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Reflection electron microscope and scanning tunneling microscope observations of CVD diamond (001) surfaces
Diamond and Related Materials, 2 (1993) 1271 1276
1271
Reflection electron microscope and scanning tunneling microscope observations of CVD diamond (001) surfaces* H. Sasaki, M. Aoki and H. Kawarada School of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku, Tokyo 169 (Japan) (Received August 31, 1992; accepted in final form December 7. 19921
Abstract Reflection electron microscopy (REM) and scanning tunneling microscopy have been applied to the estimation of the surface flatness of diamond (001) surfaces and to the 2 x 1 reconstructed structure. The macroscopic surface flatness observed by REM has been improved by using CO as a source gas and by boron-doping.
1. Introduction In recent years, many studies on diamond films synthesized by CVD methods have been carried out, but the growth mechanism, i.e. the surface reaction on diamond, has not been understood completely. It has been considered that the (001) 2 x 1 structure of diamond [1] holds the key to crystal growth like the Si (001) surface [2]. In this study, we have investigated the surface flatness and the 2 x 1 reconstructed structure of the diamond (001) surface in order to consider the surface reaction on diamond. Observations of diamond (001) surfaces have been made by several authors. For example, homo-epitaxial diamond (001) surfaces have been observed to be smooth and reconstructed to the 2 x 1 structure [3]. The textured growth of (001) surfaces on silicon substrates under the condition of high concentrations of CH4 has been reported [4]. These facts suggest that the diamond (001) surface appears spontaneously, and the reason is considered to be as follows. As carbon has a larger surface energy than other materials, the appearance of high order index surfaces is suppressed and the surfaces tend to be of lower order index. As the surface energy has a definite minimum value on the (001) surface in general, the (001~ surface is expected to become atomically flat. However, microscopic observations have been made by only a few authors [5, 6]. Additionally, methods for the advancement of surface flatness have not been fully developed. The diamond (001) surface reconstructs to a 2 x 1/1 x 2 structure like other diamond group crystals *Paper presented at Diamond 1992, Heidelberg, September 4, 1992.
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such as silicon and germanium. This structure was reported on natural diamond by Lurie in 1977 [1] using low energy electron diffraction (LEED). In epitaxially grown CVD diamond films, it has been observed using R H E E D and scanning tunneling microscopy (STM) [3, 5, 6]. In the R H E E D observations [3, 5], the zero order Laue zone was observed, but the half-order Laue zone indicating smoother 2 x 1 surfaces was not.. The observed surfaces are not smooth enough to have long reciprocal rods in the reciprocal space. Moreover, detail of the dimer structure has not been confirmed and some models have been proposed. The reflection electron microscopy (REM) images are very sensitive to surface morphology in comparison with those of scanning or transmission electron microscopy, and single atom height steps can be resolved using phase contrast caused by aberration of focus, e.g. ref. 7. STM is very attractive for its atomic level observation of the surface. We used REM for macroscopic observation and STM for microscopic observation to estimate the improved surface smoothness and its correlation with the 2 x 1/1 × 2 reconstructed structure.
2. Experimental methods Diamond homo-epitaxial films were grown by the conventional microwave plasma-assisted CVD method on high-pressure synthetic type Ib diamond (001) substrates. The total flow of source gas was 200 s.c.c.m., and the total pressure was 35 Torr. The concentrations of source gases were C H 4 / H 2 = 5 v o l . % or C O / H 2 = 5 vol.%. Boron doping was carried out by B2H 6 during the deposition. Boron/carbon (B/C) ratios in the gas
1993 -
Elsevier Sequoia. All rights reserved
H. Sasaki et al. / Surface structure of epitaxial CVD diamond (001)
1272
phase were 50, 1000, 2000 and 4000 p.p.m. The substrate temperatures during deposition were about 850 °C, and deposition time was normally 3 h. RHEED and REM observations were performed using conventional transmission electron microscopy (JEOL JEM-100CX). Specimens were mounted on bent single slit grids for REM observations. During all observations, the accelerating voltage was 100 keV and the pressure in the column was about 2-4 x 10 - 6 Torr. STM observations were performed using a Seiko I. SAM-3000 at atmospheric pressure.
