Structural and electrical properties of ZnO nanorods and Ti ... - xafs

APPLIED PHYSICS LETTERS 96, 051908 共2010兲

Structural and electrical properties of ZnO nanorods and Ti buffer layers C.-H. Kwak,1 B.-H. Kim,2 C.-I. Park,2 S.-Y. Seo,1 S.-H. Kim,1 and S.-W. Han3,a兲 1

Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea 2 Department of Physics and Institute of Fusion Science, Chonbuk National University, Jeonju 561-756, Republic of Korea 3 Division of Science Education, Institute of Fusion Science, and Institute of Science Education, Chonbuk National University, Jeonju 561-756, Republic of Korea

共Received 12 November 2009; accepted 12 January 2010; published online 4 February 2010兲 Vertically-well-aligned ZnO nanorods were synthesized on Ti buffer layers by a metal-organic chemical-vapor deposition process. Structural analyses demonstrated that the ZnO nanorods were well-aligned in the c-axis and ab-plane. Transmission electron microscopy 共TEM兲 showed that the Ti buffer layer was amorphous and interdiffused into the ZnO nanorods. Energy-dispersive spectroscopy 共EDS兲 analysis revealed the Ti buffer layers to be slightly oxide. Extended x-ray absorption fine structure confirmed the TEM and EDS results. The I-V characteristic measurements showed a 20-fold increase in current density with the Ti buffer layer, suggesting excellent electrical contact between the Ti buffer layer and ZnO nanorods. © 2010 American Institute of Physics. 关doi:10.1063/1.3308498兴 ZnO nanostructures have been studied widely for practical applications to nanoscale electronics and photonics on account of their excellent physical and mechanical properties.1 Furthermore, ZnO is formed easily into vertically aligned one-dimensional nanostructures that are very useful in functional nanodevice applications, including ultraviolet light emitting diodes 共UV-LEDs兲, gas sensors, biosensors, actuators, and solar cells.2–7 ZnO nanorods have been investigated mainly using single nanorods. However, stability, sufficiently long life time, and reproducibility are required for practical device applications. The mass production of single nanowire devices is an additional difficulty. A few attempts of bundle nanorod applications have been reported.3–5,7,8 Vertically aligned nanorods synthesized directly on an electrode with good ohmic contact will be very useful in practical nanodevice applications. A Au/Ti double layer is used widely to produce ohmic contact in ZnO semiconducting device applications.9,10 This study examined the growth of uniformly and vertically aligned ZnO nanorods on Ti buffer layers. The structural properties of the nanorods and buffer layer were investigated using a variety of analysis techniques. The electrical properties of the ZnO nanorods/Ti buffer layer were characterized by I-V characteristic measurements. Ti buffer layers were deposited on Al2O3 substrates with thickness of 15 and 40 nm using a radio-frequency 共rf兲 sputtering process at room temperature and growth chamber pressure of approximately 10−3 Torr. The rf power was 120 W and a Ti metal target was used. After the Ti buffer layers had been deposited on the substrate at room temperature, the Ti/ Al2O3 substrates were moved to a metal-organic chemical vapor deposition 共MOCVD兲 chamber in air. Subsequently, vertically aligned ZnO nanorods were synthesized on Ti/ Al2O3 substrates at 450 ° C using a MOCVD process.11 The growth rate of the nanorods was approxia兲

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mately 0.4 ␮m per minute, regardless of the buffer layer thickness. The structural and interfacial properties of the ZnO nanorods and Ti buffer layers were examined using various measurements, including field-emission scanning electron microscopy, x-ray diffraction, and x-ray absorption fine structure measurements. The environments of the Ti atoms in the buffer layers were investigated using fluorescence x-ray absorption fine structure 共XAFS兲 measurements. The XAFS measurements at the Ti K edge 共4965 eV兲 were performed by selecting the incident x-ray energy with a three-quarters tuned Si共111兲 double monochromator at the 3C1 beamline of the Pohang Light Source 共PLS兲 at room temperature. The incident x-rays were kept at 45° from the substrate surface and a seven-element Ge solid-state detector was used to select the Ti K␣ radiation only. Field-emission scanning electron microscopy 共FE-SEM兲 demonstrated that the nanorods were vertically-well-aligned with an average size of approximately 70 nm, as shown in Fig. 1共a兲. The field-emission transmission electron micros-

FIG. 1. 共Color online兲共a兲 FE-SEM image of the ZnO nanorods grown on a 15 nm thick Ti buffer-layer. 共b兲 FE-TEM image of the ZnO nanorods/15 nm Ti buffer layer/ Al2O3 substrate. 关共c兲–共f兲兴 EDS results measured from the ZnO nanorods/40 nm Ti buffer layer/ Al2O3 at Al, O, Zn, and Ti K edges, respectively.

