Some news indicates that a high-tech company in a certain country has developed a new type of substrate material that matches the GaN lattice and can grow GaN well. (Note: It is very difficult to prepare GaN bulk single crystals, so the GaN mentioned here is epitaxial layer, and reveals one of the meanings of the existence of epitaxial layers. Then why is there a difference between the substrate and the epitaxial layer? What is the significance of the existence of the epitaxial layer?

The Origin of the Epitaxial Wafer

Wafer preparation includes two major links: substrate preparation and epitaxial process. The substrate is a wafer made of semiconductor single crystal materials. The substrate can directly enter the wafer manufacturing process to produce semiconductor devices; also it can be processed by epitaxial processing to produce epitaxial wafers. Epitaxy refers to the process of growing a new single crystal on a single crystal substrate that has been carefully processed by cutting, grinding, polishing, etc.

The material of new single crystal and the substrate can be the same or different (homogeneous epitaxy or heterogeneous epitaxy). Because the new single crystal layer grows according to the crystal phase of the substrate, it is called an epitaxial layer (the thickness is usually a few microns, taking silicon as an example: the meaning of silicon epitaxial growth is on a silicon single crystal substrate with a certain crystal orientation. Grow a layer of crystals with the same crystal orientation and different thickness of the crystal lattice structure and good integrity), and the substrate with the epitaxial layer is called the epitaxial wafer (epitaxial wafer = epitaxial layer + substrate). The device made on the epitaxial layer is positive epitaxy. If the device is made on the substrate, it is called reverse epitaxial, and the epitaxial layer only plays a supporting role.

Homogeneous Epitaxy & Heterogeneous Epitaxy

What Problem Does the Epitaxial Layer Solve?

Problem:

Bulk single crystal materials are difficult to meet the growing needs of various semiconductor devices. Therefore, it accelerates the development of thin layer single crystal material epitaxial growth. So what effects does epitaxial technology have on the progress of materials?

For silicon, when the silicon epitaxial growth technology started, it was really a time that the silicon high-frequency and high-power transistors are difficult to manufacture. From the perspective of transistor principle, in order to obtain high frequency and high power, the breakdown voltage of the collector area must be high; the series resistance must be small. That is, the saturation pressure drop should be small. The former requires high electrical resistivity of materials in the collector area, while the latter requires low electrical resistivity of materials. The two contradict each other. Reducing the thickness of the collector area material will make the series resistance decrease, but the silicon wafer will be too thin and fragile to be processed. Reducing the resistivity of the material will contradict the first requirement. So the development of epitaxial technology solves this difficulty successfully.

Solution with epitaxial process:

Grow a high-resistivity epitaxial layer on a substrate with extremely low resistance, and fabricate the device on the epitaxial layer, so that the high-resistivity epitaxial layer ensures a high breakdown voltage in the tube, while the low-resistance substrate reduces the resistance of the substrate, reducing the saturation voltage drop. Thereby, the contradiction between the two is solved.

In addition, the gas phase epitaxy, liquid phase epitaxy and other epitaxy technologies of GaAs and other molecular compound semiconductor materials such as III-V, II-VI and other molecular compound semiconductor materials have been greatly developed, and they have become an indispensable process technology for most microwave devices, optoelectronic devices, power devices, etc., especially molecular beam and metal organic vapor phase epitaxy in thin layers, superlattices, quantum wells, strained superlattices, and atomic-level thin films.

In applications, almost all wide-bandgap semiconductor devices are made on the epitaxial layer, and the silicon carbide wafer itself only serves as the substrate. Furthermore, the control of the epitaxial layer is an important part of the wide-bandgap semiconductor industry.

7 Skills of Epitaxial Technology

1. A high (low) resistance epitaxial layer can be epitaxially grown on a low (high) resistance substrate.
2. An N(P) type epitaxial layer can be epitaxially grown on a P(N) type substrate to directly form a PN junction. There is no problem of compensation when making a PN junction on a single crystal substrate by a diffusion method.
3. In combination with mask technology, the epitaxial growth is carried out in the designated area, which creates conditions for the production of integrated circuits and devices with special structures.
4. The type and concentration of doping can be changed as needed during the epitaxial growth process. The change in concentration can be abrupt or slow.
5. It can grow heterogeneous, multi-layer, multi-component compound and ultra-thin layer with variable composition.
6. The epitaxial growth can be carried out at a temperature lower than the melting point of the material, the growth rate is controllable, and the epitaxial growth of the atomic-scale thickness can be realized.
7. It is possible to grow single crystal materials that cannot be drawn, such as GaN wafer, single crystal layers of tri- or quaternary compounds, etc.

Epitaxial Layers and Epitaxial Processes

In a word, the epitaxial layer is easier to obtain a perfect and controllable crystal structure than the substrate, which is more conducive to the application and development of the material.

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