H. S. Paek

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.

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.

Characterization of optical and crystal qualities in InxGa1–xN/InyGa1–yN multi-quantum wells grown by MOCVD

Abstract We have investigated the optical and structural qualities of In x Ga 1− x N/In y Ga 1− y N multi-quantum wells (MQWs) on sapphire substrates using atomic force microscope (AFM), high resolution X-ray diffraction (HRXRD), and photoluminescence (PL). In 0.08 Ga 0.92 N/ In 0.02 Ga 0.98 N five MQWs were grown by metalorganic chemical vapor deposition with three different well growth rates. We found from HRXRD and AFM that the interface roughness of MQWs was improved and the density of threading dislocation with screw component was decreased from 1.2×10 9 /cm 2 to 2.5×10 7 /cm 2 with decreasing the growth rate. Moreover, PL measurements revealed that the MQWs grown with a lower growth rate represented higher PL intensity, narrower line width and less energy shift in power dependent PL. This implies that lower growth rate allows adatoms on the surface to have longer time to arrive at two-dimensional step ledges of growth front and thereby enhances crystal quality compared with higher growth rate, leading to enhanced optical and crystal quality of MQWs.

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.

Characterization of a-plane InGaN multiple-quantum wells grown on maskless lateral epitaxially overgrown a-plane GaN

We investigated the properties of nonpolar a-plane InGaN∕GaN multiple-quantum wells (MQWs) grown on maskless lateral epitaxial overgrowth (LEO) a-plane GaN∕r-sapphire. Many surface defects with asymmetric V-shape were observed on a-plane InGaN MQWs grown on the defective regions which were seed and coalescence regions. In the low defect regions, the surface defect density of a-plane InGaN MQWs was ∼1.0×107∕cm2, which was higher than that of conventional c-plane LEO-GaN, by measuring atomic force microscope and scanning tunneling electron microscope. The cathode luminescence intensity distribution of a-plane InGaN MQWs was significantly dependent on the distribution of surface asymmetric V-defect. Therefore, we suggest that the optical properties of a-plane InGaN active layer were affected by the asymmetric V-defects which were generated by interaction between the epitaxial defects and the limit of InGaN growth kinetics.

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.