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The nanometer-scale selective overgrowth of Ge over Si islands on Si(001 ) windows in ultrathin SiO2 films
Surface Science 496 (2002) L7±L12
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Surface Science Letters
The nanometer-scale selective overgrowth of Ge over Si islands on Si(0 0 1) windows in ultrathin SiO2 ®lms Yoshiki Nitta 1, Motoshi Shibata 2, Ken Fujita 1, Masakazu Ichikawa * Joint Research Center for Atom Technology, Angstrom Technology Partnership (JRCAT-ATP), c/o National Institute for Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-0046, Japan Received 26 June 2001; accepted for publication 3 October 2001
Abstract We performed nanometer-scale overgrowths of Ge over islands of Si that had been grown on Si(0 0 1) windows in ultrathin silicon-dioxide ®lms. Growth was observed in real time by scanning tunneling microscopy. Si window areas with sizes of 10±30 nm were fabricated by using the thermal decomposition of the oxide to form voids. The selective epitaxial growth (SEG) of Si was then carried out by the introduction of disilane gas (Si2 H6 ). Faceted islands of Si grew in the voids during this stage. SEGs of Ge were then carried out by the introduction of germane gas (GeH4 ). Ge/Si hetero-islands grew with {1 0 5} facets. We then performed further overgrowths, of Si, on these Ge/Si hetero-islands to form structures with embedded Ge. The shape of the islands then changed from a pyramidal shape with {1 0 5} facets to a shape with {1 1 3} facets. Low-temperature photoluminescence spectra obtained from this sample showed a broad peak, which originated from the Ge islands embedded within the Si islands. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Oxidation; Epitaxy; Silicon; Germanium; Silicon oxides
1. Introduction SiGe/Si heterostructures have promising electronic and optical properties and the formation of quantum wires and dots using such structures has been intensively studied [1]. This material system is * Corresponding author. Tel: +81-298-54-2621/61-2590; fax: +81-298-54-2577/61-2577. E-mail address: [email protected] (M. Ichikawa). 1 Present address: Semiconductor Laboratory, Oki Electric Industry Co., Ltd., 550-5 Higashiasakawa, Hachioji, Tokyo 193-8550, Japan. 2 Present address: Advanced Technology Laboratory, Matsushita Electric Industry Co., Ltd. 3-10-1 Higashi-mita, Tama-ku, Kawasaki 214-8501, Japan.
one of the model systems for growth in the Stranski±Krastanov (S±K) mode. The S±K growth is often used to form self-assembled quantum dot structures, but the sizes and positions of the grown structures are randomly distributed on the substrates. Several researchers [2±4] have tried to form self-assembled structures at orderly and predictable positions by the growth of Ge on islands of Si. They have grown Ge on patterned islands of Si to obtain an orderly arrangement of islands of Ge along the h1 0 0i edges of the Si islands. The overgrowth of Si layers on Ge islands grown by S±K growth has also been studied on Si(1 0 0) substrates. Sutter and Lagally have investigated the growth of Si on islands of Ge [5].
