IOP PUBLISHING

JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 22 (2010) 264008 (8pp)

doi:10.1088/0953-8984/22/26/264008

Laser-combined scanning tunnelling
microscopy for probing ultrafast transient
dynamics
Yasuhiko Terada, Shoji Yoshida, Osamu Takeuchi and
Hidemi Shigekawa1
Institute of Applied Physics, CREST-JST, University of Tsukuba, Tsukuba 305-8573, Japan

Received 29 December 2009, in final form 10 March 2010
Published 14 June 2010
Online at stacks.iop.org/JPhysCM/22/264008
Abstract
The development of time-resolved scanning tunnelling microscopy (STM), in particular,
attempts to combine STM with ultrafast laser technology, is reviewed with emphasis on
observed physical quantities and spatiotemporal resolution. Ultrashort optical pulse technology
has allowed us to observe transient phenomena in the femtosecond range, which, however, has
the drawback of a relatively low spatial resolution due to the electromagnetic wavelength used.
In contrast, STM and its related techniques, although the time resolution is limited by the circuit
bandwidth (∼100 kHz), enable us to observe structures at the atomic level in real space. Our
purpose has been to combine these two techniques to achieve a new technology that satisfies therequirements for exploring the ultrafast transient dynamics of the local quantum functions inorganized small structures, which will advance the pursuit of future nanoscale scientificresearch in terms of the ultimate temporal and spatial resolutions.
(Some figures in this article are in colour only in the electronic version)

at a frequency exceeding its electrical bandwidth. In the fieldof ultrafast laser technology, there are numerous time-resolved
optical spectroscopy methods that circumvent the problem of
the bandwidth limit. The optical pump–probe technique is a
representative example (figure 1), which has been reviewed bymany groups [2, 3]. In the pump–probe technique, a sequenceof paired laser pulses illuminates a sample. First (pump)pulses are used to excite the sample. After a certain delaytime, the same area of the sample is illuminated by second(probe) pulses, and an optical response to probe pulses, suchas reflectivity, transmissivity, and scattering light, is measured
as a function of delay time. These optical properties dependon the excited state of the sample, and therefore, the time
evolution of these quantities gives a measure of the relaxation
process of the excited state. One only has to measure a
DC component, and the detector needs neither to be operated
fast nor to have a wide bandwidth. The time resolution of
the pump–probe technique is determined only by the opticalpulse width and thus can be improved up to the femtosecond
range. Other time-resolved techniques include the streak
camera, optical Kerr gate, and up-conversion. An exampleof their recent applications is the analysis of time-resolved
photoluminescence from a sample optically excited by an

1. Introduction
Making smaller and faster functional objects is a key factor forthe development of nanoscale science and technology. The sizeof semiconductor devices, for example, decreases down to thenanoscale range. The device operation is currently dominatedby atomic-scale properties, such as dopant distribution andsurface/interface fluctuation, and thus is controlled by atomicscale fabrication technology [1]. Accordingly, the operation
speed exceeds GHz, and the understanding of carrier dynamics
on an ultrashort timescale from the picosecond to nanosecond
range is required. Therefore, developing devices with novel
functions cannot be achieved without accurate evaluationof carrier dynamics under operation, in conjunction with
exploring device physics. This has led to an increasing demand
for a microscopic technique that allows the investigation ofcarrier dynamics at high spatial and temporal resolutions,in accordance with the surrounding potential, for example,
modulated by atomic defects, dopant atoms, and local
structures.
In conventional methods of measurement, one cannot use
a detector to observe an ultrafast dynamic process occurring
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Y Terada et al

