Band gap and band offsets for ultrathin (HfO2) x(SiO2)1-x dielectric films on Si (100)

Article abstract of DOI:10.1063/1.2355453

Energy band profile of ultrathin Hf silicate dielectrics, grown by atomic layer deposition, was studied by using x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. The band gap energy only slightly increases from 5.52 eV fo

Full text of DOI:10.1063/1.2355453

Band gap and band offsets for ultrathin ( Hf O 2 ) x ( Si O 2 ) 1 − x dielectric films on Si
100)
(
H. Jin, S. K. Oh, H. J. Kang, and M.-H. Cho
Citation: Applied Physics Letters 89, 122901 (2006); doi: 10.1063/1.2355453
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APPLIED PHYSICS LETTERS 89, 122901 ͑2006͒
Band gap and band offsets for ultrathin „HfO … „SiO …
dielectric
2
x
2 1−x
films on Si „100…
a͒
H. Jin, S. K. Oh, and H. J. Kang
BK21 Physics Program and Department of Physics, Chungbuk National University,
Cheongju 361-763, Korea
M.-H. Cho
Nano Surface Group, Korea Research Institute of Standard and Science, Daejeon 305-606, Korea
͑
Received 11 May 2006; accepted 30 July 2006; published online 19 September 2006͒
Energy band profile of ultrathin Hf silicate dielectrics, grown by atomic layer deposition, was
studied by using x-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy.
The band gap energy only slightly increases from 5.52 eV for ͑HfO2͒0.75͑SiO ͒
to 6.10 eV for
2 0.25
͑
HfO2͒0.25͑SiO ͒ , which is much smaller than 8.90 eV for SiO . For ultrathin Hf silicate
2 0.75 2
dielectrics, the band gap is mainly determined by the Hf 5d conduction band state and the O 2p
valence band state. The corresponding conduction band offsets are in the vicinity of 1 eV, which
satisfies the minimum requirement for the carrier barrier heights. © 2006 American Institute of
Physics. ͓DOI: 10.1063/1.2355453͔
Rapid progress in complementary metal-oxide semicon-
ductor integrated circuit technology demands for greater
integrated circuit functionality and performance, which in
turn requires an increased circuit density. This rapid shrink-
age of transistor feature size has forced the size of gate
x-ray photoelectron spectroscopy ͑XPS͒ analysis. XPS
analysis can provide us important information on the valence
band offset.
Hf silicate gate dielectric thin films, whose stoichiometry
was given by ͑HfO2͒x ͑SiO2͒͑1−x͒, were deposited by atomic
layer deposition. Prior to thin film deposition, p-type Si͑100͒
oxides to be less than subnanometers. Nevertheless, SiO as
2
gate dielectric will soon reach its fundamental limit owing to
substrate was cleaned chemically by the Radio Corporation
1
,2
9
an unacceptably high leakage current, which implies that
of America ͑RCA͒ method. SiO and HfO2 films were
2
SiO should be replaced by other high-k gate dielectrics
grown at temperatures below 280 °C using tris͑dimethylami-
do͒silane and tetrakis͑ethylmethylamido͒hafnium, respec-
2
eventually. Among the numerous alternative high-k materi-
3
–5
als, HfO , ZrO , and their silicates have received much
tively, as precursors. O vapor served as the oxygen source
2
2
3
1
0
attention as viable candidate materials. Especially, high-k
alloys are proposed as promising gate dielectrics. The oxy-
gen in Hf silicate is covalently bonded and it does not have
high ion diffusivity, whereas oxygen ion diffusivity in tran-
sition metal oxides is high because of their ionic bond
and N2 was supplied as the purge and carrier gas. The
SiO2-incorporated Hf–Si–O was grown by the alternating
ALD process, repeating many cycles for HfO2 and SiO2 to
mix SiO2 content in a controlled manner. HfO2 mole fraction
was varied by amounts of x=0.75, 0.5, and 0.25. The thick-
1
ness of ͑HfO ͒ ͑SiO ͒ films was 3 nm. The film compo-
nature. By considering all of the requirements, Hf silicate
2 x
2 1−x
seems to be a reasonable candidate material, and, in fact, Hf
silicate has a variety of desirable properties, which includes
reasonable stability against crystallization and phase separa-
tion, superior interface thermal stability in contact with either
a Si substrate or a poly-Si gate electrode, and excellent elec-
sition was determined by utilizing XPS. Subtracting the Shir-
ley background to correct for effects of inelastic electron
11
scattering and considering the sensitivity factor, corre-
sponding ratios of Hf/Si are found to be 3:1, 1:1, and 1:3.
