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Influence of high-pressure hydrogen treatment on structural and electrical properties of ZnO thin films
Applied Surface Science 256 (2010) 6770–6774
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of high-pressure hydrogen treatment on structural and electrical properties of ZnO thin films Chunye Li a , Hongwei Liang a,∗ , Jianze Zhao a , Qiuju Feng a,c , Jiming Bian a , Yang liu a , Rensheng Shen a , Wangcheng Li b , Guoguang Wu b , G.T. Du a,b,∗∗ a b c
School of Physics and Optoelectronic Technology of Dalian University of Technology, Dalian 116024, People’s Republic of China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130023, People’s Republic of China School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 31 March 2010 Received in revised form 21 April 2010 Accepted 21 April 2010 Available online 29 April 2010 Keywords: ZnO MOCVD High-pressure H2 Zn(OH)2
a b s t r a c t ZnO thin films were treated by high-pressure hydrogen (H2 ). Scanning electron microscope (SEM) images show that the surface morphology of ZnO films has been changed significantly by H2 treatment. X-ray diffraction patterns show that the Zn(OH)2 phases formed after H2 treatment. The X-ray photoelectron spectroscopy results indicate that H atoms were doped into the surface of ZnO by forming H–O–Zn bond. The phenomenon shows that it is easy to form O–H bond in ZnO rather than H interstitial atom under high-pressure hydrogen circumstance. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO), a wide-band-gap semiconductor, has gained wide interest in recent years due to its large exciton binding energy. It is a candidate material for both visible and ultraviolet light emitting diodes (LEDs) and laser diode. However, it is difficult to achieve p-type ZnO due to the high activation energy of acceptors, low solubility of acceptor dopants and self-compensating process during acceptor doping [1–3]. In addition, Van de Walle, based on first-principles density functional calculations, suggested that hydrogen will be unexpectedly incorporated into ZnO as a shallow donor during deposition process [4]. Since then, many groups have reported experiment results which indicate the existence of hydrogen shallow donors in ZnO. Cox et al. have confirmed the prediction that interstitial hydrogen atom acts as a shallow donor in ZnO by experiment [5]. Ohashi et al. have pointed out that hydrogen doping could improve the ultraviolet emission efficiency [6]. In these experiments, ZnO samples are usually treated with H ions rather than H2 . In order to gain a deeper understanding of the hydrogen molecules in ZnO surfaces, the influence on ZnO films of treat-
∗ Corresponding author. Tel.: +86 411 84707865; fax: +86 411 84707865. ∗∗ Corresponding author at: School of Physics and Optoelectronic Technology of Dalian University of Technology, Dalian 116024, People’s Republic of China. Tel.: +86 411 84707865; fax: +86 411 84707865. E-mail addresses: [email protected] (H. Liang), [email protected] (G.T. Du). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.087
ment by high-pressure hydrogen at room temperature (RT) were investigated. 2. Experiment Undoped ZnO films were deposited on c-plane sapphire by Metal Organic Chemical Vapor Deposition (MOCVD). The schematic structure of this system was reported previously [7]. c-Plane sapphires were chosen as substrates because ZnO and sapphire have almost same hexagonal close-packed crystal structure. Diethylzinc (DEZn) and high purity O2 were chosen as precursors of Zn and O, respectively. The growth temperature was fixed at 500 ◦ C. Bubbled diethylzinc (DEZn) was introduced into the reaction chamber using high purity Ar (99.999%) as carrier gas which flow was set at 10 sccm. To investigate the influence of hydrogen on the ZnO films with different stoichiometric ratio, the DEZn source flow rate was fixed and three ZnO samples: samples A, C and E were grown with different O2 fluxes at 600, 400 and 200 sccm, respectively. The time of deposition is 1 h, and the thickness of these films was ∼1 m. Each sample cut into two pieces. One was put into a dry H2 gas cylinder with 90 MPa H2 pressure for 40 days while the other was put in a vacuum chamber for comparison. The high-pressure H2 treated pieces of samples A, C and E are signed as samples B, D and F, respectively. The details of experiment were listed in Table 1. The electrical properties, structural properties, surface morphology and chemical state of ZnO films were investigated. The electrical properties were measured by Hall system (Bio-Rad
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Table 1 The experiment details of the six samples prepared at 500 ◦ C. Experiment condition The flux of O2 (sccm) High-pressure treated H2
Sample A 600 Without
Sample B 600 40 days
Sample C 400 Without
Sample D 400 40 days
Sample E 200 Without
Sample F 200 40 days
Table 2 The Hall measurements results of the untreated and hydrogen treated samples.
