K. H. Li

Ultralow-threshold electrically injected AlGaN nanowire ultraviolet lasers on Si operating at low temperature

Ultraviolet laser radiation has been adopted in a wide range of applications as diverse as water purification, flexible displays, data storage, sterilization, diagnosis and bioagent detection. Success in developing semiconductor-based, compact ultraviolet laser sources, however, has been extremely limited. Here, we report that defect-free disordered AlGaN core-shell nanowire arrays, formed directly on a Si substrate, can be used to achieve highly stable, electrically pumped lasers across the entire ultraviolet AII (UV-AII) band (∼320-340 nm) at low temperatures. The laser threshold is in the range of tens of amps per centimetre squared, which is nearly three orders of magnitude lower than those of previously reported quantum-well lasers. This work also reports the first demonstration of electrically injected AlGaN-based ultraviolet lasers monolithically grown on a Si substrate, and offers a new avenue for achieving semiconductor lasers in the ultraviolet B (UV-B) (280-320 nm) and ultraviolet C (UV-C) (<280 nm) bands.

InGaN light‐emitting diodes with indium‐tin‐oxide sub‐micron lenses patterned by nanosphere lithography

Close‐packed micro‐lenses with dimensions of the order of wavelength have been integrated onto the indium‐tin‐oxide (ITO) layer of GaN light‐emitting diodes employing nanosphere lithography. The ITO lens arrays are transferred from a self‐assembled silica nanosphere array by dry etching, leaving the semiconductor layer damage‐free. An enhancement of up to 63.5% on optical output power from the lensed light‐emitting diode (LED) has been observed. Lens‐patterned LEDs are also found to exhibit reduced emission divergence. Three‐dimensional finite‐difference time‐domain simulations performed for light extraction and emission characteristics are found to be consistent with the observed results.

III-nitride Light-emitting Diode with Embedded Photonic Crystals

A photonic crystal has been embedded within an InGaN/GaN light-emitting diode structure via epitaxial lateral overgrowth of a p-type GaN capping layer. The photonic crystal is a hexagonal-closed-packed array of nano-pillars patterned by nanosphere lithography; the capping layer planarizes the disconnected pillars to form a current-injection device. Optical properties of the nanostructures and devices are extensively studied through a range of spectroscopy techniques and simulations. Most significantly, the emission wavelengths of embedded photonic crystal light-emitting diodes are nearly invariant of injection currents, attributed to partial suppression of the built-in piezoelectric in the quantum wells.

Tunable clover-shaped GaN photonic bandgap structures patterned by dual-step nanosphere lithography

The fabrication of close-packed clover-shaped photonic crystal structure on GaN by dual-step nanosphere lithography is demonstrated. By shrinkage of spheres prior to pattern transfer, a non-closed-packed clover-shaped photonic bandgap (PBG) structure, as designed by modified 3D finite-difference time-domain simulation, is also realized. The PBG of the close-packed and non-close-packed clover-shaped structures is verified through optical transmission spectroscopy, found to agree well with simulated results. A threefold enhancement in photoluminescence (PL) intensity is observed from the optimized structure, when the PBG is tuned to overlap with the emission band of the InGaN/GaN multi-quantum wells. From time-resolved PL measurements, shortened decay lifetimes are observed.

High-Q whispering-gallery mode lasing from nanosphere-patterned GaN nanoring arrays

A hexagonal-close-packed ordered array of nanorings was fabricated on GaN with a modified nanosphere lithography process. The spheres initially served as etch masks for the formation of closed-packed nanopillars. The spheres were then shrunk and, with a layer of oxide deposited, the roles of the spheres became masks for liftoff. The final etch produced nanorings with wall widths of 140 nm. Photopumped lasing with splitting modes was observed at room temperature, with a low lase threshold of ∼10 mJ/cm2 and high quality factor of ∼5000, via whispering-gallery modes. The resonant frequencies were verified through finite-difference time-domain simulations.

Analysis of micro-lens integrated flip-chip InGaN light-emitting diodes by confocal microscopy

A hexagonally close-packed microlens array has been integrated onto the sapphire face of a flip-chip bonded InGaN light-emitting diode (LED). The micro-optics is formed by etching a self-assembled monolayer of 1-μm silica microspheres coated on the sapphire substrate, producing hemispherical sapphire lenses. Without degrading electrical characteristic, the light output power of the lensed LED is increased by more than a quarter compared with the unlensed LED. Enhanced light extraction via micro-optics is verified by rigorous coupled wave analysis. The focusing behavior of the micro-lenses, as well as the emission characteristics of the lensed LED, is studied by confocal microscopy.

Observation of enhanced visible and infrared emissions in photonic crystal thin-film light-emitting diodes

Photonic crystals, in the form of closed-packed nano-pillar arrays patterned by nanosphere lithography, have been formed on the n-faces of InGaN thin-film vertical light-emitting diodes (LEDs). Through laser lift-off of the sapphire substrate, the thin-film LEDs conduct vertically with reduced dynamic resistances, as well as reduced thermal resistances. The photonic crystal plays a role in enhancing light extraction, not only at visible wavelengths but also at infrared wavelengths boosting heat radiation at high currents, so that heat-induced effects on internal quantum efficiencies are minimized. The observations are consistent with predictions from finite-difference time-domain simulations.

Confocal microscopic analysis of optical crosstalk in GaN micro-pixel light-emitting diodes

The optical crosstalk phenomenon in GaN micro-pixel light-emitting diodes (LED) has been investigated by confocal microscopy. Depth-resolved confocal emission images indicate light channeling along the GaN and sapphire layers as the source of crosstalk. Thin-film micro-pixel devices are proposed, whereby the light-trapping sapphire layers are removed by laser lift-off. Optical crosstalk is significantly reduced but not eliminated due to the remaining GaN layer. Another design involving micro-pixels which are completely isolated is further proposed; such devices exhibited low-noise and enhanced optical performances, which are important attributes for high-density micro-pixel LED applications including micro-displays and multi-channel optical communications.

AlGaN nanowire ultraviolet lasers on Si

In this work, we demonstrate electrically injected ultraviolet lasers by spontaneously formed AlGaN nanowires on Si. The optical cavity is formed due to the Anderson localization of light.

Ultralow threshold electrically injected AlGaN nanowire ultraviolet lasers on Si

Ultraviolet (UV) lasers are of paramount importance for applications in water purification, diagnosis and bio-agent detection. Here we report that, with the use of dislocation-free AlGaN nanowires formed directly on Si substrate, electrically injected UV emission in the wavelength range from 319 nm to 335 nm can be readily achieved, which is the shortest wavelength range ever reported for electrically injected semiconductor lasers. In this work, catalyst-free AlGaN nanowire arrays are grown directly on Si substrate by radio frequency plasma-assisted molecular beam epitaxy (MBE). Our detailed calculation shows that such vertically aligned randomly distributed sub-wavelength scale nanowire array can sustain random lasing action. Various lasing peaks from 319 nm to 335 nm can be measured from such AlGaN nanowire samples under electrical injection. The threshold is measured to be in the range of tens of A/cm2 at cryogenic temperature, which is significantly smaller than the commonly reported GaN-based quantum well lasers. The measured linewidth is as narrow as 0.2 nm.