According to Auger depth profile, not shown here, the thin
film is distributed uniformly. XPS and REELS measurements
were performed with a VG ESCALAB 210 equipment. The
chemical states of thin silicate films were obtained using
monochromatic Al K␣ x-ray source ͑1486.6 eV͒. The bind-
ing energies of the spectra were calibrated by the position of
1
,6,7
trical properties.
In this letter, we made Hf silicate ͑HfO ͒ ͑SiO ͒ thin
2 ͑1−x͒
2
x
films with various hafnium contents via atomic layer depo-
sition ͑ALD͒, and focused on determining the band gap and
band offsets for 3 nm ͑HfO ͒ ͑SiO ͒
dielectric thin films
2
x
2 ͑1−x͒
12
the substrate Si 2p peak at 99.3 eV. REELS was measured
on Si ͑100͒.
with the primary electron energy of 1000 eV for excitation
and with the constant analyzer pass energy of 20 eV. The full
width at half maximum of the elastic peak was 0.8 eV.
Figure 1͑a͒ shows the Hf 4f photoelectron spectra ob-
tained from the as-deposited films with the ratio of Hf to Si
given by 3:1, 1:1, and 1:3. The comparison of the peak in-
tensity shows the change of the Hf contents in films. Hf 4f7/2
peaks in Hf silicates are located at 17.9, 18.1, and 18.3 eV,
on the binding energy scale for the three different composi-
tion films. Wilk et al. reported Hf 4f peaks in Hf silicate
Reflection energy electron loss spectroscopy ͑REELS͒
was shown to be a good tool for us to investigate the band
8
gap for ultrathin dielectric films, because the low energy
electron energy loss spectrum reflects the valence and con-
duction band structures of the samples. Hence, we make use
of REELS analysis to investigate the band gap of Hf silicate
films with differing compositions. In order to interpret the
composition dependence of the band alignment in a Hf sili-
cate dielectric layer, we combined REELS analysis with
1
3
appearing at ϳ18.2 eV, which is about 1.0 eV higher than
a͒Electronic mail: hjkang@cbu.ac.kr
that in HfO . It is due to the fact that the charge transfer out
2
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003-6951/2006/89͑12͒/122901/3/$23.00 89, 122901-1 © 2006 American Institute of Physics
55.33.16.124 On: Wed, 26 Nov 2014 14:26:42
0
1

122901-2
Jin et al.
Appl. Phys. Lett. 89, 122901 ͑2006͒
FIG. 1. ͑Color online͒ Hf 4f ͑a͒ and Si 2p ͑b͒ photoelectron spectra for
nm Hf silicate: ͑HfO ͒ ͑SiO ͒ with x equal to 0.75, 0.5, and 0.25.
3
2
x
2 1-x
of Hf is larger in Hf silicate than in HfO because the elec-
2
tronegativities of both Si and O are larger than that of Hf. So
these spectra indeed indicate the existence of silicate struc-
ture. It is observed that the Hf 4f doublet is somewhat
FIG. 2. ͑Color online͒ REELS spectra for 3 nm HfO , SiO , and Hf silicate
2
2
thin films, using primary electron energy of 1000 eV. The band gap energy
Eg is defined as the threshold energy of band-to-band excitation.
broader with more SiO incorporated, while Hf silicate with
2
x=0.75, having high HfO2 fractions, shows sharper Hf
peaks.
of a loss signal spectrum to the background level. The cross-
ing point gives the band gap value.
