The first generation of semiconductor technology was developed based on semiconductor materials such as Si and Ge, which served as the material foundation for the development of transistors and integrated circuit technology. The first generation of semiconductor materials laid the foundation for the electronic industry in the 20th century and are the fundamental materials for integrated circuit technology.

The second-generation semiconductor materials mainly include gallium arsenide, indium phosphide, gallium phosphide, indium arsenide, aluminum arsenide and their ternary compounds. The second-generation semiconductor materials are the foundation of the optoelectronic information industry, and have developed related industries such as lighting, display, laser, photovoltaic, etc. on this basis. They are widely used in contemporary information technology and optoelectronic display industry.

Representative materials of third-generation semiconductor materials include gallium nitride, silicon carbide, etc. Due to their wide bandgap, high electron saturation drift velocity, high thermal conductivity, and large breakdown field strength, they are ideal materials for preparing high-power density, high-frequency, and low loss electronic devices. Silicon carbide power devices have the advantages of high energy density, low energy consumption, and small size, and have broad application prospects in fields such as new energy vehicles, photovoltaics, rail transit, and big data. Gallium nitride RF devices have the advantages of high frequency, high power, wide frequency band, low power consumption, and small size, and have broad application prospects in fields such as 5G communication, the Internet of Things, and military radar. In addition, gallium nitride based power devices have been widely used in the low-voltage field. In addition, emerging gallium oxide materials in recent years are expected to complement existing SiC and GaN technologies, with potential applications in low-frequency and high-voltage fields.

Compared with second-generation semiconductor materials, third-generation semiconductor materials have wider bandgap widths (the typical bandgap width of the first generation semiconductor material Si is about 1.1eV, the typical bandgap width of the second generation semiconductor material GaAs is about 1.42eV, and the typical bandgap width of the third generation semiconductor material GaN is above 2.3eV), stronger radiation resistance, stronger resistance to electric field breakdown, and higher temperature resistance. Third generation semiconductor materials with wider bandgap are particularly suitable for producing radiation resistant, high-frequency, high-power, and high-density integrated electronic devices. Their applications in microwave and radio frequency devices, LEDs, lasers, power devices, and other fields have attracted much attention, and they have shown broad development prospects in mobile communications, smart grids, rail transportation, new energy vehicles, consumer electronics, as well as ultraviolet and blue-green light devices.

圖片來源：CASA,，浙商證券研究所

圖表 1 GaN功率器件時長規(guī)模及預測

GaN is a direct bandgap semiconductor, with a bandgap width of approximately 3.26 eV in the wurtzite structure at room temperature. GaN materials mainly have three crystal structures, namely wurtzite structure, sphalerite structure, and rock salt structure. Among them, the wurtzite structure is the most stable crystal structure. Figure 2 shows the hexagonal wurtzite structure of GaN. The wurtzite structured GaN material belongs to a hexagonal dense packing structure, with 12 atoms per unit cell, including 6 N atoms and 6 Ga atoms. Each Ga (N) atom is bonded to the nearest 4 N (Ga) atoms and stacked in ABABAB... order along the [0001] direction [2].

圖表 2 纖鋅礦結構GaN晶胞圖

It seems that homoepitaxy on GaN substrates is the optimal choice for GaN epitaxy. However, due to the high bond energy of GaN, its corresponding decomposition pressure is about 4.5GPa when the temperature reaches the melting point of 2500 ℃. When the decomposition pressure is lower than this pressure, GaN does not melt but directly decomposes. This makes mature substrate preparation techniques such as the Czochralski method unsuitable for the preparation of GaN single crystal substrates, making GaN substrates difficult to mass produce and costly. Therefore, the substrates commonly used in GaN epitaxial growth currently include Si, SiC, sapphire, etc. [3].

