Ultramicroscopy 42-44 (1992) 864-870 North-Holland ~ ~

Si(111) 7 × 7 domain boundaries on vicinal surfaces

Hideyuk i T a n a k a , M a s a h a r u U d a g a w a , M a k o t o I toh , Tosh ih i ro U c h i y a m a , Yosh io W a t a n a b e , T a t s u o Y o k o t s u k a and I sao Sumi ta Matsushita Research Institute Tokyo, Inc., 3-10-1, Higashirnita, Tama-ku, Kawasaki, Kanagawa 214, Japan

Received 12 August 1991

Scanning tunneling microscopy ~s used to investigate how domain boundaries on Si(111)7×7 are affected by step configuration. For making the configuration of step change, we prepare surfaces having misorientation different from (111). On the one surface misoriented towards [110], steps run in a zig-zag pattern, which causes terraces to be constricted. On every constricted site, a domain boundary exists. On the other surface misoriented towards [112], steps run parallel and consist of several atomic layers. Domain boundaries on this surface run in a zig-zag pattern. Discussion is focused on surface energy of the stepped surface containing domain boundaries.

1. Introduction

The semiconductor industry is in quest of tech- nology for atomic-layer control of crystal surfaces to design quantum well and other sophisticated devices. In this technology, understanding of step configuration plays an important role, since a step is one of the atomic-scale structures on real surfaces.

The step configuration can be affected by de- fects, and vice versa. For example, using reflec- tion electron microscopy (REM), a screw disloca- tion is reported [1] to terminate a step on S i ( l l l ) 7 x 7 . This 7 × 7 surface, consisting of larger unit cell than that of bulk, has another kind of defects: domain boundaries. Using REM [2] and scanning tunneling microscopy (STM) [3- 6], many researchers have observed the domain boundaries on Si(111)7 × 7 and their existence has become known widely. Influence of the do- main boundaries on step configuration was re- ported by Tanishiro et al. [7], who concluded that domain boundaries made a step configuration in a zig-zag pattern. However, they did not pay attention to the misorientation direction from (111) of the observed surface. In accordance ,~ith this direction, the step configuration is known to

be transformed substantially [8]. Therefore it is important to define the misorientation direction and to study the domain boundaries on various samples miscut towards different directions.

In this paper, we report the site of domain boundaries by large-area STM observation of Si(111) 7 x 7 surfaces which have different misori- entation directions from (111). The discussion is focused on the surface energy of step configura- tion containing domain boundaries to reveal their role on vicinal surfaces.

2. Experiment

Experiments were made in ultra-high vacuum (UHV) of 7 × 10 9 Pa. The details of an STM we constructed are reported elsewhere [9,10].

Samples used were P-doped silicon wafers of vicinal (111) in commercial use. One misorienta- tion was towards [110] by 0.4 ° and another to- wards [112.] by 0.5 °. ([112.] is a direction from a faulted to an unfaulted site of a 7 x 7 unit cell and was checked by measuring the filled state with STM.) After being wiped with methanol, a sample was introduced into a chamber using a load-lock mechanism, and heated up to 800°C

0304-3991/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

H. Tanaka et al. / Si( l l l) 7 x 7 domain boundaries on vicinal surfaces 865

with direct current passing through the sample. This temperature was maintained in U H V for several hours. Finally the sample was flashed repeatedly at 1200°C, and cooled down at - 3 ° / s . All STM images shown in this paper were topo- graphically taken at a tunneling current of 50 pA.

3. Results

Vicinal (111) misoriented towards [710]

Fig. 1 shows an STM image of 0.4 X 0.4 /~m 2 area of Si(111)7 x 7 misoriented towards [110]. Along a step edge, the brighter contrast means upper terrace and the darker means lower one.

Steps observed are single atomic layer high (0.31 nm indicated by S) and double atomic layer high (0.62 nm indicated by D), forming a step configu- ration in a zig-zag pattern. In order to character- ize this configuration of steps, we divided each step into fragments 5 nm long and counted them as a function of direction where each fragment of step was oriented. The distribution of the step direction is shown in fig. 2, as well as the defini- tion of its direction. This figure clearly shows that the direction of 'S' steps has a distribution at [101] and [011], which means an S step easily runs toward these directions. On the other hand, the direction of D steps has a distribution around [112], a different feature from that of S steps. The mean direction of S a n d / o r D steps is [112]; it is

Fig. 1. Large-area STM image of Si( l l 1)7 × 7 misoriented by 0.4 ° towards [110], taken at a bias voltage of + 2.0 V to the sample. Steps observed are single atomic layer high (0.31 nm indicated by S), and double atomic layer high (indicated by D). The step configuration is a zig-zag pattern, forming constricted terraces indicated by I, J, K, L, M and N. On every site of these constricted

terraces, a domain boundary is observed.

