Hydrophilic Bonding of GaN and Diamond Substrates
Authors: Matsumae Takashi , Okita Sho , Fukumoto Shoya , Hayase Masanori , Kurashima Yuichi , Takagi Hideki 
Published: February 2024

Owing to their excellent properties including breakdown voltage and electron sheet charge densities, AlGaN/GaN high electron mobility transistors (HEMT) are gaining more attention for next-generation network. With the demand for high-power and high-frequency operation, as well as miniaturization, the power density has rapidly increased to ~105 W/cm2, which exceeds that on the surface of the sun (~103 W/cm2). Thus, an extremely severe self-heating effect degrades device performance and reliability, limits the power output capability, and reduces the device's lifetime. To overcome this problem, many researchers have studied the integration of the GaN devices with a diamond heat spreader1–3, which can provide a 3 times larger power density over the GaN device fabricated on a SiC substrate.
To bond the GaN substrate with a single crystalline diamond substrate, which has the highest thermal conductivity among solid materials, direct bonding techniques has been developed. Several GaN/diamond bonding process has been proposed4,5; the previous processes requires ultra-high vacuum conditions, which hinders the widespread applications. The purpose of this study is to design a low-temperature bonding process of the GaN and diamond substrates under atmospheric conditions.
Our research group developed the low-temperature bonding of the diamond substrate surface OH-terminated with an NH4OH/H2O2 mixture (i.e. SC1) 6,7 with other semiconductor materials (e.g. Si6,7, InP8, Ga2O3
9). In addition, the SiC substrate OH-terminated by removing the native oxide layer with HF acid could form atomic bonds with Ga2O3 through a ~1-nm-thick amorphous intermediate layer10. We hypothesize that the GaN surface functionalized by chemically removing the oxide layer can form atomic bonds with the OH-terminated diamond surface. In this study, the oxide layer on the GaN substrate was etched with an HCl acid, and then the GaN surface was bonded with the diamond surface that was functionalized with the NH4OH/H2O2 mixture.
3-mm-square diamond substrates having ~3° off angled from diamond (111) surfaces were bonded on 10-mm-square self-standing GaN substrate. The bonding process is shown in the upper side of the attached figure. The diamond substrates were dipped into a mixture of NH4OH/H2O2 at 70 °C for 10 min. It consists of 10 mL of 28% NH4OH solution, 10 mL of 30% H2O2 solution, and 50 mL of deionized water (DI). Our previous study demonstrated that the diamond surface was cleaned and functionalized with OH groups6,7. On the other hand, the GaN substrate was dipped into a 5% HCl solution (10 mL of 35% HCl acid and 60 mL of DI) at room temperature (~20°C) for 5 min. After the wet chemical processes, they were rinsed using DI and dried using a flow of N2 gas. Then, the GaN and diamond surfaces were contacted with each other in a clean room (temperature: 23 °C, relative humidity: 40%). The contacted specimen was annealed at 200 °C for 2 h under a bonding pressure of 1MPa.
The photograph of the diamond substrates bonded onto the GaN substrates is shown in the lower side of the attached figure. As the diamond substrates are transparent, the dispersion caused by the gap between the substrates can be observed. It is supposed that the edges were not bonded due to the warpage of the diamond surface. When the HCl treatment for the GaN surface was at room temperature, the interface was exfoliated by the shear force of 2.12 kgf (2.31 MPa). The MIL-STD 883 requires shear strength over 1 kgf for the 3-mm-square specimen. This indicates that sufficiently strong bonding is achieved by the proposed bonding process.
GaN device fabricated on single crystalline diamond substrate is known as one of the most efficient heat dissipation structures11. This structure requires a low-temperature GaN/diamond bonding process. We have demonstrated the bonding between the GaN and diamond surfaces at 200 °C. The GaN/diamond integration was achieved by the simple bonding process consisting of wet treatments followed by low-temperature annealing; it doesn’t include any vacuum process and fabrication process of adhesion layers. We believe that this easy bonding process would contribute to the widespread applications of the GaN/SCD integrated device that enables extremely high-power and high-frequency operation.

Figure 1