A recent article published at this site discussed the prospectus for lowering the cost of LED lighting by the transition to less-expensive wafers such as silicon (Si). This follow-up article discusses some of the technical issues surrounding this thrust in technology. Gallium nitride (GaN) is one of the top thin-film epitaxial layers used for high-brightness LEDs but it has required relatively high-cost silicon carbide or sapphire wafers to be deposited on for making devices.
One of the problems of depositing GaN on Si is the crystalline lattice mismatches between the two materials, which leads to bowing and cracking of wafers at process temperatures ultimately causing poor yield and LED performance. Several renown global materials research centers such as Imec in Belgium are addressing these issues and aiding companies in their development for eventual commercial products.
Recently, Azzurro Semiconductors, based in Germany, announced that Taiwan-based LED manufacturer, Epistar, had shifted its GaN LED manufacturing line, previously dedicated to sapphire substrates, to its 150mm GaN-on-Si materials system in just 16 weeks. The catalyst for this transition was the incorporation of GaN (filled with a high concentration of negatively charged impurities) on a stain-engineered aluminum nitride (AlN) buffer layer coated on a 150mm silicon wafer. The LED manufacturer can then grow the additional GaN layers as needed for specific LED architectures, thereafter.
The critical intellectual property for this device architecture is centered on growing the AlN buffer layer on the silicon surface in an acceptable time for high-volume manufacturing. According to Azzurro, the best record of the optical output power is comparable to LEDs grown on a sapphire substrate, but the company is still seeking to resolve issues in the film stack including voids, pits, uniformity bowing and cracking.
Part of the motivation in moving to Si wafer is the low cost of only about $25 for a six-inch wafer, depending on quality, of course. In comparison, a commercially available two-inch GaN substrate runs for approximately $1900 on the open market. A market analyst firm, Lux Research, forecasted in a study this week that GaN wafer costs will fall by 60 percent by 2020. These falling prices along with the possibility of several hundred percent in performance gains for GaN over Si means that GaN substrates may absorb and limit the traction and momentum of Si in the long-run. However, Si wafers are robust and well-understood by the mainstream semiconductor industry; plus they can be processed on depreciated equipment from older microchip fabrication sites globally, which is another major cost advantage.
A handful of companies are seeking to fabricate large-scale GaN wafers for LED applications, but they will have to overcome hefty hurdles with respect to scalability, cost, cycle time, and high defect densities to prevent the likely sustained transition to Si wafers in the years to come.
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