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Si(111)
Materials Science and Engineering B89 (2002) 188– 190
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Formation of Ge quantum dots on boron-reconstructed surface/Si(111) H. Mori *, H. Nagai, T. Yanagawa, S. Matsumoto Department of Electrical Engineering, Keio Uni6ersity, 3 -14 -1, Hiyoshi, Yokohama 223 -8522, Japan
Abstract Uniformity and arrangement of Ge quantum dots were investigated on B-reconstructed surface of Si(111). B coverage was varied from 0 to 0.35 ML on the Si(111) 7 × 7 surface, and Ge dots were formed after formation of B-reconstructed surface. 8 ML Ge was deposited on this surface and annealed at 500 °C. The size and density of Ge dots was analyzed with TEM and AFM. Optical properties of Ge dots were investigated by Photoluminescence (PL) measurement. When there was no B coverage, the lateral size and height of Ge dots were 50 and 10 nm, respectively, without uniform distribution. When an ordered 3× 3 R30°-B surface was formed, quite uniform Ge dots were formed with 35 nm lateral size and 7 nm in height, with an obvious array in one direction. Moreover, a strong PL spectrum was observed from these Ge dots at 4 K. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Self-assembled; Ge quantum dots; B reconstructed structure; 3× 3 R30°
1. Introduction Self-assembled Ge quantum dots have attracted much interest because of their possibilities of electronic and opto-electronic devices for the next generation [1,2]. Deposition of Ge on Si leads to formation of Ge islands due to lattice-mismatch, resulting in the fabrication of Ge quantum dots easily. However, there are annoying problems in this method regarding the size, uniformity, areal density and arrangement of the formed Ge dots. Therefore, many techniques were devised to improve these problems [3 – 6]. Some researchers succeeded in reducing the size and increasing the areal density of Ge dots using pre-deposition of C and B on Si [7,8]. They suggested that this was caused by the strain originated from B, as B is much smaller than Si and Ge in radius. In this case the problems with size and areal density were improved, but the problems with uniformity and arrangement still remain. In this study, to improve the problems mentioned above, we formed B-reconstructed surface that can
* Corresponding author. Tel.: + 81-45-563-1141; fax. +81-45-5661529. E-mail address: [email protected] (H. Mori).
distribute B uniformly and periodically, and formed Ge dots on it.
2. Experimental The experiments were all performed in UHV chambers equipped with STM and electron gun evaporators with a base pressure of 7× 10 − 11 torr. The samples used in this study were phosphorus-doped n-Si(111) wafers with a resistivity of 0.7 –1.3 V cm. The sample was introduced into the UHV chamber, and then degassed overnight at 600 °C. To remove oxide and hydrocarbon contaminants and create an ordered clean 7× 7 surface, the sample was flashed at 1250 °C several times. B (99.9% purity) was deposited on the cleaned surface by an electron gun evaporator from 0 to 0.35 ML, and the sample was annealed at 800 °C for a minute to form B-reconstructed surface. Then Ge dots were fabricated on the surface. The areal density of Ge dots was calculated by AFM in the tapping mode, and the size was measured by cross-sectional TEM. For cross-sectional TEM measurement, the Ge layer was covered with about 50 nm thick of Si. PL was used for evaluating the optical properties of Ge dots. For PL measurement 488 nm Ar+ laser was used at 4 K.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 8 4 7 - 9
H. Mori et al. / Materials Science and Engineering B89 (2002) 188–190
189
3. Results and discussion
3.1. Formation of B-reconstructed surface on Si(111) Fig. 1 shows the STM image after B evaporation for 1410 s and annealed at 800 °C for a minute. Ordered
3 × 3 R30° surface is formed, and B atoms are uniformly and periodically distributed on the surface. The B coverage of this surface can be estimated to be 1/3 ML at this time. When B coverage is smaller than 1/3 ML, in addition to 3 × 3 R30° structure, 7×7 and two types of ring structures (bright ring, dark ring) are observed. It is known that the ring structures are the B-related metastable structures appeared when the amount of B is less than 1/3 ML [9]. When B coverage is larger than 1/3 ML, the saturated B atoms form cluster on 3 × 3 R30°-B surface.
