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  • Editor: Christian Seassal
  • Vol. 22, Iss. S3 — May. 5, 2014
  • pp: A842–A856
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Efficiency improvement of a vertical light-emitting diode through surface plasmon coupling and grating scattering

Chun-Han Lin, Chieh Hsieh, Charng-Gan Tu, Yang Kuo, Horng-Shyang Chen, Pei-Ying Shih, Che-Hao Liao, Yean-Woei Kiang, C. C. Yang, Chih-Han Lai, Guan-Ru He, Jui-Hung Yeh, and Ta-Cheng Hsu  »View Author Affiliations


Optics Express, Vol. 22, Issue S3, pp. A842-A856 (2014)
http://dx.doi.org/10.1364/OE.22.00A842


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Abstract

The enhancement of output intensity, the generation of polarized output, and the reduction of the efficiency droop effect in a surface plasmon (SP) coupled vertical light-emitting diode (LED) with an Ag nano-grating structure located between the p-GaN layer and the wafer bonding metal for inducing SP coupling with the InGaN/GaN quantum wells (QWs) are demonstrated. In fabricating the vertical LED, the patterned sapphire substrate is removed with a photoelectrochemical liftoff technique. Based on the reflection measurement from the metal grating structure and the numerical simulation result, it is found that the localized surface plasmon (LSP) resonance induced around the metal grating crest plays the major role in the SP-QW coupling process although a hybrid mode of LSP and surface plasmon polariton can be generated in the coupling process. By adding a surface grating structure to the SP-coupled vertical LED on the n-GaN side, the output intensity is further enhanced, the output polarization ratio is further increased, and the efficiency droop effect is further suppressed.

© 2014 Optical Society of America

1. Introduction

Surface plasmon (SP) coupled light-emitting diode (LED) has been widely studied and implemented [1

1. A. Neogi, C.-W. Lee, H. O. Everitt, T. Kuroda, A. Tackeuchi, and E. Yablonovitch, “Enhancement of spontaneous recombination rate in a quantum well by resonant surface plasmon coupling,” Phys. Rev. B 66(15), 153305 (2002). [CrossRef]

20

20. C. Y. Cho, K. S. Kim, S. J. Lee, M. K. Kwon, H. Ko, S. T. Kim, G. Y. Jung, and S. J. Park, “Surface plasmon-enhanced light-emitting diodes with silver nanoparticles and SiO2 nano-disks embedded in p-GaN,” Appl. Phys. Lett. 99(4), 041107 (2011). [CrossRef]

]. In implementing such a device, efforts have been made to reduce the distance between the LED quantum wells (QWs) and the metal nanostructures for inducing SP resonance such that the SP-QW coupling strength can be maximized. In a conventional LED of the lateral configuration, surface metal nanostructures, such as metal nanoparticles [11

11. D. M. Yeh, C. F. Huang, C. Y. Chen, Y. C. Lu, and C. C. Yang, “Localized surface plasmon-induced emission enhancement of a green light-emitting diode,” Nanotechnology 19(34), 345201 (2008). [CrossRef] [PubMed]

] and gratings [14

14. K. C. Shen, C. Y. Chen, H. L. Chen, C. F. Huang, Y. W. Kiang, C. C. Yang, and Y. J. Yang, “Enhanced and partially polarized output of a light-emitting diode with Its InGaN/GaN quantum well coupled with surface plasmons on a metal grating,” Appl. Phys. Lett. 93(23), 231111 (2008). [CrossRef]

, 15

15. K. C. Shen, C. H. Liao, Z. Y. Yu, J. Y. Wang, C. H. Lin, Y. W. Kiang, and C. C. Yang, “Effects of the intermediate SiO2 layer on polarized output of a light-emitting diode with surface plasmon coupling,” J. Appl. Phys. 108(11), 113101 (2010). [CrossRef]

], on the p-GaN layer can induce significant SP coupling effects only when the p-GaN layer thickness is reduced to several tens nm. However, the thin p-GaN layer leads to ineffective current spreading and hence poor electrical property of the device. Also, the coverage of the metal nanostructures with another metal layer or a transparent conductor for current spreading purpose normally red-shifts an SP resonance peak away from the designated blue-green range. With a thick p-GaN layer, deep etching nanostructures for filling into metals can make the metals close to the QWs and induce significant SP coupling [16

16. H. S. Chen, C. F. Chen, Y. Kuo, W. H. Chou, C. H. Shen, Y. L. Jung, Y. W. Kiang, and C. C. Yang, “Surface plasmon coupled light-emitting diode with metal protrusions into p-GaN,” Appl. Phys. Lett. 102(4), 041108 (2013). [CrossRef]

]. However, such a structure normally produces current leakage. Another technique is to embed metal nanoparticles in the p-GaN layer such that they can be close to the QWs [18

18. M. K. Kwon, J. Y. Kim, B. H. Kim, I. K. Park, C. Y. Cho, C. C. Byeon, and S. J. Park, “Surface-plasmon-enhanced light-emitting diodes,” Adv. Mater. 20(7), 1253–1257 (2008). [CrossRef]

20

20. C. Y. Cho, K. S. Kim, S. J. Lee, M. K. Kwon, H. Ko, S. T. Kim, G. Y. Jung, and S. J. Park, “Surface plasmon-enhanced light-emitting diodes with silver nanoparticles and SiO2 nano-disks embedded in p-GaN,” Appl. Phys. Lett. 99(4), 041107 (2011). [CrossRef]

]. Nevertheless, this technique has the drawbacks of weak localized surface plasmon (LSP) resonance, high fabrication cost with crystal re-growth, and low planar density of metal nanoparticle for guaranteeing high re-growth quality. To reduce the distance between the metal nanostructures and QWs without involving crystal re-growth, the configuration of a vertical LED allows a thin p-GaN layer without sacrificing its electrical property and can be a good choice for fabricating SP-coupled LED.

