INSTITUTE OF PHYSICS PUBLISHINGNANOTECHNOLOGY

Nanotechnology 18 (2007) 044028 (5pp)

doi:10.1088/0957-4484/18/4/044028

Ultrafast photoinduced carrier dynamics
in GaNAs probed using femtosecond
time-resolved scanning tunnelling
microscopy
Yasuhiko Terada, Masahiro Aoyama, Hiroyuki Kondo,
Atsushi Taninaka, Osamu Takeuchi and Hidemi Shigekawa1
Institute of Applied Physics, 21st COE, CREST-JST, University of Tsukuba,
Tsukuba 307-8573, Japan

Received 16 August 2006, in final form 5 December 2006
Published 21 December 2006
Online at stacks.iop.org/Nano/18/044028
Abstract
The combination of scanning tunnelling microscopy (STM) with opticalexcitation using ultrashort laser pulses enables us, in principle, tosimultaneously obtain ultimate spatial and temporal resolutions. We havedeveloped the shaken-pulse-pair-excited STM (SPPX-STM) and succeeded
in detecting a weak time-resolved tunnelling current signal from a
low-temperature-grown GaNAs sample. To clarify the underlying physics in
SPPX-STM measurements, we performed optical pump–probe reflectivity
measurements with a wavelength-changeable ultrashort-pulse laser. By
comparing the results obtained from the two methods with an analysis basedon the nonlinear relationship between the photocarrier density and tunnellingcurrent, we obtained a comprehensive explanation that the photocarrier
dynamics is reflected in the SPPX-STM signal through the surface
photovoltage effect.
(Some figures in this article are in colour only in the electronic version)

Among the numerous approaches previously reported
concerning time-resolved STM [1–7], the shaken-pulse-pairexcited STM (SPPX-STM) has turned out to be promisingfor detecting weak time-resolved tunnelling currents at ahigh spatial resolution [8–13]. In SPPX-STM measurements,
the sample surface under the STM tip is illuminated by asequence of ultrashort laser pulse pairs and the variation in thetunnelling current is measured as a function of the delay time
between the two pulses in each pulse pair. When the transientcurrent has a nonlinear relationship with the laser intensity, theaverage tunnelling current depends on the delay time. Thus,
although we cannot detect an ultrafast transient current using
STM because of the limited bandwidth of the current–voltage
converter, ∼100 kHz, we can observe the temporal changeof the photoinduced phenomena through the change in the
average tunnelling current. The observed time-resolved current
involves various complicated physical processes induced ina sample. Furthermore, SPPX-STM essentially requires a

1. Introduction
At present, ‘smaller’ and ‘faster’ are the main concepts innanoscale science and technology. Indeed, important andinteresting phenomena in various systems, such as those infunctional materials, electronic devices, signal transfer inbiosystems and chemical reactions, have been observed onscales from several tens of a nanometre to subnanometres inspace and from several tens of a picosecond to subpicoseconds
in time. However, it is extremely difficult to simultaneously
obtain spatial and temporal resolutions on these scales.
Therefore, to determine the possibility of realizing such
resolutions, it is necessary to develop a new method, that is,laser-combined scanning tunnelling microscopy (STM) which
will enable us to observe the dynamics of electronic structuresat the ultimate spatial and temporal resolutions.
1 http://dora.ims.tsukuba.ac.jp

0957-4484/07/044028+05$30.00

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Y Terada et al

Table 1. Parameters derived from SPPX-STM and OPPR measurements.
SPPX-STMOPPR

τiOPPR (ps)

Bi

i =1
i =2
a
b

Ai (pA)

τiSTM (ps)(1 mWa )48.6 ± 4.9−21.4 ± 1.3

118 ± 7550 ± 300(−2.02 ± 0.02) × 10−4
(9.57 ± 0.01) × 10−5

τiOPPR (ps)(Averageb )7.59 ± 0.04
406 ± 2

5. 3 ± 1 . 6
400 ± 40

Pump intensity.
Average of the values for all pump intensities in figure 4.

highly nonlinear relationship between the tunnelling current
and laser intensity. Accordingly, the interpretation of SPPXSTM signals is not straightforward [12]. The extraction of
the ultrafast physical properties from observed time-resolved
signals requires a detailed understanding of the physicalprocesses that contribute to tunnelling. In this study, we
examined how the ultrafast photocarrier decay process isreflected in the SPPX-STM spectra.

2. Experimental details
Figure 1. SPPX-STM signal with the fitting function. Adjusted
fitting parameters are summarized in table 1.

