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Electrical and Optical Performance of Sublimation-Grown Long GaN Nanowires Jianye Li,*,† Zhi Yang,‡ and Hui Li§ Department of Physical Chemistry, UniVersity of Science and Technology Beijing, China, College of Chemistry and Chemical Engineering, Yunnan Normal UniVersity, China, and Institute of Microstructure and Properties of AdVanced Materials, Beijing UniVersity of Technology, China ReceiVed: March 31, 2010; ReVised Manuscript ReceiVed: July 19, 2010
Through a facile sublimation method, high-quality GaN nanowires with a length of several hundred micrometers were densely grown on amorphous substrates. The morphology and single-crystalline hexagonal structure of the GaN nanowires were characterized. Photoluminescence of the nanowires was studied, and a near-bandgap emission of wurtzite GaN was observed, indicating good optical quality of the GaN nanowires. Individual GaN nanowire devices were fabricated, and their photoconductivity and electrical transport properties were investigated. The results reveal that the sublimation-grown GaN nanowires possess outstanding UV sensitivity and an n-type gating effect. 1. Introduction Wurtzite GaN is a technologically important III-V semiconductor and well known for its excellent optoelectronic properties with a direct wide band gap of 3.4 eV at room temperature, high mobility, and excellent thermal stability.1–6 It is of great interest for broad commercial applications, such as UV or blue emitters, detectors, high-speed field-effect transistors, high-temperature/high-power electronic devices, and so forth.1–6 GaN nanowires are promising building blocks for potential applications of nanodevices with excellent performance, such as lasers,7 LEDs,8 solar cells,9 and piezoelectric nanogenerators.10 In the past decade, extensive research has been focused on the hexagonal GaN nanowires for their wide prospects of understanding fundamental optical properties, developing novel nanotechnological applications, and exploiting their significant potential for nano-optoelectronics and nanobiological devices.1–15 The reported synthetic schemes for GaN nanowires include template growth,1 laser-assisted catalytic growth,2 chemical vapor deposition,3 hydride vapor phase epitaxy,4 metal-organic chemical vapor deposition,5 and molecular beam epitaxy,6 etc. Among all methods, the commonly used chemical vapor deposition (CVD) is the most facile, simplest, and cheapest way to grow GaN nanowires.3,16–18 Generally, the precursor of CVDgrown GaN nanowires includes metal gallium.3,16–18 We found that the growth with such a precursor has two shortcomings. First, because of the very low melting point (29.7646 °C) and high boiling point (2204 °C) of gallium,19 the precursor gallium is liquid and spherical during the growing process. Consequently, a GaN layer is prone to form on the liquid gallium spheres and blocks the further reaction of gallium (with ammonia). As a result, the yield of this simple CVD growth is low because only a small portion of the gallium reacted. Second, the GaN nanowires grown from this method usually are heavily doped metallic nanowires, and the heavy doping was due to the excess Ga content.17,18 * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Science and Technology Beijing. ‡ Yunnan Normal University. § Beijing University of Technology.
We found a facile way to boost the yield and quality of the grown GaN nanowires, and it is merely replacing gallium with gallium nitride as the precursor. GaN nanowires could grow on LaAlO3 single-crystal substrates by sublimating ball-milled GaN powders,20 but the high price of the LaAlO3 single-crystal substrates restricts the large-scale growth of the nanowires. Through improving the sublimation method, several hundred micrometers long single-crystalline GaN nanowires with a high quality were grown on a large scale on amorphous substrates, and the optical and electrical performances of the GaN nanowires were investigated. Herein, we report the results. Long nanowires are very important for the facile and large-scale fabrication of nanodevices. We believe that this simple way for growing submillimeter long GaN nanowires on a large scale will facilitate the future applications of GaN nanowires. 2. Experimental Section The growth of the long GaN nanowires was carried out by a system consisting of a 3.6 ft long horizontal fused-quartz tube inside a tube furnace, and the growth procedure is as follows. First, wurtzite GaN powders with a purity of 99.99% were mechanically deformed via a ball-milling process.20 The asmilled GaN powders and nickel(II) nitrate-coated fused quartz substrate were then loaded into a fused quartz boat. The boat was placed inside the horizontal fused-quartz tube with the precursor located at the furnace center, the highest temperature zone of the furnace. The catalyst-coated amorphous substrate was placed ∼5 mm upstream from the precursor. The coated substrate was prepared by dipping the substrates in a 0.01 M nickel(II) nitrate solution of ethanol and then dried in air. Before heating, the quartz tube was pumped at first, then filled with high-purity argon, and then repumped. Such an operation was repeated three times. After that, the quartz tube was heated under the flow of high-purity argon of about 50 sccm. When 560 °C was reached, a flow of ammonia of about 15 sccm was switched on, and the purity of the ammonia was 99.99%. When 900-930 °C was reached, the temperature was kept constant for ∼3 h. The power was then switched off, and the furnace was allowed to cool. To protect the as-grown GaN nanowires from decomposing, the flow of ammonia continued during the cooling
10.1021/jp102880p 2010 American Chemical Society Published on Web 09/16/2010
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Figure 1. (a) Low-magnification FESEM image of the submillimeter long GaN nanowires grown on the edge area of an amorphous substrate. Inset: low-magnification FESEM image of the densely grown long GaN nanowires in the central region of the substrate. (b) High-magnification FESEM image of the GaN nanowires.