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Small islands of Ge on Si(1 0 0) disappeared during growth of Si, while larger islands grew and developed (1 0 0) top facets. Liu et al. have studied the eect of Si overgrowths on the structural and photoluminescence (PL) properties of Ge islands and strained Ge layers grown on Si(1 0 0) [6]. In the above cases, however, it is dicult to precisely control the size and position of Ge islands. Moreover the typical size of each island is of the order of 100 nm, and this is not attractive for application of quantum eects. Selective epitaxial growth (SEG), on the other hand, makes it possible to form nanoislands with a desired size and at desired positions. Kim et al. have reported the selective growth of Ge islands on Si windows in SiO2 ®lms [7]. The windows were formed by using conventional electron-beam lithography. They found that the growth mode of the Ge islands was strongly dependent on the size of the Si windows. Their report, however, only covered those cases where the size of the Si window was greater than 90 nm. We have described, in earlier reports, the formation of nanostructures by the nanometer-scale (20 nm) SEG of Si and Ge islands on Si(0 0 1) windows in ultrathin ®lms of SiO2 [8±10]. The crystalline ®lms in these cases were initially grown in a layer-by-layer fashion and then the pyramidal islands were formed on the windows. In order to get functional properties from the islands, the overgrowth on the islands is further required to form hetero-nanoisland structures. In this paper, we report on the real-time observation, by scanning tunneling microscopy (STM), of the growth of hetero-nanoislands; overgrowths of Ge on Si nanoislands and of Si on heteronanoislands of Ge/Si. 2. Experimental The experiments were performed in a hightemperature STM chamber with a dierential pumping system for Si2 H6 and GeH4 [11]. The sample used in this study was cut from a welloriented n-type Si(0 0 1) wafer. The sample was cleaned, then an ultrathin ®lm of SiO2 (0.3 nm thick) was produced by exposing the clean Si(0 0 1)
surface to molecular oxygen at 2:7 10 4 Pa and 630±730 °C for 5±10 min. The sample was then kept at 720±750 °C to decompose the oxide, until the diameters of the resulting voids reached 10±30 nm. After these voids had been formed, nanoislands of Si were selectively grown by introducing disilane gas (Si2 H6 ) at the substrate temperature of 540 °C. We have reported that the sidewalls of these Si islands were transformed from {1 1 13} facets to such steeper facets as {1 1 9} and {1 1 3} [8,9]. In this study, Si islands with {1 1 9} facets were selected as the initial Si islands. The Ge crystals were then selectively grown by introducing germane gas (GeH4 ), at 410 °C, to the chamber. The respective dose rates of Si2 H6 and GeH4 were ®xed at 1:6 1014 and 4 1014 s 1 cm 2 . After the SEG of Ge on the Si islands, the sample temperature was raised to 540 °C and Si was then selectively grown to form islands with an embedded-Ge structure. The selective growth was observed, in real time, with STM. Line-by-line background subtraction and high-pass ®ltering were applied to all of the STM images. 3. Results and discussion Fig. 1 shows the same area (a) after the SEG of Si and (b) 500 min after the start of the selective growth of Ge. The voids were initially rectangular with their bases parallel to the h1 1 0i direction, re¯ecting the crystal structure of the Si(0 0 1) substrate. The 3D islands grown by Si SEG were polyhedrons and their bases were also parallel to the h1 1 0i direction (Fig. 1(a)). GeH4 gas was then supplied as the source for the growth of Ge on these islands of Si, at the substrate temperature of 410 °C. Fig. 1(b) shows that the shapes of all of the islands were changed by the Ge overgrowths, and that selectivity was preserved over the whole sample during this growth process. The line at the top of each polyhedron was parallel to the h1 1 0i directions, and this indicates that the facets of these islands were rotated from the initial facets of Si nanoislands by 45°. Fig. 2 shows the transformations of the islands during the selective growth of Ge on the nanoislands of Si. These are magni®ed images of the
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Fig. 1. STM images of the same Si(0 0 1) surface (a) after Si SEG and (b) 500 min after the selective growth of Ge on the islands of Si had been started. The substrate temperature was 410 °C during growth. The ultrathin SiO2 ®lms remains ouside the Si island-grown areas. The detailed shape transformation of the island indicated by an arrow is shown in Fig. 2.
crystals indicated by the arrows in Fig. 1. Images were acquired (a) after the SEG of Si, and (b) 102 min after and (c) 501 min after the start of Ge selective overgrowth. The initial nanoislands of Si have mainly the (0 0 1) and {1 1 9} planes as mentioned above (Fig. 2(a)). In this image, doublelayered steps (DB steps) are aligned along the h1 1 0i directions at all sidewalls, and the distance between the steps shows that these are {1 1 9} planes. The SEG proceeded in a layer-by-layer fashion in the (0 0 1) plane in its initial stages (Fig. 2(b)). {1 0 5} facets formed at the corners of the Si islands. At the same time, h1 1 0i-related facets formed on the sidewalls, which intersect with the (0 0 1) plane along [1 1 0] lines, such as {1 1 7}, {1 1 5} and {1 1 3}. However {1 0 5} facets grew much faster than the h1 1 0i-related facets. In Fig. 2(b), {1 1 5} facets are seen on each sidewall but their area is small compared with that of the {1 0 5} facets. The {1 0 5} facets ®nally expand to become dominant and the islands take on a hutlike shape that is composed of {1 0 5} and {1 1 3} facets (Fig. 2(c)). Fig. 2(d) shows the pro®les revealing the transformation of the island shape along the lines indicated in (a)±(c). The Ge/Si nanoisland with about 4 nm height grew on the Si window area. We have not observed the interface structure using transmission electron microscopy. This island is thought, however, to be coherent to the underlying Si layer, since {1 0 5} faceted Ge island is known to be coherent to the Si layer [12].
