A semiconductor device includes an epitaxial substrate. The epitaxial substrate includes a substrate. A strain relaxed layer covers and contacts the substrate. A III-V compound stacked layer covers and contacts the strain relaxed layer. The III-V compound stacked layer is a multilayer epitaxial structure formed by aluminum nitride, aluminum gallium nitride or a combination of aluminum nitride and aluminum gallium nitride.

A semiconductor device includes a doped substrate and a seed layer in direct contact with the substrate. The seed layer includes a first seed sublayer having a first lattice structure. The first seed layer is doped with carbon. The seed layer further includes a second seed sublayer over the first see layer, wherein the second seed layer has a second lattice structure. The semiconductor device further includes a graded layer in direct contact with the seed layer. The graded layer includes a first graded sublayer including AlGaN having a first Al:Ga ratio; a second graded sublayer including AlGaN having a second Al:Ga ratio different from the first Al:Ga ratio; and a third graded sublayer over including AlGaN having a third Al:Ga ratio different from the second Al:Ga ratio. The semiconductor device includes a channel layer over the graded layer. The semiconductor device includes an active layer over the channel layer.

An epitaxial structure includes a substrate, a nucleation layer, a buffer layer, and a nitride layer. The nucleation layer is disposed on the substrate, and the nucleation layer consists of a plurality of regions in a thickness direction, wherein a chemical composition of the region is Al.sub.(1−x)In.sub.xN, where 0≤x≤1. The buffer layer is disposed on the nucleation layer, and a thickness of the nucleation layer is less than a thickness of the buffer layer. The nitride layer is disposed on the buffer layer, wherein a roughness of a surface of the nucleation layer in contact with the buffer layer is greater than a roughness of a surface of the buffer layer in contact with the nitride layer.

A semiconductor laminate at least including: a base; a buffer layer; and a crystalline metal oxide semiconductor film containing at least one metal element and having a corundum structure, the semiconductor laminate having the buffer layer on a main surface of the base directly or via another layer, the semiconductor laminate having the crystalline metal oxide semiconductor film on the buffer layer. The buffer layer is a laminate structure of a plurality of buffer films each with a different composition, and at least two buffer films of the plurality of buffer films have a film thickness of 200 nm or more and 650 nm or less.

A SiC epitaxial wafer includes a SiC substrate and a SiC epitaxial layer disposed on the SiC substrate. The SiC epitaxial layer includes a high carrier concentration layer and two low carrier concentration layers having lower carrier concentration than the high carrier concentration layer, and being in contact with a top surface and a bottom surface of the high carrier concentration layer to sandwich the high carrier concentration layer. A difference in carrier concentration between the high carrier concentration layer and the low carrier concentration layers is 5×10.sup.14/cm.sup.3 or more and 2×10.sup.16/cm.sup.3 or less.

A semiconductor structure (100) comprising: a substrate (102), a first layer (106) of Al.sub.XGa.sub.YIn.sub.(1-X-Y)N disposed on the substrate, stacks (107, 109) of several second and third layers (108, 110) alternating against each other, between the substrate and the first layer, a fourth layer (112) of Al.sub.XGa.sub.YIn.sub.(1-X-Y)N, between the stacks, a relaxation layer of AlN disposed between the fourth layer and one of the stacks,
and, in each of the stacks: the level of Ga of the second layers increases from one layer to the next in a direction from the substrate to the first layer, the level of Ga of the third layers is constant or decreasing from one layer to the next in said direction, the average mesh parameter of each group of adjacent second and third layers increasing from one group to the next in said direction, the thickness of the second and third layers is less than 5 nm.

According to one embodiment, a nitride semiconductor includes a nitride member. The nitride member includes a first nitride region including Al.sub.x1Ga.sub.1-×1N (0 < x1 ≤ 1), a second nitride region including Al.sub.x2Ga.sub.1-x2N (0 ≤ x2 < 1), and an intermediate region being between the first nitride region and the second nitride region. In a first direction from the first nitride region to the second nitride region, an oxygen concentration in the nitride member has a peak value at a first position included in the intermediate region. The peak value is 4.9 times or more a first oxygen concentration in the first nitride region. A second carbon concentration in the second nitride region is higher than a first carbon concentration in the first nitride region.

A semiconductor device includes an epitaxial substrate. The epitaxial substrate includes a substrate. A strain relaxed layer covers and contacts the substrate. A III-V compound stacked layer covers and contacts the strain relaxed layer. The III-V compound stacked layer is a multilayer epitaxial structure formed by aluminum nitride, aluminum gallium nitride or a combination of aluminum nitride and aluminum gallium nitride.

A method for producing a nitride semiconductor wafer, in which a nitride semiconductor thin film is grown on a silicon single crystal substrate by vapor phase growth, includes, by using a silicon single crystal substrate having a resistivity of 1000 Ω.Math.cm or more, an oxygen concentration of less than 1×10.sup.17 atoms/cm.sup.3 and a thickness of 1000 μm or more, growing the nitride semiconductor thin film on the silicon single crystal substrate by vapor phase growth. As a result, a method produces a nitride semiconductor wafer in which plastic deformation and warpage are suppressed even in the case of a high-resistivity, ultra-low oxygen concentration silicon single crystal substrate, which is promising as a support substrate for high frequency devices.

A heterostructure, includes: a substrate; and a buffer layer that includes a plurality of layers having a composition Al.sub.xIn.sub.yGa.sub.1-x-yN, where x≤1 and y≥0; wherein the buffer layer has a first region that includes at least two layers, a second region that includes at least two layers, and a third region that includes at least two layers.