A composite substrate includes a polycrystalline ceramic substrate, a silicon substrate directly bonded to the polycrystalline ceramic substrate, a seed crystal film formed on the silicon substrate by vapor phase process and made of a nitride of a group 13 element, and a gallium nitride crystal layer grown on the seed crystal film by flux method.

A composite substrate includes a polycrystalline ceramic substrate, a silicon substrate directly bonded to the polycrystalline ceramic substrate, a seed crystal film formed on the silicon substrate by vapor phase process and made of a nitride of a group 13 element, and a gallium nitride crystal layer grown on the seed crystal film by flux method.

An oxychloride infrared nonlinear optical crystal and the preparation method and use thereof, the optical crystal has a general chemical formula of Pb.sub.2+xOCl.sub.2+2x, therein 0<x<0.139 or 0.141<x<0.159 or 0.161<x0.6. The crystal is non-centrosymmetric, belongs to orthonormal system with space group of Fmm2, cell parameter is a=35.4963(14)0.05 , b=5.8320(2)0.05 , c=16.0912(6)0.05 . The crystal is prepared by high temperature melt method or flux method. The crystal has a strong second harmonic generation efficiency of 4 times that of KDP (KH.sub.2PO.sub.4) tested by Kurtz method, it is phase machable, transparent in the range of 0.34-7 m. The laser damage threshold is 10 times that of the current commercial infrared nonlinear optical crystal AgGaS.sub.2. No crystalline water exists in lead oxychloride, and it is stable in the air and has good thermal stability.

An underlying substrate including a seed crystal layer of a group 13 nitride, wherein projections and recesses repeatedly appear in stripe shapes at a principal surface of the seed crystal layer, and the projections have a level difference of 0.3 to 40 m and a width of 5 to 100 m, and the recesses have a bottom thickness of 2 m or more and a width of 50 to 500 m.

An underlying substrate including a seed crystal layer of a group 13 nitride, wherein projections and recesses repeatedly appear in stripe shapes at a principal surface of the seed crystal layer, and the projections have a level difference of 0.3 to 40 m and a width of 5 to 100 m, and the recesses have a bottom thickness of 2 m or more and a width of 50 to 500 m.

A Group-III element nitride semiconductor substrate includes a first surface and a second surface. A minimum value of a specific resistance in the first surface is 1?10.sup.7 ?.Math.cm or more, and the minimum value of the specific resistance in the first surface is 0.01 or more times as large as a maximum value of the specific resistance in the first surface.

A Group-III element nitride semiconductor substrate includes: a first surface; and a second surface. The Group-III element nitride semiconductor substrate has a thickness of 200 ?m or more. In one embodiment, the number N of times of light-and-dark switching in a line segment having a length of 2 mm, which is drawn in a crossed-Nicols image obtained by observation of a region including a central portion of a surface of the first surface with a polarizing microscope, is 50 or more.

The purpose of quartz homeotypes grown epitaxially on beta quartz for use in pressure sensors or accelerometers is to be able to drastically cut down production costs on otherwise expensive or time-consuming to grow crystals that are necessary in various industrial applications. This is done via epitaxial growth of quartz homeotypes across the whole surface of a sample of beta quartz, an easily accessible and high temperature capable crystal. This invention also applies to the epitaxial application of piezoelectric material atop a piezoelectric crystal for the purpose of altering its piezoelectric coefficient and the epitaxial application of a piezoelectric crystal atop a host crystal for the purpose of increasing its insulation resistance.

An oriented alumina substrate for epitaxial growth according to an embodiment of the present invention includes crystalline grains constituting a surface thereof, the crystalline grains having a tilt angle of 1 or more and 3 or less and an average sintered grain size of 20 m or more.

An oriented alumina substrate for epitaxial growth according to an embodiment of the present invention includes crystalline grains constituting a surface thereof, the crystalline grains having a tilt angle of 1 or more and 3 or less and an average sintered grain size of 20 m or more.