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X-ray photoelectron spectroscopy study of ZnO films grown by metal-organic chemical vapor deposition
Journal of Crystal Growth 252 (2003) 180–183
X-ray photoelectron spectroscopy study of ZnO films grown by metal-organic chemical vapor deposition Yuantao Zhang*, Guotong Du, Xinqiang Wang, Wancheng Li, Xiaotian Yang, Yan Ma, Baijun Zhao, Hongjun Yang, Dali Liu, Shuren Yang College of Electronic Science and Engineering, State Key Laboratory on Integrated Optoelectronics, Jilin University, Jiefang Road 119, Changchun 130023, People’s Republic of China Received 24 November 2002; accepted 16 December 2002 Communicated by R. Kern
Abstract The ZnO films were deposited on (0 0 1)Si substrate by metal-organic chemical vapor deposition (MOCVD). Annealing was performed in air for 60 min at 8001C. The X-ray diffraction patterns of the samples showed sharp diffraction peaks for ZnO(0 0 2), which indicated that the films were highly c-axis oriented. Zn and O elements in the asdeposited ZnO film were investigated and compared with those in the annealed ZnO film by using X-ray Photoelectron Spectroscopy (XPS). XPS spectra showed that ZnO films changed from Zn-rich to O-rich after being annealed. r 2003 Elsevier Science B.V. All rights reserved. PACS: 81.05.Dz; 82.80.Pv; 81.15.Gh Keywords: A3. Metalorganic molecular beam epitaxy; B1. Oxides; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide (ZnO) is a wide-gap semiconductor with a band gap of 3.36 eV at room temperature. ZnO is extensively studied material because of its potential applications in various fields, such as gas sensor, solar cells [1], photodetectors [2], light emitting diodes (LEDs) [3] and laser systems, etc. The ZnO films are epitaxially grown on sapphire or Si substrates by several methods such as MBE *Corresponding author. Tel.: +86-431-8499063; fax: +86431-8923907. E-mail address: [email protected] (Y. Zhang).
[4–6], Sputtering [7,8], and metal-organic chemical vapor deposition (MOCVD) [9–11]. Recently, high quality ZnO films grown by MOCVD on Si substrates were reported [12]. Because silicon is not only of interest for the integration of optoelectronic devices but also cheaper and easier to cleave compared with sapphire, it is interesting that high quality ZnO films can be deposited on Si substrates. MOCVD offers high growth rate and growth efficiency, large area uniformity and different doping processes [13]. Diethyl zinc (DEZn) and O2 have been commonly used as precursors in MOCVD, but their reaction is vigorous and will result in the
0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(02)02481-8
Y. Zhang et al. / Journal of Crystal Growth 252 (2003) 180–183
181
given by comparing and studying the X-ray Photoelectron Spectroscopy (XPS) data of asdeposited and annealed films.
formation of white powders and degradation of film quality. So in our system two special injectors were employed to carry precursors into the reactor separately and the film was deposited at low pressure to reduce the pre-reaction. Schematic diagrams of separate injectors and reactor have been reported in our previous papers [14,15]. In this paper, ZnO films were grown on (0 0 1)Si substrates by MOCVD. Thermal annealing was used to increase the quality of the epitaxy film. The chemical state of Zn and O in the ZnO films was
2. Experiments ZnO films were deposited on (0 0 1) silicon by MOCVD with a rotating disk vertical reactor. The substrate was placed onto a substrate holder which could be rotated at a speed within a maximum of
as-deposited sample
25000
Intensity (a.u.)
20000
15000
10000
5000
0 20
25
30
35
40
45
50
55
60
2θ (deg)
(a)
35000
annealed sample
30000
Intensity (a.u.)
25000 20000 15000 10000 5000 0
(b)
2θ (deg)
Fig. 1. (a) X-ray diffraction spectrum of as-deposited ZnO film and (b) X-ray diffraction spectrum of annealed ZnO film.
