Large-size (1.7 × 1.7 mm²) β-Ga₂O₃ field-plated trench MOS-type Schottky barrier diodes with 1.2 kV breakdown voltage and 10⁹ high on/off current ratio

Beta gallium oxide (β-Ga₂O₃) has become of interest in the power electronics industry especially for electric vehicles, power conditioning, power distribution, and switching applications. It has a high bandgap (4.5–4.9 eV), ^1–4) high theoretical breakdown electric field (6–8 MV cm⁻¹), ⁵⁾ and Baliga's figure of merit (FOM) exceeding that of Si, SiC and GaN. Its bulk fabrication cost is lower than that of SiC and GaN. In the past, several reports have been published on the development of Ga₂O₃ Schottky barrier diodes (SBDs) with breakdown voltages (V_BR) ranging from 100 to 1000 V. ^6–14) Among the candidate device structures, trench MOS-type Schottky barrier diodes (MOSSBD) have been intensively studied because they give a V_BR higher than 1.2 kV with a specific on-resistance (R_on,sp) lower than 20 mΩ · cm². ^15–17) Some attempts have been made to improve the V_BR of Ga₂O₃ diodes, such as through the use of planar SBDs incorporating a field plate, ^18,19,23,25) guard ring, ²⁰⁾ NiO junction barrier Schottky (JBS) diode, ^21,22) or hetero-junction barrier Schottky diode (HJD). ²⁴⁾ For high-power switching applications, large rectifiers are used to produce a high current while sustaining high V_BR. Several papers have described large (≥ 1.0 × 10^–2 cm²) planar SBDs that have simple structures and that can deliver high forward current (I_F) with a high V_BR of > 700 V. ^21,23,24) However, there are no reports on diodes that can deliver a high I_F of > 1 A at low forward voltages ranging from 1.5 to 2.0 V with V_BR greater than 1.2 kV, which is required for high power applications such as an AC 400 V input circuit. In this paper, we report a field-plated MOSSBD with a size of 1.7 × 1.7 mm² that exhibits an I_F of 2 A at 2 V forward voltage and a leakage current of 5.7 × 10^–10 A at −1.2 kV reverse voltage (on/off current ratio of >10⁹) with an ideality factor of 1.05 and wafer-level R_on,sp of 17.1 mΩ · cm².

Figures 1(a) and 1(b) show a schematic cross-sectional image and scanning electron microscopy (SEM) image of the MOSSBD. The devices were fabricated on a 2 inch diameter β-Ga₂O₃ epitaxial wafer. The wafer structure contains a 12 μm thick epitaxial layer with a donor concentration of 1.5 × 10¹⁶ cm⁻³ grown by halide vapor phase epitaxy on an n-type β-Ga₂O₃ (001) substrate. In the line and space region, the mask size of the mesa top is 1.2 μm and that of the trench is 0.8 μm, but the final trench bottom width is 0.25 μm because of the tapered shape of the trench sidewall, as shown in the SEM image [Fig. 1(b)]. Because the field-plate SiO₂ on the large mesa top is completely etched during the resist etch-back process, a MOS field-plate structure is created on the stair shaped oxide in the wide trench region, as shown in Fig. 1(a).

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Figure 2 shows a schematic image of the process flow, which is as follows: (1) the trench is patterned with a line and space pitch of 2 μm by i-line stepper lithography and BCl₃-based dry etching. (2) 500 nm thick SiO₂ is deposited as a field-plate dielectric by plasma-enhanced-chemical vapor deposition (PECVD), which is then wet-etched using two masks that create a stair shape. After formation of the field-plate dielectric, 50 nm thick PECVD SiO₂ and 20 nm thick atomic-layer-deposited Al₂O₃ are successively deposited as the capacitor dielectric. (3) Resist coating and etch-back are used to remove the dielectric from the mesa top. In the resist etch-back process, the dry-etching step finishes during the Al₂O₃ etching step, which is followed by wet etching to completely remove the dielectric from the mesa top. As shown in Fig. 1(b), MOS dielectric is etched back and only the sidewall and the bottom of the trench are covered with dielectric. (4) Ni/Au anode electrode is deposited by e-beam evaporation and patterned by wet-etching; then, the back Ti/Ni/Au cathode metal is deposited by e-beam evaporation. Planar SBDs are also fabricated on the large mesa top area where the deposited field-plate SiO₂ and SiO₂/Al₂O₃ dielectric have been removed by the etch-back process. A total of four masks are used in this process.

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The main differences from the previous work on a field-plated MOSSBD ¹⁷⁾ are utilization of a stepper as a lithography tool that is extendible to mass production, the self-aligned etch-back process in removing dielectric from the mesa top that improves uniformity and reproducibility, and the field-plate formation process in which the SiO₂/Al₂O₃ dielectric is formed on the field-plate SiO₂ to make it easy optimize the thermal process of high-k dielectric, analogously to the gate-last process for making high-k metal gates in logic devices. ²⁶⁾

All the data on the trench MOSSBDs reported in this paper were obtained from devices that had a [010] trench orientation, which was found to give the highest I_F due to it having the lowest interface charge density on the (100)-like trench sidewall. ²⁹⁾

Figure 3 shows the on-wafer measured forward current–voltage (I–V) characteristics of the trench MOSSBD measuring 1.7 × 1.7 mm². The ideality factor (n) is 1.05, which is close to the ideal value for Schottky diodes. I_F reaches 2 A at a supply voltage of 2 V. A pulsed I–V measurement system, with a pulse width of 1 ms and 1% duty cycle, was used for the on-wafer high-current measurements higher than 1 A.