3.2. Ge dots on the B-reconstructed surface Fig. 1. STM image of 3× 3 R30°-B reconstructed surface (7 × 7 nm).
Fig. 2. Areal density of Ge quantum dots as a function of the coverage of boron.
Fig. 3. Cross-sectional TEM image of Ge dot structure on 3× 3 R30°-B reconstructed surface (8 ML Ge, 500 °C).
Fig. 2 shows the areal density of Ge dots as a function of the B coverage from 0.1 to 0.35 ML. The Ge coverage is 8 ML and annealed at 500 °C for all samples. We investigated two types of samples, one had B deposition on Si without annealing (not reconstructed) and the other with annealing (reconstructed). Obviously from this figure, the maximum density of 1.53×1010 cm − 2 is obtained at 1/3 ML of B that formed ordered 3 × 3 R30° surface. On the other hand, the density of Ge dots on amorphous-B is the highest at 0.25 ML of B, which largely agrees with the result by Zhou et al. However, the uniformity of Ge dots does not improve very much in the case of amorphous-B sample. Fig. 3 shows the cross-sectional TEM image of a Ge dot formed on 3 × 3 R30°-B reconstructed surface. This sample was prepared with 8 ML of Ge at 500 °C. The size of Ge dot on 3 × 3 R30°-B reconstructed surface remarkably decreases to about 35 nm in lateral size and 7 nm in height, while Ge dot on bare Si is 50 and 7 nm, respectively. Fig. 4 shows the AFM image of these Ge dots. In addition to the size reduction of the Ge dots on 3 × 3 R30°-B reconstructed surface, the uniformity of these dots is improved significantly. The standard deviation of these dots is 15.1, whereas for the case of Ge dots on Si is 25.9. Moreover, these dots obviously have an array in one direction, while the Ge dots on Si distribute at random. The direction of this array is considered to be parallel to either terrace steps or directions along atoms of 3 × 3 R30°-B surface. However, it is impossible to decide to array direction at present. The results of the size reduction and increase of the areal density by using pre-deposition of B on Si agree with past reports. The presence of B reduces the strain caused by lattice mismatch between Si and Ge, as B
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H. Mori et al. / Materials Science and Engineering B89 (2002) 188–190
however, B atoms are distributed on the surface uniformly and periodically. Consequently, all Ge atoms are influenced equally, followed by the formation of smaller size, high density and quite uniform Ge dots with an obvious array in one direction. Fig. 5 shows PL spectra of these three kinds of samples. For Ge dots on Si and on amorphous-B, the remarkable PL spectra are not obtained except for the luminescence due to phosphorus bound exciton. For Ge dots on
3 × 3 R30°-B reconstructed surface, however, the spectrum related to Ge dots is also observed around 0.77 eV. It is known that the emission related to bound exiton from Ge substrate at 0.74 eV [10]. It is considered that this result is due to quite uniform Ge dots on 3 × 3 R30°-B reconstructed surface and carrier confinement in the Ge dots efficiently.
4. Conclusion
Fig. 4. AFM image of Ge dots on 3× 3 R30°-B reconstructed surface (8 ML Ge, 500 °C, 4× 4 mm).
We investigated the formation of Ge quantum dots on
3 × 3 R30°-B reconstructed surface. The Ge dots are formed on 3 × 3 R30°-B reconstructed surface with smaller size and higher density. These Ge dots also have quite uniformity and an array in one direction. Moreover, these Ge dots are active optically and a strong PL spectrum is observed at 4 K.
Acknowledgements The authors would like to thank T. Mitani of Central Research Center, Keio University, for the maintenance of STM and measuring TEM, and M. Tajima of Institute of Space and Astronautical Science, for PL measurement.
