Structural and optical characteristics of GaN/ZnO coaxial nanotube heterostructure arrays for light-emitting device applications

We report on the structural and optical characteristics of position-controlled GaN/ZnO coaxial nanotube heterostructure and GaN/InxGa1−xN coaxial nanotube quantum structure arrays for light-emitting diode (LED) applications. The GaN/ZnO nanotube heterostructures were fabricated by growing a GaN layer on the entire surface of position-controlled ZnO nanotube arrays using low-pressure metal-organic vapour-phase epitaxy. As determined by transmission-electron microscopy (TEM), an abrupt and coherent interface between the core ZnO and the GaN overlayer was observed. The optical characteristics of heteroepitaxial GaN/ZnO nanotube heterostructures were also investigated using cathodoluminescence (CL) spectroscopy. This position-controlled growth of high-quality single crystalline GaN/ZnO coaxial nanotube heterostructures allowed the fabrication of artificial arrays of high-quality GaN-based coaxial quantum structures by the heteroepitaxial growth of GaN/InxGa1−xN multiple quantum wells along the circumference of the GaN/ZnO nanotubes. The optical and structural characteristics of the position-controlled GaN/InxGa1−xN coaxial nanotube quantum structures were investigated by using CL spectroscopy and TEM analysis, respectively. The green LED microarrays were successfully fabricated by the controlled heteroepitaxial coaxial coatings of GaN/InxGa1−xN coaxial nanotube quantum structures and the outermost Mg-doped p-type GaN layer onto the GaN/ZnO coaxial nanotube heterostructures, presumably implying that the position-controlled growth of high-quality GaN/ZnO coaxial nanotube heterostructure arrays provides a general and rational route of integrating vertical nanodevices for nanoscale electronics and optoelectronics.


Introduction
For the past several decades, monolithically integrated devices have been fabricated using a top-down process consisting of thin film deposition, lithography and etching [1]. Preparing high-quality thin films by heteroepitaxy [2]- [4] is the first step in fabricating high-quality nanodevices using the top-down method. However, even a small mismatch in lattice constant or thermal-expansion coefficient between the thin film and substrate leads to film quality deterioration through the formation of structural defects and cracks [4,5]. In addition, films can easily be contaminated and damaged during the lithography and etching processes [6]. These difficulties resulting from material incompatibility between film and substrate in the top-down approach may be circumvented by selectively growing nanostructures on nano-scale sites. Nanoscale devices, such as light-emitting diodes (LEDs), have also been fabricated using the bottom-up method [7]- [10], where various one-dimensional (1D) semiconductor nanomaterials, including nanorods, nanowires, nanobelts and nanotubes, have been synthesized for device fabrications [11]- [15]. However, despite extensive research on bottom-up methods, it is still very difficult to control the positioning of nanomaterials for nanodevice integration. 3 Accordingly, in growing 1D nanostructures, using both top-down and bottom-up processes, the development of a method for precisely controlling the position and dimension is crucial.
Position control of nanostructures has been achieved by the selective growth of 1D nanostructures on specifically patterned sites using either metal catalyst-assisted [16] or catalyst-free methods [15,17,18]. The next challenge is to fabricate position-controlled 1D nanomaterial heterostructures with compositional modulations along either the axial or radial directions [19,20]. These heterostructures could greatly increase the versatility in designing novel electronic and optoelectronic nanodevices [7,21,22]. For 1D heterostructured nanodevice applications, nanomaterial heterostructures must have a clean, abrupt interface with a very low interfacial defect concentration as the effects of interfacial defects on nanomaterial properties and nanodevice characteristics become more significant with a high interface-to-volume ratio of the nanomaterial heterostructure. However, the structural and optical characteristics of positioncontrolled 1D nanomaterial heterostructures have rarely been studied [19,23].
