We describe the electrical properties of atomic layer deposited TiO&lt;sub&gt;2&lt;/sub&gt;/Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; bilayer gate oxides which simultaneously achieve high gate capacitance density and low gate leakage current density. Crystallization of the initially amorphous TiO&lt;sub&gt;2&lt;/sub&gt; film contributes to a significant accumulation capacitance increase (∼33%) observed after a forming gas anneal at 400 °C. The bilayer dielectrics reduce gate leakage current density by approximately one order of magnitude at flatband compared to Al&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;3&lt;/sub&gt; single layer of comparable capacitance equivalent thickness. The conduction band offset of TiO&lt;sub&gt;2&lt;/sub&gt; relative to InGaAs is 0.6 eV, contributing to the ability of the stacked dielectric to suppress gate leakage conduction

Ahn, J.

Eizenberg, M.

Geppert, I.

Gunji, M.

Holland, M.

McIntyre, P.C.

Thayne, I.

English

Jaesoo Ahn

Irina Geppert

Marika Gunji

Martin Holland

Iain Thayne

Moshe Eizenberg

Paul C. McIntyre

Crossref

Applied Physics Letters

Titania/alumina bilayer gate insulators for InGaAs metal-oxide-semiconductor devices

Enlighten: Publications

Enlighten – Research publications by members of the University of Glasgow 
http://eprints.gla.ac.uk 
 
 
 
 
 
 
 
Ahn, J., Geppert, I., Gunji, M., Holland, M., Thayne, I., Eizenberg, M., 
and McIntyre, P.C. (2011) Titania/alumina bilayer gate insulators for 
InGaAs metal-oxide-semiconductor devices. Applied Physics Letters, 99 
(23). p. 232902. ISSN 0003-6951 
 
http://eprints.gla.ac.uk/64787/ 
 
Deposited on: 23 July 2012 
 
 
Titania/alumina bilayer gate insulators for InGaAs metal-oxide-
semiconductor devices
Jaesoo Ahn, Irina Geppert, Marika Gunji, Martin Holland, Iain Thayne et al. 
 
Citation: Appl. Phys. Lett. 99, 232902 (2011); doi: 10.1063/1.3662966 
View online: http://dx.doi.org/10.1063/1.3662966 
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i23 
Published by the American Institute of Physics. 
 
