Hetero-Integration of β-Ga2O3 and Diamond Substrates by Hydrophilic Bonding Technique
Authors: Matsumae Takashi , Kurashima Yuichi , Takagi Hideki , Umezawa Hitoshi , Tanaka Koji , Ito Toshimitsu , Watanabe Hideyuki , Higurashi Eiji 
Published: December 2020

Beta-phase gallium oxide (β-Ga2O3) has a wide bandgap of ~4.9 eV and a high breakdown voltage of 8 MV/cm. Moreover, high-quality, large-size, and low-cost β-Ga2O3 substrates can be obtained by melt-growth techniques, such as Czochralski (CZ), Floating Zone (FZ), Edge-defined film-fed growth (EFG) techniques. Owing to these features, β-Ga2O3 is a possible candidate for the next-generation power electronics material. However, the primary obstacle for this application is the heat dissipation problem due to low thermal conductivity of ~10–30 W/m/k.
A possible solution is to integrate β-Ga2O3 power electronics with high-thermal-conductivity materials. Our research group has developed a hydrophilic direct bonding technique of semiconductor material and diamond heat spreader[1], which has extraordinary high thermal conductivity. A diamond (111) surface treated with a H2SO4/H2O2 mixture was functionalized with OH groups, and the OH-terminated diamond substrate can form direct bonding with a OH-terminated Si substrate by a dehydration reaction. OH termination using oxygen plasma irradiation allows a strong bonding of semiconductor substrates, such as Si, SiO2, and Al2O3 (the oxide of group 13 element as Ga2O3)[2]. The purpose of this study is to form direct bonding of an oxygen-plasma-activated β-Ga2O3 substrate and the H2SO4/H2O2-treated diamond substrate by the hydrophilic direct bonding technique, as illustrated in Fig. 1.
For the bonding experiment, thin β-Ga2O3 films were exfoliated along the (100) plane using β-Ga2O3 bulk crystal (from Novel Crystal Technology) with a thermal release tape. The thickness of the β-Ga2O3 film was ~10 µm. The cleaved β-Ga2O3 surface was irradiated by oxygen plasma at 200 W and 60 Pa for 30 s using our reactive ion etching (RIE) equipment. Meanwhile, diamond substrates with a 4° off-angled (111)-oriented surface (from EDP) were treated with a H2SO4/H2O2 (4:1) mixture at 75 °C for 10 min. Subsequently, they were cleaned with an NH3/H2O2/H2O (1:1:5) mixture at 75 °C for 10 min. The β-Ga2O3 and diamond surfaces were contacted with each other under atmospheric conditions. The contacted specimens were stored with desiccant for around three days and then annealed at 250 °C for 24 h for bond formation.
Figure 2 shows the photograph of the β-Ga2O3 film bonded on the diamond substrate. As the β-Ga2O3 film is transparent, Newton’s rings were observed where the surfaces were not bonded. The nano-structure of the β-Ga2O3/diamond interface was investigated by a transmission electron microscope (TEM). The focused ion beam (FIB) was introduced from the β-Ga2O3 side to fabricate the ultra-thin cross-sectional specimen of the bonding interface. Figure 3 shows the TEM image around the bonding interface. The β-Ga2O3 [010] and diamond [110] directions were slightly twisted because of manual alignment. It revealed that the β-Ga2O3 and diamond surfaces were atomically bonded without interfacial voids or cracks. The β-Ga2O3 and diamond lattices were rarely damaged except for the outermost surface of the β-Ga2O3 film; The distortion is possibly due to oxygen plasma irradiation, interfacial deformation, and/or thermal stress. Generally, oxide layers having low thermal conductivity were generated at the bonding interface when semiconductor substrates formed direct bonding under atmospheric conditions. However, this study realized the direct bonding without any intermediate oxide layers because β-Ga2O3 is an oxide material and diamond never develops an oxide layer. Thus, the bonding interface is almost ideal for efficient heat dissipation from a β-Ga2O3 power device. Moreover, it can facilitate the development of future electronic devices using the β-Ga2O3/diamond interface.
[1]. T. Matsumae, Y. Kurashima, H. Umezawa, and H. Takagi, Scr. Mater., 175, 24–28 (2020).
[2]. T. Suni, K. Henttinen, I. Suni, and J. Mäkinen, J. Electrochem. Soc., 149, G348 (2002) http://jes.ecsdl.org/cgi/doi/10.1149/1.1477209.

Figure 1