Electrical performance of Al2O3 gate dielectric films deposited by atomic layer deposition on 4H-SiC

Article abstract of DOI:10.1063/1.2805742

Stoichiometric and pure Al2 O3 gate dielectric films were grown on n -type 4H-SiC by a thermal atomic layer deposition process. The electrical properties of both amorphous and epitaxial Al2 O3 films were studied by capacitance-voltage and current-voltage measurements of metal-oxide-semiconductor capacitors. A dielectric constant of 9 and a flatband voltage shift of +1.3 V were determined. A leakage current density of 10-3 A cm2 at 8 MVcm was obtained for the amorphous Al2 O3 films, lower than that of any high- κ gate oxide on 4H-SiC reported to date. A Fowler-Nordheim tunneling mechanism was used to determine an Al2 O3 4H-SiC barrier height of 1.58 eV. Higher leakage current was obtained for the epitaxial γ- Al2 O3 films, likely due to grain boundary conduction.

Full text of DOI:10.1063/1.2805742

Electrical performance of Al 2 O 3 gate dielectric films deposited by atomic layer
deposition on 4 H - Si C
Carey M. Tanner, Ya-Chuan Perng, Christopher Frewin, Stephen E. Saddow, and Jane P. Chang
Citation: Applied Physics Letters 91, 203510 (2007); doi: 10.1063/1.2805742
View online: http://dx.doi.org/10.1063/1.2805742
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APPLIED PHYSICS LETTERS 91, 203510 ͑2007͒
Electrical performance of Al O gate dielectric films deposited by atomic
2
3
layer deposition on 4H-SiC
Carey M. Tanner and Ya-Chuan Perng
Department of Chemical and Biomolecular Engineering, UCLA, Los Angeles, California 90095, USA
Christopher Frewin and Stephen E. Saddow
Department of Electrical Engineering, University of South Florida, Tampa, Florida 33620, USA
a͒
Jane P. Chang
Department of Chemical and Biomolecular Engineering, UCLA, Los Angeles, California 90095, USA
͑
Received 1 July 2007; accepted 16 October 2007; published online 13 November 2007͒
Stoichiometric and pure Al O gate dielectric films were grown on n-type 4H-SiC by a thermal
2
3
atomic layer deposition process. The electrical properties of both amorphous and epitaxial Al O
2
3
films were studied by capacitance-voltage and current-voltage measurements of metal-
oxide-semiconductor capacitors. A dielectric constant of 9 and a flatband voltage shift of +1.3 V
were determined. A leakage current density of 10− A/cm at 8 MV/cm was obtained for the
3
2
amorphous Al O films, lower than that of any high-␬ gate oxide on 4H-SiC reported to date. A
2
3
Fowler-Nordheim tunneling mechanism was used to determine an Al O /4H-SiC barrier height of
2
3
1
.58 eV. Higher leakage current was obtained for the epitaxial ␥-Al O films, likely due to grain
2 3
boundary conduction. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2805742͔
3
The integration of high dielectric constant ͑high-␬͒ gate
oxides in SiC power metal-oxide-semiconductor field-effect
transistors ͑MOSFETs͒ has gained more attention in recent
breakdown field of SiC. Second, the large bandgap of Al O
2
3
thin films ͑E =7.0 eV͒ relative to other high-␬ oxides, such
as HfO2 and La O , potentially enables adequate barrier
heights at the interface with 4H-SiC ͑E =3.26 eV͒.
n-type 4H-SiC ͑0001͒ substrates ͑8° off-axis͒ were ob-
tained from Cree and cleaned to remove particles and native
oxide. Hydrogen etching was used to prepare the samples for
subsequent SiC epitaxial growth. Epitaxial growth was per-
formed in a horizontal hot-wall chemical vapor deposition
g
2
3
1
4
literature. The vast majority of research has focused on a
g
SiO gate dielectric since SiC can be thermally oxidized to
2
