Coordination and interface analysis of atomic-layer-deposition Al2O3 on Si(001) using energy-loss near-edge structures

Article abstract of DOI:10.1063/1.1629397

The inteface layer was studied in tems of Si chemical bonding based on SiL₂₃ ELNES. In the Si substrate, the SiL₂₃ edge is at 100 eV for the midpoint of edge onset, and is chemically shifted near the interface by about 5 eV, which is

Full text of DOI:10.1063/1.1629397

Coordination and interface analysis of atomic-layer-deposition Al 2 O 3 on Si(001)
using energy-loss near-edge structures
K. Kimoto, Y. Matsui, T. Nabatame, T. Yasuda, T. Mizoguchi, I. Tanaka, and A. Toriumi
Citation: Applied Physics Letters 83, 4306 (2003); doi: 10.1063/1.1629397
View online: http://dx.doi.org/10.1063/1.1629397
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APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 21
24 NOVEMBER 2003
Coordination and interface analysis of atomic-layer-deposition Al2O3
on Si„001… using energy-loss near-edge structures
K. Kimotoa) and Y. Matsui
Advanced Materials Laboratory, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba,
Ibaraki 305-0044, Japan
T. Nabatame
MIRAI Project, Association of Super-Advanced Electronics Technologies (ASET), 16-1 Onogawa, Tsukuba,
Ibaraki 305-8569, Japan
T. Yasuda
MIRAI Project, Advanced Semiconductor Research Center, National Institute of Advanced Industrial Science
and Technology (AIST), 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan
T. Mizoguchi and I. Tanaka
Department of Material Science & Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan
A. Toriumi
Department of Materials Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan, and MIRAI project, AIST, Japan.
͑Received 3 March 2003; accepted 26 September 2003͒
The coordination and interface of Al2O3 formed on Si͑001͒ by atomic layer deposition ͑ALD͒ were
studied using electron energy-loss spectroscopy in a transmission electron microscope. Al
energy-loss near-edge structures ͑ELNESs͒ were interpreted using first-principles calculations. The
Al L23 ELNESs show two peaks at 78.2 and 79.7 eV, which originate from tetrahedrally and
octahedrally coordinated aluminum, respectively. The depth profile of coordination in ALD
Al2O3 /Si was investigated. While both tetrahedrally and octahedrally coordinated Al atoms exist in
the ALD Al2O3 , the former is dominant near the interface. Aluminum silicate was detected near the
interface, and it may cause the difference in aluminum coordination. © 2003 American Institute of
Physics. ͓DOI: 10.1063/1.1629397͔
The development of high dielectric constant ͑high-k͒
materials is one of the critical subjects for future comple-
mentary metal-oxide-semiconductor ͑CMOS͒ devices.1
Al2O3 is a prospective high-k material as a replacement for
SiO2 , because of its large band gap, and its thermal and
chemical stability. The electrical properties of CMOS de-
vices directly depend, not only on the Al2O3 film, but also on
the interface between Al2O3 film and Si substrate. Therefore,
the microstructure and crystallographic studies of amorphous
Al2O3 film are required, such as the determination of the Al
coordination and its depth profile with high-spatial resolu-
tion. Al2O3 ultrathin films on Si substrates have been ana-
lyzed using x-ray photoelectron spectroscopy,2 secondary ion
mass spectrometry,3 and other methods.3–5
We applied electron energy-loss spectroscopy ͑EELS͒ in
a transmission electron microscope ͑TEM͒,6 which is effec-
tive for the characterization of ultrathin dielectrics for CMOS
devices in terms of high spatial resolution.7–9 Energy-loss
near-edge structure ͑ELNES͒ in an EEL spectrum is sensitive
to the valence and the coordination of specific elements.10,11
In the present study, the Al coordination and interface of
Al2O3 /Si are investigated using TEM EELS and the first-
principles calculations of ELNES.
