Applied Physics Express 6 (2013) 016601
http://dx.doi.org/10.7567/APEX.6.016601

Direct Probing of Transient Photocurrent Dynamics in p-WSe2
by Time-Resolved Scanning Tunneling Microscopy
Shoji Yoshida, Yasuhiko Terada, Munenori Yokota, Osamu Takeuchi, Yutaka Meray, and Hidemi ShigekawaGraduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan
E-mail: hidemi@ims.tsukuba.ac.jp
Received November 22, 2012; accepted December 13, 2012; published online December 28, 2012
We have carried out time-resolved scanning tunneling microscopy on a layered semiconductor with an indirect bandgap, p-WSe2 , and thedynamics of nonequilibrium photocurrent generated by ultrashort-pulse-laser excitation was analyzed. The photocurrent dynamics reflecting theflow of excited photocarriers at the surface, which is determined by the balance between the diffusion and tunneling rates, was successfullyprobed. Furthermore, the excess minority carriers transiently trapped at the surface for a few nanoseconds, which produce a transient surface
photovoltage and cannot be detected by conventional methods, were directly observed and evaluated.
# 2013 The Japan Society of Applied Physics

he development of semiconductor physics anddevices in the past few decades has been progressing
along with the innovations, in atomically controlled
fabrication technologies. With the reduction of dimensions,the characterization of performances on the nanoscale has
become ever more crucial for the further development offunctional materials and devices. Owing to their characteristics, scanning tunneling microscopy/spectroscopy (STM/STS) and related techniques are promising; therefore,
attractive methods based on their characteristics have beendeveloped to realize the analysis of local information, suchas dopant proﬁle, carrier distribution, local conductivity,
potential variations, surface photovoltage (SPV), and local
spin eﬀects.1–6) To obtain a complete understanding of suchfunctions, however, the analysis of carrier dynamics under anonequilibrium state is important, and signiﬁcant eﬀort has
been devoted to realize time-resolved (TR) microscopy/
spectroscopy.7,8)
Recently, a new microscopy technique, shaken-pulse-pairexcited STM (SPPX-STM), which enables the probing of
ultrafast carrier dynamics in nanometer-scale structures hasbeen developed.9) The high spatial resolution of STM has beencombined with the temporal resolution of an ultrashort-pulse
laser in the femtosecond range on the basis of the optical
pump–probe (OPP) method. A nonequilibrium carrier distribution is generated by ultrashort laser pulses and its
relaxation processes are probed using the STM tunnelingcurrent. By applying the method to GaAs-based structures,
fundamental carrier dynamics in semiconductors, such as
photocarrier recombination and the eﬀect of the innerpotential on carrier dynamics, were successfully probed andvisualized.9–11) To increase the generality of this method ofmicroscopy and its applicability to various types of materials
and structures, additional techniques are desired. For example, as an application of TR-STM based on a similar method tothe analysis of a molecular electronic structure, two-photon
absorption measurement was used together with TR-STM.12)
For further advancement, the development of direct techniques for detecting photocurrent in transient dynamics on thenanoscale is expected to play an important role in determining
the optical characteristics of materials and devices.
In this study, we have carried out SPPX-STM measurements on the transient photocurrent dynamics in a layeredTyOn leave from Department of Applied Physics, The University of Tokyo.

Fig. 1. Typical STM image of WSe2 (set point: Vs ¼ þ2:0 V,
It ¼ 100 pA, 5:75 nm  5:75 nm).

semiconductor, WSe2 . Layered compounds have attracted
fundamental and practical interest in various ﬁelds. Inaddition to the current interest in device applications, forexample, ﬁeld-eﬀect transistors,13–15) layered materials
can be used as a model system for the analysis of carrier
dynamics because of their advantageous surfaces withminimal roughness, dangling bonds and defect states.
Among the layered materials, the tunneling characteristicsof photocurrent in the stationary state are well understoodfor WSe2 ,16,17) which allows detailed analysis of the results
obtained for transient properties.
Figure 1 shows a typical STM image of WSe2 . As
expected, the surface is stable and no atomic scale defects18)
were observed in the measurement area, even after laser
excitation. First, we obtained current to bias voltage (I–V )curves by measurement using the light-modulated STS
method2,3) to examine the characteristic properties of photocurrent in the stationary state. Ti:sapphire laser pulses with apulse width of 140 fs (central wavelength, 800 nm; averageintensity, 1 W, and 0.35 mW intensity at the sample) and arepetition rate of 90 MHz were chopped at 100 Hz, and two
I–V curves with and without laser excitation were obtained
simultaneously. Under these conditions, the results werethe same as those obtained with a continuous laser. A fresh
WSe2 surface was prepared by exfoliating a lightly p-doped
sample, and measurements were carried out in air andvacuum (<5:0  108 Pa), both of which produced the same
results because of the high stability of the sample surface.
Figure 2 shows the results, where blue and red lines
indicate the I–V curves obtained without and with laser

