Eur. Phys. J. Special Topics 222, 1161–1175 (2013)© EDP Sciences, Springer-Verlag 2013
DOI: 10.1140/epjst/e2013-01912-2

THE EUROPEAN
PHYSICAL JOURNAL
SPECIAL TOPICS

Review

Optical pump-probe scanning tunnelingmicroscopy for probing ultrafast dynamics on
the nanoscale
S. Yoshida, Y. Terada, M. Yokota, O. Takeuchi, H. Oigawa and H. Shigekawaa
Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571,
Japan
Received 18 March 2013 / Received in final form 12 May 2013
Published online 15 July 2013
Abstract. The development of a method for exploring the ultrafast
transient dynamics in small organized structures with high spatial resolution is expected to be a basis for further advances in current science and technology. Recently, we have developed a new microscopy
technique by combining scanning tunneling microscopy (STM) withultrashort-pulse laser technology, which enables the visualization of ultrafast carrier dynamics even on the single-atomic level. A nonequilibrium carrier distribution is generated using ultrashort laser pulses
and its relaxation processes are probed by STM using the opticalpump-probe method realized in STM by the pulse-picking technique.
In this paper, the fundamentals of the new microscopy technique are
overviewed.

1 Introduction
The development of current physics and devices has been progressing with the innovation of atomically controlled fabrication technologies. The understanding and controlof quantum dynamics, such as the quantum transition and transport in organizedsmall structures, are expected to be key factors in realizing further advances. However, for example, with the miniaturization of functional devices consisting of composite materials, quantum dynamics, which has been analyzed by techniques providing
spatially and/or temporally averaged information, does not provide a suﬃciently detailed description for the analysis and design of macroscopic functions. Atomic-scale
defects, for example, have markedly changed the entire situation: defects, which wereonce considered as a problem to be avoided, are now actively designed and controlled
to realize desired functions. The ﬂuctuation in the distribution of dopant materials
governs the characteristic properties of macroscopic functions of materials and devices. Therefore, the evaluation of transient quantum dynamics on the nanoscale isof great importance.
Scanning tunneling microscopy (STM) has been one of the most promising techniques widely used for the analysis of such properties because of its high spatialae-mail: http://dora.bk.tsukuba.ac.jp

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Fig. 1. Schematic illustrations of a) OPP method and b) laser-combined STM, and c) to f)
their probing mechanisms.

resolution [1–8]. However, since its temporal resolution is low, approximately in the
millisecond range, the development of high-time-resolution STM has been an attractive target since its invention. Considerable eﬀort has been devoted to achieving thisand various methods and techniques have been developed [9–18].
Recently, time-resolved STM combined with ultrashort-pulse laser technology has
been realized [19–29]. A nonequilibrium carrier distribution is generated using ultrashort laser pulses and its relaxation processes are probed by STM using the opticalpump-probe (OPP) method realized in STM with the pulse-picking technique. In thispaper, the fundamentals of the new microscopy technique and the basic techniques
for measurement are overviewed.

2 Optical pump-probe STM
2.1 Method
Figure 1 shows schematic illustrations of (a) the OPP method and (b) laser-combined
STM. In the OPP method (Fig. 1(a)), the sample is illuminated with a sequence of
paired laser pulses with a certain delay time td . The ﬁrst pulses are used to excite thesample (and are therefore called pump pulses) and the second pulses are used as aprobe (and are thus called probe pulses) to observe the relaxation of the excited state
induced by the ﬁrst (pump) pulses [30]. As the signal, the reﬂectivity, for example, ofthe probe pulses is measured as a function of delay time using a photodetector. SinceElectronic States and Phases Induced by Electric or Optical Impacts1163

