Synthesis and characterization of ZnO thin film grown

Synthesis and characterization of ZnO thin film grown by electron beam evaporation D. C. Agarwal, R. S. Chauhan, Amit Kumar, D. Kabiraj, F. Singh, S. A. Khan, D. K. Avasthi, J. C. Pivin, M. Kumar , J. Ghatak, and P. V. Satyam Citation: Journal of Applied Physics 99, 123105 (2006); doi: 10.1063/1.2204333 View online: http://dx.doi.org/10.1063/1.2204333 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/99/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in The post-growth effect on the properties of Cu2ZnSnS4 thin films J. Renewable Sustainable Energy 7, 011203 (2015); 10.1063/1.4908063 Transitions of bandgap and built-in stress for sputtered HfZnO thin films after thermal treatments J. Appl. Phys. 114, 084503 (2013); 10.1063/1.4819232 Study of cadmium sulfide thin films as a window layers AIP Conf. Proc. 1476, 178 (2012); 10.1063/1.4751590 Optical and electrical properties of transparent conducting B-doped ZnO thin films prepared by various deposition methodsa) J. Vac. Sci. Technol. A 29, 041504 (2011); 10.1116/1.3591348 Transparent conductive and near-infrared reflective Ga-doped ZnO/Cu bilayer films grown at room temperature J. Vac. Sci. Technol. A 29, 03A115 (2011); 10.1116/1.3570864

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JOURNAL OF APPLIED PHYSICS 99, 123105 共2006兲

Synthesis and characterization of ZnO thin film grown by electron beam evaporation D. C. Agarwala兲 and R. S. Chauhan Department of Physics, R. B. S. College, Agra 282 002, India

Amit Kumar, D. Kabiraj, F. Singh, S. A. Khan, and D. K. Avasthi Inter-University Accelerator Centre, P.O. Box 10502, New Delhi 110 067, India

J. C. Pivin CSNSM, IN2P3-CNRS, Batiment 108, F-91405 Orsay Campus, France

M. Kumar Department of Physics, Allahabad University, Allahabad 211 002, India

J. Ghatak and P. V. Satyam Institute of Physics, Sachivalaya Marg, Bhubanashwar 751 005, India

共Received 28 November 2005; accepted 16 April 2006; published online 22 June 2006兲 Highly transparent, conducting, highly oriented, and almost single phase ZnO films have been deposited by simple e-beam evaporation method, and the deposition parameters were optimized. The films were prepared by 共a兲 evaporation of ZnO at different substrate temperatures and 共b兲 evaporation of ZnO at room temperature and subsequent annealing of the films in oxygen ambient at different temperatures. The characterizations of the film were performed by optical absorption spectroscopy 共UV-visible兲, Fourier transform infrared spectroscopy, resistivity measurement, transmission electron microscopy 共TEM兲, photoluminescence, and x-ray diffraction measurement. Absorption spectra revealed that the films were highly transparent and the band gap of the pre- and postannealed films was in good agreement with the reported values. The band gap of the films increases on increasing the substrate temperature as well as annealing temperature, whereas the resistivity of the film decreases with substrate temperature and increases with annealing temperature. Fourier transform infrared spectroscopy of ZnO films confirms the presence of Zn–O bonding. X-ray diffraction, electron diffraction, and TEM images with high resolution and Raman spectra of the films showed the formation of crystalline ZnO having wurtzite structure. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2204333兴 I. INTRODUCTION

The semiconductors Si and Ge based electronic devices cannot be used at high temperature due to small band gap of these materials. Therefore, wider band gap semiconductors are required for high temperature devices. Transparent semiconducting ZnO thin films have attracted considerable attention from both fundamental and application points of view, primarily because of their useful properties. Zinc oxide is n-type semiconductor of wide interest exhibiting excellent optical, electrical, catalytic, gas sensing properties and has great technological applications in various fields.1–6 The notable properties of ZnO are its wide band gap of 3.3 eV at room temperature and high exciton binding energy 共60 meV兲 compared to those of ZnS 共20 meV兲 and GaN 共21 meV兲.7–11 Due to its wide band gap, ZnO can be used for short wavelength light emitting devices such as light emitting diodes and laser diodes, surface acoustic wave 共SAW兲 bandpass filters, optical waveguides, and laser deflectors using piezoelectric or piezo-optic properties. It is also used in gas sensors, varistor, and as transparent conducting oxide. ZnO has proved to be an interesting alternative for the commonly a兲

