Nanoscale Characterization of Carrier Dynamic and Surface

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Nanoscale characterization of carrier dynamic and surface passivation in InGaN/GaN multiple quantum wells on GaN nanorods Weijian Chen, Xiaoming Wen, Michael Latzel, Martin Heilmann, Jianfeng Yang, Xi Dai, Shujuan Huang, Santosh Shrestha, Robert John Patterson, Silke H. Christiansen, and Gavin J. Conibeer ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11675 • Publication Date (Web): 31 Oct 2016 Downloaded from http://pubs.acs.org on November 1, 2016

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ACS Applied Materials & Interfaces

Nanoscale characterization of carrier dynamic and surface passivation in InGaN/GaN multiple quantum wells on GaN nanorods Weijian Chen, † Xiaoming Wen, †* Michael Latzel, ‡§ Martin Heilmann, ‡ Jianfeng Yang, † Xi Dai, †

Shujuan Huang, † Santosh Shrestha, † Robert Patterson, † Silke Christiansen, ‡∥# and Gavin Conibeer†

†

Australian Centre for Advanced Photovoltaics, School of Photovoltaic and Renewable Energy Engineering, UNSW Australia, Sydney 2052, Australia ‡

Max Planck Institute for the Science of Light, Günther-Scharowsky-Str. 1/Bau 24, 91058 Erlangen, Germany

§

Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), Institute of Optics, Information and Photonics, Staudtstr. 7/B2, 91058 Erlangen, Germany ∥

Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Institute of Nanoarchitectures for Energy Conversion, Hahn-Meitner-Platz 1, 14109 Berlin, Germany #

Freie Universität Berlin, Department of Physics, Arnimallee 14, 14195 Berlin, Germany

ACS Paragon Plus Environment

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KEYWORDS Nanoscale optical characterisation; GaN; Multiple quantum well; Nanorods; Two-photon excitation; Surface passivation

ABSTRACT

Using advanced two-photon excitation confocal microscopy, associated with time-resolved spectroscopy, we characterize InGaN/GaN multiple quantum wells on nanorod heterostructures and demonstrate the passivation effect of a KOH treatment. High-quality InGaN/GaN nanorods were fabricated using nanosphere lithography as a candidate material for light emitting diode devices. The depth- and time-resolved characterization at the nanoscale provides detailed carrier dynamic analysis helpful for understanding the optical properties. The nanoscale spatially resolved images of InGaN quantum well and defects were acquired simultaneously.

We

demonstrate that nanorod etching improves light extraction efficiency and a proper KOH treatment has been found to reduce the surface defects efficiently and enhance the luminescence. The optical characterization techniques provide depth-resolved and time-resolved carrier dynamics with nanoscale spatially-resolved mapping, which is crucial for a comprehensive and thorough understanding of nanostructured materials and provides novel insight into the improvement of materials fabrication and applications.

INTRODUCTION III-Group nitride-based heterostructures have been widely studied for optoelectronics applications.1–5 Various kinds of nanostructures are often fabricated by directional growth and post-growth treatment.6–8 Because structural and composition variation are the essential


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characteristics of such materials, precise spatial characterization is of crucial importance for explaining the structure-properties relationship. Meanwhile, carrier dynamics in such heterostructures directly determines their optoelectronic efficiency.3,9–11 Such that XY-plane and depth resolved carrier dynamics analysis on nanoscale dimension is essential, but tough to obtain due to limitations of the techniques. Cathodoluminescence (CL) provides a nanoscale study of luminescence response.7,12–14 However, CL usually uses tilted sample holder to allow both electron beam excitation and CL collection, resulting in sophisticated analysis.12 Special specimen treatments are often required and even damage samples permanently.13,14 Photoluminescence (PL) is more often carried out to examine photoexcited carrier dynamics of quantum confined materials.15,16 But a direct comparison between the nanostructured devices and their planar counterparts is challenging due to limited spatial resolution. Three-dimensional PL characterizations with carrier lifetime monitoring have rarely been reported. Previously, characterizations of cylinder-like structures (e.g. a single nanowire) along the axial direction have only been done by detaching a single unit from its array and thus could not be applied at the ensemble level.16,17 Thus x-y-depth PL characterization techniques with nanoscale resolution are valuable characterization techniques for temporal and spectral carrier dynamics analysis in a heterostructure ensemble. Such temporally and 3D spatially resolved characterization can be provided by laser scanning confocal microscopes, in combination with fluorescence lifetime imaging (FLIM) and two-photon excitation.18,19 FLIM characterizes the variation of PL lifetime, as well as intensity, which indicates the distribution of photoexcited carrier dynamics.20 Confocal microscopy has been widely used for 3D intravital imaging by adding a pinhole at the focus plane to eliminate out-of-focal light. However, for solid state material, depth-resolution is not possible because all incoming light gets


