APPLIED PHYSICS LETTERSVOLUME 85, NUMBER 15

11 OCTOBER 2004

Probing subpicosecond dynamics using pulsed laser combined scanningtunneling microscopy
Osamu Takeuchi, Masahiro Aoyama, Ryuji Oshima, Yoshitaka Okada, Haruhiro Oigawa,Nobuyuki Sano, and Hidemi Shigekawaa)
Institute of Applied Physics, 21st COE, NANO Project, University of Tsukuba, Tsukuba 305-8573, Japan

Ryuji Morita and Mikio Yamashita
Department of Applied Physics, Hokkaido University, Sapporo 060-8628, Japan(Received 10 May 2004; accepted 18 August 2004; publisher error corrected 5 November 2004)
Time-resolved tunneling current measurement in the subpicosecond range was realized byultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pairmethod. A low-temperature-grown GaNxAs1−x 共x = 0.36% 兲 sample exhibited two ultrafast transient
processes in the time-resolved tunnel current signal, whose lifetimes were determined to be
0.653± 0.025 and 55.1± 5.0 ps. These values are of the same order of magnitude as those measured
in the conventional pump–probe reflectivity measurement. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1804238]
delay time between the two pulses, td. To decrease broadband
noise, the delay time of the two pulses td is modulated with asmall amplitude ⌬td at frequency ␻, and the tunneling current is detected by a lock-in amplifier. Since the tunnelingcurrent It responds to the modulation as

Smaller and faster are the key words in the progress ofcurrent nanoscience and technology. Thus, for further advances, a method of exploring the ultrafast transient dynamics of the local quantum functions in organized small structures is eagerly desired. Ultrashort optical pulse technology
in the near-infrared to ultraviolet region has allowed us toobserve transient phenomena in the femtosecond range, theoptical-monocycle region,1,2 which, however, has a drawback of a relatively low spatial resolution due to electromagnetic wavelength. On the other hand, scanning tunneling microscopy (STM), although its time resolution is limited bycircuit bandwidth 共⬃100 kHz兲, enables us to observe spatialdynamics at the atomic level in real space.3 Therefore, theintegration of ultrashort optical technology with STM has
been one of the most exciting goals since their invention.4–7
Pioneering works were performed by Hamers et al.,4–7 whichhave attracted the extensive interest of researchers in variousfields. However, there remain critical problems which have
prevented the achievement of the laser-combined STM measurement, such as the displacement current due to the stray
capacitance of the tunneling gap and photoelectrons produced by multiple photoabsorption.4–7 In such cases, since alarge area is included in the processes, the superior spaceresolution of STM cannot be utilized. In particular, the thermal expansion of the STM tip by photoillumination causesmuch large noise in the tunneling current, making the measurement difficult.
Here, we show the results of the time-resolved tunnelingcurrent measurement in the subpicosecond range, which canadvance the development of future research in terms of ultimate temporal and spatial resolutions.
A schematic of the measurement system is shown in Fig.
1. We adopted the recently developed shaken-pulse-pairexcited STM (SPPX-STM) method, which realizes highly
sensitive measurement free from the thermal expansion effect of the tip and sample.8 The tunneling junction is directly
illuminated by a sequence of laser pulse pairs and averagetunneling current, It共td兲, is measured as a function of thea)

Author to whom correspondence
dora.bk.tsukuba.ac.jp0003-6951/2004/85(15)/3268/3/$22.00

shouldbeaddressed;

It共td + ⌬td sin ␻t兲 = It共td兲 + ⌬td sin ␻t

dIt+ O共⌬t2d兲,dtd

共1兲the coefficient ⌬td共dIt / dtd兲 of the term sin ␻t is obtained bylock-in detection. Since the laser intensity is not modulated
in this system, thermal expansion and shrinking of the STM
tip does not occur. Then, with the numerical integration of
the dIt / dtd signal, It共td兲 can be obtained, conserving the spatial resolution of STM and high accuracy.
In the pulse-pair-excited STM measurement, the first laser pulse in each pulse pair acts as the pump pulse to excite
and modulate the electronic structure of the sample surface,which might cause deviation in the tunneling current. Then,the tunnel current is again deviated by the second laser pulse.
In the case of a short delay time between the two pulses, theexcited state induced by the first pulse partially remains
when the second pulse impinges on the sample. Thus, the

http://FIG. 1. (Color) Schematic of the system setup.
3268© 2004 American Institute of Physics

