Single Nanowire Light-Emitting Diodes Using Uniaxial and Coaxial

Feb 24, 2014 - We report the controlled synthesis of InGaN/GaN multiple quantum well (MQW) .... Integrated Photonic Platform Based on InGaN/GaN Nanowi...
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Single Nanowire Light-Emitting Diodes Using Uniaxial and Coaxial InGaN/GaN Multiple Quantum Wells Synthesized by Metalorganic Chemical Vapor Deposition Yong-Ho Ra, Rangaswamy Navamathavan, Hee-Il Yoo, and Cheul-Ro Lee* Semiconductor Materials Process Laboratory, School of Advanced Materials Engineering, Research Center for Advanced Materials Development (RCAMD), Chonbuk National University, Deokjin-dong 664-14, Jeonju 561 756, South Korea ABSTRACT: We report the controlled synthesis of InGaN/ GaN multiple quantum well (MQW) uniaxial (c-plane) and coaxial (m-plane) nanowire (NW) heterostructures by metalorganic chemical vapor deposition. Two kinds of heterostructure NW light-emitting diodes (LEDs) have been fabricated: (1) 10 pairs of InGaN/GaN MQW layers in the c-plane on the top of nGaN NWs where Mg-doped p-GaN NW is axially grown (2) pGaN/10 pairs of InGaN/GaN shell structure were surrounded by n-GaN core. Here, we discuss a comparative analysis based on the m-plane and the c-plane oriented InGaN/GaN MQW NW arrays. High-resolution transmission electron microscopy studies revealed that the barrier and the well structures of MQW were observed to be substantially clear with regular intervals while the interface regions were extremely sharp. The c-plane and m-plane oriented MQW single NW was utilized for the parallel assembly fabrication of the LEDs via a focused ion beam. The polarization induced effects on the c-plane and m-plane oriented MQW NWs were precisely compared via power dependence electroluminescence. The electrical properties of m-plane NWs exhibited superior characteristics than that of c-plane NWs owing to the absence of piezoelectric polarization fields. According to this study, high-quality m-plane coaxial NWs can be utilized for the realization of high-brightness LEDs. KEYWORDS: InGaN/GaN, multiple quantum wells, heterostructure, nanowires, light-emitting diodes

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their large surface-to-volume ratio, which makes a great improvement for the excellent performance of the GaN based optoelectronic device applications.14−17 Nanostructure geometry offers a unique platform for epitaxial alloy growth because of its strain-relieving properties. A reduced TD and strain distribution in the nanostructures also leads to a weaker piezoelectric polarization field. NW light-emitting diodes (LEDs) take advantage of 1D shape-related feature including the enhancement of extraction-based light guiding and polarization. The NW length may be used to integrate radial or coaxial heterostructures that benefit from free-surface strain relaxation. Group III-nitride NWs have also proven to be especially robust nanomaterials, which are merging as versatile building blocks for photonic devices and power transistor applications.1,18−21 The morphology of the vertically oriented GaN NWs grown by MBE methods allows for high-efficiency lightemitting diodes. LEDs produced from GaN NWs are favorable because they possess superior output coupling efficiency properties. They have the potential to increase the quantum and wall plug efficacy of LEDs. GaN NW-based photonic

aN is an important III−V semiconductor compound material that has been extensively studied and applied in diverse fields due to its excellent properties such as its thermal features, chemical stability, tunable band gap, high mobility, high power, high frequency, and photoelectric sensitivity.1−5 GaN-based nanowires (NWs) especially constitute promising candidates for the next generation of highly efficient and largescale solid state lighting devices.6−10 As one-dimensional (1D) semiconductor nanostructures, GaN NWs possess numerous optimal properties for applications in nanoscale electronics and optoelectronics, the most significant of which include a remarkably wide bandgap and hexagonal wurtzite structure with hexagonal geometry.11,12 Further, the wide bandgap tenability of InGaN ternary alloy is of great interest because it offers the potential of using a single stable crystalline material to obtain bandgap energies from ultraviolet to infrared by varying the In content appropriately.13 In addition, a multibandgap NW heterostructure facilitates the efficient matching and utilization of high-efficiency optoelectronic nanodevices. As a first step to realize the potential impact of these novel structures and the development of a process for fabricating dislocation-free structures, position- and diameter-controlled nanostructure arrays are necessary. It is well-known that the density of threading dislocations (TDs) and defects can be significantly reduced by fabricating III−V nanostructures due to © 2014 American Chemical Society