the reﬂectivity of the probe pulse reﬂects the change in the excited state, such as the
production of photocarriers and electronic structures (Fig. 1(c)), if the reﬂectivity Rof the probe pulse is measured as a function of delay time, the relaxation induced by
the pump pulse can be observed through the change in the reﬂectivity of the probe
pulse (Fig. 1(d)). When the carriers excited by the pump pulse remain in the excitedstate, the excitation by the probe pulse is suppressed, that is, absorption bleachingoccurs. In Fig. 1(c), in consideration of absorption bleaching as an example mechanism, the excitation by the probe pulse is drawn smaller than that by the pump pulse.
Since the average intensity of the probe pulses is measured as the signal, ultrafast
dynamics, which is much faster than the time resolution of the photodetector, can beprobed. To probe a weak signal, ΔR < 0.1%, a lock-in detection method is used withchopped light as the reference.
In laser-combined STM, the sample surface beneath the microscope tip is similarly illuminated with a sequence of paired laser pulses with a certain delay time td :however, for the signal, the change in tunneling current ΔI is measured as a functionof td (Fig. 1(b)). The intensities of the pump and probe pulses are adjusted to be
the same. The optical pulses give rise to current pulses in the raw tunneling current I ∗ (Fig. 1(e)), reﬂecting the excitation and relaxation of the sample. When td
is suﬃciently long compared with the relaxation time of the excited state, the paired
optical pulses with the same intensity independently induce two current pulses withthe same height in the raw tunneling current. In contrast, when td is short and theprobe pulse illuminates a sample excited by the pump pulse, the second current pulse
may have a diﬀerent height, depending on td . The signal ΔI therefore also depends
on td , because the height diﬀerence in the second current pulse changes the temporally averaged value of the tunneling current. Accordingly, the relaxation dynamics
of the excited state in the target material, for example, the decay of carrier density
after excitation by a pump pulse, can be probed by STM (Fig. 1(f)). In principle, thespatial and temporal resolutions of this microscopy technique are limited by thoseof the probes used, i.e., the tunneling current (atomic scale) and optical pulse width(femtosecond range).
2.2 Probing system
Since the signal, i.e., the photoinduced change in the tunneling current, is weak, weneed to use the lock-in detection technique. However, if the laser intensity is modulated for reference, thermal expansion of the STM tip and sample causes a criticalproblem, because a change in the tip-sample distance of only 1 Å produces a one order change in the tunneling current, making measurement impossible [31]. To realize
this microscopy technique, we adopted delay-time modulation, instead of intensity
modulation, for the reference in the lock-in detection method, which does not change
the intensity of the laser, enabling the measurement without thermal expansion. We
call this microscopy technique shaken-pulse-pair-excited STM (SPPX-STM).
Figure 2 shows a schematic of the two laser-combined STM systems we havedeveloped. In the ﬁrst system, as shown in Fig. 2(a), we mechanically controlled
the optical length with mirrors which is the technique ordinarily used in the OPP
method, to change the delay time [19, 20]. Although the ﬁrst signal was successfully
obtained with this system, several aspects of the mechanical control had to be improved to achieve further advances. Namely, to measure a weak signal accurately, forexample, stable alignment of the laser beam to keep the laser spot accurately focused
immediately below the STM tip, high-frequency modulation to reduce the signal-tonoise (SN) ratio of the tunneling current and a wide range of delay times to enablemeasurement of dynamics consisting of complex components with various time constants were required.

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Fig. 2. Schematic illustrations of laser-combined STM systems: a) first system, with sinewave modulation and b) second system with rectangular modulation of delay time.

To satisfy these requirements, in the second system shown in Fig. 2(b), we introduced the discrete modulation of the delay time based on a pulse-picking techniqueusing a Pockels cell, which, for the ﬁrst time, allowed STM to be used to stably probe
carrier dynamics with various time constants in small organized structures over awide range of time scales [24].
In the second system, pulse trains are generated by two synchronized Ti:sapphirelasers with a 90 MHz repetition rate (11 ns intervals) and a pulse width of 140 fs.
The relative timing of the two pulse trains, which provides a delay time that ranges
from 0 to 11 ns, is controlled by the synchronizing circuit. Each train is guided toa pulse picker that can selectively transmit one pulse per 90 pulses, resulting in the
reduced repetition rate of 1 MHz. The pulse selection enables the production of anadditional longer delay time that can be adjusted in multiples of 11 ns. Consequently,
td can be adjusted continuously from zero to a large value as needed. Furthermore,
the rectangular modulation of td allows the measurement of the absolute value ofΔI(td ) = I(td ) − I(∞), where I(∞) is the tunneling current for a delay time suﬃciently long for the excited state to be relaxed. The delay time modulated by changing the picking time of the probe pulse between t1d (=td ) and t2d (=∞), as shown in
Fig. 3(a).
By using rectangular modulation with an amplitude larger than the relaxation
time (Fig. 2(b)) instead of sine-wave modulation (Fig. 2(a)) for the lock-in detection,
we can measure the absolute value of the signal ΔI(td ) as described above, which
increases the signal intensity tenfold compared with the diﬀerential value obtained bysine-wave modulation. In addition, since the modulation frequency can be increased
from 20 Hz to 1 kHz or more by digital control, the SN ratio can also be improved

