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Ga2O3 Nanoribbons: Synthesis, Characterization, and Electronic Properties Lei Fu, Yunqi Liu,* Ping’an Hu, Kai Xiao, Gui Yu, and Daoben Zhu* Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China Received May 16, 2003. Revised Manuscript Received July 3, 2003
Semiconductive Ga2O3 nanoribbons have been synthesized via hydrogen-assisted thermal evaporation at 1000 °C. The width of the nanoribbons could be controlled by adjusting the content of hydrogen in a mixture of carrier gases. The room-temperature photoluminescence spectrum reveals that there exists a strong blue emission band centered at 430 nm. The blue emission is mainly attributed to the oxygen vacancies in the Ga2O3 nanoribbons. We have fabricated a Schottky junction diode, utilizing individual Ga2O3 nanoribbons, and studied its electrical transport properties. The measured current-voltage characteristic exhibited clear rectifying behavior with a turn-on voltage of 0.5 V and a rectification ratio of 2.5 × 104 at (2 V.
Introduction One-dimensional (1D) nanostructures, such as nanotubes and nanowires, have attracted much attention because of their importance in learning the fundamental concepts of the roles of both dimensionality and quantum size effect, and their potential applications serving as the building blocks for electronic/optical nanodevices and functional materials.1,2 Now, the studies of nanosized materials have been focused on two-dimensional oxide nanoribbons (nanobelts),3,4 which have been considered as an ideal system for fully investigating dimensionally confined transport phenomena and an ideal base to build functional devices. Monoclinic gallium oxide (β-Ga2O3) is a wide-band gap semiconductor (Eg ) 4.9 eV). It exhibits conduction and luminescence properties, and thus has potential applications in optoelectronic devices including flat-panel displays, solar energy conversion devices, optical limiter for ultraviolet, and high-temperature stable gas sensors.5,6 Recently, β-Ga2O3 nanoribbons have been synthesized by various methods, including an arc-discharge method based on the mechanism of catalytic step growth,7 physical deposition via the vapor-solid (VS) process,8,11-13 a vapor-liquid-solid (VLS) method,9,10 and so on. How* To whom correspondence should be addressed. Tel: +86-1062613253. Fax: +86-10-62559373. E-mail:
[email protected]. (1) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (2) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (3) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1948. (4) Wang, Z. L. Adv. Mater. 2003, 15, 432. (5) Edwards, D. D.; Mason, T. O.; Goutenoir, F.; Poeppelmeimer, K. R. Appl. Phys. Lett. 1997, 70, 1706. (6) Ogita, M.; Saika, N.; Nakanishi, Y.; Hatanaka, Y. Appl. Surf. Sci. 1999, 142, 188. (7) Lee, Y. H.; Choi, Y. C.; Kim, W. S.; Park, Y. S.; Lee, S. M.; Bae, D. J. Adv. Mater. 2000, 12, 746. (8) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Phys. Chem. B 2002, 106, 902. (9) Liang, C. H.; Meng, G. W.; Wang, G. Z.; Wang, Y. W.; Zhang, L. D. Appl. Phys. Lett. 2001, 78, 3202. (10) Chang, K. W.; Wu, J. J. Appl. Phys. A 2003, 76, 629.
