, can cover the emission wavelengths of both emitters, SP couplings with the two emitters can simultaneously occur for enhancing the efficiencies of all the aforementioned three processes. The combination of those enhancement effects can significantly increase the overall color conversion efficiency. In the case of using Ag NPs for inducing SP resonance, due to the combination of homogeneously and in-homogeneously broadening effects of size-distributed Ag NPs, the SP resonance spectrum can be quite broad, up to 150 nm in full-width at half-maximum, for simultaneously coupling with the two emitters. Also, the resonance peak can be controlled by the average size of NPs and the refractive index of the surrounding medium. By controlling these parameters, we can adjust the relative SP coupling strengths at the emission wavelengths of the two emitters for maximizing the overall color conversion efficiency.
In this study, we spin-coat red-emitting QDs on a blue-emitting quantum-well (QW) LED for converting the QW-emitted blue photon into QD-emitted red light. By fabricating Ag NPs in an interlayer between the LED structure and the coated QDs for inducing localized surface plasmon (LSP) resonance, we demonstrate the SP coupling effects for enhancing QW emission, QD emission, and QD absorption. Besides LED performance characterizations, optical properties, like IQE, are analyzed for supporting the interpretations of the LED performances. In this paper, we describe the sample structures and fabrication procedures in section 2. The fabricated LED performances are reported in section 3. Then, the optical characterization results are presented in section 4. Discussions about the SP coupling effects on color conversion mechanisms are made in section 5. Finally, conclusions are drawn in section 6.
2. Sample structures and fabrication procedures
To understand the SP coupling effects on the enhancement of color conversion efficiency, we prepare five LED samples for comparison, as schematically demonstrated in Figs. 1(a)-1(e). Sample A shown in Fig. 1(a) represents a reference sample, which is fabricated with the fundamental LED epi-structure. The epi-structure consists of a p-i-n structure, including three periods of InGaN/GaN QW emitting light around 465 nm in wavelength as the intrinsic active layer. The p-type layers include a 15-nm p-AlGaN electron blocking layer, a 40-nm p-GaN current spreading layer, and a 10-nm p+-GaN contacting layer. In sample B shown in Fig. 1(b), a 40-nm Ga-doped ZnO (GaZnO) transparent conducting layer is deposited onto the fundamental LED epi-structure. The GaZnO layer is grown with molecular beam epitaxy at 250 °C in substrate temperature [27,28]. Then, in sample C shown in Fig. 1(c), red-emitting CdSe/ZnS QDs are spin-coated on the structure of sample B three times with the QD weight percentage in toluene solution at 10%. In each time of spin-coating, two drops of QD solution are placed on device top followed by spinning with 500 rpm for 20 s and then 1000 rpm for 30 s, and baking at 100 °C for 2 min. Next, in sample D shown in Fig. 1(d), Ag NPs are fabricated on a 10-nm GaZnO interlayer, which is deposited on the fundamental LED epi-structure. After Ag NP fabrication, another GaZnO layer of 30 nm in thickness is deposited on the top. In this sample, QDs are also spin-coated on the top of the 30-nm GaZnO layer following the same procedure for sample C. Finally, the structure of sample E, as shown in Fig. 1(e), is the same as that of sample D except that no QD is coated on the top. Figure 2(a) shows a scanning electron microscopy (SEM) image demonstrating the layout of LED devices in one of samples D. Here, in each device, the central circular area corresponds to the p-contact. The cross-sectional SEM image of a device around the circled region is shown in Fig. 2(b). Here, the layer between the two horizontal dashed red lines corresponds to the QD layer, which is estimated to be 500-700 nm in thickness. The large bright spots below the QD layer in Fig. 2(b) correspond to the QD clusters attached to the sidewalls of the device mesa.
Fig. 1 (a)-(e): Schematic demonstrations of the structures of samples A-E, respectively. Circular yellow dots represent Ag NPs. Triangular red dots stand for QDs.
Fig. 2 (a): SEM image showing the layout of LED devices in one of samples D. (b): cross-sectional SEM image of a device around the circled region in part (a).
