Phonon detection using scanning tunneling microscope light emission
Y. Uehara, S. Katano, S. Watanabe, and *M. Kuwahara
Research Institute of Electrical Communication, Tohoku University* Photonics Research Institute, National Institute of Advanced Industrial Science and Technology	 ABSTRACT
We have found that the phonon energy of Sb2Te3 can be determined from scanning tunnelingmicroscope (STM) light emission spectroscopy combined with picosecond laser pulse irradiation ofthe tip-sample gap. STM light emission (STM-LE) from Sb2Te3 is excited by electronic transitions
taking place in the local electronic structure having a direct gap-like shape with a band gap of around
1.6 eV. Hence, a single peak with a narrow spectral width (original peak hereafter) is seen at the gap
energy in the spectra measured in the conventional (i.e., no laser pulse irradiation) configuration.
When the tip - sample gap in the STM is irradiated with picosecond laser pulses, several peaks areadded to the original. These peaks including the original are separated from the adjacent peak(s) by anequal photon energy. The amount of the separation read from the spectra agrees with one of thereported phonon energies of Sb2Te3 , showing that one can determine phonon energy of Sb2Te3 bySTM-LE spectroscopy.
Keywords STM, light emission spectroscopy, vibrational spectroscopy, chalcogenide alloys
INTRODUCTION
Some chalcogenide alloys including Sb2Te3 attract strong attention because of their potential fordata-storage devices with ultra-high capacity.
Optical storage devices using thesuper-resolution-readout phenomenon [1][2] found in these materials will have a capacity much higher
than that of current blu-ray disks. Recently, these materials have been intensively studied for the use
in resistive layers of phase change random access memory (PCRAM), which is one of the candidates
for the next generation of nonvolatile electrical memory. In order to advance the development of suchdevices, it is highly desirable that properties of the underlying material are explored and understoodwith the spatial resolution comparable to targeted sizes of read and write units. Vibrational
spectroscopy is a powerful tool for characterizing materials. However, the spatial resolution ofconventional ways, such as Raman and infrared spectroscopy, is limited to submicrometers due to thediffraction limit of optical measurements. Much higher spatial resolution is required for the intendedapplications.
When electrons (or holes) are tunneled from the tip of a STM into the sample, light emission isexcited (hereafter we call this emission STM-LE). Since the electron tunneling beam from the STM
tip is well focused up to an atomic scale and the beam energy and current are very low, one can obtainthe light emission spectra of individual targeted nanostructures on the surface without modifying them.
By correlating the observed spectra with the size and shape of the structure as determined by the usual
STM operation, one can obtain material information about individual nanostructures. Recently wefound that vibrational energies of individual adsorbates and surface nanostructures are determinedfrom their STM-LE spectra (e.g., [3][4]), showing that vibrational spectroscopy is possible bySTM-LE spectroscopy with the spatial resolution of STM. In this paper, we focus on thedetermination of phonon energies of Sb2Te3 by STM-LE spectroscopy.

EXPERIMENTS
A Sb2Te3 film with thicknesses of 100 nm was deposited on a quartz substrate in an Ar gas
atmosphere using a radio frequency (RF) magnetron sputtering apparatus, and was annealed at 200°C

for 2 min for crystallization in an Ar gas atmosphere. Then the sample was set in the STM, which ishoused in an ultra-high vacuum (UHV) chamber. The STM was operated in a constant current mode
at a tip and sample temperature of 80 K. The tip was made of a tungsten wire by an electrochemicalmethod.
Figure 1 shows a schematic illustration of the experimental setup for the present experiments.
The tip - sample gap of the STM is irradiated with a mode-locked picosecond Ti : sapphire laser with arepetition rate of 80 MHz and a pulse width less than 2 picoseconds. The STM light is analyzed by
the conventional intensified charge coupled device (CCD) camera attached to a spectrograph. All theSTM–LE spectra shown below are averages of 5–10 spectra taken for a tunneling current of 2 nA withan accumulation time of 100 s.

