- Email: [email protected]
Effects of thermal annealing temperature and duration on hydrothermally grown ZnO nanorod arrays
Applied Surface Science 255 (2009) 5861–5865
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of thermal annealing temperature and duration on hydrothermally grown ZnO nanorod arrays X.Q. Zhao a, C.R. Kim a, J.Y. Lee a, C.M. Shin a, J.H. Heo a, J.Y. Leem a, H. Ryu a,*, J.H. Chang b, H.C. Lee c, C.S. Son d, B.C. Shin e, W.J. Lee e, W.G. Jung f, S.T. Tan g, J.L. Zhao h, X.W. Sun g,h a
Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Obang-dong, Gimhae, Gyeongnam 621-749, Republic of Korea Major of Nano Semiconductor, Korea Maritime University, #1 Dongsam-dong, Yeongdo-Ku, Busan 606-791, Republic of Korea Department of Mechatronics Engineering, Korea Maritime University, #1 Dongsam-dong, Yeongdo-Ku, Busan 606-791, Republic of Korea d Department of Electronic Materials Engineering, Silla University, Gwaebeop-dong, Sasang-gu, Busan 617-736, Republic of Korea e Department of Nano Engineering, Dong-Eui University, 995 Eomgwangno, Busanjin-gu, Busan 614-714, Republic of Korea f School of Advanced Materials Engineering, Kookmin University, 861-1, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea g Institute of Microelectronics, 11 Science Park Road, Science Park II, Singapore 117685, Singapore h School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 December 2008 Received in revised form 8 January 2009 Accepted 8 January 2009 Available online 20 January 2009
In this study, the effects of thermal annealing temperature and duration on ZnO nanorod arrays fabricated by hydrothermal method were investigated. The annealed ZnO/Si(1 1 1) substrate was used for ZnO nanorod array growth. The effects of annealing treatment on the structural and optical properties were investigated by scanning electron microscopy, X-ray diffraction, and room-temperature photoluminescence measurements. With the annealing temperature of 750 8C and the annealing duration of 10 min, both the structural and optical properties of the ZnO nanorod arrays improved significantly, as indicated in the X-ray diffraction and photoluminescence measurement. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.05.Dz 81.16.Be 81.40.Ef Keywords: ZnO Hydrothermal synthesis Annealing treatment
1. Introduction II–VI semiconductor zinc oxide (ZnO) has attracted much attention due to its unique properties and versatile applications [1]. ZnO has a stable wurtzite structure with lattice spacing a = 0.325 nm and c = 0.521 nm. The structure of ZnO can be described as a number of alternating planes composed of tetrahedrally coordinated O2 and Zn2+ ions, which are stacked alternately along the c-axis. The oppositely charged ions produce ¯ positively charged (0 0 0 1)-Zn and negatively charged (0 0 0 1)-O polar planes, which result in a normal dipole moment and spontaneous polarization along the c-axis and different surface energy with the different crystallographic orientations. The excellent optical properties of ZnO such as a wide band gap (3.37 eV) and large exciton binding energy (60 meV) at room
* Corresponding author. Tel.: +82 55 320 3874; fax: +82 55 320 3631. E-mail address: [email protected] (H. Ryu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.022
temperature endow ZnO as a promising material for roomtemperature ultraviolet lasers [2] and field-emission devices [3]. Besides, ZnO is one of the most widely applied oxide gas-sensing materials. The sensing mechanism of ZnO belongs to the surfacecontrolled type. The oxygen vacancies on the oxide surfaces are electrically and chemically active. Because of this mechanism, the ZnO nanostructures with high surface-to-volume ratios used as sensing materials has been regarded as one of the most efficient methods for fabricating high-efficiency gas sensors. Moreover, its piezoelectric property originating from its non-centrosymmetric structure makes it suitable for electromechanical sensor or actuator applications [4]. Various ZnO nanostructures such as nanorods and nanowires have been synthesized by hydrothermal method [5–7]. Song et al. [5] presented that well-aligned ZnO nanorod arrays were grown on ZnO-buffered Si substrate in pH value ranging from 10.3 to 10.9. Yadav and Pandey [6] reported that ZnO nanorods were synthesized by an organic-free hydrothermal process. Li et al. reported that long ZnO nanowires with high aspect ratio of up to
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
5862
1000 were grown on zinc substrate in NaOH aqueous solution by hydrothermal method [7]. The well-aligned ZnO rod arrays were grown on annealed ZnO-buffered Si(1 0 0) with different annealing temperatures. It was found that the good quality of hydrothermally grown ZnO can be obtained on ZnO buffer layer annealed at 750 8C, as reported in our previous work [8]. Therefore, in this study, ZnO buffer layer was annealed prior to main ZnO growth in order to obtain higher quality of hydrothermally grown ZnO. It is well known that hydrothermally grown ZnO has many defects due to its low growth temperature and that thermal treatment is an efficient method to reduce the defects and improve the qualities of ZnO [9,10]. In this study, thermal treatment was done to hydrothermally grown ZnO samples. The effects of annealing temperature and duration would be mainly discussed here. 2. Experimental procedures In this experiment, ZnO-buffered Si(1 1 1) [ZnO/Si(1 1 1)] was used for the growth of ZnO nanorod arrays. The ZnO buffer layer with a thickness of around 40 nm was deposited on Si(1 0 0) substrate at the growth temperature of 500 8C by metal-organic chemical vapor deposition method. The detailed deposition condition was described elsewhere [11]. Prior to the growth of ZnO nanorod, the ZnO/Si(1 1 1) was annealed at 750 8C for 2 min in nitrogen atmosphere with the chamber pressure of 1 Torr.
