Bandgap engineering in perovskite oxides: Al-doped SrTiO3Agham B. Posadas, Chungwei Lin, Alexander A. Demkov, and Stefan Zollner Citation: Applied Physics Letters 103, 142906 (2013); doi: 10.1063/1.4824023 View online: http://dx.doi.org/10.1063/1.4824023 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/14?ver=pdfcov Published by the AIP Publishing Articles you may be interested in On the variations of optical property and electronic structure in heavily Al-doped ZnO films during double-stepgrowth process Appl. Phys. Lett. 104, 021913 (2014); 10.1063/1.4862201 Band-engineered SrTiO3 nanowires for visible light photocatalysis J. Appl. Phys. 112, 104322 (2012); 10.1063/1.4767229 Dopant-induced bandgap shift in Al-doped ZnO thin films prepared by spray pyrolysis J. Appl. Phys. 112, 083708 (2012); 10.1063/1.4759208 Enhanced leakage current properties of Ni-doped Ba 0.6 Sr 0.4 TiO 3 thin films driven by modified band edgestate J. Appl. Phys. 107, 024109 (2010); 10.1063/1.3291124 Spectral-resolved microprobe cathodoluminescence investigations of Al-doped single-crystalline Ba 0.6 Sr 0.4TiO 3 thin films Appl. Phys. Lett. 87, 181914 (2005); 10.1063/1.2125109
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Bandgap engineering in perovskite oxides: Al-doped SrTiO3
Agham B. Posadas,1 Chungwei Lin,1 Alexander A. Demkov,1,a) and Stefan Zollner2
1Department of Physics, The University of Texas, Austin, Texas 78712, USA2Department of Physics, New Mexico State University, Las Cruces, New Mexico 88003, USA
(Received 24 July 2013; accepted 13 September 2013; published online 1 October 2013)
The ability to modulate the bandgap of a material without altering its functional properties is
crucial for fabricating heterojunctions for device applications. Here, we explore experimentally and
theoretically the effect of the substitution of Ti with Al on the bandgap of perovskite oxide SrTiO3.
We grow Al-doped SrTiO3 films directly on Si(100) and show from electron energy loss spectra
that the bandgap is increased by approximately 0.3 eV over undoped SrTiO3. This bandgap
increase is confirmed by spectroscopic ellipsometry measurements on identical films grown on
LaAlO3 substrates, which show a 0.3 eV blue shift in the steep increase of the absorption edge.
Current vs. voltage measurements show a reduction in leakage current by six orders of magnitude
at a field of 1 MV/cm. We use density functional theory to explain how Al replacing Ti modifies
the conduction band edge density of states resulting in the experimentally observed increase in the
bandgap. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824023]
The integration of functional perovskite oxides on semi-
conductors such as silicon and germanium presents possibil-
ities for achieving both improved performance in standard
field effect transistors, as well as obtaining hybrid electronic
devices where environmental sensitivity can be directly
coupled to the charge density in the semiconductor.1–5
SrTiO3 was originally envisioned as a replacement gate
dielectric for scaled CMOS technology because of its very
high dielectric constant of �300 at room temperature.6,7
However, one critical issue that prevented this technology to
be developed is the absence of the conduction band offset
between Si and SrTiO3 (STO), making SrTiO3 unsuitable for
use as a gate dielectric.8–10 Many workers have proposed
methods to increase the conduction band offset of STO with
Si, including insertion of high band offset buffer layers11–13
and modifying the interface oxygen stoichiometry.14,15 Here,
we report direct epitaxial growth of Al-doped SrTiO3 on Si
by molecular beam epitaxy (MBE), with Al concentrations
of 10% and 20%, and demonstrate using spectroscopic
ellipsometry (SE) and electron energy loss spectroscopy
(EELS) that the bandgap of Al-doped SrTiO3 is increased by
�0.3 eV over undoped STO. We further demonstrate using
leakage current measurements of capacitor structures that
Al-doping decreases the leakage current of STO on Si by
about six orders of magnitude at a field of 1 MV/cm. Using
density functional theory (DFT) and tight-binding (TB) anal-
ysis of the effect of Al substitution for Ti in STO, we attrib-
ute the bandgap increase to the narrowing of the conduction
band width, resulting in the bottom of the conduction band
being shifted higher in energy, away from the valence band
top.
