Diamond Homoepitaxial\nGrowth Technology toward Wafer\nFabrication, Atomically Controlled Surfaces, and Low\nResistivity

ConspectusStrong covalent bonds provide\ndiamond with superior properties\nsuch as higher thermal conductivity, electron/hole mobilities, and\nwider bandgap than those of other semiconductors. This makes diamonds\npromising for next-generation power devices, optoelectronics, quantum\ntechnologies, and sensors. However, there are still challenges in\nrealizing practical diamond electronic applications. Key issues include\ncontrolling the microwave plasma chemical vapor deposition (MPCVD)\ngrowth process to achieve a large size, smooth surfaces, and desired\nconductivity. Standard semiconductor processing techniques like polishing\nand ion implantation also need improvement for diamonds. This Account\noutlines three MPCVD growth technologies being investigated at Kanazawa\nUniversity to address these challenges.First, growth rate enhancement\ntechnology was demonstrated for\nfabricating diamond wafers. By optimizing the reactor design, electric\nfields, gas composition, and substrate positioning, record high diamond\nCVD growth rates over 250 μm/h were achieved without nitrogen\naddition while maintaining excellent crystal quality. Furthermore,\nnitrogen addition and optimized CVD growth conditions enabled higher\ngrowth rates up to 432 μm/h. A 0.1 mm thick freestanding diamond\nplate was fabricated using the growth enhancement technology, exhibiting\ncrystallinity comparable to high-pressure, high-temperature (HPHT)\nsubstrates and superior to commercial CVD substrates, as evidenced\nby X-ray diffraction measurements. Scaling up to larger areas remains\na key challenge.Second, diamond surfaces were controlled at\nthe atomic level by\na growth mode adjustment. There are three main growth modes for homoepitaxial\ndiamond (111): lateral growth, 2D island growth, and 3D growth. By\nprecisely controlling growth parameters like methane concentration\nand misorientation direction/angle, the three different growth modes\nfrom lateral to 3D could be accessed on HPHT Ib (111) mesa substrates.\nThe lateral growth mode was extended from micrometer mesa to millimeter\nsubstrate scales. It enabled atomically flat diamond surfaces over\nfull substrates through optimized lateral growth conditions.Finally, the growth technique was expanded to impurity doping technology\ntoward conductivity control. Heavily boron-doped diamond films ([B]\n> 10<sup>20</sup> cm<sup>–3</sup>) were grown at 30 μm/h,\nabout 5× faster than previous reports. Freestanding plates with\ncontrollable resistivity from 100 Ω cm (semiconductor) to 10<sup>–2</sup> Ω cm (metallic) were fabricated by varying\nthe boron doping level. Delta-doped layers with alternating high and\nlow boron concentrations were fabricated using a lateral growth mode.\nAtomically flat surface was maintained even with delta-doping layers.\nDelta doping enabled 12× higher carrier concentration and 7×\nhigher mobility compared to uniform doping. Furthermore, lateral growth\nembedding within lightly nitrogen-doped layers demonstrated the precise\n3D positioning of heavily boron-doped regions. By combining lateral\ngrowth mode control with modulated doping levels, deterministic doping\nfor 3D device architecture fabrication became possible with optimized\nelectronic properties while retaining atomically flat surfaces.Homoepitaxial growth technologies could help overcome current bottlenecks\nand enable the use of diamond in advanced electronic devices and\napplications.