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Fig. 3. a) Schematic illustration of pulse picking. b) Power spectrum of STM tunnelingcurrent.

tenfold as shown by dotted lines in the power spectrum of STM (Fig. 3(b)). Therefore,
the SN ratio is improved 100-fold, and therefore, for the same signal level, the measurement time is greatly reduced (by about four orders of magnitude). Accordingly,
this method reduces the measurement time and hence enables the spatial mapping oftime-resolved signals, which has been desired for a long time [24–26].
Figure 4 shows examples of spectra obtained by SPPX-STM for various samples
with a wide range of lifetimes [24]. The upper spectra were obtained by the OPP
method, and there is good agreement between both sets of results. With using the
new microscopy technique, carrier dynamics can be measured over a wide range oftime scales using a single microscope, in principle, from the femtosecond. The carrier
dynamics in small organized structures consisting of composite materials can now be
evaluated with the spatial resolution of STM.

3 Two measurement methods
In scanning tunneling spectroscopy (STS), there are two typical methods of obtaining
spectra: (1) One is to acquire a full spectrum I(V ) by measuring the tunneling current
I as a function of bias voltage V at each point on the sample surface. (2) The otheris a type of constant-height-mode measurement, i.e., the tunneling-current map ofI(V ) for a certain bias voltage V is measured over the sample surface by scanning the
STM tip, keeping the tip-sample distance constant. By repeating the measurement forvarious bias voltages, a full series of I(V ) maps, which provides a spectrum I(V ) at

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Fig. 4. Time-resolved spectra obtained for LT-GaAs, GaNAs, n-GaAs, and H-terminated Si
obtained by SPPX-STM. The upper spectra were obtained by the OPP method, and thereis good agreement between both sets of results.

each point over the sample surface, can be obtained. These two modes, here referred
to as the (1) direct spectrum acquisition (DSA) mode and (2) constant voltage map(CVM) mode, are used depending on the purpose of the experiment.
In time-resolved microscopy, similarly to in STS, there are also two typical methods of obtaining spectra, where the bias voltage V in the case of STS is replaced by
the delay time td . Here, they are referred to as the (1) direct spectrum acquisition(DSA) mode and (2) constant delay-time map (CDM) mode, after those for STS described above.
Figure 5 shows examples of spectra obtained in the two modes [25]. The
sample is a semiconductor heterostructure consisting of materials with diﬀerent
lifetimes, GaAs/Al0.5 Ga0.5 As/LT-GaAs, where LT-GaAs denotes low-temperaturegrown GaAs. A layer of LT-GaAs (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 and contained
high-density defects that act as recombination sites for carriers. Thus, the carrier
lifetime in LT-GaAs is much shorter than that in GaAs [32]. The lifetimes of photocarriers in GaAs and LT-GaAs obtained by the OPP method are 4.8 ns and 4.0 ps,
respectively. Since the excitation energy (1.55 eV) is lower than the gap energy of
AlGaAs (2.3 eV), no signal is expected from AlGaAs.
Measurement can be carried out wherever required because the microscopy technique is STM. Figure 5(a) shows an STM image of a cleaved surface of the sample,
where the interfaces between the three layers are clearly recognizable. The typical
spectra ΔI(td ) obtained over the areas of GaAs and LT-GaAs in the DSA modeare shown in Figs. 5(b) and 5(c), respectively. As expected, the LT-GaAs region exhibits an ultrafast decay component with a time constant of 4.0 ps, while the GaAs
region exhibits decay with a time constant of 4.8 ns. These values are consistent withthe recombination lifetimes determined by OPP measurement, 1.5 ps and 2.7 ns for
the LT-GaAs and GaAs samples, respectively. The slight diﬀerence is considered tooriginate from the fact that STM probes the local electronic structure, which is anadvantage of STM.

