- Email: [email protected]
Growth and characterization of N-polar and In-polar InN films by RF-MBE
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 2780–2782
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and characterization of N-polar and In-polar InN films by RF-MBE T. Yamaguchi a,, D. Muto b, T. Araki b, Y. Nanishi b a b
Research Organization of Science & Engineering, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan Department of Photonics, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
a r t i c l e in f o
a b s t r a c t
Available online 17 January 2009
The structural and electrical properties of N- and In-polar InN films grown directly on substrates were investigated. The twist contribution of directly grown InN films, which was defined by the estimated X-ray rocking curve’s full-width at half-maximum (XRC’s FWHM) of InN (1 01¯ 0) reflection, varied considerably by at least 40 arcmin for both N- and In-polar InN films, depending on growth conditions. In contrast, the tilt contribution, defined by the XRC’s FWHM of InN (0 0 0 2) reflection, was roughly fixed at 1.5–2.5 arcmin for N-polar InN films and 8–12 arcmin for In-polar InN films. For samples with the same twist contribution, In-polar InN films had larger tilt contribution than N-polar InN films. Nevertheless, electrical properties of In-polar InN had much better properties than those of N-polar InN films. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.05.cm 81.15.Hi 73.61.Ey Keywords: A1. High-resolution X-ray diffraction A3. Molecular beam epitaxy B1. Nitrides
1. Introduction InN is a promising material for applications such as infrared light emitters and high-speed electronic devices [1,2]. The recent development of growth techniques using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) has dramatically improved the crystal quality of InN [3–11]. In addition, several specific features of InN growth have been found recently [3,4]. An important feature is the large difference between growth temperatures of N-polar InN (550–600 1C) and In-polar InN (450–500 1C) [5,6]. Although the growth and characterization of N- [7–9] and In- [10,11] polar InN have been widely studied separately, there are few comparative studies of the structural and electrical properties in these two properties. In this paper, the structural and electrical properties of directly grown InN films (without a low-temperature InN (LT-InN) buffer layer) on substrates are described, with emphasis on the comparison of N- and In-polar films.
2. Experimental procedure InN films were prepared by RF-MBE equipped with conventional effusion cells for group III elements (EpiQuest, Inc, model RC2100INR). Active nitrogen was generated using a commercialized plasma source (SVT Associates, model 6.03). N- and In-polar InN films were grown at 500 and 450 1C, respectively, directly Corresponding author.
E-mail address: [email protected] (T. Yamaguchi). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.034
on substrates. The substrate for the N-polar films was nitridated Al2O3 (0 0 0 1), whereas that for In-polar films was metal organic chemical vapor deposition (MOCVD)-grown GaN/Al2O3 (0 0 0 1). The samples were investigated ex-situ by X-ray diffraction (XRD) and Hall effect measurements. In the XRD measurement, tilt and twist contributions were investigated. The tilt and twist contributions were defined by X-ray rocking curve’s full-width at half-maximum (XRC’s FWHMs) values of InN (0 0 0 2) (FWHM002) and InN (1 0 1¯ 0) (FWHM100) reflections, respectively. As it was difficult to measure the FWHM100 value directly, owing to the thickness of the films, we used the method of Srikant et al. [12] to estimate this value.
3. Results and discussion 3.1. Structural and electrical properties of N-polar InN film Table 1 summarizes the film-thickness dependence of FWHM002, FWHM100, carrier density (n), and electron mobility (m) of N-polar InN films grown directly at the growth rate of 500 nm/h. It can be seen in Table 1 that FWHM100 value decreases with an increase of film thickness, whereas FWHM002 value is almost constant. It can also be seen that the carrier density decreases and electron mobility increases with the increase of film thickness. These improvements in the electrical properties can be explained by the improvement of the twist contribution. Only the improvement of twist contribution is a specific feature of directly grown InN films: In InN films with a LT-InN buffer layer, both the tilt and twist contributions have improved with an
ARTICLE IN PRESS T. Yamaguchi et al. / Journal of Crystal Growth 311 (2009) 2780–2782
increase of film thickness [4]. From these results, we expect that the electrical properties of directly grown InN films are directly related to the twist contribution. Table 1 Film-thickness dependence of FWHM002, FWHM100, carrier density (n) and electron mobility (m) of directly grown N-polar InN films. Film thickness
250 nm
500 nm
1.0 mm
FWHM002 (arcmin) FWHM100 (arcmin) n (cm 3) m (cm2/V s)
1.7 63.8 5.9 1018 950
2.3 55.8 4.8 1018 1080
2.4 48.4 4.6 1018 1090
In Fig. 1, the correlation between tilt and twist contributions is plotted for directly grown N-polar InN films with different thicknesses (250 nm–1.0 mm) and growth rates (250–750 nm/h). It was found that the twist contribution varied considerably from 45 to 75 arcmin. In contrast, the tilt contribution was fixed at 1.5–2.5 arcmin. Fig. 2 shows the FWHM100 value dependence on the electrical properties of directly grown N-polar InN films. Carrier density decreases and electron mobility increases with the decrease of FWHM100 value, indicating better crystal quality. These trends support our expectation that the electrical properties of directly grown InN films are correlated to the twist contribution.
