Growth Mechanism of 2-Inch High-Quality Heteroepitaxial Diamond Free-Standing Wafers on Sapphire for High-Power Diamond FETs

Diamond semiconductors, owing to their wide bandgap of 5.47 eV and superior properties compared to SiC and GaN, can be used to develop high-power devices. Recently, we have demonstrated high-quality two-inch-diameter heteroepitaxial diamond free-standing wafer with X-ray rocking curve (XRC) FWHM of 98 arcsec for (004) diffraction and threading dislocation density of 1~2x10⁷ cm^-2 [1,2].<br/>First, we investigated why on the sapphire substrate high-quality heteroepitaxial diamond can be grown, in comparison with on conventional MgO substrate by out-of-plane and in-plane X-ray diffraction measurements of the growth-direction and in-plane lattice constants of respective diamond, Ir, layers. We have found that residual <i>tensile</i> stress and <i>convex</i> crystal curvature in diamond on Ir/sapphire were much less than on conventional Ir/MgO, and residual <i>compressive</i> stress and <i>convex</i> crystal curvature in Ir buffer layer grown on sapphire substrate were much less than those on MgO substrate. The reason for these is that coefficient of thermal expansion of sapphire substrate (<i>a</i>: 4.2x10^-6 K^-1, <i>c</i>: 5.3x10^-6 K^-1) is much less than that of MgO substrate (12.8x10^-6 K^-1). Further, we have observed diamond nucleus-coalescence process starts at the earlier stage on Ir/sapphire than on Ir/MgO. This will lead to a lower dislocation density in diamond on Ir/sapphire than Ir/MgO. Finally, we have fabricated diamond FETs on the heteroepitaxial diamond grown on Ir/sapphire, which showed both high drain current of 288 mA/mm and high off-state drain voltage of 2608 V, consequently Baliga figure of merits, the available output power density, of 345 MW/cm².<br/>[1] S.-W. Kim, M. Kasu <i>et al.</i>, Appl. Phys. Lett. <u>117</u>, 202102 (2020).<br/>[2] S.-W. Kim, M. Kasu <i>et al.</i>, Appl. Phys. Express <u>14</u>, 115501 (2021).