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Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method
Applied Surface Science 254 (2007) 164–166 www.elsevier.com/locate/apsusc
Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method A. Nakagawa a,*, F. Masuoka a, S. Chiba a, H. Endo b, K. Megro b, Y. Kashiwaba c, T. Ojima a, K. Aota a, I. Niikura a, Y. Kashiwaba a a Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Iwate Industrial Research Institute, 3-35-2 Iiokashinden, Morioka 020-0852, Japan c Sendai National College of Technology, 4-16-1 Ayashi-Chuo, Sendai 989-3128, Japan b
Available online 19 July 2007
Abstract Photoluminescence (PL) spectra of nitrogen-doped ZnO films (ZnO:N films) grown epitaxially on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method were measured at 5 K. In PL spectra, free exciton emission at about 3.375 eV was very strong and emissions at 3.334 and 3.31 eV were observed. These two emissions are discussed in this paper. The nitrogen concentration in ZnO:N films measured by secondary ion mass spectroscopy was 1019–20 cm 3. Current–voltage characteristics of the junction consisting of an n-type ZnO single crystal substrate and ZnO:N film showed good rectification. Also, ultraviolet radiation and visible light were emitted from this junction under a forward bias at room temperature. It is therefore thought that ZnO:N films have good crystallinity and that doped nitrogen atoms play a role as acceptors in ZnO:N films to form a good pn junction. From these phenomena and the excitation intensity dependency of PL spectra, emissions at 3.334 and 3.31 eV were assigned to neutral acceptor-bound exciton (A0X) emission and a donor–acceptor pair (DAP) emission due to doped nitrogen, respectively. # 2007 Elsevier B.V. All rights reserved. PACS : 78.55.Et; 78.55. m; 81.05.Dz; 81.15 z Keywords: ZnO; Plasma-assisted reactive evaporation; Homoepitaxial growth; Nitrogen doping; Photoluminescence
1. Introduction ZnO is an attractive material for ultraviolet light-emitting and blue light-emitting devices (LEDs, LDs) due to a direct wide band gap of 3.37 eVand large exciton binding energy of 60 meV. However, it is difficult to realize p-type ZnO because of the strong compensation effect of ZnO. Recently, light emission was observed from a ZnO p–i–n junction fabricated on ScAlMgO4 substrates by doping of nitrogen using the repeated temperature modulation method [1]. However, it is more attractive to fabricate ZnO films by homoepitaxial growth on ZnO single crystal substrates, since there are no mismatches of a lattice
* Corresponding author. Tel.: +81 19 621 6906; fax: +81 19 621 6906. E-mail address: [email protected] (A. Nakagawa). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.107
constant and a thermal expansion coefficient. Recently, ZnO LED was fabricated by growing a nitrogen-doped p-type ZnO film on a single crystal ZnO wafer using plasma-assisted metal organic chemical vapor deposition [2]. We have also studied an LED with a ZnO homojunction consisting of nitrogen-doped ZnO films (ZnO:N films) deposited on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method (PARE method), and we have observed rectification characteristics of the diodes and the emission of lights from the junction under a forward bias. However, details of the photoluminescence (PL) characteristics of ZnO:N films grown on ZnO single crystals have not been clarified. In this paper, we report the PL characteristics at a low temperature of ZnO:N films homoepitaxially deposited on ZnO single crystal substrates by using the PARE method and the role of nitrogen in the ZnO:N films.
