Japanese Journal of Applied
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Laser-combined multiprobe microscopy and its
application to the materials with atomic layer
thickness
To cite this article: Hiroyuki Mogi et al 2022 Jpn. J. Appl. Phys. 61 SL1011

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Japanese Journal of Applied Physics 61, SL1011 (2022)
https://doi.org/10.35848/1347-4065/ac6a3cLaser-combined multiprobe microscopy and its application to the materials withatomic layer thickness
Hiroyuki Mogi1, Zi-han Wang1, Ibuki Kuroda1, Yuhei Takaguchi2, Yasumitsu Miyata2, Atsushi Taninaka1,3,
Yusuke Arashida1, Shoji Yoshida1, Osamu Takeuchi1, and Hidemi Shigekawa1*
1Faculty of pure and applied sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan
Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan
3
TAKANO Co., Ltd., Miyano-mura, Kamiina-gun, Nagano 399-4301, Japan2*

E-mail: hidemi@ims.tsukuba.ac.jp

Received January 31, 2022; revised April 1, 2022; accepted April 24, 2022; published online June 23, 2022
We have developed a method of evaluating the photo-induced carrier dynamics of nanostructures by combining optical technologies withmultiprobe microscopy techniques. Using multiple probes, measurement can be carried out even for a small sample without complicatedpretreatments, such as attaching electrode structures. Using transition metal dichalcogenides as a sample and a continuous laser or an ultrashortpulse laser as the light source, we demonstrated analyses of the carrier dynamics related to trap levels in a millisecond to second time domain andthe ultrafast photoexcited carrier dynamics in the picosecond region. © 2022 The Author(s). Published on behalf of The Japan Society of Applied
Physics by IOP Publishing Ltd

1. Introduction

The development of electronics to date has been promoted by
the miniaturization of functional devices. Molecular/atomiclevel devices are expected to be realized in the future.
However, as the size of devices decreases, it becomes more
important to control phenomena such as carrier scattering due
to nanoscale crystal irregularities and atomic-scale impurities.
Therefore, there has been a strong desire for the developmentof measurement methods that can locally evaluate theseeffects. Scanning tunneling microscopy (STM) is one of thepromising candidates for this purpose. STM was developed
by Binnig and Rohrer in 19821) and has contributed tonumerous achievements in nanoscale science. In STM, a very
sharp tip is brought close (within about 1 nm) to the surface
of a conductive material or atoms/molecules adsorbed on thesurface. Then, by measuring the tunnel current ﬂowing
between the sample and the tip, atomic structures or local
electronic states can be analyzed at the nanoscale. However,
only physical information immediately below the tip can beobtained using an ordinary STM system with a single tip.
Therefore, multiprobe (MP) microscopy techniques wereintroduced,2–7) which can accurately control the positions ofmultiple probes on the μm- to -nm scale. For example, local inplane electrical conduction, including anisotropy, can bedirectly measured. Furthermore, by combining MP microscopy
with optical technologies, it becomes possible to investigate
local photoinduced characteristics of materials.5,8–10) In particular, when an MP microscopy system is combined with apump-probe method using an ultrashort pulse laser, timeresolved measurement is possible even in the fs region8,11–15)
while maintaining the spatial resolution of STM. Therefore,
laser-combined MP microscopy has become a particularly
powerful measurement method for evaluating photoinduced
properties in a wide range of time and space regions.8)
Layered materials such as transition metal dichalcogenides(TMDCs) have recently been attracting considerable

attention.16–18) TMDCs are two-dimensional sheetlike materials
having a structure in which transition metal atoms are sandwiched between chalcogen atoms (S, Se, Te). They have a wide
range of functions due to the variety of choices of elements. For
example, a TMDC which has a 2H structure with Mo or W as
the transition metal is a semiconductor with a bandgap in thevisible light region19) and a TMDC with Zr or Hf is a widebandgap semiconductor.20) It has also been reported that Nbcontaining TMDCs exhibit superconductivity.21) Besides,
TMDC semiconductors with Mo or W have electron mobilities
comparable to that of Si,22,23) their monolayer (ML) ﬁlms havea direct-transition-type band structure and show a strong light–
matter interaction. Accordingly, TMDC semiconductors arebeing actively studied for applications to logic electronic circuits
and optoelectronics/photonics devices.
Here, using our developed laser-combined MP microscopy, the following three experiments were conducted on
TMDC semiconductors with an atomic layer thickness: (i)transport characteristic evaluation using two probes, (ii) msscale trap capture/emission analysis using a continuous lightsource, and (iii) ps-scale dynamics evaluation with a femtosecond pulse laser.
2. Laser-combined multiprobe microscopy

