Hindawi Publishing Corporation
Advances in Optical Technologies
Volume 2011, Article ID 510186, 9 pages
doi:10.1155/2011/510186

Research Article
Laser-Combined Scanning Tunneling Microscopy on the Carrier
Dynamics in Low-Temperature-Grown GaAs/AlGaAs/GaAs
Yasuhiko Terada, Shoji Yoshida, Osamu Takeuchi, and Hidemi Shigekawa
Institute of Applied Physics, University of Tsukuba, Tsukuba 307-8573, Japan
Correspondence should be addressed to Hidemi Shigekawa, hidemi@ims.tsukuba.ac.jp
Received 31 May 2011; Accepted 26 August 2011
Academic Editor: Qingming Luo
Copyright © 2011 Yasuhiko Terada et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
We investigated carrier recombination dynamics in a low-temperature-grown GaAs (LT-GaAs)/AlGaAs/GaAs heterostructure bylaser-combined scanning tunneling microscopy, shaken-pulse-pair-excited STM (SPPX-STM). With the AlGaAs interlayer as abarrier against the flow of photocarriers, recombination lifetimes in LT-GaAs of 4.0 ps and GaAs of 4.8 ns were successfully
observed separately. We directly demonstrated the high temporal resolution of SPPX-STM by showing the recombination lifetimeof carriers in LT-GaAs (4.0 ps) in the range of subpicosecond temporal resolution. In the carrier-lifetime-mapping measurement,a blurring of recombination lifetime up to 50 nm was observed at the LT-GaAs/AlGaAs boundary, which was discussed inconsideration of the screening length of the electric field from the STM probe. The eﬀect of the built-in potential on the signal,caused by the existence of LT-GaAs/AlGaAs/GaAs boundaries, was discussed in detail.

1. Introduction
The development of semiconductor physics and devices has
been progressing with the innovation of atomically controlled fabrication technologies [1–3]. However, the carrier
dynamics in materials have been analyzed only by evaluating
macroscopic functions, or with techniques that provide spatially and/or temporally averaged information. Microscopic
carrier dynamics has not been directly observed, although
it is fundamental for the device functions. Therefore, there
has been an increasing desire for developing a microscopic
technique that allows the investigation of carrier dynamics athigh spatial and temporal resolutions.
Laser-combined scanning tunneling microscopy (STM)
is a promising candidate, because the tunneling junction
itself responds on a time scale of a few femtoseconds
and thereby ultrafast dynamics can principally be studied
by STM with ultrashort pulse laser techniques [4–12].
Recently, we have reported a novel microscopy techniquethat enables the visualization of subpicosecond carrier
dynamics in nanometer-scale structures [13], that is, shakenpulse-pair-excited scanning tunneling microscopy (SPPXSTM) with a novel delay-time modulation method basedon a pulse-picking technique [14–21]. A nonequilibrium
carrier distribution is generated with ultrashort laser pulses
and its relaxation processes are probed using STM by apump-probe technique. We proposed a model clarifying
the physical origin of the signal for semiconductors andverified it experimentally. The SPPX-STM signal consists of
two components, that is, a faster one reflecting the decay
process of bulk-side photocarriers and a slower one reflecting
the decay process of surface-side photocarriers. Bulk-side
carrier lifetimes in various semiconductor materials weresuccessfully measured using tunneling current, and the holecapture rate by the gap states formed by the nanometer-scale
Co islands on GaAs was directly obtained with an atomic
spatial resolution through the analysis of the surface-side
carrier dynamics.
In this paper, we focus on the SPPX-STM measurement on the bulk-side carrier dynamics in a nanostructure consisting of materials with diﬀerent recombination
lifetimes, namely, a low-temperature-grown GaAs (LTGaAs)/AlGaAs/GaAs heterostructure. The factors determining the spatial resolution of SPPX-STM for the measurementof bulk-side decay are discussed in detail.

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Advances in Optical Technologies

frep
Laser (master)

1/N

frep /N

Delay time td

Pulse
pickerSynchronization
Laser (slave)

Pulse
pickerfmod

Reference
ΔI (in-phase)

Lock-in
amplifierI

Optical delay
generator(optional)

UHV-STM

Vs

SignalI-V
converter

Figure 1: Schematic of SPPX-STM.

