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Nanoscale probing of transient carrier dynamics modulated in a GaAs–PIN
junction by laser-combined scanning tunneling microscopy
Shoji Yoshida, Yasuhiko Terada, Ryuji Oshima, Osamu Takeuchi and Hidemi Shigekawa*
Received 21st October 2011, Accepted 13th December 2011
DOI: 10.1039/c2nr11551d

The modulation of carrier dynamics in a GaAs–PIN junction after
photoexcitation by an ultrashort-pulse laser was probed by shakenpulse-pair-excited scanning tunneling microscopy (SPPX-STM),
which enables nanoscale mapping of time-resolved STM images.
The effect of the built-in potential on the carrier dynamics, diffusionand drift, which cannot be probed by the optical pump-probe technique, was successfully visualized in real space.
Study of the nonequilibrium transport of carriers in semiconductors
is of great importance not only from fundamental viewpoints but also
as a basis for the development of electronic devices.1,2 With the size
reduction of structures in current electronic devices, differences in theelectronic properties caused, for example, by the structural nonuniformity of each element have an ever increasing effect on macroscopic
functions. The direct observation of device characteristics provides us
with a basis for the macroscopic analysis of results.
In a previous work, the doping characteristics and carrier transport
properties in a GaAs p–n junction were visualized with 10 nm
spatial resolution by light-modulated scanning tunneling spectroscopy (LM-STM).3,4 The characteristic properties under the operating
conditions, which had previously been analyzed on the basis of
empirical electric properties, were directly visualized on the nanoscale.
The obtained results provided a solid basis for elucidating themechanism of the carrier transport properties predicted by themacroscopic analysis. However, LM-STM enables the imaging of
only stationary states. Therefore, to achieve further advances incharacterization, a method of exploring the transient dynamics of
local quantum functions in small organized structures is highly
desirable.
STM has an excellent spatial resolution on the subangstrom scale.
However, since its temporal resolution is limited by the circuit
bandwidth (100 kHz),5–9 increasing its potential by, for example,
combining its characteristics with those of other techniques is desired.
A promising approach is to control the material conditions in STM
measurement using optical techniques. Ultrashort-optical-pulse
technology has enabled the observation of transient phenomena withfemtosecond duration, i.e., the optical-monocycle region, although
this has the drawback of a relatively low spatial resolution limited by
Institute of Applied Physics, University of Tsukuba, Tsukuba, 305-8573,
Japan; Web: http://dora.bk.tsukuba.ac.jp. E-mail: hidemi@ims.tsukuba.
ac.jp; Fax: +81-29-853-5276; Tel: +81-29-853-5276

This journal is ª The Royal Society of Chemistry 2012

the electromagnetic wavelength.1,2,10 Therefore, realizing themeasurement of time-resolved tunneling current in the sub-picosecond range by combining STM with an ultrashort-pulse laser isa challenge that will lead to the simultaneous realization of ultimate
spatial and temporal resolutions.11–21
In their pioneering work, Hamers and co-workers analyzed surface
photovoltage. By changing the repetition rate of laser pulses, theyobserved a corresponding change in the surface photovoltage (SPV),
which they considered to be proportional to the photocarrier
density.17,18 Through the comparison of experimental and calculated
results, they successfully determined the carrier lifetime in Si to be
1 ms. Another approach to obtaining a better time resolution is to use
the pump-probe method. Recently, a new microscopy technique,shaken-pair-pulse-excited STM (SPPX-STM), was developed and thereal-space imaging of nanoscale transient carrier dynamics witha wide range of lifetimes was realized.21–30 Since the probe in SPPXSTM is tunneling current, the effect of local electronic structures oncarrier dynamics can be observed; the effect of gap states on the holecapture rate was visualized for a Co nanoparticle/GaAs system on thenanoscale.29
In this paper, we present results obtained by SPPX-STM for the
carrier dynamics in a GaAs–PIN junction. Variation in local potential essentially influences carrier dynamics in small organized structures: therefore, understanding this effect on the nanoscale isextremely important for the future development of semiconductor
devices. Here, we demonstrate the first real-space imaging of carrier
dynamics modulated in the built-in potential of a GaAs–PIN
junction.
First we briefly explain SPPX-STM and its basic mechanism.
Fig. 1 shows a schematic illustration of SPPX-STM. In the conventional optical pump-probe reflectivity (OPPR) method, a sample isilluminated with a train of paired laser pulses with a certain delay time
between the pulse pairs. The first pulse is used to excite the sample
surface, and the reflectivity of the second pulse is measured asa function of delay time. Thus, the signal is the change in thereflectivity of the second pulse, and the information is averaged overthe photoilluminated area. In SPPX-STM, on the other hand, thesample surface below the STM tip is similarly excited by a pairedpulse train, but the signal is the tunneling current as a function ofdelay time. Namely, the tunneling current is measured by changing
the delay time. How does this work?
Fig. 2 shows the relationship between the tentative raw tunnelingcurrent I* induced by a pair of laser pulses and the delay time td
Nanoscale, 2012, 4, 757–761 | 757