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Current scalability was investigated using devices of different sizes. Figure 4 shows the measured forward current density–voltage (J–V) characteristics of the fabricated MOSSBDs with sizes of 0.3 mm in diameter and 1.7 × 1.7 mm² (square). The ideality factors of both MOSSBDs are almost the same, 1.05, which is close to the ideal value for a Schottky junction. The turn-ON voltages (V_ON) of the MOSSBDs are almost the same, 0.76 V, and both MOSSBDs exhibit forward current densities (J_F) larger than 70 A cm⁻² at an applied forward voltage (V_F) of 2.0 V. Although J_F decreases from 110 to 70 A cm⁻² at a V_F of 2.0 V and R_on,sp increases from 13.4 to 17.1 mΩ · cm² as the device size increases from 0.3 mm in diameter to 1.7 × 1.7 mm² (square), there is a possibility that the measured value of the large MOSSBDs was affected by parasitic resistance derived from the on-wafer measurement system. The inset of Fig. 4 shows the forward I–V curve of the same MOSSBD with the area of 1.7 × 1.7 mm² (square), as measured by a curve tracer after it was diced and packaged into a TO-247 package. The R_on,sp extracted from the inset decreased to 12.3 mΩ · cm², almost the same level as that of the small pattern of 0.3 mm in diameter. From the forward I–V measurement of the packaged MOSSBD, it appears that current scalability proportional to the area can be applied to the large MOSSBD. We believe that the large-size MOSSBD with low turn-ON voltage and high current density has a great potential for power device applications.

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Figure 5 shows the measured reverse J–V characteristic of the field-plated MOSSBD (denoted as FP-MOSSBD) with a 1.7 × 1.7 mm² pattern. For comparison, the measured reverse J–V characteristic of a planar SBD with the same pattern and fabricated on the same wafer and the calculated reverse J–V curve of the planar SBD based on the thermionic field emission (TFE) model ^9,27) are also plotted. The J–V calculation using the TFE model used a barrier height qϕ_B of 1.23 eV, where qϕ_B was calculated using the following equations: ^9,28)

$$\begin{eqnarray}&&J={J}_{{\rm{S}}}\exp \left[qV/\left(nkT\right)\right];\,V\,\gg \,kT/q,\end{eqnarray} \tag{ 1 }$$

$$\begin{eqnarray}&&q{\phi }_{{\rm{B}}}=kT\,\mathrm{ln}\,\left({A}^{\ast }{T}^{2}/{J}_{{\rm{S}}}\right),\end{eqnarray} \tag{ 2 }$$

where q is the electron charge, k is the Boltzmann constant, T is the absolute temperature, and A* is the effective Richardson constant (41.1 A.cm⁻².K⁻²). ⁶⁾ J_S is the saturation current density, which was estimated to be 9.7 × 10^–15 A cm⁻² from an extrapolation of the forward J–V characteristic to V_F of 0 V in the inset in Fig. 5.

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As shown in Fig. 5, a remarkable enhancement in V_BR was obtained by equipping the MOSSBD with a field plate that reduces the electric field near that Schottky interface as well as at the anode edge. The leakage current of 20 nA cm⁻² is low even at a reverse voltage of −1.2 kV.

In order to fully understand the achievements of this work, the key parameters of the reported large-size (≥ 1 × 10^–2 cm²) Ga₂O₃ vertical rectifiers ^21,23–25) are compared in Table I. All the diodes except for the HJD have Schottky contacts; therefore, they show low V_ON and high I_F at a V_F of 2 V. However, because of the high electric field near the Schottky interface, the V_BRs of the JBS and FP-SBD are lower than the 1100 V that is required for AC 400 V industry power applications. On the other hand, the V_BR of the FP-MOSSBD exceeds 1100 V, owing to the reduction in the electric field near the Schottky interface by adopting the trench MOS structure. ^15–17) In addition, the reverse leakage current of the FP-MOSSBD near V_BR is much lower than those of the other diodes. Low V_ON and high I_F combined with high V_BR and extremely small leakage current (on/off current ratio = 3.5 × 10⁹) indicate the superiority of the FP-MOSSBD for high-voltage (AC 400 V) power applications. Its V_BR and R_on,sp are 1200 V and 17.1 mΩ · cm², respectively. Its FOM for power devices (FOM = V_BR ²/R_on,sp) is the highest (84 MW cm⁻²) among the large-size diodes with Schottky contacts.

In conclusion, large-size trench MOSSBDs with field-plate were successfully fabricated on a 2 inch β-Ga₂O₃ wafer by using processes suited to mass production, such as i-line stepper lithography and photoresist etch-back. The fabricated field-plated trench MOSSBD, measuring 1.7 × 1.7 mm², exhibited an I_F of 2 A at V_F of 2 V and leakage current of 5.7 × 10^–10 A at a −1.2 kV reverse voltage (on/off current ratio of > 10⁹). These results indicate that the developed trench MOSSBD with a field plate is promising for practical high-voltage (AC 400 V) power applications.

Acknowledgments