Conversion between Hexagonal GaN and β-Ga2O3 Nanowires and

NANO LETTERS

Conversion between Hexagonal GaN and β-Ga2O3 Nanowires and Their Electrical Transport Properties

2006 Vol. 6, No. 2 148-152

Jianye Li, Lei An, Chenguang Lu, and Jie Liu* Department of Chemistry, Duke UniVersity, Durham, North Carolina 27708 Received July 2, 2005; Revised Manuscript Received November 10, 2005

ABSTRACT We have observed that the hexagonal GaN nanowires grown from a simple chemical vapor deposition method using gallium metal and ammonia gas are usually gallium-doped. By annealing in air, the gallium-doped hexagonal GaN nanowires could be completely converted to β-Ga2O3 nanowires. Annealing the β-Ga2O3 nanowires in ammonia could convert them back to undoped hexagonal GaN nanowires. Field effect transistors based on these three kinds of nanowires were fabricated, and their performances were studied. Because of gallium doping, the as-grown GaN nanowires show a weak gating effect. Through the conversion process of GaN nanowires (gallium-doped) f Ga2O3 nanowires f GaN nanowires (undoped) via annealing, the final undoped GaN nanowires display different electrical properties than the initial galliumdoped GaN nanowires, show a pronounced n-type gating effect, and can be completely turned off.

Wurtzite structure gallium nitride (hexagonal GaN), a particularly important III-V semiconductor with a direct band-gap of 3.4 eV, is an ideal material as an ultraviolet (UV) or blue light emitter, photodetector, high-speed field effect transistor, and high temperature/high power electronic device. 1,2 As the potential building blocks for nanoelectronic, nanooptical, and nanomechanical devices, nanowires have received considerable attention from the scientific and engineering communities.3 Among them, GaN nanowires have attracted much attention in the past few years because of their potential for realizing photonic and biological nanoscale devices such as blue light emitting diodes, shortwavelength UV nanolasers, and biochemical sensors.3-15 The reported synthetic schemes for GaN nanowires including template growth,4 laser ablation,5,8 metal-organic chemical vapor deposition,9,12,15 hydride vapor epitaxy,7 and chemical vapor deposition (CVD).3,6,10,16 Among all methods, CVD through the direct reaction of gallium and ammonia3,6,10,16 is the simplest and cheapest way to obtain GaN nanowires. In this report, hexagonal-structure GaN nanowires were synthesized by the reaction of gallium and ammonia in a simple CVD process10 and they were found to be galliumdoped. After annealing in air, the doped GaN nanowires were transformed to β-Ga2O3 nanowires successfully, and the β-Ga2O3 nanowires were converted back to undoped hexagonal GaN nanowires via annealing in ammonia. During the conversion process, the nanowires retain their morphol* To whom correspondence should be addressed. E-mail: jliu@ chem.duke.edu. 10.1021/nl051265k CCC: $33.50 Published on Web 12/31/2005

© 2006 American Chemical Society

ogies and crystalline structures. Using an e-beam lithography technique, individual nanowire field effect transistors (FETs) of the three different kinds of nanowires were fabricated. The electrical transport properties of the devices were examined, and significant variation in resistivity and gating effects of the three kinds of nanowires were observed. To the best of our knowledge, this is the first comparative study of the three different kinds of nanowires through the conversion process. The hexagonal GaN nanowires used in this experiment were synthesized through a CVD procedure, and the details of the nanowires’ growth were reported elsewhere.10 The growth of initial gallium-doped GaN nanowires was carried out in a horizontal quartz tube inside a tube furnace. As the source material, elemental gallium (99.999%, Alfa Aesar) was put in the center of the tube furnace. The substrate was placed downstream with a distance of about 10 cm from the gallium source. During the growth, gallium and ammonia (99.99%, National Specialty Gases) reacted to form GaN nanowires according to the following reaction: Ga + NH3 f GaN + 3/2H2 The as-grown GaN nanowires are Ga-doped and the color of the gallium-doped nanowires is darker than the normal GaN nanowires.7 β-Ga2O3 nanowires were obtained by thermal oxidation of the as-grown gallium-doped GaN nanowires at 900 °C in air for 4 h. The final non-gallium-doped GaN nanowires were obtained by annealing the converted β-Ga2O3 nanowires at

