e-Journal of Surface Science and Nanotechnology

10 February 2006

Conference - ISSS-4 -e-J. Surf. Sci. Nanotech. Vol. 4 (2006) 192-196

Tip-induced band bending and its eﬀect on local barrier height measurement studied
by light-modulated scanning tunneling spectroscopy∗
Shoji Yoshida, Junichi Kikuchi, Yuya Kanitani, Osamu Takeuchi, Haruhiro Oigawa, and Hidemi Shigekawa†
Institute of Applied Physics, COE, CREST-JST,
University of Tsukuba, Tsukuba 305-8573, Japan(Received 13 November 2005; Accepted 19 January 2006; Published 10 February 2006)
Local barrier height (LBH) of Si(001) surface was studied using light-modulated scanning tunneling spectroscopy(LM-STS), which enables the observation of the tip-sample-dependent LBH with or without photoillumination
simultaneously. The bias voltage and tip-sample distance dependence of LBH were comprehensively understoodby the tip-induced band bending (TIBB), which inﬂuences the scanning tunneling microscopy and spectroscopy(STM/STS) in measurement of the local electronic structures of semiconductors. A marked decrease in surface
photovoltage caused by photocarrier tunneling at shorter tip-sample distance was also shown. On the basis of theseresults, a method to measure LBH free of TIBB is discussed. [DOI: 10.1380/ejssnt.2006.192]
Keywords: Scanning Tunneling Microscopy; Si(001); local barrier height; surface photovoltage; tip-induced band bendingI.

INTRODUCTION

As the scale of target materials decreases, scanning tunneling microscopy and spectroscopy (STM/STS) has beenplaying an important role as a powerful method of analyzing physical and chemical phenomena because of its
high potential to probe the local electronic structures ofmaterials. Actually, since its development, a considerable
number of studies have been devoted to the utilization of
STM/STS for analyzing the characteristics of electronic
structures such as local band structures [1] and local barrier height (LBH) [2–4].
However, as has been pointed out, STM/STS parameters, such as bias voltage, tunneling current andtip-sample distance, inﬂuence the sample characteristics[5, 9]. One of the critical problems is the tip-induced
band bending (TIBB). As is discussed in this paper, it
is diﬃcult to accomplish spectroscopic analysis on semiconductor surfaces by conventional methods because thebias voltage between the tip and the sample induces band
bending in the sample surface underneath the STM tip[6].
Figure 1 illustrates the characteristics of TIBB depending on tip-sample distance. The measurement conformation among the STM tip, tunnel gap and semiconductor surface forms a metal-insulator-semiconductor (MIS)
structure. When the tip-sample distance is suﬃciently
large, since the bias voltage is mostly applied to the tunnel gap between the sample and the tip, the band bending in the sample surface is small as shown in Fig. 1(c).
However, as the tip-sample distance decreases, the bias
voltage begins to impinge on the inner part of the sample
surface, resulting in the induction of TIBB as shown in
Figs. 1(b) and 1(a). The TIBB increases with the total
applied bias voltage. Although this eﬀect is well recognized and suggested by some experimental results [6], its
characteristic properties have not yet been well examined∗ Thispaper was presented at International Symposium on Surface Science and Nanotechnology (ISSS-4), Saitama, Japan, 14-17
November, 2005.
† Corresponding
author:hidemi@ims.tsukuba.ac.jp;
Web:http://www.dora.ims.tsukuba.ac.jp

