Yik-Khoon Ee

Polarization engineering via staggered InGaN quantum wells for radiative efficiency enhancement of light emitting diodes

Staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition are demonstrated as improved active region for visible light emitters. Theoretical studies indicate that InGaN QW with step-function-like In content in the quantum well offers significantly improved radiative recombination rate and optical gain in comparison to the conventional type-I InGaN QW. Experimental results of light emitting diode (LED) structure utilizing staggered InGaN QW show good agreement with theory. Polarization band engineering via staggered InGaN quantum well allows enhancement of radiative recombination rate, leading to the improvement of photoluminescence intensity and LED output power.

Light Extraction Efficiency and Radiation Patterns of III-Nitride Light-Emitting Diodes With Colloidal Microlens Arrays With Various Aspect Ratios

The fabrication studies of silica/polystyrene (PS) colloidal microlens arrays with various aspect ratios were performed on the III-nitride light-emitting diodes (LEDs). The use of colloidal-based microlens arrays led to significant enhancement in light extraction efficiency for III-nitride LEDs. In varying the aspect ratios of the microlens arrays, the engineering of various PS thicknesses was employed by using high-temperature treatment and redeposition process. The effects of PS thickness on the light extraction efficiency and far-field emission patterns of InGaN quantum-well (QW) LEDs were studied. The total output powers of microlens LEDs with various PS thicknesses exhibited 1.93-2.70 times enhancement over that of planar LEDs, and the use of optimized PS layer thickness is important in leading the enhancement of the light extraction efficiency in large angular direction.

Metalorganic Vapor Phase Epitaxy of III-Nitride Light-Emitting Diodes on Nanopatterned AGOG Sapphire Substrate by Abbreviated Growth Mode

Metalorganic vapor phase epitaxial (MOVPE) growth of GaN on nanopatterned AGOG sapphire substrates was performed, and characteristics of the light-emitting diode (LED) devices grown on patterned sapphire and planar substrates were compared. The nanopatterned sapphire substrates were fabricated by a novel process (AGOG) whereby aluminum nanomesas were epitaxially converted into crystalline Al2O3 via a two-stage annealing process. The GaN template grown on the nanopatterned sapphire substrate was done via an abbreviated growth mode, where a 15-nm thick, low-temperature GaN buffer layer was used, without the use of an etch-back and recovery process during the epitaxy. InGaN quantum wells (QWs) LEDs were grown on the GaN template on the nanopatterned sapphire, employing the abbreviated growth mode. The optimized InGaN QW LEDs grown on the patterned AGOG sapphire substrate exhibited a 24% improvement in output power as compared to LEDs on GaN templates grown using the conventional method. The increase in output power of the LEDs is attributed to improved internal quantum efficiency of the LEDs.

Self-Consistent Analysis of Strain-Compensated InGaN–AlGaN Quantum Wells for Lasers and Light-Emitting Diodes

Strain-compensated InGaN-AlGaN quantum wells (QW) are investigated as improved active regions for lasers and light emitting diodes. The strain-compensated QW structure consists of thin tensile-strained AlGaN barriers surrounding the InGaN QW. The band structure was calculated by using a self-consistent 6-band kmiddotp formalism, taking into account valence band mixing, strain effect, spontaneous and piezoelectric polarizations, as well as the carrier screening effect. The spontaneous emission and gain properties were analyzed for strain-compensated InGaN-AlGaN QW structures with indium contents of 28%, 22%, and 15% for lasers (light-emitting diodes) emitting at 480 (500), 440 (450), and 405 nm (415 nm) spectral regimes, respectively. The spontaneous emission spectra show significant improvement of the radiative emission for strain-compensated QW for all three structures compared to the corresponding conventional InGaN QW, which indicates the enhanced radiative efficiency for light emitting diodes. Our studies show the improvement of the optical gain and reduction of the threshold current density from the use of strain-compensated InGaN-AlGaN QW as active regions for diode lasers.

Enhancement of light extraction efficiency of InGaN quantum wells light emitting diodes using SiO2/polystyrene microlens arrays

Yik-Khoon Ee, Ronald A. Arif, Nelson Tansu, Pisist Kumnorkaew, and James F. Gilchrist Citation: Applied Physics Letters 91, 221107 (2007); doi: 10.1063/1.2816891 View online: http://dx.doi.org/10.1063/1.2816891 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/91/22?ver=pdfcov Published by the AIP Publishing

Investigation of the deposition of microsphere monolayers for fabrication of microlens arrays.