Here, we report on the structural and optical characteristics of position-controlled ZnO/GaN coaxial nanotube heterostructure arrays for LED applications. Among numerous semiconductor nanomaterials, ZnO nanostructures with high crystallinity and optical qualities have been widely studied for optoelectronic nanodevice applications. Very recently, both the position and dimension of ZnO nanotubes have been precisely controlled using conventional lithography and selective area metal-organic vapour-phase epitaxy (MOVPE) [15,24]. However, since reliable and reproducible growth of p-type ZnO is still a formidable task [25], realization of high-brightness ZnO-based p-n homojunction LEDs has yet to be obtained [26,27]. Conversely, in III-nitride semiconductor heterostructures, controlled n-and p-type dopings and compositions have been achieved, which yielded high-brightness blue LEDs and even short-wavelength laser diodes [28]. Meanwhile, nanostructure LEDs can be fabricated using the epitaxial coating of nitride LED structures on well position-controlled ZnO nanostructures [7]. In this paper, the structural defects and optical characteristics of GaN/ZnO coaxial nanotube heterostructures and GaN/In x Ga 1−x N coaxial nanotube quantum structures, which were investigated using electron microscopy and cathodoluminescence (CL) spectroscopy, respectively, are described.

Experimental
GaN/ZnO coaxial nanotube heterostructure arrays were grown on GaN/c-Al 2 O 3 substrates using catalyst-free, low-pressure MOVPE. Figure 1 depicts schematics of the overall procedure for fabricating these position-controlled coaxial nanotube heterostructures. The positioncontrolled nanostructures were fabricated by selectively growing the ZnO nanotubes only on the hole patterns of the substrates. First, ZnO nanotube arrays were prepared on c-Al 2 O 3 substrates with a GaN seed layer and a hole array-patterned 50-nm-thick amorphous SiO 2 thin film as a growth-mask layer (figures 1(a) and (b)); details of the selective growth are described elsewhere [15]. Immediately following the ZnO nanotube array preparation, GaN layers were grown along the circumferences of the nanotube arrays using MOVPE ( figure 1(c)). For the reactants of GaN, trimethylgallium and ammonia, with typical flow rates of 1.5 and 1000 sccm, respectively, were used. The typical GaN growth temperature was 600 • C. The typical thickness of the GaN layer grown for 30 min was ∼200 nm. After the preparation of GaN/ZnO coaxial nanotube heterostructure arrays, three-period GaN/In 0.24 Ga 0.76 N multiple quantum wells (MQWs) were heteroepitaxially deposited on the circumferential surface of GaN/ZnO coaxial nanotube heterostructure arrays using a commercial nitride MOVPE system (figure 1(d)) [7]. For GaN/In 0.24 Ga 0.76 N MQWs, the 3-nm-thick quantum well (QW) and 13-nm-thick quantum barrier (QB) were grown at 720 and 820 • C, respectively, with the expectation of green colour emission. For In x Ga 1-x N QW layers, trimethylindium and trimethylgallium as well as ammonia were employed as reactants. Subsequently, an Mg-doped p-GaN layer with a thickness of 120 nm was deposited on top of the last GaN QB at 950 • C using a commercial nitride MOVPE system. For p-type doping of GaN, biscyclopentadienyl magnesium was employed.
Coaxial nanotube heterostructure LED microarrays were fabricated by making Ohmic contacts on both the p-GaN surface of the coaxial nanotube heterostructures and the heavily doped n-GaN seed layer. To fabricate metal contacts on the n-GaN seed layer, a Ti/Au (50/100 nm) layer was first evaporated onto an n-GaN layer. Next, in order to fill the gaps between individual nanotubes and isolate two different metal electrodes, a spin-on-glass (SOG) layer was coated by the spin coating method and cured at 425 • C. For metal contacts on p-GaN, wet chemical etching of the SOG layer was performed in a buffered-oxide etchant to expose the tip surface of the nanotubes; then an Ni/Au (10/10 nm) layer was deposited on the exposed p-GaN surface by using electron-beam evaporation. It should be noted that the conformal coating of continuous contact layers allowed the occurrence of uniform current injection in the entire nanotubes surface.
The morphology and structural characteristics of GaN/ZnO coaxial nanotube heterostructures and GaN/In x Ga 1−x N coaxial nanotube quantum structures were investigated using field-emission scanning-electron microscopy (FE-SEM; Philips XL30SFEG) and highresolution transmission-electron microscopy (HR-TEM; FEI Tecnai G 2 F20). For TEM imaging and electron diffraction analysis of the GaN/In x Ga 1−x N coaxial nanotube quantum structure, samples were milled cross-sectionally with 30 kV-accelerated Ga ions using a focused ion beam machine (NOVA 200 Nanolab, FEI Company) in the dual beam mode. The acceleration voltage of Ga ions was decreased from 30 to 5 kV at the finishing stage in order to reduce the damage of the sample and inevitable contamination with Ga ions. The compositional line profile of the GaN/In x Ga 1−x N coaxial nanotube quantum structure along its radial direction was obtained from energy-dispersive x-ray spectroscopy (EDX) in the scanning-TEM (STEM) mode of the TEM facility.
