The first generation of semiconductor materials generally refers to silicon (Si) and germanium (Ge) elements, which laid the foundation for the electronics industry in the 20th century. The second generation semiconductor materials mainly refer to compound semiconductor materials, such as gallium arsenide (GaAs), indium phosphide (InP), gallium phosphide (GaP), indium arsenide (InAs), aluminum arsenide (AlAs) and their alloy compounds Etc., which laid the foundation for the 20th century information optoelectronics industry.
The third generation of wide band gap semiconductor materials generally refers to gallium nitride (GaN), silicon carbide (SiC), aluminum nitride (AlN), diamond and other materials, which have a large band gap, strong radiation resistance, breakdown electric field The characteristics of good strength and high temperature resistance can overcome the disadvantages of traditional semiconductors and enable the equipment to work normally under extremely harsh conditions. Therefore, the materials of wide band gap semiconductors can play an important role in the field of microelectronics and have a broad application market.
Forbidden band width is an important characteristic parameter of semiconductors. According to different energy band structures of semiconductor materials, semiconductor materials can be divided into two types: wide and narrow forbidden bands. If the band gap width of the semiconductor material is less than 2.3eV, it is called a narrow band gap semiconductor. Representative materials are GaAs, Si, Ge, and InP; if the band gap width of the semiconductor material is greater than or equal to 2.3eV, it is called a wide band gap semiconductor Representative materials include GaN, SiC, AlN and aluminum gallium nitride (AlGaN). The larger the forbidden band width of the semiconductor material, the greater the energy required for its electrons to transition to the conduction band, so that the higher the temperature and voltage the material can withstand, the less likely it is to become a conductor.
The wide band gap semiconductor material is very suitable for the manufacture of radiation-resistant, high-frequency, high-power and high-density integrated electronic devices, which have good radiation resistance and chemical stability, high saturation electron drift speed and thermal conductivity, excellent Characteristics such as electrical performance. In recent years, the rapidly developing wide band gap semiconductor materials represented by GaN and SiC are solid-state light sources and "cores" of power electronics and microwave radio frequency devices. , New energy vehicles, consumer electronics and other fields have broad application prospects, and are expected to become the key new materials supporting the development of information, energy, transportation, defense and other industries. The research and development of related technologies for wide band gap semiconductor materials are becoming the global semiconductor industry New strategic heights. SiC and GaN are relatively mature materials in the third generation of wide band gap semiconductor materials. This paper mainly studies these two types of materials.
SiC material and its preparation process
SiC has unique physical and electrical properties. It can produce SiO2 through thermal oxidation process. At the same time, during the oxidation process, the element C is released in the form of gas to prepare high-quality SiO2, and then SiC can be used to produce excellent performance metals – Oxide-Semiconductor (Metal-Oxide-Semiconductor, MOS) transistor.
(1) Structure and characteristics of SiC materials
SiC is a compound composed of Si elements and C elements in the main group IV, and C atoms and Si atoms are connected in the form of covalent bonds. The basic structural unit of SiC is a silicon-carbon tetrahedron, which is interconnected to form various closely packed structures. The stacking order of Si—C diatomic layers is different, resulting in SiC having multiple crystal structures. Among them, the homomorphic polymorphs of SiC mainly include zinc blende (Zincblende) structure, wurtzite (Wurtzite) structure and diamond (Diamond) structure. The wurtzite structure of SiC is α-SiC, and the cubic sphalerite structure of SiC is β-SiC, which exhibits a multi-type structure according to different crystal stacks, of which β-SiC (3C-SiC) and α-SiC (2H- SiC, 4H-SiC, 6H-SiC, 15R-SiC) are more representative. For different crystal structures, the band gap width is also different.
