Self-assembled InAs-GaAs Quantum Dot Research Articles

Size anisotropy inhomogeneity effects in state-of-the-art quantum dot lasers

We describe a high angle annular dark field scanning transmission electron microscopy study of a self-assembled InAs-GaAs quantum dot (QD) laser sample providing insight into the micro-structure of the QD ensemble. A size distribution anisotropy of the QDs is observed in the two orthogonal (110) planes, and this structural information is used to develop a density of states model for the QD ensemble which is shown to be in strong agreement with a range of optical spectroscopic measurements. This link between the micro-structure and optical properties allows routes to QD device simulation. We go on to discuss how changes to the micro-structure would affect the density of states and hence laser performance.

Spatial regularity of InAs-GaAs quantum dots: quantifying the dependence of lateral ordering on growth rate

The lateral ordering of arrays of self-assembled InAs-GaAs quantum dots (QDs) has been quantified as a function of growth rate, using the Hopkins-Skellam index (HSI). Coherent QD arrays have a spatial distribution which is neither random nor ordered, but intermediate. The lateral ordering improves as the growth rate is increased and can be explained by more spatially regular nucleation as the QD density increases. By contrast, large and irregular 3D islands are distributed randomly on the surface. This is consistent with a random selection of the mature QDs relaxing by dislocation nucleation at a later stage in the growth, independently of each QD’s surroundings. In addition we explore the statistical variability of the HSI as a function of the number N of spatial points analysed, and we recommend N > 103 to reliably distinguish random from ordered arrays.

Self-Heating Effect on the Two-State Lasing Behaviors in 1.3-µm InAs–GaAs Quantum-Dot Lasers

Experimental and theoretical study of the self-heating effect on the two-state lasing behaviors in 1.3-µm self-assembled InAs–GaAs quantum dot (QD) lasers is presented. Lasing spectra under different injected currents, light–current (L–I) curves measured in continuous and pulsed regimes as well as a rate-equation model considering the current heating have been employed to analyze the ground-state (GS) and excited-state (ES) lasing processes. We show that the self-heating causes the quenching of the GS lasing and the ES lasing by the increased carrier escape rate and the reduced maximum modal gain of GS and ES.

Quantum dot superluminescent diodes emitting at 1.3 /spl mu/m

We report ridge-waveguide superluminescent diodes based on five stacks of self-assembled InAs-GaAs quantum dots. Devices with output powers up to 10 mW emitting around 1.3 /spl mu/m are demonstrated. Spectral analysis shows a broad emission peak (26-nm full-width at half-maximum) from the dot ground state at low injection, and an additional peak from the excited state at higher bias. Temperature characteristics in the range 10/spl deg/C-80/spl deg/C are also reported. The experimental curves are in good agreement with simulations performed using a traveling-wave rate equation model.

Optically driven spin memory in n-doped InAs-GaAs quantum dots.

We show that the spin state of the resident electron in an n-doped self-assembled InAs-GaAs quantum dot can be written and read using nonresonant, circularly polarized optical pumping. A simple theoretical model is presented and accounts for the remarkable dynamics producing counterpolarized photoluminescence.

Electronic structure and many-body effects in self-assembled quantum dots

A detailed model for the electronic properties of self-assembled InAs/GaAs quantum dots (SADs) is presented, with emphasis on inter-level transitions and many-body effects. The model is based on the self-consistent solution of three-dimensional Poisson and Schrödinger equations within the local (spin-) density approximation. Nonparabolicity of the band structure and a continuum model for the strain between GaAs and InAs results in position- and energy-dependent effective mass. The electronic spectra of SADs of various shapes have been determined with intraband level transitions and mid-infrared optical matrix elements. Shell structures obeying Hund's rule for various occupation numbers in pyramidal SADs agree well with recent capacitance measurements. It is shown that many-body interactions between orbital pairs of electrons are determined in a first approximation by classical Coulomb interaction.

Self-assembled InAs-GaAs quantum-dot intersubband detectors

The use of self-assembled InAs-GaAs quantum dots in photoconductive intersubband detectors in the far-infrared is presented. Far-infrared absorption is observed in self-assembled quantum dots in the 6-18-/spl mu/m range for subband-subband and subband-continuum transitions. Photoconductive quantum-dot intersubband detectors were fabricated and demonstrate tunable operating wavelengths between 6-18 /spl mu/m using subband-subband or subband-continuum transitions. The use of AlAs barriers allows further tuning to shorter wavelengths of 3-7 /spl mu/m. Subband-continuum quantum dot intersubband detectors show encouraging normal incidence performance characteristics at T=40 K, with responsivities of 10-100 mA/W, detectivities of 1-10 /spl times/10/sup 9/ cm/spl middot/Hz/sup 1/2//W and large photoconductive gain up to g=12 for a ten-layer quantum-dot heterostructure. With improvements in device structure, self-assembled quantum dots can be expected to provide intrinsic normal incidence broad-band detectors with advantages over quantum wells.