2. Experimental Methods
2.1. SPPX-STM Measurement. Figure 1 shows a schematicof the SPPX-STM system with the pulse-picking technique,the details of which are described elsewhere [13]. In short,each of two (pump and probe) pulse trains generated by
two synchronized Ti:sapphire lasers (Mira and Chameleon,
Coherent Inc, central wavelength = 800 nm, repetition rate =90 MHz, pulse width = 140 fs) was guided to a pulse
picker, which consisted of an ultrafast Pockels cell andpolarizers. The delay time between the two pulse trains wascontinuously varied from zero to the pulse interval (∼11 ns)
by the synchronizing circuit (Synchrolock, Coherent Inc.), or
varied from zero to 1 ns by an ordinary optical delay line.
The pulse picker selectively transmitted typically one pulse
per every 90 pulses, resulting in a pulse train with a reduced
repetition rate of f p (typically 1 MHz). Then, the two pulse
trains were arranged onto the same optical axis and guided
into the STM (VT-STM, Omicron Nanotechnology GmbH).
The light spot, which had a diameter of less than 10 µm andan average intensity of up to several milliwatts, was focusedon a sample surface below the STM tip using lenses placedoutside the UHV chamber (base pressure < 1 × 10−8 Pa).
All the experiments discussed in this paper were carried out
at room temperature. In most cases, SPPX-STM signal isso weak that lock-in detection with a modulation technique
is required for observation. In the conventional opticalpump-probe experiments, the laser-intensity modulation is
generally used, but this is not directly applicable to SPPXSTM because it produces thermal expansion and shrinkingof the STM tip in synchronization with modulation, andconsequently results in a large change in tunneling current.
A promising way to overcome this diﬃculty is to modulatedelay time instead of laser intensity. Delay time modulation was achieved by changing the pulse-picking timing
periodically at a frequency of ∼1 kHz. The delay time wasmodulated in a rectangular form between a relatively smallvalue td (1 ps–1 µs) and a very large value (0.5 µs–severalµs), by which the ultrafast phenomena reached the steadystate. Synchronously with the delay time modulation, thetunneling current signal from the STM preamplifier with abandwidth of ∼10 kHz was phase-sensitively detected using
a lock-in amplifier. By virtue of the rectangular modulationof the delay time, the output of the lock-in amplifier was
proportional to the diﬀerence between the tunneling current
at the two delay times. In particular, when the larger delay
time was set to be suﬃciently long for transient phenomena
to reach the steady state, the signal can be regarded as
proportional to ΔI = I(td ) − I(∞). The sign of td was defined
to be positive when the pump pulse illuminated the sample
first. Unlike the optical pump-probe technique, here, pumpand probe pulses were adjusted to have the same intensity
to produce a symmetric ΔI curve with respect to the origin[17].
2.2. LM-STS Measurement. Light-modulated scanning tunneling spectroscopy (LM-STS) was used for analyzing theband structure of the sample [19, 21–25]. In LM-STM
measurement, the sample under STM is intermittently
illuminated by an electrically chopped laser source and thesample bias voltage is swept to obtain tunneling current versus sample bias voltage (I-Vs ) curves underilluminated anddark conditions simultaneously, which provides information
for analyzing the band structure of the sample. In STM on asemiconductor sample in the dark condition, the bias voltage
applied between the tip and the sample penetrates into thesample within the screening length and causes, so called, tipinduced band bending (TIBB). When the sample is photoilluminated, the redistribution of photocarriers reduces theband bending, that is, surface photovoltage (SPV). The
amount of SPV, thus TIBB, is estimated from the shift of
the two I-Vs curves, providing information about the carrier
density due to doping in the dark condition.
2.3. Sample Preparation. A layer of LT-GaAs (1 µm) was
grown at 250◦ C by molecular beam epitaxy on a barrier
layer of Alx Ga1−x As (x = 0.5) (1 µm) grown on an undoped