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Fig. 5. a) STM image of a heterostructure consisiing of GaAs, AlGaAs, and LT-GaAs (setpoint bias voltage Vt = 5.5 V and tunneling current It = 45 pA). b) and c) Time-resolved
spectra obtained in DSA mode for GaAs and LT-GaAs. d) Time-resolved spectra obtainedin CDM mode along the line in a).

Figure 5(d) shows the spectra obtained in the area of STM image on the top by
the CDM mode. Here, instead of measuring a full spectrum by changing the delay
time (DSA mode), the STM tip was scanned with the delay time ﬁxed at 0.3 ps,
0.6 ps and so forth. Namely, the lines in Fig. 5(d) show the carrier density at eachdelay time. The carrier density decreases with increasing delay time. When the STM
tip is scanned over the entire surface required, two-dimensional (2D) maps of thetime-dependent signal can be obtained. The spectrum and its decay constant are
obtained from the full series of 2D maps by ﬁtting the change in density at eachpoint.

4 Fitting procedure and time resolution
To obtain a decay constant, we numerically ﬁtted ΔI(td ) spectra using a model
function developed as follows. The sample response was assumed to be exponential or
double-exponential decay. Then, the sample response was convoluted with an opticalpulse proﬁle and the impulse response of low-pass ﬁltering obtained using the lock-in
ampliﬁer.
A ﬁtting example is shown in Fig. 6, where the spectra were taken for the LTGaAs sample shown in Fig. 4. SPPX-STM signals were obtained by repeated upward(red line) and downward (blue line) scans of the delay time. Owing to the ﬁnite lock-intime constant, the peak position shifts toward the scan direction of the delay in each
spectrum. The best-ﬁt curves are obtained for the two spectra using a model function
with the same parameters, as shown by solid lines in Fig. 6. The ﬁtting error (upper
part in the ﬁgure) is small and comparable to the noise level. The decay constant forthe spectrum was found to be 2.4 ps.

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Fig. 6. Spectra obtained by repeated upward and downward scans of delay time for an LTGaAs sample and the best-fit curves (lower part). The fitting error is plotted in the upper
part.

When the range of delay is much wider than the optical pulse width and the delaytime scan speed is set at a suﬃciently low value, the decay constant is not strongly
aﬀected by the pulse width and the lock-in time constant, and the ﬁtting of a spectrum
using a simple exponential or double exponential function without any convolution is
suﬃcient. In such a case, accurate analysis gives the same decay constant.
The time resolution of the microscopy technique is determined by the opticalpulse width, the accuracy of the delay time, and the time constant of the lock-in
ampliﬁer. First, the optical pulse width is 140 fs in our setup, which provides a time
resolution of ∼200 fs, determined by the overlap of two optical pulses, that is, theautocorrelation width T0 . Second, the accuracy of the delay time is mainly determinedby that of the mirror position in the optical line and is on the order of 10 fs, which
is negligibly small compared with the other factors determining the time resolution.
Third, low-pass ﬁltering using the lock-in ampliﬁer also degrades the time resolution.
The spectra shown in Fig. 6 were obtained at a lock-in time constant Tc of 300 ms
and a delay-time scan speed v of 0.54 ps/s, and the corresponding degradation in thetime resolution was estimated to be vT c ∼160 fs. This value is comparable to T0 . We
carefully adjusted this value to optimize both the SN ratio and the time resolution[19,20].
On the basis of the above considerations, the time resolution of this microscopy
technique is likely to be determined by the ﬁrst factor, which is the optical pulse
width. This is conﬁrmed using the experimental data; as shown in Fig. 6, the ﬁtting
error (upper part in the ﬁgure) is small and comparable to the noise level. If there areother mechanisms that result in the degradation of time resolution, the signal proﬁlemay be further deformed and may not give a good ﬁt. These results indicate thatthe time resolution of this microscopy technique is limited only by the optical pulse
width, which is 140 fs in the present case.

