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Luminescent properties of ZnO thin films grown epitaxially on Si substrate
CRYS=461=C Mamatha=Venkatachala=BG
Journal of Crystal Growth 214/215 (2000) 294}298
Luminescent properties of ZnO thin "lms grown epitaxially on Si substrate A. Miyake *, H. Kominami, H. Tatsuoka , H. Kuwabara , Y. Nakanishi , Y. Hatanaka Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan Graduate School of Electronic Science and Technology, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan
Abstract ZnO thin "lms were grown on Si(1 1 1) substrates by employing an epitaxial ZnS thin "lm as a bu!er layer. The structural and luminescent properties of the ZnO thin "lms have been investigated in view of the application to opto-electronic devices due to near-ultraviolet emission by exciton the binding energy of which is about 60 meV. When the epitaxial ZnS bu!er layer was grown on the Si(1 1 1) substrate at a substrate temperature of 2003C by electron beam evaporation, the epitaxial ZnO "lm was successfully grown on the ZnS/Si(1 1 1) layer with the orientation of (0 0 0 2), [1 1 2 0]ZnO#(1 1 1), [1 1 0]ZnS#(1 1 1), [1 1 0]Si(1 1 1) at a substrate temperature of 4003C. An excitonic emission with a peak at 3.35 eV at 20 K was successfully obtained by exciting at 325 nm of He}Cd laser. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 61.14.Hg; 68.55.!a; 78.55.!m Keywords: ZnO; Epitaxial growth; Exciton; PL; Thin "lm; RHEED
1. Introduction In recent years, wide band-gap semiconductor materials have been attracting a great deal of attention for their use in blue light emitting and short-wavelength laser diodes. ZnO, a well-known bluish-green emitting wide-gap (II}VI) semiconductor, is gaining importance for the possible application of semiconductor laser because of its ultraviolet
* Corresponding author. Tel.: #81-53-478-1311; fax: #8153-478-1346. E-mail address: [email protected] (A. Miyake).
emission at room temperature (RT). It has a wide band gap (3.37 eV) and a high exciton binding energy (60 meV) which allow e$cient UV emission from the exciton and make it suitable for UV laser-emitting devices. Moreover, ZnO is thermally and chemically stable in air. Hence, various studies are carried out on ZnO bulk crystals and thin "lms, for the UV-laser emission at room temperature [1]. For an e$cient and stable ultraviolet emission from the exciton, stoichiometry of the ZnO is essential. In order to achieve this, epitaxial growth of ZnO is necessary. In earlier studies, high-quality ZnO thin "lms were grown by plasma-enhanced
0022-0248/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 0 9 5 - 6
Crys=461=CM=VVC=BG
A. Miyake et al. / Journal of Crystal Growth 214/215 (2000) 294}298
molecular beam epitaxy (MBE) and laser MBE on sapphire(0 0 0 1) substrates [2,3]. In this paper, we are reporting the structural and luminescent properties of ZnO thin "lms grown by vacuum evaporation techniques on Si(1 1 1) substrates by employing ZnS as a bu!er layer. Here we have used Si instead of sapphire as substrates, because of its inherent advantages, viz., inexpensive and conductive. To avoid the oxidation of Si during ZnO deposition, ZnS is employed as a bu!er layer between ZnO and Si. The growth of ZnS epitaxial "lms on the Si(1 1 1) substrates is not di$cult because their lattice mismatch is about 0.3% [4]. Further, it helps to grow ZnO epitaxially because of the fact that both have same tetrahedral structure.
295
Fig. 1. XRD curves of ZnO thin "lms as-deposited and annealed at several temperatures for 1 h in air.
