X-ray scattering in multilayer structures with quantum dots

The scattering of X-rays on multilayer semiconductor structures with quantum dots was studied. Equations describing coherent and diffuse scattering in superlattices with an arbitrary number of layers in the period were obtained. Numerical simulation of coherent and diffuse scattering of X-rays from laser superlattices with different number of periods was performed to analyze the experimental data. A physical interpretation of the angular displacement of the coherent peaks from the maxima of diffuse scattering observed in the experiment is given.     

Coherent and diffuse X-ray scattering from a multicomponent superlattice with quantum dots

GaAs-AlAs superlattices with InAs quantum dots grown by molecular beam epitaxy have been studied by high-resolution X-ray diffractometry. It has been experimentally revealed that the maxima of the superstructural diffuse scattering from the quantum dots do not coincide with the angular positions of the coherent superstructural satellites. A statistical theory of diffraction on the superlattice taking into account the spatial correlation of the quantum dots has been developed to explain this effect. It has been shown within this theory that the peak positions of the diffuse component can differ from the maxima of the coherent scattering. X-ray scattering by the multicomponent superlattice have been numerically simulated and the calculation results have been compared with the experimental data.     

Modelling x-ray scattering from quantum dots using Keating energy-minimised structures

In a traditional analysis of surface x-ray diffraction data, the surface unit cell can be defined by a small set of parameters, and fitting of experimental data is accomplished using well-established procedures. A quantum dot (QD), however, may contain as many as 20,000 atoms, so a different approach to data analysis is required. A method for modelling a quantum dot and relaxing the structure by minimising the Keating energy is presented, and the simulation of x-ray scattering from such models is described. A method is then developed for simulating the alloying of InAs and GaAs inside the QDs using intermediate Keating parameters. This removes the need to relax and average multiple models with randomly distributed atoms, which would increase the computation time significantly.     

Elastic light scattering by semiconductor quantum dots of arbitrary shape

Elastic light scattering by low-dimensional quantum objects without a change in the frequency is theoretically investigated in terms of the quantum perturbation theory. The differential cross section of resonance light scattering from any excitons in any quantum dots is calculated. It is demonstrated that, when the light wavelengths considerably exceed the quantum-dot size, the polarization and angular distribution of the scattered light do not depend on the shape, the size, or the configuration of quantum dots. In this case, the total light scattering cross section is independent of the quantum-dot size. If the radiative damping of an exciton exceeds the nonradiative damping, the total light scattering cross section at resonance is of the order of the light wavelength squared. The radiative damping associated with the long-range exchange interaction between electrons and holes is calculated for any excitons and any quantum dots.     

Micro-Raman scattering by laser-modified structures with Ge/Si quantum dots

Structures with self-assembled Ge/Si quantum dots grown by molecular-beam epitaxy are exposed to pulsed radiation of a picosecond laser. Changes in the vibrational spectrum of nanostructures under an external action are studied by Raman spectroscopy. An analysis of the Raman spectra measured with a micron spatial resolution along the exposed region indicates a mixing of Ge and Si atoms and a change in the induced mechanical stresses in quantum dots.     

Raman scattering in multilayer structures with CdTe quantum dots in ZnTe

Raman spectra in superlattices composed of layers of self-assembled CdTe quantum dots separated by ZnTe barriers are investigated. As the barrier thickness increases, a high-frequency shift of all peaks is observed, which is explained by a decrease in the lattice constant averaged over the volume of the entire structure. Peaks are found at a CdTe TO mode frequency of 140 cm^?1 and also at 120 cm^?1. The first peak is assigned to the symmetric Coulomb (interface) mode of the quantum dot material, and the low-frequency peak is assigned to the symmetric mode of the phonons captured in the quantum dot. This combination of modes in structures with quantum dots has not been observed previously.     

Effect of spatial correlation of quantum dots on X-ray diffuse scattering

A statistical kinematical theory of x-ray diffuse scattering by crystals with nanometer quantum dots is constructed. The theory takes into account the mutual correlation of the spatial arrangement of quantum dots.     