Jin et al. have reported that Ge dots grow at the corners of the Si islands and that the reason for this is tensile stress at the corners of the Si islands [4]. This phenomena is also observed in the SEG of Ge in voids. In that case, several 3D islands appeared at the peripheries of the voids, where steps had been formed during the formation of the voids [10]. However, in the present work, the selective epitaxy of the Ge overgrowths proceeded twodimensionally and the small hut-like islands seen in the SEG of Ge in voids were not visible. This is presumably due to a preference for the 2D growth of {1 0 5} facets at the corners of the nanoislands of Si, because the islands are small. In fact, the growth of {1 0 5} facets on this island was completed within 180 min, much more quickly than the growth of the h1 1 0i-related facets. After the completion of the {1 0 5} facets, the h1 1 0i-related facets grew steeper and {1 1 3} facets were ®nally formed. We continued to supply the GeH4 for 2 h after the formation of the hut-like islands, but no particular change was visible. This may have been because the {1 0 5} facets are chemically stable, which is the case for the SEG of Ge [10]. To form structures with embedded Ge, Si2 H6 gas was then supplied to these Ge nanoisland, at the substrate temperature of 540 °C. Before the Si overgrowths were formed, the islands had {1 0 5} and {1 1 3} facets as described above (Fig. 3(a)). After growth of the Si had started, the {1 1 3} and (0 0 1) facets grew rapidly and the islands became
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Fig. 2. STM images of the selective growth of Ge over Si islands on Si(0 0 1). The island shown is indicated with an arrow in Fig. 1. Images were acquired (a) before growth and (b) 102 and (c) 501 min after growth had started. The substrate's temperature was 410 °C. The lower illustrations show the facet structures which appear in the islands. (d) Pro®les showing the transformation of the island shape along the lines indicated in (a)±(c).
higher (Fig. 3(b)). The polyhedron seen here is similar to that reported by Sutter and Lagally [5]. In their case, however, the islands became ¯atter and the areas of their bases became larger, and they developed (1 0 0) top facets. However, in the case of SEG, while the (1 0 0) top facets appeared, they grew smaller as the islands grew higher, because the expansion of the island was restricted by the surrounding ®lm of SiO2 . This restriction was
originated from the potential barrier (3 eV) at the boundary between SiO2 and Si window areas, which con®ned Si adatoms in the Si window area [8,9]. After the overgrowth of Si, the Ge islands became pyramidal with wide {1 1 3} facets and narrow (0 0 1) top facets with 7-nm height. It is likely that alloying of Si and Ge occurred at the interface because the two substances would probably have been intermixed during the overgrowth.
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Fig. 3. STM images of the selective growth of Si over islands of Ge on Si(0 0 1). The arrow indicates the same island in each image. Images were acquired (a) before growth and (b) 2h and (c) 4 h after growth had started. The substrate temperature was 540 °C. The lower illustrations show the facet structures which appear in the island.