3. Results and discussion
3.1. Interpretation of RHEED pattern of (001)-2 x 1 To analyze the RHEED patterns, it is important to consider the arrangement of reciprocal space and the Ewald sphere (Ewald construction). In this section, the Ewald construction of the diamond (001) surface will be explained. Figure 1 shows a top view of the 2 x 1/1 × 2 reconstructed diamond (001) surface. In this figure, 2 x 1 and 1 x 2 domains (A and B), where the filled circles are dimer atoms, are divided by a single atomic height step. The dimer rows in one domain extend to the direction perpendicular to the dimer rows in another domain. As the distance between two dimer rows is twice the lattice constant of an ideal (001) surface, the corresponding reciprocal lattice points with half periodicity in the I-110] direction appear in reciprocal space. The reciprocal points of the reconstructed diamond (001) surface are shown in Fig. 2. As a surface becomes smoother, the corresponding reciprocal lattice points extend in the direction perpendicular to the surface and become reciprocal rods. The length of the rods is proportional to the terrace size of
0
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•
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0
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[010]
0 ,
0
0
"
,
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100]
Fig. 2. Reciprocal lattice of diamond (001) surface 2 x 1 reconstructed structure. Reciprocal lattice points due to this structure ( 0 ) have a periodicity of fundamental points half as long as that of the [I10] direction because the lattice constant of the 2 × 1 structure is twice that of the fundamental lattice to the [1 I0] direction in real space.
the (001) surface. Hence, we can estimate the surface flatness from the length of the reciprocal lattice rods. Figure 3 shows the arrangement of the Ewald sphere and reciprocal space in the [110] incident direction. The intersecting circular points of the Ewald sphere and the reciprocal rods appear as a RHEED pattern. Figure 4(a) shows a schematic diagram of the RHEED pattern on a diamond (001) surface in the [110] incident direction. The open and filled circles represent the fundamental and superstructure reciprocal lattice points respectively. The superstructure spots are observed in
[001] (a)
E ~ +00
i / ~ = =
)i~..
~
S
Loili
or
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[110] (b)
L~
E
/~ 5.04A.....~/ [°il1 P
~A
Single step
Fig. I. Ball and stick model of diamond (00l) 2 × 1/1 x 2 reconstructed surface. Distance between two dimer rows is 5.04 A (2/x/2 × lattice constant). O, dimer atoms; O, bulk atoms.
oo
[ilO1
[11Ol
Fig. 3. Ewald construction of a diamond (001) surface of RHEED observation in the [110] incident direction. E, Ewald sphere; S, shadow edge; Ko, wave vector of the incident beam; L m Nth order Laue zone; 0, glancing angle. In this case, the 008 spot is on the Ewald sphere (glancing angle 0 is about 2.4 ° when the accelerating voltage is 100 kV). In the zeroth and half-order Laue zones, the spots due to the 2 × 1 structure are observed because the reciprocal lattice rods are elongated O, reciprocal lattice points on Ewald sphere; O, reciprocal lattice points due to surface atoms; O, reciprocal lattice points due to 2 x 1/1 × 2 reconstructed structure; 0 , other reciprocal lattice points.
H. Sasaki et al. / Surface structure of epitaxial CVD diamond (001)
xO 0"0"0"0"0 O*O-O*O*O (a)
0.0.0-0.0
[1~Olj [ioo1 V_.. [1101
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confirmed that the surface of the film reconstructs to a 2 x 1/i x 2 structure. The faint spots of L1/2 (half-order Laue zone) in the RHEED pattern can be observed only in the [100] incident direction. This is because the length of the L1/2 rods in the [100] incident direction is less than the length of the L~/2 rods in the [110] incident direction, This suggests that reciprocal lattice rods of non-doped films do not elongate so much. In Fig. 4(c), the zero-order Laue zone cannot be seen because it is hidden behind a very bright background when the glancing angle is small. The spots belonging t o L1/2 are clearly observed. This result indicates that the reciprocal lattice points due to the 2 x 1 structure elongated more than for those of the non-doped film and suggests that the surface of the boron-doped film is smoother than the non-doped one. The spots of L1/2 in Fig. 4(c) are very streaky. This indicates that the half-order reciprocal lattice points form thin sheets. We call these sheets "reciprocal lattice sheets". The streaky spots of L~/2 and the presence of a very bright background will be discussed later. 3.3. R E M images
(b)
(c) Fig. 4 (a) Schematic diagram of the RHEED pattern in the [110] incident direction and two-dimensionalreciprocal lattice of the diamond (001) surface. (b) and (c), RHEED patterns of the non-doped and boron-doped (B/C=2000 p.p.m.) homo-epitaxial diamond films respectively. The spots of the Laue zone L~/2are observed in (c) but not in (b). the zero order Laue zone. When the surface is flat, i.e. the reciprocal lattice rods elongate, RHEED spots due to the superstructure are observed as a half-order Laue zone El, 23.2. Observed R H E E D patterns
Figures 4(b) and 4(c) show RHEED patterns of nondoped and boron-doped homo-epitaxial diamond films respectively. Both patterns were taken in the [110] incident direction. In Fig. 4(b), (0,1/2) streaky spots are seen in the zero-order Laue zone. As this pattern was also observed in the [1T0] incident direction, it is
