- Email: [email protected]
GaN multiple quantum wells
Accepted Manuscript Investigation on surface-plasmon-enhanced light emission of InGaN/GaN multiple quantum wells
Zhenzhong Yu, Qiang Li, Qigao Fan, Yixin Zhu PII:
S0749-6036(18)30084-3
DOI:
10.1016/j.spmi.2018.03.034
Reference:
YSPMI 5564
To appear in:
Superlattices and Microstructures
Received Date:
15 January 2018
Revised Date:
13 March 2018
Accepted Date:
14 March 2018
Please cite this article as: Zhenzhong Yu, Qiang Li, Qigao Fan, Yixin Zhu, Investigation on surfaceplasmon-enhanced light emission of InGaN/GaN multiple quantum wells, Superlattices and Microstructures (2018), doi: 10.1016/j.spmi.2018.03.034
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ACCEPTED MANUSCRIPT
Investigation on surface-plasmon-enhanced light emission of InGaN/GaN multiple quantum wells Zhenzhong Yu*, Qiang Li, Qigao Fan，Yixin Zhu
School of Internet of Things Engineering, Jiangnan University, Wuxi 214122, China
Abstract: We demonstrate surface-plasmon (SP) enhanced light emission from InGaN/GaN near ultraviolet (NUV) multiple quantum wells (MQWs) using Ag thin films and nano-particles (NPs). Two types of Ag NP arrays are fabricated on the NUV-MQWs, one is fabricated on p-GaN layer with three different sizes of about 120, 160 and 240 nm formed by self-assembled process, while the other is embedded close to the MQWs. In addition, the influence of the surface plasmon polariton (SPP) and localized surface plasmon (LSP) in NUV-MQWs has been investigated by photoluminescence (PL) measurement. Both PL measurements and theoretical simulation results show that the NUV light would be extracted more effectively under LSP mode than that of SPP mode. The highest enhancement of PL intensity is increased by 324% for the sample with NPs embedded in etched p-GaN near the MQWs as compared with the bare MQWs, also is about 1.24 times higher than the MQW sample covered with Ag NPs on the surface, indicating strong surface scattering and SP coupling between Ag NPs and NUV-MQWs. Keywords: InGaN/GaN MQWs, localized surface plasmon, Ag nanoparticles, near
Email addresses: [email protected] (Z. Z. Yu) 1
ACCEPTED MANUSCRIPT ultraviolet photoluminescence. 1. Introduction Gallium nitride (GaN) is a key component of today’s semiconductor for solidstate lighting with spectrum from ultraviolet to visible wavelengths, it has been attracted significant attention to numerous applications, such as solid-state lighting, display backlight units, white light pump sources and high-density optical data storage [1-4]. Due to their abundant applications, improvements in the efficiency of light emission diodes (LEDs) are sustainable acquired. Two main approaches are applied to improve the external quantum efficiency (EQE): increasing the internal quantum efficiency (IQE) and/or improving the light extraction efficiency (LEE). The breakthroughs in its IQE (approaching 100%) have already been reported by many groups [5, 6]. Thus, it’s extremely difficult to further improve the IQE. Several techniques have been applied to improve LEE, such as patterned substrates [7, 8] as well as thin-film [9], rough-surface LEDs [10], quantum dots [11] and metal assisted LEDs [12]. The LEE improvement is mostly based on novel geometrical optical designs, which doesn’t change the properties of spontaneous emission [13]. The metal assisted LEDs are based on the surface plasmons (SPs) including localized surface plasmon (LSP) and surface plasmon polariton (SPP). SPs can be induced for producing a strong electric field enhancement in the vicinity of the metal/dielectric interface at an appropriate frequency and improvement in light emitting devices [1416]. SPs can increase the density of states (DOS) of electrons and holes, which are subject to the modulation of the intensified EM field by SP-QW (quantum well) 2
ACCEPTED MANUSCRIPT coupling, and this is widely known as Purcell effect. In addition, electron–hole recombination gives rise to SPs instead of photons, and this new decay channels induce a significant improvement of spontaneous emission rate, which is expected to compensate for the loss of efficiency droop in LEDs at high current injection [14, 17]. Recently, the coupling effect between multiple quantum wells (MQWs) and SPP or LSP with metal layer and nanoparticles (NPs) for light emission enhancement has been extensively investigated. Langhammer et al. reported LSPR were tunable over the visible to near ultraviolet (NUV) by varying the size of the fabricated Al nanodisks [18]. Lin et al. found that the spontaneous emission was enhanced in wavelength from 350 to 500nm with Ag NPs [19]. Kao et al. fabricated Ag nanotriangle array on a 40-nm-thick p-type GaN layer beneath the p-pad of the LED, demonstrated that the light output power of LED with Ag NPs was 15.4% higher than the counterpart without Ag NPs at an injected current of 20 mA, which can be attributed to the coupling effect between MQW and LSP [20]. To the best of our knowledge, relevant works previously published mainly focused on coupling effect between InGaN MQWs and LSP or SPP, in this work, by using self-assembled Ni NPs as etching mask and inductively coupled plasma reactive ion etching (ICP-RIE) process, the p-GaN layer on InGaN/GaN MQWs were partially etched for MQWsLSP coupling, while the other samples were left for Ag film or Ag NPs fabrication on the wafer surface. After that, the influence of SPP and LSP effect on the enhancement of light emission for InGaN/GaN NUV-MQWs could be investigated and compared. 2. Methods 3
