- Email: [email protected]
Layered heteroepitaxial growth of germanium on Si(015) observed by scanning tunneling microscopy
.:.. :::.,::.i.:i.:‘:,i.:;:..~.~‘.::::,:. :I .:,..~.
:,.. :.-..::,,.>,..‘. j::
,...
..‘?
. . . :, .: .‘.A
:.. ;:,..
?‘,: . .
.,
..“, .(
;‘.:’ :. :
. . . ,,,/
.. ::
:
:
:
..’
surface sciencb
.., !: . ..i:.:_.. :::>:.; ,,... : .... ,..,:: ......:::j .. “...‘:-‘:::.:::_:::::_:!i_.i::.:::: :.:,: :y.(.:._,” .,:.” ELSD’IER
Surface
Science 301 (1994) 214-222
Layered heteroepitaxial growth of germanium on Si( 015) observed by scanning tunneling microscopy M. Tomitori ‘I Department ” Department
‘T*, K. Watanabe
‘, M. Kobayashi
‘, F. Iwawaki
‘, 0. Nishikawa
’
of Materials Science and Engineering, Interdisciplinary
Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227. Jupan of Electronics, Faculty of Engineering, Kanazawa Institute of Tech&o&y, 7-l Ohgigaoku, Nonoichl, Kanuzawa 921, Japun (Received
2 July 1993; accepted
for publication
17 September
1993)
Abstract Germanium
growth on a Si(O15) substrate was examined by scanning tunneling microscopy (STM) on an atomic on a Si(OO1) substrate showed the Stranski-Krastanov growth mode, where the dcpositcd films grew layer-by-layer up to a few ML followed by three-dimensional island growth with (015) facets. Thus the Gc films were deposited on the Si(O15) substrate in this study. As expected, the deposited Ge layers exhibited the layered growth mode up to thicker than 10 ML at a growth temperature of 400°C. The surface and the step structures, which seemed deeply related to the layered growth mechanism, were observed with STM and discussed in detail.
scale. The Ge deposition
1. Introduction Recently, heteroepitaxial growth of Ge on Si has attracted much interest for the fabrication of optoelectronic devices using highly developed Sitechnology. However, a 4.2% lattice mismatch between Ge and Si induces the strain in the deposited heteroepitaxial layers. Then the growth mode is complicated and it is difficult to obtain geometrically abrupt interfaces. For example, the heteroepitaxial growth of Ge on Si(OO1) substrates exhibits the Stranski-Krastanov growth mode, in which the initial deposited layers grow
* Corresponding
author
0039~6028/94/$07.00 0 1994 Elsevier SSDI 0039.6028(93)E0528-3
Science
two-dimensionally up to several layers and then the three-dimensional island growth starts on the deposited layers 111. Thus the surfaces covered with Ge become rough after several ML deposition even if the substrates are very flat. Some workers reported the Ge heteroepitaxial growth on Si(OO1) by LEED, RHEED, STM and other techniques, and observed the surface roughening with three-dimensional island growth [2-101. MO et al. [5] and lwawaki et al. [6] observed the growth of the so-called hut clusters with four facets of the (015) planes using the STM. MO et al. speculated that the hut cluster was an intermediate phase into the formation of macroscopic clusters with the (113) facets, which were higher and larger than the hut clusters and seemed stable at higher coverages. However, on
B.V. All rights reserved
M. Tomitori et al. /Surface
the {015) facets, less defects are found and the regular zigzag pattern of the 2 X 1 reconstruction is depicted in the STM images, independent of the amount of deposited Ge. Furthermore, after annealing the Ge deposited Si(OO1) substrates with macroscopic clusters at 500°C the {01.5) facets appeared again on the foot of the enlarged macroscopic clusters [lo]. These facts suggest that the (01.5) surface of Ge on Si can be stable, and the Si(O15) substrate can be suitable to obtain flat surfaces of epitaxially grown layers of Ge. This report presents the results of Ge heteroepitaxial growth on Si(O15) observed by scanning tunneling microscopy on an atomic scale.
2. Experimental An ultrahigh vacuum STM with a base pressure of 6 x lo-” Torr was employed for the present study [ll]. Samples cut from n-type Si(O15) wafers were used as substrates for Ge deposition. The resistivity was 0.02-0.05 R. cm. The substrates were cleaned at a heat stage in the STM vacuum chamber by resistively heating at about 1200°C for about 15 s after degassing at 500-700°C. After the substrate cleaning, the substrate temperature was gradually cooled and held at 4OO”C, then Ge atoms were deposited by heating Ge rods wounded with a tungsten-rhenium filament. The deposition rate and the amount were monitored with a quartz thickness monitor. A typical deposition rate was about 1 ML/min. After the deposition, the sample was immediately cooled to room temperature and transferred to the STM sample stage to observe the surface topography by the STM. The images were depicted at a tip voltage of 2.0 V and tunneling currents of 0.2 to 1 nA.