920 °C in ammonia for 5 h and the reaction according to the following schemes:17,18 2NH3 f N2 + 3H2

(1)

Ga2O3 + 2H2 f Ga2O +2H2O

(2)

Ga2O + 2NH3 f 2GaN + H2O + 2H2

(3)

The overall crystal structure and phase purity of the three kinds of nanowire bulk samples were characterized using X-ray powder diffraction (XRD) (Rigaku Multiflex X-ray diffractometer with Cu KR radiation at room temperature). The morphologies of the samples were imaged by field emission scanning electron microscopy (FESEM) (Philips FEI XL30SFEG). Nanowire devices were fabricated by an e-beam lithographic technique described as follows. Suspensions of the initial as-grown gallium-doped GaN nanowires in acetone were dispersed on n-type silicon wafers with 1000-nm thermal oxide (Silicon Quest), and the underlying silicon was used as a global back gate. After that, some wafers with the dispersed GaN nanowires were annealed in air to convert the gallium-doped GaN nanowires to β-Ga2O3 nanowires, and then half of the wafers with the converted β-Ga2O3 nanowires were annealed in ammonia to convert the β-Ga2O3 nanowires back to non-gallium-doped GaN nanowires. After conversion, devices were fabricated on the three kinds of wafers using standard e-beam lithography (Philips FEI XL30SFEG) at 30 kV. Cr/Au (20/50 nm) was used as the metal for the electrodes. The devices were images before the measurement to ensure a single nanowire between two metal electrodes. Electrical transport measurements were performed at room temperature on a home-built system that contained two source-measure units for source-drain and source-gate voltages (Keithley 2400 source meters) and a computercontrolled Labview interface. XRD measurements were performed on bulk samples to assess the overall crystal structure and phase purity of the products. Figure 1a shows a typical powder XRD pattern of the as-grown GaN nanowires, and the diffraction peaks in Figure 1a were indexed to a hexagonal wurtzite structure of GaN. The data are in good agreement with the reported values of hexagonal GaN with lattice constants of a ) 0.3189 nm and c ) 0.5185 nm (2000 Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 50-0792) (see the Supporting Information for details). The inset of Figure 1a is the FESEM image of the as-grown GaN nanowires. Figure 1b is the XRD pattern of the product obtained from annealing the sample with Figure 1a at 900 °C in air for 4 h. The diffraction peaks shown in Figure 1b are in good agreement with those of monoclinic structure Ga2O3 (or β-Ga2O3) with lattice constants a ) 1.222 nm, b ) 0.3038 nm, c ) 0.5807 nm, R ) 90°, β ) 103.82°, and γ ) 90° (2000 Joint Committee on Powder Diffraction Standards (JCPDS) Card No. 41-1103) (see the Supporting Information for details). The XRD result reveals that β-Ga2O3 is obtained, Nano Lett., Vol. 6, No. 2, 2006

Figure 1. Room-temperature XRD (using Cu KR radiation) patterns of (a) as-grown GaN nanowires; (b) β-Ga2O3 nanowires obtained through annealing the sample of Figure 1a in air at 900 °C for 4 h; and (c) GaN nanowires from annealing the sample of Figure 1b at 920 °C in ammonia for 5 h. The nonindexed peaks in the three XRD patterns are fundamentally the same, and all are from the substrates. The FESEM images (insets) in Figure 1a-c correspond to Figure 1a-c, respectively, and indicate the morphologies of the nanowires retained during the changing process.

and it also shows a highly pure monoclinic structure Ga2O3 phase of the product. The inset of Figure 1b is the FESEM 149

Figure 2. A source-drain current (Isd) vs source-drain voltage (Vsd) curve measured at room temperature from a single nanowire field effect transistor of the initial gallium-doped GaN nanowire (black) and a single nanowire device of a converted β-Ga2O3 nanowire obtained via annealing the initial gallium-doped GaN nanowires in air (red). Inset: a curve of source-drain current (Isd) vs gate voltage (Vg) from the initial gallium-doped GaN nanowire device.

