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Uniform formation process of self-organized InAs quantum dots
Journal of Crystal Growth 237–239 (2002) 1301–1306
Uniform formation process of self-organized InAs quantum dots Koichi Yamaguchi*, Toshiyuki Kaizu, Kunihiko Yujobo, Yoshikuni Saito Department of Electronic Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan
Abstract Uniform InAs quantum dots (QDs) were successfully grown by molecular beam epitaxy using the conventional Stranski–Krastanov growth mode. Two self-size-limiting (SSL) behaviors of the island size played an important role in achieving the uniform formation of the InAs QDs. In the initial growth stage of the 2-dimensional (2D) InAs platelets, the compressive strain at the step edge of the 2D islands induced the first SSL behavior of their lateral size. The growth mode transition from 2D to 3D rapidly occurred around 20 nm in the lateral size. The 3D dots were surrounded by four {1 3 6} facets, which provided the second SSL feature. The uniform InAs QDs, grown through two SSL mechanisms, revealed a narrow photoluminescence linewidth of o20 meV. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Hi; 85.40.Ux; 68.35.Bs Keywords: A1. Nanostructures; A3. Molecular beam epitaxy; B1. Arsenates; B2. Semiconducting III-V materials
1. Introduction The Stranski–Krastanov (SK) growth mode using lattice mismatch has great ability in the fabrication of coherent quantum dots (QDs), such as InAs/GaAs system [1]. However, the formation mechanism of SK QDs has not been understood clearly. In addition, large fluctuation of the dot size frequently appears in the conventional SK growth. So far, various growth techniques have been developed to reduce inhomogeneous broadening of the dot size: stacked growth [2], punctuated growth [3] and strain-reduced capping growth [4]. However, we have recently demonstrated uniform InAs QDs by molecular beam
epitaxy (MBE) using conventional SK growth mode under low arsenic pressure and low growth rate conditions [5,6]. Suppression of the size fluctuation is attributed to the self-size-limiting (SSL) behavior during the island formation. In this work, the uniform InAs QDs were grown by MBE via SK growth mode, and the uniform formation process of the InAs 2-dimensional (2D) and 3D islands was investigated. Two SSL behaviors in the 2D and 3D island formation are important processes for getting the narrow size distribution and provide a sharp photoluminescence (PL) linewidth of o20 meV.
2. Experimental procedure *Corresponding author. Tel.: +81-424-43-5149; fax: +81424-43-5149. E-mail address: [email protected] (K. Yamaguchi).
Following a 200-nm-thick GaAs buffer layer, coherently strained InAs dots were grown on semi-
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 5 1 - 6
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insulating GaAs(0 0 1) substrates by conventional solid-source MBE at 5001C. The InAs coverage was changed from 1.2 to 3.5 monolayers (ML). The substrate temperature was calibrated by using the Ga-oxide desorption temperature of 5801C as a reference. The arsenic beam equivalent pressure was changed from 3 10 7 to 6 10 7 Torr. The InAs growth rate of 0.17 ML/s (or 0.035 ML/s) was calibrated based on the critical thickness of the SK-growth mode transition from 2D to 3D, which was measured by using reflection highenergy electron diffraction (RHEED). The RHEED patterns after growing InAs dots were also studied minutely as a function of the azimuth of the sample. The samples grown for PL measurements were capped with a 100-nm-thick GaAs layer grown at 4501C under an arsenic pressure of 6 10 7 Torr. For PL studies, Ar+ laser was used for excitation (25 mW) and the sample temperature was changed from 14 to 300 K. The luminescence was detected using a cooled InGaAs photodiode. The uncapped InAs dots were evaluated using an atomic force microscope (AFM) in the contact mode.
