Electric dipole effect in PdCoO 2 /β-Ga 2 O 3 Schottky diodes for high-temperature operation

Abstract

High-temperature operation of semiconductor devices is widely demanded for switching/sensing purposes in automobiles, plants, and aerospace applications. As alternatives to conventional Si-based Schottky diodes usable only at 200°C or less, Schottky interfaces based on wide-bandgap semiconductors have been extensively studied to realize a large Schottky barrier height that makes high-temperature operation possible. Here, we report a unique crystalline Schottky interface composed of a wide-gap semiconductor β-Ga 2 O 3 and a layered metal PdCoO 2 . At the thermally stable all-oxide interface, the polar layered structure of PdCoO 2 generates electric dipoles, realizing a large Schottky barrier height of ~1.8 eV, well beyond the 0.7 eV expected from the basal Schottky-Mott relation. Because of the naturally formed homogeneous electric dipoles, this junction achieved current rectification with a large on/off ratio approaching 10 ⁸ even at a high temperature of 350°C. The exceptional performance of the PdCoO 2 /β-Ga 2 O 3 Schottky diodes makes power/sensing devices possible for extreme environments.

Full text

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License 4.0 (CC BY-NC).
Electric dipole effect in PdCoO
2
/b-Ga
2
O
3
Schottky
diodes for high-temperature operation
T. Harada
1
*, S. Ito
1
, A. Tsukazaki
1,2
High-temperature operation of semiconductor devices is widely demanded for switching/sensing purposes in
automobiles, plants, and aerospace applications. As alternatives to conventional Si-based Schottky diodes
usable only at 200°C or less, Schottky interfaces based on wide-bandgap semiconductors have been extensively
studied to realize a large Schottky barrier height that makes high-temperature operation possible. Here, we
report a unique crystalline Schottky interface composed of a wide-gap semiconductor b-Ga
2
O
3
and a layered
metal PdCoO
2
. At the thermally stable all-oxide interface, the polar layered structure of PdCoO
2
generates
electric dipoles, realizing a large Schottky barrier height of ~1.8 eV, well beyond the 0.7 eV expected from
the basal Schottky-Mott relation. Because of the naturally formed homogeneous electric dipoles, this junction
achieved current rectification with a large on/off ratio approaching 10
8
even at a high temperature of 350°C.
The exceptional performance of the PdCoO
2
/b-Ga
2
O
3
Schottky diodes makes power/sensing devices possible
for extreme environments.
INTRODUCTION
Recent requirements for Schottky junctions are demanding, particularly
for switching/sensing device applications under harsh operating
conditions, including high current densities, frequencies, and tempera-
tures (1,2). However, conventional Si-based Schottky diodes suffer
from serious current leakage at more than 200°C because of the small
Schottky barrier height (~0.9 eV) limited by the narrow bandgap of Si
(~1.1 eV) (1). To realize a higher-temperature operation, alternative
Schottky interfaces with a large Schottky barrier height should be devel-
oped on the basis of wide-bandgap semiconductors. The energy barrier
height f
b
is shown in an ideal band diagram for a metal-semiconductor
Schottky interface (Fig. 1A) according to the Schottky-Mott relation:
f
b
=f
m
−c
s
,wheref
m
is the work function of a metal and c
s
is
the electron affinity of the semiconductor (3). The typical rectifying
current-voltage characteristics of a Schottky junction, schematically
shown in Fig. 1B, can be formulated by a simple thermionic emission
model, as in Eq. 1.
J¼A**T2eðfb=kBTÞeðqV =nkBTÞ1
hi
ð1Þ
where Jis the current density, A
**
is the effective Richardson constant,
Tis the absolute temperature, qis the elementary charge, k
B
is the
Boltzmann constant, nis the empirical ideality factor, and Vis the ap-
plied bias voltage. The ideal reverse current density J
s
(Jis for V<0)is
defined as the saturation value J
s
=A
**
T
2
e
−(f
b
/k
B
T), as highlighted in
Fig. 1B, providing the fundamental lower bound of the reverse current
density achievable at a given temperature (1). The benchmark for
high-temperature operation of Schottky diodes is exemplified by the
temperature dependence of J
s
at f
b
= 1.0, 1.4, and 1.8 eV (Fig. 1C). For
high-temperature operation, the thermally excited J
s
must be sup-
pressed by a large barrier height to achieve a large on/off ratio together
with high forward current. For example, a f
b
>1.4 eV is required to
maintain J
s
below 10
−6
A/cm
2
at 250°C, which is a typical temperature
in automobile engines.
Semiconductors that are capable of realizing a high f
b
, beyond
1.4 eV, include wide-bandgap materials, such as SiC, GaN, and Ga
2
O
3
(4). In particular, Ga
2
O
3
devices have recently attracted considerable
attention owing to their wide bandgap of approximately 5 eV, the
high-temperature stability of the material, and commercial availability
of large single crystals (5). Diode operation has been demonstrated
in Schottky junctions, composed of polycrystalline Pt (6,7) and Ni
(6,8–10) and oxidized metals (11–13) (see table S1 for polycrystalline
metals and table S2 for oxidized metals). The finite range of available
work functions in elemental metal electrodes, 4.0 to 5.6 eV (14), limits
the achievable Schottky barrier height in the Schottky-Mott framework:
f
b
=f
m
−c
s
. For stable operation, choosing a thermally stable Schottky
contact is crucial to avoid interfacial reactions and maintain the f
b
value at high temperature (15).
