Quantum Dot Injection Lasers Research Articles

Tunable single and dual mode operation of an external cavity quantum-dot injection laser

We investigate quantum-dot (QD) lasers in an external cavity using Littrow and Littman configurations. Here, we report on a continuously tunable QD laser with a broad tuning range from 1047 to 1130 nm with high stability and efficient side mode suppression. The full-width at half-maximum of the laser line is 0.85 nm determined mainly by the quality of the external grating. This laser can be operated in a dual-mode modus, where the mode-spacing can be tuned continuously between 1.1 and 34 nm. Simultaneous emission of the two laser modes is shown by sum frequency generation experiments.

Self-assembling quantum dots for optoelectronic devices on Si and GaAs

The formation of self-assembling quantum dots for the different strained material systems Ge/Si, InAs/GaAs and InP/GaInP in molecular beam epitaxy is discussed. Intense 1.55 and 1.3 μm room temperature photoluminescence is achieved for stacked Ge dots in Si and single InAs dot layers in GaAs, respectively. Small InP/Ga 0.52In 0.48P quantum dots emit in the visible red spectrum. Strain-induced vertical alignment, size modification and interdiffusion for stacked dot layers are studied. A blue shift of the ground state transition energy is observed for relatively large size stacked Ge and InAs dots. This is ascribed to enhanced strain driven intermixing in vertically aligned islands. For extremely small and densely stacked InP and InAs dots the stronger confinement causes a red shift of the ground state emission. InP/Ga 0.52In 0.48P quantum dot injection lasers with coupled three-fold stacked InP dots are presented. Ground state lasing is observed at room temperature with a maximum output power up to 250 mW. The threshold current density at 300 K is j thr =2.3 kA/ cm 2 , and an external quantum efficiency of η ext=8.5% is measured for a 2 mm long laser cavity.

Self-assembling InAs and InP quantum dots for optoelectronic devices

Stranski–Krastanov growth in molecular beam epitaxy allows the preparation of self assembling InAs and InP quantum dots on GaAs and Ga 0.52In 0.48P buffer layers, respectively. InAs dots in GaAs prepared by slow growth rates and low temperature overgrowth provide intense photoluminescence at the technologically important wavelength of 1.3 μm at room temperature. Strain induced vertical alignment, size modification and material interdiffusion for stacked dot layers are studied. A blue shift of the ground state transition energy is observed for the slowly deposited stacked InAs dots. This is ascribed to enhanced strain driven intermixing in vertically aligned islands. For very small densely stacked InP and InAs dots the reduced confinement shift causes a red shift of the ground state emission. The InP quantum dots show intense and narrow photoluminescence at room temperature in the visible red spectral range. First InP/Ga 0.52In 0.48P quantum dot injection lasers are prepared using threefold stacked InP dots. We observe lasing at room temperature in the wavelength range between 690–705 nm depending on the size of the stacked InP dots.

Self-assembling nanostructures and atomic layer precise etching in molecular beam epitaxy

We report on the preparation of 10 nm lateral size semiconductor structures based on island formation in strained layer growth in molecular beam epitaxy. Red light emitting InP quantum dot injection lasers are presented. They contain densely stacked layers of self-assembled InP quantum dots embedded in a Ga 0.51In 0.49P wave guide layer. In the second part of this contribution we report on a new atomic layer precise etching technique in MBE, which allows improved interface control for the preparation of semiconductor nanostructures. The etching process involves AsBr 3 exposure of a GaAs or AlGaAs surface. Switching between atomic layer precise growth and etching is possible within a few seconds.

Gain characteristics of quantum-dot injection lasers

The current dependence of the optical gain in lasers based on self-organized InGaAs quantum dots in a AlGaAs/GaAs matrix is investigated experimentally. A transition from lasing via the ground state of quantum dots to lasing via an excited state is observed. The saturated gain in the latter case is approximately four times greater than for the ground state. This result is attributable to the fourfold degeneracy of the excited level of quantum dots. The effect of the density of the quantum-dot array on the threshold characteristics is investigated. A lower-density array of dots is characterized by a lower threshold current density in the low-loss regime, because the transmission current is lower, while dense quantum-dot arrays characterized by a high saturated gain are preferable at high threshold gains.

Injection lasers with vertically aligned InP/GaInP quantum dots: Dependence of the threshold current on temperature and dot size

We report on threefold-stacked vertically aligned InP/GaInP quantum dot injection lasers emitting in the visible part of the spectrum (690–705 nm) with a low threshold current density of jth=172 A/cm2 at 90 K showing a thermally activated increase towards higher temperatures. We derived an activation energy for this behavior, which is found to be just one half of the energetic distance between the dot transition energy and the wetting layer band gap. Thus, we identify thermal evaporation of carriers out of the dots and into the wetting layer states as the process responsible for the increase in the threshold current. The nonradiative carrier lifetime in the wetting layer (τnrWL) is estimated to be approximately 250–400 ps.

