Tan Sakong

Room-temperature GaN vertical-cavity surface-emitting laser operation in an extended cavity scheme

A GaN-based vertical-cavity surface-emitting laser (VCSEL) has been demonstrated in an extended cavity structure. A VCSEL device had a long extended cavity, which consisted of a sapphire substrate as well as a GaN epilayer and had an integrated microlens on one side. High-reflection dielectric mirrors were deposited on both sides of the laser cavity. The laser was optically pumped and operated at room temperature. The VCSEL device lased at a low threshold excitation intensity of 160 kW/cm2. In contrast to a conventional microcavity-VCSEL structure, the VCSEL operated in multiple longitudinal modes with mode spacing consistent with its physical thickness.

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.

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.

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.

High power AlInGaN-based blue-violet laser diodes

High power and high efficiency AlInGaN-based laser diodes with 405 nm were fabricated for the post-DVD applications. Magnesium doped AlGaN/GaN multiple quantum barrier (MQB) layers were introduced into the laser diode structure, which resulted in considerable improvement in lasing performances such as threshold current and slope efficiency. Asymmetric waveguide structure was used in order to improve the characteristics of laser diodes. Aluminum content in the n-cladding layer was varied in connection with the vertical beam divergence angle and COD level. By decreasing Al content in the n-cladding layer, the vertical divergence angle was reduced to 17 degree and the COD level was enhanced to over 300mW. The maximum output power reached as high as 470 mW, the highest value ever reported for the narrow-stripe GaN LDs. In addition, the fundamental transverse-mode operation was clearly demonstrated up to 500 mW-pulsed output power.

Comprehensive study of time-lapsed peak shift in InGaN quantum well structures: Discrimination of localization effect from internal field effect

Time-lapsed emission peak shift behaviors in blue-light-emitting InGaN multiple quantum well (MQW) laser diodes with different well widths are systematically investigated by means of excitation power-dependent, time-resolved optical analysis. By investigating the main emission peak shift as a function of both time evolution and excitation power density, the amount of time-lapsed emission peak shift can be differentiated by two contributions: the excitation power dependent and independent ones. The authors conclude that the power-dependent (power-independent) time-lapsed peak shift can be attributed to the internal electric-field (carrier localization) effect present in vertical growth (lateral in-plane) direction of InGaN MQW laser diode structures.