Rather than using the polycrystalline electrodes, we chose a crys-
talline interface, where a metal is rigidly integrated with a semi-
conductor. To achieve a large Schottky barrier height beyond the
limitation of the Schottky-Mott rule, we used a polar interface of oxide
heterostructures, combining Ga
2
O
3
with a polar layered metal PdCoO
2
(Fig. 1D). The layered delafossite structure of PdCoO
2
with alternat-
ing Pd
+
and [CoO
2
]
−
sublattices has two remarkable features (16,17):
an out-of-plane polarity and high in-plane conductivity (18–20). The
out-of-plane polarity, given by the alternating charged Pd
+
and
[CoO
2
]
−
sublattices of PdCoO
2
, has recently been revealed to mod-
ulate the surface physical properties of PdCoO
2
bulk single crystals
(21,22). This polar ionic stacking can effectively induce an electric di-
pole layer that enhances the Schottky barrier height at the interface.
The high in-plane conductivity of PdCoO
2
, comparable to that of Au,
supports its applicability as a Schottky contact metal. The hexagonal
lattice oxygen atoms onthe surface of PdCoO
2
(0001) match those of
b-Ga
2
O
3
(−201), which makes it possible to form a functional
interface that leverages the polar nature of PdCoO
2
.
RESULTS
We fabricated heterostructures of 20-nm-thick PdCoO
2
/b-Ga
2
O
3
(a commercial n-type substrate with a nominal donor density of 7.8 ×
10
17
cm
−3
) by pulsed-laser deposition. The caxis–oriented growth
was observed in typical x-ray diffraction patterns (fig. S1A). The
1
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan.
2
Center for Spintronics Research Network (CSRN), Tohoku University, Sendai
980-8577, Japan.
*Corresponding author. Email: t.harada@imr.tohoku.ac.jp
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lattice arrangement at the interface was imaged with a high-angle an-
nular dark-field scanning transmission electron microscope (HAADF-
STEM), as shown in Fig. 1 (E and F). The layered crystal structure of
PdCoO
2
was seen, and no threading dislocations were apparent (Fig.
1E), indicating an epitaxial relationship of PdCoO
2
[0001]//b-Ga
2
O
3
[−201], although the in-plane lattice mismatch was large at approxi-
mately 3%, as estimated from the O-O distances of 2.83 Å for PdCoO
2
and 2.93 Å for b-Ga
2
O
3
(fig. S2). An enlarged HAADF-STEM image
(Fig. 1F) shows that the [CoO
2
]
−
layer corresponds to the initial layer
of PdCoO
2
on the b-Ga
2
O
3
. The wave function of the Pd
+
conducting
layer is depicted in the inset of Fig. 1F, producing a polar interface
between PdCoO
2
/b-Ga
2
O
3
and the highly anisotropic in-plane con-
ductivity in PdCoO
2
. The in-plane conductivity of the PdCoO
2
/
b-Ga
2
O
3
is approximately 6.3 × 10
4
S/cm at room temperature (fig.
S1B), the value of which is high enough to achieve sufficient current
spreading inthe contact pad. Here,only the polarity originating from
the PdCoO
2
layer is considered, because the b-Ga
2
O
3
(−201) surface is
likely to be reconstructed to a stable nonpolar structure before the
PdCoO
2
deposition. The abrupt interface of PdCoO
2
/b-Ga
2
O
3
pro-
vides a suitable platform for exploiting interfacial dipole effects with
minimized extrinsic contributions, i.e., defects.
To investigate the Schottky characteristics of the PdCoO
2
/b-Ga
2
O
3
junctions, we patterned the PdCoO
2
thin films into circle-shaped de-
vices using a water-soluble templating process (23). Typical J-Vchar-
acteristics (Fig. 2A) showed clear rectification with a resistance ratio
>10
9
and a reverse current density as low as the measurement limit of
10
−8
A/cm
2
at 220°C (blue). Applying Eq. 1 to the forward-bias region
made it possible to evaluate the Schottky barrier height f
b
(the symbol
C
Temperature (°C)
5004003002001000
10−10
10−8
10−6
10−4
10−2
1
10−1
10−3
10−5
10−7
10−9
−50
Room
temp
= 1.0 eV
b
= 1.8 eV
b
= 1.4 eV
b
Saturation current density Js (A/cm2)
On
Off
J||
0Forward biasReverse bias
V
Js
BA
Evac
Metal
Semi-
conductor
EF
EC
b
Block
m
s
e-
F
d-s
1 nm
-
-
+
+Pd+
Pd+
Pd+
Pd+
-Ga2O3
D
E
20 nm
V
A
c
PdCoO2
Pd
O
Co
Ga
c
a
b
-Ga2O3
[CoO2]−
[CoO2]−
[CoO2]−
[CoO2]−
[CoO2]−
Fig. 1. Schottky junctions based on a layered PdCoO
2
and b-Ga
2
O
3
for high-temperature operation. (A) Energy band diagram o f a metal-semiconductor interface
with a Schottky barrier of f
b
.f
m
, work function of metal; c
s
, electron affinity of sem iconductor; E
vac
, vacuum level; E
F
, Fermi energy; E
C
, conduction band minimu m; V, bias
voltage. The electron is depicted as a red circle. (B) Typical current-voltage characteristic of Schottky junction showing the current rectification behavior under forward
(on-state) or reverse (off-state) bias voltage. The saturation current density J
s
is noted in red. (C) Temperature dependence of the saturation current density J
s
with
Schottky barrier heights of 1.0 eV (blue), 1.4 eV (green), and 1.8 eV (red) calculated by the thermionic emission model using the Richardson constant of b-Ga
2
O
3
,
A** = 41.1 A/cm
2
(24). (D) Schematic image for th e characterization of the PdCoO
2
/−b-Ga
2
O
3
Schottky junction. (E) HAA DF-STEM image of the PdCoO
2
/b-Ga
2
O
3
interface.