Advances in Optical Technologies

3

GaAs substrate, which was annealed at 700◦ C for 60 s in H2(5%)/Ar ambient. The energy bandgaps are 1.43 eV for GaAs
and LT-GaAs and 1.97 eV for AlGaAs. The excitation photon
energy was 1.55 eV (wavelength = 800 nm), and thus, nophotocarriers are generated in AlGaAs. The heterojunctions
were type I, and the AlGaAs interlayer is expected to act as abarrier that prevents photocarriers generated in the adjacent
LT-GaAs and GaAs layers from mixing with each other.
The sample was cleaved in ultrahigh vacuum (UHV), andthe cleaved surface, including GaAs/AlGaAs and AlGaAs/LTGaAs interfaces, was used for measurements. The sample wasclamped by a metallic holder, and thereby, the sample bias
voltage was applied through both the ends of the layered
structure as shown in Figures 1 and 3.

3. Results and Discussion
3.1. Band Structure Investigated by LM-STS. Figure 2 shows
an STM image of the cleaved surface. The LT-GaAs/AlGaAs
and AlGaAs/GaAs interfaces, which extend in the longitudinal direction in this figure, are clearly distinguishable andthey were used to determine the tip location. Then, we
performed LM-STS measurement to investigate the band
structure of the sample (Figure 3). As described later, its
detailed understanding is essential for interpreting the timeresolved signals (ΔI) obtained by SPPX-STM. For example,
not only the local potential below the STM tip but also the
distribution of the potential over the sample, such as an innerpotential, influences the result of SPPX-STM measurement,
and LM-STM is useful to discriminate such eﬀects from eachother.
As described in Section 2.2, I-V curves under illuminatedand dark conditions are simultaneously obtained by LMSTM measurement, which provides the information foranalyzing the band structure of the sample. First, the I-Vs
curve in the dark for GaAs (blue line in Figure 3(b)) reveals
that the tunneling current does not flow unless a bias voltage
much larger than the bandgap energy is applied. This isbecause large TIBB occurs at both positive and negative bias
voltages [20, 22–25]. This tendency is also present for AlGaAs
and LT-GaAs (Figures 3(c) and 3(d)). These results indicatethat the impurity density is extremely low for all the layers
and the materials are semi-insulating, which is reasonablebecause the sample is undoped. Meanwhile, although theonset voltage is 2.5 V for Vs > 0 for all regions, for Vs <0 the onset voltage is −2.5 V for GaAs, whereas it is notobserved at Vs > −3 V for both AlGaAs and LT-GaAs.
This indicates that for AlGaAs and LT-GaAs, the electron
density is extremely low compared with the hole density,and consequently, these layers exhibit a p-type-like feature
although they are undoped.
Second, the I-Vs curves show that a larger current flows
under the illuminated condition for GaAs and LT-GaAs (redlines in Figures 3(b) and 3(d)) than under the dark condition(especially for Vs < 0). This is because the band, which isbent in the dark owing to TIBB, relaxes towards the flatband state, resulting in the generation of SPV [23, 25–28].
The change in current is larger for Vs < 0 than for Vs > 0.

Figure 2: STM image of cleaved surface of GaAs/AlGaAs/LT-GaAs
sample. Vs = 5.5 V, I = 45 pA.