Before Si overgrowth, the tunneling current vs. the sample bias (I±V ) spectra measured for the Ge/ Si islands showed that the energy gap was about 0.6 eV, which corresponds to the band gap of bulk Ge. While the I±V spectra measured for the Si/Ge/Si islands showed the energy gap was about 1.0 eV, which corresponds to the band gap of bulk Si. We had observed the energy gap of about 1 eV on clean Si(0 0 1) surfaces at the same tunneling condition. Moreover, as hydrogen atoms decomposed from Si2 H6 are considered to play a role of surfactant to suppress Ge surface segregation. These suggest that the top surface is very nearly pure Si. After the Si selective epitaxial overgrowth, lowtemperature photoluminescence (PL) measurements of the sample were performed (Fig. 4). Excitation was provided by an Ar laser and PL signals were collected by a liquid-nitrogen cooled Ge detector. The several peaks from 1.02 to 1.15 eV in Fig. 4 are the non-phonon and phononrelated peaks of Si [13]. The broad peak at around 0.9 eV is thought to originate from the threedimensionally embedded Ge, since this peak was not observed from the samples of Ge/Si nanoislands. Kim et al. reported similar PL peaks for Si-capped Ge islands on Si windows [7]. Liu et al. also reported that similar PL peaks were obtained
Fig. 4. PL spectra from a buried-Ge sample, measured at 4.2 K. The emission originating from the buied 3D island of Ge is visible.
from 3D islands of Ge in a Si/Ge/Si stacked structure [6]. The broadness of this peak is thought
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to be due to scattering in the distribution of the sizes of the Si/Ge/Si islands in Si windows. If, therefore, Si windows of uniform size were formed by irradiation with the ®eld-emission electron beam from an STM tip [8±10], the sizes and positions of the nanoislands would be well controlled, and a PL peak with higher intensity and narrower width would be expected.
4. Summary The nanometer-scale selective epitaxy of Ge overgrowths on nanoislands of Si on Si(0 0 1) was achieved with the use of ultrathin SiO2 ®lms. In the initial stage, SEG of Ge proceeded in a layer-bylayer fashion in the (0 0 1) plane, and {1 0 5} facets were formed at the corners of the islands. At the same time, h1 1 0i-related facets grew on the sidewalls of the islands. {1 0 5} facets grew much more quickly than the h1 1 0i-related facets. The growth of these {1 0 5} facets ®nally became dominant and the islands took on a hut-like shape. Si was then grown over these islands of Ge, with the intention of embedding islands of Ge in islands of Si. After the growth of Si, the Ge/Si islands became pyramidal with {1 1 3} facets. Low-temperature PL spectrum obtained from this sample showed a broad peak at 0.9 eV. This peak originated in the islands of Ge that had been three-dimensionally con®ned in the islands of Si.
Acknowledgements This work, partly supported by New Energy and Industrial Technology Development Organization (NEDO), was performed by the Joint Research Center for Atom Technology (JRCAT) under an agreement between the National Institute for Advanced Industrial Science and Technology (AIST) and the Angstrom Technology Partnership (ATP). References [1] K. Eberl, O.G. Schmidt, R. Duschl, O. Kienzle, E. Ernst, Y. Rau, Thin Solid Films 369 (2000) 33. [2] T.I. Kamins, R.S. Williams, Appl. Phys. Lett. 71 (1997) 1201. [3] L. Vescan, J. Cryst. Growth 194 (1998) 173. [4] G. Jin, J.L. Liu, S.G. Thomas, Y.H. Luo, K.L. Wang, Appl. Phys. Lett. 75 (1999) 2752. [5] P. Sutter, M.G. Lagally, Phys. Rev. Lett. 81 (1998) 3471. [6] J.P. Liu, J.Z. Wang, D.D. Huang, J.P. Li, D.Z. Sun, M.Y. Kong, J. Cryst. Growth 207 (1999) 150. [7] E.S. Kim, N. Usami, Y. Shiraki, Appl. Phys. Lett. 72 (1998) 1617. [8] M. Shibata, Y. Nitta, K. Fujita, M. Ichikawa, Phys. Rev. B61 (2000) 7499. [9] M. Shibata, Y. Nitta, K. Fujita, M. Ichikawa, J. Cryst. Growth 220 (2000) 449. [10] Y. Nitta, M. Shibata, K. Fujita, M. Ichikawa, Surf. Sci. 462 (2000) L587. [11] K. Fujita, Y. Kusumi, M. Ichikawa, Surf. Sci. 380 (1997) 66. [12] G.M. Ribeiro, A.M. Bratkovski, T.I. Kamins, D.A.A. Ohlberg, R.S. Williams, Science 279 (1998) 353. [13] H. Sunamura, N. Usami, Y. Shiraki, S. Fukatsu, Appl. Phys. Lett. 66 (1995) 3024.