Science 301 (1994) 214-222
215
as mentioned previously. Fig. 1 shows a typical STM image of Ge on Si(OO1) with hut clusters grown at 500°C by 6.5 ML Ge deposition. The layered grown area of the Ge overlayers with missing dimer rows can be also seen clearly between the hut clusters [5-101. The hut clusters have four facets with the {015} plane indices, where regular zigzag patterns are observed, corresponding to a 2 x 1 reconstruction with respect to the ideal surface lattice of the (015) plane. Noticeably few defects are found on the facets, while a few straight step lines denoted by arrows are seen on the facets. This image implies that the hut cluster grows uniformly in the step-flow mode on the (015) facets. Thus the (015) surfaces of the Ge-Si hetero-structure are expected to be considerably stable for the growth. The schematic model of the zigzag pattern in the {015} facets proposed by several workers is shown in Fig. 2a, where 2 Ge-dimers on the small terraces of the (001) plane and the single-atomic height steps appear repeatedly [5,6]. The (001) plane intersects the (015) plane at an angle of 11.3”, as shown in Fig. 2b. In the top view, the dashed rectangle and the dotted lines indicate a
3. Results and discussion 3.1. {OlS} facets of hut cluster and Si(O15)
substrate The heteroepitaxial substrates shows the
growth of Ge on Si(OO1) Stranski-Krastanov mode,
Fig. 1. STM image of hut clusters wiih the (c15) facets of Ge on Si(OO1).The scanning area is 720 AX 680 A. The deposited amount of Ge was 6.5 ML and the growth temperature was 500°C. The arrows indicate the step lines on the (015) facets.
M. Tomitori et al./Surface
216
unit cell of the 2 x 1 reconstruction and the boundaries between the small (001) terraces, respectively. The direction of the 2 dimer long row is rotated by 90” across the single atomic height step. Accordingly to Chadi’s notation of step types on the (001) surface [12], the S, step with 2 dimers and the non-bonded S, step appear alternatively along the [loo] orientation on the (01.5) surface to form kinks; the underneath atom at
Non-bond:!
(4
[II: Top View
Science 301 (1994) 214-222
the edge of the non-bonded S, step does not bind with an atom on the lower terrace, then the underneath atom has a dangling bond. On the other hand, the non-bonded S, step is unstable compared with the re-bonded S, step of the Si(OO1) surface. The top layer atoms on the nonbonded S, step edge are brightly imaged as protruding areas in the STM images. Then the neighboring protruding areas appear zigzag. Fur-
Se Step
2x1 Unit Cell II
r
[05i
001
]
Side View
Dangling Bond
Fig. 2. (a) Schematic model of the atomic arrangement on the (015) facet, composed of 2 dimer long rows on the small (001) terrace truncated with single atomic height steps. (b) Ideal (015) surface. The top-layer atoms, denoted by black circles. are partially missed and the remained atoms with the dangling bonds form dimers. Cc) Typical high resolution STM image of Ge on Si(OlS). The positions denoted by alphabetical characters correspond to those shown in (a).
M. Tomitori et al. /Surface
thermore, this model is supported by the high resolution STM images showing individual dimers on the small (001) terraces of the (015) substrate, as shown in Fig. 2c. Fig. 3 shows an STM image of a SK0151 substrate cleaned by resistively heating at 1200°C for 15 s. The zigzag pattern, the same as that on the facet of the hut cluster, is partially observed on the flat area. The other patterns seem to be similar to the zigzag pattern, but the number of the dimers forming the row on the small (001) terraces is scattered, and it disturbs the regularly arranged pattern. The left part of Fig. 3 is the upper terrace region, and many rugged step lines are found, which run almost from the lower to the upper side in Fig. 3. The terrace widths of this substrate are not so wide, less than 100 A on average. This substrate is slightly misoriented by about 0.6” with respect to the (01.5) axis toward the [OSi] orientation. The regularity of the zigzag pattern can be improved by Si deposition on this substrate, and it can also widen the terrace width. However, Ge atoms were deposited on the SK0151 substrate right after cleaning the substrate without Si deposition to simplify the subsequent experimental process.
Fig. 3. STM image of the Si(O15) substrate. is 480 AX 450 A.