The crystal orientatio n is represented by arrows. (F) Enlarg ed image of the HAADF-STEM image of the Pd CoO
2
/b-Ga
2
O
3
interface. The anisot ropic Y
d-s
orbital proposed for
the conduction band of PdC oO
2
is schematically shown (20). Ri ght: Corresponding crystal model. The alte rnating Pd
+
and [CoO
2
]
−
charged layers are shown based on the
nominal ionic charges in the bulk PdCoO
2
. The actual charge state at the interface can be modified by electronic reconstruction with screening charges.
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f
bJV
is used to specify the measurement technique: J-Vmeasurement)
and the ideality factor n, which were approximately 1.85 eV and 1.04,
respectively, at 220°C, using the A** =41.1A/cm
2
of b-Ga
2
O
3
(24).
These values are close to the corresponding values of 1.78 eV and
1.06 characterized at room temperature. Such rectifying properties
were maintained at 350°C (Fig. 2A, red) together with a large on/off
ratio of approximately 10
8
and a reverse current density of 10
−6
A/cm
2
.
Moreover, the weak temperature dependences of f
bJV
(inset of Fig. 2A)
and the ideality factor (fig. S3A) indicate the homogeneous Schottky
barrier height at the interface (see Materials and Methods for a detailed
discussion). The value of f
bJV
is mainly dominated by the activation
process across the lowest-energy barrier region. The reverse current
density (V=−20 V) at high temperature (Fig. 2B) is compared with
the ideal J
s
lines (Fig. 1C) and data from previous studies (25–30).
Owing to the large f
bJV
,thevalueof|J
−20V
| at the PdCoO
2
/b-Ga
2
O
3
junction (Fig. 2B, red squares) is suppressed at the measured high tem-
peratures. We compare our f
bJV
values with metal/b-Ga
2
O
3
junctions
of previous studies (table S1), where f
bJV
is plotted as a function of the
metal work function f
m
(Fig.2C).Thelargedeviationofthereported
f
bJV
values for the specific metal/b-Ga
2
O
3
junctions probably arises be-
cause of the differences in interface quality, e.g., a partial Fermi-level
pinning at the surface that can occur (31), even when the same metal
contacts are used. As measured by ultraviolet photoelectron spectrosco-
py(fig.S4),thePdCoO
2
film is plotted at a work function of 4.7 eV in
Fig. 2C. The f
bJV
value of 1.8 eV for PdCoO
2
/b-Ga
2
O
3
is located above
the empirical trend of the reported values in the elemental metal/
b-Ga
2
O
3
junctions (gray dotted line), whose barrier height lies in the
range of 1.0 to 1.5 eV (table S1). Although partial oxidation of metals
is known to increase f
bJV
(11–13), the work function and crystal
structures/orientations are unknown for the oxidized states of the poly-
crystalline film. The plot shown in Fig. 2C, which is based on the exper-
imentally determined values of f
bJV
and f
m
, indicates that f
bJV
of
PdCoO
2
/b-Ga
2
O
3
is strongly influenced by the interfacial effects
existing at the abrupt interface with well-defined crystal orientation,
asshowninFig.1(EandF).
In addition to the J-Vcharacteristics, we performed capacitance
(C) measurements 1/C
2
−V(Fig. 3A) to determine the Schottky bar-
rier height at the interface. The gradients of linear fits to the data in
Fig. 3A for two device sizes (diameter D= 100 and 200 mm) indicate
that the built-in potential at the Ga
2
O
3
qV
bi
and the donor density N
D
are 2.0 eV and 3 × 10
17
cm
−3
, respectively. This N
D
value is comparable
to the nominal donor concentration of a commercial substrate. The
junction capacitance measured with V= 0 V is independent of the
AC frequency (f) over a broad range from 10
2
to 10
6
Hz, with a neg-
ligible deep trap-state capacitance C
trap
(f) (inset of Fig. 3B), which
suggests the potential application of this junction in high-frequency
switching elements.
The band diagram for the PdCoO
2
/b-Ga
2
O
3
interface is depicted
in Fig. 3C based on the experimental characteristics discussed above.