Al-doped SrTiO3 was grown on both Si(001) and
LaAlO3(001) substrates using MBE. Sr, Ti, and Al are all de-
posited using effusion cells operated at approximately 425,
1780, and 1010 �C, respectively, resulting in a flux of about
1 monolayer per minute. The film stoichiometry is controlled
by shuttering the metal sources for the appropriate length of
time during the growth. The metal fluxes are calibrated using
a quartz crystal thickness monitor with feedback from in situx-ray photoelectron spectroscopy (XPS) measurements of
the film composition. During the growth, the samples are
monitored by reflection high energy electron diffraction
(RHEED) using an electron energy of 18 keV at a glancing
angle of �3�. Growth on Si is performed at a substrate tem-
perature of 500–550 �C under an oxygen partial pressure of
4–5� 10�7 Torr. These particular growth conditions have
previously been found to result in a SiOx layer thickness of
<1 nm for undoped STO as measured by cross-section trans-
mission electron microscopy. For the growth of Al-doped
STO, the Sr shutter is held constantly open while the Ti and
Al shutters are opened alternately to achieve the desired Al
doping concentration. The total growth rate is �4 A per min.
Prior to growth, prime n-type Si substrates (cut into 20� 20
mm2 pieces) are degreased and then exposed for 15 min to
UV/ozone. The native oxide is removed using Sr-assisted de-
oxidation,16 followed by deposition of 1=2 monolayer of Sr
Zintl template at 550 �C.17 For growth on LaAlO3, 5 mm sin-
gle crystal substrates from CrysTec Gmbh are used. The sub-
strates are degreased and then loaded into the growth
chamber for outgassing (650 �C in vacuum for 30 min)
before the growth of Al-doped STO.
After film growth, Al-doped STO samples are character-
ized using in situ XPS to determine their composition and
electron energy loss spectrum. For film composition analysis,
150 A-thick samples are used to ensure no photoelectron sig-
nal from the substrate is detectable. XPS is performed using
a monochromated Al Ka x-ray source and a VG Scienta
R3000 electron energy analyzer. The analyzer is calibrated
using the Ag 3d5/2 core level, which is defined to be
368.28 eV. This results in the Fermi level of Ag being
located at 0.00 eV. High resolution spectra of the Sr 3d, Ti
2p, O 1s, and Al 2p core levels are measured using a pass
energy of 100 eV and a slit width of 0.4 mm. The scan is per-
formed using 50 meV steps and a dwell time of 157 ms per
step, with each spectrum measured three times and summed.
For composition determination, the integrated intensities ofa)Electronic mail: [email protected]
0003-6951/2013/103(14)/142906/4/$30.00 VC 2013 AIP Publishing LLC103, 142906-1
APPLIED PHYSICS LETTERS 103, 142906 (2013)
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each region (after subtraction of a Shirley background) are
modified using an energy exponent of 0.78, to account for
the kinetic energy dependence of the sampling depth.18 This
energy exponent results in a 1:1 Sr/Ti ratio for commercial
single crystalline STO substrates using our instrument.
Wagner relative sensitivity factors are then used to calculate
the composition of the films.19 In addition to core level spec-
tra, O 1s energy loss spectra were also measured for the Al-
doped STO samples to determine zone-averaged bandgaps.
