- Email: [email protected]
Horizontal growth of epitaxial (1 0 0) β-FeSi2 templates by metal–organic chemical vapor deposition
ARTICLE IN PRESS
Journal of Crystal Growth 287 (2006) 694–697 www.elsevier.com/locate/jcrysgro
Horizontal growth of epitaxial (1 0 0) b-FeSi2 templates by metal–organic chemical vapor deposition Kensuke Akiyamaa,, Satoru Kanekoa, Yasuo Hirabayashia, Takashi Suemasub, Hiroshi Funakuboc a
Kanagawa Industrial Technology Research Institute, 705-1 Shimoimaizumi, Ebina-shi, Kanagawa 243-0435, Japan b Institute of Materials Science, University of Tsukuba, Tennohdai, Tsukuba, Ibaraki 305-8573, Japan c Department of Innovative and Engineered Materials, Tokyo Institute of Technology, G1-405, 4259 Nagatsuta-cho, Midori-ku, Yokohama 228-8505, Japan Available online 1 December 2005
Abstract An epitaxial b-FeSi2 thin film with continuous flat surface was grown by metal–organic chemical vapor deposition on (1 0 0) Si substrate with an aggregated b-FeSi2 template. The horizontal growth on the aggregated b-FeSi2 template made smaller voids among islands and became the continuous film in metal–organic chemical vapor deposition (MOCVD)-overgrowth process. The higher growth rate on the sidewall planes than on the (1 0 0) b-FeSi2 plane is considered to be the origin of the horizontal growth. r 2005 Elsevier B.V. All rights reserved. PACS: 68.55; 81.15; 81.15.Kk Keywords: A3. Metal–organic chemical vapor deposition; A3. Thin film; B1. b-FeSi2
1. Introduction Semiconducting b-FeSi2 has attracted much attention over the past decade as one of the promising materials for infrared optoelectronic devices. It emits light of about 1.55 mm suitable for silica optical fiber communications [1]. Moreover, it can be grown epitaxially on Si substrates: (1 0 0)-oriented and (1 1 0)/(1 0 1)-oriented b-FeSi2 are grown on (1 0 0) Si and (1 1 1) Si, respectively. Growth of high crystal quality b-FeSi2 film having a flat surface and a good hetero-junction with Si is essential for the use of this material in optoelectronic devices. However, an epitaxial b-FeSi2 film with a flat surface was reported to be difficult to grow by vapor deposition, because it aggregates into islands [2] or evolves into a-FeSi2 [3] or cubic g-FeSi2 [4]. The aggregation of the epitaxial bFeSi2 film is considered to be due to the lattice mismatch strain or the difference in surface energy between the film and the substrate [5,6]. Recently, it was reported that epitaxial b-FeSi2 film with flat surface was grown on (1 0 0) Corresponding author. Tel.: +81 46 236 1500; fax: +81 46 236 1525.
E-mail address: [email protected] (K. Akiyama). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.097
Si substrate [7,8] and (1 1 1) Si substrate [9,10] by vapor phase epitaxy using an epitaxial b-FeSi2 template prepared by solid phase epitaxy. A (1 0 0)-oriented b-FeSi2 was grown as a flat and continuous film, while the aggregated template showed the surface of the Si substrate [7]. On the other hand, a (1 1 1) Si substrate covered by a template over the whole surface was necessary to the formation of a (1 0 1)/(1 1 0)-oriented b-FeSi2 film with flat surface [10]. Therefore, the density of the template for making flat surface was different between (1 0 0)- and (1 0 1)/(1 1 0)oriented b-FeSi2 film. In this study, we investigated the growth mechanism of (1 0 0)-oriented b-FeSi2 thin films prepared on (1 0 0) Si by metal–organic chemical vapor deposition (MOCVD). 2. Experiments An n-type floating-zone (FZ) (1 0 0) Si substrate with a 20-nm-thick (1 0 0)-oriented b-FeSi2 template prepared by reactive deposition epitaxy (RDE) was used as a substrate. An ion pumped MBE system equipped with an electron gun evaporation source for iron was used for the b-FeSi2
ARTICLE IN PRESS K. Akiyama et al. / Journal of Crystal Growth 287 (2006) 694–697