Received: December 26, 2013 Revised: February 10, 2014 Published: February 24, 2014 1537

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ments with the 325 nm line of a He−Cd laser as an excitation source at room temperature. The cathodoluminescence (CL) mapping was performed in FE-SEM system equipped with backscattered/CL detector. Time-resolved photoluminescence (PL) spectroscopy was employed to study the optical properties of these MQWs. The excitation wavelength and pumping power were 290 nm and 10 mW, respectively. The MQW NWs were removed from the Si(111) substrate by the sonication process and then collected in the transmission electron microscopy (TEM) grid. The NW samples for the high-resolution transmission electron microscopy (HR-TEM) analysis were prepared by coating Pt using a dual beam-focused ion beam (FIB, Quanta 3D FEG) technique with a resolution of 7 nm @ 30 kV and a beam current of 65 nA. The morphology of NWs was analyzed by using an HR-TEM (JEM 2010, JEOL, Japan) imager at an operating voltage of 200 kV. To study the electrical properties, Ni/Au and Ti/Au metal contacts were made on both the top and bottom regions MQW NWs to serve as p- and n-metal electrode contacts to have good Ohmic contact on the samples. Finally, the current−voltage (I− V) and electroluminescence (EL) characteristic were measured for the MQW NW heterostructure by using a semiconductor parameter analyzer (4200−SCS, Keithley). Figure 1 presents the InGaN/GaN MQW NWs grown at m(coaxial) and c- (uniaxial) plane on Si doped n-GaN NWs on

crystal LEDs are capable of directly emitting light at specific angles and increasing light brightness without consuming extra power.22−24 Significant efforts have been made to fabricate the NW-based LEDs’ size and quality InGaN/GaN MQW nanostructure growth to get high structural and optical properties for efficient device functions. Recently, a number of III−V nitride-based LEDs such as radial and coaxial multiple quantum well (MQW) NW heterostructures have been reported.25−28 At the same time, a variety of studies based on III−V compound semiconductor GaAs/AlGaAs core-multishell heterostructure NWs have been reported.29−32 Qian et al. reported the controlled synthesis of AlN/GaN MQW radial NW heterostructure by using metalorganic chemical vapor deposition (MOCVD) and their optical properties.33 More recently, Riley et al. analyzed the three-dimensional mapping of quantum wells in a GaN/InGaN core−shell NW LEDs array and estimated that In composition variation within the nonpolar quantum wells was the primary cause of the broad light emission spectrum.34 In another study, Zhou et al. reported the band edge modulation and light emission in InGaN NWs due to surface state and microscopic In distribution.35 Although there have been few reports available based on the vertically aligned NW LEDs, it is still a very challenging issue to fabricate uniaxial and coaxial MQW NWs with uniform diameter and vertical alignment on Si(111) substrate due to process complications. The demonstration and comparison of c- and m-plane oriented single NW heterostructure LEDs in terms of electroluminescence and polarization field effects have not been reported yet. Herein, we demonstrate the growth of high-quality uniaxial (cplane) and coaxial (m-plane) InGaN/GaN MQW NW heterostructures arranged on Si(111) substrates and the fabrication of a single NW LED by using focused ion beam (FIB) through a parallel assembly technique. In the present study, we conducted a comparative and detailed investigation of the uniaxial (c-plane) and coaxial (mplane) growth of GaN/InGaN MQW NWs on Si(111) substrates. The morphological, optical, and electrical characteristics of the resultant heterostructure MQW NWs were analyzed in detail. After the deposition of MQW NW heterostructures, they were conveniently detached from the growth substrate. The polarization-induced effects on c-plane and m-plane oriented MQW NWs were precisely analyzed. The c-plane and m-plane oriented MQW single NW were chosen and then parallel assembly fabrication of the NW was performed by using the focused ion beam (FIB). Finally, we investigated the electrical characteristics of the ensemble of coaxial and uniaxial single NW LEDs, with the comparative analysis of these devices discussed in detail. Uniaxially and coaxially aligned InGaN/GaN MQW NWs were grown via MOCVD using Au catalysts with GaN seeded on Si(111) substrates as described previously.36 The c-plane and m-plane oriented heterostructure NWs were deposited at the optimized growth conditions by adjusting the growth temperature and working pressure for respective NWs heterostructures. The NWs were grown using the precursors trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH3) for Ga, In, and N, respectively. H2 is used as the carrier gas. The surface morphology of the resultant structures were characterized by using field-emission scanning electron microscopy (FE-SEM, Hitachi S-7400, Hitachi, Japan) with an operating voltage of 15 kV. The band-edge emission of the NWs structures were characterized by using PL measure-

