Substrate materials: the cornerstone of semiconductor lighting

Semiconductor lighting is a solid-state lighting semiconductor light-emitting diode light source model based on high technology is one of the most promising field in twenty-first Century, has become another leap after incandescent lamp, fluorescent lamp lighting in the history of mankind. As the cornerstone of the development of semiconductor lighting industry, substrate is the core technology of semiconductor lighting industry. The choice of substrate material directly determines the manufacturing route of LED chip.

The following factors should be considered in the evaluation of substrate materials:

1 the structure of the substrate and the epitaxial film are matched: the crystal structure of the epitaxial material and the substrate material is the same or similar, the lattice parameter mismatch is small, the crystallization performance is good, and the defect density is low;

2, coefficient of thermal expansion of the substrate and the epitaxial film: the matching coefficient of thermal expansion is very important, epitaxial film and substrate material on the thermal expansion coefficient is too large may not only make the epitaxial film quality decreased, but also in the work process of equipment due to the damage of the device caused by fever;

3, the chemical stability of substrate and epitaxial film substrate materials have good chemical stability, not easy to decompose in the epitaxial growth temperature and atmosphere and corrosion, not because of the chemical reaction with the epitaxial film of the EPI film quality decline;

4 the degree of difficulty and the cost of material preparation: taking into account the needs of the development of industrialization, the preparation of substrate material requirements are simple, the cost is not high. The substrate size is not less than 2 inches.

At present, the substrate material used for GaN based LED is more, but only two kinds of substrates can be used for commercialization. Other such as GaN, Si, ZnO substrate is still in the stage of development, there is still a distance from the industrialization.

Gan:

For the ideal substrate for GaN growth is GaN single crystal materials, can greatly improve the crystal quality of epitaxial film, reduce the dislocation density, improve device service life, improve the luminous efficiency, improve the device working current density. However, it is very difficult to prepare GaN single crystal, so far there is no effective method.

Zinc oxide

The reason why ZnO has become a candidate substrate for the GaN epitaxy is that they have very striking similarities. The crystal structure is the same, the lattice identification is very small, the band gap is close (the band discontinuity value is small, the contact barrier is small). However, ZnO as a GaN epitaxial substrate is a fatal weakness in the epitaxial growth of GaN in the temperature and atmosphere of easy decomposition and corrosion. At present, ZnO is not used in the manufacture of semiconductor optoelectronic devices or high temperature electronic devices, is the main material quality is not up to the level of the device and P type doping problem has not been solved, suitable for the growth of ZnO based semiconductor materials equipment has not been developed.

Sapphire

The most common substrate for GaN growth is Al2O3. The utility model has the advantages of good chemical stability, no absorption of visible light, moderate price and relatively mature manufacturing technology. Although the thermal conductivity of the device is not exposed to the lack of obvious deficiencies in the current work, the problem is very prominent in the current operation of power devices.

Sic:

As a substrate material, SiC is widely used in the second place after sapphire, and there are no third kinds of substrates for commercial production of GaN LED. The SiC substrate has good chemical stability, good electrical conductivity, good thermal conductivity, does not absorb visible light, but problems are also very prominent, such as the price is too high, the crystal quality is difficult to achieve Al2O3 and Si so good, the mechanical performance is relatively poor, in addition, the SiC substrate absorption at 380 nm under ultraviolet light, not suitable for research and development of 380 nm ultraviolet LED. Due to the good conductivity and thermal conductivity of the SiC substrate, it can solve the problem of heat dissipation of the power GaN LED device, so it plays an important role in the field of semiconductor lighting technology.

Compared with sapphire, the lattice matching between SiC and GaN epitaxial film is improved. In addition, SiC has a blue light-emitting properties, but also for low resistance materials, can be made of electrodes, so that the device before the packaging of the external extension of the film to be fully tested, enhance the competitiveness of SiC as a substrate material. The layered structure of SiC and easy cleavage between the substrate and the epitaxial film can obtain high quality cleavage surface, which greatly simplifies the structure of the device; but at the same time because of its layered structure appeared on the surface of the substrate is the introduction of a large number of defects to the epitaxial film step.

The target to achieve the luminous efficiency in the hope that the GaN substrate LED, realize low cost, will lead to the realization of high efficiency, large area, single lamp high power through the GaN substrate, and greatly improve the technology driven simplification and yield. Semiconductor lighting once become a reality, its significance is no less than Edison invented incandescent lamp. Once the breakthroughs in key technologies such as the substrate, the industrialization process will be made considerable progress.