Author to whom correspondence should be addressed; [email protected]

0021-8979/2006/99共12兲/123105/6/$23.00

used indium-doped tin oxide 共ITO兲 transparent electrodes owing to lower absorption in UV, higher stability in hydrogen plasma, lower cost and higher availability, and lack of toxicity. Hence ZnO is useful material for optoelectronics applications because it exhibits semiconducting as well as piezoelectric properties. ZnO films can be deposited by a variety of techniques, such as sol-gel chemistry,12 spray pyrolysis,13 metal organic chemical vapor deposition,14 molecular beam epitaxy,11 pulsed laser deposition,15,16 reactive thermal evaporation,17 and sputtering.18 The properties of grown thin films and interfaces depend on the deposition process and deposition parameters. Every deposition technique has its own advantages and disadvantages. Thermal evaporation is widely used and well-known technique generally to deposit films with smoother surface than other techniques and less or no damage at the substrate interface. Sufficiently conducting ZnO films for device application can be obtained by e-beam evaporation process with controlled conditions and subsequent annealing. The ZnO has a hexagonal-close-packed wurtzite structure which shows a strong 具001典 preferred orientation, perpendicular to the substrate, related to a minimum of the surface energy.19 It is known that ZnO displays three major PL peaks as mentioned in the literature: an UV

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near band-edge emission peak around 380 nm, a green emission around 510 nm, and a red emission around 650 nm.20 Band-edge emission is sharp and strong while the other two emissions are weak and broad. Normally ZnO thin films show n-type conduction due to oxygen vacancies and interstitial Zn ions which act as donors in the ZnO lattice. The red and green emissions are attributed to oxygen vacancies and Zn ions in the interstitial position. In most polycrystalline thin films, defect related deep level emissions dominate the PL spectra, which precludes various applications such as UV luminescent devices and unconventional polycrystalline layers, known as “random laser” recently demonstrated in polycrystalline ZnO thin films.21,22 Because the center energy of the green, yellow, and orange emissions is smaller than the band gap energy of ZnO film, these emissions must be related to a localized deep level in the band gap. The origin of the defect related deep level photoluminescence 共PL兲 band has been investigated for a long time. However, due to the complexity of the microstructure of ZnO, there is still no satisfactory consensus.22–24 Large exciton energy and good luminescence properties make the ZnO useful for optical applications. In the present work, a study on the defect emission in the thermally evaporated ZnO film and effect of postannealing and substrate temperature on the structural, optical, and electrical properties of the ZnO film is performed. II. EXPERIMENTAL WORK A. Synthesis

The ZnO thin films 共⬃100 nm兲 were deposited by thermal evaporation of ZnO powder in the form of a pellet on unheated Si and quartz. The substrates were washed with trichloroethylene, acetone, and alcohol. The distance from source to substrate and source to quartz crystal was of 15 cm. The vacuum before evaporation was of 5 ⫻ 10−7 torr, during evaporation at room temperature was of 8 ⫻ 10−6 torr, and during evaporation at higher substrate temperature was of 6 ⫻ 10−5 torr. The deposition rate was 0.1– 0.4 nm/ s. After deposition at RT, the films were black. The black color may be due to the oxygen deficiency and large number of defects. Therefore, these films were annealed in oxygen ambient at temperatures of 450, 600, and 800 ° C for 2 h. After annealing, the films were totally transparent. Films were also deposited on quartz, Si, and C-coated transmission electron microscopy 共TEM兲 grids keeping the substrate temperature at 100, 200, 300, and 400 ° C. Other deposition parameters were the same as described above. After deposition the films were transparent. The summary of the sample codes and deposition conditions for the preparation of ZnO films with band gap and resistivity is given in Table I.

TABLE I. Summary of the sample codes and deposition conditions for the preparation of ZnO films with band gap and resistivity.

