Structural, electronic, and dielectric properties of ultrathin zirconia films on silicon

Article abstract of DOI:10.1063/1.1864235

As high-permittivity dielectrics approach use in metal-oxide-semiconductor field-effect transistor production, an atomic level understanding of their dielectric properties and the capacitance of structures made from them is being rigorously pursued. We and others have shown that crystal structure of Zr O2 films have considerable effects on permittivity as well as band gap. The as-deposited films reported here appear amorphous below a critical thickness (~5.4 nm) and transform to a predominantly tetragonal phase upon annealing. At much higher thickness the stable monoclinic phase will be favored. These phase changes may have a significant effect on channel mobility.

Full text of DOI:10.1063/1.1864235

Structural, electronic, and dielectric properties of ultrathin zirconia films on silicon
S. Sayan, N. V. Nguyen, J. Ehrstein, T. Emge, E. Garfunkel, M. Croft, Xinyuan Zhao, David Vanderbilt, I. Levin,
E. P. Gusev, Hyoungsub Kim, and P. J. McIntyre
Citation: Applied Physics Letters 86, 152902 (2005); doi: 10.1063/1.1864235
View online: http://dx.doi.org/10.1063/1.1864235
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/86/15?ver=pdfcov
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APPLIED PHYSICS LETTERS 86, 152902 ͑2005͒
Structural, electronic, and dielectric properties
of ultrathin zirconia films on silicon
S. Sayan, N. V. Nguyen, and J. Ehrstein
Semiconductor Electronics Division, National Institute of Standards and Technology,
Gaithersburg, Maryland 20899
T. Emge and E. Garfunkel
Department of Chemistry, Rutgers University, Piscataway, New Jersey 08854
M. Croft, Xinyuan Zhao, and David Vanderbilt
Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854
I. Levin
Ceramics Department, National Institute of Standards and Technology, Gaithersburg, Maryland 20899
E. P. Gusev
IBM Thomas J. Watson Research Center, Semiconductor Research and Development Center (SRDC),
Yorktown Heights, New York 10598
Hyoungsub Kim and P. J. McIntyre
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
͑Received 25 May 2004; accepted 17 December 2004; published online 4 April 2005͒
As high-permittivity dielectrics approach use in metal-oxide-semiconductor field-effect transistor
production, an atomic level understanding of their dielectric properties and the capacitance of
structures made from them is being rigorously pursued. We and others have shown that crystal
structure of ZrO2 films have considerable effects on permittivity as well as band gap. The
as-deposited films reported here appear amorphous below a critical thickness ͑ϳ5.4 nm͒ and
transform to a predominantly tetragonal phase upon annealing. At much higher thickness the stable
monoclinic phase will be favored. These phase changes may have a significant effect on channel
mobility. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1864235͔
As the effective gate oxide thickness scales towards
1 nm, the use of SiOxNy dielectric layers become less viable
due to high tunneling currents.1,2 One solution being exten-
sively explored is to increase capacitance by replacing
SiO2-based dielectrics with higher permittivity ones such as
HfO2 and ZrO2.2 The dielectric constant values in the litera-
ture vary from 16 to 453–7 for HfO2 and 18 to 358–10 for
ZrO2. Similarly, the reported band gap values for these ma-
terials are in the range of 5.1 to 6.0 eV, with ZrO2 reported
to have a smaller band gap.4,8,11,12
A variety of factors, such as degree of crystallinity,
roughness, chemical homogeneity, and stoichiometry, can ef-
fect the dielectric properties and leakage currents across the
dielectric layers.2 There have been many reports on the elec-
trical properties of zirconia films, but detailed reports on mi-
crostructural characterization along with electrical character-
istics are scarce.13
The known crystal phases of ZrO2 are monoclinic,
orthorhombic, cubic, and tetragonal ͑the latter three phases
are metastable under ordinary temperatures and pressures͒.
Film thickness, stress, grain size, and impurities may lead to
changes in the relative of these phases. It is therefore quite
reasonable to expect different electronic and dielectric re-
sponses for ultrathin films of different thicknesses, some of
which may be in metastable states. In addition, films pre-
pared by different methods ͑chemical vapor deposition,
atomic layer deposition, or physical vapor deposition͒ can
result in quite different physical and electronic properties.