80
80 FWHM100 (arcmin)
FWHM100 (arcmin)
2781
70
60
50
N-polar In-polar
70 60 50 40 30
40 0
2 4 FWHM002 (arcmin)
0
6
2
4
6
8
10 12 14 16 18 20
FWHM002 (arcmin)
Fig. 1. Correlation between tilt and twist contributions of directly grown N-polar InN films.
Fig. 3. Correlation between tilt and twist contributions of directly grown N- and In-polar InN films.
8.0
7.0
Carrier density (1018/cm3)
Carrier density (1018/cm3)
8.0
6.0 5.0 4.0 3.0 50
55 60 65 70 FWHM100 (arcmin)
75
In-polar
6.0 5.0 4.0 3.0 2.0 30
35
40
45 50 55 60 65 FWHM100 (arcmin)
70
75
1200 1600 Electron mobility (cm2 / Vs)
Electron mobility (cm2 /Vs)
45
N-polar 7.0
1100 1000 900
N-polar In-polar
1500 1400 1300 1200 1100 1000 900
800 45
50
55 60 65 70 FWHM100 (arcmin)
75
Fig. 2. FWHM100 value dependence on (a) carrier density and (b) electron mobility for directly grown N-polar InN films.
30
35
40
45 50 55 60 65 FWHM100 (arcmin)
70
75
Fig. 4. FWHM100 value dependence on (a) carrier density and (b) electron mobility for directly grown N- and In-polar InN films.
ARTICLE IN PRESS 2782
T. Yamaguchi et al. / Journal of Crystal Growth 311 (2009) 2780–2782
3.2. Comparison between N- and In-polar InN films Fig. 3 shows the correlation between tilt and twist contributions for both directly grown N-polar and In-polar InN films. The growth rate of In-polar InN films were fixed at 500 nm/h. The thickness was 500–750 nm. As can be seen in Fig. 3, the twist contribution varied considerably in both N- and In-polar InN films. FWHM002 of In-polar films was slightly smaller than that of N-polar films. This would be related to the lattice mismatches between InN and the underlayer (In-polar: GaN, N-polar: nitridated Al2O3). On the other hand, tilt contribution was almost fixed for each polarity. FWHM002 of N-polar films was very small value of 2–3 arcmin, whereas that of In-polar films was much larger (8–12 arcmin). This would be due to the different growth temperature (N-polar: 500 1C, In-polar: 50 1C). These results indicate that N-polar InN films had a smaller tilt contribution, compared with In-polar InN films with the same twist contribution as N-polar InN films. Nevertheless, electrical properties of In-polar InN were much better than those of N-polar InN, for samples with the same twist contribution, as can be seen in Fig. 4. This difference in electrical properties for the different polarities cannot be explained only by the correlation between residual electron concentration and density of dangling bonds at edge-component threading dislocations [11]. A possible explanation would be the different impurity or point defect incorporations for different polarity, as has been found for GaN [13]. Further investigation on this polarity dependence is in progress.
4. Conclusions In conclusion, both N- and In-polar InN films were grown directly on the substrates (N-polar: on nitridated Al2O3 (0 0 0 1), In-polar: on MOCVD-grown GaN template on Al2O3 (0 0 0 1)) by RF-MBE. The twist contribution of directly grown InN films varied
considerably depending on growth conditions in both N- and In-polar InN, whereas the tilt contribution was almost fixed in each N- and In-polar InN. For N- and In-polar InN films with the same twist contribution, In-polar InN film had a larger tilt contribution. Nevertheless, In-polar InN showed better electrical properties.