866 H. Tanaka et al. / S i ( l l l ) 7 × 7 domain boundaries on uicinal surfaces

50

rr" I,,I,,I

z 5O

I r I

DOUBLE S T E P [ ~

SINGLE STEP

/ 0 ~ I I ~ --

[1011 []i21 [0i l 1

O R I E N T A T I O N Fig. 2. Distribution of step-running direction. Steps shown in fig. 1 are cut into fragments 5 nm long and distributed as a function of their direction defined by the inset. A double- atomic-layer step (double step) has a distribution around [112], the same direction as the mean from single-atomic-layer step (single step) and /o r double step. On the other hand, a single step has a sharp distribution at [101] and [011], deviat- ing from the mean direction. Because of this deviation, the

step configuration becomes a zig-zag pattern.

The way of contrasting the upper terrace with the lower one is the same as in fig. 1. The step configuration observed is simple, running towards [110] in parallel. Although a single-atomic-layer step (0.31 nm high) can be seen, most of the steps are more than several atomic layers high. This clustering of steps, called step bunching, is con- sidered to be intrinsic to the vicinal S i ( l l l ) mis- oriented towards [117.][11-13]. Even on this sur- face having no constricted terrace site, we found

consistent that the sample observed is (111) mis- oriented towards [110].

More interesting is that terraces are con- stricted by steps approaching each other at some sites, indicated by I to N. From STM images taken with atomic resolution, a domain boundary is observed at all of these constricted sites, where the length of a domain boundary becomes short. One of the images taken at these sites is fig. 3, showing a domain boundary found at site M. We have already taken many STM images of domain boundaries, some of which have no reconstructed structure and are filled with atoms in disorder as shown in this figure. Now that we have made clear the site of a domain boundary on the sur- face, it is easy to find it, without seeking it at random.

3.2. Vicinal (111) misoriented towards [113]

Fig. 4 shows an STM image of 0.4 × 0.4 /xm a area of S i ( l l l ) 7 × 7 misoriented towards [112].

Fig. 3. Atom-resolved STM image of the domain boundary located at M in fig. 1, taken at a bias voltage of + 1.0 V to the sample. The domain boundary, seen on the middle terrace, has a disordered structure. Inset shows misfit of phase of 7 × 7 periodicity between the left side (L) and the right one (R) on

the middle terrace.

H. Tanaka et al. / Si( l l l) 7 × 7 domain boundaries on vicinal surfaces 867

domain boundar ies at the sites indicated by O to R.

In fig. 5 we show a domain boundary found at site Q. This domain boundary runs in a zig-zag pattern. Along one of directions forming this pattern, i.e. a shorter diagonal of a 7 × 7 unit cell, an ordered structure within the domain boundary is clearly observed as shown in the magnified image. If the domain boundary runs along only one of these diagonals, the pat tern will become straight with an o rdered boundary structure, which appears to have lower surface energy than a disordered one locally. However, this straight pat tern is not the shortest course between neigh- boring steps; the total energy of the domain boundary becomes high. To reduce the total en- ergy of it, the zig-zag pat tern helps the domain boundary to shorten.

4. Discussion

Energet ics regarding step configurat ions has been discussed [14]. The surface structures taken into considerat ion were usually terraces, steps and kinks. In this section, we consider the step configurat ions containing additional factors, i.e., domain boundaries , to discuss surface energy of step configurat ions on the vicinal Si(111) misori- ented toward [110].

Just as a crystal surface has stable planes called facets, a step has stable directions in which it runs. On this vicinal (111) surface, the easy direc- tions of a single step are [101] and [011], as shown in fig. 2. These directions deviate f rom their mean direction, [112], by as much as 30 °. There- fore the step configurat ion becomes a zig-zag pattern. Tanishiro et al. [7] concluded that the

Fig. 4. Large-area STM image of Si(lll)7x7 misoriented by 0.5 ° towards [112], taken at a bias voltage of +2.0 V to the sample. Steps running parallel and step bunching are observed. Step heights measured are 0.31, 2.6, 3.3 and 1.6 nm, respectively. Although

no constricted terrace is observed, domain boundaries are found on the sites indicated by O, P, Q and R.

868 H. Tanaka et al. / S i ( l l l ) 7 × 7 domain boundaries on vicinal surfaces

zig-zag pattern was due to domain boundaries. However, the step configuration of the vicinal (111) misoriented towards [117.], shown in fig. 4, is not a zig-zag pattern even though domain bound- aries exist. The reason for the zig-zag pattern is that easy directions to steps do not coincide with their mean direction, being forced by miscut from (111).

