Photon Factory Activity Report 2019 #37 (2020)

BL-11D/2019G060

Surface-sensitive chemical analysis of ultrathin Hf/Si(557) filmby photoelectron - Auger-electron coincidence spectroscopy
Takuhiro Kakiuchi1,*, Yuki Yamamoto1, and Kazuhiko Mase2, 3Faculty of Science, Ehime University, 2-5 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan2
Photon Factory, KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
3
SOKENDAI, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan

2 Experiment
A clean Si(557)-7×7 with single domain (SD) structure
was prepared by direct current heating at 1520 K in
ultrahigh vacuum (UHV) achieved at 4.0 × 10-8 Pa. An
ultrathin Hf film with a thickness of ~2.0 layers (1 layer =2.4 Å [4]) was deposited on a clean Si(557)-7×7 surface.
Si 2p photoelectron spectrum and Si-L23VV-Si-2p
APECS spectrum were measured with a handmade
electron-electron-ion coincidence analyzer at BL-11D in
PF, KEK. The photon energies (h) at 112 and 130 eV were
irradiated to sample for ionization of Si 2p core level. The
escape depth (ED) of Si 2p photoelectron (EDPE) was
estimated to be 20-30 Å at h = 112 eV. In addition, theescape depth of Si-2p-Si-L23VV PEACS (EDPEACS) can beobtainedby
1⁄𝐸𝐷PEACS = 1⁄(𝐸𝐷𝑃𝐸 ∙ cos𝜃PE ) +1⁄(𝐸𝐷𝐴𝐸 ∙ cos𝜃AE ) [5]. Here, the EDAE is the ED of Si
L23VV Auger-electron, and the PE (AE) is the detection
angle of photoelectron (Auger-electron). In this study, the
EDPEACS was estimated to be 1.3 Å at h = 130 eV.
3 Results and Discussions
Figure 1 shows (a) the conventional Si 2p
photoelectron spectrum measured at h = 112 eV and (b)the Si-L23VV-Si-2p APECS spectrum measured at h = 130
eV, respectively. These are drawn by solid circles. Somefitting curves are drawn by solid lines (2p3/2) and dashedlines (2p1/2). The spectrum in Fig.1 (a) is mainly composedof bulk Si component, on the other hand, the spectrum in
Fig. 1 (b) is composed of some surface components ofSi(557)-7×7 and segregation components of Hf silicides(HfSi and HfSi2). These results indicate that islands of Hfsilicides were formed on bare Si(557)-7×7 substrate after
annealing over 1073 K. Our consideration is consistent
with the observation in scanning electron microscope.
PEACS intensity (arb. unit)

1 Introduction
Drastic downsizing of the ultrathin SiO2 gate dielectric
film (< 10 Å) for the metal-oxide-semiconductor field
effect transistor (MOSFET) has caused a critical leakage
current [1]. Therefore, the identification of an alternative
SiO2 dielectric was an immediate issue to keep thedevelopment of semiconductor industry by following
Moore’s law. Hafnium dioxide (HfO2, Hafnia) is apromising candidate for the alternative gate dielectric dueto (1) a high dielectric constant (k, ~25), (2) a stability incontact with silicon, (3) a large bandgap (> 4-5 eV), (4) alow density of defects at the interface between HfO2 andsilicon, (5) an equivalent physical thickness to SiO2 layer
less than 10 Å, and so on [1-3].
In addition, an idea of the 3-dimensional structure of the
MOSFET is attracting to increase the number of devices in
an integrated circuit. This indicates that the behaviors ofhafnium (Hf) species on various facets of silicon substrate
become to be meaningful to control the stable operation.
In this study, we prepared ultrathin Hf film on a cleanSi(557)-7×7 surface. After annealing at 1073 K, weinvestigated the chemical states at interface and surface byusing a conventional photoelectron spectroscopy and a Si
2p photoelectron - Si L23VV Auger-electron coincidence
spectroscopy (Si-2p-Si-L23VV PEACS).

1.0

Photoelectron count (kcps)

1

4.0

Fitting line
PEACS result
EDPEACS = 1.3 Å(b) PEACS

0.8
0.6
0.4
0.2

Pedestal atom

Bulk

Adatom

HfSi2
HfSi

Si1+

0.0
2

1

(a) XPS
3.0

0

Bulk

Fitting line
XPS result

2.0

-1

-2

EDPE = 20-30 Å

Pedestal atom
HfSi2

1.0

HfSi

0.0
2

1
0
-1
Relative binding energy (eV)-2Fig. 1: Si 2p photoelectron peaks obtained by (a) aconventional Si 2p photoelectron spectroscopy and (b) aSi-2p-Si-LVV PEACS.
Acknowledgement
This work was supported by a Grant-in-Aid for
Scientific Research (No. 17K06030) from the Japanese
Ministry of Education, Culture, Sports, Science andTechnology. We thank the PF staff for supporting our
experiment.
References[1] A. I. Kingon et al., Nature 406, 1032 (2000).
[2] D. G. Schlom et al., MRS Bull. 27, 198 (2002).
[3] G. D. Wilk et al., J. Appl. Phys. 89, 5243 (2001).
[4] X. Zhao et al., Phys. Rev. B 65, 233106 (2002).
[5] T. Kakiuchi et al., Surf. Sci. 604, L27 (2010).
* kakiuchi.takuhiro.mc@ehime-u.ac.jp