A. Nakagawa et al. / Applied Surface Science 254 (2007) 164–166
2. Experimental Three kinds of samples, non-doped ZnO single crystal, nitrogen-doped ZnO single crystal and nitrogen-doped ZnO films, were prepared to study the role of nitrogen in ZnO by PL characteristics. c-Plane ZnO single crystal substrates of 0.5 mm in thickness were sliced from non-doped ZnO single crystals grown by a hydrothermal method, and both sides of the substrates were polished to obtain a mirror-like surface. Then the substrates were annealed in air for 2 h at 850 8C. The conduction type of this substrate was n-type. Nitrogen-doped ZnO single crystals were also grown by the same hydrothermal method by adding KNO2 in mineralizers, and the substrates were processed in the same way. ZnO:N films were deposited on Zn-face of non-doped ZnO single crystal substrates using the PARE method. High-purity Zn metal (>99.9999%) and oxygen (>99.99995%) and nitrogen (>99.995%) gases were used. Zn vapor evaporated from the crucible passed through the plasma area of the mixed gases of oxygen and nitrogen excited by radio frequency (RF) of 13.56 MHz, and then ZnO:N films of 0.3–0.5 mm in thickness were formed on the non-doped ZnO single crystal substrates. The substrate temperature, crucible temperature, deposition rate, pressure in the chamber and RF power applied to the plasma were 400–650 8C, 330–500 8C, 0.1–1 nm/s, 0.6– 0.8 Pa and 30–250 W, respectively. 3. Results and discussion The nitrogen concentrations measured by secondary ion mass spectroscopy (SIMS) were 1019–20 cm 3 in ZnO:N films, while the nitrogen concentration was under the noise level (under 1017 cm 3) in the non-doped ZnO single crystal substrate. The PARE method is an effective method for doping nitrogen into ZnO films. Fig. 1 shows typical near-band-edge PL spectra from a ZnO:N film deposited on a non-doped ZnO single crystal substrate (a), a nitrogen-doped ZnO single crystal (b) and a nondoped ZnO single crystal (c) measured at 5 K (He–Cd,
Fig. 1. Near-band-edge photoluminescence spectra from a ZnO:N film deposited on a non-doped ZnO single crystal substrate (a), a nitrogen-doped ZnO single crystal (b) and a non-doped ZnO single crystal (c) measured at 5 K (He– Cd, 325 nm).
165
325 nm). The spectrum in (c) has peaks at 3.374, 3.362, 3.290, 3.322 and 3.218 eV, which correspond to peaks in typical spectra of ZnO crystals [3]. Peaks at 3.373, 3.362, 3.290 and 3.218, and 3.322 eV have been assigned by many researchers to emissions from free-exciton (FE), neutral donor-bound excitons (D0X), phonon replicas and two-electron satellite (TES) of D0X, respectively. All of these peaks were also observed in spectra (a) and (b). Peaks at 3.334 and about 3.310 eV were observed in spectra from ZnO:N films and nitrogen-doped ZnO single crystals but not in spectra from non-doped ZnO single crystals. Fig. 2 shows PL spectra of a ZnO:N film measured under different excitation intensities at 5 K. The peak of 3.310 eV showed a red shift with decrease in excitation intensity, although other peaks did not shift. Hence, the peak of 3.310 eV is thought to be due to a donor–acceptor pair (DAP) emission. Some groups have reported that peaks in the region of about 3.33 eV are due to TES of D0X [3–5]. On the other hand, some groups have reported that these peaks might be emission related to acceptors [6,7]. Therefore, the origin of the emission at 3.334 eV has not been clarified yet. In this study, diodes with an Au/ZnO:N film/n-type ZnO single crystal/Al structure were also fabricated using ZnO:N films. The diameter of an Au electrode is 1 mm. The electron concentration and mobility of the n-type ZnO single crystal measured by the van der Pauw method were about 1017/cm3 and 102 cm2/Vs, respectively. The hole density of the p-type ZnO:N layers estimated from junction capacitances of the diodes was 1014–1016 /cm3. The contacts of the Au/ZnO:N film and Al/ntype ZnO single crystal were ohmic. The diode showed good rectification characteristics, as can been seen in Fig. 3, and light emissions of ultraviolet light and visible light under forward bias (Au electrode is positive to Al electrode) at room temperature. This shows that a ZnO:N film deposited on an ntype ZnO single crystal formed a pn homojunction. The value of full width of half maximum (FWHM) of the peak of 3.334 eV observed in spectra of nitrogen-doped ZnO single crystals, as shown in Fig. 1, was 0.6 meV. On the other hand, the value of FWHM of D0X peak of the crystals was 1.1 meV. Therefore, we think that the peak of 3.334 eV is not related to D0X emission. Also, the peak was observed in spectra of the nitrogen-doped ZnO single crystals and ZnO:N films but
Fig. 2. PL spectra of a ZnO:N film measured at 5 K under different excitation intensities.
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A. Nakagawa et al. / Applied Surface Science 254 (2007) 164–166
The intensities of emissions at 3.334 and 3.310 eV tend to increase but the intensity of emission from D0X tends to decrease with increase in flow rate of nitrogen gas from 0.2 to 0.4 sccm. Therefore, it is also thought that these peaks of 3.334 and 3.310 eVare caused by nitrogen. Again, the peaks at around 3.380 eV assigned to emission from free excitons are dominant. It is thought that the ZnO:N films have good crystallinity. 4. Conclusions
Fig. 3. Current–voltage characteristics of diodes with an Au-Ni/ZnO:N film/ntype ZnO single crystal/Al structure.
