*4.3. Challenges of Epitaxially Growing GaN on Diamond*

The GaN epitaxy allows the deposition of the GaN layers directly onto high thermally conductive diamond substrates without the need of any dielectric layer. However, the required AlN nucleation and stress-relief AlGaN/GaN stacked layers with lower κ will themselves hamper the flow of the heat from the top HEMT structures to the back of the diamond substrates. In addition, if the epitaxy is performed on SCD substrates, the area will be limited to a few mm<sup>2</sup> . This means that the true benefit of GaN epitaxy is doubtful when compared with the direct bonding of the HEMT structures and the diamond substrates.

A breakthrough has been achieved with the ELO of the GaN layers on PCD substrates, thus overcoming the small area availability of the SCD substrates. Nevertheless, if the GaN is deposited directly on the PCD substrate, as proposed by Webster et al. [124], the low thermally conductive stress-relief layers will once again compromise the flow of heat. In the approach followed by Ahmed et al. [137], on the other side, the GaN is initially deposited on a Si substrate and selectively deposited PCD stripes replace the SiN stripes used in [124] for the ELO step. The final integration of the ELO grown GaN epilayers with the thick diamond heat spreaders will require a few more steps: (i) the MOCVD of the HEMT structures, (ii) the removal of the Si substrate and AlGaN/GaN stress-relief layers, and (iii) the direct CVD of the diamond film. While (ii) allows placing the diamond directly in contact with the GaN epilayers, (iii) will require the deposition of a dielectric layer on the exposed GaN, as in the case of direct diamond CVD. The structure of the final GaN-on-diamond wafers fabricated using this method will be similar to the structure of GaN-on-diamond wafers described in Section 3.1, but with a difference, since the diamond stripes embedded in the ELO GaN can be overgrown by the thick diamond without the need of a protective dielectric layer. Despite this improvement, the evaluation of the potential of this approach needs to take into consideration that the fabrication procedure is significantly more complex than that of standard GaN-on-diamond wafers.