image of the annealing-obtained β-Ga2O3 product and it reveals that the nanowire morphologies are retained. Figure 1c shows the XRD pattern of the product obtained by annealing the β-Ga2O3 nanowires at 920 °C in ammonia for 5 h. The positions of the XRD peaks fit to those of the hexagonal-structure GaN very well (see the Supporting Information for details). The presence of strong diffraction peaks relative to the background signal suggests that the annealed products changed back to a pure hexagonal GaN phase. The inset of Figure 1c is the FESEM image of the GaN nanowires converted from β-Ga2O3 nanowires in ammonia, and it indicates that the nanowires keep their nanowire morphologies during the conversion process. The nonindexed peaks in the XRD patterns in Figure 1 are all from the substrate and are basically the same in all of the figures. Further structural characterization of the nanowires was performed by transmission electron microscopy (TEM). The high-resolution TEM image reveals the initial as-grown GaN nanowire with a single-crystalline hexagonal structure (see the Supporting Information for details). Similar to the structures of MoS2 nanowires obtained through thermal conversion by Li et al.,19 the β-Ga2O3 nanowires and final non-gallium-doped GaN nanowires are polycrystalline. Figure 2 shows a typical source-drain current versus source-drain voltage (Isd-Vsd) curve (black) obtained from a single nanowire FET of gallium-doped GaN nanowires. The length of the nanowire between two electrodes is about 8 micrometers, and the diameter is about 50 nm. The inset of the Figure 2 is a curve of Isd versus gate voltage (Vg) for the same GaN nanowire device measured at Vsd ) 0.1V, and the current increases only 17% when the Vg varies from -56 to +56 V, showing that the gallium-doped GaN nanowire has only a weak n-type gating effect. 150

Figure 3. (a) Gate-dependent Isd-Vsd curves measured at room temperature from a single nanowire field effect transistor (FET) of the final non-gallium-doped GaN nanowire. The non-gallium-doped GaN nanowire was obtained by annealing the converted β-Ga2O3 nanowire in ammonia. The gate voltages for each Isd-Vsd datum are indicated. Inset: an FESEM image of the final non-galliumdoped GaN single nanowire FET; the scale bar is 2 µm. (b) Isd vs Vg recorded at different Vsd at room temperature from the same non-gallium-doped GaN single nanowire FET in Figure 3a.

The red linear curve in Figure 2 is the Isd-Vsd data of an individual nanowire device made of the converted β-Ga2O3 nanowire obtained from annealing GaN nanowires in air. The diameter of the wire is about 90 nm, and the length between two electrodes is 12 micrometers. When Vsd changed from -0.1 to 0.1 V, the current is zero, and the current measured at higher Vsd ((2 V) is also zero (see the Supporting Information for details). This is because the β-Ga2O3 is intrinsically an insulator with a wide band gap of 4.8 eV at room temperature.20 The data in Figure 3 were measured from a single nanowire FET of the non-gallium-doped GaN nanowires converted from β-Ga2O3. The linear curves in Figure 3a correspond to a set of typical room-temperature Isd-Vsd data of the individual GaN nanowire device recorded at different gate voltages. The conductance of the GaN nanowire Nano Lett., Vol. 6, No. 2, 2006

Figure 4. Room-temperature Isd vs Vg curves recorded at Vsd ) 1 and 1.5 V from the single nanowire FET made from GaN nanowires converted from β-Ga2O3. Inset: Isd vs Vg curves from the as-grown gallium-doped GaN single nanowire device recorded at Vsd ) 1 and 1.5 V, respectively.

increases with increasingly positive Vg, and decreases with increasingly negative Vg. From the gate-dependence of the Isd-Vsd curves, the non-gallium-doped GaN nanowire can be identified as an n-type semiconductor. In other words, the transport through the GaN nanowires is dominated by negative carries. This n-type behavior in nominally undoped GaN is believed to be due to the nitrogen vacancies and/or oxygen impurities.8 The inset of Figure 3a is an FESEM image of the single GaN nanowire FET. The diameter of the GaN nanowire is about 70 nm, and the length between the two electrodes is about 8 micrometers. The transport characteristics of the GaN nanowire device in Figure 3a were also examined. Figure 3b shows the results of source-drain current versus Vg recorded at different source-drain voltages. It can be deduced from Figure 3b that the device operates as an n-channel metal-oxidesemiconductor FET.8 Figure 4 shows the Isd versus Vg curves recorded at Vsd ) 1 and 1.5 V on the devices made from these two types of GaN materials. It indicates that the GaN nanowires converted from Ga2O3 display different electrical properties than the as-prepared gallium-doped GaN nanowires and show a pronounced n-type gating effect. We have also found that the distance between the Ga metal source and the substrate for nanowire growth have a strong effect on the doping level in the nanowires. The further the substrate is from the gallium source, the more gallium component found in the as-grown GaN nanowires. When the substrate was over 20 cm from the source, the color of the as-grown nanowires became grayish yellow rather than pure yellow. The energy-dispersive X-ray (EDX) result revealed the nanowires with grayish-yellow color were highly gallium-doped (see the Supporting Information for details), and this kind of nanowire showed lower resistivity and nearly no gating effect. This kind of heavily gallium-doped GaN nanowires was also found by Kim et al. from their CVD grown GaN nanowires.21 When converting the β-Ga2O3 nanowires back to GaN nanowires, the β-Ga2O3 nanowires Nano Lett., Vol. 6, No. 2, 2006