Fig. 1 (a) Schematic illustration of SPPX-STM. Pulse trains are generated by two synchronized Ti:sapphire lasers with a pulse width of 140 fs (central
wavelength, 800 nm; average intensity, 1 W) at a repetition rate of 90 MHz. The detected signal is DI ¼ I(td)  I(N), where I(td) and I(N) are thetunneling currents for a delay time td and that sufficiently long for the excited state to be relaxed, respectively. (b) Sine- and rectangular-wave
modulations of delay time.

Fig. 2 Relationship between the tentative raw tunneling current induced
by a pair of laser pulses I* and the delay time between the two pulses(left), producing the delay-time dependence of the signal, i.e., thetemporally averaged tunneling current I (right).

between the two pulses (Fig. 2, left). When the delay time is large, thechange in the tunneling current induced by each of the two laser
pulses independently contributes to the signal, i.e., the temporally
averaged tunneling current I, and the magnitudes of the change are
the same. For a delay time shorter than the relaxation time of theexcited state induced by the first pulse, if nonlinear interference exists
between the excitations generated by the two pulses, the tunnelingcurrent induced by the second pulse depends on the delay time. In
such a case, the signal I also changes as a function of delay time.
Thus, the dynamics of the electronic structure of the target material
can be probed using the tunneling current with the time resolution of
the pulse width, i.e., with femtosecond resolution.
Since a time-resolved STM signal is weak, the use of a lock-in
detection method is essential. Excitation is modulated at a certain
frequency and the corresponding change in tunneling current ismeasured. To avoid the effects of thermal expansion and thedisplacement current caused by photoexcitation, the delay time
between the pump and probe laser pulses is modulated instead of thelaser intensity.
In the first system, the delay time was mechanically controlled bymoving the mirror position to change the optical length, which was
758 | Nanoscale, 2012, 4, 757–761

modulated in the form of a sine wave and the differential value dI/dt
was measured.23 In the second-generation system, as shown in Fig. 1,
the pulses are selectively transmitted using a pulse picker, and thedelay time is controlled digitally.29 With this system, repetitive
switching (rectangular modulation) between two delay times whose
difference is larger than the relaxation time of the photocarriers is
possible. The modulation frequency for lock-in detection was 20 Hz
when the optical length was mechanically changed by controlling themirror position, which can be increased to 1 kHz or more through theelectronic control of the delay time. Therefore, the measurement ishardly affected by low-frequency fluctuations in the laser intensity
and tunneling current. In addition, the modulation becomes free frommechanical noise, which causes a critical problem in the case of
optical-length control by moving the mirror position, thus providing
a high signal-to-noise ratio. Furthermore, in the case of rectangularmodulation, we can carry out accurate measurement and directly
obtain the absolute value of the signal instead of the differential value,
which is obtained by sine-wave modulation, i.e., DI ¼ I(td)  I(N),
where I(td) and I(N) are the average tunneling currents for a delay
time td and that sufficiently long for the excited state to be relaxed,
respectively (Fig. 1(b)).29
The nonlinearity between two excitations by paired pulses depends
on the material under observation. In SPPX-STM on a semiconductor, a nanoscale metal–insulator–semiconductor (MIS)
structure is formed by the STM tip, the tunneling gap and the sample.
Under a reverse bias voltage, tip-induced band bending (TIBB) is
induced owing to the leakage of the bias voltage. With photoexcitation, the redistribution of photocarriers reduces TIBB, i.e., SPV,
which subsequently relaxes to the original state through two
processes. One is the decay of the photocarriers on the bulk side via
recombination, drift and diffusion (bulk-side decay). Absorption
bleaching on the bulk side, similar to that observed in the conventional OPPR technique, produces a change in the SPV, resulting ina change in tunneling current that depends on the delay time, thereby,
we can probe the decrease in the density of photoexcited carriers ina semiconductor sample, i.e., bulk-side decay. The other is the decayof carriers trapped at the surface (surface-side decay) that appears
following the bulk-side decay, which was discussed in detail in theanalysis of the gap state effect observed for the Co nanoparticle/GaAs
system.29
This journal is ª The Royal Society of Chemistry 2012

