Control of Schottky barrier heights (SBHs) at metal/semiconductor interfaces is a critically important technique to design switching properties of semiconductor devices. In this study, we report the systematic variations of SBHs in metal/PdCoO2/β-Ga2O3 junctions with an increase in the thickness of the PdCoO2 insertion layer. The PdCoO2 insertion layer consists of ionic Pd+ and [CoO2] sublattices alternatingly stacked along the normal of the Schottky interface. This polar layered structure of PdCoO2 spontaneously induces interface dipoles that increase the SBH in β-Ga2O3 devices. We fabricated Schottky junctions composed of metal/PdCoO2/β-Ga2O3 (−201) with the PdCoO2 thickness of 0–20 nm. With an increase in the PdCoO2 thickness, we observed a systematic shift of current density–voltage (JV) characteristics to larger forward driving voltage. The shift of JV characteristics indicates the enhancement of SBH by insertion of the PdCoO2 layer, which was confirmed by the capacitance measurement as the consistent shift of the built-in potential. These results demonstrate a controllable SBH in a wide range of 0.7–1.9 eV driven by a decisive contribution of the interface dipole effect. The Schottky junctions based on β-Ga2O3 with variable barrier heights could fit a wide range of applications, with the significant merits of optimizable switching properties.

Schottky junctions composed of metals and wide-bandgap semiconductors are essential elements for power electronics.1 Among the various wide-bandgap semiconductors,2 such as GaN, SiC, diamond, and Ga2O3, an oxide semiconductor Ga2O3 has significant advantages including a wide bandgap close to 5 eV, ability to fabricate high-quality single crystals by a mass-production-compatible melt-growth technique, and controllable conductivity.3 Schottky junctions on Ga2O3 substrates have been investigated using various contact electrodes such as elemental metals,4–10 partially oxidized metals,11–13 and crystalline oxide metals,14 some of which are promising structures with sufficient rectification properties for switching device applications. The current density–voltage (JV) property of a Schottky junction is characterized by a forward threshold voltage VF and reverse leakage current density JR, which are governed by a SBH.1 A smaller VF and lower JR are preferable for the minimization of the power dissipation by Joule heating during repeated device operation. However, VF and JR normally exhibit a trade-off relationship: a smaller VF with higher JR or a larger VF with lower JR. To meet the requirements of operation conditions such as operation temperature, voltage, frequency, and duty cycle, it is necessary to optimize the SBH for minimization of the total power dissipation in on-state PLon=δSJFonVF, off-state PL(off) = (1 − δ)SJRVR, turn-off transient PLturn-off=0.5τoffSJFonVRf, and turn-on transient PLturn-on=0.5τonSJFonVRf.1 Here, S is the junction area, JFon is the on-state forward current density, VR is the reverse bias voltage, δ is the duty cycle, τon(off) is the turn-on (off) transient duration, and f is the operation frequency. As predicted by the Schottky–Mott relation for an ideal Schottky junction, an n-type SBH is determined by the difference between the work function ϕm of the contact metal and the electron affinity χs of the semiconductor.15 However, it is well known that the SBH in a realistic device often does not follow the Schottky–Mott relation due to the extrinsic origin of Fermi-level pinning, even when the contact metal is properly selected to control the SBH.16 As alternative approaches, rather than just replacing the contact metals, atom implantation,17 alloying,18 bilayer structures,19 and the interface dipole effect20,21 have been studied in conventional semiconductor devices. In this study, we exploit the interface dipole effect by insertion of the ionic layered metal PdCoO2 to control SBHs in β-Ga2O3 Schottky junctions [Fig. 1(a)]. At the interface of PdCoO2/β-Ga2O3, owing to ionic layer stacking {… Pd+/[CoO2]/Pd+/[CoO2] …[Fig. 1(a)]},22 large interface dipoles (∼1.1 eV) are naturally formed.14 By inserting ultrathin PdCoO2 between various metals (Pt, Ni, Cr, and Ti) and β-Ga2O3, we demonstrate the systematic control of SBHs in a wide range of 0.7 eV–1.9 eV by tuning the PdCoO2 thickness.

FIG. 1.

(a) The cross-sectional device structure of the metal/PdCoO2/β-Ga2O3 Schottky junction. The layered crystal structure of PdCoO2 is shown with Pd+ and [CoO2] sublattices. (b) The optical microscopy image of the sample. Four different top metals (Pt, Ni, Cr, and Ti) were deposited on different areas using a shadow mask. The region enclosed by the red dotted line does not contain top metals and thus corresponds to the bare PdCoO2/β-Ga2O3 devices.

FIG. 1.