2. Experimental procedure The Si(1 1 1) substrates were cleaned with organic solvents with ethanol for 5 min, acetone for 10 min and ethanol for 5 min in order. Then the surface oxidized layer was etched with H SO : H O ("3 : 1) for 10 min, HF : H O ("1 : 20) for about 1 min, HCl : HNO ("3 : 1) for 10 min and then with HF : H O ("1 : 20) for about 1 min and "nally washed with distilled water. ZnS "lms were grown epitaxially with a thickness of about 50 nm on the cleaned surface of the Si(1 1 1) substrates by vacuum evaporation employing 3 N-ZnS pellets at a substrate temperature of 2003C. Then the ZnO thin "lms were deposited using 4 N-ZnO pellets. The substrate temperatures were maintained at 2003C, 4003C and 6003C. After the deposition, the "lms were annealed at 800}10003C for 1 h in air. The thin-"lm structural properties were characterized by X-ray di!raction (XRD) and re#ection high-energy electron di!raction (RHEED). Luminescent properties were characterized at the excitation of 325 nm of He}Cd laser at 20 K and RT. 3. Results and discussion 3.1. Structural properties Fig. 1 shows XRD curves of the "lms deposited at a substrate temperature of 4003C and annealed
Fig. 2. RHEED patterns of ZnS thin "lms deposited on Si(1 1 1) at a substrate temperature of 2003C. (a) beam#[1 1 0]ZnS, (b) beam#[2 1 1]ZnS.
at various temperatures in air. From this "gure, we can see that there are two peaks in the pattern at 283 and 34.43. The "rst one indicates the growth of ZnS and the latter one is due to the di!raction of ZnO(0 0 0 2) plane. When the thickness of ZnO is increased, the peak due to the ZnS disappears.
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Fig. 3. RHEED patterns of ZnO thin "lms deposited on ZnS/Si(1 1 1) at substrate temperatures of (a), (a ) 2003C, (b), (b ) 4003C, (c), (c ) 6003C. (a), (b), (c) beam#[1 1 2 0]ZnO, (a ), (b ), (c )beam#[1 0 1 0]ZnO.
Fig. 4. RHEED patterns of ZnO thin "lms deposited at a substrate temperature of 4003C (a) before and (b) after annealing at 10003C for 1 h in air.
Accordingly, all the "lms have the orientation along the c-axis of wurtzite structure. The fullwidth at half-maximum (FWHM) of the (0 0 0 2) re#ection peak is decreased with an increase in the intensity with respect to the annealing temper-
atures. It clearly shows that the FWHM of the as-deposited "lm is large and decreased to one third when annealed at 10003C. Figs. 2}4 show a RHEED pattern of the ZnS "lm deposited on Si(1 1 1) and the ZnO "lms deposited
Crys=461=CM=VVC=BG
A. Miyake et al. / Journal of Crystal Growth 214/215 (2000) 294}298
on the ZnS/Si(1 1 1), respectively. The incident direction of electron beam is parallel to [1 1 0] Si(1 1 1) and to [2 1 0] Si(1 1 1). These "gures show that both "lms are grown epitaxially, although the growth mode may not be layer due to spotty patterns. The pattern indicated in Fig. 2 shows that the epitaxial ZnS "lm is grown with the orientation of (1 1 1), [1 1 0]ZnS#(1 1 1) [1 1 0]Si, with (1 1 1) micro twin. Moreover, Fig. 3 shows that the epitaxial ZnO "lm can be successfully grown on the Si substrate at a substrate temperature over 4003C with the orientation of (0 0 0 2), [1 1 2 0]ZnO#(1 1 1), [1 1 0]ZnS#(1 1 1), [1 1 0]Si. Figs. 3 and 4 show that the crystallinity of ZnO improved by increasing the substrate and annealing temperatures.
297
Fig. 6. PL spectra of ZnO thin "lms deposited on Si(1 1 1) at 4003C and annealed at 8003C for 1 h in air, measured at RT.