Among the various crystal types of SiC, the bond energy of 3C-SiC is the lowest, the lattice free energy is the highest and it is easy to nucleate, but it is in a metastable state, which has the characteristics of lower stability and solid phase transfer. Under the conditions close to equilibrium, when the annealing temperature is 1200 ℃ and 2000 ℃, 3C-SiC will undergo a phase transition, partly transformed into 6H-SiC and 4H-SiC, in which the bond energy order of the three crystal forms is 3C-SiC <6H-SiC <4H-SiC, the smaller the bond energy is, the more unstable it is, and the easier the phase change will be under the influence of external conditions. Therefore, by changing the external conditions, 3C-SiC can undergo phase transformation and become other crystal forms. At present, 4H-SiC material is mostly used. Its band gap width is 3.2eV, which is about 3 times the width of the Si band gap, and its thermal conductivity is high. Therefore, it is mostly used in the field of high-temperature and high-power microelectronic devices.
(2) Preparation of SiC crystals
There are various polytypes (polycrystalline systems) of SiC, and their physical property values are also different. There are many kinds of defects in SiC crystals, these defects will reduce its own quality. Common types of crystal defects are microtubules, dislocations, stacking faults, inclusions, polymorphism, etc., as shown in Figure 3. Crystal defects have greatly hindered the application of SiC devices. Among these defects, microtube defects have the most serious consequences. Any microtube defects in the working area of the SiC device may cause the device to fail.
Although some electronic components can be fabricated directly on the substrate material without using an epitaxial layer, high-quality SiC devices still require the use of high-quality epitaxial materials to prepare active regions. Therefore, the low defect SiC epitaxial growth technology has an important influence on the quality of SiC devices. With the continuous improvement of SiC power device manufacturing requirements and withstand voltage levels, its epitaxial materials continue to develop toward low defects and thick epitaxy. At present, CREE, Dow Corning in the United States and Showa Denko in Japan are the industrialized companies that mass-produce SiC epitaxial materials.
GaN material and its preparation process
In theory, the breakdown electric field strength of GaN materials (about 3 × 106V / cm) is close to that of SiC materials, but due to the influence of semiconductor technology, material lattice mismatch and other factors, the voltage tolerance of GaN devices is usually around 1000V. The safe operating voltage is usually below 650V. With the overcoming of various technical difficulties and the development of advanced processes, GaN will surely be used as a preparation material for a new generation of high-efficiency power devices.
(1) GaN material structure and characteristics
GaN is a III-V direct bandgap wide band gap semiconductor. The band gap width of the wurtzite structure at room temperature is 3.26 eV. GaN has three crystal structure forms, namely wurtzite structure, sphalerite structure and rock salt structure. Among them, the wurtzite structure is the most stable crystal structure in the group III nitride, the sphalerite structure exists in the form of metastable phase, and the rock salt structure is generated under high pressure conditions. The wurtzite structure of GaN materials has excellent physical properties that other semiconductors do not have, such as chemical resistance, super hardness, and ultra high melting point. Therefore, GaN-based semiconductor devices have excellent voltage resistance, heat resistance, and corrosion resistance. characteristic. Figure 4 shows the hexagonal wurtzite structure of GaN and GaN single crystal.
(2) Preparation of GaN crystal
GaN has a large covalent bond energy (E = 876.9kJ / mol). At a melting point of 2500 ° C, the decomposition pressure is about 4.5GPa. When the decomposition pressure is lower than 4.5GPa, GaN decomposes without melting. Therefore, some typical equilibrium methods (such as the Tira method and Bridgeman directional solidification method, etc.) are no longer suitable for the growth of GaN single crystals.
At present, only some special methods can be used to prepare single crystals, mainly including sublimation method, high temperature and high pressure method, melting crystallization method and hydride vapor phase epitaxy method. Among them, the first three methods have strict requirements on equipment and processes, which is difficult to achieve large-scale single crystal production and cannot meet commercial requirements. The Hydride Vapor-phase Epitaxy (HVPE) method is the mainstream of current research .
Most uniform substrates that can provide GaN commercially are produced by this method. This technology has the advantages of simple equipment, low cost and fast development speed. Using metal-organic chemical vapor deposition (Metal-organic Chemical Vapor Deposition, MOCVD) technology can grow a uniform, large-size thick film as a substrate. At present, this technology has become the most effective method for preparing epitaxial thick films, and the grown thick films can be polished or laser-stripped off the substrate as a substrate for the homogeneous epitaxial growth device structure.