3. Results and discussion 3.1. 2D platelets In order to achieve high uniformity in the dot size, it is important to enhance the surface migration of the indium adatoms. As reported previously, the low arsenic pressure and the low growth rate conditions provided the long migration length and resulted in the narrow size distribution of the InAs dot [5]. Fig. 1 shows a typical AFM image of the InAs islands with 1.8 ML coverage, grown under low arsenic pressure (6 10 7 Torr) and low growth rate (0.17 ML/s) conditions. In this image, many 2D islands (labeled A) and many small 3D dots (labeled B) appear on the top of a step edge of the InAs wetting layer. The step height is about 1 ML. The top of a step edge acts as a preferential site for the island formation because the strain of the islands, formed on the top of a step edge, is more released. The different energetic stability of
Fig. 1. AFM image of the InAs 2D islands with the indium coverage of 1.8 ML, arsenic pressure is 6 10 7 Torr, and the growth rate is 0.17 ML/s.
the island formation due to a different nucleation site induces inhomogeneous broadening of the island size. Actually, there is a tendency that the size of the islands formed on the step edge is larger than that on the terrace area. Fig. 2 shows the relationship between the height and the [1 1 0] lateral size of the InAs islands, grown at an arsenic pressure of 6 10 7 Torr and a growth rate of 0.17 ML/s. The InAs coverage was changed from 1.6 to 2.5 ML. When the InAs coverage was more than about 1.8 ML, the growth mode rapidly transited from the 2D growth to the 3D growth. In Fig. 2, region I corresponds to the formation of the 2D islands (platelets) with a constant height of 2 or 3 ML. As the growth proceeds, the 2D platelets quickly change to the 3D pyramidal dots, which are denoted as region II. In addition, one can see that the growth mode transition from 2D to 3D occurs around 20 nm in the lateral size of the 2D platelets. That is, the lateral size of the 2D platelets is limited at about 20 nm. As the lateral size of the 2D platelet increases, the strain energy becomes high because of the misfit. Particularly, the strain energy of adatom is emphasized at the island edge, compared with that at the terrace sites [7]. Therefore,
K. Yamaguchi et al. / Journal of Crystal Growth 237–239 (2002) 1301–1306
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Fig. 3. AFM image of the InAs 3D island (a) and its schematic top and (1 3 0) cross-sectional views (b).
Fig. 2. Relationship between the height and the [1 1 0] lateral size of the InAs islands, arsenic pressure is 6 10 7 Torr, and the growth rate is 0.17 ML/s.
adatom attachment probability at the 2D-platelet edge is suppressed for the large platelets, and as a result, the lateral size of the platelet is automatically limited by the compressive strain at the island edge. Such SSL behavior induces the narrow size distribution. After this SSL process, the height of the platelets increases rapidly, and then the 3D dots are formed. In order to further reduce the size distribution of the 2D platelets, it is needed to control the nucleation site (i.e. the top of a step edge) and to enhance the surface migration of indium adatoms for finding the preferential incorporation site. Needless to say, the narrow size distribution of the 2D platelets will provide the formation of the uniform 3D dots. 3.2. 3D pyramidal dots In region II of Fig. 2, almost every one of the large 3D dots has a similar pyramidal shape, and its aspect ratio is almost constant with a value of 0.24, which is related with the lateral size along the [1 1 0] direction. Fig. 3 shows a typical AFM image of the 3D dot (a) and its schematic top and crosssectional views (b). The four facets appear on the side walls of the dot. From the [3 1% 0] crosssectional AFM images, it was found that a tilt
Fig. 4. RHEED patterns just after the 3D island formation, taken along the [1% 3 0] azimuth.
angle of the facet is about 321. In addition, RHEED pattern for the [3 1% 0] azimuth, just after the dot formation, shows chevrons directed at an angle of 291 from the [0 0 1] direction as shown in Fig. 4. These structural data of the 3D dot reveal the evidence of {1 3 6} facets. When the InAs coverage is more than about 2 ML, the {1 3 6} facets appear on the side walls. In this situation, the improvement of the dot size suddenly stops except incoherent dots due to coalescence. Fig. 5 shows the average lateral size (a) and average height (b) of the coherent 3D dots as a function of the InAs coverage. The lateral size and height are clearly limited at more than about 2 ML coverage. Therefore, the above SSL behavior of the 3D dots is caused by the facet formation of the {1 3 6} plane. Once the stable facet is formed on the side wall, indium adatoms are hardly incorporated at the facet. The SSL behavior during the dot formation results in high uniformity in the dot size and shape. The detailed surface structure of the {1 3 6} facet has not been clarified, but it is
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Fig. 6. Schematic diagrams of the formation process of the InAs 3D islands.