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S. Yoshida et al.
(a)

(c)

(e)

(b)

(d)

(f)

Fig. 2. I–V curves obtained by light-modulated STS without (blue) andwith (red) photoillumination (set point: Vs ¼ 1:5 V, It ¼ 100 pA).

excitation, respectively. A large diﬀerence was observed
between the two cases. Figure 3 shows schematics of themetal-insulator-semiconductor (MIS) structures used toexplain the observed results. For a positive bias voltage(forward bias voltage), the characteristic of conductivity
with illumination (not shown) is the same as that under thedark condition [Fig. 3(a)], because photoelectrons drift tothe bulk side. In contrast, for a negative sample bias voltage(reverse bias voltage), since electrons photoexcited in the
conduction band induce tunneling, diﬀerent characteristics
are observed under the dark [Fig. 3(b)] and photoexcited[Figs. 3(c) to 3(f )] conditions. Here, Figs. 3(c) and 3(d) andFigs. 3(e) and 3(f ) represent the time evolutions of theexcited states induced by a pump pulse with a low and highintensity, respectively. The band structure appearing underillumination with a continuous laser can also be represented
by Fig. 3(e) or 3(f ).
Let us consider the case of a negative sample bias
voltage in more detail. Under the dark condition [Fig. 3(b)],minority carriers, i.e., electrons, at the surface conduction
band of WSe2 provided from the bulk side are extracted
by tunneling into the STM tip instead of accumulating.
Therefore, an inversion layer is not formed, resulting in
downward bending of the surface Fermi level of WSe2 .
This nonequilibrium change induces a large amount of
tip-induced band bending (TIBB) and prohibits electron
tunneling from the valence band of WSe2 , producing theobserved strong rectiﬁcation (blue line in Fig. 2).
Under an illuminated condition using a continuous laser,
the negative sample bias-voltage region is divided into twoparts, as can be observed in the I–V curve in Fig. 2: (1)a photovoltatic region (1:5 5 Vs 5 0:8 V in Fig. 1,
Vs : sample bias voltage) and (2) a photoamperic region(Vs 5 1:5 V in Fig. 1). Until the conduction band edge
reaches the Fermi level (photovoltatic region), the bias
voltage is mainly applied to the tunnel gap, therefore, underphotoillumination, the SPV, and thereby the tunnelingcurrent, strongly depends on the bias voltage. When thenegative bias voltage is further increased, the tunneling gap
conductance becomes higher than that in the space charge
region of WSe2 owing to the reduction of the tunnelingbarrier height for electrons (photoamperic region). Under
such a condition, the tunneling current mainly consists ofphotogenerated carriers from the bulk side which accumulatebeneath the STM tip. Therefore, the photocurrent depends on

Fig. 3. Schematic illustrations of band structures: (a) and (b) are forpositive and negative sample bias voltages under the dark condition,
respectively, while (c) and (d), and (e) and (f ) represent the time evolutionsof the excited states induced under a negative sample bias voltage by apump pulse with a low and high intensity, respectively. JD : diﬀusion
current, JV : current from valence band, JC : photocurrent of minority
carriers. Since bandgap of WSe2 is 1:2 eV, JV is negligible comparedwith JC .

the photointensity and volume of TIBB, where photogenerated carriers drift toward the STM tip. Since the region ofTIBB gradually increases as the bias voltage increases, thetunneling current slowly increases with the bias voltage.
These results are in good agreement with those reported in aprevious paper.16,17) No current is observed for a negative
bias voltage under the dark condition, indicating that thephotocurrent is directly detected as a signal via tunneling,making it suitable for SPPX-STM measurement and analysis.
To analyze the dynamics of the photocurrent measured
in the stationary state shown in Fig. 2, we carried out SPPXSTM measurement to obtain information on the photocarrier
dynamics in the temporal space. In SPPX-STM, the sample
surface below the STM tip is excited by a sequence of paired
pulses, similarly to the conventional OPP method, and thechange in the tunneling current is measured as a functionof the delay time between the pump and probe pulses. A
paired laser-pulse train was generated by two synchronized
Ti:sapphire lasers used above. The repetition rate wasreduced to 1 MHz with a pulse picker system using pockels
cells. The delay time was modulated at 1 kHz for thelock-in detection of the weak signal. The detected signal isI ¼ Iðtd Þ  Ið1Þ, where Iðtd Þ and Ið1Þ are the tunnelingcurrents for a delay time of td and a delay time suﬃciently
long for the excited state to be relaxed, respectively. Therefore, the observed change in I directly provides information about the dependence of the photocarrier density on
the delay time after photoexcitation.11)
Figure 4 shows a series of SPPX-STM spectra obtainedfor diﬀerent laser intensities. To enhance the eﬀect of SPV
to increase the magnitude of the signal, measurement wascarried out under a photovoltatic condition (Vs ¼ 1:5 eV)in consideration of the result in Fig. 2. The original laser
intensity was 0.9 mW, which was reduced to IL using ﬁlters.

