APPLIED PHYSICS LETTERSVOLUME 84, NUMBER 18

3 MAY 2004

Light-modulated scanning tunneling spectroscopy for nanoscale imaging
of surface photovoltage
Osamu Takeuchi, Shoji Yoshida, and Hidemi Shigekawaa)
Institute of Applied Physics, 21st century COE, University of Tsukuba 1-1-1 Tennodai, Tsukuba,
Ibaraki 305-8573, Japan

共Received 16 May 2003; accepted 12 March 2004; published online 20 April 2004兲
Light-modulated scanning tunneling spectroscopy 共LM–STS兲 is proposed as a useful method forinvestigating spatially resolved surface photovoltage 共SR–SPV兲. LM–STS provides thedependences of SR–SPV on bias voltage under constant tip-sample distance simultaneouslywith the entire dark/illuminated I – V curves. With this method, it is shown that SPV of a metallic
Si共111兲 surface can be bias-dependent and SPV at zero bias voltage for Si共001兲 can be
tip-sample-distance-dependent under conditions of small tip-sample distance and high illumination
intensity. The importance of the experimental condition for interpreting experimentally obtained
SR–SPV was suggested. © 2004 American Institute of Physics. 关DOI: 10.1063/1.1737063兴
Since the invention of scanning tunneling microscopy
共STM兲, many studies have been devoted to the utilization of
STM for measuring surface photovoltage 共SPV兲.1–10 In general, SPV is defined as the change in surface potential of asemiconductor material caused by superband gap illumination and it provides important electric properties of the material, such as the polarity and magnitude of surface band
bending, the recombination rate of photocarriers, and surface
conductivity.11 Thus, acquiring spatially resolved SPV 共SR–
SPV兲 simultaneously with an atomic topograph by STM willlead to a greater understanding of the electric functions of
nanodevices on the basis of their atomic structures.
Since the pioneering work by Hamers and Markert,2 several experimental methods in two categories have been developed, namely, for measuring intrinsic 共zero bias兲 SPV2–5
and SPV under finite bias voltages,6 –9 using STM. It is assumed that SPV alters the effective sample bias voltage at
the tunnel junction. Hence, the lateral shift of I – V characteristics of the junction caused by the illumination is measured as SR–SPV. In the measurement of the intrinsic SPV,
the disappearance of the differential tunnel conductancedI/dV at low bias voltages enhanced the small error in tunnel current detection, resulting in a large noise/artifact of theobtained intrinsic SPV. Thus, SR–SPV measurement is now
often performed under finite bias voltages. In these cases,
however, the local band structure beneath the STM probe canbe affected by the strong electric field and high current density required for the STM observation, in addition to theeffect of the contact potential difference between the probe
and the sample. Hence, the result of previous studies areapparently conflicting to each other. For better understandingof the obtained SR–SPV, the measurement conditions must
be carefully taken into consideration.
First, Cahill et al. reported that SPV measurement is sensitive to the tunnel current setting but not to the bias voltage;
they interpreted this result using the concept of the charging
effect.6 According to it, the tunnel current setting determines
the amount of space charge under the STM probe and governs the deviation of SPV from the intrinsic value. However,a兲Electronic mail: http://dora.ims.tsukuba.ac.jp/

McEllistrem et al. reported that SPV is strongly dependent
on bias voltage when the surface has a lower density of statesat the Fermi level.7 It was suggested that, since the low metallic surface state does not screen the electric field of the
STM probe, the penetration of the electric field into the subsurface region affects surface potential under a dark condition. On the other hand, Sommerhalter et al. insisted thatboth tunnel current and bias voltage affect surface potential
when the surface has a negligible surface state density.10 In
particular, they suggested that the tip-sample distance does
not greatly affect the surface potential under a dark condition, but affects that under an illuminated condition.
In consideration of the importance of the SR–SPV study
for the development of future electronic devices, suitable experimental conditions for SR–SPV measurement and a procedure for the analysis of the obtained results must be established. In order to clarify the disagreements among theprevious results, exhaustive studies of the SPV dependence
on experimental conditions will be required. In this letter, we
propose a simple experimental method, light-modulated
scanning tunneling spectroscopy 共LM–STS兲, as a powerful
method for such studies. LM–STS has two advantages. First,
it gives the SPV dependence on experimental conditions
complementary to what was obtained in previous methods. It
measures the SPV dependence on bias voltage under the condition of constant tip-sample distance, in contrast to previous
methods in which the condition of constant tunnel current is
adopted. Second, it provides not only SR–SPV but also thecomplete illuminated and dark I–V curves simultaneously,
providing a deeper insight into the sample properties. We
apply the method to a metallic Si(111)-7⫻7 surface and asemiconductive Si(001)-2⫻1 surface, and revealed properties of SR–SPV. SR–SPV for the metallic Si(111)-7⫻7 surface was strongly dependent on bias voltage, particularly inthe low bias voltage regime. The SPV dependence on the
tip-sample distance at zero bias voltage obtained forSi(001)-2⫻1 revealed a fundamental difficulty of measuring the intrinsic SPV by STM potentiometry.
LM–STS was performed as follows. Mechanicallychopped illumination was provided from 45° off-normal to
the surface and focused onto a spot of ⬃0.05 mm diameter at