experimentally because these issues are directly related to
the STM/STS measurement mechanism itself.
A useful way of examining TIBB is to measure surface
photovoltage (SPV). When the tip-sample distance is not
suﬃciently large and the STM/STS measurement is performed in the dark, TIBB appears as shown in Fig. 2(a),where a p-type semiconductor sample is used. When thesample below the STM tip is photoilluminated with a sufﬁcient intensity, the introduction of photoexcited carriers
reduces the band bending, resulting in the ﬂat-band condition shown in Fig. 2(b). This band bending change
in the MIS structure is deﬁned as a SPV. The eﬀective
bias voltage applied between the STM tip and the sample
surface (tunnel gap) increases by the amount of SPV under photoillumination as shown in Fig. 2, resulting in thelowering of the net barrier height (BH) for the tunnelingof electrons at the valence band maximum by SPV/2.
Since the pioneering work by Hammers et al. [7], thistechnique has been developed and extensively applied tothe analysis of semiconductor devices [6–8]. However,since the tip-sample distance cannot be maintained constant when the bias voltage is changed [5], these conventional methods cannot be used for the detailed analysis
of TIBB.
In this study, we analyzed TIBB on a Si(001) surface
by measuring LBH using light-modulated scanning tunneling spectroscopy (LM-STS) [9]. Since this method enables the observation of the tip-sample-dependent LBH
with or without photoillumination simultaneously, we canprecisely analyze the tip-sample dependence of TIBB.

II.

EXPERIMENTAL

Measurements were performed under ultrahigh vacuum(T=300 K) for a Si(100) 2 × 1 clean sample surface (ptype, 10 Ωcm). A laser diode (635 nm) or a He-Cd laser(442 nm), mechanically chopped at 100 Hz, was used forphotoillumination to perform LM-STS.
Figure 3 is an example of LM-STS for SPV measurement. After setting the tip position under the conditionsof V = −3.0 V and I = 1 nA, an I-V curve was obtained
under chopped photoillumination [9]. The tunneling current oscillates with the chopping frequency between two
envelopes that correspond to the I-V curves obtained un-

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ISSN 1348-0391 

192

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FIG. 1: Characteristics of tip-induced band bending (TIBB) depending on tip-sample distance.

FIG. 2: Schematic illustrations of MIS structures in cases (a)without and (b) with photoillumination.

der photoilluminated (red) and dark (blue) conditions.
Bias-dependent SPV is determined by calculating the lateral shift of the two I-V curves as related to bias voltage
for the I-V curve under the dark condition (blue one),which is presented in Fig. 3 (b). The central part ofthe spectrum (-1.5 to +1.5V in Fig. 3) is missing due
to the diﬃculty in calculating the shift of the two I-V
curves for that region. The existence of the bias-voltage
dependence of SPV is clear. For the forward bias (positive
sample bias) voltage condition, the SPV is small because
the Fermi level is close to the valence band edge.
By combining LM-STS with LBH measurement, wehave developed a new method to observe the tip-sample
dependence of LBH with or without photoillumination simultaneously. Figure 4 is an example of the tip-sample
distance dependence of the LM-STS spectrum. The IZ curve was obtained under chopped photoillumination.
With a similar process to that employed for SPV analysis, two I-Z curves under photoilluminated (red) and dark
(blue) conditions are obtained simultaneously from the
LM-STS spectrum as shown in Fig. 4. Thus, each valueof LBH is calculated as2Φtip + Φsample − Vh2d(ln I)
LBH =
=
. (1)
2
32π 2 mdz
Here, Φtip , Φsample , V and h represent the work functionsof the STM tip (W ) and the Si sample, the applied bias

FIG. 3: LM-STS spectrum obtained for Si(001) sample.

FIG. 4: I-Z curve (Z: tip-sample distance) measured underchopped photoillumination. Horizontal axis represents the distance of tip retraction from the set point.

voltage and Planck constant, respectively.

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Yoshida, et al.

FIG. 5: Tip-sample distance and bias-voltage dependences of LBH obtained using two diﬀerent tips ((a), (c) and (e) for theﬁrst tip and (b), (d) and (f) for the other) under dark (blue) and photoilluminated (red)) conditions, respectively. Horizontal
axis represents the distance of tip retraction from the set point.

III.

