Energy-band alignment of Al2O3 and HfAlO gate dielectrics deposited by atomic layer deposition on 4H-SiC

Article abstract of DOI:10.1063/1.3291620

Energy band alignment and band gap of Al₂O₃ and HfAlO films grown by atomic layer deposition on 4H-SiC were determined using x-ray photoelectron spectroscopy. Al₂O₃ exhibited a symmetric band profile with a conduction band offset (ΔE_C) of 1.88 eV and a valence band offset (ΔE_V) of 1.87 eV. HfAlO yielded a smaller ΔE_C of 1.16 eV and ΔE_V of 1.59 eV. The higher dielectric constant and higher effective breakdown field of HfAlO compared to Al₂O₃, coupled with sufficient electron and hole barrier heights, makes it a potential dielectric for use on 4H-SiC, and provokes interest in further investigation of HfAlO/ 4H-SiC properties.

Full text of DOI:10.1063/1.3291620

Energy-band alignment of Al2O3 and HfAlO gate dielectrics deposited by
atomic layer deposition on 4H–SiC
Rahul Suri, Casey J. Kirkpatrick, Daniel J. Lichtenwalner, and Veena Misra
Citation: Appl. Phys. Lett. 96, 042903 (2010); doi: 10.1063/1.3291620
View online: http://dx.doi.org/10.1063/1.3291620
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APPLIED PHYSICS LETTERS 96, 042903 ͑2010͒
Energy-band alignment of Al2O3 and HfAlO gate dielectrics deposited
by atomic layer deposition on 4H–SiC
Rahul Suri,1 Casey J. Kirkpatrick,1 Daniel J. Lichtenwalner,2 and Veena Misra1,a͒
1Department of Electrical and Computer Engineering, North Carolina State University,
Raleigh, North Carolina 27695, USA
2Department of Materials Science and Engineering, North Carolina State University,
Raleigh, North Carolina 27695, USA
͑Received 7 October 2009; accepted 18 December 2009; published online 28 January 2010͒
Energy band alignment and band gap of Al2O3 and HfAlO films grown by atomic layer deposition
on 4H–SiC were determined using x-ray photoelectron spectroscopy. Al2O3 exhibited a symmetric
band profile with a conduction band offset ͑⌬EC͒ of 1.88 eV and a valence band offset ͑⌬EV͒ of
1.87 eV. HfAlO yielded a smaller ⌬EC of 1.16 eV and ⌬EV of 1.59 eV. The higher dielectric
constant and higher effective breakdown field of HfAlO compared to Al2O3, coupled with sufficient
electron and hole barrier heights, makes it a potential dielectric for use on 4H–SiC, and provokes
interest in further investigation of HfAlO/4H–SiC properties. © 2010 American Institute of
Physics. ͓doi:10.1063/1.3291620͔
Silicon carbide ͑SiC͒ has attractive properties such as a
wide band gap ͑ϳ3.25 eV for 4H–SiC͒ and a high break-
down electric field ͑ϳ2–3 MV/cm͒ suitable for metal-
oxide-semiconductor ͑MOS͒ devices for high-power applica-
tions. However, the choice of gate dielectric is critical to
avail these properties effectively. Because of the limitations
imposed by the traditional SiO2 dielectric, the search for an
appropriate high-k dielectric on SiC has received significant
attention recently. By virtue of a higher dielectric constant
͑k͒, the electric field across the gate dielectric is suppressed
by a factor of ͑3.9/k͒ compared to SiO2 for the same sub-
strate field. This reduction in the dielectric electric field is
necessary to preclude the severe dielectric reliability issues
associated with thermally oxidized SiO2 on SiC.1 Thus,
high-k dielectric on SiC offers flexibility to operate the de-
vice at a substrate electric field closer to the SiC breakdown
field. Additionally, interface state density ͑Dit͒ is rather high
for SiO2 on SiC, thus device channel mobility is low.2 Alter-
native dielectrics are also sought to passivate the interface by
reducing the Dit thereby resulting in improved channel mo-
bility, and to increase the threshold voltage in order to reduce
the off-state leakage current in SiC MOS field effect
transistors.3
Although a high k is a necessary requirement of a gate
dielectric for SiC MOS devices, maximizing k does not nec-
essarily lead to optimum performance. While HfO2 has a
high k of ϳ20–25, recent experiments by Tanner et al.4 and
Mahapatra et al.5 have shown that it results in an unaccept-
ably small conduction band offset of 0.54–0.7 eV between
high-k and 4H–SiC. Al2O3 is a more favorable candidate
than HfO2 due to its large measured conduction band offset
of 2.05 eV on 4H–SiC;6 however, Al2O3 has a moderate k of
ϳ9. Intermixing of Al2O3 with HfO2 to form HfAlO has
several potential advantages for gate stacks on SiC. First,
addition of HfO2 boosts the dielectric constant of Al2O3 and
results in sufficient conduction and valence band offsets on
Si.7 Second, HfAlO alloy has a higher crystallization tem-
perature than that of HfO2, thus, minimizing the leakage
paths in the dielectric.8 This suggests that HfAlO could yield
a thermally stable and reliable interface on SiC with a suit-
ably high k and appropriate band offsets. In this study we
determine the energy band alignment of Al2O3 and HfAlO
on 4H–SiC using x-ray photoelectron spectroscopy ͑XPS͒.
