Nanostructures of Indium Gallium Nitride Crystals Grown on Carbon Nanotubes

Nanostructure (NS) InGaN crystals were grown on carbon nanotubes (CNTs) using metalorganic chemical vapor deposition. The NS-InGaN crystals, grown on a ~5-μm-long CNT/Si template, were estimated to be ~100–270 nm in size. Transmission electron microscope examinations revealed that single-crystalline InGaN NSs were formed with different crystal facets. The observed green (~500 nm) cathodoluminescence (CL) emission was consistent with the surface image of the NS-InGaN crystallites, indicating excellent optical properties of the InGaN NSs on CNTs. Moreover, the CL spectrum of InGaN NSs showed a broad emission band from 490 to 600 nm. Based on these results, we believe that InGaN NSs grown on CNTs could aid in overcoming the green gap in LED technologies.


Crystallographic characterizations of InGaN NSs on CNTs/Si template. The structural and
compositional properties of the InGaN NSs on CNT/Si template were characterized by high-resolution X-ray diffraction (HR-XRD). Figure 3 shows the high-resolution ω -2θ scan of InGaN NSs on CNT/ Si template. Three peaks appear at 34.5°, 36.9°, and 49.8°. No peaks related to other components are observed at the resolution of the X-ray diffractometer. The peaks at 34.5° and 36.9° are indexed to the (002) GaN and (101) GaN planes, while the peak at 49.8° may be indexed to (004) C originating from the CNT. This implies that the crystal planes of the GaN columns consist of (002) GaN and (101) GaN . However, we believe that (002) GaN was the main crystallographic plane, rather than (101) GaN , because the (002) GaN peak is higher in intensity.
The inset of Fig. 3 shows the magnified (002) GaN peak, which presents a very narrow full width at half maximum (FWHM) of 381.8 arc-seconds. This indicates that the GaN crystals mainly align with [0001] GaN , which is consistent with the hexagonal GaN columns observed in Fig. 1(d). Furthermore, the two low-angle side peaks and the asymmetric (002) GaN peak, shown by the three arrow symbols in the inset, could be related to low In compositions of InGaN NSs. The XRD intensities of the two low-angle side peaks are very low, yet the peaks clarify the crystallinity of InGaN NSs, whereas the low-angle shoulder of the asymmetric (002) GaN peak clearly represents the InGaN NSs with low In contents. From these results, despite the relatively low XRD intensity, the InGaN-related peaks located at low angles indicate that the In phases of the InGaN NSs may be separated into at least two components. We surmise that the In composition of the InGaN NSs may be different for each NS. The average grain size D of the InGaN NSs was estimated from the width of the diffraction peaks by the Debye-Scherer equation: D = (0.89λ )/(β cosθ ), where β is the FWHM of the diffraction peak, θ is the angle of diffraction, and λ  is the wavelength of the X-ray radiation 28 . The calculated average size of InGaN NSs is 89.9 nm, slightly smaller than that shown by the SEM results. The InGaN NSs dimension evaluated by XRD FWHM is the lower bound for the InGaN NSs, implying some strain in these grains.

