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Nonuniformities in free-standing GaN substrates
Materials Science and Engineering B79 (2001) 16 – 19 www.elsevier.com/locate/mseb
Nonuniformities in free-standing GaN substrates H.M. Kim a,*, J.E. Oh b, T.W. Kang a b
a Quantum-Functional Semiconductor Research Center, Dongguk Uni6ersity, 26 Pildong-3ga, Joong-gu, Seoul 100 -715, South Korea School of Electrical and Computer Engineering, Center for Electronic Materials and Components, Hanyang Uni6ersity, 1271 Sa-1dong, Ansan, Kyungki-do 425 -791, South Korea
Received 9 May 2000; received in revised form 22 September 2000; accepted 27 September 2000
Abstract In this study, we report that free-standing GaN substrates grown by the hydride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions with low crystal and optical quality located close to the top (near as-grown surface) and bottom (near interface between GaN/sapphire) regions of substrate cross-section. We considered that the origins of these nonuniformities were surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. We used cathodoluminescence imaging and spectroscopy for analyzing these nonuniform regions. The low quality regions with high electron concentration are easily visualized using cathodoluminescence (CL). The coexistence of regions with low- and high quality allows us to explain the concurrent evidence of high substrate quality in double crystal X-ray diffraction and photoluminescence (PL). © 2001 Elsevier Science S.A. All rights reserved. Keywords: GaN; HVPE; Cathodoluminescence; Nonuniformity; Cross-section; Free-standing Keywords: 81.05.Ea; 81.15.Kk; 68.55.Ln; 78.60.Hk
1. Introduction The demand for GaN substrates for homoepitaxy has still not been satisfied because it is difficult to grow large-volume single crystals due to the highmelting point and the high-equilibrium vapor pressure of N2 at the growth temperature [1]. For a long time, there have been discussions about obtaining bulk GaN crystals by using solid-solution growth at highpressure [2,3], bulk-like thick-film GaN by using the hydride vapor-phase epitaxy (HVPE) method [4,5], and more recently defect-free thick-films by using selective-area growth techniques [6]. However, freestanding GaN substrates are not satisfactory because their size is still limited to a few millimeter, which is a key factor in hindering progress in GaN-based devices. Among the above-mentioned methods, the HVPE technique is one of the promising approaches * Corresponding author. Tel.: + 82-2-22603952; fax: +82-222603945. E-mail address: [email protected] (H.M. Kim).
to obtain high-quality and large-area bulk-like substrates for homo-epitaxy [4,5]. However, understanding of growth protocols leading to high quality films is still far from complete. Only recently, publications begun to emerge [7], which explain high quality of HVPE films by a predominantly lateral growth mode. While high quality films with low electron concentrations about 1016 cm − 3 have been achieved [7], many authors report electron concentrations in the range of 1019 cm − 3. Structural and optical properties often indicate, however, that films are of high quality, with the full width at half maximum (FWHM) of rocking curve of 280–430 arcsec [8] and the photoluminescence (PL) linewidth as low as 1 meV [9]. Such small linewidth cannot be observed in materials with high electron concentration. We were able to find direct evidence of large-scale, highly defective regions surrounded by high quality material in HVPE GaN substrate. Here, we report results on analyzing nonuniformities of free-standing GaN substrates by low-temperature cathodoluminescence (CL) measurement.
0921-5107/01/$ - see front matter © 2001 Elsevier Science S.A. All rights reserved. PII: S0921-5107(00)00543-2
H.M. Kim et al. / Materials Science and Engineering B79 (2001) 16–19
17
2. Experimental procedure Free-standing GaN substrates used in this study were grown in a modified HVPE system using a vertical quartz reactor. The Ga source was GaCl formed in the Ga reaction zone by reacting HCl gas with liquid gallium 800–900°C. The HCl flow rate was varied from 20–200 ml min − 1, and the best layers were obtained at the growth rate of 1.5 mm min − 1. The nitrogen source was NH3, delivered at a rate of 2000 ml min − 1, while nitrogen was used as a carrier gas. Growth was carried out at 1050°C on c-plane sapphire, without a buffer layer. Grown GaN thick film was backside polished for removing sapphire substrate and then 230-mm thick GaN free-standing substrate was fabricated. In order to study free-standing GaN substrate inhomogeneities, we carried out low temperature cathodoluminescence spectroscopy and imaging. A JEOL 6330F field emission scanning electron microscope was used, equipped with a MONOCL2 CL Oxford Instruments attachment. The CL spectra and images were taken at 80 K with an accelerating voltage of 15 kV and an probe current of 0.12 nA. At this voltage, light is generated in the interaction volume within the first micrometer from the surface. The spatial resolution of images is determined by the interaction volume and the diffusion length and is also in the range of a single micrometer. We were able to examine the top regions of the substrates as well as their bottom regions.