template preparation. The base pressure of the MBE system was less than 1 10 10 Torr and it was kept below 3 10 9 Torr during the evaporation of iron. The deposition rate of iron and the substrate temperature were 0.6 nm/min and 470 1C, respectively. A continuous film of (1 0 0)-oriented b-FeSi2 film was grown epitaxially on the (1 0 0) Si substrate [11]. For b-FeSi2 deposition on this template by MOCVD (MOCVD-overgrowth), iron pentacarbonyl [Fe(CO)5] and silane (SiH4) were used as iron and silicon sources, respectively. A liquid Fe(CO)5 source was sealed in a bubbler and was carried to the reactor by H2 gas. The growth rate and substrate temperature were 3.3 nm/min and 750 1C, respectively. The operating pressure in the reactor was maintained at 1 10 3 Torr. The MOCVD reactor was vertical and cold-wall-type reactor with silicon carbide heater, which was homemade. The Si/Fe atomic ratios of the films were maintained at close to 2 by adjusting the input gas flow rates of the iron and silicon sources, and the ratios were cross-checked by X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectrometry (XRF) and Rutherford backscattering (RBS) using standard samples. The crystallographic structure of the films was characterized by the X-ray diffraction patterns (XRD, PaNalytical MRD) using CuKa radiation. The observation of microstructure and estimation of the film thickness were carried out by scanning electron microscopy (SEM). In addition, the average surface roughness (Ra) was estimated by atomic force microscopy (AFM). 3. Results and discussion
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Fig. 1. XRD y–2y spectra of MOCVD-overgrown films on (1 0 0)-oriented b-FeSi2 templates. MOCVD-overgrowth thickness: (a) 0 nm (annealed at 750 1C; the same process as the MOCVD-overgrowth without deposition); (b) 20 nm; (c) 100 nm, and (d) 200 nm.
Fig. 2. Dependence of rocking curve FWHM of b-FeSi2 800 diffraction peak on the total film thickness of MOCVD-overgrown film.
3.1. Formation of flat surface film Fig. 1 shows the XRD y–2y profiles for (a) 20-nm-thick template on (1 0 0) Si substrate and for MOCVD-overgrown layer thicknesses (b) 20, (c) 100 and (d) 200 on the template. All the films showed only diffraction peaks originating from (1 0 0) b-FeSi2, and those peak intensities increased as the MOCVD-overgrowth layer thickness increased. By pole figure measurement, an epitaxial growth of b-FeSi2 was ascertained for all films; (1 0 0) b-FeSi2J(1 0 0) Si with /0 1 0S//0 0 1S b-FeSi2J/0 1 1S Si. Moreover, the decrease of the full-width at halfmaximum (FWHM) of rocking curve corresponding to the b-FeSi2 800 diffraction peak shown in Fig. 2 suggests the increase of the degree of crystal orientation perfection as the MOCVD-overgrown layer thickness increased. Ra of MOCVD-overgrowth film is plotted in Fig. 3 as a function of total film thickness (corresponding to the sum of the thickness of the template plus MOCVD-overgrown layer). The initial Ra value of the 20-nm-thick template (asdeposited) was 1.5 nm, as shown in dashed line in Fig. 3, and increased to 19 nm by annealing at 750 1C. This is the same temperature as that used for the MOCVD-overgrowth. This shows the possibility that the b-FeSi2
Fig. 3. Dependence of Ra value on the total film thickness of MOCVDovergrown film. A dashed line indicates the value for the as-deposited 20-nm-thick template.
template had coalesced into islands before the MOCVD overgrowth started. However, the Ra value decreased almost linearly with film thickness increase, and reached about 4 nm at the total film thickness of 220 nm. Fig. 4 shows the plan-view and cross-sectional SEM images for (a) and (b) 20-nm-thickness b-FeSi2 template heat treated at 750 1C (the same process as the MOCVDovergrowth without deposition); (c) and (d) 20-nm-thick overgrown film; (e) and (f) 100-nm-thick overgrown film;
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K. Akiyama et al. / Journal of Crystal Growth 287 (2006) 694–697
Fig. 4. SEM images of the MOCVD-overgrown b-FeSi2 films on the templates. MOCVD-overgrowth thickness: (a),(b) 0 nm (after heat-treatment, corresponding to just before MOCVD-overgrowth); (c),(d) 20 nm; (e),(f) 100 nm; (g),(h) 200 nm.
