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LetterOptical Doping: Active Control of Metal#Insulator Transition in Nanowire
Yasuhiko Terada, Shoji Yoshida, Atsushi Okubo, Ken
Kanazawa, Maojie Xu, Osamu Takeuchi, and Hidemi Shigekawa
Nano Lett., 2008, 8 (11), 3577-3581 • DOI: 10.1021/nl801350p • Publication Date (Web): 07 October 2008
Downloaded from http://pubs.acs.org on November 18, 2008

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NANO
LETTERS

Optical Doping: Active Control ofMetal-Insulator Transition in Nanowire

2008Vol. 8, No. 113577-3581

Yasuhiko Terada, Shoji Yoshida, Atsushi Okubo, Ken Kanazawa, Maojie Xu,
Osamu Takeuchi, and Hidemi Shigekawa*
Institute of Applied Physics, CREST-JST, UniVersity of Tsukuba, Tsukuba 307-8573,
Japan
Received May 11, 2008; Revised Manuscript Received August 19, 2008

ABSTRACTThe reversible control of metal-insulator transition (MIT) in In/Si(111) nanowires is demonstrated by tuning the band filling of the onedimensional surface state by optical doping. The control of MIT is carried out by regulating the Fermi level in the surface state around thehalf-filled position, depending on the carrier density introduced at the interface. We successfully achieved the reversible and active control
of MIT via the charge doping by regulating the intensity of photoexcitation. This method is widely applicable to other low-dimensional systems
and makes MIT more controllable and suitable for use in nanowires as an active element in future architectures of nanosized functional
devices as well as nanoscale interdevice wiring.

One-dimensional (1D) nanowires are nanoscale building
blocks that can efficiently transport electrical charges that
constitute information in nanoelectronics. Recently, they haveattracted considerable interest in terms of their use as anactive element in the future architectures of nanosized
functional devices as well as nanoscale interdevice wiring.
An important direction in this context is to exploit the uniqueproperties of nanowires arising from the reduced dimension
to provide a function that cannot be performed by existing
devices. In designing such devices, the modulation andcontrol of unique 1D electronic structures by, for example,
external electric, magnetic, or optical field are essential for
obtaining various functions, as has been reported for the casesof the field-modulated carrier transport in carbon nanotube
transistors,1 switching of semiconductor nanowire emitters,2and photovoltaic power sources in silicon nanowires.3
In this paper, we demonstrate a new methodology tomodify the electronic properties of a quasi-1D nanowire
formed on a semiconductor, that is, the active control ofmetal-insulator transition (MIT) by optical doping. In
general, a quasi-1D metal shows a characteristic Peierls-typetransition at low temperatures, which is accompanied by amarked change in electric conductance.4,5 The transition
temperature is known to be governed by the balance between
the degree of Fermi surface contour nesting and the strengthof electron-phonon coupling. However, the thermal control
of MIT, which is related to electron-phonon coupling, isunfavorable to device application because, for example, of
low switching speed and lack of local controllability. Here,* To whom correspondence should be addressed. http://dora.bk.
tsukuba.ac.jp.
10.1021/nl801350p CCC: $40.75
Published on Web 10/07/2008

