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Influence of precipitates on GaN epilayer quality
Materials Science and Engineering B75 (2000) 214 – 217 www.elsevier.com/locate/mseb
Influence of precipitates on GaN epilayer quality Junyong Kang a,*, Qisheng Huang a, Zhanguo Wang b b
a Department of Physics, Xiamen Uni6ersity, Xiamen 361005, PR China Laboratory of Semiconductor Materials Sciences, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing 100083, PR China
Abstract GaN epilayers grown on pre-nitridated (0001) sapphire substrates by metallorganic vapor phase epitaxy were investigated by wavelength dispersive X-ray spectroscopy and energy dispersive X-ray spectroscopy. Precipitates were observed to mainly consist of O impurity whose strengths were weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in B11–20\ directions of the epilayers grown on substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. Yellow luminescence of the epilayers was imaged by cathodoluminescence. The distribution similarity between the cathodoluminescence and the precipitates suggested that the precipitates were responsible for the yellow luminescence band. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Precipitate; GaN; WDS; TEM; Cathodoluminescence
1. Introduction GaN and its related compounds are potential semiconductors for fabrications of optoelectronic devices in the spectral regions from blue to near ultraviolet. The recent breakthroughs in the growth process [1 –4] have led to extensive studies of applications for high-brightness blue light emitting diodes, blue laser diodes, visible-blind detectors, high-temperature and high-power transistors. In spite of these, imperfect coalescence at the initial stage of GaN growth is still influencing the device properties. An understanding of the imperfect coalescence is of increasing importance for optimization of device performance. The imperfect coalescence was studied by different experimental techniques and shown to bring about the formations of threading dislocations [5,6], boundaries [7], V shaped defects [8,9]. However, little is known about influence of the imperfect coalescence on precipitate formation and epilayer properties. In this work, GaN epilayers were studied by a wavelength dispersive X-ray spectrometer (WDS) and an energy dispersive X-ray spectrometer (EDS). The precipitate distributions and structures were observed by WDS and a transmission electron microscope (TEM). * Corresponding author.
Yellow luminescence from precipitates was imaged by cathodoluminescence (CL). Influence of the precipitates on mechanical and optical properties of the epilayers were investigated.
2. Experiment GaN epilayers under study were grown on (0001) sapphire substrates by metallorganic vapor phase epitaxy (MOVPE). The substrate surface was first nitridated in a H2 and NH3 mixed gas at 1150°C within 20 min to prepare different samples. A GaN buffer layer about 20 nm thick was deposited at 550°C and then GaN was grown at 1050°C by feeding trimethylgallium and NH3 gases into a reactor. None of the epilayer was intentionally doped. As-grown epilayers had different surface morphologies consisting of both flat and hexagonal crystallites. The surface morphologies with a number of tiny indentations were grown on substrate pre-nitridated for 3 min. The surfaces with many hexagonal hillocks were grown on substrate pre-nitridated for longer periods. The epilayers with 4–6 mm thickness were transparent. To remove the surface morphology effect on detection, the epilayer surfaces were optically polished down
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 6 6 - 4
J. Kang et al. / Materials Science and Engineering B75 (2000) 214–217
215
to about 4 mm thickness. Precipitate compositions and distributions in GaN epilayers were first determined by WDS spectroscopy in an electron probe microanalyzer (EPMA, JEOL JXA-8900R). TEM specimens were mechanically ground down to about 70 mm thickness and dimpled down to about 5 mm thickness from the substrate side. To observe the precipitates in GaN near the
Fig. 2. Typical plane-view TEM images of precipitates observed above (0001) plane with the primary beam energy of 200 keV. (a) Lattice images of precipitates; (b) a bright-field image of larger precipitates.
epilayer-substrate interface, the specimens were thinned with Ar+ ions at 1.5 kV and an incident angle of 2° from the GaN side and of 3° from the substrate side. Precipitate composition analyses were also carried out on an EDS with a Si (Li) detector and an ultrathin window in an electron microscope (JEOL JEM-2010F). Luminescence in the wavelength range of 400–650 nm was imaged with a CL detector (MP34020) in a scanning electron microscope (SEM JEOL JSM-5300). 3. Results and discussion
Fig. 1. Typical plane-view WDX images of O (a); C (b); and Al (C); impurities mapped at the primary beam energy of 25 keV in a polished epilayer. The brightness represents the relative impurity level.