Figure 1. Schematic diagram of c-plane (uniaxial) oriented (a) and mplane (coaxial) oriented MQW NWs, tilt-view FE-SEM image of cplane oriented (c) and m-plane oriented (d) InGaN/GaN MQW NW grown on Si(111) substrate. The insets show the top-view of the uniaxial and coaxial NWs.

Si(111) substrates. Figure 1a,b shows the schematic illustrations of coaxial and uniaxial NWs grown on Si(111) substrates. As shown in Figure 1a, after growing 10 pairs of InGaN/GaN MQW layers in the c-plane direction of a polar surface on the top of n-GaN NWs, Mg doped p-GaN NW is vertically grown thereon. Vertical MQW NWs grown in this method have many advantages compared to the conventional thin films.26,37−39 On the other hand, for the convenience of device processing a good vertical alignment of InGaN/GaN MQW NWs would be 1538

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Figure 2. (a) Panchromatic FE-SEM image, (b) CL mapping image, (c) CL spectrum, and (d) CL spectrum of n- and p-GaN of an individual cplane oriented p-GaN/InGaN/GaN/n-GaN NW. (e) Panchromatic FE-SEM image, (f) CL mapping image of cross-sectional and vertical, (g) CL spectrum of an individual m-plane oriented p-GaN/InGaN/GaN/n-GaN NW.

core−shell type MQW and the related p-GaN layer can dramatically increase the injected current at this region. Figure 1c shows the tilt-view FE-SEM image of the as-grown vertical cplane InGaN/GaN MQW NWs. The NWs are highly dense and vertically grown along ⟨0001⟩ direction on Si(111) substrate. Also, the NWs are observed to be clear without any contamination while the surface is smooth. The length and diameter of the NWs are determined to be 2 μm and 150−200 nm, respectively. It is interestingly noted that there is no coalescence of the NWs with respect to the substrate surface. The inset of Figure 1c shows a hexagonal shaped top-view of the NWs. This result implies that the GaN is grown along the cplane direction in a hexagonal structure. Figure 1d shows the tilt-view FE-SEM image of the m-plane InGaN/GaN MQW NWs grown parallel to n-GaN core. These m-plane NWs are also very dense and the surface morphology is observed to be very smooth without any particles. The NW diameter is in the range of 250−300 nm and length of 1.8 μm. The inset of Figure 1d shows the top side of the NW possessing a faceted surface and is enclosed by six sidewalls with a hexagonal structure. Furthermore, the hexagonal top-shape of the NW is not changed even after m-plane MQW growth. The length of cplane MQW NW is increased due to the vertical growth of the

necessary. Generally, GaN, which is grown on the Si substrate, has structural differences due to the differences in both materials (Si, cubic; GaN, hexagonal). Therefore, growing GaN thin films on Si substrate is accompanied by many crackings and dislocations due to the difference in thermal expansion coefficient (57%) and large lattice mismatch (17%).40,41 In addition, it is not possible to grow high-quality crystals without any additional intermediate layer and/or buffer layer caused by low nucleation of GaN on Si substrate. However, our approach on GaN NWs growth does not require any layer grown on a Si substrate and is not affected by cracking and dislocation. It is expected that InGaN/GaN MQW layer grown via NW with such advantages has high-quality crystalline structure, and thus the luminous efficiency improves. Figure 1b shows the schematic diagram of the coaxial InGaN/GaN MQW NWs on n-GaN core along m-plane direction where finally p-GaN layer is grown. InGaN/GaN MQW is grown all along n-GaN core NWs. With these morphological features, it is expected that there is no decrease of efficiency by quantum-confined Stark effect (QCSE) because the coaxially grown nonpolar mplane MQW NW is less affected by the piezoelectric polarization effects compared with that of vertical polar surface c-plane MQW NW.6,34,42 Further, the large surface area at the 1539