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Surface Science Letters
The nanometer-scale selective overgrowth of Ge over Si islands on Si(0 0 1) windows in ultrathin SiO2 ®lms Yoshiki Nitta 1, Motoshi Shibata 2, Ken Fujita 1, Masakazu Ichikawa * Joint Research Center for Atom Technology, Angstrom Technology Partnership (JRCAT-ATP), c/o National Institute for Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-0046, Japan Received 26 June 2001; accepted for publication 3 October 2001
Abstract We performed nanometer-scale overgrowths of Ge over islands of Si that had been grown on Si(0 0 1) windows in ultrathin silicon-dioxide ®lms. Growth was observed in real time by scanning tunneling microscopy. Si window areas with sizes of 10±30 nm were fabricated by using the thermal decomposition of the oxide to form voids. The selective epitaxial growth (SEG) of Si was then carried out by the introduction of disilane gas (Si2 H6 ). Faceted islands of Si grew in the voids during this stage. SEGs of Ge were then carried out by the introduction of germane gas (GeH4 ). Ge/Si hetero-islands grew with {1 0 5} facets. We then performed further overgrowths, of Si, on these Ge/Si hetero-islands to form structures with embedded Ge. The shape of the islands then changed from a pyramidal shape with {1 0 5} facets to a shape with {1 1 3} facets. Low-temperature photoluminescence spectra obtained from this sample showed a broad peak, which originated from the Ge islands embedded within the Si islands. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Scanning tunneling microscopy; Oxidation; Epitaxy; Silicon; Germanium; Silicon oxides
1. Introduction SiGe/Si heterostructures have promising electronic and optical properties and the formation of quantum wires and dots using such structures has been intensively studied [1]. This material system is * Corresponding author. Tel: +81-298-54-2621/61-2590; fax: +81-298-54-2577/61-2577. E-mail address: [email protected] (M. Ichikawa). 1 Present address: Semiconductor Laboratory, Oki Electric Industry Co., Ltd., 550-5 Higashiasakawa, Hachioji, Tokyo 193-8550, Japan. 2 Present address: Advanced Technology Laboratory, Matsushita Electric Industry Co., Ltd. 3-10-1 Higashi-mita, Tama-ku, Kawasaki 214-8501, Japan.
one of the model systems for growth in the Stranski±Krastanov (S±K) mode. The S±K growth is often used to form self-assembled quantum dot structures, but the sizes and positions of the grown structures are randomly distributed on the substrates. Several researchers [2±4] have tried to form self-assembled structures at orderly and predictable positions by the growth of Ge on islands of Si. They have grown Ge on patterned islands of Si to obtain an orderly arrangement of islands of Ge along the h1 0 0i edges of the Si islands. The overgrowth of Si layers on Ge islands grown by S±K growth has also been studied on Si(1 0 0) substrates. Sutter and Lagally have investigated the growth of Si on islands of Ge [5].
0039-6028/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 6 0 2 8 ( 0 1 ) 0 1 6 5 0 - 8
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Small islands of Ge on Si(1 0 0) disappeared during growth of Si, while larger islands grew and developed (1 0 0) top facets. Liu et al. have studied the eect of Si overgrowths on the structural and photoluminescence (PL) properties of Ge islands and strained Ge layers grown on Si(1 0 0) [6]. In the above cases, however, it is dicult to precisely control the size and position of Ge islands. Moreover the typical size of each island is of the order of 100 nm, and this is not attractive for application of quantum eects. Selective epitaxial growth (SEG), on the other hand, makes it possible to form nanoislands with a desired size and at desired positions. Kim et al. have reported the selective growth of Ge islands on Si windows in SiO2 ®lms [7]. The windows were formed by using conventional electron-beam lithography. They found that the growth mode of the Ge islands was strongly dependent on the size of the Si windows. Their report, however, only covered those cases where the size of the Si window was greater than 90 nm. We have described, in earlier reports, the formation of nanostructures by the nanometer-scale (20 nm) SEG of Si and Ge islands on Si(0 0 1) windows in ultrathin ®lms of SiO2 [8±10]. The crystalline ®lms in these cases were initially grown in a layer-by-layer fashion and then the pyramidal islands were formed on the windows. In order to get functional properties from the islands, the overgrowth on the islands is further required to form hetero-nanoisland structures. In this paper, we report on the real-time observation, by scanning tunneling microscopy (STM), of the growth of hetero-nanoislands; overgrowths of Ge on Si nanoislands and of Si on heteronanoislands of Ge/Si. 2. Experimental The experiments were performed in a hightemperature STM chamber with a dierential pumping system for Si2 H6 and GeH4 [11]. The sample used in this study was cut from a welloriented n-type Si(0 0 1) wafer. The sample was cleaned, then an ultrathin ®lm of SiO2 (0.3 nm thick) was produced by exposing the clean Si(0 0 1)
