To reduce ungrown region or abnormal grain growth region in growing a Group III nitride semiconductor through a flux method. A seed substrate has a structure in which a Group III nitride semiconductor layer is formed on a ground substrate as a base, and a mask is formed on the Group III nitride semiconductor layer. The mask has a plurality of dotted windows in an equilateral triangular lattice pattern. A Group III nitride semiconductor is grown through flux method on the seed substrate. Carbon is placed on a lid of a crucible holing the seed substrate and a molten mixture so that carbon is not contact with the molten mixture at the start of crystal growth. Thereby, carbon is gradually added to the molten mixture as time passes. Thus, ungrown region or abnormal grain growth region is reduced in the Group III nitride semiconductor crystal grown on the seed substrate.

A single-crystal diamond including: a growth striation parallel to a main growth surface; and a low-index crystal plane, wherein the growth striation in at least part of the single-crystal diamond has an off-cut angle with respect to the low-index crystal plane.

A single-crystal diamond, wherein the single-crystal diamond has a nitrogen content based on the number of atoms of more than 0.1 ppm and 50 ppm or less, the single-crystal diamond has a boron content based on the number of atoms of 0.1 ppm or less, the single-crystal diamond has an average of a phase difference per unit thickness of 20 nm/mm or less, and the phase difference has a standard deviation of 10 nm/mm or less.

A single-crystal diamond, wherein the single-crystal diamond has a nitrogen content based on the number of atoms of more than 0.1 ppm and 50 ppm or less, the single-crystal diamond has a boron content based on the number of atoms of 0.1 ppm or less, the single-crystal diamond has an average of a phase difference per unit thickness of 20 nm/mm or less, and the phase difference has a standard deviation of 10 nm/mm or less.

A single crystal diamond having a half width of an x-ray diffraction rocking curve of 20 seconds or less, a half width of a peak at a Raman shift of 1332 cm.sup.?1 to 1333 cm.sup.?1 (inclusive) in a Raman spectroscopic spectrum of 2.0 cm.sup.?1 or less, an etch pit density of 10,000/cm.sup.2 or less, a content of nitrogen based on number of atoms of 0.0001-0.1 ppm (inclusive), and a content of .sup.13C based on number of atoms of 0.01-1.0% (inclusive).

An alumina substrate wherein an AlN layer is formed on a surface of the alumina substrate and a rare earth elements-containing layer and/or rare earth elements-containing regions is/are formed in the interior of the AlN layer or in the interface between the AlN layer and the alumina substrate.

An alumina substrate wherein an AlN layer is formed on a surface of the alumina substrate and a rare earth elements-containing layer and/or rare earth elements-containing regions is/are formed in the interior of the AlN layer or in the interface between the AlN layer and the alumina substrate.

We provide methods and apparatus for preparing crystalline-clad and crystalline core optical fibers with minimal or no breakage by minimizing the influence of thermal stress during a liquid phase epitaxy (LPE) process as well as the fiber with precisely controlled number of modes propagated in the crystalline cladding and crystalline core fiber via precisely controlling the diameter of crystalline fiber core with under-saturated LPE flux. The resulting crystalline cladding and crystalline core optical fibers are also reported.

We provide methods and apparatus for preparing crystalline-clad and crystalline core optical fibers with minimal or no breakage by minimizing the influence of thermal stress during a liquid phase epitaxy (LPE) process as well as the fiber with precisely controlled number of modes propagated in the crystalline cladding and crystalline core fiber via precisely controlling the diameter of crystalline fiber core with under-saturated LPE flux. The resulting crystalline cladding and crystalline core optical fibers are also reported.

There is provided a method for manufacturing a nitride crystal substrate, including: arranging a plurality of seed crystal substrates made of a nitride crystal in a planar appearance, so that their main surfaces are parallel to each other and their lateral surfaces are in contact with each other; growing a first crystal film using a vapor-phase growth method on a surface of the plurality of seed crystal substrates arranged in the planar appearance, and preparing a combined substrate formed by combining the adjacent seed crystal substrates each other by the first crystal film; growing a second crystal film using a liquid-phase growth method on a main surface of the combined substrate so as to be embedded in a groove that exists at a combined part of the seed crystal substrates, and preparing a substrate for crystal growth having a smoothened main surface; and growing a third crystal film using the vapor-phase growth method, on the smoothed main surface of the substrate for crystal growth.