Whispering-gallery mode lasing in GaN microdisks

Abstract

The wide range of wavelengths covered by the Gallium nitride (GaN) group of materials is probably the major reason for its growing use by the semiconductor industry. GaN light emitting diodes are used extensively for illumination and display purposes,1 while UV emitters have been employed in a wide range of biomedical applications.2 High-performance GaN transistors have also been demonstrated. The laser diodes emitting at 405nm and found in the latest high-density DVD players are also GaN-based. Shorter wavelengths allow data to be packed more tightly, so it becomes possible to fit more data on a disc even though it is the same size as a CD/DVD. GaN laser diodes are currently fabricated in the conventional edge-emitting geometry, where the laser light propagates parallel to the wafer surface of the semiconductor. However, the newer surface-emitting versions, where the light propagates in the direction perpendicular to the semiconductor surface, offer more advantages. The first is their lower fabrication cost since the chips can be tested on-wafer, unlike edge-emitters that can only be tested at the end of the fabrication process. They also offer beams with lower divergence due to large output apertures, low power consumption, and wavelength tuning. However, the transition from edge to surface emission is not trivial in GaN structures. In principle, a surface-emitting laser requires a vertical cavity with top and bottom mirrors. In practice, high quality GaN materials are grown on sapphire substrates, which makes the insertion of bottom mirrors inherently difficult. One common solution is the deposition of AlGaN/GaNdistributed Bragg reflector stacks between the device structure and the sapphire substrate.3 Alternatively, undercut microdisk lasers that address mirror insertion challenges have also been fabricated using selective photoelectrochemical etching techniques.4 Recently, we have also developed a microdisk fabrication method based on GaN grown on Si substrates. Pivoted GaN Figure 1. Scanning-electron-microscope images showing (a) the cross section of a single pivoted microdisk and (b) a microdisk array.