This also ensures that the GaAs and LT-GaAs layers are ptype-like [29, 30]. Here, however, the change in current due
to photo illumination is also observed at Vs > 0, which
is not supposed to occur for intentionally p-doped samples
because negligible band bending is produced for the forward
bias condition. In the present case, the density of majority
carriers (hole) is quite low as described above. Thus, TIBB
and the resulting SPV are noticeable even under a forward
bias condition.
In contrast, for AlGaAs, the tunneling current under
the illuminated condition is almost the same as that in
the dark (Figure 3(c)). This indicates that no photocarriers
are generated and the AlGaAs layer works well as a barrier
against the flow of photocarriers from the adjacent LT-GaAs
and GaAs layers, as expected. If photocarriers had flowedfrom GaAs or LT-GaAs into AlGaAs, TIBB in AlGaAs would
have been reduced, as observed at the pn junction of GaAs
in previous studies [23–25]. The slight changes seen in the
red and blue curves in Figure 3(c) are due to the eﬀect ofphotovoltage at the LT-GaAs/AlGaAs/GaAs interfaces, which
will be discussed in Section 3.4.
3.2. Time-Resolved Signals for Each Region. Figure 4 shows
time-resolved signals obtained in each region. The ΔI(td )curve for GaAs (Figure 4(a)) exhibits an exponential-decay
component with the time constant of 4.8 ns. The origin of
the time-resolved signals is discussed in detail in [13]. In
short, for nearly semi-insulating samples, TIBB occurs in thedark at both positive and negative bias voltages. Figure 5(a)shows the band diagram in the depth direction under the
positively biased condition. As the sample is illuminatedby the optical pulse, photogenerated minority carriers are
redistributed immediately, TIBB vanishes, and the band
becomes flat, resulting in an increase in the tunneling current(Figure 5(b)). This is followed by carrier decay in thebulk side, where the band still remains flat (Figure 5(c)).
The ΔI(td ) signals shown in Figure 4 reflect the bulk-side
decay process through the absorption bleaching mechanism

4

Advances in Optical Technologies
0.1
GaAs
STM tip
I (nA)

La
ser

0

I
−0.1
−2

Vs
(a)

0
Vs (V)
(b)

0.1

0.1
AlGaAsLT-GaAsI (nA)I (nA)

2

0

−0.1

0

−0.1
−2

0

2

−2

0
Vs (V)Vs (V)(c)2(d)

Figure 3: LM-STS measurement and results. (a) Schematic of measurement setup. (b)–(d) I-Vs spectra obtained for (b) GaAs (200 nm fromthe GaAs/AlGaAs interface), (c) AlGaAs (200 nm from the GaAs/AlGaAs interface), and (d) LT-GaAs (150 nm from the AlGaAs/LT-GaAs
interface). Red and blue curves correspond to the curves with and without illumination, respectively. Oscillation between the two curves isdue to the illumination being switched on and oﬀ at 100 Hz. Vs = 4 V, I = 50 pA.

[31, 32]. A long time later, the surface-side carriers decay(Figure 5(d)) and the band relaxes to the original state. This
process is also visible in SPPX-STM measurement; however,since it is a much slower process than the bulk-side carrier
decay, it appears merely as an oﬀset within the short delaytime range, as shown in Figure 4.
The sign of ΔI(td ) is negative, because the probing
process is based on the mechanism of absorption bleaching.
The reduction in current gives a measure of the photocarrier
density remaining at the illumination of the second pulse.
As shown in Figures 4(a) and 4(c), the bulk-side carrier
decay time is 4.0 ps for LT-GaAs, which is much shorter
than that for GaAs (4.8 ns). This is interpreted as follows:
LT-GaAs contains dense defects that act as sites for carrier
recombination [31, 32] and thus has an ultrashort carrier
lifetime appearing in ΔI [33, 34]. The decay constants are
consistent with the recombination lifetimes obtained by theconventional optical pump-probe technique [13].
The result indicates that AlGaAs works well as a barrier
layer. Otherwise, part of the photocarriers in GaAs may flow
to the LT-GaAs layer. SPPX-STM probes not the recombination rate itself but the temporal and spatial changes

in the carrier density. Thus, it is important to know andmanipulate the band structure of the sample to maximize theadvantageousness of the SPPX-STM measurement.
3.3. Lifetime Mapping. To examine the possibility of lifetime
mapping, we measured line profiles of ΔI across the LTGaAs/AlGaAs interface by slowly scanning the STM tip(∼10 nm/s) with the delay time fixed (Figure 6(a)). The
profiles are presented with 0.5 pA steps for clarity. First, ΔI
at the longest delay (td = +29 ps) shows an oﬀset, which
is attributed to the decay component with a long lifetime
shown in Figure 4. Second, for shorter delay times, ΔI
in AlGaAs shows almost the same value independently of
delay time, while ΔI in LT-GaAs markedly decreases as
the delay time decreases, especially at delay times from
+0.3 to +4.6 ps. The change in ΔI for LT-GaAs on an
ultrashort picosecond timescale is attributed to the change
in the density of bulk-side carriers. No ultrafast response isobserved for AlGaAs because no photocarriers are generated
as described in Section 3.1. The reason why ΔI for AlGaAs
has the oﬀset will be described in Section 3.4. In this way, themicroscopy technique has an advantage in visualizing carrier

