GaN-based materials have gained high interest for the fabrication of green, blue and UV light emitting diodes (LEDs) and laser diodes (LDs). Recent remarkable progress towards green laser diodes by several groups has accelerated the field. Here we discuss aspects of our approach to fabricate green laser diodes, namely epitaxial ridge re-growth, metal waveguiding, and focused ion beam (FIB) coupling mirror formation.
The conventional way to form a laser current injection ridge is by etching after photolithography. However, there is ample evidence of ion damage to the active region by the high energy ions in ICP/RIE or CAIBE etching. A promising alternative is the selective re-growth of p-GaN and p-AlGaN cladding layer ridge within the openings of a SiO<SUB>2</SUB> mask. With good optimization, the re-growth results in smoother sidewalls of the ridge than either of the etching processes. Cracking typically observed in AlGaN layers of high AlN fraction on GaN are found to be readily suppressed in the re-growth process of the μm-wide ridge stripes. At the present stage of development, however, a higher forward voltage of 20 V is observed when compared to etched ridge samples at 8 V at the same current of 8 mA (equivalent a current density of 102 A/cm<SUP>2</SUP>) Inconsistency of the p-GaN doping in the re-grown layers is a possible reason, and further optimization is warranted.
Optical characterization of a LD structure usually is hindered by the upper cladding layers and p-contact regions. It therefore seems desirable to complete an LD structure only after the active QW region proves promising as determined by optical gain characterization. To this end, LD structures employing a metal mirror are a suitable supplement to complete the optical resonator. Here we employ a layer of silver on top of the p-layers of the ridge and find a substantial reduction in forward voltage and an increase in light output power. The silver mirror enhances the light output power, particularly at the shorter wavelength portion of the emission spectrum. The FWHM of this peak decreases as the current increases, making this the most likely wavelength of anticipated lasing emission.
Crystal cleaving is the most straight forward approach for laser mirror formation. Yet, for accurate length control of the cavity, a combination of ICP/RIE etching with focused ion beam milling is a desired alternative for the formation of the second mirror. Here we demonstrate results of various development stages of the laser mirror formation.
This work was supported by a DARPA VIGIL program through the United States Air Force AFRL/SNH under FA8718-08-C-0004. This work was also supported by the National Science Foundation (NSF) Smart Lighting Engineering Research Center (# EEC-0812056).