Nanoscale phase change on Ge2Sb2Te5
thin films induced by optical near fields
with photoassisted scanning tunnelingmicroscope
Cite as: Appl. Phys. Lett. 117, 211102 (2020); https://doi.org/10.1063/5.0032573
Submitted: 09 October 2020 . Accepted: 08 November 2020 . Published Online: 24 November 2020
Kanta Asakawa, Dang-il Kim, Shotaro Yaguchi, Mikito Tsujii,
Yusuke Arashida, Shoji Yoshida, Hidemi Shigekawa,

Masashi Kuwahara,Appl. Phys. Lett. 117, 211102 (2020); https://doi.org/10.1063/5.0032573© 2020 Author(s).

Katsumasa Yoshioka,

Keisuke Kaneshima,

Ikufumi Katayama, and

Jun Takeda

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Nanoscale phase change on Ge2Sb2Te5 thin films
induced by optical near fields with photoassisted
scanning tunneling microscope
Cite as: Appl. Phys. Lett. 117, 211102 (2020); doi: 10.1063/5.0032573
Submitted: 9 October 2020 . Accepted: 8 November 2020 .
Published Online: 24 November 2020
Kanta Asakawa,1,2,a)Dang-il Kim,1 Shotaro Yaguchi,1 Mikito Tsujii,1 Katsumasa Yoshioka,1
1
Keisuke Kaneshima,
Yusuke Arashida,1,3 Shoji Yoshida,3 Hidemi Shigekawa,3 Masashi Kuwahara,4Ikufumi Katayama,1and Jun Takeda1,a)AFFILIATIONS
1

Department of Physics, Graduate School of Engineering Science, Yokohama National University, Yokohama 240-8501, Japan2

Department of Applied Physics, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588, Japan

3

Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan
Sensing System Research Center, National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8562, Japan

4a)Authors to whom correspondence should be addressed: jun@ynu.ac.jp and asakawa@go.tuat.ac.jp

ABSTRACTA scanning probe microscope coupled with either femtosecond laser pulses or terahertz pulses holds great promise not only for observing
ultrafast phenomena but also for fabricating desirable structures at the nanoscale. In this study, we demonstrate that a few-nanometer-scale
phase change can be non-thermally stored on the Ge2Sb2Te5 surface by a laser-driven scanning tunneling microscope (STM). An atomicallyﬂat Ge2Sb2Te5 surface was irradiated with the optical near-ﬁeld generated by introducing femtosecond laser pulses to the STM tip-sample
junction. The STM topographic images showed that few-nanometer-scale mounds appeared after irradiation. In addition, tunneling conductance spectra showed that the bandgap increased by 0.2 eV in the area of 5  5 nm2. These indicate that the nanoscale crystal-to-amorphous
phase change was induced by the STM-tip-induced near ﬁeld. Our approach presented here offers an unprecedented increase in the recording density of optical storage devices and is, therefore, expected to facilitate the development of next-generation information technology.
Published under license by AIP Publishing. https://doi.org/10.1063/5.0032573

In optical storage devices such as DVD-RAM, information isrecorded on phase change materials (PCMs) of which structuralphases can be optically switched and detected.1–3 PCMs have recently
attracted much attention as candidate materials for phase change random access memory (PCRAM),4,5 which is a next generation nonvolatile electrical memory that utilizes the resistance contrast. Among
PCMs, Ge2Sb2Te5 (GST) is a representative example: it has two crystalline phases, the metastable face-centered cubic (fcc) and the stablehexagonal close-packed (hcp) phases, and an amorphous phase.6,7 The
fcc phase can be thermally transformed into the amorphous phase via
the liquid phase by laser irradiation. The amorphized region can beconversely crystallized in the fcc structure by weak heating. This enables us to repeatedly write and erase the information.
The recording capacity of optical storage is determined by thesize and the positional uncertainty of the amorphous/crystal-phasespots, which are in general limited by the diffraction limit of light. So