The dislocation density of the hydride vapor phase epitaxial layer decreases as the thickness of the epitaxial layer increases. Therefore, as long as the thickness of the epitaxial layer reaches a certain value, the crystal quality can be improved. Large diameter independent GaN wafers with high crystal quality and good reproducibility can be prepared by HVPE and Void-assisted separation (VAS), as shown in Figure 5. Using a porous GaN template with a surface covered with titanium nitride (TiN) nanomesh, a thick GaN layer was grown by HVPE. During the HVPE growth process, this growth technique created many small gaps between the GaN layer and the template. When the GaN layer It is easy to separate from the template after growth, and independent GaN wafers are obtained. These wafers have a large diameter, a mirror surface, no cracks, and a low dislocation density.
In addition, the MOCVD-GaN / sapphire substrate pretreatment process can be used to prepare GaN thick films. The main process is to deposit a layer of SiO2 with a thickness of about 500nm on the MOCVD-GaN / sapphire substrate by plasma chemical vapor deposition, and then use an electronic vapor machine to vapor-deposit and forge a layer of Ti with a thickness of about 20nm on the substrate. After annealing, self-assembled Ni nanoclusters were formed on the surface of SiO2, which was used as a lithography mask. After photolithography, the substrate is placed in hot HNO3 and oxidizing etchant. After removing Ti and SiO2, a layer of SiO2 is deposited by reactive ion etching technology to remove SiO2 on the surface to form a layer of GaN nanopillars wrapped with SiO2 at the edges. Finally, GaN is grown on the surface by the HVPE method. During the cooling process, GaN self-exfoliates. Figure 6 is a schematic diagram of the process of preparing GaN single crystal by HVPE and nano-cluster self-stripping technology.
The above method can not only realize the self-stripping of the substrate, but also form a special structure, which can buffer the growth rate of the crystal, thereby improving the quality of the crystal and reducing internal defects. However, these pretreatment methods are relatively complex, will waste a lot of time, and increase the cost of GaN single crystal.
(3) GaN hetero-substrate epitaxy technology
Due to the high dissociation pressure of N when GaN grows at high temperature, it is difficult to obtain large-sized GaN single crystal materials. Therefore, the preparation of epitaxial GaN films on heterogeneous substrates has become the main method for studying GaN materials and devices. Currently, GaN epitaxial growth methods are: HVPE, molecular beam epitaxy (MBE), atomic beam epitaxy (ALE) and MOCVD. Among them, MOCVD is one of the most widely used methods.
Currently, most commercial devices are based on heteroepitaxial, the main substrate is sapphire, AlN, SiC and Si. However, the lattice mismatch and thermal mismatch between these substrates and materials are very large. Therefore, the existence of greater stress and higher dislocation density in the epitaxial material is not conducive to the improvement of device performance.
1. Epitaxial growth of GaN-based heterostructures on SiC substrates
Since the thermal conductivity of SiC is much higher than that of GaN, Si and sapphire, the lattice mismatch between SiC and GaN is very small. SiC substrate can improve the heat dissipation characteristics of the device and reduce the junction temperature of the device. However, the wettability of GaN and SiC is poor, and it is difficult to obtain a smooth film by directly growing GaN on a SiC substrate. AlN has a low migration activity on the SiC substrate and good wettability with the SiC substrate. Therefore, AlN is usually used as the nucleation layer of the GaN epitaxial thin film on the SiC substrate, as shown in Figure 8.
Many studies have shown that by optimizing the growth conditions of the AlN nucleation layer, the crystal quality of the CaN film can be improved. However, the GaN film grown on the GaN nucleation layer still has large dislocation density and residual stress. The thermal expansion coefficient of AlN is much greater than that of GaN, and the GaN film grown on AlN has a large residual tensile stress during cooling. Tensile stress will accumulate to a certain extent and release the stress in the form of cracks.
In addition, the migration activity of AlN is low, and it is difficult to form a continuous film, resulting in a large dislocation density of the GaN thin film grown on AlN. Cracks and dislocations in the GaN film can cause device performance degradation or even failure. Due to the small lattice mismatch, once the wetting layer and crack problems are resolved, the quality of GaN crystals on SiC substrates is better than that of GaN crystals on Si and sapphire substrates. Therefore, the GaN heterostructure on SiC substrates 2DEG has better transport performance.
