- Email: [email protected]
Growth of high-quality InN films by insertion of high-temperature InN buffer layer
ARTICLE IN PRESS
Journal of Crystal Growth 275 (2005) e1321–e1326 www.elsevier.com/locate/jcrysgro
Growth of high-quality InN films by insertion of high-temperature InN buffer layer T. Yamaguchia,, M. Kurouchia, H. Naoib, A. Suzukic, T. Arakia, Y. Nanishia a Department of Photonics, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan Center for Promotion of the COE Program, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan c Research Organization of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan b
Available online 8 December 2004
Abstract For the growth of further-improved InN film on a sapphire substrate, the effects of the conventional initial growth process, which is substrate nitridation and the deposition of a low-temperature InN buffer layer prior to growth, were carefully reinvestigated. Although the deposition of the low-temperature InN buffer layer realized a flat surface, the degradation of the crystallinity simultaneously resulted. The insertion of a high-temperature InN buffer layer solved this problem, and a high-quality InN film with a small tilt-distribution and a very flat surface was realized. r 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 61.10.Nz; 61.14.Hg Keywords: A1. Crystal structure; A1. Reflection high-energy electron diffraction; A1. Substrate; A1. X-ray diffraction; A3. Molecular beam epitaxy; B1. Nitrides
1. Introduction InN is expected to be a very attractive material for new applications in future photonic and electronic devices. In recent years, the number of reports regarding InN and InGaN with high In composition has markedly increased. The reasons for this increase are the realization of high-quality Corresponding author. Fax: +49 421 2184581.
E-mail address: [email protected] (T. Yamaguchi).
single-crystalline InN films and the subsequent discussion on band gaps in these high-quality films. The band gap of InN has been recently recognized to be less than 0.67 eV [1], much smaller than the previously reported value of approximately 1.9 eV [2,3], by investigating the obtained high-quality InN films. For the decision of the real band-gap value of InN and the realization of InN-based devices, however, further improvement in crystal quality is required. The initial methods prior to growth including substrate nitridation and buffer-layer deposition
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.106
ARTICLE IN PRESS e1322
T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
have very important effects on the growth of highquality InN films with a flat surface on a sapphire substrate [4]. A possible way to improve the quality of the InN films is to grow thick films in addition to using these initial growth methods, which is reported in Refs. [4,5]. Another possible way of improving film quality is to improve the initial growth methods themselves. In this paper, the effects of the initial growth methods have been carefully reinvestigated. The insertion of a hightemperature InN (HT-InN) buffer layer in the initial growth process has also been proposed as a method of improving film quality furthermore.
2. Experimental procedure InN films were grown on sapphire (0 0 0 1) substrates by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). Substrate nitridation and the deposition of a low-temperature InN (LT-InN) buffer layer were carried out prior to the growth; these are conventional methods widely used in the initial growth process. Substrate nitridation and the deposition of LT-InN buffer layer were carried out at 550 and 300 1C, respectively. InN films were then grown at 550 1C. The details of our growth system and the general growth procedure including the initial growth process have been described in our previous report [4]. The new process of inserting an HT-InN buffer layer before the deposition of an LT-InN buffer layer was also performed. The HT-InN buffer layer was grown at 530–550 1C under N-rich conditions. This is because N-rich growth conditions commonly make the InN film well-oriented to the c-axis although the film exhibits a three-dimensional growth mode, when the film is grown directly on sapphire [6]. The excellent c-axis orientation of the HT-InN buffer layer was utilized for the improvement of the initial growth process, as discussed afterward. The thickness of the HT-InN buffer layer was approximately 10 nm. As evaluation methods, in situ reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) (PANalytical X’Pert MRD), scanning electron microscopy (SEM) (Hitachi S-4300SE) and atomic force
microscopy (AFM) (Digital Instruments NanoScope IIIa) were used.