3.3. Uniform InAs QDs Fig. 5. Average lateral size (a) and average height (b) of the InAs 3D islands as a function of the InAs coverage. Arsenic pressure is 6 10 7 Torr, and the growth rate is 0.17 ML/s.
possible to consider that a combination of a few low Miller index planes constructs the energetically stable surface as the {1 3 6} facets. From the above results, the dot formation process is schematically shown in Fig. 6. At the initial growth stage, the 2D platelets are formed, and the lateral size is limited by the compressive strain at the island edge. Following the SSL of the 2D platelets the height rapidly increases. In this stage, the stable facet starts to form on the side walls of the 3D islands. The {1 3 6} facet appears and then covers the side wall as a pyramidal dot. Here the second SSL mechanism due to the facet formation happens everywhere. If the surface migration of indium adatoms is enhanced, two SSL behaviors smoothly occur and suppress the size fluctuation of the dots.
The SSL feature has been often observed for the dot formation under the low arsenic pressure and low growth rate conditions. We have demonstrated the formation of the uniform InAs QDs grown at a low arsenic pressure of 3 10 7 Torr and a low growth rate of 0.035 ML/s [5]. Fig. 7 shows the AFM image of their uniform QDs. The dot densities of the coherent and coalescent dots are 3.5 1010 and 4.7 108 cm 2, respectively. Standard deviation was 1.7 nm (about 4%) for the lateral size and 0.7 nm (about 8%) for the height. After the GaAs capping growth, the PL spectrum from their QDs was measured. Fig. 8 shows the typical PL spectrum (a) and the temperature dependence of the PL linewidth (b). The PL spectrum fitted with a Gaussian curve has a peak energy position of 1.04 eV and the narrowest linewidth of 18.6 meV at 14 K. The narrow PL linewidth is almost maintained from 14 K to room temperature. At room temperature, the wavelength of the PL peak was 1.28 mm.
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Fig. 7. AFM image of the uniform InAs dots grown at an arsenic pressure of 3 10 7 Torr and a growth rate of 0.035 ML/s.
The capping growth of the GaAs is also an important process to decide the inhomogeneous broadening of the dot size. Recently, we observed that during the capping growth the size shrinkage of the InAs QDs is enhanced for the large dots. Therefore, high uniformity in the dot size is further expected by the controlled capping growth. The results of a detailed investigation of this process will be reported elsewhere [8].
Fig. 8. PL spectrum of uniform InAs QDs (a) and the temperature dependence of the PL linewidth.
4. Conclusion The uniform formation process of the InAs islands was investigated in the MBE growth via SK mode under the low arsenic pressure and low growth rate conditions. The growth mode rapidly transited from 2D to 3D at the critical InAs coverage. During each formation process of the 2D and 3D islands, the SSL behavior was clearly observed for the above growth conditions. The first SSL feature of the 2D platelets was attributed to compressive strain at the island edge. The second SSL mechanism was caused by the facet formation of the 3D pyramidal dots. Both SSL
behaviors played an important role in achieving the narrow size distribution of the InAs QDs. As a result, a narrow PL linewidth of o20 meV was successfully obtained.
References [1] D. Leonard, K. Pond, P.M. Petroff, Phys. Rev. B 50 (1994) 11687. [2] N.N. Ledentsov, J. Bohrer, D. Bimberg, I.V. Kochnev, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, A.O. Kosogov, S.S. Ruvimov, P. Werner, U. Gosele, Appl. Phys. Lett. 69 (1996) 1095.
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[3] I. Mukhametzhanov, Z. Wei, R. Heitz, A. Madhukar, Appl. Phys. Lett. 75 (1999) 85. [4] K. Nishi, H. Saito, S. Sugou, J-S. Lee, Appl. Phys. Lett. 74 (1999) 1111. [5] K. Yamaguchi, K. Yujobo, T. Kaizu, Jpn. J. Appl. Phys. 39 (2000) L1245.
[6] T. Kaizu, K. Yamaguchi, Jpn. J. Appl. Phys. 40 (2001) 1885. [7] A-L. Barabasi, Appl. Phys. Lett. 70 (1997) 2565. [8] K. Yamaguchi, Y. Saito, R. Ohtsubo, Eighth International Symposium on the Formation of Semiconductor Interfaces, 2001.
