Materials Physics
Heterostructures of Oxides and Semiconductors
- Growth and Structural Studies
Beamline
17B1 W20 X-ray Scattering beamline
Part I: Epitaxial single crystal growth of γ-Al2O3 and Sc2O3 on Si (111) and
structural studies using high-resolution X-ray diffraction
Part II: Thermodynamic stability of Ga2O3 (Gd2O3)/GaAs interface
Authors
M. Hong and J. R. Kwo
National Tsing Hua University,
Hsinchu, Taiwan
1. Epitaxial single crystal growth of γ-Al2O3 and Sc2O3 on Si (111)
and their structural properties
(a) γ-Al2O3
H. Y. Lee and C. H. Hsu
National Synchrotron Radiation
Research Center, Hsinchu, Taiwan
Hetero-epitaxial growth between insulators and semiconductors is
always fascinating in science and important in technology. For example,
growth of single crystal GaN on sapphire has been essential for producing blue lasers and LED’s. Epitaxial growth of insulators on Si is
another example, on which a subsequent single-crystalline growth of
other semiconductors such as GaN5 may integrate high-power microwave
devices or lasers with the most advanced Si-based electronic devices.
In this work, we report the attainment of very high-quality cubic γAl2O3 single crystal films with thickness as thin as 3.8 nm. The oxide films
have (111) as the
_ normal in parallel with (111) of the
_ Si substrate and the
films have [4 4 0] in-plane axis in parallel with [2 20] of the Si substrate.
The rocking scans at γ-Al2O3 (222) position of films 3.8 and 11 nm shows
a low FWHM of 0.6 and 0.3 degree, respectively. Atomic force microscopy
(AFM) and x-ray reflectivity (XRR) all show a very smooth surface about
0.1~0.2 nm. The oxide/Si interface is also atomically smooth of 0.1~0.2
nm as studied using XRR and cross-sectional TEM.
In contrast to the previous efforts using MBE with precursors or
gases, a high-purity sapphire was employed in this work. E-beam evaporation was used due to the high melting point of sapphire, and the
deposited species (incoming flux) consist of only Al2O3 molecules or
clusters.
Si (111) wafers were put into a multi-chamber MBE/UHV system,
after being cleaned with an RCA method and an HF dip. The experimental procedure was described
oxide RHEED patterns along
_ earlier. Streaky
_
the in-plane axes of [1 1 0] and [11 2 ] (shown in Fig. 1(b)) of Si were
observed after growth of oxide 1nm thick, indicating that a smooth single crystal γ-Al2O3 film formed on the Si (111) and with in-plane alignment between the oxide film and Si substrate.
Single crystal X-ray measurements were carried out on a four-circle
triple-axes diffractometer, using a 12 kW rotating anode Cu K-alpha
source. A pair of graphite crystals is used to monochromatize and analyze the X-ray beam with a resolution of 0.01 Å-1 along the longitudinal
and 0.005 Å-1 along the transverse directions respectively.
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Materials Physics
The interface was found to be atomically sharp
and smooth, as shown in Fig. 3(a), whose micrograph
was taken with the electron beam directing along
[112]Si. In the inset of Fig. 3 (a), the orientation relation in cross-sectional direction _(Si_ [112]) was found
__
to be Si (112) // Al2O3 (224), Si [ 1 11] // Al2O3 [ 2 22].
Electron diffraction patterns from the plan-view of
the hetero-structure (Fig. 3 (b)) show Si (111) // Al2O3
(222), Si [220] // Al2O3 [440].
The cubic γ-Al2O3 and Si have significantly different atomic structures and lattice constants. The
lattice constant of γ-Al2O3 is 7.91Å and that of Si is
5.42 Å. Matching the two lattices over a unit cell
dimension will result in a >30% lattice mismatch. It is
intriguing that a highly ordered epitaxial growth was
obtained in an unusually large mismatch for a hetero-epitaxial system.
Fig. 1: 1 In-situ RHEED patterns of Si (111) substrate (a) and_of γ-Al2O3 (111)
film 1.0 nm thick
_
(b) along [1 10] and [11 2] axes. Note that the
surface normal (111) γ-Al2O3 film
_ is parallel with
(111) Si and the in-plane
axe
[4
4
0] of γ-Al2O3 is
_
in parallel with [2 20] of Si substrate.
A single crystal X-ray theta two-theta scan along
substrate surface of Si (111) on the oxide film 3.8 nm
thick is shown in Fig. 2, in which a broad peak near
40 degree coincides with the (222) reflection of the
cubic gamma phase of Al2O3. The broadness of the
peak is caused by the thin thickness of the oxide
films. The relatively strong oscillation at small angle
reflectivity on all of our oxide films grown on Si (111)
indicates that the film thickness is highly uniform
and smooth.
(a)
The single crystalline nature of the γ-Al2O3 film
is further studied by scans along the major zone axes
in reciprocal lattice. We find that γ-Al2O3 (004) peak
lies on the same zone axis with the Si (004) reflection,
proving that the film is single crystalline and is
aligned with the Si (111).