The efficiency droop effect (EDE) of an LED means the efficiency decrease with increasing injection current. It has been attributed to a few causes. Two most widely discussed attributions are the effect of polarization field [21

21. M. F. Schubert, J. Xu, J. K. Kim, E. F. Schubert, M. H. Kim, S. Yoon, S. M. Lee, C. Sone, T. Sakong, and Y. Park, “Polarization-matched GaInN/AlGaInN multi-quantum-well light-emitting diodes with reduced efficiency droop,” Appl. Phys. Lett. 93(4), 041102 (2008). [CrossRef]

28

28. G.-B. Lin, D. Meyaard, J. Cho, E. F. Schubert, H. Shim, and C. Sone, “Analytic model for the efficiency droop in semiconductors with asymmetric carriertransport properties based on drift-induced reduction of injection efficiency,” Appl. Phys. Lett. 100(16), 161106 (2012). [CrossRef]

] leading to carrier overflow from the QWs and the defect-assisted Auger recombination in the QWs [29

29. K. T. Delaney, P. Rinke, and C. G. Van de Walle, “Auger recombination rates in nitrides from first principles,” Appl. Phys. Lett. 94(19), 191109 (2009). [CrossRef]

36

36. F. Bertazzi, X. Zhou, M. Goano, G. Ghione, and E. Bellotti, “Auger recombination in InGaN/GaN quantum wells: A full-Brillouin-zone study,” Appl. Phys. Lett. 103(8), 081106 (2013). [CrossRef]

]. Either effect becomes strong particularly when the injected carrier density is high. The effect of polarization field can be reduced by carefully designing the LED structure. However, the Auger recombination is a fundamental phenomenon in such a semiconductor material and cannot be avoided unless the carrier density in the QWs is reduced. In the SP coupling process of an SP-coupled LED, the carrier energy is effectively transferred into the SP mode for creating an alternative emission channel. In this transfer process, electrons and holes recombine without radiation such that the carrier density can be reduced. In other words, by transferring energy into the SP mode for radiation, the carrier density in the QWs can be maintained at a low level. Therefore, the EDE can be reduced in an SP-coupled LED [13

13. C. F. Lu, C. H. Liao, C. Y. Chen, C. Hsieh, Y. W. Kiang, and C. C. Yang, “Reduction of the efficiency droop effect of a light-emitting diode through surface plasmon coupling,” Appl. Phys. Lett. 96(26), 261104 (2010). [CrossRef]

].

In this paper, we demonstrate the enhancement of output intensity, the generation of polarized output, and the reduction of the EDE in an SP-coupled vertical LED, in which an Ag nano-grating structure is formed between the p-GaN layer and the wafer bonding metal for inducing SP coupling with the InGaN/GaN QWs. The vertical LED is fabricated based on photoelectrochemical (PEC) liftoff of sapphire substrate [37

37. C. H. Lin, C. Y. Chen, C. H. Liao, C. Hsieh, Y. W. Kiang, and C. C. Yang, “Sapphire substrate liftoff with photoelectrochemical etching for vertical light-emitting diode fabrication,” IEEE Photon. Technol. Lett. 23(10), 654–656 (2011). [CrossRef]

, 38

38. C. Hsieh, H. S. Chen, C. H. Liao, C. Y. Chen, C. H. Lin, C. H. Lin, S. Y. Ting, Y. F. Yao, H. T. Chen, Y. W. Kiang, and C. C. Yang, “Photoelectrochemical liftoff of patterned sapphire substrate for fabricating vertical light-emitting diode,” IEEE Photon. Technol. Lett. 24(19), 1775–1777 (2012). [CrossRef]

]. This liftoff method has the advantages of higher liftoff speed (parallel process), less damage to the epitaxial layer, higher yield, and lower cost, when compared with the conventional laser-lift technique. To further increase LED output intensity and output polarization ratio, and to further reduce the EDE, we fabricate a surface grating with PEC etching on the n-GaN layer [39

39. C. H. Lin, C. Y. Chen, D. M. Yeh, and C. C. Yang, “Light extraction enhancement of a GaN-based light-emitting diode through grating-patterned photoelectrochemical surface etching with phase mask interferometry,” IEEE Photon. Technol. Lett. 22(9), 640–642 (2010). [CrossRef]

, 40

40. C. H. Lin, C. G. Tu, H. S. Chen, C. Hsieh, C. Y. Chen, C. H. Liao, Y. W. Kiang, and C. C. Yang, “Vertical light-emitting diodes with surface gratings and rough surfaces for effective light extraction,” Opt. Express 21(15), 17686–17694 (2013). [CrossRef] [PubMed]

]. The use of PEC etching for fabricating a surface grating has the advantages of simple process, low cost, and mass production, particularly when it is combined with phase mask interferometry. Two epitaxial structures of different p-GaN layer thicknesses are used for comparing the vertical LED performances. The roles LSP and surface plasmon polariton (SPP) play in the SP-QW coupling process are discussed. In section 2 of this paper, the sample structures, designations, and fabrication procedures are presented. Then, the characterization results of the SP-coupled vertical LEDs only with metal gratings are reported in section 3. Next, the characterization results of the vertical LEDs with both metal gratings on the p-GaN layer and surface gratings on the n-GaN layer are shown in section 4. The SP coupling mechanism and other important issues are discussed in section 5. Finally, conclusions are drawn in section 6.