Measurement of the band gap using the XPS O 1s energy
loss spectra allows for the construction of the energy band
diagram and determination of the viability of the gate dielec-
tric.
4H–SiC substrates with an n-epilayer ͑doping ϳ4
ϫ1015 cm−3͒ received from Cree, Inc. were dipped in 1%
HF solution for native oxide removal. The samples were
transferred to an atomic layer deposition ͑ALD͒ chamber
where Al2O3 and HfAlO dielectrics were deposited at
200 °C. HfAlO was formed by alternating three cycles of
Al2O3 and three cycles of HfO2 starting with Al2O3.9 The
precursors used were ͓Al͑CH3͒3͔ and H2O for Al2O3 and
͕Hf͓N͑CH3͒2͔4͖ and H2O for HfO2. Three sets of samples
were prepared for band offset measurement using XPS.
These include ͑i͒ clean SiC ͑HF dipped͒, ͑ii͒ thin oxide/SiC
͑20 Å of Al2O3 or 24 Å of HfAlO͒, and ͑iii͒ thick ͑bulk͒
oxide ͑150 Å of Al2O3 or HfAlO͒. All dielectrics received a
post-deposition anneal ͑PDA͒ in Ar using rapid thermal an-
nealing ͑RTA͒ at 600 °C for 1 min. MOS capacitors were
fabricated by photolithography using a tungsten top gate. A
post-metallization anneal was done using RTA in Ar at
700 °C for 1 min. Ex situ XPS was performed using a Kra-
tos Axis Ultra spectrometer equipped with a monochromatic
Al K␣ x-ray source ͑1486.6 eV͒ with a pass energy of 20 eV
and a 90° take-off angle ͑normal to surface͒.
Figure 1 shows the O 1s and Al 2p core level spectra for
thick ͑bulk͒ Al2O3 and HfAlO films to compare the O and Al
chemical bonding states in the two dielectrics without inter-
ference from the substrate. The O 1s peak position in the
case of Al2O3 in Fig. 1͑a͒, occurs at 531.2 eV and corre-
sponds to O–Al bonding.7 The peak shifts to a lower binding
energy of 530.8 eV in the case of HfAlO indicative of inter-
mixing of Al2O3 and HfO2 layers and formation of Hf–O–Al
bonding. The peak at 532.4 eV is due to the O–C bonding
from surface-adsorbed hydrocarbons.7 The Al 2p spectrum
in Fig. 1͑b͒ exhibits a similar shift in the position of Al–O
a͒
Electronic mail: vmisra@ncsu.edu.
0003-6951/2010/96͑4͒/042903/3/$30.00
96, 042903-1
© 2010 American Institute of Physics
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042903-2
Suri et al.
Appl. Phys. Lett. 96, 042903 ͑2010͒
FIG. 1. ͑Color online͒ XPS spectra for ͑a͒ O 1s core level and ͑b͒ Al 2p
core level from thick ͑bulk͒ Al2O3 and HfAlO films.
bonding peak from 74.8 eV for Al2O3 to 74.4 eV for HfAlO.