RESULTS AND DISCUSSION
Let us first see conventional STM-LE spectra of the W tip - Sb2Te3 sample gap being notirradiated with the laser pulses. The STM light emission spectrum shown in Fig. 2(a) was observed in
the spectral range from 1.55 eV to 1.65 eV for a sample bias voltage of +2.3 V with respect to the tip.
A single peak with a narrow spectral width is found at a photon energy of about 1.6 eV. No STM-LE
was observed for the reversed bias polarity. From this bias-polarity dependence along with thetheoretical electronic structure of Sb2Te3 reported by Zhitang et al. [6], the mechanism of the STM
light emission is understood as depicted in Fig. 3(a). Sb2Te3 has a local electronic structure having a
direct gap-like shape, i.e., the structure consisting of upper and lower bands being convex downward
and upward, respectively, with a band gap of about 1.6 eV. When the sample bias polarity is positive
with respect to the tip, electrons are injected from the tip into the sample and then immediately relax tothe bottom of the upper band. Holes are created in the lower band through impact ionization processes
initiated by the injected electrons. As a result, STM-LE is excited via recombination of the electrons
and holes. It should be noted that the gap energy of Sb2Te3 determined by conventional methods isless than 0.5 eV. That is to say, this material electrically functions as a narrow gap semiconductor,
showing that the gap detected by STM-LE spectroscopy does not affect its electric properties.
However, the optical responses (i.e., its dielectric function) in the visible spectral range [7] aredominantly determined by the electronic transitions in the gap; this gap strongly affects its opticalproperties.
Next let us see STM-LE spectra of the tip-Sb2Te3 sample gap being irradiated with the laser. In
this configuration, the bias polarity dependence of STM-LE varies from that of the conventional
STM-LE described just above; while STM-LE comes to be detected for both polarities, the signal levelis higher for the negative polarity than for the positive. Fig. 2(b) is the STM light emission spectrum
for a bias voltage of – 2.3 V. Several peaks appear in addition to the original. These peaks including
the original are separated from the adjacent peak(s) by an equal photon-energy. The amount of theseparation read from the spectra is 18 meV. This value agrees with one of phonon energies of Sb2Te3[8].
Before elucidating the origin of these multiple peaks, we review our previous work concerning
temporal behaviors of STM-LE from the W tip - Au sample gap irradiated with the picosecond laser
pulses. [9] In this configuration, the STM-LE becomes a pulse train in synchronization with that of
the incident laser as shown in Fig. 4. The pulse width of the STM-LE is the same as that of the laser
pulse. Here it should be noted that no thermal effect makes such a pulse train. This is because therepetition rate of the laser (80MHz) is so high that any thermal effect cannot follow the laser pulses.
Figure 4 shows that the picosecond laser pulses induce a pulsed tunneling current, by which the pulsed
STM-LE is excited.
On the basis of the knowledge described just above, the mechanism responsible for exiting theSTM-LE in Fig. 2(b) is concluded as depicted in Fig. 3. While the STM-LE is still excited via therecombination process that is the same as that of the conventional STM-LE, the carriers are created ina different manner for the negative bias polarity; holes are created in the lower band through theelectron tunneling from the sample to the tip for this polarity. Supply of electrons to the upper band iscarried out via ionization processes initiated by the incident laser pulses (see Fig. 3). In addition, theincident laser pulses must excite a high density of phonons around the tip-sample gap in the sample.
The electrons and holes relaxed to the bottom of the upper band and the top of the lower band,
respectively, must interact with the phonons before the recombination. Then phonon replicas of theemission at the photon energy of the gap must be created. As a result, the multiple peaks with thephoton-energy separation corresponding to the phonon energy appear in the STM-LE spectra.
CONCLUSION
We have observed the STM-LE spectra of the W tip - Sb2Te3 sample gap irradiated with thepicosecond laser pulses. The multiple peaks separated from the adjacent peak(s) by the equal energy
are found in the STM-LE spectra. Since the amount of the separation agrees with one of the phonon
energies of Sb2Te3, we conclude that that the phonon energy of this material can be determined bySTM-LE spectroscopy combined with picosecond laser pulse irradiation of the tip-sample gap.
ACKNOWLEDGEMENT
This research was supported in part by a Grant-in-Aid for Scientific Research from the Ministry ofEducation, Culture, Sports, Science, and Technology.
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[2]
[3]
[4]
[5]
[6]
[7]

[8]
[9]

REFERENCES
K. Yasuda, M. Ono, K. Aratani, A. Fukumoto, and M. Kaneko, Jpn. J. Appl. Phys. 32, 5210
(1993).
J. Tominaga, T. Nakano, and N. Atoda, Appl. Phys. Lett. 73, 2078 (1998).
Y. Uehara and S. Ushioda, Phys. Rev. Lett. 92, 066102 (2004).
S. Katano, S. Ushioda, and Y. Uehara, J. Phys. Chem. Lett. 1, 2763-2768 (2010)
Y. Uehara, M. Kuwahara, S. Katano, and S. Ushioda, Solid State Commun. 149, 1902 (2009).
Jialin Yu, Zhitang Song, Bo Liu, Songlin Feng, Bomy Chen, Jpn. J. Appl. Phys. I 46, 4215
(2007).
Masashi Kuwahara, Osamu Suzuki, Yuzo Yamakawa, Paul Fons, Takashi Yagi, Naoyuki
Taketoshi, Kouichi Tsutsumi, Michio Suzuki, Junji Tominaga, and Tetsuya Baba, Proceedingsof the 1st International Symposium on Thermal Design and Thermophysical Property forElectronics 70 (Tsukuba, 18-20, June, 2008).
G. C. Sosso, S. Caravati, and M. Bernasconi, J. Phys. Condens. Mat. 21, 095410 (2009).
Y. Uehara, A. Yagami, K. Ito, and S. Ushioda, Appl. Phys. Lett. 76, 2487 (2000).

STM light emission

Ar ion laser

ps-laser
Tip

Vo

Ti:Sapphirelaser

Laser-induced
tunneling
Sample (Sb2Te3)

UHV
LTSTM
Optical detection
systemFig. 1

Intensified CCD camera
combined withSpectrographExperimental setup. The left
picture is a top view of thesystem. The upper right shows
optical relations.

Intensity(a) w/o laser
irradiation(a) w/o laser irradiation
Sb2Te30.5

Relaxation

01.55
1.6
1.65
Photon Energy (eV)Intensity

1(b) w laser
irradiation(b) w laser irradiation

18 meV(a) STM-LE

Intensity

100-1030(b) laser

20
10001000
2000
Sweep time (ps)Sb2Te3Tip

Lightionization!ω ~ 1.6 eVTunneling

STM light emission
spectra observed when
the laser irradiation is (a)off and (b) on. multiple
peaks are seen only in(b).

20!ω ~ 1.6 eV

Impactionization

0
1.55
1.6
1.65
Photon Energy (eV)Fig. 2Tip

Fig. 3Fig. 4

STM light emission mechanisms
operated when the laser irradiation is(a) off and (b) on.

Temporal behaviors of (a) STM light emission
from the W tip-Au sample gap irradiated with thepicosecond laser pulses and (b) the picosecond
laser pulses themselves.
These spectra were
measured with a streak camera operated in a mode
whose temporal resolution is 100 picoseconds.