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Properties of ZnO Thin Films Grown on Si (100) Substrates by Pulsed Laser Deposition
J. Mater. Sci. Technol., 2010, 26(11), 973-976.
Properties of ZnO Thin Films Grown on Si (100) Substrates by Pulsed Laser Deposition Young Rae Jang1) , Keon-Ho Yoo1)† and Seung Min Park2) 1) Department of Physics, Kyung Hee University, Seoul 130-701, Korea 2) Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea [Manuscript received September 17, 2009, in revised form February 11, 2010]
ZnO thin films were grown on Si (100) substrates by pulsed laser deposition using a ZnO target. The substrate temperature was varied in the range of room temperature to 800◦ C, and the oxygen partial pressure of 0.1333 Pa (1 mTorr) to 1333 Pa (10 Torr). The properties of the resulting films were investigated by photoluminescence (PL), grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM). Based on the ultraviolet (UV, ∼380 nm) to visible emission ratio in the PL spectrum, the optimum growth conditions were determined to be 600◦ C and 133.3 Pa (1 Torr), respectively. The oxygen 1s peak in the XPS spectrum was decomposed into two peaks. The peak at lower binding energy increased in intensity with the oxygen partial pressure from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr) while the other peak decreased. The GIXRD curve at the optimum condition showed strong two peaks (002) and (103). A strong correlation between the (103) peak intensity and the UV emission intensity was found. KEY WORDS: ZnO; Pulsed laser deposition; Photoluminescence
1. Introduction In recent years, ZnO has received enormous interest, because it has various superior properties applicable to device, such as wide direct band gap of 3.3 eV at room temperature (RT), large exciton binding energy of about 60 meV, high optical transparency in the visible range, good electrical conductivity, excellent piezoelectricity, and so on[1–3] . Thus it has been widely applied to transparent electrode[4] , optical wave guide[5] , solar cell[6] , gas sensor[7] , light emitting diode[8] , surface acoustic wave device[9] , etc. ZnO thin films have been grown by various techniques such as RF sputtering[10] , chemical vapor deposition[11] , molecular beam epitaxy[12] , and pulsed laser deposition (PLD)[13–17] . Compared with other techniques, PLD has an advantage of a convenient control of the stoichiometry of the film just by varying the composition of the target or the ambient † Corresponding author. Prof., Ph.D.; Tel.: +82 2 961 0213; Fax: +82 2 957 8408; E-mail address: [email protected] (K.H. Yoo).
gas pressure[18] . The photoluminescence (PL) spectrum of ZnO usually consists of two main peaks, near-band-edge ultraviolet (UV) emission (∼380 nm) and deep-level visible emission[19–24] . As for the two peaks of PL spectrum of ZnO, the UV emission is from the crystalline ZnO and the visible emission is from defects. Therefore the ratio of the UV emission to the visible emission can be a good standard for the quality of the film, especially for the optoelectronic applications. In this paper, the ratio of the UV emission to the visible emission is used as standard for high quality of ZnO thin films. The origin of the visible emission is still debatable; some authors attribute it to zinc interstitials[19,20] while others attribute it to oxygen vacancies[21–23] , or to strain-related structural defects[24] . ZnO thin films have been grown on Al2 O3 substrate due to its hexagonal symmetry[25] . Si is, however, at present the prime material in the semiconductor industry with mature process technology. It
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Fig. 1 PL spectra of the ZnO thin films grown at different substrate temperatures
certainly has a lot of advantages if high quality ZnO thin films could be grown on a bare Si substrate. In this study, ZnO thin films were grown on Si (100) substrates by PLD technique in a broad range of the substrate temperature and the oxygen partial pressure. The properties of the resulting films were investigated by using PL, grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM). The optimum growth conditions were determined based on the UV to visible emission ratio in PL spectra. 2. Experimental ZnO thin films were prepared by PLD on Si (100) substrates using a commercially available ZnO target (2.54 cm diameter, 99.995% purity). At 133.3 Pa (1 Torr) oxygen partial pressure, the substrate temperature was varied from RT to 800◦ C. The targetto-substrate distance was 23 mm. A customized PLD chamber was pumped down using a turbo pump, and the base pressure was 1.332×10−5 Pa (1×10−7 Torr). Oxygen was used as the ambient gas, and its pressure was varied from 0.1333 Pa (1 mTorr) to 1333 Pa (10 Torr) at substrate temperature of 600◦ C. The pulsed laser was a frequency tripled Nd:YAG laser (λ=355 nm) with 10 Hz repetition rate and the energy density of 1 J/cm2 . The laser beam was line-focused by using a cylindrical lens and the target was rotated with a speed of 18 r/min to avoid repeated ablation from the same spot. PL was measured at RT by using a continuous wave He-Cd laser (λ=325 nm) as the excitation source. The power density was about 0.5 W/cm2 . In each measurement a ZnO reference sample was measured, and the PL intensity of ZnO thin films was calibrated by the reference sample intensity to compensate the run-to-run variation of signal intensity even though such variation was very rare. XPS (Sigma Drobe of Thenmo VG) was carried out for the chemical structure analysis of the films.
