Jpn. J. Appl. Phys. Vol. 38 (1999) pp. 3833–3836
Part 1, No. 6B, June 1999c
°1999
Publication Board, Japanese Journal of Applied Physics

Electronic Structure of the Si(100) Surface A Defects
Analyzed by Scanning Tunneling Spectroscopy at 80 K
Yasuyuki S AINOO1 , Tomohiko K IMURA1 , Ryuji M ORITA1 , Mikio YAMASHITA2 ,Kenji H ATA1 and Hidemi S HIGEKAWA1,2, ∗1 Institute of Applied Physics and CREST, Japan Science Technology Cooperation, University of Tsukuba, Tsukuba 305-8573, Japan
2 Department of Applied Physics, CREST, Japan Science Technology Cooperation, Hokkaido University, Sapporo, 060-8628, Japan
3 Department of Chemistry and Biotechnology, Graduate School Engineering, University of Tokyo, Tokyo 113-8656, Japan(Received February 23, 1999; accepted for publication March 30, 1999)

The A defects on the Si(100) surface can be classified as A1 , A2 , and A3 at low temperatures. We carried out scanning tunneling microscopy observations and scanning tunneling spectroscopy measurements at ∼80 K to study their electronic structures.
We found that the A1 defect is semiconductive similar to the A defect at room temperature (RT), while the A2 and A3 defects
exhibited states in the surface band gap at 80 K. On comparing these results with the theoretical models, we concluded that
the A1 defect correspond to the Rebonded vacancy model. The broken vacancy model and the twisted vacancy models are thepossible candidates for the A2 and A3 defects, respectively.
KEYWORDS: scanning tunneling microscopy, scanning tunneling spectroscopes, surface electronic phenomena, silicon, surface
defects

1. Introduction
One of the prominent features of the Si(100) surface is theatomic level defects. From scanning tunneling microscopy(STM) studies, typical defects on this surface were classified by Hamers and Kohler1) as types A, B, and C. Amongthem, the A defect seems to have the simplest structure; asingle vacancy defect, as observed by STM in both filled
and empty state images. Some structural models have been
proposed from the theoretical studies, and the dimer vacancy(DV) model is most widely accepted.2–5) However, the A defect structure still remains to be determined.
The DV model denotes a structure with a single dimer vacancy which is formed by the simple removal of a dimer froma perfectly reconstructed Si(100) surface. Wang et al. proposed two structures for the DV model.3) When a single dimer
is removed, the second layer atoms can either move apart,or move together to form bonds across the introduced gap,
was first suggested by Pandey.4) Moreover, Owen et al. proposed that the DV model is divided into three subcategories:5)(1) Rebonded model (R-DV model, Fig. 1(a)): second layer
atoms form two bonds across the gap. (2) Twisted model (TDV model, Fig. 1(b)): a single bond is formed across the gap,
and two dangling bonds remain. (3) Broken model (B-DV
model, Fig. 1(c)): second layer atoms do not form any bonds,
and they relax away from the center of the defect.
According to the ab intio total-energy study of the DV defect by Wang et al., the R-DV model has no states in thelower half of the surface band gap. Hamers and Kohler experimentally performed current-voltage (I-V) measurements
at RT, and obtained the result that the A defect has a clear surface band gap, although it is somewhat reduced in magnitude
compared to the surface band gap. Owen et al. also demonstrated that the low-bias STM image is in good agreementwith the simulated STM image, based on the R-DV model.
Based on this consistency observed between theoretical andexperimental results, the R-DV model is widely accepted to
explain the structure of the A defect.
On the other hand, it is well known that defects influence∗ E-mail address: hidemi@ims.tsukuba.ac.jp, http://dora.ims.tsukuba.ac.jp

the characteristic configuration of the surrounding dimers. At
RT, symmetric dimers are observed mostly on the Si(100) surface. From the total-energy calculations, a bucked dimer isfound to be more stable than symmetric dimers. In fact, aphase transition occurs around 200 K, and buckled dimers begin to be observed. Yokoyama and Takayanagi.6) reported
that symmetric dimers are induced, even at ∼140 K, on bothsides of the A defect. This result also seems to support thestructure of the R-DV model.
However, from the detailed analysis of the effect of theA defect on the surrounding dimer configurations at 80 K,
Uchikawa et al. found that the A defect can be classified as
A1 , A2 , and A3 at ∼80 K.7) However, their structures and relation to the A defect have not yet been clarified. In orderto determine the structures, information about their electronic
structures is essential.
In this study, we conducted STM observations and scanning tunneling spectroscopy (STS) measurements at ∼80 K.
We found that (1) the A1 defect has a clear surface band gap,
although it is somewhat reduced in magnitude compared tothe surface band gap, and (2) the A2 and A3 defects have statesin the surface band gap. Therefore, we concluded that the A1
defect corresponds to the rebonded vacancy model, and theA2 and A3 defects correspond to the broken vacancy modeland the twisted vacancy model, respectively.
2. Experimental
Si samples were phosphorous-doped with a conductivityof 0.005 Ä cm. After the samples were prebaked at ∼700◦ C
for ∼12 h, they were flashed once to 1250◦ C for 30 sec, followed by a slow cool down. The base pressure was kept under 5 × 10−8 Pa during flashing. An electrochemically etchedtungsten tip was used for the STM observations.
3. Result and Discussion
3.1 Characteristics of the A1 , A2 , and A3 defects
First, we show the A1 , A2 , and A3 defects which were observed at ∼80 K. STM images and schematics of the stickand ball structures of the A1 , A2 , and A3 defects are shownin Figs. 2(a), 2(b), and 2(c), respectively. The A1 defect, asshown in Fig. 2(a), suppresses buckling on both its sides, and