3. Results and discussion Fig. 1 shows the FWHM of the (0002) diffraction measured for 230-mm thick substrates. The narrowest linewidth of 70.3 arcsec was found for GaN substrates. Fig. 2 shows the PL spectrum of same sample. The
Fig. 1. X-ray rocking curve of the free-standing GaN substrates.
Fig. 2. PL spectrum of the free-standing GaN substrates.
spectra of samples grown under optimized conditions are typical of good-quality films with low-electron concentration, in the range of 1016 cm − 3 are dominated by donor bound exciton (D0X) transitions at about 3.465 eV. The FWHM of the D0X lines in the best samples was about 1.94 meV. The character of the PL spectra varies across the specimen. In some locations the spectrum is composed of bound exciton features only, but a weak broad band underlying the sharp excitonic transitions can be observed over most of the film area [10]. The band extends to energies above the band gap and it has been identified as due to free electron recombination. Such spectra are typical of degenerate n-type. A scanning electron microscopy (SEM) image and panchromatic CL images of the substrate cross-section are shown in Fig. 3. We observed cross-section of the substrate divided by four regions. Region A is a top region of the substrate, region B and C are middle regions of the substrate and region D is a bottom region (GaN film and sapphire substrate interface). Normally another authors reported that top region of the substrate is good quality than other regions of the substrate but our sample did not identify. For our sample, top region shows low quality compared with middle regions. We considered that the origin of this result was a degenerate surface reconstruction by residual gas reaction on the surface of grown films after ending of growth process [11]. A bright layer built of individual columnar overgrowths is clearly visible near the substrate bottom region (D region). The columns are very closely packed so a continuous bright layer is formed near the substrate bottom (GaN/sapphire interface). Most columns are about 5-mm thick and vary in length of 5–20 mm. The CL spectra taken in four regions across the substrate cross-section are shown in Fig. 4. We show the CL spectrum taken from the top region (region A) to bottom region (region D). The
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H.M. Kim et al. / Materials Science and Engineering B79 (2001) 16–19
Oxygen and silicon may also be incorporated due to reduction of quartz chamber wall. Molnar et al. [13] show that while oxygen and silicon are dominant impurities in their films, their total concentration is insufficient to account for the observed electron concentration. We, therefore, tentatively attribute high electron concentration to structural defects, such as nitrogen vacancies, which are commonly quoted as one of the major sources of electron in undoped GaN. Further studies are necessary to conclusively identify reasons for surface degeneration in the top regions and high electron concentration in bottom defective regions of free-standing GaN substrate grown by HVPE.
4. Conclusions
Fig. 3. SEM image and panchromatic CL image of the cross-section of the free-standing GaN substrate.
spectrum shows the excitonic emission peak. The spectrum taken in the top region (region A) is slightly broader than that of middle regions (region B, region C). This result as above-mentioned is based on a degenerate surface reconstruction by undesired residual gas reaction after crystal growth [11]. The spectrum taken in the bottom region (region D) is distinctly broader, asymmetric than middle regions. Such spectra are typical of degenerate semiconductors [12]. The electron concentration found using [7] is 2.5×1019 cm − 3. Reasons for the increased electron concentration in the bright regions remain undetermined. One possible source is contaminating impurities. Incorporation of such impurities should be uniform during film growth.
In conclusion, we presented direct images of regions with low- and high-quality region in free-standing GaN substrates. The low quality regions are easily observed in panchromatic CL images. The defective layers form near the top and bottom regions. We considered that the origins of these nonuniformities in substrate were degenerate surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. Growth procedures targeted at fabrication of GaN substrates should, therefore, minimize these residual gas reaction and columnar overgrowths. CL spectra taken in the region of the defective layers are indicative of a highly degenerate material. CL spectra taken in the regions away from these regions, for example, in the middle of the substrate, reveal only narrow exciton emission lines. These results allow us to consistently explain simultaneous evidence for high quality GaN growth.
Acknowledgements This work was supported by Korea Science and Engineering Foundation through the Quantum-Functional Semiconductor Research Center at Dongguk University.
References
Fig. 4. CL spectra of selected regions — a spectrum taken from the region A to region D.
