Diamond Epilayers with Subnanometer Surface Roughness for Enhanced Device Performance

Diamond’s exceptional properties make it a promising material for electronics, optoelectronics, and quantum technologies. The development of high-quality and smooth diamond epilayers has emerged as a critical advancement for achieving superior performance and reliability of diamond-based devices. Here, we report the growth of high-quality diamond epilayers with subnanometer surface roughness on two types of commercial substrates, high-pressure high-temperature (HPHT) and chemical vapor deposition (CVD) substrates, via an optimized microwave plasma CVD process. By combining hydrogen plasma etching with elevated growth pressure (∼180 Torr), moderate microwave power (1–1.2 kW), and a practical CH4/H2 ratio (1%), we achieve epilayers exhibiting surface roughness as low as 0.2 nm and improved crystalline quality. Temperature-dependent Hall measurements reveal a clear correlation between surface quality and carrier mobility: epilayers grown on HPHT substrates exhibit four times higher mobility than those grown on CVD substrates due to reduced scattering from surface defects. Diamond field-effect transistors (FETs) fabricated on the epilayer exhibited enhanced performance, with a 7-fold increase in maximum drain current, and 2 orders of magnitude higher on/off ratios compared to those fabricated directly on substrates. This improvement is attributed to higher carrier mobility, resulting from reduced scattering caused by surface defects and surface roughness. These findings not only highlight the transformative potential of ultrasmooth diamond epilayers in advancing diamond electronics but also provide a robust framework for future developments in high-performance photonics and quantum technologies.