Figure 1(a) shows the system we have developed. Four probes
can be used in the ultrahigh vacuum chamber (∼10–8 Pa). Eachprobe scanner can be equipped with an ordinary STM probe or aconductive AFM (cAFM) cantilever as a contact probe for theelectrode. Using a cAFM probe for contact can minimize
damage during the contact process, even for ultrathin ﬁlm
samples such as TMDC MLs. The probes and sample can bealigned by observing the optical microscope (resolution ∼1μm), and repeated local conduction measurements can be
performed on a small sample without the need to form
electrodes by lithography. It is also possible to introduce liquid
He into the cryotank and carry out an experiment in a lowtemperature environment down to 10 K.

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Fig. 1. (Color online) Time-resolved multiprobe STM system. (a)Schematic diagram of the system. td: delay time, Δtd modulation width ofdelay time. (b) Examples of measurement using multiple probe system: (i)conduction dynamics (upper), (ii) carrier dynamics in TMDC (lower left),and (iii) carrier dynamics in nanostructures (lower right).

For photoexcitation, light beams are introduced andfocused through the objective lens of the optical microscope.
It is possible to irradiate any position without being hinderedby the MP structures. If the probe/sample position ﬂuctuateswith respect to the light irradiation point, the effective lightintensity at the measurement point will change, making theuse of a vibration isolation system important. The eddy
current damper in the chamber and the active air spring
system enable stable measurements. A continuous light laserthat can be electrically turned on and off or an ultrashortpulse laser with the modulation system we developed8) wasused as the light source in this study. By combining theselaser systems with the MP system, the measurement of local
photoinduced dynamics under operating conditions was
realized. Examples of experimental setups are shown in
Fig. 1(b). In this work, all probes were used in contact
with the sample.
3. Evaluation of transport characteristics using
multiple probes

Back-gated FET (BG-FET) structures have been used toinvestigate the carrier transport characteristics of low-dimensional semiconductors/semimetals of interest.24,25) A general
structure is a three-terminal device in which a channelmaterial is placed on a substrate such as SiO2/Si, and ﬁxed
electrodes are formed using a lithography process. However,since various chemical treatments are performed using a
resist mask and a solvent in the lithography processes, someresidues remain on the sample surface. Such residues greatly
affect the conduction characteristics of low-dimensional
semiconductors. Also, the spatial resolution of the analysis
using the system is limited.
On the other hand, in measurements using MP techniques,some of the probes can be used to contact the sample without
the need to form ﬁxed electrodes. This has the advantage thatno pretreatments are required and that the conduction
characteristics can be measured while freely changing theelectrode positions. Furthermore, the spatial resolution is in
the nanoscale range.
Figures 2(a) and 2(b) show the sample and experimental
setup used in our study, respectively. As shown in the optical
microscopy image in Fig. 2(a), a ML/bilayer (BL) WSe2 ﬁlm
grown by chemical vapor deposition (CVD) on a SiO2/Si
substrate26) was used for the sample. A cAFM cantilever(spring constant 0.2 N m−1, PtIr5 coating) was placed incontact with the sample. It has been reported that when an
electrode is placed on WSe2 channel, the structure becomes a
Schottky-type FET.27,28) Information on Schottky barriersand carrier densities can be obtained by measuring the gate
voltage (Vgs) dependence of the drain-source current (Ids). As
shown in Fig. 2(b), the position of the source probe was
ﬁxed, and the Ids image was measured while scanning theposition of the drain probe across the ML/BL interface. The
red cross in Fig. 2(a) shows the source probe position, andthe drain probe was scanned for measurement in the red point
indicated by the red arrow.
Figure 2(c) shows the Ids (current between source anddrain) image obtained in the red square point in Fig. 2(a)under the conditions of Vgs = 50 V and Vds = 9 V, where Vgs
is the gate voltage applied between the gate and the source,
and Vds is the bias voltage applied between the drain and thesource. A discontinuous change in current was clearlyobserved at the ML/BL interface. To analyze this characteristic in more detail, we stopped the scan every 120 nm while
scanning the same area and swept the gate voltage Vgs from−80 V to +80 V to measure the current Ids ﬂowing between
the source and the drain. Through the process, we obtained
the Ids–Vgs property. The typical Ids–Vgs curves obtained in
the ML and BL regions are shown in Fig. 2(d) on semilogarithmic axes. The broken line in the ﬁgure shows thenoise level of the current. Both graphs behave exponentiallyin the region of Vgs = 0–50 V. This indicates that the sample
is an n-channel-type FET and that the main carriers areelectrons. The slope of the semi-log plot is almost the samefor the two curves, but the increase in Vgs in the ML region isshifted to the positive side by about 10 V from that in the BL
region.
It has been reported that Schottky-type FETs exhibit
exponential Ids–Vgs characteristics when TMDC MLs of up
to several layers are used as samples.27,28) This could not be
explained by the conventional model but is now understoodto involve a mechanism that considers the effect of theelectric ﬁeld applied between the source and gate electrodes
whereby electrons entering the channel from the electrode are

