A. F. Tsatsul’nikov

QUANTUM DOT LASERS: BREAKTHROUGH IN OPTOELECTRONICS

Abstract Semiconductor heterostructures with self-organized quantum dots (QDs) have experimentally exhibited properties expected for zero-dimensional systems. When used as active layer in the injection lasers, these advantages help to strongly increase material gain and differential gain, to improve temperature stability of the threshold current, and to provide improved dynamic properties. Molecular beam epitaxy (MBE) represents a developed technology well suited for fabrication of self-organized QDs. Optimization of deposition parameters can ensure that the self-organized islands are small (∼10 nm), have a similar size and shape and form dense arrays. Saturation material gain is as high as 150000 cm −1 compared with QW values of about 3000 cm −1 . Maximum differential gain reported for QD lasers approaches 10 −12 cm 2 and exceeds the QW laser values by about three orders of magnitude. Direct observation of relaxation oscillations reveals present cut-off frequencies close to 10 GHz. High internal (>96%) and differential (70%) efficiencies at 300 K are realized. Using the novel concept of electronically-coupled QDs and oxide-defined 10 μm apertures, CW lasing with J th =180 A/cm 2 , is realized in surface-emitting QD lasers (300 K). Wall-plug efficiencies are up to 16%. Total currents as low as 68 μA are measured for 1μm apertures. GaAs-based lasers for the 1.3 μm range with low J th (65 A/cm 2 ) at room temperature (RT) are realized using InAs/InGaAs/GaAs QDs obtained by activated spinodal decomposition. In stripes the lasing occurs via the QD ground state ( J th =90 A/cm 2 ) for cavity lengths L >1 mm (uncoated). Differential efficiency is 55% and internal losses are 1.5 cm −1 . A characteristic temperature near RT is 160 K. 3W CW operation at RT is achieved. The recent progress in lasers based on self-organized MBE QDs already made it possible to fabricate devices with dramatically improved characteristics as compared to recent QW devices for the most important commercial applications.

Long-wavelength lasing from multiply stacked InAs/InGaAs quantum dots on GaAs substrates

An InAs quantum dot (QD) array covered by a thin InGaAs layer was used as the active region of diode lasers grown by molecular beam epitaxy on GaAs substrates. The wavelength of the ground-state transition in such heterostructures is in the 1.3 μm range. In the laser based on the single layer of QDs, lasing proceeds via the excited states due to insufficient gain of the ground level. Stacking of three QD planes prevents gain saturation and results in a low threshold (85 A/cm2 in broad-area 1.9-mm-long stripe) long-wavelength (1.25 μm) lasing at room temperature via the QD ground state with relatively high differential efficiency (>50%).

Effect of matrix on InAs self-organized quantum dots on InP substrate

InAs self-organized quantum dots in In0.53Ga0.47As and In0.52Al0.48As matrices have been grown on InP substrates by molecular beam epitaxy. The dot size in InGaAs has been found to be 3–4 times larger, but the areal density about an order of magnitude smaller than that in InAlAs. Low-temperature photoluminescence (PL) of the InAs/InGaAs quantum dots is characterized by a narrow (35 meV) PL line as compared to that of InAs/InAlAs quantum dots (170 meV). Quantum dot formation increases the carrier localization energy as compared to quantum well structures with the same InAs thickness in a similar manner for both InAs/InGaAs and InAs/InAlAs structures. The effect of the barrier band gap on the optical transition energy is qualitatively the same for quantum well and quantum dot structures. The results demonstrate a possibility of controlling the quantum dot emission wavelength by varying the matrix composition.

High-power continuous-wave operation of a InGaAs/AlGaAs quantum dot laser

A 1 W continuous-wave laser operation via the ground state of vertically coupled InGaAs quantum dots (VCQDs) in an AlGaAs matrix is demonstrated. VCQDs are directly revealed in transmission electron microscopy images of the laser structure. Ninety-six percent internal quantum efficiency is realized. The laser gain maximum shifts significantly with drive current towards higher photon energies in agreement with the relatively broad size distribution of VCQDs.

Room-temperature photopumped InGaN/GaN/AlGaN vertical-cavity surface-emitting laser

We report photopumped room-temperature surface-mode lasing at 401 nm in a InGaAlN vertical-cavity surface-emitting laser grown on a sapphire substrate using metal–organic vapor-phase epitaxy. A 2λ cavity was formed by a quarter-wave Al0.15Ga0.85N/GaN distributed Bragg reflector on the one side of the active layer and a GaN–air interface on the other. A multilayer structure composed of 12-fold-stacked ultrathin InGaN insertions in a GaN matrix served as an active layer providing ultrahigh material gain and making possible vertical lasing without use of the upper Bragg reflector.

