- Email: [email protected]
Nitrogen-doped ZnO prepared by capillaritron reactive ion beam sputtering deposition
Applied Surface Science 256 (2010) 4153–4156
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nitrogen-doped ZnO prepared by capillaritron reactive ion beam sputtering deposition Liang-Chiun Chao *, Yu-Ren Shih, Yao-Kai Li, Jun-Wei Chen, Jiun-De Wu, Ching-Hwa Ho Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 August 2009 Received in revised form 2 January 2010 Accepted 1 February 2010 Available online 6 February 2010
Nitrogen-doped ZnO thin films have been prepared by reactive ion beam sputtering deposition utilizing a capillaritron ion source. X-ray diffraction (XRD) analysis of the as-deposited film exhibits a single strong ZnO (002) diffraction peak centred at 34.408. Post-growth annealing causes increase of grain size and decrease of c-axis lattice constant. Micro-Raman spectroscopy analysis of the as-deposited film shows strong nitrogen-related local vibration mode at 275, 582, 640 and 720 cm 1, whereas the E2 mode of ZnO at 436 cm 1 can barely be identified. Annealing at 500–800 8C causes decrease of 275, 582, 640 and 720 cm 1 and increase of 436 cm 1 intensity, indicating out-diffusion of nitrogen and improvement of ZnO crystalline quality. Unlike un-doped ZnO, the surface roughness of nitrogen-doped ZnO deteriorates after annealing, which is also attributed to the out-diffusion of nitrogen. A nitrogen concentration of 1021/cm3 was observed while type conversion from n-type to p-type was not achieved, which is likely due to the formation of ZnI–NO or (N2)O that act as donor/double donors. ß 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO Nitrogen Raman scattering
1. Introduction ZnO is an intensively studied wide band gap semiconductor material due to its potential application in UV light-emitting and transparent conducting thin films. The band gap of ZnO is 3.37 eV at room temperature and the exciton-binding energy is 60 meV, which makes ZnO an ideal candidate for UV light-emitting applications [1]. The n-type conductivity of un-intentionally doped ZnO is not due to the presence of native point defects [2], but impurities such as hydrogen [3], zinc interstitial–nitrogen complex [4], carbon–nitrogen complexes or (N2)O [5] are responsible for the n-type conductivity. P-type ZnO may be obtained by doping ZnO with group I or group V elements that usually suffer from low hole mobility and low hole concentration [6,7]. Nitrogen is one of the most promising p-type dopant due to its lowest ionization energy. Besides, nitrogen form similar bond length as oxygen that it does not form antisite defects, which may compensate acceptors [6]. Nitrogen-doped ZnO has been prepared utilizing N2 and NH3 [8–11] by MBE (molecular beam epitaxy) and CVD (chemical vapor deposition). Ion beam sputtering deposition (IBSD) is an essential thin film deposition technique in the semiconductor industry. Comparing with diode magnetron sput-
* Corresponding author at: Department of Electronic Engineering, National Taiwan University of Science and Technology, #43 KeeLung Road, Sec. 4, Taipei, 106, Taiwan, ROC. Tel.: +886 2 2737 6369; fax: +886 2 2737 6424. E-mail address: [email protected] (L.-C. Chao). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.001
tering, thin films deposited with IBSD have superior surface quality, strong adherence and high hardness. Besides, hydrocarbon contamination in ZnO deposited by IBSD is less significant than that prepared by CVD. Capillaritron is a versatile and economic ion source that does not require sophisticated power supply systems [12]. Argon and nitrogen are passed simultaneously through the same capillary nozzle to produce ions and radicals for sputtering and doping, respectively. IBSD utilizing capilaritron ion source may be an alternative route to depositing reliable p-type nitrogendoped ZnO. 2. Experiment Nitrogen-doped ZnO has been deposited by IBSD utilizing a home-made capillaritron ion source on Si (100) substrates. Argon and nitrogen were controlled by separate mass flow controllers and were passed simultaneously through the same capillary nozzle. A ZnO target (99.99%) was positioned at 30 mm downstream of the capillaritron ion source, while Si substrates were positioned at 45 mm away from the target. The relative angle of the Si substrate and the target were adjusted so that the deposition rate was optimized. The base pressure and working pressure were 4 10 6 and 2 10 3 Torr, respectively. The substrate was resistant heated and ZnO thin films were deposited at 300 8C. Nitrogen partial flow rates N2/(N2 + Ar) varied from 0 to 40%, while the anode current of the capillaritron was fixed at 750 mA. Postgrowth annealing was performed at 500–800 8C in flowing argon ambient for 3 minutes. For Hall effect measurement, SiO2/Si
4154
L.-C. Chao et al. / Applied Surface Science 256 (2010) 4153–4156
Fig. 1. FE-SEM micrographs of (a) as-deposited un-doped ZnO, (b) un-doped ZnO annealed at 800 8C, (c) as-deposited nitrogen-doped ZnO and (d) nitrogen-doped ZnO annealed at 800 8C. Nitrogen-doped and un-doped ZnO films were deposited at 300 8C. The nitrogen partial flow rate of nitrogen-doped ZnO was 40%. Annealing was performed in flowing argon environment for three minutes.