ever, all the reported methods need catalysts (arcdischarge method7 and VLS method9,10) or induct heteroatoms (N8,13, As9, C11,12), and the nanoribbons prepared were contaminated with the residues of the catalysts. Therefore, it is desirable to synthesize Ga2O3 nanoribbons with high purity both for the fundamental research and for the potential application of these products in electronic devices. But it is difficult to directly synthesize the oxide nanoribbons by simply evaporating oxide powder because the melting point of most oxides is very high. Such is the case with Ga2O3, it requires a high processing temperature of above 1300 °C. Here, we introduce a convenient hydrogen-assisted thermal evaporation method to synthesize nanoribbons of the semiconductor Ga2O3 at a lower temperature and ambient pressure. The synthetic reaction was carried out in a quartz tube furnace using high-purity Ar mixed with H2 as carrier gas at 1000 °C, and Ga2O3 as source. We found that the width of the nanoribbons could be controlled by adjusting the content of the hydrogen in the carrier gases. The products are pure, structurally uniform, single crystalline, and in high yield; furthermore this method does not need any catalyst, substrate, vacuum condition, or preprocessing. Experimental Section A horizontal quartz tube (outer diam. 20 mm; length 120 cm) was mounted inside a high-temperature tube furnace. Analytical grade Ga2O3 powder (0.5 g, purity greater than 99.99%) was placed in an alumina boat (15 × 50 mm), which was then loaded into the central region of the quartz tube. A carrier gas of high-purity Ar premixed with H2 was kept flowing at a rate of 50 sccm. The temperature of the furnace central region was increased at a rate of 30 °C min-1 to 1000 °C, and then maintained at this temperature for 4 h. A (11) Gundiah, G.; Govindaraj, A.; Rao, C. N. Chem. Phys. Lett. 2002, 351, 189. (12) Wu, X. C.; Song, W. H.; Huang, W. D.; Pu, M. H.; Zhao, B.; Sun, Y. P.; Du, J. J. Chem. Phys. Lett. 2000, 328, 5. (13) Zhang, J.; Jiang, F. H. Chem. Phys. 2003, 289, 243.
10.1021/cm0343655 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/27/2003
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temperature gradient was established from the center to the end of the quartz tube. Before the experiment started, a movable thermocouple was inserted into the tube to measure the temperature along the tube. After the furnace was cooled to room temperature, a snow-white wool-like product was deposited on the inner wall of the quartz tube, where the temperature was measured to be approximately 900 °C. The yield of the product was ca. 60-70% (the content of the hydrogen in carrier gas is 10%), as determined from the weights of the original reagents and the products. The collected products were characterized by X-ray diffraction (XRD, Rigaku Dmax2000, Cu KR), room-temperature FT-Raman (RFS100/ S, Bruker), scanning electron microscopy (SEM, JSM-6700F equipped with an Oxford energy-dispersive X-ray detector, EDX, INCA300), and transmission electron microscopy (TEM, JEOL-2010 F, at 300 kV). Room-temperature photoluminescence (PL) spectra were recorded with a Hitachi luminescence spectrometer (F-4500) using a xenon discharge lamp as the excitation source. Individual Ga2O3 nanoribbon Schottkyjunction diodes were fabricated using a probe point extension focused-ion-beam (FIB) System (IDS P2X, 30 keV Ga ions, 10 nm nominal spot diameter) from Schlumberger Automatic Corporation. The electrical properties were measured using a probe station (Wentworth, MP1008) and a semiconductor parameter analyzer (HP 4140B) at room temperature in the air.
Results and Discussion The hydrogen-assisted thermal evaporation resulted in a snow-white wool-like product in very high yield. Figure 1 reveals that the product consists of a large quantity of ribbonlike nanostructures with typical lengths in the range of several tens to several hundreds of micrometers. The in-situ energy-dispersive X-ray spectrum (EDX) shows only the elements of Ga and O, indicating the formation of gallium oxide. Figure 2 shows an X-ray diffraction (XRD) spectrum of the nanoribbons. The diffraction peak positions in the pattern can be indexed to a monoclinic structure, in good agreement with the reported data of bulk β-Ga2O3 powder (a0 ) 5.8 Å, b0 ) 3.04 Å, c0 ) 12.23 Å, β0 ) 103.42 Å, and JCPDS 11-370). However, the relative intensity is different, for which the strongest peaks of the bulk β-Ga2O3 powder are (004), (104h ), (200), (111), and (122 h ), whereas for the β-Ga2O3 nanoribbons the strongest peak is only (004). Figure 3 shows room-temperature FT-Raman scattering spectra of the nanoribbons and the bulk β-Ga2O3 powders used as a reference. Most of the positions of the nanoribbons will correspond to those of the β-Ga2O3 powders, although the peaks become narrower due to good crystallinity. Several sharp peaks at 112, 145, 169, and 266 cm-1 were observed in addition to the previously reported peaks at 201, 253, 321, 345, 417, 476, 630, 653, and 767 cm-1.7 The purity and the composition of the β-Ga2O3 nanoribbons were further investigated by X-ray photoelectron spectroscopy. Figure 4 displays both the photoelectron and Auger electron peaks for gallium and oxygen. From this spectrum, the ratio Ga/O is estimated to be about 1:1.57. In the inset of Figure 4, the highresolution Ga 3d core level spectrum is shown with the curve-fitted results. The binding energy of the Ga 3d core level is ∼21.2 eV. The binding energy positions of the Ga 3d as well as Ga 3p3 (106.5 eV) and O 1s (532.2 eV) core levels are well consistent with the reported data
Figure 1. SEM image of the wool-like product recorded at 15 kV without surface coating: (a) content of the hydrogen in the carrier gases is 10%; (b) content of the hydrogen in the carrier gases is 15%; (c) content of the hydrogen in the carrier gases is 35%.