The scanning
area
Science 301 (I 9941 214-222
217
3.2. Layered growth of Ge on Si(O15) Germanium atoms were deposited on the Si(O15) substrates at a substrate temperature of 400°C by the amount of 0.3 to 10 ML. The STM images are shown in Figs. 4a-4f. The surface of the Ge overlayers grown on the (015) substrate exhibits the same 2 X 1 reconstruction as on the facets of the hut cluster, which appears regularly zigzagged in the STM images. The deposited Ge overlayers up to 10 ML on the SK0151 substrate show the two-dimensional layered growth and the similar zigzag reconstructed pattern on the surfaces. This is a noticeable contrast with the growth of Ge films on Si(OO1) substrates; at less than a few monolayers of the deposited Ge on Si(OOl), the strain due to the lattice mismatch induces missing dimer rows surrounding the Ge dimer rows, forming belt structures or patch structures, and at higher coverage the growth mode drastically changes to the hut cluster formation, the three-dimensional growth mode [5-101. Introduction of the missing dimers, which appear to break the dimer rows, reduces the strain piled up by forming Ge dimers in a line in the Ge overlayers. Furthermore, the atoms underneath the missing dimer can also rebond each other to reduce the number of the dangling bonds and to form a stable structure. From the viewpoint of the strain release for the Ge deposition on the Si(O15) substrate, the structure of the (015) surface has the advantage that it consists of short dimer rows on small (001) terraces. The strain piled along the dimer row can be easily released out at the step edge of the small (001) terraces for every 2 dimers. This strain release mechanism is similar to the introduction of the missing dimer rows, but the atoms underneath the step edge do not rebond and have one dangling bond per atom. Furthermore, as the deposited amount increases, the defect density on the deposited (015) surface is reduced and the terraces are widened. For example, comparing Fig. 4a, the Ge coverage of 0.3 ML, with the images of the higher Ge coverages in Figs. 4b-4f, it is clear that the defects are decreased with the increase in Ge coverage, where the defects are depicted as dark areas, which break the regularity of the zigzag pattern
in some places. The extension of the terrace width accompanies step-bunching with several atomic height steps and straightening step lines at the Ge coverage of higher than 3 ML, in Figs. 4d-4f, while the Si(O15) substrate and the low coverage surfaces exhibit a lot of the single atomic height steps and the rugged step lines, as pointed out previously. These results indicate the suitabil-
ity of the (015) substrate for the Ge heteroepitaxial growth to obtain the abrupt and low-defect Ge-Si interface. For a coverage of Ge higher than 10 ML, 3D island growth with complicated facets has been observed on the Si(O15) substrate at a Ge covcrage of about 13 ML at 400°C as shown in Fig. 5. The initial nucleation of the islands has not been
Fig. 4. (a) STM image of Ge overlayer on the Si(O15) substrate. The deposited amount of Ge is 0.3 ML at a growth temperature of 400°C. The scanning area is 720 & X 680 A as wide as that in (b)-(e). (h) 0.5 ML. Cc) I ML. (d) 3 ML. (e) 5 ML. (f) IO ML. The scanning area is 2000 A X 2000 A.
M. Tomitori CI ul. /Surface
Science 301 (1994) 214-222
219
Fig. 4. (continued)
found in the STM images, where the islands suddenly seem to grow large, but the same 2 X 1 reconstructed pattern remains on the flat area of the (015) surface between the islands. However, even at a Ge coverage thicker than about 15 ML,
Fig. 5. STM image of a 13 ML Ge film grown on the Si(O15) substrate at 400°C. The large 3D clusters grow on the layered-grown Ge film. The scanning area is 2400 AX 2300 A.
our Ge 2D the
preliminary STM observation shows that the deposited overlayer grows layer-by-layer with islands at 3OO”C,on which the surface exhibits same zigzag pattern, as shown in Fig. 6. The
Fig. 6. STM image of a 15 ML Ge overlayer grown on the Si(O15) substrate at 300°C. It shows the 2D island growth with tie zigzag pattern on the surface. The scanning area is 720 Ax680 A.
220
M. Tomitori et al. /Surface
critical thickness of the deposited Ge, which exhibits the 3D growth on the (015) surface, probably increases at 300°C; the {015} facets of the hut clusters on the Si(OO1)substrate remain at 300°C after the clusters coalesce each other at a Ge coverage as thick as 8 ML, although at higher temperatures new types of clusters without the {015} facets grow after the hut clusters almost cover the whole surface as thick as 5 ML at 400°C [lo]. This implies that the (015) surface of the deposited Ge on the SK0151 substrate is stabler at 300°C than at 400°C.
Science 301 (1994) 214-222
r
\ ‘I
3.3. Step structures and model of Ge growth on (015)
Here we consider the step structures and the growth model qualitatively to explain the Ge layered growth and the reproducibility of the 2 X 1 zigzag pattern on the Si(O15) substrate, based on the simple surface model in Fig. 2. Since the Ge overlayer on the (015) substrate grows at 400°C in the step flow mode as shown in the STM images above, key points to the growth mechanism can be in the step structure and the height. Various types of step structures come out on the (0151 surfaces. For example, Fig. 7 shows the step lines in the STM image of a 3 ML Ge deposited Si(O15) surface shown in Fig. 4d, where the left-lower part of the figure is the higher terrace region. The regular zigzag pattern on the terraces and the typical step lines can be found; the step lines run almost along the [OSil, [45i], [loo], [451] orientations, and the minimum step height estimated from the tip tracing profiles is about 0.5 A, corresponding to bhe interplanar spacing of the (015) plane, 0.55 A. In Fig. 7, the terrace heights are indicated by the number! in a unit of the single atomic step height, 0.55 A. All the step heights can be expressed by the multiples of the unit height, and single or double step heights are dominant for the 3 ML Ge deposition. Arrows in Fig. 4d show a type of the double height steps. This is the same step structure as seen on the (015) facets of the hut clusters indicated by the arrows in Fig. 1. This type of step along the [OSi] orientation seems to be deeply concerned with the layered growth on the (015)
Fig. 7. Trace of step lines surface shown in the STM indicate the terrace heights step height. 0.55 A. The terrace.