Fig. 2 GIXRD curves of the ZnO thin films grown at different substrate temperatures
The surface of the film was etched with on argon ion gun for 1 min. The XPS spectrum was fitted by Gaussian curve and the binding energy was calibrated by using the carbon 1s peak (284.6 eV) as a reference. The morphological and structural properties of the ZnO thin films were investigated by FESEM (S-4300 of Hitachi) and GIXRD (X Pert PRO of PANalytical), respectively. The angle of incidence in GIXRD was fixed at 2 deg. 3. Results and Discussion Figure 1 shows the PL spectra of the ZnO thin films grown at different substrate temperatures from RT to 800◦ C. At the temperatures of RT and 200◦ C, both UV and visible emission are very small. At 400 and 800◦ C, the visible PL peak is dominant, while at 600◦ C an intense UV peak is observed with little visible luminescence. Based on the UV to visible emission ratio, as calculated by the integrated intensities of the two peaks, it is concluded that 600◦ C is the optimum substrate temperature. Figure 2 shows the GIXRD curves of the same samples as shown in Fig. 1. All the peaks are of wurtzite structure ZnO (Joint Committee for Powder Diffraction Studies (JCPDS), No. 36-1451), and a dominant (002) peak appears in all samples, which indicates the c-axis preferential growth of ZnO. The (002) peak is the strongest at the film grown at 600◦ C, which has the largest UV to visible emission ratio, showing that this film has the best crystallinity. At 600◦ C, the (103) peak has the second highest intensity. It is noteworthy that the ZnO film with excellent UV to visible emission ratio in PL exhibits not only the (002) peak but also other peaks such as (103) peak
Y.R. Jang et al.: J. Mater. Sci. Technol., 2010, 26(11), 973–976
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Fig. 4 FESEM images of the ZnO thin films grown at oxygen partial pressures of (a) 0.1333 Pa (1 mTorr) and (b) 133.3 Pa (1 Torr)
Fig. 3 (a) Oxygen 1s peaks of the XPS spectra of the ZnO thin films grown at different oxygen partial pressures, and (b) their Gaussian fitting
in GIXRD. Figure 3 shows the XPS spectra of the ZnO thin films grown at different oxygen partial pressures from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr). Each of the oxygen 1s peaks in Fig. 3(a) could be fitted by two Gaussian peaks, P1 and P2 as shown in Fig. 3(b). The P1 peak taking the major portion of the spectrum could be attributed to the binding energy forming wurtzite structure ZnO, and the P2 peak to the binding energy forming defective structure due to lack of oxygen[26] . Even though there is a slight shift of the position of P1, we focus on the intensity change; the P1 intensity increases as the P2 intensity decreases from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr) as shown in the inset of Fig. 3(b). As oxygen partial pressure increases, collisions between the fluxes ablated from the target and oxygen molecules in the ambient gas will be enhanced[27] . The crystalline grain gets bigger by this collision process before the flux is deposited on the surface. The film morphologies as observed in FESEM images at 0.1333 Pa (1 mTorr) and 133.3 Pa (1 Torr) are shown in Fig. 4(a) and 4(b), respectively. The grains at 133.3 Pa (1 Torr) are clearly seen and are larger in size than those at 0.1333 Pa (1 mTorr). The oxygen partial pressure dependence of the PL spectra is shown in Fig. 5. The intensity of the UV emission is very small at 0.1333 Pa (1 mTorr), slightly increases at 1.333 Pa (10 mTorr) and 13.33 Pa