Figure 2. Room-temperature experimental XRD pattern of the long GaN nanowires measured with Cu KR radiation (top) and the standard XRD data from 2000 JCPDS No. 50-0792 (lower).
process. After cooling, the as-milled GaN powders disappeared completely and light yellow layers on the substrate were observed. 3. Results and Discussion Figure 1a is a low-magnification field emission scanning electron microscopy (FESEM) image of the GaN nanowires grown on the edge area of the amorphous substrate. It reveals that long GaN nanowires with lengths on the order of hundreds of micrometers were obtained. The inset of Figure 1a is a lowmagnification FESEM image of the densely grown long GaN nanowires in the central region of the substrate. Figure 1b is a high-magnification FESEM image of the GaN nanowires. It shows that the nanowires have smooth surfaces, a uniform diameter along the axial direction, and diameters on the order of 10 nm. The overall crystal structure and phase purity of the nanowires were assessed through X-ray powder diffraction (XRD) measurements. Figure 2 shows a typical powder XRD pattern of the long GaN nanowires deposited on the amorphous substrate. The diffraction peaks in the pattern can be indexed to a hexagonal structure, and the data are in good agreement with the reported standard data of hexagonal GaN with lattice
Li et al.
Figure 3. HRTEM image taken from a GaN nanowire, and the GaN nanowire grows along the [100] direction. Inset: SAED pattern recorded along the [011] direction.
constants of a ) 0.318 nm and c ) 0.518 nm (Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 50-0792). The strong diffraction peaks relative to the background signals suggested that the nanowires had a pure hexagonal GaN phase. The XRD result reveals that the nanowires are composed of pure hexagonal structure GaN. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were used to further characterize the structure of the as-grown GaN nanowires. First, the GaN nanowires were removed from the substrate to form a suspension in isopropanol by an ultrasonic cleaner. A drop of the suspension was then put on a TEM grid. Dried in air, the grid was ready for measurement. Figure 3 is a latticeresolved HRTEM image taken from a GaN nanowire, and it clearly reveals an interplanar spacing of 0.276 nm for (100) atomic planes. The GaN nanowires grow along the [100] direction. The inset of Figure 3 is a SAED pattern of a nanowire recorded along the [011] direction. The pattern can be indexed based on a hexagonal cell with lattice parameters of a ) 0.32 nm and c ) 0.52 nm, consistent with the above XRD measurements. Both HRTEM and SAED results confirm the successful growth of the single-crystalline wurtzite GaN nanowires. As-milled GaN powders have a very high specific surface area and are metastable in thermal annealing process, which makes it easy to vaporize and recondense to grow GaN nanowires. Milling GaN powders before thermal annealing is the key to completely sublimating and converting the GaN powders to the nanowires at this relatively low temperature because the reported sublimation temperature for GaN in ammonia is above 1150 °C.21 A growth mechanism for the long GaN nanowires is proposed as follows. At the furnace center with a temperature zone of 900-930 °C, the catalyst precursor nickel(II) nitrate decomposes to produce nickel(II) oxide particles on the substrate,22 and ammonia decomposes to produce nitrogen atom and hydrogen atom.23 The N atom is active and protects the GaN from decomposing at high temperature. The hydrogen reduces the nickel(II) oxide to form nickel particles/droplets on the
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Figure 4. Room-temperature photoluminescence spectrum of the long GaN nanowires.
