Gallium nitride (GaN) nanowire (NW) light emitting diodes (LEDs) are promising candidates for microdisplay applications due to smaller dimensions and potential for novel integration approaches. For the commonly adopted top-down GaN NW fabrication, the required dry etching steps tend to result in surface states, leading to reduced radiative recombination rates in LEDs. To passivate the surface and tune the diameter of the NWs, hydroxyl-based chemicals such as potassium hydroxide (KOH) are widely used to treat the surface of these nanostructures. However, studies on the effects of temperature, concentration, and the damage recovery aspects of hydroxyl etching of GaN NWs are very scarce. These etching parameters are of great importance for device performance. Here, these effects are explored thoroughly with a focus on the correlation of InGaN/GaN NW LED performances to KOH etching temperature, concentration, and time, together with a fundamental crystallographic analysis. The KOH concentration resulting in total removal of the NW base tapering and a collimated etch profile for InGaN NW LEDs was found to be 0.8 wt. % at a temperature of 45 °C. A 20 min etch at 23 °C with a 0.1 wt. % KOH concentration will remove surface states from a top-down fabricated NW LED to recover up to 90% of the peak photoluminescence (PL) intensity lost by the dry etch step. The oscillation behavior in PL intensity with regard to the KOH etch time has been demonstrated in InGaN/GaN NW LEDs for the first time, which will shed light upon the design and passivation of these devices for microdisplays.

No abstract

We report on the realization of top-down fabricated, electrically driven, deep-ultraviolet (DUV) AlGaN micropillar array light emitting diodes (LEDs) with high output power density. Ordered arrays of micropillars with the inverse-taper profile were formed from an AlGaN epitaxial stack (epistack) using a Ni-masked Cl2 plasma dry etch and KOH-based wet etching. Following deposition of the n-contact, polydimethylsiloxane was spin-coated and etched-back to reveal the tips of the pillars to allow for formation of the p-contact. The DUV LEDs were tested at the wafer-level using a manual probe station to characterize their electrical and optical properties, revealing stable electroluminescence at 286 nm with a narrow 9-nm linewidth. Optical output power was found to be linearly related to current density, with output power densities up to 35 mW/cm2, comparable to the results reported for epitaxially grown DUV nanowire LEDs. Simulations revealed that the inverse-taper profile of the micropillars could lead to large enhancements in light extraction efficiency (ηEXT) of up to 250% when compared to micropillars with vertical sidewalls. The realization of ordered, electrically driven, top-down fabricated micropillar DUV LEDs with competitive output power represents an important step forward in the development of high-efficiency, scalable DUV emitters for a wide range of applications.

Deep ultraviolet (DUV) AlGaN light-emitting diodes (LEDs) are promising alternatives for production of DUV light, offering many advantages over mercury arc lamps. In this work, AlGaN nanowires with an inverse taper profile were demonstrated through a wet etching process, enabling removal of the nanowires from the growth substrate in a novel peeling process to form flexible devices. AlGaN nanowires with taper angles of ∼22° were obtained following a 70 min etch in AZ400K. Nanowire taper angle was studied as a function of etch time and nanowire top diameter. Nanowires with inverse taper were then embedded in a flexible polymer layer and removed from their growth substrate, which could enable development of high-efficiency flexible micro-LEDs. Released nanowires embedded within the polymer liftoff layer exhibit strain relaxation induced redshift due to reduction in piezoelectric polarization electric field intensity. The inverse taper structure was found to promote enhanced light extraction from the nanowire. The demonstrated flexible DUV emitters with inverse taper are shown to improve the device efficiency and allow for realization of flexible emitters through a novel fabrication process for the first time.

Color tunable micro light emitting diodes (µLEDs) are proposed and realized, making use of V-grooves to vary the Indium content during growth. The V-grooves make use of semi-polar crystal planes and strain relaxation to provide distinct regions of low to high Indium concentration which are simultaneously integrated. The differing Indium content provides emission from 425 to 640 nm. µLEDs ranging from 2 to 500 µm are demonstrated to showcase the concept.