"Gallium nitride (GaN)-on-diamond shows promise as a next-generation semiconductor material"Jianbo Liang

Scientists from Osaka City University, Tohoku University, Saga University, and Adamant Namiki Precision Jewel Co in Japan have developed a gallium nitride (GaN)-on-diamond semiconductor material that can withstand heat of up to 1,000?, in research that could bring about the next generation of highly conductive semiconductors and high-power devices.

GaN-on-diamond offers potential as an innovative semiconductor material due to the wide bandgap of both materials, which allows for high conductivity, while the high thermal conductivity of diamond makes it an excellent heat-spreading substrate. As reported in the journal Advanced Materials [Liang et al. Adv. Mater. (2021) DOI: 10.1002/adma.202104564], this is the first time that direct bonding of GaN and diamond has been achieved.

Increasing the power of electronic devices is limited by the need to identify highly conductive semiconductors that can withstand the high temperature processes required for their manufacture. Previous attempts at producing a GaN-on-diamond structure by combining the two components with some form of transition layer were constrained by this layer interfering with diamond’s thermal conductivity.

However, by fusing the two elements together without an intermediate layer, called “wafer direct bonding”, this problem can be overcome, although to develop a sufficiently high bonding strength such a structure usually needs to be heated to extremely high degrees in a post-annealing process. This can produce cracks in dissimilar materials due to the thermal expansion mismatch.

The team had previously used surface activated bonding (SAB) to fabricate interfaces with diamond at room temperature to provide high thermal stability. Applying the method to GaN and diamond produced bonding was stable even when heated to 1,000?. SAB offers such strong bonds between different materials at room temperature by atomically cleaning and activating the bonding surfaces when brought into contact with each other.

After applying SAB to the GaN-on-diamond material, the team tested the stability the bonding site, or heterointerface. On increasing the annealing temperatures, a decrease in the layer thickness was shown due to a direct conversion of amorphous carbon into diamond at the bonding interface. After annealing at 1,000?, the layer decreased in size, which suggests the intermediate layer can be fully removed by optimizing the annealing process.

With no peeling being observed at the heterointerface after annealing, the findings indicate the GaN/diamond heterointerface can withstand harsh fabrications processes, with the temperature rise in GaN transistors being suppressed by a factor of four. It is expected that the potential applications of such transistors in high-power applications such as radars and inverters will increase. The team now hope to improve the device’s performance and simplify its heat dissipation structure to make the system smaller and lighter.

Attached cross-sectional TEM images of as-bonded and 1,000?-annealed GaN/Diamond interfaces
Attached cross-sectional TEM images of as-bonded and 1,000?-annealed GaN/Diamond interfaces