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Post growth thermal annealing of GaN grown by RF plasma MBE
Journal of Crystal Growth 227–228 (2001) 415–419
Post growth thermal annealing of GaN grown by RF plasma MBE Wei Li, Aizhen Li* State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China
Abstract Si-doped GaN with a carrier concentration of 6 1017 cm3 was grown on c-plane sapphire substrates by molecular beam epitaxy (MBE) equipped with a nitrogen RF plasma source. The thermal annealing process was performed in a nitrogen atmosphere with a tungsten–halogen lamp as the thermal source. Double crystal X-ray diffraction (DCXRD) and 77 K photoluminescence (PL) have been employed to investigate the effects of thermal annealing on Si-doped GaN films. Both annealing temperature and annealing time have been varied to investigate the influence of different annealing processes on the sample. The DCXRD results show that the dislocation density in the sample decreases after thermal annealing. 77 K PL of GaN annealed longer than 300 s at 9508C shows a new peak at 378 nm. The integrated intensity of the peak increases dramatically with increasing annealing time. The excitation power dependence of the 378 nm peak indicates that the emission was associated with donor-acceptor pair (DAP) transitions, and a similar peak has been observed in highly Si-doped GaN with a carrier concentration of 9 1019 cm3. We attribute the acceptor in DAP to SiN. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.E; 81.15.H; 81.40.G; 78.55.C Keywords: A1. Characterization; A1. Doping; A3. Molecular beam epitaxy; B1. Nitrides
1. Introduction GaN is an extremely promising material for short-wavelength optical and high temperature electronic device applications. The growth, processing issues and electronic properties of GaN-based devices are important concerns which have been intensely addressed in the recent literature [1–3]. Even though significant progress has been made in these areas, much more work is still needed in *Corresponding author. Tel.: +86-21-62511070; fax:+8621-62513510. E-mail address: [email protected] (A. Li).
order to make these electronic devices operate with high efficiency and reliability. One of the fundamental issues, for promoting practical applications of GaN-based devices, is the crystalline quality of GaN. Defects, such as dislocations, stacking faults, and microtwins, are common in GaN. Thermal annealing has been reported to improve the crystalline quality of lattice-mismatched systems [4–6]. However, few reports mentioned the application of thermal annealing in GaN epitaxial layers grown by RF Plasma assisted MBE. In this paper, we report post-growth thermal annealing of GaN grown by RF plasma MBE.
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 3 5 - 7
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W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419
2. Experimental details GaN was grown on c-plane of sapphire substrates in an MBE system equipped with an EPI Unibulb nitrogen RF plasma source. Ga and N2 are used as sources and Si as n-type dopant. Prior to epitaxial growth of GaN, the substrate was nitridized for 1 hour, then a GaN 20 nm buffer layer was grown at low temperature (6008C). The temperature of the Si source was varied to achieve doping densities in the range of 1017–9 1019 cm3. The thermal annealing process was performed in a nitrogen atmosphere with a tungsten–halogen lamp. GaN decomposed at temperature as high as 1300 K, when the reaction 2GaN(s) ! 2Ga(g)+N2 occurs [7]. To avoid decomposition of GaN during annealing, the highest annealing temperature was 10008C in our experiment. The samples used in this study were Si-doped GaN with a carrier concentration of 6 1017 cm3. In order to minimize possible variability between different growth runs, only one sample was used and it was divided into several pieces, which were then annealed for different annealing time or annealing temperature. Photoluminescence (PL) spectra were obtained by a Jobin Yvon-Spex THR1000 monochromator with a 1200 g/mm grating blazed at 400 nm. A 325 nm He–Cd laser with a maximum power of 60 mW was used as the excitation light. The luminescence was dispersed in the spectrometer and detected by a photo-multiplier-tube (PMT). Double crystal X-ray diffraction (DCXRD) of GaN (0 0 0 2) was carried out on a Philips X’Pert MRD system with a goniometer accuracy of 0.00018.