To compare the SP coupling effects under the different conditions of different LSP resonance spectra, we control the Ag NP geometry for achieving three different LSP resonance peak positions in spectrum. The SEM images in Figs. 3(a)-3(c) show the Ag NPs fabricated by depositing Ag films of 1, 1.3, and 1.6 nm in thickness, respectively, followed by thermal annealing at 100, 130, and 200 °C, respectively, for 30 min. The slightly increasing Ag NP size from Fig. 3(a) to 3(c) leads to the increasing LSP resonance peak wavelength from ~500, to ~540, and then to ~580 nm (precisely from 502, to 543, and then to 578 nm), as shown in Fig. 4, in which the depression in each transmission spectrum corresponds to LSP resonance. For clarity, samples D and E with the Ag NPs for inducing different LSP resonance peaks are designated as D/E-500, D/E-540, and D/E-580. The three vertical dashed lines in Fig. 4 indicate the QW and QD emission wavelengths at ~465 and ~630 nm, respectively, and the photoluminescence (PL) excitation laser wavelength at 405 nm (to be discussed later). It is noted that at a certain wavelength, the depth of a transmission spectrum corresponds to the LSP resonance strength.
Fig. 3 (a)-(c): SEM images of Ag NPs for generating LSP resonances with peak wavelengths around 500, 540, and 580 nm, respectively.
Fig. 4 Transmission spectra of the three Ag NP structures shown in Figs. 3(a)-3(c) illustrating the LSP resonances with peak wavelengths around 500, 540, and 580 nm. The three vertical dashed lines indicate the QW and QD emission wavelengths, and the PL excitation laser wavelength.
3. LED performances
Figures 5(a) 5(d) show the EL spectra of samples C, D-500, D-540, and D-580, respectively, when injected current increases from 3 through 100 mA. The spectral features around 465 and 630 nm correspond to the QW and QD emissions, respectively. Because the used planar QD density is not high enough, the QD emission intensity is lower than that of QW in each sample. Compared to sample C, the QW emission intensity of sample D-500 is significantly increased. Also, compared to sample C, the QD emission intensities of samples D-540 and D-580 are clearly enhanced. Figure 6 shows the normalized integrated EL intensities of QW emission as functions of injected current (L-I curves) of all the samples under study. Here, the data are normalized with respect to the intensity at 100 mA of sample A. Figure 7 shows the normalized integrated EL intensities of QD emission as functions of injected current of the samples with coated QDs. Here, the data are normalized with respect to the intensity at 100 mA of sample C. The values of the normalized integrated EL intensities of QW and QD emissions are listed in rows 2 and 3 of Table 1, respectively. In QW emission, due to the enhanced current spreading and light extraction through the 40-nm GaZnO deposition, the EL intensities of samples B and C become higher than that of sample A by 5-6% [28]. With SP coupling in samples D-xxx and E-xxx, their EL intensities are further enhanced. Among the three samples of D-xxx or E-xxx, the EL intensities of D/E-500 are the highest, followed by D/E-540, and then D/E-580. This variation trend is consistent with the change of LSP resonance strength at 465 nm (see Fig. 4). Without QD absorption in samples E-xxx, their EL intensities are systematically higher than the individual counterparts of samples D-xxx. It is noted that even with QD absorption in sample C, its EL intensity is still slightly higher than that of sample B. This result can be attributed to the increased extraction efficiency of QW emitted light caused by the QD scattering on the top surface of sample C. The light extraction factor can also be applied to the comparison of EL intensity between samples D-xxx and E-xxx. However, the enhanced QD absorption through SP coupling in samples D-xxx results in lower EL intensities, when compared with those of samples E-xxx. The issue of enhanced QD absorption will be discussed in detail in section 5. In QD emission, although the LSP resonance strength at 630 nm is the strongest in sample D/E-580, followed by D/E-540 and then D/E-500 (see Fig. 4), as shown in row 3 of Table 1, the enhanced QD emission intensity is the highest in sample D-540, followed by D-580 and then D-500. The different variation trends imply that the enhanced QD emission is not simply caused by the SP-coupling induced emission enhancement at 630 nm.