Fig. 5 PL spectra of the ZnO thin films grown at different oxygen partial pressures
(100 mTorr), reaches the maximum at 133.3 Pa (1 Torr), and decreases at 1333 Pa (10 Torr). Based on this result, the optimum oxygen pressure is determined to be 133.3 Pa (1 Torr). This is consistent with the XPS and FESEM analyses, which show that the crystallinity is the best at oxygen partial pressure of 133.3 Pa (1 Torr). The GIXRD curves at oxygen partial pressures in the range of 1.333 Pa (0.01 Torr) to 1333 Pa (10 Torr) are shown in Fig. 6. The (002) and (103) peaks are clearly shown at and above 13.33 Pa (100 mTorr), but other small (101), (102), (112), and (004) peaks are shown only in the 133.3 Pa (1 Torr) sample. Since high quality ZnO thin films are usually believed to have a single peak (002)[28] , the presence of multi peaks at the optimum oxygen pressure of 133.3 Pa (1 Torr) is interesting. It is also interesting to note that the intensity of the (103) peak shown in the inset
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Fig. 6 GIXRD curves of the ZnO thin films grown at different oxygen partial pressures. The (103) peaks were magnified in the inset
of Fig. 6, and the UV PL peak in Fig. 5 show almost the same oxygen partial pressure dependence; the intensity increased as the oxygen pressure increased up to 133.3 Pa (1 Torr), and then decreased at 1333 Pa (10 Torr). This correlation between the intensity of the UV PL peak and that of the (103) XRD peak needs further investigation. The small (101), (102), (112), and (004) peaks appearing at 133.3 Pa (1 Torr) are not seen at 1333 Pa (10 Torr), while the dominant (002) peak increases monotonously up to 1333 Pa (10 Torr). This may indicate that the increase of oxygen partial pressure enhances the c-axis growth, but the crystallinity or the stoichiometry of the film becomes poor if the oxygen is oversupplied as in the 1333 Pa (10 Torr) sample. 4. Conclusion We reported the properties of the ZnO thin films grown on Si (100) substrate by PLD. Two major growth parameters, the substrate temperature and the ambient gas pressure, were varied and the resulting films were characterized by PL, XPS, FESEM, and GIXRD. Based on the UV to visible emission ratio in PL, the optimum substrate temperature and oxygen partial pressure were determined to be 600◦ C and 133.3 Pa (1 Torr), respectively. The oxygen 1s peak of XPS spectrum was decomposed into two Gaussian peaks. The peak positioned at lower binding energy increased in intensity as the oxygen partial pressure increased from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr) while the other peak decreased. The FESEM images showed that the grains are clearly seen at 133.3 Pa (1 Torr) sample and are much larger in size than those at 0.1333 Pa (1 mTorr). The GIXRD curve of the optimum sample grown at 133.3 Pa (1 Torr) showed strong (002) and (103) peaks as well as small (101), (102), (112) and (004) peaks. Interestingly, it was found that there is a strong correlation between the (103) peak intensity and the UV emission intensity. REFERENCES [1 ] H.Y. Xu, Y.C. Liu, Y.X. Liu, C.S. Xu, C.L. Shao and