and (g) and (h) 200-nm-thick overgrown film. In crosssectional observation, samples were observed from Si /0 1 0S (b-FeSi2 /1 1 0S//1 0 1S) direction. It is found that the 20-nm-thick template layer was aggregated into islands and the Si substrate was exposed partially, as shown Figs. 4(a) and (b), when it was heat treated at 750 1C. This is in good agreement with the previous report on the aggregation of the RDE template after annealing at 850 1C for 1 h in ultrahigh vacuum [12]. However, the size of these aggregated b-FeSi2 islands became larger as the thickness of the MOCVD-overgrown layer increased, and the voids between grains became smaller. At the edge of the b-FeSi2 islands, facet planes were formed as shown in Figs. 4(d) and (f), and the angle between the facet plane and film surface was about 501. Moreover, these voids disappeared after the 200 nm thick MOCVD-overgrowth, and a continuous and flat surface film was finally observed as shown in Figs. 4(g) and (h).
Fig. 5. Schematic diagram of crystal growth of MOCVD-overgrown b-FeSi2 films on aggregated templates.
3.2. Growth mechanism Fig. 5 shows a schematic diagram of the MOCVDovergrowth mechanism on aggregated (1 0 0) b-FeSi2 template. At the early stage of MOCVD-overgrowth, facet planes are present at the edges of aggregated b-FeSi2 grains. From the angle observed by the cross-sectional
SEM image, these facet planes may be assigned as (1 0 1) and (1 1 0) b-FeSi2 planes, whose angles to (1 0 0) b-FeSi2 plane are 51.81 and 52.01, respectively. Faster growth on these facet planes than on the (1 0 0) b-FeSi2 facets can be considered to reduce the voids between islands, eventually leading to a continuous film. After a continuous film with
ARTICLE IN PRESS K. Akiyama et al. / Journal of Crystal Growth 287 (2006) 694–697
smooth surface has resulted from the coalescence of grains, the MOCVD-overgrown film is considered to grow only vertically. Such horizontal growth was not observed in overgrowth on an (1 0 0) b-FeSi2 template by MBE [13]: the film became rough with increasing growth temperature, and clear aggregation was observed for the b-FeSi2 grown at 700 1C or higher temperatures, in spite of the fact that the MBE-overgrown films grown at 470 1C showed a smooth surface. In MOCVD-overgrowth, the H2 that is used as a carrier gas may enhance the migration of iron and silicon species at the surface of b-FeSi2 grains. Such a horizontal growth of epitaxial film has been reported as epitaxial lateral overgrowth by metal–organic vapor phase epitaxy of GaAs [14] or GaN [15]. The reason for the horizontal growth on aggregated (1 0 0) b-FeSi2 islands may be the same as that for the epitaxial lateral overgrowth: the bFeSi2 facet planes appearing at the early stage of MOCVDovergrowth might be atomically rough surfaces with higher densities of steps or kinks than the (1 0 0) b-FeSi2 plane. The rough surface and higher density of step or kink generally result in an increase of trapping probability. Actually, this horizontal growth was not observed in case of (1 0 1)/(1 1 0)-oriented b-FeSi2 films overgrown by MOCVD, as the authors previously reported [10]. This can be attributed to the higher growth rate on the (1 0 1)/ (1 1 0)-planes than on other planes. 4. Conclusion We confirmed the formation of (1 0 0)-oriented b-FeSi2 films with flat surfaces by MOCVD on aggregated b-FeSi2 template layers. Horizontal growth on the aggregated templates formed continuous and smooth surface films.
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The facet planes at the edges of the islands of the aggregated template have higher growth rate, than the (1 0 0) b-FeSi2 plane. This higher growth rate of the sidewall planes is considered to be the origin of the horizontal growth.