2. Sample structures, designations, and fabrication procedures

The used epitaxial structures are grown on c-plane, patterned sapphire substrate (PSS) with metalorganic chemical vapor deposition. The PSS is fabricated through the etching of the mixed solution of hot sulfuric and phosphoric acids to form a periodic pattern (16 μm in period) of triangular-striped grooves [38

38. C. Hsieh, H. S. Chen, C. H. Liao, C. Y. Chen, C. H. Lin, C. H. Lin, S. Y. Ting, Y. F. Yao, H. T. Chen, Y. W. Kiang, and C. C. Yang, “Photoelectrochemical liftoff of patterned sapphire substrate for fabricating vertical light-emitting diode,” IEEE Photon. Technol. Lett. 24(19), 1775–1777 (2012). [CrossRef]

]. The inverted-triangular groove of 10 μm in basal length and 6 μm in height has two slant facets in the {10-12} and {10-15} planes. The PSS is used for the coalescence overgrowth of an un-doped GaN layer with the aforementioned triangular-striped grooves on sapphire preserved. The lateral overgrowth leads to coalescence after a GaN layer of ~1.5 μm in thickness is grown. Then, a green-emitting LED structure is overgrown on the GaN layer, including a 2-μm n-GaN layer, a five-period InGaN/GaN QW structure, a 15-nm p-AlGaN layer, and a p-GaN layer. Two epitaxial structures of different p-GaN layer thicknesses at 60 and 120 nm are prepared for comparison. Including the p-GaN layer, p-AlGaN layer, and a GaN barrier (~10 nm in thickness), the distance between the top QW and the epitaxial surface is 85 (145) nm in the epitaxial structure of thin (thick) p-GaN.

Before wafer-bonding a sample onto Si substrate for PEC liftoff, a one-dimensional (1-D) grating is fabricated on the p-GaN layer through an inductively coupled plasma reactive ion etching (ICP RIE) process with a patterned photoresist mask, which is formed through the exposure to the interference fringe of a Lloyd’s interferometer [40

40. C. H. Lin, C. G. Tu, H. S. Chen, C. Hsieh, C. Y. Chen, C. H. Liao, Y. W. Kiang, and C. C. Yang, “Vertical light-emitting diodes with surface gratings and rough surfaces for effective light extraction,” Opt. Express 21(15), 17686–17694 (2013). [CrossRef] [PubMed]

]. On the epitaxial structure, with either thin or thick p-GaN layer, three samples of different grating periods around 360, 520, and 680 nm are fabricated. All the gratings have 50% in duty cycle. On the epitaxial structure of 60 nm in p-GaN thickness, metal gratings with groove depth around 15 nm are formed for fabricating samples B-D. Also, metal gratings with groove depth around 65 and 15 nm are formed on the epitaxial structure of 120 nm in p-GaN thickness for fabricating samples B’-D’ and B’s-D’s, respectively. The more precise grating periods and groove depths of various samples are listed in rows 3 and 4 of Table 1.

Table 1. Sample assignments, parameters, and their characterization results of samples A-D, A’-D’, and B’s-D’s. The relative output intensities are obtained when injection current is 120 mA. The numbers within the parentheses show the ratios with respect to the values of the individual reference samples (samples A and A’). IQE: internal quantum efficiency.

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The samples without grating in the thin and thick p-GaN epitaxial structures are assigned as samples A and A’, respectively. Samples A and A’ are the reference samples for comparison to observe the effect of SP coupling. After the gratings are formed, Ag is deposited with electron-gun evaporation onto the grating surface to completely fill up the grooves. Before Ag deposition, the samples are dipped in an HCl solution (HCl: H2O = 1:10) for 10 min to slightly etch the grating surfaces for removing the shallow damaged layer caused by ICP RIE. The thickness of the flat-surface Ag layer beyond the grating crest is ~200 nm. Then, a 2-μm thick Au layer is deposited on the top for wafer-bonding the samples onto p-Si substrate. After Ag deposition, the grating on the p-GaN layer is also referred to as a metal grating.

Fig. 1 (a) Schematic demonstration of the device structure of samples B-D and B’-D’. The designated coordinate system with the x-axis along the grating groove direction and the z-axis pointing at the Si substrate is also shown. (b) Schematic demonstration of the device structure of samples CB-CD and C’B-C’D. (c) Schematic demonstration of the device structure of samples Bt-Dt and B’t-D’t. The U-turn arrow at the bottom indicates the measurement scheme of reflection spectra.
The PEC liftoff process is undertaken after wafer bonding. In this process, the preserved triangular-striped grooves (now become tunnels) allow the electrolyte, i.e., 0.05 M KOH, to flow across the sample in the lateral dimension for accelerating the PEC etching of a thin GaN layer at the GaN/sapphire interface for effective PSS liftoff. The details of the PEC liftoff process can be found in [38

38. C. Hsieh, H. S. Chen, C. H. Liao, C. Y. Chen, C. H. Lin, C. H. Lin, S. Y. Ting, Y. F. Yao, H. T. Chen, Y. W. Kiang, and C. C. Yang, “Photoelectrochemical liftoff of patterned sapphire substrate for fabricating vertical light-emitting diode,” IEEE Photon. Technol. Lett. 24(19), 1775–1777 (2012). [CrossRef]

]. The PEC liftoff of a sample of 0.7 cm x 0.7 cm in lateral dimension requires ~50 min. After PEC liftoff, the residual un-doped GaN layer is removed through a polishing process to expose the n-GaN layer with a flat N-polar surface. Figure 1(a) schematically shows the device structure after the polishing process. Here, we also show the designated coordinate system with the x-axis along the grating groove direction and the z-axis vertically pointing at the Si substrate.
Fig. 2 (a) and (b): SEM and AFM images, respectively, of the grating on the p-GaN layer of sample B’. (c) and (d): SEM and AFM images, respectively, of the surface grating on the n-GaN layer of sample CB.
Figures 2(a) and 2(b) show the scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of the grating on the p-GaN layer of sample B’ before Ag deposition. Here, one can see the almost vertical etching sidewalls in the grating grooves.