3. Results and discussion 3.1. Effects and issues of the initial growth process The growth of InN has been more difficult than that of other group III nitride semiconductors on sapphire (0 0 0 1) substrates. One of the reasons is that InN easily forms a multidomain structure due to its rotation within the c-plane on the sapphire substrate, and both ½1 1 2¯ 0InN ==½1 1 2¯ 0sapphire orientation and ½1 0 1¯ 0InN ==½1 1 2¯ 0sapphire orientation are considered as the main epitaxial relationship of the a-axis [7–9]. Substrate nitridation brings the film with only the ½1 0 1¯ 0InN == ½1 1 2¯ 0sapphire orientation irrespective of growth methods and growth conditions. Substrate nitridation also has an effect for the crystallinity of the LT-InN buffer layer. The buffer layer deposited without substrate nitridation is revealed to be mostly c-axis-oriented polycrystalline by the RHEED pattern [10]. For the buffer layer deposited after nitridation, on the other hand, the pattern always shows streaks [10,11], which indicate the presence of single crystals. In addition, the crystallinity of the InN film overgrown on the buffer layer without nitridation is always inferior to that of the film grown with nitridation. Therefore, substrate nitridation is also very important to improve the crystallinity of the buffer layer and the subsequent InN film. In regard to the effects of the LT-InN buffer layer, it is well known to bring the InN film with a flat surface [4,11]. When the InN film is grown directly on a nitridated substrate, which means that LT-InN buffer layer is not used in the initial process, the film has a three-dimensional structure in which a small tilt distribution can be realized. Increasing the thickness of the LT-InN buffer layer is an easy way to obtain a flat surface, on the other hand, the tilt distribution of the InN film deteriorates, as shown in Fig. 1; that is, there is trade-off between tilt distribution and surface flatness for the deposition of LT-InN buffer layer.
ARTICLE IN PRESS T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
e1323
Fig. 1. Dependences of XRC–FWHM of InN (0 0 0 2) reflection and AFM RMS on the thickness of the LT-InN buffer layer.
This is due to the poor crystal quality of the LT-InN buffer layer. 3.2. Insertion of HT-InN buffer layer The HT-InN buffer layer should have excellent c-axis orientation, because the growth of this HTInN buffer layer means the same as the direct InN growth on a nitridated substrate. The crystal quality of the LT-InN buffer layer is expected to be improved by the underlying HT-InN buffer layer, because of its excellent c-axis orientation. Although the HT-InN buffer layer has a threedimensional growth mode, the deposition of the LT-InN buffer layer over the HT-InN buffer layer and its subsequent annealing is expected to yield an excellent surface morphology with a flat surface, as shown in Fig. 2. Fig. 3 shows the RHEED pattern of each step; after HT-InN buffer-layer growth, after LT-InN buffer-layer deposition and after annealing of the LT-InN buffer layer, compared with the conventional process without HT-InN buffer-layer growth. After the growth of the HT-InN buffer layer, the RHEED pattern shows that the layer has a three-dimensional structure. Although the surface is still rough just after the deposition of the LT-InN buffer layer, a flat surface is realized after annealing, like the LT-InN buffer layer deposited using the conventional process [11]. In addition,
Fig. 2. Schematic of HT-InN buffer-layer deposition.
the cubic phase is not present in the LT-InN buffer layer, although it is sometimes present in the case of conventional process, as shown in the RHEED pattern. This might be because the HT-InN buffer layer is grown under N-rich conditions and at a high temperature; the details of these mechanisms are now being studied. Fig. 4 shows the surface morphologies of both InN films grown by inserting the HT-InN buffer layer and by the conventional process. The film with the HT-InN buffer layer had a very flat surface like that grown by the conventional method. The surface roughness obtained by AFM measurements was both less than 2 nm for 5 5 mm2 of these films. Fig. 5 shows the X-ray rocking curve (XRC) for InN (0 0 0 2) reflection of both films shown in
ARTICLE IN PRESS e1324
T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
Fig. 3. RHEED patterns of each step; after 10-nm-thick HT-InN buffer layer growth, after 30-nm-thick LT-InN buffer layer deposition and after annealing of the LT-InN buffer layer.