References
Fig. 5. PL spectra of Ge dots (8 ML Ge, 500 °C) (a) Ge/Si; (b) Ge/amorphous-B/Si; (c) Ge/ 3× 3 R30°-B/Si.
atoms are much smaller than Si and Ge in radius. The surface energy of Ge atoms becomes lower and the surface diffusions of Ge atoms are suppressed. Thus, the smaller sizes of Ge dots are formed with high density. In the case of amorphous-B (without annealing), B atoms are not distributed on the surface uniformly, resulting that all Ge atoms are not influenced equally. Therefore, the uniformity of Ge dots does not improve very much. In case of 3 × 3 R30°-B reconstructed surface,
[1] J.M. Moison, F. Houzay, F. Barthe, L. Leprince, E. Andre, O. Vatel, Appl. Phys. Lett. 63 (1994) 1196. [2] O.P. Pchelyakov, Y.u. B. Bolkhovityanov, A.V. Dvurechenskii, A.I. Nikiforov, A.I. Yakimov, B. Voigtlander, Thin Solid Films 367 (2000) 75. [3] P. Schittenhelm, M. Gail, J. Brunner, J.F. Nutzel, G. Abstreiter, Appl. Phys. Lett. 67 (1995) 1292. [4] G. Woehl, C. Schoellhorn, O.G. Schmidt, K. Brunner, K. Eberl, O. Kienzle, F. Ernst, Thin Solid Films 321 (1998) 86. [5] O.G. Schmidt, C. Lange, K. Eberl, O. Kienzle, F. Ernst, Thin Solid Films 336 (1998) 248. [6] N. Usami, Y. Shiraki, Thin Solid Films 369 (2000) 108. [7] O.G. Schmidt, C. Lange, K. Eberl, O. Kienzle, F. Ernst, Appl. Phys. Lett. 71 (1997) 2340. [8] X. Zhou, B. Shi, Z. Jiang, W. Jiang, D. Hu, D. Gong, Y. Fan, X. Zhang, X. Wang, Y. Li, Thin Solid Films 369 (2000) 92. [9] T. Yamamoto, S. Ohara, K. Ezoe, H. Kuriyama, K. Ishii, S. Matsumoto, Appl. Surf. Sci. 130 (1998) 1. [10] K. Terashima, T. Ikarashi, D. Tweet, K. Miyanaga, T. Tatsumi, M. Tajima, Appl. Phys. Lett. 65 (1994) 601.
www.elsevier.com/locate/mseb
Formation of Ge quantum dots on boron-reconstructed surface/Si(111) H. Mori *, H. Nagai, T. Yanagawa, S. Matsumoto Department of Electrical Engineering, Keio Uni6ersity, 3 -14 -1, Hiyoshi, Yokohama 223 -8522, Japan
Abstract Uniformity and arrangement of Ge quantum dots were investigated on B-reconstructed surface of Si(111). B coverage was varied from 0 to 0.35 ML on the Si(111) 7 × 7 surface, and Ge dots were formed after formation of B-reconstructed surface. 8 ML Ge was deposited on this surface and annealed at 500 °C. The size and density of Ge dots was analyzed with TEM and AFM. Optical properties of Ge dots were investigated by Photoluminescence (PL) measurement. When there was no B coverage, the lateral size and height of Ge dots were 50 and 10 nm, respectively, without uniform distribution. When an ordered 3× 3 R30°-B surface was formed, quite uniform Ge dots were formed with 35 nm lateral size and 7 nm in height, with an obvious array in one direction. Moreover, a strong PL spectrum was observed from these Ge dots at 4 K. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Self-assembled; Ge quantum dots; B reconstructed structure; 3× 3 R30°
1. Introduction Self-assembled Ge quantum dots have attracted much interest because of their possibilities of electronic and opto-electronic devices for the next generation [1,2]. Deposition of Ge on Si leads to formation of Ge islands due to lattice-mismatch, resulting in the fabrication of Ge quantum dots easily. However, there are annoying problems in this method regarding the size, uniformity, areal density and arrangement of the formed Ge dots. Therefore, many techniques were devised to improve these problems [3 – 6]. Some researchers succeeded in reducing the size and increasing the areal density of Ge dots using pre-deposition of C and B on Si [7,8]. They suggested that this was caused by the strain originated from B, as B is much smaller than Si and Ge in radius. In this case the problems with size and areal density were improved, but the problems with uniformity and arrangement still remain. In this study, to improve the problems mentioned above, we formed B-reconstructed surface that can
* Corresponding author. Tel.: + 81-45-563-1141; fax. +81-45-5661529. E-mail address: [email protected] (H. Mori).
distribute B uniformly and periodically, and formed Ge dots on it.
