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## Creator

[Takayuki Harada](https://orcid.org/0000-0002-8657-2258), [Takuro Nagai](https://orcid.org/0000-0001-5239-3334), [Kohei Sasaki](https://orcid.org/0000-0002-8923-7703)

## Rights

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Takayuki Harada, Takuro Nagai, Kohei Sasaki; High Schottky barrier formation in tilted-dipole PdCoO2/β-Ga2O3 (001) interfaces. Appl. Phys. Lett. 1 June 2026; 128 (22): 222101 and may be found at https://doi.org/10.1063/5.0332733.[In Copyright](http://rightsstatements.org/vocab/InC/1.0/)

## Other metadata

[High Schottky barrier formation in tilted-dipole PdCoO2/                                          <i>β</i>                                        -Ga2O3 (001) interfaces](https://mdr.nims.go.jp/datasets/247f9f76-5250-4fee-b7f6-22415721dc3e)

## Fulltext

High Schottky barrier formation in tilted-dipole PdCoO2/β-Ga2O3 (001) interfaces1   1  2  3 High Schottky barrier formation in tilted-dipole PdCoO2/β-Ga2O3 4 (001) interfaces 5  6 Takayuki Harada1,a), Takuro Nagai2, Kohei Sasaki3 7  8 1 International Center for Materials Nanoarchitectonics (MANA), National Institute for 9 Materials Science, Tsukuba, Ibaraki 305-0044, Japan. 10 2 Electron Microscopy Unit, National Institute for Materials Science; Tsukuba, Ibaraki 11 305-0044, Japan. 12 3 Novel Crystal Technology Inc., 2-3-1 Hirosedai, Sayama, Saitama 350-1328, Japan 13  14 aAuthor to whom correspondence should be addressed: HARADA.Takayuki@nims.go.jp 15   16 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327332  Abstract 17 We report the growth and Schottky junction characteristics of metallic delafossite PdCoO2/-18 Ga2O3 (001) heterostructures. The PdCoO2 thin films predominantly grow with the epitaxial 19 relationship PdCoO2 (006) // -Ga2O3 (202), forming a high-quality oxide–oxide interface. 20 Despite a 24° tilt between the PdCoO2 surface polarization axis and the -Ga2O3 (001) surface 21 normal, a large Schottky barrier height of bJV > 1.7 eV was achieved. This value is comparable 22 with that reported for PdCoO2/-Ga2O3 (2̅ 01) where the PdCoO2 surface polarization axis is 23 perpendicular to the interface. The PdCoO2/-Ga2O3 (001) Schottky junctions showed a large on-24 off ratio of ~ 108 at 573 K. These results demonstrate the feasibility of delafossite-type electrodes 25 for -Ga2O3 (001) heterostructures with high-quality homoepitaxial -Ga2O3 layers.  26 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327333  -Ga2O3 is an ultra-wide-bandgap (UWBG) semiconductor with an energy gap of 27 approximately 4.6–5.0 eV, belonging to the monoclinic system (C2/m).1 It is estimated to have a 28 high critical electric field (~8 MV·cm-1), high chemical stability, and melt-growth capability that 29 enables large-diameter single-crystal wafers in a potentially low-cost scheme.1 These properties 30 make -Ga2O3 a promising candidate for next-generation power devices such as Schottky barrier 31 diodes (SBDs) and field-effect transistors (FETs). 32 High-temperature electronics is one of the promising applications of -Ga2O3 where the large 33 band gap of -Ga2O3 may help suppress thermal excitation of carriers.2 In high-temperature 34 operation of -Ga2O3 devices, large Schottky barrier height (SBH) has merit to suppress leakage 35 and better reverse blocking properties. To this end, oxide electrodes with intrinsically high work 36 functions and/or strong interfacial dipoles have been explored.3,4 Among them, delafossite-type 37 metallic oxides ABO2 such as PdCoO2, PdCrO2, PdRhO2, and PtCoO2 exhibit metallic 38 conductivity and layered crystal structures shown in Fig. 1(a), consisting of alternating A+ and 39 [BO2]− layers stacked along the c-axis.5,6,7 These materials enable surface dipole-driven 40 modulation of electronic states.8 At ABO2–semiconductor interfaces, the surface dipole 41 significantly influences the band