stroboscopically. The other type, developed initially by
Bostanjoglo [17] and subsequently by others [18, 19], issuitable for irreversible processes and records images in asingle-shot mode. UEM provides opportunities for studying
dynamical changes in structural and morphological features(such as dislocations, impurity particles, grain boundaries,
and phase boundaries) with a high spatiotemporal resolution
both in real and reciprocal spaces, in many fields frommaterial science to biology. The resolution of UEM depends
on microscopic setups and has been discussed in detail by
Gahlmann et al [20]. In the single-electron pulsed mode,each electron pulse, which has only one electron on average,
eliminates the space charge effect and enables us to obtaina spatial resolution equivalent to that of conventional TEM.
The temporal resolution is determined by the spread in theinitial kinetic energy of the photoelectrons and the strengthof the acceleration field. In the single-shot mode, in whichhigh electron beam currents must be used, both the spatial
and temporal resolutions are degraded by the electron–electron
scattering effects due to Coulomb repulsion (space charge
effect), which occurs when the electrons come closer to eachother both before and after scattering by the specimen.
As described above, there have been numerous excellent
time-resolved microscopic techniques achieving a highspatiotemporal resolution, with a wide field of application.
However, it is still a challenge to attain an atomic-scale
resolution in real-space imaging of ultrafast phenomena. Timeresolved scanning tunnelling microscopy (STM) is a promising
candidate, because the tunnelling junction itself responds ona timescale of a few femtoseconds and principally ultrafast
dynamics can be studied by STM. However, the measurement
bandwidth of an STM preamplifier is typically 100 kHz and
10 MHz at best [21–25], and faster dynamics is unavailable bystandard STM.
We have aimed at developing a new microscopic techniquewith a high spatiotemporal resolution, by combining STM
with ultrafast laser technology.
In principle, a newly
developed microscopic technique, shaken-pulse-pair-excited
STM (SPPX-STM) [26–32] inherits the spatial resolution of
STM and the femtosecond temporal resolution of the ultrashortpulse laser. By exciting a tunnelling junction between an
STM tip and a sample by a sequence of paired pulses,
and by detecting tunnelling current that depends on delay
time between the pulses in a pair, one can analyse the time
evolution of carrier density in a localized (nanoscale) region at
subpicosecond ranges.
In this paper, we briefly review attempts to improve
resolutions by combining STM with ultrafast laser technology
and describe the features and critical issues. Furthermore, weshow the development of SPPX-STM and its principle andapplication.

Figure 1. Fundamentals of the optical pump–probe technique.
(a) Experimental setup of pump–probe reflectivity measurements;
HM denotes half-mirror. (b) Typical signal curve showing R/R
versus delay time ( R : reflectivity).

ultrashort pulse [4]. On the basis of these ultrafast opticaltechniques, various time-resolved microscopy methods havebeen developed.
Despite the ultrahigh temporal resolution achieved, thespatial resolution of real-space imaging using these opticaltechniques is determined by the diffraction limitation, andin general, it cannot be high. Several attempts have been
made to overcome this limitation and achieve a high spatial
resolution. The first example of such attempts is a combinationof ultrafast laser technology with scanning near-field optical
microscopy (SNOM). Examples of its recent applications
are the analyses of spin-dependent photoluminescence using
luminescence intensity correlation (with resolutions of 130 fs,
125 nm) [5], photoluminescence of a polymer blend using
time-correlated single-photon counting (60 ps, 70 nm) [6], andpump–probe transmittance revealing carrier dynamics (250 fs,
150 nm) [7].
The second example of such attempts is the use ofphotoelectron emission microscopy (PEEM) to overcome thediffraction limit due to the photoemitted electrons, resulting ina high spatial resolution. Indeed, by combining time-resolved
two photon photoemission with PEEM, Schmidt et al [8] first
demonstrated a spatial resolution of about 20 nm. The same
approach was used by several other research groups [9–14].
The third example is the use of ultrafast electron
microscopy (UEM) that utilizes an ultrashort electron pulse,
instead of an optical pulse, as a probe. The emerging areaof UEM has been reviewed by King et al [15]. An opticalpump pulse is used to excite a sample. Then, a delayed
optical pulse illuminates a photoactivated electron source togenerate a probe electron pulse, and transmission images arestroboscopically recorded at a sequence of delay times. The
following are the two types of ultrafast electron microscopes:
a single-electron pulsed mode, developed by Zewail [16],is used for imaging reversible processes and records images

2. Development of laser-combined STM for high time
resolution
In analogy with optical pump–probe techniques, the limitation
due to the circuit bandwidth is circumvented by the useof paired pulses.
Since the invention of STM [33],2

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Y Terada et al

Figure 2. (a) Scheme of the optical pump–probe technique. (b)–(e) Various combinations of STM with ultrashort pulse laser technology.