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Titania/alumina bilayer gate insulators for InGaAs
metal-oxide-semiconductor devices
Jaesoo Ahn,1 Irina Geppert,2 Marika Gunji,1 Martin Holland,3 Iain Thayne,3
Moshe Eizenberg,2 and Paul C. McIntyre1,a)
1Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA
2Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
3Nanoelectronics Research Centre, University of Glasgow, Glasgow G12 8LT, United Kingdom
(Received 1 August 2011; accepted 26 October 2011; published online 5 December 2011)
We describe the electrical properties of atomic layer deposited TiO2/Al2O3 bilayer gate oxides
which simultaneously achieve high gate capacitance density and low gate leakage current density.
Crystallization of the initially amorphous TiO2 film contributes to a significant accumulation
capacitance increase (33%) observed after a forming gas anneal at 400 C. The bilayer dielectrics
reduce gate leakage current density by approximately one order of magnitude at flatband compared
to Al2O3 single layer of comparable capacitance equivalent thickness. The conduction band offset
of TiO2 relative to InGaAs is 0.6 eV, contributing to the ability of the stacked dielectric to suppress
gate leakage conduction.VC 2011 American Institute of Physics. [doi:10.1063/1.3662966]
Because silicon semiconductor channels are reaching
their scaling limits, III-V compound semiconductor channels
coated by deposited high-k dielectrics are the subject of
intense interest for high performance metal-oxide-semicon-
ductor (MOS) devices beyond the 22 nm technology node.1
Unlike SiO2/Si technology, however, one of the main chal-
lenges in developing III-V based semiconductors is the diffi-
culty of preparing high quality gate oxides with low
interface defect densities.2 Atomic-layer-deposited (ALD)
Al2O3 is known to provide a relatively low interface defect
density and an unpinned Fermi level on InGaAs (100) sub-
strates.3,4 A particularly interesting approach is to use (1) an
InGaAs channel surface that is initially capped with an As2
layer that can be thermally desorbed prior to gate oxide dep-
osition3 and (2) forming gas anneals after gate electrode dep-
osition in order to passivate interface traps and border traps
in the ALD-Al2O3 layer.
5 However, Al2O3 is a moderate-k
dielectric compared to other higher-k oxides which have
been studied as SiO2 replacements,
6 and this hinders further
scaling of the gate oxide capacitance density. Oxide materi-
als with high dielectric constant such as TiO2 have a low
conduction band offset relative to a Si substrate;7 therefore,
they are not as effective in reducing leakage current as are
SiO2 and Al2O3. It is difficult to find a single oxide which
satisfies all requirements for end-of-roadmap MOS gate
dielectrics (high dielectric constant, low interface trap den-
sity, high thermal stability, etc.), making bilayer gate dielec-
trics an interesting option.8 In this letter, we report electrical
properties of ALD-TiO2/Al2O3 bilayer dielectrics with com-
parison to ALD-Al2O3 single layer dielectrics. We also dis-
cuss structural changes in the TiO2 layer after forming gas
anneal and the spectroscopic measurement of the band align-
ment of TiO2 and Al2O3 relative to n-In0.53Ga0.47As.
Epitaxial n-type In0.53Ga0.47As(100) channel layers of
500 nm thickness and Si doping concentration of
2.0 1016 cm3 were grown on heavily doped n-type InP
substrates by molecular beam epitaxy (MBE). The epilayers
were covered in-situ with an approximately 80 nm thick
amorphous As2 capping layer to protect the channel surface
from uncontrolled oxidation and contamination during wafer
transfer from the MBE chamber to the ALD reactor. The As2
capping layer was thermally desorbed at 380 C under high
vacuum in the ALD reactor prior to gate oxide deposition.
ALD-Al2O3 layers were deposited at a substrate temperature
of 270 C, using trimethylaluminum (TMA) and water vapor,
with a TMA pulsing first in the sequence. To fabricate the
bilayer structure, ALD-TiO2 deposition was performed at
150 C with a H2O/tetrakis(dimethylamino) titanium process
immediately after Al2O3 deposition, without a vacuum
break. Platinum gate electrodes of 50 nm thickness were e-
beam evaporated through a shadow mask, and wafer back
side contacts of 50 nm Au/20 nm Ti were deposited to reduce
the contact resistance. Post-metallization forming gas (5%
H2/95% N2) anneal (FGA) was done for 30min at 400
C.
Multi-frequency (1 kHz to 1MHz) capacitance-voltage
(C-V) measurements in the dark were performed on 7 nm
TiO2/2 nm Al2O3 bilayer MOS capacitors. The C-V charac-
teristic of the as-deposited TiO2/Al2O3/n-In0.53Ga0.47As bilayer
dielectric (Fig. 1(a)) exhibits large frequency dispersion
FIG. 1. (Color online) Multi-frequency (1 kHz–1MHz) C-V curves from (a)
as-deposited and (b) forming gas annealed Pt/7 nm TiO2/2 nm Al2O3/n-
In0.53Ga0.47As MOSCAPs.
a)Author to whom correspondence should be addressed. Electronic mail:
pcm1@stanford.edu.
0003-6951/2011/99(23)/232902/3/$30.00 VC 2011 American Institute of Physics99, 232902-1
APPLIED PHYSICS LETTERS 99, 232902 (2011)
