GaN, AlN, InN and their alloys are new materials of GaN system. The evaluation of substrate materials should comprehensively consider its factors, and finding a more suitable substrate is an important goal of developing GaN-based technology. The evaluation of substrate materials should comprehensively consider the lattice matching, thermal expansion coefficient matching, chemical stability matching, and the difficulty and cost of material preparation. At present, the widely used epitaxial substrate material is InN. The development of self-supporting homoepitaxial substrate is of great significance for the development of nitride semiconductor lasers with independent intellectual property rights, high-power and high-brightness LEDs for semiconductor lighting and high-power microwave devices. The manuscript "Evaluation Factors and R&D of Nitride Substrate Materials" introduces some contents of evaluation factors and R&D of Nitride Substrate Materials. GaN is a material with direct band gap, and its optical transition probability is one order of magnitude higher than that of indirect band gap. Therefore, wide band-gap GaN-based semiconductors have shown broad application prospects in short-wavelength light-emitting diodes, lasers, ultraviolet detectors and high-temperature microelectronic devices. For environmental protection, it is still very suitable to be an environmental protection material system.
1994 Nicha company of Japan made a breakthrough in GaN/Al2O3, and 1995 GaN devices were commercialized for the first time. 1998, the market scale of gan-based light-emitting diode LED was 500 million dollars, and in 2000, the market scale was expanded to1300 million dollars. According to authoritative experts' prediction, the growth period of GaN-based LED and its Al2O3 substrate in the international market will reach 50 years. GaN-based LED and its Al2O3 substrate have unique excellent physical and chemical properties and long-term durability. It is estimated that the market scale of GaN-based devices will be expanded to 3 billion US dollars in 2005, and the market scale of Al2O3 substrates for GaN-based devices will be expanded to 500 million US dollars.
The development classification of semiconductor lighting industry shows several main stages, and each stage can form a distinctive industrial chain:
(1) the first stage
The first stage (special lighting era, before 2005) includes: instrument description; Golden display, indoor and outdoor advertising; Traffic lights, signal lights, Peugeot lights, car lights; Indoor ever-burning lamps, ceiling lamps, color-changing lamps, lawn lamps; Urban landscape building outline lights, bridges, expressway, tunnel guiding street lamps and so on.
(2) the second stage (lighting era, 2005~20 10), including CD, DVD and H-DVD optical storage; Laser gold display; Entertainment, bar code, printing and image recording; Medical laser; Open up a new field of fixed lighting, derive a new lighting industry, lay the foundation for general lighting applications, and so on. (3) The third stage The third stage (general lighting era, 20 10 years later) includes the application of the above two stages, and it has also entered the general lighting market in an all-round way, occupying 30-50% of the market share.
Up to now (the first stage, special lighting era), small and medium power blue light emitting diodes (LEDs), green LEDs, white LEDs, blue-purple LEDs and so on. It has been mass-produced and has entered the commercial market. High power blue light emitting diode (led), laser diode (LD) and all-band InN-GaN will bring new and greater business opportunities, such as optical storage and optical communication. To realize the practicality and commercialization of high-power blue light emitting diode (LED), laser diode (LD) and all-band InN-GaN, suitable substrate materials are needed. Therefore, with the development of GaN materials and devices, it is necessary to find a substrate material that matches GaN to further improve the quality of epitaxial films.
In addition, in basic research and long-term planning, the development of science and technology increasingly needs to combine materials of different systems, which is called heterojunction materials. The application of covariant substrate can limit the defects of lattice and thermal mismatch to the substrate, which lays the foundation for opening up a new material system. Many preparation technologies of covariant substrates have been proposed, such as self-supporting substrate, bonding and torsion bonding, heavy lattice transition layer, SOI and VTE substrate technology. It is predicted that in the next 10~20 years, the preparation technology of large-size covariant substrates will make a breakthrough and be widely used in the growth of large mismatch heterojunction materials and the manufacture of related photoelectric devices.
Countries all over the world have invested a lot of manpower, financial resources and material resources, and are at the commanding heights in this field, hoping to make a breakthrough in GaN-based high-power devices.