Resistivity ( cm) Hall mobility (cm2 V−1 s−1 ) Carrier concentration (cm−3 )
Sample A
Sample B
Sample C
Sample D
Sample E
Sample F
152.2 0.05 8.25 × 1017
10.30 0.12 5.18 × 1018
293.90 0.13 1.66 × 1017
54.48 0.17 6.85 × 1017
0.39 1.84 8.71 × 1018
0.33 2.02 9.30 × 1018
Fig. 1. The XRD patterns of the six ZnO samples.
HL5800) using Van de Pauw configuration at RT. The crystalline quality of the ZnO films was determined by X-ray diffraction (Shimadzu XRD-6000 system with Cu Ka1 radiation k = 0.15406 nm). The surface morphology of the ZnO films is characterized by field emission scanning electron microscope (SEM) (HITACHI S-4800). To investigate the chemical state of oxygen in ZnO films, X-ray photoelectron spectroscopy (XPS) measurements were conducted using an ESCALAB Mark II X-ray photoemission spectrometer with Mg Ka radiation source. 3. Results and discussions The resistivity, Hall mobility, and carrier concentration of six samples were measured by Hall effect measurements at RT. The results are listed in Table 2. Comparing the samples with and without H2 treatment, we could find that the carrier concentrations of H2 treated samples are bigger than that of untreated samples. And the mobility has also been improved a little after treatment by H2 . Seung et al obtained similar results with post-deposition hydrogen doping [8]. The crystalline quality and crystal orientation of ZnO thin films were investigated by the X-ray diffraction (XRD). The measurement was conducted with the 2 diffraction angle from 20◦ to 80◦ continuously. The XRD patterns are shown in Fig. 1. Besides the sapphire (0 0 6) diffraction peak located at 41.68◦ , the dominated ZnO (0 0 2) and (0 0 4) diffraction peaks are located at 34.41◦ and 72.54◦ , respectively. The ZnO (1 0 1) diffraction peak is also observed which indicates that the ZnO films are the wurtzite structure with strong c-axis oriented. The full width at half maximum (FWHM) of the ZnO (0 0 2) diffraction peak is 0.22◦ , suggesting that
the ZnO film has good crystal quality. Moreover, a new diffraction peak appears at 20.48◦ in XRD pattern of H2 treated samples as shown in Fig. 2. There are only Zn, O, H elements in samples, and this peak is unrelated to ZnO, so this peak suppose to be related to Zn(OH)2 phase. From powder diffraction file (No. 380385), the Zn(OH)2 (1 1 0) diffraction peak position is 20.149◦ . The position is slightly less than 20.48◦ . The movement may be due to stress between ZnO and Zn(OH)2 . Later, the validity of this assumption will be demonstrated. The surface morphology of the six ZnO samples was in Fig. 3. Obviously, the as-grown samples are consisted of dense grains. It is noted that the surface morphology of H2 treated ZnO samples B, D and F are greatly different from as-grown samples A, C and E. After H2 treatment, the surface of the ZnO films changed from grain to network structure, as shown in Fig. 3b and d. Sample B has very different surface morphology comparing with sample A. Then the difference is more obvious between samples C and D. The surface of sample F is completely different from sample E. Because all these samples are treated at RT with different gaseous environment, the change on surface morphology was not influenced by temperature. So we suppose that atoms migrate in the surface and a new chemical reaction happened during the H2 treatment process. To investigate the reaction between ZnO and H2 , the variation of the O 1s chemical bonding states is measured by XPS. The results of six ZnO samples are shown in Fig. 4. For untreated samples, each XPS spectrum can be well fitted by two Gaussian line shapes, locating at about 529.9 and about 531.6 eV, respectively. While the XPS spectrum of H2 treated samples can be well fitted by two