R. Mu: Appl. Phys. B, 2005, 80, 871. [2 ] V. Vaithianathan, Y.H. Lee, B.T. Lee, S. Hishita and S.S. Kim: J. Cryst. Growth, 2006, 287, 85. [3 ] L. Chen, Z.Q. Chen, X.Z. Shang, C. Liu, S. Xu and Q. Fu: Solid State Commun., 2006, 137, 561. [4 ] F. Michelotti, A. Belardini, M.C. Larciprete, M. Bertolotti, A. Rousseau, A. Ratsimihety, G. Schoer and J. Mueller: Appl. Phys. Lett., 2003, 83, 4477. [5 ] E.J. Ibanga, C. Le Luyer and J. Mugnier: Mater. Chem. Phys., 2003, 80, 490. [6 ] K. Matsubara, P. Fons, K. Iwata, A. Yamada, K. Sakurai, H. Tampo and S. Niki: Thin Solid Films, 2003, 431-432, 369. [7 ] H. Nanto, T. Minami and S. Takata: J. Appl. Phys., 1986, 60, 482. [8 ] H. Kim, J.S. Horwitz, W.H. Kim, S.B. Qadri and Z.H. Kafafi: Appl. Phys. Lett., 2003, 83, 3809. [9 ] S.Y. Chu, T.Y. Chen and W. Water: J. Vac. Sci. Technol. A, 2004, 22, 1087. [10] D.K. Hwang, H.S. Kim, J.H. Lim, J.Y. Oh, J.H. Yang, S.J. Park, K.K. Kim, D.C. Look and Y.S. Park: Appl. Phys. Lett., 2005, 86, 151917. [11] Y.I. Alivov, E.V. Kalinina, A.E. Cherenkov, D.C. Look, B.M. Ataev, A.K. Omaev, M.V. Chukichev and D.M. Bagnall: Appl. Phys. Lett., 2003, 83, 4719. [12] Y.M. Lu, H.W. Liang, D.Z. Shen, Z.Z. Zhang, J.Y. Zhang, D.X. Zhao, Y.C. Liu and X.W. Fan: J. Lumin., 2006, 119-120, 228. [13] Z.Q. Chen, S. Yamamoto, A. Kawasuso, Y. Xu and T. Sekiguchi: Appl. Surf. Sci., 2005, 244, 377. [15] M. Zerdali, S. Hamzaoui, F.H. Teherani and D. Rogers: Mater. Lett., 2006, 60, 504. [16] D.J. Rogers, F.H. Teherani, T. Monteiro, M. Soares, A. Neves, M. Carmo, S. Pereira, M.R. Correia, A. Lusson, E. Alves, N.P. Barradas, J.K. Morrod, K.A. Prior, P. Kung, A. Yasan and M. Razeghi: Phys. Status Solidi (c), 2006, 3, 1038. [17] N. Gopalakrishnan, B.C. Shin, H.S. Lim, G.Y. Kim and Y.S. Yu: Physica B, 2006, 376-377, 756. [18] J.S. Ha, C.H. Bae, S.H. Nam, S.M. Park, Y.R. Jang, K.H. Yoo and K. Park: Appl. Phys. Lett., 2003, 82, 3436. [19] E.G. Bylander: J. Appl. Phys., 1978, 49, 1188. [20] M. Liu, A.H. Kitai and P. Mascher: J. Lumin., 1992, 54, 35. [21] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt and B.E. Gnade: J. Appl. Phys., 1996, 79, 7983. [22] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim and S.Y. Lee: Thin Solid Films, 2003, 443, 5. [23] Q.P. Wang, D.H. Zhang, Z.Y. Xue and X.J. Zhang: Opt. Mater., 2004, 26, 23. [24] Y.R. Jang, K.H. Yoo and S.M. Park: J. Korean Phys. Soc., 2008, 53, 110. [25] C. Liu, S.H. Chang, T.W. Noh, J.H. Song and J. Xie: Phys. Status Solidi (b), 2007, 244, 1528. [26] P.F. Zhang, X.L. Liu, H.Y. Wei, H.B. Fan, Z.M. Liang, P. Jin, S.Y. Yang, C.M. Jiao, Q.S. Zhu and Z.G. Wang: J. Phys. D: Appl. Phys., 2007, 40, 6010. [27] T. Okada, B.H. Agung and Y. Nakata: Appl. Phys. A, 2004, 79, 1417. [28] A.K. Pradhan, L. Douglas, H. Mustafa, R. Mundle, D. Hunter and C.E. Bonner: Appl. Phys. Lett., 2007, 90, 072108.
