Growth and fabrication of nitride-based distributed feedback laser diodes

Nitride laser diodes, with their short wavelength emission (≈400 nm), are attractive for many applications, such as optical storage, laser printing, lighting, medical applications and chemical sensing. Initial demonstrations in 1996 were possible due to key breakthroughs in materials growth during the late 1980's. UCSB became the first university research group to grow and fabricate a pulsed laser diode in the (Al,Ga,In)N material system in 1997. These devices were gain-guided pulsed operation laser diodes employing dry etched facets to form the optical cavity. These dry etched facets have reduced reflection due to roughness and angles deviating from vertical. Because of these limitations, we investigated the use of distributed feedback grating mirrors to create the optical cavity. This technique also provides a narrow spectral width. Conventional distributed feedback laser diodes employ regrown semiconductor gratings with small index differences. This work investigates the use of an overgrown dielectric (Si3N4), which provides a large index difference. The dielectric grating is embedded within the semiconductor crystal via lateral epitaxial overgrowth. Distributed feedback laser diodes were demonstrated, with a 5-fold reduction in the spectral emission width at moderate pumping levels. Two device designs were investigated, which placed the grating either above or below the laser diode active region. Locating the grating above the active region exhibited threshold current densities slightly reduced from comparable etched cavity laser diodes. However, the active region experienced some degradation due to the grating fabrication process, with increased thresholds for both cavity designs. The voltage was dramatically increased due to placement of the Si3N4 grating within the p-type material. Placement of the grating below the active region eliminated the voltage increase as well as the degradation of the active region during the grating fabrication process. However, the quality of the active region was poor due to the placement above the nonplanar overgrowth. This reduced yields significantly and resulted in higher thresholds. A distributed feedback laser diode with an embedded (high index difference) dielectric grating (EDG) was demonstrated for the first time. Laser diodes with the grating located above the active region show promise, with comparable thresholds to etched facet designs and decreased spectral width. The limitations evidenced in this work could be overcome with alternate processing techniques and dielectric material.