In SiC single crystal production by the solution process, an alloy of silicon (Si) and a metallic element M that increases the solubility of carbon (C) is pre-impregnated into a SiC sintered body having a relative density of 50 to 90%, following which Si and M are placed in a SiC crucible made of the SiC sintered body and the Si and M within the SiC crucible are melted, forming a Si—C solution. With heating, SiC from the SiC sintered body dissolves into the Si—C solution, efficiently supplying Si and C to the Si—C solution. As a result, Si and C are supplied uniformly and in the proper amount from all areas of contact between the SiC crucible and the Si—C solution, enabling a high-quality SiC single crystal to be stably produced over a long time at a rapid growth rate.

A layer of a crystal of a group 13 nitride selected from gallium nitride, aluminum nitride, indium nitride and the mixed crystals thereof has an upper surface and a bottom surface. The upper surface includes a linear high-luminance light-emitting part and a low-luminance light-emitting region adjacent to the high-luminance light-emitting part. The high-luminance light-emitting part includes a portion extending along an m-plane of the crystal of the group 13 nitride. A normal line to the upper surface has an off-angle of 2.0° or less with respect to <0001> direction of the crystal of the nitride of the group 13 element.

A synthetic single crystal diamond containing 100 ppm or more and 1500 ppm or less of nitrogen atoms, in which the synthetic single crystal diamond contains aggregates each composed of one vacancy and two to four nitrogen atoms present adjacent to the vacancy, a ratio b/a of a length b of a short diagonal line to a length a of a long diagonal line of diagonal lines of a Knoop indentation in a <110> direction in a {001} plane of the synthetic single crystal diamond is 0.08 or less, and the Knoop indentation is formed by measuring Knoop hardness in the <100> direction in the {001} plane of the synthetic single crystal diamond according to JIS Z 2251: 2009 under conditions of a temperature of 23° C.±5° C. and a test load of 4.9 N.

A synthetic single crystal diamond containing 100 ppm or more and 1500 ppm or less of nitrogen atoms, in which the synthetic single crystal diamond contains aggregates each composed of one vacancy and two to four nitrogen atoms present adjacent to the vacancy, a ratio b/a of a length b of a short diagonal line to a length a of a long diagonal line of diagonal lines of a Knoop indentation in a <110> direction in a {001} plane of the synthetic single crystal diamond is 0.08 or less, and the Knoop indentation is formed by measuring Knoop hardness in the <100> direction in the {001} plane of the synthetic single crystal diamond according to JIS Z 2251: 2009 under conditions of a temperature of 23° C.±5° C. and a test load of 4.9 N.

A synthetic single crystal diamond containing nitrogen atoms at a concentration of 100 ppm or more and 1500 ppm or less based on atom numbers, in which the synthetic single crystal diamond contains aggregates each composed of one vacancy and three substitutional nitrogen atoms present adjacent to the vacancy, and a Raman shift λ′ cm.sup.−1 of a peak in a first-order Raman scattering spectrum of the synthetic single crystal diamond and a Raman shift λ cm.sup.−1 of a peak in a first-order Raman scattering spectrum of a synthetic type IIa single crystal diamond containing nitrogen atoms at a concentration of 1 ppm or less based on atom numbers show a relationship of the following formula 1,

λ′−λ≥0  Formula 1.

Provided is a synthetic single-crystal diamond containing nitrogen. In an X-ray absorption fine structure thereof, a ratio I.sub.405/I.sub.412 between an intensity I.sub.405 of a peak which appears at an energy of 405±1 eV and has a full width at ¾ maximum of 3 eV or more and an intensity I.sub.412 of a peak which appears at an energy of 412±2 eV is less than 1.5

Provided is a synthetic single-crystal diamond containing nitrogen. In an X-ray absorption fine structure thereof, a ratio I.sub.405/I.sub.412 between an intensity I.sub.405 of a peak which appears at an energy of 405±1 eV and has a full width at ¾ maximum of 3 eV or more and an intensity I.sub.412 of a peak which appears at an energy of 412±2 eV is less than 1.5

Provided is a synthetic single crystal diamond containing nitrogen atoms at a concentration of more than 600 ppm and 1500 ppm or less. The Raman shift λ′ (cm.sup.−1) of a peak in a primary Raman scattering spectrum of the synthetic single crystal diamond and the Raman shift λ (cm.sup.−1) of a peak in a primary Raman scattering spectrum of a synthetic type IIa single crystal diamond containing nitrogen atoms at a content of 1 ppm or less satisfy the following expression (1):
λ′−λ≥−0.10  (1).

Provided is a synthetic single crystal diamond containing nitrogen atoms at a concentration of more than 600 ppm and 1500 ppm or less. The Raman shift λ′ (cm.sup.−1) of a peak in a primary Raman scattering spectrum of the synthetic single crystal diamond and the Raman shift λ (cm.sup.−1) of a peak in a primary Raman scattering spectrum of a synthetic type IIa single crystal diamond containing nitrogen atoms at a content of 1 ppm or less satisfy the following expression (1):
λ′−λ≥−0.10  (1).

A self-supporting substrate includes a first nitride layer grown by hydride vapor deposition method or ammonothermal method and comprising a nitride of one or more element selected from the group consisting of gallium, aluminum and indium; and a second nitride layer grown by a sodium flux method on the first nitride layer and comprising a nitride of one or more element selected from the group consisting of gallium, aluminum and indium. The first nitride layer includes a plurality of single crystal grains arranged therein and being extended between a pair of main faces of the first nitride layer. The second nitride layer includes a plurality of single crystal grains arranged therein and being extended between a pair of main faces of the second nitride layer. The first nitride layer has a thickness larger than a thickness of the second nitride layer.