Structural properties of epitaxial γ- Al2O3 (111) thin films on 4H-SiC (0001)

Article abstract of DOI:10.1063/1.2435978

Al2 O3 thin films were grown on 4H-SiC (0001) by thermal atomic layer deposition and were crystallized to the γ- Al2 O3 phase by rapid thermal annealing in N2 at 1100 °C. The films were found to be chemically stable during processing based on x-ray photoelectron spectroscopy. The change in film structure was initially confirmed by reflection high-energy electron diffraction. As shown by high-resolution transmission electron microscopy images, the abrupt interface of the as-deposited films with the 4H-SiC substrate was preserved during crystallization, indicating no interfacial reaction. Selected area electron diffraction and synchrotron-based x-ray diffraction established an epitaxial relationship of γ- Al2 O3 (111) ∥ 4H-SiC (0001) and in-plane orientation of γ- Al2 O3 (1 1- 0) ∥ 4H-SiC (11 2- 0). No other alumina phases or orientations were observed and no in-plane misorientation was observed in the 27 A Al2 O3 films. The full width at half maximum of the γ- Al2 O3 (222) rocking curve is 0.056°, indicating a lack of mosaic spread and a high-quality crystalline film. Twinning around the γ- Al2 O3 [111] axis was the only defect observed in these films.

Full text of DOI:10.1063/1.2435978

Structural properties of epitaxial γ - Al 2 O 3 (111) thin films on 4 H - Si C (0001)
Carey M. Tanner, Monica Sawkar-Mathur, Jun Lu, Hans-Olof Blom, Michael F. Toney, and Jane P. Chang
Citation: Applied Physics Letters 90, 061916 (2007); doi: 10.1063/1.2435978
View online: http://dx.doi.org/10.1063/1.2435978
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APPLIED PHYSICS LETTERS 90, 061916 ͑2007͒
Structural properties of epitaxial ␥-Al2O3 „111… thin films on 4H-SiC „0001…
Carey M. Tanner and Monica Sawkar-Mathur
Department of Chemical and Biomolecular Engineering, University of California, Los Angeles,
California 90095
Jun Lu and Hans-Olof Blom
Ångström Laboratory, Uppsala University, SE-75121 Uppsala, Sweden
Michael F. Toney
Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park,
California 94025
Jane P. Changa͒
Department of Chemical and Biomolecular Engineering, University of California, Los Angeles,
California 90095
͑Received 25 October 2006; accepted 25 December 2006; published online 8 February 2007͒
Al2O3 thin films were grown on 4H-SiC ͑0001͒ by thermal atomic layer deposition and were
crystallized to the ␥-Al2O3 phase by rapid thermal annealing in N2 at 1100 °C. The films were
found to be chemically stable during processing based on x-ray photoelectron spectroscopy. The
change in film structure was initially confirmed by reflection high-energy electron diffraction. As
shown by high-resolution transmission electron microscopy images, the abrupt interface of the
as-deposited films with the 4H-SiC substrate was preserved during crystallization, indicating no
interfacial reaction. Selected area electron diffraction and synchrotron-based x-ray diffraction
ʈ
established an epitaxial relationship of ␥-Al2O3 ͑111͒ 4H-SiC ͑0001͒ and in-plane orientation of
¯
¯
ʈ
␥-Al2O3 ͑110͒ 4H-SiC ͑1120͒. No other alumina phases or orientations were observed and no
in-plane misorientation was observed in the 27 Å Al2O3 films. The full width at half maximum of
the ␥-Al2O3 ͑222͒ rocking curve is 0.056°, indicating a lack of mosaic spread and a high-quality
crystalline film. Twinning around the ␥-Al2O3 ͓111͔ axis was the only defect observed in these
films. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2435978͔
The development of epitaxial high-␬ dielectrics has the
potential to improve the performance of SiC power metal-
oxide-semiconductor field effect transistors due to enhanced
dielectric strength, chemical stability, and reduced defects.1,2
Thermal SiO2 is presently the most widely studied gate oxide
material on SiC and its performance has been optimized by
various nitridation methods.3,4 However, commercialization
remains limited by low channel mobility and reliability
issues.5 Several intrinsic properties of Al2O3 make it a viable
candidate as an alternative gate dielectric material in SiC-
based devices. The higher dielectric constant of Al2O3 ͑␬
=10͒ compared to that of SiO2 ͑␬=3.9͒ allows device opera-
tion at a higher electric field, taking advantage of the high
breakdown field of SiC.6 The larger band gap of Al2O3 thin
films ͑Eg=7.0 eV͒ relative to that of other high-␬ oxides,
such as HfO2 and La2O3, allows adequate band offsets to be
maintained at the interface with 4H-SiC ͑Eg=3.26 eV͒.7
Alumina is also well known for its stability in several phases,
and epitaxy of ␥-Al2O3 thin films has been previously dem-
onstrated on Si.8–10 The ␥-Al2O3 phase has a defective cubic
atoms, and based on the hexagonal arrangement of O atoms
with 2.80 Å spacing, the mismatch in bonding with
4H-SiC ͑0001͒ is 8.8%. In this letter, we describe the syn-
thesis and structural characterization of high-quality epitaxial
alumina thin films, specifically the ␥-Al2O3 phase, on
4H-SiC.