Sample identity Z0 Z1 Z2 Z3 Z4 Z5 Z6 Z7

Substrate temperature 共°C兲

Annealing temperature 共°C兲

Room 100 200 300 400 Room Room Room

No No No No No 450 600 800

temperature

temperature temperature temperature

Band gap 共eV兲

Resistivity 共⍀ cm兲

1.51 3.08 3.13 3.22 3.26 3.18 3.22 3.26

14.65⫻ 10−2 12.1⫻ 10−2 4.70⫻ 10−2 3.65⫻ 10−2 2.54⫻ 10−2 9.7 28 86

Mechelle 900 monochromator system with a Si charge coupled detector was used in measurements. Optical absorption spectra were recorded with the conventional two-beam method using U-3300 UV-VIS spectrophotometer of Hitachi. The Zn–O bonding and vibrational modes were studied by Fourier transform infrared spectroscopy 共FTIR兲 and Raman spectroscopy. The FTIR spectra were recorded in the range of 400– 4000 cm−1, with a spectral resolution of 4 cm−1. The Raman spectra of the films were recorded using DILOR XY Raman spectrometer with spectral resolution of 1 cm−1 in CSNSM, Orsay. The nature of formed phases was studied by means of TEM, electron diffraction, and x-ray diffraction 共XRD兲. The XRD spectra of ZnO films were recorded with Brucker AXS x-ray diffractometer using Cu K␣ at IUAC, New Delhi. High resolution transmission electron microscopy 共HRTEM兲 images of the ZnO film deposited at 300 ° C were taken with a JEOL-2010-UHR instrument operated at 200 kV at Institute of Physics 共IOP兲, Bhubaneswar. The point to point resolution of HRTEM was 0.19 nm. The resistivity measurements of the films at room temperature by the Vander-Pauw method were also measured at IUAC, New Delhi. III. RESULTS AND DISCUSSION A. XRD analysis

Figure 1共a兲 shows the x-ray diffraction spectra of ZnO films deposited at different substrate temperatures. The film Z0 showed polycrystalline nature, with 共100兲, 共002兲, and 共101兲 peaks of hexagonal ZnO at 31.75°, 34.35°, and 36.31°. As the substrate temperature is increased, the fiber texture

B. Sample characterization

Photoluminescence 共PL兲 and UV-VIS absorption measurements were carried out to characterize the optical properties of films at Inter-University Accelerator Centre 共IUAC兲, New Delhi. PL spectra were recorded under excitation with the 325 nm line of He–Cd laser with a power of 27 mW. A

FIG. 1. XRD spectra of ZnO thin film 共a兲 at different substrate temperatures and 共b兲 at different annealing temperatures.


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along the c axis 共002兲共peak at 34.43°兲 becomes the most prominent peak while the other two peaks disappear. This shows the improvement in the fiber texturing of the films along the c axis with substrate temperature. The high substrate temperature enables the atoms to move to their stable orientation.25 A new peak 共103兲 has been observed at higher substrate temperature of 300 ° C. The appearance of the 共103兲 orientation at higher temperature is due to some atoms relocating at higher states because nuclei with this orientation become stable when providing thermal energy for compensating this higher surface energy. The d value calculated for the 共002兲 peak remains constant at around 0.264 nm with increase in the substrate temperature from RT to 300 ° C. Earlier reports indicated that the films deposited by dc magnetron sputtering at a substrate temperature of 323 K had a preferred orientation along the 共002兲 plane, a random orientation at 473– 573 K, and again a preferred orientation along the 共002兲 crystal plane at 623– 753 K 共Refs. 26 and 27兲, whereas in the case of rf sputtered films it was noticed that the films formed at 313 K exhibited a mixed orientations while those formed at 473 K exhibited single phase of ZnO with the 共002兲 orientation. The average grain sizes of the films deposited at different substrate temperatures were calculated using the Scherrer’s formula: D=

0.9␭ , ␤ cos ␪

FIG. 2. ZnO film deposited at 300 ° C substrate temperature. 共a兲 HRTEM image, 共b兲 electron diffraction pattern.

polar semiconductor with 共001兲 planes being Zn terminated ¯ 兲 being O terminated. These two crystallographic and 共001 planes have opposite polarity and hence have different surface relaxation energies, resulting in high growth rate along the c axis.28 The value of d spacing was calculated from the HRTEM image and electron diffraction pattern. The calculated values of d spacing for the 共002兲 plane are 0.269 and 0.271 nm, respectively, which are in good agreement with XRD results.

