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SIMS analysis of impurities and nitrogen isotopes in gallium nitride thin films
Applied Surface Science 252 (2006) 7265–7268 www.elsevier.com/locate/apsusc
SIMS analysis of impurities and nitrogen isotopes in gallium nitride thin films Hajime Haneda a,c,*, Takeshi Ohgaki b, Isao Sakaguchi a, Haruki Ryoken c, Naoki Ohashi a, Atsuo Yasumori b a
Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan b Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan c Kyushu University, 6-1 Kasuga-kouen, Kasuga 816-8580, Japan Received 12 September 2005; accepted 15 February 2006 Available online 3 May 2006
Abstract Gallium nitride thin films were deposited on sapphire or zinc oxide substrates with a molecular beam epitaxial method. Thin films with a Ga14N/ Ga N/Ga14N isotopic heterostructure were also grown. A CAMECA-type secondary ion mass spectrometry (SIMS) was employed to analyze impurities such as substrate elements. Nitrogen isotopes were also analyzed. Some samples were annealed in a nitrogen atmosphere, and the diffusivities of the elements and isotopes were evaluated. Although the crater surface after using Cs+ ions as the primary ion beam became smoother than after using O2+, preferential sputtering was observed. It is concluded that this preferential sputtering causes the isotope distribution to be abnormal. Elements of the substrates diffused into the thin films. # 2006 Elsevier B.V. All rights reserved. 15
Keywords: SIMS; Diffusion; GaN; Nitrogen; Aluminum; Zinc
1. Introduction Galluim nitride (GaN) and its alloys with indium nitride (InN) are promising materials for optelectronic device applications [1]. Light-emitting diodes of GaN-related alloys are now commercially available as a result of the great progress in crystalline quality and doping control. Most of the commercial GaN-based diodes are prepared on sapphire (aAl2O3) or silicon carbide (SiC) substrates. Several trials to grow high-crystalline GaN films have been carried out using various substrates of neither a-Al2O3 nor SiC [2]. We have already proposed that zinc oxide (ZnO) be used as a substrate [3]. The understanding of the diffusion process in solids is of great interest, for both technical and scientific reasons. As the thickness of thin films is decreasing, it has become increasingly important to have an understanding and the ability to control diffusion-related phenomena. Diffusion can be destructive to the delicate film structures because of transport of dopants in
* Corresponding author. Tel.: +81 29 860 4665; fax: +81 29 855 1196. E-mail address: [email protected] (H. Haneda). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.179
one layer into adjacent layers or the substrate. Secondary ion mass spectrometry (SIMS) depth profiles are generally affected by preferential sputtering or the edge effect. Thus, the makers of isotopes would be very useful for clarifying the effect of the preferential sputtering and the edge. For this report, we analyzed the isotope profile of nitrogen in a Ga14N/Ga15N/ Ga14N isotopic heterostructure by means of D-SIMS and evaluated the diffusivities of elements in the substrate. 2. Experimental 2.1. Thin film deposition The subtrates were well-polished (0 0 0 1) ZnO single crystals and well-polished & ð1 1 2 0Þ (A-face) and (0 0 0 1) (C-face) a-Al2O3 ones. Ga14N/Ga15N/Ga14N (about 400 nm/ 400 nm/400 nm) heterostructure thin films were deposited by molecular beam epitaxy (MBE). A Knudsen cell (K-cell) with a tantalum crucible was used to evaporate the Ga (99.9999%). A nitrogen radical beam, generated by an rf (13.56 MHz) discharge cell, was irradiated on the growing film. Pure nitrogen gas (99.9999%) was introduced into the plasma
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discharge cell, and the total pressure in the growth chamber was kept at 3.5–4.0 105 Torr during the deposition. The gas line was evacuated to 1 105 Torr before changing from natural nitrogen gas to 96% 15N-enriched nitrogen gas, and then the same evacuation procedure was carried out during changing process form 96% 15N-enriched nitrogen gas to the natural nitrogen gas. The substrate temperature was maintained at 1073 K during the deposition. The interfacial structure and crystallinity of the GaN thin films were determined by XRD (u– 2u, rocking curve, and reciprocal space mapping) and TEM. The films’ surface textures were observed by atomic force microscopy. 