First, the Schottky-Mott relation, f
b
=f
m
−c
s
, was adopted to esti-
mate f
b
to be approximately 0.7 eV, based on the work function f
m
for PdCoO
2
(4.7 eV; fig. S4) and the reported electron affinity c
s
of
b-Ga
2
O
3
(4.0 eV) (32). Although minor effects, such as energy lower-
ing by an image force at the interface and the Fermi energy in Ga
2
O
3
,
might make an additional contribution to the estimated value of f
b
(see Materials and Methods), it is difficult to explain the large mis-
match between 0.7 eV and the experimentally evaluated result of
1.8eV.AsshowninFig.3C,thevacuumlevelshiftedbyD≈1.1 eV,
which contributed to the large f
bJV
. This shift is attributed to the polar
nature of PdCoO
2
, which is composed of Pd
+
and [CoO
2
]
−
.Theforma-
tion of a polar interface [CoO
2
]
−
/Ga
2
O
3
(STEM image in Fig. 1F and
bottom of Fig. 3C) caused f
b
to increase to 1.8 eV through electric di-
pole effects. The interface dipole caused by the PdCoO
2
polar layered
structure agrees well with the calculated surface potential at O- and
–20–18–16–14–12–10 –8 –6 –4 –2 0 2 4
Voltage (V)
|J | (A/cm2)
102
10–10
10–8
10–6
10–4
10–2
1
10
10–1
10–3
10–5
10–7
10–9
A
700600500400300200
Temperature (K)
2.0
1.8
1.6
1.4
1.2
(eV)
b
JV
2.2
D = 200 m
T = 227 °C
T = 356 °C
In air
108
B
3002001000–100
10–1
10–10
10–8
10–6
10–4
10–2
|J-20V| (A/cm2)
10–3
10–5
10–7
10–9
Temperature (°C)
400
1
Ni [28]
Pt [30]
TiN [26]
Cu [27]
= 1.0 eV
b
= 1.8 eV
b
= 1.4 eV
b
PdCoO
2
W [25]
Ni [25]
6.05.55.04.5
Metal work function (eV)
m
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
Schottky barrier height (eV)
b
JV
Pd/ -Ga2O3
(010)
Pt/ -Ga2O3(–201)
(100)
(010)
(001)
Ni/ -Ga2O3
(–201)
(100)
(010)
(001)
Ir/ -Ga2O3
(–201)
(010)
(–201)
(100)
Au/ -Ga2O3
Cu/ -Ga2O3
(–201)
(–201)
W/ -Ga2O3
PdCoO2/ -Ga2O3 (–201)
Room temp.
C
(010)
Pt [29]
Fig. 2. High-temperature operation of the PdCoO
2
/b-Ga
2
O
3
Schottky junctions with a large barrier height. (A) Current-voltage characteristics of the PdCoO
2
/
b-Ga
2
O
3
Schottky junction with a diameter of 200 mm at 227°C (blue) and 356°C (red). The gray lines are the linear fitting for the forward-bias region. The temperature
dependence of the Schottky barrier height is plotted in the inset. (B) Temperature-dependent reverse current density under the bias voltage of −20 V |J
−20V
| plotted
together with the reported values (25–30). The ideal J
s
in Fig. 1C is also shown for f
b
= 1.0 eV (blue), 1.4 eV (green), and 1.8 eV (red). (C) Comparison of the Schottky
barrier height with the reported values for elemental metal Schottky junctions (see the Supplementary Materials for the data and references used). Perpendicularly
spread line data correspond to the range of the reported Schottky barrier height. The different colors correspond to the different surface orientations of the b-Ga
2
O
3
layers. The linear trend from the reported values is shown as a broken line. The large red square corresponds to the data obtained for PdCoO
2
/b-Ga
2
O
3
.
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Pd-terminated PdO (111), which predicts an energy shift of 1.2 eV (33).
The contributions of this interfacial dipole model to f
bJV
are analogous
to the barrier height control achieved in SrRuO
3
/Nb:SrTiO
3
Schottky
junctions by the insertion of [AlO
2
]
−
or [LaO]
+
, which increase and
decrease, respectively, the Schottky barrier height from its original
level at ~1.3 eV to 1.8 and 0.7 eV (34). In the PdCoO
2
/b-Ga
2
O
3
in-
terfaces, the interface dipole is naturally activated owing to the unique
polar layered structure and the [CoO
2
]
−
initial layer favored by the
crystal growth of PdCoO
2
electrodes (Fig. 1F).
We examined the uniformity and reproducibility of the junc-
tion properties by measuring arrays of PdCoO
2
circular devices on
b-Ga
2
O
3
with various junction areas (from 100 to 1000 mm), as shown
in the sample picture (Fig. 4A). A large f
bJV
of approximately 1.8 eV
was obtained, irrespective of the diameterof the devices (Fig. 4B). This
result contrasts with the expected inhomogeneous f
bJV
in typical large
Schottky junctions owing to the high probability of pinhole-generating
regions. Moreover, 27 devices were characterized with D= 100 mmto
confirm the uniformity of operation. The J-Vdata were consistent, as
showninFig.4C.Ahistogramoff
bJV
indicated a reproducible value of
f
bJV
= 1.76 eV with a narrow distribution of approximately 0.045 eV.
Unlike the broad distribution of f
bJV
values for the polycrystalline
metal/b-Ga
2
O
3
junction, as summarized in Fig. 2C, the highly reproduc-
ible f
bJV
could result from the layered structural features and the all-
oxide high-quality interface with the homogeneous [CoO
2
]
−
/Ga
2
O
3
polar stacks energetically favored during the thin-film growth (Fig. 1,
E and F). Hexagonal interfaces of layered PdCoO
2
on other semicon-
ductors, such as SiC and GaN, could also benefit from this interfacial
electric dipole effect.