Fig. 4. The surface morphologies of InN by SEM. (a) InN/LTInN/HT-InN/sapphire, (b) InN/LT-InN/HT-InN/sapphire. The thickness of InN films is approximately 500 nm.
Fig. 4. Table 1 shows the full-width at halfmaximum (FWHM) of the XRC. It was 7.7 arcmin for the film with the HT-InN buffer layer, compared with 22.8 arcmin for the film grown using the conventional process with the same film thickness. Fig. 6 shows the film thickness dependence of XRC (0 0 0 2) FWHM for the film with the HT-InN buffer layer compared with that in the case of the film grown by the conventional process. The crystallinity of the InN films with the HT-InN buffer layer was always superior to that of the film with the same thickness grown by the conventional method. It was found that the excellent c-axis orientation of the HT-InN buffer layer improved the c-axis orientation of the LT-InN buffer layer and subsequent InN film. The HT-InN buffer layer was verified, as a result, to have the effect of
Fig. 5. The profiles of XRC for InN (0 0 0 2) reflection of both films shown in Fig. 4.
improving InN crystal quality. The concept of a high-temperature buffer layer is expected to apply also in the epitaxial growth of other materials. Table 2 shows the values of the twist distribution corresponding to the ð1 0 1¯ 0Þ reflection for both films shown in Table 1. These values were calculated by referring to the report by Heinke
ARTICLE IN PRESS T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326 Table 1 The values of XRC (0 0 0 2) FWHM for 500-nm-thick InN films grown with and without the HT-InN buffer layer
XRC (0 0 0 2)
InN/LT-InN/HTInN/Sap
InN/LT-InN/Sap (Conventional process)
7.7 arcmin
22.8 arcmin
e1325
improvement of twist distribution, it might be necessary to use underlying layer with smaller lattice mismatch such as an InN-based alloy semiconductor.
4. Summary In conclusion, the effects and the issues of the conventional initial growth process in the growth of InN on sapphire were clearly shown. When an InN film was grown without an LT-InN buffer layer on sapphire, the film always exhibited a three-dimensional growth mode. A flat InN film surface was easily obtained when the thickness of the buffer layer was increased. On the other hand, degradation of the structural film quality also resulted from increasing the thickness of the buffer layer. To solve this problem, the insertion of an HT-InN buffer layer prior to the deposition of the LT-InN buffer layer was proposed. The insertion of the HT-InN buffer layer realized a high-quality InN film, particularly with small tilt-distribution, maintaining a surface flatness.
Fig. 6. Dependence of the XRC–FWHM of InN (0 0 0 2) reflection on film thickness.
Table 2 The values of XRC (10–10) FWHM for 500-nm-thick InN films grown with and without the HT-InN buffer layer
XRC (10–10)
InN/LT-InN/HTInN/Sap
InN/LT-InN/Sap (Conventional process)
60.1 arcmin
58.9 arcmin
et al. [12] The values were almost the same; the improvement of the twist distribution by inserting the HT-InN buffer layer was not observed. For further improvement of the InN film quality, we have to solve the problem of twist distribution [1]. This should be due to the large lattice mismatch over 10%, which is for the AlN layer formed on the substrate surface by nitridation. For the
Acknowledgements The authors thank N. Teraguchi of Sharp Corp. and Y. Saito of Toyoda Gosei for fruitful discussions. The authors also thank D. Muto of Ritsumeikan Univ. for the assistance of the evaluations. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research (B) #13450131, Academic Frontier Promotion Project and The 21st Century COE Program. References [1] Y. Nanishi, Y. Saito, T. Yamaguchi, F. Matsuda, T. Araki, H. Naoi, A. Suzuki, H. Harima, T. Miyajima, Mater. Res. Soc. Symp. Proc. 798 (2004) Y12.1.1. [2] K. Osamura, K. Nakajima, Y. Murakami, Solid State Commun. 11 (1972) 617. [3] T.L. Tansley, C.P. Foley, J. Appl. Phys. 59 (1986) 3241. [4] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2003) 2549.