substrates were used instead and nitrogen-doped ZnO were deposited under identical conditions. The surface morphology of the films was investigated using a field emission scanning electron microscope (FE-SEM, Jeol1 JSM-6500F) operating at 15 keV. Atomic force microscopy (Veeco CP II) measurements were completed in nondestructive tapping mode with 225 kHz drive frequency. The crystallinity of the ZnO thin films were analyzed by X-ray diffraction (XRD) utilizing a 12 kW Rigaku D/MAX 8 X-ray diffractometer (Cu Ka, l = 0.1541 nm). Raman spectroscopy analysis was performed using an argon laser at 514.5 nm with a power of 10 mW in backscattering configuration. Secondary ion mass spectroscopy (SIMS) depth profiles of the nitrogen-doped ZnO was analyzed in a CAMECA IMS-4F system utilizing a Cs+ primary ion beam at 13.5 keV. Hall effect measurements were carried out at room temperature utilizing four-probe Van der Pauw method. Ohmic contacts were obtained by depositing indium contacts on nitrogen-doped ZnO thin films.
annealing. The c-axis constant and grain size of un-doped and nitrogen-doped ZnO are shown in Fig. 3b and 3c, respectively. Fig. 3b indicates that the c-axis lattice constant of nitrogen-doped ZnO is larger than that of un-doped ZnO, suggesting the substitution of nitrogen molecule for oxygen site (N2)O, which acts as double donors [15]. After annealing, recrystallization causes the grain size of both nitrogen-doped and un-doped samples to increase from 15 to 25 nm. Fig. 4 shows Raman spectra of nitrogen-doped and un-doped ZnO deposited at 300 8C. From Fig. 4a, four peaks were identified at 275, 582, 640 and 720 cm 1, which are due to the local vibrational modes (LVMs) of nitrogen-doped ZnO [16,17]. The E2 (high) mode of un-doped ZnO at 436 cm 1 and E2 (high) mode of Si substrate at 310 and 520 cm 1 [18] can also be identified. Un-doped ZnO samples do not contain any nitrogen-related LVMs, indicating that nitrogen is successfully incorporated into ZnO films by capillaritron IBSD. Fig. 4b shows the 275, 582, 640 and 720 cm 1 peak
3. Results and discussion Fig. 1 shows FE-SEM micrographs of un-doped and nitrogendoped ZnO with/without post-growth annealing. Un-doped ZnO (Fig. 1a) exhibits a smooth surface morphology even after annealing at 800 8C (Fig. 1b). Comparing with un-doped samples, as-deposited nitrogen-doped ZnO (Fig. 1c) exhibits a larger grain size while after annealing, grain size increases slightly as well (Fig. 1d). The surface root-mean-square roughness (Rrms) of undoped ZnO remains below 1.5 nm, while the Rrms of nitrogendoped ZnO increases from 3.6 to 6.0 nm as annealing temperature increases to 800 8C (Fig. 2), which is consistent with previous results [13]. The increase in surface roughness after annealing is likely due to out-diffusion of nitrogen. Both nitrogen-doped and un-doped ZnO exhibit preferred orientation along the c-axis. Fig. 3a shows XRD patterns of as-deposited and annealed nitrogendoped ZnO. The nitrogen-doped ZnO was deposited at 300 8C with nitrogen partial flow rate of 40%. As the annealing temperature increases, the (002) diffraction peak position shifts to larger angles [14], indicating a decreased c-axis lattice constant due to
Fig. 2. Surface root-mean-square roughness (Rrms) of un-doped and nitrogen-doped ZnO after annealing.