Properties of ZnO Thin Films Grown on Si (100) Substrates by Pulsed Laser Deposition Young Rae Jang1) , Keon-Ho Yoo1)† and Seung Min Park2) 1) Department of Physics, Kyung Hee University, Seoul 130-701, Korea 2) Department of Chemistry, Kyung Hee University, Seoul 130-701, Korea [Manuscript received September 17, 2009, in revised form February 11, 2010]
ZnO thin films were grown on Si (100) substrates by pulsed laser deposition using a ZnO target. The substrate temperature was varied in the range of room temperature to 800◦ C, and the oxygen partial pressure of 0.1333 Pa (1 mTorr) to 1333 Pa (10 Torr). The properties of the resulting films were investigated by photoluminescence (PL), grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM). Based on the ultraviolet (UV, ∼380 nm) to visible emission ratio in the PL spectrum, the optimum growth conditions were determined to be 600◦ C and 133.3 Pa (1 Torr), respectively. The oxygen 1s peak in the XPS spectrum was decomposed into two peaks. The peak at lower binding energy increased in intensity with the oxygen partial pressure from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr) while the other peak decreased. The GIXRD curve at the optimum condition showed strong two peaks (002) and (103). A strong correlation between the (103) peak intensity and the UV emission intensity was found. KEY WORDS: ZnO; Pulsed laser deposition; Photoluminescence
1. Introduction In recent years, ZnO has received enormous interest, because it has various superior properties applicable to device, such as wide direct band gap of 3.3 eV at room temperature (RT), large exciton binding energy of about 60 meV, high optical transparency in the visible range, good electrical conductivity, excellent piezoelectricity, and so on[1–3] . Thus it has been widely applied to transparent electrode[4] , optical wave guide[5] , solar cell[6] , gas sensor[7] , light emitting diode[8] , surface acoustic wave device[9] , etc. ZnO thin films have been grown by various techniques such as RF sputtering[10] , chemical vapor deposition[11] , molecular beam epitaxy[12] , and pulsed laser deposition (PLD)[13–17] . Compared with other techniques, PLD has an advantage of a convenient control of the stoichiometry of the film just by varying the composition of the target or the ambient † Corresponding author. Prof., Ph.D.; Tel.: +82 2 961 0213; Fax: +82 2 957 8408; E-mail address: [email protected] (K.H. Yoo).
gas pressure[18] . The photoluminescence (PL) spectrum of ZnO usually consists of two main peaks, near-band-edge ultraviolet (UV) emission (∼380 nm) and deep-level visible emission[19–24] . As for the two peaks of PL spectrum of ZnO, the UV emission is from the crystalline ZnO and the visible emission is from defects. Therefore the ratio of the UV emission to the visible emission can be a good standard for the quality of the film, especially for the optoelectronic applications. In this paper, the ratio of the UV emission to the visible emission is used as standard for high quality of ZnO thin films. The origin of the visible emission is still debatable; some authors attribute it to zinc interstitials[19,20] while others attribute it to oxygen vacancies[21–23] , or to strain-related structural defects[24] . ZnO thin films have been grown on Al2 O3 substrate due to its hexagonal symmetry[25] . Si is, however, at present the prime material in the semiconductor industry with mature process technology. It
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Y.R. Jang et al.: J. Mater. Sci. Technol., 2010, 26(11), 973–976
Fig. 1 PL spectra of the ZnO thin films grown at different substrate temperatures
certainly has a lot of advantages if high quality ZnO thin films could be grown on a bare Si substrate. In this study, ZnO thin films were grown on Si (100) substrates by PLD technique in a broad range of the substrate temperature and the oxygen partial pressure. The properties of the resulting films were investigated by using PL, grazing incidence X-ray diffraction (GIXRD), X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FESEM). The optimum growth conditions were determined based on the UV to visible emission ratio in PL spectra. 2. Experimental ZnO thin films were prepared by PLD on Si (100) substrates using a commercially available ZnO target (2.54 cm diameter, 99.995% purity). At 133.3 Pa (1 Torr) oxygen partial pressure, the substrate temperature was varied from RT to 800◦ C. The targetto-substrate distance was 23 mm. A customized PLD chamber was pumped down using a turbo pump, and the base pressure was 1.332×10−5 Pa (1×10−7 Torr). Oxygen was used as the ambient gas, and its pressure was varied from 0.1333 Pa (1 mTorr) to 1333 Pa (10 Torr) at substrate temperature of 600◦ C. The pulsed laser was a frequency tripled Nd:YAG laser (λ=355 nm) with 10 Hz repetition rate and the energy density of 1 J/cm2 . The laser beam was line-focused by using a cylindrical lens and the target was rotated with a speed of 18 r/min to avoid repeated ablation from the same spot. PL was measured at RT by using a continuous wave He-Cd laser (λ=325 nm) as the excitation source. The power density was about 0.5 W/cm2 . In each measurement a ZnO reference sample was measured, and the PL intensity of ZnO thin films was calibrated by the reference sample intensity to compensate the run-to-run variation of signal intensity even though such variation was very rare. XPS (Sigma Drobe of Thenmo VG) was carried out for the chemical structure analysis of the films.