The epilayer compositions from beryllium to uranium were investigated by WDS at the primary beam energy of 25 keV. WDS spectra exhibited the characteristic X-ray of Ga and N. Besides these, characteristic X-rays of oxygen, carbon, and aluminum impurities were observed. The typical plane-view distributions of O, C, and Al impurities were shown in Fig. 1. O impurities agglomerated pronouncedly in B11–20\ directions, as shown in (a). C impurities were also observed to agglomerate, as shown in (b). Al impurities were only recognized to agglomerate in few regions, as shown in (c). Therefore, we believe that the defects were precipitates mainly consisting of O impurities.
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J. Kang et al. / Materials Science and Engineering B75 (2000) 214–217
The precipitates were further confirmed in TEM specimens with EDS at the primary beam energy of 20 keV There were a few white contrast regions arranged along B 11 – 20 \ directions in TEM specimens. Similar to the WDS results, the EDS spectra of the white regions exhibited the strong characteristic X-ray of O elements though its energy of 525 eV was within the absorption range of the ultra-thin window. On the low energy side of the O characteristic X-ray, a weak shoulder was visible, which seems attributable to the low densities of N. The intensity of the C characteristic X-ray varied from defect to defect. The Al characteristic X-ray disappeared due to fewer precipitates containing Al impurity that could only be detected by measuring the white defects individually. These results also showed that the O impurity was the most dominant impurity for the precipitates. Furthermore, the high O density accompanied with very low N density suggested that most of N host atoms are substituted by O impurities in the precipitates. The mechanical properties of the precipitates were investigated in the TEM specimens. There were a few open core precipitates in TEM images. The number of open-core precipitates was more in specimens
Fig. 3. Typical plane-view CL images measured at the primary beam energy of 25 keV in the epilayers grown on the substrates pre-nitridated for 3 min (a) and for longer periods (b), respectively. The brightness represents the relative intensities of luminescence.
thinned with Ar+ ions at higher accelerating voltages and/or larger incident angles. The precipitate images appeared as white contrast regions, which indicated that the thickness of the precipitate was thinner than that of its surrounding matrix so that its transmission electron intensity was stronger. The surface morphologies were also measured by atomic force microscopy in the TEM specimens. The indentations with several angstroms in depth were observed. This result further supported that the milling rates were higher at the precipitates. Therefore, we believed that the strength of the GaN epilayer was weaker at the precipitate sites. The images of the precipitate appeared coherent, as shown in Fig. 2(a). The intersection between the precipitates and GaN matrix appeared circular for smaller ones and more or less hexagonal for larger ones. The larger precipitates were often observed in epilayers grown on the substrates pre-nitridated for longer periods where the precipitates distributed more sparsely and inhomogeneously. They often to join to cracks in the TEM specimens, as shown in (b). The lattice mismatch between GaN epilayer and sapphire substrate is known to be large and thus to bring about the strong compressive stress in GaN epilayers. The compressive stress will result in plastic deformation at the precipitates due to their weaker strength and concentrate on edge angles of the precipitates [10]. This stress will be stronger in the larger precipitate and in the regions with low precipitate densities due to fewer precipitates to reduce the compressive stress [6]. Therefore, the crack is favorable to nucleate and grow at edge angles of the larger precipitates. The photoluminescence spectra of GaN epilayers at room temperature exhibited a narrow emission peak near in the wavelength range of 360–385 nm with a peak at 370 nm and a broad yellow emission band in the wavelength range of 470–700 nm centered around 560 nm. Since only the yellow luminescence band was within the sensitivity range of the CL detector, the CL image thus showed the intensity distribution of the yellow luminescence band. The CL images were detected at the primary beam energy of 25 keV in the polished epilayers with optical flat surfaces. The images exhibited contrast with a number of dark and bright tortuous regions and with several bright spots and lines parallel to B11–20\ directions in the samples grown on the substrates pre-nitridated for 3 min and for longer periods, as shown in Fig. 3(a) and (b), respectively. These images were similar to the precipitate distributions in relevant samples observed above. The similarity suggested that the precipitates were responsible for the yellow luminescence band in our samples.
J. Kang et al. / Materials Science and Engineering B75 (2000) 214–217
The precipitates consisted of O and C impurities in our samples. Previous experimental results had shown that there was an unknown O-related deep acceptor level in GaN [11,12]. Carbon was also known as an unintentional dopant in GaN [13] and has been observed to significantly enhance the yellow luminescence of recombination from a shallow donor to a deep acceptor induced by a C-related defect [14]. For these reasons, it is possible for the precipitates to responsible for the stronger yellow CL intensity.