The ZnO:N films deposited on Zn-faces of ZnO single crystal substrates using the plasma-assisted reactive evaporation method and nitrogen-doped ZnO single crystal showed PL peaks of 3.310 and 3.334 eV. The peak of 3.310 eV was blueshifted with increase in excitation intensity. Again, diodes with a ZnO:N film/n-type ZnO single crystal junction showed rectification characteristics and emission of ultraviolet light and visible light. Therefore, we assigned peaks of 3.334 and 3.310 eV of the nitrogen-doped ZnO films and nitrogen-doped ZnO single crystal to neutral acceptor-bound excitons (A0X) and donor–acceptor pair (DAP) emissions, respectively. Free exciton emission at about 3.380 eV was also dominant. It is thought that ZnO:N films have good crystallinity and that nitrogen atoms play as a role acceptors in ZnO:N films. Acknowledgement This work was supported in part by Iwate Prefecture, Japan. References
Fig. 4. PL spectra of ZnO:N films deposited on the ZnO single crystal substrates under various flow rates of nitrogen gas flowing into a reactive chamber measured at 5 K.
not in spectra of the non-doped ZnO substrate. Therefore, we think that a peak of 3.334 eV is neutral acceptor-bound excitons (A0X). Fig. 4 shows PL spectra measured at 5 K of ZnO:N films deposited on non-doped ZnO single crystal substrates under various flow rates of nitrogen flowing into a reactive chamber.
[1] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2005) 42. [2] W.Z. Xu, Z.Z. Ye, Y.J. Zeng, L.P. Zhu, B.H. Zhao, L. Jiang, J.G. Lu, H.P. He, S.B. Zhang, Appl. Phys. Lett. 88 (2006) 173506–173511. [3] H. Alves, D. Pfisterer, A. Zeuner, T. Rieman, J. Christen, D.M. Hofman, B.K. Meyer, Opt. Mater. 23 (2003) 33. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [5] S. Yamauchi, Y. Goto, T. Hariu, J. Cryst. Growth 260 (2004) 1. [6] H. Kato, M. Sano, K. Miyamoto, T. Yao, Phys. Status Solidi B 241 (2004) 612. [7] I.H. Choi, J. Korean Phys. Soc. 47 (2005) 696.
Photoluminescence properties of nitrogen-doped ZnO films deposited on ZnO single crystal substrates by the plasma-assisted reactive evaporation method A. Nakagawa a,*, F. Masuoka a, S. Chiba a, H. Endo b, K. Megro b, Y. Kashiwaba c, T. Ojima a, K. Aota a, I. Niikura a, Y. Kashiwaba a a Iwate University, 4-3-5 Ueda, Morioka 020-8551, Japan Iwate Industrial Research Institute, 3-35-2 Iiokashinden, Morioka 020-0852, Japan c Sendai National College of Technology, 4-16-1 Ayashi-Chuo, Sendai 989-3128, Japan b
Available online 19 July 2007
Abstract Photoluminescence (PL) spectra of nitrogen-doped ZnO films (ZnO:N films) grown epitaxially on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method were measured at 5 K. In PL spectra, free exciton emission at about 3.375 eV was very strong and emissions at 3.334 and 3.31 eV were observed. These two emissions are discussed in this paper. The nitrogen concentration in ZnO:N films measured by secondary ion mass spectroscopy was 1019–20 cm 3. Current–voltage characteristics of the junction consisting of an n-type ZnO single crystal substrate and ZnO:N film showed good rectification. Also, ultraviolet radiation and visible light were emitted from this junction under a forward bias at room temperature. It is therefore thought that ZnO:N films have good crystallinity and that doped nitrogen atoms play a role as acceptors in ZnO:N films to form a good pn junction. From these phenomena and the excitation intensity dependency of PL spectra, emissions at 3.334 and 3.31 eV were assigned to neutral acceptor-bound exciton (A0X) emission and a donor–acceptor pair (DAP) emission due to doped nitrogen, respectively. # 2007 Elsevier B.V. All rights reserved. PACS : 78.55.Et; 78.55. m; 81.05.Dz; 81.15 z Keywords: ZnO; Plasma-assisted reactive evaporation; Homoepitaxial growth; Nitrogen doping; Photoluminescence
1. Introduction ZnO is an attractive material for ultraviolet light-emitting and blue light-emitting devices (LEDs, LDs) due to a direct wide band gap of 3.37 eVand large exciton binding energy of 60 meV. However, it is difficult to realize p-type ZnO because of the strong compensation effect of ZnO. Recently, light emission was observed from a ZnO p–i–n junction fabricated on ScAlMgO4 substrates by doping of nitrogen using the repeated temperature modulation method [1]. However, it is more attractive to fabricate ZnO films by homoepitaxial growth on ZnO single crystal substrates, since there are no mismatches of a lattice
* Corresponding author. Tel.: +81 19 621 6906; fax: +81 19 621 6906. E-mail address: [email protected] (A. Nakagawa). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.07.107
constant and a thermal expansion coefficient. Recently, ZnO LED was fabricated by growing a nitrogen-doped p-type ZnO film on a single crystal ZnO wafer using plasma-assisted metal organic chemical vapor deposition [2]. We have also studied an LED with a ZnO homojunction consisting of nitrogen-doped ZnO films (ZnO:N films) deposited on n-type ZnO single crystal substrates by using the plasma-assisted reactive evaporation method (PARE method), and we have observed rectification characteristics of the diodes and the emission of lights from the junction under a forward bias. However, details of the photoluminescence (PL) characteristics of ZnO:N films grown on ZnO single crystals have not been clarified. In this paper, we report the PL characteristics at a low temperature of ZnO:N films homoepitaxially deposited on ZnO single crystal substrates by using the PARE method and the role of nitrogen in the ZnO:N films.