Uniform formation process of self-organized InAs quantum dots Koichi Yamaguchi*, Toshiyuki Kaizu, Kunihiko Yujobo, Yoshikuni Saito Department of Electronic Engineering, University of Electro-Communications, 1-5-1 Chofugaoka, Chofu-shi, Tokyo 182-8585, Japan
Abstract Uniform InAs quantum dots (QDs) were successfully grown by molecular beam epitaxy using the conventional Stranski–Krastanov growth mode. Two self-size-limiting (SSL) behaviors of the island size played an important role in achieving the uniform formation of the InAs QDs. In the initial growth stage of the 2-dimensional (2D) InAs platelets, the compressive strain at the step edge of the 2D islands induced the first SSL behavior of their lateral size. The growth mode transition from 2D to 3D rapidly occurred around 20 nm in the lateral size. The 3D dots were surrounded by four {1 3 6} facets, which provided the second SSL feature. The uniform InAs QDs, grown through two SSL mechanisms, revealed a narrow photoluminescence linewidth of o20 meV. r 2002 Elsevier Science B.V. All rights reserved. PACS: 81.05.Ea; 81.15.Hi; 85.40.Ux; 68.35.Bs Keywords: A1. Nanostructures; A3. Molecular beam epitaxy; B1. Arsenates; B2. Semiconducting III-V materials
1. Introduction The Stranski–Krastanov (SK) growth mode using lattice mismatch has great ability in the fabrication of coherent quantum dots (QDs), such as InAs/GaAs system [1]. However, the formation mechanism of SK QDs has not been understood clearly. In addition, large fluctuation of the dot size frequently appears in the conventional SK growth. So far, various growth techniques have been developed to reduce inhomogeneous broadening of the dot size: stacked growth [2], punctuated growth [3] and strain-reduced capping growth [4]. However, we have recently demonstrated uniform InAs QDs by molecular beam
epitaxy (MBE) using conventional SK growth mode under low arsenic pressure and low growth rate conditions [5,6]. Suppression of the size fluctuation is attributed to the self-size-limiting (SSL) behavior during the island formation. In this work, the uniform InAs QDs were grown by MBE via SK growth mode, and the uniform formation process of the InAs 2-dimensional (2D) and 3D islands was investigated. Two SSL behaviors in the 2D and 3D island formation are important processes for getting the narrow size distribution and provide a sharp photoluminescence (PL) linewidth of o20 meV.
2. Experimental procedure *Corresponding author. Tel.: +81-424-43-5149; fax: +81424-43-5149. E-mail address: [email protected] (K. Yamaguchi).
Following a 200-nm-thick GaAs buffer layer, coherently strained InAs dots were grown on semi-
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 2 0 5 1 - 6
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K. Yamaguchi et al. / Journal of Crystal Growth 237–239 (2002) 1301–1306
insulating GaAs(0 0 1) substrates by conventional solid-source MBE at 5001C. The InAs coverage was changed from 1.2 to 3.5 monolayers (ML). The substrate temperature was calibrated by using the Ga-oxide desorption temperature of 5801C as a reference. The arsenic beam equivalent pressure was changed from 3 10 7 to 6 10 7 Torr. The InAs growth rate of 0.17 ML/s (or 0.035 ML/s) was calibrated based on the critical thickness of the SK-growth mode transition from 2D to 3D, which was measured by using reflection highenergy electron diffraction (RHEED). The RHEED patterns after growing InAs dots were also studied minutely as a function of the azimuth of the sample. The samples grown for PL measurements were capped with a 100-nm-thick GaAs layer grown at 4501C under an arsenic pressure of 6 10 7 Torr. For PL studies, Ar+ laser was used for excitation (25 mW) and the sample temperature was changed from 14 to 300 K. The luminescence was detected using a cooled InGaAs photodiode. The uncapped InAs dots were evaluated using an atomic force microscope (AFM) in the contact mode.
