Nanoscale
View Article Online

PAPER

Published on 11 July 2013. Downloaded by RSC Internal on 27/08/2013 14:53:28.

Cite this: DOI: 10.1039/c3nr02433d

View Journal

Bases for time-resolved probing of transient carrier
dynamics by optical pump–probe scanning tunnelingmicroscopy
Munenori Yokota,a Shoji Yoshida,a Yutaka Mera,†b Osamu Takeuchi,aHaruhiro Oigawaa and Hidemi Shigekawa*a
The tangled mechanism that produces optical pump–probe scanning tunneling microscopy spectra fromsemiconductors was analyzed by comparing model simulation data with experimental data. The

Received 11th May 2013
Accepted 9th July 2013

nonlinearities reﬂected in the spectra, namely, the excitations generated by paired laser pulses with adelay time, the logarithmic relationship between carrier density and surface photovoltage (SPV), and theeﬀect of the change in tunneling barrier height depending on SPV, were examined along with thedelay-time-dependent integration process used in measurement. The optimum conditions required to

DOI: 10.1039/c3nr02433d

realize reliable measurement, as well as the validity of the microscopy technique, were demonstrated forwww.rsc.org/nanoscale

the ﬁrst time.

Introduction
With the current progress in manufacturing quantum functions
in small organized structures, probing of quantum dynamics
with high temporal and spatial resolutions has never been morecrucial for further advances in nanoscale science and technology. Scanning tunneling microscopy (STM) has an excellent
spatial resolution and has been widely used in atomic-scale
research since its invention.1,2 However, since its temporal
resolution is generally limited to the millisecond range, thedevelopment of a new microscopy technique has been desired.
Considerable eﬀort has been devoted to achieving this, andvarious methods and techniques have been developed.3–18
Recently, a microscopy technique that simultaneously realizes the spatial resolution of STM and the temporal resolutionof ultrashort-pulse laser technology has been developed.19–30
The combination of STM with optical technology has advantages that enable the analysis of photoinduced dynamics on thenanoscale as well as the realization of ultrafast time-resolved
microscopy.
In laser-combined STM, a nonequilibrium carrier distribution is generated using ultrashort laser pulses, and its relaxation
processes are probed by STM using the optical pump–probe(OPP) method realized in STM. For a semiconductor sample,
two components, i.e., bulk-side decay and surface-side decay,
which respectively reect the decay of photocarriers on the bulk
side and the decay of minority carriers tentatively trapped at theaUniversity of Tsukuba, Tsukuba 305-8571, Japan. E-mail: hidemi@ims.tsukuba.ac.jp;
Web: http://dora.bk.tsukuba.ac.jp; Fax: +81-29-853-5276; Tel: +81-29-853-5276

b

The University of Tokyo, Department of Applied Physics, Tokyo, Japan† On leave from the University of Tokyo.

This journal is ª The Royal Society of Chemistry 2013

surface, are observed.23–26 By applying the microscopy technique, fundamental carrier dynamics in semiconductors weresuccessfully probed and visualized, using the two-type decay
modes depending on purpose. For the bulk-side decay, thedecay constants obtained for various samples showed good
agreement with those obtained by the OPP method,25–27,31 andthe eﬀect of the built-in potential on the photoexcited carrier
dynamics,27 as well as the direct detection of photoexcited
carrier diﬀusion,28 was successfully observed. For the surfaceside decay, which enables probing of local dynamics at thesurface, even a single-atomic-level analysis of the carrier
dynamics via in-gap states has been demonstrated.25,29
Despite these successes, however, even the conditions for theobservation of the two components have not yet been claried
because of the tangled mechanism of the OPP-STM measurement on a semiconductor. In this study, the mechanism of
producing OPP-STM spectra from semiconductors was analyzed
by comparing model simulation data with experimental data.
The optimum conditions required to realize reliable measurement were demonstrated, for the rst time, with the validity ofthe microscopy technique.
First, the fundamental mechanism of OPP-STM is reviewed. In
OPP-STM, the sample surface beneath the STM tip is illuminatedwith a sequence of paired laser pulses with a certain delay time td
as in the OPP method, and the change in tunneling current DI ismeasured as a function of td. Here, we also call the two pulses
pump and probe pulses, whose intensities are adjusted to be thesame. The optical pulses give rise to current pulses in the raw
tunneling current I*, reecting the excitation and relaxation of
the sample. When td is suﬃciently long compared with therelaxation time of the excited state, the paired optical pulses withthe same intensity independently induce two current pulses with

NanoscaleView Article Online

Published on 11 July 2013. Downloaded by RSC Internal on 27/08/2013 14:53:28.