The REM images in the present experiment have been obtained by using a spot (normall~,/specular spot) in each RHEED pattern to form real images. Reflected electron beams are focused by an objective lens, and form a RHEED pattern at the back focal plane. As the specular spot is only selected for the imaging of REM by the OL aperture, the regions corresponding to the spot are visualized in the REM image. As the intensity of the spot in the RHEED pattern is sensitive to the glancing angle, the REM image is sensitive to the Bragg condition. Thus, the bright regions in the REM image correspond well to the area satisfying the Bragg condition. When the surface is rough, there are many areas not satisfying the Bragg condition, and these areas are imaged dark in contrast to the areas satisfying the Bragg condition, even if the roughness is observed by SEM to be very small. Figure 5(a) shows the REM image of a synthetic single-crystalline Ib type diamond (001) surface. In this image, many sharp stripes like polishing lines [8] are seen, so the surface is very rough. Figures 5(b) and 5(c) are the REM images of nondoped homo-epitaxial diamond films made from CH4/H a and CO/Ha respectively. In these images, the many sharp stripes observed in Fig. 5(a) disappear, and smoother areas appear. The surfaces of epitaxial films are flat in comparison with the surface of the substrate. The surface shown in Fig. 5(c) forms a valley-and-hill structure and is smoother than the surface in Fig. 5(b). This indicates that the surface roughness of the film made from CO/Ha is less than the surface made from CHa/H2. Figure 5(d) shows the REM image of boron-doped
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H. Sasaki et al. / Surface structure of epitaxial CVD diamond (001)
non-doped film. We also confirm that the increase in the B/C ratio causes the improvement in flatness of the surfaces when the ratio is 1000-2000 p.p.m., but when the B/C ratio is greater ,than 4000 p.p.m., the surfaces become very rough. The improvement in flatness of the surface by boron doping was also confirmed not only by R H E E D observation but also by REM observation.
5pm (a)
3.4. S T M images
Figure 6(a) is a low magnification STM image of a diamond (001) surface. In this image, 2 x 1 and 1 × 2 domains are seen clearly. The distance between steps is less than about 100 A. The terrace sizes are too small to be resolved in the REM images shown above. Figure 6(b) is a high magnification STM image of dimer rows. Each dimer is observed as an oval in the image and the dimer atoms are not resolved. The dimer structure images do not change when the tip bias is changed.
0.Spm
5pm 3.5. Flatness o f diamond (001)
(b)
0.5pm
5pm II
(c)
0.5pm
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Fig. 5. REM images on the diamond (001) surfaces: (a) substrate immersed in H2 plasma for 15 min; (b) non-doped homo-epitaxial diamond films by CH4/H2 (= 5 vol.%); (c) non-doped homo-epitaxial diamond films by CO/H2 (=5vo1.%); (d) boron-doped homoepitaxial diamond films by CO/H2 (=5vo1.%). The B/C ratio is 2000 p.p.m. film. Some flat terrace-like regions satisfying the Bragg condition well are seen in this image. These terrace-like regions were not observed on non-doped films. The surface of the boron-doped film is smoother than the
It is confirmed by REM observation that the surface flatness was improved after crystal growth. The appearance of high order index surfaces with many kinks are suppressed because of the high surface energy of carbon. Hence, during crystal growth adsorbates migrate on the surface and settle down preferentially at the kinks. The low order index surfaces, e.g. (001) surface, tend to appear as a result. Since the (001) surface reconstructs to the 1 × 2/2 x 1 structure and lowers the surface energy on the surface, other low order index surfaces, e.g. the (111) surface, tend not to appear. It is also confirmed that the film made from CO/H2 was smoother than the film made from CH4/H2. This is because oxygen dissociated from CO eliminates the nondiamond phase and two-dimensional nuclei on the surface during deposition; thus the surface became smoother. This assumption coincides with other reports, e.g. ref 9, which conclude that the quality of the diamond films made from CO/H2 is better than those made from CH4/H2. The improvement in flatness by boron doping is confirmed by R H E E D observation. The role of boron is presently unknown. We speculate that the introduction of boron has some effects on the relaxation of strain field on the surfaces. Now we consider the real space model corresponding to the diffraction pattern shown in Fig. 4(c). The origin of the reciprocal lattice sheets is many long and narrow islands parallel to the direction of the dimer rows. A schematic diagram of the surface is shown in Fig. 7. In this model, dimer rows extend further in the direction of SB type steps (steps perpendicular to the dimer rows of the upper terrace) than SA type steps (steps parallel to the dimer rows of the upper terrace), and these dimer rows form islands on the surface. These isolated dimer rows were sometimes observed on the surfaces of
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H. Sasaki et al. / SurJhce structure of epitaxial CVD diamond (001) 1×2
/2×1
[0i.,/oll [11o]
F/ Fig. 7, Schematic illustration of the 2 × 1/1 × 2 structure of borondoped diamond films derived from RHEED patterns. In this model, islands composed of a few dimer rows extend longer parallel to the dimer rows on the terraces.