Figure 2 shows the REELS spectra for Hf silicate as a
function of composition. For comparison, REELS spectra for
1
6
The Si 2p photoelectron spectra also reveal the forma-
tion of a silicate film. In Fig. 1͑b͒, the Si 2p electron peak
shifted to a higher binding energy with reduced Hf content.
The Si 2p peak located at 99.3 eV originates from the Si
ultrathin HfO and SiO dielectric films were shown. The E
2
2
g
1
2
values are 5.50 ͑Ref. 17͒ and 8.90 eV for the ultrathin HfO2
substrate. Other peaks, centered around 102–102.7 eV,
and SiO dielectric films, respectively, and they show a slight
originate from the Hf silicate film, and they are associated
with Hf–O–Si bond. Hf silicate peak position is located at a
lower energy compared with Si–O bond at 103.3 eV. The
2
change with increasing Hf content. The obtained band gap
values are shown in Table I. Here we marked the E values
g
shift to low binding energy moving from Si4+ in SiO to Si
*
of HfO and SiO for comparison. For HfO thin film, we
2 2 2
2
can see the bulk plasmon peak at 16 eV and the surface
in silicate oxidation state should be due to the change of Si’s
second-nearest neighbor from Si to Hf by introducing more
plasma peak at 9 eV, whereas for SiO the bulk plasmon
2
1
4
appears at 23 eV with a shoulder at 15 eV. With decreasing
Hf composition, the plasmon peak shape becomes more
Hf atoms. In addition, since Hf has a lower Pauling’s elec-
tronegativity compared to that of Si, there is an enhanced
charge transfer from Hf atom to silicate Si–O bond as com-
pared to the O–Si–O situation. This results in a shift of the Si
SiO -like. In addition, the intensity for the HfO characteris-
2
2
tic loss peak also decreases upon decreasing Hf composition.
Thereby, from the energy loss spectra, we can observe the
phase change for the films with different compositions. The
variation of composition changes the electronic structure,
which in turn affects the band gap value. So the change in Eg
for these Hf silicate films mainly originates from the slight
difference in electronic structure.
2
p core level in Si–O–Hf to a lower binding energy relative
to SiO and the shift increasing with the number of Hf
2
second-nearest neighbors. All this chemical information, ob-
tained via the XPS analysis, shows the composition depen-
dence of bond states and assures the deposition of Hf silicate.
This excessive energy shift caused by the incorporation
of HfO into SiO indicates that charge changes, induced by
It is well known for SiO2 that the conduction band mini-
mum ͑CBM͒ states are formed from the Si extended s states,
whereas the valence band maximum ͑VBM͒ states are
2
2
incorporation of HfO , make SiO with different covalences.
2
2
From the XPS peak energy shift, we can argue that the level
of the Hf d state depends sensitively on the charge transfer to
the surrounding O atom.
1
8
formed from the nonbonding O 2p state. For the transition
metal HfO2, the CBM states are mostly composed of Hf
1
9
In order to investigate the change in the band gap, owing
to composition variation in ultrathin Hf silicate films, we
have made use of the REELS and found the electronic struc-
localized 5d state, and the VBM states are mostly com-
posed of O 2p states. In Hf silicate dielectric films, when an
extended s state in SiO is compared to localized d states in
2
1
5
ture near the band gap at the atomic level. We can deter-
mine the band gap values of dielectric films from an energy
loss signal, which is obtained by drawing a linear fit line
with a maximum negative slope from a point near the onset
HfO , their energy will be affected by the energy level of the
2
lowest conduction band state. It is also known that the
2
0
coordination-based mechanism plays an important role in
electronic structure. When introducing HfO into SiO , the
2
2
TABLE I. Band gap and band offset values for the ultrathin gate oxide films.
