Monocliniclike local atomic structure in amorphous ZrO2 thin film

Article abstract of DOI:10.1063/1.3497077

The local atomic structure and electronic structure of amorphous ZrO ₂ (a-ZrO₂) thin film were examined using the ZrK - and OK-edge x-ray absorption spectroscopy and x-ray photoelectron spectroscopy. It was found that a monoclinic local structure is stabilized in several nanometers-thick a-ZrO₂ films due to the structural disorder. The distinct local structure in a-ZrO₂ from the ordinary tetragonal ZrO₂ (t-ZrO₂) films results in different electronic structure with a decrease in the band gap by 0.5 eV. The reduced band gap and dielectric constant of a ZrO₂ suggest inferior gate leakage current performances compared to the t-ZrO₂ films.

Full text of DOI:10.1063/1.3497077

Monocliniclike local atomic structure in amorphous ZrO 2 thin film
Deok-Yong Cho, Hyung-Suk Jung, Jeong Hwan Kim, and Cheol Seong Hwang
Citation: Applied Physics Letters 97, 141905 (2010); doi: 10.1063/1.3497077
View online: http://dx.doi.org/10.1063/1.3497077
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APPLIED PHYSICS LETTERS 97, 141905 ͑2010͒
Monocliniclike local atomic structure in amorphous ZrO2 thin film
Deok-Yong Cho,1,a͒ Hyung-Suk Jung,2 Jeong Hwan Kim,2 and Cheol Seong Hwang2,b͒
1CSCMR and FPRD, Department of Physics and Astronomy, Seoul National University,
Seoul 151-747, Republic of Korea
2WCU Hybrid Materials Program, Department of Materials Science and Engineering, and Inter-university
Semiconductor Research Center, Seoul National University, Seoul 151-744, Republic of Korea
͑Received 30 August 2010; accepted 15 September 2010; published online 5 October 2010͒
The local atomic structure and electronic structure of amorphous ZrO2 ͑a-ZrO2͒ thin film were
examined using the Zr K- and O K-edge x-ray absorption spectroscopy and x-ray photoelectron
spectroscopy. It was found that a monoclinic local structure is stabilized in several nanometers-thick
a-ZrO2 films due to the structural disorder. The distinct local structure in a-ZrO2 from the ordinary
tetragonal ZrO2 ͑t-ZrO2͒ films results in different electronic structure with a decrease in the band
gap by 0.5 eV. The reduced band gap and dielectric constant of a-ZrO2 suggest inferior gate leakage
current performances compared to the t-ZrO2 films. © 2010 American Institute of Physics.
͓doi:10.1063/1.3497077͔
A high dielectric constant ͑k͒ is essential for gate oxides
in metal-oxide-semiconductor field effect transistors because
it prevents the current leakage through the gate oxide. Fol-
lowing the intensive research to find oxides with higher k
and reasonably large band gaps, the Hf-based oxide system is
currently being used as the gate dielectric in several high
performance logic chips.1 Recently, Zr-based oxides are also
drawing an increasing amount of attention because they are
expected to exhibit similar electrical performances with a
more superior interface, which may lead to an improvement
in interface related reliability properties compared to the Hf-
based oxides.2–4 Thus it is undoubtedly important to study
their electronic structure theoretically and experimentally.