3. Characterization results of the LEDs only with metal gratings

Fig. 3 Output spectra of sample C’ when injection current is increased from 5 through 120 mA.
Fig. 4 Normalized output intensities as functions of injection current for samples A-D, A’-D’, CB-CD, and C’B-C’D.
Figure 3 shows the output spectra of LED sample C’ when injection current is increased from 5 through 120 mA. The electroluminescence spectral peaks are around 545 nm. In Fig. 4, we show the normalized output intensities as functions of injection current (L-I curves) for samples A-D, A’-D’, and other samples to be described later. Among samples A-D (A’-D’), sample C (C’) has the highest output intensity, followed by sample B (B’) and then sample D (D’). The output intensity of reference sample A (A’) is the lowest. Between the two groups of sample (A-D and A’-D’), samples A’-D’ always have higher output intensities, when compared with their corresponding samples of the similar grating periods (samples A-D). The output intensity difference between the corresponding samples in the two groups increases as the output intensity of individual LED increases.
Fig. 5 Normalized angle-dependent output intensities of samples A-D and A’-D’ in the y-z plane when injection current is 120 mA. The 0 and 90 degrees correspond to the -z and y directions, respectively.
Figure 5 shows the normalized angle-dependent intensities of samples A-D and A’-D’ in the y-z plane when injection current is 120 mA. The 0 and 90 degrees in Fig. 5 correspond to the -z and y directions [see Fig. 1(a)], respectively. Among different samples, the levels of angle-dependent intensity in Fig. 5 follow the same order as that in Fig. 4. Here, no clear diffraction pattern can be seen.

Fig. 6 Variations of relative EQE with injection current of samples A-D, A’-D’, CB-CD, and C’B-C’D.
Figure 6 shows the variations of relative external quantum efficiency (EQE) with injection current of samples A-D, A’-D’, and other samples to be described later. The EQEs are normalized to the maximum level of a sample to be described later (sample C’B) at 5 mA in injection current. Here, the EQEs of samples A and A’ show significant droop effects. Their EQEs drop by ~83% as injection current increases from 5 through 120 mA. Compared with samples A and A’, the EQEs of samples D and D’ are reduced at low injection current. However, their droop effects become less significant. By increasing injection current from 5 through 120 mA, the EQEs drop by only ~69%. In samples B and B’, the EQEs at low injection current are significantly increased, when compared with samples D and D’. However, their droop effects are comparable to those of samples D and D’. In the concerned injection current range, their EQEs drop by 67-69%. The EDE of sample C (C’) is the weakest among samples A-D (A’-D’). The EQE of sample C (C’) drops only by 61 (57) % in the concerned injection current range.

Fig. 7 Polarization ratios as functions of injection current of samples A-D, A’-D’, CB-CD, C’B-C’D, and E. The polarization ratio is defined as the output intensity in the y-polarization over that in the x-polarization.
Figure 7 shows the polarization ratios as functions of injection current of samples A-D, A’-D’, and other samples to be described later. The polarization ratio is defined as the ratio of the output intensity in the y-polarization over that in the x-polarization. The curves in Fig. 7 are generally flat, indicating that the polarization ratio does not vary with injection current. The polarization ratios of ~1 in samples A and A’ indicate that the output of these two samples are un-polarized, as expected. Among samples A-D (A’-D’), the level of polarization ratio increases following the same order as that of output intensity, as shown in Fig. 4. However, between the corresponding samples of groups A-D and A’-D’, the polarization ratios of group A-D are higher than those of group A’-D’. This trend is opposite to that of LED output intensity shown in Fig. 4. In Fig. 7, the polarization ratio of an extra vertical LED sample, i.e., sample E, is also shown for comparison. This sample does not have a grating structure between the p-GaN layer and the wafer-bonding metal, i.e., no metal grating. However, it has a surface grating on the n-GaN layer with a grating period of 163.8 nm and a groove depth of 227.7 nm. Here, one can see that the polarization ratio of sample E is only ~1.07, which is significantly smaller than those of samples B-D and B’-D’, implying that the SP coupling induced by the metal grating in samples B-D and B’-D’ produces a strong polarization effect on LED output. In other words, although a dielectric grating structure can produce a certain (weak) polarization effect, SP coupling based on a 1-D metal grating structure can generate a more significant polarization effect.

Fig. 8 Relations between current and applied voltage of samples A-D and A’-D’. The insert shows the magnification of a certain portion to differentiate those curves.
Figure 8 shows the relations between current and applied voltage (I-V curves) of samples A-D and A’-D’. Here, one can see that the leakage currents are negligibly small in all samples. The turn-on voltages of all the samples are around 3.5 V. From the insert of this figure, one can see that the device resistance levels of the reference samples (samples A and A’) are the lowest among the samples of the same epitaxial structure. Then, the device resistance increases with grating period among the samples with metal gratings. The device resistance levels of samples A-D are slightly lower than those of samples A’-D’, indicating that a thinner p-GaN layer does not necessarily lead to a higher resistance level in a vertical LED.

