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High-temperature growth of heteroepitaxial InSb films on Si(1 1 1) substrate via the InSb bi-layer
ARTICLE IN PRESS Journal of Crystal Growth 311 (2009) 1692–1695
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
High-temperature growth of heteroepitaxial InSb films on Si(111) substrate via the InSb bi-layer M. Mori a,, M. Saito b, K. Nagashima a, K. Ueda a, T. Yoshida a, K. Maezawa a a b
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan Venture Business Laboratory, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
a r t i c l e in f o
a b s t r a c t
Available online 21 October 2008
To achieve the high-temperature growth of heteroepitaxial InSb films on the InSb bi-layer, we studied the influence of substrate temperature of first layer deposition (Ts1) on the two-step growth procedure. Although the growth at higher Ts1 of 240 and 280 1C is difficult to achieve using the usual procedure due to the desorption of In atoms from the InSb bi-layer, it can be realized by means of the adsorption of excess Sb atoms onto an initial InSb bi-layer prepared via O7 O3-In surface reconstruction. The hightemperature growth of 301-rotated InSb films at 420 1C was demonstrated on a Si(111) substrate with a InSb bi-layer. The electron mobility of the InSb film grown at 420 1C was about 20,000 cm2/V s at RT. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.05.Ea 81.15.Hi 61.10.Nz Keywords: A1. X-ray diffraction A3. Molecular beam epitaxy B2. Semiconducting materials
1. Introduction Indium antimonide (InSb) has been widely noticed as a candidate material for ultra-fast, very-low-power device, because of the highest electron mobility and saturation velocity of all semiconductors. The heteroepitaxial growth of InSb on Si has attracted much interest from the viewpoint of integration of InSbbased devices and Si-LSI. However, it is very difficult to grow heteroepitaxial InSb on Si, because of the large lattice mismatch of about 19.3% between them. To solve this difficulty, another material, such as GaAs, has been often used to reduce the lattice mismatch [1–3]. We have paid attention to the surface reconstructions induced by In and Sb atoms on the Si substrate at the initial stage of the growth of InSb films [4–7], and found that the use of a suitable reconstructed surface by atoms composing InSb is a good candidate to solve the lattice mismatch problem [7,8]. We have reported the heteroepitaxial growth of 301-rotated InSb films on an InSb bi-layer (Si(111)-2 2-InSb surface reconstruction). The InSb bi-layer is prepared by the adsorption of 1 monolayer (ML) Sb atoms onto In-induced surface reconstruction such as O3 O3-In, 2 2-In [7] and O7 O3-In [8] on a Si(111) substrate. Due to this rotation, the large lattice mismatch between Si and InSb is nominally reduced to about 3.3%. However, a Si(111)-2 1-Sb surface phase was formed during the substitution process by the desorption of In atoms from the InSb bi-layer [9]. The InSb films grown on the 2 1-Sb surface phase do not rotate and have poor crystal quality. Because the area covered by
the 2 1-Sb surface phase depends on the initial In coverage on the Si surface, In-induced surface reconstruction with higher In coverage such as O7 O3-In is necessary for the growth of fully rotated InSb films with higher crystal quality [8]. The growth of InSb films on the InSb bi-layer is performed by a two-step growth procedure. In this procedure, the substrate was first held at 180–200 1C (Ts1) to grow the first layer to prevent the desorption of In atoms from the InSb bi-layer, and then rose to 350 1C for the growth of the second layer [7]. The growth temperature for the second layer (Ts2) is also restricted due to degradation of the first layer at higher temperature. The electron mobility of the 1.1 mm InSb film grown via O7 O3-In was about 16,300 cm2/V s at RT [8]. In order to grow the InSb films with higher electron mobility, it is necessary to increase the growth temperature for the improvement of crystal quality. The deterioration of the first layer at higher temperature may be caused by the temperature gap between Ts1 and Ts2. So, we tried to increase the Ts1 to prevent the deterioration of the first InSb layer at higher temperature. Many groups have reported high electron mobility InSb films grown at over 400 1C [1,2]. Our target temperature for Ts2 is 420 1C, which is the optimal temperature of the direct growth of InSb films on the Si(111) substrate for Ts2 in our laboratory. In the present paper, we report the trial of the increase of Ts1 and the heteroepitaxial growth of InSb films grown on the InSb bi-layer at a higher temperature of 420 1C. 2. Experiments
Corresponding author. Tel./fax: +8176 445 6728.