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absorbed within a thin layer at the top (Figure 1A). Two-photon laser scanning microscopy (TPLSM) provides a means of depth resolved PL study.21,22 Two-photon absorption (TPA) is a third-order nonlinear optical process in which two incident low energy photons are simultaneously absorbed and excite excitons. The cross section of TPA is usually several orders of magnitude smaller than that of linear absorption at low light intensities and is proportional to the square of the incident photon density.23 Therefore, significantly improved TPA can be achieved by using an extremely temporally and spatially bound excitation volume produced by femtosecond laser pulses and a highly focused beam, as shown in Figure 1B. Such small excitation volume and high penetration incident light facilitate depth resolution by stage movement relative to the con-focus plane. Figure 1C describes the setting of the confocal twophoton microscopy.

Figure 1. Jablonski energy diagrams and excitation structure description of (A) one-photon and (B) two-photon excitation; and (C) two-photon excitation experiment setting (APD = avalanche


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photodiode). In (A), incident light with photon energy higher than the material bandgap is used, where absorption is linear and the excitation volume is predominantly at the top surface. In contrast, (B) two-photon excitation allows penetration into the sample and an ultrashort laser is used to produce a well-defined excitation volume. III-Group nitride based material has been proved to have a strong non-linear optical effect thus is suitable for non-linear optical analysis.24,25 TPA-induced PL from bulk GaN allows crosssectional analysis in the depth dimension, which is useful for mapping spatial distribution of carrier dynamics and diagnosing complex structures. Studies of radiative relaxation processes using TPA in InGaN multiple quantum wells (MQW) planar structure have also been reported.26 The depth-of-focus resolution of the TPA has been experimentally investigated in GaN bulk material.27 However, nanostructures of III-Group nitride based MQWs have rarely been studied by TPA. A comprehensive study of one- and two-photon induced PL in InGaN MQWs nanostructures could enhance the understanding of GaN-based nanostructures with different post-growth treatments. In this study, densely packed InGaN/GaN multiple quantum wells (MQW) on top of GaN nanorods were fabricated for depth- and time-resolved characterization. Using one- and twophoton confocal based FLIM and time-correlated single photon counting (TCSPC), we characterized the spectral and temporal PL response of the InGaN MQW heterostructures and GaN in nanorod arrays. This technique has the capability to study the carrier dynamics across the ensemble of the nanorods as well as in a single rod, which provides valuable information for device optimization. It is shown that KOH wet etching can be an effective passivation to reduce surface defect trapping and thus enhance the PL efficiency. EXPERIMENTAL SECTION


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Sample fabrication The planar InGaN/GaN layer stacks were obtained from Aixtron SE, which were grown on cplane sapphire by metalorganic vapor phase epitaxy (MOVPE). The MQWs comprise of ten pairs of 2.5 nm In0.18Ga0.82N quantum wells and 15 nm GaN barriers, which are sandwiched between a 6.4 µm GaN layer and a 260 nm thick p-GaN capping layer. The bottom GaN layer consists of a 3.9 µm buﬀer layer and a 2.5 µm n-doped GaN layer. For nanosphere lithography, silica nanospheres with a diameter of 700nm±4% (Thermo Scientiﬁc) are densely placed on the surface of the planar MQW by Langmuir-Blodgett (LB) deposition (Kibron Microtrough). This monolayer worked as a mask for nanosphere lithography using inductively coupled plasma-reactive ion etching (ICP-RIE) (Oxford Instruments Plasmalab 100). Firstly the sphere monolayer was shrunk down to a diameter of 500nm in a CF4/SF6/H2 plasma. Then the MQW-GaN layer was etched into nanorod array using a Cl2/Ar gas mixture. By dipping the sample in 5 % HF for two mins, the on-top nano-spheres were removed. Finally, 1.2µm high nanorods with a 500 nm diameter and a 700 nm period in a hexagonal array on a GaN layer of around 6µm thickness are fabricated, as shown in Figure 2.