References [1] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [2] J.E. Mahan, K.M. Geib, G.Y. Robinson, R.G. Lomg, Y. Xinghua, G. Bai, M.-A. Nicolet, M. Nathan, Appl. Phys. Lett. 56 (1990) 2126. [3] J. Chevrier, P. Stocker, L.T. Vinh, J.M. Gay, J. Derrien, Europhys. Lett. 22 (1993) 449. [4] H.-U. Nissen, E. Mu¨ller, H.R. Deller, H. von Ka¨nel, Phys. Stat. Sol. A 150 (1995) 395. [5] D.J. Eaglemann, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [6] T. Suemasu, F. Hasegawa, J. Jpn. Assoc. Crystal Growth 25 (1998) 46. [7] K. Akiyama, T. Kimura, T. Suemasu, F. Hasegawa, Y. Maeda, H. Funakubo, Jpn. J. Appl. Phys. 43 (2004) L551. [8] T. Suemasu, N. Hiroi, T. Fujii, K. Takakura, F. Hasegawa, Jpn. J. Appl. Phys. 38 (1999) L878. [9] T. Suemasu, M. Tatauji, C. Li, Y. Ozawa, M. Ichida, F. Hasegawa, Jpn. J. Appl. Phys. 43 (2004) L930. [10] K. Akiyama, S. Kaneko, Y. Hirabayashi, H. Funakubo, J. Crystal Growth, accepted. [11] M. Tanaka, Y. Kumagai, T. Suemasu, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) 3620. [12] T. Suemasu, M. Tanaka, T. Fujii, S. Hashimoto, Y. Kumagai, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) L1225. [13] N. Hiroi, T. Suemasu, K. Takakura, N. Seki, F. Hasegawa, Jpn. J. Appl. Phys. 40 (2001) L1008. [14] T. Nishinaga, T. Nakano, S. Zhang, Jpn. J. Appl. Phys. 27 (1988) L964. [15] A. Kimura, C. Sasaoka, A. Sakai, A. Usui, Mater. Res. Soc. Symp. Proc. 482 (1998) 119.
Journal of Crystal Growth 287 (2006) 694–697 www.elsevier.com/locate/jcrysgro
Horizontal growth of epitaxial (1 0 0) b-FeSi2 templates by metal–organic chemical vapor deposition Kensuke Akiyamaa,, Satoru Kanekoa, Yasuo Hirabayashia, Takashi Suemasub, Hiroshi Funakuboc a
Kanagawa Industrial Technology Research Institute, 705-1 Shimoimaizumi, Ebina-shi, Kanagawa 243-0435, Japan b Institute of Materials Science, University of Tsukuba, Tennohdai, Tsukuba, Ibaraki 305-8573, Japan c Department of Innovative and Engineered Materials, Tokyo Institute of Technology, G1-405, 4259 Nagatsuta-cho, Midori-ku, Yokohama 228-8505, Japan Available online 1 December 2005
Abstract An epitaxial b-FeSi2 thin film with continuous flat surface was grown by metal–organic chemical vapor deposition on (1 0 0) Si substrate with an aggregated b-FeSi2 template. The horizontal growth on the aggregated b-FeSi2 template made smaller voids among islands and became the continuous film in metal–organic chemical vapor deposition (MOCVD)-overgrowth process. The higher growth rate on the sidewall planes than on the (1 0 0) b-FeSi2 plane is considered to be the origin of the horizontal growth. r 2005 Elsevier B.V. All rights reserved. PACS: 68.55; 81.15; 81.15.Kk Keywords: A3. Metal–organic chemical vapor deposition; A3. Thin film; B1. b-FeSi2
1. Introduction Semiconducting b-FeSi2 has attracted much attention over the past decade as one of the promising materials for infrared optoelectronic devices. It emits light of about 1.55 mm suitable for silica optical fiber communications [1]. Moreover, it can be grown epitaxially on Si substrates: (1 0 0)-oriented and (1 1 0)/(1 0 1)-oriented b-FeSi2 are grown on (1 0 0) Si and (1 1 1) Si, respectively. Growth of high crystal quality b-FeSi2 film having a flat surface and a good hetero-junction with Si is essential for the use of this material in optoelectronic devices. However, an epitaxial b-FeSi2 film with a flat surface was reported to be difficult to grow by vapor deposition, because it aggregates into islands [2] or evolves into a-FeSi2 [3] or cubic g-FeSi2 [4]. The aggregation of the epitaxial bFeSi2 film is considered to be due to the lattice mismatch strain or the difference in surface energy between the film and the substrate [5,6]. Recently, it was reported that epitaxial b-FeSi2 film with flat surface was grown on (1 0 0) Corresponding author. Tel.: +81 46 236 1500; fax: +81 46 236 1525.