Appl. Phys. Lett. 117, 211102 (2020); doi: 10.1063/5.0032573
Published under license by AIP Publishingfar, many attempts have been examined to reduce the spot size. Byheating a small area of the GST surface with gold antennas in a 5 lm
gap, the size of amorphous marks can be reduced below the diffraction
limit.8 Sato et al. demonstrated that nanoscale amorphous marks canbe created by applying voltage pulses from the tip of a scanning probe
microscope (SPM).9,10 However, the heat generated in small areas
often damages the sample and its structural conﬁguration, and thesmallest mark size is still limited to 10 nm, which can be explainedusing a thermodynamic model. In addition, the recording speed ofconventional optical storage devices and PCRAMs is limited by thermal processes including melting, quenching, and crystallization.11
To overcome these problems, a technique should be developed to
non-thermally induce the phase change in small areas.
It has been reported that GST can be amorphized non-thermallyby femtosecond laser irradiation.12–14 The mechanism of the nonthermal amorphization is explained by the ﬂipping of the Ge atom

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Applied Physics Lettersfrom the octahedral symmetry site to the tetrahedral symmetry site
assisted by the optical excitation of electrons in the valence band to the
conduction band.15,16 The non-thermal ultrafast amorphization of
GST has been achieved with various center wavelengths ranging from
800 to 1500 nm12–14,17 presumably because the optical excitation ofvalence band electrons only requires the photon energy larger than the
bandgap. Implementation of this non-thermal ultrafast amorphization
with an ultrashort pulsed laser is expected to minimize the heating
effect, which is critical for realizing high-density and high-speedrecording.
Recently, SPMs combined with either ultrashort laser pulses or
terahertz pulses have been extensively developed to image electronic
properties of materials with high spatiotemporal resolutions.18–21
These techniques allow us not only to track the motion of single molecules but also to coherently control the motion of electrons.22–24 The
tip-enhanced electric near ﬁeld produced by irradiating the tip-sample
junction with laser pulses is strongly conﬁned and can be utilized tofabricate nanoscale structures.25–29 In this study, we demonstrate theutilization of the optical near ﬁeld generated by irradiating a scanningtunneling microscope (STM) tip with femtosecond laser pulses forcreating nanoscale amorphous marks, which will contribute to realizing the high-density recording technique.
Figure 1 shows a schematic of the experimental apparatus. The
STM and scanning tunneling spectroscopy (STS) experiments wereconducted at room temperature with an ultra-high vacuum (UHV)
STM system with a base pressure lower than 1  107 Pa. A mechanically sharpened PtIr wire was used as a probing tip. Laser pulses with
the incidence angle with respect to the sample surface of 45 , the center wavelength of 1035 nm, and the pulse duration of 300 fs generated
by a ﬁber laser ampliﬁer were focused to the tip-sample junction by alens in the STM measurement chamber. The repetition rate and the
ﬂuence of each pulse were 100 kHz and 107–127 mJ/cm2, respectively.
We carefully conﬁrmed that the ﬂuence was enough to induce thecrystal-to-amorphous phase change on GST without ablation. The
beam waist diameter of the incident laser pulses was designed to be
10 lm, which was veriﬁed using a charge-coupled device (CCD)

FIG. 1. Schematic of the photoassisted STM. When the laser pulses were applied
to a tip-sample junction, the tip was lifted by 5 nm far from the sample before thelaser irradiation, to avoid tip crash due to thermal expansion of the tip.

Appl. Phys. Lett. 117, 211102 (2020); doi: 10.1063/5.0032573
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camera. The tip was retracted by 5 nm from the surface during laser
irradiation to avoid tip crash due to the thermal expansion of the tip.
Since the amorphization induced by a single-laser-pulse irradiation
occurred with much less efﬁciency, we implemented the irradiation of
2000–10 000 laser pulses with a time interval of 10 ls, which ensured
high producibility of the photoassisted amorphization. The relatively
long time interval also may minimalize the thermal process for thephotoassisted amorphization.
GST thin ﬁlms with a 100–300 nm thickness, which is thickenough to prevent ablation,14 were sputter-deposited onto a cleaved
highly oriented pyrolytic graphite (HOPG) surface at room temperature. The deposition rate was set to 0.714 nm/s. After taking out fromthe deposition chamber into air, the samples were dipped in distilledwater for one minute to remove the oxidized layer.30,31 After loading
the samples into the STM chamber, the samples were annealed in
UHV for crystallization and to ﬂatten the surfaces atomically.32
Notably, to obtain the fcc phase, the temperature should not exceed
300  C during the whole procedure as GST transforms into the hcp
phase at 310  C.