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Figure 2. (A) The top view SEM image of the nanorod array; (B) the 30° tilted SEM image of the nanorod sidewall; (C) schematic description of the nanorods sample: nanorods are 1.2µm high with a 500nm diameter, 10 stacks of InGaN/GaN MQWs are located on the top. Two such nanorod array samples were prepared for this study: one is as etched while the other is processed with an additional KOH wet etching (8.5 % KOH solution at room temperature for 75 min) for surface defect removal. The wet etched samples were rinsed in ultrapure water and blown dry with nitrogen. Characterisation Steady-state PL spectrum and photoluminescence excitation (PLE) spectrum were measured on a FluoroMax-4 spectrofluorometer (Horiba), using a 150W Ozone-free xenon arc lamp as the excitation source. Two-photon PL spectrum and images were carried out in a modified Leica TCS SP5 microscope. The excitation source is a mode-locked Ti:Sapphire laser (Mai Tai) producing 150fs pulses at repetition rate of 80MHz and wavelength was selected at 730nm corresponding to the


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excitation for above-band-gap (365nm) of GaN. The laser light was focused through an oil immersion objective (NA 1.4). The excitation volume was first focused at the top of the sample and moved into the sample subsequently with a step size of 100nm. The PL decay traces and fluorescence lifetime images were measured on a Micro Time 200 (Picoquant) confocal microscopy using TCSPC technique with one-photon excitation of 405nm laser at 5 MHz repetition rate. The use of 405nm laser guarantees an excitation of the InGaN MQWs only. The laser light was focused through a water immersion objective (NA 1.2). The excitation power density was around 200mW/cm2. All PL measurements were undertaken at room temperature. RESULTS AND DISCUSSION Figure 3 compares the PL spectra for the InGaN/GaN MQW nanorod by one-photon excitation (365nm or 380nm). A clear PL peak at 440 nm is assigned to the PL of InGaN MQW and a broad PL peak at 540 nm originates from defects of GaN, well-known as “yellow luminescence (YL)”. The origin of YL band emission has been previously attributed to impurity and point defects generated during material growth.14,28–31 Here the YL emission is much stronger when excited at the bandgap of GaN (365nm) than that by below-bandgap light (380nm).


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Figure 3. Steady PL spectra of the InGaN/GaN MQW nanorod sample, excited at 365nm and 380nm, for above and below GaN bandgap excitation, respectively. Typical height-dependent TPA-induced PL images of the nanorods are shown in Figure 4. The excitation wavelength is at 730 nm to excite at the bandgap of GaN bandgap for the PL of the InGaN MQWs and YL of GaN simultaneously. For Figure 4, the two PL emission bands were detected, corresponding to the PL of the InGaN MQWs and the YL of the GaN, respectively. The strongest MWQ PL was detected and a clear rod distribution could be recognized when focused at the top of nanorods (Top left of Figure 4). Besides, the PL intensity shows good uniformity across the broad nanorod array, which indicates the high fabrication quality of the nanorod etching. When the focal plane shifted to the depth of the GaN nanorods, although the rod distribution could still be differentiated, the intensity was much weaker and the rods became blurred (Top middle and top right of Figure 4). Note the PL of the InGaN MQWs at 440nm can only originate from the top of the nanorods where the MQWs are physically located. Because the focal point has a much higher photon density and thus much higher TPA coefficient, the charge carriers can only be generated at the point of focus. Thus the detection of PL from InGaN MQWs when focused below the MQW can be explained by carrier diffusion from GaN into the MQWs.