Advances in Optical Technologies

5

AlGaAs

GaAs

0

0
−200

ΔI (fA)ΔI (fA)

−200

−400

−600

−400

−600

−800
−10

0
Delay time (ps)

−800

10

−10

0
Delay time (ps)10(b)
LT-GaAs
0−200

ΔI (fA)ΔI (fA)

0

−100

−400

−600

−200

−800

0

5

10

−10

Delay time (ns)0Delay time (ps)

(a)

(c)

10

Figure 4: Results of time-resolved measurement (ΔI versus td curves) for (a) GaAs (400 nm from the GaAs/AlGaAs interface), (b) AlGaAs(180 nm from the AlGaAs/GaAs interface), and (c) LT-GaAs (75 nm from the AlGaAs/LT-GaAs interface) regions. Vs = 5.5 V, I = 45 pA.

dynamics on the ultrasmall scale compared with the optical
wavelength.
Next, we focus on a strange phenomenon appearing near
the interface. The ultrafast component observed in LT-GaAs
is also observed in AlGaAs near the boundary. It spatiallydecays in the AlGaAs region from the boundary with adecay constant of ∼50 nm. Such an ultrafast phenomenon
is unlikely to occur in AlGaAs because of no photocarrier
generation in this region and no photocarrier inflow fromthe LT-GaAs region. Therefore, this component is considered
to reflect the carrier dynamics in LT-GaAs. In fact, theultrafast component observed in AlGaAs has the same decay
constant as that in LT-GaAs within the experimental error.
This indicates the existence of a mechanism that degrades thespatial resolution of recombination lifetime measurement.
In the following, we assume that the blurring is caused by
the spatial broadening of TIBB and discuss its eﬀect on themeasurement.
In the dark, the band bends owing to the electric fieldfrom the STM tip. This TIBB broadens in the in-plane
direction as well as in the depth direction. Here, let us
consider the case in which the STM tip is located in the
AlGaAs region close to the interface. When the sample ispositively biased, the band bends upward (Figure 6(b)-(i)).
When the sample is illuminated, the photocarriers generated
in LT-GaAs are separated by the electric field due to TIBB,
and flatten the band in the region (Figure 6(b)-(ii)). As aresult, the finite SPV generated beneath the STM tip increases
and ΔI measured at the position becomes sensitive to thecarrier decay in the LT-GaAs region. Even when the tip is
located in the AlGaAs region, if the electric field beneaththe STM tip expands to the LT-GaAs region, ΔI is aﬀectedby the change in TIBB (or SPV) at the LT-GaAs side. This
expectation is consistent with the experimental result that the
ΔI in the AlGaAs region near the AlGaAs/GaAs interface has
the same ultrashort decay constant as that in LT-GaAs.
Consequently, the spatial resolution of the bulk-side
carrier density measured by SPPX-STM is determined by
the size of the regions over which TIBB occurs, that is, thescreening length of the sample. In the present case, the lengthis 50 nm. Generally, the screening length is dependent onvarious factors such as the carrier density. The screening
length increases as the doping becomes lower. In the present
case, the undoped AlGaAs has low carrier density, which is
not a favorable condition for the spatial measurement.

6Advances in Optical Technologies

VL

SPV
EF
Excitation

CB

VB
Tip

SampleVacuum(a)(b)

Bulk-side
Carrier decay

Surface-side
Carrier decay

(c)

(d)

Figure 5: Time evolution of in-depth band diagram (a) before excitation, (b) immediately after excitation, (c) during bulk-side carrier
decay, and (d) during surface-side carrier decay. VL, CB, VB, and EF denote the vacuum level, conduction band, valence band, and Fermi
level energy, respectively. Blue and red circles are electrons and holes, respectively.