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throughout the applied bias range and the C-V curves stretch
out through depletion, which shows that as-grown oxides con-
tain a large density of border traps and interface defects. Post-
metallization FGA effectively removes and/or passivates most
of the interface defect states present at the oxide/semiconductor
interface, which, as a result, reduces C-V stretch-out and fre-
quency dispersion as shown in Fig. 1(b). In addition, the flat-
band voltage was shifted close to the ideal value (0.5 eV,
given by the work function difference between gate metal and
semiconductor), which is indicative of reduction of the oxide
trapped charges. The post-metallization FGA effect on passi-
vating defect sites has previously been reported to occur in
Al2O3 single layer capacitors.
5,9 The key feature of the bilayer
capacitors, which is different from single layer dielectrics, is
that the maximum accumulation capacitance density of bilayer
gate oxide increases from 1.5lF/cm2 to 2.0lF/cm2 after FGA,
whereas no such effect of FGA has been observed for the sin-
gle layer Al2O3 MOSCAPs investigated.
10
The leakage current density across the bilayer gate
dielectrics was also compared to that of Al2O3 single layers.
Figure 2 plots the gate leakage current density measured at
flatband voltage as a function of the capacitance-derived
equivalent SiO2 thickness (CET). The plot clearly shows that
the addition of the TiO2 layer gives about one order of mag-
nitude reduction of the gate leakage current at a given CET.
The addition of a physically thicker but higher-k TiO2 layer
creates little change in gate capacitance density but greatly
suppresses gate leakage current. An electrostatic dipole
formed at the TiO2/Al2O3 interface can also inhibit leakage
current. In Al2O3/TiO2/n-GaAs devices, a dipole created
between the TiO2 and Al2O3 layers has been reported to
enhance the leakage current compared to TiO2/n-GaAs by
reducing the effective metal/semiconductor barrier height.11
It can be inferred that the reverse oxide stack order (TiO2/
Al2O3) should generate an opposite dipole, and this can
increase the barrier height and as a result lower the leakage
current density.
Microstructural changes were investigated in a Tecnai
G2 transmission electron microscope (TEM). Figure 3(a)
shows a cross-sectional TEM micrograph of the Pt/TiO2/
Al2O3/In0.53Ga0.47As stack after the FGA at 400
C for
30min. It is difficult to resolve the interface between the two
dielectrics, because both have an amorphous structure, with
similar densities and average atomic numbers. After the
FGA, areas with lattice fringes are observed in the TiO2 films
in both cross-sectional TEM (Fig. 3(a)) and the plan-view
TEM (Fig. 3(b)), indicating full or partial crystallization of
the TiO2 layer. Analysis of selected area electron diffraction
(SAED) patterns reveals that the crystalline TiO2 is predomi-
nantly in the anatase phase (Fig. 3(c)). In-situ TEM analyses
of the same bilayer dielectric combination on a Ge substrate
show that the crystalline anatase phase TiO2 becomes evi-
dent at an annealing temperature of 300 C and the crystal-
line rutile phase starts to form at 400 C.
The total accumulation capacitance density, Cacc, is
determined by several different capacitance factors, as
expressed in
1
Cacc
¼ 1
CAl2O3
þ 1
CTiO2
þ 1
CS þ Cit : (1)
Here, CAl2O3 and CTiO2 are the capacitances of the Al2O3 and
TiO2 layers, respectively, CS is the substrate capacitance,
and Cit is the interface trap capacitance. The dielectric con-
stant of anatase phase TiO2 is reported to be as high as 78,
12
and the rutile phase TiO2 can have a dielectric constant of up
to 170 depending on crystalline orientation,13 while the k-
value of the ALD-grown amorphous TiO2 was previously
found to be approximately 32.14 Crystallization of the TiO2
film is expected to contribute to the accumulation capaci-
tance increase after FGA, which was observed only in the
TiO2-containing samples. Another factor that can increase
the accumulation capacitance is densification of the oxide
layers during FGA, which makes the oxide layers physically
thinner. Fast interface traps can also affect the measured
accumulation capacitance through the interface trap capaci-
tance, Cit. Quantification of these various effects of FGA on
Cacc remains a topic of future work. The CET extracted for
the bilayer MOSCAP is <1.7 nm after FGA, which includes
the contributions of both the InGaAs substrate and of the ca-
pacitance of interface traps and near-interface border traps
on Cacc. However, both bilayer and single-layer MOSCAPs
have an ALD-Al2O3/InGaAs interface with very similar ther-
mal history and, therefore, we may assume that the trap con-
tributions are similar for each. We attribute most of the FGA
effect on Cacc seen in Fig. 1 to TiO2 microstructural changes,
including crystallization.
FIG. 2. (Color online) Plot of leakage current density versus CET for the
Al2O3 single layers () and TiO2/Al2O3 bilayers (n). The leakage currents
were measured at the flatband voltage.
FIG. 3. (Color online) (a) Cross-sectional TEM image and (b) plan-view
