Device characteristics of long-wavelength lasers based on self-organized quantum dots

The current state of the field of semiconductor lasers operating in the spectral range near 1.3 ??m and with an active region represented by an array of self-organized quantum dots is reviewed. The threshold and temperature characteristics of such lasers are considered; the problems of overcoming the gain saturation and of an increase in both the differential efficiency and emitted power are discussed. Data on the response speed under conditions of direct modulation and on the characteristics of lasers operating with mode synchronization are generalized. Nonlinear gain saturation, the factor of spectral line broadening, and the formation of broad gain and lasing spectra are discussed.     

A novel nonvolatile memory based on self-organized quantum dots

A novel type of memory based on self-organized quantum dots (QDs) is presented, which merges the advantages of the most important semiconductor memories, the dynamic random access memory (DRAM) and the Flash. A nonvolatile memory with fast access times and good endurance (>10¹⁵ write/erase cycles) as an ultimate solution seems possible. A storage time of 1.6 s at 300 K in InAs/GaAs QDs with an additional Al_0.9Ga_0.1As barrier is demonstrated and a retention time of 10⁶ years is predicted for GaSb QDs in an AlAs matrix. A minimum write time of 6 ns is obtained for InAs/GaAs QDs. This value is already in the order of the access time of a DRAM cell and at the moment limited by the RC low pass of the device. An erase time of milliseconds is shown in first measurements on GaSb/GaAs QDs at . Faster write/erase times below even at room temperature are expected for improved device structures.     

Electroluminescence of injection lasers based on vertically coupled quantum dots near the lasing threshold

Electroluminescence spectroscopy has been used in a wide range of temperatures (77–300 K) and driving current densities to study a laser heterostructure based on vertically coupled self-assembled InGaAs quantum dots (QD). It has been found that lasing occurs via the QD ground state in the entire temperature range. The temperature-independent position of the emission peak corresponding to the second excited state in QDs is explained.     

Vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots

Molecular beam epitaxy-grown 0.98-/spl mu/m vertical-cavity surface-emitting lasers (VCSELs) with a three-stack submonolayer (SML) InGaAs quantum-dot (QD) active region and fully doped Al/sub x/Ga/sub 1-x/As-GaAs DBRs was studied. Large-aperture VCSELs demonstrated internal optical losses less than 0.1% per one pass. Single-mode operation throughout the whole current range was observed for SML QD VCSELs with the tapered oxide apertures diameter less than 2 /spl mu/m. Devices with 3-/spl mu/m tapered-aperture showed high single-mode output power of 4 mW and external quantum efficiency of 68% at room temperature.     

Optically pumped intersublevel MidInfrared lasers based on InAs-GaAs quantum dots

We propose an optically pumped laser based on intersublevel transitions in InAs-GaAs pyramidal self-assembled quantum dots. A theoretical rate equations model of the laser is given in order to predict the dependence of the gain on pumping flux and temperature. The energy levels and wave functions were calculated using the 8-band k/spl middot/p method where the symmetry of the pyramid was exploited to reduce the computational complexity. Carrier dynamics in the laser were modeled by taking both electron-longitudinal optical phonon and electron-longitudinal acoustic phonon interactions into account. The proposed laser emits at 14.6 /spl mu/m with a gain of g/spl ap/ 570 cm/sup -1/ at the pumping flux /spl Phi/=10/sup 24/ cm/sup -2/ s/sup -1/ and a temperature of T=77 K. By varying the size of the investigated dots, laser emission in the spectral range 13-21 /spl mu/m is predicted. In comparison to optically pumped lasers based on quantum wells, an advantage of the proposed type of laser is a lower pumping flux, due to the longer carrier lifetime in quantum dots, and also that both surface and edge emission are possible. The appropriate waveguide and cavity designs are presented, and by comparing the calculated values of the gain with the estimated losses, lasing is predicted even at room temperature for all the quantum dots investigated.     

Injection lasers based on InGaAs quantum dots in an AlGaAs matrix

Arrays of vertically coupled InGaAs quantum dots (QDs) in an AlGaAs matrix have been used in injection lasers. Increase in the band gap of a matrix material by replacement of a GaAs matrix with an AlGaAs one led to dramatic increase in quantum dot localization energy. By using this approach, we reduced the thermal population of the matrix and wetting layer states and thus decreased room temperature threshold current density to 63 A/cm², increased differential efficiency up to 65%, and achieved room temperature continuous wave operation with output power of 1 W. Negative characteristic temperature has been observed in temperature dependence of threshold current density of these lasers in some temperature range. A qualitative explanation assuming a transition from non-equilibrium to Fermi population of QD states is proposed.     

