Photon Factory Activity Report 2019 #37 (2020)

BL-7C/ 2018G066

Surface elemental imaging of Ge quantum dots on a Si surface by XANAM
Shushi Suzuki1, Shingo Mukai2, Wang Jae Chun3, Masaharu Nomura4,
Syuntarou Fujimori1’, Mitsuhisa Ikeda1’, Katsunori Makihara1’,
Seiichi Miyazaki1’, and Kiyotaka Asakura2
1
Nagoya University, Nagoya, Aichi 464-8603, Japan
2
Hokkaido University, Sapporo, Hokkaido 001-0021, Japan
3
KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
4
International Christian University, Mitaka, Tokyo 181-8585, Japan
1. Introduction
In recent years, the importance of surface/interface
elemental analysis at the single-digit nanoscale has
increased with the demand for the application of single
atoms, clusters, and nanocrystals to electronic devices. Upto date, we developed X-ray aided noncontact atomic
force microscopy (XANAM), scanning probe microscopy
combined with synchrotron X-ray designed for elemental
analysis at the nanoscale. Previously, NC-AFM studies
showed that the tip-surface force involves a component ofcovalent bond force in addition to van der Waals force,electrostatic force. According to the idea, we expected thatthe covalent bond force could be a sign of an elemental
species on surfaces, with changing its force strength by Xray absorption, at which electron transition from a corelevel of sample’s atom to the electron states in the covalentbond. (Fig. 1). We reported the presence of the X-ray
induced change on the surfaces of gold, nickel, andgermanium (Ge) by irradiating the X-rays tuned at thecorresponding X-ray absorption edge energies of Au-L3,Ni-K, and Ge-K, respectively [1]. Further, the componentsof the tip-surface force were analyzed by the calculations
based on the NC-AFM theory. The results indicated thatat least two force components of the covalent bond force
and the electrostatic force were involved in the signals, sothat we thought it proved at last, the above hypothesis that
the X-ray induced change could be used for the elemental
analysis.
However, there has been an issue with the spatial
resolution of this technique, XANAM as microscopy. The
lower limit of the spatial resolution has not been clear yet
due to the less measurement efficiency to acquire enoughdata points. The evidence is essential for futureapplications of XANAM to the research fields ofsemiconductors.

Fig. 1 Basic concept of XANAM

Here, we applied XANAM to Ge quantum dots
prepared on a silicon substrate to examine the lower limitof the spatial resolution of XANAM.
2. Experimental
The experiments were performed at BL-7C of thePhoton Factory, Institute of Material Structure Science,High Energy Accelerator Organization (KEK-IMS-PF). A
noncontact atomic force microscope was operated in an
ultrahigh vacuum (UHV) chamber equipped withberyllium windows for X-ray penetration. The data wasrecorded by Nanonis control systems (SPECS ZurichGmbH). An AFM-probe was fabricated by using a quartz
tuning fork with a similar configuration of the qPlus sensor[2]. The typical resonance frequency of the sensor was ~30
kHz. As the force signal, frequency shift (Δf) of the AFMprobe from the resonance frequency was recorded. All themeasurements were carried out under X-rays with theenergies around the Ge-K absorption edge. The data wasrecorded by Nanonis control systems (Nanonis, SPECS
Zurich GmbH).
3. Results and Discussion
XANAM imaging was carried out on Ge quantum dots
on a Si substrate. Variation of the force in threedimensions was measured by force volume spectroscopy,
which is composed of the spectroscopic data of tip-surface
force taken in two-dimensions on the surface. The data
resolution in the horizontal direction was determined by
the number of points at which the spectroscopy of Δf wasFig. 2 A force volume data of Δf (x, y, z) obtainedfor Ge quantum dots (X-ray energy: 11103 eV,Volume size: 80 × 80 × 3.0 nm3, Δf range: -5 ~ +1Hz)

