Single Quantum Dot Structures Research Articles

Electromagnetically induced grating in double quantum dot system

Electromagnetically induced grating (EIG) in the ladder-plus-Y double quantum dot system is modeled and the diffraction grating properties are explored in this structure. A high transmission function is obtained under the high pump field. This function is reduced under increasing the probe field due to the Kerr effect. The phase of this function depends on pumping. It is shown that the application of another two fields from the wetting layer (WL)-quantum dot (QD) type is more efficient in obtaining very high diffraction intensity. So, EIG with high diffraction intensity is obtained under a four-field application. Note that, WL-QD field effect is not studied earlier. Neglecting the WL effect reduces the transmission by five times. The diffraction intensity of this system is six times higher than that obtained from a single QD structure.

Effect of Coupled Quantum-Dot Insertion on the Radiative Recombination Probability of Wurtzite InGaN/GaN Quantum Dots

Light emission charactersitcs of single and coupled wurtzite (WZ) InGaN/GaN quantum-dot (QD) structures were investigated by using an effective mass theory. Strain components in the well of the coupled QD are found to be smaller than those in the well of the single QD structure. In particular, the reduction effect in ϵzz is dominant and, as a result, the polarization component Izz for the strain tensor ϵzz is largely reduced for the coupled QD. The coupled QD structure shows much larger peak intensity than the single QD structure because the interband transition probability between electrons and holes is enhanced owing to the reduced internal field.

Electronic states and intraband terahertz optical transitions in InGaAs quantum rods

Strain-dependent eight-band k·p method is used to analyze the electronic structure and intraband optical transitions in self-assembled InGaAs quantum rods in the terahertz range. The calculation of absorption spectra for the growth- and in-plane-polarized radiation shows some similarities to those of quantum well and single quantum dot structures, augmented with contribution from transitions between the dot and quantum well states. The influence of rod height on the electronic structure and the intraband absorption spectra is also investigated. It is found that the energy of maximal terahertz absorption can be tailored by the rod height for both in-plane and in-growth polarized radiation.

Plane wave methodology for single quantum dot electronic structure calculations

The development of the plane wave methodology for the calculation of the electronic structure of single quantum dots within the framework of multiband envelope function theory was presented. The methodology developed enables one to use a small embedding box, sufficient to eliminate electronic coupling, without introducing the artificial interaction with periodically replicated array of quantum dots caused by periodic boundary conditions. The appropriate formulas for Fourier transforms of strain tensor components on the embedding box that eliminate the strain field of the neighboring dots were derived. The expressions that enable the evaluation of Coulomb integrals in inverse space without the introduction of artificial electrostatic interactions with surrounding dots were presented. It was also shown how symmetry can be exploited to further reduce the computational effort in the case of quantum dots of symmetric shape. Numerical results illustrating the application of the methods to the calculation of single-particle states, as well as the configuration interaction calculation of exciton, biexciton, and negative trion states in InAs∕GaAs quantum dots were given.

Influence of lateral electric fields on multiexcitonic transitions and fine structure of single quantum dots

The quantum-confined Stark effect of excitonic states in self-assembled (In,Ga)As∕GaAs quantum dots was studied by microphotoluminescence spectroscopy. A similar Stark-shift behavior for excitons, biexcitons, and a charged state was observed. Investigations suggest the absence of a permanent dipole moment in the lateral quantum dot plane. Values of the polarizability could be derived for all the investigated states. Furthermore, high-resolution Fabry-Pérot interferometry was applied to resolve the excitonic fine structure splitting and to investigate the influence of a lateral electric field. For a single dot, the splitting could be tuned to zero, thus affording the possibility to create electrically controlled entangled photon pairs.