ARTICLE IN PRESS e1326
T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
[5] H. Lu, W.J. Schaff, L.F. Eastman, J. Wu, W. Walukiewicz, D.C. Look, R.J. Molnar, Mat. Res. Soc. Symp. Proc. 743 (2003) 317. [6] T. Yamaguchi, Y. Saito, K. Kano, T. Araki, N. Teraguchi, A. Suzuki, Y. Nanishi, Proceedings of International Conference on Indium Phosphide and Related Materials PII-41, 2002, p. 643. [7] A. Wakahara, A. Yoshida, Appl. Phys. Lett. 54 (1989) 709. [8] T. Yamaguchi, Y. Saito, K. Kano, T. Araki, N. Teraguchi, A. Suzuki, Y. Nanishi, Phys. Stat. Sol. (b) 228 (2001) 17.
[9] T. Yamaguchi, T. Araki, Y. Saito, T. Maruyama, Y. Nanishi, N. Teraguchi, A. Suzuki, Inst. Phys. Conf. Ser. 170 (2002) 765. [10] T. Yamaguchi, Y. Saito, K. Kano, T. Muramatsu, T. Araki, Y. Nanishi, N. Teraguchi, A. Suzuki, Mater. Res. Soc. Symp. Proc. 693 (2002) 195. [11] Y. Saito, N. Teraguchi, A. Suzuki, T. Araki, Y. Nanishi, Jpn. J. Appl. Phys. 40 (2001) L91. [12] H. Heinke, V. Kirchner, S. Einfeldt, D. Hommel, Appl. Phys. Lett. 77 (2000) 2145.
Journal of Crystal Growth 275 (2005) e1321–e1326 www.elsevier.com/locate/jcrysgro
Growth of high-quality InN films by insertion of high-temperature InN buffer layer T. Yamaguchia,, M. Kurouchia, H. Naoib, A. Suzukic, T. Arakia, Y. Nanishia a Department of Photonics, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan Center for Promotion of the COE Program, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan c Research Organization of Science and Engineering, Ritsumeikan University, 1-1-1 Noji-Higashi, Kusatsu, Shiga 525-8577, Japan b
Available online 8 December 2004
Abstract For the growth of further-improved InN film on a sapphire substrate, the effects of the conventional initial growth process, which is substrate nitridation and the deposition of a low-temperature InN buffer layer prior to growth, were carefully reinvestigated. Although the deposition of the low-temperature InN buffer layer realized a flat surface, the degradation of the crystallinity simultaneously resulted. The insertion of a high-temperature InN buffer layer solved this problem, and a high-quality InN film with a small tilt-distribution and a very flat surface was realized. r 2004 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 61.10.Nz; 61.14.Hg Keywords: A1. Crystal structure; A1. Reflection high-energy electron diffraction; A1. Substrate; A1. X-ray diffraction; A3. Molecular beam epitaxy; B1. Nitrides
1. Introduction InN is expected to be a very attractive material for new applications in future photonic and electronic devices. In recent years, the number of reports regarding InN and InGaN with high In composition has markedly increased. The reasons for this increase are the realization of high-quality Corresponding author. Fax: +49 421 2184581.
E-mail address: [email protected] (T. Yamaguchi).
single-crystalline InN films and the subsequent discussion on band gaps in these high-quality films. The band gap of InN has been recently recognized to be less than 0.67 eV [1], much smaller than the previously reported value of approximately 1.9 eV [2,3], by investigating the obtained high-quality InN films. For the decision of the real band-gap value of InN and the realization of InN-based devices, however, further improvement in crystal quality is required. The initial methods prior to growth including substrate nitridation and buffer-layer deposition
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.11.106
ARTICLE IN PRESS e1322
T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
have very important effects on the growth of highquality InN films with a flat surface on a sapphire substrate [4]. A possible way to improve the quality of the InN films is to grow thick films in addition to using these initial growth methods, which is reported in Refs. [4,5]. Another possible way of improving film quality is to improve the initial growth methods themselves. In this paper, the effects of the initial growth methods have been carefully reinvestigated. The insertion of a hightemperature InN (HT-InN) buffer layer in the initial growth process has also been proposed as a method of improving film quality furthermore.