[1] C.D. Thurmond, R.A. Rogan, J. Electrochem. Soc. 119 (1972) 622. [2] R. Madar, G. Jacob, J. Hallais, R. Fruchart, J. Cryst. Growth 31 (1975) 197. [3] S. Porowski, I. Grzegory, J. Cryst. Growth 178 (1997) 174. [4] T. Detchprohm, K. Hiramatsu, H. Amano, I. Akasaki, Appl. Phys. Lett. 61 (1992) 2688.
H.M. Kim et al. / Materials Science and Engineering B79 (2001) 16–19 [5] Y.V. Melnik, K.V. Vassilevski, I.P. Nikitina, A.I. Babanin, V.Y. Davydov, V.A. Dmitriev, MRS Internet J. Nitride Semicond. Res. 2 (1997) 39. [6] A. Usui, H. Sunakawa, A. Sakai, A.A. Yamaguchi, Jpn. J. Appl. Phys. 36 (1997) L899. [7] R.J. Molnar, W. Go¨tz, L.T. Romano, N.M. Johnson, J. Cryst. Growth 178 (1997) 147. [8] L.T. Romano, R.J. Molnar, B.S. Krusor, G.A. Anderson, D.P. Bour, P. Maui, Mater. Res. Soc. Symp. Proc. 423 (1996) 245. [9] L. Eckey, A. Hoffmann, R. Heitz, I. Broser, B.K. Meyer, T. Detchprom, K. Hiramatsu, H. Amano, I. Akasaki, Mater. Res. Soc. Symp. Proc. 395 (1996) 589. [10] N.R. Perkins, M.N. Horton, Z.Z. Bandic, T.C. McGill, T.F.
.
19
Kuech, Mater. Res. Soc. Symp. Proc. 395 (1996) 243. [11] In this study residual gas in the quartz chamber was exhausted at atmosphere pressure after growth. But top surface of GaN film was clear when residual gas was exhausted compulsorily by rotary pump after growth. So we considered that top surface of GaN film had a low quality because residual gas in the chamber react on the as-grown surface of GaN film during cool-down process. [12] B. Arnaudov, T. Paskova, E.M. Goldys, I.G. Ivanov, R. Yakimova, S. Evtimova, A. Henry, B. Monemar, J. Appl. Phys. 85 (1999) 7888. [13] R.J. Molnar, K.B. Nichols, P. Maki, E.R. Brown, I. Melngailis, Mater. Res. Symp. Proc. 378 (1995) 479.
Nonuniformities in free-standing GaN substrates H.M. Kim a,*, J.E. Oh b, T.W. Kang a b
a Quantum-Functional Semiconductor Research Center, Dongguk Uni6ersity, 26 Pildong-3ga, Joong-gu, Seoul 100 -715, South Korea School of Electrical and Computer Engineering, Center for Electronic Materials and Components, Hanyang Uni6ersity, 1271 Sa-1dong, Ansan, Kyungki-do 425 -791, South Korea
Received 9 May 2000; received in revised form 22 September 2000; accepted 27 September 2000
Abstract In this study, we report that free-standing GaN substrates grown by the hydride vapor-phase epitaxy (HVPE) are found to contain nonuniform regions with low crystal and optical quality located close to the top (near as-grown surface) and bottom (near interface between GaN/sapphire) regions of substrate cross-section. We considered that the origins of these nonuniformities were surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. We used cathodoluminescence imaging and spectroscopy for analyzing these nonuniform regions. The low quality regions with high electron concentration are easily visualized using cathodoluminescence (CL). The coexistence of regions with low- and high quality allows us to explain the concurrent evidence of high substrate quality in double crystal X-ray diffraction and photoluminescence (PL). © 2001 Elsevier Science S.A. All rights reserved. Keywords: GaN; HVPE; Cathodoluminescence; Nonuniformity; Cross-section; Free-standing Keywords: 81.05.Ea; 81.15.Kk; 68.55.Ln; 78.60.Hk
1. Introduction The demand for GaN substrates for homoepitaxy has still not been satisfied because it is difficult to grow large-volume single crystals due to the highmelting point and the high-equilibrium vapor pressure of N2 at the growth temperature [1]. For a long time, there have been discussions about obtaining bulk GaN crystals by using solid-solution growth at highpressure [2,3], bulk-like thick-film GaN by using the hydride vapor-phase epitaxy (HVPE) method [4,5], and more recently defect-free thick-films by using selective-area growth techniques [6]. However, freestanding GaN substrates are not satisfactory because their size is still limited to a few millimeter, which is a key factor in hindering progress in GaN-based devices. Among the above-mentioned methods, the HVPE technique is one of the promising approaches * Corresponding author. Tel.: + 82-2-22603952; fax: +82-222603945. E-mail address: [email protected] (H.M. Kim).