2. Experimental The experiments were all performed in UHV chambers equipped with STM and electron gun evaporators with a base pressure of 7× 10 − 11 torr. The samples used in this study were phosphorus-doped n-Si(111) wafers with a resistivity of 0.7 –1.3 V cm. The sample was introduced into the UHV chamber, and then degassed overnight at 600 °C. To remove oxide and hydrocarbon contaminants and create an ordered clean 7× 7 surface, the sample was flashed at 1250 °C several times. B (99.9% purity) was deposited on the cleaned surface by an electron gun evaporator from 0 to 0.35 ML, and the sample was annealed at 800 °C for a minute to form B-reconstructed surface. Then Ge dots were fabricated on the surface. The areal density of Ge dots was calculated by AFM in the tapping mode, and the size was measured by cross-sectional TEM. For cross-sectional TEM measurement, the Ge layer was covered with about 50 nm thick of Si. PL was used for evaluating the optical properties of Ge dots. For PL measurement 488 nm Ar+ laser was used at 4 K.
0921-5107/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 8 4 7 - 9
H. Mori et al. / Materials Science and Engineering B89 (2002) 188–190
189
3. Results and discussion
3.1. Formation of B-reconstructed surface on Si(111) Fig. 1 shows the STM image after B evaporation for 1410 s and annealed at 800 °C for a minute. Ordered
3 × 3 R30° surface is formed, and B atoms are uniformly and periodically distributed on the surface. The B coverage of this surface can be estimated to be 1/3 ML at this time. When B coverage is smaller than 1/3 ML, in addition to 3 × 3 R30° structure, 7×7 and two types of ring structures (bright ring, dark ring) are observed. It is known that the ring structures are the B-related metastable structures appeared when the amount of B is less than 1/3 ML [9]. When B coverage is larger than 1/3 ML, the saturated B atoms form cluster on 3 × 3 R30°-B surface.
3.2. Ge dots on the B-reconstructed surface Fig. 1. STM image of 3× 3 R30°-B reconstructed surface (7 × 7 nm).
Fig. 2. Areal density of Ge quantum dots as a function of the coverage of boron.
Fig. 3. Cross-sectional TEM image of Ge dot structure on 3× 3 R30°-B reconstructed surface (8 ML Ge, 500 °C).
Fig. 2 shows the areal density of Ge dots as a function of the B coverage from 0.1 to 0.35 ML. The Ge coverage is 8 ML and annealed at 500 °C for all samples. We investigated two types of samples, one had B deposition on Si without annealing (not reconstructed) and the other with annealing (reconstructed). Obviously from this figure, the maximum density of 1.53×1010 cm − 2 is obtained at 1/3 ML of B that formed ordered 3 × 3 R30° surface. On the other hand, the density of Ge dots on amorphous-B is the highest at 0.25 ML of B, which largely agrees with the result by Zhou et al. However, the uniformity of Ge dots does not improve very much in the case of amorphous-B sample. Fig. 3 shows the cross-sectional TEM image of a Ge dot formed on 3 × 3 R30°-B reconstructed surface. This sample was prepared with 8 ML of Ge at 500 °C. The size of Ge dot on 3 × 3 R30°-B reconstructed surface remarkably decreases to about 35 nm in lateral size and 7 nm in height, while Ge dot on bare Si is 50 and 7 nm, respectively. Fig. 4 shows the AFM image of these Ge dots. In addition to the size reduction of the Ge dots on 3 × 3 R30°-B reconstructed surface, the uniformity of these dots is improved significantly. The standard deviation of these dots is 15.1, whereas for the case of Ge dots on Si is 25.9. Moreover, these dots obviously have an array in one direction, while the Ge dots on Si distribute at random. The direction of this array is considered to be parallel to either terrace steps or directions along atoms of 3 × 3 R30°-B surface. However, it is impossible to decide to array direction at present. The results of the size reduction and increase of the areal density by using pre-deposition of B on Si agree with past reports. The presence of B reduces the strain caused by lattice mismatch between Si and Ge, as B
190
H. Mori et al. / Materials Science and Engineering B89 (2002) 188–190
however, B atoms are distributed on the surface uniformly and periodically. Consequently, all Ge atoms are influenced equally, followed by the formation of smaller size, high density and quite uniform Ge dots with an obvious array in one direction. Fig. 5 shows PL spectra of these three kinds of samples. For Ge dots on Si and on amorphous-B, the remarkable PL spectra are not obtained except for the luminescence due to phosphorus bound exciton. For Ge dots on
3 × 3 R30°-B reconstructed surface, however, the spectrum related to Ge dots is also observed around 0.77 eV. It is known that the emission related to bound exiton from Ge substrate at 0.74 eV [10]. It is considered that this result is due to quite uniform Ge dots on 3 × 3 R30°-B reconstructed surface and carrier confinement in the Ge dots efficiently.