L.-C. Chao et al. / Applied Surface Science 256 (2010) 4153–4156
Fig. 3. (a) XRD patterns, (b) c-axis lattice constant and (c) grain sizes of nitrogendoped and un-doped ZnO. Nitrogen-doped and un-doped ZnO films were deposited at 300 8C. The nitrogen partial flow rate of nitrogen-doped ZnO was 40%. Annealing was performed in flowing argon environment for three minutes.
intensity normalized to E2 (high) mode of ZnO at 436 cm 1. As nitrogen partial flow rate increases, normalized intensity of 275, 582, 640 and 720 cm 1 increase as well [19], indicating that increased amount of nitrogen is being incorporated into ZnO. To study the effect of annealing, nitrogen-doped ZnO prepared utilizing nitrogen partial flow rate of 40% while annealing was performed at 500–800 8C in flowing argon ambient for three minutes. Fig. 4c shows normalized intensity variation of 275, 582, 640 and 720 cm 1 after annealing. From Fig. 4c, the normalized intensities of nitrogen-related LVMs drop, indicating improved crystalline quality of ZnO (increase of 436 cm 1 intensity) and outdiffusion of nitrogen. Since negatively charged complex ions bound with oxygen are found to be the most reliable species in evaluating the nitrogen concentration in ZnO [20], SIMS depth profile analysis was performed utilizing a Cs+ primary ion beam while 70Zn16O was used as the matrix signal and 14N16O was measured as the
4155
Fig. 4. Raman spectra of (a) nitrogen-doped and un-doped ZnO, (b) normalized scattering intensity of nitrogen-related LVMs relative to the E2 (high) mode of ZnO at 436 cm 1 and (c) normalized scattering intensity of nitrogen-related LVMs after annealing. Nitrogen-doped and un-doped ZnO films were deposited at 300 8C. The nitrogen partial flow rate of nitrogen-doped ZnO was 40%. Annealing was performed in flowing argon environment for three minutes.
nitrogen signal. Fig. 5 shows SIMS depth profile results of nitrogendoped ZnO deposited with 40% nitrogen flow rate at 300 8C. Fig. 5 shows that before annealing, the ratio of 14N16O /70Zn16O is 0.2, indicating a nitrogen concentration of 1021/cm3 [20]. Annealing at 800 8C causes the ratio of 14N16O /70Zn16O to drop to 0.1 indicating out-diffusion of nitrogen. Hall effect measurement of all the sample does not yield a conversion from n-type to p-type. The carrier concentration of the 40% nitrogen-doped samples remain in the range of 1 1019 with or without annealing. Because of target charging and ionic emission, ZnO-deposited utilizing compound targets is usually nonstoichiometric [21] and the asdeposited ZnO by IBSD is likely to be zinc rich. Since nitrogen is incorporated into ZnO, the formation of ZnI–NO acts as donors. Besides, XRD analysis indicates that the lattice constant of nitrogen-doped ZnO is larger than un-doped ZnO, suggesting the formation of (N2)O, which acts as double donors.
4156
L.-C. Chao et al. / Applied Surface Science 256 (2010) 4153–4156
Acknowledgement This research work was supported by the National Science Council of Republic of China under contract no. NSC 98-2112-M011-001.
