Base material on which single-crystal diamond is grown comprised of a base substrate, bonded single-crystal MgO layer, and heteroepitaxial film, and method for manufacturing a single-crystal diamond substrate on the base material

A method for manufacturing a single-crystal diamond substrate, comprising at least:

• a step of preparing a base substrate that consists of a material having a linear expansion coefficient smaller than that of MgO and not smaller than 0.5×10−6/K;
• a step of bonding a single-crystal MgO substrate having a thickness of 200 to 1000 μm to the prepared base substrate through a bonding layer obtained by forming on a bonding interface a film of an element or compound selected from the group consisting of gold (Au), platinum (Pt), titanium (Ti), chromium (Cr), iridium (Ir), rhodium (Rh), silicon (Si), and silicon oxide (SiO2), or a laminated film formed of these films, the bonding layer having a thickness of 0.001 to 1000 μm, and thinning the single-crystal MgO substrate by mechanical polishing to obtain a single-crystal MgO layer;
• a step of heteroepitaxially growing a film constituted of any one of an iridium film, a rhodium film, and a platinum film on the bonded single-crystal MgO layer;
• a step of heteroepitaxially growing single-crystal diamond on the heteroepitaxially grown film;
• and
• a step of separating the heteroepitaxially grown single-crystal diamond to obtain a single-crystal diamond substrate.