Evaluation factors and research progress of nitride substrate materials: GaN, AlN, InN and their alloys are new materials. To evaluate substrate materials, it is necessary to comprehensively consider its factors and find a more suitable substrate, which is an important goal of developing GaN-based technology. At present, the widely used epitaxial substrate materials are: InN；; α-al2o 3(000 1)； 6H-SiC； MGA L2 o 4( 1 1 1)； Lialo 2 and Koryo 2; MgOsi； GaAs( 1 1 1) and so on.
ⅲ-ⅴ compounds, such as GaN, AlN and InN, have two crystal forms: one is cubic sphalerite structure, and the other is hexagonal wurtzite structure. Gallium nitride, aluminum nitride and indium nitride with wurtzite structure are the research hotspots centered on blue light radiation, and they are mainly solid solutions of gallium nitride, aluminum nitride and indium nitride. The band gap of these materials is direct transition type, so they have high quantum efficiency. The band gap of solid solutions made of gallium nitride, aluminum nitride and indium nitride with different compositions and proportions can vary from 2.2eV to 6.2eV, so making light-emitting devices from these solid solutions is the development direction of optoelectronic integrated materials and devices.
(1) Inn and Gan
Because heteroepitaxial nitride films usually bring a lot of defects, defects will damage the performance of devices. Like GaN, if the defects can be greatly reduced by homoepitaxial growth on InN, the performance of the device will be greatly improved.
Self-supporting homoepitaxial GaN, AlN and al GaN substrates are the most likely substrate materials to be applied first.
Sapphire (α-Al2O3) and 6H-SiC
α-Al2O3 single crystal, namely sapphire crystal. (000 1) sapphire is the most commonly used epitaxial substrate material in InN at present. The matching directions are: inn (001)/α-Al2O3 (001), inn [110]//α-Al2O3 [165438]. Because the surface of the substrate is nitrided into AlON, Inn and InN before the growth of the thin film, and the hexagonal lattice structure of α-Al2O3 (000 1) rotates 30, the mismatch is slightly reduced compared with the original 29%. Although the mismatch rate between (000 1) sapphire and InN lattice is as high as 25%, due to its hexagonal symmetry, the melting point is 2050℃, and the highest working temperature can reach 1900℃, which has good high-temperature stability and mechanical properties. In addition, it is still the most widely used substrate material because of its more research, mature production technology and low price.
The application of 6H-SiC as substrate material is second only to sapphire. Compared with sapphire, the lattice matching between 6H-SiC and InN epitaxial film is improved. In addition, 6H-SiC is a low-resistance material with blue luminescence, which can be used to make electrodes, making it possible to completely test the epitaxial film before packaging, thus enhancing the competitiveness of 6H-SiC as a substrate material. Because the layered structure of 6H-SiC is easy to cleave, a high-quality cleavage plane can be obtained between the substrate and the epitaxial film, which will greatly simplify the structure of the device. But at the same time, due to its layered structure, there are often steps on the surface of the substrate, which introduces a large number of defects into the epitaxial film.
(3) magnesia-alumina spinel
MgAl2O4 crystal, namely magnesium aluminate crystal. MgAl2O4 crystal is a kind of crystal material with high melting point (2 130℃) and high hardness (Mohs hardness 8). It belongs to the face-centered cubic crystal system, the space group is Fd3m, and the lattice constant is 0.8085nm. MgAl2O4 crystal is an excellent sound transmission medium material, with low acoustic attenuation in the microwave section, and the insertion loss of microwave delay line made of MgAl2O4 crystal is small. MgAl2O4 crystal has good lattice matching performance with Si, and its expansion coefficient is similar to that of Si, so the deformation and distortion of epitaxial silicon film are small, and the speed of making large-scale ultra-high-speed integrated circuits is faster than that of sapphire. In addition, MgAl2O4 crystal is used as superconducting material abroad, and the effect is very good. In recent years, people have done a lot of research on epitaxial substrate materials of MgAl2O4 crystals for GaN. MgAl2O4 crystal has good lattice matching and thermal expansion matching, and the lattice mismatch rate between (11) plane MgAl2O4 crystal and GaN is 9%, which has excellent thermal stability, chemical stability and good mechanical properties. MgAl2O4 crystal is one of the most suitable substrate materials for GaN at present, and high-quality crystals have been successfully epitaxial on MgAl2O4 substrate. In addition, the most attractive feature of MgAl2O4 substrate is that the laser end face can be obtained by cleavage.