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Figure 3. (a) Schematic illustration, low- and high-magnification HR-TEM image, selective area diffraction pattern and lattice image of c-plane oriented p-GaN/InGaN/GaN/n-GaN NW. (a) Schematic illustration, low- and high-magnification HR-TEM image, energy dispersive X-ray profile, selective area diffraction pattern, and lattice image of m-plane oriented p-GaN/InGaN/GaN/n-GaN NW.

MQW layer, whereas the length of m-plane MQW NW is the same and its diameter is increased. To investigate the optical properties of these m-plane and cplane oriented InGaN/GaN MQW NWs, CL spectra were acquired for the individual NW structures. Figure 2 shows the CL data of the resultant MQW NWs. Figure 2a shows the FESEM image of a single InGaN/GaN MQW NW grown along cplane on Si(111) substrate that was separated by sonication. Diameters of the entire NW structure including p-GaN top, InGaN/GaN MQW structure and n-GaN bottom were observed to be uniform. As shown in Figure 2b, the p-GaN, MQW region and n-GaN regions were clearly distinguished as recorded by the overall CL mapping image of a single NW. Further, it was observed that the p-GaN region was slightly darker when compared to the n-GaN region. This is attributed to the effects of Mg doping in p-GaN region. The CL spectrum of the InGaN/GaN MQW NW region exhibited a stronger emission than the n-GaN and p-GaN regions as shown in Figure 2c. We measured the CL spectrum for many other individual NWs to confirm the presence of more reliable MQW structure. From the CL data, the band edge emission peak of InGaN was observed to be at 2.95 eV (419 nm) and that of GaN at 3.38 eV (366 nm). Any other additional peaks were not detected. From this result, we confirmed that the c-plane oriented InGaN/GaN MQW NW was well-grown with good optical characteristics. Furthermore, to analyze the origin of slightly different luminescence at CL mapping corresponding to p- and n-GaN regions we measured partial-CL spectra for the respective regions. These results are presented in Figure 2d using log-scale CL spectrum to observe insightful detail. The nGaN revealed the band emission edge at 3.38 eV and that of p-

GaN at 3.32 eV. The defect-related yellow luminescence (YL) peak was hardly detected in the n-GaN region, whereas a broad band of YL peak was detected in the p-GaN region. The intensity of the band edge emission of n-GaN region was 15% higher than that of p-GaN region. We infer that YL was in a deep level formed by defects such as a vacancy and impurities by Mg doping in the p-GaN. Because of this effect, the GaN peak was also red shifted and band edge emission was decreased. Figure 2e shows the FE-SEM image of the single mplane oriented InGaN/GaN MQW NW grown on the lateral surface of the n-GaN core. The NW diameter is bigger than that of the c-plane oriented NW due to lateral growth. Also the diameter of the m-plane NW was observed to be uniform from top to bottom with clear surfaces. During the final stage of the experiment, the pyramidal-shaped top region of NW was intentionally grown as r-plane by changing the growth conditions to avoid the growth of the c-plane.26 Figure 2f shows the CL mapping image of the m-plane oriented MQW NW that is measured along the ⟨0001⟩ zone axis after the FIB cutting at the middle region of the NW. The shell InGaN/GaN MQW and the core n-GaN with hexagonal-shaped structures showed well-separated CL emission. Six-sided layers of m-plane oriented MQWs emitted uniform thickness and exhibited a strong emission. The right-hand side of Figure 2f shows the CL mapping image of MQW measured at a wavelength of 425 nm. The luminescence of the MQW is emitted from the overall region. This result suggests that the MQW layer is uniformly grown at the overall regions of the core NW. Figure 2g shows the CL spectrum of m-plane MQW NWs. The band edge emission peak of InGaN is centered at 2.91 eV (425 nm) and for the GaN appears at 3.38 eV (366 nm). Comparing the CL 1540

