Diamond Homoepitaxial Growth Technology toward Wafer Fabrication, Atomically Controlled Surfaces, and Low Resistivity

Abstract

Conspectus Strong covalent bonds provide diamond with superior properties such as higher thermal conductivity, electron/hole mobilities, and wider bandgap than those of other semiconductors. This makes diamonds promising for next-generation power devices, optoelectronics, quantum technologies, and sensors. However, there are still challenges in realizing practical diamond electronic applications. Key issues include controlling the microwave plasma chemical vapor deposition (MPCVD) growth process to achieve a large size, smooth surfaces, and desired conductivity. Standard semiconductor processing techniques like polishing and ion implantation also need improvement for diamonds. This Account outlines three MPCVD growth technologies being investigated at Kanazawa University to address these challenges. First, growth rate enhancement technology was demonstrated for fabricating diamond wafers. By optimizing the reactor design, electric fields, gas composition, and substrate positioning, record high diamond CVD growth rates over 250 μm/h were achieved without nitrogen addition while maintaining excellent crystal quality. Furthermore, nitrogen addition and optimized CVD growth conditions enabled higher growth rates up to 432 μm/h. A 0.1 mm thick freestanding diamond plate was fabricated using the growth enhancement technology, exhibiting crystallinity comparable to high-pressure, high-temperature (HPHT) substrates and superior to commercial CVD substrates, as evidenced by X-ray diffraction measurements. Scaling up to larger areas remains a key challenge. Second, diamond surfaces were controlled at the atomic level by a growth mode adjustment. There are three main growth modes for homoepitaxial diamond (111): lateral growth, 2D island growth, and 3D growth. By precisely controlling growth parameters like methane concentration and misorientation direction/angle, the three different growth modes from lateral to 3D could be accessed on HPHT Ib (111) mesa substrates. The lateral growth mode was extended from micrometer mesa to millimeter substrate scales. It enabled atomically flat diamond surfaces over full substrates through optimized lateral growth conditions. Finally, the growth technique was expanded to impurity doping technology toward conductivity control. Heavily boron-doped diamond films ([B] > 1020 cm–3) were grown at 30 μm/h, about 5× faster than previous reports. Freestanding plates with controllable resistivity from 100 Ω cm (semiconductor) to 10–2 Ω cm (metallic) were fabricated by varying the boron doping level. Delta-doped layers with alternating high and low boron concentrations were fabricated using a lateral growth mode. Atomically flat surface was maintained even with delta-doping layers. Delta doping enabled 12× higher carrier concentration and 7× higher mobility compared to uniform doping. Furthermore, lateral growth embedding within lightly nitrogen-doped layers demonstrated the precise 3D positioning of heavily boron-doped regions. By combining lateral growth mode control with modulated doping levels,...