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3646

Appl. Phys. Lett., Vol. 84, No. 18, 3 May 2004

Takeuchi, Yoshida, and Shigekawa

FIG. 1. 共Color兲 Typical result of LM–STS measurement on n-type Si共111兲
共30 ⍀ cm兲. 共a兲 Magnified plot of raw STS data under chopped illumination
共red兲 and its upper/lower envelopes 共dotted black兲. 共b兲 Entire spectrum. 共c兲
Extracted SPV spectrum.

FIG. 2. 共Color兲 SPV spectra 共left column兲 and histograms of their y sections
共right column兲 of p-type Si共001兲 共0.03 ⍀ cm兲. Tip-sample distance wasvaried by setting the bias voltage of the servo condition with a constantcurrent setpoint of 1 nA; 共a兲 0.3 共b兲 0.4 共c兲 0.5 and 共d兲 0.6 V were used.

the tunnel junction. The problematic thermal expansion/
shrinkage of the STM tip 共electrochemically etched tungsten
wire, ␾0.25 mm兲 was almost negligible with a chopping frequency higher than 40 Hz and an illumination power lessthan 1 mW 共HeCd laser, 325 and 441 nm兲. Note that whether
the thermal effect exists or not can be judged by looking at
the temporal evolution of tunnel current.12 STM topographs
could be stably obtained under these conditions, althoughsmall modulation, ⬃0.02 nmrms depending on the illumination and feedback conditions, could be observed due to theintermittent SPV effect. In order to obtain an SPV spectrum,
the scanning was stopped and a conventional STS measurement was performed under the chopped illumination. Part ofa typical result for an n-type Si(111)-7⫻7 sample 共30⍀ cm兲 is shown in Fig. 1共a兲. The I – V curve 共red兲 oscillates
widely between the two virtual I – V curves 共dotted black兲,corresponding to the dark and illuminated conditions. The
SPV spectrum is acquired by plotting the lateral separation
between these two virtual curves against the bias voltage
under the dark condition, as shown in Figs. 1共b兲 and 1共c兲.
In this work, I – V curve consisting of two thousand
points was obtained in 1–2 s with a chopping frequency of⬃40 Hz. The tunnel current for 30%– 80% of each
illuminated/dark period was averaged to extract theilluminated/dark current, which are shown by the blue andgreen circles, respectively, in Fig. 1共b兲. The gate time wasintroduced to eliminate artifacts due to the finite transient
time of illumination during which the laser power changes
from 0% to 100% and vice versa, from the displacement
current as a result of capacitance change of the tunnel junction, and from the recovery time of the metal–insulator–
semiconductor 共MIS兲 junction at the tunnel gap.13 Finally,
the illuminated/dark I – V curves for calculating SPV, which
are shown by the dotted black lines in Fig. 1共a兲, are obtainedby interpolating the measured points using the smoothing

spline function. In our case, the precision of the I – V curves
was limited by a few percent of relative error due to mechanical vibration and a few picoamperes of absolute error
due to broadband noise in the current signal. These errors in
the tunnel current, together with the magnitude of dI/dV,
determine the detection limit of SPV. When the tip-sample
distance is longer, dI/dV in the low-bias-voltage regime becomes too small. Accordingly, SPV could be measured only
for the high-bias-voltage regime in such cases.
The left-hand column of Fig. 2 shows SPV spectra of ap-type Si(001)-2⫻1 surface 共0.03 ⍀ cm兲 with various tipsample distances. The distances were determined by the
servo action of conventional STM electronics under chopped
illumination with changing the bias voltage from 共a兲 0.3 to
共d兲 0.6 V with a fixed reference current of 1.0 nA. For eachtip-sample distance, 25 spectra were measured at random
points on the surface. The dotted lines represent individual
SPV spectra and the solid black ones represent the averaged
ones. It can be seen that, even under a same servo condition,
the SR–SPV spectra scatter widely. This is not because of
the spatial variety of SPV but mainly because of the uncertainty in determining the tip-sample distance. The uncertainty arises from the fact that the tip-sample distance fluctuates when the servo action is active because it tries tocompensate the intermitted SPV effect. Since each STS measurement is done asynchronously to the illumination, the tipsample distance differs from spectrum to another. To visualize the amount of fluctuation, the dotted spectra are coloredby the tunnel current at a bias voltage of ⫺0.5 V. The color
scale of red to violet corresponds to 70%–130% of the tunnel current compared with the averaged value. In the righthand column of Fig. 2, the SR–SPV at zero bias voltage
共intrinsic SR–SPV兲 is summarized in histograms.
In the remainder of this letter, we discuss the above re-