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Figure 4. Schematic illustration showing the parallel assembly fabrication of (a) c-plane oriented p-GaN/InGaN/GaN/n-GaN NW LED and (b) mplane oriented p-GaN/InGaN/GaN/n-GaN NW LED. Top-view FE-SEM image showing the fabricated (a) c-plane oriented p-GaN/InGaN/GaN/ n-GaN NW LED and (b) m-plane oriented p-GaN/InGaN/GaN/n-GaN NW LED.

image of the c-plane oriented MQW NW clearly revealed barriers and well structures. Figure 3b shows the schematic illustration of InGaN/GaN MQW NWs grown along m-plane and a high-magnification STEM image of NWs measured along the ⟨0001⟩ zone axis. The MQW was formed in correspondence to the schematic structure while white line InGaN well maintained the shape of hexagonal structure during formation of the 10 pairs of InGaN/GaN MQW. The MQW structures appeared to be very clear and the interface between the layers were also uniform. The p-GaN layer also wrapped up the MQW structure. The thickness of the InGaN well was about 3 nm, the GaN barrier was about 15 nm. The thickness of p-GaN layer was about 30 nm. From HR-TEM lattice image data, any defect diffused through the stacking faults and misfit dislocations in the InGaN/GaN MQW was not detected. These results suggested that the growth of the MQW layers was not generated by the atomic mixing at the interface between the barrier and well. These data demonstrated the possibility to improve the internal quantum efficiency. To confirm the presence of the In content in the InGaN layer, the line profile HR-TEM EDX measurement was performed at the region marked as A in Figure 3b. The line graph inserted in Figure 3b shows that the In concentration uniformly exists in the InGaN well region, and there is no In present in the GaN barrier region. Also, from the data of SAED pattern, we confirmed that the growth direction of the MQW layer was the ⟨1100⟩ zone axis of the m-plane direction. From these results, we demonstrate that the c-plane and m-plane MQW NWs were uniformly grown and were of high quality NW without any defect. In order to evaluate the electrical properties of the c-plane and m-plane oriented MQW NWs with highly crystalline nature; we have adapted a parallel assembly fabrication by choosing a single NW. First, we have separated NWs from the Si(111) substrate by using sonication. Subsequently, the NWs were dispersed on the electrode grid before an SiO2 layer was formed with a thickness of 50 nm. From this grid, an individual NW was chosen and then parallel assembly fabrication of the NW was performed by using FIB. Figure 4a shows the schematic illustration of the parallel assembly fabrication using c-plane MQW NWs. The InGaN/GaN MQW region was positioned at the center after checking the crystalline nature of

emission spectra of m-plane and c-plane, the intensity of GaN peak corresponding to the m-plane MQW NWs is lower than that of the peak intensity of the c-plane MQW NW. On the other hand, the intensity of InGaN peak corresponding to the m-plane MQW is considerably stronger than the InGaN intensity of the c-plane MQW. This result is attributed to the difference of the MQW structure (m-plane and c-plane) existing in the active region. In the case of c-plane NW, the MQW region is limited by the diameter size of the NWs. Further, the exposure region of p-GaN and n-GaN is larger when compared to the MQW region; therefore, the band edge emission intensity of GaN is strong. However, the m-plane grown MQW NW is entirely wrapped by the n-GaN core structure; therefore, the emission of the InGaN appears from a large surface area when compared to the c-plane MQW. Because of this reason, the CL intensity of InGaN was observed to be higher than that of GaN intensity in m-plane oriented MQW NW. From CL results, we conclude that the LED emission efficiency of the mplane oriented MQW NWs is superior to that of c-plane MQW NWs. The TEM images were recorded to analyze insightful structural characteristics and the presence of InGaN/GaN MQW heterostructures. Figure 3 shows the dark-field Zcontrast TEM and HR-TEM images of the c-plane and m-plane oriented MQW NW. Figure 3a shows the schematic illustration of InGaN/GaN MQW NWs grown along c-plane and low- and high-magnification STEM image of NW measured along the ⟨21-10⟩ zone axis. As shown in the low-magnification image, the NW is vertically grown on the Si(111) substrate. Further, it can be observed that the coalescence did not occur between the adjacent NWs. As shown in the high-magnification image, the NW consisted of n-GaN bottom, InGaN/GaN MQWs, and pGaN top region. Ten pairs of InGaN well and GaN barrier were grown on n-GaN NW. The barrier and the well structures of MQW were observed to be very clear with regular intervals and the interface regions were very sharp. Also, it was found that the p-GaN top region was well-grown on MQW layer. The thickness of the InGaN wells was about 3 nm, while GaN barriers were about 15 nm. The selective area electron diffraction (SAED) patterns proved that the NW was hexagonal wurtzite type structure and the ⟨0001⟩ growth direction of the MQW layer was in the c-plane direction. The HR-TEM lattice 1541