Energy ͑eV͒
HfO2
Hf0.75Si0.25O2
Hf0. 5Si0.5O2
Hf0.25Si0.75O2
SiO2
Eg
5.50
2.86
5.52
3.38
5.70
3.42
6.10
3.81
8.90
4.36
⌬
Ev
⌬
E
1.54
1.04
1.18
1.19
3.44
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122901-3
Jin et al.
Appl. Phys. Lett. 89, 122901 ͑2006͒
2
1
also obtained by the method described in the reference.
Values for the band alignment are summarized in Table I for
these ultrathin gate oxide films. ⌬E for these Hf silicate
c
films are in the vicinity of 1 eV. It satisfies the minimum
requirement for the barrier heights of over 1 eV in devices.
We note that the conduction band offsets for Hf silicate films
have nothing to do with the composition change, even
though there is a slight increase in the band gap with the
increasing of SiO contents. This implies that the increase in
2
the band gap is counteracted by the increase in the valence
band offset. The barrier heights for electrons and holes are
asymmetric in these ͑HfO ͒ ͑SiO ͒ dielectrics.
2
x
2 1-x
In summary, the band alignment evolution of Hf silicates
as a function of composition was demonstrated by REELS
and XPS analyses. It shows slight dependence on the com-
pound composition and increases slightly with decreasing Hf
contents. It was found that the band gap slightly increases
FIG. 3. ͑Color online͒ Valence band spectra for 3 nm HfO , SiO , and Hf
2
2
silicate thin films. The dotted line denotes the top of the Si valence band
energy E ͑Si͒. The valence band offset energy ⌬E is defined as the energy
v
v
difference between the valence band maximum of the dielectrics and
that of Si.
from 5.52 eV for ͑HfO2͒0.75͑SiO ͒
dielectric thin film to
2 0.25
6
.10 eV for ͑HfO2͒0.25͑SiO ͒
dielectric thin film. For the
2 0.75
Hf–Si–O bonds are formed by breaking up of the continuous
covalent network. It is clear that the nonbonding O 2p state
is on the top of the valence band, and thus the contribution of
Hf and Si to the density of states for the conduction band is
proportional to their composition. For Hf-rich silicate film,
Hf-rich silicate film, E is mainly determined by the Hf 5f
g
states and O 2p states. The band gap values can be controlled
with appropriate Hf metal composition to have the optimum
value. REELS analysis turns out to be a powerful and con-
venient method to obtain information on electronic structure
near the band gap of a film. We found that the Hf silicates are
promising candidates for alternative gate dielectric materials
because they have adequate band gaps to ensure a sufficient
conduction band offset to Si. Thus we anticipate that the Hf
silicates will be used as alternative gate dielectrics for ad-
vanced Si devices in the near future.
the band gap value is smaller than that for SiO and is similar
2
to HfO . It means that the energy level of the d electron is
2
lower than the s electron energy level, and then, as more and
more Hf atoms are added to the SiO , the Hf induced states
2
would fill in and broaden to become the lowest conduction
band of Hf silicate. The E in ͑HfO ͒ ͑SiO ͒ is domi-
g
2 0.75
2 0.25
nated by the O 2p and Hf 5d states. The slight increase in Eg
for ͑HfO2͒0.25͑SiO ͒ could be due to the low density of Hf
This work was supported by Korea Research Foundation
grant ͑KRF-2003-005-C00015͒.
2 0.75
5
d electronic states in the film and the overlap of the Si 3s
state and the localized Hf 5d state, which was caused by O
coordination changes within the alloy.
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2
7
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0
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v
3
.81 eV for ͑HfO2͒0.75͑SiO ͒ , ͑HfO ͒ ͑SiO ͒ , and
2 0.25 2 0.5 2 0.5
20͑
2001͒.
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This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:
dition, the corresponding conduction band offsets ͑⌬E ͒ are
Phys. Lett. 87, 212902 ͑2005͒.
c
1
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