Properties of Optical Resonant Modes in III-NitrideSemiconductor Micro-Cone Cavities

Arrays of III-nitride semiconductor micro-cone cavities with a base diameter of 3.3μm were fabricated by ion beam etching. The micro-cones consist of a 58nm thick multiple quantum wells of In_0.22Ga_0.78N/In_0.06Ga_0.94N as well as a 1.5μm thick epilayer of GaN. Optical resonant modes from a single micro-cone could be clearly observed in the photoluminescence spectra at temperature up to 200 K under a pumping power density two orders of magnitude lower than that for the III-nitride semiconductor micro-disk or micro-ring cavity. Using a novel optical ray tracing method, we have figured out four main types of optical resonant cavities inside the three-dimensional micro-cone, including two Fabry-Perot (F-P) mode types as well as two Whispering Gallery mode types. The three corresponding mode spacings among the four agree perfectly with the experimental results. The advantages of this new class of micro-cavity over the other micro-cavities are discussed. These findings are expected to have impact on the design of the ultraviolet/blue micro-cavity laser diodes.