S. L. Liew

p-doped 1.3 μm InAs/GaAs quantum-dot laser with a low threshold current density and high differential efficiency

A modification of the thickness of the low-growth-temperature component of the GaAs spacer layers in multilayer 1.3μm InAs∕GaAs quantum-dot (QD) lasers has been used to significantly improve device performance. For a p-doped seven-layer device, a reduction in the thickness of this component from 15to2nm results in a reduced reverse bias leakage current and an increase in the intensity of the spontaneous emission. In addition, a significant reduction of the threshold current density and an increase of the external differential efficiency at room temperature are obtained. These improvements indicate a reduced defect density, most probably a combination of the selective elimination of a very low density of dislocated dots and a smaller number of defects in the thinner low-growth-temperature component of the GaAs spacer layer.

Interwell intersubband electroluminescence from Si/SiGe quantum cascade emitters

The quantum cascade laser provides one potential method for the efficient generation of light from indirect materials such as silicon. While to date electroluminescence results from THz Si/SiGe quantum cascade emitters have shown higher output powers than equivalent III–V emitters, the absence of population inversion within these structures has undermined their potential use for the creation of a laser. Electroluminescence results from Si/SiGe quantum cascade emitters are presented demonstrating intersubband emission from heavy to light holes interwell (diagonal) transitions between 1.2 THz (250 μm) and 1.9 THz (156 μm). Theoretical modeling of the transitions suggests the existence of population inversion within the system.

Optical transitions in type-II InAs∕GaAs quantum dots covered by a GaAsSb strain-reducing layer

The excitation power dependence of the ground and excited state transitions in type-II InAs-GaAs0.78Sb0.22 quantum dot structure has been studied. Both transitions exhibit a strong blueshift with increasing excitation power but their separation remains constant. This behavior indicates a carrier-induced electric field oriented predominantly along the growth axis, which requires the holes to be localized in the GaAsSb above quantum dots. An accelerated blueshift of the ground state emission is observed once the excited state in the dots starts to populate. This behavior can be explained by a smaller spontaneous recombination coefficient for the excited state transition.

Effects of growth temperature on the structural and optical properties of 1.6μm GaInNAs∕GaAs multiple quantum wells

We have investigated the effects of growth temperature on the properties of 1.6 {mu}m GaInNAs/GaAs multilayer quantum wells (MQWs). Strong room-temperature optical efficiency is obtained at 1.58 {mu}m for the sample grown at 375 deg. C. However, the photoluminescence intensities with emission at similar wavelength are dramatically degraded for the samples grown at 350 and 400 deg. C. Structural investigations show that compositional modulation and defects occurred in the sample grown at 400 deg. C and possible point defects within the MQWs grown at 350 deg. C. Based on these observations, the mechanism of effects of growth temperature on near-1.55-{mu}m GaInNAs/GaAs MQWs is discussed.

Effects of growth temperature on the structural and optical properties of 1.6 μm GaInNAs/GaAs multiple quantum wells

We have investigated the effects of growth temperature on the properties of 1.6 {mu}m GaInNAs/GaAs multilayer quantum wells (MQWs). Strong room-temperature optical efficiency is obtained at 1.58 {mu}m for the sample grown at 375 deg. C. However, the photoluminescence intensities with emission at similar wavelength are dramatically degraded for the samples grown at 350 and 400 deg. C. Structural investigations show that compositional modulation and defects occurred in the sample grown at 400 deg. C and possible point defects within the MQWs grown at 350 deg. C. Based on these observations, the mechanism of effects of growth temperature on near-1.55-{mu}m GaInNAs/GaAs MQWs is discussed.

Si/SiGe quantum-cascade emitters for terahertz applications

The quantum cascade laser provides one possible method of realizing high efficiency light emission from indirect band gap materials such as silicon. Strain-symmetrized Si/SiGe samples designed to investigate the intersubband properties of quantum wells are examined. Electroluminescence data from Si/SiGe quantum-cascade staircases demonstrating edge emission from heavy-hole to heavy-hole transitions and light-hole to heavy-hole transitions are presented. In surface-normal emission only light-hole to heavy-hole electroluminescence is observed at ( wavelength) as predicted by theory. Modulation-doped SiGe quantum well samples are also investigated to determine the underlying physics in the system. Results of picosecond time resolved studies of the dynamics of the intersubband transitions using a free electron laser are presented which show approximately constant relaxation times of below .

Control of polarization and mode mapping of small volume high Q micropillars

We show that the polarization of the emission of a single quantum dot embedded within a microcavity pillar of elliptical cross section can be completely controlled and even switched between two orthogonal linear polarizations by changing the coupling of the dot emission with the polarized photonic modes. We also measure the spatial profile of the emission of a series of pillars with different ellipticities and show that the results can be well described by simple theoretical modeling of the modes of an infinite length elliptical cylinder.

Investigating the Capping of InAs Quantum Dots by InGaAs

The aim of this work is to investigate the capping of InAs quantum dots by InGaAs using atomic force microscopy in plan-view geometry and correlate this topographical information with scanning transmission electron microscopy observations that elucidate the microstructure and chemistry of these quantum dots. Preferential growth of the Ga-rich cap around dot islands may be due to the unfavourable lattice parameter associated with high In concentrations at dot apices.

In search of a Si/SiGe THz quantum cascade laser

While electroluminescence has been demonstrated at terahertz frequencies from Si/SiGe quantum cascade emitters, to date no laser has been achieved due to poor vertical confinement of the optical mode. A method of increasing the vertical confinement of the optical mode for a Si/SiGe quantum cascade laser is demonstrated using silicon-on-silicide technology. Such technology is used with epitaxial growth to demonstrate a strain-symmetrised 600 period Si/SiGe quantum cascade interwell emission and the polarisation is used to demonstrate the optical confinement. Electroluminescence is demonstrated at /spl sim/3 THz (/spl sim/100 /spl mu/m) from an interwell quantum cascade emitter structure. Calculated model overlap and waveguide losses for ridge waveguides are comparable to values from GaAs quantum cascade lasers demonstrated at terahertz frequencies.

Towards a Si/SiGe Quantum Cascade Laser for Terahertz Applications

While electroluminescence has been demonstrated at terahertz frequencies from Si/SiGe quantum cascade emitters, to date no laser has been achieved due to poor vertical confinement of the optical mode. A method of increasing the vertical confinement of the optical mode for a Si/SiGe quantum cascade laser is demonstrated using silicon-on-silicide technology. Such technology is used with epitaxial growth to demonstrate a strain-symmetrised 600 period Si/SiGe quantum cascade interwell emission and the polarisation is used to demonstrate the optical confinement. Electroluminescence is demonstrated at similar to 3 THz (similar to 100 mu m) from an interwell quantum cascade emitter structure. Calculated model overlap and waveguide losses for ridge waveguides are comparable to values from GaAs quantum cascade lasers demonstrated at terahertz frequencies. The effects of high doping levels in Si/SiGe quantum cascade structures is also investigated with impurity emission demonstrated rather than intersubband emission for the highest doping levels used in the cascade active regions.