Oxidized metal Schottky contact with high-κ dielectric field plate for low-loss high-power vertical β-Ga₂O₃ Schottky diodes

Gallium oxide in β-phase (β-Ga₂O₃) has emerged as a promising candidate for next-generation power electronics with a unique combination of advantageous properties, such as its large bandgap ∼4.8 eV, high projected breakdown field ∼8 MV/cm, efficient, shallow n-type dopants, and overall scalable, low-cost, melt-grown native substrates, which are not possible with other ultrawide-bandgap semiconductors.^1–5 These advantageous features have led to continued β-Ga₂O₃ device development with increasingly improved performance metrics. Particularly, vertical β-Ga₂O₃ devices have been an extensive area of research, being the preferred geometry for high-power electronics.^2–17 Vertical devices can facilitate higher current capability, better field management, scaling feasibility, and enhanced thermal management compared to their lateral counterparts.^2,10,18 However, due to the absence of a p–n homojunction in β-Ga₂O₃, alternative rectifying junctions have been realized for vertical β-Ga₂O₃ devices in the form of Schottky barrier, p–n heterojunction, or metal–insulator–semiconductor (MIS) diodes.^{2–17,19–21} Among these, the vertical n-type Schottky barrier diode (SBD) remains the primary rectifying device for β-Ga₂O₃ with the desirable high-quality interface, whereas the performance and reliability of heterojunction p–n or MIS diodes are often compromised by the heterojunction interface, non-native p-oxide, or dielectric properties leading to non-ideal forward transport, trap-assisted leakage, and high on-resistance.^19–23 

To design these vertical β-Ga₂O₃ Schottky diodes for high-power switching operation, the goal is to achieve high breakdown voltage with substantially low leakage current to minimize off-state power dissipation and a low on-state specific resistance. However, in practical β-Ga₂O₃ Schottky diodes, the blocking voltage is often limited by the reverse leakage current rather than the β-Ga₂O₃ breakdown properties.^22–24 The origin of this leakage current has been actively investigated in recent years in both β-Ga₂O₃ materials and devices. For example, the unwanted leakage path has been attributed to the presence of stacking fault, line shaped defects, or other forms of crystalline defects in β-Ga₂O₃ epitaxial layers and substrates.^25–27 From the device perspective, the non-ideal leakage current appears in β-Ga₂O₃ Schottky diode characteristics in the form of tunneling current through the Schottky barrier, as well as edge leakage current due to the electric field crowding near contact edges.^22–24 While the edge leakage can be suppressed by field termination at contact peripheries, minimizing Schottky barrier tunneling current requires a high barrier material for the anode contact.

Unfortunately, in β-Ga₂O₃, the metal Schottky barrier height (SBH) is limited to a maximum value of ∼1.6 eV in (010) orientation, defined by the partial Fermi level pinning.^28,29 Moreover, metal/β-Ga₂O₃ SBH (ϕ) has a strong orientation dependence, revealed as ϕ₍₀₁₀₎ > ϕ₍₋₂₀₁₎/ϕ₍₀₀₁₎ > ϕ₍₁₀₀₎.^29,30 The anisotropy of the β-Ga₂O₃ crystal structure, surface properties, and therefore degree of Fermi level pinning are responsible for the orientation dependence of SBH.²⁹ This has also yielded a commensurately higher leakage current and lower breakdown voltage in the $2̄01$ orientation compared to the (010).²⁹ 

However, it has been reported that the SBH on β-Ga₂O₃ can be increased to ≥2 eV using oxidized metal contacts.^23,31–34 The fundamental PtO_x SBDs on $2̄01$ β-Ga₂O₃ substrates also demonstrated improved reverse blocking compared to Pt.²³ Moreover, unlike conventional metals, oxidized metals do not exhibit orientation dependence in SBH, which has been attributed to the modulation of Fermi level pinning effects by interfacial oxygen vacancy passivation due to in situ oxidation of metals.^32–34 Despite such possibility of oxidized metals to enhance reverse blocking in different scalable planes of β-Ga₂O₃, their application in field-terminated high-voltage β-Ga₂O₃ devices is yet to be reported.