CL measurements were performed to investigate the optical characteristics of nanotube heterostructures. A CL facility (Gatan MonoCL3+) attached to the SEM (Hitachi S-4300) was employed. CL images and spectra were measured at 80 K and room temperature using a 10-kV electron beam. The spectral resolution of the employed high-resolution CL measurement system was as accurate as ±8 meV. Details of the CL measurements are described elsewhere [19]. Figure 2 shows the FE-SEM images of position-controlled ZnO nanotube and GaN/ZnO nanotube heterostructure arrays, which reveal the general morphology of nanostructures after the procedures shown in figures 1(b) and (c), respectively. As depicted in figure 2(a), the ZnO nanotube dimensions were quite uniform, with a mean diameter of 400 ± 40 nm and a length The microscopic crystal structure of GaN/ZnO coaxial nanotube heterostructures was investigated using HR-TEM. In figure 3(a), an HR-TEM image of a GaN/ZnO nanotube heterostructure coated for 1 min shows that the heterostructure consisted of an approximately 7-nm-thick GaN layer. The typical GaN growth rate on the ZnO nanotube side walls was therefore estimated to be ∼7 nm min −1 . The low growth rate makes it possible to accurately control GaN layer thickness with growth time.

Structural and optical characteristics of GaN/ZnO coaxial nanotube heterostructures
Further investigation of possible defect formation in the nanotube heterostructures was carried out. Figure 3 figure 3(b). This indicates that a single crystalline wurtzite GaN layer was epitaxially coated onto the ZnO nanotube. In figure 3(c), the inverse FFT image of the diffraction pattern also confirms the defect-free single crystallinity of the nanotube heterostructure. Figure 3(d) displays an enlarged image of the selected GaN area from the HR-TEM image of a GaN/ZnO nanotube heterostructure in figure 3(a), indicating a highly ordered lattice structure of the outer GaN layer. From this image, the lattice spacing between adjacent planes was measured as ∼0.26 nm, corresponding to the d-spacing of GaN(0001) planes.
The inset of figure 3(b) shows two peaks corresponding to the (1012) diffraction planes of ZnO and GaN, which are resolved through the difference in lattice constants along the radial direction. However, the (0002) reflection along the growth direction does not show peak splitting. These results indicate that strain exists at the GaN/ZnO interface, although the atomic configurations are not under any constraint in the radial direction. Similar behaviour was observed for GaN/ZnO coaxial nanorod heterostructures, where a GaN layer was coated on solid ZnO nanorods [29]. As reported for GaN/ZnO core-shell nanorods [29], stress due to the lattice mismatch in the axial direction may cause edge dislocations with a Burgers vector. Hence, interfaces of the hollow nanotube heterostructures were thoroughly investigated in the long length scale of several hundred nanometres from the HR-TEM images of several nanotube heterostructures. However, no structural defect was found. The different behaviour in the defect formation in nanorods and nanotube heterostructures is presumably attributed to the hollow structure of GaN/ZnO nanotube heterostructures. In comparison with a solid ZnO core nanorod in the nanorod heterostructures, nanotubes offer a larger circumference, so the strain induced at the interface should be reduced.
The optical characteristics of GaN/ZnO coaxial nanotube heterostructure arrays were individually investigated at high spatial resolution. Figure 4 shows the CL spectra of a single GaN/ZnO coaxial nanotube heterostructure. As shown in the inset of figure 4(a), the room temperature CL spectrum shows strong ultraviolet emission consisting of two distinct CL peaks at 3.28 and 3.39 eV that are attributed to near-band-edge (NBE) emissions from ZnO and GaN, respectively. Also, no deep-level green or yellow emission from either ZnO or GaN was observed in the room temperature CL spectrum, indicating the high optical quality of the GaN/ZnO coaxial nanotube heterostructures (figure 4(a)). Additionally, the CL spectrum measured at 80 K shown in figure 4(b) presents dominant CL peaks at 3.35 and 3.44 eV that are ascribed to excitonic emissions from the core ZnO nanotube and outer GaN layer, respectively. A further high-resolution CL spectrum measured in the range of 3.0-3.6 eV shows an additional CL emission at 3.31 eV with an energy difference of ∼40 meV from the excitonic emission of ZnO, similar to the two-electron satellite (TES) separation energy for excitons in ZnO [19,30].
