Growth of Epitaxial Needlelike ZnO Nanowires on ...

Journal of The Electrochemical Society, 152 共1兲 G95-G98 共2005兲

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0013-4651/2004/152共1兲/G95/4/$7.00 © The Electrochemical Society, Inc.

Growth of Epitaxial Needlelike ZnO Nanowires on GaN Films Yung-Kuan Tseng,a Chih-Ta Chia,b Chien-Yih Tsay,c Li-Jiaun Lin,a Hsin-Min Cheng,a Chung-Yi Kwo,a and I-Cherng Chena,z a

Materials Research Laboratories and cElectronics Research and Service Organization, Industrial Technology Research Institute, Hsinchu, 310 Taiwan b Department of Physics, National Taiwan Normal University, Taipei, 100 Taiwan Epitaxial needlelike ZnO nanowires were grown vertically over an entire epi-GaN/sapphire substrate at 550°C by low-pressure vapor phase deposition without employing any metal catalysts. A two-step oxygen injection process is the key of successful synthesis. The length of ZnO wires was up to 3.0 ␮m. The diameters of the roots and tips of the ZnO nanowires were around 80-100 and 15-30 nm, respectively. X-ray diffraction showed the epitaxial orientation relationship between ZnO and GaN as 关 001兴 ZnO // 关 001兴 GaN along the normal to the plane, and 关 100兴 ZnO // 关 100兴 GaN along the in-plane direction, consistent with the selective area electron diffraction pattern taken at the ZnO/GaN heterointerface. High-resolution transmission electron microscopy confirmed that nanowire was a single crystal. A room-temperature photoluminescence spectrum of the wires revealed a low concentration of oxygen vacancy in the ZnO nanowires and showed high optical quality. © 2004 The Electrochemical Society. 关DOI: 10.1149/1.1825953兴 All rights reserved. Manuscript received June 14, 2004. Available electronically December 2, 2004.

Semiconductor nanowires and nanorods have been attracting much attention in recent years, especially in mesoscopic research and the potential applications in manufacturing nanodevices. The main reasons include their interesting photonic and electronic properties, and being able to be the important building block for interconnects of transistors, junctions of metal-semiconductors, and the tips of emitters. Many studies have been carried out for Si and III-V systems.1-5 However, research on the oxide systems, including SnO2 , 6 SiO2 , 7 GeO2 , 8 ZnO,9 ITO,10 and Al2 O3 , 11 have just been conducted recently. Among them, ZnO is an intrinsic n-type semiconductor with a wide bandgap 共3.30 eV兲 and a large exciton energy 共60 meV兲. It conducts transparently and presents some interesting optoelectronic features, such as emitting short-wavelength light and room-temperature lasing. ZnO nanowires also could be used as good field electron emitters and chemical sensors because they have the characteristics of high-aspect-ratio structure, negative electron affinity, and chemical stability. Recently, optical and electronic devices based on 1-D ZnO nanostructure were also reported.12,13 To apply ZnO nanowires in the optoelectronic field, they should be grown directly on substrates at the temperature below 550°C for the compatibility of microelectronic manufacturing. Furthermore, they should be grown well oriented and could be patterned. Some efforts consisted with these requirements have been reported. Tseng et al.14 demonstrated the feasibility of selected-area growth of ZnO nanowires at low temperature 共500°C兲. Satoh et al.15 have grown highly oriented ZnO whiskers on (001)␣-Al2 O3 substrates at 550°C by chemical vapor deposition 共CVD兲. Huang et al.16 reported the gasphase synthesis of well-aligned ZnO nanowires on patterned Au catalyst with 共110兲 sapphire substrates by the vapor liquid solid 共VLS兲 reaction at 900-925°C. Park et al.17 used metallorganic vapor phase epitaxy to grow ZnO nanoneedles vertically on Si共111兲 at 400-500°C without metal catalyst. Growth on fused silica has also been reported by Wu et al.18 Despite the successful reports of direct growth of ZnO nanowires on these common substrates, using low lattice mismatch buffer layer, especially epitaxial GaN, is a good way to deposit the ZnO nanowires on various material substrates. ZnO and GaN not only have a Wu¨rtzite hexagonal structure with a low lattice mismatch 关 (a ZnO ⫺ a GaN)/a GaN ⫽ 1.6% 兴 , 19 but also have a small thermal mismatch (6.51 ⫻ 10⫺6 K⫺1 for ZnO and 5.59 ⫻ 10⫺6 K⫺1 for GaN兲. Furthermore, the use of a GaN buffer also offers the advantage of having the controllability of electrical conductivity in a broad range, which allow for greater flexibility in device design. Therefore, this combination is very appropriate to