Fig. 5 (a)-(d): EL spectra of samples C, D-500, D-540, and D-580, respectively, with injected current increased from 3 through 100 mA.
Fig. 6 EL intensities of QW emission as functions of injected current of all the samples under study.
Fig. 7 QD emission intensities as functions of injected current of the four samples with QD deposition.
Table 1. LED performance characterization and optical measurement results of all the samples under study.
Figure 8 shows the relation between injected current and applied voltage (I-V curves) of all the samples under study. One can see the reasonable turn-on voltage around 3 V. The insert of Fig. 8 shows the magnified I-V curves between 3.7 and 3.9 V for clearly differentiating those curves. The differential resistance values of all the samples under study are listed in row 4 of Table 1. By comparing samples A, B, and E-xxx, we can see that the current spreading effect of the GaZnO layer reduces resistance. From the comparisons between samples B, C, and D-xxx, one can see that the application of QDs on the top leads to slightly higher resistance. Then, by adding Ag NPs inside the GaZnO layers, the resistance values are further slightly increased as can be seen in the comparison between samples C and D-xxx or between samples B and E-xxx. Figure 9 shows the plots of reversed current in log scale versus applied voltage of all the samples under study. Here, the nine curves are close to each other. The reversed current levels of all the samples up to 10 V in reversed voltage are around 50 μA, indicating the low current leakage in all the fabricated LED samples.
Fig. 8 I-V curves of all the samples under study. The insert shows the magnified I-V curves in the applied voltage range between 3.7 and 3.9 V.
Fig. 9 Semi-log plots for the reversed current under reversed-bias voltage of all the samples under study.
4. Optical characterizations
To further understand the mechanisms behind the LED performances described above, we perform temperature-dependent and time-resolved PL (TRPL) measurements. The IQE of a sample can be obtained by taking the ratio of integrated PL intensity at 300 K over that at 10 K. The temperature-dependent PL measurement is excited by a 405-nm InGaN laser of 6 mW in power from the sapphire side. The PL signal is also collected from the sapphire side. It is noted that the QW IQE in this study is defined for a system including the QW structure and coated QDs. In other words, QD absorption affects the evaluation of QW IQE. The IQEs of QW and QD emissions in all the samples under study are listed in rows 5 and 6, respectively, of Table 1. Here, the numbers inside the parentheses correspond to the ratios of IQEs with respect to the individual reference samples, i.e., sample A for QW emission and sample C for QD emission. In QW emission, after the deposition the 40-nm GaZnO layer in sample B, its IQE is slightly reduced due to the change of the Fabry-Perot oscillation condition. In other words, the PL excitation intensity at the QW position is slightly reduced because of the change of sample thickness. Compared to sample A, the IQE of sample C is significantly reduced. Although this reduction is partly caused by the aforementioned change in the Fabry-Perot oscillation condition, it is mainly attributed to QD absorption. Because QD absorption is not significantly reduced with increasing temperature, the similar QD absorptions at 10 and 300 K results in the reduction of measured QW IQE. This attribution can also be applied to the generally higher IQEs in samples E-xxx, when compared with those of samples D-xxx. Without QD in samples E-xxx, their IQEs are higher than that of sample B due to SP coupling. With the strongest (weakest) SP resonance at 465 nm in sample E-500 (E-580), its IQE is the highest (lowest). The IQE variation trend among samples D-xxx is the same as that among samples E-xxx. However, the IQEs of samples D-xxx are significantly lower than those of samples E-xxx due to QD absorption. SP coupling leads to the higher QW IQEs in samples D-xxx, when compared to sample C. This attribution is also true for QD IQE variation, as shown in row 6 of Table 1. However, similar to that of QD emission intensity shown in row 3 of Table 1, the QD IQE variation does not follow the order of LSP resonance strength at 630 nm. Among samples D-xxx, the IQE is the highest in sample D-540. Figure 10 shows the TRPL intensity profiles of QW and QD emissions in all the samples under study. The TRPL measurement is excited by the second-harmonic (390 nm) of a femtosecond Ti:sapphire laser. Its signal is collected and analyzed with a photon counter. The PL decay times are obtained by fitting the slopes within the time range of 0.1-3 ns in the semi-log plots shown in Fig. 10. The PL decay times of the samples for QW and QD emissions are listed in rows 7 and 8, respectively, of Table 1.
