In some embodiments, a semiconductor structure includes: a first epitaxial oxide semiconductor layer; a metal layer; and a contact layer adjacent to the metal layer, and between the first epitaxial oxide semiconductor layer and the metal layer. The contact layer can include an epitaxial oxide semiconductor material. The contact layer can also include a region comprising a gradient in a composition of the epitaxial oxide semiconductor material adjacent to the metal layer, or a gradient in a strain of the epitaxial oxide semiconductor material over a region adjacent to the metal layer.

A nitride semiconductor structure includes a Group III nitride semiconductor portion and a Group II-IV nitride semiconductor portion. The Group III nitride semiconductor portion is single crystalline. The Group III nitride semiconductor portion has a predetermined crystallographic plane. The Group II-IV nitride semiconductor portion is provided on the predetermined crystallographic plane of the Group III nitride semiconductor portion. The Group II-IV nitride semiconductor portion is single crystalline. The Group II-IV nitride semiconductor portion contains a Group II element and a Group IV element. The Group II-IV nitride semiconductor portion forms a heterojunction with the Group III nitride semiconductor portion. The predetermined crystallographic plane is a crystallographic plane other than a (0001) plane.

A semiconductor structure can include a substrate comprising a first in-plane lattice constant, a graded layer on the substrate, and a first region of the graded layer comprising a first epitaxial oxide material comprising a second in-plane lattice constant. The graded layer on the substrate can include (Al.sub.x1Ga.sub.1−x1).sub.y1O.sub.z1, wherein x1 is from 0 to 1, wherein y1 is from 1 to 3, wherein z1 is from 2 to 4, and wherein x1 varies in a growth direction such that the graded layer has the first in-plane lattice constant adjacent to the substrate and a second in-plane lattice constant at a surface of the graded layer opposite the substrate. In some cases, a semiconductor structure includes a first region comprising a first epitaxial oxide material; a second region comprising a second epitaxial oxide material; and the graded region located between the first and the second regions.

In some embodiments, a semiconductor structure includes: a first epitaxial oxide semiconductor layer; a metal layer; and a contact layer adjacent to the metal layer, and between the first epitaxial oxide semiconductor layer and the metal layer. The contact layer can include an epitaxial oxide semiconductor material. The contact layer can also include a region comprising a gradient in a composition of the epitaxial oxide semiconductor material adjacent to the metal layer, or a gradient in a strain of the epitaxial oxide semiconductor material over a region adjacent to the metal layer.

In some embodiments, a semiconductor structure includes: a first epitaxial oxide semiconductor layer; a metal layer; and a contact layer adjacent to the metal layer, and between the first epitaxial oxide semiconductor layer and the metal layer. The contact layer can include an epitaxial oxide semiconductor material. The contact layer can also include a region comprising a gradient in a composition of the epitaxial oxide semiconductor material adjacent to the metal layer, or a gradient in a strain of the epitaxial oxide semiconductor material over a region adjacent to the metal layer.

An optoelectronic semiconductor component has a first semiconductor layer of a p-conductivity type, a second semiconductor layer of an n-conductivity type and also an n-doped current distribution layer containing ZnSe and adjoining the second semiconductor layer.

An optoelectronic semiconductor component has a first semiconductor layer of a p-conductivity type, a second semiconductor layer of an n-conductivity type and also an n-doped current distribution layer containing ZnSe and adjoining the second semiconductor layer.

In some embodiments, a semiconductor structure includes: a first region comprising a first epitaxial oxide material; a second region comprising a second epitaxial oxide material; and a chirp layer located between the first and the second regions. The chirp layer can include alternating layers of a plurality of wide bandgap epitaxial oxide material layers (WBG layers) and a plurality of narrow bandgap epitaxial oxide material layers (NBG layers), wherein thicknesses of the NBG layers and the WBG layers change throughout the chirp layer. The WBG layer can comprise (Al.sub.x1Ga.sub.1−x1).sub.y1O.sub.z1, wherein x1 is from 0 to 1, wherein y1 is from 1 to 3, and wherein z1 is from 2 to 4. The NBG layer can comprise (Al.sub.x2Ga.sub.1x−2).sub.y2O.sub.z2, wherein x2 is from 0 to 1, wherein y2 is from 1 to 3, and wherein z2 is from 2 to 4, and wherein x1 and x2 are different from one another.

In some embodiments, a semiconductor structure includes: a first epitaxial oxide semiconductor layer; a metal layer; and a contact layer adjacent to the metal layer, and between the first epitaxial oxide semiconductor layer and the metal layer. The contact layer can include an epitaxial oxide semiconductor material. The contact layer can also include a region comprising a gradient in a composition of the epitaxial oxide semiconductor material adjacent to the metal layer, or a gradient in a strain of the epitaxial oxide semiconductor material over a region adjacent to the metal layer.

An electro-absorption modulator of the invention is an electro-absorption modulator which is formed on an InP substrate and modulate incident light according to a voltage applied to that modulator, and which comprises a light absorbing layer for absorbing a portion of the incident light by using an electric field generated by the applied voltage; wherein the light absorbing layer is comprised of a ternary or more complex III-V semiconductor mixed crystal that does not contain Al but contains Bi.