Gaussian line shapes, locating at about 528.9 and about 531.6 eV, respectively. The peak located at 529.9 eV is corresponding to O–Zn bond, while the peak located at 531.6 eV, could be attributed to O–H bond [9]. These results suggested that H ions were introduced into the surface of ZnO samples because of that surface analysis characteristic of XPS. It was not conflicted with XRD result, which had still dominant ZnO peak. And XRD result above-mentioned simultaneously showed the H ions were not introduced into the bulk of ZnO. For sample A, it is deposited in O-rich conditions and the ratio of O to Zn is bigger than samples C and E. We think the sample A has a number of Zn vacancies, and Woo-Jin Lee et al. mentioned that under O-rich condition, the Zn vacancies easy formed [10]. These Zn vacancies are occupied by H ions which are caused through the reaction of ZnO and H2 O in air [11]. For samples C and E, with the ratio of O to Zn decreasing, the lower energy peak at 529.9 eV is relatively stronger to the higher energy peak at 531.6 eV. It suggested that with decreasing the ratio of O to Zn, the probability of H ions occupying Zn vacancies is less. From XPS results of H2 treated samples B, D and F, the reaction of H2 and ZnO is ample, and the higher energy peak corresponding to O–H bond is major peak due to H2 treatment with long time. It was worth noting that the lower energy peak had a slight shift about 1 eV between samples B, D
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Fig. 2. The XRD patterns of the six ZnO samples.
Fig. 3. SEM images of the six ZnO samples.
C. Li et al. / Applied Surface Science 256 (2010) 6770–6774
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Fig. 4. The XPS spectra in O (1s) region for the six ZnO samples.
and sample F. It may be due to a number of H ions occupying Zn vacancies forming O–H band, weakening O–Zn bond and causing the lower energy peak shift slightly. The details are investigated in progress. Ntep et al. reported that H2 could act as a sublimation activator of ZnO [12]. Kleinwechter et al. also observed same reaction when ZnO were exposed in H2 rich atmospheres at high temperature (T > 400 K) [13]. Ohnishi et al. mentioned that when ZnO films are exposed to the deoxidation gas of H2 , and the following reaction will happen [14]: ZnO + H2 → Zn + H2 O ZnO + H2 O → Zn(OH)2 From the XPS spectra of samples A, C, E, it can be found that the higher energy peak decreases gradually to a shoulder with decreasing O2 flux. But there is no obvious change of the lower energy peak. We can conclude that O–Zn bonding is major structure of O in undoped ZnO films. Nevertheless in the spectra of samples B, D and F, only the higher energy peaks exist. This indicates H atom is introduced into ZnO films and H–O–Zn bonding is formed [15,16]. This is corresponding to the O–H peak observed in Fig. 2 which is related to Zn(OH)2 . So, it is suggested that the hydrogen could be introduced into ZnO easily to form Zn(OH)2 . 4. Conclusion In summary, some of ZnO films obtained by MOCVD were treated in high-pressure H2 . A new chemical reaction happened during the H2 treatment process. The XPS results show that H atom have been introduced into ZnO surface by forming O–H bond. These could identified the peaks around 20◦ in XRD pattern is related to Zn(OH)2 phase. These results demonstrate that H could introduce into ZnO films easily by forming O-H bond in high-pressure H2 at RT.
Acknowledgments This work was supported by NSFC (Grant Nos. 60976010, 10804040, 60877020 and 50532080), China Postdoctoral Science Foundation (No. 20081081), Youth Teacher Cultivation Fund by Dalian University of Technology, and Doctoral Scientific Research Starting Foundation of Liaoning Province (No. 20081081).