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Appl. Phys. Lett., Vol. 85, No. 15, 11 October 2004

amount of tunneling current deviation due to the second
pulse can be different from that caused by the first pulse, anddependent on the delay time. Although such ultrashort spikes
in the tunneling current cannot be resolved by any amplifierof present STM systems, if the deviation also changes thetotal amount of tunnel current for each pulse pair, this change
can be detected as the average value of the tunneling current.
Thus, we can observe the ultrafast transient phenomenon bymeasuring the time-averaged tunnel current, as the probe signal, as a function of the delay time.
In such measurement, the relationship between theexcite/relax phenomena of interest and the time-resolved signal in the tunnel current is rather complex. However, thismethod combines and conserves the ultimate temporal resolution of the optical pump–probe approach and the ultimate
spatial resolution of STM. Namely, SPPX-STM satisfies therequirement for exploring the ultrafast dynamics of the local
quantum functions occurring in organized small structures.
Experiments were performed on low-temperature-grown
GaNxAs1−x (x = 0.36%; grown at 450 ° C and annealed at600 ° C), which is an excellent candidate for future fast information transfer systems and highly efficient solar cells,
similar to other III–V–N compound semiconductors.9,10
For comparison, we first examined the dynamics of theGaNxAs1−x sample by the conventional optical pump–probe
method. The system setup was similar to that shown in Fig.
1; Ti:sapphire excitation laser of 300 mW, central wavelength of 800 nm, bandwidth of 30 nm, pulse width of 25 fs,
and oscillation frequency of 80 MHz. The intensity of thepump pulses was adjusted to ⬃5 mW focused into several
micrometers. The polarizations of the pump and probe pulses
were set perpendicular to each other. The sample was firstexcited by the intense pump pulse and the reflectivity justafter pumping was measured by the probe pulse with six
times lower intensity of the pump pulse. The intensity of thereflected probe pulse was measured as a function of the delay
time between pumping and probing. In the spectrum, two
exponential components with decay times of 0.8 and 10 ps
were obtained. These were respectively attributed to the lifetimes of the intra- (faster one) and interband (slower one)carrier relaxation processes, similar to the case of GaAs.11,12
For the SPPX-STM measurement, performed in air, aweak feedback was applied to the STM z-piezo with a cutoff
frequency less than 0.2 Hz. The sample bias voltage and thereference current were set at −2.0 V and 100 pA in order toreduce the surface photovoltage effect by forming a forward
bias condition. The incident angle of the laser pulses,
s-polarized, was 40°–50°, and the light intensity at the tip–sample gap was ⬃10 mW focused into ⬃50 ␮m. The delay
time was swept as a triangular function at ⬃200 s periodicity
with a sinusoidal modulation of 0.5 ps at ⬃20 Hz. The time
constant of the lock-in-amplifier was 100 ms and the decay
amount was −24 dB/ oct.
Figure 2(a) shows the raw signals and the fitting results
as functions of delay time. The up and down scans represent
the data obtained for the increasing and decreasing paths ofthe delay time, respectively. There is a slight shift of the two
signals due to the existence of the lock-in time constant. The
delay-time-dependent tunneling current, obtained by the numerical integration of the data in Fig. 2(a), is shown in Fig.
2(b). The vertical axis represents the amount of change in thetunneling current flowing from the tip to the sample. Here,the left-hand side of the graph is adjusted to the zero point

Takeuchi et al.

3269

FIG. 2. (Color) (a) Derivative tunneling current as a function of delay time
for a GaNxAs1−x 共x = 0.36% 兲 sample annealed at 600 ° C. (b) Tunnelingcurrent obtained by numerical integration of the data in (a). The best-fit
curve is drawn together.

for simplicity, since the absolute value cannot be determinedby this experiment. There exist two exponential components
with fast 共⬃1.0 ps兲 and slow 共⬃50 ps兲 decay times for eachside of the x axis. The curves for td ⬍ 0 and td ⬎ 0 wereconfirmed to have similar shapes at various intensity ratios,i.e., the time constant and the intensity ratio of the two decay
components were always the same for both sides. The asymmetry of the graph for the negative and positive delay times
is due to the difference between the light intensities of thetwo pulses (1:2 in this case; intense pulse impinging first for
td ⬍ 0 region). The difference between the baselines for thepositive and negative sides indicates the existence of a longtime-scale physical process in the system, the origin of
which is not yet clear.
The two decay times obtained by the SPPX-STM measurement are in similar ranges as those obtained by the conventional pump–probe reflectivity measurement, which suggests that the system is probing the same ultrafast transientphenomena. In the conventional optical pump–probe
method, the density of the hot electrons excited by the first
pulse is reflected in the change of the reflectivity of the second pulse, because the existence of the hot electrons preventsthe following excitation of electrons into the same energy
states. The change in the reflectivity observed by the optical
pump–probe method, for a pump intensity two orders ofmagnitudes higher than that used in the SPPX-STM experiment, is on the order of ⌬R / R = 10−5 – 10−6. Thus, the amountof change expected to appear in the average tunneling current upon the change in the reflectivity is obviously too small
to be detected by the SPPX-STM measurement. Moreover, in
the real measurement, a higher tunnel current was observed
when the delay time was shorter despite the photoexcitationby the second pulse being smaller. Since the hot electrons

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3270

Takeuchi et al.