 2008 American Chemical Society

we introduced a new method to control MIT, namely, we
modified the nesting of the Fermi surface contour byregulating the Fermi level position using optical doping. For
the sample, a typical quasi-1D metal system formed on asemiconductor, that is, an indium quasi-1D nanowire arrayself-organized on a Si(111) surface6-10 was used. The opticalcontrol of the In Fermi level position was realized byregulating the amount of photocarriers transferred to the In
nanowires from the Si substrate at the interface. This method
is suitable for the dynamical control of MIT in nanowires at
a high switching speed. In addition, with the combinationof the optical control with the electrical field applied by ascanning tunneling microscopy (STM) tip, the local controllability of MIT within a narrow area was also demonstrated.
Since the optical control method is widely applicable to other1D metal/semiconductor systems, it is expected to open up
new possibilities for the development of future nanoscale
devices, such as optical switching, memory, and sensors.
An In/Si(111) sample was prepared by the in situ deposition of In onto a Si(111) 7 × 7 clean surface at 700 K underultrahigh vacuum (1 × 10-8 Pa). Angle-resolved photoemission,6,7 STM,6,8-10 and reflection high-energy electron diffraction (RHEED)6 studies have revealed that the MIT inthis system occurs with the structural change from the hightemperature metallic 4 × 1 phase to the low-temperatureinsulating 8 × 2 phase at a transition temperature (Tc) ofapproximately 125 K. The In/Si-4 × 1 structure has a quasi1D metallic band, and the MIT is associated with theformation of charge density wave (CDW) with a double
periodicity along the In nanowires. Because of the perfectFermi surface nesting with 2kF ) π/a0 (a0 ) 0.384 nm), theCDW phase is strongly locked with the lattice potential andstabilized.10
A variable-temperature STM was used with a chemicallyetched tungsten tip. For the optical control of MIT, an opticalbeam from a laser diode (635 nm, 1 mW) was focused onto
the sample surface below the STM tip with a spot diameterof 0.02 µm, and was turned on and off to provide a dark
condition and an illuminated condition, respectively. The
electric properties of the reconstructed structures were
determined by the current-voltage (I-V) curve taken by STS
measurement. Light-modulated scanning tunneling spectroscopy (LM-STS)11,12 was used to investigate the band structure
for analyzing the mechanism of the light-intensity-dependent
phase transition.
First, we show the characteristics of the In nanowire array
formed on a Si(111) surface, which has been veiled until
now. Figure 1a is an STM image of the In/Si(111) surface(n-type, 0.001 Ωcm) obtained at 61 K under dark conditionat the sample bias voltage of Vs ) +0.4 V. Surprisingly, thesurface is dominated by the 4 × 1 phase with a metallic
feature, as shown by the I-V curve (red line in Figure 1c),even at 61 K, which is much lower than the Tc of 125 K
reported in the literature.9 A previous study9 also showed
the coexistence of metallic and insulating domains. However,since the temperature was only slightly below Tc, in the rangeof 100-125 K, it was concluded that the entire surface wasdivided into domains of different sizes with different effective
Tc values owing to the random distribution of defects andsteps. In contrast, in the case we observed here the area ofthe metallic phase is much larger (Figure 1a), which strongly
suggests the existence of some inherently different mechanism. Here we attribute the presence of the metallic phaseat 61 K to the decrease in Tc. Indeed, the surface undergoes
MIT upon further cooling as shown below.
Figure 1b shows an STM image of the In/Si(111) surface
at 47 K under dark condition. Since the conductivity of the
Si substrate decreases, a higher bias voltage, Vs ) -1.0 V,
was used for measurement. Here, the surface is completely
dominated by the insulating 8 × 2 phase. The insulating
feature of the 8 × 2 phase was confirmed by I-V curvemeasurement (blue line in Figure 1c), and the double
periodicity of the 8 × 2 phase along the nanowires is
attributed to the formation of the insulating CDW. Figure
1d shows the temperature dependence of the area ratio of
the four domains: (1) 4 × 1 metallic domain, (2) crossoverdomain, (3) 8 × 2 insulating CDW domain, and (4) defects.
Here the crossover domain corresponds to the defect-induced
structure discussed in detail in ref 8. From Figure 1d, Tc isestimated to be 47 K, where the metallic phase vanishes.
The origin of the decrease in Tc is the shift of the Fermi
level in the In band from its half-filled (HF) position.13,14
The Fermi level shift is caused by charge transfer, which is
expected to occur in a metal/semiconductor system, but has
not been discussed until now for low-dimensional metal/
semiconductor systems. Figure 1e shows the band structureof the STM tip/tunneling junction/In layer/Si substrate system
under dark condition, where the HF position is indicated byan arrow and the In surface band is drawn with the absence
3578

Figure 1. STM images of In nanowires on n-type Si(111) obtained
under dark condition at (a) 61 K (Vs ) +0.4 V) and (b) 47 K (Vs) -1.0 V). (c) I-V curves obtained for the 4 × 1 and 8 × 2
structures. The setpoint current and voltage were 200 pA and Vs )-0.6 V, respectively. (d) Ratio of the areas with different phases
as a function of sample temperature, obtained from a series of STM
images for different temperatures. (e) Schematic illustration of theband structure profile of STM tip/In surface layer/n-type Si substrate
for 0 < Vs under dark condition. EF denotes the Fermi level positionof the tip and Si. The half-filled (HF) position is indicated by anarrow. The band gap opens if the system becomes insulating, which
is not shown in the figure for simplicity. (f) SPV as a function of
Vs obtained for the relative laser intensity of 12.5%. (see Supporting
Informaton, Figure S1 for details).

of potential band gap opening for simplicity. Here, µ is
defined as the Fermi level of the In band measured from the
HF position. At the metal/semiconductor interface, chargetransfer occurs to equalize the Fermi levels in both sides,
the In layer and Si substrate in this case, resulting in band
bending, as shown in Figure 1e. To analyze the band
structure, surface photovoltage (SPV) was measured using
LM-STS.11,12 As shown in Figure 1f, a positive SPV clearly
exists even at zero-bias voltage (see Supporting Informaton,
Figure S1 for details). Thus, the Si band bends upward inthis system under dark condition.12,15 The upward band
bending is a result of the electron transfer from the substrate
to the surface layer. Therefore, the In layer is negatively
charged, which causes a shift of µ from the HF position in
the direction in which electrons are excessively supplied (µNano Lett., Vol. 8, No. 11, 2008