TEM image of TiO2/Al2O3 bilayer stack. (c) Selected area electron diffrac-
tion pattern showing anatase phase predominant.
232902-2 Ahn et al. Appl. Phys. Lett. 99, 232902 (2011)
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In order to investigate the effects of interface energy
barriers in the bilayer structure to suppress gate leakage con-
duction, the band alignment of the TiO2/Al2O3/
In0.53Ga0.47As system was determined by x-ray photoelec-
tron spectroscopy measurements. The valence band offsets
of the Al2O3 and TiO2 films to InGaAs estimated by meas-
uring the energy difference between the valence electrons
ejected from the Al2O3 and TiO2 films (second slope in Fig.
4(a)) and from the InGaAs wafer are 3.86 0.2 eV and
2.36 0.2 eV, respectively. The valence band offsets meas-
ured in our experiments are smaller than those calculated in
a recent report,15 but in agreement with experimental results
from other researchers for individual Al2O3 and TiO2 layers
on InGaAs substrate.16 Next, the bandgaps of the Al2O3 and
TiO2 films were determined by the O 1 s core level spectrum,
as shown in Fig. 4(b). The bandgap is the energy difference
between the O-Al or O-Ti peak and the beginning of the
each energy loss region (the region with higher binding ener-
gies). The extracted band gap values are 3.66 0.2 eV for
TiO2 and 6.76 0.2 eV for Al2O3, which are in good agree-
ment with literature for amorphous phase Al2O3 and TiO2.
17
Based on these findings, we can plot the band alignment dia-
gram of the TiO2/Al2O3/InGaAs system (Fig. 4(c)), where
the conduction band offset was calculated by the equation
DEC¼EG,oxideEG,InGaAsDEV. The conduction band off-
sets of the TiO2 and Al2O3 with respect to InGaAs are
0.66 0.2 and 2.26 0.2 eV, respectively. This result indicates
the ability of the physically thicker bilayer dielectric to
reduce gate leakage conduction when TiO2 is stacked on the
Al2O3/InGaAs MOS structure.
In conclusion, we have demonstrated 1.7 nm CET with
1.9 106 A/cm2 leakage current density at the flatband
voltage with TiO2/Al2O3 bilayer dielectrics. Adding a TiO2
layer on Al2O3/InGaAs MOSCAPs can lower the gate leak-
age current by one order of magnitude, which is attributable
in part to the 0.6 eV conduction band offset of TiO2 relative
to InGaAs, as well as possible oxide/oxide dipole effects.
The bilayer dielectrics show a significant increase of accu-
mulation capacitance density after FGA, and the crystalliza-
tion of the TiO2 layer appears to be responsible for the
capacitance increase.
The authors acknowledge support from Semiconductor
Research Corporation through the Non-Classical CMOS
Research Center (Task ID 1437.005), the Stanford Initiative
in Nanoscale Materials and Processes (INMP), and the US-
Israel Binational Science Foundation.
1R. Chau, B. Doyle, S. Datta, J. Kavalieros, and K. Zhang, Nature Mater.
6(11), 810 (2007).
2R. M. Wallace, P. C. McIntyre, J. Kim, and Y. Nishi, MRS Bull. 34(07),
493 (2009).
3E. J. Kim, E. Chagarov, J. Cagnon, Y. Yuan, A. C. Kummel, P. M.
Asbeck, S. Stemmer, K. C. Saraswat, and P. C. McIntyre, J. Appl. Phys.
106(12), 124508 (2009).
4D. Lin, N. Waldron, G. Brammertz, K. Martens, W.-E. Wang, S. Sioncke,
A. Delabie, H. Bender, T. Conard, W. H. Tseng et al., ECS Trans. 28(5),
173 (2010).
5E. J. Kim, L. Wang, P. M. Asbeck, K. C. Saraswat, and P. C. McIntyre,
Appl. Phys. Lett. 96(1), 012906 (2010).
6P. W. Peacock and J. Robertson, J. Appl. Phys. 92(8), 4712 (2002).
7J. Robertson, Eur. Phys. J. Appl. Phys. 28(3), 265 (2004).
8S. Swaminathan, M. Shandalov, Y. Oshima, and P. C. McIntyre, Appl.
Phys. Lett. 96(8), 082904 (2010).
9B. Shin, J. R. Weber, R. D. Long, P. K. Hurley, C. G. Van de Walle, and
P. C. McIntyre, Appl. Phys. Lett. 96(15), 152908 (2010).
10B. Shin, J. B. Clemens, M. A. Kelly, A. C. Kummel, and P. C. McIntyre,
Appl. Phys. Lett. 96(25), 252907 (2010).
11J. Hu, Appl. Phys. Lett. 99(9), 092107 (2011).
12T. Busani and R. A. B. Devine, Semicond. Sci. Technol. 20(8), 870
(2005).
13U. Diebold, Surf. Sci. Rep. 48(5–8), 53 (2003).
14S. Swaminathan and P. C. McIntyre, Electrochem. Solid-State Lett. 13(9),
G79 (2010).
15L. Lin, J. Robertson, and S. J. Clark, Microelectron. Eng. 88(7), 1461
(2011).
16N. V. Nguyen, O. A. Kirillov, W. Jiang, W. Wang, J. S. Suehle, P. D. Ye,
Y. Xuan, N. Goel, K. W. Choi, W. Tsai et al., Appl. Phys. Lett. 93(8),
082105 (2008); R. Engel-Herbert, Y. Hwang, J. LeBeau, Y. Zheng, and S.
Stemmer, MRS Proc. 1155, C13 (2009).
17D. M. King, X. Du, A. S. Cavanagh, and A. W. Weimer, Nanotechnology
19 (44), 445401 (2008); K. Y. Gao, Th. Seyller, L. Ley, F. Ciobanu, G.
Pensl, A. Tadich, J. D. Riley, and R. G. C. Leckey, Appl. Phys. Lett.
83(9), 1830 (2003).
FIG. 4. (Color online) (a) Valence band spectra obtained from the InGaAs
wafer, Al2O3/InGaAs, and TiO2/Al2O3/InGaAs samples. (b) Oxygen 1s
energy loss spectra for the TiO2/Al2O3/InGaAs sample. The inset is a decon-
voluted O 1s core-level spectrum of this sample. (c) Band alignment dia-
gram of TiO2/Al2O3/InGaAs system.
232902-3 Ahn et al. Appl. Phys. Lett. 99, 232902 (2011)
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Enlighten

Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk        Ahn, J., Geppert, I., Gunji, M., Holland, M., Thayne, I., Eizenberg, M., and McIntyre, P.C. (2011) Titania/alumina bilayer gate insulators for InGaAs metal-oxide-semiconductor devices. Applied Physics Letters, 99 (23). p. 232902. ISSN 0003-6951  http://eprints.gla.ac.uk/64787/  Deposited on: 23 July 2012   Titania/alumina bilayer gate insulators for InGaAs metal-oxide-semiconductor devicesJaesoo Ahn, Irina Geppert, Marika Gunji, Martin Holland, Iain Thayne et al.  Citation: Appl. Phys. Lett. 99, 232902 (2011); doi: 10.1063/1.3662966 View online: http://dx.doi.org/10.1063/1.3662966 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v99/i23 Published by the American Institute of Physics.  Related ArticlesSelf-heating enhanced charge trapping effect for InGaZnO thin film transistor Appl. Phys. Lett. 101, 042101 (2012) Hybrid graphene/organic semiconductor field-effect transistors APL: Org. Electron. Photonics 5, 158 (2012) Reduction of persistent photoconductivity in ZnO thin film transistor-based UV photodetector Appl. Phys. Lett. 101, 031118 (2012) Hybrid graphene/organic semiconductor field-effect transistors Appl. Phys. Lett. 101, 033309 (2012) On the link between electroluminescence, gate current leakage, and surface defects in AlGaN/GaN high electronmobility transistors upon off-state stress Appl. Phys. Lett. 101, 033508 (2012)  Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors Downloaded 23 Jul 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsTitania/alumina bilayer gate insulators for InGaAsmetal-oxide-semiconductor devicesJaesoo Ahn,1 Irina Geppert,2 Marika Gunji,1 Martin Holland,3 Iain Thayne,3Moshe Eizenberg,2 and Paul C. McIntyre1,a)1Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA2Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel3Nanoelectronics Research Centre, University of Glasgow, Glasgow G12 8LT, United Kingdom(Received 1 August 2011; accepted 26 October 2011; published online 5 December 2011)We describe the electrical properties of atomic layer deposited TiO2/Al2O3 bilayer gate oxideswhich simultaneously achieve high gate capacitance density and low gate leakage current density.Crystallization of the initially amorphous TiO2 film contributes to a significant accumulationcapacitance increase (33%) observed after a forming gas anneal at 400 C. The bilayer dielectricsreduce gate leakage current density by approximately one order of magnitude at flatband comparedto Al2O3 single layer of comparable capacitance equivalent thickness. The conduction band offsetof TiO2 relative to InGaAs is 0.6 eV, contributing to the ability of the stacked dielectric to suppressgate leakage conduction.VC 2011 American Institute of Physics. [doi:10.1063/1.3662966]Because silicon semiconductor channels are reachingtheir scaling limits, III-V compound semiconductor channelscoated by deposited high-k dielectrics are the subject ofintense interest for high performance metal-oxide-semicon-ductor (MOS) devices beyond the 22 nm technology node.1Unlike SiO2/Si technology, however, one of the main chal-lenges in developing III-V based semiconductors is the diffi-culty of preparing high quality gate oxides with lowinterface defect densities.2 Atomic-layer-deposited (ALD)Al2O3 is known to provide a relatively low interface defectdensity and an unpinned Fermi level on InGaAs (100) sub-strates.3,4 A particularly interesting approach is to use (1) anInGaAs channel surface that is initially capped with an As2layer that can be thermally desorbed prior to gate oxide dep-osition3 and (2) forming gas anneals after gate electrode dep-osition in order to passivate interface traps and border trapsin the ALD-Al2O3 layer.5 However, Al2O3 is a moderate-kdielectric compared to other higher-k oxides which havebeen studied as SiO2 replacements,6 and this hinders furtherscaling of the gate oxide capacitance density. Oxide materi-als with high dielectric constant such as TiO2 have a lowconduction band offset relative to a Si substrate;7 therefore,they are not as effective in reducing leakage current as areSiO2 and Al2O3. It is difficult to find a single oxide whichsatisfies all requirements for end-of-roadmap