Letter pubs.acs.org/NanoLett
Yellow Luminescence of Polar and Nonpolar GaN Nanowires on r‑Plane Sapphire by Metal Organic Chemical Vapor Deposition Shengrui Xu,* Yue Hao, Jincheng Zhang, Teng Jiang, Linan Yang, Xiaoli Lu, and Zhiyu Lin State Key Discipline Laboratory of Wide Band-Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, China S Supporting Information *
ABSTRACT: We have grown horizontal oriented, high growth rate, well-aligned polar (0001) single crystalline GaN nanowires and high-density and highly aligned GaN nonpolar (11−20) nanowires on r-plane substrates by metal organic chemical vapor deposition. It can be found that the polar nanowires showed a strong yellow luminescence (YL) intensity compared with the nonpolar nanowires. The different trends of the incorporation of carbon in the polar, nonpolar, and semipolar GaN associated with the atom bonding structure were discussed and proved by energy-dispersive X-ray spectroscopy, suggesting that C-involved defects are the origin responsible for the YL in GaN nanowires. KEYWORDS: GaN, nanowire, yellow luminescence, polar, nonpolar compounds. Finally, GaN nanowires were grown at 700 °C, 40 Torr. Before the growth of polar GaN nanowires, the (112̅0) nonpolar a-plane GaN was gown on r-plane sapphire. Further details about the growth conditions, structure, and properties of nonpolar a-plane GaN grown have been published previously.10,11 Then, 10 nm Ti was deposited onto the nonpolar aplane GaN using electron beam evaporation and was nitrided in an atmosphere of ammonia and hydrogen for 10 min at 1020 °C to form TiN. The growth condition was the same with the nonpolar nanowires. The nanowires were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL). The content of C was investigated using energy-dispersive X-ray spectroscopy (EDS). Figure 1a shows a typical SEM image of the polar nanowires, where the most striking feature is that they demonstrate a high degree of horizontal alignment along c-axis (polar direction). The direction of nanowires can also be easily determined through the direction of the triangle pit which is the typical character of a-plane GaN.10 It can be seen that the nanowires are perpendicular to the N plane (0001)̅ of the triangle pit, further proving that the nanowires are along the c-axis. The high average growth rate of nearly 50 μm/h can be obtained from the length of nanowires and the growth time, which is much faster than the traditional method of growing polar nanowires.12 Figure 1b shows SEM images of the as grown high-density and highly aligned GaN nanowire array catalyzed
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aN-based nanorods and nanowires have attracted considerable attention as promising nanoscale building blocks for future electrical, optical, or optoelectronic devices.1−5 Because of its anisotropic and polar nature, GaN exhibits direction-dependent properties.6 The impurity-related emission is an important deep level phenomenon in GaN. For example, the origin of the well-known yellow luminescence (YL) band centered at 2.2−2.3 eV in the luminescence spectra of GaN is associated with the impurities and complex.7−9 However, the influence of the polar direction on luminescence properties in GaN nanowire is lacking. In fact, comparison of the luminescence properties between different polar directions GaN nanowires will probably be helpful to further seek after the intrinsic mechanism of impurity incorporation and YL in GaN from a different view. As a commercialized technique for the growth of III-nitride films, metal organic chemical vapor deposition (MOCVD) may be the most promising method for the controlled growth of IIInitride nanowires for nanodevice applications with an advantage over in situ doping and heterostructure growth. In this work, we report the growth of highly aligned GaN nanowires with polar [0001] and nonpolar [112̅0] growth orientation on r-plane sapphire via MOCVD. In this study, the growth method proposed by Li (ref 5) was used to grow nonpolar (112̅0) GaN nanowires. One nanometer Ni catalyst was deposited onto the sapphire substrate using electron beam evaporation. After chemical cleaning, r-plane sapphire substrates were loaded into the chamber. The GaN nanowires were subsequently grown on the Ni-coated sapphire in a cold-wall showerhead MOCVD reactor at a V/III ratio of 400. Hydrogen was used as the carrier gas and triethylgallium, trimethylaluminium, and ammonia (NH3) were used as source © XXXX American Chemical Society
Received: April 26, 2013 Revised: June 26, 2013
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single-crystal structure and the c-axis growth direction of the GaN nanowire, as shown in Figure 3b. The [0001] direction is parallel to the long axis of the nanowires, indicating that the [0001] direction is the growth direction of the GaN nanowires. Selected area electron diffraction measurements shown in Figure 3c also confirm that the nanowires grow along the [0001] direction. No threading dislocations and basal plane stacking fault are found, but the YL of the polar nanowire are very strong. The results of the TEM and PL indicate the edge dislocation has a less effect on the YL of the GaN nanowire. Moreover, it is likely that the impurity-related defect should play an important role in the YL emission in nanowire. It should be noted that most of the polar nanowires have a nonpolar side facet (m-plane) as shown in Figure 4. This facet assignment also agrees with previous work where it was ascertained that the facets bounding GaN nanowire were the {11̅00} family (m-plane).11 The nonpolar nanowires have isosceles-triangle cross sections with one polar (0001) plane and two semipolar {110̅ 1} side facets (semipolar).5 It is wellknown that under n-type conditions the energetically C related defect by far most stable site is CN.15 For the polar N-face (0001̅) GaN, the carbon atom attempts to replace the N to form the CN, however, every time there is a fat gallium atom giving an effective protection to the N under it. In comparison with the N-face (0001̅) condition, the Ga-face (0001) GaN did not have a protection for the N. So the Ga-face GaN contained significantly higher concentrations of carbon than the N-face.11 For the nonpolar m-plane GaN (the side of the polar GaN nanowire), there is no protection effect of the Ga atom to the N atom being free of the attack of the carbon atom, (Supporting Information Figure 2) so the nonpolar GaN must have the highest concentrations of carbon. This is consistent with the SIMS result.11 The situation of the semipolar {11̅01} plane (the side of the nonpolar GaN nanowire) GaN is nearly the same with the N plane, so the semipolar {11̅01} plane must have lower C concentration than the nonpolar m-plane GaN. On the basis of the principle of mentioned above, the polar (0001) nanowire with the nonpolar side facets have higher C concentration than the nonoplar nanowire with the polar and semipolar side facets. Xu et al. have found that the c-plane GaN film grown by MOCVD free of VGa-involved defects has a very strong YL and was proved by the monoenergetic positron annihilation spectroscopy,16 which excludes the effects of gallium vacancies. Furthermore, in previous studies of hydride metal−organic vapor phase GaN epilayers (no carbon) we found that the YL is very weak; even in the nonpolar a-plane GaN with high dislocation and point defects, other literature
Figure 1. SEM images showing (a) highly aligned polar GaN nanowires array catalyzed by Ti, and (b) high-density and highly aligned GaN nanowire array catalyzed by 1 nm Ni.