References [1] C.H. Park, S.B. Zhang, S.-H. Wei, Origin of p-type doping difficulty in ZnO: the impurity perspective, Physical Review B 66 (2002) 073202. [2] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Characterization of homoepitaxial p-type ZnO grown by molecular beam epitaxy, Applied Physics Letters 81 (2002) 1830–1832. [3] K.-K. Kim, H.-S. Kim, D.-K. Hwang, J.-H. Lim, S.-J. Park, Realization of p-type ZnO thin films via phosphorus doping and thermal activation of the dopant, Applied Physics Letters 83 (2003) 63–65. [4] C.G. Van de Walle, hydrogen as a cause of doping in zinc oxide, Physical Review Letters 85 (2000) 1012–1015. [5] S.F.J. Cox, E.A. Davis, S.P. Cottrell, P.J.C. King, J.S. Lord, J.M. Gil, H.V. Alberto, R.C. Vilão, J. Piroto Duarte, N. Ayresde Campos, A. Weidinger, R.L. Lichti, S.J.C. Irvine, Experimental confirmation of the predicted shallow donor hydrogen state in zinc oxide, Physical Review Letters 86 (2001) 2601–2604. [6] N. Ohashi, T. Ishigaki, N. Okada, T. Sekiguchi, I. Sakaguchi, H. Haneda, Effect of hydrogen doping on ultraviolet emission spectra of various types of ZnO, Applied Physics Letters 80 (2002) 2869–2871. [7] G. Du, Y. Ma, Y. Zhang, T. Yang, Preparation of intrinsic and N-doped p-type ZnO thin films by metalorganic vapor phase epitaxy, Applied Physics Letters 87 (2005) 213103. [8] S.Y. Myong, K.S. Lim, Improved electrical stability and UV emission of zinc oxide thin films prepared by combination of metalorganic chemical vapor deposition technique and post-deposition hydrogen doping, Organic Electronics 8 (2007) 51–56. [9] L.-J. Meng, C.P. Moreira de Sá, M.P. dos, Santos, Study of the structural properies of ZnO thin films by X-ray photoelectron spectroscopy, Applied Surface Science 78 (1994) 57–61. [10] W.-J. Lee, J. Kang, K.J. Chang, Electronic structure of phosphorus dopants in ZnO, Physica B 376–377 (2006) 699–702. ˇ zek, J. Kuriplach, [11] G. Brauer, W. Anwand, D. Grambole, J. Grenzer, W. Skorupa, J. Cíˇ I. Procházka, C.C. Ling, C.K. So, D. Schulz, D. Klimm, Identification of Znvacancy–hydrogen complexes in ZnO single crystals: a challenge to positron annihilation spectroscopy, Physical Review B 79 (2009) 115212.
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[12] J.-M. Ntep, M. Barbé, G. Cohen-Solal, F. Bailly, A. Lusson, R. Triboulet, ZnO growth by chemically assisted sublimation, Journal of Crystal Growth 184/185 (1998) 1026–1030. [13] H. Kleinwechter, C. Janzen, J. Knipping, H. Wiggers, P. Roth, Formation and properties of ZnO nano-particles from gas phase synthesis processes, Journal of Material Science 37 (2002) 4349–4360. [14] S. Ohnishi, Y. Hirokawa, T. Shiosaki, A. Kawabata, chemical vapor deposition of singe-crystalline ZnO film with smooth surface on intermediately sputtered
ZnO thin film on sapphire, Japanese Journal of Applied Physics 17 (5) (1978) 773–778. [15] Y.F. Lu, H.Q. Ni, Z.H. Mai, Z.M. Ren, The effects of thermal annealing on ZnO thin films grown by pulsed laser deposition, Journal of Applied Physics 88 (2000) 498–502. [16] H. Xu, W. Fan, A.L. Rosa, R.Q. Zhang, Th. Frauenheim, Hydrogen and oxygen adsorption on ZnO nanowires: a first-principles study, Physical Review B 79 (2009) 073402.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Influence of high-pressure hydrogen treatment on structural and electrical properties of ZnO thin films Chunye Li a , Hongwei Liang a,∗ , Jianze Zhao a , Qiuju Feng a,c , Jiming Bian a , Yang liu a , Rensheng Shen a , Wangcheng Li b , Guoguang Wu b , G.T. Du a,b,∗∗ a b c
School of Physics and Optoelectronic Technology of Dalian University of Technology, Dalian 116024, People’s Republic of China State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130023, People’s Republic of China School of Physics and Electronic Technology, Liaoning Normal University, Dalian 116029, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 31 March 2010 Received in revised form 21 April 2010 Accepted 21 April 2010 Available online 29 April 2010 Keywords: ZnO MOCVD High-pressure H2 Zn(OH)2