Low-temperature-grown GaNx As1−x (x = 0.36%, grown
at 720 K) was used as a sample. The energy bandgap
E g was estimated to be 1.34 eV (923 nm) from thephotoluminescence spectra measured at 300 K. Each of themode-locked Ti:sapphire laser pulses (pulse width <130 fs,
central wavelength: 800 nm, repetition rate: 80 MHz for
SPPX-STM and 90 MHz for optical pump–probe reflectivity(OPPR) measurements) was divided into two pulses to form a
train of pulse pairs (pump and probe pulses) with a delay time.
We controlled the delay time between pump and probe pulses
by changing the optical length of the delay system.
For SPPX-STM measurements [8, 10], the sample surface
under the STM tip was illuminated at the average intensity of
0.8 mW and the spot diameter of ∼10 μm. Thus the averagetunnelling current was measured as a function of the delay timeτ . For simplicity, the pump and probe powers were adjusted
to be the same to avoid the asymmetry in the SPPX-STM
signal against τ = 0 [12]. The STM set-point tunnellingcurrent and sample bias voltage were 200 pA and 406 mV,
respectively. The delay time was modulated at 31 Hz with
an amplitude of 6 ps, and the corresponding change in thetunnelling current, d(I )/dτ with respect to the delay time,
was measured by the lock-in detection technique. The time
constant of the lock-in amplifier was 100 ms and the decay
slope was −24 dB/oct. The time-resolved tunnelling currentI was obtained by integrating the raw signal.
For OPPR measurements, the average power of the probe
beam was set to be 1/10 that of the pump beam, both ofwhich were focused to a 10 μm diameter spot on the sample
surface. The pump beam was chopped at 80 Hz and thecorresponding change in probe reflectivity was measured as afunction of the delay time using a silicon photodiode and lockin amplifier. The maximum density of photoexcited carriers
was estimated to be 2.8 × 1017 cm−3 from the pump power
density of 950 mW cm−2 and the absorption coefficient of
3.4 × 104 cm−1 .

3. Results and discussion
3.1. SPPX-STM signal
3.1.1.
SPPX-STM spectra.
First, let us examine thephotoinduced dynamics observed using SPPX-STM. Figure 1
is the raw data of the differentiated tunnelling currentd(I )/dτ (blue/dotted) and the numerically integrated timeresolved tunnelling current I (red/solid). Since the integral
constant cannot be determined in the experiment, the left
end of the I curve was taken to be zero. Figure 1 also
shows the fitting function (black/dashed–dotted) consistingof the sum of two exponential functions and a constant,
i Ai exp(−τ/τiSTM ) + const (i = 1, 2), that gives the best
fit to the I curve. Parameters adjusted to fit are summarizedin table 1. Since the I curve is sufficiently symmetric against
τ = 0, we hereafter consider only the positive region τ > 0.
The I curve consists of two decay components thatreflect the ultrafast phenomena induced by laser illumination.
The faster component (τ1STM = 118 ps) has a positive
amplitude (A1 > 0), while the slower one (τ2STM = 550 ps)
has a negative amplitude (A2 < 0). The characteristicof the faster component (Type-1) is similar to that observed
previously [8–10], but the slower one (Type-2) was observed
for the first time because the time constant τ2STM (550 ps) wasmuch longer than the scan range limit of the delay time in theprevious experimental setup. The characteristic properties of
the two components were similar for both negative and positive
sample bias voltages.
In the next section, we will consider how the photocarrier
decay process is reflected in the delay time dependence of I .
3.1.2. What can be observed in the SPPX-STM signal? As
discussed in the previous paper [12], a nonlinear responseof the transient tunnelling current to the laser intensity2