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grow well-oriented ZnO nanowires on the buffer layers of various optical and electrical properties for further constructive applications. In our study, epitaxial needlelike ZnO nanowires were grown vertically over the entire GaN/sapphire at 550°C by low-pressure vapor phase deposition 共LPVPD兲 without employing any metal catalysts. A two-step oxygen injection strategy results in the successful synthesis. The X-ray diffraction 共XRD兲 ␻-rocking curve and ␾-scan techniques showed that the ZnO nanowires were grown epitaxially along the c-axis direction on the GaN/sapphire substrates. Highresolution transmission electron microscopy 共HRTEM兲 was used to confirm that the nanowires were single crystals. The photoluminescence 共PL兲 spectra measured at room temperature indicated that the nanoneedles were of high optical quality. Experimental Our synthesis was performed by LPVPD. Zinc vapor source is Zn metal powder with the purity of 99.9% from Strem Chemicals. High-quality epitaxial GaN共001兲 buffer layers of up to 2 ␮m thick were grown on sapphire共001兲 substrates by metallorganic vapor phase epitaxy 共MOVPE兲. The GaN/sapphire substrates and zinc vapor source in an alumina boat were inserted into the quartz tube and placed in the middle of the furnace. The distance between the Zn source and the substrate was 20 mm. The zinc vapor source was placed upstream. Both were located at the same horizontal level to make sure that they were in the same temperature region. A constant stream of argon flowed through the reaction system. A mechanical pump was used to evacuate the system to maintain the pressure inside the quartz tube at about 10 Torr. A programmable temperature controller was used to keep the heating ramp and furnace temperature in the range of ⫾1°C. The heating ramp was set to be 20°C/min. The reaction gases were directed into reaction system in two steps by mass flow controllers. The first step was to lead the argon flow of 54 sccm into the reaction system as the experiment began. As soon as the furnace temperature reaching 420°C, oxygen flow of 0-3 sccm was added into the argon flow as the second step until the end of the experiments. The crystalline structure of the samples was analyzed using a Panalytical X’Pert MRD highresolution triple axis X-ray diffractometer and a transmission electron microscopy 共TEM; JEOL, JEM-2000FX, operated at 200 KV兲. The morphology and size distribution were characterized using a field-emission scanning electron microscope 共FESEM; LEO 1530, operated at 5 keV兲. A Jobin Yvon-Spex fluorolog-3 spectrophotometer was used to conduct the photoluminescence studies. Results and Discussion Our two-step oxygen injection strategy was proposed for inhibiting the formation of Ga2 O3 . 20 Because of the different crystal structure, poor crystalline and large lattice misfits from ZnO and