RESULTS AND DISCUSSION

Now, let us examine TIBB by LM-STS. The tip-sample
distance dependence of LBH was measured by the method
explained in section 2 using several tungsten tips.
Figure 5 shows the tip-sample distance of LBH obtained
for two diﬀerent tips ((a), (c) and (e) for the ﬁrst tip, and(b), (d) and (f) for the other tip) under dark (blue) andphotoilluminated (red) conditions, respectively. To examine the bias dependence of LBH, we obtained I-Z curves
at various bias voltages, three of which are shown here.
With this method, since LBH with or without photoilumination can be obtained simultaneously, the tip-sample
dependence of TIBB can be analyzed as the eﬀect of SPV
taken into consideration together. After setting the STM
tip with the conditions of sample bias voltage (-2 V) andtunneling current (1 nA), we wait for a while with a weak
feedback under a chopped light to settle the tip position
just to measure 1 nA (center of the two I-V curves inFig. 4). Then, feedback is turned oﬀ completely and thebias voltage is changed to that used in the experiment.
With this process, we can keep the same tip-distance of194

the set point throughout the measurements for diﬀerent
bias voltages. Thus, LM-STS spectra were obtained as
the tip-sample distance was increased from the set point.
Due to the steep change in the tunneling current depending on the tip-sample distance, the change in distance was limited to 0.1∼0.2 nm for each measurement.
The tip-sample distance dependence was slightly diﬀerent
between these two tips, which is probably caused by thediﬀerence in the tip shape that changes the set point of
its position determined by the tunneling current and bias
voltage. Taking advantage of this variation, we were able
to observe the tip-sample distance over a wider range,
despite the limited distance change of 0.1 nm for one tip.
Note that the LBH in Fig. 5 is not a real one, but anapparent LBH, which is deduced from the derivative of
I-Z curves (Eq. (1)). If there is no TIBB, LBH should
be constant because the slope of I-Z curve should be constant. In contrast, the decrease in LBH in Fig. 5, with thedecrease in the tip-sample distance, indicates the increase
in TIBB that lowers the intensity of tunneling current resulting in a deviation from the exponential function in Eq.
(1) [10].

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Let us see the results obtained under dark conditionﬁrst. In Fig. 5(b), the LBH is almost ﬂat at a large
distance (0.08-0.1 nm) and is approximately 4.5 eV, indicating that there is little TIBB. The decrease in LBH
with further decrease in the tip-sample distance indicatesthe increase in TIBB. In Fig. 5(a), the LBH is lower thanthat in Fig. 5(d), even in the large-distance region, whichsuggests that Fig. 5(a) shows the change obtained at a
tip-sample distance shorter than that shown in Fig. 5(d).
Therefore, Figs. 5(a), 5(c) and 5(e), and Figs. 5(b), 5(d)and 5(f) are considered to show the changes in TIBB appearing in the regions shown in Figs. 1(c) and 1(b), andFigs. 1(b) and 1(a), respectively.
As shown in Figs. 5(b), 5(d) and 5(f), the decrease in
LBH is sharper with higher bias voltages. The sharper
decrease in LBH for the case of -2.5 V compared withthat for the lower bias voltages indicates that the electricﬁeld impinges on the semiconductor surface for a higher
bias voltage, resulting in a sharper tip-sample distancedependent LBH with higher bias voltage. Actually, forthe shorter tip-sample cases (Figs. 5(a), 5(c) and 5(e)),
the change in LBH in the dark (blue line) is smaller forthe high bias voltage (Fig. 5(e)) than that with lower bias
voltages (Figs. 5(a) and 5(c)). This can be attributed tothe earlier appearance of the pinning eﬀect due to a larger
TIBB for a higher bias voltage. However, when the change
in TIBB is weakened or stopped, for example, as the bandedge meets the Fermi level, the LBH will increase andhave again a higher value corresponding to the value ofthe ﬁxed TIBB. Therefore, although the change in TIBB
is slow, the low LBH still indicates the existence of a slightchange in TIBB.
When the sample below the STM tip is photoilluminated with a suﬃcient intensity, the introduction of photoexcited carriers reduces the band bending, resulting inthe ﬂat-band condition as shown in Fig. 2(b). Therefore,
the decrease in LBH under photoilluminated condition,with the decrease in the tip-sample distance, indicates adecrease in the eﬀect of photoillumination depending on
the tip-sample distance.
LBH obtained under photoillumination is almost ﬂat
at a large distance (0.05-0.1 nm) as expected. This resultclearly shows that the decrease in LBH with decrease in
the tip-sample distance in this region observed in the dark
is caused by TIBB. Decrease in LBH begins with further
decrease in the tip-sample distance, which indicates thatthe decrease in the eﬀect of photoillumination to reduce
TIBB begins near this distance.
Slope in the decrease of LBH is almost the same regardless of photoillumination for the low bias voltages (Figs.
5(a) and 5(c)). However, as shown in Fig. 5(e), the LBH
obtained at -2.5 V under photoillumination sharply decreases with the decrease in the tip-sample distance, andis close to the same value for the LBH in the dark at
shorter distance. The result indicates that the decrease
in LBH under photoilluminated condition, with the decrease in the tip-sample distance, is sharper for the higher
bias voltage. Since SPV is expected to be larger for thehigher bias voltage, some other mechanism is necessary to
explain the lowering in the photoillumination eﬀect. To
see this point, we examined the tip-sample dependence of
SPV for the cases shown in Figs. 5(a) 5(c) and 5(e).
Figure 6 is the result. The tip-sample distance was

