GaN Multiple Quantum Well Nanowire Arrays on

May 23, 2013 - From the I−V curve, the current is leniently increased with applied voltage as 20 mA at 3.2 V, 35 mA at 3.6 V, 65 mA at 4.4. V, and 8...
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Coaxial InxGa1−xN/GaN Multiple Quantum Well Nanowire Arrays on Si(111) Substrate for High-Performance Light-Emitting Diodes Yong-Ho Ra, R. Navamathavan, Ji-Hyeon Park, and Cheul-Ro Lee* Semiconductor Materials Process Laboratory, School of Advanced Materials Engineering, Engineering College, Research Center for Advanced Materials Development (RCAMD), Chonbuk National University, Deokjin-dong 664-14, Chonju 561-756, Korea ABSTRACT: We report the growth of high-quality nonpolar (m-plane) and semipolar (r-plane) multiple quantum well (MQW) nanowires (NWs) for high internal quantum efficiency light emitting diodes (LEDs) without polarization. Highly aligned and uniform InxGa1−xN/GaN MQW layers are grown coaxially on the {1−100} sidewalls of hexagonal c-axis nGaN NWs on Si(111) substrates by a pulsed flow metal− organic chemical vapor deposition (MOCVD) technique. The photoluminescence (PL) measurements reveal that the wavelength and intensity of an MQW structure with various pairs (2−20) are very stable and possess composition-dependent emission ranging from 369 to 600 nm. The cathodoluminescence (CL) spectrum of individual InxGa1−xN/GaN MQW NW is dominated by band-edge emission at 369 and 440 nm with a relatively homogeneous profile of parallel alignment. Highresolution transmission electron microscopy (HR-TEM) studies of coaxial InxGa1−xN/GaN MQW NWs measured along the ⟨0001⟩ and ⟨2−1−10⟩ zone axes reveal that the grown NWs are uniform with six nonpolar m-plane facets without any dislocations and stacking faults. The p-GaN/InxGa1−xN/GaN MQW/n-GaN NW coaxial LEDs show a current rectification with a sharp onset voltage at 2.65 V in the forward bias. The linear enhancement of power output could be attributed to the elimination of piezoelectric fields in the InxGa1−xN/GaN MQW active region. The superior performance of coaxial NW LEDs is observed in comparison with that of thin film LEDs. Overall, the feasibility of obtaining low defect and strain free m-plane coaxial NWs using pulsed MOCVD can be utilized for the realization of high-power LEDs without an efficiency droop. These kinds of coaxial NWs are viable high surface area MQW structures which can be used to enhance the efficiency of LEDs. KEYWORDS: InxGa1−xN/GaN, multiple quantum wells, nanowires, coaxial heterostructure, pulsed flow, MOCVD fields, and thus have the potential of improved efficiencies unlike c-axis oriented devices.10−16 Additionally, NW LEDs take advantage of one-dimensional shape-related features such as the enhancement of light extraction based on light guiding and polarization.17 Previous works toward the fabrication of LED devices based of NW demonstrated that the growth of radial or longitudinal orientation of heterostructures on plasmaenhanced chemical vapor deposition (PECVD) grown on SiN templates via metal−organic chemical vapor deposition (MOCVD).11 The fabrication of m-plane core−shell InGaN/ GaN MQW on GaN wires has been also demonstrated by metal−organic vapor phase epitaxy (MOVPE) in which the NW’s diameter appears to be tapered at the bottom.18 Although there have been few reports available based on the vertically aligned NW LEDs,19−21 it is still a very challenging issue to fabricate the coaxial NWs with a uniform diameter on the substrate due to process complications. At the same time, coaxial GaN/AlGaN, GaN/AlN/AlGaN, n-GaN/InGaN/pGaN, and InGaN/GaN NW heterostructures were grown, and related nanoscale electronic devices have been re-

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ide band gap III nitride-based nanostructures such as nanodots, nanorods, and nanowires (NWs) are promising for the achievement of low defects with potential applications in short wavelength optoelectronic device applications.1−5 The InGaN ternary alloy is of great interest because it offers the potential of using a single stable crystalline material to obtain band gap energies from UV to IR by tuning the In content appropriately.6,7 This large range of band gaps has made InGaN a desirable material for color tunable lightemitting diodes (LED) and laser diodes (LD). In comparison with bulk multiple quantum well (MQW) structures, NW heterostructure growth has the potential advantage of being dislocation- and strain-free. Significant efforts have been made to control the growth of InGaN/GaN MQW nanostructures in terms of size and quality to get high structural and optical properties for efficient device functions. However, the LEDs based on InGaN/GaN structures that are typically grown along the polar c-axis suffer from the strong polarization-related internal electric fields that limit radiative recombination efficiency due to the spatial separation of holes and electrons.8,9 To reduce these quantum confined Stark effects, the development of nonpolar and semipolar nanostructure growth is employed. In particular, the LED devices based on nonpolar growth orientations, including the (1−100) m-plane and (1− 102) r-plane, are free from polarization-related internal electric © XXXX American Chemical Society

