Kh Li

Air-spaced GaN nanopillar photonic band gap structures patterned by nanosphere lithography

We report on the fabrication of ordered hexagonal arrays of air-spaced GaN nanopillars by nanosphere lithography. A self-assembled two-dimensional silica nanosphere mask was initially formed by spin-coating. Prior to pattern transfer to the GaN substrate, a silica-selective dry etch recipe was employed to reduce the dimensions of the nanospheres, without shifting their equilibrium positions. This process step was crucial to be formation of air-spaced hexagonal arrays of nanospheres, as opposed to closed-packed arrays normally achieved by nanosphere lithography. This pattern is then transferred to the wafer to form air-spaced nanopillars. By introducing air gaps between pillars, a photonic band gap (PBG) in the visible region can be opened up, which is usually nonexistent in closed-packed nanopillar arrays. The PBG structures were designed using the plane wave expansion algorithm for band structure computations. The existence and positions of band gaps have been verified through optical transmittance spect...

InGaN light-emitting diodes with indium-tin-oxide photonic crystal current-spreading layer

Photonic crystal patterns on the indium tin oxide layer of an InGaN/GaN light-emitting diode are fabricated via nanosphere lithography in combination with dry etching. The silica spheres acting as an etch mask are self-assembled into a hexagonal closed-packed monolayer array. After etching, the photonic crystal (PhC) pattern is formed across the indium-tin-oxide (ITO) films so that the semiconductor layers are left intact and thus free of etch damages. Despite slight degradation to the electrical properties, the ITO-PhC light-emitting diodes (LEDs) exhibit enhancements of their optical emission power by as much as 64% over an as-grown LED. The optical performances and mechanisms of the photonic crystal LEDs are investigated with the aid of rigorous coupled wave analysis and finite-difference time-domain simulations.

Optical and Thermal Analyses of Thin-Film Hexagonal Micro-Mesh Light-Emitting Diodes

Vertical thin-film light-emitting diodes (LEDs) with integrated micro-mesh arrays are reported. By removing the sapphire substrate through laser lift-off, vertical current conduction becomes possible, improving current spreading capability and thus electrical properties. Compared with the as-grown device, the thin-film micro-mesh LEDs emits 61% more optical power, attributed to enhanced light extraction through the micro-mesh, evidence of which is provided by confocal imaging. At 100 mA, the enhancement factor rises to >;100% attributed to low junction temperatures due to efficient heat conduction as verified by infrared thermometric imaging.

Mechanism of optical degradation in microstructured InGaN light-emitting diodes

While the enhancement of light extraction efficiency from microstructured InGaN light-emitting diodes (μLED) has been firmly established, there is concern over the effect of microstructuring on the device lifetimes. A study on the electrical characteristics and reliability of μLED arrays has been carried out. Despite improved optical performance, expanded device sidewalls served to accelerate the rate of optical degradation, adversely affect the lifetimes of devices. Through current-voltage plots and noise spectrum measurements, vertical current conduction along the plasma-damaged sidewalls was identified as the key degradation mechanism.

Polarized Emission From InGaN Light-Emitting Diodes With Self-Assembled Nanosphere Coatings

The polarization behavior of light emission from InGaN light-emitting diodes (LEDs) with nanosphere opal coatings has been studied. The close-packed nanosphere opal films are self-assembled with 220-nm polystyrene nanospheres onto the LEDs. The optical transmission properties of transverse electric and transverse magnetic polarized light have been measured as a function of detection angle; an integrated p/s ratio of 2.2 has been obtained at a detection angle of 70°. The polarization of light propagating through the opal film is strongly related to the photonic bandgap of the 3-D photonic crystal and is also dependent upon the angle of incidence. Theoretical calculations by the transfer matrix method are found to be consistent with the measured results.