5. Discussions
Although the variations of the QW-emitted EL intensities and IQEs among either samples D-xxx or E-xxx follow the order of SP resonance strength at 465 nm, those of the QD-emission intensities and corresponding IQEs of samples D-xxx do not. The highest QD emission intensity and IQE in sample D-540 among samples D-xxx is attributed to the enhanced absorption of QDs at the QW emission wavelength (~465 nm). As mentioned earlier, SP coupling can lead to three enhancement effects, including QW emission at ~465 nm, QD emission at ~630 nm, and QD absorption at ~465 nm, as schematically demonstrated in Fig. 11. Here, at the QW emission wavelength, the induced LSP resonance field of the Ag NPs simultaneously couple with QW and QD (a near-field process), which are represented by a continuous arrow and a dashed arrow, respectively, for enhancing QW emission and QD absorption. The simultaneous couplings of QW and QD with the LSP resonance implies the direct energy transfer from QW into QD. Although part of the energy of QD emission comes from QD absorption of QW emitted photons, which is a far-field process, the aforementioned near-field process of direct energy transfer plays a crucial role in QD emission enhancement. The excited electrons in a QD then relax to its emission upper level, as demonstrated by the curved arrow. The induced LSP resonance field at the QD emission wavelength can couple with the QD for enhancing its emission. Enhanced QD emission can result in faster electron relaxation from the absorption upper level into the emission upper state because of more available empty upper states such that QD absorption can be further enhanced. Therefore, QD absorption enhancement can be larger when the SP couplings at both QD absorption and emission wavelengths are strong. Figure 12 shows the absorption spectrum of the used QD, indicating the strong absorption of QD at the QW emission wavelength. Among the three Ag NP structures, although the one leading to LSP resonance peak around 500 nm can result in enhanced QW emission and QD absorption in sample D-500, its LSP resonance around 630 nm is weak and hence QD emission enhancement is weak. Also, although the Ag NP structure leading to LSP resonance peak around 580 nm can result in strong QD emission enhancement, its LSP resonance around 465 nm is weak and hence the enhancements of QW emission and QD absorption are weak. In these two cases, the overall effects for enhancing QD emission are not as strong as the Ag NP structure leading to LSP resonance peak around 540 nm, in which both of its LSP resonances at 465 and 630 nm are reasonably strong for simultaneously increasing QW emission, QD absorption, and QD emission. Therefore, with this Ag NP structure, we can observe the highest QD emission intensity and IQE among samples D-xxx. Hence, for enhancing color conversion efficiency, LSP resonance with its peak located around the middle between the short and long emission wavelengths can be the optimized condition.
Fig. 11 Schematic demonstration of the three enhancement effects of SP coupling, including QW emission and QD absorption (in the left portion), and QD emission (in the right portion).
Fig. 12 Absorption spectrum of the used QD. The three vertical dashed lines indicate the QW and QD emission wavelengths, and the PL excitation laser wavelength.