Received: March 11, 2013 Revised: May 7, 2013

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Figure 1. Schematic diagram for growing coaxial InxGa1−xN/GaN/n-GaN MQW NW heterostructures on Si(111) substrates.

ported.22−27 Here, we describe the fabrication of high-quality coaxial (nonpolar) InGaN/GaN MQW structures with a uniform diameter on Si(111) substrates in our present study. This is more advantageous for enhanced sidewall emission. Vertically aligned coaxial NWs have been demonstrated by two main strategies. The first strategy relies on the close lattice match between the Si(111) substrate and the n-GaN NWs. The second strategy obtains and confines the coaxial InGaN/GaN growth on the GaN NWs. Here, we focus on the n-GaN NWs’ growth via a vapor−liquid−solid (VLS) method to obtain highdensity and highly aligned coaxial NWs on Si(111) substrate and, subsequently, grow coaxial InGaN/GaN multiple quantum well (MQW) structures on the core n-GaN NWs. We demonstrate that, by dynamically adjusting the growth pressure during shell structure, it is possible to obtain heterostructure coaxial InGaN/GaN NWs with a uniform diameter on Si(111) substrates using a pulsed flow MOCVD technique. At the same time, the coaxial InGaN/GaN MQW/n-GaN NWs on Si(111) are virtually free of extended defects despite the very large lattice and thermal mismatch between these materials. We further report on the growth of high-quality coaxial InxGa1−xN/GaN MQW heterostructure NWs on the sidewall of c-axis n-GaN NWs via a simple procedure by using the MOCVD technique. The coaxial structure provides an m-plane (nonpolar) quantum well for ideal flat band tunneling. The resultant heterostructure NW consists of an n-type GaN at core and is surrounded by InxGa1−xN/GaN MQW sheath, as shown in Figure 1. The morphological, structural, optical, and electrical characteristics of the grown InxGa1−xN/GaN MQW/n-GaN NWs are analyzed in detail. Finally, we fabricate the coaxial NW light emitting diodes (LEDs) array which consist of an n-GaN core, an InxGa1−xN/GaN MQW structure, and a p-GaN outer shell. Coaxially aligned InxGa1−xN/GaN MQW shell/n-GaN core NWs are grown on Si(111) substrates by using MOCVD with a horizontal quartz reactor. Figure 1 shows the schematic diagram of the experimental process flow for growing coaxial InxGa1−xN/GaN MQW/n-GaN NW structures on Si(111) substrates. This process involves an axial elongation of nanodroplets’ catalyzed growth followed by controlled heterostructure deposition. These nanodroplets act as a nucleation seed for growing n-GaN NWs, in which the utilized Au catalyst no longer exists at the top of the NWs during the VLS process. By virtue of this, a high-quality LED device structure can be fabricated without any additional processes. First, a thin layer of Au with a thickness of 10 nm was deposited on Si(111) substrates followed by the predeposition of trimethylgallium (TMGa) was performed for 30 s at 600 °C. Then the substrate was annealed at 650 °C for 10 min with a

reactor pressure of 600 Torr to form Au+Ga nanodroplets. We grew Si doped n-type core GaN NWs on the Si(111) substrates with Au+Ga nanodroplets at a growth temperature of 950 °C. The precursors for Ga and N are TMGa and ammonia (NH3), respectively. H2 is used as the carrier gas. The growth pressure is maintained at 600 Torr. After this process, InGaN/GaN MQW NWs shell structures are subsequently grown on n-GaN NWs using TMGa, trimethylindium (TMIn), and NH3 as the Ga, In, and N sources, respectively. For this, we utilize the pulsed flow precursor method to grow the MQW structures of InxGa1−xN wells and GaN barrier NWs at relatively reduced growth temperatures of 630 and 710 °C, respectively, for different periods. The precursors TMIn+TMGa and NH3 are simultaneously allowed into the chamber for 3 min each. These experiments are repeated to obtain coaxial InxGa1−xN/GaN MQW heterostructures for about 2, 5, 10, 15, and 20 pairs of wells and barriers. By varying the number of pulses, we can easily tune the period and thickness of the MQW structures. Also the In composition in the InxGa1−xN/GaN MQW NWs structure is conveniently varied by adjusting the growth temperature from 750 to 550 °C. To obtain the coaxial NWs with uniform diameter from bottom to top, we dramatically reduce the growth pressure during the growth of the MQW structures maintained at 200 Torr. Coaxial InxGa1−xN/GaN MQW structures are formed uniformly on the n-GaN core at this reduced growth pressure. At the final stage, Mg-doped ptype GaN NWs are grown on the top of MQW NWs at the growth temperature of 950 °C. The surface morphology of the resultant coaxial InxGa1−xN/ GaN MQW/n-GaN NWs is analyzed by using field-emission scanning electron microscopy (FE-SEM, Hitachi S-7400, Hitachi, Japan) with an operating voltage of 15 kV. The Xray diffraction (XRD) measurements are performed using a Rigaku diffractometer with Cu Kα radiation. The InxGa1−xN/ GaN MQW/n-GaN NWs are 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 is 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 is analyzed by using an HR-TEM (JEM 2010, JEOL, Japan) imager at an operating voltage of 200 kV. The band-edge emissions of the InxGa1−xN/GaN MQW/nGaN NWs structures are characterized by using PL measurements with the 325 nm line of a He−Cd laser as an excitation source at room temperature. Cathodoluminescence (CL) mapping is performed in the FE-SEM system equipped with a backscattered/CL detector. B

