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Be and Mg co-doping in GaN
ARTICLE IN PRESS
Journal of Crystal Growth 301–302 (2007) 414–416 www.elsevier.com/locate/jcrysgro
Be and Mg co-doping in GaN A. Kawaharazukaa,, T. Tanimotob,c, K. Nagaib,c, Y. Tanakab,c, Y. Horikoshib,c a
Institute for Biomedical Engineering, ASMeW, Waseda University, 513 Waseda-Tsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan b School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan c Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishi-Waseda, Shinjuku-ku, Tokyo 169-0051, Japan Available online 9 January 2007
Abstract We investigate co-doping of Be and Mg in GaN by molecular beam epitaxy with radio frequency plasma for nitrogen source. The compressive strain accumulated during the growth due to high doping of Be, which has shorter bond length than that of Ga, is compensated for by adding Mg, since its bond length is longer than that of Ga. Photoluminescence spectra reveal that both Be and Mg atoms are mainly incorporated in Ga sites and act as acceptors. The energy shift of the donor–acceptor pair emission associated with Be suggests possible correlation between co-doped Be and Mg acceptors. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 61.72.Vv; 61.10.Nz Keywords: A1. Defects; A1. Doping; A1. X-ray diffraction; A3. Molecular beam epitaxy; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction Group III-nitride semiconductors have been intensively studied [1,2] for the application to optoelectronic devices that operate in the blue and ultraviolet (UV) wavelength region [3–6]. Highly efficient blue, green and UV light emitting diodes (LEDs) as well as laser diodes (LDs) are in practical use. Recently, application of group III-nitrides to white-light source [7] received considerable attention. Enhancement of the efficiency of p-type doping is a very important issue for such device applications to further improve the device performance. Mg is widely used as ptype dopant in III-nitride systems despite the fact that its activation energy of 200 meV [8,9] is much higher compared with room temperature. In contrast, Be is advantageous as p-type dopant since its activation energy of 100 meV [10–12] is much lower than that of Mg. However, activation ratio of Be-acceptor is much lower than that expected from the activation energy. This is probably due to the formation of point defects [13] caused by strain accumulated during growth because of the shorter bond length of Be [14] than that of Ga. Corresponding author.
E-mail address: [email protected] (A. Kawaharazuka). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.109
Therefore it is essential to reduce the strain of the epitaxial layer to achieve higher activation ratio of acceptors. Recent progress in the metal–organic vapor phase epitaxy (MOVPE) makes it possible to grow GaN single crystal with low dislocation density, which can be used as a template for further epitaxial growth [15,16]. The GaN template substrate is advantageous since homoepitaxial growth significantly reduce the strain and defects, which may be introduced due to the lattice mismatch. Therefore, strain caused by doping can be clearly investigated. In addition, polarity control required in the growth on sapphire substrate, is not necessary. We investigate co-doping of Be and Mg in GaN [17] to solve the problem due to strain. Both of them act as acceptors in GaN. The compressive strain due to Be, which has the shorter bond length than that of Ga, can be compensated for by adding Mg, since it has the longer bond length [14] than that of Ga and introduces tensile strain in GaN. 2. Experimental procedure We employ molecular beam epitaxy with radio frequency plasma for nitrogen source (RF–MBE). As a substrate, we
ARTICLE IN PRESS A. Kawaharazuka et al. / Journal of Crystal Growth 301–302 (2007) 414–416