A. Nakagawa et al. / Applied Surface Science 254 (2007) 164–166
2. Experimental Three kinds of samples, non-doped ZnO single crystal, nitrogen-doped ZnO single crystal and nitrogen-doped ZnO films, were prepared to study the role of nitrogen in ZnO by PL characteristics. c-Plane ZnO single crystal substrates of 0.5 mm in thickness were sliced from non-doped ZnO single crystals grown by a hydrothermal method, and both sides of the substrates were polished to obtain a mirror-like surface. Then the substrates were annealed in air for 2 h at 850 8C. The conduction type of this substrate was n-type. Nitrogen-doped ZnO single crystals were also grown by the same hydrothermal method by adding KNO2 in mineralizers, and the substrates were processed in the same way. ZnO:N films were deposited on Zn-face of non-doped ZnO single crystal substrates using the PARE method. High-purity Zn metal (>99.9999%) and oxygen (>99.99995%) and nitrogen (>99.995%) gases were used. Zn vapor evaporated from the crucible passed through the plasma area of the mixed gases of oxygen and nitrogen excited by radio frequency (RF) of 13.56 MHz, and then ZnO:N films of 0.3–0.5 mm in thickness were formed on the non-doped ZnO single crystal substrates. The substrate temperature, crucible temperature, deposition rate, pressure in the chamber and RF power applied to the plasma were 400–650 8C, 330–500 8C, 0.1–1 nm/s, 0.6– 0.8 Pa and 30–250 W, respectively. 3. Results and discussion The nitrogen concentrations measured by secondary ion mass spectroscopy (SIMS) were 1019–20 cm 3 in ZnO:N films, while the nitrogen concentration was under the noise level (under 1017 cm 3) in the non-doped ZnO single crystal substrate. The PARE method is an effective method for doping nitrogen into ZnO films. Fig. 1 shows typical near-band-edge PL spectra from a ZnO:N film deposited on a non-doped ZnO single crystal substrate (a), a nitrogen-doped ZnO single crystal (b) and a nondoped ZnO single crystal (c) measured at 5 K (He–Cd,
Fig. 1. Near-band-edge photoluminescence spectra from a ZnO:N film deposited on a non-doped ZnO single crystal substrate (a), a nitrogen-doped ZnO single crystal (b) and a non-doped ZnO single crystal (c) measured at 5 K (He– Cd, 325 nm).
