Applied Physics Express 6 (2013) 032401
http://dx.doi.org/10.7567/APEX.6.032401

Single-Atomic-Level Probe of Transient Carrier Dynamics
by Laser-Combined Scanning Tunneling Microscopy
Shoji Yoshida, Munenori Yokota, Osamu Takeuchi, Haruhiro Oigawa, 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 February 5, 2013; accepted February 21, 2013; published online March 5, 2013
The first application of laser-combined time-resolved scanning tunneling microscopy (STM) to single-atomic-level analysis was demonstrated. The
dynamics of photoinduced holes, transiently trapped at the surface and recombined with the electrons tunneling from the STM tip to the in-gap
states associated with single-(Mn,Fe)/GaAs(110) structures, was successfully probed on the atomic level for the first time. In addition, lightmodulated scanning tunneling spectroscopy (LT-STS) was performed for energy–space analysis in conjunction with time-resolved measurement
and shown useful for developing the laser-combined STM techniques for further advances. # 2013 The Japan Society of Applied Physics

he understanding and control of quantum dynamics,
such as transition and transport in atomically controlled structures, are key factors for continuing thecurrent advances of nanoscale science and technology.
Scanning tunneling microscopy (STM) has been one of
the most promising techniques widely used for the analysis
of such properties because of its high spatial resolution.1,2)
However, since its temporal resolution is low, approximately
in the millisecond range, high-time-resolution STM has
been an attractive target since its development.3–8) Recently,
spin-polarized STM (SP-STM) has been combined with
the inelastic excitation of electrons, and spin dynamics in asingle atom has been successfully analyzed.9) With the electronic control of excitation, a subnanosecond time resolution
was realized. Moreover, STM combined with ultrashortpulse laser technology, such as the optical pump–probe
method, has enabled us to study ultrafast dynamics withpulse-width time resolution, in principle, in the femtosecond range simultaneously with the atomic resolution of
STM.10–16) Subnanoscale analyses on semiconductors by thismicroscopy technique have been reported with high time
resolution provided by the ultrashort-pulse laser: however,single-atomic-level analysis taking advantage of STM has
not yet been directly demonstrated until now.
In this paper, we show the results obtained by the ﬁrst
application of a laser-combined STM technique to thesingle-atomic-level analysis of transient photoexcited carrier
dynamics. We also show the eﬀect of an in-gap state, that is,
the acceptor level formed by single-(Mn,Fe)/GaAs(110)structures, on the carrier dynamics directly probed for theﬁrst time.
In the time-resolved STM developed by the combinationof STM with the optical pump–probe method using an
ultrashort-pulse laser (Ti:sapphire laser with a pulse width of
140 fs in our case), the sample surface below the STM tip is
illuminated with a train of paired optical pulses (i.e., pumpand probe pulses, as in the optical pump–probe method), but
the signal is tunneling current instead of, for example, thereﬂectivity of the probe pulse. Namely, tunneling current ismeasured as a function of the delay time td between pumpand probe pulses. Although the signal is tunneling current,
we call the two pulses in a pair the pump and probe pulses,
similarly to in the optical pump–probe method.

TyOn leave from Department of Applied Physics, The University of Tokyo,
Bunkyo, Tokyo 113-8656, Japan.

To detect a weak time-resolved signal without the thermal
expansion problem, the delay time td , instead of the laser
intensity, is modulated and used as the reference for the
lock-in detection method.12) With the modulation of td
between td1 and td2 using a pulse picker, the in-phase component obtained by the lock-in detection of the tunnelingcurrent gives Iðtd1 ; td2 Þ  Iðtd1 Þ  Iðtd2 Þ. As td2 is set to a value
larger than the relaxation time of the probed dynamics,
Iðtd1 ; td2 Þ can be approximated as Iðtd1 Þ  Iðtd1 Þ  Ið1Þ,
where Ið1Þ is the tunneling current for a delay time suﬃciently long for the excited state to be relaxed. Therefore,Iðtd1 Þ is accurately obtained through the lock-in detectionof I by sweeping td1 . In addition, since the modulation canbe performed at a high frequency (1 kHz in our case), themeasurement is less aﬀected by low-frequency ﬂuctuations
in laser intensity and tunneling current.
Accordingly, this method reduces the measurement timeand hence enables the spatial mapping of time-resolved
signals. Furthermore, since STM imaging is carried out
simultaneously with the spectroscopic measurement, when
the STM tip is being kept above a target position using atechnique such as atom tracking,17) the measurement is
expected to be possible even for a single-atomic-levelstructure, which is the method adopted in this study.
As the sample for this work, we chose a single-Mn/
GaAs(110) structure, which is known to form an in-gap
state, that is, an acceptor level.18–21) Transition metals insemiconductors play essential roles in producing magnetism
in dilute magnetic semiconductors (DMSs).18) Recently,
atomic-scale analyses have been carried out by STM toinvestigate the localized electronic structures of (Zn,Mn,
Fe,Co)/GaAs(110), and the wavefunction mapping of in-gap
states provided a detailed understanding of their characteristics.19) However, carrier dynamics via their electronic
structures, for example, has not yet been analyzed on theatomic scale. Since the existence of in-gap states signiﬁcantly aﬀects the carrier dynamics in materials, understanding of such properties on the atomic level is extremely
important. Among the DMSs, Ga1x Mnx As is the most wellknown prototypical example in this ﬁeld, providing fundamental mechanisms. The Mn atoms in GaAs, for example,
act as acceptors providing holes that are considered tomediate the ferromagnetic interaction between Mn 3d5 -core
spins. If spin dynamics can be measured by microscopy, forexample, in combination with circularly polarized light,
magnetic phenomena can be included in the discussion.