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GaN, Ga2 O3 is not good for the deposition of high quality ZnO overlays on epitaxial GaN substrates. It is easy to form the Ga2 O3 interlayer for the formation enthalpy of Ga2 O3 (⌬H Ga2O3 ⫽ ⫺1096.936 kJ/mol21), which is much larger in the magnitude than that of GaN and ZnO (⌬H ZnO ⫽ ⫺356.465 kJ/mol21 and ⌬H GaN ⫽ ⫺114.911 kJ/mol at 427°C,21 respectively兲. Therefore, in the low-temperature synthesis process, the GaN buffer layer would compete with the zinc atoms on oxygen. The surface of the GaN layer would be transformed to the poor crystalline Ga2 O3 . This made the nucleation of ZnO difficult. Also, Ga2 O3 may adverse the deposition of crystalline ZnO overlays. This could be in view of a different crystal structure and large lattice misfits from ZnO and GaN. Based on a comparison of the orientation relationship and lattice constants,22 the lattice misfits along the 关010兴共spacing, b ⫽ 3.038 Å) and 关100兴共spacing, a ⫽ 12.22 Å) axes of Ga2 O3 against GaN for the 关100兴共spacing, a ⫽ b ⫽ 3.186 Å) and the 关120兴共spacing, 共2 )兲 a ⫽ 1.105 Å) directions are ⫺4.7 and 10.7%, respectively. Lattice misfits of Ga2 O3 and ZnO along the same directions are ⫺6.5 and 8.6%, respectively. We therefore designed a two-step oxygen injection process to suppress the formation of gallium oxide. We did not lead oxygen into the reaction system until the reaction system reached 420°C. However, the vapor pressure of zinc element is about 10⫺1 Torr as the temperature is 400°C.23 This could deposit a thin zinc film on the substrate in the low-pressure argon atmosphere. As the oxygen was directed into the reaction system, the thin zinc film could be oxidized to form ZnO nucleus seeding layer and prevent the GaN layer from contacting with oxygen molecules directly to form Ga2 O3 phase. This simple process provided no opportunities for Ga2 O3 to form, but promoted the growth of ZnO nanowires. Using the two-step oxygen injection process, ZnO nanowires with high density were uniformly grown over the entire substrate. Figure 1a shows the image viewed at the oblique direction. Figure 1b is the side image. It shows that the wires with similar diameter and sharp tip were grown on the GaN buffer layer. The length of ZnO wires was around 2.8-3.0 ␮m. The diameters at the nanowire root and the wire tip were in the range of 80-100 and 15-30 nm, respectively. Figure 1c shows the cross-sectional image observed by TEM. The ZnO nanowires were grown vertically on the GaN film and were of almost the same height. The same height and the homogeneous size 共as the Fig. 1b shown兲 indicate that the nucleation of ZnO nanowire growth centers was very fast. This also could explain the formation of ZnO film on the GaN layer was the roots of these nanowires stuck together closely to form a film. If the ZnO film was formed prior to the ZnO nanowires, the nucleus sites on the film for growing the ZnO nanowires would form inconsistently. Then, the distribution of the height should be broad. The inset in the Fig. 1c shows a selective area electron diffraction pattern 共SAED兲 taken at the ZnO/GaN heterointerface. Those well-aligned spots clearly established the epitaxial nature of the heterostructure. From this we determined the epitaxial orientation relationship between ZnO and GaN as 关 001兴 ZnO // 关 001兴 GaN along the normal to the plane, and 关 100兴 ZnO // 关 100兴 GaN along the in-plane direction. In addition, we observed the split of 关12-2兴 diffraction spot, as indicated by the arrows. It should be made by the lattice mismatch between ZnO and GaN. The crystallographic properties of the films and nanowires were investigated by XRD. As shown in Fig. 2a, only the peaks corresponding to the 共00l兲 reflection family of ZnO, GaN, and the 共006兲 plane of the ␣-Al2 O3 substrate appeared in the XRD profiles of ZnO nanowires/GaN/sapphire sample. The 共00l兲 peaks of ZnO and GaN were not resolved due to a very good lattice matching. This indicates that the ZnO film and the nanowires were preferentially in the c-axis direction. Meanwhile, XRD ␻-scan for the 共002兲 ZnO/GaN peak was also performed to check the degree of alignment to the normal direction of the surface. The narrow full width at half-maximum 共fwhm兲 width was 0.138° in the XRD ␻-scan curve implies that the

Figure 1. 共a兲 FESEM image observed at the oblique direction. 共b兲 FESEM side view of the ZnO nanowires grown on the epitaxial GaN film/sapphire substrate. 共c兲 TEM image and the corresponding selective-area electron diffraction pattern 共SAED兲 taken at the ZnO/GaN heterointerface along the 关100兴 zone axis.

c axes of ZnO nanowires/共001兲 epi-GaN were along the normal direction of the substrate surface 共the inset in Fig. 2a兲. Because the reflection of GaN film might be strong to cover that of ZnO nanowires, the in-plane alignment of ZnO nanowires was further confirmed by the ␻ scan for the high index ZnO plane, 共006兲. As shown in the


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Figure 3. HRTEM image of an individual ZnO nanowire showing its 关001兴 growth direction. The inset shows the corresponding SAED pattern from the nanowires.