共1兲

where ␭, ␤, and ␪ are x-ray wavelength 共1.541兲, full width of half maximum 共FWHM兲 of the 共002兲 peak, and the Bragg diffraction angle. The average grain size of the films increases from 10.5 to 17 nm with increase in the substrate temperature, as expected, due to the improvement in the crystallinity of the films. Figure 1共b兲 shows the XRD pattern of ZnO films deposited at RT and annealed in oxygen ambient for 2 h at different temperatures. The XRD patterns for the postannealed ZnO films indicated that they possess polycrystalline hexagonal wurtzite crystal structure with no preferred orientation. XRD spectra of the annealed films shows that the crystallinity of the film improves with increasing the annealing temperature up to 600 ° C as the intensity of the 共100兲, 共002兲, and 共101兲 peaks increases and FWHM decreases. When annealing temperature is further increased to 800 ° C, the intensity of the 共002兲 peak decreases due to the deterioration of crystallinity in this orientation and the intensity of the 共100兲 and 共101兲 peaks increased. The x-ray diffraction studies revealed that the higher substrate temperature of 300 ° C is the optimum condition to generate highly c-axis oriented zinc oxide films.

C. Raman spectroscopy

The resulting Raman spectrum of Z3 film obtained at room temperature is shown in Fig. 3. The Raman active phonon modes for the wurtzite structure of ZnO expected from the group theory are A1 + 2E2 + E1. The characteristic mode of ZnO wurtzite phase at around 433 cm−1 共Ref. 29兲 is observed for ZnO films which also show prominent peak at 34.5° in the XRD spectrum. Other than the characteristic mode of ZnO, few second order modes are also observed. The modes at 233 and 299 cm−1 are related to B1共high兲-B2共low兲 and the mode at 822 cm−1 is related to the B1共high兲 + B1共low兲 phonon mode frequencies.30 The peak at 630 cm−1 has been assigned to E1-LO mode.31 The E1-LO mode at phonon frequency of

B. Transmission electron microscopy

In addition to the XRD analysis, we have also confirmed the crystallinity of the film using TEM analysis. Films with thickness of 50 nm were grown directly on carbon-coated TEM grids. Figures 2共a兲 and 2共b兲 showed the HRTEM lattice image and the electron diffraction patterns of ZnO film deposited at substrate temperature of 300 ° C. The electron diffraction clearly demonstrated that the ZnO film grew along the c axis which is consistent with the XRD results. ZnO is a

FIG. 3. Raman spectra of ZnO thin film deposited at 300 ° C substrate temperature.


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FIG. 4. 共a兲 FTIR spectra of ZnO thin film deposited at different conditions. 共b兲 Variation of FWHM of 408 peak of the ZnO thin film at different substrate temperatures.

630 cm−1 was theoretically calculated by Zaoui and Sekkal using the atomistic calculations based on an interatomic pair potential with in the shell model approach. D. FTIR spectra

FTIR was employed to reveal the Zn–O bonding. FTIR spectra of ZnO films on Si substrate as shown in Fig. 4共a兲 show a strong peak at 408 cm−1 and two weak peaks at 513 and 567 cm−1. Andres-Verges et al.32 theoretically calculated a strong IR band at 406 and one of small intensity at 580 for slab-type ZnO particles 共c / a Ⰶ 1兲. Hayashi et al.33 also recorded the IR spectra of ZnO and compared them with the calculated spectra. ZnO show three distinct absorption peaks located between the bulk TO-phonon frequency 共␻T储兲 and LO-phonon frequency 共␻L⬜兲. The present FTIR results are in good agreement with the reported values, confirming the ZnO phases over the whole temperature range. The peak at 408 cm−1 of the films deposited at different substrate temperatures is fitted by Gaussian distribution. The FWHM of the peak corresponding to the 408 cm−1 vibration decreases with increase in the substrate temperature but no significant change in the peak position is observed with the variation of substrate temperature. The variation in FWHM of the peak corresponding to the 408 cm−1 vibration, shown in Fig. 4共b兲, shows that Z0 film has larger FWHM which indicates wider distribution of vibrational energy of the ZnO molecule. As substrate temperature is increased the FWHM decreases, showing that distribution of vibrational energy becomes narrower, indicating that the crystallinity of the film is improved on increasing the substrate temperature. There are some re-

FIG. 5. Absorption spectra of ZnO thin film 共a兲 at different substrate temperatures and 共b兲 at different annealing temperatures.