3. Results and discussion 3.1. 2D platelets In order to achieve high uniformity in the dot size, it is important to enhance the surface migration of the indium adatoms. As reported previously, the low arsenic pressure and the low growth rate conditions provided the long migration length and resulted in the narrow size distribution of the InAs dot [5]. Fig. 1 shows a typical AFM image of the InAs islands with 1.8 ML coverage, grown under low arsenic pressure (6 10 7 Torr) and low growth rate (0.17 ML/s) conditions. In this image, many 2D islands (labeled A) and many small 3D dots (labeled B) appear on the top of a step edge of the InAs wetting layer. The step height is about 1 ML. The top of a step edge acts as a preferential site for the island formation because the strain of the islands, formed on the top of a step edge, is more released. The different energetic stability of
Fig. 1. AFM image of the InAs 2D islands with the indium coverage of 1.8 ML, arsenic pressure is 6 10 7 Torr, and the growth rate is 0.17 ML/s.
the island formation due to a different nucleation site induces inhomogeneous broadening of the island size. Actually, there is a tendency that the size of the islands formed on the step edge is larger than that on the terrace area. Fig. 2 shows the relationship between the height and the [1 1 0] lateral size of the InAs islands, grown at an arsenic pressure of 6 10 7 Torr and a growth rate of 0.17 ML/s. The InAs coverage was changed from 1.6 to 2.5 ML. When the InAs coverage was more than about 1.8 ML, the growth mode rapidly transited from the 2D growth to the 3D growth. In Fig. 2, region I corresponds to the formation of the 2D islands (platelets) with a constant height of 2 or 3 ML. As the growth proceeds, the 2D platelets quickly change to the 3D pyramidal dots, which are denoted as region II. In addition, one can see that the growth mode transition from 2D to 3D occurs around 20 nm in the lateral size of the 2D platelets. That is, the lateral size of the 2D platelets is limited at about 20 nm. As the lateral size of the 2D platelet increases, the strain energy becomes high because of the misfit. Particularly, the strain energy of adatom is emphasized at the island edge, compared with that at the terrace sites [7]. Therefore,
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Fig. 3. AFM image of the InAs 3D island (a) and its schematic top and (1 3 0) cross-sectional views (b).
Fig. 2. Relationship between the height and the [1 1 0] lateral size of the InAs islands, arsenic pressure is 6 10 7 Torr, and the growth rate is 0.17 ML/s.
adatom attachment probability at the 2D-platelet edge is suppressed for the large platelets, and as a result, the lateral size of the platelet is automatically limited by the compressive strain at the island edge. Such SSL behavior induces the narrow size distribution. After this SSL process, the height of the platelets increases rapidly, and then the 3D dots are formed. In order to further reduce the size distribution of the 2D platelets, it is needed to control the nucleation site (i.e. the top of a step edge) and to enhance the surface migration of indium adatoms for finding the preferential incorporation site. Needless to say, the narrow size distribution of the 2D platelets will provide the formation of the uniform 3D dots. 3.2. 3D pyramidal dots In region II of Fig. 2, almost every one of the large 3D dots has a similar pyramidal shape, and its aspect ratio is almost constant with a value of 0.24, which is related with the lateral size along the [1 1 0] direction. Fig. 3 shows a typical AFM image of the 3D dot (a) and its schematic top and crosssectional views (b). The four facets appear on the side walls of the dot. From the [3 1% 0] crosssectional AFM images, it was found that a tilt
Fig. 4. RHEED patterns just after the 3D island formation, taken along the [1% 3 0] azimuth.
angle of the facet is about 321. In addition, RHEED pattern for the [3 1% 0] azimuth, just after the dot formation, shows chevrons directed at an angle of 291 from the [0 0 1] direction as shown in Fig. 4. These structural data of the 3D dot reveal the evidence of {1 3 6} facets. When the InAs coverage is more than about 2 ML, the {1 3 6} facets appear on the side walls. In this situation, the improvement of the dot size suddenly stops except incoherent dots due to coalescence. Fig. 5 shows the average lateral size (a) and average height (b) of the coherent 3D dots as a function of the InAs coverage. The lateral size and height are clearly limited at more than about 2 ML coverage. Therefore, the above SSL behavior of the 3D dots is caused by the facet formation of the {1 3 6} plane. Once the stable facet is formed on the side wall, indium adatoms are hardly incorporated at the facet. The SSL behavior during the dot formation results in high uniformity in the dot size and shape. The detailed surface structure of the {1 3 6} facet has not been clarified, but it is
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Fig. 6. Schematic diagrams of the formation process of the InAs 3D islands.