In [13], we measured the carrier capture rate at theatomic resolution. We could achieve such a high resolution
because we observed the surface-side carrier decay rather
than the bulk-side carrier decay. In that case, the decay
process was dominated by the direct recombination ofphotocarriers and tunneling electrons at the localized stateon the surface, and therefore, the spatial resolution of thetunneling current, that is, the atomic resolution, was realized.
3.4. Eﬀect of Built-In Potential. As shown in Figure 6(a), ΔI
at td = +29 ps is not zero even in AlGaAs. Moreover, theresult shown in Figure 4(b) reveals that the decay time of thiscomponent is also long. This slow component is attributed
to the carrier dynamics in the built-in potential generated
at the LT-GaAs/AlGaAs/GaAs interfaces as discussed below.
Figure 7 shows a band diagram in the in-plane direction.
Since we did not carry out doping by design for this sample,
it is diﬃcult to determine the detailed band structures, forexample, the length where the band bends (depletion width)for each layer. Therefore, the depletion width in AlGaAs
was determined here in accordance with the result shownin Figure 6(a). The depletion widths for other layers should
be longer, but since the exact dimensions are not important
here, they were simply drawn to be the same as that for
AlGaAs. In addition, the band structure at each interface,which depends on the relation of the Fermi levels in bothsides, influences the sign of the time-resolved signal ΔI, as
discussed in [13], but does not aﬀect the following discussion
on ΔI. The diagram in Figure 7 is drawn in consideration of
the IV curves and the sign of the signal observed.
In the dark (Figure 7(a)), holes drift to the AlGaAs/GaAs
interface and the area where the acceptors remain isnegatively charged. Consequently, the built-in potential isformed at the interface in such a direction that the AlGaAs
band is raised. Since the sample is biased on the bothends of the layered sample (Figures 1 and 3), this built-in
potential energetically shifts the bands in the AlGaAs [25].
Furthermore, a similar built-in potential is generated at the
LT-GaAs/GaAs interface through the same mechanism.

Advances in Optical Technologies
AlGaAs7
LT-GaAs0+ 0.3 ps
−0.5

ΔI (pA)+ 0.6 ps−1

+ 4.6 ps(i)
AlGaAs(ii)
AlGaAsLT GaAs−1.5

LT GaAs+ 11 ps−2+ 29 ps−200

0
Position (nm)

Excitation

200
TIBB

(a)

(b)

Figure 6: (a) Line profiles of ΔI at diﬀerent td with the tip scanned across the AlGaAs/LT-GaAs interface. Each profile is oﬀset by 0.5 pA
steps for clarity. (b) Time evolution of in-plane band diagram near the interface: (i) before excitation and (ii) immediately after excitation.
Vs = 5.5 V, I = 45 pA.

VL

Built-in potential

CB

Photoexcitation

VB
GaAs

AlGaAsDrift

LT-GaAs(a)(b)

Relaxation(c)Figure 7: Eﬀect of built-in potential on the band diagram: (a) before excitation, (b) immediately after excitation, (c) relaxation due to the
decay of the photocarrier density at the interface (from dark brown lines to light brown lines).