References
Fig. 5. SIMS depth profile of nitrogen-doped ZnO deposited at 300 8C. The nitrogen partial flow rate was 40%. Annealing was performed in flowing argon environment for three minutes at 600 and 800 8C.
4. Conclusions Nitrogen-doped ZnO has been successfully deposited by IBSD utilizing a capillaritron ion source. The as-deposited film exhibit compressive stress while annealing causes decrease of the c-axis lattice constant. Raman spectroscopy analysis clearly identifies four additional peaks located at 275, 582, 640 and 720 cm 1, which are attributed to the LVMs of nitrogen-doped ZnO. Annealing causes decrease of the scattering intensity of nitrogen-related LVMs and increase of the E2 (high) mode of ZnO, indicating out-diffusion of nitrogen and improved crystalline quality of ZnO. SIMS analysis indicates that the concentration of nitrogen is 1021/cm3. Type conversion from n-type to p-type was not observed, likely due to the presence of ZnI–NO and (N2)O, which act as donor/double donors.
[1] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Sagawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2004) 42. [2] A. Janotti, C.G. Van de Walle, Phys. Rev. B 76 (2007) 165202. [3] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. [4] D.C. Look, G.C. Farlow, P. Reunchan, S. Limpijumnong, S.B. Zhang, K. Nordlund, Phys. Rev. Lett. 95 (2005) 225502. [5] C.L. Perkins, S.H. Lee, X. Li, S.E. Asher, T.J. Coutts, J. Appl. Phys. 97 (2005) 034907. [6] C.H. Park, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202. [7] V. Vaithianathan, Y.H. Lee, B.T. Lee, S. Hishita, S.S. Kim, J. Cryst. Growth 287 (2006) 85. [8] K. Iwata, P. Fons, A. Yamada, K. Matsubara, S. Niki, J. Cryst. Growth 209 (2000) 526. [9] A. Kaschner, U. Haboeck, M. Strassburg, M. Strassburg, G. Kaczmarczyk, A. Hoffmann, C. Thomsen, A. Zeuner, H.R. Alves, D.M. Hofmann, B.K. Meyer, Appl. Phys. Lett. 80 (2002) 1909. [10] X. Wang, S. Yang, J. Wang, M. Li, X. Jiang, G. Du, X. Liu, R.P.H. Chang, J. Cryst. Growth 226 (2001) 123. [11] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [12] J.F. Mahoney, J. Perel, A.T. Forrester, Appl. Phys. Lett. 38 (1981) 320. [13] F. Yakuphanoglu, Y. Caglar, S. Ilican, M. Caglar, Physica B 394 (2007) 86. [14] H.W. Kim, N.H. Kim, Phys. Status Solidi A 201 (2004) 235. [15] B. Yao, D.Z. Shen, Z.Z. Zhang, X.H. Wang, Z.P. Wei, B.H. Li, Y.M. Lv, X.W. Fan, L.X. Guan, G.Z. Xing, C.X. Cong, Y.P. Xie, J. Appl. Phys. 99 (2006) 123510. [16] L.L. Kerr, X. Li, M. Canepa, A.J. Sommer, Thin Solid Films 515 (2007) 5282. [17] H.Q. Le, S. Tripathy, S.J. Chua, Appl. Phys. Lett. 92 (2008) 141910. [18] F. Xiu, Z. Yang, D. Zhao, J. Liu, K.A. Alim, A.A. Balandin, M.E. Itkis, R.C. Haddon, J. Cryst. Growth 286 (2006) 61. [19] J.B. Wang, H.M. Zhong, Z.F. Li, W. Lu, Appl. Phys. Lett. 88 (2006) 101913. [20] M. Sumiya, S. Fuke, A. Tsukazaki, K. Tamura, A. Ohtomo, M. Kawasaki, H. Koinuma, J. Appl. Phys. 93 (2003) 2562. [21] J.P. Nair, I. Zon, M. Oron, R. Popovitz-Biro, Y. Feldman, J. Appl. Phys. 92 (2002) 4784.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Nitrogen-doped ZnO prepared by capillaritron reactive ion beam sputtering deposition Liang-Chiun Chao *, Yu-Ren Shih, Yao-Kai Li, Jun-Wei Chen, Jiun-De Wu, Ching-Hwa Ho Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei, 106, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 August 2009 Received in revised form 2 January 2010 Accepted 1 February 2010 Available online 6 February 2010
Nitrogen-doped ZnO thin films have been prepared by reactive ion beam sputtering deposition utilizing a capillaritron ion source. X-ray diffraction (XRD) analysis of the as-deposited film exhibits a single strong ZnO (002) diffraction peak centred at 34.408. Post-growth annealing causes increase of grain size and decrease of c-axis lattice constant. Micro-Raman spectroscopy analysis of the as-deposited film shows strong nitrogen-related local vibration mode at 275, 582, 640 and 720 cm 1, whereas the E2 mode of ZnO at 436 cm 1 can barely be identified. Annealing at 500–800 8C causes decrease of 275, 582, 640 and 720 cm 1 and increase of 436 cm 1 intensity, indicating out-diffusion of nitrogen and improvement of ZnO crystalline quality. Unlike un-doped ZnO, the surface roughness of nitrogen-doped ZnO deteriorates after annealing, which is also attributed to the out-diffusion of nitrogen. A nitrogen concentration of 1021/cm3 was observed while type conversion from n-type to p-type was not achieved, which is likely due to the formation of ZnI–NO or (N2)O that act as donor/double donors. ß 2010 Elsevier B.V. All rights reserved.