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Much weaker PL intensity is detected for GaN YL band, but a clear rod distribution can still be recognized (bottom left of Figure 4). As the excitation volume was focused deeper into the sample, PL from the GaN YL band also became weaker. Here we demonstrate that TPLSM can be used to resolute semiconductor nanostructures in depth dimension as a non-contact and nondamaging characterisation technique.

Figure 4. Depth-resolved two-photon images of the as-etched nanorods with detection (A) at 440 nm, and (B) at 540nm, for PL of InGaN MWQs and GaN YL band emission, respectively. The focus position from left to right images is moving from top to the bottom of the nanorod. A thin layer of defects on the etched surface of GaN nanorod sidewall is resulted from the ion bombardment during the rod etching process.32,33 The surface-to-volume ratio also increased by a factor of 800 after rod etching in this case. Thus proper surface passivation is of great importance for the purpose of enhancing the PL intensity and suppressing surface defect trapping. KOH


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solution is a crystal facet-selective etchant of GaN which is proposed here to remove GaN surface defects.34,35 Figure 5 compares the FLIMs on the MQWs PL at 440nm of as-etched and KOH treated nanorod samples. Compared to the as-etched nanorod (Figure 5A), the KOHtreated nanorods evidently exhibit a higher average PL intensity and longer PL lifetimes (Figure 5B).

Figure 5. FLIM images of (A) as-etched nanorod and (B) KOH-treated nanorod over a 5×5 µm2 area. Greyscale images are PL intensity only while the colored FLIM are superposed with a PL lifetime. Figure 6 shows the averaged PL decay across the whole the FLIM scanned area. It is evident that the PL decay of the KOH treated nanorod exhibits a lower decay rate. This result demonstrates that after KOH treatment, the non-radiative recombination in the nanorods is suppressed and the radiative recombination enhanced, which results in improved quantum yield. The measurement for the single nanorod provides valuable insight into the photoexcited carrier dynamics in individual nanorods. Figure 7 shows the PL decay traces of the selected nanorods on each sample, ordered from the darkest to the brightest rod. The PL decay can be well fitted


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with bi-exponential function and the time constants of fast ( τ1 ) and slow ( τ 2 ) components are extracted: I (t ) = A1 exp( −t / τ 1 ) + A2 exp( −t / τ 2 )

The fitting parameters are tabulated in Table 1, where effective lifetime is given by Wen et al. as36 τ eff = ( A1 * τ 1 + A2 * τ 2 ) / ( A1 + A2 )

Figure 6. PL decay traces of the nanorods samples averaged over the area indicated in Figure 5. From Table 1, KOH-treated nanorods exhibit a longer decay lifetime than those in the asetched nanorod sample. Rods with medium luminescence intensity and lifetime are the representative rod, as observed in Figure 5. The medium τ eff from KOH-treated nanorods (4.41 ns) is markedly larger than that of as-etched nanorods (1.69 ns), which is even larger than the τ eff of an as-etched rod having the brightest PL (3.55 ns). The same fitting methodology is

applied to the PL decay traces of the nanorods samples averaged over the area in Figure 6, showing that the τ eff of KOH-treated nanorods is 3.54 ns while that in as-etched nanorods is 1.67


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ns. This provides substantial evidence that the KOH passivation reduces the defect trapping significantly.

Figure 7. PL decay traces of 3 individual nanorods on (A) as-etched nanorod and (B) KOHtreated nanorod samples. Fitting curves of bi-exponential function are also provided as the dashed curves. The embedded FLIM image indicates the locations of the nanorods under the PL decay measurement. In the as-etched nanorods, both the intensity and the lifetime of the FLIM (Figure 5A) exhibit a minor fluctuation from rod to rod. However, there is a more prominent inhomogeneity of the intensity and lifetime across the KOH-treated nanorod (Figure 5B). The higher PL intensity rods exhibit the longer lifetime, which further confirms the better passivation will result in longer lifetime and thus higher PL intensity. Here we emphasize that both as-etched and KOH treated


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nanorods exhibit similar excellent PL homogeneity. However, using FLIM which is based on the PL quantum yield and lifetime, we can evidently find inhomogeneity across the rod array, particularly in KOH treated nanorods. This result suggests that the nanorods were not yet fully passivated across the whole area, which also suggests the KOH treated nanorods can be further improved for better quantum yield. Therefore, further investigation for uniformly passivating each nanorod and improving their performance is nevertheless required.