NanoscaleFig. 1 Schematic illustrations of 1D band structures in OPP-STM measurement of
the bare surface ((a) to (d)) and the surface with the in-gap state (e).

the same height in I*. In contrast, when td is short and the probe
pulse illuminates a sample excited by the pump pulse, the second
current pulse may have a diﬀerent height, depending on td. The
signal DI therefore also depends on td, because the heightdiﬀerence in the second current pulse changes the temporally
averaged value of the tunneling current. Accordingly, the relaxation dynamics of the excited state in the target material, forexample, the decay of carriers aer excitation by a pump pulse,
can be probed by STM.
How does the mechanism work in OPP-STM on semiconductors? Fig. 1 shows schematic illustrations of the onedimensional (1D) model of a tunnel junction. Under a reverse
bias voltage condition, i.e., a positive bias voltage is applied to ann-type semiconductor, the so-called tip-induced band bending(TIBB) occurs owing to the leakage of the electric eld applied
between the STM tip and the sample (Fig. 1(a)).33–37 When thesurface is photoilluminated, the redistribution of photocarriers
reduces the electric eld and changes the surface potential(surface photovoltage: SPV), and thus increases the eﬀective bias
voltage applied to the tunnel junction (Fig. 1(b)).9–11,35–37 Consequently, the photoexcitation increases the transient tunnelingcurrent due to the reduction in the tunneling barrier. The excitedstate subsequently relaxes to the original state via two processes:
the decay of photocarriers on the bulk side via recombination,
diﬀusion and dri (bulk-side decay with decay constant sb,
Fig. 1(c)), and the decay of minority carriers transiently trapped at
the surface during photoexcitation (surface-side decay with decay
constant ss, Fig. 1(d)). When a probe pulse arrives during therelaxation processes, an additional SPV is induced, whichchanges the total magnitude of the tunneling current depending
on td, providing time-resolved spectra.23–29 Therefore, bulk-side
decay and surface-side decay can be probed by measuring thetunneling current as a function of delay time.25

Experimental
To observe these processes in detail, we measured the laser
intensity dependence of OPP-STM spectra. A clean GaAs(110)

NanoscalePapersurface (Si-doped, 1  1017 cm3) was prepared by cleaving a GaAs
sample in a vacuum (5  107 Pa). In the OPP-STM system wehave developed, pulse trains are generated by two synchronized
Ti:sapphire lasers with a 90 MHz repetition rate (11 ns intervals)
and a pulse width of 140 fs. The relative timing of the two pulse
trains, which provides a delay time that ranges from 0 to 11 ns, iscontrolled by the synchronizing circuit. Each train is guided to apulse picker that can selectively transmit one pulse per 90 pulses,
resulting in the reduced repetition rate of 1 MHz. The pulse
selection enables the production of an additional longer delay
time that can be adjusted in multiples of 11 ns. Consequently, td
can be adjusted continuously from zero to a large value as needed.
Furthermore, the rectangular modulation of td allows themeasurement of the absolute value of DI(td) ¼ I(td)  I(N), where
I(N) is the tunneling current for a delay time suﬃciently long forthe excited state to be relaxed.25 Delay-time modulation as thereference signal for lock-in detection, instead of laser-intensity
modulation, was used to clear thermal eﬀects such as thermal
expansion of the STM tip and the sample.20,25

Results and discussion
Fig. 2(a) shows a series of laser-intensity dependent spectra
obtained with a laser repetition rate of 1 MHz and a maximum

Fig. 2 (a) Laser-intensity dependence of OPP-STM spectra obtained for theGaAs(110) sample (set point: Vs ¼ 1 V, It ¼ 100 pA). The laser intensity was
normalized with the maximum value of 4 mW. Amplitudes (b) and decay constants(c) were obtained by the exponential ﬁtting of the spectra in (a) (typical six spectra
are shown in (a)). (d) Decay constant as a function of additional laser intensity.