use GaN grown on sapphire by MOVPE (GaN template). The thickness of the GaN template layer is about 2 mm. After the thermal cleaning with nitrogen plasma irradiation, GaN is directly grown on the template substrate without introducing any buffer layer. Substrate temperature of 820 C, Ga beam equivalent pressure of 8:0 10 7 Torr, nitrogen gas flow rate of 8 sccm and RF-plasma power of 470 W are used for all the growth. The doping concentrations of Be and Mg are varied by changing the temperature of the effusion cells. The strain of the epitaxial layer is evaluated by using X-ray diffractometry (XRD). We employ photoluminescence (PL) spectroscopy to observe the microscopic behavior of dopant atoms in the crystals. 3. Results and discussion First, we investigate the effect of Be-doping by changing the temperature of Be effusion cell from 750 to 850 C. Nominal concentration of Be is 1:2 1020 cm 3 at the cell temperature of 800 C. We evaluate the strain of the epitaxial layer by using 2y-scan of XRD. In addition to the main peak, which corresponds to the (0 0 0 2) reflection of the GaN template, a shoulder is observed in the higher diffraction angle side of the spectra, as shown in Fig. 1. This diffraction corresponds to the Be-doped GaN epitaxial layer since the intensity of the shoulder increases with increasing Be-cell temperature. This result is a clear evidence that the Be-doped GaN has indeed shorter lattice constant than that of undoped GaN. We estimate the Be concentration from the obtained lattice constant of Bedoped GaN by using the theoretically calculated bond length between Be at Ga site and N assuming Vegard’s law [14,18]. The lattice constants of the Be-doped GaN epitaxial layer are extracted from the observed XRD spectra by separating the spectra with two Gaussian curves. Resulting Be concentration of 1:6 1020 cm 3 roughly
Fig. 1. XRD 2y-scan spectra obtained from Be-doped GaN grown on template substrate for different Be concentrations. Effusion cell temperatures of Be are 850, 800 and 750 C, respectively.
415
agrees with that expected from the cell temperature. Therefore, it is expected that doped Be atoms mainly substitute Ga. Next, we investigate the effect of co-doping of Mg to Bedoped GaN. The temperatures of Mg and Be cell are 280 and 800 C, respectively. The shoulder observed in the XRD spectra is significantly reduced by co-doping Mg, as shown in Fig. 2. The lattice constant of co-doped layer is closer to the substrate in spite of the increase in the total number of dopant atoms. This result clearly indicates that the strain caused by Be-doping is effectively compensated for by introducing Mg as co-dopant. We expect that a shoulder appears in the lower diffraction side of the spectrum obtained from the Mg-doped GaN. However, no significant structure is observed in the spectrum. This discrepancy may be explained by taking into account the strain of the template GaN layer itself. Note that our XRD measurement shows that the GaN template is still compressively strained. We perform PL spectroscopy measurement at a low temperature to examine the microscopic characteristics of dopant atoms. The spectrum obtained from the Be-doped GaN shows the prominent emission line at 3.37 eV as shown in Fig. 3. This line is assigned to the donor–acceptor pair (DAP) emission attributed to the Be acceptor since the energy difference from the band-edge is close to the activation energy of the Be acceptor of about 100 meV [12]. In contrast, the PL spectrum of the Mg-doped sample shows the emission line from the donor bound exciton (DB) at 3.46 eV and DAP emission due to the Mg acceptor at 3.28 eV [19]. The energy difference of 200 meV between DB and DAP emissions coincides with the activation energy of the Mg acceptor. Peaks observed on the lower energy side of the DAP emission are its longitudinal optical (LO) phonon replicas. Compared with these spectra, both Be- and Mg-doped characteristics are clearly observed in the spectrum obtained from the co-doped sample. This result reveals that both co-doped Mg and Be atoms
Fig. 2. XRD 2y-scan spectra obtained from Be-doped, Be and Mg codoped, and Mg-doped GaN grown on template substrate.
ARTICLE IN PRESS 416
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4. Conclusion We have demonstrated Be and Mg co-doping in GaN. The strain due to the high Be doping is significantly reduced by introducing Mg as co-dopant. It is revealed from the PL spectra that both Be and Mg act as acceptors in GaN. The energy shift of the Be DAP line of the codoped sample suggests possible correlation between Be and Mg acceptors.
Acknowledgments This work is funded by 21st century COE ‘‘Practical Nano-Chemistry’’ from the Ministry of Education, Science, Sports and Culture, Japan, and by the Grant-inaid for Scientific Research (A) (17206031) from Japan Society for the Promotion of Science (JSPS).
