Measurement of the band offsets between amorphous LaAlO3 and silicon

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Measurement of the band offsets between amorphous LaAlO3and silicon
L. F. Edge and D. G. Schloma)
Department of Materials Science and Engineering, Pennsylvania State University,
University Park, Pennsylvania 16802
S. A. Chambers
Fundamental Science Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352
E. Cicerrella and J. L. Freeoufb)
Department of Electrical and Computer Engineering, Oregon Health & Sciences University,
Beaverton, Oregon 97006
B. Holla ¨nder and J. Schubert
Institut fu ¨r Schichten und Grenzfla ¨chen ISG1-IT and Center of Nanoelectronic Systems for Information
Technology, Forschungszentrum Ju ¨lich GmbH, D-52425 Ju ¨lich, Germany
?Received 26 September 2003; accepted 29 November 2003?
The conduction and valence band offsets between amorphous LaAlO3and silicon have been
determined from x-ray photoelectron spectroscopy measurements. These films, which are free of
interfacial SiO2, were made by molecular-beam deposition. The band line-up is type I with
measured band offsets of 1.8?0.2 eV for electrons and 3.2?0.1 eV for holes. The band offsets are
independent of the doping concentration in the silicon substrate as well as the amorphous LaAlO3
film thickness. These amorphous LaAlO3films have a bandgap of 6.2?0.1 eV. © 2004 American
Institute of Physics. ?DOI: 10.1063/1.1644055?
There is currently an extensive research effort to find
alternative gate dielectrics to replace SiO2in metal-oxide-
semiconductor field-effect transistors ?MOSFETs? as SiO2
approaches its fundamental limits. LaAlO3is a promising
alternative gate dielectric for the replacement of SiO2in sili-
con MOSFETs.1–3Single-crystalline LaAlO3has a dielectric
constant of 24.1?0.2 measured at 145 GHz ?Ref. 4? and has
an optical bandgap of 5.6 eV.5Amorphous LaAlO3thin films
on silicon have an estimated dielectric constant of 20–27.6–8
It has also been shown that single-crystalline LaAlO3is
stable in contact with silicon under standard MOSFET pro-
cessing conditions of 1026°C for 20 s.9The band offsets
between LaAlO3and silicon have been predicted to be in the
range of 1.0 to 2.1 eV for electrons and 1.9 to 3.5 eV for
holes.10,11All of these properties meet the requirements for
an alternative gate dielectric as suggested by the Interna-
tional Technology Roadmap for Semiconductors ?ITRS?.12
Although LaAlO3/Si shows many promising properties,
there are no published papers that have experimentally deter-
mined the band offsets. It is critical that the high-K gate
dielectric have conduction band offsets ?CBOs? and valence
band offsets ?VBOs? of at least 1 eV for both electrons and
holes from the silicon.12In this letter, we report an experi-
mental determination of the bandgap, CBOs, and VBOs of
amorphous LaAlO3on silicon.
Amorphous LaAlO3 thin films with the following
thicknesses:1310, 20, 40, and 150 Å were grown by
molecular-beam deposition in an EPI 930 molecular-beam
epitaxy chamber modified for the growth of oxides,14and
shipped to Pacific Northwest National Laboratory ?PNNL?
for the x-ray photoelectron spectroscopy ?XPS? measure-
ments. The films were grown on n- and p-type Si ?001? wa-
fers. The native SiO2on the silicon wafer was thermally
removed in UHV at a temperature of 900°C, measured with
an optical pyrometer. The films were grown using elemental
sources. Lanthanum, aluminum, and molecular oxygen
?99.994% purity? at a background pressure of 6?10?8Torr
were codeposited at a thermocouple temperature of ?100°C
onto the clean 2?1 Si surface. The lanthanum and alumi-
num fluxes were each 2?1013atoms/cm2s. Several 1000-Å-
thick amorphous LaAlO3films have been analyzed by Ruth-
erford backscattering spectrometry ?RBS? ?1.4 MeV He?,
170° scattering angle?. It indicated that the films are stoichio-
metric, with a ratio of La:Al?1?0.05. A 3000-Å-thick
amorphous LaAlO3film on silicon was used to determine the
bandgap of amorphous LaAlO3using a far-UV spectroscopic
ellipsometer.5
Once the films were at PNNL in UHV, the O1s core-
level spectra indicated the presence of hydroxyls ?OH? on the
films. Polar-angle-dependent XPS measurements on a 150 Å
LaAlO3/Si film confirmed that the OH layer was only on the
surface of the films and was not distributed within the entire
film or at the LaAlO3/Si interfacial region. Nuclear reaction
analysis ?NRA? was used to corroborate the presence of hy-
drogen. A series of in situ annealing steps were completed
using NRA. NRA indicated that the hydrogen content was
completely removed by a 10 min anneal at 400°C in UHV.