GaN nanostructures and HFET structures selectively grown on silicon substrates by ammonia-MBE

Silicon substrate can be used as a highly effective pseudomask to perform selective growth of GaN by ammonia molecular beam epitaxy (MBE). This can be exploited to fabricate novel photonic and electronic device structures on Si substrates. Growth of GaN micro-pyramids as templates for fabricating InGaN quantum dots, and growth of AlGaN/GaN HFET mesa patterns using the selective growth approach is reported. The facet growth characteristics of the selectively grown GaN micro-pyramids are investigated. μ-photoluminescence (μ-PL) measurements resolving emissions from InGaN quantum dots and/or wells on a single GaN pyramid have been performed, but have found no evidence of formation of InGaN single quantum dot at the apex of the GaN pyramid. Possible improved growth approaches for achieving such single quantum dot structures are discussed. Success in selective growth of the HFET mesa pattern and especially the insulating carbon-doped GaN buffer layer is achieved, demonstrating a promising approach for potential integration of nitride devices with silicon circuits.

Site-controlled InAs single quantum-dot structures on GaAs surfaces patterned by in situ electron-beam lithography

We studied a site-control technique for InAs quantum dots (QDs) on GaAs substrates using a combination of in situ electron-beam (EB) lithography and self-organized molecular-beam epitaxy. In small, shallow holes formed on prepatterned mesa structures by EB writing and Cl2 gas etching, QDs were selectively formed, without any formation on the flat region between the patterned holes. The density of the QDs in each hole was dependent on the hole depth, indicating that atomic steps on the GaAs surfaces act as migration barriers to In adatoms. In an array of holes including 5–6 monolayer steps, a single QD was arranged in each hole.

Temperature and Power Dependence of Exciton Spectra in Quantum Dots

We employ photoluminescence spectroscopy with micrometer spatial resolution to study different types of single quantum dot structures. On dots formed by well width fluctuations in a narrow GaAs/AlGaAs quantum well, we measure linewidths around 0.1 meV at 7 K. Increasing the temperature to 50 K, a broadening up to 0.3 meV is observed. The temperature dependence varies between different dots and also between the different lines of a single dot. The energy position of the lines is remarkably stable against variation of the excitation power. On a single dot fabricated by laser induced intermixing of a GaAs/AlGaAs well, an occupation of states more than 30 meV above the ground state has been achieved by quasi-resonant excitation with picosecond pulses; but still the luminescence lines hardly shift. On the other hand, a sizeable power dependence is found for a quantum dot with a lateral electric field applied by an interdigital gate structure. This shows that the very small power dependence of the line positions is related to the symmetry of the lateral confinement potential.

Single quantum dots as scanning tunneling microscope tips

We report the use of single quantum dot structures as tips on a scanning tunneling microscope (STM). A single quantum dot structure with a diameter of less than 200 nm and a height of 2 μm was fabricated by reactive ion etching. This dot was placed on a 40 μm-high mesa and mounted on the tip of a STM. The topography of large structures such as quantum wires or gold test substrates is clearly resolved with such a tip. To check the transport properties of the tip, quantum dot arrays were fabricated on resonant tunneling double barrier structures using the same process parameters. Conventional tunneling spectroscopy clearly resolved the 0D states in our samples. Using a metal substrate as second electrode such STM tips can be used to perform high resolution energy spectroscopy on single dots and free standing wire structures.

Time resolved spectroscopy of single quantum dot structures

We report on photoluminescence measurements with temporal and spatial resolution on a series of individual quantum dot structures. Excitation is performed resonantly with very small laser intensities corresponding to between 200 and 1 exciton per pulse in the dot. Structures with different lateral confinement are investigated, as a function of excitation intensity and wavelength. In all cases, the luminescence appears rapidly, with a rise time of about 10ps. The experimental results indicate an important contribution of Coulomb scattering to the energy relaxation process. In the sample of maximum lateral confinement, we observe a strong, systematic variation of the spectrum with the excitation intensity. Additional transitions appear up to more than 30 meV above the fundamental PL line, which show no measurable population transfer to lower energy states. This is attributed to the formation of a degenerated Fermi gas of excitons.