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Growth and characterization of ZnO thin films grown by pulsed laser deposition Sang Hyuck Baea, Sang Yeol Leea,*, Beom Jun Jinb, Seongil Imc aDepartment of Electrical and Computer Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea bDepartment of Metallurgical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea cDepartment of Physics, Yonsei University, 134 Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea Received 3 August 1999; accepted 9 November 1999 Abstract ZnO thin films on (0 0 1) sapphire substrates have been deposited by pulsed laser deposition technique using a Nd:YAG laser with the wavelength of 355 nm at an oxygen pressure of 350 mTorr. In order to investigate the effect of the deposition conditions on the properties of ZnO thin films, the experiment has been performed at various substrate temperatures in the range of 200–7008C. According to XRD, (0 0 2) textured ZnO films of high crystalline quality have been obtained by pulsed laser deposition technique. However, the intensity of UV emission is mostly dependent upon the stoichiometry of ZnO films, rather than the crystalline quality. # 2001 Elsevier Science B.V. All rights reserved. Keywords: ZnO; PLD; XRD; UV emission; PL; Hall measurement 1. Introduction ZnO is a wide-bandgap II–VI semiconductor and might be a good candidate for an electroluminescence device like a UV laser diode. ZnO films also have potential applications for surface acoustic wave devices (SAW) [1–3] and low loss optical wave-guides [4]. There are many reports concerning the growth of ZnO by several techniques, such as molecular beam epitaxy (MBE), radio frequency (rf) sputtering and chemical vapor deposition (CVD) [5–7]. In present study, pulsed laser deposition (PLD) technique was used to grow ZnO thin films. The PLD has advantages of being able to employ a relatively high oxygen partial pressure and also to achieve high-quality crystalline films with relatively high deposition rate at low temperatures because the high energy (�100 eV) of the ablated particles is obtained in the laser produced plume [8]. According to X-ray diffraction (XRD) analysis, all the films deposited in the work showed c-axis oriented textures and the ZnO films are almost epitaxially grown. In this work, we have tried to investigate the effect of substrate temperature in PLD on the structural and optical- emission properties of ZnO films and optimize the growth temperature for high-quality ZnO films at a fixed oxygen pressure. 2. Experiment The PLD technique has been used for the deposition of ZnO films. The chamber was evacuated by a turbomolecular pump to a base pressure of 1� 10ÿ6 Torr. For the epitaxial growth of ZnO thin films, Applied Surface Science 169–170 (2001) 525–528 * Corresponding author. Tel.: ‡82-2-2123-2776; fax: ‡82-2-364-9770. E-mail address: sylee@yonsei.ac.kr (S.Y. Lee). 0169-4332/01/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 5 2 - 2 1 cm� 1 cm (0 0 1) a-Al2O3 substrates were used. The laser energy density was fixed at 2.5 J/cm2. We have used a ceramic ZnO target (1 in. diameter, 99.999% purity). Pulsed Nd:YAG laser at a wave- length of 355 nm and pulse duration of 6 ns was focused onto a rotating target at a repetition rate of 5 Hz. A substrate holder was placed at 50 mm from the target. The ZnO films were deposited at a fixed oxygen pressure of 350 mTorr and different substrate temperatures 200, 400, 500, 600 and 7008C. Prior to the deposition, sapphire substrates were ultrasonically degreased in acetone and methanol for 3 min. The thickness of ZnO films were measured by 2 MeV He‡ backscattering spectrometry (RBS). The optical prop- erties of the ZnO thin films were characterized by photoluminescence (PL) with an Ar ion laser as a light source using an excitation wavelength of 351 nm and a power of 100 mW. All spectra were taken at room temperature by using a grating spectrometer and a photomultiplier detector. Electrical properties were investigated by van der Pauw Hall measurements. Structural properties of the ZnO films were investi- gated by the XRD. 3. Results and discussion XRD spectra of ZnO films deposited by PLD at the substrate temperatures of 200–7008C at the fixed 350 mTorr oxygen pressure show only (0 0 2) ZnO peak and (0 0 6) substrate Al2O3 peak as shown Fig. 1. This means that (0 0 2) textured film is formed in an effective equilibrium state which gives enough surface mobility to impinge atoms at growth temperatures above 2008C. The full width at half maximum (FWHM) of 2y values reveals the crystallinity of the film [9]. The increase of the substrate temperature enhances the crystallinity of film, as indicated by the decrease of FWHM in Table 1. Large textured grains are obtained as increasing the substrate temperature. Electrical properties were measured by van der