propagating along a transmission line, and gs (t) is a timedependent conductance at the detection gap. Due to theexistence of gs (t) associated with the detection gate switch, themeasured signal Ic originates from the coupling between the tip
wire and the stripline induced by the geometric capacitance.
The tip capacitance has a spatial extent of the order of
10 μm, and hence the spatial resolution of PG-STM also
remains on the same order, incapable of detecting atomicscale deviation in a time-dependent voltage process, such asa transient photovoltage process. As Groeneveld et al pointedout [38], one may still expect a high resolution for detecting
laser-generated picosecond acoustic waves that lead to a timedependent current, but an experimental demonstration has not
yet been reported.
Another approach based on PG-STM has been made by
the combination with the junction mixing technique [39–41].
As shown in figure 2(c), optical pump and probe pulses areused to illuminate a pair of photoconductive switches. Whenthe pulse allows conduction across one of the switches, avoltage pulse is launched onto the metallic transmission line
addressed by STM. The voltage pulses V1 and V2 regulate thevoltage between the STM tip and the sample, and modulate(transient) tunnelling current,

many researchers have attempted to realize a high temporal
resolution together with the atomic resolution of STM by
combining STM with the pump–probe technique, but theymostly have succeeded only in revealing the difficulties
in achieving the STM spatial resolution [34].
Some
problems and limitations in pioneering time-resolved STM,
photoconductively gated STM (PG-STM), and pulse-excited
STM, and also the further development of SPPX-STM arediscussed in this section.
2.1. Photoconductively gated STM (PG-STM)
A concept based on a fast photoconductive gate switch inserted
into a detector circuit of STM current has been realized anddeveloped by several groups [35–37]. The scheme is called
photoconductively gated STM (PG-STM). As in optical pump–probe techniques, a sequence of paired optical pulses is used.
A pump pulse is used to excite a sample, and then a probe
pulse is used to illuminate a photoconductive gate to switch
on the detector circuit (figure 2(b)). Thereby, the tunnellingcurrent is gated and the ultrafast process is stroboscopicallymeasured. This concept was mostly tested for detecting shortvoltage pulses propagating along a transmission line fabricatedon a substrate, and the highest temporal resolution achieved
thus far is around 1 ps [37]. In PG-STM, however, it was
revealed that the detected signal originated from the geometriccapacitance between the tip and the sample [38]. The signal
measured by PG-STM is given by the averaged current of
1
Ic =
T

 T

I (t) = I (Vb ) +dI(V1 (t + td ) + V2 (t))
dV

1 d2 I(V1 (t + td ) + V2 (t))2 + · · · ,2 dV 2where Vb denotes the applied DC voltage and td denotes thedelay time between optical pulses.
When the I –V curve has the nonlinearity I = β(V +γ V 3 ) + O(V 4 ), the tunnelling current induced by the two
applied bias voltages provided by a pair of optical pulses canbe written as I (t) = I (V1 (t + td ) + V2 (t)) = I (V1 (t + td )) +I (V2 (t)). Thus, the measured time-resolved tunnelling current+

gs (t  )V (t  ) dt  ,0which integrates the periodic current pulses (T : repetition
time of the ultrashort pulse laser) from the tip to form aquasi-dc current. Here, V (t) is the time-dependent voltagethat develops on the tip wire due to the short voltage pulses
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J. Phys.: Condens. Matter 22 (2010) 264008

is given by

Itr (td ) = I (t) =

3βγ
TL

 ∞

Y Terada et al

distance, the thermal effect should seriously interfere with thecurrent [34, 44–46]. To avoid this, they retracted the tip andmeasured the displacement current instead of the tunnellingcurrent. By adopting this excellent idea, they demonstrated
the application of pair-excited STM to probe carrier dynamics.
Since displacement current was probed, the spatial resolution
was about 1 μm, and the temporal resolution was determinedby the repetition period of laser pulses, which is of the order of
10 ns.

[V1 (t + td )V22 (t)0+ V12 (t + td )V2 (t)] dt,
where TL is the repetition period of pulse trains.
In the pioneering work reported by Nunes and Freeman [38], the technique was tested for detecting voltage
pulses propagating along a transmission line, and the temporal
resolution was several picoseconds. Since the capacitive
coupling linearly depends on voltage, this component does
not contribute to the time-resolved signal, in contrast with
PG-STM. Therefore, the spatial resolution is expected to becomparable to STM, and the best resolution achieved thus far
is 1 nm [41].
The time-resolved measurement with STM is seemingly
realized. The time-resolved signal Itr , however, reflects thechange in tunnelling current owing to the voltage pulse.
Therefore, what Itr reflects is not the carrier dynamics
beneath the STM tip but the carrier dynamics around thegate switches. When one scans the tip and records Itr , theobtained time-resolved image does not reflect the variation ofcarrier dynamics in the probed area but effects the variation ofelectronic states, βγ in d I /dV [41].