On-axis n-type 4H-SiC ͑0001͒ substrates were treated
with a chemical mechanical polishing process ͑NovaSiC͒ on
the Si face to remove the disordered surface layer. The
samples were then outgassed at 300 °C in vacuum prior to
deposition. Al2O3 thin films of 25–500 Å thickness were
deposited by thermal atomic layer deposition at 200 °C us-
ing trimethylaluminum and water vapor. Alternating gas
pulses of 15 s were introduced at an operating pressure of
2ϫ10−4 Torr and separated by 60 s evacuation steps. The
¯
spinel structure with the same space group ͑Fd3m͒ as
MgAl2O4.11 Based on literature reports,12,13 we constructed a
model of the ␥-Al2O3 structure in preparation for a theoret-
ical study of the electronic structure of the bulk and its in-
terface with 4H-SiC.14 Schematics of the ␥-Al2O3 ͑111͒ and
4H-SiC ͑0001͒ surfaces are shown in Fig. 1. There are six
repeating inequivalent ␥-Al2O3 ͑111͒ planes of either Al or O
FIG. 1. Schematics of the ␥-Al2O3 ͑111͒ and 4H-SiC ͑0001͒ surfaces. The
atomic spacing of the O atoms and Si atoms, respectively, indicates an 8.8%
lattice mismatch at the interface.
a͒
Electronic mail: jpchang@ucla.edu
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0003-6951/2007/90͑6͒/061916/3/$23.00 90, 061916-1 © 2007 American Institute of Physics
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061916-2
Tanner et al.
Appl. Phys. Lett. 90, 061916 ͑2007͒
FIG. 2. RHEED and cross-sectional HRTEM images of Al2O3 on 4H-SiC.
͑a͒ As-deposited film is amorphous and no interfacial layer is observed. ͑b͒
Following postdeposition N2 annealing at 1100 °C for 4 min, the film crys-
tallizes and an abrupt interface with the substrate is observed.
FIG. 3. ͑a͒ Selected area electron diffraction pattern of a 200 Å ␥-Al2O3
film on 4H-SiC. ͑b͒ Radial scan along the specular direction of a 27 Å
␥-Al2O3 film on 4H-SiC ͑0001͒. The ␥-Al2O3 ͑222͒ peak is evident and no
other diffraction peaks related to the film are observed. The inset shows the
interference fringes of the ␥-Al2O3 ͑222͒ peak in log scale. ͑c͒ Rocking
curve of the ␥-Al2O3 ͑222͒ reflection. The peak is resolution limited. ͑d͒ Phi
growth rate was 0.88 0.03 Å/cycle. Film thickness was de-
termined by spectroscopic ellipsometry and confirmed by
high-resolution transmission electron microscopy ͑HRTEM͒
and x-ray diffraction ͑XRD͒ analyses. In situ x-ray photo-
electron spectroscopy ͑XPS͒ measurements were performed
using an ESCAlab system with a monochromatic Al K␣
͑1486.6 eV͒ x-ray source. In situ reflection high-energy elec-
tron diffraction ͑RHEED͒ was performed using a Staib In-
struments unit equipped with a 15 keV electron gun. To crys-
tallize the films, post-deposition rapid thermal annealing
͑RTA͒ was performed ex situ in a N2 ambient at 1100 °C for
4 min with a 30 s ramp-up from room temperature. The
Al2O3 atomic structure and interface were examined by
HRTEM and selected area electron diffraction ͑SAED͒ using
an FEI Tecnai F30 ST microscope at 300 kV. The film struc-
ture was also analyzed by synchrotron XRD ͑␭=1.3245 Å͒
using a four-circle diffractometer at beam line 7-2 at the
Stanford Synchrotron Radiation Laboratory. The diffracted
beam resolution was set by 1 mm ͑vertical͒ and 2 mm ͑hori-
zontal͒ slits, providing a resolution of ⌬2␪=0.1°. Scans were
taken along the specular direction and the theta, chi, and phi
axes to obtain detailed information about the film crystallin-
ity and orientation.
¯
¯
scans around the ␥-Al2O3 ͑440͒ and 4H-SiC ͑1120͒ reflections.
chemical shifts coincide with film crystallization, which was
observed by RHEED after this step. No further binding en-
ergy shift was observed after the 1100 °C annealing step. No
suboxide or metallic peaks were observed, and the full
widths at half maxima ͑FWHMs͒ of the Al 2p and O 1s
peaks were 1.55 and 1.90 eV, respectively, and remained un-
changed within 0.05 eV throughout the series of annealing
steps. Thus, these Al2O3 films were found to be chemically
stable during annealing up to 1100 °C.
