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.
We demonstrate long cavity (60.5 ) GaN-based vertical-cavity surface-emitting lasers (VCSELs) with a topside curved monolithic GaN lens, a buried tunnel junction (BTJ) current aperture, and a bottomside nanoporous GaN distributed Bragg reflector (DBR). Under pulsed operation (1% duty cycle, 1μs pulse), a VCSEL with a 9μm diameter aperture had a threshold current density of 6.6kA/cm2, a maximum output power of 3mW, and a differential efficiency of 5.6% for a lasing mode at 411nm. Under CW operation, the threshold current density was 7.3kA/ cm2, the differential efficiency was 2.8%, and a peak output power of 1.1mW at rollover was reached. Preliminary farfield patterns are presented and discussed.
High-efficiency green laser diodes remain a challenge for researchers and manufacturers due to several factors intrinsic to the InAlGaN material system. Notably, the high indium content quantum wells (QW) are sensitive to thermal damage in the form of indium segregation, particularly when the QWs are exposed to the high-temperature growth steps of the ptype AlGaN cladding layers in MOCVD. This paper presents device results from blue distributed feedback (DFB) laser diodes grown entirely by MOCVD compared to material grown by MOCVD through the active region and overgrown with low-temperature p-AlGaN using remote plasma chemical vapor deposition (RPCVD). Preliminary data demonstrate comparable performance from MOCVD-only and MOCVD with RPCVD p-cladding, showing a full-width at half maximum emission of 3 pm and side mode suppression ratio of 19 dB. RPCVD enables the growth of high-quality ptype AlGaN layers at temperatures below the thermal damage threshold of In-rich QWs, leading to longer wavelength devices. The degree of thermal damage to the In-rich green QWs is assessed using fluorescence microscopy to directly compare the RPCVD-enhanced p-AlGaN growths at various growth temperatures over all-MOCVD n-side and green quantum wells.
GaN based edge emitting laser diodes typically use AlGaN or InGaN for mode confinement in waveguide cladding layers. Defect formation, high voltage, and lifetime issues limit the possible thickness and composition. Nano-porous GaN is a lattice matched, high index contrast material under investigation to replace AlGaN or InGaN for optical confinement. This opens up new designs to improve power and efficiency in GaN laser diodes. Electrically injected lasers have been fabricated using nano-porous GaN cladding, leading to a reduction in threshold current density at a cost to efficiency. Methods to reduce excess loss and improve heat dissipation will be discussed.
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