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0031-8949%2F2006%2FT126%2F002

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Physica Scripta
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This content was downloaded from IP address 129.59.95.115 on 27/10/2017 at 09:22
INSTITUTE OF PHYSICS PUBLISHING
PHYSICA SCRIPTA
Phys. Scr. T126 (2006) 6–9
doi:10.1088/0031-8949/2006/T126/002
Annealing study of H2O and O3 grown
Al2O3 deposited by atomic layer
chemical vapour deposition on
n-type 4H-SiC
Marc Avice1 , Ulrike Grossner1 , Ola Nilsen2 , Jens S Christensen1 ,
Helmer Fjellvåg2 and Bengt G Svensson1
1
Physics Department and Centre for Materials Science and Nanotechnology (SMN),
University of Oslo, Box 1048, Blindern, Oslo 0316, Norway
2
Chemistry Department and Centre for Materials Science and Nanotechnology (SMN),
University of Oslo, Box 1033, Blindern, Oslo 0315, Norway
E-mail: marc.avice@fys.uio.no
Received 1 October 2005
Accepted for publication 5 January 2006
Published 17 August 2006
Online at stacks.iop.org/PhysScr/T126/6
Abstract
Al2 O3 has been grown by atomic layer chemical vapour deposition on HF cleaned n-type
4H-SiC using either H2 O or O3 as an oxidant. After post-deposition annealing at high
temperature (1000◦ C) in argon atmosphere for different durations (1, 2 and 3 h), bulk and
interface properties of the films were studied by capacitance–voltage (CV), current–voltage
(IV) and secondary ion mass spectrometry (SIMS) measurements. Electrical measurements
show a decreasing shift of the flatband voltage indicating a diminution of the negative oxide
charges with increasing annealing time. The SIMS measurements reveal accumulation of
boron, sodium and potassium at the Al2 O3 /SiC interface but the accumulation decreases with
annealing at 1000◦ C where also out diffusion of silicon into the Al2 O3 film takes place.
PACS numbers: 77.55.+f, 73.55.Qv, 78.66.Nk, 73.60.Ng
1. Introduction
2. Experimental
As a native oxide, SiO2 has been used as a gate oxide for
SiC metal-oxide-semiconductor (MOS) devices. However, the
SiC/SiO2 interface formed has a density of interface states
one or two orders of magnitude higher than the Si/SiO2
interface. Al2 O3 is one of the alternative dielectric candidates
among the pure metal oxides exhibiting a large dielectric
constant (ε ∼ 10), a large band gap (∼ 9 eV) and is believed
to be the most chemically and thermodynamically stable
one [1, 2]. Several studies of Al2 O3 deposited on Si have
reported the presence of carbon contamination, metal clusters
and defect states like Al–Al and OH bonds in the Al2 O3 bulk
grown with water as an oxidant [3]. By growing the oxide
using ozone as an oxidant instead of water, some groups have
reported films showing a leakage-current density which is one
to two orders of magnitude lower and these structures also
exhibit a smaller flatband voltage [4].
Al2 O3 films were prepared by atomic layer chemical vapour
deposition on Si-face, n-type 4H-SiC samples with a 10 µm
thick epi-layer (doping level 2 × 1015 cm−3 ) on a highly
doped substrate (doping level 1 × 1018 cm−3 ), oriented 8◦
off the (0001) direction, purchased from Cree Research Inc.
The films were grown with water or ozone as an oxidant
and tri-methyl-aluminium (TMA) as a precursor at a reactor
temperature of 300◦ C. The 4H-SiC samples were cleaned
prior to the oxide deposition using hydrofluoric acid followed
by rinsing in de-ionized water and in situ ozone treatment just
before the deposition. The oxide thickness was determined
by x-ray reflectometry to be ∼100 nm. Annealing treatments
were carried out in Ar atmosphere at 1000◦ C during 1, 2 and
3 h. Ni contacts were formed by thermal evaporation through
a shadow mask providing circular dots with a diameter of
0.3 mm. As an Ohmic back side contact, a eutectic InGa
0031-8949/06/126006+04$30.00
© 2006 The Royal Swedish Academy of Sciences
Printed in the UK
6
Annealing study of H2 O and O3 grown Al2 O3 deposited
a
b
c
d
e
f
Figure 1. CV measurements of annealed Al2 O3 /SiC capacitors for both H2 O (1, 2 and 3 h-annealing: a, c, e) and O3 growth
(1, 2 and 3 h-annealing: b, d, f).
mixture was used. Secondary ion mass spectrometry (SIMS,
Cameca IMS 7F) analysis was performed for B and Si using
oxygen as sputtering ions with an impact energy of 10 keV.