FIG. 6: Tip-sample distance dependence of SPV. The green
spectrum was obtained at the position of 0 nm in Figs. 5(a),5(c) and 5(d). Blue and green spectra were obtained at two
diﬀerent positions of -0.35 nm (closer) and 0.24 nm (far) fromthe set point. Horizontal axis represents the distance of tip
retraction from the set point.

varied by setting the bias voltage of the servo condition
with a constant tunneling current set point of 1 nA. The
green spectrum was obtained at the position of 0 nm in
Figs. 5(a), 5(c) and 5(e). Blue and green spectra wereobtained at two diﬀerent positions of -0.35 nm (closer) and
0.24 nm (far) from the set point. The central part of each
spectrum is missing due to the diﬃculty in calculating theshift of the two I-V curves for that region as explained in
Section 2.
In general, with the increase in TIBB, the SPV increases as the tip-sample distance decreases as expectedfrom the mechanism shown in Fig. 1. However, the SPV
decreases with the distance in Fig. 6, which indicatesthat the eﬀect of photoillumination is reduced as the tipsample distance decreases.
Since SPV is realized by the photoexcited carriers, theobserved change in SPV may be caused by the eﬃcienttunneling of the photoexcited carriers at a shorter tipsample distance that results in a suppression of the SPV
eﬀect [9]. As assumed by the results of LBH, change in
SPV is larger at higher bias voltage. This may be dueto the fact that tunneling of the photoexcited carriers ismore eﬀective for the higher bias voltage because of thedecrease in LBH.
To examine the eﬀect of photoillumination on LBH, wemeasured the tip-sample distance dependence of LBH bychanging photoillumination intensity. A setpoint was determined as -2 V and 1 nA. Figure 7 shows the results,
where the tunnel gap is photoilluminated continuously
without chopping. In such a condition, since thermal expansion eﬀect can be reduced, we can increase the intensity of photoillumination compared to the case of chopped
light.
Low-intensity conditions (12.5, 25%) are similar to
those used for the other experiments in this paper, where
the observed LBHs are very low. This is caused by the efﬁcient tunneling of the photoexcited carriers at a shortertip-sample distance that results in a suppression of the
SPV eﬀect. Actually, with the increase in photoillumina-

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195Volume 4 (2006)

Yoshida, et al.

