Page 24 - PEN eBook July 2022

Semiconductors Semiconductors

to the defects and poor crystallinity. The thick buffer with low thermal conductivity adds significant
thermal resistance to the heat dissipation path from the device to the substrate, as most of the
heat is generated within the active layer at the top. Defect and boundary scatterings within the

transition layer, at the interface between the substrate and transition layer, and by near-interfacial
disorder contribute together to large thermal resistance.

Though there are choices of substrates that can be used for growing GaN epi, some are not
foundry-friendly, whereby CMOS processes are used. Another reason is that the lithography tools
and other tools for making CMOS devices that are state of the art are available only on larger-scale
wafers. Hence, GaN-on-Si with wafer sizes of up to 12 inches has advantages. GaN-on-sapphire at
6 inches is relatively inexpensive; however, many foundries do not accept sapphire, and its thermal Figure 1: GaN manufacturing
conductivity is poor.
WHAT NEW TECHNOLOGIES CAN HELP SOLVE THESE PROBLEMS?

To grow high-quality GaN, expensive substrates such as bulk GaN and SiC are required. Therefore, Until now, there has been no easy way to remove the SiC or Si substrate from this device structure,
the production cost for the device manufacturing is significantly higher than Si-based electronics. To and hence, the device was very expensive.
achieve cost-effective state-of-the-art GaN power device performance while efficiently managing
the generated heat, the epi layer could be removed from the substrate, enabling substrate reuse, The invention of remote epitaxy and 2D material-based layer transfer (2DLT) technology at MIT
and directly bonded to a heatsink to improve device thermal performance. However, existing made it possible to grow the compound materials through the 2D material. Once grown, it can be
removal processes such as those involving photoelectrochemical etching, mechanical spalling, lifted off to release the substrate from it and reuse.

and laser interface decomposition suffer from slow processing speed and/or significant surface
roughening/cracking, limiting the process yield and practicality of substrate reuse. Therefore, With this technology, one can create the GaN epi layer and lift it off from the expensive SiC
the process cost of these conventional methods typically exceeds GaN substrate cost, limiting substrate and transfer it onto a low-cost substrate. This will free up the SiC substrate to be reused
manufacturing. in the next GaN epi wafer growth (see Figure 1).

When the device needs better quality, in terms of dislocation density, thermal properties, and The advantages of remote epitaxy and 2DLT solutions are instantaneous liftoff of the GaN thin film
higher frequencies that are needed for high-voltage devices in power for automotive, RF, and without any polishing or other post-processing step. No polycrystalline or amorphous regions will
data-center applications, GaN-on-SiC tends to be the way to go. However, GaN-on-SiC is an be introduced by the bonding or exfoliation process. No nucleation layer with poor crystallinity is
expensive solution. Once the good-quality GaN epi layer is grown on the SiC substrate, you will get required, so it is possible to obtain ultrathin (<200 nm) GaN freestanding membranes. This is not

a better GaN device for power and RF applications. The drawback is that SiC substrates are very possible with any other existing technology.
expensive. The SiC substrate is no longer needed after the GaN epi layer is grown on top of it.
The new age of GaN has started. Remote epitaxy and 2DLT are enabling the technology to scale
To summarize: the GaN to a larger size, improve the quality by reducing the dislocation density, and help manage
thermal properties at low costs.
▶ Large GaN wafers of current technology have higher dislocation density (poor
crystallinity).
Reference

▶ GaN-on-Si wafers tend to use very thick buffers and interlayers to manage the stress,
making it difficult to manage thermal conductivity.
▶ PowerUP Expo Virtual Conference

▶ Most other substrates are very expensive, and scaling to larger wafers is not an option.

28 JULY 2022 | www.powerelectronicsnews.com JULY 2022 | www.powerelectronicsnews.com 29