2.3. Shaken-pulse-pair-excited STM (SPPX-STM)
The scheme for applying the pump–probe technique directly
to a tip–sample junction is called paired-pulse-excited STM(PPX-STM). In PPX-STM, pump and probe pulses arecollimated coaxially and illuminate the junction, and the DC
current through the junction is measured as a function ofdelay time (figure 2(d)). The first attempt at PPX-STM wasreported by Pfeiffer et al [47]. They observed a laser-induced
time-resolved current on tantalum and GaAs(110) in ultrahighvacuum (UHV). The observed current was independent of
the tip–sample distance, indicating that the current does not
originate from tunnelling. From the nonlinear dependenceof the signal on optical intensity, they concluded that thesignal originates from multiphoton photoelectron emission.
They demonstrated a temporal resolution of the pulse duration(subpicoseconds).
A high spatial resolution requires the isolation of thetime-resolved tunnelling current from unwanted backgroundcurrents, such as multiphoton photoemission, and more
seriously, interference arising from thermal expansion of thetip. The modulation of optical intensity, as used in the
optical pump–probe technique, may be useful for detecting
an extremely weak time-resolved tunnelling current (∼100 fA
when the STM feedback current is set to be ∼100 pA). In
the tunnelling regime, however, it results in serious thermalinterference [34, 44–46] comparable to the STM feedback
current and spoils the measurement.
This major drawback has been overcome by modulating
the delay time instead of optical intensity and by using lockin detection. This idea has been realized recently and named
SPPX-STM [26–32]. In SPPX-STM, the optical intensity isnearly unchanged at the modulation frequency, and hence thethermal interference is markedly suppressed. This new scheme
realizes pump–probe-based STM and allows the detection oftime-resolved tunnelling current. In the next section, we
discuss the principle of SPPX-STM and show how carrier
dynamics is probed by tunnelling current.

2.2. Pulse-excited STM (PX-STM)
The idea of directly exciting a tip–sample junction by opticalpulses was realized first by Hamers et al [42, 43]. In
this pulse-excited STM (PX-STM), a sequence of single
pulses illuminates the junction, and the current through thejunction is recorded as a function of the pulse repetition
period (interval between pulses) (figure 2(d)). They measured
carrier relaxation on Si(111)-(7 × 7) with 10 ns and 1 μm
resolutions. As described later in detail, for a semiconductor
sample, tip-induced band bending (TIBB) occurs beneath the
STM tip under a certain voltage condition. When a pulse
excites the sample, TIBB relaxes as a result of photocarrier
redistribution, leading to a change in the surface potential
or surface photovoltage (SPV). SPV relaxes as photocarriers
decay, accompanied by a change in the displacement current
originating from capacitive coupling between the tip and thesample. As a result of the narrow circuit bandwidth, themeasured signal is the time-averaged displacement current,
which, however, depends on the repetition period. SPV has asaturation tendency when sufficient numbers of photocarriers
are generated. Therefore, as the pulse repetition period
decreases and becomes smaller than the relaxation time for
SPV, SPV becomes almost saturated all the time, resulting ina change in the time-averaged displacement current. Thus,
the dependence of displacement current on repetition period
reflects the relaxation processes of SPV and photocarrier
density.
In order to increase sensitivity, Hamers et al modulated
the optical intensity and detected the synchronized signal by
the lock-in technique. However, intensity regulation causes
a thermal expansion and shrinkage of the tip and sample.
Since the tunnelling current is sensitive to the tip–sample

3. Basis and results of SPPX-STM
3.1. Principle
Figure 3 shows how SPPX-STM works. The surface of thesample beneath the STM tip is illuminated with a sequenceof paired laser pulses with a certain delay time td , and thetunnelling current I is measured as a function of td . The opticalpulses give rise to current pulses in the raw tunnelling currentI ∗ , reflecting the excitation and relaxation of the sample.
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Y Terada et al

Figure 3. Relationship between delay time and transient tunnelling current I ∗ (left), and the corresponding time-averaged tunnelling current I
measured as a function of delay time (right).

