Effective-mass theory for InAs/GaAs strained coupled quantum dots

Effects of an External Magnetic Field on the Interband and Intraband Optical Properties of an Asymmetric Biconvex Lens-Shaped Quantum Dot

The theoretical investigation of interband and intraband transitions in an asymmetric biconvex lens-shaped quantum dot are considered in the presence of an external magnetic field. The selection rules for intraband transitions are obtained. The behaviors of linear and nonlinear absorption and photoluminescence spectra are observed for different temperatures and magnetic field strengths. The second and third harmonic generation coefficients as a function of the photon energy are examined both in the absence and presence of an external magnetic field.

Experimental Study of Effective Mass and Spin-Orbital Energy of the Al2O3/NiO Nanoheterostructure Material

The effective mass of the materials relies on exciton dynamics which is greatly dominated by energy gap of the materials. We examined the effective mass for the Al₂O₃/NiO nanoheterostructure material through polarization in the dielectric study. The mean optical effective mass was calculated to be m_opt^* = 5.69645 × 10^-20 m_op/m₀ in the UV-visible region (1 × 10⁶ to 5.4 × 10⁶ Hz). From the group and phase velocity values, we observed that Al₂O₃/NiO is an anisotropic material. The electron spin-orbit energy splitting associated to electron in the valence band was identified using the Gaussian-confining 3D potential under normalized angular momentum (n, l = 0, 1, 2, 3). The 1S (¹S_1/2) and 2S (²S_1/2) orbital ionization energies were calculated to be -6.08 and -5.99 eV for AlO₃/NiO. The orbital ionization energies were established from the Bohr radius a_B = 5.29 × 10^-11 m and donor Rydberg constant R_y = 1.097 × 10⁷ m^-1 of the identical hydrogen atom. Our study gives insights into the exciton dynamics and calculation of orbital energy for the nanoheterostructure materials.

Electronic properties of an exciton in CdTe/CdSe/CdTe/CdSe type-II nano-heterostructure

In this study, we have carried out a detailed theoretical investigation on the binding energy of an exciton in type-II CdTe/CdSe core/shell/well/shell (CSWS) nanocrystal quantum dot (NCQD) in the strong confinement region. The calculations are based on the effective mass approximation, and the coulombic interaction between electron and hole is introduced using Hartree approximation. With these theoretical basis, the coupled Poisson-Schrodinger equations are solved in a self consistent iterative manner. In strong confinement regime, the binding energy variation with core radius in type-II NCQD shows a peak. And this peak widens for larger well width and inner shell thickness. Our study suggests that, this anomalous behavior of exciton binding energy is due to an effect called 'positional flip of exciton', caused by the faster tunneling of hole to the inner layer in comparison with electron. Our results can be applied in laser and optoelectronic engineering for designing more efficient optoelectronic devices.

Extraction of optical constants using multiple reflections in the terahertz emitter-sample hybrid structure

To extract optical constants of nontransmitted samples in the terahertz (THz) spectral region, we employ a THz emitter-sample hybrid structure where THz pulses are generated at the emitter surface and multiply reflected at the interface between the THz emitter and the sample. Since each THz electric field profile appears well separated in a time domain, we could obtain the amplitude and phase spectra for each pulse from the Fourier transform, and determine the optical constants of the sample numerically based on the Fresnel equations. We applied this technique for doped semiconductors, and found that obtained optical constants are in good agreement with the values determined by using other conventional THz techniques.

Quantum Dot Infrared Photodetectors: Photoresponse Enhancement Due to Potential Barriers

Potential barriers around quantum dots (QDs) play a key role in kinetics of photoelectrons. These barriers are always created, when electrons from dopants outside QDs fill the dots. Potential barriers suppress the capture processes of photoelectrons and increase the photoresponse. To directly investigate the effect of potential barriers on photoelectron kinetics, we fabricated several QD structures with different positions of dopants and various levels of doping. The potential barriers as a function of doping and dopant positions have been determined using nextnano(3) software. We experimentally investigated the photoresponse to IR radiation as a function of the radiation frequency and voltage bias. We also measured the dark current in these QD structures. Our investigations show that the photoresponse increases ~30 times as the height of potential barriers changes from 30 to 130 meV.