4. Conclusion
Fig. 4. AFM image of Ge dots on 3× 3 R30°-B reconstructed surface (8 ML Ge, 500 °C, 4× 4 mm).
We investigated the formation of Ge quantum dots on
3 × 3 R30°-B reconstructed surface. The Ge dots are formed on 3 × 3 R30°-B reconstructed surface with smaller size and higher density. These Ge dots also have quite uniformity and an array in one direction. Moreover, these Ge dots are active optically and a strong PL spectrum is observed at 4 K.
Acknowledgements The authors would like to thank T. Mitani of Central Research Center, Keio University, for the maintenance of STM and measuring TEM, and M. Tajima of Institute of Space and Astronautical Science, for PL measurement.
References
Fig. 5. PL spectra of Ge dots (8 ML Ge, 500 °C) (a) Ge/Si; (b) Ge/amorphous-B/Si; (c) Ge/ 3× 3 R30°-B/Si.
atoms are much smaller than Si and Ge in radius. The surface energy of Ge atoms becomes lower and the surface diffusions of Ge atoms are suppressed. Thus, the smaller sizes of Ge dots are formed with high density. In the case of amorphous-B (without annealing), B atoms are not distributed on the surface uniformly, resulting that all Ge atoms are not influenced equally. Therefore, the uniformity of Ge dots does not improve very much. In case of 3 × 3 R30°-B reconstructed surface,
[1] J.M. Moison, F. Houzay, F. Barthe, L. Leprince, E. Andre, O. Vatel, Appl. Phys. Lett. 63 (1994) 1196. [2] O.P. Pchelyakov, Y.u. B. Bolkhovityanov, A.V. Dvurechenskii, A.I. Nikiforov, A.I. Yakimov, B. Voigtlander, Thin Solid Films 367 (2000) 75. [3] P. Schittenhelm, M. Gail, J. Brunner, J.F. Nutzel, G. Abstreiter, Appl. Phys. Lett. 67 (1995) 1292. [4] G. Woehl, C. Schoellhorn, O.G. Schmidt, K. Brunner, K. Eberl, O. Kienzle, F. Ernst, Thin Solid Films 321 (1998) 86. [5] O.G. Schmidt, C. Lange, K. Eberl, O. Kienzle, F. Ernst, Thin Solid Films 336 (1998) 248. [6] N. Usami, Y. Shiraki, Thin Solid Films 369 (2000) 108. [7] O.G. Schmidt, C. Lange, K. Eberl, O. Kienzle, F. Ernst, Appl. Phys. Lett. 71 (1997) 2340. [8] X. Zhou, B. Shi, Z. Jiang, W. Jiang, D. Hu, D. Gong, Y. Fan, X. Zhang, X. Wang, Y. Li, Thin Solid Films 369 (2000) 92. [9] T. Yamamoto, S. Ohara, K. Ezoe, H. Kuriyama, K. Ishii, S. Matsumoto, Appl. Surf. Sci. 130 (1998) 1. [10] K. Terashima, T. Ikarashi, D. Tweet, K. Miyanaga, T. Tatsumi, M. Tajima, Appl. Phys. Lett. 65 (1994) 601.