Table 1. Best fitted PL decay times of as-etched and KOH-treated nanorods using bi-exponential model (Unit: ns) As-Etched Rod

τ1

τ2

τ eff

KOH-Treated Rod

τ1

τ2

τ eff

1(Darkest)

0.7

3.0

1.0

4(Darkest)

1.6

6.1

2.6

2(Medium)

1.4

4.1

1.7

5(Medium)

2.2

10.2

4.4

3(Brightest)

2.7

7.9

3.6

6(Brightest)

2.4

11.2

4.8

To better understand the carrier dynamics within in a nanorod, further PL spectra measurements were performed on as-etched nanorods. Figure 8A shows the PLE spectra of the MWQ PL at 440nm and GaN YL band emission at 540nm. The PLE spectrum of 540nm peaks at 365nm which indicates that the GaN YL band emission originates from the GaN band-edge excitation. In other words, the carriers at the conduction band of GaN excited by 365 nm have a relatively large chance of getting trapped by the defect states. That of the MWQ PL at 440nm spans a very broad wavelength, suggesting the MWQ PL band can be excited by a wide range excitation. By excitation at 365nm (near bandgap of GaN), both MQW and defect PL can be


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obtained. When a below GaN bandgap excitation is used, the PL from MQW dominates with negligible contribution from the defects. For 730nm two-photon excitation, the carriers will be generated at the band edge of the GaN, similar to that of 365 nm one-photon excitation. Solely taking into account the energy aspect, it is expected that the GaN YL band emission can be similarly observed as 365 nm excitation in Figure 3. However, it is striking to find that the intensity of GaN YL band emission is dramatically lower than the MWQ PL by TPA at 730nm in Figure 4. For better understanding such phenomena, it is necessary to emphasize a spatial aspect other than just the excitation energy. For one-photon excitation, the majority of absorption happens at the GaN capping layer and MQW layers on top of rods. In contrast, for two-photon excitation, absorption occurs spanning from the capping layer to the bottom of the nanorods, depending on the focusing plane and the material thickness, rather than solely in the top capping and MQW layer. Thus carriers are mainly photoexcited below the MQW layer, especially when the excitation light is focused deep into the nanorods. The weak YL band emission by TPA at 730nm strongly suggests that the GaN YL band emission originates from the GaN capping layer and possibly the barriers layer between InGaN MQWs. Depth scanning steady PL measurement was undertaken to verify this possibility, using TPA at 730nm, as shown in Figure 8B. The defect-related PL spectra at different z-position are normalized to the MQW PL band. The PL intensity of the GaN YL band is evidently stronger when the excitation laser beam is focused at the top of the rod rather than deeper into the nanorod.


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Figure 8. (A) PLE spectra of 440 nm and 530 nm bands, and (B) two-photon PL spectra excitation at 730nm at different z-position of InGaN/GaN MQW nanorod sample. CONCLUSION We have demonstrated that one- and two-photon optical imaging associated with TCSPC techniques can be a powerful tool for characterizing nanorods. Using these techniques, we have obtained depth-resolved nanoscale images based on PL intensity and lifetime, which provides not only information about the uniformity of the PL efficiency, but more importantly the carrier dynamics for individual and ensembles of nanorods. Two-photon excitation microscopy provides depth-resolved fluorescence characterization of the InGaN/GaN MQW nanorods and