This journal is ª The Royal Society of Chemistry 2013

View Article Online

Published on 11 July 2013. Downloaded by RSC Internal on 27/08/2013 14:53:28.

Paper
delay time of 500 ns. The spectra are symmetric with respect tothe td ¼ 0 axis, indicating that the intensities of the pump andprobe laser pulses were adjusted well to the same value. The
laser intensity was changed using lters and normalized withrespect to the maximum value of 4 mW.
For high intensities (1 and 0.5), a fast component, whichcorresponds to the bulk-side decay, was clearly observed near td¼ 0 in addition to the slow component, while only the slow
component (surface-side decay) was observed for low intensities(&0.2). The spectra were well tted with a combination of two
exponential functions, each with a single decay constant, andthe amplitudes and decay constants obtained by tting areshown in Fig. 2(b) and (c), respectively, as a function of thelogarithm of laser intensity.
For the fast component, we carried out tting using thespectra measured with the synchrolock system (not shown here)
to obtain accurate values. For the spectra obtained at low laser
intensities (&0.05 in Fig. 2(a)), a slope still remained at 500 ns,
indicating that the dynamics were not in a steady state even at
500 ns. To make the tting more accurate, the measurements
were carried out again with a repetition rate of 0.5 MHz andtd,max ¼ 1000 ns, which is the reason for the two curves showing
ss in Fig. 2(b).
The surface-side decay was observed over the entire range oflaser intensities with a broad maximum (Fig. 2(b)) and its decay
constant ss decreased with increasing laser intensity (Fig. 2(c)),
while the bulk-side decay emerged only for high laser intensities
and its decay constant sb showed no apparent laser intensity
dependence (Fig. 2(b) and (c)).
From the model discussed above, a change in laser intensitychanges the magnitude of SPV, which, therefore, is considered
to play basic roles in both the modes. In fact, to reect bulk-side
decay, a fast response of SPV to carrier dynamics is necessary,and therefore, a high laser intensity that attens TIBB is
required, as experimentally observed. On the other hand,surface-side decay can be observed over the entire laser intensity range in which SPV is produced, as expected, but its decay
constant is considered to change continuously with the relaxation of SPV in a wider range with increasing laser intensity,
instead of the exponential decay observed in the spectra.9,10
To observe the process for producing ss in more detail, we
carried out measurements with the control of the SPV baselineby adjusting the extinction rate of the Pockels cell, with thelaser intensity xed at a certain value. Namely, the sample
surface was additionally illuminated with optical pulses otherthan the pump and probe pulses (89 of 90 pulses) with atransmission rate of RT. Fig. 2(d) shows the result, where three
STM spectra with exponential tting are shown together. Each
OPP-STM spectrum shows exponential decay with a single
decay constant, which decreases with increasing RT. How does
this work?
To understand the observed results, we carried out simulations
on OPP-STM spectra by considering ve key factors: (1) absorption
bleaching, (2) the logarithmic relationship between carrier density
and SPV, (3) the change in tunneling barrier height, (4) the integration process for repeated signals in measurements, and (5)control of the SPV baseline with additional illumination. The

This journal is ª The Royal Society of Chemistry 2013

Nanoscaleeﬀects of these factors were introduced as follows: (1) Neﬀ(t) ¼
Nex[1 p ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ(nb(t)/n+ An (t)/n ],9–11 (3),ﬃ 0)], (2) VSPV(t) ¼ (1/kBT)ln[1
pﬃﬃﬃ pﬃﬃﬃﬃﬃﬃsﬃ - dop
I*fI0 f=f0 ðVs  VTIBB þ VSPV Þ exp  ð f2 2m=hÞz , (f ¼ WÐÐ+ c  Vs + VTIBB  VSPV), and (4) DI(td) ¼ I*(td, t)dt  I*(N, t)dt,
where Nex: carrier density excited by the rst pump pulse, Neﬀ(t):carrier density excited by the optical pulses at t, n0: parameter for
modulating the eﬀect of laser pulses, nb(t): bulk-side carrier density
ndop: dopant density (1  1017 cm3), I0: current, W: work functionof the STM tip (4.54 eV for the tungsten tip), c: electron aﬃnity(4.07 eV for GaAs), f: tunnel barrier height, f0: normalizing factor,
Vs: bias voltage, VTIBB: tip-induced band bending, A: constant, kB:
Boltzmann's constant, T: temperature, m: electron mass, ħ: Dirac's
constant, and z: tip sample distance (1 nm). (5) The additional
illumination was adjusted by changing the RT of the optical pulses
arriving at the sample every 10 ns.
We considered two decay modes for ns(t): (1) exponential
decay for a high laser intensity via recombination in bulk-side
decay (decay rate: 1/sb) and (2) a thermal activation processdepending on SPV [decay rate: 1/sd(t), sd(t) ¼ sd exp(VSPV(t)/kBT)], where sd is the relaxation lifetime under a dark condition
with the activation energy of VTIBB induced by the sample bias
voltage Vs.
The diﬀerential equation for ns(t) is 0dns ðtÞ
ns ðtÞ
n0  nb ðtÞ¼
dt
sd expð  VSPV ðtÞ=kB TÞn00ns ðtÞ
t
:
exp 