Data in Fig. 1 show annealing temperature dependence of both FWHM of X-ray rocking curves and estimated dislocation density in annealed GaN. The ramp-up time to the annealing temperature was 15 s. The samples were annealed for 35 s and then cooled down to room temperature. A mosaic model is used to calculate dislocation densities of a crystal. According to this theory, a crystal is composed of many mosaics, which are separated by grain boundaries. Defect in a mosaic are neglected. Taking assumption of the mosaic model, the dislocation density in the GaN film shown in Fig. 1 was calculated using the following equation [8]: DB ¼ ba =ð2p ln 2Þ1=2 st;
ð1Þ
where DB is the dislocation density and ba is the rocking curve broadening caused by diffraction angular rotation at dislocations, which equals approximately to the FWHM of the X-ray rocking curve. The size of the column in the mosaic model is t and can be obtained from atomic force microscopy (AFM) of the sample and s is the Burgers vector. As shown in Fig. 1, the FWHM of the rocking curve decreases when the annealing temperature increases. This decrease of FWHM indicates an improvement in crystal quality due to decrease of dislocation density. Using Eq. (1) with s ¼ a=3h1 1 2 0i and t ¼ 0:3 mm, the dislocation
3. Results and discussion The full width at half maximum (FWHM) of the DCXRD rocking curve and the 77 K near-bandedge luminescence of the GaN sample used for annealing is 262 arcsec and 22 meV, respectively. The peak intensity ratio of the near-band-edge transition compared to the yellow luminescence at 2.2 eV is 16, indicating high quality of the epitaxial film.
Fig. 1. Estimated dislocation density and the value of the full width at half maximum (FWHM) of the XRD for the as-grown and annealed GaN layers, as a function of annealing temperature.
W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419
density in the as-grown GaN is 1 109 cm2 and decreases to 8.7 108 cm2 after annealing at 10008C for 35 s. Clearly, high temperature rapid thermal annealing (RTA) have improved the structural quality of GaN. The annealing time dependence of FWHM of XRD is shown in Fig. 2. The annealing temperature was fixed at 9508C. The improvement of crystal quality has been observed after rapid thermal annealing. However, with an increase of the annealing time, the decrease of FWHM of XRD became less obvious. When the sample was annealed for 600 s, the FWHM became almost the same as that of the as-grown GaN. Annealing time longer than 600 s made the crystal quality become worse. Thermal annealing may affect various point defects such as vacancies, interstitials, and hydrogen complex. The 77 K PL of GaN annealed at 9508C for different time was measured (Fig. 3). A new peak at 378 nm appeared in the sample annealed for longer than 300 s. The integrated intensity of the peak increased dramatically with increasing of the annealing time. The excitation power in PL measurement was varied in order to clarify the origin of this new peak (Fig. 4). We can see clearly that the new peak continuously redshifted with decreasing excitation power. It is well
417
Fig. 3. 77 K photoluminescence of GaN annealed at 9508C with different time (the same scale).
Fig. 4. Excitation density dependence of 77 K photoluminiscence of GaN after annealing at 9508C for 35 min. The blue shift of the peak with longer wavelength can be clearly seen with increasing excitation density.
Fig. 2. The annealing time dependence of FWHM of XRD for GaN layers. The annealing temperature was fixed at 9508C.
known that the donor–acceptor pair DAP emission blueshifts with increasing excitation power due to recombinations of closer pairs [9]. Therefore, the excitation power dependence of the new peak indicates that the emission is associated with DAP transitions. A similar DAP emission was observed in highly Si-doped GaN. When the carrier concentration in Si-doped GaN became as high as 9 1019 cm3, a
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W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419
energy in the material and lead to equilibrium state. The formation of SiN can decrease the total energy by compensating the increasing total energy caused by SiGa. As a result, SiN tends to form in the annealed GaN. Other point defects, however, can be ruled out as the source of the DAP transition because of their unsuitable energy level position [10] or their negligible concentration [11]. The formation of SiN in highly doped GaN can also be explained by the amphoteric nature of Si. Although Ga site is preferred when Si was doped into GaN, Si on N site will surely form due to the short of Ga site in extremely high Si concentration. Theoretical studies of defects and impurities also showed that SiN can act as acceptor in GaN [12].
4. Conclusion
Fig. 5. (a) 77 K PL of a highly Si-doped GaN whose carrier concentration is 9 1019 cm3; (b) 77 K PL of an undoped GaN.
In conclusion, using XRD, we have demonstrated that post growth rapid thermal annealing can improve the crystal quality of a GaN epilayer. A new peak due to DAP emission appeared in the 77 K PL of the GaN annealed at 9508C for longer than 300 s. Similar DAP emission can also be seen in highly Si-doped GaN. The origin of the acceptor was proposed to be SiN.