(a) The cross-sectional device structure of the metal/PdCoO2/β-Ga2O3 Schottky junction. The layered crystal structure of PdCoO2 is shown with Pd+ and [CoO2] sublattices. (b) The optical microscopy image of the sample. Four different top metals (Pt, Ni, Cr, and Ti) were deposited on different areas using a shadow mask. The region enclosed by the red dotted line does not contain top metals and thus corresponds to the bare PdCoO2/β-Ga2O3 devices.

Close modal

For precise evaluation of the Schottky barrier height, avoiding defect formation during the device patterning is important. Here, we chose a lift-off process using water-soluble sacrificial templates to fabricate the Schottky junctions with the diameter D = 200 µm, following our previous works.14,23 This process is free from a high-energy plasma bombardment that could generate deep trapping sites in β-Ga2O3.24 Commercially available unintentionally n-type doped β-Ga2O3 (−201) substrates of nominal electron concentration n = 6.2 × 1017 cm−3 (Novel Crystal Technology, Inc.) were cleaned by an acidic solution [water:(30%–35.5% H2O2):(95% H2SO4) = 1:1:4] for 5 min and then rinsed with water. The LaAlO3/BaOx sacrificial templates were patterned on the β-Ga2O3 substrates using conventional photolithography and pulsed-laser deposition (PLD) at room temperature. The c-axis-oriented PdCoO2 thin films were grown by PLD at a substrate temperature of 700 °C under an oxygen pressure of 150 mTorr.25 The thickness of PdCoO2 (d) was controlled by the number of ablation laser pulses. A growth rate (nm/pulse) was estimated by the Laue fringes in the x-ray diffraction pattern of a PdCoO2 reference sample grown on c-Al2O3.25 The 40-nm top metals (Pt, Ni, Cr, and Ti) were deposited independently with a shadow mask on different regions of one sample, as shown in Fig. 1(b). The Ni, Cr, and Ti layers were deposited by electron-beam evaporation, and the Pt layer was deposited by sputtering. After the deposition of the top metals, the samples were immersed in de-ionized water under ultrasound to remove the LaAlO3/BaOx water-soluble template together with the unnecessary part of the top metal/PdCoO2 layers. To obtain Ohmic contacts, Al wires were wedge-bonded directly to the top surface of the β-Ga2O3 substrate. The bonded Al wires were then covered by mechanically pressed In, which gives the contact resistance in the order of 10 Ω. Two-probe JV characteristics were measured at 298 K using an Agilent 4155C semiconductor parameter analyzer and triaxial needle probers. As shown in Fig. 1(a), the forward direction corresponds to a positive bias applied to the top metal and ground connection with the In/Al Ohmic contact. We characterized JV properties of approximately 10 devices for each combination of a top metal material and a different PdCoO2 thickness d. The triangular domains with smooth surface terraces in PdCoO2 thin films give thickness inhomogeneity with the root-mean-square (rms) roughness below 1 nm (Fig. S1). Capacitance vs voltage (CV) characteristics were measured by using an Agilent E4980A precision LCR meter using 1-kHz AC modulation voltage with an amplitude of 0.1 V.

The systematic change in the JV characteristics of the Schottky junctions by varying d is presented in Fig. 2. By using the thermionic emission model (1),15 the SBH ϕbJV was determined from the exponential region of the forward-bias JV characteristics,

J=A**T2eϕbJV/kBTeqV/nkBT1,
(1)

where A** is the Richardson constant, T is the temperature, kB is the Boltzmann constant, q is the elementary charge, V is the applied voltage, and n is the ideality factor. By fitting with A** of β-Ga2O3 of 41.1 A/cm2K2,26 the room-temperature ϕbJV and n were obtained. Following the general analysis for SBH in Schottky junctions,27 the ideal SBH for JV (ϕb0JV) was extrapolated at n = 1 in n dependence of ϕbJV [Figs. S2(a) and S2(b)]. For the metal/β-Ga2O3 junctions (d = 0 nm), ϕb0JV was dependent on the work function of the top metal. Pt (ϕm = 5.64 eV)28 and Ni (ϕm = 5.15 eV)29 effectively formed large SBHs of about 1.3 eV with low leakage current density under low forward bias and reverse bias conditions [Figs. 2(a) and 2(b), respectively], while Cr (ϕm = 4.5 eV)28 and Ti (ϕm = 4.33 eV)28 formed junctions with high leakage current density [Figs. 2(c) and 2(d), respectively]. With an increase in the thickness of PdCoO2 insertion layers, the forward JV curves shifted to higher voltages for all the top metals. The JV characteristics of Pt, Ni, and Ti for d = 20 nm junctions almost overlapped with that of the bare PdCoO2 (20 nm)/β-Ga2O3 junction (gray lines), indicating that the Schottky property was governed by the PdCoO2/β-Ga2O3 interface. Moreover, the systematic parallel shift of the forward JV curves directly reflects the increase in ϕbJV with maintaining the comparable ideality factor. Regarding the negative bias region, the JR of the Pt and Ni junctions [Figs. 2(a) and 2(b)] was below the measurement limit. The finite JR at −1 V in the Cr and Ti junctions for d = 0 nm was effectively suppressed by the insertion of the 3-nm PdCoO2 layer, as shown in Figs. 2(c) and 2(d), respectively, which could originate from the increase in ϕbJV. For d > 3 nm, in both Cr and Ti junctions, the JR was below the measurement limit, as in the Pt and Ni junctions (Fig. S3 of the supplementary material). These results show that the ϕbJV values of all junctions are enhanced by insertion of PdCoO2 layers, approaching almost comparable values for sufficiently large d.