FIG. 6. Variation in band gap of ZnO thin films with substrate and annealing temperatures.

ports available which show that the higher the FWHM of the FTIR absorption peak, the smaller is the grain size.34,35 A crystallinity can be defined from the damping constant36 共␥兲 which is defined according to the relation

冉冊

1 w −1=f , T共w兲 wTO

共2兲

where T共w兲 is the experimental transmittance and the FWHM of the function f共w / wTO兲 is the measure of the damping constant which defines the damping in optical vibrational modes and measure of crystallinity. The FWHM of the peak at 408 cm−1 also decreases for the films Z5 – Z7, giving a clue that the crystallinity increases on annealing of the RT deposited samples. E. Optical absorption study

The dependence of absorption of the films deposited on quartz on wavelength is shown in Figs. 5共a兲 and 5共b兲. The transmittance shows a strong temperature dependence. Films deposited at room temperature 共RT兲 have a significant, continuous absorption in the visible range, demonstrating the presence of a noticeable amount of metallic zinc and consid-

FIG. 7. PL spectra of ZnO thin film 共a兲 at different substrate temperatures and 共b兲 at different annealing temperatures 共the intensity of the film grown at RT and postannealed at 450 ° C was multiplied by 10兲.


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erable amount of defects in the RT deposited films. The metallic zinc can be incorporated in the film as droplets or as coherent domains of metallic zinc with zinc oxide. The band edge becomes sharper on increasing the substrate temperature. The transmittance is poor in as-deposited state and it increases with increasing the substrate temperature or postdeposition annealing. The increase in the transmittance in the films grown at higher substrate temperatures or in postannealed films in oxygen is likely to be related, in part, to the decrease in defects and increase in the ZnO to Zn ratio. The absorption spectra of annealed films show sharper band edge, as also those of ZnO films deposited on heated substrate. The increase of transmittance is attributed to the intake of the oxygen in the oxygen deficient ZnO films. The absorbed oxygen removes the oxygen vacancies, hence reducing the density of these donorlike defects. From the optical absorption spectra, the band gap of ZnO films is determined using the Tauc’s procedure of plotting 共␣h␯兲2 vs h␯ and extrapolating the linear portion of absorption edge to the energy axis. The band gap of the films Z0 – Z7 was given in Table I. These values show that the band gap is small in films deposited at room temperature and increases with increase in annealing temperature or substrate temperature. The small band gap in the case of deposition at RT may be attributed to the energy levels in the intrinsic gap due to the defects such as oxygen vacancies and Zn ions at the interstitial sites. The increase in band gap energy upon increasing the substrate temperature or annealing temperature is due to the decrease in the amount of defects. F. Photoluminescence spectra

PL measurements were performed at room temperature with the 325 nm line of a continuous wave He–Cd laser. The band-edge emission could not be observed because the used filter of the laser light was cutting the wavelength less than 400 nm. PL spectra of ZnO films on Si deposited at different substrate temperatures are shown in Fig. 2共a兲. The film Z0 deposited at RT shows yellow emission centered around 570 nm. It is attributed to the transition between conduction band to defects states or defects states to valence band. Therefore, the intensity variation of the defect related emission may result from the variation of the intrinsic defects in ZnO film, such as zinc vacancy VZn, oxygen vacancy VO, interstitial zinc Zni, interstitial oxygen Oi, and antisite oxygen OZn. This may be due to the presence of Zn interstitial and oxygen vacancies in the film. On increasing the substrate temperature, the intensity of the PL emission increases with redshift centered at 615 nm. This red PL is related with band emission from deep interband levels, which are identified as oxygen vacancies or interstitial zinc. Since oxygen vacancies have lower formation energy than the zinc interstitial, they should be more abundant. The redshift and increase in the intensity of the photoluminescence are likely to be due to the desorption of oxygen from the film surface at higher substrate temperature. It causes the formation of oxygen vacancies and removal of other defects states such as antisite Zn and oxygen in the film. Figure 2共b兲 shows the PL spectra of as-deposited and annealed ZnO films on Si. In as-deposited

