Gallium nitride (GaN) ultraviolet (UV) laser diodes (LDs) show tremendous promise for optical communications, data storage, and medical applications due to their compact size and higher efficiency compared to gas lasers. Typically, GaN UV LDs utilize a symmetric waveguide structure surrounding a multiple quantum well (MQW) active region for optical confinement. By increasing the thickness of these waveguides, device performance can be enhanced by reducing absorption losses. However, thin waveguides offer decreased carrier losses and improved electrical performance. These two competing effects can be balanced through the use of an asymmetric waveguide structure, composed of a thin upper waveguide and thick lower waveguide, in order to minimize both carrier (hole) losses as well as optical losses. Here, we demonstrated an edge-emitting ridge waveguide UV GaN LD emitting at ~392 nm. Mirror facets were fabricated through reactive ion etch and potassium hydroxide wet etch. These LD structures with InGaN/GaN MQWs and AlGaN cladding layers were grown via metalorganic chemical vapor deposition on a patterned sapphire substrate and utilize an asymmetric 100 nm thick upper unintentionally doped GaN (uGaN) waveguide and 500 nm thick lower uGaN waveguide structure. We have successfully demonstrated a LD device with 1000 μm cavity length with a lasing threshold of 2.2 A, and 111.8 mW per facet peak optical output power with a differential efficiency of 3%. This demonstration paves the way for GaN LDs with improved differential efficiency at high current densities through the use of optimized asymmetric waveguide structures.
Surface properties are important for structures such as micropillars and nanowires, which are critical for emerging devices including μLEDs, nano-lasers, and vertical power transistors due to increased surface to volume ratios. Fabrication of III-Nitride micropillars can be realized through a top-down approach, where structures are defined through lithography and reactive ion etching (RIE). While effective at forming these micropillar structures, RIE etching leaves behind roughened, non-vertical sidewalls. This surface damage increases non-radiative recombination, forms current leakage paths, and can severely degrade device performance. However, damage can be removed through a follow-up wet etch in potassium hydroxide (KOH) solution. KOH acts as a crystallographic etchant, preferentially exposing vertical <1-100> m-planes, producing smooth, vertical sidewalls. Here, we investigate KOH wet etch passivation for 2.5 μm diameter top-down fabricated GaN micropillars utilizing different temperatures and solution concentrations, and the effects of a Ni etch mask present during wet etching. We observed an average etch rate of 11.67 nm/min for micropillars etched in 60% AZ400k solution compared to 9.44 nm/min for micropillars etched in 20% AZ400k solution, both at a temperature of 80°C. At a constant 40% AZ400k concentration, an average etch rate of 14.39 nm/min for micropillars etched at 90°C are observed compared to 9.89 nm/min for micropillars etched at 70°C. Micropillars with a Ni etch mask present during KOH etching have an average etch rate of 9.46 nm/min compared to 12.83 nm/min for those without a Ni mask. The effects of KOH etching work to further optimize the performance of GaN-based micropillar and nanowire devices.
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