Recently, the high-κ and extreme-κ dielectrics have also gained great interest in β-Ga₂O₃ devices.^{7,11,20,21,35} The higher permittivity of the dielectric compared to that of β-Ga₂O₃ leads to a lower electric field inside the dielectric, which translates to an increased effective barrier width for tunneling and improves field termination.^7,20,21,35

Hence, we report here the integration of oxidized metal (PtO_x) Schottky contact with high-permittivity dielectric field-plate in vertical (001) β-Ga₂O₃ diodes to combine leakage management at both surfaces and edges. For high-κ dielectric, we have demonstrated here the use of ZrO₂ because of its favorable properties of ∼1.2 eV conduction band offset with β-Ga₂O₃, breakdown field ≥3 MV/cm, and higher reported dielectric constant of >20.^36–38 The field-plate with ZrO₂ enabled increase in breakdown voltage with improvement in edge termination and allowed the peak electric field to appear inside β-Ga₂O₃. Moreover, the higher Schottky barrier height of PtO_x contacts provided reduced leakage, higher breakdown voltage, and a significantly reduced total power dissipation for all duty cycles ≤0.5 compared to that of Pt due to the ∼10⁵ times lower off-state loss. Thus, our design approach demonstrates a pathway to integrate high barrier Schottky contact with high-κ dielectric field-plate in β-Ga₂O₃ diodes to improve their reverse blocking capability for low-loss, high-power switching operation.

The vertical SBDs were fabricated on commercially available halide vapor phase epitaxy (HVPE)-grown (001) n⁻ β-Ga₂O₃ of ∼8.5 µm homoepitaxy (Si-doped, doping ∼2 × 10¹⁶ cm⁻³) on a ∼640 µm thick Sn-doped (∼7.5 × 10¹⁸ cm⁻³) substrate.^39,40 The Schottky diodes had circular 100 µm diameter Pt or PtO_x anode contacts, whereas the backside Ohmic contacts were formed with Ti/Au metal stacks [Fig. 1(a)]. The device fabrication started with substrate backside etching with BCl₃ reactive-ion etching (RIE), followed by Ohmic metal (Ti/Au) deposition by e-beam evaporation. The Ohmic metal was later annealed by rapid thermal annealing (RTA) in N₂ ambient at 470 °C for 1 min. Next, the PtO_x Schottky contacts were fabricated by the lift-off process, where the PtO_x was formed with reactive sputtering of Pt at 60 W with an Ar:O₂ flux ratio of 10:10 sccm, followed by a sputtered Pt cap deposition. For comparison, the regular Pt Schottky contacts were also fabricated using the same mask and process flow, with the Pt metal deposited by conventional electron beam evaporation. The mesa isolation was later formed by BCl₃ RIE etching to form an etch depth of ∼450 nm and a subsequent wet etch of HCl and HF to remove dry etch damage.^2,4,8 Afterward, the field plate structures were fabricated using ZrO₂ as field-oxide, which was reactively sputtered with a thickness of ∼215 nm. The ZrO₂ was then patterned with photoresist and opened by dry etch with BCl₃ RIE and wet etch with HF to expose the Pt cap/PtO_x or Pt Schottky contact. Finally, the field plates were formed with Pt/Au by a lift-off process with field-plate length (L_FP) of 15 µm. Some diodes were kept as regular SBDs (without field-plate) in the same samples to characterize basic Schottky contact properties and evaluate the performance of field plates. Additionally, metal–oxide–semiconductor capacitor (MOSCAP) diode was also fabricated on a diced piece of the same wafer to extract the dielectric constant of ZrO₂. The dielectric constant of ZrO₂ was extracted to be ∼26 from capacitance–voltage (C–V) measurements.