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(a)

(b)

(c)

Fig. 1. Schematic illustration of nanoscale MIS structure formed by STM
tip, tunneling barrier and semiconductor sample (n-type in this case). A gap
state formed by a metal atom deposited on the surface is drawn. The holesthat are trapped at the surface decay via recombination based on the balance
between the tunneling rate W and the hole-capture rate Cp .

From these standpoints, the single-Mn/GaAs(110) structure
is considered a good sample for the application of lasercombined STM to establish the technique of single-atomiclevel analysis. For comparison, measurements were also
carried out on a single-Fe/GaAs(110) structure.
Figure 1 shows a schematic diagram of the band structure,that is, the nanoscale metal–insulator–semiconductor (MIS)
structure, formed by the STM tip, tunneling barrier andn-type semiconductor sample with an in-gap state. Under areverse bias voltage condition between the STM tip and thesample, i.e., a positive sample bias voltage for the n-typesemiconductor sample, the so-called tip-induced band
bending (TIBB) is induced.22,23) When the sample surface
below the STM tip is photoilluminated, the redistribution ofphotoexcited carriers, i.e., transient trapping of minority
carriers (holes in this case) decreases the level of TIBB. The
decrease in TIBB is called the surface photovoltage (SPV).
SPV changes the tunneling barrier height, resulting in thechange in STM tunneling current.
Because of the lack of counter carriers (electrons in thiscase) at the surface, the density of trapped holes decreases via
certain processes such as thermionic emission, resulting in along time for the relaxation of SPV, typically 200 ns.12) If
there exists an in-gap state and tunneling current (electrons)
directly ﬂows from the STM tip into the in-gap state, however, the holes are recombined with the electrons at the ingap state. There are two limiting factors, i.e., tunneling rate(W) and hole-capture rate (Cp ). When the tunneling current
is suﬃcient (W  Cp ), the hole-capture rate becomes thelimiting factor of the recombination process. In this case, the
SPV induced by the pump pulse relaxes with the delay time
due to the decay of the trapped holes with the decay constant  1=Cp . On the other hand, the number of additional holes
trapped at the surface by the probe-pulse excitation, whichfurther increases the SPV, depends on the SPV remaining
at the excitation (delay) time. Therefore, the time-resolved
STM signal, which probes the change in tunneling current as
a function of delay time, Iðtd1 Þ, gives the hole-capture rate
as Cp  1= (see Ref. 12 for detail).
Samples were prepared by the deposition of a small
amount of Mn or Fe metal on a GaAs(110) cleaved surface(n-type: Si-doped, 1  1017 cm3 ) at room temperature. To
analyze the recombination of the transiently trapped holes

Fig. 2. (a) STM image of Mn/GaAs(100) surface (setpoint bias voltage
Vs ¼ 2:0 V and tunneling current It ¼ 100 pA). Single Mn atoms areindicated by arrows. (b) Magniﬁed image of single-Mn/GaAs structure.
(c) Schematic illustration of structural model for single-Mn/GaAs(110)structure.