Pauw Hall measurements. The ZnO films grown under conditions of substrate temperature of 200–7008C at the fixed 350 mTorr oxygen pressure showed all n- type conduction with carrier concentration of the order of 1018–1019/cm3. The resistivity of films grown under above conditions was varied from (1.32–2:18†� 10ÿ2 Ocm. Fig. 1. XRD spectra obtained from ZnO films deposited at the substrate temperatures of (a) 2008C, (b) 4008C and (c) 7008C at a fixed oxygen pressure of 350 mTorr. Table 1 FWHM of ZnO thin films deposited at the substrate temperatures of 200–7008C at the fixed oxygen pressure of 350 mTorr Substrate temperature (8C) 200 400 500 600 700 FWHM (degree) 0.212 0.188 0.141 0.118 0.118 526 S.H. Bae et al. / Applied Surface Science 169–170 (2001) 525–528 Fig. 2 shows the PL of ZnO films grown at the substrate temperatures of (a) 2008C, (b) 4008C, (c) 5008C, (d) 6008C and (e) 7008C at a fixed oxygen pressure of 350 mTorr. UV PL characteristics of ZnO films showed strong relation to the substrate tempera- ture. At the substrate temperature lower than 2008C, Zn and O2 molecules could not have enough energy to form stoichiometric ZnO film. The ZnO films show the most intense UV luminescence as they are depos- ited at 4008C since the stoichiometry of the ZnO films has been improved as increasing the substrate tem- perature to 4008C. As substrate temperature increases above 4008C, the intensity of UV emission decreases. This result reveals that the ZnO films grown at 4008C are more stoichiometric than any other films grown above 4008C. In order to explain the present results, it may be reasonable to consider the effect of the Zn evaporation. Its melting point is only 693 K [10]. If the substrate temperature becomes too high during the PLD process, the Zn and O vacancies possibly increase due to Zn evaporation leading to inferior stoichiometric ZnO. In the present PLD work, the substrate temperature of 4008C is found to be an optimum deposition temperature to keep the best stoichiometry when the oxygen pressure is fixed at 350 mTorr. Compared to the XRD results of Fig. 1, it is also interesting to note that emitted UV intensity is not always dependent upon the textured grain size but upon the stoichiometry of ZnO. Besides the UV emission, earlier work by Stude- ninkin et al. [11] indicated that ZnO exhibited other PL bands centered around 510 and 640 nm, labeled green and yellow bands. In this work, the ZnO films exhibited similar emission behavior, show- ing UV (380 nm) and broad blue-green PL bands (480–540 nm). The green luminescence of ZnO is related to oxygen vacancies in the films according to Vanheusden et al. [12]. In our case, these defect- related luminescence shows the highest intensity from the samples deposited at such high temperature of 6008C. 4. Conclusion Highly c-axis oriented ZnO films on sapphire have been deposited at a fixed 350 mTorr oxygen pressure by the PLD technique. According to XRD result, it is found that substrate temperature plays an important role for improving crystal quality of ZnO films. The optical properties were measured by PL. The intensity of UV emission is mostly dependent upon the stoichiometry of ZnO films, rather than the textured grain size. The ZnO film grown at 4008C substrate temperature showed the highest UV PL intensity, while the most intense blue-green PL was obtained from the samples grown at a higher tem- perature of 6008C. Fig. 2. Photoluminescence spectra obtained from films deposited at the substrate temperatures of (a) 2008C, (b) 4008C, (c) 5008C, (d) 6008C and (e) 7008C at a fixed oxygen pressure of 350 mTorr. S.H. Bae et al. / Applied Surface Science 169–170 (2001) 525–528 527 Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation made in the program year of 1998. References [1] T. Mitsuyu, S. Ono, K. Wasa, J. Appl. Phys. 51 (1980) 2646. [2] Y. Ito, K. Kushida, H. Kanda, H. Takeuchi, K. Sugaware, H. Onozato, Ferroelectrics 134 (1992) 325. [3] T. Yamazaki, S. Wada, T. Noma, T. Suzuki, Sens. Actuators B 13/14 (1993) 594. [4] W.W. Wenas, A. Yamada, K. Takahashi, J. Appl. Phys. 70 (1991) 7119. [5] Y. Chen, D.M. Bagnall, Z. Zhu, T. Sekiuchi, K. Park, K. Hiraga, T. Yao, S. Koyama, M.Y. Shen, T. Goto, J. Cryst. Growth 181 (1997) 165. [6] K.B. Sundaram, A. Khan, Thin Solid Films 295 (1997) 87. [7] F.T.J. Smith, Appl. Phys. Lett. 43 (1983) 1108. [8] W.S. Hu, Z.G. Liu, J. Sun, S.N. Zhu, Q.Q. Xu, D. Feng, Z.M. Ji, J. Phys. Chem. Solids 58 (1997) 953. [9] J.P. Zheng, H.S. Kwok, Thin Solid Films 232 (1993) 99. [10] C. Kittel, Introduction to Solid State Physics, 7th Edition, Wiley, Inc., New York, 1996. [11] S.A. Studeninkin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [12] K. Vanheusden, C.H. Seager, W.L. Waren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403. 528 S.H. Bae et al. / Applied Surface Science 169–170 (2001) 525–528