2. Experimental procedure InN films were grown on sapphire (0 0 0 1) substrates by radio-frequency plasma-excited molecular beam epitaxy (RF-MBE). Substrate nitridation and the deposition of a low-temperature InN (LT-InN) buffer layer were carried out prior to the growth; these are conventional methods widely used in the initial growth process. Substrate nitridation and the deposition of LT-InN buffer layer were carried out at 550 and 300 1C, respectively. InN films were then grown at 550 1C. The details of our growth system and the general growth procedure including the initial growth process have been described in our previous report [4]. The new process of inserting an HT-InN buffer layer before the deposition of an LT-InN buffer layer was also performed. The HT-InN buffer layer was grown at 530–550 1C under N-rich conditions. This is because N-rich growth conditions commonly make the InN film well-oriented to the c-axis although the film exhibits a three-dimensional growth mode, when the film is grown directly on sapphire [6]. The excellent c-axis orientation of the HT-InN buffer layer was utilized for the improvement of the initial growth process, as discussed afterward. The thickness of the HT-InN buffer layer was approximately 10 nm. As evaluation methods, in situ reflection high-energy electron diffraction (RHEED), X-ray diffraction (XRD) (PANalytical X’Pert MRD), scanning electron microscopy (SEM) (Hitachi S-4300SE) and atomic force
microscopy (AFM) (Digital Instruments NanoScope IIIa) were used.
3. Results and discussion 3.1. Effects and issues of the initial growth process The growth of InN has been more difficult than that of other group III nitride semiconductors on sapphire (0 0 0 1) substrates. One of the reasons is that InN easily forms a multidomain structure due to its rotation within the c-plane on the sapphire substrate, and both ½1 1 2¯ 0InN ==½1 1 2¯ 0sapphire orientation and ½1 0 1¯ 0InN ==½1 1 2¯ 0sapphire orientation are considered as the main epitaxial relationship of the a-axis [7–9]. Substrate nitridation brings the film with only the ½1 0 1¯ 0InN == ½1 1 2¯ 0sapphire orientation irrespective of growth methods and growth conditions. Substrate nitridation also has an effect for the crystallinity of the LT-InN buffer layer. The buffer layer deposited without substrate nitridation is revealed to be mostly c-axis-oriented polycrystalline by the RHEED pattern [10]. For the buffer layer deposited after nitridation, on the other hand, the pattern always shows streaks [10,11], which indicate the presence of single crystals. In addition, the crystallinity of the InN film overgrown on the buffer layer without nitridation is always inferior to that of the film grown with nitridation. Therefore, substrate nitridation is also very important to improve the crystallinity of the buffer layer and the subsequent InN film. In regard to the effects of the LT-InN buffer layer, it is well known to bring the InN film with a flat surface [4,11]. When the InN film is grown directly on a nitridated substrate, which means that LT-InN buffer layer is not used in the initial process, the film has a three-dimensional structure in which a small tilt distribution can be realized. Increasing the thickness of the LT-InN buffer layer is an easy way to obtain a flat surface, on the other hand, the tilt distribution of the InN film deteriorates, as shown in Fig. 1; that is, there is trade-off between tilt distribution and surface flatness for the deposition of LT-InN buffer layer.
ARTICLE IN PRESS T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
e1323
Fig. 1. Dependences of XRC–FWHM of InN (0 0 0 2) reflection and AFM RMS on the thickness of the LT-InN buffer layer.