E-mail address: [email protected] (K. Akiyama). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.10.097
Si substrate [7,8] and (1 1 1) Si substrate [9,10] by vapor phase epitaxy using an epitaxial b-FeSi2 template prepared by solid phase epitaxy. A (1 0 0)-oriented b-FeSi2 was grown as a flat and continuous film, while the aggregated template showed the surface of the Si substrate [7]. On the other hand, a (1 1 1) Si substrate covered by a template over the whole surface was necessary to the formation of a (1 0 1)/(1 1 0)-oriented b-FeSi2 film with flat surface [10]. Therefore, the density of the template for making flat surface was different between (1 0 0)- and (1 0 1)/(1 1 0)oriented b-FeSi2 film. In this study, we investigated the growth mechanism of (1 0 0)-oriented b-FeSi2 thin films prepared on (1 0 0) Si by metal–organic chemical vapor deposition (MOCVD). 2. Experiments An n-type floating-zone (FZ) (1 0 0) Si substrate with a 20-nm-thick (1 0 0)-oriented b-FeSi2 template prepared by reactive deposition epitaxy (RDE) was used as a substrate. An ion pumped MBE system equipped with an electron gun evaporation source for iron was used for the b-FeSi2
ARTICLE IN PRESS K. Akiyama et al. / Journal of Crystal Growth 287 (2006) 694–697
template preparation. The base pressure of the MBE system was less than 1 10 10 Torr and it was kept below 3 10 9 Torr during the evaporation of iron. The deposition rate of iron and the substrate temperature were 0.6 nm/min and 470 1C, respectively. A continuous film of (1 0 0)-oriented b-FeSi2 film was grown epitaxially on the (1 0 0) Si substrate [11]. For b-FeSi2 deposition on this template by MOCVD (MOCVD-overgrowth), iron pentacarbonyl [Fe(CO)5] and silane (SiH4) were used as iron and silicon sources, respectively. A liquid Fe(CO)5 source was sealed in a bubbler and was carried to the reactor by H2 gas. The growth rate and substrate temperature were 3.3 nm/min and 750 1C, respectively. The operating pressure in the reactor was maintained at 1 10 3 Torr. The MOCVD reactor was vertical and cold-wall-type reactor with silicon carbide heater, which was homemade. The Si/Fe atomic ratios of the films were maintained at close to 2 by adjusting the input gas flow rates of the iron and silicon sources, and the ratios were cross-checked by X-ray photoelectron spectroscopy (XPS), X-ray fluorescence spectrometry (XRF) and Rutherford backscattering (RBS) using standard samples. The crystallographic structure of the films was characterized by the X-ray diffraction patterns (XRD, PaNalytical MRD) using CuKa radiation. The observation of microstructure and estimation of the film thickness were carried out by scanning electron microscopy (SEM). In addition, the average surface roughness (Ra) was estimated by atomic force microscopy (AFM). 3. Results and discussion
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Fig. 1. XRD y–2y spectra of MOCVD-overgrown films on (1 0 0)-oriented b-FeSi2 templates. MOCVD-overgrowth thickness: (a) 0 nm (annealed at 750 1C; the same process as the MOCVD-overgrowth without deposition); (b) 20 nm; (c) 100 nm, and (d) 200 nm.
Fig. 2. Dependence of rocking curve FWHM of b-FeSi2 800 diffraction peak on the total film thickness of MOCVD-overgrown film.