Thus, both the O 1s and Al 2p spectra reveal the formation
of a composite HfAlO alloy after annealing, rather than a
multilayered Al2O3–HfO2 structure. XPS quantitative analy-
sis was employed to determine the chemical composition of
the films. The atomic fractions in Al2O3 were found to be Al
͑37 %͒—O ͑63 %͒ rather than the values of Al ͑40 %͒—O
͑60 %͒ in the case of stoichiometric Al2O3. For HfAlO, the
composition was determined to be Hf ͑14.7 %͒—Al ͑17.6
%͒—O ͑67.7 %͒. It is important to note the elemental frac-
tions since the energy band gap and band offsets of HfAlO
are expected to vary with the Al and Hf content.7
To determine the valence band offset at the high-k/SiC
interface using XPS, Kraut’s method was employed.10 Ac-
cording to this method, the energy separation between the
valence band edge and core level of the substrate remains
unchanged before/after dielectric deposition. The valence
band offset ͑⌬EV͒ was determined using the relation,
FIG. 2. ͑Color online͒ Determination of valence band offset at Al2O3 /SiC
interface using XPS. ͑a͒ Si 2p core level spectra from clean SiC and thin
Al2O3 /SiC samples, ͑b͒ Al 2p core level spectra from thin Al2O3 /SiC and
bulk Al2O3 samples, and ͑c͒ Valence band spectra from clean SiC and bulk
Al2O3 samples.
literature.5 Figure 3 presents the XPS spectra for the
measurement of valence band offset at the HfAlO/SiC
interface. Like in the case of Al2O3, Figs. 3͑a͒ and 3͑b͒ result
SiC
Si 2p
in E
−ETSih2inp_Oxide/SiC=EAThli2np_Oxide/SiC−EBAul l2kp_Oxide=0. From
Fig. 3͑c͒, the position of the VBM was found to be 1.84 eV
for clean SiC and 3.43 eV for bulk HfAlO yielding a valence
band offset of ⌬EV=1.59 eV for HfAlO/SiC.
Figure 4͑a͒ shows the XPS O 1s energy loss spectra for
bulk Al2O3 and HfAlO films. The onset of single particle
⌬EV = ͑ESSiiC2p − E
͒ − ͑ESi 2p
Thin_Oxide/SiC − E
͒
SiC
VBM
Thin_Oxide/SiC
Al 2p
Bulk_Oxide
VBM
− ͑EAl 2p
Bulk_Oxide − E
͒.
͑1͒
Si 2p3/2 and Al 2p3/2 were chosen as the reference core lev-
els for the SiC substrate and dielectric, respectively. Valence
band maximum ͑VBM͒ was found by linear extrapolation of
the leading edge of the valence band spectra. All of the spec-
tra for thin oxide/SiC samples were referenced to the Si 2p3/2
peak from clean SiC sample at 100.4 eV to compensate for
charging effects. For the thick ͑bulk͒ dielectric samples, the
spectra was calibrated against the adventitious carbon C 1s
peak at 285.0 eV to correct for differential charging.11 Fig. 2
presents the XPS spectra for measurement of band offset at
the Al2O3/SiC interface. Figure 2͑a͒ shows Si 2p3/2 spectra
for clean SiC and Al2O3/SiC samples with the peaks aligned
at 100.4 eV. A slight shoulder at 103.0 eV is due to Si–O
bonding from a thin interfacial SiO2 growth likely during
PDA. Figure 2͑b͒ shows Al 2p spectra of thin Al2O3/SiC
and bulk Al2O3 samples. The peak due to Al–O bonding is
positioned at 74.8 eV for both samples. From Figs. 2͑a͒ and
SiC
Thin_Oxide/SiC
−ETSih2inp_Oxide/SiC=E
Al 2p
2͑b͒, it is clear that E
Si 2p
−EBAul l2kp_Oxide=0. Therefore, the valence band offset given by
͑1͒ is modified as, ⌬EV=EBVuBlMk_Oxide−E
. Figure 2͑c͒
SiC
VBM
shows the valence band edge spectra for clean SiC and bulk
Al2O3 samples. By linear extrapolation, the position of the
VBM was found to be 1.84 eV for clean SiC and 3.71 eV for
bulk Al2O3. The valence band offset of Al2O3/SiC was de-
termined to be ⌬EV=1.87 eV. The result is in general agree-
ment with a previously reported value of 1.59 eV in
FIG. 3. ͑Color online͒ Determination of valence band offset at HfAlO/SiC
interface using XPS. ͑a͒ Si 2p core level spectra from clean SiC and thin
HfAlO/SiC samples, ͑b͒ Al 2p core level spectra from thin HfAlO/SiC and
bulk HfAlO samples, and ͑c͒ Valence band spectra from clean SiC and bulk
HfAlO samples.
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042903-3
Suri et al.