182
Y. Zhang et al. / Journal of Crystal Growth 252 (2003) 180–183
1000 rpm. The best vacuum of our chamber was about 1 10 3 Pa. The precursors were diethyl zinc (DEZn) and O2. Since DEZn reacted vigorously with O2 even at room temperature, DEZn and O2 were introduced into the chamber, respectively, by two injectors to avoid pre-reaction. By this method, uniform and high quality ZnO film could be obtained. The growth conditions of our samples were as follows: The chamber pressure was about 1 Torr. The N2 flow was 600 sccm and O2 flow 80 sccm, Ar was made to pass through the bubbler at 2 sccm and an additional Ar flow of 8 sccm was introduced into DEZn line immediately at the exit of the bubbler to reduce the transit time of DEZn to the reactor. The temperature of DEZn bubbler was 101C and that of growth was 6001C. Annealing was performed in air for 60 min at 8001C. We used SIEMENS D 5005 X-ray Diffractometer to investigate crystal quality and crystal structure. XPS measurements were carried out by using an ESCALAB Mark II X-ray photoemission spectrometer with monochromatic MgKa! radiation source, to identify the chemical state of Zn, O and their ratio. The XPS analysis chamber was about 1 10 7 Pa. In order to avoid the influence of surface absorption in the atmosphere, Ar ion etching was performed about 30 min with an etching rate of 0.5 nm/min.
XPS spectra of O 1s. We could observe that O 1s peak position of the as-deposited ZnO film was at 531.1 eV, which indicated the film was in the oxygen deficient state [16]. However O 1s peak position of the annealed ZnO film was at 531.5 eV. We could find that oxygen binding energy shifted towards the large energy direction, which was attributed to increase in the oxygen atoms of the annealed ZnO film. Part of the increased oxygen atoms exist as Zn–O bond, and the rest as free oxygen. XPS spectra of Zn 2p are shown in Fig. 3. From the spectra, we could find the binding energy of Zn 2p was at 1022.8 eV in the as-deposited ZnO film, which confirmed Zn enrichment in the as-deposited ZnO film [16]. Zn 2p peak position changed to 1021.2 eV on being annealed in air. The zinc binding energy shifted towards small energy direction, which showed that more zinc atoms were bound to oxygen atom. Fig. 4 shows XPS spectra of Zn LMM. The shift of Zn LMM peak position also showed that more zinc atoms were bound to oxygen atoms. In Zn LMM spectra, the shoulder peak at about 262.3 eV became weaker with annealing. This weak shoulder peak was believed to be the result of Zn interstitials or Zn metal. The weaker shoulder indicated that the content of Zn in free state decreased greatly when annealed. From the statistical results of XPS, we could obtain that the atom ratio of O:Zn equaled,
3. Results and discussion Fig. 1(a) showed the X-ray diffraction (XRD) spectrum of ZnO film deposited at 6001C. In Fig. 1(a), the high ZnO(0 0 2) diffraction peak can be observed and not any other ZnO diffraction peak, which indicates that the sample was strongly c-axis oriented. The XRD spectrum of ZnO film annealed in the atmosphere is shown in Fig. 1(b). By comparing Fig. 1(b) with Fig. 1(a), we could see not only the strong ZnO(0 0 2) diffraction peak but also the decreasing of the value of full-width at half-maximum (FWHM) for (0 0 2) diffraction peak, which meant that the quality of ZnO film was improved after being annealed. XPS spectra of the as-deposited and annealed ZnO films are shown in Figs. 2–4. Fig. 2 shows
O 1s
Annealed
As-deposited
536
534
532
530
528
526
Binding Energy (eV) Fig. 2. XPS spectra of O1s in as-deposited and annealed ZnO films.
Y. Zhang et al. / Journal of Crystal Growth 252 (2003) 180–183
183
value while the annealed film showed better quality. Having been annealed, O 1s and Zn 2p peak shifted slightly and the weak shoulder peak of Zn LMM spectra became less sharp. The XPS spectra were used to confirm the stoichiometry of ZnO film. The result showed ZnO film changed from Zn-rich to Orich after being annealed, also, that crystal quality was improved through annealing.
Zn 2p Annealed
As-deposited
Acknowledgements
1060
1050
1040
1030
1020
Binding Energy (eV)
This work was supported by NSFC-RGC (No. 59910161983) and NSFC (No. 60177007, 60176026) and 863 project with contract number of 2001AA311130.
Fig. 3. XPS spectra of Zn 2p in as-deposited and annealed ZnO films.