sb
sb

(1)

Here, the term [(n00  nb(t))/n00 ] delays the activation process(rst term) compared with the second term (n00 is a constant).
The contribution of the second term for a high laser intensity
decreases with exp(t/sb). For the other parameters, thefollowing values were used: Vs ¼ 1 V and VTIBB ¼ 0.3 V (calculated using Feenstra's formula38 for Vs).
First, to examine the model discussed above, the laser
intensity dependences of the two components shown in Fig. 2
were calculated using eqn (1) with the transmission rate RT of
1%, which was the condition used in our previous experimentand that shown in Fig. 2 because the lowest RT value is 1% for
our system. Simulations were carried out at eight points corresponding to those in Fig. 2. Fig. 3 shows the results obtained,
which exhibit excellent agreement with the experimental
results shown in Fig. 2, despite the tangled mechanism withvarious nonlinearities in the measurement steps, supporting
the validity of the model. Since a high laser intensity isnecessary for measuring the bulk-side decay and there is alimitation of the laser power, the bulk-side decay was observedfor only two laser intensities, as shown in Fig. 2. However,according to the simulation, the amplitude ratio of the bulkside decay for the third laser intensity to that for the second
one is 1/25, as shown in Fig. 3(b). The fact that the bulk-side
decay for the third laser intensity could not be observed
experimentally, as shown in Fig. 2(b) and (c), is consistent withthe simulation results. For laser intensities higher than thosein Fig. 2 and 3, the measurement was beyond the limitation of
our laser system. More complex and interesting phenomena

NanoscaleView Article Online

Published on 11 July 2013. Downloaded by RSC Internal on 27/08/2013 14:53:28.

Nanoscale

Paper

Fig. 4 Simulation results of ﬁve transmission rates RT for four processes in OPPSTM measurement: (a) ns(t), (b) SPV(t), (c) I*(t), and (d) DI(t). (e) Transmission ratedependence of sb. The parameters used were Vs ¼ 1 V, I0 ¼ 50 pA, Nex ¼ 2  1014cm3, n0 ¼ 1018 cm3, sd ¼ 1 ms, n00 ¼ 2  1015 cm3, sb ¼ 10 ns and A ¼ 105.

Fig. 3 Results of simulations carried out for comparison with experimental
results shown in Fig. 2. Simulations shown in (a) to (c) were carried out at eight
points corresponding to those in (a) to (c) (six of them are shown in (a)). The
dependence of the additional illumination intensity of the surface side decay
shown in (d) was calculated for ﬁve intensities and compared with the experimental results shown in Fig. 2(d). The parameters used were Vs ¼ 1 V, I0 ¼ 50 pA,
Nex ¼ 2  1014 cm3, n0 ¼ 1018 cm3, sd ¼ 1 ms, n00 ¼ 2  1015 cm3, sb ¼ 10 ns
and A ¼ 105.