圖表 3 GaN與常用襯底材料的參數(shù)

Sapphire has stable chemical properties, is inexpensive, and has a high maturity in large-scale production industry. Therefore, it has become one of the earliest applications in semiconductor device engineering and one of the widely used substrate materials. As one of the commonly used substrates for GaN epitaxy, the main problems that sapphire substrates need to solve are:

? Due to the significant lattice mismatch between sapphire (Al2O3) and GaN (about 15%), the defect density at the interface between the epitaxial layer and the substrate is very high. In order to reduce its adverse effects, complex pretreatment must be carried out on the substrate before the epitaxial process begins. Before growing GaN epitaxial on sapphire substrate, strict cleaning treatment of the substrate surface is required to remove pollutants, residual polishing damage, etc., and generate steps and step surface structures. Then, the substrate surface is nitrided to change the wetting characteristics of the epitaxial layer. Finally, a thin AlN buffer layer (usually 10-100nm thick) needs to be deposited on the substrate surface and annealed at low temperature to prepare for the final epitaxial growth. However, compared with homoepitaxial growth, the dislocation density in GaN epitaxial films grown on sapphire substrates is still relatively high (about 1010 cm-2, while the dislocation density in silicon homoepitaxial or gallium arsenide homoepitaxial growth is basically zero, or between 102 and 104 cm-2). The higher defect density reduces carrier mobility, shortens minority carrier lifetime, and lowers thermal conductivity, all of which can lower device performance [4];

? The thermal expansion coefficient of sapphire is greater than that of GaN, so biaxial compressive stress will be generated in the epitaxial layer during the process of reducing the deposition temperature to room temperature. For thick epitaxial films, this stress may cause cracking of the thin film or even the substrate;

?Compared with other substrates, sapphire substrates have lower thermal conductivity (about 0.25W * cm-1 * K-1 at 100 ℃) and poorer heat dissipation performance;

?Sapphire substrates have poor conductivity, which hinders their integration and application with other semiconductor devices.

Although the defect density of GaN epitaxial layer grown on sapphire substrate is relatively high, it does not seem to significantly reduce the photoelectric performance of GaN based blue-green LED. Therefore, sapphire substrate is still a commonly used substrate for GaN based LED.

With the development of more GaN devices such as lasers or other high-density power devices, the inherent defects of sapphire substrates have become increasingly prominent, limiting their applications. In addition, with the development of SiC substrate growth technology, cost reduction, and the maturity of GaN epitaxial technology on Si substrates, more research on growing GaN epitaxial layers on sapphire substrates has gradually shown a cooling trend.

Compared with sapphire, SiC substrate (4H - and 6H - crystal types) has a smaller lattice mismatch with GaN epitaxial layer (3.1%, equivalent to [0001] oriented epitaxial film), higher thermal conductivity (about 3.8W * cm-1 * K-1), etc. In addition, the conductivity of SiC substrate also allows for electrical contacts to be made on the back of the substrate, which helps simplify the device structure. The existence of these advantages is attracting more and more researchers to work on GaN epitaxy on silicon carbide substrates.

However, avoiding the growth of GaN epitaxial layers directly on SiC substrates also faces a series of disadvantages, mainly including:

? The surface roughness of SiC substrate is much higher than that of sapphire substrate (sapphire roughness 0.1nm RMS, SiC roughness 1nm RMS). SiC substrate has high hardness and poor processing performance. This roughness and residual polishing damage are also one of the sources of defects in GaN epitaxial layer.

? SiC substrate has a high density of screw dislocations (dislocation density 103-104cm-2), which may propagate to the GaN epitaxial layer and reduce device performance;

? The atomic arrangement on the substrate surface induces the formation of stacking faults (BSFs) in GaN epitaxial layers. For epitaxial GaN on SiC substrates, there are multiple possible atomic arrangement orders on the substrate, resulting in inconsistent initial atomic stacking order in the epitaxial GaN layer, which can easily lead to stacking faults. Stacking faults (SFs) introduce built-in electric fields along the c-axis, leading to issues such as leakage in in-plane carrier separation devices;

? The thermal expansion coefficient of SiC substrate is smaller than that of AlN and GaN, which leads to thermal stress accumulation between the epitaxial layer and the substrate during the cooling process. Waltereit and Brand predicted based on their research results that this problem can be alleviated or solved by growing GaN epitaxial layer on a thin, coherent strain AlN nucleation layer;

? The problem of poor wettability of Ga atoms is that when directly growing GaN epitaxial layers on SiC surfaces, due to the poor wettability between the two atoms, GaN is prone to 3D island like growth on the substrate surface. Introducing a buffer layer is the most commonly used solution to improve the quality of epitaxial materials in GaN epitaxy. Introducing AlN or AlxGa1 xN buffer layers can effectively improve SiC surface wettability, making the GaN epitaxial layer grow in two dimensions. In addition, it can also regulate stress and block substrate defects from extending to GaN epitaxy;

? The SiC substrate preparation technology is not mature, the substrate cost is high, and there are few suppliers with limited supply.