E-mail address: [email protected] (M. Mori). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.10.043
All the depositions were carried out in an OMICRON molecular beam epitaxy (MBE) chamber with a base pressure of about
ARTICLE IN PRESS M. Mori et al. / Journal of Crystal Growth 311 (2009) 1692–1695
2 10 8 Pa, equipped with reflection high-energy electron diffraction (RHEED). The substrate with a dimension of about 15 4 0.6 mm3 was obtained from mirror-polished p-type Si(111) wafer with a resistivity of about 20 O cm. The substrates were degassed at about 600 1C for 12 h, followed by a flushannealing at 1250 1C, and then slowly cooled in the chamber. This process gave a clean (7 7) surface, as confirmed by RHEED. Highpurity (6 N) elemental indium and antimony were used as source materials and evaporated from each PBN K-cell. The substrate temperature was monitored by an infrared pyrometer. Prior to the growth of InSb films, the InSb bi-layer was prepared by the following process. First, a Si(111)-O3 O3-In surface phase was prepared by the deposition of 0.33 ML-In atoms onto the clean Si(111)-7 7 surface at 450 1C. After cooling down to RT, additional In atoms of 0.87 ML (total In coverage was 1.2 ML) were deposited onto the O3 O3-In surface to obtain O7 O3-In surface reconstruction. The InSb bi-layer was obtained by 1 ML-Sb deposition onto this surface at about 180 1C. The growth of InSb films was then performed by the two-step growth procedure on the In-Sb bi-layer. In the procedure, the substrate temperature was first held at 200, 240 and 280 1C to grow the first InSb layer for 5 min, and then rose to 350 1C for the beginning of the second layer deposition. In the case of the samples grown at 240 and 280 1C, the Sb coverage for the preparation of the InSb bi-layer was increased to 3 ML, because of the appearance of the O3 O3-Sb phase during increasing
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substrate temperature to Ts1. It indicates the desorption of In atoms from the InSb bi-layer, and phase transition from the 2 1-Sb phase due to higher temperature. The InSb films grown on the O3 O3-Sb phase do not rotate similar to the 2 1-Sb phase. The thickness of the first layer was about 300 A˚. While elevating Ts for the second layer growth, the growth was interrupted by closing the shutter. The evolution of the RHEED pattern operating at 15 kV was observed in the /1¯ 1 0S and /2¯ 11S azimuths of the Si substrate during the deposition. The total film thickness measured by the interferometer was about 1.1 mm. For structural analysis, the InSb films were characterized by X-ray diffraction (XRD) using Cu-Ka1 radiation. In the XRD measurement, we took the f scan pattern to know the in-plane orientation of the film. In this case, the f axis was parallel to the normal direction of the Si(111) substrate, and the w axis was tilted by about 70.51 to detect the equivalent (111) planes ((11¯ 1), (1¯ 11), (111¯)), while keeping the 2y/o axis to the Bragg angle of InSb(111) or Si(111) peaks. The measurement of electric properties of the samples was carried out using the van der Pauw method.
3. Results and discussion Fig. 1 compares the RHEED patterns of InSb films after the first and the second layer deposition. These images were taken along
Fig. 1. RHEED patterns after first layer (on the left) and second layer (on the right) deposition at (a, b) 200 1C, (c, d) 240 1C and (e, f) 280 1C. These patterns were taken along the /1¯ 1 0S azimuth of the Si substrate.