yield SiO , a property that makes it unique among wide
2
bandgap semiconductors. The performance of these
1
0
SiO /SiC MOSFETs has been improved by various nitrida-
2
tion methods, but their commercialization remains limited by
a low channel mobility due to a high interface state density
11
system to produce an 8–10 ␮m specular, single crystalline
layer of 4H-SiC. Al O thin films of 200–300 Å thickness
2
near the conduction band. In addition, since the electric field
2
3
across an interface scales inversely with the dielectric con-
were deposited by thermal atomic layer deposition ͑ALD͒ at
200 °C using trimethylaluminum and water vapor. Alternat-
ing precursor pulses of 15 s were introduced at 2
ϫ10− Torr and separated by 60 s evacuation steps. Film
thickness was determined by spectroscopic ellipsometry over
a range of 280–760 nm. In situ x-ray photoelectron spectros-
copy ͑XPS͒ was performed with a monochromatic Al K␣
͑1486.6 eV͒ x-ray source. In situ reflection high-energy elec-
tron diffraction ͑RHEED͒ was performed with a 15 keV
electron gun. To determine the impact of film structure on the
electrical performance, some of the films were crystallized
stant of the material, the low dielectric constant of SiO ͑␬
2
=
3.9͒ relative to that of 4H-SiC ͑␬=10͒ results in an electric
4
field in SiO that is 2.5 times higher than that in SiC. This
2
inequity requires device operation at an electric field far be-
low the SiC breakdown field in order to avoid premature
3
SiO breakdown. Thus, the high breakdown field of SiC
2
͑
3.0 MV/cm͒ is severely underutilized, minimizing one of
the material’s major advantages for high-power applications.
Additionally, since the blocking voltage of the power MOS-
FET scales with the square of the electric field, the device’s
blocking voltage capability is dramatically reduced for a
by post-deposition rapid thermal annealing in a N ambient
2
given on-resistance. The limitations imposed by SiO prompt
at 1100 °C for 4 min with a 30 s ramp up from room tem-
perature. MOS capacitors were fabricated with evaporated Al
gate electrodes and patterned using standard photolitho-
graphic methods. An evaporated Ti/Pt gate electrode was
also tested and similar results were obtained. Capacitance-
voltage ͑C-V͒ measurements were performed using an HP
4284A precision LCR meter at 1 MHz with a voltage sweep
rate of 98 mV/s. Current-voltage ͑I-V͒ measurements were
performed using an HP 4145A semiconductor parameter ana-
lyzer with a voltage sweep rate of 64 mV/ms. At least 20
devices were measured with consistent behavior, and repre-
sentative data sets were selected for this manuscript.
2
the search for a high-␬ gate oxide to enable MOSFET opera-
tion near the SiC breakdown field while maintaining a sig-
nificantly lower field in the oxide. Numerous high-␬ gate
dielectric materials and stacks have been investigated on
4
5
6
SiC, including Al O , oxidized Ta Si, SiO /TiO ,
2
3
2
2
2
7
8
9
Gd O , SiO /HfO , and AlN. Several intrinsic properties
2
3
2
2
of Al O make it a viable candidate as a gate dielectric ma-
2
3
terial in SiC-based devices. First, the higher dielectric con-
stant of Al O ͑␬=10͒ compared to SiO allows device op-
2
3
2
eration at a higher electric field, taking advantage of the high
a͒Author to whom correspondence should be addressed. Electronic mail:
jpchang@seas.ucla.edu
The Al O films were found to be stoichiometric with
negligible carbon incorporation based on in situ XPS analy-
2 3
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003-6951/2007/91͑20͒/203510/3/$23.00 91, 203510-1 © 2007 American Institute of Physics
69.230.243.252 On: Wed, 17 Dec 2014 20:37:47
0
1