When td is sufficiently long, the paired optical pulses withthe same intensity independently induce two current pulses
with the same height in I ∗ (A in figure 3). In contrast,
when td is short and the second pulse illuminates the sample
in the excited state caused by the first pulse, the second
current pulse may have a different height, depending on td (Band C in figure 3). Thereby, the signal I also depends on
td , because the height difference in the second current pulsechanges the time-averaged value of the tunnelling current.
Accordingly, the relaxation dynamics of the excited carriersin the target material, namely, the decay of carrier density after
the excitation by the first optical pulse, can be probed by STM.
With the modulation of td between td1 and td2 , the in-phasecomponent obtained by the lock-in detection of the tunnellingcurrent gives I (td1 , td2 ) ≡ I (td1 ) − I (td2 ). As td2 is set to avalue larger than the relaxation time of the probed dynamics,
I (td1 , td2 ) can be approximated as I (td1 , td2 ) ≡ I (td1 )−I (∞),
where I (∞) is the tunnelling current for a delay time that is
sufficiently long for the excited state to be relaxed. Therefore,I (td1 ) is obtained through the lock-in detection of I bysweeping td1 .
Similar to a pump–probe technique, SPPX-STM has the
temporal resolution of the pulse duration (potentially down toseveral femtoseconds) and a spatial resolution comparable tothat of STM. The mechanism by which dynamical processes
triggered by optical irradiation appear in the tunnelling current
varies with the material system. In the following, the results
and mechanism for semiconductors are discussed.

Figure 4. Results for the GaNx As1−x ( x = 0.36%) sample. (a) SPPX
signal and exponential fit to data (solid line). Excitation energy was
1.38 eV (wavelength of 900 nm), averaged intensity was 5 mW,
sample bias was 4 V, and feedback current was 200 pA.
(b) Delay-time dependence of reflectivity change measured by the
optical pump–probe technique, and fit to data (solid line). Excitationenergy was 1.55 eV (wavelength of 800 nm), averaged intensity was
1 mW for the pump and 0.1 mW for the probe.

3.2. Example of signal
An example of the I dependence on delay time measured
by SPPX-STM is shown in figure 4(a). The sample used wasa low-temperature-grown GaN x As1−x (x = 0.36%, grown at
720 K). The band gap energy was estimated to be 1.34 eV(923 nm) from the photoluminescence spectra. A mode-locked
Ti:sapphire oscillator provides 140 fs pulses at a repetition

rate of 90 MHz. The delay-time-dependent current is clearlyseen at a positive bias voltage, as shown in figure 4(a); I
approaches zero as delay time increases. The best fit to an
exponential function gives a decay constant of 440 ps. At anegative bias voltage, in contrast, I is observed to be almost
zero, regardless of delay time.
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Y Terada et al

the tunnel junction. Consequently, the illumination increases
I ∗ due to the reduction in the potential barrier for tunnellingcurrent.
The excited state subsequently relaxes to the original statethrough two processes. One is the decay of the photocarrierson the bulk side (bulk-side decay) via recombination, drift,and diffusion (figure 5(c)). The other is the decay ofthe carriers trapped at the surface (surface-side decay) via
thermionic emission and recombination. Because of the lackof counterpart carriers near the surface, the surface-side decay
constant is larger than the bulk-side decay constant. In
probing bulk-side decay in a semiconductor sample, the spatial
resolution is affected by the potential below the STM tip,
while surface-side decay enables us to observe the effectsof, for example, atomic-scale defects on the recombination
processes. Both of these decay processes appear in the SPPXSTM signal [48].
Here, we focus on the faster component reflecting thebulk-side carrier density. The first component, bulk-side decay,
is probed through the mechanism of absorption bleaching,which typically occurs in pump–probe measurements. Whencarriers excited by the first optical pulse remain in the
excited state, the absorption of the second optical pulse issuppressed [2]; the number of carriers excited by the second
optical pulse decreases, depending on the number of remaining
carriers. Thus, the second current pulse induced in I ∗decreases and depends on td , reflecting the decay of the excited
carriers after the first pulse excitation (B and C in figure 3).
Accordingly, I (td ) gives a measure of the bulk-side carrier
density.
Figure 4(b) shows the result of optical pump–probereflectivity measured in the same sample. In general, thetime evolution of pump–probe reflectivity reflects the decayof photogenerated carriers, and the time constant corresponds
to the bulk recombination lifetime. The time constant derived
from the exponential fit to data in figure 4(b) is 410 ps, which isnear the time constant in I (td ), and therefore, I (td ) shownin figure 4(a) corresponds to the fast component, that is, thedecay of the bulk-side carrier density.

Figure 5. Time evolution of band structure for an n-typesemiconductor, and optical response in tunnelling current. (a) Before
pulse illumination (dark state). (b) Immediately after pulse
illumination. (c) During photocarrier relaxation.