Flexible free-standing III-nitride thin films for emitters and displays

The majority of a GaN light-emitting diode (LED) is released from its sapphire substrate through selective-area laser lift-off to form a freely suspended light emitter. By virtue of being suspended in air without supporting substrates, the ultrathin crystalline and crack-free film possesses flexibility and bendability. The free-standing LEDs benefit from significant relaxation of strain, evident from red-shifting of the E2(high) phonon frequencies as measured by Raman spectroscopy toward those of strain-free free-standing GaN substrates. The phonon frequencies remain invariant upon bending of the film; this indicates that the properties of the flexible device will not be dependent on the bending curvatures. The observation of pronounced spectral blue-shifts from the photoluminescence (PL) spectrum from the flexible regions further confirms the occurrence of strain relaxation in the quantum wells. Being free-standing and thus lacking a direct heat-sinking pathway, emissions from the different regions of the suspended film can be affected by thermal effects to different extents, which are investigated by long-wave infrared thermometry. Heat accumulation is determined to be most severe at the far end of the flexible stripe at higher currents, leading to reduced efficiencies and electroluminescence (EL) spectral red-shifts. Based on this architecture, a monolithic 3 × 4 dot-matrix microdisplay prototype is demonstrated, comprising three adjacent flexible stripe emitters with four individually addressable pixels on each stripe. This proof-of-concept demonstration opens up new opportunities for GaN optoelectronics for a wide range of flexible display and visual applications.

Whispering gallery mode lasing in optically isolated III-nitride nanorings.

III-nitride nanorings fabricated from a combination of hybrid-nanosphere-lithography and laser lift-off processes is demonstrated. Being formed on an interfacial metallic layer optically coupling between the optical ring and its substrate is eliminated, maximizing optical confinement of whispering gallery resonant mode within the ring cavity. The tapered cross-sectional profile also promotes coupling of emitted light into resonant modes. Optically pumped lasing with a dominant peak at 421.5 nm is observed at room temperature, with threshold energy density of ∼6.5  mJ/cm2. Etch-induced sidewall roughness causes scattering of light at the interface to diminish confinement, and is also responsible for the mode-splitting effect according to finite-difference time-domain simulations.

1-µm Micro-Lens Array on Flip-Chip Light-Emitting Diode

The fabrication of hexagonally close-packed micro-lens array on sapphire face of flip-chip bonded LED by nanosphere lithography is demonstrated. Self-assembled silica spheres serve as an etch mask to transfer hemispherical geometry onto the sapphire. The optical and electrical properties are evaluated. Without degrading the current?voltage (I?V) properties, the lensed LED shows an enhancement of 27.8% on light output power, compared with unpatterned LED. The emission characteristic is also investigated by performing finite-difference time-domain simulation, which is found to be consistent with the experimental results.

Influence of strain on emission from GaN-on-Si microdisks

Strain-relaxation effects in AlN-buffered GaN/InGaN microdisks pivoted on Si posts of varying radii have been studied by micro-Raman spectroscopy and scanning near-field optical spectroscopy (SNOS). With increasing undercut beneath the microdisks by chemical wet-etching, the mitigation of biaxial tensile stress is found to be dependent on the contact areas between the Si posts and GaN microdisks. Strain-relaxation reduces the quantum-confined Stark effect (QCSE) in the quantum wells (QWs), leading to an 18.3% enhancement in InGaN QW internal quantum efficiency (IQE). Light out-coupling is also improved in the suspended regions owing to reduced optical absorption at AlN/Si interface compared to the central region. Meanwhile, spectral blue-shifts of ~45.6 meV are observed from the near-field photoluminescence (nf-PL) spectrum towards the edge of the microdisk. Such localization of strain relaxation can be exploited for precise strain engineering of the microdisks. The emission wavelengths of the microdisks can be stabilized by balancing strain-relaxation effects with thermal effects.

Polarized emission from InGaN light-emitting diode with self-assembled opal coating

The polarization behaviors of light emission from InGaN light-emitting diodes (LEDs) with nanosphere opal coatings have been studied. The close-packed nanosphere opal films are self-assembled with 220 nm polystyrene nanospheres onto the LEDs. Optical transmission properties of the TE and TM polarized light have been measured as a function of detection angle; an integrated p/s ratio of 2.16 has been obtained at the detection angle of 70°. The polarization of light propagating through the opal film is strongly related to the photonic bandgap of the three-dimensional photonic crystal and is also dependent upon the angle of incidence. Theoretical calculations by the transfer matrix method are found to be consistent with the measured results.