Acknowledgements
4. Conclusions
References
GaN epilayers were investigated by WDS and EDS techniques. The precipitates were observed to mainly consist of O impurity whose strength was weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in B 11–20\ directions of the epilayers grown on the substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. The CL intensity distributions of the yellow band were similar to the precipitate distributions, which suggested that the precipitates were responsible for the yellow luminescence band. Although the precipitates had been known to influence the mechanical and optical properties of GaN epilayers, they would be possibly reduced by purging the growth materials.
.
217
The authors would like to thank Professor T. Ogawa of the Department of Physics of Gakushuin University for making experiment convenient. This work was partly supported by the High Technology Research and Development Program of China, the National Natural Science Foundation of China, and the Natural Science Foundation of Fujian Province of China.
[1] H. Amano, N. Sawaki, I. Akasaki, et al., Appl. Phys. Lett. 48 (1986) 353. [2] I. Akasaki, T. Kozowa, K. Hiramatsu, et al., J. Lumin. 40–41 (1988) 121. [3] S. Nakamura, N. Iwasa, M. Senoh, et al., Jpn. J. Appl. Phys. 31 (1992) 1258. [4] A. Usui, H. Sunakawa, A. Sakai, et al., Jpn. J. Appl. Phys. 36 (1997) L899. [5] W. Qian, M. Skowrongski, M. De Graef, et al., Appl. Phys. Lett. 66 (1995) 1252. [6] J. Kang, T. Ogawa, Appl. Phys. Lett. 71 (1997) 2304. [7] B.N. Sverdlov, G.A. Martin, H. Morkoc, et al., Appl. Phys. Lett. 67 (1995) 2063. [8] Z. Liliental-Weber, Y. Chen, S. Ruvimov, et al., Phys. Rev. Lett. 79 (1997) 2835. [9] J. Kang, T. Ogawa, J. Mater. Res. 14 (1999) 1. [10] N.J. Petch, Prog. Met. Phys. 5 (1954) 1. [11] B-C. Chung, M. Gershenzon, J. Appl. Phys. 72 (1992) 651. [12] J.C. Zolper, R.G. Wilson, S.J. Pearton, et al., Appl. Phys. Lett. 68 (1996) 1945. [13] G.-C. Yi, B.W. Wessels, Appl. Phys. Lett. 70 (1997) 357. [14] T. Ogino, M. Aoki, Jpn. J. Appl. Phys. 19 (1980) 2395.
Influence of precipitates on GaN epilayer quality Junyong Kang a,*, Qisheng Huang a, Zhanguo Wang b b
a Department of Physics, Xiamen Uni6ersity, Xiamen 361005, PR China Laboratory of Semiconductor Materials Sciences, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing 100083, PR China
Abstract GaN epilayers grown on pre-nitridated (0001) sapphire substrates by metallorganic vapor phase epitaxy were investigated by wavelength dispersive X-ray spectroscopy and energy dispersive X-ray spectroscopy. Precipitates were observed to mainly consist of O impurity whose strengths were weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in B11–20\ directions of the epilayers grown on substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. Yellow luminescence of the epilayers was imaged by cathodoluminescence. The distribution similarity between the cathodoluminescence and the precipitates suggested that the precipitates were responsible for the yellow luminescence band. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Precipitate; GaN; WDS; TEM; Cathodoluminescence
1. Introduction GaN and its related compounds are potential semiconductors for fabrications of optoelectronic devices in the spectral regions from blue to near ultraviolet. The recent breakthroughs in the growth process [1 –4] have led to extensive studies of applications for high-brightness blue light emitting diodes, blue laser diodes, visible-blind detectors, high-temperature and high-power transistors. In spite of these, imperfect coalescence at the initial stage of GaN growth is still influencing the device properties. An understanding of the imperfect coalescence is of increasing importance for optimization of device performance. The imperfect coalescence was studied by different experimental techniques and shown to bring about the formations of threading dislocations [5,6], boundaries [7], V shaped defects [8,9]. However, little is known about influence of the imperfect coalescence on precipitate formation and epilayer properties. In this work, GaN epilayers were studied by a wavelength dispersive X-ray spectrometer (WDS) and an energy dispersive X-ray spectrometer (EDS). The precipitate distributions and structures were observed by WDS and a transmission electron microscope (TEM). * Corresponding author.
Yellow luminescence from precipitates was imaged by cathodoluminescence (CL). Influence of the precipitates on mechanical and optical properties of the epilayers were investigated.