It is noted that because sample E (without a metal grating) was prepared for a different purpose, it is unreasonable to compare its output intensity and device resistance with other samples. However, its polarization ratio can still be used for showing that without SP coupling, a surface grating can produce only a small polarization ratio (1.07). We choose the small grating period at ~164 nm and the large groove depth at ~228 nm for sample E because the grating scattering is stronger with a smaller period or a larger depth [40

40. C. H. Lin, C. G. Tu, H. S. Chen, C. Hsieh, C. Y. Chen, C. H. Liao, Y. W. Kiang, and C. C. Yang, “Vertical light-emitting diodes with surface gratings and rough surfaces for effective light extraction,” Opt. Express 21(15), 17686–17694 (2013). [CrossRef] [PubMed]

]. With the small grating period and the large groove depth in sample E, its output polarization ratio is maximized for demonstrating that the effect of grating scattering is significantly weaker than that of SP coupling on output polarization. It is noted that the polarization ratio data in Table 1 are obtained by taking the average over the injection current range between 5 and 120 mA.

The SP-coupling process for enhancing the output intensities of those metal-grating LED samples can be further confirmed by comparing the performances of samples B-D and B’-D’ with those of samples B’s-D’s. The device structures of samples B’s-D’s are similar to those of samples B-D except that they are fabricated with the epitaxial structure of thick p-GaN layer (120 nm). The device parameters and characterization results of samples B’s-D’s are also summarized in Table 1. From row 3 of Table 1, one can see that the metal grating periods of samples B’s-D’s are similar to the corresponding samples of groups B-D and B’-D’. However, as shown in row 4 of Table 1, the metal groove depths of samples B’s-D’s are only around 15 nm. In other words, the distance between the metal grating crest and the first QW in samples B’s-D’s is as large as ~130 nm. In this situation, the SP coupling is expected to be weak. As shown in rows 5 and 6 of Table 1 for the relative output intensities, the output enhancements of samples B’s-D’s are indeed significantly smaller, when compared with samples B-D and B’-D’. Also, as shown in row 7, the polarization ratios of samples B’s-D’s are close to unity (smaller than that of sample E). The noticeable output intensity enhancements (with respect to sample A’) and slight output polarization of samples B’s-D’s can partly be caused by the non-resonant scattering of metal gratings. With such scattering, the light extraction efficiencies of samples B’s-D’s become higher than that of the reference sample (A’). Because the grating scattering strength increases with decreasing grating period, the output enhancement and polarization ratio in samples B’s-D’s decrease with increasing metal grating period [40

40. C. H. Lin, C. G. Tu, H. S. Chen, C. Hsieh, C. Y. Chen, C. H. Liao, Y. W. Kiang, and C. C. Yang, “Vertical light-emitting diodes with surface gratings and rough surfaces for effective light extraction,” Opt. Express 21(15), 17686–17694 (2013). [CrossRef] [PubMed]

]. Nevertheless, the slightly increased IQEs in samples B’s-D’s (11.9, 10.9, and 11.3%, respectively), when compared with that of sample A’ (10.6%), indicate that weak SP coupling may still exist in these samples even though the distance between the metal grating crest and the first QW is as large as ~130 nm. The comparison in LED performance between samples B’s-D’s and B’-D’ shows that the SP coupling can be effective only when the aforementioned distance is small enough, such as ~70 nm in samples B-D and ~80 nm in samples B’-D’. The distance dependence is one of the keys to identifying SP coupling. In observing such a dependency, LED output intensity, polarization ratio, and IQE are important parameters.

4. Characterization results of the LEDs with both metal and surface gratings

To enhance light extraction, surface gratings are fabricated on the N-polar surface of the n-GaN layer in the vertical LEDs, as schematically demonstrated in Fig. 1(b). The surface gratings are fabricated based on the technical combination of PEC etching and Lloyd’s interferometry [40

40. C. H. Lin, C. G. Tu, H. S. Chen, C. Hsieh, C. Y. Chen, C. H. Liao, Y. W. Kiang, and C. C. Yang, “Vertical light-emitting diodes with surface gratings and rough surfaces for effective light extraction,” Opt. Express 21(15), 17686–17694 (2013). [CrossRef] [PubMed]

]. In other words, the interference fringes of a Lloyd’s interferometer is formed on the N-polar surface of the n-GaN layer of a vertical LED device, which is immersed in the PEC electrolyte (KOH), for controlling the spatially-resolved PEC etching depth and hence forming the periodic grooves. Since in either sample group of B-D or B’-D’, the metal grating of ~520 nm in period leads to the highest output intensity, the parameters of samples C and C’ are chosen for fabricating new samples with surface gratings.

Table 2. Sample assignments, parameters, and their characterization results of samples C, C’, CB-CD, and C’B-C’D. The relative output intensities are obtained when injection current is 120 mA. The numbers within the parentheses show the ratios with respect to the values of the individual reference samples (samples C and C’ here).

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As shown in Table 2, three new samples are prepared based on the parameters of either sample C or C’, including the conditions with the surface gratings of ~160 nm (sample CB or C’B), ~360 nm (sample CC or C’C), and ~560 nm (sample CD or C’D) in period. The groove depths of the surface gratings in all the six new samples are around 225 nm. Figures 2(c) and 2(d) show the SEM and AFM images of the surface grating on sample CB. The grating morphology based on PEC etching is quite different from that based on dry etching shown in Figs. 2(a) and 2(b).