The ellipsometric angles of pure and Al-doped SrTiO3
films on LaAlO3 were measured from 0.8 to 6.5 eV at three
angles of incidence (65�, 70�, and 75�) on a J. A. Woollam
Co. variable-angle-of-incidence rotating-analyzer ellipsome-
ter, which was equipped with a computer-controlled Berek
waveplate compensator. The optical constants of the films
were then determined from the ellipsometric angles by solv-
ing Fresnel’s equations at each wavelength, using the meas-
ured film thickness and surface roughness obtained by x-ray
reflectivity (Philips X’Pert). A 10% error in thickness causes
about a 10% error of the refractive index and the absorption
coefficient; and also about a 20 meV error in the onset of the
strong absorption (better than electron energy loss spectros-
copy). To avoid complications due to substrate critical point
absorption effects, films grown on LaAlO3 were used for the
ellipsometric measurements. The dispersion of the LaAlO3
substrate was taken from the literature.20
Current vs. voltage (I-V) measurements of capacitor
structures were performed on both the undoped and 20% Al-
doped STO samples using a Keithley 4200 Semiconductor
Characterization System. 90 A-thick samples were grown on
heavily doped n-type Si. 500 A-thick gold contacts were de-
posited through a shadow mask using a sputter coater with
an array of circular holes of 500 lm in diameter. After con-
tact deposition, the samples were heated in air to 250 �C for
20 min. For the back contact, the backside of the Si sub-
strates were scraped with a scalpel and placed on a copper
block using a thin layer of liquid In-Ga eutectic alloy in
between. The copper block was grounded and the bias was
applied to the top electrode using a whisker probe tip.
The samples were characterized using RHEED during
and after the growth. RHEED patterns along the h120i zone
axis for both the 10% and 20% Al-doped STO samples on Si
after the growth of 20 unit cells (78 A) are shown in Fig. 1.
Both images show sharp, well-defined streaks indicating the
high degree of crystalline order in the films, similar to what
we have reported for undoped STO on Si.21 Immediately af-
ter growth, the films are transferred in situ to the XPS
analysis chamber where core level and electron energy loss
spectra are measured. The Al 2p binding energy is found to
be 74.68 eV and the O 1s binding energy is found to be
530.62 eV. An additional weak feature in the O 1s spectrum
appears at a binding energy of 532.66 eV, which is not found
in undoped STO. Both the Sr 3d and Ti 2p spectra of the Al-
doped films are similar to that obtained for undoped STO.
Analysis of the stoichiometry shows an Al concentration of
12% and 23%, respectively, for the nominally 10% and 20%
doped samples. There is also a measurable deficiency in oxy-
gen of �5–10% suggesting compensated doping. Sheet re-
sistance measurements at room temperature indicate that the
Al-doped samples are highly insulating with a sheet resist-
ance larger than 109 X/square (the measurement limit of our
instrument).
The bandgap was determined from the onset of the
energy loss spectra for O 1s photoelectrons.22 Because plas-
mon excitations for dielectrics have characteristic energies
larger than the lowest valence band to conduction band tran-
sitions, the bandgap value for dielectrics can be determined
by the onset of energy-loss spectrum of a core level peak. To
avoid complications due to spin-orbit pairs and multiplet sat-
ellites, the O 1s loss spectrum, which has neither, is typically
used for this type of measurement. Because of the relatively
large angular acceptance in transmission mode (30�) and the
large wave vector for O 1s photoelectrons, the bandgap value
measured by this method is averaged over the entire
Brillouin zone.23 The onset of the energy-loss spectrum was
defined by linearly extrapolating the segment of maximum
slope to the background level. Fig. 2(a) shows the O 1s
energy loss spectra for undoped and Al-doped STO. The
core level spectrum was fitted to either two (undoped) or
three (Al-doped) components and subtracted from the energy
loss spectra for clarity. A Tougaard background24 was used
to model the spectral background, which was also subtracted
from the measured signal. The overall features of the spectra
are consistent with published results for bulk single crystal
STO.25 For undoped SrTiO3, we find the onset of the loss
features to be at an energy of 3.7 eV. For both 10% and 20%
Al-doped samples, the onset is shifted to lower kinetic
energy (higher loss energy) by about 0.3 eV to a value of
4.0 eV with an uncertainty of about 0.1 eV. The energy loss
spectra show that substituting Al for Ti in STO results in an
increase in the zone-averaged bandgap.
Fig. 2(b) shows the calculated absorption coefficient
from the ellipsometry measurements as a function of photon
energy for the three samples. The data for bulk SrTiO3 are
also shown for comparison.26 The solid and dotted lines
show the absorption coefficient for bulk and thin-film
SrTiO3, respectively. The dashed lines show the Al-doped
SrTiO3 absorption coefficient. Ellipsometry confirms that the
Al doping causes a shift in the absorption edge of STO to
higher energies by about 0.3 eV. There is also a decrease in
the maximum of the absorption coefficient with Al doping.