3.3. Origin of the signal
What process appears in the time-resolved signal I (td )?
The measured signal is clearly the time-resolved tunnellingcurrent, because the time-resolved signal sharply decreases
with the tip–sample distance [27, 48]. Since the techniqueused does not involve any gate switch in the measurement
system, the time-dependent conductance at the detection gap,
gs (t) in the equation in section 2.1, is constant; thereby, themeasured signal does not exhibit the effect of the geometriccapacitance between the tip and the sample. This rules out
other possibilities that may appear in time-resolved signals of,for example, photoemission and those related to displacement
current. The thermal expansion effect of the STM tip also does
not contribute to the signal, as expected, because the signal isalmost zero at negative bias voltages for the sample shown infigure 4. Therefore, what physical phenomena occurring in thesample are reflected in the time-resolved tunnelling current?
In SPPX-STM on a semiconductor, a nanoscale metal–
insulator–semiconductor (MIS) junction is formed by the STM
tip, tunnelling gap, and sample (figure 5(a)). When a reverse
bias voltage is applied to the junction, TIBB occurs in thesurface region owing to the leakage of the electric field into thesample [34, 49, 50]. With pulse excitation, the redistribution ofphotocarriers reduces the electric field and changes the surface
potential, and increases the effective bias voltage applied to

3.4. Application
As described above, SPPX-STM allows the study of transient
photocarrier dynamics by the same mechanism as the
optical pump–probe technique. Taking advantage of atomicscale structural investigation using STM, SPPX-STM has
the potential to probe spatial information on local carrier
dynamics.
Here, we show spatially dependent carrier recombination
in a semiconductor heterostructure consisting of materials withdifferent lifetimes, GaAs/Al0.5 Ga0.5 As/LT-GaAs (figure 6),
where LT-GaAs represents low-temperature-grown GaAs. The
sample was prepared as follows. A layer (1 μm) was grown at
250 ◦ C by molecular beam epitaxy (MBE) on a barrier layer of
AlGaAs (1 μm) grown on an undoped GaAs substrate, which
was annealed at 700 ◦ C for 60 s in H2 (5%)/Ar ambient, andcontained high-density defects that act as recombination sites
for carriers. Thus, its carrier lifetime is much shorter than6

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Y Terada et al

Figure 6. SPPX-STM measurement for GaAs/AlGaAs/LT-GaAs heterostructure. (a) Schematic of measurement. (b) STM image of thecleaved surface. (c) SPPX-STM signals observed at each region.

that of GaAs. The AlGaAs barrier layer has a band gap larger
than the optical excitation energy (1.55 eV). Figure 6(b) shows
an STM image of the sample cleaved in UHV. The interfacesof the three layers are clearly recognized. Figure 6(c) shows
the SPPX-STM spectra, I (td ), taken at certain points in
the GaAs, AlGaAs, and LT-GaAs regions. As expected, the
LT-GaAs region exhibits an ultrafast decay component witha time constant of 4.0 ps, while the GaAs region exhibits atime constant of 4.8 ns. These values are consistent withthe recombination lifetimes determined by the optical pump–probe reflectivity measurement, 1.5 ps and 2.7 ns for the LTGaAs and GaAs samples, respectively. The negative signof the signals, which corresponds to the decrease in I , is ingood agreement with the absorption bleaching mechanism.
These results support the mechanism proposed above. For
AlGaAs, there is no delay-time dependence in I (td ), because
no photocarrier generation occurs in this area.
Here, we gave an example of an SPPX-STM study on
carrier recombination; other carrier dynamics such as drift,
diffusion, and surface-side decay, which also govern thedevice operation, can be investigated as well [48], indicating
a potential for probing various types of dynamics.

process as an example, but other dynamical processes, suchas carrier drift and diffusion, can be imaged as well, making
SPPX-STM applicable for the analysis of various composite
materials in future devices. SPPX-STM is also applicable tosystems where a response in tunnelling current has a nonlinear
dependence on optical intensity. We hope that this newtechnique will open a window to the further advancement of
future research in nanoscale science and technology.

Acknowledgment
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 (Yong Scientists B).

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4. Summary
We reviewed a wealth of time-resolved microscopy methods
based on ultrafast laser technology.
The features anddrawbacks of each developed microscopy method werediscussed, particularly from the viewpoint of spatial andtemporal resolutions. Then, a new microscopy technique,
SPPX-STM, which realizes a high spatiotemporal resolution
by combining STM with the pump–probe technique andhas the potential for imaging of ultrafast dynamics on a
femtosecond timescale with a spatial resolution based on STM,
was explained in detail. For semiconductor samples, the time
evolution of photocarrier density is reflected in the signal of
SPPX-STM, I (td ). We showed a carrier recombination
7

J. Phys.: Condens. Matter 22 (2010) 264008

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