First, without domain boundaries, we consider ground states of step configurations. Although

the steps we observed appear to be far from the ground states, it is helpful to consider these states in order to reveal the role of defects. In the ground states, single steps probably run only along the easy directions, [101] and [011]. And we inter- pret step configurations comprising these single steps as the solid-on-solid (SOS) model [14]. The surface energy of the SOS model is determined by the number of dangling bonds. Since reduction of the number of dangling bonds is considered to

Fig. 5. Atom-resolved STM images of the domain boundary located at Q in fig. 4, taken at a bias voltage of + 2.0 V to the sample. Inset shows misfit between the left-side domain (L) and the right one (R). The domain boundary running between neighboring steps has a zig-zag pattern. At some sites where the domain boundary run along a shorter diagonal of a 7 x 7 unit cell, a

reconstructed structure can be seen as shown in magnified image.

H. Tanaka et aL / S i ( l l l ) 7 × 7 domain boundaries on uicinal surfaces 869

(a)

(b)

(¢)

(d)

(e)

tOil]

ril0] d01]

Fig. 6. Different step configurations having the same step density on vicinal (111) misoriented towards [110]. Based on the solid-on-solid model, these configurations degenerate in the ground states. By taking domain boundaries into account, this degeneracy is resolved and the surface energy becomes (a), (b)> (c)> (d)=(e) . (a) A fine zig-zag, macroscopically seen as a parallel step configuration. (b) A zig-zag in co-phase between neighboring steps. (c) A zig-zag in counter-phase, causing terraces to be constricted. (d) A zig-zag containing narrower constricted sites than that of (c). (e) An extended zig-zag with lower density of constricted sites than that of (d).

stabilize Si(111)7 × 7 [15], the application of the SOS model will be realistic. Provided that the steps have dangling bonds, the energy of the stepped surface is governed by step density. Hence step configurations shown in fig. 6, which have the same step density, will have the same energy and degeneracy. These degenerating zig- zag step configurations have many kink sites, where the running direction of a step changes. A kink is generally considered to raise surface en- ergy by extending a step and raising step density. However, in these step configurations, the kink does not make a step long. Note that the kinks in this figure do not give rise to the surface energy. This degeneracy is resolved by domain bound- aries as follows. From the fact that we found a shorter domain boundary preferred, a domain boundary is estimated to have higher energy than a 7 × 7 arrangement. Therefore, surface energy is raised by step configurations which have long length and high number density of the domain boundaries. However, in the case of step configu-

rations showing zig-zag patterns between neigh- boring steps in co-phase in figs. 6a and 6b, we cannot pinpoint the site of domain boundaries to know their number density. We can only expect that zig-zag patterns of counter-phase, as shown in figs. 6c-e, where a domain boundary becomes substantially short, will lower the surface energy in comparison with that of the co-phase, on the assumption that the order of number density of domain boundaries is the same in any step config- urations. On the contrary, the number density of domain boundaries in figs. 6c -e is clear. In addi- tion to the consideration of length of a domain boundary, we obtain the relation of surface en- ergy. What we must point out here is that each domain boundary in figs. 6d and 6e is short enough to ignore the surface energy of it. Al- though the number density of domain boundaries in fig. 6d is higher than that in 6e, we can ignore the energy rise of 6d from 6e. Consequently, the relation of surface energy in fig. 6 is

(a) , (b) > (c) > (d) = (e) . (1)

Since surface energy in fig. 6d is equal to that in 6e, the step configuration that figs. 6d and 6e remain degenerating is realized. Fig. 1 is inter- preted as an excited state from these ground states, keeping the zig-zag pattern between neigh- boring steps in counter-phase.

We showed that a domain boundary plays an important role in determination of step configu- ration on the vicinal S i ( l l l ) surface misoriented towards [110]. Our discussion, however, was fo- cused on the ground states, ignoring double steps and local defects that cause kinks. By taking these factors into consideration, the step configu- ration will be understood more precisely.

5. Conclusions

We have investigate the influence of S i ( l l l ) 7 × 7 domain boundaries on the step configura- tion, using STM. On the vicinal (111) misoriented towards [110], steps ran zig-zag, causing terraces to be constricted. Domain boundaries were ob- served at these constricted sites. On the vicinal

870 H. Tanaka et al. / S i ( l l l ) 7 x 7 domain boundaries on vicinal surfaces

(111) misoriented towards [11,2], steps ran paral- lel. Domain boundaries on this surface ran be- tween neighboring steps in a zig-zag pattern. Sur- face energy of the stepped surface containing domain boundaries was discussed to explain the zig-zag configuration of steps.

Acknowledgements

Our STM combined with a field ion micro- scope (FIM) was developed in collaboration with Prof. Sakurai and Asst. Prof. Hashizume of To- hoku University. We would like to express our gratitude for their collaboration. Also we thank Prof. Iwasaki of Osaka University and Mr. Niwa of Matsushita Electric Industrial Co. for their stimulating discussions, and Shin-Etsu Handotai Co., Ltd. and Showa Denko for providing silicon wafers.

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