alignment between the ABO2 and the semiconductor.4 Previous 42 studies have demonstrated that PdCoO2, PdCrO2, and PdRhO2 electrodes on -Ga2O3 (2̅01) have 43 dipole-driven enhancement of SBH.4,9-11 In particular, large SBH of ~1.8 eV has been obtained in 44 PdCoO2/-Ga2O3 (2̅01) interface.4 The interface dipole arises from the alternating stacking of 45 ionic Pd+ and [CoO2]− layers.7,12 Consistent with the interface dipole picture, scanning tunneling 46 microscopy on bulk crystals has revealed that the work function of PdCoO2 is strongly dependent 47 on the local termination layer of the cleaved surface, indicating the values for Pd-terminated 48 surface mPd = 4.7 eV and for CoO2-terminated surfaces mCoO2 = 7.8 eV.13 The high mCoO2 could 49 be consistent with the experimentally observed high SBH in PdCoO2/-Ga2O3 (2̅01) interface 50 where PdCoO2 is dominantly terminated with CoO2 layer at the interface.4,9,10,14 51 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327334  PdCoO2 thin films have been grown by pulsed laser deposition,15,16 molecular beam epitaxy,17 52 solution-based process,18 and reactive sputtering19 mostly on c-plane Al2O3 substrates. So far, 53 growth of PdCoO2 on -Ga2O3 has been limited to (2̅01) orientations.4,9,10 The (2̅01) orientation 54 of -Ga2O3, however, tends to introduce twins in epitaxial layers, which can degrade device 55 breakdown performance.20 The -Ga2O3 (001), on the other hand, allows high-quality epitaxial 56 growth of homoepitaxial -Ga2O3 and is more suitable for high-power application.20 In this study, 57 we investigate the growth of PdCoO2 on -Ga2O3 (001) and evaluate its interface structure and 58 Schottky properties. 59 On n-type -Ga2O3 (001) substrates doped with Sn (ND ~ 1×1018 cm−3), an 11-m thick Cl-60 doped -Ga2O3 epitaxial layer (ND ~ 1×1016 cm−3) was grown by halide vapor phase epitaxy 61 (HVPE).21 PdCoO2 thin films (~30 nm) were subsequently deposited by pulsed laser deposition 62 (PLD)15 at a substrate temperature of 660 °C under an oxygen pressure of 150 mTorr. The fourth 63 harmonic of an Nd:YAG laser was used to ablate a spark plasma–sintered PdCoO2 target with the 64 repetition rate of 5 Hz. The thickness of the PdCoO2 films was controlled by the number of 65 Nd:YAG laser pulses, based on the deposition rate determined on c-plane sapphire substrates. 66 The crystal structure of the PdCoO2 thin films was characterized by x-ray diffraction (XRD). 67 For selected samples, Au/Ni layers were deposited by electron-beam evaporation on the 68 PdCoO2/-Ga2O3 heterostructures to enhance lateral current spreading in the Schottky contact. 69 Circular mesa structures with diameters of D = 100–300 m were defined by photolithography 70 and reactive ion etching (RIE) using a mixture of Ar and BCl3 gases. After RIE, the samples were 71 immersed in 35 wt% HCl for 5 min to remove possible damaged layers on the sidewalls of the 72 mesas,22 followed by rinsing in deionized water and isopropanol. 73 Ohmic contacts were formed by depositing Au (200 nm)/Ti (50 nm) on the backside of the 74 Sn-doped -Ga2O3 substrates, resulting in a vertical Schottky diode configuration. Current 75 density–voltage (J–V) characteristics were measured using an Agilent 4155B semiconductor 76 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327335  parameter analyzer with tungsten probes connected via triaxial cables. Capacitance–voltage (C–77 V) measurements were carried out using an Agilent E4980A LCR meter by applying a 1 kHz AC 78 signal superimposed on a DC voltage. For breakdown measurements, ohmic contacts to the -79 Ga2O3 top surface were formed by aluminum wire bonding with indium contacts.9 80 Figures 1(b) and 1(c) show x-ray diffraction (XRD) data for a PdCoO2/-Ga2O3 (001) 81 heterostructure. XRD φ–χ scans of the PdCoO2 (006) reflection, shown in Fig. 1(b), reveal that 82 the PdCoO2 (003L) planes are tilted with respect to the -Ga2O3 (001) planes. Here, φ = χ = 0° 83 corresponds to the alignment where the a-axis of the -Ga2O3 (001) substrate is parallel to the 84 projection of the x-ray detector arm on the substrate surface. The PdCoO2 (006) reflection is 85 observed at φ ≈ 90° and χ ≈ 24°, indicating a 24° tilt of the PdCoO2 (006) plane from the -Ga2O3 86 (001) plane. These angles correspond to the dominant macroscopic epitaxial relationship of 87 PdCoO2 (006) // -Ga2O3 (202). Consistently, the 2θ–ω scan along the PdCoO2 (006) reflection 88 in Fig. 1(c) shows both the PdCoO2 (003L) and -Ga2O3 (202) reflections. The tilted c-axis of the 89 PdCoO2 thin film results in a faceted surface morphology composed of PdCoO2 c-plane facets, as 90 confirmed by atomic force microscope (AFM) (Fig. S5). This epitaxial relationship can be 91 understood based on the similar triangular arrangement of oxygen atoms on the PdCoO2 (006) 92 and -Ga2O3 (202) surfaces (Fig. S3), which provides a structural basis for the observed alignment. 93 To examine the microscopic interfacial structure, high-angle annular dark field scanning 94 transmission electron microscope (HAADF-STEM) image was captured around the PdCoO2/-95 Ga2O3 interface, which is shown in Fig. 2. Due to Z-contrast, atomic columns with higher atomic 96 numbers appear brighter in Fig. 2. Overall, a highly oriented crystalline interface was observed, 97 showing chemical affinity between PdCoO2 and -Ga2O3. Alternating bright Pd and dark Co 98 layers were clearly resolved, consistent with the crystal structure of PdCoO2 shown in Fig. 99 1(a).This layered structure extended continuously near to the interface with -Ga2O3 and also near 100 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327336  to the surface (Fig. S6). Some regions near the interface showed weak contrast, likely due to local 101 lattice tilting, interface roughness, and/or structural reconstruction. The presence of disorder near 102 the interface may locally alter the interfacial structure from the macroscopic epitaxial relationship 103 between PdCoO2 (006) and -Ga2O3 (202), which could lead to spatial variation in the Schottky 104 barrier height. 105 Closer inspection revealed stacking faults in the PdCoO2 lattice, where twin boundaries (TBs) 106 are shown by green lines in Fig. 2. PdCoO2 belongs to the rhombohedral R3̅m structure (3R 107 polytype), but local 2H polytype regions were occasionally observed at stacking faults,7 resulting 108 in 180° rotated domains around the c-axis of PdCoO2. Although the impact of these stacking faults 109 on Schottky performance remains unclear, the Pd⁺/[CoO2]⁻ stacking order was preserved, 110 suggesting minimal influence on the interface dipole. 111 The current density–voltage (J–V) characteristics of a circular PdCoO2/-Ga2O3 junction (D 112 = 100 μm) measured at varied temperature between T = 298 K and 573 K are shown in Fig. 3(a). 113 The PdCoO2/-Ga2O3 junction showed rectifying J-V characteristics with reverse current density 114 as low as the noise level even at T = 573 K. The large on-off ratio ~108 at T = 573 K demonstrates 115 suitability of the PdCoO2/-Ga2O3 (001) junction for diodes for high-temperature operation. The 116 forward J-V characteristics are shown in Fig. 3(b). We compare the forward J–V characteristics 117 with the thermionic emission model: 118 𝐽 = 𝐴∗∗𝑇2𝑒−(𝜙𝑏𝐽𝑉/𝑘𝑇)[𝑒(𝑞𝑉/𝑛𝑘𝑇) − 1]       (eq. 1) 119 where A** is the effective Richardson constant, T is the temperature, q is the elementary charge, 120 and k is the Boltzmann constant. Here, A** ≃ 41.1 Acm−2K−2 for the theoretical effective mass m*/ 121 m0 = 0.342 of -Ga2O3.23 Linear fitting of the forward J–V slope in the low J region (black lines 122 in Fig. 3(b)) and comparison with the eq. 1 yielded bJV = 1.74 eV and an ideality factor n = 1.06 123 at T = 298 K, indicating that the electrical transport is well described by thermionic emission. 