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Figure 5. (a) I−V characteristics of c- and m-plane-oriented NW LEDs and (b) the EL emission intensity of c- and m-plane-oriented NW LEDs at an injection current of 25 μA. Schematic diagram illustrating the propagation of generated light in the NW at (c1) c-plane oriented InGaN/GaN NW LED and (c2) m-plane oriented InGaN/GaN NW LED.

resultant rectification properties of the typical p−n junction diodes were compared. With increasing applied voltage, the current was stably increased. The measured I−V characteristics at room temperature showed a sharp onset voltage at 2.6 and 2.2 V for the c-plane and m-plane MQW NW LEDs, respectively, in the forward bias with relatively negligible leakage currents at the reverse bias. The low forward voltage is attributed to the low resistive contact between the p-GaN and metal by Ohmic contact. The high injection current in the MQW NWs is generated due to the small work function. The m-plane-oriented MQW NW LED showed improved characteristics than the c-plane NW LED. The amount of injected current along p-GaN region at core−shell type NW LED is higher than that of c-plane NW LED. This is due to the increased surface area of m-plane NW LEDs and hence their enhanced electrical characteristics. Figure 5b shows the EL spectra c-plane- and m-planeoriented MQW NW LEDs measured at the injection current of 25 μA. The comparative EL emission intensities of both LEDs are presented here. The c-plane-oriented NW LED showed a blue emission wavelength peak at 417.5 nm and the m-planeoriented NW LED showed an emission wavelength peak at 425 nm. Furthermore, the EL emission intensity of m-plane LED showed a 28.6% improved emission than that of c-plane LED. The enhanced emission efficiency is attributed to the absence of piezoelectric polarization of the m-plane-oriented NW LED.26,27,43,44 In other words, the InGaN/GaN MQWs grown on c-plane have a polarization fundamentally due to the structural nature of the GaN; however, in the case of MQW grown on m-plane, the piezoelectric field effect did not occur due to the noninfluence of polarization.6 Because of this

the end portions of the NW. The ends of the p-GaN and nGaN region were trimmed by using an FIB. In order to maximize the current injection of the p-GaN and n-GaN, regions of the NW were coated with Ohmic contact electrodes. It is possible to reduce the electron tunneling with a barrier by choosing the metal with a higher semiconductor doping concentration of n- and p-type through the Ohmic contact. Here, we used Ti/Ni and Ni/Au electrodes as the n- and p-type metal contacts, respectively. Figure 4c shows the FE-SEM image of the parallel assembly of LED that is fabricated by utilizing a single c-plane-oriented MQW NW. The p- and ncontacts are carefully formed without any damage to the NW. Figure 4b shows the schematic illustration of an m-plane oriented MQW NW LED fabricated via parallel assembly technique. A similar fabrication process has been adapted for the m-plane oriented MQW NWs. For a suitable electrode contact over the n-type region, NW was carefully milled to remove p-GaN and MQW as shown in Figure 4b. Figure 4d shows the FE-SEM image of the LED fabricated from a single m-plane oriented MQW NW via parallel assembly. As can be seen from the FE-SEM image, the n-GaN core was clearly separated from the shell structure. For the sake of comparison, the fabricated c-plane and m-plane oriented NW LEDs were almost within the same dimension. The same parameters of the electrical characteristics were investigated for these NW LEDs. To observe the performance of these LEDs, we compared the electrical properties of the c-plane- and m-plane-oriented MQW NW LEDs fabricated via parallel assembly process. Figure 5a shows the I−V characteristics of c-plane- and mplane-oriented NW LEDs. Two kinds of LEDs were subjected to the applied bias of −5 to 7 V at room temperature while the 1542

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Figure 6. The EL emission intensity of (a) c-plane oriented NW LED and (b) m-plane oriented NW LED, as a function of injection current from 5 to 50 μA. Insets showing the EL emission of NW LED.