Nanotechnology 18 (2007) 044028

Y Terada et alis necessary to produce a delay-time-dependent averagetunnelling current (I curve). The nonlinear dependenceof tunnelling current on laser intensity is classified into two
steps: (i) nonlinearity between the physical parameters of
the samples and laser intensity and (ii) nonlinearity between
the tunnelling current and the physical parameters. Here,the physical parameters include photocarrier density, plasmon
density, electron temperature, electric polarization, lattice
vibration amplitude and any other optically induced parametersthat influence the tunnelling current.
Among the possible processes, the excitation andrelaxation of photocarriers are considered to dominantly affectthe I curve. We hereafter focus on the photocarrier
dynamics. In the previous study, the relationship between
the decay of carrier density in real time and the I curve
was treated with a simple model, which revealed that I
curves exhibit different behaviours for the two different typesof nonlinearity: (i) divergent and (ii) saturation-prone [12].
The divergent and saturation-prone nonlinearities are
expressed here as It (t) ∝ n(t)k (k > 1) and It (t) ∝ n(t)k (k <1), respectively, where It is the transient tunnelling current
and n is the photocarrier density. Under the assumption of
exponential decay of carrier density, n(t) ∼ exp(−t/τdecay ),the divergent and saturation-prone nonlinearities result incharacteristics similar to those of the faster (Type-1) and slower(Type-2) components observed for the SPPX-STM signal infigure 1, respectively. For the divergent nonlinearity I has its
maximum at τ = 0 and decreases with increasing τ , whereas
for the saturation-prone nonlinearity I is minimum at τ = 0
and increases with τ .
The Type-2 process is related to surface photovoltage(SPV). Under the STM measurement conditions on asemiconductor sample, the STM configuration has a metal–
insulator–semiconductor (MIS) structure in which the STM tip
induces a band bending in the sample surface in the dark [14].
When the surface is photo-illuminated, the band is flattened
because of the spatial redistribution of photoexcited carriers.
The resulting change in surface potential is referred to as
SPV, which can be measured by STM with a high spatial
resolution [14–17]. The SPV modifies the effective bias
voltage at the tunnel junction, hence the relationship between
the photocarrier density and SPV has a strong nonlinearity.
The nonlinearity between the photocarrier density and SPV
is saturation-prone because the SPV readily saturates with anincreasing laser intensity. Therefore, the decay process ofthe photocarriers appears in the I curve with the Type-2
characteristic.
For the Type-1 component, the dynamics may berelated, for example, to multi-excitation of carriers, hot-carrier
temperature, emission and reabsorption of hot phonons, theplasmon effect, carrier diffusion and drift. However, it is still
under consideration and the mechanism of the Type-1 decay isleft open for future investigation.
In the next section, we will discuss the results obtained
from OPPR analysis.

(a)

(b)

Figure 2. Schematic illustrations of the (a) cooling and (b) decay
processes of the photoexcited carriers.

distribution on the subpicosecond timescale and are thencooled via phonon emission on a timescale of a few
picoseconds [18]. The carrier cooling or energy relaxation
occurs through various phonon scattering mechanisms whichtransfer the excess energy to the lattice, and the main one is thescattering of longitudinal optical (LO) phonons (figure 2(a)).
In addition to the carrier cooling, the carrier density decays
on the same or longer timescale through various processes:
diffusion, recombination or trapping in deep states or defects(figure 2(b)). Although both the carrier cooling and decay
may involve various processes, in the simplest picture under
the assumption of a parabolic energy band structure, all theeffects related to the cooling and decay can be incorporated
in the phenomenological carrier cooling and decay times, τcooland τdecay , respectively. In this approach, the lifetime ofphotocarriers is identical to τdecay .
This simple approach, however, may not be valid for
the GaNAs sample because the incorporation of nitrogencauses a change in the modification of the conduction band
structure [19–21]: narrowing of the bandgap, increase in
the effective electron mass and conduction-band splitting into
E− and E+ bands. Although the former two effects canbe incorporated in the parabolic approximation by adopting
suitable values for the model parameters, the third one cannot
be included; that is, the band splitting would produce different
decay processes for photocarriers instead of a single decay time
τdecay .
Therefore the parabolic approximation is reasonable only
when the photon energy is below the bandgap energy for theE+ band, 1.74 eV [20], which is satisfied for the photon energy
region (1.32–1.72 eV) of our experiment. In order to confirm
this point, we performed OPPR measurements for various laser
wavelengths (figure 3(a)). The band structure of the sample isilluminated by the cross section of OPPR traces (figure 3(b)),
the ‘photon energy dependence of OPPR spectra’, where each
spectrum shows a derivative-like feature at the photon energy
corresponding to the red-shifted bandgap E g (=1.34 eV). In
addition to the bandgap of GaNAs, a small derivative feature
is observed at 1.43 eV that might be attributed to the band
structure of the GaAs substrate 1 μm below the GaNAs
layer. No derivative-like feature other than these two features
exists with excitation energies between 1.32 eV (940 nm) and1.72 eV (720 nm) as expected. The decay process could
therefore be analysed using a single decay time τdecay , and thelifetime experimentally obtained was almost photon-energyindependent in this region, as will be described in the next
section.

4. Results of OPPR analysis
4.1. Band structure of GaNAs sample
In direct bandgap semiconductors such as GaAs, thephotoexcited carriers form a hot, thermalized energy
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Y Terada et al

(a)

(a)

(b)

(b)

(c)

Figure 3. (a) OPPR spectra obtained for various laser wavelengths.
(b) Cross sections of the OPPR traces in (a).
Figure 4. (a) OPPR traces obtained for various pump intensities. The
excitation energy is 1.55 eV (800 nm) and the fitting functions (blacklines) are shown together. (b) Results of the simulation with respect
to the procedure reported in [20]. (c) Pump intensity dependence of
the amplitudes and decay times of the two components of the OPPR
signal.