2. Epitaxial growth of GaN-based heterostructures on Si substrates
Currently, the cost of GaN-based power electronic devices is still very expensive compared to Si devices. The only way to solve the cost problem is to use epitaxial Si substrates to fabricate GaN-based heterostructures, and then use complementary metal oxide semiconductor (Complementary Metal Oxide Semiconductor, CMOS) technology to fabricate GaN-based devices, making the device cost-effective over Si devices. But compared to SiC and sapphire substrates, epitaxial GaN on Si substrates is much more difficult. The lattice mismatch rate of GaN (0001) and Si (111) is as high as 16.9%, and the thermal expansion coefficient mismatch (thermal mismatch) is as high as 56%. Therefore, the epitaxial growth of GaN and its heterostructure on Si substrates face severe challenges in terms of stress control and defect control.
Differences in the lattice constants of the epitaxial layer materials will result in high-density dislocation defects at the interface of the Si and GaN epitaxial layers. During epitaxial growth, most dislocations will penetrate the epitaxial layer, which seriously affects the crystal quality of the epitaxial layer. However, due to the inconsistency of the thermal expansion coefficients of the two layers, the internal stress of the entire epitaxial layer accumulated during the cooling process after high-temperature growth was large, warpage occurred and the epitaxial layer cracked. As the size of the substrate increases, this phenomenon of warpage and cracking will become more and more obvious.
At present, the insertion layer and the buffer layer are widely used to solve the stress problem of GaN heteroepitaxial on the Si substrate. There are currently three mainstream stress adjustment schemes:
(A) Low temperature AlN insertion layer structure;
(B) GaN / AlN superlattice structure;
(C) AlGaN buffer layer structure
The insertion layer technology is to introduce one or more thin insertion layers to adjust the internal stress state of the epitaxial layer and balance the tensile stress of the epitaxial layer caused by thermal mismatch and lattice mismatch during cooling. Currently, low temperature AlN is used as the insertion Layer to adjust the stress state, as shown in Figure 9 (a).
The buffer layer technology provides compressive stress to adjust the stress balance in the epitaxial film. Currently commonly used are AlGaN gradient buffer and AlN / (Al) GaN superlattice buffer, as shown in Figure 9 (b) and (c). All of the above methods can provide compressive stress to balance the tensile stress of Si-based GaN, so that the entire system tends to stress balance. Of course, these methods cannot completely solve the stress problem. The stress adjustment mechanism of the buffer layer is not yet clear and needs further exploration and optimization.
In addition, it has been reported that the surface activation bonding (SAB) low temperature bonding process is used to transfer the GaN layer to the SiC and Si substrates, and the GaN-on-Si structure and the GaN-on-SiC structure are directly bonded at room temperature. , The surface of the wafer is activated by an argon (Ar) ion beam source. After surface activation, the two wafers will be bonded together. Compared with the quality of heteroepitaxial layers grown on Al2O3 (sapphire) and SiC substrates, the quality and electrical properties of GaN-based heterostructures on Si substrates are still very different. In particular, residual stress and local trap states exist in the GaN epitaxial layer on the Si substrate. These stress and defect control issues have not been fundamentally resolved, resulting in material and device reliability issues are particularly prominent. Therefore, how to prepare GaN-based heterostructures on high-quality Si substrates is still one of the core issues in this field.
Conclusion
The development of high-frequency, high-power, radiation-resistant, high-density integrated wide-bandgap semiconductor electronic devices requires excellent materials for basic support. High-quality SiC and GaN devices require the use of epitaxial materials to prepare active regions. Therefore, low-defect substrates and high-quality epitaxial layers play a crucial role in device performance. In recent years, the manufacturing requirements and withstand voltage levels of SiC and GaN power devices have been continuously improved. The defect density of the substrate and heterostructures (GaN-on-SiC, GaN-on-Si) and the stress balance state inside the epitaxial film are both Higher requirements are put forward. At present, the use of AlN as a transition layer, superlattice buffer layer, etc. provides compressive stress, and then adjusts the internal stress of the epitaxial layer to balance the state. In the future, a lot of work needs to be explored and optimized for stress control .