Fig. 2 GIXRD curves of the ZnO thin films grown at different substrate temperatures
The surface of the film was etched with on argon ion gun for 1 min. The XPS spectrum was fitted by Gaussian curve and the binding energy was calibrated by using the carbon 1s peak (284.6 eV) as a reference. The morphological and structural properties of the ZnO thin films were investigated by FESEM (S-4300 of Hitachi) and GIXRD (X Pert PRO of PANalytical), respectively. The angle of incidence in GIXRD was fixed at 2 deg. 3. Results and Discussion Figure 1 shows the PL spectra of the ZnO thin films grown at different substrate temperatures from RT to 800◦ C. At the temperatures of RT and 200◦ C, both UV and visible emission are very small. At 400 and 800◦ C, the visible PL peak is dominant, while at 600◦ C an intense UV peak is observed with little visible luminescence. Based on the UV to visible emission ratio, as calculated by the integrated intensities of the two peaks, it is concluded that 600◦ C is the optimum substrate temperature. Figure 2 shows the GIXRD curves of the same samples as shown in Fig. 1. All the peaks are of wurtzite structure ZnO (Joint Committee for Powder Diffraction Studies (JCPDS), No. 36-1451), and a dominant (002) peak appears in all samples, which indicates the c-axis preferential growth of ZnO. The (002) peak is the strongest at the film grown at 600◦ C, which has the largest UV to visible emission ratio, showing that this film has the best crystallinity. At 600◦ C, the (103) peak has the second highest intensity. It is noteworthy that the ZnO film with excellent UV to visible emission ratio in PL exhibits not only the (002) peak but also other peaks such as (103) peak
Y.R. Jang et al.: J. Mater. Sci. Technol., 2010, 26(11), 973–976
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Fig. 4 FESEM images of the ZnO thin films grown at oxygen partial pressures of (a) 0.1333 Pa (1 mTorr) and (b) 133.3 Pa (1 Torr)
Fig. 3 (a) Oxygen 1s peaks of the XPS spectra of the ZnO thin films grown at different oxygen partial pressures, and (b) their Gaussian fitting
in GIXRD. Figure 3 shows the XPS spectra of the ZnO thin films grown at different oxygen partial pressures from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr). Each of the oxygen 1s peaks in Fig. 3(a) could be fitted by two Gaussian peaks, P1 and P2 as shown in Fig. 3(b). The P1 peak taking the major portion of the spectrum could be attributed to the binding energy forming wurtzite structure ZnO, and the P2 peak to the binding energy forming defective structure due to lack of oxygen[26] . Even though there is a slight shift of the position of P1, we focus on the intensity change; the P1 intensity increases as the P2 intensity decreases from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr) as shown in the inset of Fig. 3(b). As oxygen partial pressure increases, collisions between the fluxes ablated from the target and oxygen molecules in the ambient gas will be enhanced[27] . The crystalline grain gets bigger by this collision process before the flux is deposited on the surface. The film morphologies as observed in FESEM images at 0.1333 Pa (1 mTorr) and 133.3 Pa (1 Torr) are shown in Fig. 4(a) and 4(b), respectively. The grains at 133.3 Pa (1 Torr) are clearly seen and are larger in size than those at 0.1333 Pa (1 mTorr). The oxygen partial pressure dependence of the PL spectra is shown in Fig. 5. The intensity of the UV emission is very small at 0.1333 Pa (1 mTorr), slightly increases at 1.333 Pa (10 mTorr) and 13.33 Pa
