Nikolai N. Ledentsov

Radiative recombination in type‐II GaSb/GaAs quantum dots

Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self‐organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan‐view transmission electron microscopy studies reveal well developed rectangular‐shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Epitaxy of nanostructures

1. Introduction.- 2. Growth and Characterization Techniques.- 3. Self-Organization Phenomena at Crystal Surfaces.- 4. Engineering of Complex Nanostructures: Working Together with Nature.- 5. Devices Based on Epitaxial Nanostructures.- 6. Conclusion.- A. Energy of a Strained Disk with Perturbed Shape.- A.1 Energy of the Disk Boundary.- A.2 Elastic Relaxation Energy of the Disk.- A.3 Evaluation of Integrals.- A.4 Stiffness of the Disk against Shape Perturbations.- B. Elastic Interaction of Two Strained Disks.- C. Stiffness of a Hexagonal Array of Interacting Strained Disks.- References.

Self-organization processes in MBE-grown quantum dot structures

InAs quantum dots in a GaAs matrix have been prepared by molecular beam epitaxy using a self-organizing mechanism. A narrow size distribution of single dots of pyramidal shape (typically with a base of 12 ± 1 nm and a height of 4–6 nm) is created as directly imaged with plan-view and cross-section transmission electron microscopy. The dots exhibit self-organized short range order and preferentially align in rows along 〈100〉. The photoluminescence of the dot ensemble has, due to fluctuations in dot size, shape and strain, a FWHM of typically 50–60 meV. However, using highly spatially and spectrally resolved cathodoluminescence it is possible to directly excite a tiny fraction of all dots (typically only 30 dots). Under these excitation conditions the spectrum changes drastically into a series of ultrasharp lines with a FWHM < 0.15 meV, each originating from a different single InAs quantum dot. This directly visualizes their δ function-like density of electronic states, especially since the lines remain sharp even for kBT⪢FWHM.

Quantum dots: lasers and amplifiers

Continuous wave room-temperature output power of ~ 3 W for edge emitters and of 1.2 mW for vertical-cavity surface-emitting lasers is realized for GaAs-based devices using InAs quantum dots (QDs) operating at 1.3 µm. Characteristic temperatures up to 170 K below 330 K are realized. Simultaneously, differential efficiency exceeds 80% for these devices. Lasers emitting up to 12 W at 1140–1160 nm are useful as pump sources for Tm3+-doped fibres for frequency up-conversion to 470 nm. Both types of lasers show transparency current densities of 6 A cm−2 per dot layer, ηint = 98% and αi around 1.5 cm−1. Long operation lifetimes (above 3000 h at 50 °C heatsink temperature at 1.5 W CW) and improved radiation hardness as compared to quantum well (QW) devices are manifested. Cut-off frequencies of about 10 GHz at 1100 nm and 6 GHz at 1300 nm and low α factors resulting in reduced filamentation and improved M2 values in single-mode operation are realized. Quantum dot semiconductor optical amplifiers (QD SOAs) demonstrate gain recovery times of 120–140 fs, 4–7 times faster than bulk/QW SOAs. The breakthrough became possible due to the development of self-organized growth in QD technology.

Carrier-induced refractive index in quantum dot structures due to transitions from discrete quantum dot levels to continuum states

The carrier-induced refractive index in quantum dot (QD) structures due to optical transitions from QD levels to continuum states is considered. It is shown that, for large photon energies, the refractive index change is given asymptotically by the Drude formula. Calculations of the linewidth enhancement factor, α, show that α∼1 due to this contribution to the total refractive index. Furthermore, for highly localized QD states, the absorption coefficient at the photon energies ∼0.8–1.0 eV due to these transitions can be on the order of 103 m−1.

InGaAs/GaAs Quantum Dot Lasers with Ultrahigh Characteristic Temperature (T 0= 385 K) Grown by Metal Organic Chemical Vapour Deposition

Low threshold current density (AlInGa)As/GaAs lasers based on InGaAs quantum dots (QDs) are grown by metal organic chemical vapour deposition (MOCVD). Quantum dots deposited at 490° C and covered with GaAs are directly revealed in the active region. On a transmission electron microscopy (TEM) image of the laser structure no large clusters or dislocations are found over a macroscopic distance. We show that the properties of QD lasers can be strongly improved if the QDs are confined by Al0.3Ga0.7As barriers and the cladding layers are grown at high temperature. Optimisation of the laser structure geometry allows extension of the range of ultrahigh temperature stability (T0=385 K) of the threshold current to 50° C.

Ordering phenomena in InAs strained layer morphological transformation on GaAs (100) surface

Initial stage of InAs pseudomorphic layer transformation (1–3 ML) on GaAs (100) singular surface may result for sequential submonolayer molecular beam epitaxy in formation of a pseudoperiodic array of InAs ‘‘wires’’ along the [001] direction. Complex parquet structures having similar anisotropy are formed on misoriented surface (3° towards [0–11] direction). Increase in growth interruption time after each growth cycle for 2 ML InAs deposited on singular surface results in decomposition of the wires into dots arranged in a 2D square lattice. Intentional substrate misorientation stabilizes the initial ordering effect along [001] and does not change the direction of anisotropy.

On ultrafast optical switching based on quantum-dot semiconductor optical amplifiers in nonlinear interferometers

It is shown that interferometers containing quantum-dot semiconductor optical amplifiers can be effective for ultrafast cross-phase modulation and digital signal processing with low dependence on the specific random data pattern.

Effect of excited-state transitions on the threshold characteristics of a quantum dot laser

The general relationship between the gain and spontaneous emission spectra of a quantum dot (QD) laser is shown to hold for an arbitrary number of radiative transitions and an arbitrary QD-size distribution. The effect of microscopic parameters (the degeneracy factor and the overlap integral for a transition) on the gain is discussed. We calculate the threshold current density and lasing wavelength as a function of losses. The conditions for a smooth or step-like change in the lasing wavelength are described. We have simulated the threshold characteristics of a laser based on self-assembled pyramidal InAs QDs in the GaAs matrix and obtained; a small overlap integral for transitions in the QDs and a large spontaneous radiative lifetime. These are shown to be a possible reason for the low single-layer modal gain, which limits lasing via the ground-state transition for short (several hundreds of micrometers) cavity lengths.

20 Gb/s 85

980 nm vertical-cavity surface-emitting lasers based on submonolayer growth of quantum dots show clearly open eyes and operate error free with bit error rates better than 10 at 25 and 85degC for 20 Gb/s without current adjustment. The peak differential efficiency only reduces from 0.71 to 0.61 W/A between 25 and 85degC; the maximum output power at 25degC is above 10 mW.