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We report on the effect of an AlGaN interlayer with a low Al composition, which is inserted between multiple quantum wells (MQWs) and an AlGaN electron blocking layer (EBL) in light-emitting diodes (LEDs). The band diagram shows that this interlayer reduces the hole barrier height between the last GaN barrier in MQWs and the AlGaN EBL to enhance the hole injection. The optical output power at 20 mA of LEDs with interlayer is increased by 20% more than that of LEDs without an interlayer. This improvement is attributed to the enhanced radiative recombination rate, which is due to a more uniform hole distribution and higher hole concentration in MQWs.

The effects of indium segregation and hydrogen on the optical and structural properties of InGaN/GaN multiple quantum wells, grown by metalorganic chemical vapor deposition were investigated. Photoluminescence and high-resolution transmission electron microscopy analysis showed that two types of indium-rich regions can be formed in the InGaN well layers. Self-assembled quantum dot-like indium-rich regions were found in the well layer grown at a normal growth temperature. These regions behaved as luminescent centers, showing a maximum indium content at the center of indium-rich region. However, randomly-distributed indium-segregated regions, which formed near the upper interface of the InGaN well layers during the subsequent high-temperature annealing process led to the degradation of the optical properties by generating defects such as misfit dislocations. The use of hydrogen during the growth interruption was found to be very effective in suppressing the formation of indium-segregated regions in the InGaN well layers.

We demonstrate the surface plasmon-enhanced blue light-emitting diodes (LEDs) using Ag nanoparticles embedded in p-GaN. A large increase in optical output power of 38% is achieved at an injection current of 20 mA due to an improved internal quantum efficiency of the LEDs. The enhancement of optical output power is dependent on the density of the Ag nanoparticles. This improvement can be attributed to an increase in the spontaneous emission rate through resonance coupling between the excitons in multiple quantum wells and localized surface plasmons in Ag nanoparticles embedded in p-GaN.

We report on the improvement of light output power of InGaN/GaN blue light-emitting diodes (LEDs) by lateral epitaxial overgrowth (LEO) of GaN using a pyramidal-shaped SiO(2) mask. The light output power was increased by 80% at 20 mA of injection current compared with that of conventional LEDs without LEO structures. This improvement is attributed to an increased internal quantum efficiency by a significant reduction in threading dislocation and by an enhancement of light extraction efficiency by pyramidal-shaped SiO(2) LEO mask.

A GaInN light-emitting diode (LED) structure is analyzed that employs a separate epitaxial growth for the p-type region, i.e., the AlGaN electron-blocking layer (EBL) and p-type GaN cladding layer, followed by wafer or chip bonding. Such LED structure has a polarization-inverted EBL and allows for uncompromised epitaxial-growth optimization of the p-type region, i.e., without the need to consider degradation of the quantum-well active region during p-type region growth. Simulations show that such an LED structure reduces electron leakage, reduces the efficiency droop, improves hole injection, and has the potential to extend high efficiencies into the green spectral region.

The forward voltage characteristics of GaInN light-emitting diodes are studied in the temperature range of 80 K to 450 K. The forward-voltage-vs.-temperature curve has a “two-slope” characteristic with a slope of dVf/dT = −1.7 mV/K and −8.0 mV/K at room temperature and cryogenic temperatures, respectively. To investigate the two-slope characteristic, we perform transmission-line-model measurements on p-type GaN and show that both p-type contact and sheet resistance decrease drastically with increasing temperature. We conclude that dVf/dT in the high-slope region is limited by p-type sheet and contact resistance, and in the low-slope region by the GaN pn junction properties.

We demonstrate a high efficiency and an improvement of the electrical properties in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) using intentionally formed V-shaped pits. Efficiency droop behaviors are measured and LEDs with V-shaped pits act like LEDs with a low dislocation density. The reverse voltage at −10 μA of LEDs with V-shaped pits shows −120 V, which is comparable to p-i-n rectifiers grown on a free-standing GaN, and reverse leakage current is decreased indicating electrical passivation of dislocation. A calculated diode ideality factor shows that electron tunneling at low forward voltage is suppressed in LEDs with V-shaped pits.

An unequivocal correlation between the onset of high injection and the onset of the efficiency droop is demonstrated in GaInN light-emitting diodes over a wide range of temperatures. The diode voltage at the onset of high injection and the voltage at the onset of the efficiency droop are correlated by the equation VHigh-injection onset + ΔV ≈ VDroop onset. The excess voltage, ΔV, determined to be 0.3 V, drops partially over the p-type neutral region. The resulting electric field sweeps electrons out of the active region and results in substantial electron leakage despite high barriers that confine the carriers to the active region.

The frequency dependence of current spreading and light output power (LOP) was investigated for InGaN/GaN light-emitting diodes (LEDs) grown on n-GaN templates with different threading dislocation densities (TDDs) that were operated using alternating-current (AC). By comparison with LEDs with low TDDs, the LEDs with high TDDs showed a strong frequency dependence for both current spreading and LOP during high-frequency AC operation. The results were attributed to a weak carrier injection into InGaN quantum wells (QWs) during high frequency AC operation, which resulted from the suppression of carrier transport induced by the carrier scattering around negatively charged dislocation cores.