In Fig. 4, we also show the normalized output intensities as functions of injection current for samples CB-CD and C’B-C’D. One can see that with surface gratings, the LED output intensities are significantly enhanced due to the increased light extraction. In each sample group (CB-CD or C’B-C’D), a smaller surface grating period leads to higher output intensity [40

40. C. H. Lin, C. G. Tu, H. S. Chen, C. Hsieh, C. Y. Chen, C. H. Liao, Y. W. Kiang, and C. C. Yang, “Vertical light-emitting diodes with surface gratings and rough surfaces for effective light extraction,” Opt. Express 21(15), 17686–17694 (2013). [CrossRef] [PubMed]

]. Also, samples C’B-C’D have higher output intensities, when compared with the corresponding samples in the group of CB-CD. Figure 6 also shows the relative EQEs of samples CB-CD and C’B-C’D. The relative EQEs are normalized to the maximum level of sample C’B. We can see that the EDE is further reduced by adding the surface grating for enhancing light extraction. Generally speaking, the EDE is reduced following the increasing trend of LED output intensity. In sample C’B, the efficiency droop is reduced to only 29% when injection current is increased from 5 to 120 mA. Figure 7 also shows the polarization ratio variations with injection current of samples CB-CD and C’B-C’D. One can see that in either C or C’ sample group, the fabrication of surface grating further increases the output polarization ratio. The polarization ratio increases with increasing LED output intensity. It is interesting to note that the polarization ratio increment from sample C to CB (or from sample C’ to C’B) is about the same as that from sample A or A’ to E (by 0.07), whose surface grating has about the same period and depth as those in samples CB and C’B, indicating that the increment of polarization ratio from sample C to CB (or from sample C’ to C’B) is caused by surface grating scattering.

In rows 7-10 of Table 2, we summarize the LED characterization results of samples CB-CD and C’B-C’D. For comparison, those of samples C and C’ are also shown. Here, again, the results of relative output intensity(a) and relative output intensity(b) are obtained from the extended-angle and angle-dependent measurements, respectively. In rows 7 and 8 of Table 2, the relative intensity values without parentheses are based on the normalization with respect to the intensity of sample A’ (see Table 1). The numbers within parentheses are the ratios of output intensity with respect to that of sample C (C’) in the sample group of CB-CD (C’B-C’D). Here, one can see that compared with sample A’, the output intensity of sample C’B is enhanced by 372 or 391%, depending on the measurement method. Also, compared with sample A, the output intensity of sample CB is enhanced by 301 or 292%. As mentioned earlier, the polarization ratio difference (0.07) between sample C and CB (also between samples C’ and C’B) is the same as that between sample A’ and E. In the bottom row of Table 2, we show the device resistance levels of samples CB-CD and C’B-C’D. Here, one can see that the device resistance is increased by fabricating a surface grating. The resistance increment is larger in an LED of a smaller surface grating period. Although the resistance level of sample C’ is slightly larger than that of sample C, those of samples C’B-C’D are slightly smaller than those of the corresponding samples in the group of CB-CD.

5. Discussions

Fig. 9 Dispersion curves under the conditions of flat interface, interface gratings of 360, 520, and 680 nm in grating period (Λ) at an Ag/GaN interface. The horizontal dashed line represents the QW emission energy (545 nm in wavelength).
SPP on a flat metal/dielectric interface cannot interact with incident plane wave and cannot emit photon unless a certain momentum compensation scheme is applied, such as a grating structure formed at the interface. However, a nearby radiating dipole can interact with SPP through near-field coupling. Nevertheless, SPP radiation in such a coupling process is not as strong as that of an SPP-dipole coupling system with a grating structure at the interface. Figure 9 shows the dispersion curves of SPP under the conditions of flat interface and interface grating structures of 360, 520, and 680 nm in grating period (Λ) at the Ag/GaN interface. The dispersion curves are drawn by using the experimental data for the wavelength-dependent dielectric constant of Ag [42

42. E. D. Palik, Handbook of Optical Constants of Solids (Academic, 1991).

]. The horizontal dashed line represents the QW emission energy (545 nm in wavelength). Here, one can see that the dispersion curve of flat interface is left-shifted by a range of 2π/Λ when a grating structure is applied to the interface. When the shifted dispersion curve intercepts the vertical axis (corresponding to the emission along the -z direction for optimized light extraction) around the QW emission energy, the SPP-QW coupling can lead to a significant LED output enhancement. However, as shown in Fig. 9, in the period range of the fabricated metal gratings, the shift range of the dispersion curve is not large enough for reaching the aforementioned optimized condition. The dispersion curves of the designed metal-grating devices intercept the horizontal dashed line of QW emission energy just around the light line. Therefore, it is believed that SPP coupling is not the major mechanism for producing the observed SP coupling behaviors.

Table 3. Sample assignments and parameters of samples Bt-Dt and B’t-D’t.