5 Time-resolved STM on semiconductor
5.1 Mechanism
In laser-combined STM on a semiconductor, a nanoscale metal-insulatorsemiconductor (MIS) junction is formed by the STM tip, tunneling gap, and sample.

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Fig. 7. a) Schematic illustrations of metal-insulator-semiconductor (MIS) structures formedby the STM tip, tunneling gap, and sample, and b) to d) their change under photoillumination. e) LM-STS spectrum obtained for GaAs. c) Two components observed in a spectrum:bulk-side decay (decay constant: τb ) and surface-side decay (τs ).

When a reverse bias voltage is applied to the junction, tip-induced band bending(TIBB) occurs in the surface region (Fig. 7(a)) [33–36]. When the sample is illuminated, the redistribution of photocarriers reduces the TIBB and surface photovoltage(SPV) is generated (Fig. 7(b)). The magnitudes of TIBB and SPV can be measured
by light-modulated-STS [35, 36], which provides tunneling current-bias voltage (I-V )curves with and without photoillumination simultaneously. From the shift of the two
I-V curves, SPV is obtained for each bias voltage, as shown in Fig. 7(e). When thephotoillumination intensity is suﬃciently high to ﬂatten the TIBB, the value of SPV
corresponds to that of TIBB, i.e., band bending occurs under the dark condition.
The excited state subsequently relaxes to the original state through two processes:(i) decay of the photocarriers on the bulk side (bulk-side decay) via recombination,
drift and diﬀusion (Fig. 7(c)), and (ii) decay of the carriers trapped at the surface(surface-side decay) via recombination and thermionic emission (Fig. 7(d)). Becauseof the lack of counterpart carriers near the surface, surface-side decay is slower thanbulk-side decay. Both decay processes appear in the SPPX-STM signal as follows.
The bulk-side decay is probed, for example, through the mechanism of absorption
bleaching, which is a typical mechanism observed in OPP measurement (see 2.1).
When the carriers excited by the pump pulse remain in the excited state, the absorption of the probe pulse is suppressed. In such a case, the second current pulse inducedin the raw tunnling current decreases and depends on td (Fig. 1(e)), reﬂecting the
decay of the excited carriers after the pump-pulse excitation (Fig. 1(c)). On the other1170

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hand, the surface-side decay is probed through the mechanism related to SPV. If theprobe pulse arrives during the relaxation of SPV induced by the pump pulse, the photocarriers generated by the probe pulse induce SPV again, but the total relaxation
time of the SPV depends on the delay time, resulting in change in the total tunnelingcurrent, ΔI. Accordingly, ΔI(td ) is a measure of the surface-side carrier density at
the delay time td as well as the bulk-side carrier density.
Figure 7(f) shows the two components in the spectra, i.e., bulk-side decay (τb )and surface-side decay (τs ). For the case of a p-type semiconductor, photocurrent isdirectly probed by SPPX-STM [27], which will be discussed in 5.2.
5.2 Example of bulk-side decay
We have already seen some examples of results obtained for bulk-side decay (Figs. 4
and 5) for n-type samples, as discussed in detail in 5.1. Here, we give the results
obtained for WSe2 , a p-type semiconductor [27].
Since WSe2 has an indirect band gap [37], the recombination time of photoexcited
carriers is much longer (∼ 10 μs), and therefore, the tunneling current is considered to
be governed by the balance between the two processes at the surface: the tunnelingand diﬀusion of the photocarriers from the bulk side, which have rates of Rt and rate
Rd , respectively.
Figure 8(a) shows the laser-intensity dependence of the time-resolved spectra
obtained for WSe2 . When the tip-sample distance is ﬁxed (Rt = constant), the photocarrier density varies with the laser intensity, and the associated change in thediﬀusion rate Rd governs the balance. For a low laser intensity (Rt  Rd ), the diffusion rate becomes the limiting factor in the balance, where the tunneling of excess
electrons trapped at the surface during photoexcitation is predominantly detected
as the signal (fast component, (e), (f), and (g) in Fig. 8(a)). In contrast, for a highlaser intensity (Rt  Rd ), the tunneling rate becomes the limiting factor and thediﬀusion process produces tunneling with a large decay constant (slow component,(a) in Fig. 8(a)). In the intermediate range (Rt ∼ Rd ), the excess electrons trappedat the surface ﬁrst undergo tunneling. Then, the electrons provided from the bulk
side undergo tunneling at a rate of Rd . As a result, the slow component appears in
the time-resolved spectra after the fast component (b), (c), and (d) in Fig. 8(a)).
The decay constants of the fast and slow components are ∼5 ns and ∼200 ns,
respectively, and that of the slow component is in good agreement with the carrier lifetime obtained by the OPP method, ∼200 ns, as shown in Fig. 8(b). Figures8(c) to 8(e) show the schematic illustrations of the band structures to explain thetime-evolution processes described above for a negative sample bias voltage with thecondition of Rt  Rd ((b), (c) and (d) in Fig. 8(a)). In SPPX-STM, the excess minority carriers transiently trapped at the surface for a few nanoseconds, which produce
a transient SPV and cannot be detected by conventional methods, were directly observed and evaluated.
To enable further advances in the application of laser-combined STM, the development of direct techniques for detecting photocurrent in transient dynamics on
the nanoscale, such as that shown here, is expected to play an important role indetermining the optical characteristics of materials and devices.
5.3 Example of surface-side decay
In this section, the analysis of local structures using surface-side decay is discussed. In
5.3.1, the carrier dynamics via in-gap states formed by metal nanoparticles on GaAs
is probed by SPPX-STM. A single-atomic-level probe is achieved using the techniqueElectronic States and Phases Induced by Electric or Optical Impacts