4.2. Estimation of photocarrier lifetime by OPPR analysis
The lifetime of photocarriers was determined from the
OPPR traces through fitting under the assumption of thesum of two exponential functions and a step function,
i Bi exp(−τ/τiOPPR ) + B3 (i = 1, 2). The distortion due tothe finite temporal width of laser pulses and the time constantsof lock-in detection were taken into account.
Figure 4(a) shows the OPPR traces with the excitationenergy of 1.55 eV (800 nm) and the best fits to the data.
The fitting parameters are listed in table 1, together with thevalues obtained from SPPX-STM measurement. In order to
identify the parameters with carrier cooling and decay times,
we simulated the OPPR traces according to the procedurereported by Prabhu et al [23]. Our calculation took into accountthe Coulomb enhancement factor and free carrier absorption
but not bandgap renormalization for the sake of simplicity.
We assumed a parabolic conduction and two valence bands
with the electron, light- and heavy-hole effective masses of
0.14 m 0 [22], 0.082 m 0 and 0.5 m 0 , respectively, where m 0
is the electron rest mass.
The result of the calculation with the photocarrier densityof 9.7 × 1016 cm−3 (laser intensity of 1 mW), τcool = 5.3 psand τdecay = 400 ps (figure 4(b)) is in good agreement with the
OPPR traces, indicating that the shorter time constant τ1OPPR is
attributed to the carrier cooling time τcool , while the longer one
τ2OPPR contributes to the carrier decay time τdecay .

Since it is difficult to accurately determine the spot
diameter, and hence the effective laser intensity on the samples,the photocarrier densities may differ between OPPR and
SPPX-STM measurements. This may cause a difficulty in the
direct comparison of the lifetime with the SPPX-STM time
constants if the lifetime is dependent on the laser intensity. This
is not the case, as is revealed by the decay time dependence on
the pump intensity (figure 4(c)). The values of τ2OPPR are almost
constant for different intensities. We took the average of τ2OPPR
obtained for the different intensities, 400±40 ps, as the value ofthe carrier lifetime. The lifetime obtained is almost the same asthat, 360 ± 160 ps, similarly estimated from the OPPR traces
with the excitation wavelength of 920 nm. The consistencyof the lifetime analysis using different wavelengths confirms
the validity of the estimation method, because photocarrier
lifetimes in semiconductors should be only weakly dependent
on the excitation wavelength [24].
These results indicate that the estimated photocarrier
lifetime (=400 ± 40 ps) should be related to the slower decay
component (τ2STM = 550 ps) of the I curve that also reflects
the carrier lifetime through the decay process of SPV.
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Y Terada et al

5. Comparison of photocarrier lifetime determinedby OPPR with SPPX-STM signal

Yamashita in our group at The University of Tsukuba for her
help in preparing this paper.

The photocarrier lifetime τdecay (=400 ± 40 ps) determinedfrom the OPPR measurement is close to τ2STM (=550 ps)
in the SPPX-STM signal, as expected. However, τ2STM isslightly longer than τdecay . In order to clarify this point,
we recall the fact that the exponential decay of a sample
parameter does not appear in the I curve as an exponentialfunction. Furthermore, the apparent timescale of the calculated
I curve can be different from τdecay . Therefore, the fitting
procedure with exponential functions for I curves yields
an approximation for deriving decay times of the sample
parameters. For saturation-prone nonlinearity, the apparent
decay time in the calculated I curve is longer than the
carrier lifetime, as reported in the previous paper [12].
Experimentally, τ2STM is slightly longer than the estimatedcarrier lifetime τdecay , which is in good agreement with thetheoretical results.

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6. Conclusion
We examined how the photocarrier dynamics is observedin the SPPX-STM signal (I curve), using the resultsof OPPR measurements and of the analysis based on the
nonlinear relationship between the photocarrier density and thetunnelling current. A comprehensive explanation was obtained
for the SPPX-STM signal using the model based on thephotocarrier dynamics that produces the surface photovoltage
effect. This new technique is expected to contribute to
the advance of future research in nanoscale science andtechnology, in terms of the ultimate temporal and spatial
resolutions.

Acknowledgments
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.
We thank
Professor Yoshitaka Okada and Dr Ryuji Oshima at the
University of Tsukuba for their help with sample preparationand the photoluminescence study. We also thank Ms Rie

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