References Zn LMM
Annealed
As-deposited
272
270
268
266
264
262
260
258
256
254
Binding Energy (eV) Fig. 4. XPS spectra of Zn LMM in as-deposited and annealed ZnO films.
respectively, 0.93:1 before being annealed and 1.04:1 after being annealed which showed that ZnO changed from Zn-rich to O-rich after being annealed in air. 4. Conclusion ZnO films were deposited on Si(0 0 1) substrates by low-pressure MOCVD with separate precursor injectors. XRD measurements showed that ZnO films were strongly c-oriented with narrow FWHM
[1] U. Rau, M. Schmidt, Thin Solid Films 387 (2001) 141. [2] Y. Liu, C.R. Gorla, S. Liang, et al., J. Electron. Mater. 29 (2000) 60. [3] T. Soki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [4] K. Iwata, P. Fons, A. Yamada, K. Matsubara, S. Niki, J. Crystal Growth 209 (2000) 526. [5] P. Fons, K. Iwata, S. Niki, A. Yamada, K. Matsubara, M. Watanabe, J. Crystal Growth 209 (2000) 532. [6] D. Bagnall, Y. chen, Z. Zhu, T. Yao, S. Koyama, M. Shen, T. Golo, Appl. Phys. Lett. 73 (1998) 1038. [7] S.F. Chichibu, T. Yoshida, T. Onuma, H. Nakanishi, J. Appl. Phys. 91 (2) (2002) 874. [8] Y. Zhang, G. Du, D. Liu, X. Wang, Y. Ma, J. Wang, J. Yin, X. Yang, X. Hou, S. Yang, J. Crystal Growth 243 (2002) 439–443. [9] S.Y. Myong, S.J. Baik, C.H. Lee, W.Y. Cho, K.S. Lim, Jpn. J. Appl. Phys. 36 (1997) L1078. [10] Y. Kashiwaba, F. Katahira, K. Haga, T. Sekiguchi, H. Watanabe, J. Crystal Growth 221 (2000) 431. [11] K. Ogata, T. Kawanishi, K. Maejima, K. Sakurai, Sz. Fujita, Sg. Fujita, Jpn. J. Appl. Phys. 40 (Part 2) (2001) L657. [12] K. Ogata, T. Kawanishi, K. Maejima, K. Sakurai, Sz. Fujita, Sg. Fujita, J. Crystal Growth 237–239 (2002) 553–557. [13] C.R. Gorla, N.W. Emanetoglu, S. Liang, et al., J. Appl. Phys. 85 (1999) 2595. [14] X. Wang, S. Yang, X. Yang, D. Liu, Y. Zhang, J. Wang, J. Yin, D. Liu, H.C. Ong, G. Du, J. Crystal Growth 243 (2002) 13–18. [15] X. Wang, G. Du, Ch. Gu, et al., J. Phys. D: Appl. Phys. 35 (2002) L1–L3. [16] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (2000) 134–140.
X-ray photoelectron spectroscopy study of ZnO films grown by metal-organic chemical vapor deposition Yuantao Zhang*, Guotong Du, Xinqiang Wang, Wancheng Li, Xiaotian Yang, Yan Ma, Baijun Zhao, Hongjun Yang, Dali Liu, Shuren Yang College of Electronic Science and Engineering, State Key Laboratory on Integrated Optoelectronics, Jilin University, Jiefang Road 119, Changchun 130023, People’s Republic of China Received 24 November 2002; accepted 16 December 2002 Communicated by R. Kern
Abstract The ZnO films were deposited on (0 0 1)Si substrate by metal-organic chemical vapor deposition (MOCVD). Annealing was performed in air for 60 min at 8001C. The X-ray diffraction patterns of the samples showed sharp diffraction peaks for ZnO(0 0 2), which indicated that the films were highly c-axis oriented. Zn and O elements in the asdeposited ZnO film were investigated and compared with those in the annealed ZnO film by using X-ray Photoelectron Spectroscopy (XPS). XPS spectra showed that ZnO films changed from Zn-rich to O-rich after being annealed. r 2003 Elsevier Science B.V. All rights reserved. PACS: 81.05.Dz; 82.80.Pv; 81.15.Gh Keywords: A3. Metalorganic molecular beam epitaxy; B1. Oxides; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide (ZnO) is a wide-gap semiconductor with a band gap of 3.36 eV at room temperature. ZnO is extensively studied material because of its potential applications in various fields, such as gas sensor, solar cells [1], photodetectors [2], light emitting diodes (LEDs) [3] and laser systems, etc. The ZnO films are epitaxially grown on sapphire or Si substrates by several methods such as MBE *Corresponding author. Tel.: +86-431-8499063; fax: +86431-8923907. E-mail address: [email protected] (Y. Zhang).