We investigate the effect of internal electric field in InGaN well layer of InGaN/GaN multiple quantum well light-emitting diodes (LEDs) on efficiency droop behavior. The simulation results show that the internal electric field in InGaN well layers of Ga-polar LEDs is same as the direction of external electric field by forward bias voltage, resulting in a strong efficiency droop. However, N-polar LEDs show that the efficiency droop is drastically improved due to an increase of internal quantum efficiency and carrier injection efficiency by weakening the internal electric field with increasing the forward bias voltage and decrease of electron overflow.

The effect of chip area on the temperature-dependent light-output power (LOP) in GaInN-based light-emitting diodes (LEDs) is investigated. The larger the chip size, the faster the reduction in LOP with increasing temperature becomes, indicating that increasing the size of LED chips, a technology trend for reducing the efficiency droop at high currents, is detrimental for high temperature-tolerant LEDs. In addition, it is found that regardless of chip size, the temperature-dependent LOP is identical for the LEDs operating at the same current density.

We report the effect of an electron blocking layer (EBL) on the piezoelectric field in InGaN/GaN multiple quantum well light-emitting diodes (LEDs). Electric-field-dependent ER measurements showed an enhanced piezoelectric field in LEDs with a p-AlGaN EBL compared with LEDs without EBL. In contrast, LEDs with a p-AlGaN EBL exhibited reduced blueshift and a sublinear increase of full width at half maximum in EL spectra at low current densities. These behaviors can be explained by the strong localization of injected carriers in dominant InGaN regions due to an increase of the piezoelectric field by subsequent growing EBL.

The effect of GaN grown on InN islands on InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) is investigated. The pit density of GaN grown on InN islands is decreased and the photoluminescence intensity of GaN grown on InN islands is increased compared with those of GaN without InN islands. The LEDs fabricated with GaN grown on InN islands show higher optical output power and lower reverse-bias leakage current than the LEDs without InN islands. These enhancements are attributed to the InN islands which reduce the threading dislocations and stress of GaN and MQWs.

We investigated the effect of graded Al composition in the p-type AlGaN/GaN superlattices (SLs) of InGaN/GaN multiple quantum well light-emitting diodes (LEDs) to improve their performance. The light output power and external quantum efficiency (EQE) of LEDs with Al composition grading was increased compared with those of LEDs without Al grading, indicating that the efficiency droop was reduced. The improved output power and EQE of LEDs with a graded Al composition was attributed to the increased hole injection by the reduced AlGaN barrier height and the suppression of potential spikes between the graded AlGaN and GaN layers in SLs.

We investigated InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with trapezoidal wells to improve the efficiency droop. MQW LEDs with trapezoidal wells showed a lower operating voltage and an improved efficiency droop with a low crossover current density of 5 A cm−2, which was a significant improvement over conventional LEDs that use rectangular wells. The external quantum efficiency was increased by 20% at a current density of 70 A cm−2. The improvement in efficiency droop of the MQWs with trapezoidal wells can be attributed to an increased internal quantum efficiency due to the enhanced overlap of the electron and hole wave functions at high current densities.

We report on the properties of green light-emitting diodes (LEDs) with a photonic crystal (PC) structure on p-GaN. A PC structure was fabricated by the selective area epitaxy of p-GaN using SiO2 nanopillars. The electrical characteristics of LEDs with PC were not degraded and the optical output power of green LEDs with PC was increased by 70% at 20 mA of injection current compared with that of conventional LEDs without PC. This enhancement of optical output power was attributed to the improvement in light extraction efficiency by the SiO2/p-GaN PC layer on p-GaN.

We report on Mg doping in the barrier layers of InGaN/GaN multiple quantum wells (MQWs) and its effect on the properties of light-emitting diodes (LEDs). Mg doping in the barriers of MQWs enhances photoluminescence intensity, thermal stability, and internal quantum efficiency of LEDs. The light output power of LEDs with Mg-doped MQW barriers is higher by 19% and 27% at 20 and 200 mA, respectively, than that of LEDs with undoped MQW barriers. The improvement in output power is attributed to the enhanced hole injection to well layers in MQWs with Mg-doped barriers.

We report on the properties of green light-emitting diodes (LEDs) on semipolar {1122} microfacets grown by selective area epitaxy. The green LEDs were realized by growing InGaN/GaN multiple quantum wells (MQWs) on the semipolar {1122} side facets of triangular n-GaN arrays. Excitation power- and temperature-dependent photoluminescence studies showed that InGaN/ GaN MQWs grown on semipolar {1122} planes had a high radiative recombination efficiency of carriers due to the negligibly small polarization-induced electric field compared to the MQWs on c-plane (0001). The electroluminescence of LEDs exhibited a peak wavelength in the green spectra range and a small blueshift with increasing injection current density.

The effect of an electron blocking layer (EBL) on the efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes (LEDs) is investigated. At low current density, the LEDs with a p-AlGaN EBL show a higher external quantum efficiency (EQE) than LEDs without an EBL. However, the EQE of LEDs without an EBL is higher than LEDs with an EBL as injection current density is increased. The improved EQE of LEDs without an EBL at high current density is attributed to the increased hole injection efficiency.