Figure 1 shows the O1s core-level spectra before and after
annealing. Such an annealing step was completed in the XPS
chamber for all of the films measured in this letter. High-
resolution scans of the Si2s region were completed on the
as-grown films and after annealing. The as-grown films, even
as thin as 10 Å LaAlO3/Si and exposed to air, were free of
detectable SiO2at the interface.15After annealing at 400°C
in UHV, the XPS spectra contained a small peak at ?153 eV,
indicating the formation of Si–O bonding.
A core-level photoemission-based method similar to that
a?Electronic mail: schlom@ems.psu.edu
b?Also with: Interface Studies Inc.
APPLIED PHYSICS LETTERSVOLUME 84, NUMBER 52 FEBRUARY 2004
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of Kraut et al.16,17was used to determine the VBOs.18Ap-
propriate shallow core-level peaks were referenced to the top
of the VB for bulk Si?001? and a thick ?150 Å? film of
LaAlO3on Si?001?, using a linear extrapolation method to
determine the valence band maximum ?VBM?.18–20The re-
sulting binding energy differences between the core peaks
and VBMs for the pure materials were then combined with
core-level binding energy differences for heterojunctions to
obtain the VBO as a function of thickness and doping type.
Monochromatic AlK? x rays at normal incidence with a
GammaData SES 200 analyzer were used for all measure-
ments. The XPS spectrometer was calibrated using a poly-
crystalline Au foil. The Au f7/2peak position and Fermi-edge
inflection point were determined to be 84.00?0.02 and
0.00?0.02 eV, respectively. Therefore, all of the binding en-
ergies ?BE? are accurate on an absolute scale within 0.02–
0.03 eV, over the binding energy range of 0 to 100 eV.
A thick ?150 Å? LaAlO3/Si film was used to obtain the
XPS spectra for bulk amorphous LaAlO3. The 10, 20, and
40 Å LaAlO3/Si films were thin enough to obtain XPS spec-
tra from both the LaAlO3film and the underlying silicon.
Figure 2 shows the shallow core-level and VB spectra
for bulk Si?001?, a thick ?150 Å? LaAlO3/Si, and one of the
10 Å LaAlO3/Si heterojunctions that was used to determine
the band offsets. The VB value (E?) was determined by
linearly fitting the leading edge of the VB and linearly fitting
the flat energy distribution and finding the intersection of
these two lines, as shown in Fig. 2 for silicon and the thick
LaAlO3/Si film. The energy differences between the VB
edges and the Si2p centroids (ESi 2p?E?)Siwere measured
to be 98.90?0.05 and 98.98?0.05 eV for n- and p-type sili-
con wafers, respectively.19,20These results are in good agree-
ment with the value of 98.95?0.04 eV for n-type silicon
measured by Yu et al.21For the thick ?150 Å? LaAlO3/Si,
the leading edge of the VB relative to the Fermi level ?FL?
and the energy difference between the Al2p centroid and the
leading edge of the VB (EAl 2p?E?)Thick LaAlO3, were mea-
sured to be 4.34?0.05 and 70.86?0.05 eV, respectively. The
energy difference between the Si2p centroid and the Al2p
centroid (ESi 2p?EAl 2p)LaAlO3/Siwas determined for each of
the LaAlO3/Si heterojunctions. These values were then in-
serted into the following equations to calculate the VBOs
(?E?) and CBOs (?Ec):
?E???ESi 2p?E??Si??EAl 2p?E??Thick LaAlO3
??ESi 2p?EAl 2p?LaAlO3/Si
?1?
and
?Ec?Eg?LaAlO3??Eg?Si???E?,
where Eg(LaAlO3)is the bandgap of amorphous LaAlO3and
Eg(Si)is the bandgap of silicon. The VBOs and CBOs were
determined for a series of films on n- and p-type silicon.
Table I shows a summary of the band offset results. Both the
CBO and VBO are independent of silicon doping as well as
the thickness of the amorphous-LaAlO3film.
A 3000-Å-thick film of amorphous LaAlO3was grown
on a silicon substrate to determine the bandgap of amorphous
LaAlO3on silicon ?Eg(LaAlO3)?. The sample was measured in
a spectroscopic ellipsometer system described previously.5
The initial analysis assumed that the dielectric response of
amorphous LaAlO3was identical to that previously deter-
mined for crystalline5LaAlO3. The results indicated a thick-
ness of around 3350 Å, ?10% more than the target thick-
ness. We then fixed the thickness and permitted n and k to
vary from that of single-crystalline LaAlO3to better match
the experimental results. We performed these measurements
several times under several different assumptions: ?1? assum-
ing a single layer of amorphous LaAlO3on Si, ?2? assuming
a mixed interface between the amorphous LaAlO3and the Si
substrate ?our routines always found that this was very
small?, ?3? assuming an SiO2interlayer between the amor-
?2?