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

Fig. 1. Defect structures of (a) Rebonded dimer vacancy model (R-DVmodel), (b) Twisted dimer vacancy model (T-DV model), (c) Broken dimer
vacancy model (B-DV model).

induces a symmetric dimer structure. The A2 defect does not
disturb the antiferromagnetic ordering of the buckled dimers
on both its sides (see Fig. 2(b)). The A3 defect does not suppress the buckling on both its sides, but it disturbs the antiferromagnetic ordering of the buckled dimers (see Fig. 2(c)).
Namely, the A3 defect is a phase shifter. At 80 K, A1 defects
were frequently observed. However, the A2 defect occurred
less frequently than the A1 defect. Moreover, the A3 defectwas rarely observed. The A3 defect looks to appears to occur
on a higher defect density surface.
3.2 Spectroscopic analysis of the A1 , A2 , and A3 defects
We measured the tunneling I-V characteristics of the A1 ,A2 , and A3 defects, as shown in Figs. 3(a), 3(c), and 3(e).
All I-V measurements were carried out at the center of thevacancy. The solid lines in Fig. 3 denote the I-V curves measured for the defects. In each case, the tunneling I-V characteristic of the normal dimers was measured with the same
tip apex and is denoted in Fig. 3 as a dashed line. Tunneling spectra of the dimers were taken to confirm that the STS

Y. S AINOO et al.

Fig. 2. STM images and the stick and ball structures of the A1 , A2 , andA3 defects in (a), (b) and (c), respectively. All STM images were taken at
sample bias −1.0 V and tunneling current 1.0 nA.

measurements are not influenced by the electronic structureof the tips. Here, they show a semiconductive feature with aband gap of −0.5 V, which is in accordance with previous results.1, 8) Here, we address the issue of the reliability of STS
measurements. It is well known that STS measurements are
subject to the conditions of the tunneling tip. One type of tip
apex will provide reproducible STS spectra, although a different tip apex might produce very difference results. Regarding
this point, before and after the transitions, STS measurements
were conducted on the normal dimers, and we confirmed thateach STS spectrum had a similar shape to the spectrum reported by Hamers et al.8) The spectrum of the dimers obtained by Hamers (and by us) is consistent with many experimental results and is believed to represent the true electronic
characteristics. We discussed the characteristic of each spectrum in comparison with that of the normal dimer. By thisprocedure, we can safely confirm that all of the STS measurements were conducted with the similar tip conditions and thatthe experimental results were not devalued by the possible

Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

Fig. 3. I-V and STS spectra obtained for the A1 ((a), (b)), A2 ((c), (d)) andA3 ((e), (f)), respectively. The dashed lines denote the spectra obtained forthe normal dimers taken with the same tip apex. The spectra (2)–(4) in(d) represents the typical spectra obtained for the A2 defects in different
areas. The shaded regions in (b), (d) and (f) represent the energy window in which reliable STS signals could not be obtained because of theemphasized tunneling noise.

electronic structure of the tips.
As shown in Fig. 3(a), the A1 defect always had a surface
band gap, whose width was around ∼0.4 V, slightly smaller
than that of the normal dimers. Thus, the A1 defect has asemiconductive characteristic even at 80 K. Regarding the A2
defect, the surface band-gap width seemed to be slightly scattered, however, it was much narrower than that of the A1 defect. In a specific case, the defect showed a metallic feature(see Fig. 3(c)). The A3 defect did not exhibit a surface bandgap, and it was always metallic (see Fig. 3(e)).
In order to discuss the electronic structures in more detail, the normalized tunneling conductivities (STS spectra,(dI/dV)/(I/V) versus V) in the range from −2 V to +2 V
were numerically calculated from the tunneling I-V curves inFig. 3. The corresponding STS spectra of the A1 , A2 , and A3
defects are displayed in Figs. 3(b), 3(d), and 3(f), respectively.
The normalization procedure of the tunneling current significantly emphasizes the tunneling noise (∼several pA) aroundthe Fermi level, preventing observation of the intrinsic electronic structure near the Fermi level. The shaded regions in
Fig. 3 represent the energy window in which no reliable STS
signals could be obtained because of the emphasized tunnel-