Figure 7. (a) EL emission wavelength and energy gap as a function of InGaN well thickness at an injection current of 25 μA. Insets showing the HRTEM images of c- and m-plane-oriented NW with InGaN well thickness of 2 and 12 nm, respectively and (b) plot of decay time as a function of excitation power for the c-plane-oriented and m-plane-oriented NW LEDs.

5c1,c2. As shown in the illustration, the light rays are emanating both vertically and laterally out of the NW LEDs. Light exits more uniformly and densely from the m-plane NW LED underlying the p-GaN contacts. This can be explained by the fact that light is generated in the region all along the p-GaN where the MQW heterostructures existing can provide larger volume for the electron−hole pairs to recombine in the active region and therefore light rays exiting from entire sides undergo the same recombination process along their regions of the mplane NW. Therefore, the m-plane-oriented NW LEDs tend to have more room for the light emission than c-plane-oriented NW LEDs. In order to further clarify the discrepancy raised in CL and EL data, we study the polarization field-induced characteristics of c- and m-plane-oriented NW LED via power dependence EL measurements. Figure 6 shows the room-temperature EL emission spectra of c-plane and m-plane MQW NW LEDs measured by changing the injection current of 5 to 50 μA and the insets show the respective EL emission images of the LED devices. Figure 6a shows the EL spectra of c-plane MQW NW LEDs. From the EL spectra, the emission wavelength peak of about 419 nm corresponding to the injection current of 50 μA was observed. However, the emission wavelength peaks were

phenomenon, the efficiency droop did not occur and hence the improved efficiency in m-plane-oriented NW LEDs. The EL emission of c-plane NW LED was slightly different from the CL data (as shown in Figure 2c) whereas m-plane NW LED was matched very well with the CL data as shown in Figure 2g. This result was attributed to the polarization field related with the cplane grown MQW heterostructures. It has been reported by the comprehensive theoretical model on InGaN core/multi shell NW LEDs that the carrier injection is easier for the region just below the p-contact layer.45,46 It is believed that the m-plane-oriented MQW possesses a very large surface area and the carrier injection by saturated current occurs in the entire surface area. Therefore, the light-emission area of m-plane MQW NW is expected to be the larger. However, in the case of c-plane MQW NWs, the recombination area is limited by the diameter of the NW thoroughly and the light-emission efficiency is significantly reduced compared to that of m-plane MQW NWs. The MQW structure at the mplane direction can contribute to the improvement of the efficiency by the maximization of the light-emitting area with the absence of the polarization.6 To further understand the light emission from these c-plane and m-plane-oriented NW LEDs, we present a schematic illustration as shown in Figure 1543

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Letter

Overall, the growths of m-plane and c-plane MQW NW heterostructures are very important for the realization of highinternal quantum efficiency nanoscale LEDs. Our results demonstrate that the fabricated nonpolar MQW NW LEDs are remarkable and can improve the efficiency of the LED. In conclusion, we fabricated the controlled synthesis of InGaN/GaN MQW c-plane and m-plane NW heterostructures by MOCVD. The successful growth of axial and coaxial types of LEDs device structures are demonstrated with 10 pairs of MQW structures having sharply defined wells and barriers. The surface morphology, optical, and electrical characterization of the fabricated c-plane and m-plane p-GaN/InGaN/GaN MQW/n-GaN NW LEDs heterostructures are studied by FESEM, HR-TEM, CL, I−V, and EL measurements. HR-TEM images reveal high-quality NW structures with sharp InGaN and GaN interfaces. The measured I−V characteristics at room temperature show a sharp onset voltage at 2.6 and 2.2 V for the c-plane and m-plane MQW NW LEDs, respectively. The electrical properties of m-plane NWs exhibit superior characteristics to that of c-plane NWs owing to the absence of piezoelectric polarization. The electroluminescence emission intensity of the m-plane LEDs shows improved results over about 28.6% compared to the c-plane LEDs. These kinds of cplane and m-plane NWs may allow flat band quantum structures that can improve the efficiency of LEDs. On the basis of our studies, these InGaN/GaN NWs-based heterostructures are promising structures for developing high performance LEDs.