Torres et al.'s study showed that etching SiC substrates with H2 at high temperature (1600 ℃) before epitaxy can generate more ordered step structures on the substrate surface, resulting in higher quality AlN epitaxial films than those grown directly on the original substrate surface. Xie and his team's research also shows that etching pretreatment of silicon carbide substrates can significantly improve the apparent morphology and crystal quality of GaN epitaxial layers. Smith et al. found that the penetration dislocations originating from the substrate/buffer layer and buffer layer/epitaxial layer interfaces are related to the flatness of the substrate [5].

Figure 4 TEM morphology of GaN epitaxial layer samples grown on 6H SiC substrate (0001) under different surface treatments (a) chemical cleaning; (b) Chemical cleaning+hydrogen plasma treatment; (c) Chemical cleaning+hydrogen plasma treatment+1300 ℃ hydrogen heat treatment for 30 minutes

Compared with substrates such as silicon carbide and sapphire, the preparation process of silicon substrates is mature, which can stably provide mature large-sized substrates with high cost-effectiveness. At the same time, it has good thermal and electrical conductivity, and the Si electronic device process is mature. In the future, the possibility of perfectly integrating optoelectronic GaN devices with Si electronic devices also makes GaN epitaxial growth on silicon very attractive.

However, at the same time, due to the significant difference in lattice constants between Si substrate and GaN material, heteroepitaxy of GaN on Si substrate is a typical large mismatch epitaxy, which also needs to face a series of problems:

? Surface interface energy issue. When GaN grows on a Si substrate, the surface of the Si substrate will first nitride to form an amorphous silicon nitride layer that is not conducive to high-density GaN nucleation growth. Moreover, the Si surface will first come into contact with Ga, which will corrode the surface of the Si substrate. At high temperatures, the decomposition of the Si surface will diffuse into the GaN epitaxial layer, forming black silicon spots.

?  The lattice constant mismatch between GaN and Si is significant (~17%), which leads to the formation of high-density through dislocations and significantly reduces the quality of the epitaxial layer;

?  Compared with Si, GaN has a larger thermal expansion coefficient (GaN has a thermal expansion coefficient of about 5.6 × 10-6K-1, while Si has a thermal expansion coefficient of about 2.6 × 10-6K-1), which may cause cracks in the GaN epitaxial layer during cooling to room temperature;

? Si reacts with NH3 at high temperatures to form polycrystalline SiNx. AlN is unable to form preferentially oriented crystal nuclei on polycrystalline SiNx, resulting in disordered orientation and high defects in the subsequently grown GaN layer, leading to poor crystal quality of GaN epitaxial layer and even difficulty in forming single crystal GaN epitaxial layer [6].

To address the issue of large lattice mismatch, researchers have attempted to introduce materials such as AlAs, GaAs, AlN, GaN, ZnO, SiC, etc. as buffer layers on Si substrates. In order to avoid the formation of polycrystalline SiNx and reduce its adverse effects on the crystal quality of GaN/AlN/Si (111) materials, TMAl is usually pre introduced for a certain period of time before epitaxial growth of AlN buffer layers to prevent NH3 from reacting with the exposed Si surface to form SiNx. In addition, epitaxial techniques such as graphic substrate technology can be used to improve the quality of epitaxial layers. The development of these technologies helps to suppress the formation of SiNx at the epitaxial interface, promote the two-dimensional growth of GaN epitaxial layers, and improve the quality of epitaxial layer growth. In addition, introducing AlN buffer layer to compensate for the tensile stress caused by the difference in thermal expansion coefficient, in order to avoid the formation of cracks in GaN epitaxial layer on silicon substrate. Krost's research shows that there is a positive correlation between the thickness of AlN buffer layer and strain reduction. When the thickness of buffer layer reaches 12nm, by appropriate growth scheme, epitaxial layer with a thickness of more than 6 μ m can be grown on silicon substrate without cracking phenomenon.