References
Fig. 3. PL spectra of the Be-doped, Mg-doped and co-doped GaN obtained at 10 K.
substitute Ga and act as acceptors in GaN. The PL intensity obtained from Mg-doped and co-doped samples are comparable. However, the intensity of the DAP attributed to the Be acceptor in the co-doped sample is low compared with that of Mg acceptor. Since the Mg acceptor has deeper level than that of Be, the efficiency of the PL emission is higher than that of Be. The DB line from the co-doped sample slightly shifts to the higher energy side and has narrower width compared with that of Mg-doped sample. This result also suggests that the co-doping reduces the strain of the epitaxial GaN layer. In addition, Be DAP line of the co-doped sample shifts about 20 meV toward band-edge emission line compared with that of Be-doped one. This shift may be explained by the correlation of Be and Mg acceptors and change in the depth of the acceptor level due to that [20].
[1] H. Morkoc- , Nitride Semiconductors and Devices, in: Materials Science, Springer, Berlin, Heidelberg, 1999. [2] H. Morkoc- , J. Mater. Sci. 12 (2001) 677. [3] S.N.T. Mukai, N. Iwasa, M.S.T. Yamada, J. Phys. 13 (2001) 7089. [4] S. Nagahama, T. Yamamoto, M. Sano, T. Mukai, Phys. Stat. Solidi. (a) 194 (2002) 423. [5] T. Mukai, S. Nagahama, M. Sano, T. Yamamoto, D. Morita, T. Mitani, Y. Narukawa, S. Yamamoto, I. Niki, M. Yamada, et al., Phys. Stat. Solidi. (a) 200 (2003) 52. [6] M.A. Khan, M. Shatalov, H.P. Maruska, H.M. Wang, E. Kuokstis, Jpn. J. Appl. Phys. 44 (2001) 7191. [7] F.K. Yam, Z. Hassan, Microelectronics J. 36 (2005) 129. [8] J. Pankove, J.A. Hutchby, J. Appl. Phys. 47 (1976) 5387. [9] T. Tanaka, A. Watanabe, H. Amano, Y. Kobayashi, I. Akasaki, S. Yamazaki, M. Koike, Appl. Phys. Lett 65 (1994) 593. [10] F.J. Saˆnchez, M.A.S.-G.F. Calle, E. Calleja, E. Mun˜oz, C.H. Molloy, D.J. Somerfeld, J.J. Serrano, J.M. Blanco, Semicond. Sci. Technol. 13 (1998) 1130. [11] F. Bernardini, V. Fiorentini, Appl. Phys. Lett. 70 (1994) 2990. [12] S. Sugita, Y. Watari, G. Yoshizawa, J. Sodesawa, H. Yamamizu, K.-T. Liu, Y.-K. Su, Y. Horikoshi, Jpn. J. Appl. Phys. 42 (2003) 7194. [13] C.G.V. de Walle, S. Limpijumnong, Phys. Rev. B 63 (2001) 245205. [14] C.G.V. de Walle, Phys. Rev. B 68 (2003) 165209. [15] L. Liu, J.H. Edgar, Mater. Sci. Eng. R37 (2002) 61. [16] H. Morkoc- , Mater. Sci. Eng. R33 (2001) 135. [17] F.B. Naranjo, M.A. Saˆnches-Garcıˆ a, J.L. Pau, A. Jimeneˆz, E.C.E. Mun˜oz, J. Oila, K. Saarinen, P. Hautja¨rvi, Phys. Stat. Solidi. (a) 180 (2002) 97. [18] C.K. Shih, W.E. Spicer, W.A. Harrison, A. Sher, Phys. Rev. B 31 (1985) 1139. [19] M.A. Reshchikov, H. Morkoc- , J. Appl. Phys. 97 (2005) 061301. [20] H. Katayama-Yoshida, T. nishimatsu, T. Yamamoto, N. orita, J. Phys. 13 (2001) 8901.