a b s t r a c t ZnO thin films were treated by high-pressure hydrogen (H2 ). Scanning electron microscope (SEM) images show that the surface morphology of ZnO films has been changed significantly by H2 treatment. X-ray diffraction patterns show that the Zn(OH)2 phases formed after H2 treatment. The X-ray photoelectron spectroscopy results indicate that H atoms were doped into the surface of ZnO by forming H–O–Zn bond. The phenomenon shows that it is easy to form O–H bond in ZnO rather than H interstitial atom under high-pressure hydrogen circumstance. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO), a wide-band-gap semiconductor, has gained wide interest in recent years due to its large exciton binding energy. It is a candidate material for both visible and ultraviolet light emitting diodes (LEDs) and laser diode. However, it is difficult to achieve p-type ZnO due to the high activation energy of acceptors, low solubility of acceptor dopants and self-compensating process during acceptor doping [1–3]. In addition, Van de Walle, based on first-principles density functional calculations, suggested that hydrogen will be unexpectedly incorporated into ZnO as a shallow donor during deposition process [4]. Since then, many groups have reported experiment results which indicate the existence of hydrogen shallow donors in ZnO. Cox et al. have confirmed the prediction that interstitial hydrogen atom acts as a shallow donor in ZnO by experiment [5]. Ohashi et al. have pointed out that hydrogen doping could improve the ultraviolet emission efficiency [6]. In these experiments, ZnO samples are usually treated with H ions rather than H2 . In order to gain a deeper understanding of the hydrogen molecules in ZnO surfaces, the influence on ZnO films of treat-
∗ Corresponding author. Tel.: +86 411 84707865; fax: +86 411 84707865. ∗∗ Corresponding author at: School of Physics and Optoelectronic Technology of Dalian University of Technology, Dalian 116024, People’s Republic of China. Tel.: +86 411 84707865; fax: +86 411 84707865. E-mail addresses: [email protected] (H. Liang), [email protected] (G.T. Du). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.04.087
ment by high-pressure hydrogen at room temperature (RT) were investigated. 2. Experiment Undoped ZnO films were deposited on c-plane sapphire by Metal Organic Chemical Vapor Deposition (MOCVD). The schematic structure of this system was reported previously [7]. c-Plane sapphires were chosen as substrates because ZnO and sapphire have almost same hexagonal close-packed crystal structure. Diethylzinc (DEZn) and high purity O2 were chosen as precursors of Zn and O, respectively. The growth temperature was fixed at 500 ◦ C. Bubbled diethylzinc (DEZn) was introduced into the reaction chamber using high purity Ar (99.999%) as carrier gas which flow was set at 10 sccm. To investigate the influence of hydrogen on the ZnO films with different stoichiometric ratio, the DEZn source flow rate was fixed and three ZnO samples: samples A, C and E were grown with different O2 fluxes at 600, 400 and 200 sccm, respectively. The time of deposition is 1 h, and the thickness of these films was ∼1 m. Each sample cut into two pieces. One was put into a dry H2 gas cylinder with 90 MPa H2 pressure for 40 days while the other was put in a vacuum chamber for comparison. The high-pressure H2 treated pieces of samples A, C and E are signed as samples B, D and F, respectively. The details of experiment were listed in Table 1. The electrical properties, structural properties, surface morphology and chemical state of ZnO films were investigated. The electrical properties were measured by Hall system (Bio-Rad
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Table 1 The experiment details of the six samples prepared at 500 ◦ C. Experiment condition The flux of O2 (sccm) High-pressure treated H2
Sample A 600 Without
Sample B 600 40 days
Sample C 400 Without
Sample D 400 40 days
Sample E 200 Without
Sample F 200 40 days
Table 2 The Hall measurements results of the untreated and hydrogen treated samples.