3.1. Formation of flat surface film Fig. 1 shows the XRD y–2y profiles for (a) 20-nm-thick template on (1 0 0) Si substrate and for MOCVD-overgrown layer thicknesses (b) 20, (c) 100 and (d) 200 on the template. All the films showed only diffraction peaks originating from (1 0 0) b-FeSi2, and those peak intensities increased as the MOCVD-overgrowth layer thickness increased. By pole figure measurement, an epitaxial growth of b-FeSi2 was ascertained for all films; (1 0 0) b-FeSi2J(1 0 0) Si with /0 1 0S//0 0 1S b-FeSi2J/0 1 1S Si. Moreover, the decrease of the full-width at halfmaximum (FWHM) of rocking curve corresponding to the b-FeSi2 800 diffraction peak shown in Fig. 2 suggests the increase of the degree of crystal orientation perfection as the MOCVD-overgrown layer thickness increased. Ra of MOCVD-overgrowth film is plotted in Fig. 3 as a function of total film thickness (corresponding to the sum of the thickness of the template plus MOCVD-overgrown layer). The initial Ra value of the 20-nm-thick template (asdeposited) was 1.5 nm, as shown in dashed line in Fig. 3, and increased to 19 nm by annealing at 750 1C. This is the same temperature as that used for the MOCVD-overgrowth. This shows the possibility that the b-FeSi2
Fig. 3. Dependence of Ra value on the total film thickness of MOCVDovergrown film. A dashed line indicates the value for the as-deposited 20-nm-thick template.
template had coalesced into islands before the MOCVD overgrowth started. However, the Ra value decreased almost linearly with film thickness increase, and reached about 4 nm at the total film thickness of 220 nm. Fig. 4 shows the plan-view and cross-sectional SEM images for (a) and (b) 20-nm-thickness b-FeSi2 template heat treated at 750 1C (the same process as the MOCVDovergrowth without deposition); (c) and (d) 20-nm-thick overgrown film; (e) and (f) 100-nm-thick overgrown film;
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Fig. 4. SEM images of the MOCVD-overgrown b-FeSi2 films on the templates. MOCVD-overgrowth thickness: (a),(b) 0 nm (after heat-treatment, corresponding to just before MOCVD-overgrowth); (c),(d) 20 nm; (e),(f) 100 nm; (g),(h) 200 nm.
and (g) and (h) 200-nm-thick overgrown film. In crosssectional observation, samples were observed from Si /0 1 0S (b-FeSi2 /1 1 0S//1 0 1S) direction. It is found that the 20-nm-thick template layer was aggregated into islands and the Si substrate was exposed partially, as shown Figs. 4(a) and (b), when it was heat treated at 750 1C. This is in good agreement with the previous report on the aggregation of the RDE template after annealing at 850 1C for 1 h in ultrahigh vacuum [12]. However, the size of these aggregated b-FeSi2 islands became larger as the thickness of the MOCVD-overgrown layer increased, and the voids between grains became smaller. At the edge of the b-FeSi2 islands, facet planes were formed as shown in Figs. 4(d) and (f), and the angle between the facet plane and film surface was about 501. Moreover, these voids disappeared after the 200 nm thick MOCVD-overgrowth, and a continuous and flat surface film was finally observed as shown in Figs. 4(g) and (h).
Fig. 5. Schematic diagram of crystal growth of MOCVD-overgrown b-FeSi2 films on aggregated templates.