The ion beam current was 2 nA, and the beam was rastered
over an area of 100 × 100 µm2 . Sample charging effects were
minimized by electron flooding. High frequency (1 MHz)
capacitance–voltage (CV) measurements were carried out in
the dark using a sweep rate of 0.5 V s−1 .
3. Result and discussion
CV measurements have been performed at room temperature
on as-grown and annealed Al2 O3 /SiC capacitors for both
H2 O and O3 growth. As-grown samples exhibit a flatband
voltage (VFB ) in excess of 35 V indicating a large amount of
negative oxide charge. However, after annealing at 1000◦ C for
different durations, namely 1, 2 and 3 h, the flatband voltage
7
M Avice et al
Table 1. Survey of electrical characteristics of H2 O and O3 grown Al2 O3 /SiC capacitors for different annealing durations at 1000◦ C.
VFB (V)
Q ox (q cm−2 )
Annealing
time (h)
H2 O
growth
O3
growth
H2 O
growth
O3
growth
H2 O
growth
O3
growth
0
1
2
3
>35
29
17
6
>35
12
8
6
>5 × 1013
2.78 × 1013
1.85 × 1013
5.74 × 1012
>5 × 1013
1.18 × 1013
7.58 × 1012
5.22 × 1012
—
—
6
4
—
7
6
1, 2
Figure 2. Flatband voltage for the Al2 O3 /SiC capacitors versus
annealing time at 1000◦ C. Results are shown for the samples using
both H2 O and O3 as oxidants during the Al2 O3 growth.
decreases gradually. Typical capacitance-versus-voltage plots
obtained with a measurement frequency of 1 MHz are shown
figure 1. At ∼ 0 V, a bump appears in the CV curves. This
may be caused by charges that are trapped at the SiC/oxide
interface while sweeping the voltage from inversion to
accumulation.
Here, a first CV measurement is performed and is
then immediately followed by a second one. After the first
measurement, charges are trapped and not released while
sweeping the voltage from accumulation to depletion. As a
result, the second CV measurement exhibits shifted curves
towards positive voltages. However, after a few days of
storage of the samples at room temperature, the CV curves
recover to the initial state (i.e. the first CV measurement)
indicating that the traps are located deep in the bandgap. For
the capacitors shown in figure 1(a), a very low breakdown
voltage was found and a second CV measurement was not
possible. Figure 2 shows the average flatband voltage values
versus annealing time determined for all the capacitors on
each sample. For both types of oxidants, H2 O and O3 , the
flatband voltage decreases while increasing the annealing
time. For the O3 grown samples the flatband voltage has
strongly diminished after 1 h annealing at 1000◦ C, while
the H2 O grown samples reveal a more linear diminution
with the annealing time. This behaviour may be correlated
with crystallization of the Al2 O3 layer and the re-formation
8
Hysteresis (V)
of the oxide/SiC interface. After 3 h annealing both types
of capacitors show the same flatband voltage VFB = 6 V.
However, the hysteresis of the CV curves obtained while
sweeping the gate voltage from depletion to accumulation
and then back is smaller for the O3 grown samples (1.2 V)
than for the H2 O grown samples (4 V). This indicates that the
electron traps causing the hysteresis are of a different nature
in the case of more reactive ozone growth, either being less
temperature stable or shifting to more shallow positions in
the energy gap than for the water growth. The oxide charge
density is substantially reduced after the high temperature
treatments for both types of oxidants (table 1). Here, it may be
mentioned that current–voltage (IV) measurements of the 2 h
annealed Al2 O3 /SiC capacitor reveal a lower leakage current
(J [O3 ] ∼ 0.3 µA cm−2 and J [H2 O] ∼ 0.5 µA cm−2 at 25 V)
than the 3 h annealed capacitor for both growth types, possibly
due to the formation of leakage current paths, like grain
boundaries during the annealing process since crystallization
is anticipated to occur at ∼1000◦ C [5]. The interface states
density (Dit ) was deduced using the conductance method
on the 2 h annealed capacitors [6]. The ozone grown
samples show a lower Dit (from 9 × 1012 eV−1 cm−2 close to
the conductance band edge to 1.3 × 1012 eV−1 cm−2 close
to midgap) compared to the water grown sample (from
6.25 × 1013 eV−1 cm−2 close the conduction band edge to
2.8 × 1013 eV−1 cm−2 close to midgap).