are expected for the carrier dynamics induced by higherintensity laser excitation, which will be an interesting andimportant research target of this microscopy technique in the
future.
Next, to observe the eﬀect of each step in more detail, therelationship among the four steps in the measurement involving
ns(t), VSPV(t), I*(t), and DI(t) was examined. Fig. 4(a)–(d) show theresults obtained for ve values of RT (0%, 0.1%, 0.5%, 1.0% and3%). Simulations were carried out for these RT values to determine under which condition the measurement can be optimized.
Although the spectra obtained for RT ¼ 0 could not be tted byexponential functions with a constant decay rate, it became
possible for a nite RT. This is due to the fact that the eﬀect of thelong-range relaxation of ns(t) with the time-dependent decay
constant ss(t) is reduced. When the sample surface is additionally
illuminated every 10 ns by optical pulses with transmission rate
RT, the SPV baseline increases and the magnitude of the change
in SPV decreases, depending on RT. This, in conjunction with thenonlinearities discussed above, changes the decays of SPV and I*

Nanoscaleinto exponential decay. Thereby, despite the fact that thechange in ns(t) includes a thermal excitation process, the STM
signal DI(t) is expressed by a combination of two exponentialfunctions. The RT dependence of ss shown in Fig. 3(d)expresses the change in the thermal activation process approximated by a constant decay within the narrowed decay
range in SPV. By comparing the values of sd(RT) in Fig. 2(d)and 3(d), sd can be evaluated using the relationship sd(t) ¼
sd exp(VSPV(t)/kBT).
Fig. 4(e) shows the decay constant of the bulk-side decay, sb,
as a function of RT. The decay constant slightly increases withincreasing RT, and its value (11 ns) is close to that used for thesimulations (10 ns). According to the simulation results, thebulk-side decay can be probed eﬃciently by OPP-STM with theadditional light of RT ¼ 1%. In fact, the decay constants
obtained for various samples are in good agreement with thoseobtained by the OPP method.25,27 Under high-intensity excitation, various interesting and complex phenomena are includedin the carrier dynamics, which may be considered when moredetailed theoretical analyses of OPP-STM results are carried out.
Lastly, let us analyze the case of OPP-STM analysis on an ingap state, which has been shown in recent papers.25,29 In thepresence of the in-gap state shown in Fig. 1(e), holes trapped at
the surface recombine with the electrons tunneling from the
STM tip. When suﬃcient electrons are provided, the holecapture rate, which becomes the limiting factor of the process,can be obtained even for a single atomic level.29 To satisfy theconditions, two methods were used in previous experiments: (1)the tunneling current increased while the STM tip was moved
toward the sample and (2) the laser intensity decreased.25,29
Here, the laser intensity was adjusted.

This journal is ª The Royal Society of Chemistry 2013

View Article Online

Paper

Nanoscale

The equation used for simulation is

Published on 11 July 2013. Downloaded by RSC Internal on 27/08/2013 14:53:28.

dns ðtÞ
n ðtÞ
n ðtÞ
 s
 s :¼
dt
sg
sd exp  Vspv ðtÞ=kB T(2)

Eqn (2) was obtained from eqn (1), in accordance with themeasurement condition described above, by assuming nb(t)/n00¼ 0, which produces the rst term in eqn (2), and replacing thesecond term in eqn (1) with ns(t)/sg because the laser intensity is
low and the eﬀect of the in-gap state is dominant. Here, sg is the
decay constant of ns obtained by the recombination of holes
with electrons tunneling from the STM tip via the in-gap state,
which is assumed as a constant to satisfy the condition required
for the tunneling current to be suﬃcient for the hole capture
rate to be the limiting factor of the process.
Simulations similar to those in Fig. 4 were carried out andthe results obtained for the RT dependence of sg are shown in
Fig. 5. Similar to the case in Fig. 4, tting by exponentialfunctions with a constant decay rate became possible for a nite
RT. The decay constant sg slightly decreases with increasing RT,
but its value corresponds well to the sg used in the simulation(10 ns). With the transmission rate RT of 1%, which wasconrmed to be a suitable condition for probing bulk-side
decay, sg is almost constant (10 ns) for the three laser intensities and suﬃcient for performing a reliable experiment on theanalysis of in-gap states. In conjunction with light-modulated
scanning tunneling spectroscopy,36,37 single-atomic-level
analysis is possible.29
Fig. 6 shows the experimentally obtained time-resolved OPPSTM signals for Fe nanoparticle/GaAs(110) samples. The in-gap
states formed by the structure enhance the recombination ofholes transiently trapped at the surface with electrons tunnelingfrom the STM tip at the in-gap states. Exponential spectra wereobtained regardless of the Fe nanoparticle size as predicted,
three examples of which are shown in Fig. 6, despite thecomplex processes including various nonlinearities as
described above. As the Fe base area increases, the hole capture

