BACKGROUND

Technical Field

The disclosure relates to a semiconductor structure, and more particularly, to an epitaxial structure.

Description of Related Art

Since the film formed by the epitaxy process has advantages of high purity, good thickness control, etc., it has been widely applied to the production of radio-frequency (RF) components or power components.

In the epitaxial structure adopted in the general RF component, an aluminum nitride (AlN) layer as a nucleation layer is formed on the Si-substrate before the epitaxial process is performed. However, spontaneous polarization induced by the nucleation layer material itself often occurs at the interface between the nucleation layer and the Si-substrate, or lattice mismatch between the nucleation layer and the substrate may induce piezoelectric polarization, which results in the presence of a parasitic channel. As a result, the RF loss is increased. In addition, the aluminum atoms in the nucleation layer are prone to diffusion to the surface of the Si-substrate, which results in formation of a highly conductive layer, generates leakage current, and affects the RF component characteristics.

SUMMARY

The disclosure provides an epitaxial structure which can solve the diffusion of metal atoms in a nucleation layer to a substrate as in a conventional epitaxial structure, reduce the RF loss, and thus does not affect the RF component characteristics.

An epitaxial structure of the disclosure includes a substrate, a nucleation layer, a buffer layer, and a nitride layer. The nucleation layer is disposed on the substrate. The nucleation layer consists of a plurality of regions in a thickness direction, and a chemical composition of the regions is Al(1−x)InxN, where 0≤x≤1. The x value in the chemical composition of each four of the regions consists of four sections of variation along the thickness direction. The four sections of variation comprises a first fixed region of a maximum value, a first gradient region gradually changing from the maximum value to a minimum value, a second fixed region of the minimum value, and a second gradient region gradually changing from the minimum value to the maximum value, wherein an absolute value of a gradient slope of the first gradient region and the second gradient region is 0.1%/nm to 50%/nm. The buffer layer is disposed on the nucleation layer, and the nitride layer is disposed on the buffer layer. A thickness of the nucleation layer is less than a thickness of the buffer layer. 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.

In an embodiment of the disclosure, the absolute value of the gradient slope of the first gradient region and the second gradient region is 0.5%/nm to 10%/nm.

In an embodiment of the disclosure, an initial content of the x value of the nucleation layer is 10% to 100%, an end content of the x value is 0% to 90%, and an initial content of the (1−x) value is 0% to 90%, and an end content of the (1−x) value is 10% to 100%, wherein the initial content of the x value is located on a bottom portion of the nucleation layer close to the substrate, and the end content of the x value is located on a top portion of the nucleation layer close to the buffer layer.

In an embodiment of the disclosure, the initial content of the x value of the nucleation layer is 50% to 100%, the end content of the x value is 0% to 50%, the initial content of the (1−x) value is 0% to 50%, and the end content of the (1−x) value is 50% to 100%.

In an embodiment of the disclosure, the thickness of the nucleation layer is 1 nm to 500 nm.

In an embodiment of the disclosure, the thickness of the nucleation layer is 1 nm to 50 nm.

In an embodiment of the disclosure, a number of the plurality of regions of the nucleation layer is 4 to 100.

In an embodiment of the disclosure, a number of the plurality of regions of the nucleation layer is 4 to 20.

In an embodiment of the disclosure, the roughness of the surface of the nucleation layer in contact with the buffer layer is 1 nm to 10 nm.

In an embodiment of the disclosure, the roughness of the surface of the nucleation layer in contact with the buffer layer is 1 nm to 3 nm.

Based on the above, in the epitaxial structure of the disclosure, the nucleation layer includes a plurality of regions, the chemical composition of each of the regions is Al(1−x)InxN, and the x value has different types of variation along the thickness direction. Therefore, it is possible to solve the issue of the presence of the parasitic channel resulting from large spontaneous polarization as well as piezoelectric polarization generated by lattice mismatch in the nucleation layer and the Si-substrate in the conventional epitaxial structure. Moreover, it is also possible to solve the issue of reduced interface resistance caused by the diffusion of atoms of the nucleation layer to the Si-substrate, which further reduces the RF loss without affecting the RF component characteristics.

To make the aforementioned more comprehensible, several embodiments accompanied with drawings are described in detail as follows.

