The influence of nucleation layer growth modulation on the RF loss of Si-based GaN epitaxial wafers

This paper proposes substrate nitridation as an effective method to reduce radio-frequency (RF) loss in Si-based GaN epitaxial wafers. By optimizing the process, an amorphous SiN_xlayer was formed, which effectively blocks the downward diffusion of Al atoms and suppresses the formation of a parasitic conductive channel, thereby leading to a significant reduction in RF loss. Four distinct pre-flow conditions were specifically designed to decouple and modulate the properties of the AlN/Si interface. A detailed analysis of the initial dislocation evolution behavior was conducted, comparing the nitridated substrate with conventional pre-deposited Al processes. Although the nitridation process leads to a moderate increase in threading dislocation density by promoting their parallel propagation, the proposed dislocation coalescence mechanism, supported by our experimental design and analysis, indicates that the spatial extent of individual dislocations and defects is effectively constrained. This results in a substantial improvement in the overall RF electrical characteristics. Based on this proposed process, a coplanar waveguide (CPW) transmission line was fabricated, demonstrating a low RF loss of only -0.6 dB at 40 GHz. These results underscore that the nitridation process is a highly promising pathway for enhancing the RF performance of Si-based GaN materials; more importantly, this study reveals that the advantage of an initially optimized interface must be synergistically integrated and stabilized with subsequent epitaxial processes to achieve low-loss performance in final high-electron-mobility transistor devices, which holds significant implications for the development of high-performance RF devices.