I
but the detailed configuration has not been verified. For a detailed interpretation of the STM images, ab initio molecular orbital calculations have been employed. Preliminary results indicate that the hydrogen terminated structure (monohydride model) is more stable than carbon terminated structures of triplet state or singlet state [11 ].
I
(a)
lOnm
4. Conclusions
I
(b)
II
I
1n m
Fig. 6. STM images of boron-doped diamond films (B/C = 2000 p.p.m.) at 0.1 V tip bias and lnA tunnel current: (a) is the low magnification image and (b) is the high resolution image. Dimer structures are well resolved in (b).
We have estimated the flatness of diamond (001) surfaces using R H E E D and REM which were sensitive to the surface morphology. The flatness of the surfaces has been improved by the crystal growth using CO as the source gas and by boron doping. The improvement in the surface morphology by the homo-epitaxial growth is caused by the deep energy minimum of the (001) surface. We speculate that the improvement in the surface flatness by using CO or boron doping is caused by the elimination of the non-diamond phase and twodimensional nuclei on the surface or the relaxation of surface strain field. STM has been used to investigate the 2 x 1 reconstructed structure. The 2 x 1 and 1 x 2 domains have been observed clearly. The dimer structure images have been obtained in atmospheric pressure. It is expected that the dimer configuration can be determined by more detailed STM observations associated with the image simulation by molecular orbital calculations.
Acknowledgments non-doped and boron-doped films by STM. From the observations of R H E E D oscillation on GaAs (001), the number of diffusely scattered electrons is large when such islands exist, e.g. ref. 10. These diffusely scattered electrons contribute to the bright background in the R H E E D pattern, so the model in Fig. 7 also coincides with the diffraction pattern from the viewpoint of the bright background. The image of the dimer structure is observed by STM,
The authors would like to thank Mr. K. Tsugawa, Mr. A. Yamaguchi, Dr. K. Tsukui and Professor I. Ohdomari for their collaboration and advice, and also to Professor T. Osaka of Kagami Memorial Laboratory for Materials Science and Technology for his support, Mr. N. Fujimori of Sumitomo Electric Industries, Ltd. and Mr. T. Yamashita of Tokyo Gas Co., Ltd. for their cooperation. This work was supported in part by Grants-
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H. Sasaki et al. / Surface structure of epitaxial CVD diamond (001)
in-Aid for General Scientific Research (Grant No. 04650658) and Scientific Research on Priority Areas of "Metal-Semiconductor Interfaces" (Grant No. 03216106) from the Ministry of Education and Culture of Japan and by the Miyashita Research Foundation for Material Science (1992).
References 1 P. G. Lurie and J. M. Wilson, Surf Sci., 65 (1977) 453. 2 R.J. Hamers, U. K. K6hler and J. E. Demuth, J. Vac. Sci. Technol., A,8 (1990) 195.
