- Email: [email protected]
Pulsed laser deposition of ZnO thin films for applications of light emission
Applied Surface Science 154–155 Ž2000. 458–461 www.elsevier.nlrlocaterapsusc
Pulsed laser deposition of ZnO thin films for applications of light emission Sang Hyuck Bae a , Sang Yeol Lee
a,)
, Beom Jun Jin b, Seongil Im
b
a
b
Department of Electrical and Computer Engineering, Yonsei UniÕersity, 134, Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea Department of Metallurgical Engineering, Yonsei UniÕersity, 134, Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea Received 1 June 1999; accepted 21 July 1999
Abstract ZnO is a material suitable for light emission. In order to investigate the light emission properties, ZnO thin films were deposited on Ž0001. sapphire substrates by pulsed laser deposition ŽPLD. technique using an Nd:YAG laser with a wavelength of 355 nm. The influence of the deposition parameters, such as oxygen pressure, substrate temperature and laser energy density variation on the properties of the grown film, was studied. The experiment was performed for substrate temperatures in the range 200–6008C. The deposition chamber was filled with the oxygen at working pressures between 10y6 and 5 = 10y1 Torr. According to observations, the intensity of the light emission of laser-ablated ZnO thin films increased as substrate temperatures increased from 200 to 6008C. We investigated the structural, electrical and optical properties of ZnO thin films using X-ray diffraction ŽXRD., van der Pauw Hall measurements, photoluminescence ŽPL., and Rutherford backscattering spectrometry ŽRBS.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: ZnO; PLD; XRD; RBS; PL
1. Introduction ZnO is a wide-bandgap II–VI semiconductor and has a variety of potential applications. ZnO exhibits good piezoelectric, photoelectric and optic properties, and might be a good candidate for electroluminescence devices. ZnO films can also be used for applications in surface acoustic wave devices ŽSAW. w1–3x and low-loss optical waveguides w4x. Several techniques such as molecular beam epitaxy ŽMBE., ) Corresponding author. Tel.: q82-2-361-2776; fax: q82-2364-9770. E-mail address: [email protected] ŽS.Y. Lee..
radio frequency ŽRF. sputtering, chemical vapor deposition ŽCVD. have been used to deposit Ž002. highly textured ZnO films. The pulsed laser deposition ŽPLD. technique has advantages, such as deposition in relatively high oxygen-partial pressure, crystallization of films at lower temperatures because of the higher energy of the ablated particles in the laser-produced plume, and relatively high deposition rates w5x. Zu et al. w6x and the present paper observed UV emission and green–yellow photoluminescence ŽPL. in ZnO films. In this paper, the effect of the processing parameters of PLD on the properties of light emissive ZnO thin films is systematically studied and discussed.
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 3 9 8 - 0
S.H. Bae et al.r Applied Surface Science 154–155 (2000) 458–461
459
2. Experiment We deposited ZnO films using PLD technique. The chamber was evacuated using a turbomolecular pump to a base pressure of 1 = 10y6 Torr. ZnO films were grown on 1 = 1 cm Ž0001. sapphire Ž a-Al 2 O 3 . substrate. The laser energy density was 2.5 Jrcm2 . We used a ceramic ZnO target Ž1-in. diameter, 99.999% purity.. Pulsed Nd:YAG laser with a wavelength of 355 nm and a repetition rate of 5 Hz was used. A substrate holder was placed at 50 mm from the target. The target was rotated at 2 rpm to preclude pit formation in the target and ensure uniform ablation of the target w7x. The ZnO films were deposited at various oxygen pressures of 1 = 10y6 , 1, 20, 200 and 400 mTorr, and substrate temperatures of 200, 300, 350, 400 and 6008C. Prior to deposition, sapphire substrates were ultrasonically degreased in acetone and methanol for 3 min. The thickness of ZnO films measured were about 1 mm for substrate temperatures above 2008C and oxygen pressure above base pressure, by 2 MeV Heq Rutherford backscattering spectrometry ŽRBS.. The structural properties of the sample were investigated using the X-ray diffraction ŽXRD. u –2 u method where a Ni-filtered CuK a Ž l s 1.5418 = 10y1 0 m. source was used. The optical properties of the ZnO thin films were characterized by 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 using van der Pauw Hall measurements.