The HRTEM analysis was performed to obtain atomic-
scale information about the crystallinity of the film and its
interface with the SiC substrate. A cross-sectional image of
an as-deposited 50 Å Al2O3 film, as shown in Fig. 2͑a͒, dem-
onstrates a smooth, uniform, amorphous film and an abrupt
interface with SiC. The absence of an interfacial layer may
be attributed to the low deposition temperature and the high
chemical stability of SiC. A cross-sectional image of the
crystalline Al2O3 film following RTA at 1100 °C is shown in
Fig. 2͑b͒. A single grain is observed and there is no evidence
of additional crystallites in this image. The abrupt interface
with the substrate is preserved, indicating the lack of inter-
facial reaction upon annealing. This result is promising for
device applications, since a well-ordered and lattice-matched
interface is necessary to minimize the defect density at crys-
talline oxide/semiconductor interfaces.
The as-deposited Al2O3 films were stoichiometric and
carbon incorporation was less than 2% as evaluated by in
situ XPS. These films were determined to be amorphous by
in situ RHEED, as evidenced in Fig. 2͑a͒ by the absence of
any pattern. Additionally, no Al2O3 diffraction peaks were
observed in XRD scans of the as-deposited films. Following
RTA at 1100 °C, crystallization of the films to the ␥-Al2O3
phase was observed by several techniques. Streaky RHEED
patterns, as shown in Fig. 2͑b͒ for a 50 Å Al2O3 film, were
¯ ¯
The SAED pattern for a crystalline Al2O3 film, as shown
in Fig. 3͑a͒, indicates an epitaxial relationship of ␥-Al2O3
ʈ
observed along the SiC ͓1210͔ azimuth, indicating a high-
quality crystalline film.
͑111͒
4H-SiC ͑0001͒ with an in-plane orientation of
The thermal stability of the Al2O3 films during the RTA
process was examined by XPS. An amorphous 50 Å Al2O3
film was annealed sequentially for durations of 4 min at 800,
900, 1000, and 1100 °C. After each annealing step, XPS
spectra and RHEED patterns were obtained. After the
1000 °C anneal step, a positive binding energy shift of
0.3 eV was observed in both the Al 2p and O 1s spectra. The
¯
¯
ʈ
␥-Al2O3 ͑110͒ 4H-SiC ͑1120͒. This pattern was obtained
for a 200 Å film to achieve a stronger signal compared to the
thinner films. The ␥-Al2O3 orientation was confirmed on a
macroscopic scale by synchrotron XRD and the crystalline
properties were examined in more detail. As shown in Fig.
3͑b͒ for a 27 Å film, the ␥-Al2O3 ͑222͒ peak is clearly ob-
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spectra were referenced to the substrate Si 2p3/2 peak. These
served in the specular ͑radial͒ scan at q=2.779 Å−1, on the
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061916-3
Tanner et al.
Appl. Phys. Lett. 90, 061916 ͑2007͒
shoulder of the 4H-SiC ͑0004͒ substrate peak. Therefore, the
͑111͒ lattice spacing is 4.522 Å, in good agreement with the
literature value of 4.560 Å,15 but indicating that the film is in
slight compression ͑0.84%͒ in the normal direction. The ab-
sence of additional peaks indicates the lack of other Al2O3
phases and crystallite orientations along the 4H-SiC ͑0001͒
direction. Interference fringes are observed around the ͑222͒
reflection, as shown in the inset of Fig. 3͑b͒, demonstrating
that the atomic planes are highly parallel and that the film
surface and interface are smooth. The presence of fringes
confirms a high crystalline quality and a lack of structural
defects. The film thickness was calculated from the period-
icity of the fringes and agrees well with the thickness deter-
mined by spectroscopic ellipsometry. The rocking curve of
the ␥-Al2O3 ͑222͒ reflection, as shown in Fig. 3͑c͒, has a
resolution-limited FWHM of 0.056°, indicating a lack of mo-
saic spread in the film. Figure 3͑d͒ shows the phi scans of the
␥-Al2O3 ͑111͒ films via rapid thermal annealing in N2. The
chemical stability of these films up to 1100 °C was estab-
lished by XPS. An abrupt interface with 4H-SiC was dem-
onstrated by HRTEM, and the epitaxial relationship and
quality of the crystalline films were confirmed by several
techniques.
The authors acknowledge the financial support of a
Young Investigator Award from the Office of Naval Research
and a UCLA Chancellor’s Faculty Career Development
Award. Portions of this research were carried out at the Stan-
ford Synchrotron Radiation Laboratory, a national user facil-
ity operated by Stanford University on behalf of the U.S.
Department of Energy, Office of Basic Energy Sciences.
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In summary, we have transformed amorphous Al2O3 thin
films on 4H-SiC ͑0001͒ into highly oriented epitaxial
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