Microcavities have the potential to be enormously efficient lasers. Several groups of researchers are capitalizing on a method of growing tiny pyramids of high-quality gallium nitride (GaN) that could potentially be used to create arrays of efficient electrically pumped blue-green lasers. The selective epitaxial overgrowth method can be used to grow high-quality GaN.
In 1998, Serge Bidnyk and others at Oklahoma State University (Stillwater, OK) and Honeywell (Plymouth, MN) demonstrated laser action in optically pumped GaN pyramids that measured about 15 µm across the base and 15 µm tall.1 More recently, Hongxing Jiang and Jiangyu Lin's group at Kansas State University (Manhattan, KS) and Honeywell described the optical resonance modes in GaN pyramids—work essential to optimizing and controlling lasing in these cavities.2
Forming high-quality GaN with few dislocations is difficult because an ideal substrate does not exist for the material. The lateral overgrowth method, explains Jiang, uses a sapphire substrate covered with a masking layer in which holes are opened. The GaN is grown by metal-organic chemical-vapor deposition up through the windows, then sideways across the masking layer. The lateral overgrowth results in material with much better quality. Depending on the growth conditions, the material can grow into prisms or pyramids.
Microcavities have been made in the form of rings or disks, and the optical modes in these structures tend to be radial or whispering-gallery modes. The mode structures in hexagonal pyramids—a shape dictated by the crystal lattice of GaN—are less evident. Unlike disks and rings, which are nearly two-dimensional, the pyramids are three-dimensional (3-D) cavities. The sides of the pyramid are very smooth, which gives the cavity a higher quantum efficiency. Also, unlike the disks and rings, the pyramidal cavity could be designed to emit a beam from each of its six sides. In combination with the ability to make arrays of the lasers, such properties might be useful for scanners or 3-D displays.
The next step in developing these devices, says Jiang, is enabling electrical pumping. The pyramids measure about 10 µm on a side and stand about 15 µm tall. As a result, the contacts have to be made on sides that are not parallel to the plane of the wafer.
The group is also interested in learning how the size of the pyramids alters the optical properties. To make a cavity that resonates in a single mode, says Jiang, is another challenge because it would need to be much smaller. Other potential applications for the tiny GaN pyramids include use as a solid-state UV detector. Whereas most detectors are flat—and therefore may need to turn to follow the target—a pyramid detector could remain fixed. The target's direction could be determined by detecting the signal to the different surfaces of each pyramid.
REFERENCES
- S. Bidnyk et al., Appl. Phys. Lett. 73(6), 2242 (19 Oct 1998).
- H. X. Jiang et al., Appl. Phys. Lett. 75(6), 763 (9 Aug 1999).