2.2. Depth profile of elements and nitrogen isotopes The deposited thin films were annealed at various temperatures in a silica glass tube with a flowing nitrogen atmosphere in which the dew point was under 210 K. Depth profiling of 15N and 14N and the substrate elements (Al and Zn) was carried out with a double focusing magnetic sector SIMS (CAMECA, IMS 4f). The negative secondary ions such as Ga15N, Ga14N, AlO and ZnO were analyzed using 10 keV Cs+ primary ions. The ion beam current was about 20 nA, and the raster area was 100 mm 100 mm. The positive secondary ions, such as 14N+, 15N+, Al+, Zn+, etc., were depth profiled using a 12.5 keV O2+ primary ion beam with an ion current of about 100 nA. In this case, the ion beam was raster-scanned over an area of 250 mm 250 mm. Sputtered crater profiles were measured with a Dektac profilometer. 3. Results and discussion TEM and XRD results revealed that all the films prepared in the present study were epitaxially grown on all substrates. The growth direction was parallel to the c-axis of the wurtzite structure. Fig. 1 shows typical depth profiles for an as-deposited sample, taken with the O2+ and Cs+ primary ions, respectively. The intensity of the signals from the aluminum ions for Cs+ was affected by interfering ions. Although the Cs+ gave sharper profiles than O2+, the depth profiles in the data for Cs+ tended to be asymmetrical. Fig. 2 plots the 15N isotope abundance in the annealed samples. The above-mentioned tendency became much more
Fig. 2. Depth profile of 15N in GaN thin film annealed at 1223 K for 8 h in N2 atmosphere.
pronounced in the annealed samples. Fig. 3 shows AFM images obtained for sputtered crater bottoms. Although the texture of the O2+-sputtered bottom is finer and rougher than that of the Cs+-sputtered one, many holes existed in the latter. The poor sharpness at the isotopic boundaries was concluded to be due to the large roughness shown in the O2+data. Deep holes caused large deformations in the profiles of 15N isotope abundance. An incorrect profile is a serious flaw that hampers the analysis of diffusion properties; hence, the data taken with Cs+ could not be used. Fig. 4 shows the diffusion profiles of Al ions from the interface between the GaN thin films and the substrate. These data were analyzed with a simple error function (shown as the line in the figure). The Al concentration inside the films was higher than the values that would be expected with volume diffusion. This suggests that a high diffusivity path such as subgrain boundaries exists in thin films. Temperature dependence of Al ions are plotted in Fig. 5. Zn diffusivity was also evaluated. The diffusion coefficient of Zn was 1.0 1016 cm2/s at 1203 K. Fig. 6 shows the annealing time variation of the depth profile of 15N in the thin films deposited on the sapphire with the Aface. The slope at the boundary decreased with increasing annealing duration because of the nitrogen ion diffusion. However, 15N shallowly penetrated into the 14N side of the samples without diffusion, because some 15N diffused during the deposition process and/or an apparent penetration occurred because the lower resolution of the depth profile. To obtain the diffusion coefficients of nitrogen ions, one has to use a simple
Fig. 1. Typical depth profiles of N isotopes and substrate element, Al (as-deposited sample). (A) Cs+ primary ion, negative secondary ions. (B) O2+ primary ion, positive secondary ions.
H. Haneda et al. / Applied Surface Science 252 (2006) 7265–7268
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Fig. 3. 20 mm 20 mm AFM surface images of crater bottoms sputtered by: (A) Cs+ and (B) O2+.
Fig. 4. Diffusion profile of Al ions from substrates.
Fig. 6. Annealing duration dependence on 15N abundances in GaN films deposited on A-face of sapphire. Annealing temperature: 1223 K. O2+, primary ion beam.
error function with a correction term for the apparent penetration [4]. For this purpose, we estimated the initial distribution and obtained a value, 7.6 1013, for correction term. Hence, ‘‘Dt + K’’ (K = 7.6 1013) was used instead of ‘‘Dt’’ in the error function for the diffusion analysis. Fig. 7 shows a typical result of the fitting procedure, giving an average of 4 1017 cm2/s for A-face samples at 1223 K. The values were two orders of magnitude lower than that reported by Ambacher et al. [5]. Their diffusion annealing had been carried out in vacuum, in which the number of nitrogen site vacancies should increase.