3.2. Luminescent properties Fig. 5 shows the PL spectrum of the ZnO "lm measured at 20 K, where the "lm was deposited at a substrate temperature of 4003C and annealed at 10003C for 1 h in air. It is seen that the spectrum consists of a sharp emission with a peak at about 3.35 eV and a broad band with a peak at around 2.4 eV. It clearly shows that the former emission is due to the exciton and the latter blue}green emission, a typical ZnO emission due to oxygen vacancy [5]. A background increasing in intensity at higher energy seems to be due to the tail of He}Cd laser. Fig. 6 shows the PL spectrum of the same "lm as shown in Fig. 5, measured at RT. The exciton
emission is observed even at room temperature with a peak at about 3.27 eV, although the intensity is weak. It suggests that the sharp emission at 20 K is bound exciton emission and the emission at RT is free exciton emission, because the energy value of the exciton emission is the same as the reported value [1]. From the above results, it is concluded that the exciton emission can be obtained from the ZnO "lm grown on the Si(1 1 1) substrate by vacuum evaporation technique. It is evaluated that the crystallinity and stoichiometry is insu$cient to obtain exciton emission because the visible emission is due to defect level [5]. Figs. 3 and 4 show that ZnO thin "lms were grown epitaxially from their RHEED patterns, but the patterns are not streaky. Therefore, if the oxygen vacancies in the "lm are compensated and the "lm crystallinity is improved, the exciton emission will become dominant. Detailed studies on the e!ect of the ZnS bu!er layer are in progress, because the contribution of the ZnS bu!er layer is very important to the preparation of the epitaxial ZnO with good crystallinity.
Acknowledgements Fig. 5. PL spectra of ZnO thin "lms deposited on Si(1 1 1) at 4003C and annealed at 8003C for 1 h in air, measured at 20 K.
Partial support by the Grant-in-Aid from HosoBunka Foundation is gratefully acknowledged.
Crys=461=CM=VVC=BG
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A. Miyake et al. / Journal of Crystal Growth 214/215 (2000) 294}298
References [1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [2] H. Koinuma, M. Kawasaki, M. Yoshimoto, Mater. Res. Soc. Symp. Proc. 397 (1996) 145.
[3] D.M. Bagnall, Y.F. Chen, Z. Zhuad, T. Yao, Appl. Phys. Lett. 70 (1997) 2230. [4] Y. Nakanishi, G. Shimaoka, J. Vac. Sci. Technol. A 5 (1987) 2092. [5] S. Shionoya, in: S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC Press, New York, 1998, p. 255.
Journal of Crystal Growth 214/215 (2000) 294}298
Luminescent properties of ZnO thin "lms grown epitaxially on Si substrate A. Miyake *, H. Kominami, H. Tatsuoka , H. Kuwabara , Y. Nakanishi , Y. Hatanaka Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan Graduate School of Electronic Science and Technology, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8011, Japan Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu 432-8561, Japan
Abstract ZnO thin "lms were grown on Si(1 1 1) substrates by employing an epitaxial ZnS thin "lm as a bu!er layer. The structural and luminescent properties of the ZnO thin "lms have been investigated in view of the application to opto-electronic devices due to near-ultraviolet emission by exciton the binding energy of which is about 60 meV. When the epitaxial ZnS bu!er layer was grown on the Si(1 1 1) substrate at a substrate temperature of 2003C by electron beam evaporation, the epitaxial ZnO "lm was successfully grown on the ZnS/Si(1 1 1) layer with the orientation of (0 0 0 2), [1 1 2 0]ZnO#(1 1 1), [1 1 0]ZnS#(1 1 1), [1 1 0]Si(1 1 1) at a substrate temperature of 4003C. An excitonic emission with a peak at 3.35 eV at 20 K was successfully obtained by exciting at 325 nm of He}Cd laser. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 61.14.Hg; 68.55.!a; 78.55.!m Keywords: ZnO; Epitaxial growth; Exciton; PL; Thin "lm; RHEED
1. Introduction In recent years, wide band-gap semiconductor materials have been attracting a great deal of attention for their use in blue light emitting and short-wavelength laser diodes. ZnO, a well-known bluish-green emitting wide-gap (II}VI) semiconductor, is gaining importance for the possible application of semiconductor laser because of its ultraviolet
* Corresponding author. Tel.: #81-53-478-1311; fax: #8153-478-1346. E-mail address: [email protected] (A. Miyake).