MOS gatedielectrics (high dielectric constant, low interface trap den-sity, high thermal stability, etc.), making bilayer gate dielec-trics an interesting option.8 In this letter, we report electricalproperties of ALD-TiO2/Al2O3 bilayer dielectrics with com-parison to ALD-Al2O3 single layer dielectrics. We also dis-cuss structural changes in the TiO2 layer after forming gasanneal and the spectroscopic measurement of the band align-ment of TiO2 and Al2O3 relative to n-In0.53Ga0.47As.Epitaxial n-type In0.53Ga0.47As(100) channel layers of500 nm thickness and Si doping concentration of2.0 1016 cm3 were grown on heavily doped n-type InPsubstrates by molecular beam epitaxy (MBE). The epilayerswere covered in-situ with an approximately 80 nm thickamorphous As2 capping layer to protect the channel surfacefrom uncontrolled oxidation and contamination during wafertransfer from the MBE chamber to the ALD reactor. The As2capping layer was thermally desorbed at 380 C under highvacuum in the ALD reactor prior to gate oxide deposition.ALD-Al2O3 layers were deposited at a substrate temperatureof 270 C, using trimethylaluminum (TMA) and water vapor,with a TMA pulsing first in the sequence. To fabricate thebilayer structure, ALD-TiO2 deposition was performed at150 C with a H2O/tetrakis(dimethylamino) titanium processimmediately after Al2O3 deposition, without a vacuumbreak. Platinum gate electrodes of 50 nm thickness were e-beam evaporated through a shadow mask, and wafer backside contacts of 50 nm Au/20 nm Ti were deposited to reducethe contact resistance. Post-metallization forming gas (5%H2/95% N2) anneal (FGA) was done for 30min at 400C.Multi-frequency (1 kHz to 1MHz) capacitance-voltage(C-V) measurements in the dark were performed on 7 nmTiO2/2 nm Al2O3 bilayer MOS capacitors. The C-V charac-teristic of the as-deposited TiO2/Al2O3/n-In0.53Ga0.47As bilayerdielectric (Fig. 1(a)) exhibits large frequency dispersionFIG. 1. (Color online) Multi-frequency (1 kHz–1MHz) C-V curves from (a)as-deposited and (b) forming gas annealed Pt/7 nm TiO2/2 nm Al2O3/n-In0.53Ga0.47As MOSCAPs.a)Author to whom correspondence should be addressed. Electronic mail:pcm1@stanford.edu.0003-6951/2011/99(23)/232902/3/$30.00 VC 2011 American Institute of Physics99, 232902-1APPLIED PHYSICS LETTERS 99, 232902 (2011)Downloaded 23 Jul 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsthroughout the applied bias range and the C-V curves stretchout through depletion, which shows that as-grown oxides con-tain a large density of border traps and interface defects. Post-metallization FGA effectively removes and/or passivates mostof the interface defect states present at the oxide/semiconductorinterface, which, as a result, reduces C-V stretch-out and fre-quency dispersion as shown in Fig. 1(b). In addition, the flat-band voltage was shifted close to the ideal value (0.5 eV,given by the work function difference between gate metal andsemiconductor), which is indicative of reduction of the oxidetrapped charges. The post-metallization FGA effect on passi-vating defect sites has previously been reported to occur inAl2O3 single layer capacitors.5,9 The key feature of the bilayercapacitors, which is different from single layer dielectrics, isthat the maximum accumulation capacitance density of bilayergate oxide increases from 1.5lF/cm2 to 2.0lF/cm2 after FGA,whereas no such effect of FGA has been observed for the sin-gle layer Al2O3 MOSCAPs investigated.10The leakage current density across the bilayer gatedielectrics was also compared to that of Al2O3 single layers.Figure 2 plots the gate leakage current density measured atflatband voltage as a function of the capacitance-derivedequivalent SiO2 thickness (CET). The plot clearly shows thatthe addition of the TiO2 layer gives about one order of mag-nitude reduction of the gate leakage current at a given CET.The addition of a physically thicker but higher-k TiO2 layercreates little change in gate capacitance density but greatlysuppresses gate leakage current. An electrostatic dipoleformed at the TiO2/Al2O3 interface can also inhibit leakagecurrent. In Al2O3/TiO2/n-GaAs devices, a dipole createdbetween the TiO2 and Al2O3 layers has been reported toenhance the