on a 3 ML Ge deposited Si(OO1) image of Fig. 4d. The numbers by the unit of the single atomic right-upper region is the lowest
substrate. It gradually disappeared with increase in the Ge deposition and the Ge growth can advance preferentially at the step edge. Figs. 8aSC show the STM images of this step without and with a kink, and the structure model, respectively. The lower side of the figure shows th,e upper terrace and the step height is about 1.1 A, corresponding to the double atomic height. The atomic configuration on the upper step edge appears to repeat the unit of a 2 dimer long row and a single dimer, which cross perpendicularly each other as seen in the zigzag line. The atoms underneath the single dimers on the step edge may partially rebond the atoms on the lower terrace. The Ge growth possibly advances at the kink along the [Ols] step line by a unit combined with the 2 dimer and the single dimer, but the detailed atomic structure of the kink is not clear from the STM images. Several configurations of the single dimer at the kink are observed, and it may indicate the growth process to elongate the step line. With a single atomic height, Fig. 9 shows the atomic configuration of the deposited Ge atoms represented by black circles. The adsorption sites of Ge with energetically preference are the position to reduce the number of dangling bonds, then the non-bonded A and B sites shown in Fig.
M. Tomitori et al. /Surface
221
Science 301 (1994) 214-222
bonds. The succeedingly migrating Ge atom possibly adsorbed at the A’ site to reduce the number of the dangling bonds. The adsorption at the A’ site breaks again the dimer bond at C’ site and gives rise to the extra dangling bonds at the C’ site. This site is also favorable to the succeedingly adsorbed Ge atom, and form a dimer with the non-bonded atom at the D site. Consequently, the zigzag pattern is resumed repeatedly and the step line can advance with a single atomic height in the Ge growth process. One of the noticeable features in the Ge growth on the (015) substrate is the almost defect-free structure on the surface, which is advantageous for the deposited atoms to diffuse without trapping near the defects. Moreover, the dark zigzag lines regularly arranged between the short dimer rows act as fast diffusion channels of the deposited atoms as the Si atom diffusion on the (001) surface [13]. The dynamic processes that the growth proceeds primarily along the [OlS]-ori-
[05ii
Top View r Fig. 8. (a) Enlarged STM image of the double-atomic height step with high resolution. (b) STM image of the step with kinks. (c) The schematic model of the step with the kink.
[ 1 0 0 1
0.55i
Side View 9 are suitable
atoms. and B and D bonded
for sitting for the migrating Ge When the two Ge atoms adsorbed at A sites form a dimer, the dimer bonds at C sites are broken to reproduce the nonS, step and then A’ site gets 3 dangling
Fig. 9. Schematic model of the atomic configuration on the (015) surface covered by the Ge overlayer with a single atomic height. The black circles represent the adsorption sites of the Ge atoms on the reconstructed (015) substrate. The ellipses show the atomic combination to form a dimer on the top layer, reproducing the zigzag pattern.
222
M. Tomitori et al. /Surface
ented steps suggests that the (015) wafer misoriented toward the [loo], on which the [Ol?]oriented steps are dominant, may be favorable to extend the critical thickness of the layered growth.
Science 301 (1994) 214-222
Professor Kunio Takayanagi of Tokyo Institute of Technology for valuable discussions. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
4. Conclusion References The Ge overlayer grown on the SK0151 substrate was observed by STM. It exhibits the layerby-layer growth up to 10 ML at 400°C. The defect density on the surface is reduced by the Ge deposition forming the regular zigzag reconstructed pattern. The reconstructed (015) surface is probably stable in the region close to the Ge-Si interface due to the strain release mechanism. The results indicate the suitability of the (015) substrate for the Ge heteroepitaxial growth to obtain an abrupt and low-defect Ge-Si interface. The STM observation of the facets of the initial 3D islands in the Stranski-Krastanov growth mode can be suitable for selecting a substrate for the heteroepitaxy with an abrupt interface.
Acknowledgements The authors would like to thank Associate Professor Hiroshi Tochihara of Hokkaido University for providing us with Si(O15) wafers, and
111E. Kasper and H. Jorke, J. Vat. Sci. Technol. A 10 (1992) 1927.
01 A. Asai, H. Ueba and C. Tatsuyama,
J. Appl. Phys. 58 (1985) 2577. f31 K. Miki, K. Sakamoto. T. Sakamoto, H. Okumura, N. Takahashi and S. Yoshida, J. Cryst. Growth 95 (lYX9) 444. [41S.S. lyer, P.R. Pukite, J.C. Tsang and M.W. Copel, J. Cryst. Growth 95 (1989) 439. and M.G. [51Y.-W. MO, D.E. Savage, B.S. Swartzentruber Lagally, Phys. Rev. Lett. 65 (1990) 1020. [61 F. Iwawaki, M. Tomitori and 0. Nishikawa, Surf. Sci. Lett. 253 (1991) L411. and 0. Nishikawa, Ultrami[71F. Iwawaki, M. Tomitori croscopy 42-44 (1992) 902. Hoegen [XlU. Kohler, 0. Jusko, B. Miiller, M. Horn-von and M. Pook, Ultramicroscopy 42-44 (1992) X23. [91J. Knall and J.B. Pethica, Surf. Sci. 265 (1992) 156. K. Watanabe, M. Kobayashi and 0. [lOI M. Tomitori, Nishikawa, Appl. Surf. Sci., to be published. [Ill 0. Nishikawa, M. Tomitori and F. Iwawaki, Mater. Sci. Eng. B 8 (1991) 81. [121 D.J. Chadi, Phys. Rev. Lett. 59 (1987) 1691. [131 C. Roland and G.H. Gilmer, Phys. Rev. B 46 (1992) 13428.