of Ga2O3.14 Further, a small and broad tail at a higher energy side of the main peak (at ∼24.4 eV) is possibly due to the presence of the contribution from the oxygen 2s core level. Thus, the XPS results support the conclusion that the sample is composed of only stoichiometric Ga2O3. (14) Wanger, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. E.; Muilenber, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation, Physical Electronics Division: Boston, MA, 1979.
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Figure 2. XRD pattern of the as-prepared Ga2O3 nanoribbons.
Figure 3. Room-temperature Raman spectrum of the asprepared Ga2O3 nanoribbons.
Figure 4. X-ray photoelectron survey spectrum of the Ga2O3 nanoribbons. In the inset, the Ga 3d core level spectrum is shown with the curve-fitted results.
The structure and morphology of the Ga2O3 nanoribbons were further characterized by transmission electron microscopy (TEM). Figure 5a gives a low magnification TEM image showing the ribbonlike structure of Ga2O3 (the content of the hydrogen in carrier gas is 25%). It can be observed that the nanoribbons are long and straight. The nanoribbons are single crystalline and free from dislocations. Their surfaces are
Figure 5. (a) Low-magnification TEM image of Ga2O3 nanoribbons; (b) TEM image for a typical Ga2O3 nanoribbon; (c) corresponding electron diffraction pattern; (d) HRTEM image.
clean without an amorphous layer. The bending contours are due to a bending of the ribbon with respect to electron beams. This is an electron diffraction phenom-
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enon and is most frequently observed in metal foils due to deformation and bending.15 Figure 5b shows a typical β-Ga2O3 nanoribbon with a width of 60 nm (the content of hydrogen in the carrier gas is 10%). The SAED pattern (5c) recorded can be indexed for the [101] zone axis of β-Ga2O3. Figure 5d is a high-resolution TEM (HRTEM) image of the Ga2O3 nanoribbon. The lattice plane of (100) with interlayer spacing of 0.56 nm was clearly displayed. In our experiments, the growth of Ga2O3 nanoribbons may involve the following steps. First, the reduction reaction of Ga2O3 powder by hydrogen forms Ga and H2O vapor at a higher temperature. It may be expressed as
Ga2O3(s) + 2H2(g) ) Ga2O(g) + 2H2O(g)
(1)
Ga2O3(s) + 3H2(g) ) 2Ga(g) + 3H2O(g)
(2)
Then the Ga and Ga2O vapors are transported to, and react with, the H2O, and form Ga2O3 vapor at a low temperature. The presence of H2O acts as the oxygen source during the formation of Ga2O3 vapor. The reaction may be expressed as
Ga2O(g) + 2H2O(g) ) Ga2O3(g) + 2H2(g)
(3)
2Ga(g) + 3H2O(g) ) Ga2O3(g) + 3H2(g)
(4)
Finally, the Ga2O3 vapor grows into Ga2O3 nanoribbons. In the present work, no metal catalyst was used, and no tips (or particles) were observed at the ends of the nanoribbons. The source was high-purity Ga2O3 (99.99%) with a melting point of about 1800 °C. It was found that the carrier gas, Ar with H2, was crucial to the formation of Ga2O3 nanoribbons during the thermal evaporation below its melting point. When the carrier gas was pure Ar and the other conditions were maintained the same, no Ga2O3 nanoribbons were produced. The width of the nanoribbons is in direct proportion to the content of the hydrogen in the carrier gases. This fact indicates the importance of Ga vapor generated by hydrogen reduction of Ga2O3 powder. The growth of the nanoribbons might be dominated by the VS process,8,16-17 but we cannot totally rule out the VLS process18-21 because low melting point Ga metal particles