ACCEPTED MANUSCRIPT The InGaN/GaN-based NUV-MQWs were grown on a c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). A schematic diagram of SPenhanced NUV-MQWs with Ag thin film and NPs is shown in Fig. 1. A 2-μm-thick n-GaN (Si doped) layer was grown at 1050℃ on 2μm thick undoped GaN epitaxial layer, followed by five periods of InGaN (3 nm)/GaN (12 nm) NUV-MQWs. As the SPs fringing field penetration depth is limited, which is given by L
R GRaN + Ag
2 GaN
(1)
R where GRaN and Ag are the real part of the dielectric constants of GaN and Ag, and
λ is the wavelength of the SP resonance. In this study, the GaN medium is set with dielectric constant of 6.25. The penetration depth L was estimated to be 62 nm for QW-SP coupling at the wavelength of 390 nm for NUV-LEDs. To investigate the resonance coupling effect of SPP/LSPs and MQWs in GaN-based NUV-MQWs more precisely, the thicknesses of the p-GaN and AlGaN layers were deposited as 35 and 15 nm in this paper, which are smaller than the penetration depth as calculated from equation 1. As can be seen from different regions of Fig. 1 (a), Ag layers were deposited directly on top of the p-GaN layer by magnetron sputtering. The size-tunable Ag nano-particles (NPs) were formed by thermal annealing with different thickness of Ag film at 300-320℃ in N2 ambient and annealing duration for 180 s. In order to embed NPs into the p-GaN layer near the MQWs, the nanoporous p-GaN layer were fabricated by using self-assembled Ni nanomasks with ICP-RIE system (Oxford instruments ICP 180), which was reported in our previous work [21]. After dry 4
ACCEPTED MANUSCRIPT etching, the samples were dipped into dilute hydrochloric acid (HCl) solution to remove the Ni nanomask and etching residues, and followed by the Ag deposition. Fig. 1(b) shows the scanning electron microscope (SEM) image of the Ni nano-island masks on p-GaN surface after the rapid thermal annealing process in N2 ambient, while Fig. 1(c) presents the atomic force microscopy (AFM) image of the p-GaN layer after ICP-RIE etching using Ni NP masks. The etching depth was estimated to be about 45 nm from AFM measurement. The surface morphologies were measured by AFM (Agilent 5100) and FE-SEM (JEOL JSM-7000F). The light emission of the NUV-MQWs was investigated by photoluminescence (PL) measurements, which were excited from the sample top surface and carried out by Renishaw inVia spectroscopy system under excitation of 325 nm at room temperature with a cw He-Cd laser . The PL signal is collected from the bottom of wafer through a focusing lens of 2 cm in diameter with its optical axis normal to the sample surface. In addition, the power and the focusing are same for each wafer. For the theoretical simulation of NUV-MQWs coupled with SPP and LSP, the finite-difference time-domain (FDTD, Lumerical solutions) simulation method was used to investigate the effects of Ag layers and NPs coated on NUV-MQWs. To accurate exquisite field distribution, the maximum grid size was fixed as 1×1×1nm, which is an adequate size to resolve the strongly localized field distribution. The entire region was placed by perfectly matched layers (PMLs) conditions on all the sides in order to absorb the waves, and the whole system was merged into air with dielectric constant. In this study, the 5
ACCEPTED MANUSCRIPT dielectric constant of GaN medium was set with 6.25 and Ag material was fitted into the promising experimental data [22]. In order to obtain accurate simulation profiles, the simulation structures of the Ag layer and NPs were extracted from original AFM data directly. 3. Results and Discussion Fig. 2(a) shows the surface morphology of Ag layer (3×3 μm). The root mean square roughness (RMS) of the Ag films is approximately as low as 2.9 nm, indicating smooth surfaces of Ag layer could be obtained using magnetron sputtering deposition. By changing the deposition time of the magnetron sputtering, three different thicknesses of Ag films were obtained. From the cross-sectional profiles of AFM, the thicknesses of the three Ag films are estimated to be about 10, 15 and 20 nm as the solid cut line indicated in Fig. 2(b). Fig. 3(a) shows the PL spectra of NUV-MQWs coated with Ag layer with different thickness. For the referential bare sample, the spectrum is dominated by a near ultraviolet emission peak approximately at λ=393 nm typically from the MQWs. The emission peak of bare sample at 393 nm is normalized to 1. From the PL spectra, we can conclude that the thickness of Ag layers could play an important role on the PL emission intensity of the MQWs, since the PL intensity is increased after the top Ag layer was deposited due to SP-QW coupling. More detailedly, when Ag layers are added to the top of a QW epitaxial structure, the enhanced reflection at the metal/semiconductor interface can increase the PL excitation intensity at the location of a QW and hence enhances PL emission. It can be seen that the ratio of the PL 6