Convective deposition of a monolayer of microspheres by drawing a meniscus of a suspension across a substrate is used to fabricate microlens arrays to enhance the photon extraction efficiency of light emitting diodes (LEDs). The self-assembly of a colloidal crystal within the blade-drawn thin film is dominated by capillary forces and the thickness of this crystal depends on many parameters, including the deposition rate and particle size. This study investigates these and other parameters such as angle and hydrophobicity of the deposition blade that have not previously been considered. Using a confocal laser scanning microscope, the local and long-range order of the deposited particles are evaluated by the radial distribution function, and the fraction of the number of nearest neighbors and local bond order, demonstrating the dependence of the microstructure on the deposition parameters. Our results suggest previous descriptions of the critical deposition parameters are inadequate for understanding how various processing conditions influence deposition. For instance, increasing the deposition blade angle from 20 degrees up to 90 degrees requires an increase in deposition rate to achieve a monolayer deposition. The microlens arrays were fabricated on LEDs where polystyrene and silica are coated in consecutive depositions. Heat is used to sacrifice the polystyrene layers to result in an ordered array of partially buried silica microspheres that act as lenses to scatter light from the device. Enhancement in light extraction efficiency of 2.66 times was demonstrated for InGaN-based light emitting diodes employing micron scale microlens arrays with 1 um diameter silica microspheres.

III-Nitride Photonics

The progress in III-Nitride photonics research in 2009 is reviewed. The III-Nitride photonics research is a very active field with many important applications in the areas of energy, biosensors, laser devices, and communications. The applications of nitride semiconductors in energy-related technologies include solid-state lighting, solar cells, thermoelectric, and power electronics. Several new research areas in III-Nitride photonics related to terahertz photonics, intersubband quantum wells, nanostructures, and other devices are discussed.

Light extraction efficiency enhancement of InGaN quantum wells light-emitting diodes with polydimethylsiloxane concave microstructures

Improvement of light extraction efficiency of InGaN light emitting diodes (LEDs) using polydimethylsiloxane (PDMS) concave microstructures arrays was demonstrated. The size effect of the concave microstructures on the light extraction efficiency of III-Nitride LEDs was studied. Depending on the size of the concave microstructures, ray tracing simulations show that the use of PDMS concave microstructures arrays can lead to increase in light extraction efficiency of InGaN LEDs by 1.5 to 2.0 times. Experiments utilizing 2.0 micron thick PDMS with 1.0 micron diameter of the PDMS concave microstructures arrays demonstrated 1.70 times improvement in light extraction efficiency, which is consistent with improvement of 1.77 times predicted from simulation. The enhancement in light extraction efficiency is attributed to increase in effective photon escape cone due to PDMS concave microstructures arrays.

Spontaneous Emission and Characteristics of Staggered InGaN Quantum-Well Light-Emitting Diodes

A novel gain media based on staggered InGaN quantum wells (QWs) grown by metal-organic chemical vapor deposition was demonstrated as improved active region for visible light emitters. Fermis golden rule indicates that InGaN QW with step-function like In content in the well leads to significantly improved radiative recombination rate and optical gain due to increased electron-hole wavefunction overlap, in comparison to that of conventional InGaN QW. Spontaneous emission spectra of both conventional and staggered InGaN QW were calculated based on energy dispersion and transition matrix element obtained by 6-band k middotp formalism for wurtzite semiconductor, taking into account valence-band-states mixing, strain effects, and polarization-induced electric fields. The calculated spectra for the staggered InGaN QW showed enhancement of radiative recombination rate, which is in good agreement with photoluminescence and cathodoluminescence measurements at emission wavelength regime of 425 and 500 nm. Experimental results of light-emitting diode (LED) structures utilizing staggered InGaN QW also show significant improvement in output power. Staggered InGaN QW allows polarization engineering leading to improved luminescence intensity and LED output power as a result of enhanced radiative recombination rate.

Dislocation structure of GaN films grown on planar and nano-patterned sapphire

Plane view and cross-section transmission electron microscopy (TEM) images were used to compare the density, character, and curvature of dislocations developed during metalorganic vapor phase epitaxy (MOVPE) of GaN on planar c-plane sapphire with those developed during growth on nano-patterned c-plane sapphire. Scanning electron microscopy (SEM) characterization of GaN films at different stages of growth for both types of substrates complemented the TEM investigation. GaN growth on wafers patterned with an array of submicron sapphire bumps exhibited relatively uniform nucleation and initial growth, as well as early island coalescence. It is suggested that this coalescence results in a relatively small fraction of dislocations with partial screw character at the surface of the films grown on the patterned substrate, and that this may be responsible for the improvements in carrier lifetime and device efficiency seen in earlier studies on similar sapphire substrates.