Figure 2. 共a兲 XRD profiles of ZnO nanowires/共001兲 epi-GaN/共001兲 ␣-Al2 O3 sample. The inset is XRD ␻-rocking curve of the 共002兲 peak. 共b兲 XRD ␻-rocking curve of the 共006兲 ZnO. The inset shows the ␻-scan profile. 共c兲 X-ray diffraction ␾-scan profiles of the ZnO nanowires/GaN/␣-AlO2 O3 structure: the 兵204其 plane family of ZnO nanowires 共top兲 and GaN film 共bottom兲. The inset shows the ␪-2␪ scan profile.

inset of Fig. 2b, the diffractions of 共006兲 planes of ZnO and GaN were split and could be identified. The fwhm value of the 共006兲 ZnO peak did not exceed 0.53°, which positively indicates ZnO nanow-

ires were grown with good ordering along the growth direction. The in-plane epitaxial relationship between crystalline direction of the ZnO nanowires and the GaN buffer layer was also investigated by using the ␾-scan technique, as shown in Fig. 2c. We chose the high index 兵204其 plane family for ␾-scan analysis because there exists a 2␪ difference between ZnO共204兲 and GaN共204兲 as high as 1.8° 共ZnO共204兲 is 107.425°19 and GaN共204兲 is 109.225°19兲, as the inset of Fig. 2c shows. It would be wide enough to avoid interference from each other and to help us to get the individual spectra of ZnO nanowires and GaN layer. It is clear from Fig. 2c that the well-defined peaks showed sixfold azimuthal symmetry, which is consistent with the wu¨rtzite crystal structure. In addition, the ␾-scan for the ZnO nanowires was superposed on the ␾-scan for the GaN layer. As a consequence, the epitaxial quality of the ZnO nanowires was close to that of the underlying GaN. The baseline in the ␾-scan is another indication of the quality of the epitaxy. The baseline was almost zero and revealed that the fraction of polycrystalline ZnO was negligible. Further structure characterization of the ZnO nanowires was performed by HRTEM. Figure 3 shows the lattice fringes and the corresponding selected-area electron diffraction pattern of the ZnO nanowires. These results confirmed that the nanowire was a single crystal. The lattice spacing of ⬃2.6 Å between adjacent lattice planes corresponded to the distance between two 共002兲 crystal planes. This confirmed that 关001兴 was the preferred growth direction for ZnO nanowires. The optical properties of ZnO nanowires were investigated by the room temperature photoluminescence spectroscopy. As Fig. 4 shows, the dominant peak was observed at ⬃378 nm. It is attributed to the recombination of free excitons through an exciton-exciton collision process24 because of the 3.3 eV 共376 nm兲 wide direct bandgap transition of ZnO nanowires at room temperature. The secondorder diffraction of the band-edge emission was clearly shown at 760 nm. Deep level emissions 共green-yellow bands兲 were also observed in the PL spectrum. It has been suggested that the deep level luminescence corresponds to the singly ionized oxygen vacancy in ZnO.24 According to Egelhaaf et al.,25 the defect-related luminescence is caused by the radiative transitions between shallow donors 共related to oxygen vacancies兲. The acceptor level 共Zn vacancy兲 is located at 2.5 eV below the conduction bandedge, while the donor level is known as shallow as 0.05-0.19 eV. Therefore, the weak green-yellow bands in this figure meant that there was a low concentration of oxygen vacancy in the ZnO nanowires and revealed the high quality of the ZnO nanowires. Based on the observations of the morphology and the heterointerface, the epitaxial ZnO nanowires growth was due to neither

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Figure 4. Room-temperature PL emission spectra excited at ␭ exc ⫽ 254 nm.

