Commercial gallium nitride (GaN)-based light-emitting diodes (LEDs) are typically fabricated on expensive sapphire or silicon carbide substrates that are also difficult to dice and not easily scaled to the 300 mm and beyond large-diameter wafer sizes desired by manufacturers. But researchers at Purdue University (West Lafayette, IN) have successfully fabricated LEDs using metallized silicon (Si) substrates that offer easier dicing, larger substrate sizes, and above all, much lower cost than traditional substrates.
Although Si has a 20% lattice mismatch and an approximate 35% coefficient-of-thermal-expansion mismatch with GaN, a zirconium nitride (ZrN) metal layer between Si and GaN provides an n-type ohmic contact and better lattice matching to GaN. However, because ZrN alone reacts with Si to form unwanted byproducts, an intermediate aluminum nitride (AlN) buffer layer prevents this reaction and facilitates epitaxy in the GaN/ZrN/AlN/Si heterostructures. The thermally conductive but electrically insulating AlN layer also permits electrical isolation of devices on the same wafer, an advantage when designing multiwavelength emitters. Realization of production-ready devices using this method will require resolving the remaining cracking issues that are due to thermal-expansion mismatch between Si and GaN when using GaN layers greater than 1 µm thick. Contact Timothy D. Sands at [email protected].