Acknowledgements new peak located at 375 nm appeared (Fig. 5). The excitation intensity dependence of the 375 nm peak indicates that the emission is also associated with DAP transitions. We infer that the 375 nm peak in Fig. 5 has the same origin as the 378 nm peak shown in Fig. 3. The discrepancy between the peak wavelength in the two cases is due to different spacial separation between donors and acceptors. Since the annealing temperature is much lower than the decomposition temperature of GaN, the possibility of formation of Ga-vacancy or Nvacancy is excluded. We attribute the acceptor in both DAP emissions to SiN. Because of the large lattice mismatch between GaN and the sapphire substrate, stress exists in the as-grown GaN. Thermal annealing tends to decrease the total
This work is supported in part by the Shanghai Center of Applied Physics.
References [1] R.T. Leonard, S.M. Bedair, Appl. Phys. Lett. 68 (1996) 794. [2] J.C. Zolper, M. Hageron, A.J. Howard, J. Ramer, D. Hersee, Appl. Phys. Lett. 79 (1996) 200. [3] C. Yuan, T. Salagaj, A.G. Thompson, W. Kroll, R.A. Stall, C.Y. Hwang, M. Schurman, Y. Li, W.E. Mayo, Y. Lu, S. Krishnankutty, I.K. Shmagin, R.M. Kolbas, J. Vac. Sci. Technol. B 13 (1995) 2075. [4] N. Chand, R. People, F.A. Baiocchi, K.W. Wecht, A.Y. Cho, Appl. Phys. Lett. 49 (1986) 815. [5] R.J. Matyi, J.W. Lee, H.F. Schaake, J. Electron. Mater. 17 (1988) 87.
W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419 [6] M. Yamaguchi, M. Tachikawa, Y. Itoh, M. Sugo, S. Kondo, J. Appl. Phys. 68 (1990) 4518. [7] Z.A. Munir, A.W. Sercy, J. Chem. Phys. 42 (1965) 4223. [8] Q.K. Yang, A.Z. Li, Y.G. Zhang, B. Yang, O. Brandt, K. Ploog, J. Crystal Growth 192 (1998) 27. [9] T. Ogino, M. Aoki, Jpn. J. Appl. Phys. 19 (1980) 2395.
419
[10] P. Boguslawski, E. Briggs, J. Bernholc, Phys. Rev. B 47 (1993) 13353. [11] Jorg Neugebauer, Chirs G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503. [12] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B10 (1992) 1237.
Post growth thermal annealing of GaN grown by RF plasma MBE Wei Li, Aizhen Li* State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Metallurgy, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China
Abstract Si-doped GaN with a carrier concentration of 6 1017 cm3 was grown on c-plane sapphire substrates by molecular beam epitaxy (MBE) equipped with a nitrogen RF plasma source. The thermal annealing process was performed in a nitrogen atmosphere with a tungsten–halogen lamp as the thermal source. Double crystal X-ray diffraction (DCXRD) and 77 K photoluminescence (PL) have been employed to investigate the effects of thermal annealing on Si-doped GaN films. Both annealing temperature and annealing time have been varied to investigate the influence of different annealing processes on the sample. The DCXRD results show that the dislocation density in the sample decreases after thermal annealing. 77 K PL of GaN annealed longer than 300 s at 9508C shows a new peak at 378 nm. The integrated intensity of the peak increases dramatically with increasing annealing time. The excitation power dependence of the 378 nm peak indicates that the emission was associated with donor-acceptor pair (DAP) transitions, and a similar peak has been observed in highly Si-doped GaN with a carrier concentration of 9 1019 cm3. We attribute the acceptor in DAP to SiN. # 2001 Elsevier Science B.V. All rights reserved. PACS: 81.05.E; 81.15.H; 81.40.G; 78.55.C Keywords: A1. Characterization; A1. Doping; A3. Molecular beam epitaxy; B1. Nitrides
1. Introduction GaN is an extremely promising material for short-wavelength optical and high temperature electronic device applications. The growth, processing issues and electronic properties of GaN-based devices are important concerns which have been intensely addressed in the recent literature [1–3]. Even though significant progress has been made in these areas, much more work is still needed in *Corresponding author. Tel.: +86-21-62511070; fax:+8621-62513510. E-mail address: [email protected] (A. Li).
order to make these electronic devices operate with high efficiency and reliability. One of the fundamental issues, for promoting practical applications of GaN-based devices, is the crystalline quality of GaN. Defects, such as dislocations, stacking faults, and microtwins, are common in GaN. Thermal annealing has been reported to improve the crystalline quality of lattice-mismatched systems [4–6]. However, few reports mentioned the application of thermal annealing in GaN epitaxial layers grown by RF Plasma assisted MBE. In this paper, we report post-growth thermal annealing of GaN grown by RF plasma MBE.