Y. S AINOO et al.

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ing noise.
As denoted by the solid line in Fig. 3(b), the A1 defect has
three dominant energy states at ∼0.5 V, ∼0.2 V and ∼−1.0 V.
In comparison with the STS spectrum obtained for a normal dimer, STS characteristics are almost the same in bothcases. This result is in good agreement with those obtainedby Hamers et al. as the I-V characteristic of the A defect at
RT.
The STS spectra of the A2 defects were slightly scattered
as mentioned above, which is denoted by the solid lines in
Figs. 3(d)-(2)–(4). However, in comparison with the STS
spectra of the normal dimers, STS spectra of the A2 defectgenerally showed a weaker peak at the filled state, and a larger
peak at the empty state side near the Fermi level.
As denoted by the solid line in Fig. 3(f), the STS spectraof the A3 defect did not show any characteristic peaks likethe A1 and A2 defects, and were completely metallic. The
spectral intensity was weak compared to those of the others.
Filled- and empty-state STM images of the A1 defect at∼80 K are shown in Figs. 4(a) and 4(b), respectively. As
shown in Fig. 4(a), symmetric dimers are observed on bothsides of the defect in the filled-state image. However, at thepositions of the symmetric dimers in the filled-state image,four separated atoms were observed in the empty-state STM
image as shown in Fig. 4(b).
Current imaging tunneling spectroscopy (CITS) was performed on an A1 defect, and Fig. 4(d) schematically shows
the spectra taken at the positions shown in Fig. 4(c). Numbering of the spectra correspond to the positions of which thedata were taken. The spectra were averaged over an area ofapproximately 0.16 × 0.16 nm2 .
STS spectra in Fig. 4(d) show a surface band gap of ∼0.4 V.
This is narrower compared to the value of the normal dimer,∼0.5 V, but the characteristic is clearly semiconductive. As
shown in Fig. 4(d), the dominant peak in the filled-state area
shifted toward the position observed for the normal dimer as
the measured position which is away from the defect.
Since symmetric dimers theoretically have a metallic feature, the observed semiconductive STS spectra do not agreewith this feature.
3.3 Structural models for the A1 , A2 , and A3 defects from theanalysis at 80 K
In this section, we discuss the appropriate structural models
for the A1 , A2 , and A3 defects observed at ∼80 K.
The A1 defect is a “single dimer vacancy”, and apparent
symmetric dimers are observed on both sides of the vacancy.
Moreover, the results of the I-V measurements of the A1 defect at ∼80 K shows a clear surface band gap. These results
are in good agreement with the results of the I-V measurements of the A defect at RT by Hamers and Kohler1) and thecalculated density of states performed for the “rebonded vacancy model” by Wang et al.3) Thus, we conclude that the A1
defect corresponds to the “rebonded vacancy model”.
On the other hand, buckled dimers are observed on bothsides of the A2 and A3 defects, and the A2 and A3 defects
show some density of states near the Fermi level. Thus, theA2 and A3 defects are considered to have different structuresfrom those based on the R-DV model. However, since symmetric dimers are observed on both sides of the A defect at
RT, it is possible that A2 and A3 defects have the same origin

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Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 1, No. 6B

as the A1 defect. At a low temperature such as 80 K, dimers
are buckled and have a high-ordering among them; c(4 × 2)or p(2 × 2) arrangements. Thus the configuration of dimers
on both sides of the defects must be strongly affected by thesurrounding dimer arrangement. Namely, the bonding structure in the underlying layer is expected to be affected by thestrain induced by the surrounding dimer arrangement. In thescheme of the dimer vacancy model, the structure of the BRV model has some density of states near the Fermi level,
as calculated by Wang et al.3) Therefore, taking account of
the rather free bonding condition in the underlying layer, the
B-RV model is a possible candidate for the structures of theA2 defect. In consideration of the structure of the underlying
layer, the T-RV model may be attributed to the A3 defect. In
order to discriminate between the A2 and A3 defects in moredetail, further study, including theoretical analysis, is essential.
4. Conclusion
We conducted STM observation and STS measurements at∼80 K to study the electronic structure of the A1 , A2 , and A3
defects. We found that the A1 defect has a semiconductive
characteristic similar to that observed at RT. However, the A2
and A3 defects showed the existence of some states in thesurface band. In comparison with the theoretical results by
Wang et al.,3) we concluded that the A1 defect corresponds to
the rebonded vacancy model. The broken vacancy model andthe twisted vacancy model are possible candidates for the A2
and A3 defects, respectively.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports andCulture of Japan.

Fig. 4. Filled (a) (sample bias −1.0 V and tunneling current 1.0 nA) andempty (b) (+0.6 V, 1.0 nA.) STM images of the A1 defect. (c) a stick andball structure around a A1 defect. (d) STS spectra taken at the positions
shown in (c).

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