Fig. 6 Decay constant of the density of photoinduced holes transiently trappedat the surface as a function of the Fe nanoparticle size on a GaAs(110) surface.
Three spectra with a single exponential ﬁtting curve, which were obtained above
the Fe nanoparticles shown by STM images, are shown together (the size of the Fe
nanoparticle in image A corresponds to a single Fe atom).

probability is expected to increase and thus the decay constantdecreases, which corresponds well to the obtained results.
Since the hole capture rate, for example, is considered todepend on temperature, the activation process, which isincluded in sg, is expected to be analyzed at the atomic level if
the temperature dependence of carrier dynamics is measured.32
Bulk-side decay may also show temperature dependence
depending on the type of sample used. These are interesting
research targets for the application of our microscopy
technique.

Conclusions

Fig. 5 Simulation results for analysis of hole dynamics via the in-gap state shownin Fig. 1(e): decay constant sg as a function of RT for three diﬀerent laser intensities((a) 5  1012, (b) 1  1013, (c) 3  1013 cm3). The parameters used were Vs ¼ 1 V,
I0 ¼ 50 pA, Nex ¼ 2  1014 cm3, n0 ¼ 1018 cm3, sd ¼ 1 ms and sg ¼ 10 ns.

This journal is ª The Royal Society of Chemistry 2013

The probing mechanism of OPP-STM spectra from semiconductors was examined. The conditions required to obtaincomprehensive results were demonstrated for the rst time.
When such conditions are satised, transient carrier dynamics
can be probed directly by measuring the tunneling current as afunction of delay time, in which decay constants of carrier
dynamics can be obtained by the exponential tting of thespectra without complex calibrations. In this study, we considered a GaAs(110) sample because a clean surface without in-gap
states can be prepared by cleavage, which can provide a typicalexample applicable for further experiments.39,40 Diﬀerent
materials have diﬀerent characteristics and more complex
carrier dynamics may be induced, for example, by a higherlaser-intensity excitation. Further progress in probing more
complex phenomena and a comprehensive understanding may
be realized with more detailed theoretical analyses of OPP-STM
results obtained for various materials, which is expected tomake OPP-STM more applicable and play more important roles
in further advances in nanoscale science and technology.

NanoscaleView Article Online

NanoscaleAcknowledgements
The support from Japan Society for the Promotion of Science(Grants-in-Aid for Scientic Research) is acknowledged.

Published on 11 July 2013. Downloaded by RSC Internal on 27/08/2013 14:53:28.

Notes and references
1 G. Binning, H. Rohrer, Ch. Gerber and E. Weibel, Phys. Rev.
Lett., 1982, 49, 57.
2 R. Wisendanger, Scanning Probe Microscopy and Spectroscopy,
Cambridge University Press, Cambridge, 1994.
3 H. J. Mamin, H. Birk, P. Wimmer and D. Rugar, J. Appl. Phys.,
1994, 75, 161.
4 U. Kemiktarak, T. Ndukum, K. C. Schwab and K. L. Ekinci,
Nature, 2007, 85, 450.
5 G. Nunes, Jr and M. R. Freeman, Science, 1993, 262,
1029.
6 G. M. Steeves, A. Y. Elezzabi and M. R. Freeman, Appl. Phys.
Lett., 1998, 72, 504.
7 D. Botkin, J. Glass, D. S. Chemla, D. F. Ogletree, M. Salmeronand S. Weiss, Appl. Phys. Lett., 1998, 72, 504.
8 I. Moult, M. Herve and Y. Pennec, Appl. Phys. Lett., 2011, 98,
233103.
9 R. J. Hamers and D. G. Cahill, Appl. Phys. Lett., 1990, 57,
2031.
10 R. J. Hamers and D. G. Cahill, J. Vac. Sci. Technol., B:
Microelectron. Nanometer Struct.–Process., Meas., Phenom.,
1991, 9, 514.
11 S. Grafström, J. Appl. Phys., 2002, 91, 1717.
12 S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler and
A. J. Heinrich, Science, 2010, 329, 1628.
13 C. Saunus, J. R. Bindel, M. Pratzer and M. Morgenstern, Appl.
Phys. Lett., 2013, 102, 051601.
14 U. Kemiktarak, T. Ndukum, K. C. Schwab and K. L. Ekinci,
Nature, 2007, 450, 85.
15 N. N. Khusnatdinov, T. J. Nagle and G. Nunes, Appl. Phys.
Lett., 1996, 69, 2294.
16 R. H. M. Groeneveld and H. van Kempen, Appl. Phys. Lett.,
2000, 77, 4434.
17 R. Giridharagopal, G. E. Rayermann, G. Shao, D. T. Moore,
O. G. Reid, A. F. Tillack, D. J. Masiello and D. S. Ginger,
Nano Lett., 2012, 12, 893.
18 S. Weiss, D. F. Ogletree, D. Botkin, M. Salmeron and
D. S. Chemla, Appl. Phys. Lett., 1993, 63, 2567.