Appl. Phys. Lett. 96, 042903 ͑2010͒
FIG. 5. ͑Color online͒ Capacitance-voltage and frequency dispersion char-
acteristics of MOS capacitors on 4H–SiC with ͑a͒ 115 Å of Al2O3 and ͑b͒
115 Å of HfAlO. The insets to ͑a͒ and ͑b͒ are plots of gate leakage current
density ͑Jg͒ vs the effective electric field ͑Eeff͒. Eeff was calculated using
Eeff=͑Vg −Vfb͒/EOT where Vfb is the flatband voltage.
FIG. 4. ͑Color online͒ ͑a͒ XPS O 1s energy loss spectra from bulk Al2O3
and HfAlO films as well as sapphire substrate for determination of the
energy band gap ͑b͒ Energy band diagram of Al2O3 /4H–SiC and
HfAlO/4H–SiC constructed using valence band offset and band gap values
as determined by XPS.
excitation or band-to-band transition can be used to measure
the energy band gap of the films.12 The energy gap of Al2O3
and HfAlO were determined by linear extrapolation to be 7.0
and 6.0 eV, respectively. Also, included is the spectrum for
single crystal sapphire ͑Al2O3͒ yielding a band gap of 8.7 eV.
Interestingly, while a pure crystalline Al2O3 in the form of
sapphire yields a large band gap, the value for amorphous
ALD-deposited Al2O3 film is significantly reduced. This is
mainly due to the amorphous structure of ALD-Al2O3 and
growth induced defect states within the band gap.13 The en-
ergy gap of HfAlO is significantly larger than the reported
value of 5.2–5.5 eV ͑Refs. 7 and 14͒ for HfO2 and adequate
to yield a larger than 1 eV conduction band offset on
4H–SiC.
The conduction band offset at the high-k/SiC interface
was determined by using the expression, ⌬EC=Eg͑oxide͒
−⌬EV−Eg͑SiC͒. Using Eg=3.25 eV for 4H–SiC, ⌬EC was
found to be 1.88 eV for Al2O3 and 1.16 eV for HfAlO.
Figure 4͑b͒ shows schematically the energy band alignment
of the two dielectrics on 4H–SiC. It is interesting to note
that Al2O3 on 4H–SiC results in symmetric valence and con-
duction band offsets indicating equal barrier heights for elec-
trons and holes. The conduction band offset for HfAlO of
⌬EC=1.16 eV is raised considerably above the value ͑0.54–
0.70 eV͒ ͑Refs. 4 and 5͒ reported for HfO2. Although Al2O3
results in a larger conduction band offset, HfAlO offers ac-
ceptable barrier heights greater than 1 eV for both electrons
and holes making it a suitable dielectric for SiC MOS de-
vices.
Figure 5 shows the capacitance-voltage ͑C-V͒ plots of
MOS capacitors with 115 Å of Al2O3 or HfAlO gate dielec-
trics. For the same physical thickness, the equivalent oxide
thickness ͑EOT͒ values were obtained as 3.0 nm for HfAlO
and 6.0 nm for Al2O3 from the C-V curve at 1 MHz fre-
quency. Based on the physical thickness and EOT, the dielec-
tric constant was determined to be kϳ7.5 for Al2O3 and k
ϳ15.0 for HfAlO. A large frequency dispersion in the C-V
curves was observed in the depletion regime for Al2O3; how-
ever this dispersion was absent for HfAlO suggesting a re-
duction in Dit at HfAlO/SiC interface. In addition, both di-
electrics exhibited a negative fixed charge density at the
interface revealed by the positive flatband voltage shift. De-
spite a lower conduction band offset, HfAlO resulted in a
significantly lower gate leakage ͑ϳ2 orders of magnitude at
an effective electric field of 7 MV/cm͒ and a higher effective
breakdown field of ϳ15 MV/cm compared to Al2O3 as
shown in the insets of Figs. 5͑a͒ and 5͑b͒. The effective
electric field takes into account the physical thickness and
the dielectric constant of the dielectric. The leakage reduc-
tion is attributed in part to a higher k and to a decreased
density of traps in HfAlO compared to Al2O3.15 Thus,
HfAlO exhibits superior C-V properties than Al2O3 with a
higher k, lower Dit and a higher effective breakdown field.
In conclusion, the energy band alignment of Al2O3 and
HfAlO films on 4H–SiC was experimentally determined us-
ing XPS. In addition to a sufficient conduction band offset,
the higher dielectric constant and effective breakdown elec-
tric field of HfAlO compared to Al2O3 makes it potentially
more suitable to exploit the high breakdown field of SiC for
MOS device operation. This provokes interest in further in-
vestigation of HfAlO/SiC gate stack properties.
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