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Fig. 8. a) Laser-intensity dependence of time-resolved spectra obtained for WSe2 . The
maximum laser intensity was 0.9 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. b) Time-resolved spectrum obtained by OPP method.
c) to e) Schematic illustrations of the band structures to explain the spectra in a).

of atom tracking. On the other hand, in 5.3.2, the reduction in defect density on
a semiconductor surface by (NH4 )2 S treatment, which has been used to improve
the performance of electronic devices, is examined as an example of the practicalapplication of the microscopy technique [40].
5.3.1 Single-atomic-level probing
When some metals are deposited on a semiconductor surface, nanoparticles areformed with gap states, which are considered to enhance the carrier recombination
process. We applied SPPX-STM to directly measure the hole-capture rate of (Co, Fe,
Mn)/GaAs structures on the atomic scale [24, 29]. Transition metals in semiconductors play essential roles in producing magnetism in dilute magnetic semiconductors(DMSs) [38].
Figure 9(a) shows a schematic of an MIS structure with an in-gap state. Tunneling current, electrons in this case, directly ﬂows from the STM tip into the in-gap
state, and the holes trapped at the surface during photoexcitation recombine withthe electrons at the in-gap state. There are two limiting factors in this process: tunneling rate (W ) and hole-capture rate (CP ). When the tunneling current is suﬃcient(W  CP ), the hole-capture rate becomes the limiting factor of the recombination

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Fig. 9. a) Band structure for explaining the carrier dynamics at the in-gap state formed by ametal/GaAs structure. b) Typical STM images of a single-Fe/GaAs structure. c) Schematic
model for the single-Fe/GaAs structure. d) Time-resolved spectrum obtained above Fe atomin the single-Fe/GaAs structure.

Fig. 10. Decay constants obtained by SPPX-STM for (Co, Fe, Mn)/GaAs structures as afunction of nanoparticle size. In the results for Fe and Mn, the measurements for the smallest
size were carried out on the single-atomic level.

process. In this case, the SPV induced by the pump pulse relaxes with increasing
delay time owing to the decay of the trapped holes with the decay constant τ ∼1/CP .
On the other hand, the number of additional holes trapped at the surface by probepulse excitation, which further increases the SPV, depends on the SPV remaining
after the excitation (delay) time. Therefore, time-resolved STM signal, which probes
the change in tunneling current as a function of delay time, ΔI(td ), gives the holecapture rate as CP ∼1/τ .