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Figure 2. (a) Tilt-view FE-SEM image, (b) XRD data, and (c) PL spectrum, of n-GaN NWs grown on Si(111) substrates.

Figure 3. (a) Schematic, (b) tilt-view FE-SEM image, and (c) EDX data, of coaxial InxGa1−xN/GaN/n-GaN NWs grown on Si(111) substrates.

To study the electrical properties, p-GaN epilayers are deposited on the coaxial InxGa1−xN/GaN MQW/n-GaN NW LEDs. A 100-nm thick indium tin oxide (ITO) layer is coated on the top of the p-GaN epilayer to serve as a transparent electrode and current spreading layer. Thin Au/Ni metal layers are then deposited on both the top and back sides of the Si(111) substrate to serve as p- and n-metal electrode contacts to have good ohmic contact on the samples. Subsequently, the current−voltage (I−V) and light power−current (L−I) characteristics are measured for the MQW NW heterostructure by using a semiconductor parameter analyzer (4200-SCS, Keithley). The core structures of n-GaN NWs for InxGa1−xN/GaN MQWs are grown on Si(111) substrates and are characterized accordingly as shown in Figure 2. Figure 2a shows the tilt-view FE-SEM image of Si doped n-type GaN NWs grown on Si(111) substrates. The grown n-GaN NWs are very dense, and most of them are vertically aligned on the Si(111) substrate. The size and the density of the NWs decided by controlling the Au+Ga nanodroplets and GaN NWs of hexagonal structure tend to grow perpendicular on Si(111) due to the energy stability of the c-axis direction. From the FE-SEM data, the average diameter and the length of n-GaN NWs are in the ranges of 150−180 nm and 1.4−1.5 μm, respectively. More importantly, the sidewall m-plane facets of the n-GaN NWs have very smooth surface morphology and without any nanoparticles, which are essential for the adsorption of adatoms and the growth of uniform coaxial MQW structure. No significant coalescence and aggregation of individual NWs

occur as presented in Figure 2a, which is advantageous for growing coaxial InxGa1−xN/GaN MQW structure. As shown in the inset of Figure 2a, the sidewall diameters of a single n-GaN NW are observed to be uniform from bottom to top with a pyramid-shaped top end which is conveniently utilized for semipolar r-plane growth. Further, the utilized Au +Ga nanodroplets no longer exist at the top of the n-GaN NWs as reported in our previous study28−30 which allows obtaining the growth of the r-plane facet MQW structure. The X-ray diffraction (XRD) analysis reveals that the prominent diffraction peak at 34.62° from the (002) planes of the wurtzite-type hexagonal n-GaN NWs indicates that the formed n-GaN NWs are preferentially oriented in the c-axis direction (Figure 2b). The diffraction intensity is displayed on a logarithmic scale in Figure 2b to visually check any impurities. No significant diffraction peaks of any other phases or impurities can be detected in the XRD patterns, which indicate the high-quality n-GaN NWs. The room temperature PL spectrum of the Si doped n-GaN NWs is dominated by bandedge emission at around 368.8 nm (3.36 eV), and any defect level peaks like yellow luminescence are not observed as shown in Figure 2c. The PL intensity is displayed on a logarithmic scale in Figure 2c to visually emphasize the bands with weaker intensities. The steep and intense PL spectrum with a full width at half-maximum (fwhm) of 61 meV is attributed to the wellaligned strain free n-GaN NWs on Si(111) substrates. The PL data reveal that the grown n-GaN NWs are of good optical quality with lower defects. In the next step, the active layers of InxGa1−xN/GaN MQWs are grown on the sidewalls of n-GaN NWs. Figure 3a shows the C

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Figure 4. (a) PL spectra of coaxial InxGa1−xN/GaN MQW/n-GaN NWs with different pairs of MQW and (b) normalized PL spectra of coaxial InxGa1−xN/GaN MQW/n-GaN NWs with increasing In content with 10 pairs of MQW.