The bulk hafnia ͑HfO2͒ and zirconia ͑ZrO2͒ are remark-
ably alike in terms of electrochemical properties. They have
a monoclinic crystal structure ͑m;P21/c͒ at room tempera-
ture with almost identical lattice constants.5,6 The similarity
originates from the identical valences ͑+4͒ and orbital con-
figurations ͑d0͒, along with the similar ionic radii ͑within
difference of only 0.02 Å͒ of the Hf and Zr ions.7 When in
nanoscale structures such as ultrathin films ͑which are only
several nanometers thick͒ or nanoparticles, however, their
preferred crystal structures are different from each other; the
HfO2 nanostructure still prefers the monoclinic structure
while the ZrO2 nanostructure favors a tetragonal structure
͑t;P42/nmc͒ ͑Ref. 8͒ even at room temperature.9 The stabi-
lization mechanism of the tetragonal structure has been ex-
ploited extensively but is still rather controversial; perhaps, it
could be summarized as a combination of the effects of sur-
face and stress.9,10
gate oxide is frequently desired due to its homogeneous and
isotropic electrical properties. In a HfO2 thin film, the dielec-
tric properties are rarely influenced by amorphization be-
cause the monoclinic local structure is preserved even in the
amorphous phase, which is why the k of amorphous HfO2
͑a-HfO2͒ is roughly similar to that of monoclinic HfO2
͑m-HfO2͒.14 However, this experimental study shows that in
amorphous ZrO2 ͑a-ZrO2͒ thin films, a monoclinic local
structure is re-stabilized due to the structural disorder, so the
k of the a-ZrO2 film would become similar to that of m-ZrO2
͑kϳ20͒, which is much lower than that of t-ZrO2.14 This
suggests that a-ZrO2 may have inferior dielectric perfor-
mance compared to crystalline t-ZrO2 in next-generation
high-k dielectric applications.
ZrO2 films were prepared on a Si wafer by plasma
enhanced atomic layer deposition ͑PEALD͒ using
Zr͓N͑C2H5͒͑CH3͔͒ as the Zr-precursor and plasma-
4
activated O2 as the oxygen source at a wafer temperature
of 280 °C. The film thickness was controlled by tuning the
number of the PEALD cycles and was confirmed by ellip-
sometry. The 40 and 80-Å-thick as-grown films were amor-
phous, which was confirmed by the absence of Bragg peaks
in grazing angle x-ray diffraction ͑GAXRD͒. Postdeposition-
annealing ͑PDA͒ was also performed at 950 °C in a N2
ambiance to make the films crystalline. The GAXRD con-
firmed that the PDA films crystallized with a tetragonal sym-
metry ͑P42/nmc͒. The local structures were examined by
Zr K-edge x-ray absorption spectroscopy ͑XAS͒ and the
x-ray absorption near-edge structure ͑XANES͒ was ana-
lyzed. The electronic structure and chemistry were examined
by O K-edge XAS and x-ray photoelectron spectroscopy
͑XPS͒. The Zr K- and O K-edge XAS were performed at the
3C1 and 2A beamlines in the Pohang Light Source ͑PLS͒,
respectively. The XPS was performed using a monochro-
matic Al K␣ source, after cleaning the film surfaces with a
moderate Ar-ion sputtering ͑acceleration voltage Ͻ0.5 keV͒.
Figure 1 shows the Zr K-edge XANES of the ͑a͒ PDA
and ͑b͒ as-deposited ZrO2 films.15 The overall XANES fea-
tures are almost thickness independent; the results of the 80-
Å-thick PDA and as-deposited films are quite similar to those
of the 40-Å-thick PDA and as-deposited films, respectively.
It has been reported the k value in Zr-based oxides is
mainly determined from the local atomic structure, although
the charge transfer between the adjacent clusters or structural
units also have a minor influence.11,12 The k value of t-ZrO2
is about two times larger ͑kϳ40͒ than that of m-ZrO2 ͑k
ϳ20͒ due to the difference in local structure.13 Therefore, it
is much encouraged to use tetragonal ZrO2 for Zr-based ox-
ide gate dielectrics to obtain a higher k. Also an amorphous
a͒
Electronic mail: zax@snu.ac.kr.
Electronic mail: cheolsh@snu.ac.kr.
b͒
0003-6951/2010/97͑14͒/141905/3/$30.00
97, 141905-1
© 2010 American Institute of Physics
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141905-2
Cho et al.
Appl. Phys. Lett. 97, 141905 ͑2010͒
FIG. 2. ͑Color online͒ O K-edge XAS spectra of 80 Å-thick ZrO2 and HfO2
films. t-ZrO2 ͑m-HfO2͒, tetragonal ͑monoclinic͒ PDA films; a-Zr͑Hf͒O2,
as-deposited amorphous film.
FIG. 1. Zr K-edge XANES of the ͑a͒ PDA and ͑b͒ as-deposited ZrO2 films.