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Fig. 11. a) Typical time-resolved spectrum obtained in the DAS mode over a) as-etched andb) (NH4 )2 S-treated GaAs(100) surfaces. c) Tunneling-current dependence of carrier decay
time obtained for (NH4 )2 S-treated GaAs(100) surface.

Figures 9(b) and 9(c) show typical STM images of a single-Fe/GaAs(110) structure
obtained for positive and negative bias voltages and its structural model, respectively.
A number of Fe atoms were deposited on a cleaved GaAs(110) surface to prepare thesample. According to previous studies, Fe atoms are considered to occupy the Ga
sites on the surface [39]. When the laser intensity is adjusted to satisfy the condition
W  CP , a depression appears at the Fe sites for a positive bias voltage, as shown by
the cross section drawn in the STM image, owing to the charging eﬀect of electrons.
In such a case, the STM tip position can easily be estimated if the tip is scanned
above a single Fe atom during the time-resolved STM measurement. When the STM
tip began to probe the edge of the slope of the depression, the tip was moved the backto the top of the Fe atom. Figure 9(d) shows a typical spectrum obtained above an Fe
atom in the single-Fe/GaAs structure. The decay constant obtained by SPPX-STM
was 14.3 ns.
Similar experiments were carried out on Mn/GaAs and Co/GaAs, the results ofwhich are summarized in Fig. 10 along with that for Fe as a function of the nanoparticle size. According to a recent STS measurement [39], the in-gap state of Fe is abovethat of Co in a bulk, while it is below that of Co at the surface. The in-gap state
formed by Mn is the lowest for both cases. The obtained results for the ordering of thelevels are in good agreement with the theoretical predictions. Thus, the atomic-scale
analysis of the carrier dynamics is now possible by laser-combined STM.
5.3.2 (NH4 )2 S treatment
High-density defects on semiconductor surfaces deteriorate the performance, for example, of photoelectronic devices. Considerable eﬀort has been devoted to solving thissigniﬁcant problem, and one of the most excellent methods is a surface treatment with

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The European Physical Journal Special Topics(NH4 )2 S solution. According to previous reports, (NH4 )2 S treatment eﬀectively removes the surface oxide and covers the pristine surface with sulfur atoms [40–46]. Thismethod has been widely used to prepare defect-free semiconductor surfaces/interfaces
such as for the formation of Schottky junctions and the growth of quantum dots. Here,SPPX-STM was applied to observe the eﬀectiveness of the treatment using surfaceside decay, i.e., the eﬀect of the (NH4 )2 S treatment on the minority carrier lifetimeat the GaAs(100) surface was investigated [40].
Figure 11 shows the spectra obtained in the DSA mode over (a) as-etched and (b)(NH4 )2 S-treated GaAs(100) surfaces. The decay constant obtained for the (NH4 )2 Streated surface (361 ns) is two orders longer than that for the as-etched surface (afew ns) and almost the same as that observed for a detect-free GaAs(110) surface(τs in Fig. 7(e)) [24]. This result is considered to originate from the fact that the
number of surface defects of as-etched GaAs induced by oxidation was successfully
reduced by the (NH4 )2 S treatment. This is also supported by the measurement of
the tunneling-current dependence of carrier decay time; as shown in Fig. 11(c), thedecay constant is independent of the tunneling current. Namely, the surface defects,
which accelerate the recombination of minority carriers (holes in this case) with thetunneling electrons via the in-gap states, were removed by the (NH4 )2 S treatment.
These results are in good agreement with those observed by other techniques such as
photoemission spectroscopy [46].

6 Conclusion
We have overviewed time-resolved STM, which has been developed by combining
STM with ultrashort pulse laser technology, and the fundamentals of the microscopy
technique along with experimental results. The OPP method was realized in STM
on the basis of delay-time modulation with the pulse-picking technique using a Pockels cell. Using an atom-tacking technique, single-atomic-level measurement has beenachieved. On the basis of these techniques, the spatial resolution of STM and the temporal resolution of an ultrashort pulse laser have been realized simultaneously. Direct
measurement of the photocurrent by the microscopy technique was also demonstrated.
Support from Japan Society for the Promotion of Science (Grants-in-Aid for Scientific
Research) is acknowledged.

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