substrate.8,24 At the same time, the as-milled GaN powders that located at the furnace center vaporized. As energetically favored sites for absorption of vapor-phase GaN, the hot Ni droplets on the substrates absorb the GaN vapor around them. When the absorbed GaN reached saturation, the nanowires begin to nucleate and grow. With continuous steady absorption and growth, the submillimeter long nanowires are densely grown on the substrate. The growth is via a vapor-liquid-solid (VLS) mechanism, and the VLS mechanism is very common for the growth of semiconductor nanowires.25 Two typical evidences suggest a VLS growth of the long GaN nanowires. The GaN nanowires only grow on the regions with catalyst. Catalyst particles are typically detected at tips of the nanowires, and energy-dispersive X-ray analysis indicates that the particles contain nickel metal. Figure 4 is a room-temperature photoluminescence (PL) spectrum of the long GaN nanowires with 290 nm excitation. There is only one PL emission peak centered at 3.37 eV, and the emission corresponds to the near-band-gap emission of wurtzitic GaN. The appearance of only a near-band emission implies good crystalline quality of the GaN nanowires. It indicates that the GaN nanowires grown through this sublimation method are of good optical quality and suitable for fabricating optoelectronic devices, such as UV detectors. To assess the possibility of the sublimation-grown GaN nanowires for future electrical applications, single GaN nanowire devices were fabricated by an e-beam lithographic technique,18,26 and the process is as follows. First, the GaN nanowires were removed from the substrate to form a suspension in isopropanol by ultrasonic agitation. The suspension of the GaN nanowires was dispersed on cleaned thermal oxide n-type silicon wafers that had been pretreated by hot piranha solution, a mixture of one part concentrated hydrogen peroxide and two parts concentrated H2SO4. Poly(methyl methacrylate) (PMMA) resist layers were spin-coated on the wafers, and the patterns were defined on the resist layers by electron irradiation carried out in an SEM at 30 kV. The patterns were developed with a 1:3 methyl isobutyl ketone (MIBK)/isopropanol solution. The Cr/ Au films were then thermally evaporated on the resist, and the final pattern was formed by lift-off in acetone. Figure 5a shows the room-temperature I-V curves of an individual GaN nanowire device measured with and without 254 nm UV light illumination. As is expected for semiconducting nanowires, the conductivity of the GaN nanowire increases greatly under UV illumination. The augmentation is due to the photogenerated carriers in the semiconducting GaN nanowire. The photon energy of 254 nm UV light is 4.88 eV, large enough
Figure 5. (a) Room-temperature I-V curves from an individual GaN nanowire device with and without UV illumination (the black line “before UV” overlaps with the green line “after UV”). Inset: FESEM image of the single GaN nanowire device. (b) Room-temperature Isd-Vg curve obtained from the same individual GaN nanowire device measured at Vsd ) 0.5 V. Inset: Isd-Vg curve recorded on an individual heavily doped GaN nanowire device, and the nanowire is grown from CVD with gallium as the precursor.
to excite electrons across the 3.4 eV band gap of GaN. After turning off the UV light, the conductivity of the nanowire recovers. The excellent UV response of the GaN nanowires indicates that they are an ideal candidate for applications in UV light detectors/sensors. The inset of Figure 5a is an FESEM image of the individual GaN nanowire device. Figure 5b is a room-temperature source-drain current versus gate voltage (Isd-Vg) curve obtained from the individual GaN nanowire device measured at Vsd ) 0.5 V. It can be deduced from Figure 5b that the device operates as an n-channel metaloxide-semiconductor field-effect transistor (FET).2 It indicates that the GaN nanowires display a perfect n-type gating effect and have possibilities for applications. This n-type behavior in nominally undoped GaN is due to the few intrinsic nitrogen vacancies, and the N deficiencies control the electrical properties of the GaN nanowires.17 As a contrast, the inset of Figure 5b shows an Isd-Vg curve recorded on a device fabricated from an individual CVD-grown GaN nanowire, and the growth was with metal gallium as the precursor.17,18 The device was measured at room temperature with Vsd ) 0.5 V too. Because of the excess Ga content,17,18 the CVD-grown GaN nanowires are heavily doped and metallic, show lower resistivity, and have only a weak n-type gating effect. Figure 5b reveals that the sublimationgrown GaN nanowires are “pure” and not heavily doped. 4. Conclusions High-quality single-crystalline GaN nanowires with a length of several hundred micrometers were densely grown on amorphous substrates through a facile sublimation method. The
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morphology and single-crystalline wurtzitic structure of the GaN nanowires were characterized. Photoluminescence of the nanowires was studied, and only one near-band-gap emission of wurtzite GaN was observed. It indicates that the GaN nanowires are of good optical quality and suitable for fabricating optoelectronic devices. Individual GaN nanowire devices were fabricated, and their photoconductivity and electrical transport properties were investigated. The results revealed that the sublimation-grown GaN nanowires possess excellent UV sensitivity and an ideal n-type gating effect. This facile way for growing long GaN nanowires on a large scale will facilitate the broad applications of GaN nanowires. Acknowledgment. The work is supported, in part, by the Beijing Natural Science Foundation (2093038), the Fundamental Research Funds for the Central Universities of China, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, Ministry of Education of China. References and Notes (1) Han, W.; Fan, S. S.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 1287. (2) Huang, Y.; Duan, X. F.; Cui, Y.; Lieber, C. M. Nano Lett. 2002, 2, 101. (3) Chen, X. L.; Li, J. Y.; Cao, Y. G.; Lan, Y. C.; Li, H.; He, M.; Wang, C. Y.; Zhang, Z.; Qiao, Z. Y. AdV. Mater. 2000, 12, 1432. (4) Kim, H. M.; Kim, D. S.; Park, Y. S.; Kim, D. Y.; Kang, T. W.; Chung, K. S. AdV. Mater. 2002, 14, 991. (5) Kuykendall, T.; Pauzauskie, P.; Lee, S.; Zhang, Y. F.; Goldberger, J.; Yang, P. D. Nano Lett. 2003, 3, 1063. (6) Calarco, R.; Marso, M.; Richer, T.; Aykanat, A. I.; Meijers, R.; Hart, A.; Stoica, T.; Luth, H. Nano Lett. 2005, 5, 981.
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