emission wavelength of InxGa1−xN wells and GaN barrier structures are observed to be constant regardless of the number of MQW pairs. On the other hand, the emission intensity of InxGa1−xN wells tends to increase with increasing MQW pairs. This result suggests that the InxGa1−xN/GaN MQW layers grown on the overall sidewalls of core n-GaN NWs are very uniform and the In content on the overall plane is constant. A slight shoulder-like peak is observed at around 510 nm observed at 15 and 20 pairs of MQWs, owing to the phase separation of indium concentration. To further investigate the band gap properties of coaxially aligned InxGa1−xN/GaN MQW NWs, we vary the In content (x = 0.08, 0.2, 0.29, and 0.38) of the InxGa1−xN for ten (10) pairs of MQW structure. Figure 4b shows the room temperature normalized PL spectra of the coaxial InxGa1−xN/GaN MQW/n-GaN NWs with increasing In content. The shift in emission wavelength from 400 to 598 nm is achieved by reducing the growth temperature of MQWs from 750 to 550 °C. This figure also depicts the PL spectra obtained from four different samples in which the In composition is continuously varied. It is noticed that a large tenability of the output spectrum can be obtained by varying the In content in the NWs. These results are consistent with band-edge emission from InGaN with different In compositions.19,33,34 However, the emission wavelength of the GaN remains the same at 369 nm. The line width of InxGa1−xN band-edge emission consecutively increases with increasing In concentration [InxGa1−xN (x = 0.08−0.38)], which is largely due to the increased alloy broadening with higher In content in the NWs. At the same time, it has been significantly noted that there is no defect-related yellow luminescence, which is further evidence of the good crystalline quality of the MQW NWs. Also, the intensity of the MQW peak emission is constant from 400 to 598 nm. The steep and intense PL spectrum with a narrow full width at half-maximum (fwhm) is attributed to optimized growth condition with stable emission intensity owing to the nonstacking faults and nonchemical defects in the InxGa1−xN MQWs. These results show that the MQWs grown along nonpolars have the ability to tune an InxGa1−xN emission

schematic diagram of core−shell MQW structure grown as the m-plane (1−100) and r-plane (1−102). Figure 3b shows the tilt-view FE-SEM image of the coaxial InxGa1−xN/GaN MQW structure on n-GaN NWs. Coaxially aligned and high dense InxGa1−xN/GaN MQW NW heterostructures are grown perpendicular to the c-plane of ⟨0001⟩ direction on Si(111) substrate and exhibit six nonpolar sidewalls of hexagonal structure with the ⟨1−100⟩ direction as shown in the inset of Figure 3c. However, it is observed that some of the GaN NWs are tilted and slightly inclined to the substrate. The pyramidal top sides of InxGa1−xN/GaN MQW NWs are sharper than that of the n-GaN core NWs. This is attributed to the different growth rates of r-plane and c-plane. The geometry of the wurtzite GaN-based NWs in MOCVD growth can be distinguished by crystal polarity or pyramidal-shaped NW tips corresponding to Ga-polar c-plane growth direction.31,32 The growth of semipolar is more useful than the growth at c-polar plane to reduce polarization. The surface morphology of the coaxial InxGa1−xN/GaN MQW NWs is very smooth and clear without any impurity nanoparticles. The diameters of the coaxial core−shell MQW NW heterostructures are observed to be uniform from the bottom to top and are calculated to vary from 370 to 400 nm, with the average length being 1.6 μm. Figure 3c shows the EDX spectrum of the individual coaxial InxGa1−xN/GaN MQW NW measured from the ⟨0001⟩ zone axis. It can be observed that the samples consist of only Ga, In, and N elements. The successful deposition of MQW NW heterostructures is confirmed by the FE-SEM-EDX spectrum. Figure 4 shows the optical properties of the coaxial InxGa1−xN/GaN MQW/n-GaN NWs grown on Si(111) substrates. To investigate the optical properties, we perform the PL measurement at room temperature by varying the number of MQW pairs and the concentration of In content. Figure 4a shows the PL spectra of the coaxial InxGa1−xN/GaN MQW/n-GaN NWs with different pair numbers of the MQW grown on Si(111) substrates. The PL spectra are dominated by band-edge emission at around 369 and 440 nm, corresponding to the GaN and InxGa1−xN MQW structures. The band-edge D

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Figure 5. (a) FE-SEM image of a single coaxial InxGa1−xN/GaN MQW/n-GaN NW and (b) CL spectrum recorded along the entire coaxial NW and the inset showing CL mapping image. Spatially resolved CL spectra are recorded in the region A and B at 440 and 480 nm, and their corresponding CL mapping images are shown in insets (c and d), respectively.

