N. Kirstaedter

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A/spl middot/cm/sup -2/ at 77 K and of 62 A/spl middot/cm/sup -2/ at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of /spl sim/80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to /spl sim/50 meV, ultrahigh material gain of /spl sim/10/sup 5/ cm/sup -1/, differential gain of /spl sim/10/sup -13/ cm/sup 2/ and strong nonlinear gain effects with a gain compression coefficient of /spl sim/10/sup -16/ cm/sup 3/. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is /spl sim/0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 /spl mu/m and 1.37 /spl mu/m at 300 K.

Room-temperature continuous-wave lasing from stacked InAs/GaAs quantum dots grown by metalorganic chemical vapor deposition

We report on quantum dot (QD) lasers made of stacked InAs dots grown by metalorganic chemical vapor deposition. Successful growth of defect-free binary InAs/GaAs QDs with high lateral density (dl⩾4×1010 cm−2) was achieved in a narrow growth parameter window. The room-temperature photoluminescence (PL) intensity is enhanced up to a factor of 3 and the PL peak width is reduced by more than 30% when a thin layer of In0.3Ga0.7As is deposited onto the InAs QDs. A QD laser with a single sheet of such InAs/InGaAs/GaAs QDs exhibits threshold current densities as low as 12.7 and 181 A/cm2 at 100 and 300 K, respectively. Lasers with threefold stacked QDs show ground-state lasing and allow for cw operation at room temperature.

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.

InAs–GaAs Quantum Pyramid Lasers: In Situ Growth, Radiative Lifetimes and Polarization Properties

We have realized injection lasers based on InAs–GaAs and InGaAs–GaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 A. The structures with relatively small dots (~80 A) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 Acm-2) and ultrahigh characteristic temperatures (T0=350–425 K). For operation temperatures above 100–130 K, T0 decreases and the threshold current density increases (up to 0.95–3.3 kAcm-2 at room temperature) due to carrier evaporation from QPs. Larger InAs QPs (~140 A) providing better carrier localization exhibit saturation of the ground-state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size in the range 80–140 A being close to ~1.8–2 ns, respectively. A significant decrease in radiative lifetime is realized in vertically coupled quantum dots formed by a QP shape-transformation effect. The final arrangement corresponds to a three-dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. We achieved injection lasing in such an array for the first time.

InAs/GaAs Quantum Dots Grown by Metalorganic Chemical Vapor Deposition

InAs quantum dots (QDs) have been grown by metalorganic chemical vapor deposition on exactly (001) oriented GaAs using the Stranski-Krastanow growth mode. The samples exhibit a high average dot density of 4 x 10 10 cm -2 with no defects over macroscopic areas. The QDs show bright room temperature luminescence at around 1.1 eV. Vertical dot stacks consisting of up to 5 QD sheets with various GaAs separation layer thicknesses have been produced. Transmission eletron microscope images show pronounced QD ordering in the growth direction. For thin separation layers the dot luminescence is red shifted by ∼70 meV for the stacked dots as compared to single dot sheets. A low threshold (100 A/cm 2 at 77K) separate confinement heterojunction laser with a five-fold dot stack as an active medium operating at up to room temperature is demonstrated.

Three-dimensional arrays of self-ordered quantum dots for laser applications

Abstract Semiconductor heterostructures with quantum dots (QDs) are experimentally proved to exhibit properties expected for zero-dimensional systems, e.g. ultrasharp luminescence lines up to high temperatures, massively increased exciton oscillator strength per unit volume and temperature insensitivity of the radiative lifetime. When applied to the injection lasers these advantages help to increase strongly material gain, differential gain, to improve temperature stability of the threshold current and to suppress chirp. Threshold current densities as low as 60 A/cm 2 at 300 K are obtained. Formation of QDs with properties satisfying device requirements on QD size, shape, uniformity and density became possible by utilizing self-ordering phenomena on crystal surfaces.

Semiconductor quantum dots for application in diode lasers

Abstract Recent progress in the epitaxy (molecular beam epitaxy and metal-organic chemical vapor deposition) of strained heterostructures and the use of the Stranski-Krastanow growth mode allows to create spontaneously ordered, defect-free and dense arrays of nano-size islands. Such islands act as electronic quantum dots. In superlattices the islands are ordered in vertical stacks. Using such self-ordered InGaAs/AlGaAs quantum dots we have fabricated diode lasers for which some properties are superior to those of current lasers based on quantum wells. In particular, we have demonstrated low laser threshold current and high temperature stability of the threshold.

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Relaxation oscillation frequency of self-organized stacked quantum dot lasers at room temperature

Here, shallow mesa stripe lasers with 265 /spl mu/m cavity length and 6 /spl mu/m mesa width (MOCVD sample) or 550 /spl mu/m cavity length and 8 /spl mu/m mesa width (MBE sample) are used for the resonance frequency measurement. With such a short cavity for a MOCVD sample, the emission stems from the excited states. The threshold current is 21.8 mA and the external quantum efficiency is 55.6 percent. We give the photoluminescent spectrum of our quantum dot laser.

Static and dynamic properties of (InGa)As/GaAs quantum dot lasers

Threshold current, polarization and carrier dynamics in (InGa)As/GaAs quantum dot (QD) structures are studied. Large T/sub 0/ of 425 K, a low threshold current density of /spl sim/80 A cm/sup -2/ and an external quantum efficiency of 30% are measured between 50 K-120 K.