Fig. 5 PL spectra of the ZnO thin films grown at different oxygen partial pressures
(100 mTorr), reaches the maximum at 133.3 Pa (1 Torr), and decreases at 1333 Pa (10 Torr). Based on this result, the optimum oxygen pressure is determined to be 133.3 Pa (1 Torr). This is consistent with the XPS and FESEM analyses, which show that the crystallinity is the best at oxygen partial pressure of 133.3 Pa (1 Torr). The GIXRD curves at oxygen partial pressures in the range of 1.333 Pa (0.01 Torr) to 1333 Pa (10 Torr) are shown in Fig. 6. The (002) and (103) peaks are clearly shown at and above 13.33 Pa (100 mTorr), but other small (101), (102), (112), and (004) peaks are shown only in the 133.3 Pa (1 Torr) sample. Since high quality ZnO thin films are usually believed to have a single peak (002)[28] , the presence of multi peaks at the optimum oxygen pressure of 133.3 Pa (1 Torr) is interesting. It is also interesting to note that the intensity of the (103) peak shown in the inset
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Fig. 6 GIXRD curves of the ZnO thin films grown at different oxygen partial pressures. The (103) peaks were magnified in the inset
of Fig. 6, and the UV PL peak in Fig. 5 show almost the same oxygen partial pressure dependence; the intensity increased as the oxygen pressure increased up to 133.3 Pa (1 Torr), and then decreased at 1333 Pa (10 Torr). This correlation between the intensity of the UV PL peak and that of the (103) XRD peak needs further investigation. The small (101), (102), (112), and (004) peaks appearing at 133.3 Pa (1 Torr) are not seen at 1333 Pa (10 Torr), while the dominant (002) peak increases monotonously up to 1333 Pa (10 Torr). This may indicate that the increase of oxygen partial pressure enhances the c-axis growth, but the crystallinity or the stoichiometry of the film becomes poor if the oxygen is oversupplied as in the 1333 Pa (10 Torr) sample. 4. Conclusion We reported the properties of the ZnO thin films grown on Si (100) substrate by PLD. Two major growth parameters, the substrate temperature and the ambient gas pressure, were varied and the resulting films were characterized by PL, XPS, FESEM, and GIXRD. Based on the UV to visible emission ratio in PL, the optimum substrate temperature and oxygen partial pressure were determined to be 600◦ C and 133.3 Pa (1 Torr), respectively. The oxygen 1s peak of XPS spectrum was decomposed into two Gaussian peaks. The peak positioned at lower binding energy increased in intensity as the oxygen partial pressure increased from 0.1333 Pa (1 mTorr) to 133.3 Pa (1 Torr) while the other peak decreased. The FESEM images showed that the grains are clearly seen at 133.3 Pa (1 Torr) sample and are much larger in size than those at 0.1333 Pa (1 mTorr). The GIXRD curve of the optimum sample grown at 133.3 Pa (1 Torr) showed strong (002) and (103) peaks as well as small (101), (102), (112) and (004) peaks. Interestingly, it was found that there is a strong correlation between the (103) peak intensity and the UV emission intensity. REFERENCES [1 ] H.Y. Xu, Y.C. Liu, Y.X. Liu, C.S. Xu, C.L. Shao and
