LED epitaxial wafer substrate materials basic knowledge

Substrate materials are the cornerstone of the technological development of the semiconductor lighting industry. Different substrate materials require different epitaxial growth technologies, chip processing technologies, and device packaging technologies. Substrate materials determine the development path of semiconductor lighting technology.

The choice of substrate materials mainly depends on the following nine aspects:

good structural characteristics, the crystal structure of the epitaxial material and the substrate is the same or similar, the lattice constant mismatch is small, the crystallization performance is good, and the defect density is small
Good interface characteristics, which are conducive to the nucleation of epitaxial materials and have strong adhesion

Good chemical stability, not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth

Good thermal properties, including good thermal conductivity and small thermal mismatch

Good electrical conductivity, can be made into upper and lower structures

Good optical properties, the light emitted by the manufactured device is less absorbed by the substrate

Good mechanical properties, the device is easy to process, including thinning, polishing and cutting, etc.

Low price

Large size, generally requiring a diameter of not less than 2 inches

It is very difficult to select a substrate that satisfies all nine aspects above. Therefore, the research and development and production of semiconductor light-emitting devices on different substrates can only be adapted by changing the epitaxial growth technology and adjusting the device processing technology. There are many substrate materials used for GaN research, but there are only two substrates that can be used for production, namely sapphire Al2O3 and silicon carbide SiC substrates. Table 2-4 makes a qualitative comparison of the performance of five substrate materials used for GaN growth.

The following factors must be considered when evaluating substrate materials:

Structural matching between substrate and epitaxial film: the crystal structure of epitaxial material and substrate material is the same or similar, with small lattice constant mismatch, good crystallization performance and low defect density;

Matching the thermal expansion coefficients of the substrate and the epitaxial film: Matching the thermal expansion coefficients is very important. If the thermal expansion coefficients of the epitaxial film and the substrate material differ too much, it may not only reduce the quality of the epitaxial film, but also cause damage to the device due to heat during the operation of the device.

The chemical stability of the substrate and the epitaxial film should match: the substrate material should have good chemical stability, not easy to decompose and corrode in the temperature and atmosphere of epitaxial growth, and the quality of the epitaxial film should not be reduced due to chemical reaction with the epitaxial film;

Difficulty and cost of material preparation: Considering the needs of industrial development, the preparation of substrate materials requires simplicity and not too high a cost. The substrate size is generally not less than 2 inches.

There are many substrate materials used for GaN-based LEDs, but there are only two types of substrates that can be used for commercialization, namely sapphire and silicon carbide substrates. Other substrates such as GaN, Si, and ZnO are still in the research and development stage and are still some distance away from industrialization.

Gallium nitride:

The most ideal substrate for GaN growth is GaN single crystal material, which can greatly improve the crystal quality of epitaxial film, reduce dislocation density, increase device life, improve luminous efficiency, and increase device operating current density. However, it is very difficult to prepare GaN single crystals, and there is no effective method so far.

Zinc oxide:

ZnO is a candidate substrate for GaN epitaxy because the two have very striking similarities. The two have the same crystal structure, very low lattice recognition, and similar bandgap widths (small band discontinuity and small contact barrier). However, the fatal weakness of ZnO as a GaN epitaxial substrate is that it is easily decomposed and corroded in the temperature and atmosphere of GaN epitaxial growth. At present, ZnO semiconductor materials cannot be used to manufacture optoelectronic devices or high-temperature electronic devices, mainly because the material quality does not meet the device level and the P-type doping problem has not been truly solved. Equipment suitable for the growth of ZnO-based semiconductor materials has not yet been successfully developed.

Sapphire:

The most common substrate for GaN growth is Al2O3. Its advantages are good chemical stability, no absorption of visible light, moderate price, and relatively mature manufacturing technology. Although poor thermal conductivity does not expose obvious deficiencies when the device is working at low current, it is a very prominent problem when the power device is working at high current.

Silicon carbide:

SiC is second only to sapphire in the wide application of substrate materials. Currently, there is no third substrate for commercial production of GaN LEDs. SiC substrates have good chemical stability, good electrical conductivity, good thermal conductivity, and do not absorb visible light, but their shortcomings are also prominent, such as high prices, difficult crystal quality to reach the same level as Al2O3 and Si, and poor mechanical processing performance. In addition, SiC substrates absorb ultraviolet light below 380 nanometers and are not suitable for the development of ultraviolet LEDs below 380 nanometers. Due to the beneficial electrical and thermal conductivity of SiC substrates, the heat dissipation problem of power-type GaN LED devices can be better solved, so it occupies an important position 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 blue luminescence characteristics and is a low-resistance material that can be used to make electrodes, making it possible to fully test the epitaxial film before packaging the device, enhancing the competitiveness of SiC as a substrate material. Since SiC's layered structure is easy to cleave, a high-quality cleavage surface 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, steps often appear on the surface of the substrate that introduce a large number of defects to the epitaxial film.

To achieve the goal of luminous efficiency, we must rely on GaN substrate LEDs to achieve low cost, and also use GaN substrates to achieve high efficiency, large area, and high power per lamp, as well as the simplification of process technology and greatly improved yield. Once semiconductor lighting becomes a reality, its significance is no less than Edison's invention of the incandescent lamp. Once breakthroughs are made in key technical fields such as substrates, its industrialization process will make great progress.