to obtain high-quality and large-area bulk-like substrates for homo-epitaxy [4,5]. However, understanding of growth protocols leading to high quality films is still far from complete. Only recently, publications begun to emerge [7], which explain high quality of HVPE films by a predominantly lateral growth mode. While high quality films with low electron concentrations about 1016 cm − 3 have been achieved [7], many authors report electron concentrations in the range of 1019 cm − 3. Structural and optical properties often indicate, however, that films are of high quality, with the full width at half maximum (FWHM) of rocking curve of 280–430 arcsec [8] and the photoluminescence (PL) linewidth as low as 1 meV [9]. Such small linewidth cannot be observed in materials with high electron concentration. We were able to find direct evidence of large-scale, highly defective regions surrounded by high quality material in HVPE GaN substrate. Here, we report results on analyzing nonuniformities of free-standing GaN substrates by low-temperature cathodoluminescence (CL) measurement.
0921-5107/01/$ - see front matter © 2001 Elsevier Science S.A. All rights reserved. PII: S0921-5107(00)00543-2
H.M. Kim et al. / Materials Science and Engineering B79 (2001) 16–19
17
2. Experimental procedure Free-standing GaN substrates used in this study were grown in a modified HVPE system using a vertical quartz reactor. The Ga source was GaCl formed in the Ga reaction zone by reacting HCl gas with liquid gallium 800–900°C. The HCl flow rate was varied from 20–200 ml min − 1, and the best layers were obtained at the growth rate of 1.5 mm min − 1. The nitrogen source was NH3, delivered at a rate of 2000 ml min − 1, while nitrogen was used as a carrier gas. Growth was carried out at 1050°C on c-plane sapphire, without a buffer layer. Grown GaN thick film was backside polished for removing sapphire substrate and then 230-mm thick GaN free-standing substrate was fabricated. In order to study free-standing GaN substrate inhomogeneities, we carried out low temperature cathodoluminescence spectroscopy and imaging. A JEOL 6330F field emission scanning electron microscope was used, equipped with a MONOCL2 CL Oxford Instruments attachment. The CL spectra and images were taken at 80 K with an accelerating voltage of 15 kV and an probe current of 0.12 nA. At this voltage, light is generated in the interaction volume within the first micrometer from the surface. The spatial resolution of images is determined by the interaction volume and the diffusion length and is also in the range of a single micrometer. We were able to examine the top regions of the substrates as well as their bottom regions.
3. Results and discussion Fig. 1 shows the FWHM of the (0002) diffraction measured for 230-mm thick substrates. The narrowest linewidth of 70.3 arcsec was found for GaN substrates. Fig. 2 shows the PL spectrum of same sample. The
Fig. 1. X-ray rocking curve of the free-standing GaN substrates.
Fig. 2. PL spectrum of the free-standing GaN substrates.
spectra of samples grown under optimized conditions are typical of good-quality films with low-electron concentration, in the range of 1016 cm − 3 are dominated by donor bound exciton (D0X) transitions at about 3.465 eV. The FWHM of the D0X lines in the best samples was about 1.94 meV. The character of the PL spectra varies across the specimen. In some locations the spectrum is composed of bound exciton features only, but a weak broad band underlying the sharp excitonic transitions can be observed over most of the film area [10]. The band extends to energies above the band gap and it has been identified as due to free electron recombination. Such spectra are typical of degenerate n-type. A scanning electron microscopy (SEM) image and panchromatic CL images of the substrate cross-section are shown in Fig. 3. We observed cross-section of the substrate divided by four regions. Region A is a top region of the substrate, region B and C are middle regions of the substrate and region D is a bottom region (GaN film and sapphire substrate interface). Normally another authors reported that top region of the substrate is good quality than other regions of the substrate but our sample did not identify. For our sample, top region shows low quality compared with middle regions. We considered that the origin of this result was a degenerate surface reconstruction by residual gas reaction on the surface of grown films after ending of growth process [11]. A bright layer built of individual columnar overgrowths is clearly visible near the substrate bottom region (D region). The columns are very closely packed so a continuous bright layer is formed near the substrate bottom (GaN/sapphire interface). Most columns are about 5-mm thick and vary in length of 5–20 mm. The CL spectra taken in four regions across the substrate cross-section are shown in Fig. 4. We show the CL spectrum taken from the top region (region A) to bottom region (region D). The
18
H.M. Kim et al. / Materials Science and Engineering B79 (2001) 16–19
Oxygen and silicon may also be incorporated due to reduction of quartz chamber wall. Molnar et al. [13] show that while oxygen and silicon are dominant impurities in their films, their total concentration is insufficient to account for the observed electron concentration. We, therefore, tentatively attribute high electron concentration to structural defects, such as nitrogen vacancies, which are commonly quoted as one of the major sources of electron in undoped GaN. Further studies are necessary to conclusively identify reasons for surface degeneration in the top regions and high electron concentration in bottom defective regions of free-standing GaN substrate grown by HVPE.