ACCEPTED MANUSCRIPT enhancement is not monotonous increased with the thickness of Ag film inducing by Ag absorption. The PL intensity of MQWs with 15 nm Ag film is higher than the other samples, achieving the enhancement ratio of 2.34 at the position of 392 nm compared to the as-grown sample. This result indicates that the excitons of MQWs in NUV-LEDs are effectively excited at the frequency of SP resonance for Ag films, which is further convinced by E-field distribution in Fig 3(b), which presents the Efield enhanced ( log10 E /E0 2 ) distribution on the x-y plane with 15 nm Ag layer on MQW surface. It can be seen that enhanced SP resonant field can be observed on the surface of Ag film, however, the distribution of E-field intensity shows an anomalous texture due to the surface of Ag film is not really flat. Actually, it would be difficult to extract light from the SP mode in flat metal film due to low scattering probability. Okamoto et al. achieved about 5-fold enhancement in peak (∼470 nm) PL intensity from the Ag-coated blue emitter when the distance of the active layer to the metals is ∼50 nm, but different Ag reflectivity between 390nm and 470 nm [14]. Fig. 4(a-c) show the surface morphologies (5×5 μm) of self-assembled Ag NPs with three different diameters fabricated by thermal annealing directly on p-GaN surface. It can be seen that the Ag NPs show a densely pebble-like texture. The Ag nano-islands become larger and dispersive as the initial Ag film is thicker. On the other hand, as the Ag thickness decreased, it is easier for Ag migration that results in a smaller Ag cluster and denser particle matrix under the same annealed condition. Estimated by statistical calculation from the AFM cross-sectional profiles in Fig. 4(e), the sizes of most Ag NPs for the three samples are approximately 120±10, 160±10, and 240±10 7
ACCEPTED MANUSCRIPT nm, denoted as sample A, B, and C, respectively. Fig. 5(a) shows the PL spectra of InGaN/GaN MQWs coated with dispersive Ag NPs for sample A, B and C. It can be seen that the NUV-MQW PL intensities are significantly enhanced after coated with Ag NPs than that of the Ag-film-deposited MQWs, as the PL intensity enhancement ratio shown in Fig. 5(b), verifying that LSPs are more effective than SPs for NUV-MQW light emission enhancement. The maximum PL intensity of NUV-MQWs is achieved at 392 nm with a narrow FWHM of 8 nm in sample B with 160 nm Ag NPs on MQWs wafer surface. Moreover, with the employment of the Ag NPs on p-GaN layer, an obviously blue-shift (~3 nm blue shift from as-grown sample to sample A) of the emission peak can be observed in the PL spectra, indicating the existence of an LSP coupling process [23, 24]. And by increasing the size of Ag NPs, a red-shift (~2 nm red shift from sample A to B) of the PL emission peak could be subsequently induced with gradually increased PL intensity. This would be ascribed to the absorption wavelength of the NPs towards red shifting with increase of NP size, which has been verified by many researches in Ref [19, 25, 26]. However, further increase of the NP size will cause the decline of PL intensity. Nevertheless, it is also much larger than that of the sample coated with Ag thin film (shown in Fig. 5(b)). It is clear that the peak PL intensities of the MQW emission are enhanced by 235%, 319%, and 293% for samples A, B and C, respectively, compared with the bare MQWs. The enhancement of PL would be relevant to the self-coupling of SPs on NPs, and further increased diameter of the NPs could result in stronger self-coupling of SPs, which renders invalidation of SP energy8
ACCEPTED MANUSCRIPT momentum dispersion relation [19]. It is noted that the magnitude of enhancement ration is declined for NPs with diameter about 240 nm, which is attributed to that larger metal particles dissipation can cause SP loss [27]. Previous report verified that the SP absorption wavelength of the NPs should be overlapped with the PL emission wavelength of the MQWs in the same ranges for efficient exciton-localized SP coupling [28], which is in line with the our experimental phenomenon described above. Fig. 5(c) shows the normalized PL intensity of bare sample and absorption spectra for Ag NPs with different sizes. The 160 nm sized Ag NPs is expected to result in the best performance for NUV-MQWs, because its absorption peak wavelength is much more close to the PL emission wavelength of the bare MQWs than that of the other two samples, which is in excellent agreement with the experimental result of PL enhanced spectra in Fig. 3(a). It has reported that the LSP resonance strength at the peak is expected to be weaker when compared with that of an Ag NP distribution of a uniform NP size based on the techniques of nanoimprint lithography and reactive ion etching [29]. Nevertheless, this method can be fabricated with an inexpensive approach and hence are practically useful in LED application. Fig. 5(d-f) illustrates the E-field enhanced distribution ( log10
E /E0
2
) evaluated on
the slice cutting through Ag NPs at the peak wavelength corresponding to sample A, B and C, in all of which enhanced SP resonant field can be observed on the surfaces of Ag NPs. The comparison of magnitude of the E-field intensity distribution can also be distinguished by the profiles that the Ag NPs with a diameter of 160 nm (shown in Fig. 5(e)) reveals the highest peak intensity and largest area of high intensity region, 9