:,.. :.-..::,,.>,..‘. j::
,...
..‘?
. . . :, .: .‘.A
:.. ;:,..
?‘,: . .
.,
..“, .(
;‘.:’ :. :
. . . ,,,/
.. ::
:
:
:
..’
surface sciencb
.., !: . ..i:.:_.. :::>:.; ,,... : .... ,..,:: ......:::j .. “...‘:-‘:::.:::_:::::_:!i_.i::.:::: :.:,: :y.(.:._,” .,:.” ELSD’IER
Surface
Science 301 (1994) 214-222
Layered heteroepitaxial growth of germanium on Si( 015) observed by scanning tunneling microscopy M. Tomitori ‘I Department ” Department
‘T*, K. Watanabe
‘, M. Kobayashi
‘, F. Iwawaki
‘, 0. Nishikawa
’
of Materials Science and Engineering, Interdisciplinary
Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 227. Jupan of Electronics, Faculty of Engineering, Kanazawa Institute of Tech&o&y, 7-l Ohgigaoku, Nonoichl, Kanuzawa 921, Japun (Received
2 July 1993; accepted
for publication
17 September
1993)
Abstract Germanium
growth on a Si(O15) substrate was examined by scanning tunneling microscopy (STM) on an atomic on a Si(OO1) substrate showed the Stranski-Krastanov growth mode, where the dcpositcd films grew layer-by-layer up to a few ML followed by three-dimensional island growth with (015) facets. Thus the Gc films were deposited on the Si(O15) substrate in this study. As expected, the deposited Ge layers exhibited the layered growth mode up to thicker than 10 ML at a growth temperature of 400°C. The surface and the step structures, which seemed deeply related to the layered growth mechanism, were observed with STM and discussed in detail.
scale. The Ge deposition
1. Introduction Recently, heteroepitaxial growth of Ge on Si has attracted much interest for the fabrication of optoelectronic devices using highly developed Sitechnology. However, a 4.2% lattice mismatch between Ge and Si induces the strain in the deposited heteroepitaxial layers. Then the growth mode is complicated and it is difficult to obtain geometrically abrupt interfaces. For example, the heteroepitaxial growth of Ge on Si(OO1) substrates exhibits the Stranski-Krastanov growth mode, in which the initial deposited layers grow
* Corresponding
author
0039~6028/94/$07.00 0 1994 Elsevier SSDI 0039.6028(93)E0528-3
Science
two-dimensionally up to several layers and then the three-dimensional island growth starts on the deposited layers 111. Thus the surfaces covered with Ge become rough after several ML deposition even if the substrates are very flat. Some workers reported the Ge heteroepitaxial growth on Si(OO1) by LEED, RHEED, STM and other techniques, and observed the surface roughening with three-dimensional island growth [2-101. MO et al. [5] and lwawaki et al. [6] observed the growth of the so-called hut clusters with four facets of the (015) planes using the STM. MO et al. speculated that the hut cluster was an intermediate phase into the formation of macroscopic clusters with the (113) facets, which were higher and larger than the hut clusters and seemed stable at higher coverages. However, on
B.V. All rights reserved
M. Tomitori et al. /Surface
the {015) facets, less defects are found and the regular zigzag pattern of the 2 X 1 reconstruction is depicted in the STM images, independent of the amount of deposited Ge. Furthermore, after annealing the Ge deposited Si(OO1) substrates with macroscopic clusters at 500°C the {01.5) facets appeared again on the foot of the enlarged macroscopic clusters [lo]. These facts suggest that the (01.5) surface of Ge on Si can be stable, and the Si(O15) substrate can be suitable to obtain flat surfaces of epitaxially grown layers of Ge. This report presents the results of Ge heteroepitaxial growth on Si(O15) observed by scanning tunneling microscopy on an atomic scale.
2. Experimental An ultrahigh vacuum STM with a base pressure of 6 x lo-” Torr was employed for the present study [ll]. Samples cut from n-type Si(O15) wafers were used as substrates for Ge deposition. The resistivity was 0.02-0.05 R. cm. The substrates were cleaned at a heat stage in the STM vacuum chamber by resistively heating at about 1200°C for about 15 s after degassing at 500-700°C. After the substrate cleaning, the substrate temperature was gradually cooled and held at 4OO”C, then Ge atoms were deposited by heating Ge rods wounded with a tungsten-rhenium filament. The deposition rate and the amount were monitored with a quartz thickness monitor. A typical deposition rate was about 1 ML/min. After the deposition, the sample was immediately cooled to room temperature and transferred to the STM sample stage to observe the surface topography by the STM. The images were depicted at a tip voltage of 2.0 V and tunneling currents of 0.2 to 1 nA.