Zhao and Vanderbilt performed first-principles density
functional theory calculations on all crystal phases of HfO2
and ZrO2, where it was shown that the dielectric response
and band gap of the material are strongly phase
dependent.14,15 In this contribution, we experimentally ex-
plore the issue of crystal structure in ultrathin films of ZrO2
and the effect of structure on dielectric properties.
A series of thicknesses ͑in the range of 36 to 99 Å͒ of
ZrO2 films were deposited on silicon substrates ͑on either a
chemical oxide or an oxynitride layer͒ at 300 °C by atomic
layer deposition using ZrCl4 and H2O as precursors. Some of
the samples were annealed in various ambient and tempera-
ture regimes including a 500 °C oxidation, a 400 °C form-
ing gas anneal, 600 and 800 °C rapid thermal anneals
͑RTA/N2͒, and an 800 °C vacuum anneal ͑10−9 Torr͒.
The x-ray absorption spectroscopy ͑XAS͒ measurements
were performed at Brookhaven ͑NSLS͒ in the total electron
yield mode.16
Wide-angle x-ray scattering was performed to obtain
x-ray diffraction ͑XRD͒ patterns of several ZrO2 thin-film
samples using an area detector and a rotating anode x-ray
generator equipped with
a
graphite monochromator
͑Cu K␣;␭=1.5418 Å͒.
Vacuum ultraviolet spectroscopic ellipsometry ͑VUV-
SE͒ measurements were performed on a commercial instru-
ment from 1.5 to 8.5 eV in steps of 0.02 eV. A four-phase
model consisting of silicon substrate, SiO2 interfacial oxide,
ZrO2 film, and air ambient was employed to extract the real
and imaginary part of dielectric functions ͑␧=␧1+i␧ ͒ of the
2
ZrO2 films, where the interfacial oxide thicknesses are ex-
tracted by high-resolution transmission electron microscopy.
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0003-6951/2005/86͑15͒/152902/3/$22.50 86, 152902-1 © 2005 American Institute of Physics
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152902-2
Sayan et al.
Appl. Phys. Lett. 86, 152902 ͑2005͒
FIG. 1. XRD spectra of ZrO2 films of optical thickness 49, 69, 80, and
91 Å.
FIG. 2. Zr L2-edge XAS spectra of bulk monoclinic and yttrium stabilized
cubic phases of zirconia, a representative film spectrum of tetragonal ZrO2
͑confirmed by XRD͒, and representative spectra of amorphous ZrO2 films
before and after annealing. The distinct spectral changes associated with the
tetragonal phase, induced by annealing ͑44 Å RTA 600 °C spectrum͒ or
increased film thickness ͑71 Å as-deposited͒ should be noted.
The thicknesses reported here are the ZrO2 optical film thick-
nesses.
Figure 1 presents selected XRD spectra for a series of
as-deposited ZrO2 films. For the samples with optical thick-
nesses greater than Ϸ49 Å, only the tetragonal phase can be
positively identified. Based upon the decreasing relative in-
tensity and increasing relative peak width of the ͗101͘ reflec-
tion ͑2␪=30.3° for the tetragonal phase͒, the degree of crys-
tallinity decreases with decreasing film thickness. The 49 Å
as-deposited film is found to be predominantly amorphous.
The measurements on thinner samples did not yield useful
information because of insufficient scattering.
not consistent with that of any pure single phase ͑i.e., tetrag-
onal, cubic, or monoclinic͒. The XRD measurements indicate
that thin as-deposited samples are predominantly amorphous.
The spectrum obtained for as-deposited thicker films ͑e.g.,
the 71 Å film in Fig. 2͒ is consistent with that of the tetrag-
onal phase, indicating that, as the films get thicker, the te-
tragonal structure becomes favored over an amorphous one.
Anneals were performed in various ambient and tem-
perature regimes. An amorphous-to-tetragonal phase trans-
formation was observed for the thinner fims as a result of all
annealing treatments tested, as represented by the 44-Å-thick
annealed ͑600 °C RTA͒ sample, as shown in Fig. 2.