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the /1¯ 1 0S azimuth of the Si substrate. As seen in Fig. 1(a, c and e), after the first layer deposition, the RHEED patterns showed a 2 2 periodicity with a wider line spacing. This wider line spacing in the /1¯ 1 0S azimuth indicates the 301-rotation of the films [7,8], because the RHEED pattern with the wider line spacing appears along the /2¯ 11S azimuth in the Si(111) substrate, and the angle between the /1¯ 1 0S and /2¯ 11S azimuths is 301. Fig. 1(c) shows some island patterns due to the large Sb coverage. The island patterns disappeared due to the desorption of Sb atoms from the Sb islands at higher Ts1, as shown in Fig. 1(e). The RHEED patterns after the second layer deposition show a clear 2 2 periodicity in all samples, indicating a highly ordered and smooth surface of the films. The spotty patterns shown by arrows are also observed in Fig. 1(d and f). These spots may come from small islands on the film surface. The results of the RHEED observation implied that the Sb islands prepared by the additional Sb atoms prevent the formation of the O3 O3-Sb phase, and lead to the successful growth of 301rotated InSb films at higher Ts1. Because the substitution of In and Sb atoms also occurs on the InSb bi-layer for the additional Sb atoms, there is the probability of the existence of the InSb bi-layer on top of the Sb islands. However, In atoms on the InSb bi-layer may not be able to exist any longer at higher temperature, especially at 280 1C. So, this may imply that the rotated InSb films were grown via the Sb islands. We will report the improvement of crystal orientation by additional In and/or Sb atoms onto the initial InSb bi-layer elsewhere. The InSb films showed a clear 301-rotated 2 2 pattern in the RHEED pattern. To confirm the results of RHEED observation, the XRD measurement (f scan) was performed. The f scan patterns of the (111) reflection of the samples are shown in Fig. 2. The peak positions of the Si substrate indicated by black circles are separated by 1201 intervals, indicating the three-fold symmetry of the Si(111) substrate. As shown in Fig. 2, all samples show only six intense peaks shifted by 7301 compared with those of Si(111) peaks, meaning fully 7301-rotated InSb. In spite of the growth at higher Ts1 of 240 and 280 1C, no unrotated peaks between the intense peaks are observed in Fig. 2(b and c). This means that the InSb bi-layer and/or Sb islands prepared by additional Sb atoms work effectively. We also carried out 2y/o scan of all samples (not shown here), and confirmed that all samples were heteroepitaxially grown and had no polycrystalline nature. The FWHM of InSb(111) peaks of almost all samples was about 350 arcsec, indicating good crystal quality. The wider FWHM of 520 arcsec was only shown in the sample grown at Ts1 of 240 1C. This sample showed the island patterns of Sb after the growth of the first layer. This result shows that the surface roughness of the initial stage of growth affects crystal quality of the film. This fact that the first layer can be grown at higher Ts1 for fully 301-rotated InSb encouraged us to increase Ts2. We tried to grow InSb films at Ts2 over 400 1C, and found the first layers grown at 240 and 280 1C also deteriorated at around 395 1C by observing the RHEED pattern. Although we could increase Ts2 from 350 to around 390 1C, the increase of Ts2 of 40 1C is much smaller than that of Ts1 of 80 1C. This result may indicate that the deterioration of the films was caused not only by temperature gap but also by other reasons. Murata et al. [11] reported that heteroepitaxial InSb films can be directly grown on Si(111) substrates by using a twostep growth procedure. Their optimum growth temperature was 240 1C for Ts1 and 420 1C for Ts2, respectively. The difference of the growth condition between these results is only the existence of the InSb bi-layer. The In atoms substituted by Sb are forced to float on top of the Sb ML [9]. The weak bonds between In and Sb of the InSb bi-layer may give rise to the deterioration of InSb films at higher Ts2.
Fig. 2. f scan patterns of the InSb films grown at Ts1 of (a) 200 1C, (b) 240 1C and (c) 280 1C.
Fig. 3. Temperature programming of the InSb film grown at a higher Ts2 of 420 1C.
The unsuccessful growth of InSb on the InSb bi-layer at higher Ts2 over 400 1C reminded us of the result of the heteroepitaxial InSb films grown via Ge islands [12]. In that case, after the first layer deposition at 200 1C, the substrate temperature was gradually increased to 400 1C during the second layer deposition. This growth method enabled the preparation of high crystal quality InSb at a higher temperature of 400 1C while preventing the films’ surface from roughening. We tried to apply the procedure to the growth of InSb film on the InSb bi-layer. The growth temperature program of the InSb film grown at 420 1C is shown in Fig. 3. The first InSb layer was grown at 240 1C for 5 min. Then the substrate temperature was elevated to 360 1C for the start of the second layer deposition. After starting the second layer deposition, the Ts2 was gradually increased with the increasing rate as shown in Fig. 3. The total growth time was 3 h. The film thickness of the grown InSb film was estimated to be 1.1 mm.