blue shifted from 419 to 415 nm with respect to the gradual increase of injection current from 5 to 50 μA. These results can be attributed to piezoelectric polarization and spontaneous polarization as anisotropic structural properties of GaN.47 In particular, the InGaN/GaN MQW LED grown along ⟨0001⟩ direction generates the charge offset by the electric field along the c-plane axis of polar direction. With increasing drive current, the blue shift of the wavelength peak generated by band bending with the wave function can reduce the hole and electron by the QCSE. As a result of the InGaN/GaN MQW formed on the c-plane direction, the nonradiative recombination rate of the hole and the electron is increased whereas the radiative recombination rate of the hole and the electron is decreased, and finally leads to efficiency droop due to the reduction of photon generation. As shown in Figure 6b, nonpolar MQW NW LED grown on the m-plane exhibits an EL emission wavelength peak of about 425 nm. The EL intensity is increased with increasing injection current and the EL peak position does not shift. It is significantly noted that the m-plane MQW NW LED is not affected by the polarization fields. Further, the influence of efficiency droop by the QCSE will not cause the wave function of hole and electron. Moreover, the external quantum efficiency of c-plane- and mplane-oriented MQW NW LEDs were estimated to be 25 and 35.65% at an injection current of 50 μA, respectively. This is the highest external quantum efficiency reported in a single mplane-oriented MQW NW LED compared to the previous resports.48,49 As a result of that, the m-plane MQW NW LED exhibits superior EL emission intensity compared to that of cplane MQW NW LED. Finally, the electrical property of the mplane MQW LED is vastly superior to that of c-plane MQW LED. To further investigate the results, we correlated the different InGaN well thicknesses as a function of EL emission for both cand m-plane NW LEDs. Figure 7a illustrates the dependence of EL peak wavelength and energy with respect to the InGaN well thickness at an injection current of 25 μA. Insets of Figure 7a show the HR-TEM images of c- and m-plane-oriented NW with InGaN well thickness of 2 and 12 nm, respectively. The peak wavelength of m-plane NW LED almost remains constant when the well width changed from 2 to 12 nm. This result is attributed to the reduced nonradiative recombination rate and a decreased piezoelectric effect in these m-plane-oriented MQWs.50 On the other hand, the c-plane NW LED underwent a slight blue shift with increasing well thickness due to the strain-induced QCSE existing in the MQW NW, which in is good agreement with a recent study.51 To further explore the lifetimes of carriers in c- and m-plane-oriented NW LEDs, we performed time-resolved photoluminescence (TR-PL) analysis. Figure 7b shows the dependence of decay time as a function of excitation power of c- and m-plane NW LEDs. With increasing excitation power, the decay time (2 ns) of carriers in the mplane NW LEDs is almost observed to be constant. This result implies that the piezoelectric field is greatly reduced in m-planeoriented NW LEDs. On the other hand, as the excitation power increases, the decay time becomes shorter for the c-planeoriented NW LED. The decay time of localized carriers in cplane NW LED reduced from 13 to 6 ns as the excitation power increases from 0 to 10 mW. The reduced decay time is attributed to the existence of piezoelectric field in c-plane NW LED. It is well-known that the piezoelectric field will cause a spatial separation of the electron and hole wave functions as well as a reduction of the radiative recombination rate.52−54

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +82-63-270-2304. Fax: +8263-270-2305. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (BRL. No. 2010-0019626) and by the “Human Resource Development (project name: Advanced track for Si-based solar cell materials and devices, project number: 20104010100660)” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy, and also by the Ministry of Education, Science and Technology (MEST) and National Research Foundation of Korea (NRF) through the Human Resource Training Project for Regional Innovation.

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REFERENCES

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Nano Letters

Letter

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