Keywords: ZnO Nitrogen Raman scattering
1. Introduction ZnO is an intensively studied wide band gap semiconductor material due to its potential application in UV light-emitting and transparent conducting thin films. The band gap of ZnO is 3.37 eV at room temperature and the exciton-binding energy is 60 meV, which makes ZnO an ideal candidate for UV light-emitting applications [1]. The n-type conductivity of un-intentionally doped ZnO is not due to the presence of native point defects [2], but impurities such as hydrogen [3], zinc interstitial–nitrogen complex [4], carbon–nitrogen complexes or (N2)O [5] are responsible for the n-type conductivity. P-type ZnO may be obtained by doping ZnO with group I or group V elements that usually suffer from low hole mobility and low hole concentration [6,7]. Nitrogen is one of the most promising p-type dopant due to its lowest ionization energy. Besides, nitrogen form similar bond length as oxygen that it does not form antisite defects, which may compensate acceptors [6]. Nitrogen-doped ZnO has been prepared utilizing N2 and NH3 [8–11] by MBE (molecular beam epitaxy) and CVD (chemical vapor deposition). Ion beam sputtering deposition (IBSD) is an essential thin film deposition technique in the semiconductor industry. Comparing with diode magnetron sput-
* Corresponding author at: Department of Electronic Engineering, National Taiwan University of Science and Technology, #43 KeeLung Road, Sec. 4, Taipei, 106, Taiwan, ROC. Tel.: +886 2 2737 6369; fax: +886 2 2737 6424. E-mail address: [email protected] (L.-C. Chao). 0169-4332/$ – see front matter ß 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.02.001
tering, thin films deposited with IBSD have superior surface quality, strong adherence and high hardness. Besides, hydrocarbon contamination in ZnO deposited by IBSD is less significant than that prepared by CVD. Capillaritron is a versatile and economic ion source that does not require sophisticated power supply systems [12]. Argon and nitrogen are passed simultaneously through the same capillary nozzle to produce ions and radicals for sputtering and doping, respectively. IBSD utilizing capilaritron ion source may be an alternative route to depositing reliable p-type nitrogendoped ZnO. 2. Experiment Nitrogen-doped ZnO has been deposited by IBSD utilizing a home-made capillaritron ion source on Si (100) substrates. Argon and nitrogen were controlled by separate mass flow controllers and were passed simultaneously through the same capillary nozzle. A ZnO target (99.99%) was positioned at 30 mm downstream of the capillaritron ion source, while Si substrates were positioned at 45 mm away from the target. The relative angle of the Si substrate and the target were adjusted so that the deposition rate was optimized. The base pressure and working pressure were 4 10 6 and 2 10 3 Torr, respectively. The substrate was resistant heated and ZnO thin films were deposited at 300 8C. Nitrogen partial flow rates N2/(N2 + Ar) varied from 0 to 40%, while the anode current of the capillaritron was fixed at 750 mA. Postgrowth annealing was performed at 500–800 8C in flowing argon ambient for 3 minutes. For Hall effect measurement, SiO2/Si
4154
L.-C. Chao et al. / Applied Surface Science 256 (2010) 4153–4156
Fig. 1. FE-SEM micrographs of (a) as-deposited un-doped ZnO, (b) un-doped ZnO annealed at 800 8C, (c) as-deposited nitrogen-doped ZnO and (d) nitrogen-doped ZnO annealed at 800 8C. Nitrogen-doped and un-doped ZnO films were deposited at 300 8C. The nitrogen partial flow rate of nitrogen-doped ZnO was 40%. Annealing was performed in flowing argon environment for three minutes.