3. Results and discussion The dependence of the growth rate on substrate temperature and the oxygen-partial pressure is shown in Fig. 1. The thickness of ZnO films was measured by using RBS. The deposition rate was observed to increase with the substrate temperature up to 4008C. At 200 mTorr oxygen pressure, the growth rate was higher than at base oxygen pressure. Above 4008C, the deposition rate decreased. Since Zn and O 2 molecules need enough thermal energy to react, up to 4008C the reactions occur more actively with
Fig. 1. Variation of the growth rate of ZnO films deposited at the oxygen pressures of Ža. 10y5 and Žb. 2=10y1 Torr and at various substrate temperatures in the range of 200–6008C. The thickness of film was measured by 2 MeV Heq RBS.
higher substrate temperatures. Above 4008C, Zn and O 2 molecules have too much thermal energy and O 2 molecules begin to resputter. Fig. 2 shows XRD patterns of ZnO films deposited by PLD at substrate temperatures of Ža. 600, Žb. 350, Žc. 300 and Žd. 2008C at a fixed 200 mTorr oxygen pressure. Only the Ž002. ZnO peak and Ž006. Al 2 O 3 peak were observed. The full width at the half maximum ŽFWHM. 2 u values revealed the crystallinity of the film w8x. The XRD patterns showed that ZnO films are strongly c-axis oriented. Increasing the substrate temperature enhanced the crystallinity of film, indicated by the decrease of FWHM 2 u values. The films grown at substrate temperatures higher than 2008C and at oxygen pressure higher than 200 mTorr showed good crystallinity as observed by XRD as shown in Fig. 2. The FWHM of the Ž002. ZnO peak indicates that the c-axis of the grains became uniformly perpendicular to the substrate surface by increasing the substrate temperature. Fujimura et al. w9x suggested that grains with
460
S.H. Bae et al.r Applied Surface Science 154–155 (2000) 458–461
Fig. 2. XRD spectra obtained from ZnO films deposited at substrate temperatures of Ža. 600, Žb. 350, Žc. 300, Žd. 2008C and at the fixed oxygen pressure of 200 mTorr. Only Ž002. ZnO peak and Ž006. Al 2 O 3 peak were observed.
lower surface energy will become larger as the film grows, so the film orientation develops into one crystallographic direction of the lowest surface energy. This means that the Ž002. textured film must be formed in an effective equilibrium state to give enough surface mobility to impinging atoms for temperatures above 2008C. Earlier work w10x indicates that ZnO exhibited three PL bands centered around 390, 510 and 640 nm, i.e., near ultraviolet ŽUV., and green–yellow bands. In this work, the ZnO films showed UV and broad green–yellow PL bands. The effect of the substrate temperature on the light-emission properties of ZnO films was investigated. Fig. 3 shows the PL spectra of the ZnO films deposited by PLD. The PL spectra are for films deposited at substrate temperatures of Ža. 600, Žb. 350, Žc. 300 and Žd. 2008C and at a fixed partial-oxygen pressure of 200 mTorr. The PL characteristics of ZnO thin films were found to be strongly dependent on the substrate temperature. As substrate temperature increases, strong UV luminescence was observed. The green luminescence of polycrystalline ZnO is related to the amount of oxygen vacancies in the films and the green PL originates from oxygen-deficient films as reported by Vanheusden et al. w11x. UV PL is also related to the microcrystalline structure as reported by Tang et al.
Fig. 3. PL spectra obtained from the films deposited at the substrate temperatures of Ža. 600, Žb. 350, Žc. 300 and Žd. 2008C at a fixed oxygen pressure of 200 mTorr.
w12x. The increase of the substrate temperature leads to an increase in the PL intensity. Fig. 4 shows the
Fig. 4. Ultraviolet PL spectra obtained from films deposited at the substrate temperatures of Ža. 600, Žb. 350, Žc. 300, Žd. 2008C and at a fixed partial-oxygen pressure of 200 mTorr.
S.H. Bae et al.r Applied Surface Science 154–155 (2000) 458–461
461
temperatures of ZnO films. Broad band green–yellow luminescence was also observed. Good crystalline quality has been obtained for ZnO films deposited at substrate temperatures higher than 3008C and at an oxygen pressure higher than 200 mTorr. The intensity of UV luminescence increases as the crystallinity of ZnO films increases. Strong UV emission was obtained from high-quality ZnO film deposited at a substrate temperature of 6008C and oxygen pressure of 200 mTorr. This study suggests the possibility of using ZnO thin films in light emission device applications.
Fig. 5. Substrate temperature dependence of resistivity of the ZnO films deposited at the oxygen pressure of 200 mTorr. The resistivity was measured by van der Pauw Hall measurement.