165
325 nm). The spectrum in (c) has peaks at 3.374, 3.362, 3.290, 3.322 and 3.218 eV, which correspond to peaks in typical spectra of ZnO crystals [3]. Peaks at 3.373, 3.362, 3.290 and 3.218, and 3.322 eV have been assigned by many researchers to emissions from free-exciton (FE), neutral donor-bound excitons (D0X), phonon replicas and two-electron satellite (TES) of D0X, respectively. All of these peaks were also observed in spectra (a) and (b). Peaks at 3.334 and about 3.310 eV were observed in spectra from ZnO:N films and nitrogen-doped ZnO single crystals but not in spectra from non-doped ZnO single crystals. Fig. 2 shows PL spectra of a ZnO:N film measured under different excitation intensities at 5 K. The peak of 3.310 eV showed a red shift with decrease in excitation intensity, although other peaks did not shift. Hence, the peak of 3.310 eV is thought to be due to a donor–acceptor pair (DAP) emission. Some groups have reported that peaks in the region of about 3.33 eV are due to TES of D0X [3–5]. On the other hand, some groups have reported that these peaks might be emission related to acceptors [6,7]. Therefore, the origin of the emission at 3.334 eV has not been clarified yet. In this study, diodes with an Au/ZnO:N film/n-type ZnO single crystal/Al structure were also fabricated using ZnO:N films. The diameter of an Au electrode is 1 mm. The electron concentration and mobility of the n-type ZnO single crystal measured by the van der Pauw method were about 1017/cm3 and 102 cm2/Vs, respectively. The hole density of the p-type ZnO:N layers estimated from junction capacitances of the diodes was 1014–1016 /cm3. The contacts of the Au/ZnO:N film and Al/ntype ZnO single crystal were ohmic. The diode showed good rectification characteristics, as can been seen in Fig. 3, and light emissions of ultraviolet light and visible light under forward bias (Au electrode is positive to Al electrode) at room temperature. This shows that a ZnO:N film deposited on an ntype ZnO single crystal formed a pn homojunction. The value of full width of half maximum (FWHM) of the peak of 3.334 eV observed in spectra of nitrogen-doped ZnO single crystals, as shown in Fig. 1, was 0.6 meV. On the other hand, the value of FWHM of D0X peak of the crystals was 1.1 meV. Therefore, we think that the peak of 3.334 eV is not related to D0X emission. Also, the peak was observed in spectra of the nitrogen-doped ZnO single crystals and ZnO:N films but
Fig. 2. PL spectra of a ZnO:N film measured at 5 K under different excitation intensities.
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A. Nakagawa et al. / Applied Surface Science 254 (2007) 164–166
The intensities of emissions at 3.334 and 3.310 eV tend to increase but the intensity of emission from D0X tends to decrease with increase in flow rate of nitrogen gas from 0.2 to 0.4 sccm. Therefore, it is also thought that these peaks of 3.334 and 3.310 eVare caused by nitrogen. Again, the peaks at around 3.380 eV assigned to emission from free excitons are dominant. It is thought that the ZnO:N films have good crystallinity. 4. Conclusions
Fig. 3. Current–voltage characteristics of diodes with an Au-Ni/ZnO:N film/ntype ZnO single crystal/Al structure.
The ZnO:N films deposited on Zn-faces of ZnO single crystal substrates using the plasma-assisted reactive evaporation method and nitrogen-doped ZnO single crystal showed PL peaks of 3.310 and 3.334 eV. The peak of 3.310 eV was blueshifted with increase in excitation intensity. Again, diodes with a ZnO:N film/n-type ZnO single crystal junction showed rectification characteristics and emission of ultraviolet light and visible light. Therefore, we assigned peaks of 3.334 and 3.310 eV of the nitrogen-doped ZnO films and nitrogen-doped ZnO single crystal to neutral acceptor-bound excitons (A0X) and donor–acceptor pair (DAP) emissions, respectively. Free exciton emission at about 3.380 eV was also dominant. It is thought that ZnO:N films have good crystallinity and that nitrogen atoms play as a role acceptors in ZnO:N films. Acknowledgement This work was supported in part by Iwate Prefecture, Japan. References
Fig. 4. PL spectra of ZnO:N films deposited on the ZnO single crystal substrates under various flow rates of nitrogen gas flowing into a reactive chamber measured at 5 K.
not in spectra of the non-doped ZnO substrate. Therefore, we think that a peak of 3.334 eV is neutral acceptor-bound excitons (A0X). Fig. 4 shows PL spectra measured at 5 K of ZnO:N films deposited on non-doped ZnO single crystal substrates under various flow rates of nitrogen flowing into a reactive chamber.
[1] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2005) 42. [2] W.Z. Xu, Z.Z. Ye, Y.J. Zeng, L.P. Zhu, B.H. Zhao, L. Jiang, J.G. Lu, H.P. He, S.B. Zhang, Appl. Phys. Lett. 88 (2006) 173506–173511. [3] H. Alves, D. Pfisterer, A. Zeuner, T. Rieman, J. Christen, D.M. Hofman, B.K. Meyer, Opt. Mater. 23 (2003) 33. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830. [5] S. Yamauchi, Y. Goto, T. Hariu, J. Cryst. Growth 260 (2004) 1. [6] H. Kato, M. Sano, K. Miyamoto, T. Yao, Phys. Status Solidi B 241 (2004) 612. [7] I.H. Choi, J. Korean Phys. Soc. 47 (2005) 696.