Fig. 7. Typical result of fitting procedure with a simple error function for 8 h annealing at 1223 K. Open circles are observed values and the solid line is the calculated values, respectively.
4. Conclusion
Fig. 5. Arrhenius plot of Al diffusion coefficient in GaN.
We studied the elements comprising a-Al2O3 and ZnO substrates for GaN thin films and nitrogen ions diffusion in GaN with a wurtzite structure by thermal annealing with an isotope heterostructure. The depth profiles of impurity ions and the 15N isotope were determined by means of SIMS. Al and Zn had almost same level of diffusivity. As for the
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nitrogen ions, the effect of their initial distribution had to be included in a simple error function to obtain the nitrogen diffusivity in GaN, so that the apparent penetration of the isotope can be corrected. Acknowledgement This study was partly supported by an Industrial Sport, Science and Technology (MEXT), an Industrial Technology Grant (ID04A26018) from New Energy and Industrial Technology Development Organization (NEDO), Japan.
References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Jpn. J. Appl. Phys. 37 (1998) L309. [2] K. Shimamura, E.G. Villora, K. Domen, K. Yui, K. Aoki, N. Ichinose, Jpn. J. Appl. Phys. 44 (2005) L7–L8 (Part 2). [3] T. Ohgaki, S. Sugimura, H. Ryoken, N. Ohashi, I. Sakaguchi, T. Sekiguchi, H. Haneda, Key Eng. Mater. 306 (2006) 79. [4] J. Crank, Mathematics of Diffusion, Oxford University Press, Oxford, 1975, pp. 11–27. [5] O. Ambacher, F. Freudenberg, R. Dimitrov, H. Angerer, M. Stutzmann, Jpn. J. Appl. Phys. 37 (1998) 2416 (Part 1).
SIMS analysis of impurities and nitrogen isotopes in gallium nitride thin films Hajime Haneda a,c,*, Takeshi Ohgaki b, Isao Sakaguchi a, Haruki Ryoken c, Naoki Ohashi a, Atsuo Yasumori b a
Advanced Materials Laboratory, National Institute for Materials Science, 1-1 Namiki Tsukuba, Ibaraki 305-0044, Japan b Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan c Kyushu University, 6-1 Kasuga-kouen, Kasuga 816-8580, Japan Received 12 September 2005; accepted 15 February 2006 Available online 3 May 2006
Abstract Gallium nitride thin films were deposited on sapphire or zinc oxide substrates with a molecular beam epitaxial method. Thin films with a Ga14N/ Ga N/Ga14N isotopic heterostructure were also grown. A CAMECA-type secondary ion mass spectrometry (SIMS) was employed to analyze impurities such as substrate elements. Nitrogen isotopes were also analyzed. Some samples were annealed in a nitrogen atmosphere, and the diffusivities of the elements and isotopes were evaluated. Although the crater surface after using Cs+ ions as the primary ion beam became smoother than after using O2+, preferential sputtering was observed. It is concluded that this preferential sputtering causes the isotope distribution to be abnormal. Elements of the substrates diffused into the thin films. # 2006 Elsevier B.V. All rights reserved. 15
Keywords: SIMS; Diffusion; GaN; Nitrogen; Aluminum; Zinc
1. Introduction Galluim nitride (GaN) and its alloys with indium nitride (InN) are promising materials for optelectronic device applications [1]. Light-emitting diodes of GaN-related alloys are now commercially available as a result of the great progress in crystalline quality and doping control. Most of the commercial GaN-based diodes are prepared on sapphire (aAl2O3) or silicon carbide (SiC) substrates. Several trials to grow high-crystalline GaN films have been carried out using various substrates of neither a-Al2O3 nor SiC [2]. We have already proposed that zinc oxide (ZnO) be used as a substrate [3]. The understanding of the diffusion process in solids is of great interest, for both technical and scientific reasons. As the thickness of thin films is decreasing, it has become increasingly important to have an understanding and the ability to control diffusion-related phenomena. Diffusion can be destructive to the delicate film structures because of transport of dopants in