Among the samples under study, the QW PL decay times of those samples with coated QDs are generally longer than those without QD. This difference is due to QD absorption. Right after QW is excited by the femtosecond laser, the QW energy or emitted light in the early-stage of PL decay is absorbed by QDs. However, the PL decay time of QD is significantly longer than that of QW, as shown in the bottom row of Table 1. In other words, the long-lifetime occupied upper-state of QD reduces the QD absorption of QW energy or emitted light in the later-stage of QW PL decay. Therefore, the depressed QW emission intensity in the early-stage of PL decay due to more effective QD absorption effectively results in a longer calibrated QW decay time. QD can also be directly excited by the femtosecond laser. This excitation further reduces the QD absorption in the later-stage of QW PL decay. Regarding QD PL decay time, SP couplings in samples D-xxx result in faster PL decays, when compared with sample C. Among samples D-xxx, QD PL decay time is controlled not only by the SP coupling strength at the QD emission wavelength, but also by the excitation condition. In general, a higher excitation intensity leads to a shorter PL decay time. In sample D-580, although the SP coupling at the QD emission wavelength for shortening PL decay time is stronger, its enhanced QD absorption is weaker. On the other hand, in sample D-540, although the SP coupling at the QD emission wavelength is weaker, its enhanced QD absorption is stronger, leading to an effectively stronger excitation intensity for shortening PL decay time. The competition between the two factors results in about the same QD PL decay time in samples D-540 and D-580. In sample D-500, although the enhanced QD absorption is the strongest among samples D-xxx, its SP coupling effect at the QD emission wavelength is negligibly weak, leading to the longest QD PL decay time. However, it is still shorter than that of sample C due to the enhanced QD absorption through SP coupling in sample D-500..
In characterizing the LED performances, QD emission is excited by QW emission. However, in PL measurements, QDs are partly excited by QW emission and partly excited by the PL excitation laser at 405 nm (indicated by the left vertical dashed line in Fig. 12). In other words, the effect of enhanced QD absorption at the QW emission wavelength is mixed with that of QD absorption at the laser wavelength of PL excitation. As shown in Fig. 4, the LSP resonances at 405 nm are weak. The QD excitation by the laser “dilutes” the effect of enhanced QD absorption through SP coupling at 465 nm. This mechanism can be used for explaining the smaller enhancement ratios of QD IQE in samples D-540 and D-580 with respect to that of sample C, when compared with the enhancement ratios of QD emission intensity (e.g., 15% increment of IQE versus 24% increment of emission intensity in sample D-540).
As shown in Figs. 5(a)-5(d), the QW emission intensities are significantly stronger than those of QD emission in the fabricated devices of the current study. For display application, QW emission needs to be completely converted into QD emission. This requirement can be achieved by increasing the used QD concentration. Although complete color conversion can be implemented simply by increasing the coated planar density of QD, the increase of conversion efficiency through SP coupling can help in saving excitation energy and used QD amount. As shown in Fig. 2(b), the QD layer thicknesses in samples D-xxx can be as large as 700 nm, which is larger than the SP coupling range (~100 nm). To extend the SP coupling range in the QD layer, we can place Ag NPs between multiple QD layers or simply mix Ag NPs with QDs in the coating process. Although such Ag NPs can be quite far away from the QWs, they can still induce enhanced absorption and emission of the surrounding QDs. This issue deserves further investigation. In this study, we fabricate the Ag NP structure shown in Fig. 3(b) for generating LSP resonance peak around 540 nm to demonstrate a significant improvement of QD emission intensity. The Ag NP structure can be further adjusted for optimizing its LSP resonance peak wavelength and maximizing the resonance strength and spectral width such that the QD emission intensity or color conversion efficiency can be further improved.
6. Conclusions
In summary, we have demonstrated the efficiency enhancement of color conversion from blue emission of QW into red emission of QD through SP coupling by coating CdSe/ZnS QDs onto the top of an InGaN/GaN QW LED. Ag NPs were deposited within a transparent conductive GaZnO interlayer to induce LSP resonance for simultaneously coupling with the QWs and QDs at their emission wavelengths. Such a coupling process produced three enhancement effects, including QW emission, QD absorption at the QW emission wavelength, and QD emission, leading to an overall enhancement effect of QD emission intensity. An Ag NP structure for inducing LSP resonance peak around the middle between the QW and QD emission wavelengths could generate the optimized condition for maximizing QD emission enhancement. IQE and PL decay time measurements showed consistent results with LED performance characterizations even though the QD absorption of the PL excitation laser might mix with the SP-coupling induced enhancement effect of QD absorption in PL measurement.
Funding
Ministry of Science and Technology, Taiwan, the Republic of China (MOST) (MOST 106-2221-E-002-163-MY3, MOST 105-2221-E-002-159-MY3, and MOST 106-2221-E-002-162); US Air Force Office of Scientific Research (AFOSR) (AOARD-17IOA087).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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