Science 301 (1994) 214-222
215
as mentioned previously. Fig. 1 shows a typical STM image of Ge on Si(OO1) with hut clusters grown at 500°C by 6.5 ML Ge deposition. The layered grown area of the Ge overlayers with missing dimer rows can be also seen clearly between the hut clusters [5-101. The hut clusters have four facets with the {015} plane indices, where regular zigzag patterns are observed, corresponding to a 2 x 1 reconstruction with respect to the ideal surface lattice of the (015) plane. Noticeably few defects are found on the facets, while a few straight step lines denoted by arrows are seen on the facets. This image implies that the hut cluster grows uniformly in the step-flow mode on the (015) facets. Thus the (015) surfaces of the Ge-Si hetero-structure are expected to be considerably stable for the growth. The schematic model of the zigzag pattern in the {015} facets proposed by several workers is shown in Fig. 2a, where 2 Ge-dimers on the small terraces of the (001) plane and the single-atomic height steps appear repeatedly [5,6]. The (001) plane intersects the (015) plane at an angle of 11.3”, as shown in Fig. 2b. In the top view, the dashed rectangle and the dotted lines indicate a
3. Results and discussion 3.1. {OlS} facets of hut cluster and Si(O15)
substrate The heteroepitaxial substrates shows the
growth of Ge on Si(OO1) Stranski-Krastanov mode,
Fig. 1. STM image of hut clusters wiih the (c15) facets of Ge on Si(OO1).The scanning area is 720 AX 680 A. The deposited amount of Ge was 6.5 ML and the growth temperature was 500°C. The arrows indicate the step lines on the (015) facets.
M. Tomitori et al./Surface
216
unit cell of the 2 x 1 reconstruction and the boundaries between the small (001) terraces, respectively. The direction of the 2 dimer long row is rotated by 90” across the single atomic height step. Accordingly to Chadi’s notation of step types on the (001) surface [12], the S, step with 2 dimers and the non-bonded S, step appear alternatively along the [loo] orientation on the (01.5) surface to form kinks; the underneath atom at
Non-bond:!
(4
[II: Top View
Science 301 (1994) 214-222
the edge of the non-bonded S, step does not bind with an atom on the lower terrace, then the underneath atom has a dangling bond. On the other hand, the non-bonded S, step is unstable compared with the re-bonded S, step of the Si(OO1) surface. The top layer atoms on the nonbonded S, step edge are brightly imaged as protruding areas in the STM images. Then the neighboring protruding areas appear zigzag. Fur-
Se Step
2x1 Unit Cell II
r
[05i
001
]
Side View
Dangling Bond
Fig. 2. (a) Schematic model of the atomic arrangement on the (015) facet, composed of 2 dimer long rows on the small (001) terrace truncated with single atomic height steps. (b) Ideal (015) surface. The top-layer atoms, denoted by black circles. are partially missed and the remained atoms with the dangling bonds form dimers. Cc) Typical high resolution STM image of Ge on Si(OlS). The positions denoted by alphabetical characters correspond to those shown in (a).
M. Tomitori et al. /Surface
thermore, this model is supported by the high resolution STM images showing individual dimers on the small (001) terraces of the (015) substrate, as shown in Fig. 2c. Fig. 3 shows an STM image of a SK0151 substrate cleaned by resistively heating at 1200°C for 15 s. The zigzag pattern, the same as that on the facet of the hut cluster, is partially observed on the flat area. The other patterns seem to be similar to the zigzag pattern, but the number of the dimers forming the row on the small (001) terraces is scattered, and it disturbs the regularly arranged pattern. The left part of Fig. 3 is the upper terrace region, and many rugged step lines are found, which run almost from the lower to the upper side in Fig. 3. The terrace widths of this substrate are not so wide, less than 100 A on average. This substrate is slightly misoriented by about 0.6” with respect to the (01.5) axis toward the [OSi] orientation. The regularity of the zigzag pattern can be improved by Si deposition on this substrate, and it can also widen the terrace width. However, Ge atoms were deposited on the SK0151 substrate right after cleaning the substrate without Si deposition to simplify the subsequent experimental process.
Fig. 3. STM image of the Si(O15) substrate. is 480 AX 450 A.