This is due to the poor crystal quality of the LT-InN buffer layer. 3.2. Insertion of HT-InN buffer layer The HT-InN buffer layer should have excellent c-axis orientation, because the growth of this HTInN buffer layer means the same as the direct InN growth on a nitridated substrate. The crystal quality of the LT-InN buffer layer is expected to be improved by the underlying HT-InN buffer layer, because of its excellent c-axis orientation. Although the HT-InN buffer layer has a threedimensional growth mode, the deposition of the LT-InN buffer layer over the HT-InN buffer layer and its subsequent annealing is expected to yield an excellent surface morphology with a flat surface, as shown in Fig. 2. Fig. 3 shows the RHEED pattern of each step; after HT-InN buffer-layer growth, after LT-InN buffer-layer deposition and after annealing of the LT-InN buffer layer, compared with the conventional process without HT-InN buffer-layer growth. After the growth of the HT-InN buffer layer, the RHEED pattern shows that the layer has a three-dimensional structure. Although the surface is still rough just after the deposition of the LT-InN buffer layer, a flat surface is realized after annealing, like the LT-InN buffer layer deposited using the conventional process [11]. In addition,
Fig. 2. Schematic of HT-InN buffer-layer deposition.
the cubic phase is not present in the LT-InN buffer layer, although it is sometimes present in the case of conventional process, as shown in the RHEED pattern. This might be because the HT-InN buffer layer is grown under N-rich conditions and at a high temperature; the details of these mechanisms are now being studied. Fig. 4 shows the surface morphologies of both InN films grown by inserting the HT-InN buffer layer and by the conventional process. The film with the HT-InN buffer layer had a very flat surface like that grown by the conventional method. The surface roughness obtained by AFM measurements was both less than 2 nm for 5 5 mm2 of these films. Fig. 5 shows the X-ray rocking curve (XRC) for InN (0 0 0 2) reflection of both films shown in
ARTICLE IN PRESS e1324
T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
Fig. 3. RHEED patterns of each step; after 10-nm-thick HT-InN buffer layer growth, after 30-nm-thick LT-InN buffer layer deposition and after annealing of the LT-InN buffer layer.
Fig. 4. The surface morphologies of InN by SEM. (a) InN/LTInN/HT-InN/sapphire, (b) InN/LT-InN/HT-InN/sapphire. The thickness of InN films is approximately 500 nm.
Fig. 4. Table 1 shows the full-width at halfmaximum (FWHM) of the XRC. It was 7.7 arcmin for the film with the HT-InN buffer layer, compared with 22.8 arcmin for the film grown using the conventional process with the same film thickness. Fig. 6 shows the film thickness dependence of XRC (0 0 0 2) FWHM for the film with the HT-InN buffer layer compared with that in the case of the film grown by the conventional process. The crystallinity of the InN films with the HT-InN buffer layer was always superior to that of the film with the same thickness grown by the conventional method. It was found that the excellent c-axis orientation of the HT-InN buffer layer improved the c-axis orientation of the LT-InN buffer layer and subsequent InN film. The HT-InN buffer layer was verified, as a result, to have the effect of
Fig. 5. The profiles of XRC for InN (0 0 0 2) reflection of both films shown in Fig. 4.
improving InN crystal quality. The concept of a high-temperature buffer layer is expected to apply also in the epitaxial growth of other materials. Table 2 shows the values of the twist distribution corresponding to the ð1 0 1¯ 0Þ reflection for both films shown in Table 1. These values were calculated by referring to the report by Heinke
ARTICLE IN PRESS T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326 Table 1 The values of XRC (0 0 0 2) FWHM for 500-nm-thick InN films grown with and without the HT-InN buffer layer
XRC (0 0 0 2)
InN/LT-InN/HTInN/Sap
InN/LT-InN/Sap (Conventional process)
7.7 arcmin
22.8 arcmin
e1325
improvement of twist distribution, it might be necessary to use underlying layer with smaller lattice mismatch such as an InN-based alloy semiconductor.