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discriminates between different luminescence mechanisms contributed from InGaN quantum wells and defects in GaN layers. FLIM and TCSPC measurement reveals the high quality of the nanorod and uniformity. The practical passivation effects of KOH treatment by suppressing the surface defects trapping both on a single rod and an ensemble level is confirmed. PL inhomogeneity shows that the passivation using a KOH treatment can be further improved for better performance if homogeneity can be enhanced. The combination of all these optical characterization techniques allows for non-contact and non-destructive 3D imaging with carrier dynamic analysis, which aids the diagnosis of heterostructure performance issues in optoelectronic devices and provides novel insights for fabrication improvement.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions X.Wen conceived the project; W.Chen analyzed experimental results and wrote the manuscript with assistance from X.Wen, J.Yang, and R.Patterson; W.Chen and M. Latzel performed experiments with help from M.Heilmann and D.Xi; M. Latzel fabricated samples. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENT This work is supported by the Australian Government through the Australian Renewable Energy Agency (ARENA). This work is also supported by the European Union Seventh Framework Programme [FP7/2007-2013] under grant agreement number 280566, and the German Research Foundation (DFG) within the project "Dynamics and Interactions of Semiconductor Nanowires for Optoelectronics (FOR 1616)". The authors thank DFG for financial support through the Cluster of Excellence Engineering of Advanced Materials at the University of Erlangen-Nürnberg, and the Deutscher Akademischer Austauschdienst (DAAD) for providing exchange scholarship. The authors gratefully acknowledge Aixtron SE for providing the planar MQW samples. W. Chen and J. Yang thank the China Scholarship Council (CSC) for scholarship support (No. 201403120056 and No. 201306070023). ABBREVIATIONS MQW, multiple quantum well; FLIM, fluorescence lifetime imaging; PL, photoluminescence; TCSPC, time-correlated single photon counting; TPA, two-photon absorption; ICP, inductively coupled plasma; RIE, reactive ion etching; SEM, scanning electron microscopy.

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TABLE OF CONTENTS GRAPHIC AND SYNOPSIS

Depth and time resolved characterisation of InGaN/GaN multiple quantum well (MQW) nanorods using one- and two-photon laser scanning fluorescence microscopy


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Figure 1. Jablonski energy diagrams and excitation structure description of (A) one-photon and (B) twophoton excitation; and (C) two-photon excitation experiment setting (APD = avalanche photodiode). In (A), incident light with photon energy higher than the material bandgap is used, where absorption is linear and the excitation volume is predominantly at the top surface. In contrast, (B) two-photon excitation allows penetration into the sample and an ultrashort laser is used to produce a well-defined excitation volume. Figure 1 102x59mm (300 x 300 DPI)



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Figure 2. (A) The top view SEM image of the nanorod array; (B) the 30° tilted SEM image of the nanorod sidewall; (C) schematic description of the nanorods sample: nanorods are 1.2µm high with a 500nm diameter, 10 stacks of InGaN/GaN MQWs are located on the top. Figure 2 82x85mm (300 x 300 DPI)


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Figure 3. Steady PL spectra of the InGaN/GaN MQW nanorod sample, excited at 365nm and 380nm, for above and below GaN bandgap excitation, respectively. Figure 3 64x49mm (300 x 300 DPI)



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Figure 4. Depth-resolved two-photon images of the as-etched nanorods with detection (A) at 440 nm, and (B) at 540nm, for PL of InGaN MWQs and GaN YL band emission, respectively. The focus position from left to right images is moving from top to the bottom of the nanorod. Figure 4 102x59mm (300 x 300 DPI)


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Figure 5. FLIM images of (A) as-etched nanorod and (B) KOH-treated nanorod over a 5×5 µm2 area. Greyscale images are PL intensity only while the colored FLIM are superposed with a PL lifetime. Figure 5 68x57mm (300 x 300 DPI)



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Figure 6. PL decay traces of the nanorods samples averaged over the area indicated in Figure 5. Figure 6 65x51mm (300 x 300 DPI)


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Figure 7. PL decay traces of 3 individual nanorods on (A) as-etched nanorod and (B) KOH-treated nanorod samples. Fitting curves of bi-exponential function are also provided as the dashed curves. The embedded FLIM image indicates the locations of the nanorods under the PL decay measurement. Figure 7 111x150mm (300 x 300 DPI)



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Figure 8. (A) PLE spectra of 440 nm and 530 nm bands, and (B) two-photon PL spectra excitation at 730nm at different z-position of InGaN/GaN MQW nanorod sample. Figure 8 128x200mm (300 x 300 DPI)


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Nanoscale Characterization of Carrier Dynamic and Surface

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