The word epitaxy comes from the Greek word "epi", which means "on top of...". From this word origin, we can easily understand the common expression "GaN on Si", which is the structure of gallium nitride on a silicon substrate. In the preparation process of semiconductor materials, wafer preparation is a core link, which mainly includes two key steps: substrate preparation and epitaxy process. The substrate is a wafer carefully made of semiconductor single crystal materials. It is like the cornerstone of a building and plays a vital basic supporting role in the manufacture of semiconductor devices. On the one hand, the substrate can directly enter the wafer manufacturing link for the production of various semiconductor devices; on the other hand, it can also be used as a basis for epitaxy process to produce epitaxial wafers.

Epitaxy is an extremely delicate and critical process. It refers to the process of growing a new layer of single crystal on a single crystal substrate that has undergone a series of fine processing such as cutting, grinding, and polishing. There are two different relationships between this newly grown single crystal and the substrate. One is that the new single crystal and the substrate are the same material, which is called homoepitaxial growth; the other is that the new single crystal and the substrate are different materials, which we call heteroepitaxial growth. Since this new single crystal layer is extended and grown in the direction of the crystal phase of the substrate, it is figuratively called an epitaxial layer. Its thickness is usually relatively thin, generally a few microns.

Taking silicon as an example, the significance of silicon epitaxial growth is that a layer of crystal with the same crystal orientation as the substrate, but different resistivity and thickness, and good lattice structure integrity is grown on a silicon single crystal substrate with a specific crystal orientation. When the epitaxial layer is successfully grown on the substrate, the whole is called an epitaxial wafer. Simply put, epitaxial wafer = epitaxial layer + substrate. In the actual manufacture of semiconductor devices, if the device is made on the epitaxial layer, we call it positive epitaxy; if the device is made on the substrate, it is called reverse epitaxy. In this case, the epitaxial layer mainly plays a supporting role. Let's take a closer look at the difference between homoepitaxial and heteroepitaxial. In homogeneous epitaxy, the epitaxial layer and the substrate are made of the same material, such as the common Si/Si (silicon/silicon), GaAs/GaAs (gallium arsenide/gallium arsenide), GaP/GaP (gallium phosphide/gallium phosphide) and other combinations. In heterogeneous epitaxy, the epitaxial layer and the substrate are made of different materials, such as Si/Al2O3 (silicon/aluminum oxide), GaS/Si (gallium sulfide/silicon), GaAlAs/GaAs (gallium aluminum arsenic/gallium arsenide), GaN/SiC (gallium nitride/silicon carbide), etc.

So, what key problems does the epitaxial process solve in the field of semiconductor materials? With the continuous development of semiconductor technology, simple bulk single crystal materials have become increasingly difficult to meet the needs of increasingly diverse and complex semiconductor device manufacturing. It was in this context that at the end of 1959, the thin-layer single crystal material growth technology - epitaxial growth technology came into being. Taking silicon as an example, when silicon epitaxial growth technology just appeared, the production of silicon high-frequency and high-power transistors was facing huge difficulties. From the perspective of the basic principles of transistors, in order to obtain high-frequency and high-power performance, two seemingly contradictory requirements must be met at the same time: on the one hand, the collector region breakdown voltage must be high, which requires the collector region material to have a high resistivity; on the other hand, the series resistance must be small, that is, the saturation voltage drop must be small, which in turn requires the collector region material to have a low resistivity. If the series resistance is reduced by simply thinning the collector region material, the Silicon Wafer will become too thin and easy to break, making it impossible to carry out subsequent processing; if the resistivity of the material is reduced, it will conflict with the requirement of high breakdown voltage. The emergence of epitaxial technology has successfully solved this problem that has long plagued the industry.

The specific solution is to grow a high-resistivity epitaxial layer on a substrate with extremely low resistance, and then make the device on the epitaxial layer. In this way, the high-resistivity epitaxial layer can ensure that the tube has a high breakdown voltage, and the low-resistance substrate effectively reduces the resistance of the substrate, thereby reducing the saturation voltage drop, perfectly solving the conflict between these two contradictory requirements. In addition, not only silicon epitaxy technology, but also epitaxy technologies such as vapor phase epitaxy and liquid phase epitaxy of III-V, II-VI and other molecular compound semiconductor materials such as GaAs have made great progress.

Today, these epitaxy technologies have become indispensable key process technologies in the production process of most microwave devices, optoelectronic devices, power devices, etc. In particular, molecular beam epitaxy technology and metal organic vapor phase epitaxy technology, their successful application in thin layer, superlattice, quantum well, strained superlattice, and atomic thin layer epitaxy have laid a solid foundation for the development of a new field of semiconductor research - "band engineering". In practical applications, wide bandgap semiconductor devices are almost all made on the epitaxial layer, while silicon carbide wafers themselves are usually only used as substrates. This fully demonstrates that the control of the epitaxial layer occupies a pivotal position in the wide bandgap semiconductor industry and is an important part of the development of the entire industry. Let's quote the information on the official website of Formosa Plastics to further understand the meaning of "epitaxy". "epi" means above, and "taxy" means regular arrangement. From the literal meaning, "epitaxial" is also called "epitaxial". In the early days, epitaxial wafers were mainly used to improve the quality of components such as bipolar transistors. 

With the continuous advancement of technology, it has also been widely used in Bipolar IC components and MOS processes in recent years. The reason why epitaxial technology is so important in the field of semiconductor materials is that it has seven unique skills.

First, it can epitaxially grow high (low) resistance epitaxial layers on low (high) resistance substrates, and in this way, the electrical properties of the material can be flexibly adjusted to meet the needs of different devices.

Second, it can epitaxially grow N (P) type epitaxial layers on P (N) type substrates to directly form PN junctions. This method avoids the compensation problem that may occur when making PN junctions on single crystal substrates by diffusion method, and greatly improves the quality and performance of PN junctions.

Third, epitaxial technology is combined with mask technology to selectively grow epitaxially in designated areas. This feature creates extremely favorable conditions for the production of integrated circuits and devices with special structures, making the design and manufacture of semiconductor devices more flexible and diverse.

Fourth, during the epitaxial growth process, the type and concentration of doping can be changed according to actual needs. Moreover, the change in concentration can be either abrupt or gradual. This ability to accurately control doping is crucial to optimizing the performance of semiconductor devices.

Fifth, epitaxial technology can grow heterogeneous, multi-layer, and multi-component compounds, and can achieve the growth of ultra-thin layers with variable components. This provides a rich material selection and structural design space for manufacturing high-performance, multifunctional semiconductor devices.

Sixth, epitaxial growth can be carried out at a temperature below the melting point of the material, and the growth rate is controllable. More importantly, it can achieve epitaxial growth of atomic-scale thickness, which enables the manufacture of semiconductor devices to achieve extremely high precision and performance levels.

Seventh, epitaxial technology can grow some single crystal layers that cannot be obtained by traditional pulling methods, such as GaN and ternary and quaternary compounds. This greatly expands the types and application range of semiconductor materials and provides more possibilities for the innovative development of semiconductor technology.

In summary, the substrate and epitaxial layer have a clear division of labor and an indispensable role in semiconductor materials. The existence of the epitaxial layer not only solves many problems in the manufacture of semiconductor devices, but also provides strong support for the continuous innovation and development of semiconductor technology. With the continuous advancement of semiconductor technology, epitaxial technology will continue to improve and develop, bringing us more semiconductor devices with excellent performance and powerful functions.