by 1 nm Ni on r-plane sapphire. The GaN nanowires have tapered and triangular cross sections, as was also seen in previous studies of MOCVD-grown GaN nanowires.8 PL measurement of individual GaN nanowire was collected at room temperature to evaluate their optical quality. A representative PL spectrum for the polar nanowire grown with catalyst Ti is shown in Figure 2a and features a band edge emission peak at around 365 nm and a broad YL band centered at 560 nm. The band-edge peak is slightly red shifted compared to a 363 nm band-edge of the nonstrain state. As a comparison, Figure 2b shows the PL of the nonpolar nanowires grown with 1 nm Ni. A dramatic decrease in the YL is observed. It should be noted that the two different nanowires have the same growth conditions. The defect-related YL emission is an important deep-level phenomenon in GaN. However, the origin of the deep acceptors responsible for YL is still not identified. First, the gallium vacancies (VGa) involved defects, such as the VGa− ON complex, are regarded as the candidate of the deep acceptors to enhance the YL band by some authors.7 Second, several other experimental groups also attributed the YL band to carbon impurity or a complex involving carbon.8,9 Third, some authors suggested that the yellow luminescence is related to the edge dislocation density.13,14 We investigate the possible factors could contribute to YL intensity of the different directions nanowires. TEM analysis was used to evaluate the crystalline quality of the polar nanowires with high intensity of YL. Figure 3 displays the TEM images of the GaN nanowires scratched from the substrate. The low-magnification TEM image for a single GaN nanowire is shown in Figure 3a. It is observed that the GaN nanowires are straight and gradually thinner from the roots to the top. The high-resolution TEM image of the nanowire confirms the
Figure 2. Representative PL spectra of GaN nanowires grown on r-plane sapphire (a) polar nanowire (b) nonpolar nanowire. B
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Figure 3. (a) Low-magnification TEM image of the polar GaN nanowire, (b) high-resolution TEM image of a single-crystalline wurtzite structure GaN nanowire, and (c) the single-crystal and c-direction nature of the GaN nanowires.
of the polar nanowire was significantly higher than the nonpolar nanowire as shown in Figure 5. The result of EDS is consistent with our discussion above. In summary, we have grown horizontally oriented, high growth rate, well-aligned polar single crystalline GaN nanowires and high-density and highly aligned nonpolar GaN nanowires on r-plane substrates without using patterning or a template. In addition, it can be found that the polar nanowires showed a strong YL intensity compared with the nonpolar nanowires. The different trend of the incorporation of carbon in the polar, nonpolar, and semipolar GaN associated with the atom bonding structure suggesting that C-involved defects are the origin responsible for the YL in GaN nanowires. The result of EDS further proves our discussion.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 4. Magnified SEM images of polar GaN nanowire grown on rplane sapphire with catalyst Ti.
Additional figures and information. This material is available free of charge via the Internet at http://pubs.acs.org.
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reported the same phenomenon.17,18 We believe that it is ascribed to lack of the carbon. Specifically, Wright has proved that carbon-involved defects have the deep levels suitable to produce YL.19 All of this is consistent with our conclusion of the relation of the CN to the YL. So we believe that the carbon and carbon-involved defects are the origin of the deep levels accounting for the YL in the as grown GaN nanowires. In order to evaluate the C content of the nonpolar and polar nanowires to further validate our conclusions, we performed EDS measurements. The high-resolution SEM of FEI Magellan 400L system was used, the EDS was XMax80 of Oxford. After normalizing the peak of N, it can be found that the C content
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 61204006 and 51202176), the Fundamental Research Funds for the Central Universities
Figure 5. The amplified EDS spectrum of the polar nanowire and nonpolar nanowire. C
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(Grant K50511250002), and the National Key Science and Technology Special Project (Grant 2008ZX01002-002).
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