Structural characterizations of InGaN NSs on CNTs/Si template. The microstructural proper-
ties of InGaN NSs on the CNT/Si template were analyzed by transmission electron microscopy (TEM). Figure 4(a) shows that the InGaN NSs contain several types of polygonal crystals, which could be different thermodynamically stable facets. The dimensions of the InGaN NSs are evaluated to range from 100 to 200 nm, consistent with the SEM results ( Fig. 1). Notably, the InGaN NSs are tightly bound to the CNTs, as shown in Fig. 4(b). The InGaN NSs were particularly well developed at the edges of the CNTs. This strongly suggests that InGaN NSs can be directly grown on CNTs. Furthermore, the selected area electron diffraction (SAED) pattern obtained from the interface between the InGaN NSs and the CNT shows well-defined sharp spots, corresponding to the 10 plane of wurtzite InGaN, along with diffuse rings associated with the CNTs, as shown in Fig. 4(c). However, the SAED pattern obtained from several InGaN NSs crystallites exhibits numerous weak diffraction spots combined with sharp rings of diffracted intensity, as shown in Fig. 4(d), which is characteristic of a polycrystalline substance. Based on the SAED examinations, single-crystalline InGaN NSs were clearly grown on the CNT/Si templates, although the individual crystallites were oriented completely at random with respect to each other. Figure 5 presents the scanning transmission electron microscopy (STEM) Z-contrast image and corresponding EDS maps for Ga, In, and N atoms taken from the InGaN NSs grown on the CNT/Si template. The distributions of Ga, In, N and C atoms clearly match the positions of the InGaN NSs and CNTs shown in the STEM image. However, the distributions of Si, Al, and Fe atoms are inconsistent with the STEM image of the InGaN NSs on CNT within the detection limit of the instrument, indicating that the Si, Fe, and Al atoms do not diffuse from the Si substrate and Fe/Al catalyst with the growth of  the InGaN NSs. From these results, we believe that InGaN NSs can be directly grown on CNTs without catalytic assistance. Figure 6(a,b) depict SEM and the panchromatic cathodoluminescence (CL) images, respectively, of the InGaN NSs on the CNT/Si template. The bright CL image is strongly consistent with the SEM image of the InGaN NSs. This indicates the excellent optical properties of the InGaN NSs. We measured a particularly strong panchromatic CL emission from InGaN NSs on the CNTs, as shown in Fig. 6(c). The CL wavelength and FWHM of the CL spectrum are 493.4 and 79.4 nm, respectively. In general, the emission FWHM of the InGaN active layer of GaN-based LEDs is ~25 nm at ~500 nm, which is much lower than the FWHM of our InGaN NSs on CNTs. Despite growing the InGaN NSs on the CNT template, we surmise that this broad emission spectrum could be caused by the non-uniform In distribution or the broad size distribution of the InGaN NSs. To clarify the origin of the broad emission spectrum, we measured the CL spectrum of one InGaN NSs at position A, shown in Fig. 6(d). The spectrum shows two emission peaks at 528 nm and 612 nm. This implies that the broad emission of the InGaN NSs does not mainly originate from the size distribution of InGaN NSs, but from two or more In compositions among the InGaN NSs because InGaN NSs show a few crystallographic planes, yielding the different In incorporation rate to the different crystal planes.

Discussion
To further analyze the optical properties of the InGaN NSs, we performed temperature-dependent photoluminescence (PL) measurements from 10 K to 300 K, as shown in Fig. 7. Figure 7(a) shows the PL spectra at different ambient temperatures of the InGaN NSs grown on CNTs. The room-temperature PL emission wavelength and FWHM of InGaN NSs are 510.5 nm and 99.35 nm, respectively, slightly longer and broader than those measured by the CL spectra because of the different carrier excitation densities. However, we observe no GaN-related peak at 360 nm in the low-temperature PL spectra, indicating that the optical quality of GaN is much lower than that of the InGaN NSs. Our GaN crystal was grown at 800 °C, which is much lower than the > 1000 °C growth temperature of high-quality GaN films, leading to the poor optical emission properties of the crystal. Therefore, we believe that our GaN crystal may assist the growth of high-quality InGaN NSs on CNTs as a seed layer. Based on Fig. 7(a), we have replotted the PL intensity ratios of the InGaN NSs as a function of reciprocal temperature, as shown in  Fig. 7(b). The PL intensity ratio of 300 to 10 K is 11.4%, indicating the high internal quantum efficiency (IQE) of the InGaN NSs in the green emission region. Figure 7(c,d) show the temperature-dependent PL wavelengths and FWHMs as a function of ambient temperature. With decreasing the temperature from 300 to 10 K, the PL wavelength of InGaN NSs is shifted from 510.5 to 484.4 nm by the bandgap narrowing effect. However, the FWHMs of the PL spectra increase slightly with a temperature reduction from 300 to 180 K, and fluctuate at ~104 nm at temperatures below 180 K. The In incorporation in InGaN films is significantly affected by crystallographic plane orientation because they have different surface energies 29 . As shown in Fig. 4(a,d), the InGaN NSs grown on the CNT template contain several crystallographic planes. We surmise that the different facets of our InGaN NSs may have different In contents, generating the broad emission range from 10 to 300 K. Moreover, the low-temperature PL spectra of the InGaN NSs may induce other emission peaks around localized regions in the InGaN NSs with decreased ambient temperature, resulting in increased PL FWHMs at low temperatures.
In summary, we have demonstrated the successful growth of InGaN NSs on a CNT/Si template by MOCVD. SADPs indicated that the crystallites of InGaN NSs formed in random directions, and that only individual InGaN NSs were single crystals. From HR-XRD, the InGaN NSs crystallite was mainly aligned with the (0001) plane. A CL emission of ~500 nm was observed from the InGaN NSs crystallites. Temperature-dependent PL analyses indicated that the IQE of the InGaN NSs is 11.4% for the green emission region. Furthermore, the InGaN NSs exhibited temperature-independent PL FWHM behaviors from 10 to 300 K. This may result from the large localization of In in different facets of the InGaN NSs. We suggest that InGaN NSs on CNT/Si templates are among the best candidates for achieving green and yellow emission.