emission at room temperature (RT). It has a wide band gap (3.37 eV) and a high exciton binding energy (60 meV) which allow e$cient UV emission from the exciton and make it suitable for UV laser-emitting devices. Moreover, ZnO is thermally and chemically stable in air. Hence, various studies are carried out on ZnO bulk crystals and thin "lms, for the UV-laser emission at room temperature [1]. For an e$cient and stable ultraviolet emission from the exciton, stoichiometry of the ZnO is essential. In order to achieve this, epitaxial growth of ZnO is necessary. In earlier studies, high-quality ZnO thin "lms were grown by plasma-enhanced
0022-0248/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 0 9 5 - 6
Crys=461=CM=VVC=BG
A. Miyake et al. / Journal of Crystal Growth 214/215 (2000) 294}298
molecular beam epitaxy (MBE) and laser MBE on sapphire(0 0 0 1) substrates [2,3]. In this paper, we are reporting the structural and luminescent properties of ZnO thin "lms grown by vacuum evaporation techniques on Si(1 1 1) substrates by employing ZnS as a bu!er layer. Here we have used Si instead of sapphire as substrates, because of its inherent advantages, viz., inexpensive and conductive. To avoid the oxidation of Si during ZnO deposition, ZnS is employed as a bu!er layer between ZnO and Si. The growth of ZnS epitaxial "lms on the Si(1 1 1) substrates is not di$cult because their lattice mismatch is about 0.3% [4]. Further, it helps to grow ZnO epitaxially because of the fact that both have same tetrahedral structure.
295
Fig. 1. XRD curves of ZnO thin "lms as-deposited and annealed at several temperatures for 1 h in air.
2. Experimental procedure The Si(1 1 1) substrates were cleaned with organic solvents with ethanol for 5 min, acetone for 10 min and ethanol for 5 min in order. Then the surface oxidized layer was etched with H SO : H O ("3 : 1) for 10 min, HF : H O ("1 : 20) for about 1 min, HCl : HNO ("3 : 1) for 10 min and then with HF : H O ("1 : 20) for about 1 min and "nally washed with distilled water. ZnS "lms were grown epitaxially with a thickness of about 50 nm on the cleaned surface of the Si(1 1 1) substrates by vacuum evaporation employing 3 N-ZnS pellets at a substrate temperature of 2003C. Then the ZnO thin "lms were deposited using 4 N-ZnO pellets. The substrate temperatures were maintained at 2003C, 4003C and 6003C. After the deposition, the "lms were annealed at 800}10003C for 1 h in air. The thin-"lm structural properties were characterized by X-ray di!raction (XRD) and re#ection high-energy electron di!raction (RHEED). Luminescent properties were characterized at the excitation of 325 nm of He}Cd laser at 20 K and RT. 3. Results and discussion 3.1. Structural properties Fig. 1 shows XRD curves of the "lms deposited at a substrate temperature of 4003C and annealed
Fig. 2. RHEED patterns of ZnS thin "lms deposited on Si(1 1 1) at a substrate temperature of 2003C. (a) beam#[1 1 0]ZnS, (b) beam#[2 1 1]ZnS.
at various temperatures in air. From this "gure, we can see that there are two peaks in the pattern at 283 and 34.43. The "rst one indicates the growth of ZnS and the latter one is due to the di!raction of ZnO(0 0 0 2) plane. When the thickness of ZnO is increased, the peak due to the ZnS disappears.
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Fig. 3. RHEED patterns of ZnO thin "lms deposited on ZnS/Si(1 1 1) at substrate temperatures of (a), (a ) 2003C, (b), (b ) 4003C, (c), (c ) 6003C. (a), (b), (c) beam#[1 1 2 0]ZnO, (a ), (b ), (c )beam#[1 0 1 0]ZnO.
Fig. 4. RHEED patterns of ZnO thin "lms deposited at a substrate temperature of 4003C (a) before and (b) after annealing at 10003C for 1 h in air.