124 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327337  Similar behaviors are consistently observed in other PdCoO2/-Ga2O3 (001) junctions fabricated 125 on the same chip (Fig. S4). 126 Reported bJV values for Schottky junctions on -Ga2O3 (001) are 1.09–1.46 eV for Pt 127 contacts,24,25 0.99–1.22 eV for Ni contacts,26 and 1.85 eV for PtOx contacts.27 The present PdCoO2 128 electrode thus exhibits one of the highest SBHs among reported -Ga2O3 (001) Schottky junctions, 129 likely consistent with the large work function on CoO2-terminated surface.13 The breakdown 130 voltage reached approximately Vbr = −530 V as shown in Fig. S1 without edge termination or 131 trench isolation, showing the potential of PdCoO2/-Ga2O3 (001) interface for high-voltage 132 application. Further device engineering to relieve the electric field crowding may further improve 133 the breakdown voltages.25,28 The Schottky barrier height measured by C-V characteristics is bCV 134 = 2.10 eV at T = 298 K (Fig. S2). As typically observed, bCV > bJV, reflecting that C–V 135 measurements probe the spatial average of the barrier height, while J–V characteristics are 136 dominated by the lowest-barrier regions at the interface.29 Consistently, the increase of bJV and 137 the decrease of n at higher temperature indicate the inhomogeneity of the SBH at the interface. 138 Assuming a simple Gaussian model and following the discussion by Werner and Güttler,30 the 139 standard deviation s of the Schottky barrier height can be estimated as, 140 s ≈ {2kBT(bCV − bJV)}1/2. Using this formula, s = 143 meV for the present PdCoO2/-Ga2O3 141 (001), which is comparable to the typical Schottky junctions on -Ga2O3.31 It should be noted that 142 the structural complexity of the interface, as seen in the HAADF-STEM image, limits the 143 discussion based on the simple Gaussian distribution. 144    In the J–V characteristics in Fig. 3(b), the current density saturates at higher forward biases, 145 resulting in a large on-state resistance (Ron). This behavior is likely caused by limited current 146 spreading in the PdCoO2 electrode. PdCoO2 is known to exhibit strong anisotropy in resistivity: 147 c = 1.07 mcm and ab = 2.6 cm for the c-axis direction and the ab-plane direction at T = 148 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327338  300 K.6 This gives the resistivity ratio as high as c/ab ~ 400. We patterned the PdCoO2 thin film 149 on -Ga2O3 (001) into Hall-bar geometry and measured the resistivity along different directions. 150 As shown in Fig. S7, the resistivity along the direction intersecting the PdCoO2 (006) facets is 151 consistently higher than that along the facet surfaces. Thus, the high resistivity along the c-axis 152 can hinder efficient lateral current spreading across the contact area. To examine this effect, we 153 deposited an Au (150 nm)/Ni (30 nm) current-spreading electrode on PdCoO2 and measured the 154 J-V characteristics as shown in Fig. 4(a). Without the Au/Ni layer, Ron increases with increasing 155 diode diameter D, reflecting the longer spreading distance from the needle probe contact point 156 (gray plot in Fig. 4(b)). In contrast, the Au/Ni current-spreading electrode significantly reduces 157 Ron (green plot in Fig. 4(b)), although Au/Ni thermal stability is insufficient for high-temperature 158 operation. Developing thermally stable current-spreading electrodes will therefore be important 159 to achieve both high-temperature operation and low Ron. Figure 4(c) summarizes the device-to-160 device distribution of the Schottky barrier height bJV extracted from J–V characteristics and the 161 ideality factor n, demonstrating good reproducibility of bJV > 1.7 eV and n close to unity. The 162 observed spread in bJV and n could reflect local variations in the interface structure, defect density, 163 and quality of PdCoO2. 