FIG. 2.

Room-temperature JV characteristics of the (a) Pt, (b) Ni, (c) Cr, and (d) Ti (40 nm)/PdCoO2 (d nm)/β-Ga2O3 Schottky devices. The diameters (D) of the Schottky diodes are 200 µm. The colored numbers are the d values. The JV curve for a PdCoO2 (20 nm)/β-Ga2O3 junction is also plotted (gray lines).

FIG. 2.

Room-temperature JV characteristics of the (a) Pt, (b) Ni, (c) Cr, and (d) Ti (40 nm)/PdCoO2 (d nm)/β-Ga2O3 Schottky devices. The diameters (D) of the Schottky diodes are 200 µm. The colored numbers are the d values. The JV curve for a PdCoO2 (20 nm)/β-Ga2O3 junction is also plotted (gray lines).

Close modal

The SBH was also measured by CV characteristics, as plotted in Figs. 3(a)–3(d). The CV characteristics of an ideal Schottky junction can be formulated as15 

SC2=2VbiVqεrε0ND,
(2)

where S is the junction area S = π(D/2)2, Vbi is the built-in potential, εr is the relative permittivity, ε0 is the vacuum permittivity, and ND is the donor density. Vbi is related to SBH (ϕbCV) as ϕbCV=qVbi+ECEF, where EC and EF correspond to the conduction band minimum and the Fermi level, respectively. Using the effective density of states in the conduction band NC=22πm*kBT/h23/2, the difference (ξ) of EC and EF can be calculated as ξ = ECEF = kBT In(NC/ND), where m* is the effective mass of electrons and h is the Planck constant. We calculated ϕbCV using the Vbi and ND [Figs. S4(a) and S4(b)] obtained from the fitting of CV characteristics, by employing εr = 10 (Refs. 30 and 31) and m* = 0.342 m0 for Ga2O3,26 where m0 is the electron rest mass. ND is evaluated to be 2–3 × 1017 cm−3 [Fig. S4(b)], which is comparable with the nominal electron concentration n in the Ga2O3 crystal. This indicates that the high growth temperature of PdCoO2 (∼700 °C) did not change the β-Ga2O3 dopant concentration in the depletion layer width Wd of about 60–100 nm [Fig. S4(c)]. Overall, the (S/C)2V curves [Figs. 3(a)–3(d)] clearly exhibited the increase in Vbi (the crossing point of the fitting lines and voltage axis) with an increase in the thickness of PdCoO2 insertion layers, consistent with the behaviors of JV curves shown in Figs. 2(a)–2(d). The bare PdCoO2/β-Ga2O3 junction, in contrast, did not show d dependence in JV and (S/C)2V characteristics (Fig. S5).

FIG. 3.

CV characteristics of the (a) Pt, (b) Ni, (c) Cr, and (d) Ti (40 nm)/PdCoO2 (d nm)/β-Ga2O3 Schottky devices with D = 200 µm, measured at T = 300 K. The linear fittings and the experimental data (filled circles) are plotted for various d together with those for a PdCoO2 (20 nm)/β-Ga2O3 junction (only PdCoO2, gray plots).

FIG. 3.

CV characteristics of the (a) Pt, (b) Ni, (c) Cr, and (d) Ti (40 nm)/PdCoO2 (d nm)/β-Ga2O3 Schottky devices with D = 200 µm, measured at T = 300 K. The linear fittings and the experimental data (filled circles) are plotted for various d together with those for a PdCoO2 (20 nm)/β-Ga2O3 junction (only PdCoO2, gray plots).