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Fig. 2. (Color online) (a) Optical photograph of the sample. ML: monolayer, BL: bilayer. The red cross point is the source probe position, and the drainelectrode probe was scanned in the red point indicated by the red arrow. (b) Schematic diagram of the experimental setup. (c) Ids image obtained in the redsquare point in (a). (d) Vgs dependence of Ids on ML and BL when irradiating the probe position on the source side. (e) Energy relationship between the probes
and sample. (f) Band structure in the junction form.

produced by tunneling or thermal excitation above the
Schottky barrier. This is consistent with our results asfollows.
First, the bandgaps of ML and BL WSe2 are about 2.4 eV
and 1.8 eV, respectively, and the electronegativity of the BL
is about 0.2 eV larger than that of the ML.29) Therefore, theband structure when the probe is not in contact can be
expressed as shown in Fig. 2(e). If the probe is placed incontact in this state, the Schottky barrier will be lower on thedrain side than on the source side [Fig. 2(f)]. Thus, the risingvoltage of the BL will become smaller than that of the ML
because the electronegativity in the BL region is large, andthe effective Schottky barrier is lowered accordingly. As
shown in Fig. 2(d), the slope of the semi-log plot did notchange and only a shift in Vgs that depended on the Schottky
barrier height was observed, which is also consistent with themodel.
We succeeded in showing that the change in characteristics
caused by the differences in Schottky barrier height can beevaluated with nanoscale spatial resolution by MP-STM. It
has thus become possible to evaluate the characteristics of
FETs together with the information of local structures such as
atomic-scale defects observed by STM.
4. Analysis of millisecond-range optical transient
response
4.1. Examination of the measurement method

TMDCs have a strong light–matter interaction and exhibit up
to 10% light absorption in their MLs. Therefore, TMDCs
exhibit high responsivity when used as a photodetector ascompared with three-dimensional (3D) semiconductors suchas Si. On the other hand, the response time of Si is ∼10–8 s,
whereas that of TMDC materials is 10–3–10–1 s, and the gain
bandwidth when TMDCs are used as a photodetector isalmost the same as that of the conventional 3D semiconductors. The slow optical response of TMDC-based materials isdue to the trap levels existing in the substrate and channels.
Therefore, it is very important to analyze this response so thatthe performance of TMDCs can exceed that of conventional
photodetectors. We attempted to develop a method of
measuring the dynamics of the photoconductive response at
the nanoscale using MP microscopy.
First, we carried out an experiment using the MP-STM
system combined with a continuous light source to measure
the ms-scale optical response. Figure 3(a) shows the measurement setup. After the two probes were brought in contact
with any desired point on the sample, continuous light withon-off modulation was intermittently irradiated between theprobes. The electrical connection was a BG-FET structureusing two conductive AFM cantilevers, as in the case of
Fig. 2. We introduced three laboratory-built preampliﬁers for
multiprobe measurement so that the currents ﬂowing throughthe drain probe, source probe, and gate electrode could be
detected independently. If there is a large difference in the
response bands of the three preampliﬁers, an artifact will
occur when the difference in the time response of eachpreampliﬁer is taken. To avoid such a problem, the phasecompensation capacitor of the current–voltage conversion
circuit of each preampliﬁer was adjusted to make theresponse band uniform. From the step response time, it was
conﬁrmed that the band was ∼1 kHz. A semiconductor laser(wavelength 532 nm) that can be electrically switched on and

