Development of Surface-Activated Bonding Technologies to Compensate for Shortcomings of Ga₂O₃ Devices

Remarkable intrinsic physical properties of gallium oxide (Ga₂O₃), such as an extremely large bandgap energy of 4.5 eV, a high breakdown electric field of over 8 MV/cm, exact controllability of <i>n</i>-type doing, and availability of melt-grown bulk wafers, have been drawing a great deal of attention from the semiconductor community. The increased interest led to rapid progress in Ga₂O₃ material and device technologies in this decade. However, it is famous that Ga₂O₃ has two fundamental drawbacks of a low thermal conductivity causing poor heat dissipation capacity for high-power device operation and a lack of hole-conductive <i>p</i>-type imposing a restriction on degrees of freedom in device structure design. As one possible solution for both shortcomings, we have been developing surface-activated bonding (SAB) technologies between Ga₂O₃ and dissimilar materials. In this talk, structural, thermal, and electrical properties of Ga₂O₃/poly-SiC and Ga₂O₃/Si heterostructures fabricated by SAB are presented as typical examples of the study that we have been conducting [1,2].<br/>Poly-SiC is an attractive candidate for alternative wafers of high-power Ga₂O₃ devices since it possesses a high thermal conductivity of more than 10 times larger than that of Ga₂O₃ and an electrical conductivity of over 100 S/cm. Note that these two properties are comparable with or even a little superior to those of single-crystal SiC. Furthermore, poly-SiC wafers have a great advantage compared to single-crystal SiC ones in terms of production cost. The Ga₂O₃/poly-SiC bonding substrate fabricated by SAB showed not only a large bonding strength of over 10 MPa but also high thermal and electrical conductivities at the interface. With thinning down the Ga₂O₃ part of the bonding substrate from 650 to 20 µm, the effective thermal conductivity monotonically increased from 0.2 to 1.5 W/cmK, which is comparable with the value for Si. The interface electrical resistance was also negligibly small. Therefore, poly-SiC substrates would be useful to manage heat dissipation of vertical Ga₂O₃ devices at high-power operation.<br/>To overcome the difficulty that it is nearly impossible to realize hole-conductive <i>p</i>-Ga₂O₃, we are exploring the possibility of using <i>p</i>-type layers of a different semiconductor as a substitute. Electrical properties of <i>n</i>-Ga₂O₃/<i>n</i>-Si heterostructures fabricated by SAB were first studied to extract the conduction band offset between the two semiconductors and the charged interface state density caused by the defective region that had been formed during the bonding process. Details will be given in the presentation.<br/>This work was supported in part by JSPS KAKENHI Grant Number 19H02182.<br/>[1] C. H. Lin <i>et al.</i>, Appl. Phys. Lett. 114, 032103 (2019).<br/>[2] Z. Wang <i>et al.</i>, <i>manuscript in preparation.</i>