with the electrons tunneling from the STM tip, we used ann-type GaAs substrate.
Figures 2(a) and 2(b) show a typical STM image ofthe Mn/GaAs(110) surface and a magniﬁcation of the structures indicated by arrows in Fig. 2(a), respectively. Whenthe image in Fig. 2(b) is compared with the STM images
reported in previous papers,19,20) the structures indicatedby arrows are single Mn atoms that occupy the Ga sites on
the surface, as shown in the schematic model [Fig. 2(c)].
According to the previous work referred to above, the in-gap
state formed is associated with the structure.21)
Since the sample is n-type, the acceptor level formedby Mn is occupied by electrons, and the tunneling current(electrons) from the STM tip does not ﬂow into the in-gap
state. Therefore, an additional technique is necessary toanalyze the electronic structure of this system. We introduced light-modulated scanning tunneling spectroscopy(LM-STS)24) to measure current–voltage (I–V ) curves. In
LM-STS, I–V curves are generally measured under photoillumination with chopping at 100 Hz. The holes producedby photoexcitation enable current ﬂow, thereby, enabling the
probing of the electronic structure of the sample. Accordingly,
the I–V curves under dark and photoilluminated conditions
are obtained simultaneously over both positive and negative
bias voltages. From the shift of the two I–V curves, we
can estimate the magnitude of SPV for each bias voltage.
A Ti:sapphire laser for time-resolved analysis, as well as acontinuous-wave (CW) laser, can be used with the adjustment
of intensity. Modulation at 100 Hz can be suﬃciently fast witha low photointensity to reduce the thermal expansion eﬀect of
the STM tip and sample on the LM-STS measurement.
Figure 3 shows the LM-STS spectra obtained (a) above
a single Mn atom and (b) over a bare GaAs surface, (c) acomparison between the two I–V curves obtained underphotoillumination in (a) and (b), (d) the magniﬁcation of therectangular area in (b), and (e) the maps of SPV for threebias voltages (lower three images), which are obtained fromthe LM-STS spectra measured over the topographic image(top image) (see Ref. 24 for detail). In the I–V curvemeasured above a single Mn atom under photoillumination, there is a broad shoulder with a maximum at 0:8 eV032401-2# 2013 The Japan Society of Applied Physics

Appl. Phys. Express 6 (2013) 032401

(a)

(b)

S. Yoshida et al.
(a)

(d)

(b)

(c)

Fig. 4. Schematic illustration to explain spectra in Fig. 3. Band structureswith (a) zero bias voltage, (b) positive bias voltage and (c) negative bias
voltage. Here, electron aﬃnity of GaAs: 4.07 eV, work function of the W
tip: 4.52 eV, band gap of GaAs: 1.42 eV, and acceptor level: 0:1 eV above
the valence band.

(e)

(c)

Fig. 3. Typical LM-STS spectra obtained (a) above Mn atom of singleMn/GaAs(110) structure in Fig. 2 and (b) over a bare GaAs surface.
(c) Comparison between two I–V curves obtained under photoilluminationin (a) and (b). (d) Magniﬁcation of rectangular area in (b). SPV obtained by
the I–V curves is also shown. (e) Maps of SPV obtained for three bias
voltages (lower three images) from shift of I–V curves shown in (d) overtopographic image (top). The Mn sites are indicated by arrows.

[Fig. 3(a)] and a metallic characteristic is shown, in which
the sign of tunneling current changes at 0:6 eV, asobserved in the magniﬁcation [Fig. 3(c)].
Figure 4 shows schematic illustrations to explain theobserved results. Figure 4(a) shows the band structure
without a bias voltage between the STM tip and the sample.
Band bending that equalizes the Fermi levels of both sidesof the structure even at zero bias voltage is observed.
Figures 4(b) and 4(c) show the band structures with positive
and negative bias voltages, respectively. The magnitude ofband bending depends on the photoillumination intensity.
Since holes are produced under photoillumination, tunneling current ﬂows even for a positive sample bias voltage[Fig. 4(b)]. TIBB is induced and increases with the positive
bias voltage until the acceptor level reaches the Fermi level.
In this bias-voltage region, the balance between the increasein barrier height and the increase in potential slope in thesample, which are induced by the applied bias voltage, isconsidered to form the broad shoulder with a maximum at0:8 eV in the spectrum. The peak position depends on thephotoillumination intensity that determines the value of
TIBB. After the Fermi level of the W tip reaches the conduction-band edge at the surface, electrons directly tunnel
from the STM tip to the conduction band, resulting in theincrease in tunneling current for voltages higher than the0:8 eV peak. On the other hand, for a negative sample
bias voltage, electrons, which are the majority carriers of thissample, may tunnel from the conduction band to the STM tip
when band bending becomes almost ﬂat, which corresponds
to the change in the sign of the tunneling current observedin Fig. 3(c). When the Fermi level of the W tip reaches theacceptor level and then the valence band edge, the current,i.e., electrons tunneling from the valence band to the STM
tip, emerges in the spectrum.
As shown in Fig. 3(d), SPV, i.e., the shift of I–V curves
between the cases with and without photoillumination, isobserved until the negative bias voltage reaches 2:0 V.
Furthermore, the bias-voltage dependence of SPV, shownin the spatial maps of SPV [Fig. 3(e)], clearly shows that the
SPV is lower at the Mn site. These results indicate that the
upward band bending exists even for a negative sample bias
voltage, owing to the eﬀect of the in-gap state formed by thesingle-Mn/GaAs structure.
To prepare the conditions under which the hole-capture
rate becomes the limiting factor of the process (W  Cp ), we
adjusted the laser intensity by observing the change in the
STM image of the single-Mn structure. Namely, when thecondition of W  Cp is satisﬁed, the STM image shows acentrosymmetric depression at the Mn site as observed in thechange in the STM image from Figs. 5(a) to 5(b) for Vs ¼
þ2:0 V, owing to the charging eﬀect at the Mn site. The bias
voltage for the time-resolved measurement was set at +2.0 V,
at which the injection of electrons from the STM tip to the ingap state occurs and the GaAs area can be imaged. The direct
recombination of electrons tunneling from the STM tip withholes in the valence band was negligible. In fact, when timeresolved measurement was carried out above a bare GaAs
surface, the decay constant was typically 200 ns.
In addition, to realize single-atomic-level measurement,
we adopted an atom tracking technique.17) As shown in the
cross section in Fig. 5(c) along the line in Fig. 5(b), a depression at the Mn site is traced when the measurement condition
discussed above is satisﬁed. Therefore, the STM tip position
can easily be estimated if the STM tip is scanned above thesingle Mn atom during the time-resolved STM measurement.
When the STM tip began to probe the edge of the slope of thedepression structure [Fig. 5(c)], the position of the STM tip
was controlled to move the tip back to the top of the Mn atom.
Figure 5(d) shows a typical time-resolved spectrum measured above the Mn atom in a single-Mn/GaAs(110) structure.
The decay constant  was determined by ﬁtting the spectrum