are reduced in the synthesis, which may act as catalyst for the growth. The formation of nanoribbons is a combined result of VLS and VS as well as growth kinetics. Therefore, through hydrogen reduction of oxide powder, we may also produce other oxide nanoribbons at a lower temperature, such as In2O3 and ZnO. Figure 6 shows the photoluminescence (PL) spectra of bulk β-Ga2O3 nanoribbons and bulk β-Ga2O3 powder at room temperature with the same experiment conditions. A striking PL feature showing in the Ga2O3 nanoribbons is that there exists a strong luminescence (15) Hirsch, P. B.; Howie, A.; Nicholson, R. B.; Pashley, D. W.; Whelan, M. J. Electron Microscopy of Thin Crystals; Butterworth: Newton, MA, 1965. (16) Yang, P.; Lieber, C. M. J. Mater. Res. 1997, 12, 2981. (17) Sears, G. W. Acta Metall. 1965, 3, 268. (18) Duan, X. F.; Lieber, C. M. Adv. Mater. 2000, 12, 298. (19) Duan, X. F.; Lieber, C. M. J. Am. Chem. Soc. 2000, 122, 188. (20) Han, W.; Fan, S.; Li, Q.; Hu, Y. Science 1997, 277, 1287. (21) Huang, M. H.; Wu, Y.; Feick, H.; Tran, N.; Weber, E.; Yang, P. Adv. Mater. 2001, 13, 113.
Figure 6. Room-temperature photoluminescence spectra of Ga2O3 nanoribbons (the content of the hydrogen in the carrier gases is 10%) and powders measured upon excitation at 250 nm and filter wavelength of 310 nm.
band centered at 430 nm. Compared with the PL feature of β-Ga2O3 powder, the PL of β-Ga2O3 nanoribbons increases largely in intensity and broadens saliently. It is very interesting that all the peaks (396, 450, and 467 nm) of β-Ga2O3 nanoribbons have their corresponding peaks of β-Ga2O3 powder. The blue emission occurring in β-Ga2O3 has been observed by other researchers.9-13 However, such phenomenon is first observed in our study and has not yet been reported. As for the PL mechanism of metal oxides (such as Ga2O3 and In2O3), previous studies suggested that it mainly originates from the recombination of an electron on a donor formed by oxygen vacancies and a hole on an acceptor formed by metal vacancies.22 These oxygen vacancies generally act as deep defect donors in semiconductors and would cause the formation of new donor levels in the band gap. In the photoexcitation process, the electron in a donor oxygen vacancy can be captured by the excited hole on an acceptor, and then a blue photon is emitted via the radiative recombination process.23 In our present experiments, the Ga2O3 nanoribbons are synthesized in H2 atmosphere at high temperature. Considering the insufficiency of chemical oxidation and the inevitable defects occurring in the crystallization process, a mass quantity of oxygen vacancies would be created in the growth of Ga2O3 nanoribbons. In addition, the Ga2O3 nanoribbons with high aspect ratio and rectangle-like morphology should also favor the existence of large quantities of oxygen vacancies. These oxygen vacancies would induce the formation of new energy levels in the band gap of the Ga2O3 nanoribbons. Besides oxygen vacancies, various kinds of defects such as gallium vacancies or interstitials, stacking faults, and so on, have also been introduced in the growth of nanoribbons. These defects would provide the recombination centers for luminescent emission. The very broad PL background signal (appearing from 396 to 467 nm) seems to support this fact. However, further work is needed to clarify the underlying mechanism for the interpretation of the emission of Ga2O3 nanoribbons. (22) Harwing, T.; Kellendonk, F. J. Solid State Chem. 1978, 24, 255. (23) Binet, L.; Gourier, D. J. Phys. Chem. Solids 1998, 59, 1241.