Zinc nitrate hexahydrate (Zn(NO3)2) and methenamine (HMT) with the concentration of 0.1 M and with the equal molar ratio were used as precursors to fabricate ZnO nanorod arrays. The pretreated ZnO/Si(1 1 1) was immersed into the mixed solution of Zn(NO3)2 and HMT, which was in Teflonlined steel autoclaves. The autoclaves were kept in an oven with the constant temperature of 95 8C for 10 h. After that, the autoclaves were taken out from the oven and allowed to cool down to room temperature naturally. Then the samples were taken out and washed with de-ionized water and blown dry with pure N2 gas. The hydrothermally grown samples were annealed with various temperatures ranging from 450 to 900 8C for 10 min at N2 atmosphere with the fixed chamber pressure of 1 Torr to investigate the effects of annealing temperature. Moreover, in order to study the influence of annealing duration, the samples were annealed with various durations from 2 to 30 min at the constant temperature of 750 8C in N2 atmosphere. The morphologies of ZnO nanorod arrays were examined via field-emission scanning electron microscope (FE-SEM) (HITACHI S4300SE). The crystal structures were analyzed by X-ray diffractometer (XRD) with Cu Ka radiation, and optical properties were characterized by photoluminescence (PL) measurement. The PL measurements were performed at room temperature with a He–Cd laser (l = 325 nm) as the excitation source.
Fig. 1. SEM images of as-grown and annealed ZnO at 750 and 900 8C for 10 min nanorod arrays: (a) planar view, (b) cross-section.
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
3. Results and discussion Fig. 1 illustrates the planar and cross-section views of as-grown and annealed at 750 and 900 8C for 10 min ZnO samples. It is shown from Fig. 1(a) that the ZnO nanorod arrays did not have significant change in the morphology after the thermal annealing. The mean diameter of the nanorod is about 150 nm for as-grown and about 160 nm for annealed samples. Besides, the estimated length of nanorods after annealing increases to about 120 mm from about 84 mm of as-grown sample. And, the ZnO nanorod arrays annealed at 750 8C are more compact than other samples. The SEM images evidently indicate that the ZnO nanorod arrays underwent crystal restructuring during the thermal annealing process. As other annealed samples at 450 and 600 8C have similar morphology with the sample annealed at 900 8C, the other SEM images are not shown here. Fig. 2(a and b) illustrates the effects of thermal annealing temperature and duration on the crystallinity of the hydrothermally grown ZnO, respectively. In addition, XRD spectra of the asgrown and annealed samples are shown in the inserts of Fig. 2(a) and (b), respectively. It can be seen from the XRD spectra that only one peak was detected at 34.448, suggesting the growth is along the c-axis (0 0 2) orientation. From Fig. 2(a), it is seen that the intensity of (0 0 2) peak is dependent on annealing temperatures. It reaches the maximum value after annealed at 750 8C. The position of (0 0 2) peak shifts to a little higher 2u angles from 34.448 to 34.508 after annealing, which is possibly due to the release of intrinsic strain through annealing [12]. In addition, the full width at half maximum (FWHM) values of (0 0 2) peak are 0.288, 0.298, 0.298, 0.328, and 0.288 on as-grown and annealed ZnO/Si(1 1 1) at
Fig. 2. XRD (0 0 2) peak intensity with various (a) annealing temperatures with constant duration of 10 min, (b) annealing durations with fixed temperature of 750 8C. The inserted graphs are XRD spectra.
5863
450, 600, 750, and 900 8C, respectively. These small values indicate the high crystal properties of ZnO. After annealing at the temperature of less than 750 8C, the crystal quality was monotonously enhanced as the temperature increased. This can be possibly explained that when the annealing temperature is lower than 750 8C, the energy supplied by annealing is favorable for atoms to move into proper sites, leading to the improvement in the crystal quality [13]. In this temperature region, the re-crystallization is dominant. Due to the thermal equilibrium and the principle of minimum Gibbs free energy at the most stable condition, the Zn and O atoms would move into the right sites in its crystal structure to reduce the Gibbs free energy and form higher crystal quality of ZnO. However, as the annealing temperature is further increased to 900 8C, the intensity of (0 0 2) peak is greatly decreased. This can be possibly attributed to the more violent movements in atoms at higher temperature of 900 8C. It is accepted that as the temperature increases to higher, the movements of molecules become more violent, which deteriorates the perfection of crystal lattice. Therefore, when the sample was annealed at 900 8C, its crystal quality was sharply degraded. The deterioration can also be attributed to the evaporation of Zn and O atoms. It was reported that the evaporation of ZnO thin films slightly occurred when temperature exceeded 800 8C [13,14]. Although the bulk ZnO can be decomposed at as high as 1950 8C, it is easy for nano-scale ZnO to decompose at lower temperature. Zhao et al. [8] and Kim et al. [15] reported that the thickness of ZnO thin film became much thinner after annealing at 900 8C due to the evaporation.