3 H. Shiomi, K. Tanabe, Y. Nishibayashi and N. Fujimori, Jpn. J. Appl. Phys., 29 (1990) 34. 4 Y. Sato, C. Hata and M. Kamo, Science and technology of new diamond, Proc. 1st Int. Conf. New Diamond Science and Technology, KTK, Tokyo, 1988, p. 83. 5 T. Tsuno, T. Imai, Y, Nishibayashi, K. Hamada and N. Fujimori, Jpn, J. Appl. Phys., 30 (1991) 1063. 6 H.G. Maguire, M. Kamo, H. P. Lang, E. Meyer, K. Weissendanger and H. J. Guntherodt, Diamond Relat. Mater., 1 (1992) 634. 7 K. Yagi, J. Appl. Cryst., 20 (1987) 147, 8 Z. L. Wang, J. Electron Microsc. Tech., 17 (1991) 231. 9 H. Kawarada, A. Yamaguchi, Phys. Rev. B, in press. 10 J. H. Neave, B. A. Joyce, P. J. Dobson and N. Norton, Appl. Phys., A, 31 (1983) 1. 11 K. Tsugawa, T. Hoshino, H. Kawarada and I. Ohdomari, submitted to Surf Sci..
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Reflection electron microscope and scanning tunneling microscope observations of CVD diamond (001) surfaces* H. Sasaki, M. Aoki and H. Kawarada School of Science and Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku, Tokyo 169 (Japan) (Received August 31, 1992; accepted in final form December 7. 19921
Abstract Reflection electron microscopy (REM) and scanning tunneling microscopy have been applied to the estimation of the surface flatness of diamond (001) surfaces and to the 2 x 1 reconstructed structure. The macroscopic surface flatness observed by REM has been improved by using CO as a source gas and by boron-doping.
1. Introduction In recent years, many studies on diamond films synthesized by CVD methods have been carried out, but the growth mechanism, i.e. the surface reaction on diamond, has not been understood completely. It has been considered that the (001) 2 x 1 structure of diamond [1] holds the key to crystal growth like the Si (001) surface [2]. In this study, we have investigated the surface flatness and the 2 x 1 reconstructed structure of the diamond (001) surface in order to consider the surface reaction on diamond. Observations of diamond (001) surfaces have been made by several authors. For example, homo-epitaxial diamond (001) surfaces have been observed to be smooth and reconstructed to the 2 x 1 structure [3]. The textured growth of (001) surfaces on silicon substrates under the condition of high concentrations of CH4 has been reported [4]. These facts suggest that the diamond (001) surface appears spontaneously, and the reason is considered to be as follows. As carbon has a larger surface energy than other materials, the appearance of high order index surfaces is suppressed and the surfaces tend to be of lower order index. As the surface energy has a definite minimum value on the (001) surface in general, the (001~ surface is expected to become atomically flat. However, microscopic observations have been made by only a few authors [5, 6]. Additionally, methods for the advancement of surface flatness have not been fully developed. The diamond (001) surface reconstructs to a 2 x 1/1 x 2 structure like other diamond group crystals *Paper presented at Diamond 1992, Heidelberg, September 4, 1992.
0925 9635/93/$6.00
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such as silicon and germanium. This structure was reported on natural diamond by Lurie in 1977 [1] using low energy electron diffraction (LEED). In epitaxially grown CVD diamond films, it has been observed using R H E E D and scanning tunneling microscopy (STM) [3, 5, 6]. In the R H E E D observations [3, 5], the zero order Laue zone was observed, but the half-order Laue zone indicating smoother 2 x 1 surfaces was not.. The observed surfaces are not smooth enough to have long reciprocal rods in the reciprocal space. Moreover, detail of the dimer structure has not been confirmed and some models have been proposed. The reflection electron microscopy (REM) images are very sensitive to surface morphology in comparison with those of scanning or transmission electron microscopy, and single atom height steps can be resolved using phase contrast caused by aberration of focus, e.g. ref. 7. STM is very attractive for its atomic level observation of the surface. We used REM for macroscopic observation and STM for microscopic observation to estimate the improved surface smoothness and its correlation with the 2 x 1/1 × 2 reconstructed structure.
2. Experimental methods Diamond homo-epitaxial films were grown by the conventional microwave plasma-assisted CVD method on high-pressure synthetic type Ib diamond (001) substrates. The total flow of source gas was 200 s.c.c.m., and the total pressure was 35 Torr. The concentrations of source gases were C H 4 / H 2 = 5 v o l . % or C O / H 2 = 5 vol.%. Boron doping was carried out by B2H 6 during the deposition. Boron/carbon (B/C) ratios in the gas
1993 -
Elsevier Sequoia. All rights reserved
H. Sasaki et al. / Surface structure of epitaxial CVD diamond (001)
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phase were 50, 1000, 2000 and 4000 p.p.m. The substrate temperatures during deposition were about 850 °C, and deposition time was normally 3 h. RHEED and REM observations were performed using conventional transmission electron microscopy (JEOL JEM-100CX). Specimens were mounted on bent single slit grids for REM observations. During all observations, the accelerating voltage was 100 keV and the pressure in the column was about 2-4 x 10 - 6 Torr. STM observations were performed using a Seiko I. SAM-3000 at atmospheric pressure.