ACCEPTED MANUSCRIPT which is consistent with the PL intensity enhancement in Fig. 5(a). Fig. 6(a) presents the SEM image of nanoporous arrays etched by ICP-RIE process using self-assembled Ni nano masks, and covered with Ag NPs. It should be noted that the Ag film was directly deposited on etched MQWs surface layer without any annealing process, for comparison with SPP structure, the Ag deposition conditions was consistent with 10 nm thick Ag layer in Fig. 2. The surface morphology shows densely island-like texture of the Ag layers. Fig. 6(b) shows the normalized PL intensity spectra of bare MQWs, etched MQWs and etched MQWs with Ag NPs on the surface. It appears that the peak wavelength shows a slight enhancement for the etched MQWs compared with the as grown bare MQW sample, which would be due to the strong sidewall light scattering [30, 31] on etched MQWs surface. This means that the nanoporous p-GaN surface can provide stronger surface scattering and improve the light extraction, which could also increase the PL intensity. The PL intensity of the plasmonic sample with Ag NPs covered on etched MQWs is increased by 324%, compared to the as-grown sample without Ag NPs at the peak position of around 392 nm. The large increase in the luminescence intensity after metallization is due to the LSP-MQWs coupling. It should be noted that the weak peak at 406 nm gets sharper in the PL spectra when the Ag NPs is covered. The origin of this blue band may be caused by p-GaN (Mg doped), which have been attributed to the donor-acceptor-pair and conduction-band-acceptor transitions, involving the same MgGa acceptor with an activation energy of 0.23 eV [32]. In addition, the enhancement ratio is larger than the three samples in Fig. 5(a), which can be 10
ACCEPTED MANUSCRIPT explained by the reasons of enhanced surface light scattering and SP-MQWs coupling. As SP is an evanescent wave that exponentially decays with distance from the metal/dielectric interface [23]. Only electron–hole pairs located within the nearfield of the surface can couple to the SP mode, and this penetration depth of the SP fringing field into the semiconductor is limited, which is given in equation (1). As the inset of Fig. 6(a) shown, the LSP near-field of Ag NPs can penetrate in to the MQWs, leading to a strong coupling between electron-hole pairs and SP, and the SP energy can be extracted effectively in short distance. 4. Conclusion In summary, we have demonstrated SP-enhanced light emission from InGaN/GaN NUV-MQWs covered by Ag films, self-assembled Ag NPs, and etched p-GaN embedded with Ag NPs. The thickness of Ag films and the size of Ag NPs have exerted an important impact on SP-QW coupling, the higher PL enhancements could be obtained with deposited 15 nm-thick Ag film or 160 nm-sized Ag NPs on the surface of NUV-MQWs of SPP and LSP modes, respectively. Moreover, the numerical value of the PL intensity enhancement ratios for LSP-coupling MQWs are larger than that of SP-coupling MQWs, and the PL intensity of the etched MQWs with Ag NPs is enhanced about 3.24 times as compared to the bare MQW sample, also is about 1.24 times higher than the un-etched plasmonic MQW wafer only with Ag NPs covered on the surface, indicating that SP energy can be extracted effectively in shorter distance. We conclude that the significant light emission enhancement is mainly attributed to strong surface light scattering and LSP-coupling between Ag NPs 11
ACCEPTED MANUSCRIPT and MQWs, which has promising applications for increasing the luminescence efficiency of InGaN-based visible and NUV LEDs. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51405198), Natural Science Foundation of Jiangsu Province (No. BK20130159), China Postdoctoral Science Foundation (No. 2016M590406).
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Figure Captions: Fig.1. (a) Schematic structure of SP-enhanced NUV-LEDs with Ag layer and NPs. (b) SEM image of the self-assembled Ni NPs. (c) Top-view AFM image of the p-GaN layer after dry etching by using self-assembled Ni NP masks. Inset: cross-sectional profile performed along the etched p-GaN indicated in the AFM image with white 16
ACCEPTED MANUSCRIPT dotted line. The etching depth was estimated to be about 45 nm (red dotted line).
Fig. 2. A typical 3D morphological AFM image of Ag layer (a) and cross-sectional profile performed along the Ag layers with different thickness (b).
Fig. 3. (a) Room temperature PL spectra for the NUV-LEDs with 10, 15 and 20 nm Ag layer and bare MQWs. (b) E-field profiles ( log10 E /E0 2 ) distribution on the x-y plane for simulation system with 15 nm Ag layer.
Fig. 4. (a)-(c) Typical AFM images of Ag NPs corresponding to sample A, B and C, respectively. (d) Line scan profiles performed along the Ag NPs with different sizes.
Fig. 5. (a) Room temperature PL spectra for the bare MQWs and sample A, B, C corresponding to Ag NPs with different sizes. (b) PL intensity enhancement ratio for plasmonic samples with Ag films and NPs. (c) The normalized PL intensity of bare sample and absorption spectra of the three Ag NP samples. The absorption spectra are calculated by FDTD simulation. (d-f) E-field profiles ( log
10
E /E0
2
) distribution on the x-
y plane for simulation system with Ag NPs corresponding to sample A, B and C, respectively.
Fig 6. (a) SEM image of nanoporous arrays covered with Ag NPs. The inset shows simulated local near-field profiles ( log
10
E /E0
17
2
) distribution of Ag NPs covered on
ACCEPTED MANUSCRIPT etched MQWs surface. The shape of Ag NPs is considered as a sphere in the simulation and the radius of the Al NPs is set to 30 nm. (b) PL intensity spectra of bare MQWs, etched MQWs and Ag NPs covered on the surface of etched MQWs. Fig. 1
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Highlights > Three types of Ag plasmonic nanostructures is compared to enhance MQW photoluminescence. > Ni nanomasks are used to etch the GaN surface for nanostructure fabrication. >NUV-MQW emissions are significantly increased using Ag NPs embedded in pGaN.