were heated with pure ammonia at high temperature. There was no gallium source during the conversion process. So, the final GaN nanowires are not gallium-doped. They have higher resistivity than the initial gallium-doped GaN nanowires, showing a strong n-type gating effect, and can be completely turned off. In summary, β-Ga2O3 nanowires were obtained through annealing as-grown gallium-doped hexagonal GaN nanowires in air, and the β-Ga2O3 nanowires can be converted back to non-gallium-doped hexagonal GaN nanowires via annealing in ammonia. Devices from these three kinds of nanowires were fabricated, and their electrical transport properties were measured and analyzed. Because of gallium doping, the asgrown GaN nanowires show low resistivity and weak gating effects. Through the conversion process of hexagonal GaN (doped) f Ga2O3 f GaN, the GaN nanowires obtained from annealing Ga2O3 in ammonia possess radically different electronic properties than the initial as-grown GaN nanowires. They show a pronounced n-type gating effect and can be completely turned off. This conversion method could also be applicable for other semiconductor nanowires such as InN, GaP, InP, and so forth and have potential use in fundamental research and technological applications. Acknowledgment. This work is supported in part by Grant no. 49620-02-1-0188 from AFOSR. J. Li thanks Dr. H. Li for helpful discussions. Supporting Information Available: Comparisons of the XRD spectra of the three kinds of nanowires with the standard XRD spectra, TEM images of the three kinds of nanowires, an I-V curve of the β-Ga2O3 nanowire device measured at higher Vsd ((2 V), and EDX spectra of the three kinds of nanowires. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Fasol, G. Science 1996, 272, 175. (2) Ponce, F. A.; Bour, D. P. Nature 1997, 386, 351. (3) Zhong, Z. H.; Qian, F.; Wang, D. L.; Lieber, C. M. Nano Lett. 2003, 3, 343. (4) Han, W.; Fan, S. H.; Li, Q. Q.; Hu, Y. D. Science 1997, 277, 1287. (5) Duan, X. F.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. (6) 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. (7) Kim, H.; Kim, D. S.; Park, Y. S.; Kim, D. Y.; Kang, T. W.; Chung, K. S. AdV. Mater. 2002, 14, 991. (8) Huang, Y.; Duan, X. F.; Cui, Y.; Lieber, C. M. Nano Lett. 2002, 2, 101. (9) Kuykendall, T.; Pauzauskie, P.; Lee, S.; Zhang, Y. F.; Goldberger, J.; Yang, P. D. Nano Lett. 2003, 3, 1063. (10) Li, J. Y.; Lu, C. G.; Maynor, B.; Huang, S. M.; Liu, J. Chem. Mater. 2004, 16, 1633. (11) Maynor, B. M.; Li, J. Y.; Lu, C. G.; Liu, J. J. Am. Chem. Soc. 2004, 126, 6409. (12) Kuykendall, T.; Pauzauskie, P.; Zhang, Y. F.; Goldberger, J.; Sirbuly, D.; Denlinger, J.; Yang, P. D. Nat. Mater. 2004, 3, 524. (13) Timoshkin, A. Y.; Schaefer, H. F. J. Am. Chem. Soc. 2004, 126, 12141. (14) Qian, F.; Li, Y.; Gradecak, S.; Wang, D. L.; Barrelet, C. J.; Lieber, C. M. Nano Lett. 2004, 4, 1975. 151

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NL051265K

Nano Lett., Vol. 6, No. 2, 2006