On the basis of predecessors' research, the recent research on MgAl2O4 crystal as the epitaxial substrate material of InN has also been reported in the literature. The matching directions are: inn (001)/mgal2o4 (111), inn [1/kloc-0]//mgal2o4 [/kloc-0]. The research shows that the mismatch rate between (11) plane MgAl2O4 crystal and the InN lattice is 15%, and the lattice matching performance is much better than sapphire, and the mismatch rate between (000 1) plane sapphire and the InN lattice is as high as 25%. Moreover, if the magnesium atoms located below the top oxygen atom layer occupy effective coordination lattice sites and oxygen lattice sites, it is hoped that the lattice mismatch rate will be further reduced to 7%, which is much lower than sapphire. Therefore, MgAl2O4 crystal is a promising substrate material for InN epitaxy.
(4)LiAlO2 and LiGaO2
In the past, LiAlO2 and LiGaO2 were used as epitaxial substrate materials of GaN. The mismatch between LiAlO2 and LiGaO2 and GaN epitaxial film is very small, which makes LiAlO2 and LiGaO2 very suitable for GaN epitaxial substrate materials. At the same time, as an epitaxial substrate material of g an, Koryo 2 has its unique advantages: after epitaxial growth of GaN, the substrate of Koryo 2 can be etched, leaving an epitaxial film of GaN, which will greatly facilitate the fabrication of devices. However, the lithium ion in LiGaO2 _ 2 crystal is very active and can not exist stably under ordinary epitaxial growth conditions (such as chemical atmosphere and growth temperature of MOCVD method), so its single crystal needs further study as an epitaxial substrate material of GaN. At present, LiAlO2 and LiGaO2 are rarely used as epitaxial substrate materials of InN.
(5) Magnesium oxide
MgO crystal belongs to cubic system, NaCl structure and melting point is 2800℃. Because MgO crystal is not stable enough in MOCVD atmosphere, it is rarely used, especially for InN thin films with high melting point and growth temperature.
(6)GaAs
GaAs( 1 1 1) is also the substrate material for growing InN thin films at present. When the nitridation temperature of the substrate is lower than 700℃ and the thickness of the grown InN film is less than 0.05μm, the InN film has a cubic structure; When the thickness of the grown InN film exceeds 0.2μm, the cubic structure disappears and all of them are transformed into hexagonal InN films. GaAs InN thin film (1 1 1)
The nucleation mode on the substrate is very different from that on the α-Al2O3 (00 1) substrate. The nucleation mode of InN thin film on GaAs (11) substrate is not the columnar and fibrous structure when growing InN thin film on sapphire substrate, but the surface is very flat.
(7)Si
Monocrystalline silicon is a widely used semiconductor material. It is very interesting to use Si as the substrate material of InN, because it is possible to integrate InN-based devices with Si devices. In addition, Si technology is quite mature in the semiconductor industry. It is conceivable that if the device-quality InN epitaxial film can be grown on the Si substrate, the manufacturing process of devices based on InN will be greatly simplified and the size of devices will be reduced.
(8) Zirconium boride
ZrB2 is a new substrate for nitride epitaxy first proposed by Japanese scientists at 200 1. ZrB2 is lattice matched with nitride, and has matching thermal expansion coefficient and high conductivity. Flux method and floating zone method are mainly used for growth.
The development of self-supporting homoepitaxial substrate is of great significance for the development of nitride semiconductor lasers with independent intellectual property rights, high-power and high-brightness LEDs for semiconductor lighting and high-power microwave devices.