Acknowledgements The author would like thank to Dr. M. Kaneko for a fruitful discussion. This work was supported by the MEXT through Grantin-Aids for Scientific Research in Priority Areas ‘‘Optoelectronics Frontier by Nitride Semiconductor’’ #18069012 and Scientific Research (A) #18206003.
References [1] B.E. Foutz, S.K. O’Leary, M.S. Shur, L.F. Eastman, J. Appl. Phys. 85 (1999) 7727. [2] B.R. Nag, J. Crystal Growth 269 (2004) 35. [3] H. Lu, W.J. Schaff, J. Hwang, H. Wu, G. Koley, L.F. Eastman, Appl. Phys. Lett. 79 (2001) 1489. [4] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2002) 2549. [5] K. Xu, A. Yoshikawa, Appl. Phys. Lett. 83 (2003) 251. [6] Y. Nanishi, Y. Saito, T. Yamaguchi, M. Hori, F. Matsuda, T. Araki, A. Suzuki, T. Miyajima, Phys. Status Solidi (a) 200 (2003) 202. [7] X. Wang, S.–B. Che, Y. Ishitani, A. Yoshikawa, J. Appl. Phys. (2006) 073512. [8] G. Koblmu¨ller, C.S. Gallinat, S. Bernardis, J.S. Speck, G.D. Chern, E.D. Readinger, H. Shen, M. Wraback, Appl. Phys. Lett. 89 (2006) 071902. [9] D. Muto, H. Naoi, T. Araki, S. Kitagawa, M. Kurouchi, H. Na, Y. Nanishi, Phys. Status Solidi (a) 203 (2006) 1691. [10] C.S. Gallinat, G. Koblmu¨ller, J.S. Brown, S. Bernardis, J.S. Speck, Appl. Phys. Lett. 89 (2006) 032109. [11] X. Wang, S.–B. Che, Y. Ishitani, A. Yoshikawa, Jpn. J. Appl. Phys. 45 (2006) L730. [12] V. Srikant, J.S. Speck, D.R. Clarke, J. Appl. Phys. 82 (1997) 4286. [13] M. Sumiya, K. Yoshimura, K. Ohtsuka, S. Fuke, Appl. Phys. Lett. 76 (2000) 2098.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Growth and characterization of N-polar and In-polar InN films by RF-MBE T. Yamaguchi a,, D. Muto b, T. Araki b, Y. Nanishi b a b
Research Organization of Science & Engineering, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan Department of Photonics, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan
a r t i c l e in f o
a b s t r a c t
Available online 17 January 2009
The structural and electrical properties of N- and In-polar InN films grown directly on substrates were investigated. The twist contribution of directly grown InN films, which was defined by the estimated X-ray rocking curve’s full-width at half-maximum (XRC’s FWHM) of InN (1 01¯ 0) reflection, varied considerably by at least 40 arcmin for both N- and In-polar InN films, depending on growth conditions. In contrast, the tilt contribution, defined by the XRC’s FWHM of InN (0 0 0 2) reflection, was roughly fixed at 1.5–2.5 arcmin for N-polar InN films and 8–12 arcmin for In-polar InN films. For samples with the same twist contribution, In-polar InN films had larger tilt contribution than N-polar InN films. Nevertheless, electrical properties of In-polar InN had much better properties than those of N-polar InN films. & 2008 Elsevier B.V. All rights reserved.
PACS: 61.05.cm 81.15.Hi 73.61.Ey Keywords: A1. High-resolution X-ray diffraction A3. Molecular beam epitaxy B1. Nitrides
1. Introduction InN is a promising material for applications such as infrared light emitters and high-speed electronic devices [1,2]. The recent development of growth techniques using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) has dramatically improved the crystal quality of InN [3–11]. In addition, several specific features of InN growth have been found recently [3,4]. An important feature is the large difference between growth temperatures of N-polar InN (550–600 1C) and In-polar InN (450–500 1C) [5,6]. Although the growth and characterization of N- [7–9] and In- [10,11] polar InN have been widely studied separately, there are few comparative studies of the structural and electrical properties in these two properties. In this paper, the structural and electrical properties of directly grown InN films (without a low-temperature InN (LT-InN) buffer layer) on substrates are described, with emphasis on the comparison of N- and In-polar films.