164 As a possible mechanism of high Schottky barrier formation, interface dipole effects proposed 165 in PdCoO2/-Ga2O3 (2̅ 01) may also be relevant.4 For -Ga2O3 (2̅ 01), the interface dipole of 166 PdCoO2 was oriented normal to the interface,4 whereas in the present (001) system, the dipole is 167 tilted by  = 24°. The perpendicular component (cos = 0.91) is nearly equivalent to the vertical 168 case, suggesting that the dipole effect remains non-negligible. However, if both Pd and CoO2 169 layers extend to the interface, periodic stripe-like potential modulation may arise. Although the 170 temperature dependence of bJV and n indicate barrier inhomogeneity, such behavior is commonly 171 observed in various Schottky junctions and cannot be the direct evidence for the periodic potential 172 modulation. In fact, the periodicity of this stripe is estimated to be c/3×1/cos = 0.65 nm—much 173 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.03327339  smaller than the depletion width W ≃ 520 nm corresponding to bCV = 2.10 eV and ND = 7.8×1015 174 cm−3. The large depletion width W suppresses the short-range modulation of the band bending in 175 -Ga2O3. Thus, the interface likely behaves effectively as a spatially averaged Schottky barrier 176 rather than a collection of local high barriers and leaky low barriers even if stripe-like potential 177 modulation existed. Such averaging effect of nanoscale potential distribution has been discussed 178 in SrRuO3/Nb:SrTiO3 Schottky junctions with a fractionally inserted (AlO2)−/(LaO)+ dipole 179 layer.32 The observed bCV = 2.10 eV exceeds the value predicted from the experimentally 180 observed work function dependence of b and the average work function maverage = (mPd + 181 mCoO2)/2 = 6.25 eV, expected from the work functions for Pd- (mPd = 4.7 eV)4,13 and CoO2-182 terminated surfaces (mCoO2 = 7.8 eV).13 The observed bCV and qVbi rather closely match that of 183 CoO2 termination on (2̅01) (qVbi = 2.0 eV).4 This suggests that the simple averaging model is 184 insufficient to explain the observed barrier height, and the microscopic interfacial structure likely 185 plays a crucial role. The HAADF-STEM image shows reduced Pd-layer contrast near the interface, 186 implying a locally modified layered structure distinct from bulk PdCoO2. As reported for PtOx/-187 Ga2O3 interfaces, oxygen-mediated bonding may be important at the interface.27  188 In addition, the electronic states of PdCoO2 are strongly layer-dependent. Previous studies 189 have shown that the Pd-derived states near the Fermi level are primarily associated with in-plane 190 Pd–Pd metallic bonding.33 The CoO2 layers, on the other hand, are associated with more correlated 191 electronic states involving Co 3d–O 2p hybridized states.33 Such differences could result in 192 distinct contributions of Pd- and CoO2-derived states to electronic transport across PdCoO2/-193 Ga2O3 interface. In particular, at an oxide interface where bonding is mediated by oxygen, CoO2-194 like configurations could couple more effectively to -Ga2O3 through oxygen-mediated 195 interactions. This may result in an effective barrier height that deviates from a simple spatial 196 averaging of Pd- and CoO2-terminated regions. Further theoretical and experimental studies are 197 needed to elucidate the atomic-scale interface and the potential distribution when a layered oxide 198 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.033273310  with surface dipoles contacts an oxide semiconductor at an oblique angle. 199 In summary, we fabricated heterostructures of conductive layered oxide PdCoO2 and -Ga2O3 200 (001). The PdCoO2 thin film was found to grow dominantly with the epitaxial relationship 201 PdCoO2 (006) // -Ga2O3 (202), with the PdCoO2 surface polarization direction tilted by ~24° 202 from the -Ga2O3 (001) surface normal. Nevertheless, a large Schottky barrier height exceeding 203 1.7 eV was obtained at the PdCoO2/-Ga2O3 (001) interface, enabling current rectification at 204 elevated temperatures. This demonstrates that the advantage of PdCoO2 electrodes—such as its 205 strong dipole-induced high work function and high-quality oxide–oxide interface—can be usable 206 on -Ga2O3 (001)-based heterostructures. Further optimization of device design, such as edge 207 termination25 and/or trench structures28 is needed to enable high-voltage and high-temperature 208 operation utilizing PdCoO2/-Ga2O3 crystalline heterostructures. 