How to identify the quality of LED epitaxial wafers?

 The quality of LED epitaxial wafers depends on the substrate material and epitaxial growth technology. The substrate material is 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. The substrate material determines the development path of semiconductor lighting technology. The choice of substrate material mainly depends on the following nine aspects:

　　1. Good conductivity, can be made into upper and lower structures;

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

　　3. 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;

　　4. Low price;

　　5. Good optical performance, the light emitted by the manufactured device is less absorbed by the substrate;

　　6. Large size, generally required to be no less than 2 inches in diameter.

　　7. Good interface characteristics, conducive to the nucleation of epitaxial materials and strong adhesion;

　　8. Good thermal properties, including good thermal conductivity and small thermal mismatch;

　　9. Good mechanical properties, easy processing of devices, including thinning, polishing and cutting;

　　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:

　　1. 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;

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

　　3. Difficulty and cost of material preparation: Considering the needs of industrial development, the preparation of substrate materials should be simple and the cost should not be very high. The substrate size is generally not less than 2 inches.

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

　　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.

　　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:

　　The wide application of SiC as a substrate material is second only to sapphire, and there is currently no third substrate for the commercial production of GaNLED. 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, they can better solve the heat dissipation problem of power-type GaNLED devices, so they occupy an important position in the field of semiconductor lighting technology.

　　Zinc Oxide:

　　The reason why ZnO can become a candidate substrate for GaN epitaxy is that the two have very striking similarities. The two have the same crystal structure, very small lattice recognition, and similar bandgap width (small band discontinuity value and small contact barrier). However, the fatal weakness of ZnO as a GaN epitaxial substrate is that it is easy to decompose and corrode 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.