[4–6], Sputtering [7,8], and metal-organic chemical vapor deposition (MOCVD) [9–11]. Recently, high quality ZnO films grown by MOCVD on Si substrates were reported [12]. Because silicon is not only of interest for the integration of optoelectronic devices but also cheaper and easier to cleave compared with sapphire, it is interesting that high quality ZnO films can be deposited on Si substrates. MOCVD offers high growth rate and growth efficiency, large area uniformity and different doping processes [13]. Diethyl zinc (DEZn) and O2 have been commonly used as precursors in MOCVD, but their reaction is vigorous and will result in the
0022-0248/03/$ - see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-0248(02)02481-8
Y. Zhang et al. / Journal of Crystal Growth 252 (2003) 180–183
181
given by comparing and studying the X-ray Photoelectron Spectroscopy (XPS) data of asdeposited and annealed films.
formation of white powders and degradation of film quality. So in our system two special injectors were employed to carry precursors into the reactor separately and the film was deposited at low pressure to reduce the pre-reaction. Schematic diagrams of separate injectors and reactor have been reported in our previous papers [14,15]. In this paper, ZnO films were grown on (0 0 1)Si substrates by MOCVD. Thermal annealing was used to increase the quality of the epitaxy film. The chemical state of Zn and O in the ZnO films was
2. Experiments ZnO films were deposited on (0 0 1) silicon by MOCVD with a rotating disk vertical reactor. The substrate was placed onto a substrate holder which could be rotated at a speed within a maximum of
as-deposited sample
25000
Intensity (a.u.)
20000
15000
10000
5000
0 20
25
30
35
40
45
50
55
60
2θ (deg)
(a)
35000
annealed sample
30000
Intensity (a.u.)
25000 20000 15000 10000 5000 0
(b)
2θ (deg)
Fig. 1. (a) X-ray diffraction spectrum of as-deposited ZnO film and (b) X-ray diffraction spectrum of annealed ZnO film.
182
Y. Zhang et al. / Journal of Crystal Growth 252 (2003) 180–183
1000 rpm. The best vacuum of our chamber was about 1 10 3 Pa. The precursors were diethyl zinc (DEZn) and O2. Since DEZn reacted vigorously with O2 even at room temperature, DEZn and O2 were introduced into the chamber, respectively, by two injectors to avoid pre-reaction. By this method, uniform and high quality ZnO film could be obtained. The growth conditions of our samples were as follows: The chamber pressure was about 1 Torr. The N2 flow was 600 sccm and O2 flow 80 sccm, Ar was made to pass through the bubbler at 2 sccm and an additional Ar flow of 8 sccm was introduced into DEZn line immediately at the exit of the bubbler to reduce the transit time of DEZn to the reactor. The temperature of DEZn bubbler was 101C and that of growth was 6001C. Annealing was performed in air for 60 min at 8001C. We used SIEMENS D 5005 X-ray Diffractometer to investigate crystal quality and crystal structure. XPS measurements were carried out by using an ESCALAB Mark II X-ray photoemission spectrometer with monochromatic MgKa! radiation source, to identify the chemical state of Zn, O and their ratio. The XPS analysis chamber was about 1 10 7 Pa. In order to avoid the influence of surface absorption in the atmosphere, Ar ion etching was performed about 30 min with an etching rate of 0.5 nm/min.
XPS spectra of O 1s. We could observe that O 1s peak position of the as-deposited ZnO film was at 531.1 eV, which indicated the film was in the oxygen deficient state [16]. However O 1s peak position of the annealed ZnO film was at 531.5 eV. We could find that oxygen binding energy shifted towards the large energy direction, which was attributed to increase in the oxygen atoms of the annealed ZnO film. Part of the increased oxygen atoms exist as Zn–O bond, and the rest as free oxygen. XPS spectra of Zn 2p are shown in Fig. 3. From the spectra, we could find the binding energy of Zn 2p was at 1022.8 eV in the as-deposited ZnO film, which confirmed Zn enrichment in the as-deposited ZnO film [16]. Zn 2p peak position changed to 1021.2 eV on being annealed in air. The zinc binding energy shifted towards small energy direction, which showed that more zinc atoms were bound to oxygen atom. Fig. 4 shows XPS spectra of Zn LMM. The shift of Zn LMM peak position also showed that more zinc atoms were bound to oxygen atoms. In Zn LMM spectra, the shoulder peak at about 262.3 eV became weaker with annealing. This weak shoulder peak was believed to be the result of Zn interstitials or Zn metal. The weaker shoulder indicated that the content of Zn in free state decreased greatly when annealed. From the statistical results of XPS, we could obtain that the atom ratio of O:Zn equaled,
3. Results and discussion Fig. 1(a) showed the X-ray diffraction (XRD) spectrum of ZnO film deposited at 6001C. In Fig. 1(a), the high ZnO(0 0 2) diffraction peak can be observed and not any other ZnO diffraction peak, which indicates that the sample was strongly c-axis oriented. The XRD spectrum of ZnO film annealed in the atmosphere is shown in Fig. 1(b). By comparing Fig. 1(b) with Fig. 1(a), we could see not only the strong ZnO(0 0 2) diffraction peak but also the decreasing of the value of full-width at half-maximum (FWHM) for (0 0 2) diffraction peak, which meant that the quality of ZnO film was improved after being annealed. XPS spectra of the as-deposited and annealed ZnO films are shown in Figs. 2–4. Fig. 2 shows
O 1s
Annealed
As-deposited
536
534
532
530
528
526
Binding Energy (eV) Fig. 2. XPS spectra of O1s in as-deposited and annealed ZnO films.