A
1/C2 (1022 F–2)
D = 200 m
D =100 m
qVbi = 2.0 eV
ND = 3 ×1017 cm–3
f = 1 kHz
Voltage (V)
–6 –4 –2 0 2 4
2.0
1.5
1.0
0.5
0
Ctrap(f)
Cp
Rp
0.15
0.10
0.05
0
102
Frequency f (Hz)
103104105106
C/S ( Fcm
–2)
B
C
EF
(4.7 eV)
m
(1.1 eV)
Evac
(1.8 eV)
b
JV
(0.07 eV)
(4.0 eV)
s
EC
PdCoO2
++
-
-
e–
-Ga2O3
= 0 V
Fig. 3. Energy band diagram of the PdCoO
2
/b-Ga
2
O
3
interface. (A)1/C
2
-Vplots for the devices with D= 100 and 200 mm measured with an AC frequency of 1 kHz at
room temperature. (B) Frequency dependence of the capacitance measured with 0.1-V excitation. (C) Top: Band diagram for the PdCoO
2
/b-Ga
2
O
3
Schottky junction based on
experimentally observed values. The energy difference of the conduction band bottom and the Fermi energy in b-Ga
2
O
3
(x) is calculated using x=k
B
Tln(N
C
/N
D
)and
N
C
=2(2pm
*
k
B
T/h
2
)
3/2
,wherehis the Planck constant and m
*
=0.342m
0
is the effective mass of the electron in b-Ga
2
O
3
(24). Bottom: Schematics of the PdCoO
2
/b-Ga
2
O
3
interface. A conductio n electron in the Pd
+
layer is shownin red. The bulk-likecharged layers of PdCoO
2
are schematically depicted, neglectingpossible charge reconstructions.
1 mm
AC
1
3.02.01.00
Voltage (V)
104
10–10
10–8
10–6
10–4
10–2
|J | (A/cm2)
102
4.0
N = 27
D = 100 m
T = 25 °C
B3.0
2.5
2.0
1.5
1.0
0.5
0
10–9 10–7 10–6
10–8
b
JV (eV)
D ( m)
200100 300
400 500 1000
Junction area (m2)
12
4
0
8
2.01.81.61.4
16
Count
D
= 45 meV2
= 1.76 eV
b
N = 27
D = 100 m
T = 25 °C
b
JV (eV)
Fig. 4. Uniformity and reproducibility of the large barrier height in the PdCoO
2
/
b-Ga
2
O
3
Schottky junctions. (A) Optical microscopy image of the PdCoO
2
/b-Ga
2
O
3
Schottky junction arrays. (B) Schottky barrier height obtained for the devices with
different junction sizes. (C) Current-voltage characteristics of the 27 different devices.
(D) Histogram of the Schottky barrier height for D= 100-mmdevices.TheGaussian
fitting (red curve) gives the central value f
bm
= 1.76 eV and the width of distribution
2s=45meV,wheresis the SD.
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DISCUSSION
Superior Schottky junction properties were demonstrated with large
on/off ratios, high-temperature operation at 350°C, no dependence of
C-fcharacteristics, and considerable uniformity and reproducibility.
This performance is attributed to the large f
bJV
induced by the natu-
rally formed electric dipoles at the well-regulated polar oxide interface
of PdCoO
2
and b-Ga
2
O
3
. For applications under harsh conditions,
PdCoO
2
electrodes have considerable advantages owing to their excep-
tional stability to heat (~800°C), chemicals (acids/bases, pH 0 to 14),
and mechanical stress (fig. S5), in addition to high optical transparency
(35). The abrupt interface of the layered oxides PdCoO
2
and b-Ga
2
O
3
can extend applications of semiconductor devices to hot operating en-
vironments, such as those in automobile and aerospace applications.
MATERIALS AND METHODS
Substrate preparation
For the devices with acid-cleaned b-Ga
2
O
3
, commercially available
unintentionally doped b-Ga
2
O
3
(−201) substrates with the nominal
N
D
=7.8×10
17
cm
−3
(Novel Crystal Technology Inc.) were immersed
in an acidic solution (water: 30 to 35.5%; H
2
O
2
:95%;H
2
SO
4
= 1:1:4)
for 5 min, followed by rinsing in water for 15 min.
PdCoO
2
/b-Ga
2
O
3
device fabrication
To pattern the PdCoO
2
layer by soft lithography, we used the LaAlO
3
/
BaO
x
template as a water-soluble sacrificial layer (23). First, an organic
photoresist was patterned on the b-Ga
2
O
3
substrates using a standard
photolithography process. The LaAlO
3
(~40 nm)/BaO
x
(~100 nm)
templates were then deposited by pulsed-laser deposition at room
temperature under the base pressure of ~10
−7
torr. Removing organic
photoresist by hot acetone gave the patterned LaAlO
3
/BaO
x
template
on the b-Ga
2
O
3
substrates. Just before the deposition of PdCoO
2
thin
films, the LaAlO
3
/BaO
x
/b-Ga
2
O
3
samples were put in O
2
plasma for
50 s to remove the residual photoresist. The PdCoO
2
thin films were
grown by pulsed-laser deposition (35) at a growth temperature of
700°C and an oxygen partial pressure of 150 mtorr. A KrF excimer
laser was used to alternately ablate the PdCoO
2
stoichiometric tar-
get and Pd-PdO mixed phase target. After the thin-film growth, the
LaAlO
3
/BaO
x
templates were removed by sonication in water to-
gether with the unnecessary parts ofthe PdCoO
2
to obtain the circular
PdCoO
2
electrodes with a diameter of 100 to 1000 mm.
Electrical transport measurement
Current-voltage characteristics of the Schottky junctions were measured
via two-wire configuration using a Keithley 2450 source meter. Al wires
were bonded to the top surface of the b-Ga
2
O
3
substrates to form ohmic
contacts to the b-Ga
2
O
3
substrates. A needle prober was used to contact
the PdCoO
2
and Pt top electrodes for the room temperature measure-
ment. The capacitance measurements were carried out using an Agilent
E4980A precision LCR meter with an AC modulation voltage of 0.1 V.
