Chia-En Lee

Hole injection and efficiency droop improvement in InGaN/GaN light-emitting diodes by band-engineered electron blocking layer

A graded-composition electron blocking layer (GEBL) with aluminum composition increasing along the [0001] direction was designed for c-plane InGaN/GaN light-emitting diodes (LEDs) by employing the band-engineering. The simulation results demonstrated that such GEBL can effectively enhance the capability of hole transportation across the EBL as well as the electron confinement. Consequently, the LED with GEBL grown by metal-organic chemical vapor deposition exhibited lower forward voltage and series resistance and much higher output power at high current density as compared to conventional LED. Meanwhile, the efficiency droop was reduced from 34% in conventional LED to only 4% from the maximum value at low injection current to 200 A/cm2.

Directional light extraction enhancement from GaN-based film-transferred photonic crystal light-emitting diodes

Experimental investigation of the directionality in the far-field pattern and light extraction enhancement in collected cone were performed in GaN-based film-transferred photonic crystal (PhC) light-emitting diodes (FTLEDs). Angular-resolved measurement revealed directional profile and azimuthal anisotropy in the far-field distribution with guided modes extraction. Good agreement according to Bragg’s diffraction theory and free photon band structure were achieved. The light enhancement in PhC FTLEDs compared to non-PhC FTLEDs within the collection cone angle was obtained according to measured three-dimensional far-field patterns. In a ±20° collection cone, collected light was enhanced by a factor of ∼2.4 for the collimated PhC FTLED.

Efficiency and Droop Improvement in GaN-Based High-Voltage Light-Emitting Diodes

The efficiency and electrical characteristics of GaN-based high-voltage light-emitting diodes (HV-LEDs) are investigated in detail. The spatial distribution of light output and simulation results showed that 100-V HV-LED with smaller microchips had superior current spreading. As a result, under 1-W operation, the luminous efficiency of 100-V HV-LED with smaller microchips was enhanced by 7.8% compared to that of 50-V HV-LED, while the efficiency droop behaviors were reduced from 28% in 50-V HV-LED to 25.8% in 100-V HV-LED. Moreover, smaller microchips exhibited lower series resistance and forward voltage, leading to higher wall-plug efficiency.

Further Enhancement of Nitride-Based Near-Ultraviolet Vertical-Injection Light-Emitting Diodes by Adopting a Roughened Mesh-Surface

In this letter, the nitride-based near-ultraviolet (NUV) vertical-injection light-emitting diodes (VLEDs) with roughened mesh-surface are proposed and demonstrated by a combination of pattern sapphire substrate, wafer bonding, laser lift-off, and chemical wet etching processes. With the help of adopting a roughened mesh-surface, the light-output power (at 350 mA) of the NUV-VLEDs could be further enhanced about 20% as compared with that of the conventional NUV-VLED.

Enhancement of Flip-Chip Light-Emitting Diodes With Omni-Directional Reflector and Textured Micropillar Arrays

The flip-chip light-emitting diodes (FC-LEDs) with a conductive omni-directional reflector and textured micropillar arrays were investigated. The micropillar arrays structure was formed on the bottom side of sapphire substrate by dry etching process to increase the light-extraction efficiency. The light output power of the FC-LED was increased by 65% for a 3.2-mum textured micropillar on the bottom side of the sapphire substrate. Our work offers promising potential for enhancing output powers of commercial light-emitting devices.

Far-field of GaN film-transferred green light-emitting diodes with two-dimensional photonic crystals.

Far-field distributions of GaN-based photonic crystal (PhC) film-transferred light-emitting diodes (FT-LEDs) were investigated. The thickness of the device is about 840 nm. The emission wavelength is around 520 nm. The PhC region is a square lattice with a/lambda around 0.5. Angular-resolved measurements in the Gamma-X and Gamma-M directions were made in the polarized-resolved manner. Guided mode extraction behavior in agreement with the two-dimensional free-photon band calculation was observed. In addition, the pseudo-TM behavior for the non-collinearly coupled modes was observed. The azimuthal-mapping of the angular-resolved spectra revealed the evolution of the collinearly and the non-collinearly coupled modes. Furthermore, the light enhancement of approximately 2.7x and the collimation angle about 102.3 degrees were achieved.

Nitride-Based Thin-Film Light-Emitting Diodes With Photonic Quasi-Crystal Surface

In this letter, the nitride-based thin-film light-emitting diodes (TFLEDs) with eight-fold photonic quasi-crystal (PQC) surfaces are proposed and demonstrated by a combination of wafer bonding, laser lift-off, and electron-beam lithography processes. By adopting a PQC surface, the light-output power (at 350 mA) of the PQC-TFLEDs exhibits 140% output power enhancement as compared with that of TFLEDs without a PQC surface.

Far-Field and Near-Field Distribution of GaN-Based Photonic Crystal LEDs With Guided Mode Extraction

The near-field and far-field distribution of GaN LEDs with square photonic crystal (PhC) lattice are experimentally investigated. The optical images of the near-field pattern are obtained from the guided electroluminescent light generated at the center of a special annular structure. For increasing lattice constant, symmetric patterns with varying number of petals according to the symmetry of the PhC are observed. The far-field distribution is studied with GaN-based film-transferred PhC LEDs with the thickness about 1500 nm. Angular-resolved measurements under electrical injection revealed guided modes extraction behavior and polarization based on the Braggs diffraction with the PhC lattice. Good agreement with the two-dimension free photon band structure is obtained.

High-Brightness InGaN–GaN Flip-Chip Light-Emitting Diodes With Triple-Light Scattering Layers

The flip-chip light-emitting diodes (FC-LEDs) with triple-light scattering layers were investigated comprising a top surface sapphire textured layer, an interface patterned sapphire layer, and a bottom naturally textured p-GaN layer. Such triple-textured layers are useful for light extraction efficiency enhancement. The light output power of FC-LEDs was increased 60% (at 350-mA current injection) compared to that of conventional FC-LEDs by implementing the triple-light scattering layers.

Generation of orthogonally polarized self-mode-locked Nd:YAG lasers with tunable beat frequencies from the thermally induced birefringence

The simultaneous self-mode-locking of two orthogonally polarized states in a Nd:YAG laser is demonstrated by using a short linear cavity. A total output power of 3.8 W can be obtained at an incident pump power of 8.2 W. The beat frequency Δfc between two orthogonally polarized mode-locked components is observed and measured precisely. It is found that the beat frequency increases linearly with an increase in the absorbed pump power. The origin of the beat frequency can be utterly manifested by considering the thermally induced birefringence in the Nd:YAG crystal. The present result offers a promising approach to generate orthogonally polarized mode-locked lasers with tunable beat frequency.