3.3. Uniform InAs QDs Fig. 5. Average lateral size (a) and average height (b) of the InAs 3D islands as a function of the InAs coverage. Arsenic pressure is 6 10 7 Torr, and the growth rate is 0.17 ML/s.
possible to consider that a combination of a few low Miller index planes constructs the energetically stable surface as the {1 3 6} facets. From the above results, the dot formation process is schematically shown in Fig. 6. At the initial growth stage, the 2D platelets are formed, and the lateral size is limited by the compressive strain at the island edge. Following the SSL of the 2D platelets the height rapidly increases. In this stage, the stable facet starts to form on the side walls of the 3D islands. The {1 3 6} facet appears and then covers the side wall as a pyramidal dot. Here the second SSL mechanism due to the facet formation happens everywhere. If the surface migration of indium adatoms is enhanced, two SSL behaviors smoothly occur and suppress the size fluctuation of the dots.
The SSL feature has been often observed for the dot formation under the low arsenic pressure and low growth rate conditions. We have demonstrated the formation of the uniform InAs QDs grown at a low arsenic pressure of 3 10 7 Torr and a low growth rate of 0.035 ML/s [5]. Fig. 7 shows the AFM image of their uniform QDs. The dot densities of the coherent and coalescent dots are 3.5 1010 and 4.7 108 cm 2, respectively. Standard deviation was 1.7 nm (about 4%) for the lateral size and 0.7 nm (about 8%) for the height. After the GaAs capping growth, the PL spectrum from their QDs was measured. Fig. 8 shows the typical PL spectrum (a) and the temperature dependence of the PL linewidth (b). The PL spectrum fitted with a Gaussian curve has a peak energy position of 1.04 eV and the narrowest linewidth of 18.6 meV at 14 K. The narrow PL linewidth is almost maintained from 14 K to room temperature. At room temperature, the wavelength of the PL peak was 1.28 mm.
K. Yamaguchi et al. / Journal of Crystal Growth 237–239 (2002) 1301–1306
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Fig. 7. AFM image of the uniform InAs dots grown at an arsenic pressure of 3 10 7 Torr and a growth rate of 0.035 ML/s.
The capping growth of the GaAs is also an important process to decide the inhomogeneous broadening of the dot size. Recently, we observed that during the capping growth the size shrinkage of the InAs QDs is enhanced for the large dots. Therefore, high uniformity in the dot size is further expected by the controlled capping growth. The results of a detailed investigation of this process will be reported elsewhere [8].
Fig. 8. PL spectrum of uniform InAs QDs (a) and the temperature dependence of the PL linewidth.
4. Conclusion The uniform formation process of the InAs islands was investigated in the MBE growth via SK mode under the low arsenic pressure and low growth rate conditions. The growth mode rapidly transited from 2D to 3D at the critical InAs coverage. During each formation process of the 2D and 3D islands, the SSL behavior was clearly observed for the above growth conditions. The first SSL feature of the 2D platelets was attributed to compressive strain at the island edge. The second SSL mechanism was caused by the facet formation of the 3D pyramidal dots. Both SSL
behaviors played an important role in achieving the narrow size distribution of the InAs QDs. As a result, a narrow PL linewidth of o20 meV was successfully obtained.
References [1] D. Leonard, K. Pond, P.M. Petroff, Phys. Rev. B 50 (1994) 11687. [2] N.N. Ledentsov, J. Bohrer, D. Bimberg, I.V. Kochnev, M.V. Maximov, P.S. Kop’ev, Zh.I. Alferov, A.O. Kosogov, S.S. Ruvimov, P. Werner, U. Gosele, Appl. Phys. Lett. 69 (1996) 1095.
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K. Yamaguchi et al. / Journal of Crystal Growth 237–239 (2002) 1301–1306
[3] I. Mukhametzhanov, Z. Wei, R. Heitz, A. Madhukar, Appl. Phys. Lett. 75 (1999) 85. [4] K. Nishi, H. Saito, S. Sugou, J-S. Lee, Appl. Phys. Lett. 74 (1999) 1111. [5] K. Yamaguchi, K. Yujobo, T. Kaizu, Jpn. J. Appl. Phys. 39 (2000) L1245.
[6] T. Kaizu, K. Yamaguchi, Jpn. J. Appl. Phys. 40 (2001) 1885. [7] A-L. Barabasi, Appl. Phys. Lett. 70 (1997) 2565. [8] K. Yamaguchi, Y. Saito, R. Ohtsubo, Eighth International Symposium on the Formation of Semiconductor Interfaces, 2001.