(b)
Fig. 3: Cross-sectional TEM image and electron
diffraction pattern of a 3.8 nm γ-Al 2O 3 film,
showing a sharp interface and smooth surface
(a). The electron diffraction pattern indicates
that the film is well aligned with Si substrate. The
In-plane electron diffraction pattern of the 3.8
nm γ-Al2O3 film is shown in (b).
Fig. 2: X-ray theta two-theta scan along Si (111)
surface of (111)γ-Al2O3 film 3.8 nm thick and the
mosaic scan of the γ-Al2O3 (222) peak.
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Materials Physics
Fig. 4: X-ray theta two-theta scan using synchrotron radiation on the same film shown in Fig. 1
Fig. 6: A cross-sectional HRTEM image
_ of the
Sc 2O 3/Si(111) heterostructure in [ 112 ] Si projection, showing the Sc2O3 film thickness of ~4 nm.
The edge-type misfit dislocation is indicated by “T”.
The arrow exhibits the inclined defects in Sc2O3.
Note that our recent study using synchrotron
radiation source at NSRRC has given much more
detailed information on the crystallographic structures of both the thin film and the interface. This
then will allow us to study γ-Al2O3 films with thickness in the range of 2 nm. In fact, an X-ray diffraction
normal scan with synchrotron radiation on the same
sample of Fig. 1 is shown in Fig. 4. In Fig. 1, the (222)
peak of γ-Al2O3 is indicated with a small broad peak. In
comparison, all the peaks of γ-Al2O3 are well resolved
with sharp peaks. The Pendellösung fringe oscillations
surrounding the (222) are clearly exhibited, indicating a
highly uniform film with an atomically smooth surface
and sharp oxide/Si interface.
electron beam evaporated from a Sc2O3 target. Structural and morphological studies were carried out by
X-ray diffraction and reflectivity (Fig. 5, along with
very small rocking FWHM of 0.033 at Sc2O3 (222)),
atomic force microscopy (AFM), high-resolution
transmission electron microscopy (HRTEM) (Fig. 6),
and medium energy ion scattering (MEIS) (Fig. 7),
with the initial epitaxial growth monitored by in-situ
reflection high energy electron diffraction (RHEED).
The synchrotron X-ray experiments were performed
at wiggler beamline BL17B1 at the National Synchrotron Radiation Research Center (NSRRC), Hsinchu,
Taiwan. The films have the cubic bixbyite phase with
a remarkably uniform thickness and high structural
perfection. The film surfaces are very smooth and the
oxide/Si interfaces are atomically sharp with a low
average roughness of 0.06 nm. The films are well
aligned with the Si substrate with an orientation relationship of Si (111)
// Sc2O3 (111),
and an in-plane
_
_
expitaxy of Si [ 1 1 0 ] // Sc2O3 [ 1 0 1 ].
(b) Sc2O3
High-quality single-crystal Sc 2 O 3 films a few
nanometer thick have been grown epitaxially on Si
(111) despite a huge lattice mismatch. The films were
The density of the inclined defects in our sample
was estimated to be ~1014 m-2. This value is considerably low compared to that of the line defect, misfit
dislocations (>1016 m-2), frequently observed in epitaxial heterostructures. Since neither considerable
inclined defects nor extensive dislocation networks
were observed in the current system, we would
expect that the corresponding rocking scans are little
affected by such crystallographic defects.
Fig. 5: Single crystal scans along the surface
normal in reciprocal space finds an epitaxial
growth of the Sc2O3 bixbyite phase.
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Materials Physics
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D9.5 (2005).
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(2005).
Fig. 7: Backscattering energy spectrum (130 keV
H+) of a Sc2O3 film on Si(111) in a normal incidence and substrate blocking geometry. A simulation (solid line) shows a fit for a 3.9 nm thick
crystalline film with no detectable interfacial
layer formation, to be compared to a simulation
for an amorphous film (dashed line). The inset
shows angular distributions for the Si and Sc
signals.
2. Thermodynamic stability of Ga2O3 (Gd2O3)/GaAs
interface
GaAs MOSFET offers potential advantages over
Si-based MOSFET because of the high electron
mobility, high breakdown field, and semi-insulating
substrate of GaAs. Searching and identifying electrically and thermodynamically stable insulators on
GaAs with a low interfacial density of states (Dit) has
been one of the key challenges in the compound
semiconductor devices over the past four decades.
In-situ deposition of Ga2O3(Gd2O3) dielectric film on
GaAs surfaces produced MOS diode structures with a
low D it . Subsequent employment of the Ga 2 O 3
(Gd2O3) as a gate dielectric along with an ion implantation process led to the demonstration of the first
enhancement mode GaAs MOSFETs with inversion
on semi-insulating GaAs substrates in both n- and pchannel configurations.
Figure 7 shows a MEIS backscattering energy
spectrum for an uncapped Sc2O3 film exposed to air
prior to analysis. Observation of distinct blocking
minima in Sc signal, shown in the inset in Fig. 7, and
comparison of the Sc signal just below the surface Sc
peak to a calculated yield for the same thickness
amorphous film (the yield, shown as a dashed line in
Fig. 7) indicate very good film crystalline quality.