Journal of Crystal Growth 301–302 (2007) 414–416 www.elsevier.com/locate/jcrysgro
Be and Mg co-doping in GaN A. Kawaharazukaa,, T. Tanimotob,c, K. Nagaib,c, Y. Tanakab,c, Y. Horikoshib,c a
Institute for Biomedical Engineering, ASMeW, Waseda University, 513 Waseda-Tsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan b School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan c Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishi-Waseda, Shinjuku-ku, Tokyo 169-0051, Japan Available online 9 January 2007
Abstract We investigate co-doping of Be and Mg in GaN by molecular beam epitaxy with radio frequency plasma for nitrogen source. The compressive strain accumulated during the growth due to high doping of Be, which has shorter bond length than that of Ga, is compensated for by adding Mg, since its bond length is longer than that of Ga. Photoluminescence spectra reveal that both Be and Mg atoms are mainly incorporated in Ga sites and act as acceptors. The energy shift of the donor–acceptor pair emission associated with Be suggests possible correlation between co-doped Be and Mg acceptors. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Hi; 81.05.Ea; 61.72.Vv; 61.10.Nz Keywords: A1. Defects; A1. Doping; A1. X-ray diffraction; A3. Molecular beam epitaxy; B1. Nitrides; B2. Semiconducting III–V materials
1. Introduction Group III-nitride semiconductors have been intensively studied [1,2] for the application to optoelectronic devices that operate in the blue and ultraviolet (UV) wavelength region [3–6]. Highly efficient blue, green and UV light emitting diodes (LEDs) as well as laser diodes (LDs) are in practical use. Recently, application of group III-nitrides to white-light source [7] received considerable attention. Enhancement of the efficiency of p-type doping is a very important issue for such device applications to further improve the device performance. Mg is widely used as ptype dopant in III-nitride systems despite the fact that its activation energy of 200 meV [8,9] is much higher compared with room temperature. In contrast, Be is advantageous as p-type dopant since its activation energy of 100 meV [10–12] is much lower than that of Mg. However, activation ratio of Be-acceptor is much lower than that expected from the activation energy. This is probably due to the formation of point defects [13] caused by strain accumulated during growth because of the shorter bond length of Be [14] than that of Ga. Corresponding author.
E-mail address: [email protected] (A. Kawaharazuka). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.109
Therefore it is essential to reduce the strain of the epitaxial layer to achieve higher activation ratio of acceptors. Recent progress in the metal–organic vapor phase epitaxy (MOVPE) makes it possible to grow GaN single crystal with low dislocation density, which can be used as a template for further epitaxial growth [15,16]. The GaN template substrate is advantageous since homoepitaxial growth significantly reduce the strain and defects, which may be introduced due to the lattice mismatch. Therefore, strain caused by doping can be clearly investigated. In addition, polarity control required in the growth on sapphire substrate, is not necessary. We investigate co-doping of Be and Mg in GaN [17] to solve the problem due to strain. Both of them act as acceptors in GaN. The compressive strain due to Be, which has the shorter bond length than that of Ga, can be compensated for by adding Mg, since it has the longer bond length [14] than that of Ga and introduces tensile strain in GaN. 2. Experimental procedure We employ molecular beam epitaxy with radio frequency plasma for nitrogen source (RF–MBE). As a substrate, we
ARTICLE IN PRESS A. Kawaharazuka et al. / Journal of Crystal Growth 301–302 (2007) 414–416