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To understand whether any other SP mode exists in such a metal grating structure, we fabricate metal gratings of about the same periods and depths as those in samples B-D and B’-D’ on the surface of a 4-μm un-doped GaN (u-GaN) thin film, which is grown on double-polished sapphire substrate, as schematically depicted in Fig. 1(c). Each of the GaN grating samples is covered by an Ag film of ~200 nm in thickness to form a metal grating. As described in Table 3 for the grating parameters, those grating samples are assigned as samples Bt-Dt and B’t-D’t, corresponding to samples B-D and B’-D’, respectively. Such metal gratings on GaN can be used for demonstrating the similar SP behaviors to those in the metal-grating LED samples of the similar grating parameters.
Fig. 10 Reflectance spectra from the sapphire side of samples Bt-Dt. A vertical dashed line is drawn to indicate the QW emission peak wavelength (545 nm).
Fig. 11 Reflectance spectra from the sapphire side of samples B’t-D’t. A vertical dashed line is drawn to indicate the QW emission peak wavelength (545 nm).
Figures 10 and 11 show the reflectance spectra from the sapphire side [see Fig. 1(c)] of samples Bt-Dt and B’t-D’t, respectively. The reflection measurement is undertaken with the 45-degree incidence of a TM-polarized white-light beam and the specular-reflection detection with a ~2-degree receiving angle. In either Fig. 10 or 11, a vertical dashed line is drawn to indicate the QW emission wavelength (545 nm). The reflectance depressions in those curves in Figs. 10 and 11 correspond to certain LSP resonance modes induced around the crests of the Ag gratings because the enhanced absorption of LSP resonance reduces light reflection. Note that because of the imperfect and non-uniform grating crest geometry, it is difficult to observe the detailed LSP resonance features based on this reflectance measurement. Therefore, the reflectance behaviors in Figs. 10 and 11 simply demonstrate the existence of certain LSP resonance modes around the metal grating crests at the QW emission wavelength. Also, in an SP-coupled LED, it is the near-field coupling process between SP and radiating dipoles in the QWs, instead of plane wave coupling with SP, for producing output enhancement. Hence, the reflectance behaviors in Figs. 10 and 11 cannot be used for interpreting the LSP-QW coupling strength. In other words, the relatively shallower depressions in Fig. 10, when compared with Fig. 11, do not necessarily imply weaker LSP-QW coupling in samples B-D. In either Fig. 10 or 11, there are LSP resonance modes around the QW emission wavelength. They are responsible for the SP coupling processes in the LED samples with metal gratings.

Fig. 12 Simulation results of reflectance spectra with three grating periods at Λ = 360, 520, and 680 nm when the grating groove depth is 15 nm. For comparison, the reflectance spectrum of a flat Ag/GaN interface is also shown. The QW emission wavelength at 545 nm is marked by the vertical dashed line.
Fig. 13 Simulation results of reflectance spectra with three grating periods at Λ = 360, 520, and 680 nm when the grating groove depth is 65 nm. For comparison, the reflectance spectrum of a flat Ag/GaN interface is also shown. The QW emission wavelength at 545 nm is marked by the vertical dashed line.
Fig. 14 Intensity distributions of electric field at 545 nm around the grating crests for the cases of 15 nm [parts (a)-(c) for 360, 520, and 680 nm in grating period, respectively] and 65 nm [parts (d)-(f) for 360, 520, and 680 nm in grating period, respectively] in grating groove depth.
To confirm the existence of LSP resonance modes around the metal grating crests, numerical simulations based on the finite element method (the COMSOL software) are performed. In the simulation study, based on the geometry shown in Fig. 1(c), a TM-polarized plane wave is incident upon the grating interface between a half-space of Ag [42

42. E. D. Palik, Handbook of Optical Constants of Solids (Academic, 1991).

] and another half-space of GaN (refractive index set at 2.4) at 45 degrees from the GaN side. In Figs. 12 and 13, we show the reflectance spectra with three grating periods (360, 520, and 680 nm) when the grating groove depths are 15 and 65 nm, respectively. In each figure, for comparison, the reflectance spectrum of a flat Ag/GaN interface is also shown. Here, the reflectance is defined as the percentage of the total returned power (including all directions) evaluated in a parallel plane far away from the grating interface in the GaN half-space. Therefore, its physical meaning is different from that shown in Figs. 10 and 11 based on experimental measurement. In numerical study, it is difficult to simulate this experimental condition. In Fig. 12 for the grating groove depth of 15 nm (corresponding to samples B-D), one can see a few LSP resonance features in each curve with the resonance strength increasing with increasing wavelength. Those features at shorter wavelengths correspond to higher-order LSP resonance modes. Near the QW emission wavelength (545 nm), as marked by the vertical dashed line, a higher-order LSP mode exists for each grating period. Again, the depression depths here cannot be used for interpreting the LSP-QW coupling strength. It is noted that because the grating duty cycle is fixed at 50%, the grating crest width is changed and hence the LSP resonance behavior becomes different when grating period is varied. In Fig. 13 for the grating groove depth of 65 nm (corresponding to samples B’-D’), the reflectance spectra become more complicated. However, certain resonance features can still be seen near the QW emission wavelength. The complicated spectral structures here are due to the deeper metal protrusions and hence the existence of many local resonance features. The generally lower reflectance in the spectral range between 400 and 550 nm, which is caused by high metal absorption, can be attributed to the coupling of LSP energy into SPP modes for forming certain LSP-SPP hybrid modes. As shown in Fig. 9, a 45-degree incident plane wave cannot directly excite an SPP mode. However, because of the deep protrusion structure (65 nm in depth), an excited LSP mode can generate momentum matching with an SPP mode for mutual coupling. Since an SPP mode at the Ag/GaN interface has strong dissipation in the blue range (peak around 440 nm), the metal absorption of the hybrid LSP-SPP mode leads to low reflectance in the spectral range of 400-550 nm. Therefore, it is reasonable to conclude that the output enhancements in samples B’-D’ are caused by the QW coupling with certain LSP-SPP hybrid modes. In particular, the near-field coupling of the radiating dipoles in a QW can directly excite an SPP mode. With the deep metal protrusion structure, it is very likely that the excited LSP and SPP can couple each other to form a hybrid mode. To further demonstrate the LSP resonance features around the grating crests, in Fig. 14, we show the intensity distributions of electric field at 545 nm around the grating crests for the cases of 15 nm [Figs. 14(a)14(c) for 360, 520, and 680 nm in grating period, respectively] and 65 nm [Figs. 14(d)14(f) for 360, 520, and 680 nm in grating period, respectively] in grating groove depth. In each part of Fig. 14, a horizontal line is drawn to indicate the location of the QW closest to the grating crest. Here, one can see a localized field distribution on the top of the grating crest in each case. Such a field distribution covers the QW. Therefore, it is believed that LSP plays the major role in the SP-QW coupling processes of the fabricated SP-coupled LEDs.