Zhenzhong Yu, Qiang Li, Qigao Fan, Yixin Zhu PII:
S0749-6036(18)30084-3
DOI:
10.1016/j.spmi.2018.03.034
Reference:
YSPMI 5564
To appear in:
Superlattices and Microstructures
Received Date:
15 January 2018
Revised Date:
13 March 2018
Accepted Date:
14 March 2018
Please cite this article as: Zhenzhong Yu, Qiang Li, Qigao Fan, Yixin Zhu, Investigation on surfaceplasmon-enhanced light emission of InGaN/GaN multiple quantum wells, Superlattices and Microstructures (2018), doi: 10.1016/j.spmi.2018.03.034
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Investigation on surface-plasmon-enhanced light emission of InGaN/GaN multiple quantum wells Zhenzhong Yu*, Qiang Li, Qigao Fan，Yixin Zhu
School of Internet of Things Engineering, Jiangnan University, Wuxi 214122, China
Abstract: We demonstrate surface-plasmon (SP) enhanced light emission from InGaN/GaN near ultraviolet (NUV) multiple quantum wells (MQWs) using Ag thin films and nano-particles (NPs). Two types of Ag NP arrays are fabricated on the NUV-MQWs, one is fabricated on p-GaN layer with three different sizes of about 120, 160 and 240 nm formed by self-assembled process, while the other is embedded close to the MQWs. In addition, the influence of the surface plasmon polariton (SPP) and localized surface plasmon (LSP) in NUV-MQWs has been investigated by photoluminescence (PL) measurement. Both PL measurements and theoretical simulation results show that the NUV light would be extracted more effectively under LSP mode than that of SPP mode. The highest enhancement of PL intensity is increased by 324% for the sample with NPs embedded in etched p-GaN near the MQWs as compared with the bare MQWs, also is about 1.24 times higher than the MQW sample covered with Ag NPs on the surface, indicating strong surface scattering and SP coupling between Ag NPs and NUV-MQWs. Keywords: InGaN/GaN MQWs, localized surface plasmon, Ag nanoparticles, near
Email addresses: [email protected] (Z. Z. Yu) 1
ACCEPTED MANUSCRIPT ultraviolet photoluminescence. 1. Introduction Gallium nitride (GaN) is a key component of today’s semiconductor for solidstate lighting with spectrum from ultraviolet to visible wavelengths, it has been attracted significant attention to numerous applications, such as solid-state lighting, display backlight units, white light pump sources and high-density optical data storage [1-4]. Due to their abundant applications, improvements in the efficiency of light emission diodes (LEDs) are sustainable acquired. Two main approaches are applied to improve the external quantum efficiency (EQE): increasing the internal quantum efficiency (IQE) and/or improving the light extraction efficiency (LEE). The breakthroughs in its IQE (approaching 100%) have already been reported by many groups [5, 6]. Thus, it’s extremely difficult to further improve the IQE. Several techniques have been applied to improve LEE, such as patterned substrates [7, 8] as well as thin-film [9], rough-surface LEDs [10], quantum dots [11] and metal assisted LEDs [12]. The LEE improvement is mostly based on novel geometrical optical designs, which doesn’t change the properties of spontaneous emission [13]. The metal assisted LEDs are based on the surface plasmons (SPs) including localized surface plasmon (LSP) and surface plasmon polariton (SPP). SPs can be induced for producing a strong electric field enhancement in the vicinity of the metal/dielectric interface at an appropriate frequency and improvement in light emitting devices [1416]. SPs can increase the density of states (DOS) of electrons and holes, which are subject to the modulation of the intensified EM field by SP-QW (quantum well) 2
ACCEPTED MANUSCRIPT coupling, and this is widely known as Purcell effect. In addition, electron–hole recombination gives rise to SPs instead of photons, and this new decay channels induce a significant improvement of spontaneous emission rate, which is expected to compensate for the loss of efficiency droop in LEDs at high current injection [14, 17]. Recently, the coupling effect between multiple quantum wells (MQWs) and SPP or LSP with metal layer and nanoparticles (NPs) for light emission enhancement has been extensively investigated. Langhammer et al. reported LSPR were tunable over the visible to near ultraviolet (NUV) by varying the size of the fabricated Al nanodisks [18]. Lin et al. found that the spontaneous emission was enhanced in wavelength from 350 to 500nm with Ag NPs [19]. Kao et al. fabricated Ag nanotriangle array on a 40-nm-thick p-type GaN layer beneath the p-pad of the LED, demonstrated that the light output power of LED with Ag NPs was 15.4% higher than the counterpart without Ag NPs at an injected current of 20 mA, which can be attributed to the coupling effect between MQW and LSP [20]. To the best of our knowledge, relevant works previously published mainly focused on coupling effect between InGaN MQWs and LSP or SPP, in this work, by using self-assembled Ni NPs as etching mask and inductively coupled plasma reactive ion etching (ICP-RIE) process, the p-GaN layer on InGaN/GaN MQWs were partially etched for MQWsLSP coupling, while the other samples were left for Ag film or Ag NPs fabrication on the wafer surface. After that, the influence of SPP and LSP effect on the enhancement of light emission for InGaN/GaN NUV-MQWs could be investigated and compared. 2. Methods 3