lumination can be lower than that in the dark, becauseof the eﬀect of SPV, which is slightly shown for the large
distance region in Fig. 5(b). This condition is suitable tomeasure LBH.
As has been shown, TIBB on a Si(001) surface is comprehensively examined using LM-STS, which enables theobservation of tip-sample-dependent LBH with or without photoillumination simultaneously. Analysis on thelocal electronic structures of semiconductor surfaces, forexample, with adsorbed molecules is one of the keys innanoscience and technology today. Understanding of thephotoinduced phenomena is also an important issue. We
hope this technique will lead to further development inthis ﬁeld.

IV.
FIG. 7: Tip-sample distance dependence of LBH under various
intensities of photoillumination. Horizontal axis represents the
distance of tip retraction from the set point.

tion, the LBH increases to close to the ﬂat-band condition
as shown in Fig. 2(b). This is due to the reduction of
TIBB caused by the increased photo carriers introducedby the increase in photoillumination. Therefore, although
it is not perfect, the undesirable eﬀects on LBH measurement can be suppressed by photoillumination of higher
intensity. Since tunneling of the photoilluminated carriers at a shorter tip-sample distance suppresses the SPV
eﬀect, we should measure LBH at a large distance. A
dull tip, which forms a large tip-sample distance as theset point of the tip position determined by the tunnelingcurrent and bias voltage, is suitable to measure a better
LBH.
When there is no band bending, LBH can be observed
correctly regardless of photoillumination. Even in the casewhere band bending exists, LBH can be observed almost
constant if TIBB is weak and little tip-sample distancedependent. In such a case, LBH obtained under photoil-[1] R. J. Hamers, R. M. Tromp and J. E. Demuth. Phys. Rev.
Lett. 56, 1972 (1986).
[2] R. Akiyama, T. Matsumoto, and T. Kawai. Phys. Rev. B
62, 2034 (2000).
[3] D. Ostermann, G. Walther, and K. D. Schierbaum, Phys.
Rev. B 71, 235416 (2005).
[4] H. Totsuka, S. Furuya, and S. Watanabe, Jpn. J. Appl.
Phys. 44, 5459 (2005).
[5] S. Yoshida, J. Kikuchi, Y. Kanitani, O. Takeuchi, H.
Oigawa, and H. Shigekawa, Phys. Rev. B 70, 235411(2004).

196

CONCLUSIONS

By combining light-modulated STS (LM-STS) with local barrier height (LBH) measurement, we have developed
a new method to observe the tip-sample dependence of
LBH with or without photoillumination simultaneously.
The bias voltage and tip-sample distance dependence of
LBH observed on a Si(001) surface were comprehensively
explained by the tip-induced band bending (TIBB), whichinﬂuences the local electronic structures of semiconductors. A marked decrease in surface photovoltage causedby photocarrier tunneling at shorter tip-sample distance
was also shown. On the basis of these results, a method
to measure LBH free of TIBB was discussed.

Acknowledgments

This work was supported in part by a Grant-in-Aid
for Scientiﬁc research from the Ministry of Education,
Culture, Sports, Science, and Technology of Japan. We
thank Ms. Rie Yamashita, in our group at University ofTsukuba, for her help in preparing this paper.

[6] M. McEllistrem, G. Haase, D. Chen, and R. J. Hamers.
Phys. Rev. Lett. 70, 2471 (1993).
[7] R. J. Hamers and K. Markert, Phys. Rev. Lett. 64, 1051
(1990).
[8] Y. Kuk, R. S. Becker, P. J. Silverman, and G. P. Kochanski, Phys. Rev. Lett. 65, 456 (1990).
[9] O. Takeuchi, S. Yoshida and H. Shigekawa, Appl. Phys.
Lett. 84,3645 (2004).
[10] M. Weimer, J. Kramar, and J. D. Baldeschwieler, Phys,
Rev. B 39, 5572 (1989).

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