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Fig. 3. (Color online) (a) Schematic diagram of the measurement setup. (b) Measurement sequence. (c) The light irradiation positions were set to 1 and 3
shown in (a). Measurement results of Is and Id transient responses with and without light irradiation. Vds = 10 V, Vgs = 0 V. (d) Potential distribution between
source and drain with and without light irradiation to position 1 shown in (a). (e) Circuit diagram corresponding to the conﬁguration of the measuring unit.
(f) Time dependences of Id and Is when the light irradiated to the center of the channel between the source and drain was turned on and off.

off was used as the light source. The analog modulation bandof the light source was 250 kHz, which is sufﬁciently fasterthan the response of the preampliﬁer.
Figure 3(b) shows the measurement sequence of ms opticalresponse dynamics. (i) First, Vds or Vgs is changed to the set
value. When Vds/Vgs is changed, a pulse current ﬂows to theparasitic capacitance of the system and the capacitancecomponent of the sample. Thus, we must wait for a certain
period until the current reaches a steady value. (ii) Next,
measurements are carried out repeatedly to acquire the time
response of Ids while turning the light on and off at a regularinterval. (iii) Thus, a transient response curve can be obtainedby integrating and averaging the data. Then, returning to (i),Vds/Vgs is changed to a different value and the measurement isrepeated.
First, the transient optical response of ML-WSe2 on a
SiO2/Si substrate prepared under the same conditions as theexperiment shown in Fig. 2 was analyzed. Figure 3(c) shows
the results when the light irradiation positions were set to 1
and 3 in Fig. 3(a). The experimental conditions wereVds = 10 V, Vgs = 0 V, a drain-source probe distance of∼7 μm, light intensity = 0.5 mW, and a light spot diameterof ∼2 μm. When the contact on the source side wasirradiated, both currents, Id and Is, increased when the light
was turned on, but a sharp peak occurred only in Is. On theother hand, the opposite tendency was observed when thedrain side was irradiated. The difference in noise level
between Id and Is is due to the fact that no voltage wasapplied to the preampliﬁer measuring Is, where no noise
originated from the voltage source.

When the measurement is performed in a steady-state andthere is no leakage current to the gate, Id and Is should beequivalent. However, as shown in Fig. 3(c), when the light
was on, a pulselike current peak occurred in the probe near
the light spot, whereas when the light was turned off, thecurrent slowly decayed in the probe far from the light spot.
The reason for the difference between Id and Is is considered
to be the presence of capacitance components in the channel,
which cause charging and discharging.
The origin of this phenomenon is considered to be thechange in conductivity around the probe due to lightirradiation. Figure 3(d) shows the in-plane potential distributions when a positive voltage is applied to the drain without(upper) and with light illumination at the source probe. A
nonlinear potential distribution is formed in the two-dimensional channel, and a large potential gradient is generated in
the vicinity of the probes. The resistances near the drain andsource contacts are respectively written as Rd and Rs, andcontact resistance is not considered for simplicity. When thevicinity of the source probe is irradiated, a large number ofelectron–hole pairs are generated. In this case, electrons ﬂow
to the channel side and holes ﬂow to the source probe sideowing to the in-plane electric ﬁeld produced by Vds. In
addition, screening occurs in the irradiation spot, and theresistance Rs near the probe decreases owing to the increasein carrier density. As a result, the voltage drop near the sourceprobe decreases, and the potential of the entire channel drops[Fig. 3(d), lower ﬁgure]. Thereby, the potential difference
between the gate and the channel also changes, and thecapacitance between the gate and the source is charged so

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Fig. 4. (Color online) (a) Time dependence of Id after light irradiation was stopped. (b) Vgs dependence of Id at the time of light irradiation and Id at each timeafter stopping light irradiation. (c) Vgs dependence of Is–Id obtained by subtracting the Id after 1 ms from that immediately after light irradiation was stopped.
(d) Band structures when Vgs is negative (left) and positive (right) with Vds = 0.

that a peak displacement current is generated from the sourceprobe side with low resistance. When the light is turned off,
the potential distribution returns to that before light irradiation, but Id slowly decreases because the charged electrons
are extracted from the drain side.
Figure 3(e) shows a simple equivalent circuit and theelectron path. The difference in transient response time
constant when the source and drain sides were irradiatedwith light, shown in Fig. 3(c), is considered to be due to thedifference in contact resistance. Namely, the RC couplingtime constant increased owing to the large contact resistanceof the drain probe.
Figure 3(f) shows the results obtained when the center ofthe channel between the probes was irradiated. The currents
Id and Is are almost the same, which means that when thecenter of the channel is irradiated, the potential of the entire
channel does not change owing to the effect mentioned
above, and the effect of the displacement current due tocapacitive coupling can be minimized. Therefore, to correctly
evaluate the photoconductive response in the photoexcitedmultiprobe system, it is necessary to perform photoexcitation
symmetrically with regard to the probe arrangement. This
relationship holds even when the width of the light spot islarge compared with the distance between the probes,
provided that the light excitation is carried out uniformly
on both probes. Thus, by reducing the distance between theprobes to the nm scale, it is possible to measure the light
conduction response more locally.
4.2. Application of the developed method