Effects of applied electric and magnetic fields on a donor impurity in laterally coupled quantum dots

A theoretical description of the electronic structure, optical spectrum and binding energy of a hydrogenic impurity in laterally coupled quantum discs, under applied electric and magnetic fields, is given within the framework of the effective-mass approach. Calculations are performed using the envelope-function formalism and a variational procedure, with the electric field applied in the coupling direction, the magnetic field along the growth direction, and the impurity at the center of the heterostructure. The results indicate that the anisotropy of the laterally coupled confinement potential leads to interesting relative extrema and anticrossings in the energy spectra, and that the infrared absorption spectrum is sensitive to the type of polarization and magnitude of external fields.

Interaction and Cooperative Nucleation of InAsSbP Quantum Dots and Pits on InAs(100) Substrate

An example of InAsSbP quaternary quantum dots (QDs), pits and dots-pits cooperative structures' growth on InAs(100) substrates by liquid phase epitaxy (LPE) is reported. The interaction and surface morphology of the dots-pits combinations are investigated by the high-resolution scanning electron microscope. Bimodal growth mechanism for the both QDs and pits nucleation is observed. Cooperative structures consist of the QDs banded by pits, as well as the "large" pits banded by the quantum wires are detected. The composition of the islands and the pits edges is found to be quaternary, enriched by antimony and phosphorus, respectively. This repartition is caused by dissociation of the wetting layer, followed by migration (surface diffusion) of the Sb and P atoms in opposite directions. The "small" QDs average density ranges from 0.8 to 2 × 109 cm-2, with heights and widths dimensions from 2 to 20 nm and 5 to 45 nm, respectively. The average density of the "small" pits is equal to (6-10) × 109 cm-2 with dimensions of 5-40 nm in width and depth. Lifshits-Slezov-like distribution for the amount and surface density of both "small" QDs and pits versus their average diameter is experimentally detected. A displacement of the absorption edge toward the long wavelength region and enlargement toward the short wavelength region is detected by the Fourier transform infrared spectrometry.

Energy transfer within ultralow density twin InAs quantum dots grown by droplet epitaxy

Ultralow density (approximately 10(6)/cm(2)) of twin InAs quantum dot (QD) hybrid structure was grown by a droplet epitaxy technique. The photoluminescence (PL) from ensemble and individual twin InAs QD structures showed a bimodal behavior and an energy transfer between the well-separated (approximately 190 nm) twin QDs, which was supposedly due to the special wetting ring that built the channel for exciton transfer. This research demonstrates a novel approach to fabricate lateral InAs QD pairs as the candidate for a laterally coupled QD molecule.

Influence of GaAs Substrate Orientation on InAs Quantum Dots: Surface Morphology, Critical Thickness, and Optical Properties

InAs/GaAs heterostructures have been simultaneously grown by molecular beam epitaxy on GaAs (100), GaAs (100) with a 2° misorientation angle towards [01−1], and GaAs (n11)B (n = 9, 7, 5) substrates. While the substrate misorientation angle increased from 0° to 15.8°, a clear evolution from quantum dots to quantum well was evident by the surface morphology, the photoluminescence, and the time-resolved photoluminescence, respectively. This evolution revealed an increased critical thickness and a delayed formation of InAs quantum dots as the surface orientation departed from GaAs (100), which was explained by the thermal-equilibrium model due to the less efficient of strain relaxation on misoriented substrate surfaces.

The Influence of a Continuum Background on Carrier Relaxation in InAs/InGaAs Quantum Dot

We have investigated the ultra-fast carrier dynamics in Molecular Beam Epitaxy (MBE)-grown InAs/InGaAs/GaAs quantum dots (QDs) emitting at 1.3 μm by time resolved photoluminescence (TRPL) upconversion measurements with a time resolution of about 200 fs. Changing the detection energies in the spectral region from the energy of the quantum dots excitonic transition up to the barrier layer absorption edge, we have found that, under high excitation intensity, the intrinsic electronic states are populated mainly by carriers directly captured from the barrier.