In order to identify impurities in the Al2 O3 films, SIMS
measurements have been performed on both as-grown and
3 h annealed (1000◦ C)Al2 O3 /SiC capacitors (H2 O and O3
as an oxidant) using different purities of the precursor
(TMA). Significant amounts of boron, sodium and potassium
have been found at the Al2 O3 /SiC interface [7]. Using
a high purity (99.99%) TMA the boron concentration at
the Al2 O3 /SiC interface as well as within the aluminium
oxide is substantially reduced. However, no corresponding
improvement of the electrical characteristics is obtained with
high purity TMA suggesting that boron does not affect VFB .
Similar studies have been carried out with other elements such
as nickel and phosphorus and no influence on VFB was found.
After 3 h annealing at 1000◦ C, the amount of boron at the
Al2 O3 /SiC interface decreases in the sample grown with high
purity TMA and this seems to be accompanied by a diffusion
towards the surface, figure 3. Concurrently, a redistribution of
silicon into Al2 O3 layer takes place which may possibly be
associated with the improved electrical performance; on the
other hand, for samples grown with 98% TMA using H2 O and
O3 as oxidant the silicon redistribution is more pronounced in
the latter case, figure 3, but no correlation with the electrical
performance is obtained. Further work is being pursued to
Annealing study of H2 O and O3 grown Al2 O3 deposited
Figure 3. SIMS profiles of both as-grown and annealed Al2 O3 /SiC capacitors (O3 and H2 O growth) with different purity of the precursor
(TMA) used for the Al2 O3 growth.
identify the cause for the improved electrical characteristics
with annealing at 1000◦ C.
4. Summary and conclusions
In this work, we have studied the effect of high temperature
annealing (1000◦ C) on H2 O and O3 grown Al2 O3 /SiC
capacitors. Electrical characterizations were performed by
CV and IV measurements. The interface states density Dit
decreased with annealing time and after 2 h at 1000◦ C it
was deduced to be 2.8 × 1013 eV−1 cm−2 close to midgap
for the water grown capacitors and 1.3 × 1012 eV−1 cm−2
for the ozone grown capacitors. Hence, ozone appears to
be the preferable oxidant, and this is further supported by
a smaller flatband voltage (fixed oxide charges) than the
water grown samples although the difference after 3 h at
1000◦ C is marginal. Impurities were investigated by SIMS
measurements on as-grown and annealed samples. Significant
amounts of boron, phosphorus and nickel were measured at
the Al2 O3 /SiC interface. However, these elements do notseem
to affect the electrical properties of the Al2 O3 /SiC capacitors.
Acknowledgment
Financial support by the FUNMAT program at the University
of Oslo and the Norwegian Research Council (strategic
university programme on advanced sensors) is gratefully
acknowledged.
References
[1] Groner M D, Elam J W, Fabreguette F H and George S M
2002 Thin Solid Films 413 186
[2] Afanas’ev V V, Stesmans A and Zhao C 2002 Appl. Phys. Lett.
81 1678
[3] Kim J B, Kwon D R and Chakrabarti K 2002 J. Appl. Phys.
92 6739
[4] Kim J B, Chakrabarti K and Lee J 2003 Mat. Chem. Phys.
78 733
[5] Chang Y, Ducroquet F and Gosset L G 2001 J. Phys. IV France
11 139
[6] Nicollian E H and Brews J R 1976 Metal Oxide Semiconductor
(New York: Wiley) 213
[7] Avice M et al 2006 Mater. Sci. Forum
9
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