use GaN grown on sapphire by MOVPE (GaN template). The thickness of the GaN template layer is about 2 mm. After the thermal cleaning with nitrogen plasma irradiation, GaN is directly grown on the template substrate without introducing any buffer layer. Substrate temperature of 820 C, Ga beam equivalent pressure of 8:0 10 7 Torr, nitrogen gas flow rate of 8 sccm and RF-plasma power of 470 W are used for all the growth. The doping concentrations of Be and Mg are varied by changing the temperature of the effusion cells. The strain of the epitaxial layer is evaluated by using X-ray diffractometry (XRD). We employ photoluminescence (PL) spectroscopy to observe the microscopic behavior of dopant atoms in the crystals. 3. Results and discussion First, we investigate the effect of Be-doping by changing the temperature of Be effusion cell from 750 to 850 C. Nominal concentration of Be is 1:2 1020 cm 3 at the cell temperature of 800 C. We evaluate the strain of the epitaxial layer by using 2y-scan of XRD. In addition to the main peak, which corresponds to the (0 0 0 2) reflection of the GaN template, a shoulder is observed in the higher diffraction angle side of the spectra, as shown in Fig. 1. This diffraction corresponds to the Be-doped GaN epitaxial layer since the intensity of the shoulder increases with increasing Be-cell temperature. This result is a clear evidence that the Be-doped GaN has indeed shorter lattice constant than that of undoped GaN. We estimate the Be concentration from the obtained lattice constant of Bedoped GaN by using the theoretically calculated bond length between Be at Ga site and N assuming Vegard’s law [14,18]. The lattice constants of the Be-doped GaN epitaxial layer are extracted from the observed XRD spectra by separating the spectra with two Gaussian curves. Resulting Be concentration of 1:6 1020 cm 3 roughly
Fig. 1. XRD 2y-scan spectra obtained from Be-doped GaN grown on template substrate for different Be concentrations. Effusion cell temperatures of Be are 850, 800 and 750 C, respectively.
415
agrees with that expected from the cell temperature. Therefore, it is expected that doped Be atoms mainly substitute Ga. Next, we investigate the effect of co-doping of Mg to Bedoped GaN. The temperatures of Mg and Be cell are 280 and 800 C, respectively. The shoulder observed in the XRD spectra is significantly reduced by co-doping Mg, as shown in Fig. 2. The lattice constant of co-doped layer is closer to the substrate in spite of the increase in the total number of dopant atoms. This result clearly indicates that the strain caused by Be-doping is effectively compensated for by introducing Mg as co-dopant. We expect that a shoulder appears in the lower diffraction side of the spectrum obtained from the Mg-doped GaN. However, no significant structure is observed in the spectrum. This discrepancy may be explained by taking into account the strain of the template GaN layer itself. Note that our XRD measurement shows that the GaN template is still compressively strained. We perform PL spectroscopy measurement at a low temperature to examine the microscopic characteristics of dopant atoms. The spectrum obtained from the Be-doped GaN shows the prominent emission line at 3.37 eV as shown in Fig. 3. This line is assigned to the donor–acceptor pair (DAP) emission attributed to the Be acceptor since the energy difference from the band-edge is close to the activation energy of the Be acceptor of about 100 meV [12]. In contrast, the PL spectrum of the Mg-doped sample shows the emission line from the donor bound exciton (DB) at 3.46 eV and DAP emission due to the Mg acceptor at 3.28 eV [19]. The energy difference of 200 meV between DB and DAP emissions coincides with the activation energy of the Mg acceptor. Peaks observed on the lower energy side of the DAP emission are its longitudinal optical (LO) phonon replicas. Compared with these spectra, both Be- and Mg-doped characteristics are clearly observed in the spectrum obtained from the co-doped sample. This result reveals that both co-doped Mg and Be atoms
Fig. 2. XRD 2y-scan spectra obtained from Be-doped, Be and Mg codoped, and Mg-doped GaN grown on template substrate.
ARTICLE IN PRESS 416
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4. Conclusion We have demonstrated Be and Mg co-doping in GaN. The strain due to the high Be doping is significantly reduced by introducing Mg as co-dopant. It is revealed from the PL spectra that both Be and Mg act as acceptors in GaN. The energy shift of the Be DAP line of the codoped sample suggests possible correlation between Be and Mg acceptors.
Acknowledgments This work is funded by 21st century COE ‘‘Practical Nano-Chemistry’’ from the Ministry of Education, Science, Sports and Culture, Japan, and by the Grant-inaid for Scientific Research (A) (17206031) from Japan Society for the Promotion of Science (JSPS).