Close modal

The ideal SBHs for JV ϕb0JV and the SBHs for CV ϕbCV are plotted against d in Figs. 4(a) and 4(b). For all the top metals except for Cr, ϕb0JV gradually increases with d to the maximum value of about 1.9 eV that agrees well with that of the bare PdCoO2/β-Ga2O3 (gray diamonds). This indicates that the saturated ϕb0JV for the thickest limit d = 20 nm is dominated by the PdCoO2/β-Ga2O3 interface; the work function of the top metal plays a less decisive role in the determination of ϕb0JV because of the sufficient separation from the interface. We speculate that the smaller ϕb0JV ∼ 1.6 eV observed for Cr/PdCoO2 (20 nm)/β-Ga2O3 might originate from the interfacial reaction between Cr and PdCoO2, which could affect the work function of PdCoO2. In fact, the ideality factor of the all the Cr junctions for d = 20 nm (Fig. S2) is apparently worse than that for other junctions. By contrast to the behavior of the ϕb0JV in Fig. 4(a), the ϕbCV of metal/PdCoO2/β-Ga2O3 reaches the saturation value around d = 4.7 nm, approaching the ϕbCV of PdCoO2/β-Ga2O3 (gray line) [Fig. 4(b)]. As ϕbCV generally reflects the areal mean value of SBHs in the junction area S and ϕb0JV is dominated by the lowest SBH in the junction,32 the different thickness dependences between ϕbCV and ϕb0JV suggest the existence of the in-plane inhomogeneity of the SBH for small d. This inhomogeneity could originate from the difference in PdCoO2 thickness and/or difference in electrostatic screening length between the inside and the boundary regions of the triangular PdCoO2 domains (Fig. S1). For small d, as the effect of top metals is not completely screened by the PdCoO2 layer, the interface area would consist of the inside of PdCoO2 domains generating high SBHs and the boundary regions generating low SBHs. Such in-plane fluctuation of SBHs is known to be smoothed by the band-bending effect that occurs in the length scale ∼Wd.27 Because the typical size of the boundary regions (the purple dotted line and the gray lines and arrows in Fig. S1) is smaller than the depletion layer width Wd = 60–100 nm, the low-SBH regions would be pinched-off by the neighboring high-SBH regions, as a result of the band-bending effect.16,27,33 This pinch-off effect would result in the gradual increase in ϕb0JV with an increase in d, as in Fig. 4(a).

FIG. 4.

(a) Ideal SBHs (ϕb0JV) and (b) ϕbCV plotted against d for the various metal/PdCoO2/β-Ga2O3 and PdCoO2/β-Ga2O3 (gray) Schottky junctions.

FIG. 4.

(a) Ideal SBHs (ϕb0JV) and (b) ϕbCV plotted against d for the various metal/PdCoO2/β-Ga2O3 and PdCoO2/β-Ga2O3 (gray) Schottky junctions.

Close modal

The controllable range of ϕb0JV by the insertion of the PdCoO2 layer is summarized in Fig. 5, where ϕb0JV is plotted against the work function of the top metal.28,29 By selecting the suitable combination of top metal and d, ϕb0JV can be controlled in the range of 0.7 eV–1.9 eV. The wide-range controllability would be beneficial to tune the optimal balance between VF and JR, depending on various types of Schottky diode applications. The possible energy band diagram is presented in the inset of Fig. 5. At the interface of the top metal and PdCoO2 (ϕPCO = 4.7 eV14) layers, the vacuum-level shift between the metal and PdCoO2 (Δm) is expected to compensate the difference in work function. Usually, the interface resistance caused by Δm is negligible, owing to the short potential screening lengths that allow electrons to pass through the interface by tunneling. In contrast, the dipole at the PdCoO2/β-Ga2O3 metal–semiconductor interface (Δ = 1.1 eV) effectively contributes to the large SBH ϕb0JV and ϕbCV of ∼1.9 eV.14 

FIG. 5.

ϕb0JV vs the top-metal work function ϕm for polycrystals.28,29 The range of available SBH (Δϕb0JV) is indicated by the dotted lines and the arrow. The possible band diagram for a large d is presented in the inset. Δm and Δ are the vacuum-level shifts at the metal/PdCoO2 and PdCoO2/β-Ga2O3 interfaces, respectively, caused by the interface dipoles (Δ of 1.1 eV14). ϕPCO is the work function of PdCoO2 (ϕPCO of 4.7 eV14). The electron affinity (χs of 4.0 eV35) and energy difference (ξ of about 0.08 eV) of the conduction band bottom (EC) and Fermi energy (EF) of β-Ga2O3 are presented together with the vacuum level (Evac).