2. Experiment GaN epilayers under study were grown on (0001) sapphire substrates by metallorganic vapor phase epitaxy (MOVPE). The substrate surface was first nitridated in a H2 and NH3 mixed gas at 1150°C within 20 min to prepare different samples. A GaN buffer layer about 20 nm thick was deposited at 550°C and then GaN was grown at 1050°C by feeding trimethylgallium and NH3 gases into a reactor. None of the epilayer was intentionally doped. As-grown epilayers had different surface morphologies consisting of both flat and hexagonal crystallites. The surface morphologies with a number of tiny indentations were grown on substrate pre-nitridated for 3 min. The surfaces with many hexagonal hillocks were grown on substrate pre-nitridated for longer periods. The epilayers with 4–6 mm thickness were transparent. To remove the surface morphology effect on detection, the epilayer surfaces were optically polished down
0921-5107/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 0 ) 0 0 3 6 6 - 4
J. Kang et al. / Materials Science and Engineering B75 (2000) 214–217
215
to about 4 mm thickness. Precipitate compositions and distributions in GaN epilayers were first determined by WDS spectroscopy in an electron probe microanalyzer (EPMA, JEOL JXA-8900R). TEM specimens were mechanically ground down to about 70 mm thickness and dimpled down to about 5 mm thickness from the substrate side. To observe the precipitates in GaN near the
Fig. 2. Typical plane-view TEM images of precipitates observed above (0001) plane with the primary beam energy of 200 keV. (a) Lattice images of precipitates; (b) a bright-field image of larger precipitates.
epilayer-substrate interface, the specimens were thinned with Ar+ ions at 1.5 kV and an incident angle of 2° from the GaN side and of 3° from the substrate side. Precipitate composition analyses were also carried out on an EDS with a Si (Li) detector and an ultrathin window in an electron microscope (JEOL JEM-2010F). Luminescence in the wavelength range of 400–650 nm was imaged with a CL detector (MP34020) in a scanning electron microscope (SEM JEOL JSM-5300). 3. Results and discussion
Fig. 1. Typical plane-view WDX images of O (a); C (b); and Al (C); impurities mapped at the primary beam energy of 25 keV in a polished epilayer. The brightness represents the relative impurity level.
The epilayer compositions from beryllium to uranium were investigated by WDS at the primary beam energy of 25 keV. WDS spectra exhibited the characteristic X-ray of Ga and N. Besides these, characteristic X-rays of oxygen, carbon, and aluminum impurities were observed. The typical plane-view distributions of O, C, and Al impurities were shown in Fig. 1. O impurities agglomerated pronouncedly in B11–20\ directions, as shown in (a). C impurities were also observed to agglomerate, as shown in (b). Al impurities were only recognized to agglomerate in few regions, as shown in (c). Therefore, we believe that the defects were precipitates mainly consisting of O impurities.
216
J. Kang et al. / Materials Science and Engineering B75 (2000) 214–217
The precipitates were further confirmed in TEM specimens with EDS at the primary beam energy of 20 keV There were a few white contrast regions arranged along B 11 – 20 \ directions in TEM specimens. Similar to the WDS results, the EDS spectra of the white regions exhibited the strong characteristic X-ray of O elements though its energy of 525 eV was within the absorption range of the ultra-thin window. On the low energy side of the O characteristic X-ray, a weak shoulder was visible, which seems attributable to the low densities of N. The intensity of the C characteristic X-ray varied from defect to defect. The Al characteristic X-ray disappeared due to fewer precipitates containing Al impurity that could only be detected by measuring the white defects individually. These results also showed that the O impurity was the most dominant impurity for the precipitates. Furthermore, the high O density accompanied with very low N density suggested that most of N host atoms are substituted by O impurities in the precipitates. The mechanical properties of the precipitates were investigated in the TEM specimens. There were a few open core precipitates in TEM images. The number of open-core precipitates was more in specimens
Fig. 3. Typical plane-view CL images measured at the primary beam energy of 25 keV in the epilayers grown on the substrates pre-nitridated for 3 min (a) and for longer periods (b), respectively. The brightness represents the relative intensities of luminescence.