The resistivity of the PdCoO
2
thin films was measured using a four-
probe configuration.
Effect of image force lowering
For the interface of a metal and an n-type semiconductor with the
relative permittivity of e
r
, the image force lowering Df under zero bias
can be formulated as
Df ¼qfq=4pere0½2qVbiND=ere01=2g1=2
Here, e
0
is the vacuum permittivity. Using the experimentally
determined qV
bi
and N
D
for the PdCoO
2
/b-Ga
2
O
3
,qV
bi
=2.0eV
and N
D
=3×10
17
cm
−3
, and the reported e
r
=10(36), Df was esti-
mated to be 0.08 eV.
Estimation of the spatial homogeneity of the Schottky
barrier height
We analyzed the temperature-dependent Schottky barrier height and
the ideality factor using the potential fluctuation model to estimate the
homogeneity of the barrier height (37). We considered the spatial
distribution of the Schottky barrier height (f
b
)andthebuilt-in
potential (V
bi
) by introducing the Gaussian distribution P(qV
bi
)and
P(f
b
), with an SD s
s
around the mean values 
Vbi and 
fb.
PðfbÞ¼ 1
ssffiffiffiffiffi
2p
peð
fbfbÞ2=ð2ss2Þð2Þ
PðqVbiÞ¼ 1
ssffiffiffiffiffi
2p
peðq

VbiqV biÞ2=ð2ss2Þð3Þ
As discussed by Werner and Güttler (37), the Schottky barrier
height determined by the current-voltage characteristic, f
bJV
, relates
to the 
fband s
s
as
fJV
b¼
fbs2
s
2kBTð4Þ
Capacitance-voltage (C-V) measurement probes the spatial aver-
age of the built-in potential, V
biCV
=
Vbi, which relates to the Schottky
barrier height f
bCV
to be
fCV
b¼q

Vbi þkBTlnðNC=NDÞ¼
fbð5Þ
where N
C
and N
D
denote the effective density of states in the conduction
band and the doping concentration, respectively. Here, we neglected a
possible contribution from the image force in the calculation of f
bCV
,
which did not change the estimation of s
s
.
Comparing Eqs. 4 and 5, we found
fCV
bfJV
b¼s2
s
2kBTð6Þ
We fitted the experimental data by Eq. 6, as shown in fig. S3B, to
obtain the SD s
s
= 54 meV, which is less than half of the reported value
for Pt/b-Ga
2
O
3
(s
s
= 130 meV) (38). The ideality factor n(V,T) reflects
the voltage-dependent mean barrier 
fbðVÞand the SD s2
sðVÞas
n1ðV;TÞ1¼

fbðVÞ
fbð0Þ
qV þss2ðVÞssð0Þ2
2kBTqV
Assuming that 
fbðVÞand s
s2
(V) vary linearly with the bias voltage V,
we can parameterize the voltage deformation of the barrier distribution
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using the coefficients r
2
and r
3
.

fbðVÞ
fbð0Þ¼r2qV
ss2ðVÞssð0Þ2¼r3qV
n1ðTÞ1¼r1ðTÞ¼r2þr3
2kBTð7Þ
The experimental data for the PdCoO
2
/b-Ga
2
O
3
Schottky junction
are fitted by Eq. 7, as shown in fig. S3C, to obtain the temperature-
independent coefficients r
2
=−0.073 and r
3
=−1.93 meV.
SUPPLEMENTARY MATERIALS
Supplementary material for this article is available at http://advances.sciencemag.org/cgi/
content/full/5/10/eaax5733/DC1
Fig. S1. Basic characterization of PdCoO
2
thin films grown on b-Ga
2
O
3
(−201).
Fig. S2. Relationship between work function and lattice constant for typical metal electrodes
with a (pseudo-)hexagonal lattice constant close to the representative hexagonal wide-gap
semiconductors.
Fig. S3. Temperature dependence of the Schottky barrier height and the ideality factor.
Fig. S4. Determination of the work function for PdCoO
2
using the ultraviolet photoelectron
spectroscopy.
Fig. S5. Stability of PdCoO
2
thin films in a harsh environment.
Table S1. Summary of the metal/b-Ga
2
O
3
junctions reported in literature.
Table S2. Summary of the partially oxidized metal/b-Ga
2
O
3
junctions reported in literature.
REFERENCES AND NOTES
1. B. J. Baliga, Fundamentals of Power Semiconductor Devices (Springer, 2008).
2. P. G. Neudeck, R. S. Okojie, L.-Y. Chen, High-temperature electronics—A role for wide
bandgap semiconductors? Proc. IEEE 90, 1065–1076 (2002).
3. S. M. Sze, K. K. Ng, Physics of Semiconductor Devices (Wiley, 2006).
4. J. Y. Tsao, S. Chowdhury, M. A. Hollis, D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,
S. Rajan, C. G. van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper,
K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam,
P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich,
R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback,
J. A. Simmons, Ultrawide-bandgap semiconductors: Research opportunities and
challenges. Adv. Electron. Mater. 4, 1600501 (2018).
5. M. Higashiwaki, K. Sasaki, H. Murakami, Y. Kumagai, A. Koukitu, A. Kuramata, T. Masui,
S. Yamakoshi, Recent progress in Ga
2
O
3
power devices. Semicond. Sci. Technol. 31,
034001 (2016).
6. E. Farzana, Z. Zhang, P. K. Paul, A. R. Arehart, S. A. Ringel, Influence of metal choice on
(010) b-Ga
2
O
3
Schottky diode properties. Appl. Phys. Lett. 110, 202102 (2017).