Volmer–Weber and Stranski–Krastanov InAs-(Al,Ga)As quantum dots emitting at 1.3 μm

Quantum dots (QDs) formed on GaAs(100) substrates by InAs deposition followed by (Al,Ga)As or (In,Ga,Al)As overgrowth demonstrate a photoluminescence (PL) peak that is redshifted (up to 1.3 μm) compared to PL emission of GaAs-covered QDs. The result is attributed to redistribution of InAs molecules in the system in favor of the QDs, stimulated by Al atoms in the cap layer. The deposition of a 1 nm thick AlAs cover layer on top of the InAs–GaAs QDs results in replacement of InAs molecules of the wetting layer by AlAs molecules, leading to a significant increase in the heights of the InAs QDs, as follows from transmission electron microscopy. This effect is directly confirmed by transmission electron microscopy indicating a transition to a Volmer–Weber-like QD arrangement. We demonstrate an injection laser based on this kind of QDs.

Molecular beam epitaxy growth of GaAsN layers with high luminescence efficiency

(In)GaAsN bulk layers and quantum wells usually demonstrate lower photoluminescence intensity than the nitrogen-free compositions. In the present work we have carefully optimized both conductance and operation of a nitrogen plasma source as well as growth parameters of GaAsN layers. We found conditions when incorporation of nitrogen did not lead to formation of additional nonradiative recombination. There is some minimum growth rate to obtain good crystal and optical quality of GaAsN. At growth rates below this value the pattern of reflection high energy electron diffraction turns spotty and the growth proceeds in a three-dimensional mode. This leads to a steep decrease in luminescence efficiency of the grown layer. The minimum value of growth rate depends on nitrogen content and growth temperature. Defects caused by low temperature growth are removed by post-growth annealing. We achieved the same radiative efficiency of GaAsN samples with nitrogen content up to about 1.5% grown at 520 °C as that of a reference layer of GaAs grown at 600 °C. Compositional fluctuation in the GaAsN layers leads to the S-shape temperature dependence of photoluminescence peak position. Post-growth annealing reduces compositional fluctuation.

Long-wavelength emission in structures with quantum dots formed in the stimulated decomposition of a solid solution at strained islands

When an array of strained InAs nanoislands formed on a GaAs surface is overgrown by a thin (1–10 nm) layer of an indium-containing solid solution, stimulated decomposition of the solid solution is observed. This process causes the formation of zones of elevated indium concentration in the vicinity of the nanoislands. The volume of newly formed InAs quantum dots increases as a result of this phenomenon, producing a substantial long-wavelength shift of the photoluminescence line. This effect is enhanced by lowering the substrate temperature, and it depends weakly on the average width of the band gap of the solid solution. The indicated approach has been used successfully in achieving room-temperature emission at a wavelength of 1.3 µm.

Influence of metalorganic chemical vapor deposition growth conditions on In-rich nanoislands formation in InGaN/GaN structures

The influence of different growth conditions on the In distribution in ultrathin InGaN insertions in a GaN matrix is investigated by high-resolution transmission electron microscopy and an appropriate image evaluation technique. It is demonstrated that the indium distribution represents dense arrays of In-rich nanodomains inserted in a layer with a lower indium concentration. The sizes of the In-rich regions are about 4–5 nm at a growth temperature of 720 °C. Increasing the growth temperature leads to a strong decrease in the of nanoisland density and, also, a moderate decrease in their lateral size. Increasing the trimethylindium/trimethylgallium ratio strongly increases the density of the islands, but the lateral size remains weakly effected. The observations are in agreement with a thermodynamic model of island formation including entropy effects.

The influence of heat treatment conditions on the evaporation of defect regions in structures with InGaAs quantum dots in the GaAs matrix

Structures with In(Ga)As quantum dots in the GaAs matrix obtained using molecular-beam epitaxy are investigated using photoluminescence (PL) measurements and transmission electron microscopy. The structures were subjected in situ to the procedure of the selective thermal elimination of defect regions. Based on the results of the analysis of luminescence properties, a method for evaluating the crystalline quality of structures using the measurements of PL intensity for the GaAs matrix at high temperatures (as high as 400 K) is suggested. Procedures for the elimination of defects are investigated, namely, the single-stage selective elimination of InAs defect islands at 600°C and a two-stage procedure. The latter procedure additionally includes selective overgrowth with a thin AlAs layer and high-temperature (650–700°C) heat treatment. The optimal conditions of the process, which permit the obtaining of structures with a relatively low defect density without a considerable decrease in the density of coherent quantum dots, are found.