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copy 共FE-TEM兲 images in Fig. 1共b兲 showed a clean interface between the Ti buffer layer and Al2O3 substrate. Before the ZnO nanorods were deposited, grazing incident x-ray reflectivity 共XRR兲 measurements 共not shown here兲 confirmed the smooth interface between the buffer layer and substrate with a root-mean-square 共rms兲 roughness of approximately 3.0 Å. Before the nanorods were synthesized, the rms roughness of the Ti buffer layers was less than 1.0 nm according to atomic force microscopy 共AFM兲 measurements 共data not shown兲. AFM results agreed well with the XRR results. After the nanorods were deposited, FE-TEM demonstrated that the interface between the buffer layer and ZnO nanorods showed substantial roughness. FE-TEM revealed interdiffusion at the ZnO/Ti interface, as indicated by the dark circle in Fig. 1共b兲. The interdiffusion might occur during synthesis of the ZnO nanorods. The FETEM images also revealed that the structures of the upper and lower parts of the Ti buffer layer were different. A more ordered structure was observed in the upper region compared to that in the lower region of the buffer layer. Energy dispersive spectroscopy 共EDS兲 measurements provided further evidence of interdiffusion at the ZnO/Ti interface, as shown in Fig. 1共d兲 a clean interface in the left 共Al2O3 / Ti兲 and a rough interface in the right 共Ti/ZnO兲. EDS measurements revealed oxidation of the entire Ti buffer layer. However, it was unclear if the oxygen originated from air, the ZnO nanorods, or the MOCVD growth chamber. The crystalline structural properties of the ZnO nanorods and the Ti buffer layer were examined by XRD measurements, as shown in Fig. 2共a兲. XRD showed that the ZnO nanorods on the Ti buffer layers were well aligned along the c-axis. No extra diffraction peak was observed, suggesting that the Ti buffer layer had an amorphous phase. The residual strain and the mosaicity of the nanorods were similar to those of the nanorods grown directly on sapphire.12,13 The ¯ 1兲 diffraction peak ⌽-scans of the nanorods at the ZnO 共101 showed clear sixfold symmetry, indicating that the nanorods were well-aligned in the ab-plane. A previous study reported that the nanorods were not well-aligned in the ab-plane, when the ZnO nanorods were synthesized directly on an Al2O3 substrate due to the surface roughness.12 This study showed that the Ti buffer layer with a smooth and electronrich surface enhanced the alignment of the nanorods in the ab-plane as well as in the c-axis. This result agrees well with previous reports.12,13 The XRD results are summarized in Table I. The microstructural properties of the Ti buffer layers were examined by extended XAFS 共EXAFS兲共Ref. 14兲 at the Ti K edge. Figures 2共b兲 and 2共c兲 show the magnitude of the Fourier transformed EXAFS with the k3-weight. The EXAFS data was analyzed using the UWXAFS software package.15 The peaks correspond to the atomic shells around a Ti atom. The peak positions were approximately 0.5 Å shorter than the true bond lengths because the photoelectron phase shift was not counted. The intensity of the EXAFS signal from the Ti buffer layer was considerably weaker and the position was lower than those of the Ti foil. However, the position and structures were quite different from those of the TiO2 powers. The detail structural properties can be determined by fitting the EXAFS data to the EXAFS theoretical calculation.16

FIG. 2. 共a兲 XRD of ZnO nanorods/15 nm Ti buffer layer/ Al2O3 as a function of 2␪ using Cu K␣1 radiation in air. The insets showed the ␪-rocking ¯ 1兲 peak. curve at ZnO 共0002兲 diffraction peak and a ⌽-scan of the ZnO 共101 共b兲 Magnitude of the Fourier transformed EXAFS from 共dotted line兲 Ti foil, 共light solid line兲 anatase TiO2 powder, 共dotted-dashed line兲 rutile TiO2 powder, and 共dark solid line兲 the Ti buffer layer, respectively, as functions of the distance from the probe Ti atom. 共c兲共Solid line兲 Vertically expanded EXAFS of the Ti buffer layer and 共dotted line兲 the best fit. For the Fourier transform, the Hanning window with a windowsill width of 1.0 Å−1 was used.

The best fit of the EXAFS data showed that the Ti buffer layer had an amorphous phase and oxide. These results agreed well with the TEM and EDS results. From the fits, it was found that the Ti atom had ten Ti and two oxygen atoms as the first neighboring atoms. The bond lengths of the Ti–O and Ti–Ti pairs were 2.43 and 2.89 Å, respectively. A Ti atom in Ti foil has 12 neighboring Ti atoms, six Ti atoms at 2.896 Å and six Ti atoms at 2.950 Å. The Ti–Ti bond length in the buffer layer was comparable to that of the Ti foil. However, the Debye–Waller factor 共␴2兲 of the Ti–Ti pairs in TABLE I. 共Top兲 XRD results from ZnO nanorods grown with and without a 15 nm Ti buffer layer and 共bottom兲 EXAFS results of the Ti buffer layer. ␪B is the ZnO 共0002兲 Diffraction peak position in a ␪ − 2␪ scan. Full widths at half maximum of ␪ − 2␪ 共FWHM-2␪兲 scans and FWHM of ␪-rocking were determined at the ZnO 共0002兲 diffraction peak. N, d, and ␴2 are the coordination number, the bond length, and the Debye–Waller factor of atomic pairs, respectively. S20 of 0.9 was used in the fit.