203510-2
Tanner et al.
Appl. Phys. Lett. 91, 203510 ͑2007͒
FIG. 1. The area-normalized capacitance-voltage curves ͑solid lines͒ of
2
4
H-SiC MOS capacitors ͑40ϫ200, 100ϫ200, and 200ϫ200 ␮m ͒ and an
ideal curve ͑dashed line͒ with an amorphous as-deposited 260 Å Al O gate
2
3
dielectric layer. The upper inset shows the leakage current density as a
function of electric field across the gate dielectric in capacitors with amor-
phous Al O , crystalline ␥-Al O , and amorphous HfO thin films. The
FIG. 2. X-ray photoemission spectra of ͑a͒ 4H-SiC, ͑b͒ a 200 Å Al O film,
2
3
and ͑c͒ a 25 Å Al O film. ͑d͒ The O 1s peak and inelastic scattering loss for
2
3
2
3
2
3
2
a 200 Å Al O film.
2
3
lower inset shows the Fowler-Nordheim tunneling fit for the amorphous
Al O /4H-SiC MOS capacitor. Note that a different test equipment with a
2
3
lower sensitivity at low current values was used to measure the amorphous
Al O sample.
2
3
age current of the HfO films is likely due to the low con-
2
1
8
duction band offset at the HfO /4H-SiC interface. HfO is
2
2
a promising gate oxide material on Si due to its high dielec-
tric constant ͑␬=20͒; however, its small bandgap ͑Eg
sis. The Al O film growth rate was determined to be
2
3
0.88±0.03 Å/cycle by spectroscopic ellipsometry, and was
=
5.7 eV͒ appears to be prohibitive for 4H-SiC MOS device
integration.
The J-E response of an Al/amorphous-Al O /n-4H-SiC
confirmed by high-resolution transmission electron micros-
copy ͑HRTEM͒. The as-deposited films were amorphous and
smooth as determined by in situ RHEED and atomic force
microscopy. As detailed in our earlier report, amorphous
Al O films are transformed into epitaxial ␥-Al O films by
2
3
capacitor, shown as an inset in Fig. 1, was analyzed using
Poole-Frenkel emission, Schottky emission and Fowler-
Nordheim ͑FN͒ tunneling mechanisms. The FN tunneling
2
3
2
3
1
2
rapid thermal annealing. The films are chemically stable up
to 1100 °C, as no significant interfacial layer formation was
observed by XPS or HRTEM.
1
9
mechanism was found to yield the most reliable fitting and
has been previously used to characterize SiC-based
1
7,19
2
MOS capacitors.
The model is described by J=AE
C-V measurements indicated a dielectric constant of 9
for the Al O films based on the film thickness measured by
3
exp͑−B/E͒, where A=͑q /8␲h͒͑m /m ͒͑1/␾ ͒, B=
/3hq, q is the electron charge, mSiC and mox
b
are the effective electron masses in SiC and Al O , and ␾ is
SiC
ox
b
2
3
1
/2 3/2
ellipsometry. This value is reasonable for Al O thin films
8␲͑2mox͒
␾
2
3
1
3,14
and consistent with previous reports.
As shown in Fig. 1,
2
3
b
a reasonably constant flatband voltage shift of +1.3 V rela-
tive to the ideal flatband voltage was observed for various
devices, suggesting a negative fixed charge density of
the Al O /4H-SiC barrier height. The value of m was as-
2 3 ox
2
0
sumed to be 0.2m, where m is the free electron mass. An
Al O /4H-SiC barrier height of 1.58 eV was obtained from
2
3
1
2
−2
͑
2–3͒ϫ10 cm . The source of the fixed charge is attrib-
this analysis based on the fitting between 4.4 and
8.3 MV/cm.
uted to trapped hydroxyl groups in the film due to the depo-
1
5
sition process. This flatband shift is relatively small com-
pared to other reports of Al O films grown by various
methods,
We also used XPS to determine the band offsets at the
Al2O3/SiC interface. Based on the method described by
2
3
1
6,17
21
suggesting that the thermal ALD method is bet-
Puthenkovilakam and Chang and Van de Walle and
2
2
ter suited for gate dielectric deposition.
Martin, XPS spectra were obtained for bulk 4H-SiC, a
200 Å Al2O3 film, and a 25 Å Al2O3 film grown on 4H-SiC,
as shown in Fig. 2. To determine the band alignment, first,
I-V measurements of Al/Al O /4H-SiC MOS capaci-
2
3
tors are shown as an inset in Fig. 1, where the electric field
represents the gate voltage divided by the physical film
thickness. Although hard breakdown was observed for the
SiC
the valence band maximum of bulk 4H-SiC, EV =1.92 eV,
SiC
is referenced to the Si 2p3/2 core-level binding energy, ECL
amorphous Al O3 films at an electric field ͑E͒ of
=100.40 eV. Similarly, the valence band maximum of
2
Al O
2
3
5
.2 MV/cm, a relatively low leakage current density ͑J͒ of
Al2O3, EV
=3.33 eV, is referenced to the Al 2p3/2 core-
Al O
2 3
−
3
2
level binding energy, ECL =74.30 eV. The valence bands
are then aligned based on the core-level to core-level energy
10
A/cm was observed at 8 MV/cm. However, the epi-
taxial ␥-Al O films exhibited a leakage current density of
2
3
−
3
2
difference, ⌬ECL=25.92 eV, determined from
a thin
10
A/cm at a lower electric field of E=2.5 MV/cm. The
Al O /4H-SiC sample in which both layers are sampled si-
higher leakage current is attributed to charge conduction
along the grain boundaries in the oxide, which were identi-
fied as twinned crystallites during the structural analysis of
2
3
SiC
SiC
multaneously. The valence band offset is ⌬EV=͑EV −ECL ͒
oxide
oxide
CL
−͑EV −E
͒+⌬ECL=1.59 eV. The Al2O3 bandgap was
1
2
the film by synchrotron x-ray diffraction. However, the
leakage current is far lower than that measured for amor-
determined from the onset of the inelastic scattering loss sig-
nal on the higher binding energy side of the O 1s peak for a
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phous HfO gate dielectric films on 4H-SiC. The high leak-
200 Å Al O film, as shown in Fig. 2͑d͒. The separation
2
2 3
1
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203510-3
Tanner et al.
Appl. Phys. Lett. 91, 203510 ͑2007͒
tion as a gate dielectric in SiC power MOSFET devices.
In conclusion, Al O gate dielectrics with superior elec-
2
3
trical performance were demonstrated in 4H-SiC MOS ca-
pacitors. A dielectric constant of 9 and state-of-the-art leak-
age current densities were obtained for amorphous Al O
2
3
films. Higher leakage current was obtained for the epitaxial
-Al O films, suggesting the need for further optimization
␥
2
3
of the crystalline quality of these films.
The authors acknowledge the financial support of a
Young Investigator Award from the Office of Naval Re-
search.
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17
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1
9
2
3
gests that ALD Al O films are very promising for integra-
2
3
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