Resistivity ( cm) Hall mobility (cm2 V−1 s−1 ) Carrier concentration (cm−3 )
Sample A
Sample B
Sample C
Sample D
Sample E
Sample F
152.2 0.05 8.25 × 1017
10.30 0.12 5.18 × 1018
293.90 0.13 1.66 × 1017
54.48 0.17 6.85 × 1017
0.39 1.84 8.71 × 1018
0.33 2.02 9.30 × 1018
Fig. 1. The XRD patterns of the six ZnO samples.
HL5800) using Van de Pauw configuration at RT. The crystalline quality of the ZnO films was determined by X-ray diffraction (Shimadzu XRD-6000 system with Cu Ka1 radiation k = 0.15406 nm). The surface morphology of the ZnO films is characterized by field emission scanning electron microscope (SEM) (HITACHI S-4800). To investigate the chemical state of oxygen in ZnO films, X-ray photoelectron spectroscopy (XPS) measurements were conducted using an ESCALAB Mark II X-ray photoemission spectrometer with Mg Ka radiation source. 3. Results and discussions The resistivity, Hall mobility, and carrier concentration of six samples were measured by Hall effect measurements at RT. The results are listed in Table 2. Comparing the samples with and without H2 treatment, we could find that the carrier concentrations of H2 treated samples are bigger than that of untreated samples. And the mobility has also been improved a little after treatment by H2 . Seung et al obtained similar results with post-deposition hydrogen doping [8]. The crystalline quality and crystal orientation of ZnO thin films were investigated by the X-ray diffraction (XRD). The measurement was conducted with the 2 diffraction angle from 20◦ to 80◦ continuously. The XRD patterns are shown in Fig. 1. Besides the sapphire (0 0 6) diffraction peak located at 41.68◦ , the dominated ZnO (0 0 2) and (0 0 4) diffraction peaks are located at 34.41◦ and 72.54◦ , respectively. The ZnO (1 0 1) diffraction peak is also observed which indicates that the ZnO films are the wurtzite structure with strong c-axis oriented. The full width at half maximum (FWHM) of the ZnO (0 0 2) diffraction peak is 0.22◦ , suggesting that
the ZnO film has good crystal quality. Moreover, a new diffraction peak appears at 20.48◦ in XRD pattern of H2 treated samples as shown in Fig. 2. There are only Zn, O, H elements in samples, and this peak is unrelated to ZnO, so this peak suppose to be related to Zn(OH)2 phase. From powder diffraction file (No. 380385), the Zn(OH)2 (1 1 0) diffraction peak position is 20.149◦ . The position is slightly less than 20.48◦ . The movement may be due to stress between ZnO and Zn(OH)2 . Later, the validity of this assumption will be demonstrated. The surface morphology of the six ZnO samples was in Fig. 3. Obviously, the as-grown samples are consisted of dense grains. It is noted that the surface morphology of H2 treated ZnO samples B, D and F are greatly different from as-grown samples A, C and E. After H2 treatment, the surface of the ZnO films changed from grain to network structure, as shown in Fig. 3b and d. Sample B has very different surface morphology comparing with sample A. Then the difference is more obvious between samples C and D. The surface of sample F is completely different from sample E. Because all these samples are treated at RT with different gaseous environment, the change on surface morphology was not influenced by temperature. So we suppose that atoms migrate in the surface and a new chemical reaction happened during the H2 treatment process. To investigate the reaction between ZnO and H2 , the variation of the O 1s chemical bonding states is measured by XPS. The results of six ZnO samples are shown in Fig. 4. For untreated samples, each XPS spectrum can be well fitted by two Gaussian line shapes, locating at about 529.9 and about 531.6 eV, respectively. While the XPS spectrum of H2 treated samples can be well fitted by two