surface to molecular oxygen at 2:7 10 4 Pa and 630±730 °C for 5±10 min. The sample was then kept at 720±750 °C to decompose the oxide, until the diameters of the resulting voids reached 10±30 nm. After these voids had been formed, nanoislands of Si were selectively grown by introducing disilane gas (Si2 H6 ) at the substrate temperature of 540 °C. We have reported that the sidewalls of these Si islands were transformed from {1 1 13} facets to such steeper facets as {1 1 9} and {1 1 3} [8,9]. In this study, Si islands with {1 1 9} facets were selected as the initial Si islands. The Ge crystals were then selectively grown by introducing germane gas (GeH4 ), at 410 °C, to the chamber. The respective dose rates of Si2 H6 and GeH4 were ®xed at 1:6 1014 and 4 1014 s 1 cm 2 . After the SEG of Ge on the Si islands, the sample temperature was raised to 540 °C and Si was then selectively grown to form islands with an embedded-Ge structure. The selective growth was observed, in real time, with STM. Line-by-line background subtraction and high-pass ®ltering were applied to all of the STM images. 3. Results and discussion Fig. 1 shows the same area (a) after the SEG of Si and (b) 500 min after the start of the selective growth of Ge. The voids were initially rectangular with their bases parallel to the h1 1 0i direction, re¯ecting the crystal structure of the Si(0 0 1) substrate. The 3D islands grown by Si SEG were polyhedrons and their bases were also parallel to the h1 1 0i direction (Fig. 1(a)). GeH4 gas was then supplied as the source for the growth of Ge on these islands of Si, at the substrate temperature of 410 °C. Fig. 1(b) shows that the shapes of all of the islands were changed by the Ge overgrowths, and that selectivity was preserved over the whole sample during this growth process. The line at the top of each polyhedron was parallel to the h1 1 0i directions, and this indicates that the facets of these islands were rotated from the initial facets of Si nanoislands by 45°. Fig. 2 shows the transformations of the islands during the selective growth of Ge on the nanoislands of Si. These are magni®ed images of the
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Fig. 1. STM images of the same Si(0 0 1) surface (a) after Si SEG and (b) 500 min after the selective growth of Ge on the islands of Si had been started. The substrate temperature was 410 °C during growth. The ultrathin SiO2 ®lms remains ouside the Si island-grown areas. The detailed shape transformation of the island indicated by an arrow is shown in Fig. 2.
crystals indicated by the arrows in Fig. 1. Images were acquired (a) after the SEG of Si, and (b) 102 min after and (c) 501 min after the start of Ge selective overgrowth. The initial nanoislands of Si have mainly the (0 0 1) and {1 1 9} planes as mentioned above (Fig. 2(a)). In this image, doublelayered steps (DB steps) are aligned along the h1 1 0i directions at all sidewalls, and the distance between the steps shows that these are {1 1 9} planes. The SEG proceeded in a layer-by-layer fashion in the (0 0 1) plane in its initial stages (Fig. 2(b)). {1 0 5} facets formed at the corners of the Si islands. At the same time, h1 1 0i-related facets formed on the sidewalls, which intersect with the (0 0 1) plane along [1 1 0] lines, such as {1 1 7}, {1 1 5} and {1 1 3}. However {1 0 5} facets grew much faster than the h1 1 0i-related facets. In Fig. 2(b), {1 1 5} facets are seen on each sidewall but their area is small compared with that of the {1 0 5} facets. The {1 0 5} facets ®nally expand to become dominant and the islands take on a hutlike shape that is composed of {1 0 5} and {1 1 3} facets (Fig. 2(c)). Fig. 2(d) shows the pro®les revealing the transformation of the island shape along the lines indicated in (a)±(c). The Ge/Si nanoisland with about 4 nm height grew on the Si window area. We have not observed the interface structure using transmission electron microscopy. This island is thought, however, to be coherent to the underlying Si layer, since {1 0 5} faceted Ge island is known to be coherent to the Si layer [12].