deposition ͑ALD͒ of a 5-nm-thick layer at 300 °C using
Al(CH3)3 and H2O precursors on a H-terminated Si͑001͒
wafer.12 The ALD process is gaining acceptance because it
allows controllable and uniform deposition with an accuracy
of an atomic layer. Rapid thermal annealing ͑RTA͒ was done
after the deposition in a mixture of Arϩ1% O2 at 750 °C. A
TEM specimen for cross-sectional observations was made by
mechanical thinning and ion milling. We also analyzed a few
␥-alumina specimens for ELNES interpretation: commercial
␥-alumina powder ͑Nilaco Co., 99.99%͒ and ultrafine
␥-alumina particles prepared by the direct oxidation of alu-
minum in a mixture of Ar and O2 .13 Contrary to Si coordi-
nation in silica, the Al atom can occupy both tetrahedrally
and octahedrally coordinated sites in transition aluminas,
such as ␥-alumina. The ␥-alumina ͑widely defined͒ includes
␥ ͑narrowly defined͒, ␩, ␪, and other structures. These
␥-alumina structures are basically defective spinel structures
with cation-site vacancies. The proportion of Al coordination
͑tetrahedral:octahedral͒ is reported as follows: 36:51 ͑␩͒,
32:68 ͑␥͒, and 50:50 ͑␪͒;14 however, there is still a contro-
versy about cation-site vacancies.15 Although the proportions
of Al coordination in the present specimens ͑commercial
powder and ultrafine particles͒ are not clear, they include
both tetrahedrally and octahedrally coordinated Al atoms.
A TEM ͑Hitachi, HF-3000͒ with an energy-loss spec-
trometer ͑Gatan, GIF2002͒ was used at an acceleration volt-
age of 300 kV. Due to the field-emission electron gun of the
An Al2O3 /Si specimen was prepared by atomic layer
a͒
Author to whom correspondence should be addressed; electronic mail:
kimoto.koji@nims.go.jp
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0003-6951/2003/83(21)/4306/3/$20.00 4306 © 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 83, No. 21, 24 November 2003
Kimoto et al.
4307
FIG. 2. Cross-sectional TEM image of ALD Al2O3 /Si ͑a͒ and the profile of
the TEM image contrast ͑b͒.
FIG. 1. Experimental and calculation results of Al L23 ELNES. Two experi-
mental results obtained using commercial ␥-Al2O3 powder ͑a͒ and ultrafine
␥-Al2O3 particles prepared by the direct oxidation of aluminum ͑b͒ are
shown. Calculation results are shown as the sum ͑sum͒ of two calculations,
in which tet. and oct. Al sites are excited.
͓Fig. 1͑c͔͒. In the experiment, both tetrahedral and octahedral
sites are equally excited independently, so the two calcula-
tion results ͑tet. and oct.͒ are simply summed as the calcula-
tion result ͑sum͒ for comparison with experimental results.
Calculations successfully reproduce the experimental
ELNES, including absolute energy loss of the two peaks.
Few pioneering studies on Al ELNES calculation exist,23,24
and our calculation quantitatively shows that peak T at about
78.2 eV mainly originated from the tetrahedrally coordinated
Al and peak O at about 79.7 eV from the octahedrally coor-
dinated one. These assignments are informative to interpret
various ELNESs of related materials, such as HfAlOx . The
aforementioned difference in the experimental peak ratio
(T:O) is considered to be due to the difference in the pro-
portion of Al coordination ͑tetrahedral:octahedral͒, and is
consistent with the variation of its proportion in several
␥-aluminas.14
TEM and a customized software for drift correction, a high
energy resolution of about 0.3 eV was realized in EELS.16
Al L23 and Si L23 ELNESs were recorded. We also applied a
spatially resolved technique to analyze the thin film speci-
men. By this technique, EEL spectra from the different po-
sitions were measured simultaneously using
a two-
dimensional detector. The details in the spatially resolved
EELS techniques have been reported elsewhere.9,17
Theoretical calculations of Al L23 ELNES were made us-
ing the density-functional-theory-based first-principles or-
thogonalized linear combinations of atomic orbitals
͑OLCAO͒ method.15,18 In the present study, the crystal struc-
ture of ␪-Al2O314 was used for the OLCAO calculations. We
prepared two supercells composed of 80 atoms, in which the
Al site of interest is located at the center. Since fine struc-