The spatially resolved CL characteristics of individual GaN/ZnO coaxial nanotube heterostructure were investigated by measuring monochromatic CL images at room temperature. Figure 4(c) presents a SEM image showing surface morphology and figures 4(d) and (e) present monochromatic CL images measured at photon energies of 3.28 ± 0.01 and 3.39 ± 0.01 eV, corresponding to CL peak energies from the NBE emission of ZnO and GaN, respectively. These CL images show that NBE emission of GaN originated from the nanotube structure, which strongly suggests that the GaN layer was deposited mainly on the outer surface of the core ZnO nanotubes. It is also notable that the CL spectra measured in different tubes  did not show any peak shift within a spectral resolution of the CL measurement system, indicating that the optical characteristics of each GaN/ZnO nanotube heterostructure are almost uniform.

Structural and optical characteristics of GaN/In x Ga 1−x N coaxial nanotube quantum structures
The catalyst-free MOVPE of nanostructures enables the control of both the composition and the layer thickness of nanotube heterostructures. As shown in figure 1(d), GaN/In x Ga 1−x N coaxial nanotube quantum structures were deposited along the circumference of GaN/ZnO coaxial nanotube heterostructure arrays. The tilted-view SEM image in figure 5(a) shows the morphology of a hexagon-faceted GaN/In 0.24 Ga 0.76 N coaxial nanotube quantum structure array with a typical diameter of 1.0 ± 0.1 µm and a length of 4.0 ± 0.2 µm. Figure 5(b) presents the cross-sectional STEM image and the corresponding FFT image of a GaN/In 0.24 Ga 0.76 N coaxial nanotube quantum structure. The high magnification STEM image of figure 5(b) obviously reveals that MQW layers were radially coated on the GaN/ZnO nanotube heterostructures. Three bright lines corresponding to In 0.24 Ga 0.76 N QW layers were observed alternating with clearly discriminated GaN QB layers. The corresponding FFT image of the coaxial nanotube quantum structure reveals single crystallinity with six-fold rotational symmetry in the {1010} plane (inset of figure 5(b)).
Further TEM analysis was carried out to confirm the indium composition in In x Ga 1−x N MQWs and to investigate interfacial defects. The compositional line profile along the radial direction of the GaN/In x Ga 1−x N coaxial nanotube quantum structure was obtained by employing EDX in the TEM facility. Figure 5(b) shows the EDX line profiles of gallium and indium, from which the indium concentration in In x Ga 1−x N QWs was estimated at x = 0.24 by averaging the integrated EDX profile within the QW thicknesses. Moreover, composition analysis by EDX confirmed the formation of three-period GaN/In x Ga 1−x N MQW layers on the GaN/ZnO nanotube heterostructures. Figure 5(c) displays the enlarged image of selected area from the HR-TEM image of a GaN/In 0.24 Ga 0.76 N QW in figure 5(b). The interface between GaN and In 0.24 Ga 0.76 N is clearly visible, indicating the formation of an abrupt interface. Structural defects, such as dislocations at the interface, were rarely observed.