substrates were used instead and nitrogen-doped ZnO were deposited under identical conditions. The surface morphology of the films was investigated using a field emission scanning electron microscope (FE-SEM, Jeol1 JSM-6500F) operating at 15 keV. Atomic force microscopy (Veeco CP II) measurements were completed in nondestructive tapping mode with 225 kHz drive frequency. The crystallinity of the ZnO thin films were analyzed by X-ray diffraction (XRD) utilizing a 12 kW Rigaku D/MAX 8 X-ray diffractometer (Cu Ka, l = 0.1541 nm). Raman spectroscopy analysis was performed using an argon laser at 514.5 nm with a power of 10 mW in backscattering configuration. Secondary ion mass spectroscopy (SIMS) depth profiles of the nitrogen-doped ZnO was analyzed in a CAMECA IMS-4F system utilizing a Cs+ primary ion beam at 13.5 keV. Hall effect measurements were carried out at room temperature utilizing four-probe Van der Pauw method. Ohmic contacts were obtained by depositing indium contacts on nitrogen-doped ZnO thin films.
annealing. The c-axis constant and grain size of un-doped and nitrogen-doped ZnO are shown in Fig. 3b and 3c, respectively. Fig. 3b indicates that the c-axis lattice constant of nitrogen-doped ZnO is larger than that of un-doped ZnO, suggesting the substitution of nitrogen molecule for oxygen site (N2)O, which acts as double donors [15]. After annealing, recrystallization causes the grain size of both nitrogen-doped and un-doped samples to increase from 15 to 25 nm. Fig. 4 shows Raman spectra of nitrogen-doped and un-doped ZnO deposited at 300 8C. From Fig. 4a, four peaks were identified at 275, 582, 640 and 720 cm 1, which are due to the local vibrational modes (LVMs) of nitrogen-doped ZnO [16,17]. The E2 (high) mode of un-doped ZnO at 436 cm 1 and E2 (high) mode of Si substrate at 310 and 520 cm 1 [18] can also be identified. Un-doped ZnO samples do not contain any nitrogen-related LVMs, indicating that nitrogen is successfully incorporated into ZnO films by capillaritron IBSD. Fig. 4b shows the 275, 582, 640 and 720 cm 1 peak
3. Results and discussion Fig. 1 shows FE-SEM micrographs of un-doped and nitrogendoped ZnO with/without post-growth annealing. Un-doped ZnO (Fig. 1a) exhibits a smooth surface morphology even after annealing at 800 8C (Fig. 1b). Comparing with un-doped samples, as-deposited nitrogen-doped ZnO (Fig. 1c) exhibits a larger grain size while after annealing, grain size increases slightly as well (Fig. 1d). The surface root-mean-square roughness (Rrms) of undoped ZnO remains below 1.5 nm, while the Rrms of nitrogendoped ZnO increases from 3.6 to 6.0 nm as annealing temperature increases to 800 8C (Fig. 2), which is consistent with previous results [13]. The increase in surface roughness after annealing is likely due to out-diffusion of nitrogen. Both nitrogen-doped and un-doped ZnO exhibit preferred orientation along the c-axis. Fig. 3a shows XRD patterns of as-deposited and annealed nitrogendoped ZnO. The nitrogen-doped ZnO was deposited at 300 8C with nitrogen partial flow rate of 40%. As the annealing temperature increases, the (002) diffraction peak position shifts to larger angles [14], indicating a decreased c-axis lattice constant due to
Fig. 2. Surface root-mean-square roughness (Rrms) of un-doped and nitrogen-doped ZnO after annealing.
