Substrate material is the cornerstone of the semiconductor lighting industry technology development. Different substrate materials, the need for different epitaxial growth technology, chip processing technology and device packaging technology, the substrate material determines the development of semiconductor lighting technology.
The choice of substrate material depends mainly on the following nine aspects:
Good structural characteristics, epitaxial material and the substrate crystal structure of the same or similar, lattice constant mismatch degree is small, good crystallinity, defect density is small
Good interface characteristics, is conducive to epitaxial material nucleation and strong adhesion
Chemical stability is good, in the epitaxial growth of the temperature and atmosphere is not easy to break down and corrosion
Good thermal performance, including good thermal conductivity and thermal resistance
Good conductivity, can be made up and down structure
Good optical performance, the fabric produced by the light emitted by the substrate is small
Good mechanical properties, easy processing of the device, including thinning, polishing and cutting
Large size, generally requires a diameter of not less than 2 inches
The choice of the substrate to meet the above nine aspects is very difficult. Therefore, at present only through the epitaxial growth technology changes and device processing technology to adapt to different substrates on the semiconductor light-emitting device research and development and production. There are many substrates for gallium nitride, but there are only two substrates that can be used for production, namely sapphire Al2O3 and silicon carbide SiC substrates. Table 2-4 qualitatively compares the performance of five substrates for gallium nitride growth.
Evaluation of the substrate material must take into account the following factors:
The structure of the substrate and the epitaxial film match: the epitaxial material and the substrate material crystal structure of the same or similar, lattice constant mismatch small, good crystallinity, defect density is low;
The thermal expansion coefficient of the substrate and the epitaxial film match: the thermal expansion coefficient of the match is very important, epitaxial film and substrate material in the thermal expansion coefficient difference is not only possible to reduce the quality of epitaxial film, but also in the device work process, due to heat caused Damage to the device;
The chemical stability of the substrate and the epitaxial film match: the substrate material should have good chemical stability, in the epitaxial growth temperature and atmosphere is not easy to break down and corrosion, can not because of the chemical reaction with the epitaxial film to reduce the quality of epitaxial film;
Material preparation of the degree of difficulty and the level of cost: taking into account the needs of industrial development, substrate material preparation requirements simple, the cost should not be high. The substrate size is generally not less than 2 inches.
There are currently more substrate materials for GaN-based LEDs, but there are currently only two substrates that can be used for commercialization, namely sapphire and silicon carbide substrates. Other such as GaN, Si, ZnO substrate is still in the development stage, there is still some distance from the industrialization.
The ideal substrate for GaN growth is GaN single crystal material, which can greatly improve the crystal quality of epitaxial film, reduce the dislocation density, improve the working life of the device, improve the luminous efficiency and improve the device working current density. However, the preparation of GaN single crystal is very difficult, so far there is no effective way.
ZnO has been able to become GaN epitaxial candidate substrate, because the two have a very striking resemblance. Both crystal structures are the same, the lattice recognition is very small, the forbidden band width is close (band with discontinuous value is small, contact barrier is small). However, the fatal weakness of ZnO as a GaN epitaxial substrate is easy to decompose and corrode at the temperature and atmosphere of GaN epitaxial growth. At present, ZnO semiconductor materials can not be used to manufacture optoelectronic devices or high-temperature electronic devices, mainly the quality of the material does not reach the device level and P-type doping problems have not been truly resolved, suitable for ZnO-based semiconductor material growth equipment has not yet developed successfully.
The most common substrate for GaN growth is Al2O3. Its advantages are good chemical stability, do not absorb visible light, affordable, manufacturing technology is relatively mature. Poor thermal conductivity Although the device is not exposed in the small current work is not obvious enough, but in the power of high-current device under the work of the problem is very prominent.
SiC as a substrate material widely used in the sapphire, there is no third substrate for the commercial production of GaN LED. SiC substrate has good chemical stability, good electrical conductivity, good thermal conductivity, do not absorb visible light, but the lack of aspects is also very prominent, such as the price is too high, the crystal quality is difficult to achieve Al2O3 and Si so good, mechanical processing performance is poor, In addition, SiC substrate absorption of 380 nm below the UV light, not suitable for the development of UV LEDs below 380 nm. Because of the beneficial conductivity and thermal conductivity of SiC substrate, it can solve the problem of heat dissipation of power type GaN LED device, so it plays an important role in semiconductor lighting technology.
Compared with sapphire, SiC and GaN epitaxial film lattice matching is improved. In addition, SiC has a blue luminescent properties, and a low resistance material, can make electrodes, so that the device before the packaging of the epitaxial film is fully tested to enhance the SiC as a substrate material competitiveness. Since the layered structure of SiC is easily cleaved, a high quality cleavage surface can be obtained between the substrate and the epitaxial film, which greatly simplifies the structure of the device; but at the same time, due to its layered structure, The epitaxial film introduces a large number of defective steps.
The goal of achieving luminous efficiency is to hope for the GaN of the GaN substrate to achieve low cost, but also through the GaN substrate to achieve efficient, large area, single lamp high power to achieve, as well as driven technology simplification and yield improve. Once the semiconductor lighting has become a reality, its significance as much as Edison invented incandescent. Once in the substrate and other key technology areas to achieve a breakthrough, its industrialization process will be made considerable progress.