Buffer layer No Ti

Pairs Ti–O Ti–Ti

␪B 共°兲

FWHM-2␪ 共°兲

FWHM-␪ 共°兲

34.399共2兲 34.426共3兲

0.192共5兲 0.184共7兲

3.52共2兲 2.19共1兲

N

d 共Å兲

␴2 共Å2兲

2.43共1兲 2.89共1兲

0.005共1兲 0.026共1兲

2.0共3兲 10.1共3兲


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along the c-axis and in the ab-plane. FE-TEM and EDS measurements showed that the Ti buffer layer was amorphous and interdiffused into the ZnO nanorods, which was confirmed by EXAFS. EDS and EXAFS revealed that the Ti buffer layer was oxide slightly. The I-V measurements demonstrated that the Ti buffer layer enhanced the ohmic contact of the ZnO nanorods. This strongly suggested that the Ti buffer layers can be used widely to produce ohmic contact with ZnO nanostructures. The vertically aligned ZnO nanorods on Ti buffer layers can be also applicable in various nanodevices, including sensors and LEDs.

FIG. 3. 共Color online兲 I-V characteristic curves from the ZnO nanorods grown on different buffer layers as a function of the applied voltage. The inset shows a schematic diagram of the I-V measurements.

the buffer layer was approximately three times larger than that of the Ti foil, suggesting the presence of substantial disorder in the bond length of the Ti–Ti pairs. The EXAFS results are summarized in Table I. The electric contact between the Ti buffer layer and the ZnO nanorods was examined by measuring the I-V characteristic curves. For I-V measurements, a Au/Ti bilayer was used for the ohmic contact. Figure 3 shows the ohmic behavior of the ZnO nanorods/Ti buffer layer system. This result was similar to the I-V measurements of Ti/Au/ZnO bulk substrates.9 The current increased 20-fold by introducing a Ti buffer layer. Previous study reported that a natural buffer layer formed underneath ZnO nanorods directly grown on a sapphire substrate.12 Without the Ti buffer layer, charge might pass through the natural ZnO buffer layer. Charge runs through the Ti buffer layer when a Ti buffer layer was replaced beneath the nanorods. The I-V measurements revealed that the Ti buffer layer acted as an excellent electrical bridge between the nanorods. In addition, the current density decreased with increasing Ti buffer layer thickness. The surface roughness and TiO2 region increased with increasing buffer-layer thickness. These structural changes might contribute to the current density.10 In conclusion, high quality ZnO nanorods were synthesized on Ti buffer layers. The nanorods were well-aligned

The work at Chonbuk National University was conducted under the auspices of the Basic Science Research Program through the National Research Foundation of Korea 共NRF兲 grant funded by the Korea government 共MEST兲共Grant No. KRF-2007-313-C00262兲, the NRF 共Grant No. 2009-0085915兲, and the Korea MEST through the PEFP User Program. The EXAFS data were collected at 3C1 beamline of PLS. X. Wang, J. Song, J. Liu, and Z. L. Wang, Science 316, 102 共2007兲. M. H. Huang, S. Mao, H. Feik, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, and P. Yang, Science 292, 1897 共2001兲. 3 W. I. Park and G.-C. Yi, Adv. Mater. 16, 87 共2004兲. 4 S.-H. Park, S.-H. Kim, and S.-W. Han, Nanotechnology 18, 055608 共2007兲. 5 M.-C. Jeong, B.-Y. Oh, M.-H. Ham, and J.-M. Myoung, Appl. Phys. Lett. 88, 202105 共2006兲. 6 Y. F. Hsu, Y. Y. Xi, A. B. Djurisić, and W. K. Chan, Appl. Phys. Lett. 92, 133507 共2008兲. 7 A. Umar, M. M. Rahman, M. Vaseem, and Y.-B. Hahn, Electrochem. Commun. 11, 118 共2009兲. 8 D. Whang, S. Jin, Y. Wu, and C. M. Lieber, Nano Lett. 3, 1255 共2003兲. 9 H. S. Yang, D. P. Norton, and S. J. Pearton, Appl. Phys. Lett. 87, 212106 共2005兲. 10 K. Ip, Y. W. Heo, K. H. Baik, D. P. Norton, and S. J. Pearton, Appl. Phys. Lett. 84, 544 共2004兲. 11 S.-H. Park, S.-Y. Seo, S.-H. Kim, and S.-W. Han, J. Cryst. Growth 303, 580 共2007兲. 12 S.-H. Park, S.-Y. Seo, S.-H. Kim, and S.-W. Han, Appl. Phys. Lett. 88, 251903 共2006兲. 13 S.-H. Park and S.-W. Han, J. Nanosci. Nanotechnol. 7, 2526 共2007兲. 14 S.-W. Han, Int. J. Nanotechnol. 3, 396 共2006兲. 15 E. A. Stern, M. Newville, B. Ravel, Y. Yacoby, and D. Haskel, Physica B 208-209, 117 共1995兲. 16 A. L. Ankudinov, B. Ravel, J. J. Rehr, and S. D. Conradson, Phys. Rev. B 58, 7565 共1998兲. 1 2