FIG. 2. STM images of Ge2Sb2Te5(300 nm)/HOPG surfaces prepared by dippingthem in distilled water and UHV annealing at (a) 200  C for 3 h, (b) at 220  C for
3 h, (c) at 240  C for 3 h, and (d) at 240  C for 10 h. (e) The line proﬁle of (d)whose position is indicated by the black line. (f) The atomic image of the sample
obtained by the same preparation procedure as (d) taken with Vs ¼ 50 mV and
It ¼ 20 pA.

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Figures 2(a)–2(c) show the STM images of the samples annealedfor 3 h at 200, 220, and 240  C, respectively, recorded with the sample
bias voltage (Vs) of 5 V and the tunneling current (It) of 5 pA. The
sample annealed at 240  C had a clear step-terrace structure, whereas
the structures of samples annealed at 200 and 220  C were unclear.
The areas of the terraces were enlarged by increasing the annealing
time to 10 h, as shown in Fig. 2(d). The step height was estimated as
0.34 nm from the line proﬁle as shown in Fig. 2(e), which agreed wellwith the distance between neighboring Te layers30,33 and is signiﬁcantly different from the interlayer distance of the hcp phase.16,34 The
atomic image shown in Fig. 2(f) exhibits a defect-free hexagonal arraywith a distance between atoms of 0.41 6 0.01 nm, which indicates thatthe surface is the Te-terminated GST (111) surface.30 Based on theseresults, the GST ﬁlm prepared by annealing at 240  C for 10 h wasappraised as having a crystalline phase with a fcc structure.
Figures 3(a) and 3(b) show the STM images taken before andafter the irradiation, respectively, and Fig. 3(c) shows the line proﬁlesof the location indicated by the black bars. Atomic corrugations are
faintly visible in Figs. 3(a) and 3(b) and their periodicity is consistent
with that of Te-terminated GST (111) surfaces, which was conﬁrmedby two-dimensional fast Fourier transformation. A mound-like feature
with a height of ’ 0:3 nm and a width of ’ 4 nm was formed by laser
irradiation. All the scan lines within 2 nanometers from the irradiation point exhibited a similar feature, which indicates that a nanoscale
mound was formed by laser irradiation. Figure 3(d) shows the tunneling current–voltage (I–V) curves and differential tunneling conductance (dI/dV) spectra measured at the location shown by the black
dots in (a) and (b). The difference in the position is due to the thermal
drift. The dI/dV spectra were obtained by numerically differentiating
the I–V curve measured after stabilizing the tip height at Vs¼ 100 mV and It ¼ 20 pA. The differential conductance spectrum
before laser irradiation exhibits a bandgap, and the Fermi level isFIG. 3. STM images taken (a) before and (b) after laser irradiation. (c) The line
proﬁles of the location indicated by black bars in (a) and (b). (d) I-V curve and differential tunneling conductance spectra measured at the location indicated by black
dots in (a) and (b).