In this study, we analyzed carrier dynamics through the probing of
the bulk-side decay. We first carried out OPPR measurement and
SPPX-STM on three different samples, GaAs, low-temperaturegrown GaAs (LT-GaAs) and GaNAs, and compared the results. Then
we applied SPPX-STM to the analysis of a GaAs–PIN junction toprobe the effect of the local potential landscape on carrier dynamics,
which cannot be obtained by the conventional optical pump-probe
technique. OPPR measurement was carried out in air, while for SPPXSTM measurement, samples were prepared by cleaving them ina vacuum (5  107 Pa), and experiments were carried out under anultra high vacuum (UHV) condition (2  108 Pa).
Fig. 3 shows the spectra obtained by the OPPR technique and
SPPX-STM for LT-GaAs, GaNAs and GaAs. Since the intensities ofpump and probe pulses were adjusted to be the same, the SPPX-STM
signal has a symmetry with respect to the longitudinal axis at td ¼ 0.
The value of 500 ns was chosen as td to obtain I(N). The decay
constants for each sample were obtained by single exponential curve
fitting of the SPPX-STM spectra to be 2.4 ps, 440 ps and 4.8 ns for
LT-GaAs, GaNAs and GaAs, respectively. The defects formed byexcess As atoms in the LT-GaAs sample, which were introduced
during the LT growth of the sample by molecular beam epitaxy(MBE), and the degradation of the crystal quality of the GaNAs
sample owing to the incorporation of N are considered to enhance
the recombination of photoexcited carriers in comparison with that in
GaAs.31,32 In fact, considerably shorter carrier decay was observed for
LT-GaAs and GaNAs compared with GaAs. Furthermore, thesevalues are in good agreement with those obtained by OPPR
measurement (1.5 ps for LT-GaAs, 410 ps for GaNAs and 3.3 ns for
GaAs), thus indicating the high reliability of SPPX-STM measurement. Since the signal is averaged over the photoilluminated area inthe case of OPPR measurement, the slight differences observed areconsidered to originate from, for example, the effect of local electronic structures such as gap-state density. Since surface-side decay,

Fig. 3 Comparison of the spectra obtained by OPPR and SPPX-STM
measurement for (a and b) LT-GaAs, (c and d) GaNAs and (e and f)
GaAs, respectively. The sample bias voltage and tunneling current were(a) VS ¼ +5.5 V, It ¼ 45 pA, (c) VS ¼ +4.0 V, It ¼ 200 pA, and (e)VS ¼ +3.0 V, It ¼ 500 pA, respectively. For OPPR measurement, thesetup for SPPX-STM was used but the measurement was carried out inair. The derived decay constants are: (a) 1.5 ps, (b) 2.4 ps, (c) 410 ps, (d)440 ps, (e) 3.3 ns and (f) 4.8 ns. td ¼ 500 ns was chosen for I(N).