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 7 3 5 - 7
416
W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419
2. Experimental details GaN was grown on c-plane of sapphire substrates in an MBE system equipped with an EPI Unibulb nitrogen RF plasma source. Ga and N2 are used as sources and Si as n-type dopant. Prior to epitaxial growth of GaN, the substrate was nitridized for 1 hour, then a GaN 20 nm buffer layer was grown at low temperature (6008C). The temperature of the Si source was varied to achieve doping densities in the range of 1017–9 1019 cm3. The thermal annealing process was performed in a nitrogen atmosphere with a tungsten–halogen lamp. GaN decomposed at temperature as high as 1300 K, when the reaction 2GaN(s) ! 2Ga(g)+N2 occurs [7]. To avoid decomposition of GaN during annealing, the highest annealing temperature was 10008C in our experiment. The samples used in this study were Si-doped GaN with a carrier concentration of 6 1017 cm3. In order to minimize possible variability between different growth runs, only one sample was used and it was divided into several pieces, which were then annealed for different annealing time or annealing temperature. Photoluminescence (PL) spectra were obtained by a Jobin Yvon-Spex THR1000 monochromator with a 1200 g/mm grating blazed at 400 nm. A 325 nm He–Cd laser with a maximum power of 60 mW was used as the excitation light. The luminescence was dispersed in the spectrometer and detected by a photo-multiplier-tube (PMT). Double crystal X-ray diffraction (DCXRD) of GaN (0 0 0 2) was carried out on a Philips X’Pert MRD system with a goniometer accuracy of 0.00018.
Data in Fig. 1 show annealing temperature dependence of both FWHM of X-ray rocking curves and estimated dislocation density in annealed GaN. The ramp-up time to the annealing temperature was 15 s. The samples were annealed for 35 s and then cooled down to room temperature. A mosaic model is used to calculate dislocation densities of a crystal. According to this theory, a crystal is composed of many mosaics, which are separated by grain boundaries. Defect in a mosaic are neglected. Taking assumption of the mosaic model, the dislocation density in the GaN film shown in Fig. 1 was calculated using the following equation [8]: DB ¼ ba =ð2p ln 2Þ1=2 st;
ð1Þ
where DB is the dislocation density and ba is the rocking curve broadening caused by diffraction angular rotation at dislocations, which equals approximately to the FWHM of the X-ray rocking curve. The size of the column in the mosaic model is t and can be obtained from atomic force microscopy (AFM) of the sample and s is the Burgers vector. As shown in Fig. 1, the FWHM of the rocking curve decreases when the annealing temperature increases. This decrease of FWHM indicates an improvement in crystal quality due to decrease of dislocation density. Using Eq. (1) with s ¼ a=3h1 1 2 0i and t ¼ 0:3 mm, the dislocation
3. Results and discussion The full width at half maximum (FWHM) of the DCXRD rocking curve and the 77 K near-bandedge luminescence of the GaN sample used for annealing is 262 arcsec and 22 meV, respectively. The peak intensity ratio of the near-band-edge transition compared to the yellow luminescence at 2.2 eV is 16, indicating high quality of the epitaxial film.
Fig. 1. Estimated dislocation density and the value of the full width at half maximum (FWHM) of the XRD for the as-grown and annealed GaN layers, as a function of annealing temperature.
W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419
density in the as-grown GaN is 1 109 cm2 and decreases to 8.7 108 cm2 after annealing at 10008C for 35 s. Clearly, high temperature rapid thermal annealing (RTA) have improved the structural quality of GaN. The annealing time dependence of FWHM of XRD is shown in Fig. 2. The annealing temperature was fixed at 9508C. The improvement of crystal quality has been observed after rapid thermal annealing. However, with an increase of the annealing time, the decrease of FWHM of XRD became less obvious. When the sample was annealed for 600 s, the FWHM became almost the same as that of the as-grown GaN. Annealing time longer than 600 s made the crystal quality become worse. Thermal annealing may affect various point defects such as vacancies, interstitials, and hydrogen complex. The 77 K PL of GaN annealed at 9508C for different time was measured (Fig. 3). A new peak at 378 nm appeared in the sample annealed for longer than 300 s. The integrated intensity of the peak increased dramatically with increasing of the annealing time. The excitation power in PL measurement was varied in order to clarify the origin of this new peak (Fig. 4). We can see clearly that the new peak continuously redshifted with decreasing excitation power. It is well
417
Fig. 3. 77 K photoluminescence of GaN annealed at 9508C with different time (the same scale).