3. Results and discussion
3.1. Interpretation of RHEED pattern of (001)-2 x 1 To analyze the RHEED patterns, it is important to consider the arrangement of reciprocal space and the Ewald sphere (Ewald construction). In this section, the Ewald construction of the diamond (001) surface will be explained. Figure 1 shows a top view of the 2 x 1/1 × 2 reconstructed diamond (001) surface. In this figure, 2 x 1 and 1 x 2 domains (A and B), where the filled circles are dimer atoms, are divided by a single atomic height step. The dimer rows in one domain extend to the direction perpendicular to the dimer rows in another domain. As the distance between two dimer rows is twice the lattice constant of an ideal (001) surface, the corresponding reciprocal lattice points with half periodicity in the I-110] direction appear in reciprocal space. The reciprocal points of the reconstructed diamond (001) surface are shown in Fig. 2. As a surface becomes smoother, the corresponding reciprocal lattice points extend in the direction perpendicular to the surface and become reciprocal rods. The length of the rods is proportional to the terrace size of
0
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0 ,
0
0
"
,
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100]
Fig. 2. Reciprocal lattice of diamond (001) surface 2 x 1 reconstructed structure. Reciprocal lattice points due to this structure ( 0 ) have a periodicity of fundamental points half as long as that of the [I10] direction because the lattice constant of the 2 × 1 structure is twice that of the fundamental lattice to the [1 I0] direction in real space.
the (001) surface. Hence, we can estimate the surface flatness from the length of the reciprocal lattice rods. Figure 3 shows the arrangement of the Ewald sphere and reciprocal space in the [110] incident direction. The intersecting circular points of the Ewald sphere and the reciprocal rods appear as a RHEED pattern. Figure 4(a) shows a schematic diagram of the RHEED pattern on a diamond (001) surface in the [110] incident direction. The open and filled circles represent the fundamental and superstructure reciprocal lattice points respectively. The superstructure spots are observed in
[001] (a)
E ~ +00
i / ~ = =
)i~..
~
S
Loili
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[110] (b)
L~
E
/~ 5.04A.....~/ [°il1 P
~A
Single step
Fig. I. Ball and stick model of diamond (00l) 2 × 1/1 x 2 reconstructed surface. Distance between two dimer rows is 5.04 A (2/x/2 × lattice constant). O, dimer atoms; O, bulk atoms.
oo
[ilO1
[11Ol
Fig. 3. Ewald construction of a diamond (001) surface of RHEED observation in the [110] incident direction. E, Ewald sphere; S, shadow edge; Ko, wave vector of the incident beam; L m Nth order Laue zone; 0, glancing angle. In this case, the 008 spot is on the Ewald sphere (glancing angle 0 is about 2.4 ° when the accelerating voltage is 100 kV). In the zeroth and half-order Laue zones, the spots due to the 2 × 1 structure are observed because the reciprocal lattice rods are elongated O, reciprocal lattice points on Ewald sphere; O, reciprocal lattice points due to surface atoms; O, reciprocal lattice points due to 2 x 1/1 × 2 reconstructed structure; 0 , other reciprocal lattice points.