Study of phonons in self-organized multiple Ge quantum dots

Raman scattering measurements were carried out in a self-organized multi-layered Ge quantum dot sample, which was grown using solid-source molecular-beam epitaxy, and consisted of 25 periods of 20-Å-high Ge quantum dots sandwiched by 20-nm Si spacers. The Ge-Ge optical phonon mode was found at 298.2 cm^?1, which was tuned by the phonon confinement and strain effects. Acoustic phonons related to Ge quantum dots have also been demonstrated.     

Formation and characterization of self-organized CdSe quantum dots

We have investigated the formation and characteristic of self-organized CdSe quantum dots (QDs) on ZnSe(001) surfaces with the use of photoluminescence (PL) and transmission electron microscopy (TEM). Coherent CdSe QDs are naturally formed on ZnSe surfaces, when the thickness of CdSe layers is around 2 ML. The plan-view TEM images exhibit that CdSe QDs have a relatively narrow distribution of QD size, and that the density of CdSe QDs is about 10¹⁰ cm⁻². The base structure of the CdSe dot is rhombic, which has the long axis of about 20 nm in length along direction. The temperature dependence of macro-PL spectra reveals that the behavior of self-organized CdSe QDs is quite different from that of ZnCdSe quantum well (QW), resulting from characteristic features of zero-dimensional structures of QDs. Moreover, the macro-PL results suggest the existence of QW-like continuous state lying over QD states. Micro-PL measurements show several numbers of high-resolved sharp lines from individual CdSe QDs. The linewidth broadening with temperature depends on peak energy position of the QDs. The linewidths of lower energy lines, corresponding to larger size QDs, are more temperature dependent.     

Wide range tunable laterally coupled distributed-feedback lasers based on InGaAs-GaAs quantum dots

The authors have investigated tunable distributed feedback (DFB) lasers based on InGaAs quantum dots grown by molecular beam epitaxy. Two-section tunable DFB lasers were fabricated by patterning laterally gain coupling binary superimposed gratings perpendicular to the ridge waveguide. Side-mode suppression ratios of up to 40 dB have been achieved. The tuning range covers 30 nm.     

Metastable population of self-organized InAs/GaAs quantum dots

In the present work, we report on the investigation of a p-n heterostructure with InAs/GaAs quantum dots (QD) by capacitance-voltage and deep level transient spectroscopy. We have observed controllable and reversible metastable population of the energy states of quantum dots and interface in the structure containing one plane of InAs QDs as a function of temperature of isochronous annealing as well as under bias-on-bias-off cooling conditions and white light illumination. This effect was attributed to the change in the Fermi level position due to the hole capture on self-trapped defects similar to the DX center in GaAs after isochronous annealing and white light illumination.     

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. Fiz. Tekh. Poluprovodn. 33, 1111–1114 (September 1999)     

STM probe-assisted site-control of self-organized InAs quantum dots on GaAs surfaces

A site-control technique for individual InAs quantum dots (QDs) has been developed by using scanning tunneling microscope (STM) probe-assisted nanolithography and self-organizing molecular-beam epitaxy. We find that nano-scale deposits can be created on a GaAs surface by applying voltage and current pulses between the surface and a tungsten tip of the STM, and that they act as “nano-masks” on which GaAs does not grow directly. Accordingly, subsequent thin GaAs growth produces GaAs nano-holes above the deposits. When InAs is supplied on this surface, QDs are self-organized at the hole sites, while hardly any undesirable Stranski-Krastanov QDs are formed in the flat surface region. Using this technique with nanometer precision, a QD pair with 45-nm pitch is successfully fabricated. An erratum to this article is available at .     

Pulsed laser annealing of self-organized InAs/GaAs quantum dots

The effect of pulsed laser annealing (PLA), using an excimer laser, on the luminescence efficiency of self-organized InAs/GaAs and In_0.4Ga_0.6As/GaAs quantum dots has been investigated. It is found that such annealing can enhance both the peak and integrated photoluminescence (PL) efficiency of the dots, up to a factor of 5–10 compared to as-grown samples, without any spectral shift of the luminescence spectrum. The improved luminescence is attributed to the annealing of nonradiative point and extended defects in and around the dots.     

Shape stabilization and size equalization of InGaAs self-organized quantum dots

We report a simultaneous shape stabilization and size equalization after shape transformation of InGaAs self-organized quantum dots (QDs) formed via a fractional monolayer (ML) deposition technique. The density of QD increases rapidly from an initial value of 110±10/μm² (at a total deposition of 4 ML) to 270±30/μm² (at 5 ML) and saturates at a level of 240±20/μm² (at 10 ML). At an intermediate stage of 7 ML deposition, bimodal QD height (peaked at 8.5 nm and 14.5 nm) and aspect ratio (peaked at 0.18 and 0.26) distributions occur, confirming the QD shape transformation from a shallower to a steeper shape. The eventual convergence in lateral size, height and aspect ratio is the direct result of the simultaneous QD size equalization and shape stabilization. The QD size and shape evolution is also substantiated by the low temperature (4 K) photoluminescence (PL) data taken from samples with QDs capped by GaAs.     