209  210 SUPPLEMENTARY MATERIAL 211 Figures S1-S7 are available in the supplementary material. 212  213 ACKNOWLEDGEMENTS: A part of this work was supported by ARIM of MEXT 214 (JPMXP1223NM5155), MEXT Leading Initiative for Excellent Young Researchers 215 (JPMXS0320200047), JST PRESTO (JPMJPR20AD), and Grant-in-Aid for Scientific Research 216 (B) from JSPS (24K01353). 217  218 REFERENCE 219 1 M. Higashiwaki, K. Sasaki, H. Murakami, Y. Kumagai, A. Koukitu, A. Kuramata, T. 220 Masui, and S. Yamakoshi,  Semicond. Sci. Technol. 31, 034001 (2016);  S. 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Phys. 69, 1522 (1991). 294 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.033273313  31 G. Jian, Q. He, W. Mu, B. Fu, H. Dong, Y. Qin, Y. Zhang, H. Xue, S. Long, Z. Jia, H. Lv, 295 Q. Liu, X. Tao, and M. Liu,  AIP Advances 8, 015316 (2018). 296 32 T. Yajima, M. Minohara, C. Bell, H. Y. Hwang, and Y. Hikita,  Appl. Phys. Lett. 113, 297 221603 (2018). 298 33 Q. Lu, H. Martins, J. M. Kahk, G. Rimal, S. Oh, I. Vishik, M. Brahlek, W. C. Chueh, J. 299 Lischner, and S. Nemsak,  Communications Physics 4 (1), 143 (2021). 300   301 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.033273314   302  303  304 FIG. 1. (a) Crystal structure of PdCoO2 highlighting the alternating ionic layers of Pd+ and 305 [CoO2]−. (b) XRD - mapping of PdCoO2/-Ga2O3 (001). (c) XRD 2- scan of PdCoO2/-306 Ga2O3 (001) along the reciprocal vector G003n of PdCoO2, measured in asymmetric geometry. 307 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.033273315   308  309  310 FIG. 2. HAADF-STEM image of a PdCoO2/-Ga2O3 (001) interface. The crystal model of 311 PdCoO2 is overlapped in the HAADF-STEM image. Twin boundaries (TBs) in the stacking faults 312 are indicated by green lines.   313 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.033273316   314  315  316 FIG. 3. (a) Temperature-dependent current density-voltage characteristics of a PdCoO2/-Ga2O3 317 (001) heterostructure. The noted numbers are the stage temperature (T) during the J-V 318 measurement. Inset: Schottky barrier height determined by the J-V characteristics (bJV, red 319 circles) and the ideality factor (n, blue squares) at varied temperature. (b) Forward J-V 320 characteristics of a PdCoO2/-Ga2O3 (001) heterostructure with the fitting lines at the linear region 321 (black lines). The bJV and n are determined from the fitting lines by thermionic emission model 322 (eq. 1).  323 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.033273317   324  325  326  327  328  329  330  331  332  333  334  335  336  337 FIG. 4. (a) J-V characteristics of Au/Ni/PdCoO2/-Ga2O3 (001) Schottky junction at T = 298 K. 338 The inset is the schematics of the device structure. (b) Dependence of on-state resistance (Ron) on 339 diode diameter (D) for PdCoO2-only and Au/Ni/PdCoO2 contacts. Each data point represents the 340 average of multiple devices (N ≥ 5 for PdCoO2-only and N ≥ 10 for Au/Ni/PdCoO2 contacts), and 341 the error bars indicate the standard deviation. (c) The bJV versus n plot for Au/Ni/PdCoO2/-342 Ga2O3 (001) Schottky junctions with D = 100 m at T = 298 K for the number of the devices N = 343 25, fabricated on the same wafer in a single process flow. The blue line is the linear fitting to the 344 bJV versus n plot, which intersects the n = 1.0 line at bHOM ≃ 1.85 eV. 345 This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset.PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0332733