4. Summary In conclusion, the effects and the issues of the conventional initial growth process in the growth of InN on sapphire were clearly shown. When an InN film was grown without an LT-InN buffer layer on sapphire, the film always exhibited a three-dimensional growth mode. A flat InN film surface was easily obtained when the thickness of the buffer layer was increased. On the other hand, degradation of the structural film quality also resulted from increasing the thickness of the buffer layer. To solve this problem, the insertion of an HT-InN buffer layer prior to the deposition of the LT-InN buffer layer was proposed. The insertion of the HT-InN buffer layer realized a high-quality InN film, particularly with small tilt-distribution, maintaining a surface flatness.
Fig. 6. Dependence of the XRC–FWHM of InN (0 0 0 2) reflection on film thickness.
Table 2 The values of XRC (10–10) FWHM for 500-nm-thick InN films grown with and without the HT-InN buffer layer
XRC (10–10)
InN/LT-InN/HTInN/Sap
InN/LT-InN/Sap (Conventional process)
60.1 arcmin
58.9 arcmin
et al. [12] The values were almost the same; the improvement of the twist distribution by inserting the HT-InN buffer layer was not observed. For further improvement of the InN film quality, we have to solve the problem of twist distribution [1]. This should be due to the large lattice mismatch over 10%, which is for the AlN layer formed on the substrate surface by nitridation. For the
Acknowledgements The authors thank N. Teraguchi of Sharp Corp. and Y. Saito of Toyoda Gosei for fruitful discussions. The authors also thank D. Muto of Ritsumeikan Univ. for the assistance of the evaluations. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research (B) #13450131, Academic Frontier Promotion Project and The 21st Century COE Program. References [1] Y. Nanishi, Y. Saito, T. Yamaguchi, F. Matsuda, T. Araki, H. Naoi, A. Suzuki, H. Harima, T. Miyajima, Mater. Res. Soc. Symp. Proc. 798 (2004) Y12.1.1. [2] K. Osamura, K. Nakajima, Y. Murakami, Solid State Commun. 11 (1972) 617. [3] T.L. Tansley, C.P. Foley, J. Appl. Phys. 59 (1986) 3241. [4] Y. Nanishi, Y. Saito, T. Yamaguchi, Jpn. J. Appl. Phys. 42 (2003) 2549.
ARTICLE IN PRESS e1326
T. Yamaguchi et al. / Journal of Crystal Growth 275 (2005) e1321–e1326
[5] H. Lu, W.J. Schaff, L.F. Eastman, J. Wu, W. Walukiewicz, D.C. Look, R.J. Molnar, Mat. Res. Soc. Symp. Proc. 743 (2003) 317. [6] T. Yamaguchi, Y. Saito, K. Kano, T. Araki, N. Teraguchi, A. Suzuki, Y. Nanishi, Proceedings of International Conference on Indium Phosphide and Related Materials PII-41, 2002, p. 643. [7] A. Wakahara, A. Yoshida, Appl. Phys. Lett. 54 (1989) 709. [8] T. Yamaguchi, Y. Saito, K. Kano, T. Araki, N. Teraguchi, A. Suzuki, Y. Nanishi, Phys. Stat. Sol. (b) 228 (2001) 17.
[9] T. Yamaguchi, T. Araki, Y. Saito, T. Maruyama, Y. Nanishi, N. Teraguchi, A. Suzuki, Inst. Phys. Conf. Ser. 170 (2002) 765. [10] T. Yamaguchi, Y. Saito, K. Kano, T. Muramatsu, T. Araki, Y. Nanishi, N. Teraguchi, A. Suzuki, Mater. Res. Soc. Symp. Proc. 693 (2002) 195. [11] Y. Saito, N. Teraguchi, A. Suzuki, T. Araki, Y. Nanishi, Jpn. J. Appl. Phys. 40 (2001) L91. [12] H. Heinke, V. Kirchner, S. Einfeldt, D. Hommel, Appl. Phys. Lett. 77 (2000) 2145.