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Fig. 4. RHEED pattern of the InSb film grown at 420 1C. These patterns were taken along the /1¯ 1 0S and /2¯ 11S azimuths of the Si substrate.
from the improvement of crystal quality due to the hightemperature growth. The reason for this successful growth without deterioration at higher temperature is not clear. A possible reason is the grain size of InSb crystals before starting of the high-temperature growth over 400 1C.
4. Conclusion
Fig. 5. f scan pattern of the InSb film grown at 420 1C.
The RHEED patterns of the grown InSb taken along the /1¯ 1 0S and /2¯ 11S azimuths of the Si substrate are shown in Fig. 4. As shown in Fig. 4, a clear 2 2 periodicity was observed in both azimuths, indicating the highly ordered and smooth surface of the film. The relation between the beam incident azimuth and line spacing of the first-order streaks indicates 301-rotation of the grown InSb film [7,8,10]. This result means that the InSb bi-layer also functions effectively in this growth procedure. The result of RHEED observation encouraged us to measure the XRD patterns of the InSb film. Fig. 5 shows the f scan pattern of the (111) reflection of the InSb film. The black circles show the peak position of the Si substrate. The six intense peaks shifted by 7301 with respect to the Si substrate are shown in this pattern. This 301-rotation is consistent with the result of RHEED observation. There are no peaks between the intense peaks, indicating the fully 301-rotated InSb film. The XRD pattern of the 2y/o scan (not shown here) shows that the InSb film is heteroepitaxially grown by the new growth procedure, and has no polycrystalline nature. The FWHM of the film is about 350 arcsec, indicating good crystal quality. This value is narrower than that of the sample grown by using TS1 of 240 1C. These XRD results indicate that the quality InSb film was grown at a high Ts2 of 420 1C via the InSb bi-layer without any deterioration. The results of the RHEED observation and XRD measurement were just what we hoped. We carried out the measurement of the electrical properties of the InSb film in expectation of improvement of electron mobility. The electron mobility and carrier concentration of the InSb film at RT were about 20,000 cm2/V s and about 3.6 1016/cm3, respectively. The high electron mobility of the 1.1-mm-thick InSb film on Si is better than that reported before [7,8,13]. This high electron mobility of the film may result
We tried the high-temperature growth of the 301-rotated InSb films with an InSb bi-layer on a Si(111) substrate. The fully 301rotated InSb films are grown at a higher Ts1 of 240 and 280 1C with additional Sb atoms onto the initial InSb bi-layer. The electron mobility and carrier concentration of the 1.1-mm-thick InSb films grown at a higher temperature of 420 1C were 20,000 cm2/V s and 3.6 1016/cm3 at RT, respectively. It was achieved on a Si(111) substrate using mono molecular layer (bi-layer) by atoms composing InSb.
Acknowledgements A part of this work has been supported by a Grant-in-Aid Scientific Research (19760233, 19569003) of the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Hitachi Kokusai Electric Inc., Japan.
