Crystal Quantum Dots Research Articles

Improvement on Thermoelectric Characteristics of Layered Nanostructure by Ion Beam Bombardment

ABSTRACTWe made p-type nanoscale super lattice thermoelectric cooling devices which consist of multiple periodic layers of Si1−x Gex / Si, The thickness of each layer ranges between 10 and 50 nm. The super lattice was bombarded by 5 MeV Si ion with different fluencies aiming to form nano-cluster quantum dot structures. We estimated the thermo-electric efficiency of the so fabricated devices, measuring the thin film cross plane thermal conductivity by the 3rd harmonic method, measuring the cross plane Seebeck coefficient, and finally measuring the cross plane electric conductivity before and after ion bombardment. As predicted, the thermo-electric Figure of Merit of the films increases with increasing Si ion fluencies. In addition to the effect of quantum well confinement of the phonon transmission, the nano-scale crystal quantum dots produced by the incident Si beam further adversely affects the thermal conductivity by absorbing and dissipating phonon along the lattice, and therefore further reduces the cross plane thermal conductivity, This process increases the electron density of state therefore increasing Seebeck coefficient, and the electric conductivity.

GaAs-based four-channel photonic crystal quantum dot laser module operating at 1.3 µm

A GaAs-based laser module, which combines the output of four singlemode lasers operating at 1.3 μm by using photonic crystal mirrors and combiners, has been developed. The complete device size is around 0.2 mm 2 . The lasers can be operated individually or in parallel with sidemode suppression ratios better than 20 dB.

Lateral quantum-dot replication in three-dimensional quantum-dot crystals

Single quantum dots (QDs) reproduce into pairs of QDs with increasing separation distance during the growth of a three-dimensional QD crystal. Kinetic Monte Carlo simulations—that rely on strain profiles deduced from experiment—can describe this lateral replication process, which is triggered by a distinct ridge structure that evolves during the overgrowth of two-dimensional periodic QD arrays.

Magnetically induced Bragg scattering of electrons in quantum-dot crystals

We demonstrate a novel manifestation of dynamic Bragg reflection in artificial quantum-dot crystals that is driven by the application of a magnetic field. This backscattering is coherently cascaded as the number of dots in the structure is increased, causing a superlinear damping of the electron wave function and the appearance of a series of gaps in the miniband spectrum. The evolution of the dynamic miniband structure as the magnetic field is varied gives rise to behavior analogous to a metal-insulator transition, which is manifest as a dramatic resonance in the magneto-resistance of the structures.

Growth of three-dimensional quantum dot crystals on patterned GaAs (0 0 1) substrates

The growth of a three-dimensional (3D) InAs quantum dot (QD) crystal on a patterned GaAs (0 0 1) substrate is demonstrated. The morphology of QDs grown on a surface patterned with shallow holes is studied as a function of the amount of deposited InAs. We observe that the QDs form in the patterned holes close to each other forming lateral QD bimolecules for InAs coverages below the commonly observed critical thickness of ∼1.6 monolayers. When the coverage increases, the QD bimolecules coalesce to form larger single QDs. The QDs in the holes are then capped with a Ga(Al)As spacer. The buried QD array serves as a strain template for controlling the formation site of the QDs in the second layer. By tuning the growth conditions for the second and subsequent layers, we achieve a 3D InAs QD crystal with a high degree of perfection. A detail investigation of the growth on hole patterns with different periodicities is presented.

Growth of three-dimensional quantum dot crystals on patterned GaAs (001) substrates

Growth of three-dimensional quantum dot crystals on patterned GaAs (001) substrates

Photon control of phonons in mixed crystal quantum dots

Coherent phonon oscillations in solids can be excited impulsively by a single femtosecond laser pulse whose duration is shorter than a phonon period. In the impulsive stimulated Raman scattering (ISRS) experiment, scattering of probe is monitored as a function of time with respect to pump to generate time domain spectra of coherent phonons. In this paper, we present one such study of CdSe 0.68Te 0.32 ( d∼80 A) quantum dots in glass matrix, i.e semiconductor-doped glass (SDG) RG780 from Schott, USA and the experiment was performed at Prof. Merlin's laboratory at the University of Michigan, USA. Here, we present first report of selectively driving only CdSe-like modes in these mixed crystal quantum dots using photon control with two pump beams.

Electron and phonon energy spectra in a three-dimensional regimented quantum dot superlattice

We report on theoretical investigation of the electron and phonon energy spectra in a three-dimensional regimented quantum dot superlattice. Our results are obtained by numerical solution of the Schr\"odinger and elasticity equations using the finite-difference method. The calculations are performed for a Ge/Si material system taking into account characteristic band-gap offsets, elastic stiffness constants, and other relevant parameters. Coupling among quantum dots in such a regimented structure results in formation of extended electron states and minibands, provided that the disorder in the system is small. Electron and phonon densities of states of these artificial quantum dot crystals are also calculated. We demonstrate that the acoustic-phonon dispersion in the quantum dot superlattice undergoes strong modification, which leads to emergence of quasioptical branches. These branches are much lower in energy than optical phonons in bulk semiconductors and thus may strongly affect energy relaxation processes. Other phenomena that originate from the specific electron and phonon spectra in quantum dot superlattices, such as negative differential conductivity and carrier scattering anisotropy, are also discussed.