ACCEPTED MANUSCRIPT The InGaN/GaN-based NUV-MQWs were grown on a c-plane sapphire substrate by metal-organic chemical vapor deposition (MOCVD). A schematic diagram of SPenhanced NUV-MQWs with Ag thin film and NPs is shown in Fig. 1. A 2-μm-thick n-GaN (Si doped) layer was grown at 1050℃ on 2μm thick undoped GaN epitaxial layer, followed by five periods of InGaN (3 nm)/GaN (12 nm) NUV-MQWs. As the SPs fringing field penetration depth is limited, which is given by L
R GRaN + Ag
2 GaN
(1)
R where GRaN and Ag are the real part of the dielectric constants of GaN and Ag, and
λ is the wavelength of the SP resonance. In this study, the GaN medium is set with dielectric constant of 6.25. The penetration depth L was estimated to be 62 nm for QW-SP coupling at the wavelength of 390 nm for NUV-LEDs. To investigate the resonance coupling effect of SPP/LSPs and MQWs in GaN-based NUV-MQWs more precisely, the thicknesses of the p-GaN and AlGaN layers were deposited as 35 and 15 nm in this paper, which are smaller than the penetration depth as calculated from equation 1. As can be seen from different regions of Fig. 1 (a), Ag layers were deposited directly on top of the p-GaN layer by magnetron sputtering. The size-tunable Ag nano-particles (NPs) were formed by thermal annealing with different thickness of Ag film at 300-320℃ in N2 ambient and annealing duration for 180 s. In order to embed NPs into the p-GaN layer near the MQWs, the nanoporous p-GaN layer were fabricated by using self-assembled Ni nanomasks with ICP-RIE system (Oxford instruments ICP 180), which was reported in our previous work [21]. After dry 4
ACCEPTED MANUSCRIPT etching, the samples were dipped into dilute hydrochloric acid (HCl) solution to remove the Ni nanomask and etching residues, and followed by the Ag deposition. Fig. 1(b) shows the scanning electron microscope (SEM) image of the Ni nano-island masks on p-GaN surface after the rapid thermal annealing process in N2 ambient, while Fig. 1(c) presents the atomic force microscopy (AFM) image of the p-GaN layer after ICP-RIE etching using Ni NP masks. The etching depth was estimated to be about 45 nm from AFM measurement. The surface morphologies were measured by AFM (Agilent 5100) and FE-SEM (JEOL JSM-7000F). The light emission of the NUV-MQWs was investigated by photoluminescence (PL) measurements, which were excited from the sample top surface and carried out by Renishaw inVia spectroscopy system under excitation of 325 nm at room temperature with a cw He-Cd laser . The PL signal is collected from the bottom of wafer through a focusing lens of 2 cm in diameter with its optical axis normal to the sample surface. In addition, the power and the focusing are same for each wafer. For the theoretical simulation of NUV-MQWs coupled with SPP and LSP, the finite-difference time-domain (FDTD, Lumerical solutions) simulation method was used to investigate the effects of Ag layers and NPs coated on NUV-MQWs. To accurate exquisite field distribution, the maximum grid size was fixed as 1×1×1nm, which is an adequate size to resolve the strongly localized field distribution. The entire region was placed by perfectly matched layers (PMLs) conditions on all the sides in order to absorb the waves, and the whole system was merged into air with dielectric constant. In this study, the 5
ACCEPTED MANUSCRIPT dielectric constant of GaN medium was set with 6.25 and Ag material was fitted into the promising experimental data [22]. In order to obtain accurate simulation profiles, the simulation structures of the Ag layer and NPs were extracted from original AFM data directly. 3. Results and Discussion Fig. 2(a) shows the surface morphology of Ag layer (3×3 μm). The root mean square roughness (RMS) of the Ag films is approximately as low as 2.9 nm, indicating smooth surfaces of Ag layer could be obtained using magnetron sputtering deposition. By changing the deposition time of the magnetron sputtering, three different thicknesses of Ag films were obtained. From the cross-sectional profiles of AFM, the thicknesses of the three Ag films are estimated to be about 10, 15 and 20 nm as the solid cut line indicated in Fig. 2(b). Fig. 3(a) shows the PL spectra of NUV-MQWs coated with Ag layer with different thickness. For the referential bare sample, the spectrum is dominated by a near ultraviolet emission peak approximately at λ=393 nm typically from the MQWs. The emission peak of bare sample at 393 nm is normalized to 1. From the PL spectra, we can conclude that the thickness of Ag layers could play an important role on the PL emission intensity of the MQWs, since the PL intensity is increased after the top Ag layer was deposited due to SP-QW coupling. More detailedly, when Ag layers are added to the top of a QW epitaxial structure, the enhanced reflection at the metal/semiconductor interface can increase the PL excitation intensity at the location of a QW and hence enhances PL emission. It can be seen that the ratio of the PL 6