4. Conclusions
Fig. 3. SEM image and panchromatic CL image of the cross-section of the free-standing GaN substrate.
spectrum shows the excitonic emission peak. The spectrum taken in the top region (region A) is slightly broader than that of middle regions (region B, region C). This result as above-mentioned is based on a degenerate surface reconstruction by undesired residual gas reaction after crystal growth [11]. The spectrum taken in the bottom region (region D) is distinctly broader, asymmetric than middle regions. Such spectra are typical of degenerate semiconductors [12]. The electron concentration found using [7] is 2.5×1019 cm − 3. Reasons for the increased electron concentration in the bright regions remain undetermined. One possible source is contaminating impurities. Incorporation of such impurities should be uniform during film growth.
In conclusion, we presented direct images of regions with low- and high-quality region in free-standing GaN substrates. The low quality regions are easily observed in panchromatic CL images. The defective layers form near the top and bottom regions. We considered that the origins of these nonuniformities in substrate were degenerate surface reconstruction by undesired residual gas reaction after crystal growth on the top regions and the individual columns forming an irregular layer in the bottom regions by lattice mismatch and difference of thermal expansion coefficient between GaN films and sapphire substrate. Growth procedures targeted at fabrication of GaN substrates should, therefore, minimize these residual gas reaction and columnar overgrowths. CL spectra taken in the region of the defective layers are indicative of a highly degenerate material. CL spectra taken in the regions away from these regions, for example, in the middle of the substrate, reveal only narrow exciton emission lines. These results allow us to consistently explain simultaneous evidence for high quality GaN growth.
Acknowledgements This work was supported by Korea Science and Engineering Foundation through the Quantum-Functional Semiconductor Research Center at Dongguk University.
References
Fig. 4. CL spectra of selected regions — a spectrum taken from the region A to region D.
[1] C.D. Thurmond, R.A. Rogan, J. Electrochem. Soc. 119 (1972) 622. [2] R. Madar, G. Jacob, J. Hallais, R. Fruchart, J. Cryst. Growth 31 (1975) 197. [3] S. Porowski, I. Grzegory, J. Cryst. Growth 178 (1997) 174. [4] T. Detchprohm, K. Hiramatsu, H. Amano, I. Akasaki, Appl. Phys. Lett. 61 (1992) 2688.
H.M. Kim et al. / Materials Science and Engineering B79 (2001) 16–19 [5] Y.V. Melnik, K.V. Vassilevski, I.P. Nikitina, A.I. Babanin, V.Y. Davydov, V.A. Dmitriev, MRS Internet J. Nitride Semicond. Res. 2 (1997) 39. [6] A. Usui, H. Sunakawa, A. Sakai, A.A. Yamaguchi, Jpn. J. Appl. Phys. 36 (1997) L899. [7] R.J. Molnar, W. Go¨tz, L.T. Romano, N.M. Johnson, J. Cryst. Growth 178 (1997) 147. [8] L.T. Romano, R.J. Molnar, B.S. Krusor, G.A. Anderson, D.P. Bour, P. Maui, Mater. Res. Soc. Symp. Proc. 423 (1996) 245. [9] L. Eckey, A. Hoffmann, R. Heitz, I. Broser, B.K. Meyer, T. Detchprom, K. Hiramatsu, H. Amano, I. Akasaki, Mater. Res. Soc. Symp. Proc. 395 (1996) 589. [10] N.R. Perkins, M.N. Horton, Z.Z. Bandic, T.C. McGill, T.F.
.
19
Kuech, Mater. Res. Soc. Symp. Proc. 395 (1996) 243. [11] In this study residual gas in the quartz chamber was exhausted at atmosphere pressure after growth. But top surface of GaN film was clear when residual gas was exhausted compulsorily by rotary pump after growth. So we considered that top surface of GaN film had a low quality because residual gas in the chamber react on the as-grown surface of GaN film during cool-down process. [12] B. Arnaudov, T. Paskova, E.M. Goldys, I.G. Ivanov, R. Yakimova, S. Evtimova, A. Henry, B. Monemar, J. Appl. Phys. 85 (1999) 7888. [13] R.J. Molnar, K.B. Nichols, P. Maki, E.R. Brown, I. Melngailis, Mater. Res. Symp. Proc. 378 (1995) 479.