The simulation results based on tetragonal and monoclinic local structures
were appended in ͓͑a͒ and ͑b͔͒, respectively. The spectrum of a polycrystal-
line ZrO2 powder was also attached.
⌬Fbulk = Fbulk͑t͒ − Fbulk͑m͒ Ͼ 0,
⌬Fsurf = Fsurf͑t͒ − Fsurf͑m͒ Ͻ 0,
However, the features of the PDA films are quite different
from those of the as-deposited films, as highlighted by the
arrows in Fig. 1. This indicates the PDA process rather than
the film thickness is responsible for the spectral evolution. In
order to interpret the features in detail, the theoretical spectra
⌬Ftot = Ftot͑t͒ − Ftot͑m͒ Ͻ 0,
͑1͒
where F’s are the free energies for the total, bulk, and surface
terms, and t and m in the parentheses indicate the tetragonal
and monoclinic local structures, respectively.
16
obtained by FEFF8 and appended.15 Here, the full multiple
However, it is shown in Fig. 1 that the monoclinic local
structure is stabilized in the amorphous zirconia films, even
though the as-deposited films are identical in geometric
terms with the tetragonal PDA films. The structural disorder
in the a-ZrO2 possibly enables this type of stabilization by
lowering the Fbulk͑m͒ due to the additionally increased bulk
entropy ͑Sbulk;Fbulk=Ebulk−TSbulk͒. This would allow the
scatterings within a radius of 6 Å around the photon absorber
were considered. The simulations were performed based on
the representative tetragonal and monoclinic local
structures.17–19 The simulated spectra of t-ZrO2 and m-ZrO2
well reproduce all the features in the experimental spectra of
the PDA and as-deposited films, respectively, indicating that
the PDA films have a tetragonal local structure while the
as-deposited films have a monoclinic local structure. There-
fore, the discrepancy in the experimental spectra in Figs. 1͑a͒
and 1͑b͒, denoted by arrows, can be regarded as the signa-
tures of tetragonal co-ordinations.
The intensities of the oscillatory features in the spectrum
of the 40 Å-PDA film appears to be slightly reduced com-
pared to those of the 80 Å-PDA film due to the reduced
crystallinity in the thinner film. This is because the reduced
crystallinity would enhance the structural disorder, which at-
tenuates the fine structure oscillation. The extinction of the
features in the spectra of the as-deposited films ͓Fig. 1͑b͔͒,
however, cannot be attributed to the influence of the struc-
tural disorder, since the main peak near h␯ϳ18.01 keV sur-
vived. For clarity, the spectrum of a polycrystalline ZrO2
powder ͑m; P21/c͒, which had distinct Bragg peaks in the
XRD ͑not shown here͒, was appended in the figure. The
coincidence of the XANES features clearly shows the as-
deposited films have monoclinic local structure.
ء
Ftot͑m͒ in the a-ZrO2, which is quite different from the
Ftot͑m͒ in a hypothetical m-ZrO2, to become lower than
Ftot͑t͒. The existence of a monoclinic local structure in the
a-ZrO2 thin films can be understood using this rationale.
The different local structures between the amorphous
and crystalline ZrO2 films result in distinct electronic struc-
tures. Figure 2 shows the unoccupied electronic structures of
the 80-Å-thick films probed by O K-edge XAS. From now
on, t-ZrO2 ͑a-ZrO2͒ refers to the 80-Å-thick PDA ͑as-
deposited͒ film. The spectra of 80-Å-thick crystalline m- and
a-HfO2 films, which were prepared through a similar proce-
dure to this experiment except for the precursor, are ap-
pended for comparison.15 The overall features in the spectra
of a-ZrO2 and a-HfO2 are broadened out from their respec-
tive crystalline phases due to the structural disorder. Interest-
ingly, the line shapes of the a-ZrO2 and a-HfO2 films are
very similar to each other except for an energy shift due to
the band gap difference.20 This suggests a similarity in the
local structure, since the O K-edge XAS probes the local
density of states of the O 2p wave function that hybridizes
with the neighboring Zr or Hf d/sp bands, where the hybrid-
ization symmetry is determined by the local structures. It is
well known a-HfO2 thin films have a monoclinic local
structure,21 so the results above reconfirm the monoclinic
local structure of a-ZrO2.