Jin et al. have reported that Ge dots grow at the corners of the Si islands and that the reason for this is tensile stress at the corners of the Si islands [4]. This phenomena is also observed in the SEG of Ge in voids. In that case, several 3D islands appeared at the peripheries of the voids, where steps had been formed during the formation of the voids [10]. However, in the present work, the selective epitaxy of the Ge overgrowths proceeded twodimensionally and the small hut-like islands seen in the SEG of Ge in voids were not visible. This is presumably due to a preference for the 2D growth of {1 0 5} facets at the corners of the nanoislands of Si, because the islands are small. In fact, the growth of {1 0 5} facets on this island was completed within 180 min, much more quickly than the growth of the h1 1 0i-related facets. After the completion of the {1 0 5} facets, the h1 1 0i-related facets grew steeper and {1 1 3} facets were ®nally formed. We continued to supply the GeH4 for 2 h after the formation of the hut-like islands, but no particular change was visible. This may have been because the {1 0 5} facets are chemically stable, which is the case for the SEG of Ge [10]. To form structures with embedded Ge, Si2 H6 gas was then supplied to these Ge nanoisland, at the substrate temperature of 540 °C. Before the Si overgrowths were formed, the islands had {1 0 5} and {1 1 3} facets as described above (Fig. 3(a)). After growth of the Si had started, the {1 1 3} and (0 0 1) facets grew rapidly and the islands became
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Fig. 2. STM images of the selective growth of Ge over Si islands on Si(0 0 1). The island shown is indicated with an arrow in Fig. 1. Images were acquired (a) before growth and (b) 102 and (c) 501 min after growth had started. The substrate's temperature was 410 °C. The lower illustrations show the facet structures which appear in the islands. (d) Pro®les showing the transformation of the island shape along the lines indicated in (a)±(c).
higher (Fig. 3(b)). The polyhedron seen here is similar to that reported by Sutter and Lagally [5]. In their case, however, the islands became ¯atter and the areas of their bases became larger, and they developed (1 0 0) top facets. However, in the case of SEG, while the (1 0 0) top facets appeared, they grew smaller as the islands grew higher, because the expansion of the island was restricted by the surrounding ®lm of SiO2 . This restriction was
originated from the potential barrier (3 eV) at the boundary between SiO2 and Si window areas, which con®ned Si adatoms in the Si window area [8,9]. After the overgrowth of Si, the Ge islands became pyramidal with wide {1 1 3} facets and narrow (0 0 1) top facets with 7-nm height. It is likely that alloying of Si and Ge occurred at the interface because the two substances would probably have been intermixed during the overgrowth.
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Fig. 3. STM images of the selective growth of Si over islands of Ge on Si(0 0 1). The arrow indicates the same island in each image. Images were acquired (a) before growth and (b) 2h and (c) 4 h after growth had started. The substrate temperature was 540 °C. The lower illustrations show the facet structures which appear in the island.
