Wen-Che Tsai

Effects of growth temperature on InN∕GaN nanodots grown by metal organic chemical vapor deposition

InN nanodots grown on GaN by metal organic chemical vapor deposition using conventional growth mode as well as flow-rate modulation epitaxy at various growth temperatures (550–730°C) were investigated. We found that different precursor injection schemes together with the effect of growth temperatures greatly influenced the surface morphology of InN nanodots and their photoluminescence (PL) properties. The sample grown at around 650°C showed the highest growth rate. For samples grown at higher temperatures, the residual carrier concentration was reduced and the PL efficiency was improved. Furthermore, we found that the growth of InN nanodots is still sustainable even at a temperature higher than 700°C while retaining their optical quality.

Tunable slow light device using quantum dot semiconductor laser

This investigation experimentally demonstrates a tunable slow light device using a quantum dot (QD) semiconductor laser. The QD semiconductor laser at 1.3 mum fabricated on a GaAs substrate is grown by molecular beam epitaxy. Tunable slow light can be achieved by adjusting the bias current and wavelength detuning. The slow light device operated under probe signal from 5 to 10 GHz is presented. Moreover, we also demonstrate that the tunable slow light device can be used in a subcarrier multiplexed system.

Optical properties associated with strain relaxations in thick InGaN epitaxial films.

Structural and optical properties of thick InGaN layers with strain and composition inhomogeneities are investigated. High resolution x-ray diffractions (XRD) and reciprocal space mapping (RSM) along an asymmetric axis reveal that the In composition inhomogeneity is accompanied by strain relaxations during the growth of thick InGaN layers. According to the structural analysis, the commonly observed double photoluminescence (PL) peaks have been confirmed to be associated with the strain relaxation in thick InGaN films. Temperature-dependent PL measurements further indicate that the relaxed phase in InGaN films exhibits better emission efficiency than the strained phase. Recombination dynamics reveal that the carrier localization effect is more pronounced in the relaxed phase due to the compositional pulling effect. The correlations between emission efficiency and localization effect in thick InGaN films are discussed.

Dynamic characteristics of long-wavelength quantum dot vertical-cavity surface-emitting lasers with light injection.

This investigation experimentally demonstrates the dynamic characteristics of quantum dot vertical-cavity surface-emitting lasers (QD VCSEL) without and with light injection. The QD VCSEL is fully doped structure on GaAs substrate and operates in the 1.3 mum optical communication wavelength. The eye diagram, frequency response, and intermodulation distortion are presented. We also demonstrate that the frequency response enhancement by light injection technique allows us to improve the performance of subcarrier multiplexed system.

Size-dependent strain relaxation in InN islands grown on GaN by metalorganic chemical vapor deposition

We report Raman measurements on InN islands grown on GaN by metalorganic chemical vapor deposition. The Raman frequency of the InN E2 mode is found to decrease exponentially with the island’s aspect ratio, indicating a size dependent strain relaxation during the island formation. Our results suggest that most of the strain at the InN–GaN interface have been released plastically during the initial stage of island formations. After that, the residual strain of only −3.5×10−3 is further relaxed elastically via surface islanding. The experimental data are in agreement with the strain relaxation predicted from a simplified model analysis as well as three-dimensional finite-element simulations.

Structural and optical properties of In-rich InGaN nanodots grown by metallo-organic chemical vapor deposition

The surface morphologies, alloy compositions and emission properties of In-rich InxGa1−xN nanodots (x≥0.87) grown by metallo-organic chemical vapor deposition at various growth temperatures (550–750 °C) were investigated. We found that the nucleation of InGaN dots was dominated by the surface migration of In adatoms. A higher Ga content can be achieved at lower growth temperatures due to the relatively lower migration ability of Ga adatoms. At higher growth temperatures, the InGaN dots tend to decompose into In-rich islands and a thin Ga-rich layer. These In-rich islands exhibit photoluminescence emission in the near-infrared range. Another visible emission band was also observed for samples grown at higher temperatures. The formation of a thin Ga-rich layer is likely to be responsible for the visible emission.

Tunable Slow Light using Quantum Dot VCSEL for Subcarrier Multiplexed System

We demonstrate that the slow light device can be used in a SCM system for the first time. Tunable slow light can be achieved by adjusting the bias current and wavelength detuning of QD VCSEL.

Dynamic characteristics of quantum dot VCSEL with external light injection

This investigation for the first time, experimentally demonstrates the dynamic characteristics of 1.3 mum quantum dot VCSEL without and with external light injection. The eye diagram, frequency response, and intermodulation distortion are presented.

Tunable Optical Delay using Quantum Dot VCSEL by Polarization Control

This study experimentally demonstrates the tunable optical delay using a 1.3 mum quantum dot VCSEL. Tunable optical delays of 42 ps for 10 GHz are achieved by varying the bias current and the signal polarization

Singlemode Monolithically Quantum-Dot Vertical-Cavity Surface-Emitting Laser in 1.3 μm with Side-mode Suppression ratio g 30dB

We present monolithically quantum-dot vertical-cavity surface-emitting laser (QD VCSELs) operating in the 1.3 μm optical communication wavelength. The QD VCSELs have adapted fully doped structure on GaAs substrate. The output power is ∼330 μW with slope efficiency of 0.18 W/A at room temperature. Single mode operation was obtained with side-mode suppression ratio of > 30 dB. Modulation bandwidth was also presented for the first time.