Modeling of Phonon Dispersion in a Semiconductor Quantum Dot Crystal

AbstractWe describe a model for numerical calculation of phonon spectrum in a three-dimensional regimented array of semiconductor quantum dots. Regimentation and possibility of carrier mini-band formation make this structure analogous to a crystal, e.g.quantum dot crystal. It is demonstrated that the acoustic phonon dispersion undergoes strong modification in such a structure leading to emergence of low-energy quasi-optical branches. Strong phonon spectrum modification is expected to affect carrier relaxation and transport properties.

Miniband formation in a quantum dot crystal

We analyze the carrier energy band structure in a three-dimensional regimented array of semiconductor quantum dots using an envelope function approximation. The coupling among quantum dots leads to a splitting of the quantized carrier energy levels of single dots and formation of three-dimensional minibands. By changing the size of quantum dots, interdot distances, barrier height, and regimentation, one can control the electronic band structure of this artificial quantum dot crystal. Results of simulations carried out for simple cubic and tetragonal quantum dot crystal show that the carrier density of states, effective mass tensor and other properties are different from those of bulk and quantum well superlattices. It has also been established that the properties of artificial crystal are more sensitive to the dot regimentation rather then to the dot shape. The proposed engineering of three-dimensional mini bands in quantum dot crystals allows one to fine-tune electronic and optical properties of such nanostructures.

Spatial ordering of stacked quantum dots

We investigate the growth conditions necessary to form an ordered quantum dot crystal by capping spatially ordered quantum dots and growing a new layer of dots on top of the capping layer. Performing Monte Carlo simulations and developing analytic arguments based on the stress energy function, we demonstrate the existence of an optimal capping layer thickness, external flux, and temperature for the formation of quantum dot crystals.

Investigation of the Electron Energy Spectrum in a Three Dimensional Regimented Tetragonal Quantum Dot Superlattice

ABSTRACTWe analyze the electron energy spectrum in three-dimensional regimented arrays of semiconductor quantum dots. The coupling among quantum dots results in formation of three- dimensional electron mini-bands. Changing the size of quantum dots, inter-dot distance, barrier height and regimentation, one can control the electronic band structure of this quantum dot superlattice, which can also be referred to as quantum dot crystal due to its structure and energy spectrum that resemble those of a real crystal. Results of computer simulations carried out for a tetragonal InAs/GaAs quantum dot superlattice show that the electron density of states, effective mass tensor and other properties are different from those of bulk and conventional quantum well superlattices.

Self-organized growth of three-dimensional IV–VI semiconductor quantum dot crystals with fcc-like vertical stacking and tunable lattice constant

Heteroepitaxial growth of highly lattice-mismatched semiconductor layers has evolved as a novel method for direct synthesis of self-assembled quantum dots based on the Stranski–Krastanov growth mode. In this growth mode, nano-scale three-dimensional (3D) islands are spontaneously formed on the epitaxial surface once the layer thickness exceeds a certain critical coverage. In multilayer structures, the interaction of the strained islands via their long-range elastic strain fields may lead to a vertical and lateral ordering of the dots. Here, it is shown that the elastic anisotropy and the growth orientation play a crucial role for this self-organization process. In particular, for (111)-oriented IV–VI semiconductor (PbSe/Pb 1− x Eu x Te) multilayers, the direction of the interlayer dot correlations is shown to be inclined to the surface normal. This leads to a fcc-like ABCABC… vertical dot stacking sequence, which is particularly effective for inducting a lateral ordering of the dots as well. As a result, large domains of trigonally ordered 3D quantum dot crystals are formed, and their lattice constant can be tuned continuously just by changing the thickness of the spacer between the quantum dot layers.

Self-organized growth of three- dimensional quantum-Dot crystals with fcc-like stacking and a tunable lattice constant

The self-organization of pyramidal PbSe islands that spontaneously form during strained-layer epitaxial growth of PbSe/Pb1-xEuxTe (x = 0.05 to 0.1) superlattices results in the formation of three-dimensional quantum-dot crystals. In these crystals, the dots are arranged in a trigonal lattice with a face-centered cubic (fcc)-like A-B-C-A-B-C vertical stacking sequence. The lattice constant of the dot crystal can be tuned continuously by changing the superlattice period. As shown by theoretical calculations, the elastic anisotropy in these artificial dot crystals acts in a manner similar to that of the directed chemical bonds of crystalline solids. The narrow size distribution and excellent control of the dot arrangement may be advantageous for optoelectronic device applications.

SITE-DIAGONAL DISORDER AND OFF-DIAGONAL DISO-RDER AMORPHOUS QUANTUM DOTS ON DIAMOND LATTICE

The theoretical studies about site-diagonal disorder and off-diagonal disorder amorphous quantum. dots on diamond lattice are reported. We describe the electronic structures of the amorphous models by simple tight-binding Hamiltonian, solve the Ha-miltonian equation by recursion method, and study the localization behavior from the sensitivity of eigenvalues to a change in boundary conditions. It is found that if the band edges of amorphous quantum dots are consisted of extended states, the changing tendency of the band widths with size are similar to that of crystal quantum dots; if they are consisted of localized states, the band widths do not change with size when the size is greater than a critical one, but the densities of states at the band edges increase with size. The imaginary part of dielectric function ε2（?ω） of amorphous quantum dots is also studied. The ε2（?ω） corresponding to extended state is sensitive to the change of size. The ε2（?ω） corresponding to localized state has a small value and is insensitive to the change of size when the size is greater than localization length Lloc; when the size is close to or less than Lloc; there are no difierences between localized states and extended states, and ε2（?ω） increases quickly with the decrease of size.