Quantum dot p-i-n structure in an electric field

Time-resolved photoluminescence (PL), steady-state PL, and electroluminescence (EL) techniques have been used to characterize the carrier relaxation processes and carrier escape mechanisms in self-assembled InAs/GaAs quantum dot (SAQD) p-i-n structures under reverse bias. The measurements were performed between 5 K and room temperature on a ring mesa sample as a function of bias. At 100 K, the PL decay time originating from the n = 1 SAQD decreases with increasing reverse bias from ∼3 ns under flat band condition to∼ 400 ps for a bias of −3 V. The data can be explained by a simple model based on electron recombination in the quantum dots (QDs) or escape out of the dots. The escape can occur by one of three possible routes: direct tunneling out of the distribution of excited electronic levels, thermally assisted tunneling of ground state electrons through the upper excited electronic states or thermionic emission to the wetting layer.

Erratum: Self-consistent calculation of the electronic structure and electron-electron interaction in self-assembled InAs-GaAs quantum dot structures [Phys. Rev. B 57, 4017 (1998)

Erratum: Self-consistent calculation of the electronic structure and electron-electron interaction in self-assembled InAs-GaAs quantum dot structures [Phys. Rev. B 57, 4017 (1998)

Self-consistent calculation of the electronic structure and electron-electron interaction in self-assembled InAs-GaAs quantum dot structures

We have performed a detailed self-consistent calculation of the electronic structure and electron-electron interaction energy in pyramidal self-assembled InAs-GaAs quantum dot structures. Our model is general for three-dimensional quantum devices without simplifying assumptions on the shape of the confining potential nor fitting parameters. We have used a continuum model for the strain, from which the position-dependent effective mass and band diagram are calculated. The number of electrons in the dot is controlled by applying an external voltage to a metal gate on the top of a complete multilayer device containing a single dot. In order to determine the electron occupation number in the dot that minimizes the total energy of the system, we have adopted the concept of transition state as defined by Slater for shell filling in atoms. We have calculated the exchange and correlation terms of the many-body Hamiltonian using the local (-spin) -density approximation. By accounting for spins we have been able to determine the shell structure in the pyramid and to calculate the energy differences between the various spin configurations. We have also calculated the different contributions to the total electronic energy in the dot, i.e., the single-particle energies, the exchange-correlation energy, and the classical electrostatic electron-electron repulsion energy as a function of the gate voltage and number of electrons in the dot. Comparison with recent experimental data of Fricke et al. [Europhys. Lett. 36, 197 (1996)] shows good agreement.

Self-assembled InAs/GaAs quantum dots under resonant excitation

The energy structure and the carrier relaxation in self-assembled InAs/GaAs quantum dots (SADs) is investigated by photoluminescence excitation spectroscopy (PLE) and photoluminescence (PL) at resonant excitation (below the GaAs and the wetting layer bandgap). In PLE measurements we find a clear resonance from the first excited hole state as well as resonances from a relaxation via different phonons. From a comparison of the PL-rise times in time resolved spectroscopy, we conclude on a fast electron relaxation (⩽50 ps) and a slow hole relaxation with a time constant of about 400 ps. Different relaxation paths are observed in the InAs/GaAs quantum dot system and allow us to identify the hole relaxation in the SADs as multiphonon assisted tunneling. The PL-decay time in the SADs after resonant excitation (about 600 ps) is attributed to the lifetime of the quantum dot exciton. In agreement with theoretical predictions, we find a constant lifetime of about 600 ps for temperatures below 50 K and a linear increase of the lifetime between 50 and 100 K with a slope of 26 ps/K.

Intrinsic Carrier Relaxation and the Exciton Lifetime in InAs/GaAs Quantum Dots

The carrier relaxation in self-assembled InAs/GaAs quantum dots (SADs) is investigated by photoluminescence spectroscopy (PL) at resonant excitation (below the GaAs and the wetting layer bandgap). Different relaxation paths are observed in the InAs/GaAs quantum dot system and allow to identify the hole relaxation in the SADs as multiphonon assisted tunneling. The PL decay time in the SADs after resonant excitation (about 600 ps) is attributed to the lifetime of the quantum dot exciton. In agreement with theoretical predictions, we find a constant lifetime of about 600 ps for temperatures below 50 K and a linear increase of the lifetime between 50 and 100 K with a slope of 26 ps/K.

Structural and optical characterization of self-assembled InAs-GaAs quantum dots grown on high index surfaces

The structural and the optical properties of InAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (QDs) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QD coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) spectra of GaAs (100) and (311) samples show typical QD features with PL peaks in the energy range 1.15-1.35 eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70K was observed. These results suggest that the high index substrates are promising candidates for production of high quality self-assembled QD materials for application to photonics.