2. Experimental procedure InN films were prepared by RF-MBE equipped with conventional effusion cells for group III elements (EpiQuest, Inc, model RC2100INR). Active nitrogen was generated using a commercialized plasma source (SVT Associates, model 6.03). N- and In-polar InN films were grown at 500 and 450 1C, respectively, directly Corresponding author.
E-mail address: [email protected] (T. Yamaguchi). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2009.01.034
on substrates. The substrate for the N-polar films was nitridated Al2O3 (0 0 0 1), whereas that for In-polar films was metal organic chemical vapor deposition (MOCVD)-grown GaN/Al2O3 (0 0 0 1). The samples were investigated ex-situ by X-ray diffraction (XRD) and Hall effect measurements. In the XRD measurement, tilt and twist contributions were investigated. The tilt and twist contributions were defined by X-ray rocking curve’s full-width at half-maximum (XRC’s FWHMs) values of InN (0 0 0 2) (FWHM002) and InN (1 0 1¯ 0) (FWHM100) reflections, respectively. As it was difficult to measure the FWHM100 value directly, owing to the thickness of the films, we used the method of Srikant et al. [12] to estimate this value.
3. Results and discussion 3.1. Structural and electrical properties of N-polar InN film Table 1 summarizes the film-thickness dependence of FWHM002, FWHM100, carrier density (n), and electron mobility (m) of N-polar InN films grown directly at the growth rate of 500 nm/h. It can be seen in Table 1 that FWHM100 value decreases with an increase of film thickness, whereas FWHM002 value is almost constant. It can also be seen that the carrier density decreases and electron mobility increases with the increase of film thickness. These improvements in the electrical properties can be explained by the improvement of the twist contribution. Only the improvement of twist contribution is a specific feature of directly grown InN films: In InN films with a LT-InN buffer layer, both the tilt and twist contributions have improved with an
ARTICLE IN PRESS T. Yamaguchi et al. / Journal of Crystal Growth 311 (2009) 2780–2782
increase of film thickness [4]. From these results, we expect that the electrical properties of directly grown InN films are directly related to the twist contribution. Table 1 Film-thickness dependence of FWHM002, FWHM100, carrier density (n) and electron mobility (m) of directly grown N-polar InN films. Film thickness
250 nm
500 nm
1.0 mm
FWHM002 (arcmin) FWHM100 (arcmin) n (cm 3) m (cm2/V s)
1.7 63.8 5.9 1018 950
2.3 55.8 4.8 1018 1080
2.4 48.4 4.6 1018 1090
In Fig. 1, the correlation between tilt and twist contributions is plotted for directly grown N-polar InN films with different thicknesses (250 nm–1.0 mm) and growth rates (250–750 nm/h). It was found that the twist contribution varied considerably from 45 to 75 arcmin. In contrast, the tilt contribution was fixed at 1.5–2.5 arcmin. Fig. 2 shows the FWHM100 value dependence on the electrical properties of directly grown N-polar InN films. Carrier density decreases and electron mobility increases with the decrease of FWHM100 value, indicating better crystal quality. These trends support our expectation that the electrical properties of directly grown InN films are correlated to the twist contribution.
80
80 FWHM100 (arcmin)
FWHM100 (arcmin)
2781
70
60
50
N-polar In-polar
70 60 50 40 30
40 0
2 4 FWHM002 (arcmin)
0
6
2
4
6
8
10 12 14 16 18 20
FWHM002 (arcmin)
Fig. 1. Correlation between tilt and twist contributions of directly grown N-polar InN films.
Fig. 3. Correlation between tilt and twist contributions of directly grown N- and In-polar InN films.
8.0
7.0
Carrier density (1018/cm3)
Carrier density (1018/cm3)
8.0
6.0 5.0 4.0 3.0 50
55 60 65 70 FWHM100 (arcmin)
75
In-polar
6.0 5.0 4.0 3.0 2.0 30
35
40
45 50 55 60 65 FWHM100 (arcmin)
70
75
1200 1600 Electron mobility (cm2 / Vs)
Electron mobility (cm2 /Vs)
45
N-polar 7.0
1100 1000 900
N-polar In-polar
1500 1400 1300 1200 1100 1000 900
800 45
50
55 60 65 70 FWHM100 (arcmin)
75
Fig. 2. FWHM100 value dependence on (a) carrier density and (b) electron mobility for directly grown N-polar InN films.
30
35
40
45 50 55 60 65 FWHM100 (arcmin)
70
75
Fig. 4. FWHM100 value dependence on (a) carrier density and (b) electron mobility for directly grown N- and In-polar InN films.