Fig. 3. PL spectra with various (a) annealing temperatures with constant duration of 10 min, (b) annealing durations with fixed temperature of 750 8C. The inserted graphs are PL spectra in visible emission.
5864
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
Fig. 2(b) shows that the intensity of (0 0 2) peak increases as the annealing duration increases to 10 min, then decreases as the annealing duration reaches 30 min. In the annealing process, both the re-structuring and evaporation possibly occur. When the duration is shorter than 10 min, the re-crystallization is dominant, which results in the improvement of crystallinity. However, as the duration exceeds over 10 min to 30 min, the evaporation might be dominant, which possibly deteriorates the perfection of crystal structure. For the FWHM values of (0 0 2) peak, they are all less than 0.328, indicating the high crystal quality of these samples. Fig. 3(a) and (b) illustrate the room-temperature PL spectra as a function of thermal annealing temperature and duration, respectively. The PL spectra exhibit a strong UV emission and a weak visible emission peak. The UV peak is peaking at 3.24 eV, which is attributed to excitonic-related recombination. And there is no clear shift in UV peak position after annealing. For these samples, the visible peak is hardly observed from Fig. 3. The FWHM values from all samples are ranging from 110 to 120 meV. To further study the influence of annealing temperature and duration on visible emission of ZnO, the detailed spectra in visible region are illustrated in the insert of Fig. 3. As shown in the inserted graph in Fig. 3(a), the visible peak intensity is changed with different annealing temperatures. After annealing, the intensity of visible emission is greatly decreased compared with that of asgrown sample. It can be seen that the visible emission intensity diminishes as the annealing temperature increases to 750 8C. After annealing at 600 and 750 8C, the visible emission is almost quenched, indicating that there are hardly defects in these samples. However, it re-appears as the annealing temperature is further increased to 900 8C. As for its peak position, it is shifted to 500 nm after annealing at 900 8C compared with that of the asgrown sample (centered around 550 nm). The significant peak position shift suggests different defects existing in these two samples. The origins of yellow emission (550 nm) which are common in hydrothermally grown ZnO can be attributed to interstitial oxygen (Oi) defects [16] and the presence of Zn(OH)2 on the surface [17]. The re-appearance of green luminescence (500 nm) for the sample annealed at 900 8C might be attributed to other increase of the radiative deep centers, or the change of the defect occupancy [18]. The origin of green emission is also possibly from newly formed defects such as ionized oxygen vacancies. The violent movements of molecules at 900 8C might bring in more defects into ZnO such as ionized oxygen vacancies in some crystal lattices. These ionized oxygen vacancies can capture electrons, leading to the green emission. The oxygen vacancies are possibly also resulted from the evaporation of the O atoms. The inserted graph in Fig. 3(b) depicts the visible emission of ZnO with various annealing durations. From this graph, only asgrown sample shows a clear visible emission, while for the annealed samples, the visible emission is so weak that it can be negligible. This indicates that the defects relating to visible emission can be quickly reduced by annealing at 750 8C. It is recognized that the intensity ratio of UV emission peak to visible emission peak is a good way to evaluate the optical quality. As shown in Fig. 4(a), the ratio increases as the annealing temperature increases to 750 8C, and then decreases when the temperature further increases up to 900 8C. As for the UV intensity, there is no significant difference among the as-grown sample and the samples annealed at 450 and 600 8C, and it greatly increases when the temperature exceeds 600 8C. Furthermore, there is a rapid increase in intensity with the temperature increasing from 750 to 900 8C. There are two main ways for the photon-excited nonequilibrium carriers in semiconductors to recombine: radiative and non-radiative recombination. It is accepted that the light emission intensity is determined by both of radiative and non-
Fig. 4. UV intensity and IUV/IVisible with various (a) annealing temperatures with constant duration of 10 min, (b) annealing durations with fixed temperature of 750 8C.
radiative recombinations [19]. To improve the light emission efficiency, the density of non-radiative recombination centers should be as low as possible. The non-radiative recombination centers mainly include dislocations, point defects and surface/ interface states [20]. For as-grown sample, it has many nonradiative recombination centers due to the low temperature growth, resulting in the relative weak UV emission [21]. When the samples were annealed, these non-radiative-related defects might be reduced and re-structured. In addition, during annealing process, the other potential sources of non-radiative centers such as OH , H2O and other active molecules from solutions would be driven out from ZnO nanorods, which leads to higher UV emission efficiency [21]. And with higher temperature annealing, more active molecules are driven out. Consequently, the stronger UV emission intensity is resulted in. The effects of annealing duration on optical property of ZnO are described in Fig. 4(b). The UV emission peak intensity increases as the duration increases, indicating that the annihilation of nonradiative defects in ZnO is a slow and time-consuming process. The IUV/IVisible increases as the duration increases. When the duration is increased to 10 and 30 min, the IUV/IVisible reaches infinite. It suggests that the defects in ZnO are reduced through annealing and the longer the annealing duration, the fewer the defects. 4. Conclusions The effects of annealing temperature and duration on hydrothermally grown ZnO nanorod arrays were studied in this report. It has been found that the annealing treatment can efficiently improve the properties of hydrothermally grown ZnO. The crystal quality is enhanced with the annealing temperature up to 750 8C, and then degraded as the annealing temperature is further up to 900 8C possibly due to violent movements of atoms and evaporation. As for the effects of annealing duration on crystal property,
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
the intensity of (0 0 2) peak with the duration of 10 min is much stronger than with other durations. The UV peak intensity is monotonously increased as the annealing temperature increased to 900 8C. So is the same trend with the annealing duration. And the increase extent from 10 to 30 min is a little. For the intensity ratio of UV to visible emission peak, it reaches infinite with the annealing temperature of 750 8C for 10 and 30 min. In conclusion, the optimum annealing condition is 750 8C with the annealing duration of 10 min. Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOEST) (no. R01-2007-000-20580-0).