Gaussian line shapes, locating at about 528.9 and about 531.6 eV, respectively. The peak located at 529.9 eV is corresponding to O–Zn bond, while the peak located at 531.6 eV, could be attributed to O–H bond [9]. These results suggested that H ions were introduced into the surface of ZnO samples because of that surface analysis characteristic of XPS. It was not conflicted with XRD result, which had still dominant ZnO peak. And XRD result above-mentioned simultaneously showed the H ions were not introduced into the bulk of ZnO. For sample A, it is deposited in O-rich conditions and the ratio of O to Zn is bigger than samples C and E. We think the sample A has a number of Zn vacancies, and Woo-Jin Lee et al. mentioned that under O-rich condition, the Zn vacancies easy formed [10]. These Zn vacancies are occupied by H ions which are caused through the reaction of ZnO and H2 O in air [11]. For samples C and E, with the ratio of O to Zn decreasing, the lower energy peak at 529.9 eV is relatively stronger to the higher energy peak at 531.6 eV. It suggested that with decreasing the ratio of O to Zn, the probability of H ions occupying Zn vacancies is less. From XPS results of H2 treated samples B, D and F, the reaction of H2 and ZnO is ample, and the higher energy peak corresponding to O–H bond is major peak due to H2 treatment with long time. It was worth noting that the lower energy peak had a slight shift about 1 eV between samples B, D
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Fig. 2. The XRD patterns of the six ZnO samples.
Fig. 3. SEM images of the six ZnO samples.
C. Li et al. / Applied Surface Science 256 (2010) 6770–6774
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Fig. 4. The XPS spectra in O (1s) region for the six ZnO samples.
and sample F. It may be due to a number of H ions occupying Zn vacancies forming O–H band, weakening O–Zn bond and causing the lower energy peak shift slightly. The details are investigated in progress. Ntep et al. reported that H2 could act as a sublimation activator of ZnO [12]. Kleinwechter et al. also observed same reaction when ZnO were exposed in H2 rich atmospheres at high temperature (T > 400 K) [13]. Ohnishi et al. mentioned that when ZnO films are exposed to the deoxidation gas of H2 , and the following reaction will happen [14]: ZnO + H2 → Zn + H2 O ZnO + H2 O → Zn(OH)2 From the XPS spectra of samples A, C, E, it can be found that the higher energy peak decreases gradually to a shoulder with decreasing O2 flux. But there is no obvious change of the lower energy peak. We can conclude that O–Zn bonding is major structure of O in undoped ZnO films. Nevertheless in the spectra of samples B, D and F, only the higher energy peaks exist. This indicates H atom is introduced into ZnO films and H–O–Zn bonding is formed [15,16]. This is corresponding to the O–H peak observed in Fig. 2 which is related to Zn(OH)2 . So, it is suggested that the hydrogen could be introduced into ZnO easily to form Zn(OH)2 . 4. Conclusion In summary, some of ZnO films obtained by MOCVD were treated in high-pressure H2 . A new chemical reaction happened during the H2 treatment process. The XPS results show that H atom have been introduced into ZnO surface by forming O–H bond. These could identified the peaks around 20◦ in XRD pattern is related to Zn(OH)2 phase. These results demonstrate that H could introduce into ZnO films easily by forming O-H bond in high-pressure H2 at RT.
Acknowledgments This work was supported by NSFC (Grant Nos. 60976010, 10804040, 60877020 and 50532080), China Postdoctoral Science Foundation (No. 20081081), Youth Teacher Cultivation Fund by Dalian University of Technology, and Doctoral Scientific Research Starting Foundation of Liaoning Province (No. 20081081).