H. Sasaki et al. / Surface structure of epitaxial CVD diamond (001)
xO 0"0"0"0"0 O*O-O*O*O (a)
0.0.0-0.0
[1~Olj [ioo1 V_.. [1101
1273
confirmed that the surface of the film reconstructs to a 2 x 1/i x 2 structure. The faint spots of L1/2 (half-order Laue zone) in the RHEED pattern can be observed only in the [100] incident direction. This is because the length of the L1/2 rods in the [100] incident direction is less than the length of the L~/2 rods in the [110] incident direction, This suggests that reciprocal lattice rods of non-doped films do not elongate so much. In Fig. 4(c), the zero-order Laue zone cannot be seen because it is hidden behind a very bright background when the glancing angle is small. The spots belonging t o L1/2 are clearly observed. This result indicates that the reciprocal lattice points due to the 2 x 1 structure elongated more than for those of the non-doped film and suggests that the surface of the boron-doped film is smoother than the non-doped one. The spots of L1/2 in Fig. 4(c) are very streaky. This indicates that the half-order reciprocal lattice points form thin sheets. We call these sheets "reciprocal lattice sheets". The streaky spots of L~/2 and the presence of a very bright background will be discussed later. 3.3. R E M images
(b)
(c) Fig. 4 (a) Schematic diagram of the RHEED pattern in the [110] incident direction and two-dimensionalreciprocal lattice of the diamond (001) surface. (b) and (c), RHEED patterns of the non-doped and boron-doped (B/C=2000 p.p.m.) homo-epitaxial diamond films respectively. The spots of the Laue zone L~/2are observed in (c) but not in (b). the zero order Laue zone. When the surface is flat, i.e. the reciprocal lattice rods elongate, RHEED spots due to the superstructure are observed as a half-order Laue zone El, 23.2. Observed R H E E D patterns
Figures 4(b) and 4(c) show RHEED patterns of nondoped and boron-doped homo-epitaxial diamond films respectively. Both patterns were taken in the [110] incident direction. In Fig. 4(b), (0,1/2) streaky spots are seen in the zero-order Laue zone. As this pattern was also observed in the [1T0] incident direction, it is
The REM images in the present experiment have been obtained by using a spot (normall~,/specular spot) in each RHEED pattern to form real images. Reflected electron beams are focused by an objective lens, and form a RHEED pattern at the back focal plane. As the specular spot is only selected for the imaging of REM by the OL aperture, the regions corresponding to the spot are visualized in the REM image. As the intensity of the spot in the RHEED pattern is sensitive to the glancing angle, the REM image is sensitive to the Bragg condition. Thus, the bright regions in the REM image correspond well to the area satisfying the Bragg condition. When the surface is rough, there are many areas not satisfying the Bragg condition, and these areas are imaged dark in contrast to the areas satisfying the Bragg condition, even if the roughness is observed by SEM to be very small. Figure 5(a) shows the REM image of a synthetic single-crystalline Ib type diamond (001) surface. In this image, many sharp stripes like polishing lines [8] are seen, so the surface is very rough. Figures 5(b) and 5(c) are the REM images of nondoped homo-epitaxial diamond films made from CH4/H a and CO/Ha respectively. In these images, the many sharp stripes observed in Fig. 5(a) disappear, and smoother areas appear. The surfaces of epitaxial films are flat in comparison with the surface of the substrate. The surface shown in Fig. 5(c) forms a valley-and-hill structure and is smoother than the surface in Fig. 5(b). This indicates that the surface roughness of the film made from CO/Ha is less than the surface made from CHa/H2. Figure 5(d) shows the REM image of boron-doped
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H. Sasaki et al. / Surface structure of epitaxial CVD diamond (001)
non-doped film. We also confirm that the increase in the B/C ratio causes the improvement in flatness of the surfaces when the ratio is 1000-2000 p.p.m., but when the B/C ratio is greater ,than 4000 p.p.m., the surfaces become very rough. The improvement in flatness of the surface by boron doping was also confirmed not only by R H E E D observation but also by REM observation.
5pm (a)
3.4. S T M images
Figure 6(a) is a low magnification STM image of a diamond (001) surface. In this image, 2 x 1 and 1 × 2 domains are seen clearly. The distance between steps is less than about 100 A. The terrace sizes are too small to be resolved in the REM images shown above. Figure 6(b) is a high magnification STM image of dimer rows. Each dimer is observed as an oval in the image and the dimer atoms are not resolved. The dimer structure images do not change when the tip bias is changed.