When the sample is illuminated, the laser spot (several micrometers in diameter) covers the entire LTGaAs/AlGaAs/GaAs regions, and thereby, photocarriers are
generated in LT-GaAs and GaAs (Figure 7(b)). The LT-GaAs
and GaAs bands become flat owing to the photocarrier drift(Figure 7(c), dark brown lines), and thereby, the AlGaAs
band shifts downward because both the ends of the sample
are at the same bias voltage as explained in Section 2.3 (Figures 1 and 3). This is the eﬀect of the photovoltages produced
at the LT-GaAs/AlGaAs and AlGaAs/GaAs interfaces. In thissituation, the vacuum level decreases and the voltage between
the tip and sample increases, resulting in an increase in thetunneling current. Then, the change photoinduced at thebuilt-in potential relaxes, as the photocarriers in the LT-GaAs
and GaAs regions decay (Figure 7(c), light brown lines).
The above relaxation process of the built-in potential also
appears in the time-resolved signal through the change in thetunneling current. The sign of this component changes withthe bias voltage. For Vs > 0, the tunneling current increases
owing to photocarrier generation, and hence, the current
deviation due to absorption bleaching is negative (ΔI < 0).
The sign of the signal measured for AlGaAs is consistent withthis interpretation.
In the GaAs and LT-GaAs regions, ΔI reflects the bulkside and surface-side decay processes through the change
in TIBB and SPV between the tip and the sample. In
contrast, in the AlGaAs region far from the GaAs/AlGaAs and
AlGaAs/GaAs interfaces, no photocarrier is generated andflows from the adjacent layers, and therefore, ΔI reflects not
the change in TIBB between the tip and the sample but thechange in SPV arising from the built-in potential at the twointerfaces. When the tip is located in the AlGaAs region near
the interfaces and the electric field beneath the tip reaches the
GaAs or LT-GaAs region, ΔI is aﬀected by both the fast and8slow decay processes in each region. The slow component
only appears as the oﬀset component within the ultrashorttime range as shown in Figure 4. Since the amounts of the
SPVs at the build-in potentials depend on the absorption
bleaching processes in the GaAs and LT-GaAs regions, a fastcomponent is, in principle, expected to be observed evenin the AlGaAs region as an eﬀect of the build-in potential.
The reason for the no observation of such a signal may
be explained by the fact, for example, that the ratio of thesignal intensities between fast and slow components depends
on the laser intensity, the details of which will be discussed
elsewhere.
3.5. Temporal Resolution. Here, we discuss the temporal
resolution. In the present case, we directly demonstrated
the high temporal resolution of SPPX-STM by showing therecombination lifetime of carriers in LT-GaAs (4.0 ps) in therange of subpicosecond temporal resolution (Figure 6(a)).
Results of the detailed analysis of the shape of signal ΔI(td ),which was discussed in the supplementary information in[13], showed that the temporal resolution is determinedby the duration of the excitation laser pulse (140 fs in thepresent case). The introduction of an excitation pulse witha shorter duration such as 10 fs will enable the probing offaster phenomena with higher temporal resolution.
3.6. Stability of Measurement. Finally we discuss the stabilityof SPPX-STM measurement. A slightly asymmetrical appearance in the ΔI curve was observed as shown in Figure 4(b),and the values of ΔI measured in the AlGaAs region aresmall for td = +4.6 and +11 ps (Figure 6(a)). Since the sample
used in the present study was not intentionally doped, thetunneling current hardly flowed and the measurement was
carried out with the tip-sample distance being fairly closed.
Therefore, as shown in the STM image in Figure 2, thetip was often unstable during the scan and the tunnelingcurrent was noisy, which results in instability in SPPXSTM measurement. For example, the noise increases the tipsample distance, reducing the value of ΔI. This instability is
considered responsible for the asymmetrical appearance inthe ΔI curve shown in Figure 4(b), and for the smaller valuesof ΔI measured in the AlGaAs region for td = +4.6 and +11 ps(Figure 6(a)).

4. Conclusions
We performed SPPX-STM on an LT-GaAs/AlGaAs/GaAs
heterostructure. With the AlGaAs layer as a barrier, theultrashort recombination lifetime of LT-GaAs, 4.0 ps, wasseparately obtained from that of GaAs, 4.8 ns. We directly
demonstrated the high temporal resolution of SPPX-STM byshowing the recombination lifetime of carriers in LT-GaAs(4.0 ps) in the range of subpicosecond temporal resolution.
The spatial resolution was discussed in consideration of
the screening length of the electric field from the STM
probe and the eﬀect of the built-in potential in the sample
structure. Since SPPX-STM is a novel microscopy techniqueand the measured signals are associated with a varietyof physical processes, the interpretation of data is not

Advances in Optical Technologies
straightforward currently. However, as shown in this paper,
careful physical considerations and comparison with results
obtained by other techniques provide a better understandingof the physical processes occurring in the measurement.
Understanding of the basic mechanisms will enhance thevalue of this microscopy technique. With the use of selectedwavelength and circularly polarized illumination, quantum
transitions including exited and/or spin states are expected
to be probed.

Acknowledgments
This work was supported by a Grant-in-Aid for scientificresearch from the Ministry of Education, Culture, Sports,
Science, and Technology of Japan. The authors thankProfessor Y. Hirayama, Tohoku University, for his fruitful
discussion.

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