Remarkably no interfacial Sc and O peaks are observed,
indicating a highly ordered Sc2O3 lattice structure at
the interface with Si.
Sc2O3 and Si have significantly different crystal
structures, different bonding, and lattice constants.
The bulk lattice constants of Si (5.43 Å) and Sc2O3
(9.86 Å) are mismatched by 9.2% (relative to the
doubled Si unit cell dimension). It is intriguing that a
highly ordered epitaxial growth was obtained with
this unusually large mismatch.
Previously, the oxide-GaAs interface was found
to be roughened in a high temperature (> 750˚C)
annealing for fabricating the inversion-channel GaAs
MOSFETs, in which the annealing was inevitably
needed to activate the ion implantation for ohmic
contacts at source and drain regions. For the device
processing, Ga2O3(Gd2O3) should be thermodynamically stable with GaAs at temperatures of ~ 750˚C or
above. The interfacial roughness has to be controlled
and minimized to a few Å, as was witnessed in the
case of the perfected SiO2-Si interface. However, it
was found out later that when the samples are exposed
to air, they absorb water and form hydro-oxides.
References
．S. Nakamura, et al., Appl. Phys. Lett. 64,
1687 (1994).
．R. A. Mckee, et al. and M. F. Chrisholm, Phys.
Rev. Lett. 81, 3014 (1998).
．J. Kwo, et al., Appl. Phys. Lett. 77, 130 (2000).
30
Materials Physics
for activating implanted dopant, is a must to ensure
the low Dit and to maintain a high carrier mobility in
the channel of the MOSFET.
During the annealing process, the hydro-oxides or
other contaminations in the oxides, not the pure Ga2O3
(Gd2O3), react with GaAs, resulting in rough interfaces.
Table I lists the fitting results for the samples
with the two different thermal processes. Dwelling at
300˚C for 30 min in UHV allowed the hydro-oxides to
be removed out of the air-exposed sample B, as evidenced from a rapid increase of the amount of H2O
containing in the chamber detected using RGA
(residual gas analyzer). The consequent annealing to
high temperatures let GaAs be adjacent to hydrooxide free Ga2O3(Gd2O3).
In this work, Ga 2O 3(Gd 2O 3)/GaAs heterostructures have been annealed up to ~780˚C. Studies
using X-ray reflectivity (Fig. 1) and high resolution
transmission electron microscopy (Fig. 2) have shown
that the samples annealed under UHV have maintained smooth and abrupt interfaces with the interfacial roughness being less than 0.2 nm. The oxide
remains as amorphous, an important parameter for
device consideration. Current-voltage and capacitance-voltage measurements have shown low leakage currents (10-8 to 10-9 A/cm2), a high dielectric
constant of 15, and a low interfacial density of states
(Dit) between gate dielectrics and GaAs. The attainment of a smooth interface between the gate dielectric and GaAs, even after high temperature annealing
Figure 3(a) of the J-E curves for the two samples
shows a leakage current density of Ga2O3(Gd2O3) on
GaAs of about 10-8 to 10-9 A/cm2 at low gate voltages. CV curves were obtained with frequencies varying from 1KHz to 1MHz and the dispersion in the CV
curves of different frequencies was due to the equivalent circuit of complex impedance. The dielectric
constant of Ga2O3(Gd2O3) is calculated to be about
15.11. The Dit was estimated to be less than ~1012
cm-2eV-1 using the Terman method.
Fig. 1: 1 Low angle X-ray reflectivity of Ga2O3
(Gd2O3) on GaAs annealing in UHV, with experimental data (dots), and a theoretical fit (line)
Fig. 2: High-resolution cross sectional TEM picture of Ga2O3(Gd2O3) on GaAs after air exposure
and annealing in UHV (Sample B).
Table 1: X-ray reflectivity studies on Ga2O3(Gd2O3)/GaAs
sample No.
oxide thickness
(nm)
thermal process
A
26.1
annealed to 780 C in UHV directly
0.648
0.17
B
25
Exposed to air, then put back to UHV
system, dwelled at 300 C for 30min
before annealing to 780 C
0.466
0.14
31
air-oxide surface
roughness (nm)
interfacial
roughness (nm)
Materials Physics
Experimental Station
X-ray scattering end station
References
．M. Hong, C. T. Liu, H. Reese, and J. Kwo,
"Encyclopedia of Electrical and Electronics
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573, 219 (1999).
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(2000).
．Y. L. Huang, et al., Appl. Phys. Lett. 86,
191905 (2005).
Fig. 3: (a) Leakage current density J(A/cm2) vs E
(MV/cm) for Ga 2 O 3 (Gd 2 O 3 )/GaAs samples in
different thermal processes and (b) C-V curves of
a MOS diode made of Au/Ga 2 O 3 (Gd 2 O 3 )
(25nm)/GaAs after two-frequency corrections
(Sample B)
Contact E-mail
[email protected]
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