The results of the I-V measurements are shown in Fig.
3. The undoped STO initially shows a moderate leakage of
4� 10�6 A/cm2 at þ1 V (1.1 MV/cm) bias but rapidly
increasing to 3.5� 10�2 A/cm2 at þ3 V. However, after the
first voltage sweep, the undoped STO changes to a higher
leakage condition with a leakage current of 8� 10�4 A/cm2
FIG. 1. RHEED patterns of 8-nm-thick Al-doped SrTiO3 grown on Si with a
nominal Al concentration of 10% (left) and 20% (right). Both images are
taken along the h120i zone axis of STO and show excellent crystallinity sim-
ilar to undoped STO on Si.
142906-2 Posadas et al. Appl. Phys. Lett. 103, 142906 (2013)
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at þ1 V (9� 10�2 A/cm2 at þ3 V). Additional sweeps do not
show any further changes in the leakage current of undoped
STO. The change in the leakage current in undoped STO has
been observed previously in relation to memristor applica-
tions, possibly resulting from voltage-driven movement of
oxygen vacancies.27,28 On the other hand, the 20% Al-doped
STO sample shows a very low leakage of 7� 10�10 A/cm2
at þ1 V and 8� 10�7 A/cm2 at þ3 V. This is a six orders of
magnitude reduction in leakage current at þ1 V due to the
Al-doping. The Al-doped sample also does not show any
change in the leakage current between the first sweep and
subsequent sweeps, indicating that oxygen vacancies are
strongly bound to the Al sites and are not mobile. The low
leakage and stability of the I-V measurements for Al-doped
STO shows its suitability as a current injection barrier on Si.
We have performed DFT calculations and have analyzed
the results using a TB model. The details are given in the
supplementary materials.29 Our main conclusion is that each
Al dopant effectively behaves like a Ti vacancy in the energy
range of valence and conduction bands, and its main effect
on the electronic structure is to block Ti-Ti hopping, which
reduces the conduction band width and therefore increases
the bandgap. A simple rule, based on TB analysis, to under-
stand the orbitally decomposed density of states (DOS) is
given in the supplementary materials.
From the band theory point of view, Al-doped STO is a
“hole” metal. Experimentally however, the Al-doped STO is
found to be insulating. The as-grown sample is also found to
have a significant amount of oxygen vacancies, as quantified
using in situ XPS.30 By comparing undoped STO with Al-
doped STO, we find an effective oxygen to metal ratio that is
�5–10% lower in the Al-doped films compared to undoped
films, suggesting compensation. Based on these observa-
tions, we consider theoretically configurations containing
two Al and one oxygen vacancy (OV), as shown in Fig. 4.
As discussed in Refs. 31 and 32, the two main effects of OV
are to provide two electrons and to lower one of the eg levels
of two adjacent Ti atoms. In the case of Al doping, the two
electrons provided by an OV fill two holes provided by two
Al atoms, so the whole system remains insulating. Also, the
FIG. 3. Current vs. voltage for 90 A-thick undoped and 20% Al-doped STO
on silicon. Undoped STO shows a change in state after the first voltage
sweep settling to a high leakage current of 8� 10�4 A/cm2 at þ1 V. The Al-
doped STO shows a low leakage of 7� 10�10 A/cm2 at þ1 V and does not
exhibit a change in state after the first voltage sweep.
FIG. 2. (a) O 1s photoelectron energy loss spectrum extracted from the low kinetic energy (high binding energy) side of the O 1s core level spectrum for both
undoped (dashed line) and 20% Al-doped (solid line) STO on Si. The O 1s core level components and the background (fit as a Tougaard function) were sub-
tracted from the data. The linearly extrapolated onset of the energy loss features (see inset) is located at a kinetic energy 3.7 eV below the main O 1s core level
for the undoped STO sample, and at 4.0 eV for the Al-doped sample. (b) Spectroscopic ellipsometry data for undoped and Al-doped STO plotted as absorption
coefficient versus photon energy. The solid and dotted lines show the absorption coefficient for undoped STO thin film and bulk STO, respectively. The two
dashed lines show the Al-doped STO absorption coefficient for 10% and 20% Al concentrations. The Al doping causes a shift of the absorption edge to higher
energies by about 0.3 eV. There is also a decrease in the maximum of the absorption coefficient with Al doping.