leakage current compared to TiO2/n-GaAs byreducing the effective metal/semiconductor barrier height.11It can be inferred that the reverse oxide stack order (TiO2/Al2O3) should generate an opposite dipole, and this canincrease the barrier height and as a result lower the leakagecurrent density.Microstructural changes were investigated in a TecnaiG2 transmission electron microscope (TEM). Figure 3(a)shows a cross-sectional TEM micrograph of the Pt/TiO2/Al2O3/In0.53Ga0.47As stack after the FGA at 400C for30min. It is difficult to resolve the interface between the twodielectrics, because both have an amorphous structure, withsimilar densities and average atomic numbers. After theFGA, areas with lattice fringes are observed in the TiO2 filmsin both cross-sectional TEM (Fig. 3(a)) and the plan-viewTEM (Fig. 3(b)), indicating full or partial crystallization ofthe TiO2 layer. Analysis of selected area electron diffraction(SAED) patterns reveals that the crystalline TiO2 is predomi-nantly in the anatase phase (Fig. 3(c)). In-situ TEM analysesof the same bilayer dielectric combination on a Ge substrateshow that the crystalline anatase phase TiO2 becomes evi-dent at an annealing temperature of 300 C and the crystal-line rutile phase starts to form at 400 C.The total accumulation capacitance density, Cacc, isdetermined by several different capacitance factors, asexpressed in1Cacc¼ 1CAl2O3þ 1CTiO2þ 1CS þ Cit : (1)Here, CAl2O3 and CTiO2 are the capacitances of the Al2O3 andTiO2 layers, respectively, CS is the substrate capacitance,and Cit is the interface trap capacitance. The dielectric con-stant of anatase phase TiO2 is reported to be as high as 78,12and the rutile phase TiO2 can have a dielectric constant of upto 170 depending on crystalline orientation,13 while the k-value of the ALD-grown amorphous TiO2 was previouslyfound to be approximately 32.14 Crystallization of the TiO2film is expected to contribute to the accumulation capaci-tance increase after FGA, which was observed only in theTiO2-containing samples. Another factor that can increasethe accumulation capacitance is densification of the oxidelayers during FGA, which makes the oxide layers physicallythinner. Fast interface traps can also affect the measuredaccumulation capacitance through the interface trap capaci-tance, Cit. Quantification of these various effects of FGA onCacc remains a topic of future work. The CET extracted forthe bilayer MOSCAP is <1.7 nm after FGA, which includesthe contributions of both the InGaAs substrate and of the ca-pacitance of interface traps and near-interface border trapson Cacc. However, both bilayer and single-layer MOSCAPshave an ALD-Al2O3/InGaAs interface with very similar ther-mal history and, therefore, we may assume that the trap con-tributions are similar for each. We attribute most of the FGAeffect on Cacc seen in Fig. 1 to TiO2 microstructural changes,including crystallization.FIG. 2. (Color online) Plot of leakage current density versus CET for theAl2O3 single layers () and TiO2/Al2O3 bilayers (n). The leakage currentswere measured at the flatband voltage.FIG. 3. (Color online) (a) Cross-sectional TEM image and (b) plan-viewTEM image of TiO2/Al2O3 bilayer stack. (c) Selected area electron diffrac-tion pattern showing anatase phase predominant.232902-2 Ahn et al. Appl. Phys. Lett. 99, 232902 (2011)Downloaded 23 Jul 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsIn order to investigate the effects of interface energybarriers in the bilayer structure to suppress gate leakage con-duction, the band alignment of the TiO2/Al2O3/In0.53Ga0.47As system was determined by x-ray photoelec-tron spectroscopy measurements. The valence band offsetsof the Al2O3 and TiO2 films to InGaAs estimated by meas-uring the energy difference between the valence electronsejected from the Al2O3 and TiO2 films (second slope in Fig.4(a)) and from the InGaAs wafer are 3.86 0.2 eV and2.36 0.2 eV, respectively. The valence band offsets meas-ured in our experiments are smaller than those calculated ina recent report,15 but in agreement with experimental resultsfrom other researchers for individual Al2O3 and TiO2 layerson InGaAs substrate.16 Next, the bandgaps of the Al2O3 andTiO2 films were determined