UV PL spectra of films deposited at substrate temperatures of Ža. 600, Žb. 350, Žc. 300, Žd. 2008C and at a fixed partial-oxygen pressure of 200 mTorr. This PL spectra indicate that the characteristics of PL are closely related to the microcrystalline structure of the ZnO films. Strong UV emission has been observed from ZnO film deposited at the substrate temperature of 6008C and at an oxygen pressure of 200 mTorr. The crystallinity of ZnO thin films was enhanced by increasing substrate temperature. This is confirmed by the result of XRD shown in Fig. 2. Fig. 5 shows the dependence of resistivity of the ZnO films on the substrate temperatures. The resistivity of the films deposited above the substrate temperature of 3008C decreased with increasing substrate temperature. The structural transition from amorphous to polycrystalline explains the rapid decrease in the resistivity of the films.
4. Conclusion C-axis-oriented ZnO films on sapphire have been deposited by the PLD technique. Strong UV luminescence was obtained by increasing the substrate
Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation made in the program year of 1998.
References w1x T. Mitsuyu, S. Ono, K. Wasa, J. Appl. Phys. 51 Ž1980. 2646. w2x Y. Ito, K. Kushida, H. Kanda, H. Takeuchi, K. Sugaware, H. Onozato, Ferroelectrics 134 Ž1992. 325. w3x T. Yamazaki, S. Wada, T. Noma, T. Suzuki, Sens. Actuators, B 13–14 Ž1993. 594. w4x W.W. Wenas, A. Yamada, K. Takahashi, J. Appl. Phys. 70 Ž1991. 7119. w5x W.S. Hu, Z.G. Liu, J. Sun, S.N. Zhu, Q.Q. Xu, D. Feng, Z.M. Ji, J. Phys. Chem. Solids 58 Ž6. Ž1997. 857–953. w6x P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 Ž8. Ž1997. 459–463. w7x Y.S. Jeong, S.Y. Lee, H.K. Jang, I.S. Yang, S.H. Moon, B.D. Oh, Appl. Surf. Sci. 109 Ž1997. 424–427. w8x G.L. Clark, Applied X-rays, McGraw-Hill, New York, 1932. w9x N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 Ž1993. 269–279. w10x S.A. Studeninkin, N. Golego, M. Cocivera, J. Appl. Phys. 84 Ž4. Ž1998. 2287–2294. w11x K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 Ž3. Ž1996. 403–405, 15. w12x Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 Ž25. Ž1998. 3270–3272.
Pulsed laser deposition of ZnO thin films for applications of light emission Sang Hyuck Bae a , Sang Yeol Lee
a,)
, Beom Jun Jin b, Seongil Im
b
a
b
Department of Electrical and Computer Engineering, Yonsei UniÕersity, 134, Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea Department of Metallurgical Engineering, Yonsei UniÕersity, 134, Shinchon-dong, Seodaemoon-ku, Seoul 120-749, South Korea Received 1 June 1999; accepted 21 July 1999
Abstract ZnO is a material suitable for light emission. In order to investigate the light emission properties, ZnO thin films were deposited on Ž0001. sapphire substrates by pulsed laser deposition ŽPLD. technique using an Nd:YAG laser with a wavelength of 355 nm. The influence of the deposition parameters, such as oxygen pressure, substrate temperature and laser energy density variation on the properties of the grown film, was studied. The experiment was performed for substrate temperatures in the range 200–6008C. The deposition chamber was filled with the oxygen at working pressures between 10y6 and 5 = 10y1 Torr. According to observations, the intensity of the light emission of laser-ablated ZnO thin films increased as substrate temperatures increased from 200 to 6008C. We investigated the structural, electrical and optical properties of ZnO thin films using X-ray diffraction ŽXRD., van der Pauw Hall measurements, photoluminescence ŽPL., and Rutherford backscattering spectrometry ŽRBS.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: ZnO; PLD; XRD; RBS; PL
1. Introduction ZnO is a wide-bandgap II–VI semiconductor and has a variety of potential applications. ZnO exhibits good piezoelectric, photoelectric and optic properties, and might be a good candidate for electroluminescence devices. ZnO films can also be used for applications in surface acoustic wave devices ŽSAW. w1–3x and low-loss optical waveguides w4x. Several techniques such as molecular beam epitaxy ŽMBE., ) Corresponding author. Tel.: q82-2-361-2776; fax: q82-2364-9770. E-mail address: [email protected] ŽS.Y. Lee..
radio frequency ŽRF. sputtering, chemical vapor deposition ŽCVD. have been used to deposit Ž002. highly textured ZnO films. The pulsed laser deposition ŽPLD. technique has advantages, such as deposition in relatively high oxygen-partial pressure, crystallization of films at lower temperatures because of the higher energy of the ablated particles in the laser-produced plume, and relatively high deposition rates w5x. Zu et al. w6x and the present paper observed UV emission and green–yellow photoluminescence ŽPL. in ZnO films. In this paper, the effect of the processing parameters of PLD on the properties of light emissive ZnO thin films is systematically studied and discussed.