[5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15] [16] [17]
References [18] [1] S.T. Tan, B.J. Chen, X.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98 (2005) 013505. [2] H. Cao, J.Y. Xu, D.Z. Zhang, S.H. Chang, S.T. Ho, E.W. Seelig, X. Liu, R.P.H. Chang, Phys. Rev. Lett. 84 (2000) 5584. [3] Y.W. Zhu, H.Z. Zhang, X.C. Sun, S.Q. Feng, J. Xu, Q. Zhao, B. Xiang, R.M. Wang, D.P. Yu, Appl. Phys. Lett. 83 (2003) 144. [4] A.B. Djurisˇic´, Y.H. Leung, Small 2 (8–9) (2006) 944.
[19] [20] [21]
5865
J.J. Song, S.H. Baek, J.H. Lee, S.W. Lim, J. Chem. Technol. Biotechnol. 83 (2008) 345. R.S. Yadav, A.C. Pandey, Physica E 40 (2008) 660. Z.K. Li, X.T. Huang, J.P. Liu, H.H. Ai, Mater. Lett. 62 (2008) 2507. X.Q. Zhao, C.R. Kim, J.Y. Lee, J.H. Heo, C.M. Shin, H. Ryu, J.H. Chang, H.C. Lee, C.S. Son, W.J. Lee, W.G. Jung, S.T. Tan, J.L. Zhao, X.W. Sun, Appl. Surf. Sci. 255 (2009) 4461. J.M. Lim, C.M. Lee, J. Alloys Compd. 449 (2008) 371. Z.Y. Wang, B.B. Huang, X.J. Liu, X.Y. Qin, X.Y. Zhang, J.Y. Wei, P. Wang, S.S. Yao, Q. Zhang, X.Y. Jing, Mater. Lett. 62 (2008) 2637. S.T. Tan, X.W. Sun, J.L. Zhao, S. Iwan, Z.H. Cen, T.P. Chen, J.D. Ye, G.Q. Lo, D.L. Kwong, K.L. Teo, Appl. Phys. Lett. 93 (2008) 013506. R.J. Hong, J.B. Huang, H.B. He, Z.X. Fan, J.D. Shao, Appl. Surf. Sci. 242 (2005) 346. S.T. Tan, X.W. Sun, X.H. Zhang, S.J. Chua, B.J. Chen, C.C. Teo, J. Appl. Phys. 100 (2006) 033502. I.-W. Kim, K.-M. Lee, J. J. Appl. Phys. 46 (6A) (2007) 3556. C.R. Kim, J.Y. Lee, C.M. Shin, J.Y. Leem, H. Ryu, J.H. Chang, H.C. Lee, C.S. Son, W.J. Lee, W.G. Jung, S.T. Tan, J.L. Zhao, X.W. Sun, Solid State Commun. 148 (2008) 395. X.Q. Wei, Z.G. Zhang, M. Liu, C.S. Chen, G. Sun, C.S. Xue, H.Z. Zhuang, B.Y. Man, Mater. Chem. Phys. 101 (2007) 285. H. Zhou, H. Alves, D.M. Hofmann, W. Kriegseis, B.K. Meyer, G. Kaczmarczyk, A. Hoffmann, Appl. Phys. Lett. 80 (2002) 210. Z.Q. Chen, S. Yamamoto, M. Maekawa, A. Kawasuso, X.L. Yuan, T. Sekiguchi, J. Appl. Phys. 98 (8) (2003) 4807. L.H. Quang, S.J. Chua, K.P. Loh, E. Fitzgerald, J. Cryst. Growth 287 (2006) 157. Y.G. Wang, S.P. Lau, X.H. Zhang, H.H. Hng, H.W. Lee, S.F. Yu, B.K. Tay, J. Cryst. Growth 259 (2003) 335. K.H. Tam, C.K. Cheung, Y.H. Leung, A.B. Djurisıˇc´, C.C. Ling, C.D. Beling, S. Fung, W.M. Kwok, W.K. Chan, D.L. Phillips, L. Ding, W.K. Ge, J. Phys. Chem. B 110 (2006) 20865.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of thermal annealing temperature and duration on hydrothermally grown ZnO nanorod arrays X.Q. Zhao a, C.R. Kim a, J.Y. Lee a, C.M. Shin a, J.H. Heo a, J.Y. Leem a, H. Ryu a,*, J.H. Chang b, H.C. Lee c, C.S. Son d, B.C. Shin e, W.J. Lee e, W.G. Jung f, S.T. Tan g, J.L. Zhao h, X.W. Sun g,h a
Department of Nano Systems Engineering, Center for Nano Manufacturing, Inje University, Obang-dong, Gimhae, Gyeongnam 621-749, Republic of Korea Major of Nano Semiconductor, Korea Maritime University, #1 Dongsam-dong, Yeongdo-Ku, Busan 606-791, Republic of Korea Department of Mechatronics Engineering, Korea Maritime University, #1 Dongsam-dong, Yeongdo-Ku, Busan 606-791, Republic of Korea d Department of Electronic Materials Engineering, Silla University, Gwaebeop-dong, Sasang-gu, Busan 617-736, Republic of Korea e Department of Nano Engineering, Dong-Eui University, 995 Eomgwangno, Busanjin-gu, Busan 614-714, Republic of Korea f School of Advanced Materials Engineering, Kookmin University, 861-1, Jeongneung-dong, Seongbuk-gu, Seoul 136-702, Republic of Korea g Institute of Microelectronics, 11 Science Park Road, Science Park II, Singapore 117685, Singapore h School of Electrical & Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b c
A R T I C L E I N F O
A B S T R A C T
Article history: Received 19 December 2008 Received in revised form 8 January 2009 Accepted 8 January 2009 Available online 20 January 2009