The baseline SBDs (without field-plates) were first characterized with current–voltage (J–V) and C–V measurements at room temperature to extract the Schottky contact properties of Pt and PtO_x. The diodes exhibited excellent forward transport characteristics with near unity ideality factor over several decades [Fig. 1(b)]. The higher turn-on voltage exhibited by the PtO_x diode indicates the formation of a higher Schottky barrier compared to that of Pt. Assuming the thermionic emission was the dominant current transport mechanism at room temperature, the SBH from J–V characteristics was extracted as 1.22 and 2.10 eV for the Pt and PtO_x diodes, respectively. The 1 MHz C–V measurements revealed a net doping concentration of ∼2 × 10¹⁶ cm⁻³ in the drift layer. The 1/C²–V analysis also provided SBHs of Pt and PtO_x diodes as 1.69 and 2.19 eV, respectively, which were calculated from the extracted built-in voltage (V_bi) by accounting for the Fermi level position relative to the conduction band (E_C-E_F) [Fig. 1(c)]. Hence, both J–V and C–V measurements demonstrated a consistent trend of higher SBH of ∼2.1 eV achieved with PtO_x compared to Pt.

After extracting the fundamental Schottky contact properties, we characterized field-plate SBDs of Pt and PtO_x SBDs. The forward J–V characteristics of the 100 µm diameter Pt and PtO_x diodes with field-plate are shown in Fig. 2(a). These field-plate diodes also provided forward transport properties similar to that of their respective baseline SBDs. The ideality factors of the field-plate diodes were 1.06 and 1.17, and the extracted SBHs were 1.39 and 2.03 eV for the Pt and PtO_x contacts, respectively. The minimum differential specific on-resistance (R_on,sp) of these diodes extracted from their respective forward transport properties was 2.29 and 2.36 mΩ cm² for the Pt and PtO_x contacts [Fig. 2(b)], respectively. The similar R_on,sp of Pt and PtO_x diodes indicates that R_on,sp was not compromised with PtO_x formation.

The reverse breakdown voltage (V_br) of the SBDs was subsequently characterized with the samples submerged in FC-40 Fluorinert dielectric liquid. The breakdown voltage is considered as the voltage value at which destructive breakdown was observed. To inspect the statistical validity of the breakdown voltage results, multiple diodes with field-plate were measured across the wafer for both Pt and PtO_x contacts, which are shown from two representative field-plate diodes for each contact case in Fig. 3. As shown in Fig. 3, a consistent improvement in V_br was obtained with introduced field-plate for both Pt and PtO_x diodes, indicating the efficiency of high-κ ZrO₂ to suppress edge leakage. Moreover, with PtO_x, V_br is further increased with a reduced leakage compared to the Pt SBDs for the same device structure and at a given reverse bias [Fig. 3(b)]. As seen from Fig. 3(b), with PtO_x contacts, the leakage was below the detection limit for a significantly larger range of reverse biases for diodes with and without field plate, compared to that of Pt diodes. This drastic reduction of leakage current demonstrated by PtO_x SBDs is due to the tunneling leakage suppression through their higher SBH. The maximum V_br obtained with the field-plate PtO_x diode was 882 V compared to that of 688 V with Pt, indicating an increase of V_br by more than 190 V in PtO_x diodes.

The breakdown field of the field-plate PtO_x SBDs was further analyzed using Silvaco TCAD software⁴¹ as shown in Fig. 4. With a drift layer doping of ∼2 × 10¹⁶ cm⁻³, the SBD was not fully depleted and had a non-punch-through field profile with depletion depth (W_d) ∼7.0 µm. This W_d provides an intrinsic on resistance (R_on,in) of ∼1.56 mΩ cm², which was calculated using the following equation:^2,12

Here, q is the electron charge, N_D − N_A ∼ 2 × 10¹⁶ cm⁻³ is the net doping concentration, and μ is the mobility in the drift layer. To estimate R_on,in from Eq. (1), we used a mobility of 140 cm² V⁻¹ s⁻¹ based on a prior report on HVPE-grown (001) Si-doped β-Ga₂O₃ epitaxial layers for this range of carrier concentration.⁴² Thus, the undepleted 1.5 µm epitaxy of the drift layer added a parasitic component of 0.33 mΩ cm² in the total R_on,sp of 2.36 mΩ cm².