L.-C. Chao et al. / Applied Surface Science 256 (2010) 4153–4156
Fig. 3. (a) XRD patterns, (b) c-axis lattice constant and (c) grain sizes of nitrogendoped and un-doped ZnO. Nitrogen-doped and un-doped ZnO films were deposited at 300 8C. The nitrogen partial flow rate of nitrogen-doped ZnO was 40%. Annealing was performed in flowing argon environment for three minutes.
intensity normalized to E2 (high) mode of ZnO at 436 cm 1. As nitrogen partial flow rate increases, normalized intensity of 275, 582, 640 and 720 cm 1 increase as well [19], indicating that increased amount of nitrogen is being incorporated into ZnO. To study the effect of annealing, nitrogen-doped ZnO prepared utilizing nitrogen partial flow rate of 40% while annealing was performed at 500–800 8C in flowing argon ambient for three minutes. Fig. 4c shows normalized intensity variation of 275, 582, 640 and 720 cm 1 after annealing. From Fig. 4c, the normalized intensities of nitrogen-related LVMs drop, indicating improved crystalline quality of ZnO (increase of 436 cm 1 intensity) and outdiffusion of nitrogen. Since negatively charged complex ions bound with oxygen are found to be the most reliable species in evaluating the nitrogen concentration in ZnO [20], SIMS depth profile analysis was performed utilizing a Cs+ primary ion beam while 70Zn16O was used as the matrix signal and 14N16O was measured as the
4155
Fig. 4. Raman spectra of (a) nitrogen-doped and un-doped ZnO, (b) normalized scattering intensity of nitrogen-related LVMs relative to the E2 (high) mode of ZnO at 436 cm 1 and (c) normalized scattering intensity of nitrogen-related LVMs after annealing. Nitrogen-doped and un-doped ZnO films were deposited at 300 8C. The nitrogen partial flow rate of nitrogen-doped ZnO was 40%. Annealing was performed in flowing argon environment for three minutes.
nitrogen signal. Fig. 5 shows SIMS depth profile results of nitrogendoped ZnO deposited with 40% nitrogen flow rate at 300 8C. Fig. 5 shows that before annealing, the ratio of 14N16O /70Zn16O is 0.2, indicating a nitrogen concentration of 1021/cm3 [20]. Annealing at 800 8C causes the ratio of 14N16O /70Zn16O to drop to 0.1 indicating out-diffusion of nitrogen. Hall effect measurement of all the sample does not yield a conversion from n-type to p-type. The carrier concentration of the 40% nitrogen-doped samples remain in the range of 1 1019 with or without annealing. Because of target charging and ionic emission, ZnO-deposited utilizing compound targets is usually nonstoichiometric [21] and the asdeposited ZnO by IBSD is likely to be zinc rich. Since nitrogen is incorporated into ZnO, the formation of ZnI–NO acts as donors. Besides, XRD analysis indicates that the lattice constant of nitrogen-doped ZnO is larger than un-doped ZnO, suggesting the formation of (N2)O, which acts as double donors.
4156
L.-C. Chao et al. / Applied Surface Science 256 (2010) 4153–4156
Acknowledgement This research work was supported by the National Science Council of Republic of China under contract no. NSC 98-2112-M011-001.
References
Fig. 5. SIMS depth profile of nitrogen-doped ZnO deposited at 300 8C. The nitrogen partial flow rate was 40%. Annealing was performed in flowing argon environment for three minutes at 600 and 800 8C.