The XAS near-edge structure of the L2 and L3 edges of
Zr compounds are dominated by transitions into unoccupied
final Zr 4d states ͑Fig. 2͒. Although both the L2 and L3 edges
probe the 4d density of states, we discuss the L2 edge here
because it involves only 4d3/2 final states and is less sensitive
to multiplet effects.17 The lower energy ͑less intense͒ and
higher energy ͑more intense͒ peak features in the cubic ZrO2
spectrum at the bottom of Fig. 2 are due to eg and t2g final
states, respectively. The additional splitting of the lower
symmetry monoclinic phase is reflected by the additional
Zr L2 edge spectral features ͑at around Ϸ2311.8 eV͒. In par-
ticular, the sharpening and narrowing of the splitting between
the two peak features on going from the cubic to tetragonal
phase ͑see the spectral overlay in Fig. 2 at the bottom͒ is
supported by our band structure calculations and by the
O K-edge electron energy loss spectroscopy measurements
on these phases where a Ϸ0.2 eV narrowing is observed go-
ing from the cubic to the tetragonal phase.18,19 A point to
emphasize here is that the peak splittings and relative inten-
sity in the Zr L2-edge measurements provide a convenient
tool to identify the structural changes in these ZrO2 films.
The XAS L2-edge spectra of a thin, as-deposited ZrO2
Figure 3 presents the real ͑␧ ͒ and imaginary ͑␧ ͒ part of
1
2
dielectric functions of these films. Two important trends ob-
served in the dielectric functions are the dependence on
thickness and thermal annealing. As the thickness of as-
deposited films increased above a transitional value, some-
where between 54 and 64 Å, the optical band gap deter-
mined by Tauc plots ͑not shown͒ increased from
5.1 to 5.5 eV and remained at 5.5 eV for thicker films. In
addition, the dielectric function exhibits new interband tran-
sitions ͑Ϸ6.0, 7.3, and 8.5 eV͒, and the overall magnitude
increases with thickness. The band gap of the thin samples
increase from 5.1 to 5.5 eV after being annealed at 600 °C
and also display new interband transitions similar to those
observed for thicker films ͑lower panel in Fig. 3͒.
A main finding is that there is a transition from an amor-
phous to a tetragonal phase in the thickness regime explored
here. This phase change should increase the dielectric con-
stant of the ZrO2 film to a large value, as predicted by
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film of 44 Å optical thickness are also given in Fig. 2. It is
Vanderbilt and Zhao.15,19 XRD studies of an equivalently
On: Tue, 02 Dec 2014 02:37:25

152902-3
Sayan et al.
Appl. Phys. Lett. 86, 152902 ͑2005͒
the film is too thin, then it may exhibit a predominantly
amorphous phase, whereas for intermediate thickness films
͑Ͼ5 nm͒, the more stable tetragonal phase is able to form.
͑ii͒ The change of crystal structure will also have important
consequences from a band alignment point of view. As
shown in Fig. 3, the tetragonal phase has a larger band gap
than the amorphous phase by about Ϸ0.4 eV. This will lead
to different band offsets and thereby affect charge transport
and leakage. ͑iii͒ From a metrology point of view, effects of
phase on the dielectric properties should be fully understood
for any given system.
We have shown that both amorphous and tetragonal
phases of ZrO2 can be observed in thin films. Film thickness
is a key factor regulating which phase is observed under any
set of specific conditions. Processing history ͑especially the
maximum temperature a film experiences͒ also affects phase.
In microelectronic applications, we suggest that the crystal
structure of thin high-␬ films should be both determined and
stated in comparative experimental studies of dielectric and
electrical properties. Film stress, thickness, grain-size, and
impurities may lead to stabilization or predominance of one
or more phases, which in turn result in variations in dielectric
and electronic response.
The authors would like to thank the SRC, Sematech,
NIST, and the NSF for financial support. Two authors ͑D.V.
and X.Z.͒ acknowledge NSF Grant DMR-0233925 for finan-
cial support.
FIG. 3. Dielectric functions of ZrO2 films resulting from VUV spectro-
scopic ellipsometry for as-deposited films of 44, 71, and 99 Å and for a
53 Å film after RTA at 600 °C. Real ͑top͒ and imaginary ͑bottom͒ parts of
dielectric functions.
1M. L. Green, E. P. Gusev, R. Degrave, and E. Garfunkel, J. Appl. Phys.
90, 2057 ͑2001͒.