The optical characteristics of position-controlled GaN/In x Ga 1−x N coaxial nanotube quantum structure arrays were individually investigated by measuring CL spectra. Figure 6 shows the CL spectra of a single GaN/In 0.24 Ga 0.76 N coaxial nanotube quantum structure measured at temperatures of 80 and 297 K. As shown in figure 6, four dominant CL peaks at 2.58, ∼3.0, 3.29 and 3.48 eV were observed at 80 K. Among them, the CL peak at 3.48 eV is ascribed to the neutral-donor-bound excitons from the core GaN layer. The broad CL emission peaks at ∼3.0 and 3.29 eV are attributed to band to acceptor and Mg-related donor-acceptor pair (DAP) transition from the outermost Mg-doped p-GaN layer, respectively [31]. The most dominant CL emission at 2.58 eV is assigned to GaN/In 0.24 Ga 0.76 N MQW emissions. At 297 K, three CL peaks were observed at 2.46, ∼2.9 and 3.41 eV. The CL peak at 3.41 eV is attributed to NBE emission of the core GaN layer. The very broad CL peak at ∼2.9 eV is ascribed to the band to acceptor emission of the outermost Mg-doped p-GaN layer. The most dominant CL peak centred at 2.46 eV is assigned to GaN/In 0.24 Ga 0.76 N MQW emissions, which appeared The intensity of the 80 K CL emission at 2.58 eV decreased much more slowly while the CL peak at 3.48 eV decreased rapidly on increasing the temperature. This strongly suggests that the room temperature CL emission centred at 2.58 eV originated from GaN/In 0.24 Ga 0.76 N coaxial MQWs. It is also noted that the full-width at half-maximum (FWHM) value of the CL peak of 2.46 eV was estimated to be as broad as 0.24 eV, presumably due to the broad distributions in thickness of In 0.24 Ga 0.76 N QW layers deposited on the six sidewalls of the GaN/ZnO coaxial nanotubes.

Coaxial nanotube LED microarrays consist of GaN-based p-n homojunction heterostructures with GaN/In x Ga 1−x N coaxial multiple QWs
The controlled formation of GaN/In x Ga 1−x N MQWs onto position-controlled GaN/ZnO nanotube heterostructures enabled the fabrication of coaxial nanotube LED microarrays by coating of the outermost p-type GaN layer. Electrical characteristics of the nanotube LEDs were investigated by measuring current-voltage (I -V ) characteristic curves on the LEDs. The I -V curve shown in figure 7(a) clearly exhibited nonlinear and typical rectifying behaviour with a turn-on voltage of ∼3 V and a leakage current of ∼4 × 10 −4 mA at −4 V. Above the turnon voltage, the current began to increase rapidly with bias voltage, resulting in an increase in light emission intensity. Figure 7(b) exhibits the electroluminescence (EL) spectra of nanotube LED microarrays measured at an applied current of 90 mA. The EL spectra clearly exhibited a dominant peak centred at ∼2.4 eV. Moreover, the EL emission of 2.4 eV is closely matched to the CL peak at ∼2.4 eV. Since we expected and designed the coaxial QW structures to observe green colour emission from MQW thickness and composition, the EL peak of ∼2.4 eV results from GaN/In 0.24 Ga 0.76 N MQWs. Figure 7(c) shows the microscopic magnified photographs of light emission from the coaxial nanotube LED microarrays at current levels of 40 and 120 mA. At zero applied current, no light emission was observed (not shown). Upon increasing the applied current, while most individual nanotube LEDs started to emit clear light, some individual LEDs did not emit light, presumably due to non-uniform distributions in the turnon voltage of individual coaxial nanotube LEDs. At a high current level above 100 mA, most nanotube LEDs emitted light. The EL colour was mostly green and partly blue, and the light emission was so strong that it was clearly observed with the naked eye even under normal room-illumination conditions. Although some local areas did not emit light, presumably due to failure of the contact layers, nearly the entire patterned area of nanotube LED microarrays clearly emitted light. The number of turned-on LEDs increased with increasing applied current, indicating that the turn-on voltages of individual LEDs were different from each other. The observed non-uniformities of the turn-on voltage and light intensity of individual nanotube LEDs presumably resulted from non-uniform light extraction, current injection through the metal contact layer, or both.

Summary and conclusions
Position-controlled GaN/ZnO coaxial nanotube heterostructures arrays were fabricated by the catalyst-free MOVPE method. HR-TEM analysis revealed that GaN/ZnO nanotube heterostructures were single crystalline without any significant formation of structural defects. Furthermore, the position-controlled nanotube heterostructures exhibited excellent CL characteristics. The high-quality heteroepitaxial GaN/ZnO nanotube heterostructures enabled greater tunability in the thickness and composition of the QW as well as GaNbased p-n homojunction diodes in the heterostructures, which will significantly enhance the versatility of the components for nanoscale electronics and photonics. This simple, 12 precise, well-controlled 'bottom-up' method for fabricating high-quality, position-controlled coaxial nanotube heterostructures provides a general and rational route of integrating vertical nanodevices for nanometre-scale electronics and optoelectronics.