7. K. Sasaki, M. Higashiwaki, A. Kuramata, T. Masui, S. Yamakoshi, Ga
2
O
3
Schottky barrier
diodes fabricated by using single-crystal b-Ga
2
O
3
(010) substrates. IEEE Electr. Device L. 34,
493–495 (2013).
8. T. Oishi, Y. Koga, K. Harada, M. Kasu, High-mobility b-Ga
2
O
3
(−201) single crystals grown
by edge-defined film-fed growth method and their Schottky barrier diodes with
Ni contact. Appl. Phys. Express 8, 031101 (2015).
9. K. Irmscher, Z. Galazka, M. Pietsch, R. Uecker, R. Fornari, Electrical properties of b-Ga
2
O
3
single crystals grown by the Czochralski method. J. Appl. Phys. 110, 063720 (2011).
10. J. Yang, S. Ahn, F. Ren, S. J. Pearton, S. Jang, J. Kim, A. Kuramata, High reverse breakdown
voltage Schottky rectifiers without edge termination on Ga
2
O
3
.Appl. Phys. Lett. 110,
192101 (2017).
11. S. Müller, H. von Wenckstern, F. Schmidt, D. Splith, F. L. Schein, H. Frenzel, M. Grundmann,
Comparison of Schottky contacts on b-gallium oxide thin films and bulk crystals.
Appl. Phys. Express 8, 121102 (2015).
12. S. Müller, H. von Wenckstern, F. Schmidt, D. Splith, H. Frenzel, M. Grundmann, Method of
choice for the fabrication of high-quality b-gallium oxide-based Schottky diodes.
Semicond. Sci. Technol. 32, 065013 (2017).
13. C. Hou, R. M. Gazoni, R. J. Reeves, M. W. Allen, Direct comparison of plain and oxidized
metal Schottky contacts on b-Ga
2
O
3
.Appl. Phys. Lett. 114, 033502 (2019).
14. W. M. Haynes, D. R. Lide, T. J. Bruno, CRC Handbook of Chemistry and Physics 97th Edition
(CRC Press, 2017).
15. S. J. Pearton, F. Ren, M. Tadjer, J. Kim, Perspective: Ga
2
O
3
for ultra-high power rectifiers
and MOSFETS. J. Appl. Phys. 124, 220901 (2018).
16. A. P. Mackenzie, The properties of ultrapure delafossite metals. Rep. Prog. Phys. 80,
032501 (2017).
17. R. Daou, R. Frésard, V. Eyert, S. Hébert, A. Maignan, Unconventional aspects of electronic
transport in delafossite oxides. Sci. Technol. Adv. Mater. 18, 919–938 (2017).
18. H. Takatsu, S. Yonezawa, S. Mouri, S. Nakatsuji, K. Tanaka, Y. Maeno, Roles of
high-frequency optical phonons in the physical properties of the conductive delafossite
PdCoO
2
.J. Phys. Soc. Japan 76, 104701 (2007).
19. P. J. W. Moll, P. Kushwaha, N. Nandi, B. Schmidt, A. P. Mackenzie, Evidence for
hydrodynamic electron flow in PdCoO
2
.Science 351, 1061–1064 (2016).
20. M. Tanaka, M. Hasegawa, T. Higuchi, T. Tsukamoto, Y. Tezuka, S. Shin, H. Takei, Origin of the
metallic conductivity in PdCoO
2
with delafossite structure. Ph ysica B 245,157–163 (1998).
21. V.Sunko,H.Rosner,P.Kushwaha,S.Khim,F.Mazzola,L.Bawden,O.J.Clark,J.M.Riley,
D. Kasinathan, M. W. Haverkort, T. K. Kim, M. Hoesch, J. Fujii, I. Vobornik, A. P. Mackenzie,
P. D. C. King, Maximal Rashba-like spin splitting via kinetic-energy-coupled
inversion-symmetry breaking. Nature 549, 492–496 (2017).
22. F. Mazzola, V. Sunko, S. Khim, H. Rosner, P. Kushwaha, O. J. Clark, L. Bawden, I. Marković,
T. K. Kim, M. Hoesch, A. P. Mackenzie, P. D. C. King, Itinerant ferromagnetism of the
Pd-terminated polar surface of PdCoO
2
.Proc. Natl. Acad. Sci. U.S.A. 115, 12956–12960
(2018).
23. T. Harada, A. Tsukazaki, A versatile patterning process based on easily soluble sacrificial
bilayers. AIP Adv. 7, 085011 (2017).
24. H. He, R. Orlando, M. A. Blanco, R. Pandey, E. Amzallag, I. Baraille, M. Rérat, First-principles
study of the structural, electronic, and optical properties of Ga
2
O
3
in its monoclinic
and hexagonal phases. Phys. Rev. B 74, 195123 (2006).
25. C. Fares, F. Ren, S. J. Pearton, Temperature-dependent electrical characteristics of b-Ga
2
O
3
diodes with W Schottky contacts up to 500°C. ECS J. Solid State Sci. Technol. 8,
Q3007–Q3012 (2019).
26. M. J. Tadjer, V. D. Wheeler, D. I. Shahin, C. R. Eddy Jr., F. J. Kub, Thermionic emission
analysis of TiN and Pt Schottky contacts to b-Ga
2
O
3
.ECS J. Solid State Sci. Technol. 6,
165–168 (2017).