2G. D. Wilk, R. M. Wallace, and J. M. Anthony, J. Appl. Phys. 89, 5243
͑2001͒.
produced 800 Å film that show the coexistence of similar
amounts of tetragonal and monoclinic ͑the stable phase͒
components. Recent work by Navrotsky et al.20,21 on bulk
zirconia films of varying particle size has demonstrated that
the total energy of the system is minimized by moving from
amorphous to tetragonal to monoclinic with increasing par-
ticle size. We observe the same trends with thickness. In the
thickness regime below some transitional value ͑of order
54 to 64 Å͒, the optical band gap of the as-deposited ZrO2
films ͑amorphous͒ is 5.1 eV, while above this regime it in-
creases to 5.5 eV ͑in the predominantly tetragonal phase͒.
Annealing of the amorphous films results in a transformation
to the tetragonal phase, and the band gap increases to 5.5 eV.
TEM results on similar films show the existence of tetrago-
nal crystallites at a slightly lower nominal thickness.
These findings lead one to consider several issues when
thinking of dielectric design and metrology. On one hand,
one wants a high capacitance dielectric structure, while on
the other, the film should result in a laterally smooth and
homogeneous potential in the channel. ͑i͒ Since the tetrago-
nal phase has the larger dielectric constant, it should be pre-
ferred if it can be made uniform. This can be achieved by
tailoring film thickness or annealing the gate stack, both of
which can lead to formation of the tetragonal phase. Another
route is to stabilize by impurity incorporation, as is done for
the cubic phase.22 By growing under conditions that prefer
͑or stabilize͒ the tetragonal phase, one can take advantage of
higher dielectric constant ͑lower equivalent oxide thickness͒,
larger band gap, and lower leakage. Our results imply that if
3I. Barin and O. Knacke, Thermodynamic Properties of Elements and Ox-
ides ͑Springer, Berlin, 1973͒.
4M. Balog, M. Schieber, M. Michman, and S. Patai, Thin Solid Films 41,
247 ͑1977͒.
5C. T. Hsu, Y. K. Su, and M. Yokoyama, Jpn. J. Appl. Phys., Part 1 31,
2501 ͑1992͒.
6B. H. Lee, L. Kang, R. Nieh, W.-J. Qi, and J. C. Lee, Appl. Phys. Lett. 76,
1926 ͑2000͒.
7Y.-S. Lin, R. Puthenkovilakam, and J. P. Chang, Appl. Phys. Lett. 81,
2041 ͑2002͒.
8M. Balog, M. Schieber, M. Michman, and S. Patai, Thin Solid Films 47,
109 ͑1977͒.
9B.-O. Cho, J. Wang, L. Sha, and J. P. Chang, Appl. Phys. Lett. 80, 1052
͑2002͒.
10M. Houssa, V. V. Afanas’ev, and A. Stesmans, Appl. Phys. Lett. 77, 1885
͑2000͒.
11J. Robertson, J. Vac. Sci. Technol. B 18, 1785 ͑2000͒.
12T. Nishide, S. Honda, M. Matsuura, and M. Ide, Thin Solid Films 371, 61
͑2000͒.
13S. Ramanathan, P. C. McIntyre, J. Luning, P. Pianetta, and D. A. Muller,
Philos. Mag. Lett. 82, 519 ͑2002͒.
14X. Zhao and D. Vanderbilt, Phys. Rev. B 65, 233106 ͑2002͒.
15X. Zhao and D. Vanderbilt, Phys. Rev. B 65, 075105 ͑2002͒.
16Y. Jeon, J. Chen, and M. Croft, Phys. Rev. B 50, 6555 ͑1994͒.
17F. M. F. d. Groot, Physica B 208–209, 15 ͑1995͒.
18D. W. McComb, Phys. Rev. B 54, 7094 ͑1996͒.
19X. Zhao and D. Vanderbilt, in First Principles Study of Electronic and
Dielectric Properties of ZrO2 and HfO2, ͑Materials Research Society,
Warrendale, PA, 2003͒, pp. 283–288.
20A. Demkov ͑personal communication͒.
21S. V. Ushakov, C. E. Brown, A. Navrotsky, A. Demkov, C. Wang, and
B.-Y. Nguyen, Mater. Res. Soc. Symp. Proc. 745, 3 ͑2003͒.
22K. Kita, K. Kyuno, and A. Toriumi, Appl. Phys. Lett. 86, 102906 ͑2005͒.
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