Growth of CNTs/Si template and InGaN NSs.
Al was deposited in a 10-nm-thick film on a 200-nm-thick SiO 2 /Si (001) substrate by radio-frequency magnetron sputtering, and then oxidized at 650 °C in air to form the alumina supporting film. An ultrathin (~0.5 nm) Fe film was e-beam evaporated onto the alumina specimens and subsequently thermally oxidized at 600 °C for 10 min in air.
After the e-beam evaporation process, the thickness of the catalyst film was measured by a thickness monitor using a quartz crystal microbalance. In order to grow the CNTs, the Fe/Al-deposited SiO 2 /Si (001) template was loaded into the reactor of a homemade radio-frequency (13.56 MHz) remote-plasma CVD 27 . As a source gas for the CNTs, methane gas was introduced at 60 sccm into the quartz tube reactor and the subsequent plasma (15 W) was ignited to grow the CNTs. During the growth of the CNT forest, the working temperature and pressure of the radio-frequency remote-plasma CVD were maintained at 450 °C and 64 Pa, respectively 27 . After growing the CNTs on the Fe/Al-deposited SiO 2 /Si (001) template, we loaded the template into the reactor of a MOCVD system to form the InGaN NSs on the CNT template. Trimethylgallium (TMGa), trimethylindium (TMIn), and ammonia (NH 3 ) were used as precursors for Ga, In, and N, respectively. Before growing the InGaN NSs, we grew GaN on the CNT on Fe/Al-deposited SiO 2 /Si (001) template at 800 °C. The working pressure and V/III ratio were 27 kPa and 4300, respectively. Subsequently, InGaN NSs were grown by introducing TMGa, TMIn, and NH 3 at 750 °C under a N 2 atmosphere. As a result, we achieved InGaN NSs/CNTs grown on the Fe/Al-deposited SiO 2 /Si (001) template.

Characterizations of InGaN NSs on CNTs/Si template. SEM and atomic force microscopy (AFM)
were used to observe the surface structure of the InGaN NSs on the CNT/Si template. The optical properties of the InGaN NSs on CNT/Si template were characterized by CL analysis at room temperature using a Hitachi S-4700 system installed on a field-emission scanning electron microscope (FESEM). In addition, temperature-dependent PL spectroscopy was performed using a He-Cd laser (λ = 325 nm) with an excitation power density of 2.0 kW/cm 2 . The crystallinity of the InGaN NSs on CNT/Si template was characterized by HR-XRD and electron diffraction patterns. TEM examinations were performed with a Tecnai G2 F30 S-Twin (FEI) with an accelerating voltage of 300 kV and fitted with an EDS (EDAX Genesis) to characterize the atomic structure and the compositions of the InGaN NSs on the CNT/Si template.