The Silvaco device simulation also showed that the peak field (4.14 MV/cm) appeared in β-Ga₂O₃ due to its lower permittivity compared to ZrO₂, which indicates an efficient field management approach in maximizing the field in β-Ga₂O₃ (Fig. 4). However, the breakdown hotspot was still at ZrO₂ with a dielectric peak field of ∼3.0 MV/cm, which led to its destructive failure, limiting β-Ga₂O₃ to reach its ability of 8 MV/cm. Figure 5 shows the benchmark R_on,sp vs V_br of our work and other vertical β-Ga₂O₃ SBDs.^2–17 The V_br of 882 V and R_on,sp of 2.36 mΩ cm² provide a Baliga’s figure-of-merit (BFOM) of 0.33 GW/cm², which is comparable to the state-of-the-art and recent reports.^2–17 Future design with longer field-plate lengths (L_FP ≥ 20 µm), as appears in some existing literature,^3,16 will be helpful for additional increase of the breakdown voltage by redistribution of electric field further away from the edge.

However, the major driver of the PtO_x Schottky diodes over Pt and other conventional Schottky metals is the overall reduced power dissipation during high-voltage switching operation. At low switching frequency, where switching transients can be ignored, the Schottky diode total power loss per unit area (P_L) can be quantified with the combination of on-state loss (P_ON) and off-state loss (P_OFF) for a duty cycle (D) as follows:⁴³ 

where P_ON = V_ON J_ON and P_OFF = V_OFF J_OFF

Here, the diode turn-on voltage, V_ON, is defined at the forward current density, J_ON = 100 A cm⁻².^18,44 The V_OFF and J_OFF refer to off-state voltage and off-state current density, respectively.

Using Eq. (2), we have evaluated here the total power dissipation with respect to duty cycle for both Pt and PtO_x diodes. The P_OFF was calculated at the same off-state voltage, V_OFF = −450 V and V_OFF = −650 V for non-field plate and field-plate diodes, respectively. On the other hand, P_ON was estimated for these diodes with their respective V_ON values when the forward current density reaches J_ON = 100 A cm⁻². The dissipated power as a function of duty cycle for the diodes is plotted in Fig. 6. As seen from Fig. 6(a), the off-state loss component is significantly reduced by PtO_x diodes (∼10⁵ times less) compared to Pt due to their higher SBH. Although the higher SBH also led to the slightly higher V_ON in the PtO_x diodes (2.44 V) compared to that of Pt (1.6 V) [Fig. 2(a)], the estimated total power dissipation is still orders of magnitude less in PtO_x diodes for all D ≤ 0.5, contributed by their significantly reduced off-state loss as shown in Fig. 6(b). Thus, the total loss in Pt SBDs is dominated by off-state loss whereas in PtO_x diodes, it is dominated by on-state loss, which for duty cycles ≤0.5, is comparatively much lower than the Pt SBDs. This will be fundamentally promising for β-Ga₂O₃ power diodes that are intended for high-voltage switching applications with minimal power loss and low heat dissipation.

In summary, we reported a combined integration of the PtO_x Schottky contact and high-κ dielectric field-plate that can enable low off-state power dissipation, suppressing both surface and edge leakage in vertical β-Ga₂O₃ power diodes. The higher SBH of PtO_x diodes allowed higher breakdown voltage by more than 190 V, significantly reduced leakage current, and an overall orders of magnitude lower power dissipation than that of Pt for all duty cycles ≤0.5. This demonstrates the potential of PtO_x over conventional metal Schottky contacts to develop low-loss, high-voltage power switches. Moreover, the high-κ ZrO₂ field-plate aided in reducing edge leakage and further increasing the breakdown voltage. The ZrO₂ also facilitated the peak field to appear in β-Ga₂O₃ which is promising. Future work will explore extreme permittivity dielectrics of higher breakdown properties, longer field plate lengths, and lower-doped epitaxy to maximize β-Ga₂O₃ potential in power devices.

This work was supported, in part, by AFOSR and GAME MURI under program Grant Nos. FA9550-18-1-0059 and FA9550-18-1-0479, respectively (Program Manager Dr. Ali Sayir), AFOSR Radiation Effects Center of Excellence program under Grant No. FA9550-22-1-0012 through a subcontract (OSA00000043) from Vanderbilt University, and a subcontract from Agnitron Technology through the ONR program under Grant No.N00014-16-P-2058. The work at the University of Utah was supported by the AFOSR under Award No. FA9550-21-1-0078 (Program Manager Dr. Ali Sayir). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the US Air Force. E. Farzana conveys her thanks to Akhil Mauze for dicing the wafer.