* Corresponding author. Tel.: +81 29 860 4665; fax: +81 29 855 1196. E-mail address: [email protected] (H. Haneda). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.02.179
one layer into adjacent layers or the substrate. Secondary ion mass spectrometry (SIMS) depth profiles are generally affected by preferential sputtering or the edge effect. Thus, the makers of isotopes would be very useful for clarifying the effect of the preferential sputtering and the edge. For this report, we analyzed the isotope profile of nitrogen in a Ga14N/Ga15N/ Ga14N isotopic heterostructure by means of D-SIMS and evaluated the diffusivities of elements in the substrate. 2. Experimental 2.1. Thin film deposition The subtrates were well-polished (0 0 0 1) ZnO single crystals and well-polished & ð1 1 2 0Þ (A-face) and (0 0 0 1) (C-face) a-Al2O3 ones. Ga14N/Ga15N/Ga14N (about 400 nm/ 400 nm/400 nm) heterostructure thin films were deposited by molecular beam epitaxy (MBE). A Knudsen cell (K-cell) with a tantalum crucible was used to evaporate the Ga (99.9999%). A nitrogen radical beam, generated by an rf (13.56 MHz) discharge cell, was irradiated on the growing film. Pure nitrogen gas (99.9999%) was introduced into the plasma
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discharge cell, and the total pressure in the growth chamber was kept at 3.5–4.0 105 Torr during the deposition. The gas line was evacuated to 1 105 Torr before changing from natural nitrogen gas to 96% 15N-enriched nitrogen gas, and then the same evacuation procedure was carried out during changing process form 96% 15N-enriched nitrogen gas to the natural nitrogen gas. The substrate temperature was maintained at 1073 K during the deposition. The interfacial structure and crystallinity of the GaN thin films were determined by XRD (u– 2u, rocking curve, and reciprocal space mapping) and TEM. The films’ surface textures were observed by atomic force microscopy. 2.2. Depth profile of elements and nitrogen isotopes The deposited thin films were annealed at various temperatures in a silica glass tube with a flowing nitrogen atmosphere in which the dew point was under 210 K. Depth profiling of 15N and 14N and the substrate elements (Al and Zn) was carried out with a double focusing magnetic sector SIMS (CAMECA, IMS 4f). The negative secondary ions such as Ga15N, Ga14N, AlO and ZnO were analyzed using 10 keV Cs+ primary ions. The ion beam current was about 20 nA, and the raster area was 100 mm 100 mm. The positive secondary ions, such as 14N+, 15N+, Al+, Zn+, etc., were depth profiled using a 12.5 keV O2+ primary ion beam with an ion current of about 100 nA. In this case, the ion beam was raster-scanned over an area of 250 mm 250 mm. Sputtered crater profiles were measured with a Dektac profilometer. 3. Results and discussion TEM and XRD results revealed that all the films prepared in the present study were epitaxially grown on all substrates. The growth direction was parallel to the c-axis of the wurtzite structure. Fig. 1 shows typical depth profiles for an as-deposited sample, taken with the O2+ and Cs+ primary ions, respectively. The intensity of the signals from the aluminum ions for Cs+ was affected by interfering ions. Although the Cs+ gave sharper profiles than O2+, the depth profiles in the data for Cs+ tended to be asymmetrical. Fig. 2 plots the 15N isotope abundance in the annealed samples. The above-mentioned tendency became much more
Fig. 2. Depth profile of 15N in GaN thin film annealed at 1223 K for 8 h in N2 atmosphere.