After long-term efforts by researchers, the quality of GaN epitaxial layers grown on silicon substrates has been significantly improved, and devices such as field-effect transistors, Schottky barrier ultraviolet detectors, blue-green LEDs, and ultraviolet lasers have made significant progress.

In summary, due to the fact that commonly used GaN epitaxial substrates are heteroepitaxial, they all face common problems such as lattice mismatch and large differences in thermal expansion coefficients to varying degrees. Homogeneous epitaxial GaN substrates are limited by technological maturity, and have not yet achieved large-scale production, resulting in high production costs, small substrate sizes, and unsatisfactory substrate quality. The development of new GaN epitaxial substrates and the improvement of epitaxial quality are still important factors restricting the further development of the GaN epitaxial industry.

It seems that homoepitaxy on GaN substrates is the optimal choice for GaN epitaxy, but due to

The precursors of chemical vapor deposition method are trimethyl gallium and ammonia, and the carrier gas is hydrogen gas. The typical growth temperature of MOCVD method is about 1000-1100 ℃, and the growth rate of MOCVD method is about several micrometers per hour. It can produce atomic level steep interfaces, which is very suitable for growing heterostructures, quantum wells, superlattices and other structures. Its fast growth rate, good uniformity, and suitability for large-scale and multi patch growth are commonly used in industrial production.

Ga in molecular beam epitaxy uses an element source, and active nitrogen is obtained from nitrogen gas through RF plasma. Compared with MOCVD method, MBE growth temperature is about 350-400 ℃ lower, which can avoid some pollution that may be caused by high temperature environment. The MBE system operates under ultra-high vacuum conditions, which allows it to integrate more in-situ detection methods. At the same time, its growth rate and production capacity cannot be compared to MOCVD, and it is more commonly used in scientific research [7].

Figure 5 (a) Eiko MBE schematic diagram (b) MBE main reaction chamber schematic diagram

The precursors of hydride vapor phase epitaxy are GaCl3 and NH3. Detchbrohm et al. used this method to grow GaN epitaxial layers hundreds of micrometers thick on sapphire substrates. In their experiment, a layer of ZnO was grown between the sapphire substrate and the epitaxial layer as a buffer layer, and the epitaxial layer was peeled off from the substrate surface. Compared with MOCVD and MBE, the main feature of HVPE method is its high growth rate, which is suitable for producing thick layers and bulk materials. However, when the thickness of the epitaxial layer exceeds 20 μ m, the epitaxial layer produced by this method is prone to cracking.

Akira USUI introduced graphic substrate technology based on this method. They first grew a thin 1-1.5 μ m thick GaN epitaxial layer on a sapphire substrate using MOCVD method. The epitaxial layer consists of a 20nm thick GaN buffer layer grown under low-temperature conditions and a GaN layer grown under high-temperature conditions. Then, at 430 ℃, a layer of SiO2 is deposited on the surface of the epitaxial layer, and window stripes are fabricated on the SiO2 film through photolithography process, with a stripe spacing of 7 μ m and a mask width ranging from 1 μ m to 4 μ m. After this improvement, they obtained a GaN epitaxial layer on a 2-inch diameter sapphire substrate that remained crack free and mirror smooth even when the thickness increased to tens or even hundreds of micrometers, and the defect density decreased from the traditional HVPE method's 109-1010cm-2 to about 6 × 107cm-2. They also pointed out in the experiment that when the growth rate exceeds 75 μ m/h, the surface of the sample will become rough [8].

Figure 6 Schematic diagram of graphical substrate

GaN materials began to emerge in the field of consumer electronics with their fast charging applications, as they won the Nobel Prize in Physics for blue light LEDs in 2014. In fact, they have quietly emerged in applications such as power amplifiers and RF devices used in 5G base stations that most people cannot see. In recent years, the breakthrough of GaN based automotive grade power devices is expected to open up new growth points for the GaN material application market.

The huge market demand will inevitably drive the great development of GaN related industries and technologies. With the maturity and improvement of the GaN related industry chain, the problems faced by GaN epitaxial technology will eventually be improved or overcome. In the future, people will inevitably develop more new epitaxial technologies and more excellent substrate options to choose from. At that time, people will be able to select the most suitable external research technology and substrate for different application scenarios based on their characteristics, and produce the most competitive customized products.