This journal is ª The Royal Society of Chemistry 2012

which is not considered here, has a decay constant much larger thanthat of bulk-side decay for these materials, the signal does notdecrease to zero in this time range.
Next, we applied SPPX-STM to the GaAs–PIN junction witha different potential landscape. Fig. 4 shows schematic diagrams of
the GaAs–PIN junction that we used as the sample and its built-in
potential. A p-type GaAs buffer layer (Be doping concentration: 2 1018 cm3, thickness: 250 nm), p-type GaAs (Be doping concentration: 1  1018 cm3, thickness: 500 nm), nondoped GaAs (i-GaAs,
400 nm) and an n-type GaAs substrate (Si doping concentration: 1 1018 cm3, thickness: 1000 nm) were deposited on a p-type GaAs(100)substrate (Be doping concentration: 5  1018 cm3). Au was deposited
on both sides of the sample to form an ohmic contact to enable theapplication of bias voltage to the sample. In this experiment, we useda short-circuit condition and, as shown in Fig. 4(b), a built-in
potential was induced in the sample, which was confirmed bymeasuring the SPV.
Fig. 5(a)–(d) show four images from a series of 2D maps of timeresolved signals. Instead of obtaining the full spectrum by changing
the delay time, the STM tip was scanned with the delay time fixed at(a) 0, (b) 2, (c) 4 and (d) 11 ns. Namely, the images show the carrier
density at each delay time after photoexcitation at t ¼ 0. As describedabove, since a reverse bias voltage between the STM tip and thesample is necessary to produce TIBB and its relaxation by photoexcited carriers (SPV), which are both required for the measurementof the time-resolved signal, only the left half of each figure isconsidered here because measurements were carried out under theconditions of VS ¼ 2.2 V (sample bias voltage) and It ¼ 100 pA (setpoint current). Namely, the photoexcited electron density in the
conduction band was probed since a negative sample bias voltage wasapplied in this experiment. To observe the dynamics in the right halfof the figures, it is necessary to change the sign of the bias voltage.
Since the combination of TIBB and SPV is the basic mechanism forbulk-side decay in this case, the depth information originates from thearea where the drift of photocarriers is influenced by TIBB dependingon the bias voltage applied between the tip and sample, i.e., several
tens of nanometres in the present case.
The decay constant sb is obtained from the full series of 2D maps
by fitting the change in the carrier density at each point, a map of
which is shown in Fig. 5(e). Fig. 5(f) shows the cross-section along theline in Fig. 5(e). The variation observed in the cross-section is partly
caused by the low signal-to-noise ratio owing to the probing of the
weak signal.
As described above, absorption bleaching is the mechanism forproviding the SPPX-STM signal for the bulk-side decay. The numberof photocarriers excited by the second optical pulse (probe pulse)

Fig. 4 Schematic diagrams of (a) a GaAs–PIN junction and (b) its builtin potential.

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These results clearly indicate that what is observed by SPPX-STM
is the change in the carrier density in the region where nanostructures
are probed, including the effects of diffusion and drift in the local
potential, rather than the ordinary lifetime observed by OPPR
measurement. In such a case, the spatial resolution is determined by
the physical processes in the observed sample. As has been reported in
our previous paper, local dynamics can be probed with atomic
resolution when the surface-side decay is analyzed.29 By using one ofthese two analysis methods depending on the target processes, we canobtain more detailed information on the carrier dynamics in semiconductors. Since SPPX-STM is applicable if nonlinear interference
exists between the two excitations induced by the two laser pulses, theprobing of other physical processes of various materials is possible,
which is currently being investigated.
We have carried out SPPX-STM measurements on a GaAs–PIN
structure. The direct observation of the effect of built-in-potential on
the carrier dynamics, which cannot be probed by the OPPR technique, was clearly and directly observed, showing the high potentialof this microscopy technique.

Acknowledgements
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.
Fig. 5 Time-dependent maps obtained by SPPX-STM for a GaAs–PIN
structure in Fig. 4. The bias voltage and tunneling current were 2.2 V
and 100 pA, respectively, and the STM tip was scanned over an area of
800 nm  240 nm (1000 points  30 lines) at 100 nm s1 with a fixed delay
time. (a)–(d) show the carrier densities 0, 2, 4 and 11 ns after photoexcitation at t ¼ 0, respectively. (e) Map of the decay constant (sb) obtained
from a full series of time-dependent carrier density maps. (f) Crosssection along the line in (e).

depends on the carrier density excited by the first optical pulse (pumppulse), therefore, the delay-time dependence of the signal reflects thechange in the carrier density induced by the pump pulse, similar tothat in the OPPR method. However, since the probe is tunnelingcurrent, we can obtain local information such as the effect of the local
potential on the carrier dynamics. Immediately after the first pulse
excited the sample (0 ns), photoexcited electrons were homogeneously
generated as shown in Fig. 5(a). After 2–4 ns ((b) and (c)), the electron
density started to decrease in the i-type region while most of theelectrons still remained in the p-type region. However, the electron
density decreased considerably in the entire region after 11 ns ((d)). In
the p- and n-type regions, since the band is flat, recombination anddiffusion are the main processes involved in the decay of photocarriers, similarly to that in Fig. 3. In contrast, since there is a gradientof the potential in the i-type region, it must have an effect on the
carrier dynamics; electron density is considered to rapidly decrease in
the i-type region owing to the drift of electrons toward the n-type
region in the built-in-potential. In fact, as expected, the decay
constant decreases in the i-type region as shown by the cross-sectionin Fig. 5(f). Namely, the carrier density decreases via drift rather thanby recombination in the i-type region. The slight decrease in the decay
constant observed in the p-type region near the p–i interface isconsidered to show the effect of the diffusion of photoexcited electrons toward the i-type region.
760 | Nanoscale, 2012, 4, 757–761

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