Appl. Phys. Lett., Vol. 85, No. 15, 11 October 2004

FIG. 3. (Color) Relationship between time-resolved signal intensity andaverage tunnel current, measured while varying tip–sample distance.

feel a lower tunneling barrier, the lower density of the hot
electrons should result in a smaller tunnel current. Then,
what causes the observed change in the average tunnel current in the SPPX-STM measurement?
In general, hot electrons produced by laser pulses kinetically relax first in energy space through energy-dissipating
phonon scattering until they reach local thermal equilibrium,
which takes place in the time scale of subpicoseconds topicoseoconds, depending on temperature (the kinetic stage).
Then the hydrodynamic stage, relaxation processes from thelocal thermal equilibrium due to scatterings to the global
equilibrium due to collective motion, follows with the timescale of a few tens of picoseconds. During the processes, thenonuniformity of the electron density induced locally by thelaser pulse gradually vanishes. Finally, the overall thermal
equilibrium is reached.13 When the electron density is high,the scenario is modified in some respects. Because of thesmall effective mass of GaAs, electrons easily degenerate as
the electron density becomes higher. Since the local thermoequilibrium distribution function is given by the Fermi–Diracdistribution, the relaxation time in the kinetic stage of such
hot electrons becomes longer, compared with the case of
nondegeneracy, due to the Pauli exclusion principle.13 A
rough estimation shows that the electron density excited by
the pump and/or probe pulses in the present experiments
exceeds well above 1018 cm−3, so that the electrons just after
the excitation are strongly degenerate. The relaxation time inthe kinetic stage of hot electrons excited by the second pulse
could be, therefore, varied according to when the electrons
are excited by the second pulse. In other words, as the delay
time becomes shorter (the effect of degeneracy due to thefirst pulse is stronger), the relaxation time of hot electrons
generated by the second pulse becomes longer and the deviation of the average tunneling current becomes greater.
This scenario may explain the spectrum shown in Fig. 2.
In the development of the time-resolved STM, what isalways questioned is whether the time-resolved signal is related to tunnel current or not, because this is the key issue
which determines whether the method can have the superior
spatial resolution of STM or not.4–8 Figure 3 shows the timeresolved signal intensity of SPPX-STM as a function of average tunnel current, which was measured while varying thetip–sample distance. There is a strong linear relationship.
This indicates that the time-resolved signal is indeed ob-

tained from the area just below the STM tip similar to ordinary STM measurement. Namely, the signal in Fig. 2 is thedemonstration of subpicosecond time-resolved tunneling current measurement by laser-combined STM.
Before summarizing this study, we briefly mention theanalytical procedure of the time-resolved signal obtained by
SPPX-STM experiment. First, for extracting the lifetimes of
the transient processes from a SPPX-STM spectrum, one
should not directly compare the fitting functions (two exponential and one step function) and the integrated experimental data. This is due to the fact that the latter is strongly
affected by the measurement conditions, such as the laserpulse width, amplitude of the delay time modulation, and thelock-in time constant. Thus, we must always take them into
account in the fitting procedure.15 We found that the model
function shown by the dotted line in Fig. 2(b) gives the best
fit to the experimental result, the dotted lines in Fig. 2(a).
The lifetimes, obtained in this analysis, are 0.653± 0.025 and55.1± 5.0 ps, respectively.
Second, when the delay time is as short as the pulse
width, the interference between the two pulses causes theoscillation of the illumination intensity and breaks the condition of constant excitation intensity.7,8 This causes oscillation of tunnel current via the thermal expansion of the STM
tip and/or other photoinduced current response.8,14 When thedelay time modulation is larger than the pulse width, theoscillation causes an artifact signal in the region of−⌬td ⬍ td ⬍ ⌬td. Thus, this signal can affect the observationof fast decay processes. This signal can, however, be effectively reduced by increasing ⌬td and ␻.15 Actually, this component did not obviously appear in the present experimentalresult.
In conclusion, the time-resolved tunneling current measurement in the subpicosecond range was demonstrated,which can advance the development of future research interms of ultimate temporal and spatial resolutions.
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