4. Conclusions Nitrogen-doped ZnO has been successfully deposited by IBSD utilizing a capillaritron ion source. The as-deposited film exhibit compressive stress while annealing causes decrease of the c-axis lattice constant. Raman spectroscopy analysis clearly identifies four additional peaks located at 275, 582, 640 and 720 cm 1, which are attributed to the LVMs of nitrogen-doped ZnO. Annealing causes decrease of the scattering intensity of nitrogen-related LVMs and increase of the E2 (high) mode of ZnO, indicating out-diffusion of nitrogen and improved crystalline quality of ZnO. SIMS analysis indicates that the concentration of nitrogen is 1021/cm3. Type conversion from n-type to p-type was not observed, likely due to the presence of ZnI–NO and (N2)O, which act as donor/double donors.
[1] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Sagawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2004) 42. [2] A. Janotti, C.G. Van de Walle, Phys. Rev. B 76 (2007) 165202. [3] C.G. Van de Walle, Phys. Rev. Lett. 85 (2000) 1012. [4] D.C. Look, G.C. Farlow, P. Reunchan, S. Limpijumnong, S.B. Zhang, K. Nordlund, Phys. Rev. Lett. 95 (2005) 225502. [5] C.L. Perkins, S.H. Lee, X. Li, S.E. Asher, T.J. Coutts, J. Appl. Phys. 97 (2005) 034907. [6] C.H. Park, S.B. Zhang, S.H. Wei, Phys. Rev. B 66 (2002) 073202. [7] V. Vaithianathan, Y.H. Lee, B.T. Lee, S. Hishita, S.S. Kim, J. Cryst. Growth 287 (2006) 85. [8] K. Iwata, P. Fons, A. Yamada, K. Matsubara, S. Niki, J. Cryst. Growth 209 (2000) 526. [9] A. Kaschner, U. Haboeck, M. Strassburg, M. Strassburg, G. Kaczmarczyk, A. Hoffmann, C. Thomsen, A. Zeuner, H.R. Alves, D.M. Hofmann, B.K. Meyer, Appl. Phys. Lett. 80 (2002) 1909. [10] X. Wang, S. Yang, J. Wang, M. Li, X. Jiang, G. Du, X. Liu, R.P.H. Chang, J. Cryst. Growth 226 (2001) 123. [11] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [12] J.F. Mahoney, J. Perel, A.T. Forrester, Appl. Phys. Lett. 38 (1981) 320. [13] F. Yakuphanoglu, Y. Caglar, S. Ilican, M. Caglar, Physica B 394 (2007) 86. [14] H.W. Kim, N.H. Kim, Phys. Status Solidi A 201 (2004) 235. [15] B. Yao, D.Z. Shen, Z.Z. Zhang, X.H. Wang, Z.P. Wei, B.H. Li, Y.M. Lv, X.W. Fan, L.X. Guan, G.Z. Xing, C.X. Cong, Y.P. Xie, J. Appl. Phys. 99 (2006) 123510. [16] L.L. Kerr, X. Li, M. Canepa, A.J. Sommer, Thin Solid Films 515 (2007) 5282. [17] H.Q. Le, S. Tripathy, S.J. Chua, Appl. Phys. Lett. 92 (2008) 141910. [18] F. Xiu, Z. Yang, D. Zhao, J. Liu, K.A. Alim, A.A. Balandin, M.E. Itkis, R.C. Haddon, J. Cryst. Growth 286 (2006) 61. [19] J.B. Wang, H.M. Zhong, Z.F. Li, W. Lu, Appl. Phys. Lett. 88 (2006) 101913. [20] M. Sumiya, S. Fuke, A. Tsukazaki, K. Tamura, A. Ohtomo, M. Kawasaki, H. Koinuma, J. Appl. Phys. 93 (2003) 2562. [21] J.P. Nair, I. Zon, M. Oron, R. Popovitz-Biro, Y. Feldman, J. Appl. Phys. 92 (2002) 4784.