ACCEPTED MANUSCRIPT enhancement is not monotonous increased with the thickness of Ag film inducing by Ag absorption. The PL intensity of MQWs with 15 nm Ag film is higher than the other samples, achieving the enhancement ratio of 2.34 at the position of 392 nm compared to the as-grown sample. This result indicates that the excitons of MQWs in NUV-LEDs are effectively excited at the frequency of SP resonance for Ag films, which is further convinced by E-field distribution in Fig 3(b), which presents the Efield enhanced ( log10 E /E0 2 ) distribution on the x-y plane with 15 nm Ag layer on MQW surface. It can be seen that enhanced SP resonant field can be observed on the surface of Ag film, however, the distribution of E-field intensity shows an anomalous texture due to the surface of Ag film is not really flat. Actually, it would be difficult to extract light from the SP mode in flat metal film due to low scattering probability. Okamoto et al. achieved about 5-fold enhancement in peak (∼470 nm) PL intensity from the Ag-coated blue emitter when the distance of the active layer to the metals is ∼50 nm, but different Ag reflectivity between 390nm and 470 nm [14]. Fig. 4(a-c) show the surface morphologies (5×5 μm) of self-assembled Ag NPs with three different diameters fabricated by thermal annealing directly on p-GaN surface. It can be seen that the Ag NPs show a densely pebble-like texture. The Ag nano-islands become larger and dispersive as the initial Ag film is thicker. On the other hand, as the Ag thickness decreased, it is easier for Ag migration that results in a smaller Ag cluster and denser particle matrix under the same annealed condition. Estimated by statistical calculation from the AFM cross-sectional profiles in Fig. 4(e), the sizes of most Ag NPs for the three samples are approximately 120±10, 160±10, and 240±10 7
ACCEPTED MANUSCRIPT nm, denoted as sample A, B, and C, respectively. Fig. 5(a) shows the PL spectra of InGaN/GaN MQWs coated with dispersive Ag NPs for sample A, B and C. It can be seen that the NUV-MQW PL intensities are significantly enhanced after coated with Ag NPs than that of the Ag-film-deposited MQWs, as the PL intensity enhancement ratio shown in Fig. 5(b), verifying that LSPs are more effective than SPs for NUV-MQW light emission enhancement. The maximum PL intensity of NUV-MQWs is achieved at 392 nm with a narrow FWHM of 8 nm in sample B with 160 nm Ag NPs on MQWs wafer surface. Moreover, with the employment of the Ag NPs on p-GaN layer, an obviously blue-shift (~3 nm blue shift from as-grown sample to sample A) of the emission peak can be observed in the PL spectra, indicating the existence of an LSP coupling process [23, 24]. And by increasing the size of Ag NPs, a red-shift (~2 nm red shift from sample A to B) of the PL emission peak could be subsequently induced with gradually increased PL intensity. This would be ascribed to the absorption wavelength of the NPs towards red shifting with increase of NP size, which has been verified by many researches in Ref [19, 25, 26]. However, further increase of the NP size will cause the decline of PL intensity. Nevertheless, it is also much larger than that of the sample coated with Ag thin film (shown in Fig. 5(b)). It is clear that the peak PL intensities of the MQW emission are enhanced by 235%, 319%, and 293% for samples A, B and C, respectively, compared with the bare MQWs. The enhancement of PL would be relevant to the self-coupling of SPs on NPs, and further increased diameter of the NPs could result in stronger self-coupling of SPs, which renders invalidation of SP energy8
ACCEPTED MANUSCRIPT momentum dispersion relation [19]. It is noted that the magnitude of enhancement ration is declined for NPs with diameter about 240 nm, which is attributed to that larger metal particles dissipation can cause SP loss [27]. Previous report verified that the SP absorption wavelength of the NPs should be overlapped with the PL emission wavelength of the MQWs in the same ranges for efficient exciton-localized SP coupling [28], which is in line with the our experimental phenomenon described above. Fig. 5(c) shows the normalized PL intensity of bare sample and absorption spectra for Ag NPs with different sizes. The 160 nm sized Ag NPs is expected to result in the best performance for NUV-MQWs, because its absorption peak wavelength is much more close to the PL emission wavelength of the bare MQWs than that of the other two samples, which is in excellent agreement with the experimental result of PL enhanced spectra in Fig. 3(a). It has reported that the LSP resonance strength at the peak is expected to be weaker when compared with that of an Ag NP distribution of a uniform NP size based on the techniques of nanoimprint lithography and reactive ion etching [29]. Nevertheless, this method can be fabricated with an inexpensive approach and hence are practically useful in LED application. Fig. 5(d-f) illustrates the E-field enhanced distribution ( log10
E /E0
2
) evaluated on
the slice cutting through Ag NPs at the peak wavelength corresponding to sample A, B and C, in all of which enhanced SP resonant field can be observed on the surfaces of Ag NPs. The comparison of magnitude of the E-field intensity distribution can also be distinguished by the profiles that the Ag NPs with a diameter of 160 nm (shown in Fig. 5(e)) reveals the highest peak intensity and largest area of high intensity region, 9