References [1] C.H. Park, S.B. Zhang, S.-H. Wei, Origin of p-type doping difficulty in ZnO: the impurity perspective, Physical Review B 66 (2002) 073202. [2] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Characterization of homoepitaxial p-type ZnO grown by molecular beam epitaxy, Applied Physics Letters 81 (2002) 1830–1832. [3] K.-K. Kim, H.-S. Kim, D.-K. Hwang, J.-H. Lim, S.-J. Park, Realization of p-type ZnO thin films via phosphorus doping and thermal activation of the dopant, Applied Physics Letters 83 (2003) 63–65. [4] C.G. Van de Walle, hydrogen as a cause of doping in zinc oxide, Physical Review Letters 85 (2000) 1012–1015. [5] S.F.J. Cox, E.A. Davis, S.P. Cottrell, P.J.C. King, J.S. Lord, J.M. Gil, H.V. Alberto, R.C. Vilão, J. Piroto Duarte, N. Ayresde Campos, A. Weidinger, R.L. Lichti, S.J.C. Irvine, Experimental confirmation of the predicted shallow donor hydrogen state in zinc oxide, Physical Review Letters 86 (2001) 2601–2604. [6] N. Ohashi, T. Ishigaki, N. Okada, T. Sekiguchi, I. Sakaguchi, H. Haneda, Effect of hydrogen doping on ultraviolet emission spectra of various types of ZnO, Applied Physics Letters 80 (2002) 2869–2871. [7] G. Du, Y. Ma, Y. Zhang, T. Yang, Preparation of intrinsic and N-doped p-type ZnO thin films by metalorganic vapor phase epitaxy, Applied Physics Letters 87 (2005) 213103. [8] S.Y. Myong, K.S. Lim, Improved electrical stability and UV emission of zinc oxide thin films prepared by combination of metalorganic chemical vapor deposition technique and post-deposition hydrogen doping, Organic Electronics 8 (2007) 51–56. [9] L.-J. Meng, C.P. Moreira de Sá, M.P. dos, Santos, Study of the structural properies of ZnO thin films by X-ray photoelectron spectroscopy, Applied Surface Science 78 (1994) 57–61. [10] W.-J. Lee, J. Kang, K.J. Chang, Electronic structure of phosphorus dopants in ZnO, Physica B 376–377 (2006) 699–702. ˇ zek, J. Kuriplach, [11] G. Brauer, W. Anwand, D. Grambole, J. Grenzer, W. Skorupa, J. Cíˇ I. Procházka, C.C. Ling, C.K. So, D. Schulz, D. Klimm, Identification of Znvacancy–hydrogen complexes in ZnO single crystals: a challenge to positron annihilation spectroscopy, Physical Review B 79 (2009) 115212.
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[12] J.-M. Ntep, M. Barbé, G. Cohen-Solal, F. Bailly, A. Lusson, R. Triboulet, ZnO growth by chemically assisted sublimation, Journal of Crystal Growth 184/185 (1998) 1026–1030. [13] H. Kleinwechter, C. Janzen, J. Knipping, H. Wiggers, P. Roth, Formation and properties of ZnO nano-particles from gas phase synthesis processes, Journal of Material Science 37 (2002) 4349–4360. [14] S. Ohnishi, Y. Hirokawa, T. Shiosaki, A. Kawabata, chemical vapor deposition of singe-crystalline ZnO film with smooth surface on intermediately sputtered
ZnO thin film on sapphire, Japanese Journal of Applied Physics 17 (5) (1978) 773–778. [15] Y.F. Lu, H.Q. Ni, Z.H. Mai, Z.M. Ren, The effects of thermal annealing on ZnO thin films grown by pulsed laser deposition, Journal of Applied Physics 88 (2000) 498–502. [16] H. Xu, W. Fan, A.L. Rosa, R.Q. Zhang, Th. Frauenheim, Hydrogen and oxygen adsorption on ZnO nanowires: a first-principles study, Physical Review B 79 (2009) 073402.