References [1] S.D. Wu, L.W. Guo, Z.H. Li, X.Z. Shang, W.X. Wang, O. Huang, J.M. Zhou, J. Crystal Growth 277 (2005) 21. [2] A. Okamoto, H. Geka, I. Shibasaki, K. Yoshida, J. Crystal Growth 278 (2005) 604. [3] T. Ashley, L. Buckie, S. Datta, M.T. Emeny, D.G. Hayes, K.P. Hilton, R. Jefferies, T. Martin, T.J. Phillips, D.J. Walls, P.J. Wilding, R. Chau, IEE Electron. Lett. 43 (2007) 777. [4] B.V. Rao, M. Atoji, D.M. Li, T. Okamoto, T. Tambo, C. Tatsuyama, Jpn. J. Appl. Phys. 37 (1993) L1297. [5] D.M. Li, M. Atoji, T. Okamoto, T. Tambo, C. Tatsuyama, Surf. Sci. 417 (1998) 210. [6] D.V. Gruznev, B.V. Rao, T. Tambo, C. Tatsuyama, Appl. Surf. Sci. 190 (2002) 134. [7] M. Mori, M. Saito, Y. Yamashita, K. Nagashima, M. Hashimoto, C. Tatsuyama, T. Tambo, J. Crystal Growth 301–302 (2007) 207. [8] M. Mori, M. Saito, K. Nagashima, K. Ueda, Y. Yamashita, C. Tatsuyama, T. Tambo, K. Maezawa, Phys. Status Solidi (c) 5 (2008) 2772. [9] G.E. Franklin, D.H. Rich, H. Hong, T. Miller, T.-C. Chiang, Phys. Rev. B 45 (1992) 3426. [10] M. Saito, M. Mori, K. Maezawa, Appl. Surf. Sci. 254 (2008) 6052. [11] K. Murata, N.B. Ahmad, M. Mori, T. Tambo, K. Maezawa, Fifth International Symposium on Control of Semiconductor Interfaces (ISCSI-5), 2007. [12] M. Mori, Y. Nizawa, Y. Nishi, T. Tambo, C. Tatsuyama, Thin Solid Films 333 (1998) 60. [13] S. Yamauchi, T. Hariu, H. Ohba, K. Sawamura, Thin Solid Films 316 (1998) 93.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
High-temperature growth of heteroepitaxial InSb films on Si(111) substrate via the InSb bi-layer M. Mori a,, M. Saito b, K. Nagashima a, K. Ueda a, T. Yoshida a, K. Maezawa a a b
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan Venture Business Laboratory, University of Toyama, 3190 Gofuku, Toyama 930-8555, Japan
a r t i c l e in f o
a b s t r a c t
Available online 21 October 2008
To achieve the high-temperature growth of heteroepitaxial InSb films on the InSb bi-layer, we studied the influence of substrate temperature of first layer deposition (Ts1) on the two-step growth procedure. Although the growth at higher Ts1 of 240 and 280 1C is difficult to achieve using the usual procedure due to the desorption of In atoms from the InSb bi-layer, it can be realized by means of the adsorption of excess Sb atoms onto an initial InSb bi-layer prepared via O7 O3-In surface reconstruction. The hightemperature growth of 301-rotated InSb films at 420 1C was demonstrated on a Si(111) substrate with a InSb bi-layer. The electron mobility of the InSb film grown at 420 1C was about 20,000 cm2/V s at RT. & 2008 Elsevier B.V. All rights reserved.
PACS: 81.05.Ea 81.15.Hi 61.10.Nz Keywords: A1. X-ray diffraction A3. Molecular beam epitaxy B2. Semiconducting materials
1. Introduction Indium antimonide (InSb) has been widely noticed as a candidate material for ultra-fast, very-low-power device, because of the highest electron mobility and saturation velocity of all semiconductors. The heteroepitaxial growth of InSb on Si has attracted much interest from the viewpoint of integration of InSbbased devices and Si-LSI. However, it is very difficult to grow heteroepitaxial InSb on Si, because of the large lattice mismatch of about 19.3% between them. To solve this difficulty, another material, such as GaAs, has been often used to reduce the lattice mismatch [1–3]. We have paid attention to the surface reconstructions induced by In and Sb atoms on the Si substrate at the initial stage of the growth of InSb films [4–7], and found that the use of a suitable reconstructed surface by atoms composing InSb is a good candidate to solve the lattice mismatch problem [7,8]. We have reported the heteroepitaxial growth of 301-rotated InSb films on an InSb bi-layer (Si(111)-2 2-InSb surface reconstruction). The InSb bi-layer is prepared by the adsorption of 1 monolayer (ML) Sb atoms onto In-induced surface reconstruction such as O3 O3-In, 2 2-In [7] and O7 O3-In [8] on a Si(111) substrate. Due to this rotation, the large lattice mismatch between Si and InSb is nominally reduced to about 3.3%. However, a Si(111)-2 1-Sb surface phase was formed during the substitution process by the desorption of In atoms from the InSb bi-layer [9]. The InSb films grown on the 2 1-Sb surface phase do not rotate and have poor crystal quality. Because the area covered by
the 2 1-Sb surface phase depends on the initial In coverage on the Si surface, In-induced surface reconstruction with higher In coverage such as O7 O3-In is necessary for the growth of fully rotated InSb films with higher crystal quality [8]. The growth of InSb films on the InSb bi-layer is performed by a two-step growth procedure. In this procedure, the substrate was first held at 180–200 1C (Ts1) to grow the first layer to prevent the desorption of In atoms from the InSb bi-layer, and then rose to 350 1C for the growth of the second layer [7]. The growth temperature for the second layer (Ts2) is also restricted due to degradation of the first layer at higher temperature. The electron mobility of the 1.1 mm InSb film grown via O7 O3-In was about 16,300 cm2/V s at RT [8]. In order to grow the InSb films with higher electron mobility, it is necessary to increase the growth temperature for the improvement of crystal quality. The deterioration of the first layer at higher temperature may be caused by the temperature gap between Ts1 and Ts2. So, we tried to increase the Ts1 to prevent the deterioration of the first InSb layer at higher temperature. Many groups have reported high electron mobility InSb films grown at over 400 1C [1,2]. Our target temperature for Ts2 is 420 1C, which is the optimal temperature of the direct growth of InSb films on the Si(111) substrate for Ts2 in our laboratory. In the present paper, we report the trial of the increase of Ts1 and the heteroepitaxial growth of InSb films grown on the InSb bi-layer at a higher temperature of 420 1C. 2. Experiments
Corresponding author. Tel./fax: +8176 445 6728.