of m-plane region is much more dominated than the r-plane in the coaxial MQW NW. Figure 6 shows the spatial-resolved CL mapping image and the spectrum of the individual NW along the cross-section measured at the wavelengths of 369 (n-GaN) and 440 (InxGa1−xN) nm, respectively, from the ⟨0001⟩ zone axis. From the CL image, it is seen that the n-GaN and InxGa1−xN/ GaN MQW structures appear to have a distinct color (Figure 6a and b). The shell of the InxGa1−xN/GaN MQW structure appears to be white in color unlike the n-GaN core structure which appears transparent as shown in Figure 6b. Based in Figure 6c, the CL spectrum taken at the core n-GaN NW region exhibits a prominent band-edge emission peak of bound exciton at 368.9 nm without any yellow luminescence owing to defects like VGa-SiGa+ and VGa-OGa+. The CL mapping profile measured at the 440 nm wavelength is dominated by the bandemission edge at 369 and 440.3 nm corresponding to n-GaN core and InxGa1−xN/GaN MQW shell structures, respectively (inset of Figure 6d). The luminescence distribution of the peaks at 369 and 440 nm with the well-aligned nonpolar side facets shows a relatively homogeneous profile of parallel alignment. These results demonstrate that the CL emission intensity of the two structures (369 and 440 nm corresponding to n-GaN and InxGa1−xN/GaN MQW, respectively) are quite uniform, which might be related to the homogeneous distribution of strain-free coaxially aligned InxGa1−xN/GaN MQW/n-GaN NW. Further, we analyze the InxGa1−xN/GaN

into the blue−orange region of the visible spectrum with high optical properties by low defects. To further clarify the origin of the band emissions, the wavelength-resolved CL properties of the individual coaxial InxGa1−xN/GaN MQW/n-GaN NW are investigated. For this purpose, both entire and cross-section regions of the individual NW were chosen. Figure 5 shows the CL mapping data of the entire NW. Figure 5a shows the FE-SEM image of the coaxial InxGa1−xN/GaN MQW/n-GaN NW which is sonicated from the Si(111) substrate. Figure 5b shows the CL spectrum of the individual coaxial NW, and the corresponding CL mapping image is displayed in the inset. The CL spectrum clearly exhibited two prominent peaks centered at 369 and 440 nm corresponding to n-GaN and InxGa1−xN structures, respectively. In addition to that, there exists yet another feeble peak centered at 480 nm. To check insightful features of these mixed band emissions more exclusively, we performed spatially resolved CL mapping at the wavelengths 440 and 480 nm. Figure 5c and d shows the spatial-resolved CL spectra of a single coaxial NW recorded at 440 and 480 nm in the regions A (m-plane) and B (r-plane), respectively, are shown in the dotted circles of Figure 5a. The respective CL mapping images are displayed in the inset of Figure 5c and d. The peak emission at 440 nm is attributed to the MQWs grown on the NW sidewalls. The longer emission wavelength (480 nm) is especially observed at the edge of the coaxial NW (r-plane). From these results, we confirmed that the band edge emission E

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Figure 6. CL mapping image of a single coaxial InxGa1−xN/GaN MQW/n-GaN NW with spatially resolved CL mapping at the wavelength (a) 369 and (b) 440 nm, taken along ⟨0001⟩ zone axis, and the respective CL spectra of n-GaN (c) and InxGa1−xN/GaN MQW regions in the inset, respectively, and (d) accelerating voltage-dependent CL spectra.

As can be seen from the HR-TEM image, the coaxial InxGa1−xN/GaN MQW NW grown on Si(111) on any threading dislocation and cracking caused from the substrate is hardly observed at the interface region. It is very difficult to obtain high-quality GaN structure without any buffer and intermediate layer due to large lattice mismatch (about 16.9%) and thermal mismatch (about 56%) between GaN and Si(111) substrate. High-magnification HR-TEM images of the r- and mplane InxGa1−xN/GaN MQW structures are shown in Figure 7b−d. From this data, the thickness of the InGaN well and GaN barrier grown at nonpolar (m-plane) and semipolar (rplane) are determined to be 2.5 and 10 nm and 2 and 7 nm, respectively. A significant reduction in the thickness of the InxGa1−xN/ GaN MQW structure at the r-plane is noticed unlike that at the m-plane. This is because the growth rate of the r-plane is much lower than that of the c- and m-planes. It is reported that the growth rate mainly relies on the surface energy and the stability of surface atoms.36,37 The MQWs grown on semipolar r-plane are extremely thin, possibly too thin to capture the electrons and holes for radiative recombination.38 Consequently, the formation of the semipolar r-plane MQW structure sidewall is too thin due to the lower growth rates at this plane. Figure 7e