ARTICLE IN PRESS 2782
T. Yamaguchi et al. / Journal of Crystal Growth 311 (2009) 2780–2782
3.2. Comparison between N- and In-polar InN films Fig. 3 shows the correlation between tilt and twist contributions for both directly grown N-polar and In-polar InN films. The growth rate of In-polar InN films were fixed at 500 nm/h. The thickness was 500–750 nm. As can be seen in Fig. 3, the twist contribution varied considerably in both N- and In-polar InN films. FWHM002 of In-polar films was slightly smaller than that of N-polar films. This would be related to the lattice mismatches between InN and the underlayer (In-polar: GaN, N-polar: nitridated Al2O3). On the other hand, tilt contribution was almost fixed for each polarity. FWHM002 of N-polar films was very small value of 2–3 arcmin, whereas that of In-polar films was much larger (8–12 arcmin). This would be due to the different growth temperature (N-polar: 500 1C, In-polar: 50 1C). These results indicate that N-polar InN films had a smaller tilt contribution, compared with In-polar InN films with the same twist contribution as N-polar InN films. Nevertheless, electrical properties of In-polar InN were much better than those of N-polar InN, for samples with the same twist contribution, as can be seen in Fig. 4. This difference in electrical properties for the different polarities cannot be explained only by the correlation between residual electron concentration and density of dangling bonds at edge-component threading dislocations [11]. A possible explanation would be the different impurity or point defect incorporations for different polarity, as has been found for GaN [13]. Further investigation on this polarity dependence is in progress.
4. Conclusions In conclusion, both N- and In-polar InN films were grown directly on the substrates (N-polar: on nitridated Al2O3 (0 0 0 1), In-polar: on MOCVD-grown GaN template on Al2O3 (0 0 0 1)) by RF-MBE. The twist contribution of directly grown InN films varied
considerably depending on growth conditions in both N- and In-polar InN, whereas the tilt contribution was almost fixed in each N- and In-polar InN. For N- and In-polar InN films with the same twist contribution, In-polar InN film had a larger tilt contribution. Nevertheless, In-polar InN showed better electrical properties.
Acknowledgements The author would like thank to Dr. M. Kaneko for a fruitful discussion. This work was supported by the MEXT through Grantin-Aids for Scientific Research in Priority Areas ‘‘Optoelectronics Frontier by Nitride Semiconductor’’ #18069012 and Scientific Research (A) #18206003.
References [1] B.E. Foutz, S.K. O’Leary, M.S. Shur, L.F. Eastman, J. Appl. Phys. 85 (1999) 7727. [2] B.R. Nag, J. Crystal Growth 269 (2004) 35. [3] H. Lu, W.J. Schaff, J. Hwang, H. Wu, G. Koley, L.F. Eastman, Appl. Phys. Lett. 79 (2001) 1489. [4] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2002) 2549. [5] K. Xu, A. Yoshikawa, Appl. Phys. Lett. 83 (2003) 251. [6] Y. Nanishi, Y. Saito, T. Yamaguchi, M. Hori, F. Matsuda, T. Araki, A. Suzuki, T. Miyajima, Phys. Status Solidi (a) 200 (2003) 202. [7] X. Wang, S.–B. Che, Y. Ishitani, A. Yoshikawa, J. Appl. Phys. (2006) 073512. [8] G. Koblmu¨ller, C.S. Gallinat, S. Bernardis, J.S. Speck, G.D. Chern, E.D. Readinger, H. Shen, M. Wraback, Appl. Phys. Lett. 89 (2006) 071902. [9] D. Muto, H. Naoi, T. Araki, S. Kitagawa, M. Kurouchi, H. Na, Y. Nanishi, Phys. Status Solidi (a) 203 (2006) 1691. [10] C.S. Gallinat, G. Koblmu¨ller, J.S. Brown, S. Bernardis, J.S. Speck, Appl. Phys. Lett. 89 (2006) 032109. [11] X. Wang, S.–B. Che, Y. Ishitani, A. Yoshikawa, Jpn. J. Appl. Phys. 45 (2006) L730. [12] V. Srikant, J.S. Speck, D.R. Clarke, J. Appl. Phys. 82 (1997) 4286. [13] M. Sumiya, K. Yoshimura, K. Ohtsuka, S. Fuke, Appl. Phys. Lett. 76 (2000) 2098.