J. K. Son

High-power GaN-based blue-violet laser diodes with AlGaN∕GaN multiquantum barriers

AlGaN∕GaN multiquantum barriers (MQBs) were introduced into violet AlInGaN laser diodes with an InGaN multiquantum-well structure, resulting in drastic improvements in lasing performance. Comparing with conventional AlGaN single electron blocking layer (EBL), lower threshold current of 32mA and higher slope efficiency of 1.12W∕A at room temperature has been achieved by using the AlGaN∕GaN multiquantum barrier. This improvement implies that p-type AlGaN∕GaN MQBs are more effective in suppressing the overflow of electrons than p-type AlGaN single EBL. Effective barrier heights of the MQBs should be higher than the single EBL due to the quantum effect of MQBs and the enhancement of p-type doping efficiency. Additionally, the effect of strain on InGaN multiquantum wells from the single EBL can be reduced by using the AlGaN∕GaN MQBs structure.

High-Performance Blue InGaN Laser Diodes With Single-Quantum-Well Active Layers

The authors report on the high-performance blue laser diodes (LDs) with an emission wavelength of ~448 nm employing InGaN single-quantum-well (QW) active layers. At 100-mW continuous-wave (CW) output power, operation current and voltage are, respectively, 150 mA and 5.3 V, corresponding to the wall plug efficiency of >12%, a record value for the single-mode InGaN LDs with blue wavelengths. The single QW blue LD showed normal temperature dependence of light output-current curves with the characteristic temperature of 170 K. In addition, we demonstrate a high level of catastrophic optical damage of >300 mW and long device lifetime under CW operation condition at room temperature.

Highly stable temperature characteristics of InGaN blue laser diodes

We report stable temperature characteristics of threshold current and output power in InGaN blue laser diodes emitting around 450nm. The threshold current is changed by <3mA in operation temperature range from 20to80°C, and even negative characteristic temperature is observed in a certain temperature range. This peculiar temperature characteristic is attributed to originate from unique carrier transport properties of InGaN quantum wells with high In composition, which is deduced from the simulation of carrier density and optical gain. In addition, slope efficiency is also maintained well and wall plug efficiency is even improved as temperature increases.

Evaluation of radiative efficiency in InGaN blue-violet laser-diode structures using electroluminescence characteristics

The authors analyzed radiative efficiency of InGaN laser diodes (LDs) emitting at 405nm. Based on semiconductor rate equations, the radiative efficiency is unambiguously determined by the analysis of electroluminescence characteristics. The radiative efficiency exceeds 70% even far below threshold of ∼3mA at a high temperature of 80°C. This highly radiative characteristic is attributed to reduced contribution of nonradiative recombination in LDs with low-dislocation-density active material. It is also found that the radiative efficiency is almost independent of threshold current, indicating that nonradiative recombination is not a major factor which determines lasing threshold in 405nm emitting InGaN LDs having low dislocation density.

Efficient Alternating Current Operated White Light-Emitting Diode Chip

The structure of a single light-emitting diode (LED) chip for operation under high voltage alternating current (ac) conditions was proposed. The chip was designed and fabricated as an integrated circuit with several isolated LED structures grown on a single insulated sapphire substrate. The efficient blue LED chip for operation directly from 220-V ac power source has been designed, fabricated, and analyzed. The white ac operated LED (ACOLED) chip demonstrates a value of averaged luminous flux ~ 320 lm and luminous efficiency 80 lm/W under operation from 220-V ac source with 4 W of power consumption. The ladder type of circuit and the efficient ACOLED chip configuration results in proficient operation under ac conditions.

Single-mode blue-violet laser diodes with low beam divergence and high COD level

We demonstrate GaN-based high-power single transverse-mode laser diodes (LDs) emitting at 405 nm. LD structures are designed to exhibit a high level of catastrophic optical damage and small beam divergence angle. By the control of refractive index profiles, we achieved a vertical beam divergence angle of as low as 17.5/spl deg/ and maximum output power of as high as 470 mW under continuous-wave operation condition. In addition, nearly fundamental transverse-mode operation is demonstrated up to 500-mW pulsed output power by far-field investigation.

Comparative investigation of InGaN quantum well laser diode structures grown on freestanding GaN and sapphire substrates

Comparative analysis of optical characteristics of In0.08Ga0.92N∕In0.03Ga0.97N multiquantum well (MQW) laser diode structures grown on freestanding GaN and on sapphire substrates is reported. Higher quantum efficiency, higher thermal activation energy, smaller Stokes-like shift, and shorter radiative lifetime are observed for InGaN MQWs on GaN substrate than those of the same MQWs on sapphire substrate. From time-resolved optical analysis, we find that not only an increase in nonradiative lifetime due to reduced dislocation density but also a decrease in radiative lifetime caused by suppressed piezoelectric field play an important role in enhancing optical properties of InGaN MQWs on GaN substrates.

Direct comparison of optical characteristics of InGaN-based laser diode structures grown on pendeo epitaxial GaN and sapphire substrates

Direct comparison of optical properties and carrier dynamics of InGaN multiple quantum well (MQW) laser diode structures grown on pendeo epitaxial (PE)-GaN and sapphire substrates is reported. A strong increase in quantum efficiency and a dramatic reduction in stimulated emission threshold are observed for InGaN MQWs on PE-GaN substrates as compared to MQWs on sapphire substrates. Based on temperature-dependent time-resolved optical analysis, the authors find that a significant increase in nonradiative lifetime due to suppressed dislocation density plays an important role in enhancing optical properties of InGaN MQWs grown on PE-GaN substrates, resulting in radiative-process dominant emission even at room temperature.

Recent achievements of AlInGaN based laser diodes in blue and green wavelength

AlInGaN based blue and blue-green LDs were investigated with regard to the characteristics of GaN semiconductor laser diodes. High power, single mode blue LDs with high COD level (~334mW under CW operation at 25°C, kink-free at 150mW) and long lifetime (~10000 hours under CW operation, 50mW 25°C) were achieved. No significant characteristic differences between blue LDs on LEO-GaN/sapphire and GaN substrate were observed. The blue-green LD which has the wavelength of 485 nm was successfully fabricated and demonstrated under CW operation 25°C, while it showed poor performances of LD characteristics compared to those of blue LDs. We believe that the poor performance of blue-green LDs were caused by the piezo-electric effect by lattice mismatch along C-axis of GaN, In fluctuation by lattice mismatch and In solubility limit in InGaN QWs and thermal annealing which was performed during the p-layer growth.

High-power AlInGaN-based violet laser diodes with InGaN optical confinement layers

InGaN optical confinement layers (OCLs) were introduced into blue-violet AlInGaN-based laser diodes (LDs), resulting in the drastic improvements of lasing performance. Comparing with conventional LD structure, the lowest threshold current density of 2.3kA∕cm2 has been achieved by adding 100-nm-thick InGaN OCLs which represented maximum optical confinement factor. Additionally, we observed the high quantum efficiency and the uniform emission intensity distribution of InGaN quantum wells grown on lower InGaN OCL than on typical GaN layer. Upper InGaN OCL can reduce Mg diffusion from p-type layers to InGaN active region by separating the distance between InGaN quantum wells and p-type layers.