tures in an EEL spectrum depend mainly on the neighboring
atoms, the calculation results of crystalline Al2O3 are appli-
cable for interpreting ELNESs of amorphous Al2O3 . It is
known that ‘‘core-hole effect’’ should be implemented for the
first-principles calculation of ELNESs,19–21 because the final
state is drastically different from the ground state, owing to
the interaction between the electron and the core hole. The
present first-principles calculations with the core-hole effect
can reproduce experimental ELNESs, such as so-called ‘‘ex-
citon peaks’’ observed near the threshold. Here, the core-hole
effect has been implemented in the OLCAO method as re-
ported by Mo et al.21 The initial ground state and the final
state with a core-hole at the center Al site were calculated,
and the photo absorption cross section was obtained by cal-
culating the dipole transition matrix. Absolute transition en-
ergy ͑energy-loss͒ was evaluated as the difference in total
energies in the initial and the final state. The ELNES calcu-
lation results of the OLCAO method have already shown
good agreement for silicon nitride11 and a few oxides.21,22
Figures 1͑a͒ and 1͑b͒ show the experimental results of
Al L23 ELNESs for two ␥-aluminas: ͑a͒ the commercial pow-
der and ͑b͒ the ultrafine particles. The Al L23 ELNES has two
peaks at 78.2 and 79.7 eV, as indicated by T and O. The two
spectra look similar, but the intensity ratio of peaks T to O is
slightly different. To identify the origin of the two peaks T
and O, we calculated two ELNESs in which the excited Al
Figure 2͑a͒ shows cross-sectional TEM image of ALD
Al2O3 /Si specimen, indicating an amorphous ALD Al2O3
layer. The line profile of the TEM image contrast ͓Fig. 2͑b͔͒
shows that image brightness is relatively high near the inter-
face, suggesting an interface layer. We estimated the inter-
face layer to be roughly 1.3 nm thick. Figure 3 shows
FIG. 3. Spatially resolved EEL spectra indicating Al L23 and Si L23
ELNESs. Note the depth dependence of peak intensity ratio (T:O) in
Al L23 , and the chemical shift of Si L23 near the interface. The interval of
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site is set at a tetrahedral site ͑tet.͒ or octahedral site ͑oct.͒
each EEL spectrum is 0.28 nm in depth.
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4308
Appl. Phys. Lett., Vol. 83, No. 21, 24 November 2003
Kimoto et al.
spatially-resolved EEL spectra obtained across a region from
the Si substrate to the ALD Al2O3 layer. The background in
each EEL spectrum was subtracted using the power-low
model.6 The interval of each spectrum is 0.28 nm in depth.
We discuss the Al L23 ELNES and its depth profile. The
aforementioned peaks T and O are also observed in the ALD
Al2O3 . One specific feature is that peak O is high on the
upper side of Al2O3 layer and peak T is relatively intense
near the Si substrate. Peak T near the interface is extraordi-
narily high compared to ␥-aluminas. It is known that thermal
treatment eventually transforms amorphous Al2O3 to
␥-alumina,14 but under insufficient thermal treatment such as
the present RTA, tetrahedrally coordinated Al is dominant,
especially near the interface.
We studied the interface layer in terms of Si chemical
bonding based on Si L23 ELNES ͑Fig. 3͒. In the Si substrate,
the Si L23 edge is at 100 eV for the midpoint of edge onset,
and is chemically shifted near the interface by about 5 eV,
which is the same as observed in silicates containing SiO4
tetrahedra6 or in amorphous SiO2 .9,17 Although peak inten-
sities gradually decrease toward the upper side, the Si L23
fine structure does not differ in the amorphous layer. We thus
conclude that an aluminum silicate interface layer is formed,
and Si atoms exist as SiO4 tetrahedra. It is estimated that
aluminum silicate near the interface may cause the difference
in aluminum coordination, because tetrahedrally coordinated
cation is dominant in silica. The aluminum silicate layer is
considered to be grown by the Si oxidation during the ALD
process, therefore this should be minimized to optimize the
growth conditions.12
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