MQW based on the CL measurement at a 440 nm wavelength with increasing accelerating voltage (from 5 to 25 kV) in order to confirm the absence of quantum-confined Stark effect in the InGaN/GaN MQW structure as shown in Figure 6d. With increasing acceleration voltage the electron−hole pair concentration in the MQW structure is increased; it could screen an existing piezoelectric field. Our results reveal that the insignificant shift of CL spectra is observed with increasing acceleration voltage which is attributed to the absence of piezoelectric field in the InGaN well structure. These results are consistent with depth-resolved and excitation power dependent CL study of GaN films35 and m-plane core−shell InGaN/GaN MQWs on sapphire substrate.18 Coaxial InxGa1−xN/GaN MQW NW is sectioned by FIB and measured by HR-TEM for the structure analysis. Figure 7 shows an HR-TEM image and high-angle annular dark field (ADF) images of the individual InxGa1−xN/GaN MQW NW recorded along the ⟨2−1−10⟩ zone axis. Figure 6a shows a lowmagnification HR-TEM image of about 10 pairs of coaxial MQW structures which are clearly seen and are well-formed on the core n-GaN NW. The coaxial InxGa1−xN/GaN MQW structures are uniformly grown from the bottom to top along the semipolar r-plane and nonpolar m-plane. F

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Figure 7. (a) HR-TEM image of coaxial InxGa1−xN/GaN MQW/n-GaN NW structure taken along ⟨2−1−10⟩ zone axis and the high magnified image of the respective regions (b, c, and d), annular dark field image of MQW NW heterostructures (e), and (f) HR-TEM EDX composition profile mapping for the regions labeled as A and B.

To give a more detailed analysis about the structure and the origin of InxGa1−xN MQWs from various zone axes, the crystal structure of the coaxial InxGa1−xN/GaN MQW grown on core n-GaN NW is revealed by HR-TEM diffraction patterns and lattice images recorded in the ⟨2−1−10⟩ and ⟨0001⟩ zone axes as shown in Figure 8. The MQW NWs are uniformly grown along the nonpolar m-plane. The coaxial InxGa1−xN/GaN MQW structures are clearly seen and are equally distributed on the core n-GaN NW as shown in Figure 8a. The m-plane aligned MQW layers are clear without any threading dislocations and boundary defect layers. Figure 8a1 shows the highly magnified HR-TEM lattice image of the MQW layer taken from the ⟨2−1−10⟩ zone axis. These images show the [0001] growth direction of a MQW NW, which is aligned along the NW axis (c-plane). The lattice image of the adjacent interface between the InxGa1−xN well and GaN barrier is clear without any vacancy defects and stacking faults. It is very important to prevent the occurrence of a stacking fault at the first InGaN well layer since the stacking fault generated at the first InxGa1−xN well layer can be propagated to the overall MQW layers by the diffusion of dislocation. A clearer lattice image is shown in Figure 8a2, which is taken from the inverse fast Fourier transform (IFFT) image. The interplanar spacing at the ⟨0001⟩ and ⟨01−10⟩ direction segments of HR-TEM image

shows the STEM image and ADF line profile intensity scan measurement of the m-plane InxGa1−xN/GaN MQW structure on n-GaN NWs. It is observed that the thickness of the InxGa1−xN/GaN MQW NW heterostructures are almost uniform throughout the periods. This is attributed to optimized growth parameters. The possibility to grow well-defined 2.5 nm InGaN and 10 nm GaN NWs suggests that it will be feasible to control the well-defined nonpolar MQW structures. The HR-TEM EDX mapping profile of the InxGa1−xN/GaN MQW NW region clearly confirms the spatial distributions of Ga, In, and N in the NW structure as shown in Figure 7f. The In composition of InxGa1−xN/GaN MQW layer grown at the same growth condition exhibits a slight difference in the m-plane (x = 0.16) and r-plane (x = 0.22). The increased amount of In content is also in good agreement with the CL data (Figure 5) in which it underwent longer wavelength (480 nm) emission. The difference in In composition on the two different planes has also been observed in the core−shell nanostructures.39,40 We believe that this reason is due to the difference of adatom diffusion caused by the difference in the growth rate of the mplane and r-plane. Adatom migration and In diffusion at r-plane is slower than at m-plane. This is the reason the r-plane has a lower growth rate. G

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Figure 8. HR-TEM image of coaxial InxGa1−xN/GaN MQW/n-GaN NW structure taken along; (a) ⟨2−1−10⟩ zone axis and (b) ⟨0001⟩ zone axis, (a1) lattice image of coaxial InGaN/GaN structure, (a2) IFFT lattice image of the MQW, below one showing the ADF lattice fringe, and (a3) the respective SAED patterns. (b1) Top-view of the high magnified coaxial InxGa1−xN/GaN structure, (b2) IFFT lattice image, the below one showing the magnified view, and (b3) the respective SAED patterns.