3.2. Growth mechanism Fig. 5 shows a schematic diagram of the MOCVDovergrowth mechanism on aggregated (1 0 0) b-FeSi2 template. At the early stage of MOCVD-overgrowth, facet planes are present at the edges of aggregated b-FeSi2 grains. From the angle observed by the cross-sectional
SEM image, these facet planes may be assigned as (1 0 1) and (1 1 0) b-FeSi2 planes, whose angles to (1 0 0) b-FeSi2 plane are 51.81 and 52.01, respectively. Faster growth on these facet planes than on the (1 0 0) b-FeSi2 facets can be considered to reduce the voids between islands, eventually leading to a continuous film. After a continuous film with
ARTICLE IN PRESS K. Akiyama et al. / Journal of Crystal Growth 287 (2006) 694–697
smooth surface has resulted from the coalescence of grains, the MOCVD-overgrown film is considered to grow only vertically. Such horizontal growth was not observed in overgrowth on an (1 0 0) b-FeSi2 template by MBE [13]: the film became rough with increasing growth temperature, and clear aggregation was observed for the b-FeSi2 grown at 700 1C or higher temperatures, in spite of the fact that the MBE-overgrown films grown at 470 1C showed a smooth surface. In MOCVD-overgrowth, the H2 that is used as a carrier gas may enhance the migration of iron and silicon species at the surface of b-FeSi2 grains. Such a horizontal growth of epitaxial film has been reported as epitaxial lateral overgrowth by metal–organic vapor phase epitaxy of GaAs [14] or GaN [15]. The reason for the horizontal growth on aggregated (1 0 0) b-FeSi2 islands may be the same as that for the epitaxial lateral overgrowth: the bFeSi2 facet planes appearing at the early stage of MOCVDovergrowth might be atomically rough surfaces with higher densities of steps or kinks than the (1 0 0) b-FeSi2 plane. The rough surface and higher density of step or kink generally result in an increase of trapping probability. Actually, this horizontal growth was not observed in case of (1 0 1)/(1 1 0)-oriented b-FeSi2 films overgrown by MOCVD, as the authors previously reported [10]. This can be attributed to the higher growth rate on the (1 0 1)/ (1 1 0)-planes than on other planes. 4. Conclusion We confirmed the formation of (1 0 0)-oriented b-FeSi2 films with flat surfaces by MOCVD on aggregated b-FeSi2 template layers. Horizontal growth on the aggregated templates formed continuous and smooth surface films.
697
The facet planes at the edges of the islands of the aggregated template have higher growth rate, than the (1 0 0) b-FeSi2 plane. This higher growth rate of the sidewall planes is considered to be the origin of the horizontal growth.
References [1] D. Leong, M. Harry, K.J. Reeson, K.P. Homewood, Nature 387 (1997) 686. [2] J.E. Mahan, K.M. Geib, G.Y. Robinson, R.G. Lomg, Y. Xinghua, G. Bai, M.-A. Nicolet, M. Nathan, Appl. Phys. Lett. 56 (1990) 2126. [3] J. Chevrier, P. Stocker, L.T. Vinh, J.M. Gay, J. Derrien, Europhys. Lett. 22 (1993) 449. [4] H.-U. Nissen, E. Mu¨ller, H.R. Deller, H. von Ka¨nel, Phys. Stat. Sol. A 150 (1995) 395. [5] D.J. Eaglemann, M. Cerullo, Phys. Rev. Lett. 64 (1990) 1943. [6] T. Suemasu, F. Hasegawa, J. Jpn. Assoc. Crystal Growth 25 (1998) 46. [7] K. Akiyama, T. Kimura, T. Suemasu, F. Hasegawa, Y. Maeda, H. Funakubo, Jpn. J. Appl. Phys. 43 (2004) L551. [8] T. Suemasu, N. Hiroi, T. Fujii, K. Takakura, F. Hasegawa, Jpn. J. Appl. Phys. 38 (1999) L878. [9] T. Suemasu, M. Tatauji, C. Li, Y. Ozawa, M. Ichida, F. Hasegawa, Jpn. J. Appl. Phys. 43 (2004) L930. [10] K. Akiyama, S. Kaneko, Y. Hirabayashi, H. Funakubo, J. Crystal Growth, accepted. [11] M. Tanaka, Y. Kumagai, T. Suemasu, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) 3620. [12] T. Suemasu, M. Tanaka, T. Fujii, S. Hashimoto, Y. Kumagai, F. Hasegawa, Jpn. J. Appl. Phys. 36 (1997) L1225. [13] N. Hiroi, T. Suemasu, K. Takakura, N. Seki, F. Hasegawa, Jpn. J. Appl. Phys. 40 (2001) L1008. [14] T. Nishinaga, T. Nakano, S. Zhang, Jpn. J. Appl. Phys. 27 (1988) L964. [15] A. Kimura, C. Sasaoka, A. Sakai, A. Usui, Mater. Res. Soc. Symp. Proc. 482 (1998) 119.