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(a)

(b)

(c)

(d)

Fig. 6. STM images of single-Fe/GaAs(110) structure (Vs ¼ þ2:5 V,
It ¼ 300 pA and 2:0 V, 300 pA) and time-resolved STM spectrum
obtained above Fe atom (Vs ¼ þ1:8 V, It ¼ 400 pA).

in conjunction with time-resolved measurement, which isuseful for optimizing the conditions for time-resolved measurement. An atom tracking technique is useful for realizing
reliable measurement when phenomena such as thermal
drift exist. With the techniques developed in this study, lasercombined STM is expected to enable further advances.
STM images of single-Mn/GaAs(110) structure obtained for(a) high (0.13 mW) and (b) low (0.01 mW) laser intensities (Vs ¼ 2:0 V,
It ¼ 100 pA). For a positive bias voltage, a depression is imaged at the Mnsite for a low laser intensity owing to the charging eﬀect of the Mn site.
(c) Cross sections along lines in (b). (d) Time-resolved STM spectrum
obtained above Mn atom (Vs ¼ þ2:0 V, It ¼ 150 pA).
Fig. 5.

Acknowledgment Support from the Japan Society for the Promotion of
Science (Grants-in-Aid for Scientiﬁc Research) is acknowledged.

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with an exponential function, and the obtained value is 1.6 ns.
This is the ﬁrst result obtained by the application of laser-combined-time-resolved STM to single-atomic-level analysis.
For comparison, in consideration of further advances,
we applied the microscopy technique to the analysis of asingle-Fe/GaAs structure which has a similar structure to thesingle-Mn/GaAs structure [Fig. 2(c)] with an in-gap state.19)
Figure 6 shows typical STM images of the Fe/GaAs structure [similar to Fig. 2(c)] and a time-resolved signal obtained
above the single Fe atom in the structure. The decay constantobtained by exponential ﬁtting of the spectrum is 14.3 ns,
which is ten times larger than that obtained for Mn (1.6 ns).
According to the previous results obtained by the analysis of
I–V curves of highly doped samples, the in-gap state formed
by Fe at the surface is closer to but still above that by Mn by a
few tens of millielectron volts.19) The diﬀerence in energy
level may result in the observed diﬀerence in decay constant,i.e., the capture cross section of holes: however, for further
understanding of the mechanism, more systematic experiments, as well as detailed theoretical analyses, are necessary.
In such a case, as shown here, the single-atomic-level probeof quantum dynamics is expected to provide information thatenables a sophisticated analysis based on the simpliﬁed dataof the single atomic level. As discussed above, when spin
dynamics, for example, becomes measurable, analysis of themagnetic phenomena may be possible.
In conclusion, we have succeeded in applying lasercombined-time-resolved STM to single-atomic-level analysis. The photoinduced-hole dynamics modulated by the ingap states formed by single-(Mn,Fe)/GaAs structures was
successfully probed on the single atomic level for the ﬁrst
time. LM-STS was eﬀectively used for energy–space analysis(Cambridge University Press, Cambridge, U.K., 1994).
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# 2013 The Japan Society of Applied Physics