Next, keeping the photoirradiation at the central part between
the probes, transient responses in Id and Is were measured
while changing Vgs. The light intensity was 10 μW with
Vds = 10 V. The transient response up to 600 ms wasmeasured at a sampling rate of 5 kHz after photoirradiation
was stopped at 0 ms. Figure 4(a) shows a typical result.
Figure 4(b) shows plots of Id as a function of time for various
Vgs values, which were obtained from all the measured data
shown in Fig. 4(a). Vgs was changed within the range
possible in this experiment. When Vgs higher than this range
was applied, current leakage occurred and the sample wasdestroyed. As shown in Fig. 4(a), two different features withdifferent slopes were observed in regions (i) t < 1 ms and (ii)t > 1 ms.
First, we consider the dynamics observed for (i) t < 1 ms.
For ML-WSe2, it has been reported that optical carrier© 2022 The Author(s). Published on behalf of

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Fig. 5. (Color online) Vgs dependences of the (a) drain current Id,(b) source current Is, and (c) the difference between them Is–Id on the gate
voltage and time. (d) Time-resolved spectra of Id at Vgs = +38 V, +60 V,
and +100 V, where the red lines are the experimental values and the solid
blue lines are the ﬁtting curves. (e) Fitting curves as a function of Vgs, with
the intensity shown by the color scale. All time-resolved data were used.
(f) Vgs dependence of the lifetimes of the two components.

recombination occurs on a time scale of ps to ns. Therefore,
the 1 kHz band of the preampliﬁer used is considerablyslower than the carrier recombination dynamics and all
excited carriers were already recombined 1 ms after theillumination was turned off. Therefore, to examine in detailthe photoexcited carriers Id,on–Id,1ms, the drain current Id,1ms
measured 1 ms after the light was turned off was subtracted
from the drain current Id,on measured during light irradiation
for all the data similar to those in Fig. 4(b). The results areshown in Fig. 4(c). Id,on–Id, 1ms increased as Vgs became
increasingly positive. From the Ids–Vgs characteristics in thedark state, as shown in Figs. 2(d) and 2(e), this sample isslightly of the n-type for Vgs = 0 V, and the Fermi level EF is

close to the conduction band. In the Vgs range of thisexperiment, the amount of electron doping increases as Vgs
becomes increasingly positive. It has been reported thatelectrical doping reduces the binding energy of excitons in a
ReSe2 sample formed on graphene.30) Figure 4(d) shows
schematics of the band structures for positive and negativegate voltages under the dark condition. For simplicity, thetrap levels are not shown here. Band bending due tothe Schottky junction appears between the electrodes andthe sample, and the sign of the gate voltage changes the
direction of the electric ﬁeld.31) Therefore, when the numberof free electrons increases with Vgs in the positive region, thescreening is considered to weaken the Coulomb binding force
of excitons. This effect should also be reﬂected in the change
in the intensity of the photocurrent response to Vgs in region(i) of Fig. 4(a).
Next, we consider the dynamics in (ii) t > 1 ms. Photoexcited
free carriers have already disappeared at t = 1 ms, and theemission of carriers captured by the trap levels immediately after
the light has been turned off determines the subsequent
dynamics of the current response. Such responses have been
analyzed by macroscopic measurement methods such as deeplevel transient spectroscopy32) and photoinduced current transient spectroscopy33) and other related techniques. Defect levels
have been evaluated using these methods, and the large variationin response time for photoelectric devices using MoS2 is a major
problem to be solved for their application.17,34) If the multiprobe
method can be used to investigate the transient response in time
domain (ii), it is expected to become an important technology
indispensable for device miniaturization and further performance
improvement.
When photoexcitation is stopped, the excess electrons
generated by photoexcitation are captured by the trap levelsin the bandgap. It has been reported that carriers are emitted
from the trap levels after turning off the light, and a longlived response (ms-s) is observed.17,34,35) Therefore, by
measuring the Vgs dependence of the Id and Is dynamics,
the characteristics of the trap levels in this system can beanalyzed. Figures 5(a)–5(c) show the dependences of Is, Id,
and the difference between them Is–Id on Vgs and time. The
color scale represents the intensity of each current. As can beseen, there is a region where Is–Id has a measurable value,indicating that there are holes emitted from the trap levels inthis region and that there is a transient response of the holes
in addition to that of the electrons. The value decreases as Vgs
decreases and falls below the noise level at Vgs < −90 V.
Figure 5(d) shows the spectra of Id obtained at Vgs =+ 38 V, +74 V, and +110 V, where the solid blue lines are
the ﬁtting curves for the data shown in red. The ﬁtting was
performed using a two-component exponential function, andthe agreement with the experimental results is very good.
Figure 5(e) shows the ﬁtting curves as a function of Vgs, with
the intensity shown by the color scale. A graph similar to thatshown in Fig. 5(b) was obtained. Figure 5(f) shows the Vgs
dependence of the lifetimes of the two components. The
variation in the small-Vgs region is attributed to the weaksignal and the low signal-to-noise ratio, as shown in
Fig. 5(d). The shorter lifetime (∼20 ms) is that of the holes,
and the longer one (∼200 m) is that of the electrons. The
slow component appears to become slightly longer with Vgs.