ACCEPTED MANUSCRIPT which is consistent with the PL intensity enhancement in Fig. 5(a). Fig. 6(a) presents the SEM image of nanoporous arrays etched by ICP-RIE process using self-assembled Ni nano masks, and covered with Ag NPs. It should be noted that the Ag film was directly deposited on etched MQWs surface layer without any annealing process, for comparison with SPP structure, the Ag deposition conditions was consistent with 10 nm thick Ag layer in Fig. 2. The surface morphology shows densely island-like texture of the Ag layers. Fig. 6(b) shows the normalized PL intensity spectra of bare MQWs, etched MQWs and etched MQWs with Ag NPs on the surface. It appears that the peak wavelength shows a slight enhancement for the etched MQWs compared with the as grown bare MQW sample, which would be due to the strong sidewall light scattering [30, 31] on etched MQWs surface. This means that the nanoporous p-GaN surface can provide stronger surface scattering and improve the light extraction, which could also increase the PL intensity. The PL intensity of the plasmonic sample with Ag NPs covered on etched MQWs is increased by 324%, compared to the as-grown sample without Ag NPs at the peak position of around 392 nm. The large increase in the luminescence intensity after metallization is due to the LSP-MQWs coupling. It should be noted that the weak peak at 406 nm gets sharper in the PL spectra when the Ag NPs is covered. The origin of this blue band may be caused by p-GaN (Mg doped), which have been attributed to the donor-acceptor-pair and conduction-band-acceptor transitions, involving the same MgGa acceptor with an activation energy of 0.23 eV [32]. In addition, the enhancement ratio is larger than the three samples in Fig. 5(a), which can be 10
ACCEPTED MANUSCRIPT explained by the reasons of enhanced surface light scattering and SP-MQWs coupling. As SP is an evanescent wave that exponentially decays with distance from the metal/dielectric interface [23]. Only electron–hole pairs located within the nearfield of the surface can couple to the SP mode, and this penetration depth of the SP fringing field into the semiconductor is limited, which is given in equation (1). As the inset of Fig. 6(a) shown, the LSP near-field of Ag NPs can penetrate in to the MQWs, leading to a strong coupling between electron-hole pairs and SP, and the SP energy can be extracted effectively in short distance. 4. Conclusion In summary, we have demonstrated SP-enhanced light emission from InGaN/GaN NUV-MQWs covered by Ag films, self-assembled Ag NPs, and etched p-GaN embedded with Ag NPs. The thickness of Ag films and the size of Ag NPs have exerted an important impact on SP-QW coupling, the higher PL enhancements could be obtained with deposited 15 nm-thick Ag film or 160 nm-sized Ag NPs on the surface of NUV-MQWs of SPP and LSP modes, respectively. Moreover, the numerical value of the PL intensity enhancement ratios for LSP-coupling MQWs are larger than that of SP-coupling MQWs, and the PL intensity of the etched MQWs with Ag NPs is enhanced about 3.24 times as compared to the bare MQW sample, also is about 1.24 times higher than the un-etched plasmonic MQW wafer only with Ag NPs covered on the surface, indicating that SP energy can be extracted effectively in shorter distance. We conclude that the significant light emission enhancement is mainly attributed to strong surface light scattering and LSP-coupling between Ag NPs 11
ACCEPTED MANUSCRIPT and MQWs, which has promising applications for increasing the luminescence efficiency of InGaN-based visible and NUV LEDs. Acknowledgements This work is supported by the National Natural Science Foundation of China (No. 51405198), Natural Science Foundation of Jiangsu Province (No. BK20130159), China Postdoctoral Science Foundation (No. 2016M590406).
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Figure Captions: Fig.1. (a) Schematic structure of SP-enhanced NUV-LEDs with Ag layer and NPs. (b) SEM image of the self-assembled Ni NPs. (c) Top-view AFM image of the p-GaN layer after dry etching by using self-assembled Ni NP masks. Inset: cross-sectional profile performed along the etched p-GaN indicated in the AFM image with white 16
ACCEPTED MANUSCRIPT dotted line. The etching depth was estimated to be about 45 nm (red dotted line).
Fig. 2. A typical 3D morphological AFM image of Ag layer (a) and cross-sectional profile performed along the Ag layers with different thickness (b).
Fig. 3. (a) Room temperature PL spectra for the NUV-LEDs with 10, 15 and 20 nm Ag layer and bare MQWs. (b) E-field profiles ( log10 E /E0 2 ) distribution on the x-y plane for simulation system with 15 nm Ag layer.
Fig. 4. (a)-(c) Typical AFM images of Ag NPs corresponding to sample A, B and C, respectively. (d) Line scan profiles performed along the Ag NPs with different sizes.
Fig. 5. (a) Room temperature PL spectra for the bare MQWs and sample A, B, C corresponding to Ag NPs with different sizes. (b) PL intensity enhancement ratio for plasmonic samples with Ag films and NPs. (c) The normalized PL intensity of bare sample and absorption spectra of the three Ag NP samples. The absorption spectra are calculated by FDTD simulation. (d-f) E-field profiles ( log
10
E /E0
2
) distribution on the x-
y plane for simulation system with Ag NPs corresponding to sample A, B and C, respectively.
Fig 6. (a) SEM image of nanoporous arrays covered with Ag NPs. The inset shows simulated local near-field profiles ( log
10
E /E0
17
2
) distribution of Ag NPs covered on
ACCEPTED MANUSCRIPT etched MQWs surface. The shape of Ag NPs is considered as a sphere in the simulation and the radius of the Al NPs is set to 30 nm. (b) PL intensity spectra of bare MQWs, etched MQWs and Ag NPs covered on the surface of etched MQWs. Fig. 1
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Highlights > Three types of Ag plasmonic nanostructures is compared to enhance MQW photoluminescence. > Ni nanomasks are used to etch the GaN surface for nanostructure fabrication. >NUV-MQW emissions are significantly increased using Ag NPs embedded in pGaN.