In this study, the effects of thermal annealing temperature and duration on ZnO nanorod arrays fabricated by hydrothermal method were investigated. The annealed ZnO/Si(1 1 1) substrate was used for ZnO nanorod array growth. The effects of annealing treatment on the structural and optical properties were investigated by scanning electron microscopy, X-ray diffraction, and room-temperature photoluminescence measurements. With the annealing temperature of 750 8C and the annealing duration of 10 min, both the structural and optical properties of the ZnO nanorod arrays improved significantly, as indicated in the X-ray diffraction and photoluminescence measurement. ß 2009 Elsevier B.V. All rights reserved.
PACS: 81.05.Dz 81.16.Be 81.40.Ef Keywords: ZnO Hydrothermal synthesis Annealing treatment
1. Introduction II–VI semiconductor zinc oxide (ZnO) has attracted much attention due to its unique properties and versatile applications [1]. ZnO has a stable wurtzite structure with lattice spacing a = 0.325 nm and c = 0.521 nm. The structure of ZnO can be described as a number of alternating planes composed of tetrahedrally coordinated O2 and Zn2+ ions, which are stacked alternately along the c-axis. The oppositely charged ions produce ¯ positively charged (0 0 0 1)-Zn and negatively charged (0 0 0 1)-O polar planes, which result in a normal dipole moment and spontaneous polarization along the c-axis and different surface energy with the different crystallographic orientations. The excellent optical properties of ZnO such as a wide band gap (3.37 eV) and large exciton binding energy (60 meV) at room
* Corresponding author. Tel.: +82 55 320 3874; fax: +82 55 320 3631. E-mail address: [email protected] (H. Ryu). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.01.022
temperature endow ZnO as a promising material for roomtemperature ultraviolet lasers [2] and field-emission devices [3]. Besides, ZnO is one of the most widely applied oxide gas-sensing materials. The sensing mechanism of ZnO belongs to the surfacecontrolled type. The oxygen vacancies on the oxide surfaces are electrically and chemically active. Because of this mechanism, the ZnO nanostructures with high surface-to-volume ratios used as sensing materials has been regarded as one of the most efficient methods for fabricating high-efficiency gas sensors. Moreover, its piezoelectric property originating from its non-centrosymmetric structure makes it suitable for electromechanical sensor or actuator applications [4]. Various ZnO nanostructures such as nanorods and nanowires have been synthesized by hydrothermal method [5–7]. Song et al. [5] presented that well-aligned ZnO nanorod arrays were grown on ZnO-buffered Si substrate in pH value ranging from 10.3 to 10.9. Yadav and Pandey [6] reported that ZnO nanorods were synthesized by an organic-free hydrothermal process. Li et al. reported that long ZnO nanowires with high aspect ratio of up to
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
5862
1000 were grown on zinc substrate in NaOH aqueous solution by hydrothermal method [7]. The well-aligned ZnO rod arrays were grown on annealed ZnO-buffered Si(1 0 0) with different annealing temperatures. It was found that the good quality of hydrothermally grown ZnO can be obtained on ZnO buffer layer annealed at 750 8C, as reported in our previous work [8]. Therefore, in this study, ZnO buffer layer was annealed prior to main ZnO growth in order to obtain higher quality of hydrothermally grown ZnO. It is well known that hydrothermally grown ZnO has many defects due to its low growth temperature and that thermal treatment is an efficient method to reduce the defects and improve the qualities of ZnO [9,10]. In this study, thermal treatment was done to hydrothermally grown ZnO samples. The effects of annealing temperature and duration would be mainly discussed here. 2. Experimental procedures In this experiment, ZnO-buffered Si(1 1 1) [ZnO/Si(1 1 1)] was used for the growth of ZnO nanorod arrays. The ZnO buffer layer with a thickness of around 40 nm was deposited on Si(1 0 0) substrate at the growth temperature of 500 8C by metal-organic chemical vapor deposition method. The detailed deposition condition was described elsewhere [11]. Prior to the growth of ZnO nanorod, the ZnO/Si(1 1 1) was annealed at 750 8C for 2 min in nitrogen atmosphere with the chamber pressure of 1 Torr.
