GaN based edge-emitting lasers traditionally rely on index contrast from InGaN or AlGaN cladding for modal confinement. However, defects introduced by lattice mismatch limit the feasible composition and layer thickness of alloyed cladding layers. Alternatively, porous GaN offers high refractive index contrast while remaining lattice-matched, making it a suitable candidate for cladding in green lasers. Edge-emitting laser diodes with nano-porous cladding were fabricated and reached electrical injection at 524 nm. In this work, a deep ridge waveguide structure was used to improve electrochemical etch selectivity of the porous cladding, resulting in higher efficiency and lower loss compared to shallow ridge devices.
In this work, the device performances of three laser architectures are examined. All the laser epitaxial structures are grown on freestanding m-plane GaN substrates by Metalorganic Chemical Vapor Deposition (MOCVD), with a peak emission wavelength of about 405 nm. The three laser device architectures are shallow-etch ridge design, which the ridges are defined by etching into the p-GaN layer and not through the active region, and the deep-etch ridge structure that etches through the active region with or without Atomic Layer Deposition (ALD) sidewall passivation. By utilizing ALD sidewall passivation, the optical and electrical characteristics show significant improvements than the other two device designs.
A novel approach to realize DFB gratings on GaN based laser diodes is presented and single longitudinal mode operation is achieved. For lasers with plasma-etched surface gratings, single mode operation was maintained until 900 mA and the spectral width FWHM was less than 5 pm with a SMSR of more than 29 dB. Moreover, several issues limiting the performance of semipolar III-Nitride DFB laser diodes with the etched grating are also addressed in this work. Besides these first order gratings that were formed by electron beam lithography and shallow plasma etching, an improved grating design based on dielectric teeth imbedded into ITO is described, along with the design’s impact on power and spectral performance. Particularly, by utilizing the HSQ resist, we focus on reducing the high operating voltage by imbedding the grating inside the transparent conductive oxide layer without dry etching. This new design with a non-etched imbedded grating successfully reduces the threshold voltage and achieves an output power of more than 200 mW under pulsed operation from an anti-reflection coated facet.
Several hurdles to further enhance the performance of semipolar III-Nitride laser diodes are addressed in this work. Particularly, we focused on improving their high operating voltage by thinning the p-GaN cladding layer and utilizing a transparent conductive oxide p-contact. On-wafer optical absorption measurements showed that a further reduction in voltage with thinner p-GaN was limited by increased optical loss due to increased mode overlap with the ITO/metal anode. In separate attempts to minimize bulk-related optical losses, we implemented a new design that consisted of an AlGaN electron blocking layer (EBL) placed remotely from the quantum wells (QWs) and a low p-waveguide Mg doping profile. A very low optical loss of about 2 cm-1 was extracted but the net improvement in differential efficiency was limited by lower internal injection efficiency due to carrier accumulation in the p-waveguide region. With an optimized design, that consisted of a lightly doped EBL close to the QWs and a UID p-waveguide, an improved light output power of 1.4 W at 1.5 A and a low threshold current density of 1.2 kA/cm2 were obtained.
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