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S. Yoshida et al.
(a)(b)

Fig. 5. (a) Time-resolved spectra obtained for the fast component undervarious laser intensities (set point: Vs ¼ 2:0 V, laser intensity: 0.2 mW).
(b) Decay constant  obtained from (a) as a function of the tunneling current
It adjusted by changing the tip–sample distance. The solid line indicates theﬁtting curve of the data with the function  ¼ 18=It .

Time-resolved spectra obtained under various laser intensities IL(set point: Vs ¼ 1:5 V, It ¼ 1 nA). The maximum laser intensity was0.9 mW and the relative intensities were (a) 1, (b) 0.5, (c) 0.25, (d) 0.13,(e) 0.1, (f ) 0.05, and (g) 0.003.
Fig. 4.

A fast component (decay constant 5 ns) and a slow component (decay constant 200 ns) were observed. For low
laser intensities, only the fast component appears and theintensity of the slow component increases with increasing
laser intensity.
Since WSe2 has an indirect bulk bandgap, the recombination lifetime of photoexcited carriers is signiﬁcantly long(10 s) compared with the observed values. Therefore, apossible explanation for the obtained results is that theobserved dynamics in the signal reﬂects the change in photocarrier (minority carrier) density with the balance between
the two processes at the surface, namely, the diﬀusion rateof photoexcited electrons from the bulk side, Rd , and theirtunneling to the STM tip Rt . In fact, the decay constant of
200 ns was observed by the OPP method, which supportsthat the process is considered to be diﬀusion of photocarriers
from the illuminated area.
When the tip–sample distance is ﬁxed (Rt ¼ constant),
the photocarrier density varies with the laser intensity, andthe associated change in the diﬀusion rate Rd governs thebalance. For a low laser intensity (Rt  Rd ), the diﬀusion
rate becomes the limiting factor in the balance, where thetunneling of the excess electrons trapped at the surface
during photoexcitation is detected dominantly as the signal[fast component, (e)–(g) in Fig. 4]. In contrast, for a highlaser intensity (Rt  Rd ), the tunneling rate becomes thelimiting factor and the diﬀusion process produces tunnelingwith a large decay constant [slow component, (a) in Fig. 4].
In the intermediate range (Rt  Rd ), the excess electrons
trapped at the surface ﬁrst undergo tunneling. Then, electrons provided from the bulk side undergo tunneling at a rateof Rd . As a result, the slow component appears in the timeresolved spectra following the fast component [(b)–(d) inFig. 4]. Since the signal detection is based on the mechanismof absorption bleaching similarly to the OPP method,9) thesignal intensity increases with increasing the laser power.
To analyze the process for the fast component in moredetail, we measured the tunneling current dependence of thefast component. To observe the fast component easily, inconsideration of the results in Figs. 2 and 4, measurement
was carried out under the conditions of Vs ¼ 2:0 V andIL ¼ 0:2 mW. Figure 5(a) shows the series of spectra
obtained, and the decay constant  as a function of thetunneling current is summarized in Fig. 5(b). The tunnelingcurrent was adjusted by changing the tip–sample distancebefore each measurement under illumination with the pulselaser. The decay constant  decreases with increasing
tunneling current as expected.
In the stationary-state measurement of photocurrent, as
described in a previous paper, the excess electrons, whichcause, for example, the inversion layer, cannot be observed
because they disappear as a result of tunneling.16,17) In
SPPX-STM, the number of excess electrons transiently
trapped at the surface can be observed via the dynamics ofthe fast component. As a rough estimation, the maximum
value of I gives the number of excess electrons induced
by a pair of laser pulses at 1 MHz. Therefore, the numberof excess electrons induced by a laser pulse is estimated tobe 5–40 from the results shown in Fig. 5(a). The increasein the number of excess electrons with decreasing tip–
sample distance may be caused by the increase in the area ofthe depletion layer produced by TIBB. These values arecomparable to the number of charges N estimated to exist
between a sphere of radius R and an inﬁnite plane placed at adistance of 1 nm from the sample; N ¼ 15:5 for R ¼ 10 nm
and 40.4 for R ¼ 20 nm.
In conclusion, we have carried out time-resolved STM ona p-type layered semiconductor with an indirect transition
band structure, WSe2 . The photocurrent dynamics reﬂecting
the ﬂow at the surface, which was locally balanced between
the diﬀusion and tunneling rates, was successfully probed.
The dynamics of the excess electrons trapped at the surface,which produce a transient surface photovoltage on thenanosecond scale and cannot be detected by conventional
methods, was observed and evaluated. Although we carriedout measurements on the sample surfaces without defects todevelop a solid basis for the new technique, the eﬀect ofnanostructures on the photocurrent plays an essential rolein determining the optical characteristics of materials anddevices. SPPX-STM, for example, with the manipulation
techniques of STM19) may create exciting new possibilities
for further studies.

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# 2013 The Japan Society of Applied Physics