E-mail address: [email protected] (M. Mori). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.10.043
All the depositions were carried out in an OMICRON molecular beam epitaxy (MBE) chamber with a base pressure of about
ARTICLE IN PRESS M. Mori et al. / Journal of Crystal Growth 311 (2009) 1692–1695
2 10 8 Pa, equipped with reflection high-energy electron diffraction (RHEED). The substrate with a dimension of about 15 4 0.6 mm3 was obtained from mirror-polished p-type Si(111) wafer with a resistivity of about 20 O cm. The substrates were degassed at about 600 1C for 12 h, followed by a flushannealing at 1250 1C, and then slowly cooled in the chamber. This process gave a clean (7 7) surface, as confirmed by RHEED. Highpurity (6 N) elemental indium and antimony were used as source materials and evaporated from each PBN K-cell. The substrate temperature was monitored by an infrared pyrometer. Prior to the growth of InSb films, the InSb bi-layer was prepared by the following process. First, a Si(111)-O3 O3-In surface phase was prepared by the deposition of 0.33 ML-In atoms onto the clean Si(111)-7 7 surface at 450 1C. After cooling down to RT, additional In atoms of 0.87 ML (total In coverage was 1.2 ML) were deposited onto the O3 O3-In surface to obtain O7 O3-In surface reconstruction. The InSb bi-layer was obtained by 1 ML-Sb deposition onto this surface at about 180 1C. The growth of InSb films was then performed by the two-step growth procedure on the In-Sb bi-layer. In the procedure, the substrate temperature was first held at 200, 240 and 280 1C to grow the first InSb layer for 5 min, and then rose to 350 1C for the beginning of the second layer deposition. In the case of the samples grown at 240 and 280 1C, the Sb coverage for the preparation of the InSb bi-layer was increased to 3 ML, because of the appearance of the O3 O3-Sb phase during increasing
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substrate temperature to Ts1. It indicates the desorption of In atoms from the InSb bi-layer, and phase transition from the 2 1-Sb phase due to higher temperature. The InSb films grown on the O3 O3-Sb phase do not rotate similar to the 2 1-Sb phase. The thickness of the first layer was about 300 A˚. While elevating Ts for the second layer growth, the growth was interrupted by closing the shutter. The evolution of the RHEED pattern operating at 15 kV was observed in the /1¯ 1 0S and /2¯ 11S azimuths of the Si substrate during the deposition. The total film thickness measured by the interferometer was about 1.1 mm. For structural analysis, the InSb films were characterized by X-ray diffraction (XRD) using Cu-Ka1 radiation. In the XRD measurement, we took the f scan pattern to know the in-plane orientation of the film. In this case, the f axis was parallel to the normal direction of the Si(111) substrate, and the w axis was tilted by about 70.51 to detect the equivalent (111) planes ((11¯ 1), (1¯ 11), (111¯)), while keeping the 2y/o axis to the Bragg angle of InSb(111) or Si(111) peaks. The measurement of electric properties of the samples was carried out using the van der Pauw method.