Zinc nitrate hexahydrate (Zn(NO3)2) and methenamine (HMT) with the concentration of 0.1 M and with the equal molar ratio were used as precursors to fabricate ZnO nanorod arrays. The pretreated ZnO/Si(1 1 1) was immersed into the mixed solution of Zn(NO3)2 and HMT, which was in Teflonlined steel autoclaves. The autoclaves were kept in an oven with the constant temperature of 95 8C for 10 h. After that, the autoclaves were taken out from the oven and allowed to cool down to room temperature naturally. Then the samples were taken out and washed with de-ionized water and blown dry with pure N2 gas. The hydrothermally grown samples were annealed with various temperatures ranging from 450 to 900 8C for 10 min at N2 atmosphere with the fixed chamber pressure of 1 Torr to investigate the effects of annealing temperature. Moreover, in order to study the influence of annealing duration, the samples were annealed with various durations from 2 to 30 min at the constant temperature of 750 8C in N2 atmosphere. The morphologies of ZnO nanorod arrays were examined via field-emission scanning electron microscope (FE-SEM) (HITACHI S4300SE). The crystal structures were analyzed by X-ray diffractometer (XRD) with Cu Ka radiation, and optical properties were characterized by photoluminescence (PL) measurement. The PL measurements were performed at room temperature with a He–Cd laser (l = 325 nm) as the excitation source.
Fig. 1. SEM images of as-grown and annealed ZnO at 750 and 900 8C for 10 min nanorod arrays: (a) planar view, (b) cross-section.
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
3. Results and discussion Fig. 1 illustrates the planar and cross-section views of as-grown and annealed at 750 and 900 8C for 10 min ZnO samples. It is shown from Fig. 1(a) that the ZnO nanorod arrays did not have significant change in the morphology after the thermal annealing. The mean diameter of the nanorod is about 150 nm for as-grown and about 160 nm for annealed samples. Besides, the estimated length of nanorods after annealing increases to about 120 mm from about 84 mm of as-grown sample. And, the ZnO nanorod arrays annealed at 750 8C are more compact than other samples. The SEM images evidently indicate that the ZnO nanorod arrays underwent crystal restructuring during the thermal annealing process. As other annealed samples at 450 and 600 8C have similar morphology with the sample annealed at 900 8C, the other SEM images are not shown here. Fig. 2(a and b) illustrates the effects of thermal annealing temperature and duration on the crystallinity of the hydrothermally grown ZnO, respectively. In addition, XRD spectra of the asgrown and annealed samples are shown in the inserts of Fig. 2(a) and (b), respectively. It can be seen from the XRD spectra that only one peak was detected at 34.448, suggesting the growth is along the c-axis (0 0 2) orientation. From Fig. 2(a), it is seen that the intensity of (0 0 2) peak is dependent on annealing temperatures. It reaches the maximum value after annealed at 750 8C. The position of (0 0 2) peak shifts to a little higher 2u angles from 34.448 to 34.508 after annealing, which is possibly due to the release of intrinsic strain through annealing [12]. In addition, the full width at half maximum (FWHM) values of (0 0 2) peak are 0.288, 0.298, 0.298, 0.328, and 0.288 on as-grown and annealed ZnO/Si(1 1 1) at
Fig. 2. XRD (0 0 2) peak intensity with various (a) annealing temperatures with constant duration of 10 min, (b) annealing durations with fixed temperature of 750 8C. The inserted graphs are XRD spectra.
5863
450, 600, 750, and 900 8C, respectively. These small values indicate the high crystal properties of ZnO. After annealing at the temperature of less than 750 8C, the crystal quality was monotonously enhanced as the temperature increased. This can be possibly explained that when the annealing temperature is lower than 750 8C, the energy supplied by annealing is favorable for atoms to move into proper sites, leading to the improvement in the crystal quality [13]. In this temperature region, the re-crystallization is dominant. Due to the thermal equilibrium and the principle of minimum Gibbs free energy at the most stable condition, the Zn and O atoms would move into the right sites in its crystal structure to reduce the Gibbs free energy and form higher crystal quality of ZnO. However, as the annealing temperature is further increased to 900 8C, the intensity of (0 0 2) peak is greatly decreased. This can be possibly attributed to the more violent movements in atoms at higher temperature of 900 8C. It is accepted that as the temperature increases to higher, the movements of molecules become more violent, which deteriorates the perfection of crystal lattice. Therefore, when the sample was annealed at 900 8C, its crystal quality was sharply degraded. The deterioration can also be attributed to the evaporation of Zn and O atoms. It was reported that the evaporation of ZnO thin films slightly occurred when temperature exceeded 800 8C [13,14]. Although the bulk ZnO can be decomposed at as high as 1950 8C, it is easy for nano-scale ZnO to decompose at lower temperature. Zhao et al. [8] and Kim et al. [15] reported that the thickness of ZnO thin film became much thinner after annealing at 900 8C due to the evaporation.