NanoscalePaper
19 O. Takeuchi, R. Morita, M. Yamashita and H. Shigekawa,
Jpn. J. Appl. Phys., Part 1, 2002, 41, 4994.
20 O. Takeuchi, M. Aoyama, R. Oshima, Y. Okada, H. Oigawa,N. Sano, H. Shigekawa, R. Morita and M. Yamashita, Appl.
Phys. Lett., 2004, 85, 3268.
21 O. Takeuchi, M. Aoyama and H. Shigekawa, Jpn. J. Appl.
Phys., Part 1, 2005, 44, 5354.
22 O. Takeuchi, M. Aoyama, M. Kondo, A. Taninaka, Y. Teradaand H. Shigekawa, Jpn. J. Appl. Phys., Part 1, 2006, 45, 1926.
23 Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa,
J. Phys.: Condens. Matter, 2010, 22, 264008.
24 Y. Terada, M. Aoyama, H. Kondo, A. Taninaka, O. Takeuchi
and H. Shigekawa, Nanotechnology, 2007, 18, 044028.
25 Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, Nat.
Photonics, 2010, 4, 869.
26 Y. Terada, S. Yoshida, O. Takeuchi and H. Shigekawa, Adv.
Opt. Technol., 2011, 2011, 510186.
27 S. Yoshida, Y. Terada, R. Oshima, O. Takeuchi and
H. Shigekawa, Nanoscale, 2012, 4, 757.
28 S. Yoshida, M. Yokota, O. Takeuchi, Y. Mera andH. Shigekawa, Appl. Phys. Express, 2013, 6, 016601.
29 S. Yoshida, M. Yokota, O. Takeuchi, H. Oigawa, Y. Mera andH. Shigekawa, Appl. Phys. Express, 2013, 6, 032401.
30 S. W. Wu and W. Ho, Phys. Rev. B: Condens. Matter Mater.
Phys., 2010, 82, 085444.
31 J. Shah, Ultrafast Spectroscopy of Semiconductors andSemiconductor Nanostructures, Springer, Berlin, 1999.
32 A. Richardella, D. Kitchen and A. Yazdani, Phys. Rev. B:
Condens. Matter Mater. Phys., 2009, 80, 045318.
33 R. M. Feenstra, Y. Dong, M. P. Semtsiv and W. T. Masselink,
Nanotechnology, 2007, 18, 044015.
34 R. M. Feenstra, Surf. Sci., 2009, 603, 2841.
35 M. McEllistrem, G. Haase, D. Chen and R. J. Hamers, Phys.
Rev. Lett., 1993, 70, 2471.
36 S. Yoshida, Y. Kanitani, R. Oshima, Y. Okada, O. Takeuchi
and H. Shigekawa, Phys. Rev. Lett., 2007, 98, 026802.
37 O. Takeuchi, S. Yoshida and H. Shigekawa, Appl. Phys. Lett.,
2004, 84, 3645.
38 R. M. Feenstra, J. Vac. Sci. Technol., B: Microelectron.
Nanometer Struct.–Process., Meas., Phenom., 2003, 21, 2080.
39 R. M. Feenstra, J. A. Stroscio, J. Tersoﬀ and A. P. Fein, Phys.
Rev. Lett., 1987, 58, 1192.
40 R. M. Feenstra and J. A. Stroscio, J. Vac. Sci. Technol., B:
Microelectron. Process. Phenom., 1987, 5, 923.

This journal is ª The Royal Society of Chemistry 2013