The scanning
area
Science 301 (I 9941 214-222
217
3.2. Layered growth of Ge on Si(O15) Germanium atoms were deposited on the Si(O15) substrates at a substrate temperature of 400°C by the amount of 0.3 to 10 ML. The STM images are shown in Figs. 4a-4f. The surface of the Ge overlayers grown on the (015) substrate exhibits the same 2 X 1 reconstruction as on the facets of the hut cluster, which appears regularly zigzagged in the STM images. The deposited Ge overlayers up to 10 ML on the SK0151 substrate show the two-dimensional layered growth and the similar zigzag reconstructed pattern on the surfaces. This is a noticeable contrast with the growth of Ge films on Si(OO1) substrates; at less than a few monolayers of the deposited Ge on Si(OOl), the strain due to the lattice mismatch induces missing dimer rows surrounding the Ge dimer rows, forming belt structures or patch structures, and at higher coverage the growth mode drastically changes to the hut cluster formation, the three-dimensional growth mode [5-101. Introduction of the missing dimers, which appear to break the dimer rows, reduces the strain piled up by forming Ge dimers in a line in the Ge overlayers. Furthermore, the atoms underneath the missing dimer can also rebond each other to reduce the number of the dangling bonds and to form a stable structure. From the viewpoint of the strain release for the Ge deposition on the Si(O15) substrate, the structure of the (015) surface has the advantage that it consists of short dimer rows on small (001) terraces. The strain piled along the dimer row can be easily released out at the step edge of the small (001) terraces for every 2 dimers. This strain release mechanism is similar to the introduction of the missing dimer rows, but the atoms underneath the step edge do not rebond and have one dangling bond per atom. Furthermore, as the deposited amount increases, the defect density on the deposited (015) surface is reduced and the terraces are widened. For example, comparing Fig. 4a, the Ge coverage of 0.3 ML, with the images of the higher Ge coverages in Figs. 4b-4f, it is clear that the defects are decreased with the increase in Ge coverage, where the defects are depicted as dark areas, which break the regularity of the zigzag pattern
in some places. The extension of the terrace width accompanies step-bunching with several atomic height steps and straightening step lines at the Ge coverage of higher than 3 ML, in Figs. 4d-4f, while the Si(O15) substrate and the low coverage surfaces exhibit a lot of the single atomic height steps and the rugged step lines, as pointed out previously. These results indicate the suitabil-
ity of the (015) substrate for the Ge heteroepitaxial growth to obtain the abrupt and low-defect Ge-Si interface. For a coverage of Ge higher than 10 ML, 3D island growth with complicated facets has been observed on the Si(O15) substrate at a Ge covcrage of about 13 ML at 400°C as shown in Fig. 5. The initial nucleation of the islands has not been
Fig. 4. (a) STM image of Ge overlayer on the Si(O15) substrate. The deposited amount of Ge is 0.3 ML at a growth temperature of 400°C. The scanning area is 720 & X 680 A as wide as that in (b)-(e). (h) 0.5 ML. Cc) I ML. (d) 3 ML. (e) 5 ML. (f) IO ML. The scanning area is 2000 A X 2000 A.
M. Tomitori CI ul. /Surface
Science 301 (1994) 214-222
219
Fig. 4. (continued)
found in the STM images, where the islands suddenly seem to grow large, but the same 2 X 1 reconstructed pattern remains on the flat area of the (015) surface between the islands. However, even at a Ge coverage thicker than about 15 ML,
Fig. 5. STM image of a 13 ML Ge film grown on the Si(O15) substrate at 400°C. The large 3D clusters grow on the layered-grown Ge film. The scanning area is 2400 AX 2300 A.
our Ge 2D the
preliminary STM observation shows that the deposited overlayer grows layer-by-layer with islands at 3OO”C,on which the surface exhibits same zigzag pattern, as shown in Fig. 6. The
Fig. 6. STM image of a 15 ML Ge overlayer grown on the Si(O15) substrate at 300°C. It shows the 2D island growth with tie zigzag pattern on the surface. The scanning area is 720 Ax680 A.
220
M. Tomitori et al. /Surface
critical thickness of the deposited Ge, which exhibits the 3D growth on the (015) surface, probably increases at 300°C; the {015} facets of the hut clusters on the Si(OO1)substrate remain at 300°C after the clusters coalesce each other at a Ge coverage as thick as 8 ML, although at higher temperatures new types of clusters without the {015} facets grow after the hut clusters almost cover the whole surface as thick as 5 ML at 400°C [lo]. This implies that the (015) surface of the deposited Ge on the SK0151 substrate is stabler at 300°C than at 400°C.
Science 301 (1994) 214-222
r
\ ‘I
3.3. Step structures and model of Ge growth on (015)
Here we consider the step structures and the growth model qualitatively to explain the Ge layered growth and the reproducibility of the 2 X 1 zigzag pattern on the Si(O15) substrate, based on the simple surface model in Fig. 2. Since the Ge overlayer on the (015) substrate grows at 400°C in the step flow mode as shown in the STM images above, key points to the growth mechanism can be in the step structure and the height. Various types of step structures come out on the (0151 surfaces. For example, Fig. 7 shows the step lines in the STM image of a 3 ML Ge deposited Si(O15) surface shown in Fig. 4d, where the left-lower part of the figure is the higher terrace region. The regular zigzag pattern on the terraces and the typical step lines can be found; the step lines run almost along the [OSil, [45i], [loo], [451] orientations, and the minimum step height estimated from the tip tracing profiles is about 0.5 A, corresponding to bhe interplanar spacing of the (015) plane, 0.55 A. In Fig. 7, the terrace heights are indicated by the number! in a unit of the single atomic step height, 0.55 A. All the step heights can be expressed by the multiples of the unit height, and single or double step heights are dominant for the 3 ML Ge deposition. Arrows in Fig. 4d show a type of the double height steps. This is the same step structure as seen on the (015) facets of the hut clusters indicated by the arrows in Fig. 1. This type of step along the [OSi] orientation seems to be deeply concerned with the layered growth on the (015)
Fig. 7. Trace of step lines surface shown in the STM indicate the terrace heights step height. 0.55 A. The terrace.
on a 3 ML Ge deposited Si(OO1) image of Fig. 4d. The numbers by the unit of the single atomic right-upper region is the lowest
substrate. It gradually disappeared with increase in the Ge deposition and the Ge growth can advance preferentially at the step edge. Figs. 8aSC show the STM images of this step without and with a kink, and the structure model, respectively. The lower side of the figure shows th,e upper terrace and the step height is about 1.1 A, corresponding to the double atomic height. The atomic configuration on the upper step edge appears to repeat the unit of a 2 dimer long row and a single dimer, which cross perpendicularly each other as seen in the zigzag line. The atoms underneath the single dimers on the step edge may partially rebond the atoms on the lower terrace. The Ge growth possibly advances at the kink along the [Ols] step line by a unit combined with the 2 dimer and the single dimer, but the detailed atomic structure of the kink is not clear from the STM images. Several configurations of the single dimer at the kink are observed, and it may indicate the growth process to elongate the step line. With a single atomic height, Fig. 9 shows the atomic configuration of the deposited Ge atoms represented by black circles. The adsorption sites of Ge with energetically preference are the position to reduce the number of dangling bonds, then the non-bonded A and B sites shown in Fig.