> 0). In the 1D electron system, when its band structure
has an HF state, that is, µ ) 0, the wavenumber (Q ) 2kF)
of CDW is equal to a*/2, where kF is the Fermi wavenumber
and a* is the reciprocal lattice vector.6,7 In this case, CDW
is commensurate with the lattice potential and is strongly
locked with the lattice, making the system insulating withthe gap opening in the In band. In contrast, for µ > 0 (Figure1e), Q deviates from a*/2, and is incommensurate with thelattice potential. In this case, the energy gain obtained by
the formation of an incommensurate CDW is low. Therefore,
when the deviation is small, the formation of a commensurate
CDW accompanied by the decrease in the transition temperature Tc may be energetically favored.13,14 This is the caseobserved for the current system.
Next, we demonstrate a new methodology to control MIT
by optical doping. When the In/Si surface with the band
structure shown in Figure 1e is photoilluminated with anenergy higher than the band gap of silicon, electron-holepairs are generated in the silicon substrate, and the photogenerated holes drift to the surface owing to upward band
bending, which neutralizes the excess electrons in the In band
(Figure 2b). Thus, µ approaches zero, namely, the In bandchanges closer to the HF state, resulting in the change in Tc
to the value close to the original Tc0 (∼125 K). Throughthis mechanism, optical doping is expected to increase thetotal area of insulating domains.
Figure 2a shows an STM image at 61 K under illuminatedcondition obtained over the same area shown in Figure 1a(Vs ) +0.4V). When the surface is illuminated, the 4 × 1
metallic phase (Figure 1a) changed into the surface with the
8 × 2 insulating phase (Figure 2a), and vice versa, indicatingthat MIT can be reversibly controlled by optical doping. The
phase transition may be caused by the sample temperature
increase as a result of local heating upon laser illumination.
If this is the case, the metallic high-temperature phase may
dominate under illumination. However, this is not the case;therefore, the observed change in the MIT induced by optical
illumination is not due to laser-induced heating.
To quantitatively investigate the effect of optical dopingon the phase transition, we measured the ratio of the area of
each domain, as a function of laser intensity (Vs ) +0.4 V).
As shown in Figure 3a (see also Supporting Informaton,
Figure S2), the area of the metallic (insulating) phasedecreases (increases) as the light intensity increases, but there
exists a stepwise change around 1.7%. Namely, MIT occurs
within an extremely narrow intensity range (1.1-2.5%),
indicating that MIT is highly sensitive to the light intensity.
As the light intensity increases, the amount of doping charge
increases, and eventually µ decreases. Therefore, the observed characteristic is attributed to a strong dependence of
Tc on µ. To observe the mechanism in more detail, we
performed SPV measurement. As shown in Figure 3b, anabrupt change in SPV was observed at around 1.3%. In
general, as the light intensity increases, SPV increases
linearly, followed by saturation at high intensities. At
moderate intensities, the dependence of SPV on the lightintensity is approximately expressed by SPV ) A ln(1 +CP), where A and C are constants, and P is the light
Nano Lett., Vol. 8, No. 11, 2008

Figure 2. (a) STM images of In nanowires on n-type Si(111)
obtained at 61 K and Vs ) +0.4 V under photoilluminatedconditions. Magnified images of the color-squared areas are shown
together. (b) Schematic illustration of the band structure profile of
STM tip/In surface layer/n-type Si substrate for 0 < Vs underillumination.

intensity.16 However, the abrupt change in SPV cannot be
explained by this equation. We attribute the observed
anomalous characteristic to the gap opening caused by MIT.
The gap opening results in the marked change in the densityof states (DOS) of the In band. Since the constant C depends
on the DOS, the gap opening in the In DOS is considered to
produce the abrupt change in SPV. Indeed, the intensity at
which the abrupt change occurs is close to the critical value
at which MIT occurs. At higher intensities, after the gap
opening, the SPV curve gradually saturates as in the typicalcase.
In the last part, to understand the mechanism in more detailand consider a more efficient and local control we discuss
the Vs dependence on MIT. Figure 3b shows the laser
intensity dependence of the ratio of the domain area withdifferent phases obtained for Vs ) -0.7 V. The laser intensity
at which the ratio of the area of the insulating phase becomes
3579

Figure 3. (a,c) Ratios of the domain areas with different phases asa function of laser intensity. The bias conditions are (a) +0.4 and(c) -0.7 V, respectively. (b) SPV as a function of laser intensity
measured by LM-STS for Vs ) +0.4 V (see also Supporting
Informaton, Figure S2).