FIG. 4. (a) DOS for the configuration containing two Al with one OV in
between in a linear arrangement. The position of Al is represented by the
black spheres. Only a reduction in the density of states near the bottom of the
conduction band is observed in this case. (b) DOS for the configuration con-
taining two body-diagonal Al plus one OV. The oxygen vacancy is indicated
by the “X”. The DOS of two body-diagonal Al plus two La (replacing Sr) is
also provided for comparison. In both cases the system is insulating and the
Fermi energy is in the gap. Both cases also show an enlarged bandgap.
142906-3 Posadas et al. Appl. Phys. Lett. 103, 142906 (2013)
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t2g� t2g* bands are at most only mildly affected since OV
mainly changes the eg levels.
Similar to what was found in Co-doped STO,33 OV and
Al are energetically favored to be adjacent to each other
because this arrangement facilitates charge transfer between
OV and Al. We find that the configuration separating Al and
OV is at least 500 meV higher in energy. Two of the config-
urations we have considered are shown in Fig. 4(a) [Al-OV-
Al] and (b) [2 Al along the body diagonal and OV]. Note
that the OV is chosen to be next to at least one of the Al
atoms. In both cases, the system is insulating. The linear Al-
OV-Al configuration has the lowest energy in the calcula-
tions as the OV is adjacent to two Al atoms.33 In both cases,
the Al and Ti are relaxed away from the OV, consistent with
the previous OV calculations at the local density approxima-
tion level.32 The total DOS are very similar to those without
OV, leading to either a reduction of DOS near the bottom of
the conduction band [Fig. 4(a)] or an actual bandgap increase
[Fig. 4(b)].29 It is likely that both configurations are present
experimentally since the growth is done at elevated tempera-
tures, resulting in randomization of the Al and OV distribu-
tion, which on average, leads to an increase in the bandgap
(see also Fig. S2). From the theoretical point of view, the
higher the Al concentration, the larger the resulting increase
in the bandgap. We expect the optimum Al concentration
would be the highest that can be incorporated into STO with-
out compromising the crystallinity. In the present study, the
20% Al concentration still shows excellent crystallinity but
the optimum concentration could be slightly higher.
We have grown single crystalline, epitaxial Al-doped
SrTiO3 on Si with Al concentrations of �10% and 20% and
show that there is a bandgap increase of �0.3 eV using both
electron energy loss spectroscopy and spectroscopic ellips-
ometry. I-V measurements confirm the large reduction in the
leakage current of STO on Si (six orders of magnitude) as a
result of Al-doping. Theoretical analysis suggests that Al
dopants effectively reduce Ti-Ti hopping, behaving in many
respects like a Ti vacancy. Due to this reduction of hopping,
the Al doped STO has a narrower conduction band width,
leading to an enlarged bandgap. This work shows that Al
doping is a highly effective method of increasing the
bandgap of STO and is expected to be applicable to all tita-
nates, including ferroelectric BaTiO3 and PbTiO3, poten-
tially allowing for bandgap engineering in functional
perovskite titanates integrated in semiconductor devices.
The experimental work at UT Austin is supported by the
Office of Naval Research under grant N00014-10-1-0489,
the Air Force Office of Scientific Research under grant
FA9550-12-10494, and the National Science Foundation
under grant DMR-1207342. The theoretical work is sup-
ported through Scientific Discovery through Advanced
Computing (SciDAC) program funded by U.S. Department
of Energy, Office of Science, Advanced Scientific
Computing Research and Basic Energy Sciences under
award number DESC0008877. All calculations are done at
the Texas Advanced Computing Center. The work at NMSU
was supported by the National Science Foundation under
grant DMR-1104934.
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