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Fig. 6. (Color online) Model diagrams showing the time dependence of the band structures for different Vgs values. Ec: conduction band edge, EF: Fermi
energy, Ev: valence band edge, blue arrows: current ﬂow, green arrows: electric ﬁeld produced by Vds, blue square: trap states for electrons, red square: trap
states for holes, ﬁlled electrons/holes are shown by blue and red-colored regions. The schematics on the right show the band structures for different Vds values.

Next, the results obtained for region (ii) shown in Fig. 4
are discussed. Figure 6 shows model diagrams of the timedependence of the band structures for different Vgs values. In
accordance with the discussion thus, the time axis t is divided
into the dark state, the excited state, and the time when
excitation has stopped, i.e. 0 < t < 1 ms, 1 ms < t < 30 ms,
and 30 ms < t, respectively. The gate voltage is divided into
Vgs < −90 V, where only the dynamics of photoexcitedcurrent is observed, and −90 V < Vgs, where the dynamics
of holes and electrons captured at the trap levels areobserved. The table on the left shows model diagrams ofthe carrier dynamics at trap levels under various conditions.
The ﬁgures on the right show the change in band structurecaused by Vgs.
At Vgs < −90 V, the Fermi level EF is close to the valenceband, and the trap levels at energies higher than Ef are empty,
as shown in Fig. 6. Also, almost no steady current ﬂows. Theband is convex upwards. Therefore, the photoexcited electrons and holes reach the electrode and become the current
observed in the region of t < 1 ms as shown in Fig. 4(a).
Furthermore, no trapping occurs; thus, the carrier dynamics
observed in region (ii) do not occur.
On the other hand, at −90 V < Vgs, the acceptor levels are
ﬁlled with electrons under the dark condition. When thesample is photoexcited, the trap levels become empty, andwhen photoexcitation is stopped, the electrons in the valenceband start to be trapped, thereby holes start to be emitted to
the valence band. Then, the in-plane electric ﬁeld drives theholes to the source electrode, causing the current dynamics of
Is–Id, as shown in Fig. 5(c). The emission of electrons
captured at the trap levels also occurs after the photoexcitation is stopped. A change in the number of electrons trappedin the trap levels affects the potential in the light spot area andmobility of electrons,34,36) causing a change in steadycurrent.37,38) To provide a deeper and quantitative insight,
it is necessary to evaluate the temperature dependence andother dependences. This is beyond the scope of this study andis left as a future task.
It has been shown that the laser-combined multiprobe
system enables the analysis of the ms-scale photoinduced
carrier dynamics related to trap levels. In low-dimensional
semiconductor devices, the effect of trap levels existing at thesurface and interface is large and there is a marked variation
among samples, limiting the application of TMDCs.17,34)
Laser-combined MP-STM has enabled us to locally andrepeatedly measure the photoconductive response without
difﬁcult pretreatments such as lithography, making it aneffective analysis method for solving the above problems.
Furthermore, by combining the method with the observationand imaging techniques of STM itself, it becomes possible toevaluate the dynamics of electronic states as compared with
the nanoscale structures.
5. Evaluation of ps-scale carrier dynamics by timeresolved MP microscopy

