Abstract: Deferring GaN on a silicon substrate provides a new technology platform that accelerates the expansion of GaN applications in optoelectronics and microelectronics. This article describes the development of GaN-based light-emitting diodes on silicon substrates.
Group III nitride semiconductor materials are widely used in violet, blue, green and white light-emitting diodes, violet lasers for high-density optical storage, ultraviolet light detectors, and high-power high-frequency electronic devices. However, due to the lack of a suitable substrate, currently high quality GaN films are usually grown on sapphire or SiC substrates, but both substrate parts are relatively expensive, especially silicon carbide, and are relatively small in size. Sapphire also has the disadvantage of being extremely hard and non-conductive. In order to overcome the above disadvantages, people have been continuously exploring the use of silicon as a substrate for growing GaN. Since the electroluminescence of GaN materials is not sensitive to crystal defects, it is expected that heteroepitaxial growth of Group III nitride light-emitting devices on Si substrates has significant technical advantages in terms of cost reduction.
It is also expected that the use of Si substrates will further integrate light emitters with silicon electronics in the future, which will accelerate and expand the application of gallium nitride in optoelectronics and microelectronics.
First, the advantages and disadvantages of using silicon as a GaN LED substrate
The advantage of using silicon as a GaN light emitting diode (LED) substrate is that the manufacturing cost of the LED is greatly reduced. This is not only because the price of the Si substrate itself is much cheaper than the currently used sapphire and SiC substrates, but also a larger size substrate than the sapphire and SiC substrates (for example, using a 4-inch Si wafer substrate) can be used. Increase the utilization of MOCVD to increase die yield. Like the SiC substrate, Si is also a conductive substrate. The electrodes can be drawn from both sides of the die without having to be pulled out from one side like non-conductive sapphire. This not only reduces the die area but also eliminates the need for GaN. Dry etching step of the epitaxial layer. At the same time, since silicon has a lower hardness than sapphire and SiC, the LED chip can be cut out by using a general-purpose cutting device used in LSI processing, which saves the cost of die production. In addition, since the CaAs industry is currently transitioning from 4 inches to 6 inches, the eliminated 4-inch process line can be used for GaN LED production on silicon substrates. According to estimates by Sanken Electric Company of Japan, the manufacturing cost of fabricating blue GaN LEDs using silicon substrates will be 90% lower than that of sapphire substrates and SiC substrates, and it is expected to be used in applications requiring low power emitters.
However, it is more difficult to grow GaN on a Si substrate than sapphire and SiC. Because the thermal mismatch and lattice mismatch between the two are greater. The difference in thermal expansion coefficient between silicon and GaN will cause cracking of the GaN film, and the difference in lattice constant will cause high dislocation density in the GaN epitaxial layer. The GaN LED may also have a low on-voltage and a poor crystal integrity due to a 0.5 V hetero barrier between Si and GaN, resulting in a low P-type doping efficiency, resulting in an increase in series resistance. Another disadvantage of using a Si substrate is that the absorption of visible light by the silicon reduces the external quantum efficiency of the LED. Despite this, there have been many exciting results in GaN LEDs on silicon since 1998.
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