Accordingly, all the "lms have the orientation along the c-axis of wurtzite structure. The fullwidth at half-maximum (FWHM) of the (0 0 0 2) re#ection peak is decreased with an increase in the intensity with respect to the annealing temper-
atures. It clearly shows that the FWHM of the as-deposited "lm is large and decreased to one third when annealed at 10003C. Figs. 2}4 show a RHEED pattern of the ZnS "lm deposited on Si(1 1 1) and the ZnO "lms deposited
Crys=461=CM=VVC=BG
A. Miyake et al. / Journal of Crystal Growth 214/215 (2000) 294}298
on the ZnS/Si(1 1 1), respectively. The incident direction of electron beam is parallel to [1 1 0] Si(1 1 1) and to [2 1 0] Si(1 1 1). These "gures show that both "lms are grown epitaxially, although the growth mode may not be layer due to spotty patterns. The pattern indicated in Fig. 2 shows that the epitaxial ZnS "lm is grown with the orientation of (1 1 1), [1 1 0]ZnS#(1 1 1) [1 1 0]Si, with (1 1 1) micro twin. Moreover, Fig. 3 shows that the epitaxial ZnO "lm can be successfully grown on the Si substrate at a substrate temperature over 4003C with the orientation of (0 0 0 2), [1 1 2 0]ZnO#(1 1 1), [1 1 0]ZnS#(1 1 1), [1 1 0]Si. Figs. 3 and 4 show that the crystallinity of ZnO improved by increasing the substrate and annealing temperatures.
297
Fig. 6. PL spectra of ZnO thin "lms deposited on Si(1 1 1) at 4003C and annealed at 8003C for 1 h in air, measured at RT.
3.2. Luminescent properties Fig. 5 shows the PL spectrum of the ZnO "lm measured at 20 K, where the "lm was deposited at a substrate temperature of 4003C and annealed at 10003C for 1 h in air. It is seen that the spectrum consists of a sharp emission with a peak at about 3.35 eV and a broad band with a peak at around 2.4 eV. It clearly shows that the former emission is due to the exciton and the latter blue}green emission, a typical ZnO emission due to oxygen vacancy [5]. A background increasing in intensity at higher energy seems to be due to the tail of He}Cd laser. Fig. 6 shows the PL spectrum of the same "lm as shown in Fig. 5, measured at RT. The exciton
emission is observed even at room temperature with a peak at about 3.27 eV, although the intensity is weak. It suggests that the sharp emission at 20 K is bound exciton emission and the emission at RT is free exciton emission, because the energy value of the exciton emission is the same as the reported value [1]. From the above results, it is concluded that the exciton emission can be obtained from the ZnO "lm grown on the Si(1 1 1) substrate by vacuum evaporation technique. It is evaluated that the crystallinity and stoichiometry is insu$cient to obtain exciton emission because the visible emission is due to defect level [5]. Figs. 3 and 4 show that ZnO thin "lms were grown epitaxially from their RHEED patterns, but the patterns are not streaky. Therefore, if the oxygen vacancies in the "lm are compensated and the "lm crystallinity is improved, the exciton emission will become dominant. Detailed studies on the e!ect of the ZnS bu!er layer are in progress, because the contribution of the ZnS bu!er layer is very important to the preparation of the epitaxial ZnO with good crystallinity.
Acknowledgements Fig. 5. PL spectra of ZnO thin "lms deposited on Si(1 1 1) at 4003C and annealed at 8003C for 1 h in air, measured at 20 K.
Partial support by the Grant-in-Aid from HosoBunka Foundation is gratefully acknowledged.
Crys=461=CM=VVC=BG
298
A. Miyake et al. / Journal of Crystal Growth 214/215 (2000) 294}298
References [1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [2] H. Koinuma, M. Kawasaki, M. Yoshimoto, Mater. Res. Soc. Symp. Proc. 397 (1996) 145.
[3] D.M. Bagnall, Y.F. Chen, Z. Zhuad, T. Yao, Appl. Phys. Lett. 70 (1997) 2230. [4] Y. Nakanishi, G. Shimaoka, J. Vac. Sci. Technol. A 5 (1987) 2092. [5] S. Shionoya, in: S. Shionoya, W.M. Yen (Eds.), Phosphor Handbook, CRC Press, New York, 1998, p. 255.