3. Results and discussion Fig. 1 compares the RHEED patterns of InSb films after the first and the second layer deposition. These images were taken along
Fig. 1. RHEED patterns after first layer (on the left) and second layer (on the right) deposition at (a, b) 200 1C, (c, d) 240 1C and (e, f) 280 1C. These patterns were taken along the /1¯ 1 0S azimuth of the Si substrate.
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the /1¯ 1 0S azimuth of the Si substrate. As seen in Fig. 1(a, c and e), after the first layer deposition, the RHEED patterns showed a 2 2 periodicity with a wider line spacing. This wider line spacing in the /1¯ 1 0S azimuth indicates the 301-rotation of the films [7,8], because the RHEED pattern with the wider line spacing appears along the /2¯ 11S azimuth in the Si(111) substrate, and the angle between the /1¯ 1 0S and /2¯ 11S azimuths is 301. Fig. 1(c) shows some island patterns due to the large Sb coverage. The island patterns disappeared due to the desorption of Sb atoms from the Sb islands at higher Ts1, as shown in Fig. 1(e). The RHEED patterns after the second layer deposition show a clear 2 2 periodicity in all samples, indicating a highly ordered and smooth surface of the films. The spotty patterns shown by arrows are also observed in Fig. 1(d and f). These spots may come from small islands on the film surface. The results of the RHEED observation implied that the Sb islands prepared by the additional Sb atoms prevent the formation of the O3 O3-Sb phase, and lead to the successful growth of 301rotated InSb films at higher Ts1. Because the substitution of In and Sb atoms also occurs on the InSb bi-layer for the additional Sb atoms, there is the probability of the existence of the InSb bi-layer on top of the Sb islands. However, In atoms on the InSb bi-layer may not be able to exist any longer at higher temperature, especially at 280 1C. So, this may imply that the rotated InSb films were grown via the Sb islands. We will report the improvement of crystal orientation by additional In and/or Sb atoms onto the initial InSb bi-layer elsewhere. The InSb films showed a clear 301-rotated 2 2 pattern in the RHEED pattern. To confirm the results of RHEED observation, the XRD measurement (f scan) was performed. The f scan patterns of the (111) reflection of the samples are shown in Fig. 2. The peak positions of the Si substrate indicated by black circles are separated by 1201 intervals, indicating the three-fold symmetry of the Si(111) substrate. As shown in Fig. 2, all samples show only six intense peaks shifted by 7301 compared with those of Si(111) peaks, meaning fully 7301-rotated InSb. In spite of the growth at higher Ts1 of 240 and 280 1C, no unrotated peaks between the intense peaks are observed in Fig. 2(b and c). This means that the InSb bi-layer and/or Sb islands prepared by additional Sb atoms work effectively. We also carried out 2y/o scan of all samples (not shown here), and confirmed that all samples were heteroepitaxially grown and had no polycrystalline nature. The FWHM of InSb(111) peaks of almost all samples was about 350 arcsec, indicating good crystal quality. The wider FWHM of 520 arcsec was only shown in the sample grown at Ts1 of 240 1C. This sample showed the island patterns of Sb after the growth of the first layer. This result shows that the surface roughness of the initial stage of growth affects crystal quality of the film. This fact that the first layer can be grown at higher Ts1 for fully 301-rotated InSb encouraged us to increase Ts2. We tried to grow InSb films at Ts2 over 400 1C, and found the first layers grown at 240 and 280 1C also deteriorated at around 395 1C by observing the RHEED pattern. Although we could increase Ts2 from 350 to around 390 1C, the increase of Ts2 of 40 1C is much smaller than that of Ts1 of 80 1C. This result may indicate that the deterioration of the films was caused not only by temperature gap but also by other reasons. Murata et al. [11] reported that heteroepitaxial InSb films can be directly grown on Si(111) substrates by using a twostep growth procedure. Their optimum growth temperature was 240 1C for Ts1 and 420 1C for Ts2, respectively. The difference of the growth condition between these results is only the existence of the InSb bi-layer. The In atoms substituted by Sb are forced to float on top of the Sb ML [9]. The weak bonds between In and Sb of the InSb bi-layer may give rise to the deterioration of InSb films at higher Ts2.