Red-light-emitting injection laser based on InP/GaInP self-assembled quantum dots

Red-light-emitting quantum dot injection lasers have been prepared by solid-source molecular beam epitaxy. The separate confinement heterostructure contains densely stacked layers of self-assembled InP quantum dots embedded in Ga0.51In0.49P waveguide and Si/Be-doped Al0.53In0.47P cladding layers. Edge-emitting laser diodes are processed, which show quantum dot lasing at 90 K. Thereby, the threshold current density is 172 A/cm2. The energy of the laser line is at 1.757 eV, which is very close to the peak energy of subthreshold electroluminescence.

Negative Characteristic Temperature of InGaAs Quantum Dot Injection Laser

The range of negative characteristic temperatures in temperature dependences of threshold current density of low-threshold (In, Ga)As/(Al, Ga)As quantum dot injection lasers has been observed. A model describing the decrease in threshold current density with temperature at low temperatures is proposed.

Gain and differential gain of single layer InAs/GaAs quantum dot injection lasers

We present gain measurements and calculations for InAs/GaAs quantum dot injection lasers. Measurements of the modal gain and estimation of the confinement factor by transmission electron microscopy yield an exceptionally large material gain of 6.8(±1)×104 cm−1 at 80 A cm−2. Calculations including realistic quantum dot energy levels, dot size fluctuation, nonthermal coupling of carriers in different dots, and band filling effects corroborate this result. A large maximum differential gain of 2×10−12 cm2 at 20 A cm−2 is found. The width of the gain spectrum is determined by participation of excited quantum dot states. We record a low transparency current density of 20 A cm−2. All experiments are carried out at liquid nitrogen temperature.

Ordered arrays of quantum dots: Formation, electronic spectra, relaxation phenomena, lasing

Elastic relaxation on facet edges, renormalization of the surface energy of the facets, and interaction between islands via the strained substrate are the driving forces for self-organization of ordered arrays of uniform coherent three-dimensional islands on crystal surfaces. For a (100) surface of a cubic crystal, two-dimensional square lattice of pyramid-like islands (quantum dots) with the periodicity along the directions of the lowest stiffness [010] and [001] has the minimum energy among different one-dimensional and two-dimensional arrays. For the InAs GaAs(100) system, an equilibrium array of dots of the lateral size ∼ 120–140 Å exists in a fixed range of growth parameters. The main luminescence peak at 1.1 eV, as well as peaks of excited states coincide in energy with the peaks revealed in the calorimetric absorption spectra regardless of the amount of InAs deposited (2–5 ML). Raman spectra indicate significant strain in InAs dots. The “phonon bottleneck” effect is bypassed via multi-phonon exciton and carrier relaxation. Ultranarrow lines (>0.15 meV) are observed in cathodoluminescence spectra up to high temperatures. Low threshold current density operation via zero-dimensional states and ultrahigh temperature stability of the threshold current ( T 0=450 K) are realized for a quantum dot injection laser. Increase in the gain and significant reduction in the radiative lifetime are possible via the self-organization of vertically-coupled quantum dots (VECODs) arranged in a well ordered artificial three-dimensional tetragonal lattice.

Optical spectroscopy of self-organized nanoscale hetero-structures involving high-index surfaces

Similar effects are responsible for self-organization of periodically corrugated surface structures and ordered dot arrays on crystal surfaces. Strain relaxation on facet edges may result in the appearance of periodically corrugated surfaces for lattice-matched growth. Strain relaxation on facet edges and island interaction via the strained substrate act as driving forces for the formation of ordered arrays of uniform, strained lattice-mismatched islands on a crystal surface. A pseudoperiodic square lattice is manifested for the InAs-GaAs(100) system. Less ordered dots are formed on the GaAs(100) surface with a 4 monolayer GaSb deposition. New experimental methods are applied for the characterization of faceted nanoscale structures. For GaAs-AlAs multilayer structures grown on (311)A substrates, interface corrugation results in optical anisotropy of the same sign as expected from the low symmetry growth direction, making the main origin of the anisotropy unclear. Our quantitative optical reflectance and reflectance anisotropy studies show that the interface corrugation plays an important role for thin (less than 4 nm) GaAs layers. Mesa arrays from samples with InAs quantum dots grown on (100) surface are fabricated. The photoluminescence intensity is found to depend only weakly on the mesa size (1000 nm to 250 nm). The estimated electron-hole pair capture time into the InAs dot at room temperature is less than 1 ps. We also found a weak dependence of the threshold current density on the deep mesa stripe width (down to 3 μm) in the case of room temperature operated quantum dot injection lasers.