Fig. 3. PL spectra with various (a) annealing temperatures with constant duration of 10 min, (b) annealing durations with fixed temperature of 750 8C. The inserted graphs are PL spectra in visible emission.
5864
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
Fig. 2(b) shows that the intensity of (0 0 2) peak increases as the annealing duration increases to 10 min, then decreases as the annealing duration reaches 30 min. In the annealing process, both the re-structuring and evaporation possibly occur. When the duration is shorter than 10 min, the re-crystallization is dominant, which results in the improvement of crystallinity. However, as the duration exceeds over 10 min to 30 min, the evaporation might be dominant, which possibly deteriorates the perfection of crystal structure. For the FWHM values of (0 0 2) peak, they are all less than 0.328, indicating the high crystal quality of these samples. Fig. 3(a) and (b) illustrate the room-temperature PL spectra as a function of thermal annealing temperature and duration, respectively. The PL spectra exhibit a strong UV emission and a weak visible emission peak. The UV peak is peaking at 3.24 eV, which is attributed to excitonic-related recombination. And there is no clear shift in UV peak position after annealing. For these samples, the visible peak is hardly observed from Fig. 3. The FWHM values from all samples are ranging from 110 to 120 meV. To further study the influence of annealing temperature and duration on visible emission of ZnO, the detailed spectra in visible region are illustrated in the insert of Fig. 3. As shown in the inserted graph in Fig. 3(a), the visible peak intensity is changed with different annealing temperatures. After annealing, the intensity of visible emission is greatly decreased compared with that of asgrown sample. It can be seen that the visible emission intensity diminishes as the annealing temperature increases to 750 8C. After annealing at 600 and 750 8C, the visible emission is almost quenched, indicating that there are hardly defects in these samples. However, it re-appears as the annealing temperature is further increased to 900 8C. As for its peak position, it is shifted to 500 nm after annealing at 900 8C compared with that of the asgrown sample (centered around 550 nm). The significant peak position shift suggests different defects existing in these two samples. The origins of yellow emission (550 nm) which are common in hydrothermally grown ZnO can be attributed to interstitial oxygen (Oi) defects [16] and the presence of Zn(OH)2 on the surface [17]. The re-appearance of green luminescence (500 nm) for the sample annealed at 900 8C might be attributed to other increase of the radiative deep centers, or the change of the defect occupancy [18]. The origin of green emission is also possibly from newly formed defects such as ionized oxygen vacancies. The violent movements of molecules at 900 8C might bring in more defects into ZnO such as ionized oxygen vacancies in some crystal lattices. These ionized oxygen vacancies can capture electrons, leading to the green emission. The oxygen vacancies are possibly also resulted from the evaporation of the O atoms. The inserted graph in Fig. 3(b) depicts the visible emission of ZnO with various annealing durations. From this graph, only asgrown sample shows a clear visible emission, while for the annealed samples, the visible emission is so weak that it can be negligible. This indicates that the defects relating to visible emission can be quickly reduced by annealing at 750 8C. It is recognized that the intensity ratio of UV emission peak to visible emission peak is a good way to evaluate the optical quality. As shown in Fig. 4(a), the ratio increases as the annealing temperature increases to 750 8C, and then decreases when the temperature further increases up to 900 8C. As for the UV intensity, there is no significant difference among the as-grown sample and the samples annealed at 450 and 600 8C, and it greatly increases when the temperature exceeds 600 8C. Furthermore, there is a rapid increase in intensity with the temperature increasing from 750 to 900 8C. There are two main ways for the photon-excited nonequilibrium carriers in semiconductors to recombine: radiative and non-radiative recombination. It is accepted that the light emission intensity is determined by both of radiative and non-
Fig. 4. UV intensity and IUV/IVisible with various (a) annealing temperatures with constant duration of 10 min, (b) annealing durations with fixed temperature of 750 8C.
radiative recombinations [19]. To improve the light emission efficiency, the density of non-radiative recombination centers should be as low as possible. The non-radiative recombination centers mainly include dislocations, point defects and surface/ interface states [20]. For as-grown sample, it has many nonradiative recombination centers due to the low temperature growth, resulting in the relative weak UV emission [21]. When the samples were annealed, these non-radiative-related defects might be reduced and re-structured. In addition, during annealing process, the other potential sources of non-radiative centers such as OH , H2O and other active molecules from solutions would be driven out from ZnO nanorods, which leads to higher UV emission efficiency [21]. And with higher temperature annealing, more active molecules are driven out. Consequently, the stronger UV emission intensity is resulted in. The effects of annealing duration on optical property of ZnO are described in Fig. 4(b). The UV emission peak intensity increases as the duration increases, indicating that the annihilation of nonradiative defects in ZnO is a slow and time-consuming process. The IUV/IVisible increases as the duration increases. When the duration is increased to 10 and 30 min, the IUV/IVisible reaches infinite. It suggests that the defects in ZnO are reduced through annealing and the longer the annealing duration, the fewer the defects. 4. Conclusions The effects of annealing temperature and duration on hydrothermally grown ZnO nanorod arrays were studied in this report. It has been found that the annealing treatment can efficiently improve the properties of hydrothermally grown ZnO. The crystal quality is enhanced with the annealing temperature up to 750 8C, and then degraded as the annealing temperature is further up to 900 8C possibly due to violent movements of atoms and evaporation. As for the effects of annealing duration on crystal property,
X.Q. Zhao et al. / Applied Surface Science 255 (2009) 5861–5865
the intensity of (0 0 2) peak with the duration of 10 min is much stronger than with other durations. The UV peak intensity is monotonously increased as the annealing temperature increased to 900 8C. So is the same trend with the annealing duration. And the increase extent from 10 to 30 min is a little. For the intensity ratio of UV to visible emission peak, it reaches infinite with the annealing temperature of 750 8C for 10 and 30 min. In conclusion, the optimum annealing condition is 750 8C with the annealing duration of 10 min. Acknowledgement This work was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MOEST) (no. R01-2007-000-20580-0).