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located near the top of the valence band, which indicates that the sample is a p-type semiconductor. The p-type carriers are typically attributed to excess vacancies.35–38 The few-atom-scale contrast in atomic
images can also be attributed to the spatial distribution of the densityof excess vacancies. The conduction band bottom moved upward by’ 0:2 eV after laser irradiation, indicating that the region was
amorphized by a tip-enhanced near ﬁeld.39 The formation of a moundcan also be explained by the lattice expansion of GST by amorphization.33 From the observed height of the amorphous mound and the
expansion rate of 4.6%,33 the depth of the amorphous region isroughly estimated to be 7 nm. The defect-free atomic rows are still
faintly visible after amorphization, suggesting that the amorphization
is driven non-thermally without liquidization.16,40
The height of the amorphous mound is lower than the surface
roughness, which means that it is difﬁcult to detect the amorphizedregion only from the topographic image. On the other hand, theshapes of I–V curves, which reﬂect the local density of states (DOS) of
electrons, are not disturbed by the surface morphology. The I–V curveis, thereby, more suitable for detection of the amorphous region, andthe tunneling current mapping before and after laser irradiation was
implemented. As only the conduction band bottom moves upward byamorphization39 [see Fig. 3(d)], the rising edge in the positive bias
region of the I–V curves shifts upward, leading to the decrease incurrent at the positive bias. Figure 4(a) shows a 100 nm100 nm topographic image of the GST(100 nm)/HOPG surface. The root meansquare roughness was 2.69 nm, which is one order of magnitude larger
than the height of the mound (’ 0:3 nm) formed by amorphization.
The laser pulses were introduced after settling the tip at the position
represented by the green dot. We measured I–V curves after stabilizing
the tip-sample distance at Vs ¼ 600 mV and It ¼ 70 pA at the centers of each square in the grid pattern with 5 nm intervals arranged in
the area of Fig. 4(a) before and after the irradiation. The I–V curves
before irradiation show p-type semiconducting behavior. After theirradiation, bandgap broadening was observed only at the irradiatedpoint. These results well reproduced the result shown in Fig. 3(d).
Here, we introduce the normalized tunneling current IðV
¼ 200 mVÞ=IðV ¼ 600 mVÞ for the evaluation of the area of thelaser-induced amorphization. Based on this treatment, we could
minimize the deterioration of the signal-to-noise ratio caused by the
instability of tip-sample distance.41 Figures 4(b) and 4(c) show theIðV ¼ 200 mVÞ=IðV ¼ 600 mVÞ mappings before and after
the irradiation, respectively. The small shift of the green dot indicating
the irradiated position is due to the thermal drift. A bright protrusion
corresponding to the laser-induced amorphous region clearly appears
at the spot, whose intensity (13.1) is much larger than the standard
deviation (0.34) of the background region. This demonstrates that theamorphous area could be sensitively detected with a good signal-tonoise ratio by utilizing the I–V characteristics, even if the topographic
image is not ﬂat enough to resolve the height of the amorphous
mound. We also found that the size of the laser-induced amorphous
region was comparable to the pixel size (5 nm  5 nm), which is significantly smaller than the smallest thermally created amorphous marks.9
This indicates that a nanoscale crystal-to-amorphous phase change
was induced non-thermally by the STM-tip-induced near ﬁeld andthat the tip-enhanced near ﬁeld is localized within few-nanometer
area, which is smaller than the typical tip apex radius (10–100 nm).
This is supported by simulation results showing that the near ﬁeld is117, 211102-3

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images or the change in the I–V characteristics originating from thebandgap broadening. Because both generation of optical near ﬁelds by
SPM tips and the non-thermal amorphization of GST can support awide range of wavelengths,12–14,17 our concept presented here should
work with various laser sources and may help in increasing the information storage density of memory devices.
This work was supported in part by the Grants-in-Aid for
Scientiﬁc Research (KAKENHI Nos. 17H06124, 17H06088,
18H04288, 20H00326, and 20H05662 from the Japan Society for
the Promotion of Science (JSPS). The STM topographic images
were processed using WSxM.43
DATA AVAILABILITY
The data that support the ﬁndings of this study are available
from the corresponding author upon reasonable request.
REFERENCES
1