A polarization ratio larger than unity means that the output intensity in the polarization direction perpendicular to the grating grooves is stronger than that in the direction along the grooves. This is so because with the 1-D metal stripes along the x-axis, electron oscillation and hence SPP and LSP resonances can be induced only along the y-axis. Therefore, the difference between a polarization ratio and unity roughly corresponds to the percentage of LED output caused by SP coupling, which is polarized in contrast to the un-polarized emission through the radiative recombination of carriers.

As shown in the bottom row of Table 1, the device resistance levels in the vertical LED samples with metal gratings are higher than those of the corresponding reference samples of no metal grating. This trend can be attributed to the surface damages caused by ICP RIE in forming the metal gratings. Although a wet etching process is applied to remove the shallow damage layer on the exposed surface, the damage removal can be incomplete. Among the samples with metal gratings, the increasing trend of device resistance with increasing grating period is due to the different sidewall areas of grating grooves for injecting current into p-GaN among different samples. With a smaller metal grating period, the larger sidewall Ag/GaN contacting area can lead to a relatively smaller device resistance even though its surface damage area is also larger. Meanwhile, the generally higher device resistance levels in samples B’-D’, when compared with samples B-D, can be partly caused by the longer ICP RIE process for producing larger etching depths such that more sidewall surface damages are generated. By comparing the curves for samples CB-CD and C’B-C’D with those for samples B-D and B’-D’ in Fig. 6, one can see that the addition of a surface grating to an SP-coupled vertical LED for enhancing light extraction cannot only increase the absolute EQE, but also further reduce the EDE (smaller drooping slopes). When the light extraction of an LED is low, the trapped photons can be reabsorbed for generating heat and increasing the carrier density in the QWs. With the surface grating structures, the increased light extraction can weaken such effects and hence enhance EQE, particularly when injection current is high. In other words, the EDE is further reduced.

6. Conclusions

In summary, we have demonstrated the enhancement of output intensity, the generation of polarized output, and the reduction of the EDE in SP-coupled vertical LEDs with Ag nano-grating structures located between the p-GaN layers and the wafer bonding metals for inducing SP coupling with the InGaN/GaN QWs. In fabricating the vertical LEDs, the patterned sapphire substrate was removed with the PEC liftoff technique. Based on the reflection measurements from the metal grating structures and the numerical simulation results, it was found that the LSP resonances induced around the metal grating crests played the major role in the SP-QW coupling processes although certain LSP-SPP hybrid modes could be generated in the coupling processes. By fabricating surface grating structures on the n-GaN side with the combination of Lloyd’s interferometry and PEC etching in the SP-coupled vertical LEDs, the output intensity was further enhanced, the output polarization ratio was further increased, and the EDE was further suppressed.

Acknowledgments

This research was supported by National Science Council, Taiwan, under the grants of NSC 102-2221-E-002-204-MY3, NSC 102-2120-M-002-006, NSC 101-2622-E-002-002-CC2, and NSC 102-2221-E-002-199, by Epistar Corporation, by the Excellent Research Projects of National Taiwan University (102R890951 and 102R890952), and by US Air Force Scientific Research Office under the contract of AOARD-13-4143.

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OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(240.6680) Optics at surfaces : Surface plasmons

ToC Category:
Light-Emitting Diodes

History
Original Manuscript: January 2, 2014
Revised Manuscript: March 6, 2014
Manuscript Accepted: March 30, 2014
Published: April 10, 2014

Citation
Chun-Han Lin, Chieh Hsieh, Charng-Gan Tu, Yang Kuo, Horng-Shyang Chen, Pei-Ying Shih, Che-Hao Liao, Yean-Woei Kiang, C. C. Yang, Chih-Han Lai, Guan-Ru He, Jui-Hung Yeh, and Ta-Cheng Hsu, "Efficiency improvement of a vertical light-emitting diode through surface plasmon coupling and grating scattering," Opt. Express 22, A842-A856 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S3-A842


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References

  1. A. Neogi, C.-W. Lee, H. O. Everitt, T. Kuroda, A. Tackeuchi, and E. Yablonovitch, “Enhancement of spontaneous recombination rate in a quantum well by resonant surface plasmon coupling,” Phys. Rev. B66(15), 153305 (2002). [CrossRef]
  2. K. Okamoto, I. Niki, A. Shvartser, Y. Narukawa, T. Mukai, and A. Scherer, “Surface-plasmon-enhanced light emitters based on InGaN quantum wells,” Nat. Mater.3(9), 601–605 (2004). [CrossRef] [PubMed]
  3. G. Sun, J. B. Khurgin, and R. A. Soref, “Practicable enhancement of spontaneous emission using surface plasmons,” Appl. Phys. Lett.90(11), 111107 (2007). [CrossRef]
  4. Y. Kuo, S. Y. Ting, C. H. Liao, J. J. Huang, C. Y. Chen, C. Hsieh, Y. C. Lu, C. Y. Chen, K. C. Shen, C. F. Lu, D. M. Yeh, J. Y. Wang, W. H. Chuang, Y. W. Kiang, and C. C. Yang, “Surface plasmon coupling with radiating dipole for enhancing the emission efficiency of a light-emitting diode,” Opt. Express19(S4), A914–A929 (2011). [CrossRef] [PubMed]
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