[5] [6] [7] [8]
[9] [10] [11] [12] [13] [14] [15] [16] [17]
References [18] [1] S.T. Tan, B.J. Chen, X.W. Sun, W.J. Fan, H.S. Kwok, X.H. Zhang, S.J. Chua, J. Appl. Phys. 98 (2005) 013505. [2] H. Cao, J.Y. Xu, D.Z. Zhang, S.H. Chang, S.T. Ho, E.W. Seelig, X. Liu, R.P.H. Chang, Phys. Rev. Lett. 84 (2000) 5584. [3] Y.W. Zhu, H.Z. Zhang, X.C. Sun, S.Q. Feng, J. Xu, Q. Zhao, B. Xiang, R.M. Wang, D.P. Yu, Appl. Phys. Lett. 83 (2003) 144. [4] A.B. Djurisˇic´, Y.H. Leung, Small 2 (8–9) (2006) 944.
[19] [20] [21]
5865
J.J. Song, S.H. Baek, J.H. Lee, S.W. Lim, J. Chem. Technol. Biotechnol. 83 (2008) 345. R.S. Yadav, A.C. Pandey, Physica E 40 (2008) 660. Z.K. Li, X.T. Huang, J.P. Liu, H.H. Ai, Mater. Lett. 62 (2008) 2507. X.Q. Zhao, C.R. Kim, J.Y. Lee, J.H. Heo, C.M. Shin, H. Ryu, J.H. Chang, H.C. Lee, C.S. Son, W.J. Lee, W.G. Jung, S.T. Tan, J.L. Zhao, X.W. Sun, Appl. Surf. Sci. 255 (2009) 4461. J.M. Lim, C.M. Lee, J. Alloys Compd. 449 (2008) 371. Z.Y. Wang, B.B. Huang, X.J. Liu, X.Y. Qin, X.Y. Zhang, J.Y. Wei, P. Wang, S.S. Yao, Q. Zhang, X.Y. Jing, Mater. Lett. 62 (2008) 2637. S.T. Tan, X.W. Sun, J.L. Zhao, S. Iwan, Z.H. Cen, T.P. Chen, J.D. Ye, G.Q. Lo, D.L. Kwong, K.L. Teo, Appl. Phys. Lett. 93 (2008) 013506. R.J. Hong, J.B. Huang, H.B. He, Z.X. Fan, J.D. Shao, Appl. Surf. Sci. 242 (2005) 346. S.T. Tan, X.W. Sun, X.H. Zhang, S.J. Chua, B.J. Chen, C.C. Teo, J. Appl. Phys. 100 (2006) 033502. I.-W. Kim, K.-M. Lee, J. J. Appl. Phys. 46 (6A) (2007) 3556. C.R. Kim, J.Y. Lee, C.M. Shin, J.Y. Leem, H. Ryu, J.H. Chang, H.C. Lee, C.S. Son, W.J. Lee, W.G. Jung, S.T. Tan, J.L. Zhao, X.W. Sun, Solid State Commun. 148 (2008) 395. X.Q. Wei, Z.G. Zhang, M. Liu, C.S. Chen, G. Sun, C.S. Xue, H.Z. Zhuang, B.Y. Man, Mater. Chem. Phys. 101 (2007) 285. H. Zhou, H. Alves, D.M. Hofmann, W. Kriegseis, B.K. Meyer, G. Kaczmarczyk, A. Hoffmann, Appl. Phys. Lett. 80 (2002) 210. Z.Q. Chen, S. Yamamoto, M. Maekawa, A. Kawasuso, X.L. Yuan, T. Sekiguchi, J. Appl. Phys. 98 (8) (2003) 4807. L.H. Quang, S.J. Chua, K.P. Loh, E. Fitzgerald, J. Cryst. Growth 287 (2006) 157. Y.G. Wang, S.P. Lau, X.H. Zhang, H.H. Hng, H.W. Lee, S.F. Yu, B.K. Tay, J. Cryst. Growth 259 (2003) 335. K.H. Tam, C.K. Cheung, Y.H. Leung, A.B. Djurisıˇc´, C.C. Ling, C.D. Beling, S. Fung, W.M. Kwok, W.K. Chan, D.L. Phillips, L. Ding, W.K. Ge, J. Phys. Chem. B 110 (2006) 20865.