taken from the ⟨2−1−10⟩ zone axis is about 5.196 and 2.6 Å, respectively, as illustrated in Figure 8a2. A representative SAED pattern is shown in Figure 8a3 in which (0001), (01−10), and (0−11−1) diffraction spots are present. The clear SAED spots indicate that they are of a highquality wurtzite-type hexagonal coaxial InxGa1−xN/GaN MQW NWs, and the growth direction of core n-GaN NW and sidewall InxGa1−xN/GaN MQWs is the c-plane of ⟨0001⟩ direction and m-plane of ⟨1−100⟩ direction. Figure 8b shows the HR-TEM image of coaxial InxGa1−xN/GaN MQW/n-GaN NW taken from the ⟨0001⟩ zone axis, demonstrating the well-defined hexagonal shape. Ten pairs of InxGa1−xN/GaN MQWs are clearly seen without any defects and are equally distributed on the six nonpolar m-plane. Figure 8b1 shows a magnified HRTEM image to confirm the m-plane growth of the InxGa1−xN and GaN MQW structures. The six m-plane edge is uniform with each 120°. Figure 8b2 shows the IFFT image of MQW structure with interplanar spacing of 2.7 Å at ⟨10−10⟩ direction segments taken along the ⟨0001⟩ zone axis. Dark areas in the

InxGa1−xN well layer of 2.5 nm thickness are slightly warped due to increase in the In composition. However, stacking fault does not occur, and the crystal structure is well-defined. The corresponding SAED pattern taken from the ⟨0001⟩ zone axis is shown in Figure 8b3 in which (−1100), (01−10), and (10− 10) diffraction spots are present. There are no extended defects, such as misfit dislocations and stacking faults observed in the InxGa1−xN/GaN MQW NW structure. It is evident that the MQW structure changes abruptly although there is some atomic mixing near the interface which is not observed for the sharper interface between well and barrier. To investigate the electrical properties of coaxial InxGa1−xN/ GaN MQW/n-GaN NWs grown on Si(111) substrates, we fabricate the vertically aligned LED device structure using Mg doped p-type GaN on the coaxial NWs. Figure 9a shows the schematic of the coalescence mechanism of the p-GaN layer grown on the InxGa1−xN/GaN MQW NWs. This growth method is based on the dense NW density for coalescence and the low growth pressure, which increase the Ga source for twoH

dx.doi.org/10.1021/nl400906r | Nano Lett. XXXX, XXX, XXX−XXX

Nano Letters

Letter

applied voltage as 20 mA at 3.2 V, 35 mA at 3.6 V, 65 mA at 4.4 V, and 80 mA at 5 V. The lower forward voltage can be attributed to the low resistive contact and small work function difference between Au/Ni/ITO and p-GaN, forming a high current injection in the MQW NWs. The electroluminescence (EL) spectra of the emitted light exhibits a blue emission wavelength peak of about 440 nm as shown in Figure 10b. The electroluminescence is observed to emanate from the junction of the coaxial NW LEDs, as shown in the inset of Figure 10b. The EL intensity increased with the injection current and the EL peak position did not shift noticeably, indicating the Au/Ni/ITO act as a p-contact electrode in the coaxial InGaN/GaN based LEDs in a suitable manner. Considering the geometry of our coaxial NW LEDs, one can expect that the emitted light is generated from r-plane of the coaxial NWs since it has substantial contact with p-GaN layer. Our spatially resolved CL spectra are in good agreement with this EL data in which the m-plane emission intensity is more dominant than the r-plane. It has been reported by the comprehensive theoretical model on InGaN core/multishell NW LEDs that the carrier injection is easier for that area just below to the p-contact layer.41,42 However, based on our optical properties, we believe that our coaxial NW LED device exhibits dominant EL emission from the m-plane (sidewall emitting). This result also infers that the p-GaN layer is in good contact with the sidewalls of the coaxial MQW NWs. Figure 10c shows the light output powers (L−I) of the vertically aligned coaxial MQW NW and thin film LEDs as a function of current injection. The output power curve exhibits gradual increase with increasing current which is observed to be 53 μW at 6 mA, 166 μW at 22 mA, and 250 μW at 33 mA. The output power is linearly increased with increasing current owing to the low resistive contact. The linear enhancement of output power could be attributed to the elimination of piezoelectric fields in the InxGa1−xN/GaN MQW active region. The external quantum efficiency of the LED device can be calculated by using the equation: ηext = the internal quantum efficiency (ηint) × extraction quantum efficiency (ηextrac).43 The estimated external quantum efficiency of the coaxial p-GaN/InxGa1−xN/ GaN MQW/n-GaN NW-LEDs and thin film LEDs were of

Figure 9. (a) Schematic of the LED device and (b) cross-sectional FESEM image of the LED device structure.

