D. J. Mowbray

Improved performance of 1.3μm multilayer InAs quantum-dot lasers using a high-growth-temperature GaAs spacer layer

The use of a high-growth-temperature GaAs spacer layer is demonstrated to significantly improve the performance of 1.3μm multilayer self-assembled InAs∕InGaAs dot-in-a-well lasers. The high-growth-temperature spacer layer inhibits threading dislocation formation, resulting in enhanced electrical and optical characteristics. Incorporation of these spacer layers allows the fabrication of multilayer quantum-dot devices emitting above 1.3μm, with extremely low room-temperature threshold current densities and with operation up to 105°C.

Optimizing the growth of 1.3 μm InAs/InGaAs dots-in-a-well structure

The structural and optical properties of GaAs-based 1.3 μm InAs/InGaAs dots-in-a-well (DWELL) structures have been optimized in terms of different InGaAs and GaAs growth rates, the amount of InAs deposited, and In composition of the InGaAs quantum well (QW). An improvement in the optical efficiency is obtained by increasing the growth rate of the InGaAs and GaAs layers. A transition from small quantum dots (QDs), with a high density (∼5.3×1010u200acm−2) and broad size distribution, to larger quantum dots with a low dot density (∼3.6×1010u200acm−2) and narrow size distribution, occurs as the InAs coverage is increased from 2.6 to 2.9 monolayers. The room-temperature optical properties also improve with increased InAs coverage. A strong dependence of the QD density and the QD emission wavelength on the In composition of InGaAs well has been observed. By investigating the dependence of the dot density and the high-to-width ratio of InAs islands on the matrix of InGaAs strained buffer layer (SBL), we show that the in...

New physics and devices based on self-assembled semiconductor quantum dots

Self-assembled semiconductor quantum dots (QDs) exhibit fully quantized electronic states and high radiative efficiencies. This makes them highly suitable both for fundamental physics studies of zero-dimensionality, atomic-like semiconductor systems and applications in a range of novel electro-optical devices. This review discusses recent important advances in the study and application of semiconductor QDs. Using a wide range of optical spectroscopy techniques, it is possible to obtain a detailed understanding of the electronic structure and dynamical carrier processes. Such an understanding is required for the implementation of a wide range of QD-based devices.

Electronic band structure of AlGaInP grown by solid‐source molecular‐beam epitaxy

The compositional dependence of the electronic band structure of (AlxGa1−x)0.52In0.48P lattice matched to GaAs is reported. Epitaxial layers, grown by solid‐source molecular‐beam epitaxy, with excellent structural and optical quality are obtained over the whole compositional range. Optical spectroscopic techniques are used to study the electronic band structure as a function of composition. The low‐temperature, direct excitonic band gap is found to be given by Eg(x)=1.979+0.704x eV and the lowest band gap becomes indirect for xc=0.50±0.02. The low‐temperature excitonic direct band gap of Al0.52In0.48P is measured to be 2.680 eV.

Low threshold current density and negative characteristic temperature 1.3μm InAs self-assembled quantum dot lasers

By combining optimized growth of the GaAs spacer layers and p-type modulation doping of the quantum dots, a 1.3μm emitting self-assembled quantum dot laser exhibiting both a low threshold current density and negative-T0 temperature behavior at room temperature is achieved. Spontaneous emission measurements provide no evidence for enhanced Auger recombination in doped devices. The negative T0 exhibited by the doped device is consistent with a delayed thermalization of carriers within the quantum dot ensemble.

Exciton-phonon interactions in quantum wells and superlattices

Abstract The effects of the exciton-phonon interaction on the properties of quantum wells and superlattices are reviewed. The phonon sidebands of the absorption and photoluminescence spectra, the lifetime broadening of the exciton line at room temperature, and resonant Raman scattering are discussed. Most experimental work has been performed on the GaAs/(Al,Ga)As, (In, Ga)As/(In, Al)As and (In, Ga)As/InP material systems, but results for these III–V compounds are compared, where possible, with those for the more polar II–VI semiconductors. The importance of exciton-phonon interactions for room temperature devices based on the exciton absorption, and for photoluminescence spectroscopy as a semiconductor characterisation technique, is emphasised.

Enhanced room-temperature quantum-dot effects in modulation-doped InAs/GaAs quantum dots

Modulation-doped InAs/GaAs quantum dots (QDs) show bright photoluminescence (PL) at 300 K, linear increase of PL intensity on excitation at 300 K and rather temperature insensitive PL intensity and carrier lifetime, in contrast to undoped QDs. Systematic analyses indicate that those advantageous behaviors come from the enhanced Coulomb attraction due to excess carriers in doped QDs. The stronger Coulomb interaction increases the thermal activation energy, keeps more carriers in QDs, and provides enhanced QD characteristics at room temperature.

GaInP–AlGaInP band offsets determined from hydrostatic pressure measurements

Low‐temperature (1.8 K) photoluminescence spectra of a Ga0.52In0.48P–(Al0.58Ga0.42)0.52In0.48P multiple quantum well have been measured as a function of hydrostatic pressure from 0 to 5.0 GPa. The extrapolation to zero pressure of the energy of the indirect k‐and‐real‐space barrier X to well Γ transition allows the direct determination of the valence band offset for this heterojunction system. A value of ΔEv=(0.35±0.05)ΔEg is found, in good agreement with values previously determined from the theoretical modeling of quantum well transition energies.

In situ annealing enhancement of the optical properties and laser device performance of InAs quantum dots grown on Si substrates

The addition of elevated temperature steps (annealing) during the growth of InAs/GaAs quantum dot (QD) structures on Si substrates results in significant improvements in their structural and optical properties and laser device performance. This is shown to result from an increased efficacy of the dislocation filter layers (DFLs); reducing the density of dislocations that arise at the Si/III-V interface which reach the active region. The addition of two annealing steps gives a greater than three reduction in the room temperature threshold current of a 1.3 μm emitting QD laser on Si. The active region of structures grown on Si have a room temperature residual tensile strain of 0.17%, consistent with cool down from the growth temperature and the different Si and GaAs thermal expansion coefficients. This strain limits the amount of III-V material that can be grown before relaxation occurs.

Competitive carrier interactions influencing the emission dynamics of GaAsSb-capped InAs quantum dots

The optical properties of InAs/GaAs quantum dots capped with a GaAsSb quantum well are investigated by means of power-dependent and time-resolved photoluminescence. The structure exhibits the coexistence of a type-I ground state and few type-II excited states, the latter characterized by a simultaneous carrier density shift of the peak position and wavelength-dependent carrier lifetimes. Complex emission dynamics are observed under a high-power excitation regime, with the different states undergoing shifts during specific phases of the measurement. These features are satisfactorily explained in terms of band structure and energy level modifications induced by two competitive carrier interactions inside the structure.