Sang-Heon Han

Effect of electron blocking layer on efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes

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.

Surface plasmon-enhanced light-emitting diodes using silver nanoparticles embedded in p-GaN

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.

Temperature dependent efficiency droop in GaInN light-emitting diodes with different current densities

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.

Identifying the cause of the efficiency droop in GaInN light-emitting diodes by correlating the onset of high injection with the onset of the efficiency droop

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.

Effect of Mg doping in the barrier of InGaN/GaN multiple quantum well on optical power of light-emitting diodes

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.

Improvement of efficiency droop in InGaN/GaN multiple quantum well light-emitting diodes with trapezoidal wells

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.

Improvement of efficiency and electrical properties using intentionally formed V-shaped pits in InGaN/GaN multiple quantum well light-emitting diodes

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.

Improvement of GaN-based light-emitting diodes using p-type AlGaN/GaN superlattices with a graded Al composition

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.

Improvement of light output power of InGaN/GaN light-emitting diode by lateral epitaxial overgrowth using pyramidal-shaped SiO 2

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.

Analysis of the temperature dependence of the forward voltage characteristics of GaInN light-emitting diodes

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.