dimensional (2D) growth. With increasing precursor sources, the p-GaN layers are laterally grown on MQW NWs, and finally, complete coalesced p-GaN film is grown along the cplane. Figure 9b shows the cross-sectional FE-SEM image of the p-GaN epilayer grown on InxGa1−xN/GaN MQW NWs. The thickness of grown p-GaN epilayer is about 1 μm. The upper part of p-GaN slightly appears to be of pyramidal shape due to some internal stress. Such rough surface morphology can improve external quantum efficiency via reducing total reflection through the increased scattering of emission light. Afterward, metal contacts must be made on the top and bottom of the NWs to inject holes and electrons simultaneously into the coaxial MQW NWs. To observe the performance of the coaxial MQW NWs, we compared the electrical properties of NW LEDs with that of thin film LEDs fabricated on sapphire substrate. Figure 10a shows the comparison of the I−V characteristics of the vertical type NW LEDs and thin film LEDs. The fabricated coaxial LED device structure is shown in the inset of Figure 10a. These LEDs exhibit a typical p−n junction diode. With increasing voltage, current is stably increased. The measured I−V characteristics at room temperature show a sharp onset voltage at 2.65 and 2.9 V for the coaxial NW and thin film LEDs, respectively, in the forward bias with relatively negligible leakage currents at the reverse bias. The rectification of the coaxial NW LEDs is better than that of the thin film LEDs. From the I−V curve, the current is leniently increased with

Figure 10. Electrical properties of the coaxial p-GaN/InxGa1−xN/GaN MQW/n-GaN NW-LEDs and thin film LEDs structure: (a) I−V characteristics, inset showing the coaxial NW LED structure, (b) the EL intensity of the coaxial p-GaN/InxGa1−xN/GaN MQW/n-GaN NW-LED as a function of injection currents, inset showing the EL emission from coaxial NW LED, and (c) power gain. I

dx.doi.org/10.1021/nl400906r | Nano Lett. XXXX, XXX, XXX−XXX

Nano Letters 27.94 and 18.87% at an injection of current of 50 mA, respectively, corresponding to the wavelength of 440 nm. The external quantum efficiency of the NW LEDs was notably better than that of the thin film LEDs. The results of the present investigation are more encouraging; moreover, we can expect that further increase in quantum efficiency can be realized by improving the p-GaN layer or to optimize the thickness of core/shell structures. Although our nonpolar coaxial NWs are of high quality, in order to improve electrical characteristics, a further stable fabrication process is required. These results suggest that, with more stable fabrication at metal contact, ITO contact can improve output power efficiency. By optimizing the electrode thickness for electrical conductivity and transparency, the current injection and light emission efficiency of the coaxial InGaN/GaN MQW/n-GaN NWs based LEDs on Si(111) substrate may be further enhanced. They also suggest that the growth of m-plane aligned nonpolar InxGa1−xN/GaN MQW NWs will improve radiative recombination by eliminating the piezoelectric fields which causes an efficiency droop in LED devices. In summary, the coaxial InxGa1−xN/GaN MQW/n-GaN NWs are grown by a pulsed flow MOCVD technique. The number of pairs and uniform diameter of the coaxial InxGa1−xN/GaN MQW structures on n-GaN core NWs can be obtained by adjusting the growth temperature and the working pressure. The successful growth of coaxial LED device structures are achieved with different pairs of MQW structures having sharply defined wells and barriers. The surface morphology, optical, and electrical characterization of the fabricated coaxial InxGa1−xN/GaN MQW/n-GaN NWs LED structures are studied by FE-SEM, HR-TEM, PL, CL, I−V, and L−I measurements. HR-TEM images reveal high-quality hexagonal coaxial NW structures with sharp InxGa1−xN and GaN interfaces. The results of PL and CL evidence that the grown InxGa1−xN/GaN MQW NWs are of good optical quality with lower defects. The measured CL peal at 440 nm is observed to be independent of power, which proves the absence of polarization fields in the active region. The p-GaN/InxGa1−xN/GaN MQW/n-GaN NWs LED shows a current rectification with a sharp onset voltage at 2.65 V in the forward bias. The overall electrical properties of the coaxial NW LEDs are significantly improved compared to that of thin film LEDs. The output power of the NW LED is linearly increased with increasing current. The linear enhancement of output power could be attributed to the highly uniform and defect-free n-GaN core and InxGa1−xN/ GaN MQW shell NW structures. Electrical transport measurement demonstrates that these coaxial NWs exhibit well-defined and reliable behavior of p−n junction diodes. These kinds of coaxial NWs promise to allow flat band quantum structures that can improve the efficiency of light-emitting diodes.

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ACKNOWLEDGMENTS

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REFERENCES

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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 Basic Research of the Korea Science and Engineering Foundation (NRL. R0A-2008-000-0031-0) of Korean Government (MOEHRD). This research was also financially supported 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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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Tel.: +82-63-270-2304. Fax: +82-63-270-2305. Author Contributions

Y.-H.R. and R.N. contributed equally to this work. Notes

The authors declare no competing financial interest. J

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