0169-4332r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 3 9 8 - 0
S.H. Bae et al.r Applied Surface Science 154–155 (2000) 458–461
459
2. Experiment We deposited ZnO films using PLD technique. The chamber was evacuated using a turbomolecular pump to a base pressure of 1 = 10y6 Torr. ZnO films were grown on 1 = 1 cm Ž0001. sapphire Ž a-Al 2 O 3 . substrate. The laser energy density was 2.5 Jrcm2 . We used a ceramic ZnO target Ž1-in. diameter, 99.999% purity.. Pulsed Nd:YAG laser with a wavelength of 355 nm and a repetition rate of 5 Hz was used. A substrate holder was placed at 50 mm from the target. The target was rotated at 2 rpm to preclude pit formation in the target and ensure uniform ablation of the target w7x. The ZnO films were deposited at various oxygen pressures of 1 = 10y6 , 1, 20, 200 and 400 mTorr, and substrate temperatures of 200, 300, 350, 400 and 6008C. Prior to deposition, sapphire substrates were ultrasonically degreased in acetone and methanol for 3 min. The thickness of ZnO films measured were about 1 mm for substrate temperatures above 2008C and oxygen pressure above base pressure, by 2 MeV Heq Rutherford backscattering spectrometry ŽRBS.. The structural properties of the sample were investigated using the X-ray diffraction ŽXRD. u –2 u method where a Ni-filtered CuK a Ž l s 1.5418 = 10y1 0 m. source was used. The optical properties of the ZnO thin films were characterized by 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 using van der Pauw Hall measurements.
3. Results and discussion The dependence of the growth rate on substrate temperature and the oxygen-partial pressure is shown in Fig. 1. The thickness of ZnO films was measured by using RBS. The deposition rate was observed to increase with the substrate temperature up to 4008C. At 200 mTorr oxygen pressure, the growth rate was higher than at base oxygen pressure. Above 4008C, the deposition rate decreased. Since Zn and O 2 molecules need enough thermal energy to react, up to 4008C the reactions occur more actively with
Fig. 1. Variation of the growth rate of ZnO films deposited at the oxygen pressures of Ža. 10y5 and Žb. 2=10y1 Torr and at various substrate temperatures in the range of 200–6008C. The thickness of film was measured by 2 MeV Heq RBS.
higher substrate temperatures. Above 4008C, Zn and O 2 molecules have too much thermal energy and O 2 molecules begin to resputter. Fig. 2 shows XRD patterns of ZnO films deposited by PLD at substrate temperatures of Ža. 600, Žb. 350, Žc. 300 and Žd. 2008C at a fixed 200 mTorr oxygen pressure. Only the Ž002. ZnO peak and Ž006. Al 2 O 3 peak were observed. The full width at the half maximum ŽFWHM. 2 u values revealed the crystallinity of the film w8x. The XRD patterns showed that ZnO films are strongly c-axis oriented. Increasing the substrate temperature enhanced the crystallinity of film, indicated by the decrease of FWHM 2 u values. The films grown at substrate temperatures higher than 2008C and at oxygen pressure higher than 200 mTorr showed good crystallinity as observed by XRD as shown in Fig. 2. The FWHM of the Ž002. ZnO peak indicates that the c-axis of the grains became uniformly perpendicular to the substrate surface by increasing the substrate temperature. Fujimura et al. w9x suggested that grains with
460
S.H. Bae et al.r Applied Surface Science 154–155 (2000) 458–461
Fig. 2. XRD spectra obtained from ZnO films deposited at substrate temperatures of Ža. 600, Žb. 350, Žc. 300, Žd. 2008C and at the fixed oxygen pressure of 200 mTorr. Only Ž002. ZnO peak and Ž006. Al 2 O 3 peak were observed.
lower surface energy will become larger as the film grows, so the film orientation develops into one crystallographic direction of the lowest surface energy. This means that the Ž002. textured film must be formed in an effective equilibrium state to give enough surface mobility to impinging atoms for temperatures above 2008C. Earlier work w10x indicates that ZnO exhibited three PL bands centered around 390, 510 and 640 nm, i.e., near ultraviolet ŽUV., and green–yellow bands. In this work, the ZnO films showed UV and broad green–yellow PL bands. The effect of the substrate temperature on the light-emission properties of ZnO films was investigated. Fig. 3 shows the PL spectra of the ZnO films deposited by PLD. The PL spectra are for films deposited at substrate temperatures of Ža. 600, Žb. 350, Žc. 300 and Žd. 2008C and at a fixed partial-oxygen pressure of 200 mTorr. The PL characteristics of ZnO thin films were found to be strongly dependent on the substrate temperature. As substrate temperature increases, strong UV luminescence was observed. The green luminescence of polycrystalline ZnO is related to the amount of oxygen vacancies in the films and the green PL originates from oxygen-deficient films as reported by Vanheusden et al. w11x. UV PL is also related to the microcrystalline structure as reported by Tang et al.