Before Si overgrowth, the tunneling current vs. the sample bias (I±V ) spectra measured for the Ge/ Si islands showed that the energy gap was about 0.6 eV, which corresponds to the band gap of bulk Ge. While the I±V spectra measured for the Si/Ge/Si islands showed the energy gap was about 1.0 eV, which corresponds to the band gap of bulk Si. We had observed the energy gap of about 1 eV on clean Si(0 0 1) surfaces at the same tunneling condition. Moreover, as hydrogen atoms decomposed from Si2 H6 are considered to play a role of surfactant to suppress Ge surface segregation. These suggest that the top surface is very nearly pure Si. After the Si selective epitaxial overgrowth, lowtemperature photoluminescence (PL) measurements of the sample were performed (Fig. 4). Excitation was provided by an Ar laser and PL signals were collected by a liquid-nitrogen cooled Ge detector. The several peaks from 1.02 to 1.15 eV in Fig. 4 are the non-phonon and phononrelated peaks of Si [13]. The broad peak at around 0.9 eV is thought to originate from the threedimensionally embedded Ge, since this peak was not observed from the samples of Ge/Si nanoislands. Kim et al. reported similar PL peaks for Si-capped Ge islands on Si windows [7]. Liu et al. also reported that similar PL peaks were obtained
Fig. 4. PL spectra from a buried-Ge sample, measured at 4.2 K. The emission originating from the buied 3D island of Ge is visible.
from 3D islands of Ge in a Si/Ge/Si stacked structure [6]. The broadness of this peak is thought
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to be due to scattering in the distribution of the sizes of the Si/Ge/Si islands in Si windows. If, therefore, Si windows of uniform size were formed by irradiation with the ®eld-emission electron beam from an STM tip [8±10], the sizes and positions of the nanoislands would be well controlled, and a PL peak with higher intensity and narrower width would be expected.
4. Summary The nanometer-scale selective epitaxy of Ge overgrowths on nanoislands of Si on Si(0 0 1) was achieved with the use of ultrathin SiO2 ®lms. In the initial stage, SEG of Ge proceeded in a layer-bylayer fashion in the (0 0 1) plane, and {1 0 5} facets were formed at the corners of the islands. At the same time, h1 1 0i-related facets grew on the sidewalls of the islands. {1 0 5} facets grew much more quickly than the h1 1 0i-related facets. The growth of these {1 0 5} facets ®nally became dominant and the islands took on a hut-like shape. Si was then grown over these islands of Ge, with the intention of embedding islands of Ge in islands of Si. After the growth of Si, the Ge/Si islands became pyramidal with {1 1 3} facets. Low-temperature PL spectrum obtained from this sample showed a broad peak at 0.9 eV. This peak originated in the islands of Ge that had been three-dimensionally con®ned in the islands of Si.
Acknowledgements This work, partly supported by New Energy and Industrial Technology Development Organization (NEDO), was performed by the Joint Research Center for Atom Technology (JRCAT) under an agreement between the National Institute for Advanced Industrial Science and Technology (AIST) and the Angstrom Technology Partnership (ATP). References [1] K. Eberl, O.G. Schmidt, R. Duschl, O. Kienzle, E. Ernst, Y. Rau, Thin Solid Films 369 (2000) 33. [2] T.I. Kamins, R.S. Williams, Appl. Phys. Lett. 71 (1997) 1201. [3] L. Vescan, J. Cryst. Growth 194 (1998) 173. [4] G. Jin, J.L. Liu, S.G. Thomas, Y.H. Luo, K.L. Wang, Appl. Phys. Lett. 75 (1999) 2752. [5] P. Sutter, M.G. Lagally, Phys. Rev. Lett. 81 (1998) 3471. [6] J.P. Liu, J.Z. Wang, D.D. Huang, J.P. Li, D.Z. Sun, M.Y. Kong, J. Cryst. Growth 207 (1999) 150. [7] E.S. Kim, N. Usami, Y. Shiraki, Appl. Phys. Lett. 72 (1998) 1617. [8] M. Shibata, Y. Nitta, K. Fujita, M. Ichikawa, Phys. Rev. B61 (2000) 7499. [9] M. Shibata, Y. Nitta, K. Fujita, M. Ichikawa, J. Cryst. Growth 220 (2000) 449. [10] Y. Nitta, M. Shibata, K. Fujita, M. Ichikawa, Surf. Sci. 462 (2000) L587. [11] K. Fujita, Y. Kusumi, M. Ichikawa, Surf. Sci. 380 (1997) 66. [12] G.M. Ribeiro, A.M. Bratkovski, T.I. Kamins, D.A.A. Ohlberg, R.S. Williams, Science 279 (1998) 353. [13] H. Sunamura, N. Usami, Y. Shiraki, S. Fukatsu, Appl. Phys. Lett. 66 (1995) 3024.