Fig. 4. Excitation density dependence of 77 K photoluminiscence of GaN after annealing at 9508C for 35 min. The blue shift of the peak with longer wavelength can be clearly seen with increasing excitation density.
Fig. 2. The annealing time dependence of FWHM of XRD for GaN layers. The annealing temperature was fixed at 9508C.
known that the donor–acceptor pair DAP emission blueshifts with increasing excitation power due to recombinations of closer pairs [9]. Therefore, the excitation power dependence of the new peak indicates that the emission is associated with DAP transitions. A similar DAP emission was observed in highly Si-doped GaN. When the carrier concentration in Si-doped GaN became as high as 9 1019 cm3, a
418
W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419
energy in the material and lead to equilibrium state. The formation of SiN can decrease the total energy by compensating the increasing total energy caused by SiGa. As a result, SiN tends to form in the annealed GaN. Other point defects, however, can be ruled out as the source of the DAP transition because of their unsuitable energy level position [10] or their negligible concentration [11]. The formation of SiN in highly doped GaN can also be explained by the amphoteric nature of Si. Although Ga site is preferred when Si was doped into GaN, Si on N site will surely form due to the short of Ga site in extremely high Si concentration. Theoretical studies of defects and impurities also showed that SiN can act as acceptor in GaN [12].
4. Conclusion
Fig. 5. (a) 77 K PL of a highly Si-doped GaN whose carrier concentration is 9 1019 cm3; (b) 77 K PL of an undoped GaN.
In conclusion, using XRD, we have demonstrated that post growth rapid thermal annealing can improve the crystal quality of a GaN epilayer. A new peak due to DAP emission appeared in the 77 K PL of the GaN annealed at 9508C for longer than 300 s. Similar DAP emission can also be seen in highly Si-doped GaN. The origin of the acceptor was proposed to be SiN.
Acknowledgements new peak located at 375 nm appeared (Fig. 5). The excitation intensity dependence of the 375 nm peak indicates that the emission is also associated with DAP transitions. We infer that the 375 nm peak in Fig. 5 has the same origin as the 378 nm peak shown in Fig. 3. The discrepancy between the peak wavelength in the two cases is due to different spacial separation between donors and acceptors. Since the annealing temperature is much lower than the decomposition temperature of GaN, the possibility of formation of Ga-vacancy or Nvacancy is excluded. We attribute the acceptor in both DAP emissions to SiN. Because of the large lattice mismatch between GaN and the sapphire substrate, stress exists in the as-grown GaN. Thermal annealing tends to decrease the total
This work is supported in part by the Shanghai Center of Applied Physics.
References [1] R.T. Leonard, S.M. Bedair, Appl. Phys. Lett. 68 (1996) 794. [2] J.C. Zolper, M. Hageron, A.J. Howard, J. Ramer, D. Hersee, Appl. Phys. Lett. 79 (1996) 200. [3] C. Yuan, T. Salagaj, A.G. Thompson, W. Kroll, R.A. Stall, C.Y. Hwang, M. Schurman, Y. Li, W.E. Mayo, Y. Lu, S. Krishnankutty, I.K. Shmagin, R.M. Kolbas, J. Vac. Sci. Technol. B 13 (1995) 2075. [4] N. Chand, R. People, F.A. Baiocchi, K.W. Wecht, A.Y. Cho, Appl. Phys. Lett. 49 (1986) 815. [5] R.J. Matyi, J.W. Lee, H.F. Schaake, J. Electron. Mater. 17 (1988) 87.
W. Li, A. Li / Journal of Crystal Growth 227–228 (2001) 415–419 [6] M. Yamaguchi, M. Tachikawa, Y. Itoh, M. Sugo, S. Kondo, J. Appl. Phys. 68 (1990) 4518. [7] Z.A. Munir, A.W. Sercy, J. Chem. Phys. 42 (1965) 4223. [8] Q.K. Yang, A.Z. Li, Y.G. Zhang, B. Yang, O. Brandt, K. Ploog, J. Crystal Growth 192 (1998) 27. [9] T. Ogino, M. Aoki, Jpn. J. Appl. Phys. 19 (1980) 2395.
419
[10] P. Boguslawski, E. Briggs, J. Bernholc, Phys. Rev. B 47 (1993) 13353. [11] Jorg Neugebauer, Chirs G. Van de Walle, Appl. Phys. Lett. 69 (1996) 503. [12] S. Strite, H. Morkoc, J. Vac. Sci. Technol. B10 (1992) 1237.