thinned with Ar+ ions at higher accelerating voltages and/or larger incident angles. The precipitate images appeared as white contrast regions, which indicated that the thickness of the precipitate was thinner than that of its surrounding matrix so that its transmission electron intensity was stronger. The surface morphologies were also measured by atomic force microscopy in the TEM specimens. The indentations with several angstroms in depth were observed. This result further supported that the milling rates were higher at the precipitates. Therefore, we believed that the strength of the GaN epilayer was weaker at the precipitate sites. The images of the precipitate appeared coherent, as shown in Fig. 2(a). The intersection between the precipitates and GaN matrix appeared circular for smaller ones and more or less hexagonal for larger ones. The larger precipitates were often observed in epilayers grown on the substrates pre-nitridated for longer periods where the precipitates distributed more sparsely and inhomogeneously. They often to join to cracks in the TEM specimens, as shown in (b). The lattice mismatch between GaN epilayer and sapphire substrate is known to be large and thus to bring about the strong compressive stress in GaN epilayers. The compressive stress will result in plastic deformation at the precipitates due to their weaker strength and concentrate on edge angles of the precipitates [10]. This stress will be stronger in the larger precipitate and in the regions with low precipitate densities due to fewer precipitates to reduce the compressive stress [6]. Therefore, the crack is favorable to nucleate and grow at edge angles of the larger precipitates. The photoluminescence spectra of GaN epilayers at room temperature exhibited a narrow emission peak near in the wavelength range of 360–385 nm with a peak at 370 nm and a broad yellow emission band in the wavelength range of 470–700 nm centered around 560 nm. Since only the yellow luminescence band was within the sensitivity range of the CL detector, the CL image thus showed the intensity distribution of the yellow luminescence band. The CL images were detected at the primary beam energy of 25 keV in the polished epilayers with optical flat surfaces. The images exhibited contrast with a number of dark and bright tortuous regions and with several bright spots and lines parallel to B11–20\ directions in the samples grown on the substrates pre-nitridated for 3 min and for longer periods, as shown in Fig. 3(a) and (b), respectively. These images were similar to the precipitate distributions in relevant samples observed above. The similarity suggested that the precipitates were responsible for the yellow luminescence band in our samples.
J. Kang et al. / Materials Science and Engineering B75 (2000) 214–217
The precipitates consisted of O and C impurities in our samples. Previous experimental results had shown that there was an unknown O-related deep acceptor level in GaN [11,12]. Carbon was also known as an unintentional dopant in GaN [13] and has been observed to significantly enhance the yellow luminescence of recombination from a shallow donor to a deep acceptor induced by a C-related defect [14]. For these reasons, it is possible for the precipitates to responsible for the stronger yellow CL intensity.
Acknowledgements
4. Conclusions
References
GaN epilayers were investigated by WDS and EDS techniques. The precipitates were observed to mainly consist of O impurity whose strength was weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in B 11–20\ directions of the epilayers grown on the substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. The CL intensity distributions of the yellow band were similar to the precipitate distributions, which suggested that the precipitates were responsible for the yellow luminescence band. Although the precipitates had been known to influence the mechanical and optical properties of GaN epilayers, they would be possibly reduced by purging the growth materials.
.
217
The authors would like to thank Professor T. Ogawa of the Department of Physics of Gakushuin University for making experiment convenient. This work was partly supported by the High Technology Research and Development Program of China, the National Natural Science Foundation of China, and the Natural Science Foundation of Fujian Province of China.
[1] H. Amano, N. Sawaki, I. Akasaki, et al., Appl. Phys. Lett. 48 (1986) 353. [2] I. Akasaki, T. Kozowa, K. Hiramatsu, et al., J. Lumin. 40–41 (1988) 121. [3] S. Nakamura, N. Iwasa, M. Senoh, et al., Jpn. J. Appl. Phys. 31 (1992) 1258. [4] A. Usui, H. Sunakawa, A. Sakai, et al., Jpn. J. Appl. Phys. 36 (1997) L899. [5] W. Qian, M. Skowrongski, M. De Graef, et al., Appl. Phys. Lett. 66 (1995) 1252. [6] J. Kang, T. Ogawa, Appl. Phys. Lett. 71 (1997) 2304. [7] B.N. Sverdlov, G.A. Martin, H. Morkoc, et al., Appl. Phys. Lett. 67 (1995) 2063. [8] Z. Liliental-Weber, Y. Chen, S. Ruvimov, et al., Phys. Rev. Lett. 79 (1997) 2835. [9] J. Kang, T. Ogawa, J. Mater. Res. 14 (1999) 1. [10] N.J. Petch, Prog. Met. Phys. 5 (1954) 1. [11] B-C. Chung, M. Gershenzon, J. Appl. Phys. 72 (1992) 651. [12] J.C. Zolper, R.G. Wilson, S.J. Pearton, et al., Appl. Phys. Lett. 68 (1996) 1945. [13] G.-C. Yi, B.W. Wessels, Appl. Phys. Lett. 70 (1997) 357. [14] T. Ogino, M. Aoki, Jpn. J. Appl. Phys. 19 (1980) 2395.