Thus, we have evaluated the photoinduced conduction
dynamics in the range of ms to s. Since carrier dynamics inthe ultrahigh-speed region of low-dimensional materials are
greatly affected by phenomena such as atomic-scale defects,
the evaluation of the ultrafast carrier dynamics on the nm
scale is also strongly required. Using the MP microscopy
system, the probe position can be determined with nm
accuracy, and it is possible to evaluate carrier dynamics inthe ultrahigh-speed region even for a sample placed on an

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we obtained are in good agreement with the previous
results.39,40) The origin of the dynamics in the ps region is
attributed to a mechanism involving very shallow traplevels40) and Auger processes.41) Using the laser-combined
MP microscopy system we have developed, we demonstrated
that time-resolved measurements at the ps level are possible.
6. Conclusion

By combining multiprobe microscopy techniques with optical measurement technology, we have realized a method ofinvestigating the transient response of the electronic state of asample to a light stimulus over a wide time range. Using
STM, it is also possible to analyze the electronic state incomparison with the nanoscale structure. This method is
expected to play an important role in developing new
functional materials and devices in the future.
Acknowledgments

We acknowledge the ﬁnancial support of a Grant-in-Aid for
Scientiﬁc Research from Japan Society for the Promotion of
Science and Japan (17H06088, 20H00341, 20H02605,
21H05232, 21H05234), and Japan Science and Technology
Agency (JST) CREST program (JPMJCR1875, JPMJCR16F3).

Fig. 7. (Color online) (a) Optical microscopy image of the measurement
setup. (b) Schematic diagrams of the band structure under measurement
condition. (c) Time-resolved signal obtained for CVD-grown ML WSe2 on
SiO2/Si. The time-resolved signal in the range of 0–400 ps was ﬁtted by a
two-component exponential function.

insulating material. Thus, measurement results in the msregion have been shown, but here we show results obtained
for the measurement of carrier dynamics in the ps region.
For CVD-grown ML WSe2 on SiO2/Si, the time-resolved
measurement of the ps region was performed using thelaser-combined MP-STM. Figure 7(a) shows an optical
microscopy image of the measurement setup. Two cAFM
cantilevers were brought into contact with the sample. The
distance between the probes was set to 34 μm, and one ofthem was irradiated with a femtosecond laser pulse formeasurement.
A delay time modulation method with pulse picking wasused for the optical pump-probe system for time-resolved
measurement.8,12) The light source used was a Ti:sapphirelaser with a wavelength of 800 nm, a pulse width of 150 fs,
and an average intensity of ∼30 mW. The voltages applied to
the AFM probes were Vds = 20 V and Vgs = 0 V. Figure 7(b)shows the band structure under the measurement condition.
Figure 7(b) shows the obtained time-resolved spectrum.
The time-resolved signal in the range of 0–400 ps was ﬁtted
by a two-component exponential function. The early component was 17 ± 0.4 ps and the slow component was 114 ± 4 ps.
Experiments using the conventional optical pump-probe
method have already been reported, and the two lifetimes