R. Mu: Appl. Phys. B, 2005, 80, 871. [2 ] V. Vaithianathan, Y.H. Lee, B.T. Lee, S. Hishita and S.S. Kim: J. Cryst. Growth, 2006, 287, 85. [3 ] L. Chen, Z.Q. Chen, X.Z. Shang, C. Liu, S. Xu and Q. Fu: Solid State Commun., 2006, 137, 561. [4 ] F. Michelotti, A. Belardini, M.C. Larciprete, M. Bertolotti, A. Rousseau, A. Ratsimihety, G. Schoer and J. Mueller: Appl. Phys. Lett., 2003, 83, 4477. [5 ] E.J. Ibanga, C. Le Luyer and J. Mugnier: Mater. Chem. Phys., 2003, 80, 490. [6 ] K. Matsubara, P. Fons, K. Iwata, A. Yamada, K. Sakurai, H. Tampo and S. Niki: Thin Solid Films, 2003, 431-432, 369. [7 ] H. Nanto, T. Minami and S. Takata: J. Appl. Phys., 1986, 60, 482. [8 ] H. Kim, J.S. Horwitz, W.H. Kim, S.B. Qadri and Z.H. Kafafi: Appl. Phys. Lett., 2003, 83, 3809. [9 ] S.Y. Chu, T.Y. Chen and W. Water: J. Vac. Sci. Technol. A, 2004, 22, 1087. [10] D.K. Hwang, H.S. Kim, J.H. Lim, J.Y. Oh, J.H. Yang, S.J. Park, K.K. Kim, D.C. Look and Y.S. Park: Appl. Phys. Lett., 2005, 86, 151917. [11] Y.I. Alivov, E.V. Kalinina, A.E. Cherenkov, D.C. Look, B.M. Ataev, A.K. Omaev, M.V. Chukichev and D.M. Bagnall: Appl. Phys. Lett., 2003, 83, 4719. [12] Y.M. Lu, H.W. Liang, D.Z. Shen, Z.Z. Zhang, J.Y. Zhang, D.X. Zhao, Y.C. Liu and X.W. Fan: J. Lumin., 2006, 119-120, 228. [13] Z.Q. Chen, S. Yamamoto, A. Kawasuso, Y. Xu and T. Sekiguchi: Appl. Surf. Sci., 2005, 244, 377. [15] M. Zerdali, S. Hamzaoui, F.H. Teherani and D. Rogers: Mater. Lett., 2006, 60, 504. [16] D.J. Rogers, F.H. Teherani, T. Monteiro, M. Soares, A. Neves, M. Carmo, S. Pereira, M.R. Correia, A. Lusson, E. Alves, N.P. Barradas, J.K. Morrod, K.A. Prior, P. Kung, A. Yasan and M. Razeghi: Phys. Status Solidi (c), 2006, 3, 1038. [17] N. Gopalakrishnan, B.C. Shin, H.S. Lim, G.Y. Kim and Y.S. Yu: Physica B, 2006, 376-377, 756. [18] J.S. Ha, C.H. Bae, S.H. Nam, S.M. Park, Y.R. Jang, K.H. Yoo and K. Park: Appl. Phys. Lett., 2003, 82, 3436. [19] E.G. Bylander: J. Appl. Phys., 1978, 49, 1188. [20] M. Liu, A.H. Kitai and P. Mascher: J. Lumin., 1992, 54, 35. [21] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt and B.E. Gnade: J. Appl. Phys., 1996, 79, 7983. [22] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim and S.Y. Lee: Thin Solid Films, 2003, 443, 5. [23] Q.P. Wang, D.H. Zhang, Z.Y. Xue and X.J. Zhang: Opt. Mater., 2004, 26, 23. [24] Y.R. Jang, K.H. Yoo and S.M. Park: J. Korean Phys. Soc., 2008, 53, 110. [25] C. Liu, S.H. Chang, T.W. Noh, J.H. Song and J. Xie: Phys. Status Solidi (b), 2007, 244, 1528. [26] P.F. Zhang, X.L. Liu, H.Y. Wei, H.B. Fan, Z.M. Liang, P. Jin, S.Y. Yang, C.M. Jiao, Q.S. Zhu and Z.G. Wang: J. Phys. D: Appl. Phys., 2007, 40, 6010. [27] T. Okada, B.H. Agung and Y. Nakata: Appl. Phys. A, 2004, 79, 1417. [28] A.K. Pradhan, L. Douglas, H. Mustafa, R. Mundle, D. Hunter and C.E. Bonner: Appl. Phys. Lett., 2007, 90, 072108.