In a bulk form, zirconia prefers to have an m structure
than a t structure because the bulk free energy of the m phase
is lower than that of the t ͑or cubic͒ phase.10 However, in a
nanostructural form, the surface free energy becomes com-
parable to the bulk free energy. Since the surface free energy
of the t phase is much lower than that of the m phase, the
total free energy of the t phase could be lower than that of
the m phase at the nano-scale.10 This can be formulated as
follows:
Generally, the dielectric property is determined from the
local structure and the electronic structure. The dielectric re-
sponse of media, ␧ or k=␧/␧0, can be decomposed into two
factors; an optical term and a vibronic term.22,23 The former
F
= F + F
,
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141905-3
Cho et al.
Appl. Phys. Lett. 97, 141905 ͑2010͒
trapolating the steepest slopes in the features near the band
gap. The VB edge energies almost coincided with each other
with a BE of ϳ3.2 eV while the CB edge energy of a-ZrO2
͑E−EFϳ+1.5 eV͒ differs from that of t-ZrO2 ͑E−EFϳ
+2.0 eV͒ by approximately Ϫ0.5 eV. The reduced band gap
and lower CB edge will deteriorate the performance of the
high-k gate oxide by facilitating leakage current.
In conclusion, it was confirmed that a-ZrO2 thin films
have a monoclinic local structure, so it was expected to have
a k value lower than that of crystalline t-ZrO2. The CB edge
energy of a-ZrO2 was lower than that of t-ZrO2 by ϳ0.5 eV.
Due to these reasons, the dielectric performance of the
a-ZrO2 dielectrics would be less desirable compared to
t-ZrO2 films although more uniform characteristics are ex-
pected from the a-ZrO2 dielectrics.
FIG. 3. ͑Color online͒ ͑a͒ Zr 3d and ͑b͒ O 1s XPS spectra of the 80 Å a-
and t-ZrO2 films, indicating identical chemistry.
The authors acknowledge the supports from Hynix Co.
through the system IC 2010 project of the Korean Govern-
ment, and from NRF funded by MEST through the Converg-
ing Research Center Program ͑Grant No. 2009-0081961͒ and
WCU program ͑Grant No. R31-2008-000-10075-0͒. The ex-
periments at the PLS were supported in part by MEST and
POSTECH.
is related to the optical electronic transition which is deter-
mined by the electronic structure, and the latter concerns the
phononic vibrations which are sensitive to the local structure.
Thus, the monoclinic local structure of a-ZrO2 suggests that
the k should be similar to monoclinic ZrO2, kϳ20, not
t-ZrO2, kϳ40.14
Now, the influence that local structural change during
amorphization has on the chemistry and valence bands/
conduction bands ͑VBs/CBs͒ of the films are described. Fig-
ure 3 shows the ͑a͒ Zr 3d and ͑b͒ O 1s core level XPS
spectra of the t-ZrO2 and a-ZrO2 films.15 Negligible differ-
ences between the spectra of the two films were found at
both core levels. The same binding energy ͑BE͒ and intensity
of the main peaks at both core levels suggest an identical
chemistry of the Zr4+ and O2− ions in the t-ZrO2 and a-ZrO2
films. The additional tiny features in the t-ZrO2 at BE
ϳ181 eV in Fig. 3͑a͒ and at BEϳ532 eV in Fig. 3͑b͒,
might reflect the Zr-silicide formation at the interface and the
remaining adsorbed molecules at the surface, respectively.
The identical O2− chemistry allows a direct comparison of
energies in the O K-edge XAS spectra without any uncer-
tainty on the core hole effect in the O 1s-2p photoabsorption
process. In Fig. 4, the CBs near the band gaps of the two
films were redrawn from Fig. 2, being shifted by a same
amount of energy to reproduce the band gap of t-ZrO2 to the
well known value of ϳ5.2 eV.24 The XPS VB spectra are
also appended.15 The edge energies were determined by ex-
1G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, 5243
͑2001͒.