1) G. Binnig, H. Rohrer, C. Gerber, and E. Weibel, Phys. Rev. Lett. 49, 57(1982).
2) H. Mogi, T. Bamba, M. Murakami, Y. Kawashima, M. Yoshimura,
A. Taninaka, S. Yoshida, O. Takeuchi, H. Oigawa, and H. Shigekawa, ACS
Appl. Electron. Mater. 1, 1762 (2019).
3) S. Hasegawa, J. Phys.:Condens. Matter 31, ab0bf4 (2019).
4) B. Voigtländer, V. Cherepanov, S. Korte, A. Leis, D. Cuma, S. Just, andF. Lüpke, Rev. Sci. Instrum. 89, 101101 (2018).
5) T. Nakayama, O. Kubo, Y. Shingaya, S. Higuchi, T. Hasegawa, C. S. Jiang,
T. Okuda, Y. Kuwahara, K. Takami, and M. Aono, Adv. Mater. 24, 1675
(2012).
6) M. Hirose, E. Tsunemi, K. Kobayashi, and H. Yamada, Appl. Phys. Lett.
103, 4 (2013).
7) M. Kolmer et al., Nat. Commun. 10, 1573 (2019).
8) H. Mogi et al., Appl. Phys. Express 12, 045002 (2019).
9) Y. Masaki, K. Tomita, Y. Kojima, and F. Kannari, Appl. Opt. 58, 5595
(2019).
10) R. Fujimoto, A. Kaneta, K. Okamoto, M. Funato, and Y. Kawakami, Appl.
Surf. Sci. 258, 7372 (2012).
11) H. Mogi, Z. Wang, R. Kikuchi, C. Hyun Yoon, S. Yoshida, O. Takeuchi,
and H. Shigekawa, Appl. Phys. Express 12, 025005 (2019).
12) Y. Terada, S. Yoshida, O. Takeuchi, and H. Shigekawa, Nat. Photonics 4,869 (2010).
13) S. Yoshida, Y. Aizawa, Z.-H. Wang, R. Oshima, Y. Mera, E. Matsuyama,
H. Oigawa, O. Takeuchi, and H. Shigekawa, Nat. Nanotechnol. 9, 588(2014).
14) S. Yoshida, Y. Arashida, H. Hirori, T. Tachizaki, A. Taninaka, H. Ueno,
O. Takeuchi, and H. Shigekawa, ACS Photonics 8, 315 (2021).
15) K. Yoshioka, I. Katayama, Y. Minami, M. Kitajima, S. Yoshida,
H. Shigekawa, and J. Takeda, Nat. Photonics 10, 762 (2016).
16) A. Castellanos-Gomez, Nat. Photonics 10, 202 (2016).
17) G. Konstantatos, Nat. Commun. 9, 5266 (2018).
18) D. Jariwala, V. K. Sangwan, L. J. Lauhon, T. J. Marks, and M. C. Hersam,
ACS Nano 8, 1102 (2014).
19) K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, Nat.
Mater. 12, 207 (2013).
20) K. W. Lau, C. Cocchi, and C. Draxl, Phys. Rev. Mater. 3, 074001 (2019).
21) H. Wang et al., Nat. Commun. 8, 394 (2017).
22) W. Zhang, Z. Huang, W. Zhang, and Y. Li, Nano Res. 7, 1731 (2014).
23) P. C. Shen et al., Nature 593, 211 (2021).
24) K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang,
S. V. Dubonos, I. V. Grigorieva, and A. A. Firsov, Science 306, 666(2004).

SL1011-8© 2022 The Author(s). Published on behalf of
The Japan Society of Applied Physics by IOP Publishing Ltd

Jpn. J. Appl. Phys. 61, SL1011 (2022)

H. Mogi et al.

25) B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, Nat.
Nanotechnol. 6, 147 (2011).
26) S. Li, S. Wang, D. M. Tang, W. Zhao, H. Xu, L. Chu, Y. Bando, D. Golberg,
and G. Eda, Appl. Mater. Today 1, 60 (2015).
27) A. Prakash, H. Ilatikhameneh, P. Wu, and J. Appenzeller, Sci. Rep. 7, 12596(2017).
28) W. Liu, J. Kang, D. Sarkar, Y. Khatami, D. Jena, and K. Banerjee, Nano
Lett. 13, 1983 (2013).
29) H. J. Liu, L. Jiao, L. Xie, F. Yang, J. L. Chen, W. K. Ho, C. L. Gao, J. F. Jia,
X. D. Cui, and M. H. Xie, 2D Mater. 2, 034004 (2015).
30) Z. Qiu et al., Sci. Adv. 5, eaaw2347 (2019).
31) Y. Yi, C. Wu, H. Liu, J. Zeng, H. He, and J. Wang, Nanoscale 7, 15711(2015).

32) D. V. Lang, J. Appl. Phys. 45, 3023 (1974).
33) C. Hurtes, M. Boulou, A. Mitonneau, and D. Bois, Appl. Phys. Lett. 32, 821(1978).
34) M. M. Furchi, D. K. Polyushkin, A. Pospischil, and T. Mueller, Nano Lett.
14, 6165 (2014).
35) D. Kufer and G. Konstantatos, Nano Lett. 15, 7307 (2015).
36) D. Knipp, R. A. Street, A. Völkel, and J. Ho, J. Appl. Phys. 93, 347(2003).
37) T. J. Echtermeyer et al., Nano Lett. 14, 3733 (2014).
38) H. Huang et al., Nanotechnology 27, 445201 (2016).
39) T. Godde et al., Phys. Rev. B 94, 165301 (2016).
40) L. Li and E. A. Carter, J. Am. Chem. Soc. 141, 10451 (2019).
41) Y. Li et al., Nanoscale 10, 17585 (2018).

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The Japan Society of Applied Physics by IOP Publishing Ltd