0.Spm
5pm 3.5. Flatness o f diamond (001)
(b)
0.5pm
5pm II
(c)
0.5pm
5pm (d)
0.5pm
Fig. 5. REM images on the diamond (001) surfaces: (a) substrate immersed in H2 plasma for 15 min; (b) non-doped homo-epitaxial diamond films by CH4/H2 (= 5 vol.%); (c) non-doped homo-epitaxial diamond films by CO/H2 (=5vo1.%); (d) boron-doped homoepitaxial diamond films by CO/H2 (=5vo1.%). The B/C ratio is 2000 p.p.m. film. Some flat terrace-like regions satisfying the Bragg condition well are seen in this image. These terrace-like regions were not observed on non-doped films. The surface of the boron-doped film is smoother than the
It is confirmed by REM observation that the surface flatness was improved after crystal growth. The appearance of high order index surfaces with many kinks are suppressed because of the high surface energy of carbon. Hence, during crystal growth adsorbates migrate on the surface and settle down preferentially at the kinks. The low order index surfaces, e.g. (001) surface, tend to appear as a result. Since the (001) surface reconstructs to the 1 × 2/2 x 1 structure and lowers the surface energy on the surface, other low order index surfaces, e.g. the (111) surface, tend not to appear. It is also confirmed that the film made from CO/H2 was smoother than the film made from CH4/H2. This is because oxygen dissociated from CO eliminates the nondiamond phase and two-dimensional nuclei on the surface during deposition; thus the surface became smoother. This assumption coincides with other reports, e.g. ref 9, which conclude that the quality of the diamond films made from CO/H2 is better than those made from CH4/H2. The improvement in flatness by boron doping is confirmed by R H E E D observation. The role of boron is presently unknown. We speculate that the introduction of boron has some effects on the relaxation of strain field on the surfaces. Now we consider the real space model corresponding to the diffraction pattern shown in Fig. 4(c). The origin of the reciprocal lattice sheets is many long and narrow islands parallel to the direction of the dimer rows. A schematic diagram of the surface is shown in Fig. 7. In this model, dimer rows extend further in the direction of SB type steps (steps perpendicular to the dimer rows of the upper terrace) than SA type steps (steps parallel to the dimer rows of the upper terrace), and these dimer rows form islands on the surface. These isolated dimer rows were sometimes observed on the surfaces of
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/2×1
[0i.,/oll [11o]
F/ Fig. 7, Schematic illustration of the 2 × 1/1 × 2 structure of borondoped diamond films derived from RHEED patterns. In this model, islands composed of a few dimer rows extend longer parallel to the dimer rows on the terraces.
I
but the detailed configuration has not been verified. For a detailed interpretation of the STM images, ab initio molecular orbital calculations have been employed. Preliminary results indicate that the hydrogen terminated structure (monohydride model) is more stable than carbon terminated structures of triplet state or singlet state [11 ].
I
(a)
lOnm
4. Conclusions
I
(b)
II
I
1n m
Fig. 6. STM images of boron-doped diamond films (B/C = 2000 p.p.m.) at 0.1 V tip bias and lnA tunnel current: (a) is the low magnification image and (b) is the high resolution image. Dimer structures are well resolved in (b).
We have estimated the flatness of diamond (001) surfaces using R H E E D and REM which were sensitive to the surface morphology. The flatness of the surfaces has been improved by the crystal growth using CO as the source gas and by boron doping. The improvement in the surface morphology by the homo-epitaxial growth is caused by the deep energy minimum of the (001) surface. We speculate that the improvement in the surface flatness by using CO or boron doping is caused by the elimination of the non-diamond phase and twodimensional nuclei on the surface or the relaxation of surface strain field. STM has been used to investigate the 2 x 1 reconstructed structure. The 2 x 1 and 1 x 2 domains have been observed clearly. The dimer structure images have been obtained in atmospheric pressure. It is expected that the dimer configuration can be determined by more detailed STM observations associated with the image simulation by molecular orbital calculations.
Acknowledgments non-doped and boron-doped films by STM. From the observations of R H E E D oscillation on GaAs (001), the number of diffusely scattered electrons is large when such islands exist, e.g. ref. 10. These diffusely scattered electrons contribute to the bright background in the R H E E D pattern, so the model in Fig. 7 also coincides with the diffraction pattern from the viewpoint of the bright background. The image of the dimer structure is observed by STM,
The authors would like to thank Mr. K. Tsugawa, Mr. A. Yamaguchi, Dr. K. Tsukui and Professor I. Ohdomari for their collaboration and advice, and also to Professor T. Osaka of Kagami Memorial Laboratory for Materials Science and Technology for his support, Mr. N. Fujimori of Sumitomo Electric Industries, Ltd. and Mr. T. Yamashita of Tokyo Gas Co., Ltd. for their cooperation. This work was supported in part by Grants-
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in-Aid for General Scientific Research (Grant No. 04650658) and Scientific Research on Priority Areas of "Metal-Semiconductor Interfaces" (Grant No. 03216106) from the Ministry of Education and Culture of Japan and by the Miyashita Research Foundation for Material Science (1992).
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