FIG. 5.

ϕb0JV vs the top-metal work function ϕm for polycrystals.28,29 The range of available SBH (Δϕb0JV) is indicated by the dotted lines and the arrow. The possible band diagram for a large d is presented in the inset. Δm and Δ are the vacuum-level shifts at the metal/PdCoO2 and PdCoO2/β-Ga2O3 interfaces, respectively, caused by the interface dipoles (Δ of 1.1 eV14). ϕPCO is the work function of PdCoO2 (ϕPCO of 4.7 eV14). The electron affinity (χs of 4.0 eV35) and energy difference (ξ of about 0.08 eV) of the conduction band bottom (EC) and Fermi energy (EF) of β-Ga2O3 are presented together with the vacuum level (Evac).

Close modal

Based on the possible band diagram, the systematic reduction in ϕb0JV and ϕbCV with a decrease in d can be discussed assuming the two dipoles, Δm and Δ at the top and bottom of the PdCoO2 layer. To consider the mechanism, the characteristic length scale dc was determined by a fitting curve for the average SBH ϕbCV vs d (Fig. S6), assuming an exponential decaying function. We employed the ϕbCV vs d plot for the determination of dc because it is less affected by the in-plane inhomogeneity of the SBH. The obtained dc values were 1.2 ± 0.2 nm for Pt, 1.5 ± 0.5 nm for Ni, 4.0 ± 1.4 nm for Cr, and 2.4 ± 0.8 nm for Ti, where the error bars represent the standard deviations of the fittings. As a relevant length scale to dc, the Thomas–Fermi screening length of the electrostatic potential should be taken into account, which is formulated as rTF=1/2aBohr3/n3D1/6, where the Bohr radius aBohr is 0.53 Å and n3D is the charged carrier density in the metal. Using n3D of a PdCoO2 bulk single crystal22 (∼2.45 × 1022 cm−3), rTF was estimated to be ∼0.07 nm. The experimentally obtained dc values were considerably larger than rTF ∼ 0.07 nm that was calculated by assuming the homogeneous carrier distribution n3D in PdCoO2. In the layered crystal structure of PdCoO2, mobile electrons exist mainly in the Pd+ sublattices, while the electrons in [CoO2] sublattices are localized.34 Thus, it is expected that charge screening occurs effectively in the Pd+ layer, but not in the insulating [CoO2] layer. Considering the insulating nature of [CoO2] layers, the effective screening length is expected to be in the order of the inter-Pd-layer distance (∼0.6 nm), which is much longer than rTF ∼ 0.07 nm. The estimated screening length of ∼0.6 nm, accompanied with the finite rms interface roughness < 1 nm (Fig. S1), would explain the observed dc values for Pt (dc = 1.2 ± 0.2 nm) and Ni (dc = 1.5 ± 0.5 nm). The longer dc observed for Cr (dc = 4.0 ± 1.4 nm) and Ti (dc = 2.4 ± 0.8 nm) might be due to an interfacial layer that may form between Cr (Ti) and PdCoO2. Further studies are necessary to understand the atomic-scale band diagram of the Schottky junction having a PdCoO2 insertion layer thinner than dc.

In summary, we have fabricated the stacked Schottky contacts composed of various metals (Pt, Ni, Cr, and Ti) and PdCoO2 thin films on n-type β-Ga2O3 (−201) substrates. The SBH systematically increased with the increase in the thickness of PdCoO2, thanks to the contribution of the interface dipole effect. The characteristic length dc for a change in the average SBH ϕbCV is likely to be determined by the electrostatic screening in the layered metal PdCoO2 that has mobile electrons dominantly in Pd+ layers. The wide-range controllability of the SBH, from 0.7 eV to 1.9 eV, demonstrates that the insertion of the PdCoO2 layer is useful for the development of high-performance Schottky diodes based on Ga2O3 with optimized properties.

See the supplementary material for the surface morphology of a PdCoO2/β-Ga2O3 thin film, additional datasets of the Schottky junction properties, and the determination of the characteristic length dc.

The authors thank K. Fujiwara for the experimental help with sputtering and NEOARK Corporation for the use of a maskless lithography system PALET. This work was performed under the Inter-university Cooperative Research Program of the CRDAM-IMR, Tohoku University (Proposal No. 18G0407). This work was partly supported by a Grant-in-Aid for Scientific Research (A) (Grant No. 15H02022) and a Grant-in-Aid for Early-Career Scientists (Grant No. 18K14121) from the Japan Society for the Promotion of Science (JSPS), JST CREST (No. JPMJCR18T2).

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