R. Von Gutfeld and P. Chaudhari, J. Appl. Phys. 43, 4688 (1972).
V. Weidenhof, I. Friedrich, S. Ziegler, and M. Wuttig, J. Appl. Phys. 89, 3168
(2001).
3
T. Matsunaga, J. Akola, S. Kohara, T. Honma, K. Kobayashi, E. Ikenaga, R. O.
Jones, N. Yamada, M. Takata, and R. Kojima, Nat. Mater. 10, 129 (2011).
4
S. Raoux, G. W. Burr, M. J. Breitwisch, C. T. Rettner, Y.-C. Chen, R. M. Shelby,
M. Salinga, D. Krebs, S.-H. Chen, H.-L. Lung et al., IBM J. Res. Dev. 52, 465(2008).
5
F. Rao, K. Ding, Y. Zhou, Y. Zheng, M. Xia, S. Lv, Z. Song, S. Feng, I.
Ronneberger, R. Mazzarello et al., Science 358, 1423 (2017).
6
N. Yamada, E. Ohno, K. Nishiuchi, N. Akahira, and M. Takao, J. Appl. Phys.
69, 2849 (1991).
7
I. Friedrich, V. Weidenhof, W. Njoroge, P. Franz, and M. Wuttig, J. Appl.
Phys. 87, 4130 (2000).
8
Y. Sanari, T. Tachizaki, Y. Saito, K. Makino, P. Fons, A. V. Kolobov, J.
Tominaga, K. Tanaka, Y. Kanemitsu, M. Hase et al., Phys. Rev. Lett. 121,
165702 (2018).
9
H. Satoh, K. Sugawara, and K. Tanaka, J. Appl. Phys. 99, 024306 (2006).
10
K. Tanaka, T. Gotoh, and K. Sugawara, J. Optoelectron. Adv. Mater. 6, 1133
(2004).
11
M. Wuttig and N. Yamada, Nat. Mater. 6, 824 (2007).
12
M. Hada, W. Oba, M. Kuwahara, I. Katayama, T. Saiki, J. Takeda, and K. G.
Nakamura, Sci. Rep. 5, 13530 (2015).
13
M. Konishi, H. Santo, Y. Hongo, K. Tajima, M. Hosoi, and T. Saiki, Appl. Opt.
49, 3470 (2010).
14
S. H. Møller, E. H. Eriksen, P. L. Tønning, P. B. Jensen, J. Chevallier, and P.
Balling, Appl. Surf. Sci. 476, 221 (2019).
15
X.-B. Li, X. Liu, X. Liu, D. Han, Z. Zhang, X. Han, H.-B. Sun, and S. Zhang,Phys. Rev. Lett. 107, 015501 (2011).
16
A. V. Kolobov, P. Fons, A. I. Frenkel, A. L. Ankudinov, J. Tominaga, and T.
Uruga, Nat. Mater. 3, 703 (2004).
17
J. Takeda, W. Oba, Y. Minami, T. Saiki, and I. Katayama, Appl. Phys. Lett. 104,
261903 (2014).
18
P. Klarskov, H. Kim, V. L. Colvin, and D. M. Mittleman, ACS Photonics 4,2676 (2017).
19
T. L. Cocker, V. Jelic, M. Gupta, S. J. Molesky, J. A. Burgess, G. De Los Reyes,
L. V. Titova, Y. Y. Tsui, M. R. Freeman, and F. A. Hegmann, Nat. Photonics 7,
620 (2013).
20
V. Jelic, K. Iwaszczuk, P. H. Nguyen, C. Rathje, G. J. Hornig, H. M. Sharum, J.
R. Hoffman, M. R. Freeman, and F. A. Hegmann, Nat. Phys. 13, 591 (2017).
21
Y. Terada, S. Yoshida, O. Takeuchi, and H. Shigekawa, Nat. Photonics 4, 869(2010).
22
T. L. Cocker, D. Peller, P. Yu, J. Repp, and R. Huber, Nature 539, 263 (2016).
23
K. Yoshioka, I. Katayama, Y. Minami, M. Kitajima, S. Yoshida, H. Shigekawa,and J. Takeda, Nat. Photonics 10, 762 (2016).
2

FIG. 4. (a) Topographic image of Ge2Sb2Te5(100 nm)/HOPG surface with a 100 nm
 100 nm area. (b) IðV ¼ 200 mVÞ=IðV ¼ 600 mVÞ mapping of the same
area as (a) before and (c) after laser irradiation.