Y. Zhang et al. / Journal of Crystal Growth 252 (2003) 180–183
183
value while the annealed film showed better quality. Having been annealed, O 1s and Zn 2p peak shifted slightly and the weak shoulder peak of Zn LMM spectra became less sharp. The XPS spectra were used to confirm the stoichiometry of ZnO film. The result showed ZnO film changed from Zn-rich to Orich after being annealed, also, that crystal quality was improved through annealing.
Zn 2p Annealed
As-deposited
Acknowledgements
1060
1050
1040
1030
1020
Binding Energy (eV)
This work was supported by NSFC-RGC (No. 59910161983) and NSFC (No. 60177007, 60176026) and 863 project with contract number of 2001AA311130.
Fig. 3. XPS spectra of Zn 2p in as-deposited and annealed ZnO films.
References Zn LMM
Annealed
As-deposited
272
270
268
266
264
262
260
258
256
254
Binding Energy (eV) Fig. 4. XPS spectra of Zn LMM in as-deposited and annealed ZnO films.
respectively, 0.93:1 before being annealed and 1.04:1 after being annealed which showed that ZnO changed from Zn-rich to O-rich after being annealed in air. 4. Conclusion ZnO films were deposited on Si(0 0 1) substrates by low-pressure MOCVD with separate precursor injectors. XRD measurements showed that ZnO films were strongly c-oriented with narrow FWHM
[1] U. Rau, M. Schmidt, Thin Solid Films 387 (2001) 141. [2] Y. Liu, C.R. Gorla, S. Liang, et al., J. Electron. Mater. 29 (2000) 60. [3] T. Soki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [4] K. Iwata, P. Fons, A. Yamada, K. Matsubara, S. Niki, J. Crystal Growth 209 (2000) 526. [5] P. Fons, K. Iwata, S. Niki, A. Yamada, K. Matsubara, M. Watanabe, J. Crystal Growth 209 (2000) 532. [6] D. Bagnall, Y. chen, Z. Zhu, T. Yao, S. Koyama, M. Shen, T. Golo, Appl. Phys. Lett. 73 (1998) 1038. [7] S.F. Chichibu, T. Yoshida, T. Onuma, H. Nakanishi, J. Appl. Phys. 91 (2) (2002) 874. [8] Y. Zhang, G. Du, D. Liu, X. Wang, Y. Ma, J. Wang, J. Yin, X. Yang, X. Hou, S. Yang, J. Crystal Growth 243 (2002) 439–443. [9] S.Y. Myong, S.J. Baik, C.H. Lee, W.Y. Cho, K.S. Lim, Jpn. J. Appl. Phys. 36 (1997) L1078. [10] Y. Kashiwaba, F. Katahira, K. Haga, T. Sekiguchi, H. Watanabe, J. Crystal Growth 221 (2000) 431. [11] K. Ogata, T. Kawanishi, K. Maejima, K. Sakurai, Sz. Fujita, Sg. Fujita, Jpn. J. Appl. Phys. 40 (Part 2) (2001) L657. [12] K. Ogata, T. Kawanishi, K. Maejima, K. Sakurai, Sz. Fujita, Sg. Fujita, J. Crystal Growth 237–239 (2002) 553–557. [13] C.R. Gorla, N.W. Emanetoglu, S. Liang, et al., J. Appl. Phys. 85 (1999) 2595. [14] X. Wang, S. Yang, X. Yang, D. Liu, Y. Zhang, J. Wang, J. Yin, D. Liu, H.C. Ong, G. Du, J. Crystal Growth 243 (2002) 13–18. [15] X. Wang, G. Du, Ch. Gu, et al., J. Phys. D: Appl. Phys. 35 (2002) L1–L3. [16] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Appl. Surf. Sci. 158 (2000) 134–140.