Photon Factory Activity Report 2019 #37 (2020)

measured. In surface normal, the data resolution was thesame as the data points in the tip sweep direction.
Figure 2 shows a force volume data of Δf (x, y, z)obtained for Ge quantum dots obtained at the beamline
under X-rays tuned at the Ge-K absorption edge energy of11103 eV. The data resolutions in the horizontal andvertical directions are 1.25 nm/points and 0.1 nm/points,respectively. The volume size of data is 80 × 80 × 3.0 nm3.
Compared with the previous experiments, the improvedacquisition rate of the force spectroscopy was about fourtimes, and the recording time par data was 40 min. The
color bar indicates the magnitude of Δf within the rage of-5 - +1 Hz. Horizontal images could be extracted at eachpoint along the vertical direction, resulting in 30 slices of
XANAM images from this volume data.
Figure 3 shows images of possibly two quantum dots
regenerated from the volume data at the tip-surface
separation (z) within the range of 0.4-1.2 nm. The image
size is 40 × 40 nm2. The images of (a), (b), (c), and (d)were obtained under X-ray irradiation with the energies of11053 eV, 11103 eV, 11153 eV, and 11203 eV,respectively. The bright region was observed at the centerin each image and was assigned to the quantum dots. The
dark region around them was assigned to the bare SiOx
surface. We found that the topography of quantum dots
was modulated by increasing the X-energy in theseparation range of 0.4-0.7 nm. Compared with the images
for 11053 eV and 11203 eV, the topography in images for11103 eV and 11153 eV indicated small changes in its
shape. However, the topography in the far region, morethan 0.8 nm, it showed changes depending on the X-ray
energy became ambiguous. In the far region, theelectrostatic force is dominant, compared with thecovalent bond force. Thus, it could be deduced under our
hypothesis that the change in the covalent bond force wascovered by the variation in the electrostatic force, resulting
in the vague change.
Consequently, we found that the X-ray energy
dependence of the force change at the nanoscale on the Ge
quantum dots. By estimated from the data resolution, wethought that the spatial resolution of XANAM wasachieved to at least a few nanometer scale.
4. Conclusion
We have developed XANAM to obtain elemental
information on surfaces at the nanoscale using NC-AFM
combined with SR X-ray. To evaluate the details of X-ray
induced force change and the lower limit of the spatial
resolution of XANAM as microscopy, we observed the Ge
quantum dots on the Si substrate under X-ray irradiation.
From the force volume date in three dimensions, theXANAM images of the Ge quantum dots were obtained.
Further, the X-ray energy dependence of the images wasobserved, as observed in the spectroscopic data in theprevious studies [1]. We concluded that tentatively thespatial resolution of XANAM could be achieved at least afew nanometer scale.
Acknowledgments
This work was supported financially by JSPS
KAKENHI Grant Number JP19K05266, and Tatematsu
Foundation, as well as the previous grants from the JSPS,JST, and NEDO. The experiments were carried out under
the approval of Photon Factory Advisory Committee(PAC: Proposal No. 2012G099, 2014G114, 2016G109,2018G066).
References[1] S. Suzuki, Y. Koike, K. Fujikawa, W.-J. Chun, M.
Nomura, and K. Asakura , Chem. Lett., 33, 636-637,2004.; S. Suzuki, Y. Koike, K. Fujikawa, N.
Matsudaira, M. Nakamura, W.-J. Chun, M. Nomura,and K. Asakura, Catal. Today, 117, 80-83, 2006.; S.
Suzuki, M. Nakamura, K. Kinoshita, Y. Koike, K.
Fujikawa, N. Matsudaira , W.-J. Chun, M. Nomura,and K. Asakura, J. Phys., 61, 1171-1121, 2007.;
Suzuki, S., Bull. Chem. Soc. Jpn., 88, 240-250,2015.; Please also see Photon Factory Activity
Reports 2012-2019.
[2] F. J. Giessibl, Appl. Phys. Lett. 76, 1470-1472, 2000.
[3] J. E. Sader and S. P. Jarvis, Appl. Phys. Lett., 84,1801-1803, 2004.
*shushi@chembio.nagoya-u.ac.jp

Fig. 3 Extracted-Δf (x, y) images of quantum dots
prepared on a Si substrate obtained at X-ray
energies of (a)11053 eV, (b)11103 eV, (c)11153
eV, and (d)11203 eV. (Image size: 40 × 40 nm2, Δf
range: -5 ~ +1 Hz)