pronounced in the annealed samples. Fig. 3 shows AFM images obtained for sputtered crater bottoms. Although the texture of the O2+-sputtered bottom is finer and rougher than that of the Cs+-sputtered one, many holes existed in the latter. The poor sharpness at the isotopic boundaries was concluded to be due to the large roughness shown in the O2+data. Deep holes caused large deformations in the profiles of 15N isotope abundance. An incorrect profile is a serious flaw that hampers the analysis of diffusion properties; hence, the data taken with Cs+ could not be used. Fig. 4 shows the diffusion profiles of Al ions from the interface between the GaN thin films and the substrate. These data were analyzed with a simple error function (shown as the line in the figure). The Al concentration inside the films was higher than the values that would be expected with volume diffusion. This suggests that a high diffusivity path such as subgrain boundaries exists in thin films. Temperature dependence of Al ions are plotted in Fig. 5. Zn diffusivity was also evaluated. The diffusion coefficient of Zn was 1.0 1016 cm2/s at 1203 K. Fig. 6 shows the annealing time variation of the depth profile of 15N in the thin films deposited on the sapphire with the Aface. The slope at the boundary decreased with increasing annealing duration because of the nitrogen ion diffusion. However, 15N shallowly penetrated into the 14N side of the samples without diffusion, because some 15N diffused during the deposition process and/or an apparent penetration occurred because the lower resolution of the depth profile. To obtain the diffusion coefficients of nitrogen ions, one has to use a simple
Fig. 1. Typical depth profiles of N isotopes and substrate element, Al (as-deposited sample). (A) Cs+ primary ion, negative secondary ions. (B) O2+ primary ion, positive secondary ions.
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Fig. 3. 20 mm 20 mm AFM surface images of crater bottoms sputtered by: (A) Cs+ and (B) O2+.
Fig. 4. Diffusion profile of Al ions from substrates.
Fig. 6. Annealing duration dependence on 15N abundances in GaN films deposited on A-face of sapphire. Annealing temperature: 1223 K. O2+, primary ion beam.
error function with a correction term for the apparent penetration [4]. For this purpose, we estimated the initial distribution and obtained a value, 7.6 1013, for correction term. Hence, ‘‘Dt + K’’ (K = 7.6 1013) was used instead of ‘‘Dt’’ in the error function for the diffusion analysis. Fig. 7 shows a typical result of the fitting procedure, giving an average of 4 1017 cm2/s for A-face samples at 1223 K. The values were two orders of magnitude lower than that reported by Ambacher et al. [5]. Their diffusion annealing had been carried out in vacuum, in which the number of nitrogen site vacancies should increase.
Fig. 7. Typical result of fitting procedure with a simple error function for 8 h annealing at 1223 K. Open circles are observed values and the solid line is the calculated values, respectively.
4. Conclusion
Fig. 5. Arrhenius plot of Al diffusion coefficient in GaN.
We studied the elements comprising a-Al2O3 and ZnO substrates for GaN thin films and nitrogen ions diffusion in GaN with a wurtzite structure by thermal annealing with an isotope heterostructure. The depth profiles of impurity ions and the 15N isotope were determined by means of SIMS. Al and Zn had almost same level of diffusivity. As for the
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nitrogen ions, the effect of their initial distribution had to be included in a simple error function to obtain the nitrogen diffusivity in GaN, so that the apparent penetration of the isotope can be corrected. Acknowledgement This study was partly supported by an Industrial Sport, Science and Technology (MEXT), an Industrial Technology Grant (ID04A26018) from New Energy and Industrial Technology Development Organization (NEDO), Japan.
References [1] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, K. Chocho, Jpn. J. Appl. Phys. 37 (1998) L309. [2] K. Shimamura, E.G. Villora, K. Domen, K. Yui, K. Aoki, N. Ichinose, Jpn. J. Appl. Phys. 44 (2005) L7–L8 (Part 2). [3] T. Ohgaki, S. Sugimura, H. Ryoken, N. Ohashi, I. Sakaguchi, T. Sekiguchi, H. Haneda, Key Eng. Mater. 306 (2006) 79. [4] J. Crank, Mathematics of Diffusion, Oxford University Press, Oxford, 1975, pp. 11–27. [5] O. Ambacher, F. Freudenberg, R. Dimitrov, H. Angerer, M. Stutzmann, Jpn. J. Appl. Phys. 37 (1998) 2416 (Part 1).