Nelson Tansu

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

Optimization of internal quantum efficiency (IQE) for InGaN quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered InGaN QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered InGaN QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the InGaN sub-layers will not affect the performance of the staggered InGaN QWs detrimentally. The growths of linearly-shaped staggered InGaN QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of InGaN QWs LEDs and other approaches to reduce dislocation in InGaN QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in InGaN QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer.

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.

Improvement in spontaneous emission rates for InGaN quantum wells on ternary InGaN substrate for light-emitting diodes

The spontaneous emission characteristics of green- and red-emitting InGaN quantum wells (QWs) on ternary InGaN substrate are analyzed, and the radiative recombination rates for the QWs grown on ternary substrate were compared with those of InGaN QWs on GaN templates. For green- and red-emitting InGaN QWs on In0.15Ga0.85N substrate, the spontaneous emission rates were found as ∼2.5-3.2 times of the conventional approach. The enhancement in spontaneous emission rate can be achieved by employing higher In-content InGaN ternary substrate, which is also accompanied by a reduction in emission wavelength blue-shift from the carrier screening effect. The use of InGaN substrate is expected to result in the ability for growing InGaN QWs with enhanced spontaneous emission rates, as well as reduced compressive strain, applicable for green- and red-emitting light-emitting diodes.

Low-threshold-current-density 1300-nm dilute-nitride quantum well lasers

Metalorganic chemical vapor deposition-grown In0.4Ga0.6As0.995N0.005 quantum well (QW) lasers have been realized, at an emission wavelength of 1.295 μm, with threshold and transparency current densities as low as 211 A/cm2 (for L=2000 μm) and 75 A/cm2, respectively. The utilization of a tensile-strained GaAs0.67P0.33 buffer layer and GaAs0.85P0.15 barrier layers allows a highly-compressively-strained In0.4Ga0.6As0.995N0.005 QW to be grown on a high-Al-content lower cladding layer, resulting in devices with high current injection efficiency (ηinj∼97%).

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

Efficiency-Droop Suppression by Using Large-Bandgap AlGaInN Thin Barrier Layers in InGaN Quantum-Well Light-Emitting Diodes

The electrical and optical characteristics of InGaN quantum-well light-emitting diodes with large-bandgap AlGaInN thin barriers were analyzed with the consideration of carrier transport effect for efficiency droop suppression. The lattice-matched AlGaInN quaternary alloys with different compositions, thicknesses, and positions were employed as thin barrier layers (1-2 nm) surrounding the InGaN QW in LED structures. The increased effective barrier heights of AlGaInN thin barrier led to suppression of carrier leakage as compared to conventional InGaN QW LEDs with GaN barrier only. The current work provides a comprehensive simulation taking into consideration the carrier transport in self-consistent manner, and the finding indicated the use of thin layers of AlGaInN or AlInN barriers as sufficient for suppressing the droop in InGaN-based QW LEDs. The efficiency of InGaN QW LED with the insertion of lattice-matched Al0.82In0.18N thin barrier layers showed the least droop phenomenon at high current density among the investigated LEDs. The thickness study indicated that a thin layer (<; 2 nm) of large-bandgap material in the barrier region was sufficient for efficiency droop suppression.

Growths of staggered InGaN quantum wells light-emitting diodes emitting at 520–525 nm employing graded growth-temperature profile

Three-layer staggered InGaN quantum wells (QWs) light-emitting diodes (LEDs) emitting at 520–525 nm were grown by metal-organic chemical vapor deposition by employing graded growth-temperature profile. The use of staggered InGaN QW, with improved electron-hole wave functions overlap design, leads to an enhancement of its radiative recombination rate. Both cathodoluminescence and electroluminescence measurements of three-layer staggered InGaN QW LED exhibited enhancements by 1.8–2.8 and 2.0–3.5 times, respectively, over those of conventional InGaN QW LED.