Fig. 2. f scan patterns of the InSb films grown at Ts1 of (a) 200 1C, (b) 240 1C and (c) 280 1C.
Fig. 3. Temperature programming of the InSb film grown at a higher Ts2 of 420 1C.
The unsuccessful growth of InSb on the InSb bi-layer at higher Ts2 over 400 1C reminded us of the result of the heteroepitaxial InSb films grown via Ge islands [12]. In that case, after the first layer deposition at 200 1C, the substrate temperature was gradually increased to 400 1C during the second layer deposition. This growth method enabled the preparation of high crystal quality InSb at a higher temperature of 400 1C while preventing the films’ surface from roughening. We tried to apply the procedure to the growth of InSb film on the InSb bi-layer. The growth temperature program of the InSb film grown at 420 1C is shown in Fig. 3. The first InSb layer was grown at 240 1C for 5 min. Then the substrate temperature was elevated to 360 1C for the start of the second layer deposition. After starting the second layer deposition, the Ts2 was gradually increased with the increasing rate as shown in Fig. 3. The total growth time was 3 h. The film thickness of the grown InSb film was estimated to be 1.1 mm.
ARTICLE IN PRESS M. Mori et al. / Journal of Crystal Growth 311 (2009) 1692–1695
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Fig. 4. RHEED pattern of the InSb film grown at 420 1C. These patterns were taken along the /1¯ 1 0S and /2¯ 11S azimuths of the Si substrate.
from the improvement of crystal quality due to the hightemperature growth. The reason for this successful growth without deterioration at higher temperature is not clear. A possible reason is the grain size of InSb crystals before starting of the high-temperature growth over 400 1C.
4. Conclusion
Fig. 5. f scan pattern of the InSb film grown at 420 1C.
The RHEED patterns of the grown InSb taken along the /1¯ 1 0S and /2¯ 11S azimuths of the Si substrate are shown in Fig. 4. As shown in Fig. 4, a clear 2 2 periodicity was observed in both azimuths, indicating the highly ordered and smooth surface of the film. The relation between the beam incident azimuth and line spacing of the first-order streaks indicates 301-rotation of the grown InSb film [7,8,10]. This result means that the InSb bi-layer also functions effectively in this growth procedure. The result of RHEED observation encouraged us to measure the XRD patterns of the InSb film. Fig. 5 shows the f scan pattern of the (111) reflection of the InSb film. The black circles show the peak position of the Si substrate. The six intense peaks shifted by 7301 with respect to the Si substrate are shown in this pattern. This 301-rotation is consistent with the result of RHEED observation. There are no peaks between the intense peaks, indicating the fully 301-rotated InSb film. The XRD pattern of the 2y/o scan (not shown here) shows that the InSb film is heteroepitaxially grown by the new growth procedure, and has no polycrystalline nature. The FWHM of the film is about 350 arcsec, indicating good crystal quality. This value is narrower than that of the sample grown by using TS1 of 240 1C. These XRD results indicate that the quality InSb film was grown at a high Ts2 of 420 1C via the InSb bi-layer without any deterioration. The results of the RHEED observation and XRD measurement were just what we hoped. We carried out the measurement of the electrical properties of the InSb film in expectation of improvement of electron mobility. The electron mobility and carrier concentration of the InSb film at RT were about 20,000 cm2/V s and about 3.6 1016/cm3, respectively. The high electron mobility of the 1.1-mm-thick InSb film on Si is better than that reported before [7,8,13]. This high electron mobility of the film may result
We tried the high-temperature growth of the 301-rotated InSb films with an InSb bi-layer on a Si(111) substrate. The fully 301rotated InSb films are grown at a higher Ts1 of 240 and 280 1C with additional Sb atoms onto the initial InSb bi-layer. The electron mobility and carrier concentration of the 1.1-mm-thick InSb films grown at a higher temperature of 420 1C were 20,000 cm2/V s and 3.6 1016/cm3 at RT, respectively. It was achieved on a Si(111) substrate using mono molecular layer (bi-layer) by atoms composing InSb.
Acknowledgements A part of this work has been supported by a Grant-in-Aid Scientific Research (19760233, 19569003) of the Ministry of Education, Culture, Sports, Science and Technology, Japan, and Hitachi Kokusai Electric Inc., Japan.
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