50% increases from 1.7% for Vs ) +0.4 V (Figure 3a) to
2.3% for Vs ) -0.7 V (Figure 3c). This indicates that the
MIT is more easily induced for positive Vs than for negative
Vs. Indeed, an STM image taken at Vs ) -0.7 V under dark
condition (Figure 4a) shows that the 8 × 2 insulating phasearea is smaller than that of Vs ) +0.4 V (Figure 1a). Thisdifference is attributed to Vs-dependent band bending.15 SPV
gradually increases with Vs (Figure 1f) and is larger for
positive Vs than for negative Vs, indicating that the magnitudeof the band bending is larger for positive Vs than for negative
Vs. Thus, µ is closer to zero for positive Vs (Figure 4d) thanfor negative Vs (Figure 4c). Therefore, for positive Vs thesystem is expected to be more insulating and a weaker lightintensity induces the insulating 8 × 2 phase. Figure 4b shows
the area ratio of the phases as a function of Vs. The insulatingarea with the 8 × 2 phase increases with Vs, but the totalchange is less than 20%, indicating that the effect of Vs on
MIT is not negligible but is small. This is in contrast to theoptical doping shown in Figure 3, which enables a much
more efficient control of MIT. However, since the bias
voltage can locally be applied, the combination of optical
doping with Vs has the potential to realize an efficient andlocal control of MIT.
Depending on the doping type and concentration of thesemiconductor substrate, the position of the substrate Fermi
energy shifts and, hence, the amounts of band bending andcharge remaining in the In layer change. Accordingly, the3580

Figure 4. (a) STM images of In nanowires on n-type Si(111)
obtained at 61 K and Vs ) -0.7 V under dark condition. (b) Ratiosof the domain areas with different phases under dark condition as
a function of Vs. Schematic illustrations of band structure profilesof STM tip/In surface layer/n-type Si substrate for (c) Vs < 0 and(d) 0 < Vs conditions. The HF positions are indicated by arrows.
The band gap opens if the system becomes insulating, which is
not shown in the figure for simplicity.

phase area and transition temperature, both of which dependon the charge amount, are expected to change. This mechanism was confirmed by performing similar measurements
on a p-type substrate (0.02 Ωcm), where SPV was zero
independent of Vs, indicating the absence of band bending
and thus the absence of charge transfer between the substrate
and the In layer. In fact, for this sample the transition
temperature was close to Tc0, and no change was observedfor MIT by either voltage or optical excitation. Theseobservations are consistent with the fact that the In band on
p-type Si is HF.
The optical control of MIT can be applied to other lowdimensional systems formed on a semiconductor surface. If
the doping level of the semiconductor is prepared as the band
bending occurs under dark condition, that is, the Fermi levelin the low-dimensional system differs from that of thesubstrate semiconductor, the Tc of the system is decreased
from the original value and can be modified using opticalNano Lett., Vol. 8, No. 11, 2008

doping. To realize a high efficient control, proper selectionof the doping level of the semiconductor substrate isimportant.
The electronic structures of low-dimensional structures on
semiconductor surfaces are widely studied experimentallyusing optical excitation methods, such as X-ray diffraction,
photoemission, and Raman spectroscopy. Clearly, it isimportant to consider the optical perturbation that modifies
the electronic structure of a probed low-dimensional structure
through a charging or discharging effect.
In summary, the reversible control of the MIT in In/Si(111)
nanowires was demonstrated by tuning the band filling of
the one-dimensional surface state by optical doping. We
found that the MIT in the In/Si(111) nanowire system canbe controlled by external photoexcitation. We demonstrated
that MIT is controllable by adjusting the amounts of band
bending and band filling by regulating the laser intensity andthe voltage applied between the STM tip and the substrate.
This method is widely applicable to other low-dimensional
systems and makes MIT more controllable and suitable foruse in nanowires as an active device element in futurearchitectures of nanosized functional devices as well as
nanoscale interdevice wiring.
Acknowledgment. This work was supported in part by aGrant-in-Aid for Scientific research from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan(Young Scientists B, et al.). We thank Ms. Rie Yamashita

Nano Lett., Vol. 8, No. 11, 2008

in our group at University of Tsukuba for her help in
preparing this paper.
Supporting Information Available: This material isavailable free of charge via the Internet at http://pubs.acs.org.
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