References
Fig. 3. PL spectra of the Be-doped, Mg-doped and co-doped GaN obtained at 10 K.
substitute Ga and act as acceptors in GaN. The PL intensity obtained from Mg-doped and co-doped samples are comparable. However, the intensity of the DAP attributed to the Be acceptor in the co-doped sample is low compared with that of Mg acceptor. Since the Mg acceptor has deeper level than that of Be, the efficiency of the PL emission is higher than that of Be. The DB line from the co-doped sample slightly shifts to the higher energy side and has narrower width compared with that of Mg-doped sample. This result also suggests that the co-doping reduces the strain of the epitaxial GaN layer. In addition, Be DAP line of the co-doped sample shifts about 20 meV toward band-edge emission line compared with that of Be-doped one. This shift may be explained by the correlation of Be and Mg acceptors and change in the depth of the acceptor level due to that [20].
[1] H. Morkoc- , Nitride Semiconductors and Devices, in: Materials Science, Springer, Berlin, Heidelberg, 1999. [2] H. Morkoc- , J. Mater. Sci. 12 (2001) 677. [3] S.N.T. Mukai, N. Iwasa, M.S.T. Yamada, J. Phys. 13 (2001) 7089. [4] S. Nagahama, T. Yamamoto, M. Sano, T. Mukai, Phys. Stat. Solidi. (a) 194 (2002) 423. [5] T. Mukai, S. Nagahama, M. Sano, T. Yamamoto, D. Morita, T. Mitani, Y. Narukawa, S. Yamamoto, I. Niki, M. Yamada, et al., Phys. Stat. Solidi. (a) 200 (2003) 52. [6] M.A. Khan, M. Shatalov, H.P. Maruska, H.M. Wang, E. Kuokstis, Jpn. J. Appl. Phys. 44 (2001) 7191. [7] F.K. Yam, Z. Hassan, Microelectronics J. 36 (2005) 129. [8] J. Pankove, J.A. Hutchby, J. Appl. Phys. 47 (1976) 5387. [9] T. Tanaka, A. Watanabe, H. Amano, Y. Kobayashi, I. Akasaki, S. Yamazaki, M. Koike, Appl. Phys. Lett 65 (1994) 593. [10] F.J. Saˆnchez, M.A.S.-G.F. Calle, E. Calleja, E. Mun˜oz, C.H. Molloy, D.J. Somerfeld, J.J. Serrano, J.M. Blanco, Semicond. Sci. Technol. 13 (1998) 1130. [11] F. Bernardini, V. Fiorentini, Appl. Phys. Lett. 70 (1994) 2990. [12] S. Sugita, Y. Watari, G. Yoshizawa, J. Sodesawa, H. Yamamizu, K.-T. Liu, Y.-K. Su, Y. Horikoshi, Jpn. J. Appl. Phys. 42 (2003) 7194. [13] C.G.V. de Walle, S. Limpijumnong, Phys. Rev. B 63 (2001) 245205. [14] C.G.V. de Walle, Phys. Rev. B 68 (2003) 165209. [15] L. Liu, J.H. Edgar, Mater. Sci. Eng. R37 (2002) 61. [16] H. Morkoc- , Mater. Sci. Eng. R33 (2001) 135. [17] F.B. Naranjo, M.A. Saˆnches-Garcıˆ a, J.L. Pau, A. Jimeneˆz, E.C.E. Mun˜oz, J. Oila, K. Saarinen, P. Hautja¨rvi, Phys. Stat. Solidi. (a) 180 (2002) 97. [18] C.K. Shih, W.E. Spicer, W.A. Harrison, A. Sher, Phys. Rev. B 31 (1985) 1139. [19] M.A. Reshchikov, H. Morkoc- , J. Appl. Phys. 97 (2005) 061301. [20] H. Katayama-Yoshida, T. nishimatsu, T. Yamamoto, N. orita, J. Phys. 13 (2001) 8901.