Fig. 3. PL spectra obtained from the films deposited at the substrate temperatures of Ža. 600, Žb. 350, Žc. 300 and Žd. 2008C at a fixed oxygen pressure of 200 mTorr.
w12x. The increase of the substrate temperature leads to an increase in the PL intensity. Fig. 4 shows the
Fig. 4. Ultraviolet PL spectra obtained from films deposited at the substrate temperatures of Ža. 600, Žb. 350, Žc. 300, Žd. 2008C and at a fixed partial-oxygen pressure of 200 mTorr.
S.H. Bae et al.r Applied Surface Science 154–155 (2000) 458–461
461
temperatures of ZnO films. Broad band green–yellow luminescence was also observed. Good crystalline quality has been obtained for ZnO films deposited at substrate temperatures higher than 3008C and at an oxygen pressure higher than 200 mTorr. The intensity of UV luminescence increases as the crystallinity of ZnO films increases. Strong UV emission was obtained from high-quality ZnO film deposited at a substrate temperature of 6008C and oxygen pressure of 200 mTorr. This study suggests the possibility of using ZnO thin films in light emission device applications.
Fig. 5. Substrate temperature dependence of resistivity of the ZnO films deposited at the oxygen pressure of 200 mTorr. The resistivity was measured by van der Pauw Hall measurement.
UV PL spectra of films deposited at substrate temperatures of Ža. 600, Žb. 350, Žc. 300, Žd. 2008C and at a fixed partial-oxygen pressure of 200 mTorr. This PL spectra indicate that the characteristics of PL are closely related to the microcrystalline structure of the ZnO films. Strong UV emission has been observed from ZnO film deposited at the substrate temperature of 6008C and at an oxygen pressure of 200 mTorr. The crystallinity of ZnO thin films was enhanced by increasing substrate temperature. This is confirmed by the result of XRD shown in Fig. 2. Fig. 5 shows the dependence of resistivity of the ZnO films on the substrate temperatures. The resistivity of the films deposited above the substrate temperature of 3008C decreased with increasing substrate temperature. The structural transition from amorphous to polycrystalline explains the rapid decrease in the resistivity of the films.
4. Conclusion C-axis-oriented ZnO films on sapphire have been deposited by the PLD technique. Strong UV luminescence was obtained by increasing the substrate
Acknowledgements The authors wish to acknowledge the financial support of the Korea Research Foundation made in the program year of 1998.
References w1x T. Mitsuyu, S. Ono, K. Wasa, J. Appl. Phys. 51 Ž1980. 2646. w2x Y. Ito, K. Kushida, H. Kanda, H. Takeuchi, K. Sugaware, H. Onozato, Ferroelectrics 134 Ž1992. 325. w3x T. Yamazaki, S. Wada, T. Noma, T. Suzuki, Sens. Actuators, B 13–14 Ž1993. 594. w4x W.W. Wenas, A. Yamada, K. Takahashi, J. Appl. Phys. 70 Ž1991. 7119. w5x W.S. Hu, Z.G. Liu, J. Sun, S.N. Zhu, Q.Q. Xu, D. Feng, Z.M. Ji, J. Phys. Chem. Solids 58 Ž6. Ž1997. 857–953. w6x P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 Ž8. Ž1997. 459–463. w7x Y.S. Jeong, S.Y. Lee, H.K. Jang, I.S. Yang, S.H. Moon, B.D. Oh, Appl. Surf. Sci. 109 Ž1997. 424–427. w8x G.L. Clark, Applied X-rays, McGraw-Hill, New York, 1932. w9x N. Fujimura, T. Nishihara, S. Goto, J. Xu, T. Ito, J. Cryst. Growth 130 Ž1993. 269–279. w10x S.A. Studeninkin, N. Golego, M. Cocivera, J. Appl. Phys. 84 Ž4. Ž1998. 2287–2294. w11x K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 Ž3. Ž1996. 403–405, 15. w12x Z.K. Tang, G.K.L. Wong, P. Yu, Appl. Phys. Lett. 72 Ž25. Ž1998. 3270–3272.