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were then connected to surrounding Au/Ti pads. This method has the advantage of allowing ion-beam imaging of the contacted nanoribbons on the substrate surface and then in situ depositing the Pt electrode on the nanoribbons. The electrical properties were measured using a probe station and semiconductor parameter analyzer at room temperature in air. To obtain better contact between the nanoribbon and the Pt lead, the thermal annealing was performed in a flowing Ar atmosphere at 450 °C for 20 min using a tube furnace. Figure 7b shows the current voltage (I-V) characteristic of the Ga2O3 nanoribbons before and after annealing, exhibiting clear rectifying behaviors with turn-on voltages of 1.1 and 0.5 V. No reverse bias breakdown voltage was observed until the measured voltage was up to 2 V. Before annealing, the rectification ratio [RR ) (current at -2 V)/ (current at 2 V)] is ca. 1.2 × 103 for this rectifying curve. After annealing, the rectification ratio augments to ca. 2.5 × 104, exhibiting more obvious rectifying behaviors. The observed I-V curve represents n-type Schottky diode characteristics. This means that our Ga2O3 nanoribbons should be an n-type semiconductor. The work function of Ga2O3 amounts to about 5.0 eV. Because Pt electrode has a higher work function of about 5.7 eV, the oxide/metal contact interface can show a Schottky-diode-like behavior. The photoluminescence spectrum (Figure 6) clearly reveals a deep trap broad peak centered at 430 nm, which is ascribed to donor states derived from oxygen vacancies. Such states lead to an n-type behavior. Moreover, the electrical properties of our Ga2O3 nanoribbons are very stable in air. Conclusion
Figure 7. (a) SEM image of an example of Ga2O3 nanoribbons with Pt contacts (diameter of the nanoribbon is 120 nm); and (b) current-voltage characteristics of a nanoribbon measured at room temperature showing rectifying behavior.
The Ga2O3 nanoribbons were spread on a 500-nmthick thermal oxidized silicon surface between Ti/Au pads. Using a probe point extension focused-ion-beam (FIB) system, the surface was visualized using a low beam current (4 pA) in searching for the nanoribbons. Once found, two Pt leads were patterned on the nanoribbons by ion-induced deposition of Pt from a (CH3C5H4)Pt(CH3)3 carrier gas. After depositing the Pt electrode on the nanoribbons, we used Ga+ ion plasma cleaner to clean Pt contamination on the surface of Ga2O3 nanoribbons. All of the samples were prepared with exactly the same procedures. An example of the resulting connections is shown in Figure 7a. These small leads
In summary, the semiconductor Ga2O3 nanoribbons have been synthesized at a large scale via hydrogenassisted thermal evaporation at 1000 °C. The width (ranging from 60 nm to 10 µm) of the nanoribbons could be controlled by adjusting the content of the hydrogen in the carrier gases. The room-temperature PL spectrum of the nanoribbons shows emission in the blue region, which may be attributed to the existence of oxygen vacancies formed in the growth process of nanoribbons and the recombination from the localized defect states. This blue emission suggests that the Ga2O3 nanoribbons have potential to be a blue and a UV light emitter, or other UV optical devices. We have fabricated a Schottkyjunction diode, utilizing individual Ga2O3 nanoribbons, and studied its electrical transport properties. The measured current-voltage characteristic exhibited clear rectifying behavior and no reverse bias breakdown was observed up to the measured voltage of 2 V. Acknowledgment. This work was supported by the Major State Basic Research Development Program, the National Natural Science Foundation of China (NSFC), and the Chinese Academy of Sciences. CM0343655