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Appl. Phys. Lett., Vol. 84, No. 18, 3 May 2004

sults. Figure 1 clearly shows that SR–SPV can be strongly
dependent on bias voltage even for a surface with a metallic
surface state, such as the Si(111)-7⫻7 surface. Namely, theSPV value increased linearly in the low-voltage regime( 兩 V s 兩 ⬍0.1 V) but showed saturation in the high-voltage regimes (V s ⬍⫺0.1 V and 0.1 V⬍V s ). The curves in thesethree regimes, with different slopes, were also confirmed in
SR–SPV spectra for a nonmetallic Si(001)-2⫻1 surface, as
shown in Fig. 2. These spectra cannot be simply explained interms of the charging effect because the tunnel current
changes more in the high-bias-voltage regimes than in the
low-bias-voltage regime. Instead, penetration of the electricfield due to the bias voltage into the subsurface region underdark condition and the amount of accumulated carriers underilluminated condition should be considered. Sommerhalter
et al. reported that the surface band bending of the nonmetallic p-type WS2 surface under STM measurement can bequalitatively represented by a simple one-dimensional MIS
junction model.10 The three regimes observed in the SPV
spectra for p-type Si共001兲 surface can be explained in accordance with their result as follows. In the high-positivevoltage regime (0.1 V⬍V s ), sufficient accumulation of majority carrier occurs under dark condition. Hence, the little
band bending under the dark condition results in a small
SPV. At a negative bias voltage, depletion of majority carrier
occurs and the amount of surface band bending is proportional to the bias voltage under the dark condition. In the
low-bias-voltage regime ( 兩 V s 兩 ⬍0.1 V), the band bending isfully relaxed by illumination, resulting in the linear dependence of SPV on bias voltage. At high negative bias voltages(V s ⬍⫺0.1 V), the amount of band bending relaxation saturates because of the limited backdiffusion of minority carrier.
In addition to bias voltage dependence, the SPV dependence on the tip-sample distance can also be explained by
the same model. In Fig. 2, a small tip-sample distance correspondings to a small SPV. In addition, the same tendency
was obtained in the comparison of the dotted spectra shownin the same plot; the one with a larger current showed asmaller SPV. When the tip-sample distance is small, the photogenerated carriers tunnel efficiently into the STM tip andthe accumulation of carriers at the surface region is insufficient, preventing the complete relaxation of band bending.
Furthermore, the SR–SPV at zero bias voltage was also
dependent on the tip-sample distance, as shown in the righthand column of Fig. 2. This is rather surprising, since there isno tunnel current flowing between the tip and the sample inthis situation. We conjecture that the very high tunnel transmission coefficient between the tip and the sample pins thesurface potential, preventing the band bending relaxation. In
some of the previous studies on the SPV at zero bias voltage,
shortening of the tip-sample distance was attempted in order
to increase the measurement accuracy.5,9 Our result revealed
the fundamental limitation of such approach to measure theprecise SPV at zero bias voltage.
Before summarizing the argument, we mention the highspatial resolution of LM–STS method. Figure 3 shows an
SPV mapping in a color scale overlapped on a STM topograph. Silver was deposited from a tungsten filament on aclean n-type Si共001兲 substrate 共0.005 ⍀ cm兲 at room tem-Takeuchi, Yoshida, and Shigekawa

3647

FIG. 3. 共Color兲 SPV mapping of unsaturated Ag/Si(001)-2⫻3 structure forV s ⫽2.4 V. Green lines outline the two-dimensional silver islands.

perature and annealed at 400 °C. About 40% of the surface
was covered by the two-dimensional silver islands of the 2⫻3 structure 共indicated by the green lines in Fig. 3兲 in accordance with literature.14 During the STM imaging over the30⫻30 nm area (I t ⫽1.0 nA and V s ⫽⫺2.0 V) at 80 K, 27⫻27 LM–STS spectra were taken at the grid points 共white
dots in Fig. 3兲. From each spectrum, SR–SPV value for V s⫽2.4 V was extracted and shown in the color scale. SPV
values between the grid points were calculated by linear interpolation. Nanometer scale spatial variation of SPV due tothe existence of silver islands was successfully visualized
simultaneously with obtaining an atomically resolved STM
topograph. The detail will be reported elsewhere.15
In summary, LM–STS was proposed as an easily accessible and very powerful method for investigating SR–SPV
under various experimental conditions. Unlike previous
methods, it can obtain not only the SR–SPV spectrum but
also the entire illuminated/dark I – V characteristics with aconstant tip-sample distance. With this method, it was shownthat the experimentally obtained SR–SPV is dependent on
the bias voltage for a metallic Si共111兲 surface, and on the
tip-sample distance for a semiconductive Si共001兲 surface
even at zero bias voltage.
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