Ahmed N. Noemaun

Reduction in efficiency droop, forward voltage, ideality factor, and wavelength shift in polarization-matched GaInN/GaInN multi-quantum-well light-emitting diodes

Blue light-emitting diodes (LEDs) with polarization-matched GaInN/GaInN multi-quantum-well (MQW) active regions are grown by metal-organic vapor-phase epitaxy. The GaInN/GaInN MQW structure reduces the magnitude of polarization sheet charges at heterointerfaces in the active region. The GaInN/GaInN MQW LEDs are shown to have enhanced light-output power, reduced efficiency droop, a lower forward voltage, a smaller diode ideality factor, and decreased wavelength shift, compared with conventional GaInN/GaN MQW LEDs.

The origin of the high diode-ideality factors in GaInN/GaN multiple quantum well light-emitting diodes

We report on a significant decrease in the diode-ideality factor of GaInN/GaN multiple quantum well light-emitting diodes (LEDs), from 5.5 to 2.4, as Si-doping is applied to an increasing number of quantum barriers (QBs). The minimum ideality factor of 2.4 is obtained when all QBs are doped. It is shown that polarization-induced triangular band profiles of the undoped QBs are the major cause of the high ideality factors in GaInN/GaN LEDs. Numerical simulations show excellent agreement with the measured ideality factor value and its dependence on QB doping.

Elimination of total internal reflection in GaInN light-emitting diodes by graded-refractive-index micropillars

A method for enhancing the light-extraction efficiency of GaInN light-emitting diodes (LEDs) by complete elimination of total internal reflection is reported. Analytical calculations show that GaInN LEDs with multilayer graded-refractive-index pillars, in which the thickness and refractive index of each layer are optimized, have no total internal reflection. This results in a remarkable improvement in light-extraction efficiency. GaInN LEDs with five-layer graded-refractive-index pillars, fabricated by cosputtering TiO2 and SiO2, show a light-output power enhanced by 73% and a strong side emission, consistent with analytical calculations and ray-tracing simulations.

Enhanced electron capture and symmetrized carrier distribution in GaInN light-emitting diodes having tailored barrier doping

The confinement of electrons to the active region of GaInN light-emitting diodes(LEDs) is limited by the (i) inefficient electron capture into polar quantum wells, (ii) electron-attracting properties of electron-blocking layers (EBL), (iii) asymmetry in electron and hole transport, and (iv) unfavorable p-doping in the EBL for high Al content. To counteract these mechanisms, we employ tailored Si-doping in the quantum barriers (QBs). Experiments show a 37.5% enhancement in light-output power at high currents of one-QB-doped LEDs over all-QB-doped LEDs. These results are consistent with simulations showing that QB doping can be used to symmetrize the electron and hole distribution.

Inductively coupled plasma etching of graded-refractive-index layers of TiO2 and SiO2 using an ITO hard mask

Transparent dielectric layers with varying compositions of TiO2 and SiO2, and ITO are deposited on sapphire and Si substrates by using an RF sputter system. Inductively coupled plasma (ICP) reactive ion etching (RIE) of the ITO hard mask is examined under H2, CH4, and Cl2 chemical environments. The slope of the sidewall and the etch residue on the sidewall of the ITO hard mask are controlled by the flow rates of H2, CH4, and Cl2. ICP-RIE dry etch of TiO2 and SiO2 is investigated under fluorinated environments. Comparable etch rates of TiO2 and SiO2 (ratio ≈ 2:1) and high selectivity ≫ 1 over ITO are found. Graded-refractive-index (GRIN) layers, made up of multiple dielectric layers of TiO2 and SiO2, are patterned to form cylindrical pillars by ICP etching using the ITO hard mask. Fluorine containing residues are identified on the TiO2 and SiO2 surfaces. Various etch chemistries are investigated to obtain smooth, vertical, and residue-free sidewalls of the GRIN pillars.

Growth and characteristics of GaInN/GaInN multiple quantum well light-emitting diodes

We demonstrate GaInN multiple quantum well (MQW) light-emitting diodes (LEDs) having ternary GaInN quantum barriers (QBs) instead of conventional binary GaN QBs for a reduced polarization mismatch between QWs and QBs and an additional separate confinement of carriers to the MQW active region. In comparison with GaInN LEDs with conventional GaN QBs, the GaInN/GaInN LEDs show a reduced blueshift of the peak wavelength with increasing injection current and a reduced forward voltage. In addition, we investigate the density of pits emerging on top of the MQW layer that are correlated with V-defects and act as a path for the reverse leakage current. The GaInN/GaInN MQW structure has a lower pit density than the GaInN/GaN MQW structure as well as a lower reverse leakage current. Finally, the GaInN/GaInN MQW LEDs show higher light output power and external quantum efficiency at high injection currents compared to the conventional GaInN/GaN MQW LEDs. We attribute these results to the reduced polarization mismatch ...

Optically functional surface composed of patterned graded-refractive-index coatings to enhance light-extraction of GaInN light-emitting diodes

Graded-refractive-index (GRIN) coatings, composed of multiple dielectric layers of TiO2 and SiO2 are sputter-deposited on the nitrogen-face of thin-film GaInN/GaN light emitting diodes (LEDs). The thickness and refractive index of each layer in the GRIN stack is designed to minimize light trapping inside the LED caused by total internal reflection at the semiconductor–air interface. Patterning the GRIN stack forms an optically functional surface, which converts trapped modes of light into desirable extracted modes that have preferential directions. Inductively coupled-plasma reactive-ion-etching is used to fabricate various patterns, including arrays of cylindrical pillars and diamond-shaped pillars on the GRIN coatings. In comparison to an uncoated planar reference device, the light-output power is enhanced by 131% and 104% for an array of GRIN diamond-shaped pillars and an array of GRIN cylindrical pillars, respectively. This enhancement in light-output power is comparable to N-face roughened LEDs, whic...

Emission pattern control and polarized light emission through patterned graded-refractive-index coatings on GaInN light-emitting diodes

Patterned graded-refractive-index (GRIN) coatings that offer the controllability of far-field emission pattern and polarization properties of GaInN light-emitting diodes (LEDs) are investigated. Compared with a planar reference LED, the light-output power of an LED with patterned GRIN coatings (GRIN LED) is enhanced by about 69%. Furthermore, the GRIN LED has bidirectional emission peaks at about 45° off-surface-normal and polarized light emission with the maximum polarization ratio occurring at the same angle, i.e. the intensity maximum and the polarization-ratio maximum coincide. The large off-surface-normal emission of the GRIN LED results from the strong light extraction through the sidewalls of the GRIN pillars, which is in good agreement with results predicted from ray-tracing simulations.

Encapsulation shape with non-rotational symmetry designed for extraction of polarized light from unpolarized sources

A non-rotationally symmetric encapsulation shape - which takes advantage of the low reflection coefficient for transverse magnetic polarized light near Brewsters angle - designed to enhance extraction of a particular desired linear polarization from an unpolarized source is reported. The algorithm for optimization of the shape is described. Numerical ray-tracing simulations of the encapsulation shape are performed and predict an integrated enhancement of 8.3% in the ratio of desired polarization to undesired polarization when the refractive index of the encapsulant is 1.5. Experimental measurements of fabricated encapsulant shapes agree well with numerical predictions.