2R. I. Hegde, D. H. Triyoso, S. B. Samavedam, and B. E. White, Jr., J.
Appl. Phys. 101, 074113 ͑2007͒.
3H.-S. Jung, J.-M. Park, H. K. Kim, J. H. Kim, S.-J. Won, J. Lee, S. Y. Lee,
C. S. Hwang, W.-H. Kim, M.-W. Song, N.-I. Lee, and D.-Y. Cho, Electro-
chem. Solid-State Lett. 13, G71 ͑2010͒.
4D. H. Triyoso, R. I. Hegde, J. K. Schaeffer, R. Gregory, X.-D. Wang, M.
Canonico, D. Roan, E. A. Hebert, K. Kim, J. Jiang, R. Rai, V. Kaushik, S.
B. Samavedam, and N. Rochat, J. Vac. Sci. Technol. B 25, 845 ͑2007͒.
5X. Luo, W. Zhou, S. V. Ushakov, A. Navrotsky, and A. A. Demkov, Phys.
Rev. B 80, 134119 ͑2009͒.
6P. W. Peacock and J. Robertson, Phys. Rev. Lett. 92, 057601 ͑2004͒.
7R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen.
Crystallogr. A32, 751 ͑1976͒.
8P. Aldebert and J.-P. Traverse, J. Am. Ceram. Soc. 68, 34 ͑1985͒.
9R. C. Garvie, J. Phys. Chem. 69, 1238 ͑1965͒.
10P. Southon, Structural evolution during the preparation and heating of
nanophase zirconia gels, Ph.D. dissertation, University of Technology,
2000.
11G.-M. Rignanese, F. Detraux, X. Gonze, A. Bongiorno, and A. Pas-
quarello, Phys. Rev. Lett. 89, 117601 ͑2002͒.
12G. Lucovsky and G. B. Rayner, Jr., Appl. Phys. Lett. 77, 2912 ͑2000͒.
13X. Zhao and D. Vanderbilt, Phys. Rev. B 65, 075105 ͑2002͒.
14D. Ceresoli and D. Vanderbilt, Phys. Rev. B 74, 125108 ͑2006͒.
15The spectra of 80-Å-thick PDA films were taken from D.-Y. Cho, H. S.
Jung, and C. S. Hwang, Phys. Rev. B 82, 094104 ͑2010͒.
16A. L. Ankudinov, B. Ravel, J. J. Rehr, and S. D. Conradson, Phys. Rev. B
58, 7565 ͑1998͒.
17J. Kang, E.-C. Lee, and K. J. Chang, Phys. Rev. B 68, 054106 ͑2003͒.
18M. Winterer, R. Delaplane, and R. McGreevy, J. Appl. Crystallogr. 35,
434 ͑2002͒.
19The simulated spectrum for a cubic zirconia is same as that of tetragonal
zirconia due to their similar local structures.
20J. Robertson, K. Xiong, and S. J. Clark, Thin Solid Films 496, 1 ͑2006͒.
21D.-Y. Cho, T. J. Park, K. D. Na, J. H. Kim, and C. S. Hwang, Phys. Rev.
B 78, 132102 ͑2008͒.
22G.-M. Rignanese and A. Pasquarello, J. Phys.: Condens. Matter 17, S2089
͑2005͒.
23H. Momida, T. Hamada, Y. Takagi, T. Yamamoto, T. Uda, and T. Ohno,
Phys. Rev. B 75, 195105 ͑2007͒.
FIG. 4. ͑Color online͒ VB structures measured by XPS and CB structures
measured by O K-edge XAS of the 80 Å a- and t-ZrO2 films. The band edge
energies were determined by extrapolating the steepest slopes ͑arrows͒.
24D. Tahir, E. K. Lee, S. K. Oh, T. T. Tham, H. J. Kang, H. Jin, S. Heo, J.
C. Park, J. G. Chung, and J. C. Lee, Appl. Phys. Lett. 94, 212902 ͑2009͒.
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