M. Tomitori et al. /Surface
221
Science 301 (1994) 214-222
bonds. The succeedingly migrating Ge atom possibly adsorbed at the A’ site to reduce the number of the dangling bonds. The adsorption at the A’ site breaks again the dimer bond at C’ site and gives rise to the extra dangling bonds at the C’ site. This site is also favorable to the succeedingly adsorbed Ge atom, and form a dimer with the non-bonded atom at the D site. Consequently, the zigzag pattern is resumed repeatedly and the step line can advance with a single atomic height in the Ge growth process. One of the noticeable features in the Ge growth on the (015) substrate is the almost defect-free structure on the surface, which is advantageous for the deposited atoms to diffuse without trapping near the defects. Moreover, the dark zigzag lines regularly arranged between the short dimer rows act as fast diffusion channels of the deposited atoms as the Si atom diffusion on the (001) surface [13]. The dynamic processes that the growth proceeds primarily along the [OlS]-ori-
[05ii
Top View r Fig. 8. (a) Enlarged STM image of the double-atomic height step with high resolution. (b) STM image of the step with kinks. (c) The schematic model of the step with the kink.
[ 1 0 0 1
0.55i
Side View 9 are suitable
atoms. and B and D bonded
for sitting for the migrating Ge When the two Ge atoms adsorbed at A sites form a dimer, the dimer bonds at C sites are broken to reproduce the nonS, step and then A’ site gets 3 dangling
Fig. 9. Schematic model of the atomic configuration on the (015) surface covered by the Ge overlayer with a single atomic height. The black circles represent the adsorption sites of the Ge atoms on the reconstructed (015) substrate. The ellipses show the atomic combination to form a dimer on the top layer, reproducing the zigzag pattern.
222
M. Tomitori et al. /Surface
ented steps suggests that the (015) wafer misoriented toward the [loo], on which the [Ol?]oriented steps are dominant, may be favorable to extend the critical thickness of the layered growth.
Science 301 (1994) 214-222
Professor Kunio Takayanagi of Tokyo Institute of Technology for valuable discussions. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan.
4. Conclusion References The Ge overlayer grown on the SK0151 substrate was observed by STM. It exhibits the layerby-layer growth up to 10 ML at 400°C. The defect density on the surface is reduced by the Ge deposition forming the regular zigzag reconstructed pattern. The reconstructed (015) surface is probably stable in the region close to the Ge-Si interface due to the strain release mechanism. The results indicate the suitability of the (015) substrate for the Ge heteroepitaxial growth to obtain an abrupt and low-defect Ge-Si interface. The STM observation of the facets of the initial 3D islands in the Stranski-Krastanov growth mode can be suitable for selecting a substrate for the heteroepitaxy with an abrupt interface.
Acknowledgements The authors would like to thank Associate Professor Hiroshi Tochihara of Hokkaido University for providing us with Si(O15) wafers, and
111E. Kasper and H. Jorke, J. Vat. Sci. Technol. A 10 (1992) 1927.
01 A. Asai, H. Ueba and C. Tatsuyama,
J. Appl. Phys. 58 (1985) 2577. f31 K. Miki, K. Sakamoto. T. Sakamoto, H. Okumura, N. Takahashi and S. Yoshida, J. Cryst. Growth 95 (lYX9) 444. [41S.S. lyer, P.R. Pukite, J.C. Tsang and M.W. Copel, J. Cryst. Growth 95 (1989) 439. and M.G. [51Y.-W. MO, D.E. Savage, B.S. Swartzentruber Lagally, Phys. Rev. Lett. 65 (1990) 1020. [61 F. Iwawaki, M. Tomitori and 0. Nishikawa, Surf. Sci. Lett. 253 (1991) L411. and 0. Nishikawa, Ultrami[71F. Iwawaki, M. Tomitori croscopy 42-44 (1992) 902. Hoegen [XlU. Kohler, 0. Jusko, B. Miiller, M. Horn-von and M. Pook, Ultramicroscopy 42-44 (1992) X23. [91J. Knall and J.B. Pethica, Surf. Sci. 265 (1992) 156. K. Watanabe, M. Kobayashi and 0. [lOI M. Tomitori, Nishikawa, Appl. Surf. Sci., to be published. [Ill 0. Nishikawa, M. Tomitori and F. Iwawaki, Mater. Sci. Eng. B 8 (1991) 81. [121 D.J. Chadi, Phys. Rev. Lett. 59 (1987) 1691. [131 C. Roland and G.H. Gilmer, Phys. Rev. B 46 (1992) 13428.