Origin of size distributions in ZnSe self-organized quantum dots grown on ZnS layers

Main factors which determine the size, the standard deviations which show the degree of the size fluctuations for the average dot height and diameter, and density in ZnSe self-organized quantum dots (QDs) grown on ZnS layers were studied. By lowering the growth temperature the QDs average size and its standard deviation decreased and the density increased due to the slower surface migration. With the application of the scaling theory, it was revealed that the normalized size distributions were uniquely determined by the nucleation process although the apparent standard deviations of the QD sizes were dependent on the growth temperature. The influence of surface roughness of the underneath layer on the formation of the relations of the dot height and diameter was also examined. It was shown that the fluctuation of the surface potential contributes significantly to the apparent standard deviations of ZnSe self-organized QDs sizes.     

Gain and index measurements in GaAlAs quantum well lasers

Measurements of the modal gain and group index in GaAlAs single-quantum-well (SQW) lasers are presented. The elimination of substrate emission has allowed accurate results to be obtained even in the near-bandgap and below-bandgap spectral regions. Substantial lifetime broadening is observed, and the gain smoothly goes to zero as the bandgap is approached. The group velocity index measurements indicate a dispersion of -3.44 μm^-1     

Temperature dependent optical properties of self-organized InAs/GaAs quantum dots

We report photoluminescence (PL), time-resolved PL, and PL excitation experiments on InAs/GaAs quantum dots (QDs) of different size as a function of temperature. The results indicate that both the inhomogeneous properties of the ensemble and the intrinsic properties of single QDs are important in understanding the temperature-dependence of the optical properties. With increasing temperature, excitons are shown to assume a local equilibrium distribution between the localized QD states, whereas the formation of a position-independent Fermi-level is prevented by carrier-loss to the barrier dominating thermally stimulated lateral carrier transfer. The carrier capture rate is found to decrease with increasing temperature and, at room temperature, long escape-limited ground state lifetimes of some 10 ps are estimated. PL spectra excited resonantly in the ground state transition show matching ground state absorption and emission, indicating the intrinsic nature of exciton recombination in the QDs. Finally, the PL excitation spectra are shown to reveal size-selectively the QD absorption, demonstrating the quantum-size effect of the excited state splitting.     

A dual-color injection laser based on intra- and inter-band carriertransitions in semiconductor quantum wells or quantum dots

A new type of semiconductor injection laser capable of simultaneously generating radiation in the mid-infrared (MIR) (λ~10 μm) and near-infrared (NIR) (λ~0.9 μm) spectral regions is proposed. The MIR emission is a result of intersubband (intraband) electron transitions within a three-level conduction band in a quantum well or a quantum dot. The NIR emission, on the other hand, is due to conventional interband recombination of injected electrons and holes into the conduction and valence bands, respectively. The conditions for population inversion in the intersubband emission process are determined by an appropriately engineered energy structure for a three-level system in the conduction band of a quantum well or dot structure: for the quantum-well-based system, the structure has an asymmetric funnel shape to provide long electron-phonon lifetime at the third (top) energy level. Under high carrier injection, NIR interband emission depopulates the conduction ground level of the quantum well, thereby stabilizing the electron concentration at this level-a necessary condition fur the operation of the MIR laser. This paper discusses the calculation of the population inversion conditions, the requisite gain, and threshold current for MIR laser operation. We also present a preliminary design of the laser structure with a composite waveguide that accommodates both mid- and NIR stimulated emission     

Gain measurements on GaAs-based quantum cascade lasers using atwo-section cavity technique

A two-section cavity device has been used to measure gain spectra and waveguide losses of a GaAs-based quantum cascade laser. The device operates at 8.9 μm and optical confinement is obtained by means of Al-free cladding layers. We investigated the gain characteristics in a spectral window of ~60 meV and up to 200 K. For current densities ranging from 1 to 8 kA/cm², we report a constant gain coefficient of 13 cm/kA at 4 K and 6 cm/kA at 200 K. At low temperatures and for current densities above 8 kA/cm², we observe gain saturation which we attribute to a reduced electron injection in the active region caused by space charge effects. We report a value of 22 cm ^-1 for the waveguide losses in good agreement with previous measurements