by the O 1 s core level spectrum,as shown in Fig. 4(b). The bandgap is the energy differencebetween the O-Al or O-Ti peak and the beginning of theeach energy loss region (the region with higher binding ener-gies). The extracted band gap values are 3.66 0.2 eV forTiO2 and 6.76 0.2 eV for Al2O3, which are in good agree-ment with literature for amorphous phase Al2O3 and TiO2.17Based on these findings, we can plot the band alignment dia-gram of the TiO2/Al2O3/InGaAs system (Fig. 4(c)), wherethe conduction band offset was calculated by the equationDEC¼EG,oxideEG,InGaAsDEV. The conduction band off-sets of the TiO2 and Al2O3 with respect to InGaAs are0.66 0.2 and 2.26 0.2 eV, respectively. This result indicatesthe ability of the physically thicker bilayer dielectric toreduce gate leakage conduction when TiO2 is stacked on theAl2O3/InGaAs MOS structure.In conclusion, we have demonstrated 1.7 nm CET with1.9 106 A/cm2 leakage current density at the flatbandvoltage with TiO2/Al2O3 bilayer dielectrics. Adding a TiO2layer on Al2O3/InGaAs MOSCAPs can lower the gate leak-age current by one order of magnitude, which is attributablein part to the 0.6 eV conduction band offset of TiO2 relativeto InGaAs, as well as possible oxide/oxide dipole effects.The bilayer dielectrics show a significant increase of accu-mulation capacitance density after FGA, and the crystalliza-tion of the TiO2 layer appears to be responsible for thecapacitance increase.The authors acknowledge support from SemiconductorResearch Corporation through the Non-Classical CMOSResearch Center (Task ID 1437.005), the Stanford Initiativein Nanoscale Materials and Processes (INMP), and the US-Israel Binational Science Foundation.1R. Chau, B. Doyle, S. Datta, J. Kavalieros, and K. Zhang, Nature Mater.6(11), 810 (2007).2R. M. Wallace, P. C. McIntyre, J. Kim, and Y. Nishi, MRS Bull. 34(07),493 (2009).3E. J. Kim, E. Chagarov, J. Cagnon, Y. Yuan, A. C. Kummel, P. M.Asbeck, S. Stemmer, K. C. Saraswat, and P. C. McIntyre, J. Appl. Phys.106(12), 124508 (2009).4D. Lin, N. Waldron, G. Brammertz, K. Martens, W.-E. Wang, S. Sioncke,A. Delabie, H. Bender, T. Conard, W. H. Tseng et al., ECS Trans. 28(5),173 (2010).5E. J. Kim, L. Wang, P. M. Asbeck, K. C. Saraswat, and P. C. McIntyre,Appl. Phys. Lett. 96(1), 012906 (2010).6P. W. Peacock and J. Robertson, J. Appl. Phys. 92(8), 4712 (2002).7J. Robertson, Eur. Phys. J. Appl. Phys. 28(3), 265 (2004).8S. Swaminathan, M. Shandalov, Y. Oshima, and P. C. McIntyre, Appl.Phys. Lett. 96(8), 082904 (2010).9B. Shin, J. R. Weber, R. D. Long, P. K. Hurley, C. G. Van de Walle, andP. C. McIntyre, Appl. Phys. Lett. 96(15), 152908 (2010).10B. Shin, J. B. Clemens, M. A. Kelly, A. C. Kummel, and P. C. McIntyre,Appl. Phys. Lett. 96(25), 252907 (2010).11J. Hu, Appl. Phys. Lett. 99(9), 092107 (2011).12T. Busani and R. A. B. Devine, Semicond. Sci. Technol. 20(8), 870(2005).13U. Diebold, Surf. Sci. Rep. 48(5–8), 53 (2003).14S. Swaminathan and P. C. McIntyre, Electrochem. Solid-State Lett. 13(9),G79 (2010).15L. Lin, J. Robertson, and S. J. Clark, Microelectron. Eng. 88(7), 1461(2011).16N. V. Nguyen, O. A. Kirillov, W. Jiang, W. Wang, J. S. Suehle, P. D. Ye,Y. Xuan, N. Goel, K. W. Choi, W. Tsai et al., Appl. Phys. Lett. 93(8),082105 (2008); R. Engel-Herbert, Y. Hwang, J. LeBeau, Y. Zheng, and S.Stemmer, MRS Proc. 1155, C13 (2009).17D. M. King, X. Du, A. S. Cavanagh, and A. W. Weimer, Nanotechnology19 (44), 445401 (2008); K. Y. Gao, Th. Seyller, L. Ley, F. Ciobanu, G.Pensl, A. Tadich, J. D. Riley, and R. G. C. Leckey, Appl. Phys. Lett.83(9), 1830 (2003).FIG. 4. (Color online) (a) Valence band spectra obtained from the InGaAswafer, Al2O3/InGaAs, and TiO2/Al2O3/InGaAs samples. (b) Oxygen 1senergy loss spectra for the TiO2/Al2O3/InGaAs sample. The inset is a decon-voluted O 1s core-level spectrum of this sample. (c) Band alignment dia-gram of TiO2/Al2O3/InGaAs system.232902-3 Ahn et al. Appl. Phys. Lett. 99, 232902 (2011)Downloaded 23 Jul 2012 to 130.209.6.41. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

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Titania/alumina bilayer gate insulators for InGaAs metal-oxide-semiconductor devices

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