FIG. 1. High-energy-resolution O 1s core-level spectra for a 40-Å-thick
amorphous LaAlO3film on p-Si(001) before and after annealing at 400 °C
for 10 min in UHV.
FIG. 2. Shallow core-level and VB photoelectron spectra for bare p-Si(001)
and 150 Å and 10 Å LaAlO3/Si heterojunctions. The insets show high-
resolution scans of the VB regions for the p-Si(001) and the 150 Å
LaAlO3/Si.
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TABLE I. Summary of CBOs (?Ec) and VBOs (?E?) for a series of
different thickness amorphous LaAlO3films on n- and p-type silicon.
Film
?Ec?eV??E??eV?
10 Å LaAlO3/n-Si
10 Å LaAlO3/p-Si
20 Å LaAlO3/p-Si
40 Å LaAlO3/n-Si
40 Å LaAlO3/p-Si
1.75?0.2
1.93?0.2
1.99?0.2
1.76?0.2
1.88?0.2
3.35?0.1
3.17?0.1
3.11?0.1
3.34?0.1
3.22?0.1
727 Appl. Phys. Lett., Vol. 84, No. 5, 2 February 2004Edge et al.

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phous LaAlO3and the Si substrate ?our routines always
found this essentially zero?, and ?4? assuming a rough surface
on the amorphous LaAlO3?our routines sometimes found
this value to exceed 100 Å, but the improvement in rms error
was small?. The results always indicated virtually no absorp-
tion for energies below 6.1–6.3 eV. While we note that very
small levels of absorption may be difficult to observe by this
technique, we conclude that the bandgap of this material is
6.2?0.1 eV. This value is in fairly good agreement with Lu
et al.,8who reported a bandgap of 6.55 eV for amorphous
LaAlO3on fused silica.
As a check of the XPS method, we simulated the VB
spectra of the heterojunctions by shifting and summing the
appropriately weighted spectra for Si?001? and thick ?150 Å?
LaAlO3. Both of the spectra are shown in Fig. 2. These
spectra were shifted in energy so that the Si2p and the Al2p
binding energies matched those of the heterojunctions. The
spectra were then scaled so that the Si2p and the Al2p
areas matched the heterojunctions areas. The adjusted spectra
were then summed to simulate the heterojunction VB.
Figure 3 shows the simulated VB in comparison with the
experimentally measured heterojunction for a 10-Å-thick
LaAlO3/p-Si heterojunction. There was good agreement be-
tween such simulations and the measured XPS spectra for all
heterojunctions.
Figure 4 shows the band diagram for LaAlO3on n- and
p-type silicon. Si 2p centroids in the majority of the films
fall at the BE expected for FL pinning at 0.8 eV above the
VBM, but a minority fall at other energies. This suggests FL
pinning. The experimentally measured VB and CB values are
in good agreement with the theoretical predictions of Pea-
cock and Robertson.10,11The theoretical predictions have es-
timated a CBO of 1.0 to 2.1 eV and a VBO of 1.9 to 3.5 eV
for single-crystalline-LaAlO3on silicon.10,11We have experi-
mentally measured a CBO of 1.8?0.2 eV and a VBO of
3.2?0.1 eV for amorphous LaAlO3on silicon. These band-
offset values meet the requirements for an alternative gate
dielectric as suggested by ITRS12and make amorphous
LaAlO3a candidate material for the replacement SiO2in
Si-based MOSFETs.
Two authors ?L.F.E. and D.G.S.? gratefully acknowledge
the financial support of the Semiconductor Research Corpo-
ration
?SRC?
andSEMATECH
SEMATECH FEP Center. Author ?L.F.E.? gratefully ac-
knowledges an AMD/SRC fellowship. The portion of this
work conducted at PNNL was carried out in the Environmen-
tal Molecular Sciences Laboratory, a national scientific user
facility sponsored by the Department of Energy’s Office of
Biological and Environmental Research. Two authors ?E.C.
and J.L.F.? gratefully acknowledge the support of the Na-
tional Science Foundation under Grant No. 0218288.
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FIG. 3. Simulated VB region in comparison with the measured heterojunc-
tion valence band for 10 Å amorphous LaAlO3on p-Si(001). The simulated
VB was produced by shifting and scaling the bare Si?001? spectrum and the
thick ?150 Å? LaAlO3/Si spectrum and summing the spectra.
FIG. 4. Band diagrams for amorphous LaAlO3/Si(001) heterojunctions for
both n- and p-type silicon. Energies are in eV.
728Appl. Phys. Lett., Vol. 84, No. 5, 2 February 2004Edge et al.
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