FIG. 8. Variation of resistivity of ZnO thin films with substrate and annealing temperatures.

case, a broad peak at 575 nm is observed. No significant change in the peak intensity is observed after annealing at 450 ° C but peak position shifts at 552 nm. When film is annealed at 600 and 800 ° C, the intensity of the peak increases and shifts to 552 nm. Since the concentration of oxygen vacancies and zinc interstitial ought to decrease with the annealing of the film in oxygen ambient, the oxygen interstitial 共Oi兲 and antisite oxygen OZn can be easily formed. Hence the enhancement in the emission intensity is due to the increase of antisite oxygen 共OZn兲 and oxygen interstitial 共Oi兲.

G. Resistivity measurement

ZnO films deposited on any substrate exhibited n-type conductivity,37 in which the electrical conductivity is due to the excess zinc presumably located interstitially within the lattice and oxygen vacancies. Electrical resistivity of all films was measured using the Vander-Pauw four-probe method at room temperature. Various deposition conditions resulted in dramatic changes in film resistivity. The variation in resistivity with substrate temperature and annealing temperature is shown in Fig. 8. The resistivity of the film strongly depends upon the substrate temperature as well as annealing temperature. Resistivity of the film decreases with substrate temperature and increases with annealing temperature. The decrease in resistivity with increasing the substrate temperature can be attributed to the desorption of oxygen from the surface at higher substrate temperature and thus increase in the concentration of the oxygen vacancies. The increase in resistivity on annealing the film in oxygen ambient can be attributed to the absorption of oxygen and thus reducing the concentration of oxygen vacancies and interstitial zinc, which reduced the concentration of free carrier produced by zinc interstitial and oxygen vacancies from the film.


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IV. CONCLUSION

Undoped conducting ZnO thin films have been deposited by e-beam evaporation at different deposition conditions. The results of absorption spectroscopy, photoluminescence, and FTIR characterization show that the optical properties of the ZnO films can be controlled by increasing the substrate temperature or postdeposition annealing temperature. The band gap of the films can be engineered from 1.51 to 3.26 eV by annealing temperature. It shows higher transparency in the visible range and band gap close to the expected value. In the case of thermally evaporated film, it has been observed that substrate temperature has a strong effect on the optical as well as electrical properties. Optical and electrical properties are also improved by annealing of the film in oxygen ambient after the deposition. The film conductivity increases with increasing the postannealing temperature, whereas it decreases with increasing the substrate temperature. The structural properties improve with substrate temperature and annealing temperature. We were able to deposit the highly transparent and conducting film of ZnO, oriented along the c axis. ACKNOWLEDGMENTS

One of the authors 共D.C.A.兲 is thankful to IUAC, New Delhi for providing the financial and program support under the UFUP project for this work and would like to express his deepest appreciation to S. R. Abhilash, Target Laboratory, IUAC, New Delhi for the help during the deposition of ZnO thin films. Another author 共D.K.A.兲 likes to give sincere thanks to the Department of Science and Technology for providing the XRD system at IUAC, New Delhi, under the IRHPA project to boost the research activities. Z. W. Pang, Z. R. Dai, and Z. L. Wang, Science 291, 1947 共2001兲. H. Rensmo, K. K. Lindstrom, L. N. Wang, and M. Muhammed, J. Phys. Chem. B 101, 2598 共1997兲. 3 M. H. Huang, Y. Y. Wu, H. Feick, N. Tran, E. Weber, and P. D. Yang, Science 292, 1897 共2001兲. 4 H. J. Muhr, F. Krumeich, U. P. Schonholzer, F. Bieri, M. Niederberger, L. J. Gaucklerand, and R. Nesper, Adv. Mater. 共Weinheim, Ger.兲 12, 231 共2000兲. 5 C. H. Liu, W. C. Yiu, F. C. K. Au, J. K. Ding, C. S. Lee, and S. T. Lee, Appl. Phys. Lett. 83, 3168 共2003兲. 1 2

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