The length of ZnO wires was up to 3.0 ␮m. The diameters of the roots and tips of the ZnO nanowires were in the range of around 80-100 and 15-30 nm, respectively. The nanowires checked by the XRD confirmed the epitaxial orientation relationship between ZnO and GaN as 关 001兴 ZnO // 关 001兴 GaN along the normal to the plane, and 关 100兴 ZnO // 关 100兴 GaN along the in-plane direction. It was consistent with the SAED taken at the ZnO/GaN heterointerface. HRTEM confirmed that the nanowire was a single crystal. A room-temperature PL spectrum of the wires revealed a low concentration of oxygen vacancy in the ZnO nanowires and showed high optical quality. Furthermore, the two-step oxygen injection strategy proposed in this study is the key of successful synthesis of epitaxial ZnO nanowires. In the first step, a thin Zn film covered on the GaN layer. This Zn film played a crucial role to prevent the formation of Ga2 O3 and being oxidized to form nucleus seeds with the epitaxial alignment of epi-GaN buffer layer in the second step. Based on the epitaxial 1D structure, nanoelectronics, nanophotonics, and nanosensors will create many constructive applications. The Industrial Technology Research Institute assisted in meeting the publication costs of this article.

vapor-liquid-solid 共VLS兲26 nor screw-dislocation mechanism 共Frank mechanism兲.27 The VLS growth uses nanosized metal clusters as the catalyst to absorb the gas-phase reactants and to form eutectic alloy droplets. As the concentration of reactant in the liquidized droplets being oversaturated, the precipitation begins and the 1D structure will be formed. In our study, neither transition metals were used as catalysts nor did additional metal particles appear on the top or bottom of the nanowires, which is a characteristic of VLS growth. Therefore, it should not follow the VLS mechanism. Frank mechanism is based on the screw dislocation, which emerges at the growth interface and provides a self-perpetuation step for the addition of new layers. Therefore, a crystal has a conic tip with spiral morphology at the end of 1D structure and a screw dislocation, which is parallel to the axis of 1D structure. The screw-dislocation was also not found in our ZnO nanowires. However, the crystal structure of the ZnO nanowires have been identified to have a close relationship with that of the epitaxial GaN layer in both vertical and in-plane alignment. This means that nucleation at the initial stage might have a crucial role. Furthermore, the ZnO nanowires were grown with almost the same height 共Fig. 1c兲 and the homogeneous size 共Fig. 1b兲. These indicate that the nucleation was at the same time 共instantaneous兲. What happened to induce the nucleation in such a short time? We proposed that it is due to the injection of oxygen into reaction system. The oxygen made the thin zinc films be oxidized and form zinc oxide nuclei all over the entire substrate. During the formation of nuclei, zinc oxide nuclei were grown with an epitaxial relationship with GaN layer since ZnO and GaN are lattice matched. Then, the ZnO nuclei kept on growing under 550°C. The growth of ZnO is consistent with the model of idealized ZnO crystal growth proposed by Li et al.28 For thermodynamic stability of growth habit, the growth rate along 具001典 direction is faster than that along 具101典 and 具100典. The deposition based on GaN共001兲 plane and the growth along the normal direction of the substrate surface 共具001典 direction兲 were faster than the lateral one and resulted in that the crystals had a high aspectratio shape. Furthermore, Wu et al.29 reported that adjusting the vapor flux of the zinc source can control the diameter of ZnO nanorods. Therefore, the gradually exhausted Zn metal powder and thermodynamic stability of growth habit could explain the formation of the special needlelike ZnO nanowires in our study. Conclusions By adopting the two-step oxygen injection process, we successfully grew needlelike ZnO nanowires uniformly and vertically with high density over the entire epi-GaN/sapphire substrate at 550°C.

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