localized within a few nanometers from the tip center due to the ﬁeld
enhancement by the tip-sample gap.29,42 It is well known that theamorphous-to-crystalline phase change can be introduced by either
weak heating6,7 or irradiating with intense terahertz pulses.8 These will
be conducted in the future to examine the rewritable characteristics ofthe nanoscale phase change of GST.
In conclusion, we have demonstrated that STM coupled withfemtosecond laser pulses can be utilized to non-thermally create
nanometer-scale amorphous marks on GST surfaces. The amorphous
region can be detected through either the changes in the topographic

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24

K. Yoshioka, I. Katayama, Y. Arashida, A. Ban, Y. Kawada, K. Konishi, H.
Takahashi, and J. Takeda, Nano Lett. 18, 5198 (2018).
25
S. Huang, M. Hong, Y. Lu, B. Lukyanchuk, W. Song, and T. Chong, J. Appl.
Phys. 91, 3268 (2002).
26
A. Chimmalgi, D. J. Hwang, and C. P. Grigoropoulos, Nano Lett. 5, 1924
(2005).
27
A. Chimmalgi, C. Grigoropoulos, and K. Komvopoulos, J. Appl. Phys. 97,
104319 (2005).
28
A. Chimmalgi, T. Choi, C. Grigoropoulos, and K. Komvopoulos, Appl. Phys.
Lett. 82, 1146 (2003).
29
Y. Chen, Y. Xu, D. Xie, J. Jiang, and Y. Yue, Appl. Therm. Eng. 148, 129 (2019).
30
C. Pauly, M. Liebmann, A. Giussani, J. Kellner, S. Just, J. Sanchez-Barriga, E.
Rienks, O. Rader, R. Calarco, G. Bihlmayer et al., Appl. Phys. Lett. 103, 243109(2013).
31
Z. Zhang, J. Pan, Y. L. Foo, L. W.-W. Fang, Y.-C. Yeo, R. Zhao, L. Shi, and T.C. Chong, Appl. Surf. Sci. 256, 7696 (2010).
32
H. Dieker and M. Wuttig, Thin Solid Films 478, 248 (2005).
33
T. Nonaka, G. Ohbayashi, Y. Toriumi, Y. Mori, and H. Hashimoto, Thin Solid
Films 370, 258 (2000).

Appl. Phys. Lett. 117, 211102 (2020); doi: 10.1063/5.0032573
Published under license by AIP Publishing

ARTICLE

scitation.org/journal/apl

34

Y. Pan, Z. Li, and Z. Guo, Crystals 9, 136 (2019).
J. Kellner, G. Bihlmayer, V. Deringer, M. Liebmann, C. Pauly, A. Giussani, J.
Boschker, R. Calarco, R. Dronskowski, and M. Morgenstern, Phys. Rev. B 96,
245408 (2017).
36
M. Wuttig, D. L€
usebrink, D. Wamwangi, W. Wełnic, M. Gilleßen, and R.
Dronskowski, Nat. Mater. 6, 122 (2007).
37
A. H. Edwards, A. C. Pineda, P. A. Schultz, M. G. Martin, A. P. Thompson, H.
P. Hjalmarson, and C. J. Umrigar, Phys. Rev. B 73, 045210 (2006).
38
T. Schafer, P. M. Konze, J. D. Huyeng, V. L. Deringer, T. Lesieur, P. Muller,
M. Morgenstern, R. Dronskowski, and M. Wuttig, Chem. Mater. 29, 6749
(2017).
39
T. Kato and K. Tanaka, Jpn. J. Appl. Phys., Part 1 44, 7340 (2005).
40
K. Makino, J. Tominaga, and M. Hase, Opt. Express 19, 1260 (2011).
41
A. Stȩpniak, M. Caminale, A. Leon Vanegas, H. Oka, D. Sander, and J.
Kirschner, AIP Adv. 5, 017125 (2015).
42
X. Shi, N. Coca-L
opez, J. Janik, and A. Hartschuh, Chem. Rev. 117, 4945
(2017).
43
I. Horcas, R. Fernandez, J. Gomez-Rodriguez, J. Colchero, J. G
omez-Herrero,and A. Baro, Rev. Sci. Instrum. 78, 013705 (2007).
35

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