27. D. Splith, S. Müller, F. Schmidt, H. von Wenckstern, J. J. van Rensburg, W. E. Meyer,
M. Grundmann, Determination of the mean and the homogeneous barrier height of
Cu Schottky contacts on heteroepitaxial b-Ga
2
O
3
thin films grown by pulsed laser
deposition. Phys. Status Solidi 211,40–47 (2014).
28. J. Yang, F. Ren, S. J. Pearton, A. Kuramata, Vertical geometry, 2-A forward current
Ga
2
O
3
Schottky rectifiers on bulk Ga
2
O
3
substrates. IEEE Trans. Electron Devices 65,
2790–2796 (2018).
29. M. Higashiwaki, K. Konishi, K. Sasaki, K. Goto, K. Nomura, Q. T. Thieu, R. Togashi,
H. Murakami, Y. Kumagai, B. Monemar, A. Koukitu, A. Kuramata, S. Yamakoshi,
Temperature-dependent capacitance–voltage and current–voltage characteristics of
Pt/Ga
2
O
3
(001) Schottky barrier diodes fabricated on n
−
-Ga
2
O
3
drift layers grown
by halide vapor phase epitaxy. Appl. Phys. Lett. 108, 133503 (2016).
30. K. Konishi, K. Goto, H. Murakami, Y. Kumagai, A. Kuramata, S. Yamakoshi, M. Higashiwaki,
1-kV vertical Ga
2
O
3
field-plated Schottky barrier diodes. Appl. Phys. Lett. 110, 103506
(2017).
31. F. Ren, J. C. Yang, C. Fares, S. J. Pearton, Device processing and junction formation needs
for ultra-high power Ga
2
O
3
electronics. MRS Commun. 9,77–87 (2019).
32. M. Mohamed, K. Irmscher, C. Janowitz, Z. Galazka, R. Manzke, R. Fornari, Schottky barrier
height of Au on the transparent semiconducting oxide b-Ga
2
O
3
.Appl. Phys. Lett. 101,
132106 (2012).
33. J. Rogal, K. Reuter, M. Scheffler, Thermodynamic stability of PdO surfaces. Phys. Rev. B 69,
075421 (2004).
34. T. Yajima, Y. Hikita, M. Minohara, C. Bell, J. A. Mundy, L. F. Kourkoutis, D. A. Muller,
H. Kumigashira, M. Oshima, H. Y. Hwang, Controlling band alignments by artificial
interface dipoles at perovskite heterointerfaces. Nat. Commun. 6, 6759 (2015).
35. T. Harada, K. Fujiwara, A. Tsukazaki, Highly conductive PdCoO
2
ultrathin films for
transparent electrodes. APL Mater. 6, 046107 (2018).
36. H. Hoeneisen, C. A. Mead, M.-A. Nicolet, Permittivity of b-Ga
2
O
3
at low frequencies.
Solid-State Electron. 14, 1057–1059 (1971).
37. J. H. Werner, H. H. Güttler, Barrier inhomogeneities at Schottky contacts. J. Appl. Phys. 69,
1522–1533 (1991).
38. G. Jian, Q. He, W. Mu, B. Fu, H. Dong, Y. Qin, Y. Zhang, H. Xue, S. Long, Z. Jia, H. Lv, Q. Liu,
X. Tao, M. Liu, Characterization of the inhomogeneous barrier distribution in a Pt/(100)
b-Ga
2
O
3
Schottky diode via its temperature-dependent electrical properties. AIP Adv.
8, 015316 (2018).
Acknowledgments: This work is a cooperative program (proposal no. 16G0404) of the
CRDAM-IMR, Tohoku University. We thank the NEOARK Corporation for lending a mask-less
lithography system PALET. Funding: This work was supported, in part, by a Grant-in-Aid
for Specially Promoted Research (no. 25000003), a Grant-in-Aid for Scientific Research (A)
(no. 15H02022), and a Grant-in-Aid for Early-Career Scientists (no. 18K14121) from the Japan
Society for the Promotion of Science (JSPS), JST CREST (JPMJCR18T2), the Mayekawa Houonkai
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Foundation, and the Tanaka Foundation. Author contributions: T.H. and A.T. designed
the experiments. T.H. prepared the samples, performed transport measurements, and
analyzed the data. S.I. captured the HAADF-STEM image. T.H. and A.T. wrote the manuscript.
Competing interests: The authors declare that they have no competing interests. Data
and materials availability: All data needed to evaluate the conclusions in the paper are
present in the paper and/or the Supplementary Materials. Additional data related to this paper
may be requested from the authors.
Submitted 3 April 2019
Accepted 25 September 2019
Published 18 October 2019
10.1126/sciadv.aax5733
Citation: T. Harada, S. Ito, A. Tsukazaki, Electric dipole effect in PdCoO
2
/b-Ga
2
O
3
Schottky
diodes for high-temperature operation. Sci. Adv. 5, eaax5733 (2019).
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Schottky diodes for high-temperature operation
3
O
2
-Gaβ/
2
Electric dipole effect in PdCoO
T. Harada, S. Ito and A. Tsukazaki
DOI: 10.1126/sciadv.aax5733
(10), eaax5733.5Sci Adv
ARTICLE TOOLS http://advances.sciencemag.org/content/5/10/eaax5733
MATERIALS
SUPPLEMENTARY http://advances.sciencemag.org/content/suppl/2019/10/10/5.10.eaax5733.DC1
REFERENCES http://advances.sciencemag.org/content/5/10/eaax5733#BIBL
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