Raman scattering of light by optical phonons in Si-Ge-Si structures with quantum dots

Raman scattering of light by optical phonons in Si-Ge-Si structures with pseudomorphic germanium quantum dots has been investigated. Resonance amplification of the scattering intensity on E ₀ (Γ₇−Γ₈) transitions has been observed. It is shown that as a result of the formation of the layer of germanium quantum dots, the resonance energy is ∼0.3 eV higher than in the two-dimensional case. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 3, 203–207 (10 August 1996)     

Diffuse X-ray scattering from several platinum chain compounds

Values of the Fermi wavevector for several platinum based one-dimensional conductors were determined from diffuse X-ray scattering measurements. The values were compared with those expected from the chemical compositions. The importance of conclusive values of this parameter is stressed and the coupling between Peierls instability and other structural instabilities is discussed.     

X-ray diffuse scattering from incommensurable structures

X-Ray diffuse scattering from a series of examples where the reason for the existence of incommensurable features is fairly well understood, is described. They include in particular non stoichiometric compounds such as [DIPS Φ₄(l₃)0.76] and such as intercalated graphite RbC₂₄. and quasi one dimensional conductors. A particular emphasis is given on various 1 - D conductors and to the relation of the incommensurability to the characteristics of the electron conduction bands.     

Solution processed microcavity structures with embedded quantum dots

The authors report the fabrication of a one-dimensional microcavity structure embedded with colloidal CdSe/ZnS core/shell quantum dots using solution processing. The microcavity structures were fabricated by spin coating alternating layers of polymers of different refractive indices (poly-vinylcarbazole—PVK, and poly-acrylic acid—PAA) to form the distributed Bragg reflectors (DBRs). Greater than 90% reflectivity was obtained using ten periods of the structure. The one-dimensional microcavity was formed by sandwiching a λ/n thick defect layer between two such DBRs. The emission of the quantum dots from the microcavity structure demonstrated directionality following the cavity mode dispersion and spectral narrowing. Room temperature time-resolved photoluminescence measurements carried out on this structure showed significant reduction in the photoluminescence decay time which is attributed primarily to nonradiative mechanism originating in the presence of the PVK host matrix. The photoluminescence decay time of the quantum dots was found to be 1000 ps while for the quantum dots embedded in the polymer host and the microcavity were 400 and 150 ps, respectively.     

Helical quantum states in HgTe quantum dots with inverted band structures

We investigate theoretically the electron states in HgTe quantum dots (QDs) with inverted band structures. In sharp contrast to conventional semiconductor quantum dots, the quantum states in the gap of the HgTe QD are fully spin-polarized and show ringlike density distributions near the boundary of the QD and spin-angular momentum locking. The persistent charge currents and magnetic moments, i.e., the Aharonov-Bohm effect, can be observed in such a QD structure. This feature offers us a practical way to detect these exotic ringlike edge states by using the SQUID technique.     

Electronic structures of alloy quantum dots with nonuniform composition

In this study, the significant effect of the nonuniform composition in alloy quantum dots (QDs) on electronic structure is analyzed in depth. The equilibrium composition profiles in experimentally observed dome and barn shaped GeSi/Si QDs are determined by combining the finite element method and the method of moving asymptotes. Due to the composition variation, the total band edge of heavy hole is dominated by the band offset and spin-orbit coupling rather than the strain effect. The numerical results reveal that the wave function of heavy hole trends to be localized in the Ge-rich region at the top of the large QD. Moreover, the size effect gradually compensates the composition effect as the size of QD decreases.     

Impurity absorption of light in structures with quantum dots

The impurity absorption of light in a quantum dot with a parabolic potential profile is considered within the framework of the model of a zero radius potential in the effective mass approximation. The sensitivity of the effect of position disorder to the size factor at the transition from a quantum well to a quantum dot is revealed. The spectral dependence of the coefficient of impurity absorption of light is investigated with account of the spread in size of quantum dots. It is shown that the account of spread in size results in smearing of discrete absorption lines. The impurity absorption edge depends on the parameters of quantum dots and the depth of the impurity level.     

Diffuse scattering of modulated structures and quasicrystals

The method of Fourier transforms is applied to the calculation of sharp (coherent) and diffuse (incoherent) scattering of modulated structures and quasicrystals. It is shown that the fundamental difference between both may best be described by introducing different distribution functions of atoms, before applying modulations of the structure.     

Quantum dots in quantum well structures

Recent progress toward fabricating and characterizing quantum dots in III–V quantum well structures is reviewed. Quantum dots made by use of lithography and etching, including deep-etched, barrier-modulated, strain-induced and interdiffused quantum dots, are described. Quantum dots fabricated by growth, including natural quantum dots, dots on patterned substrates, and self-assembled dots, are discussed. Dot sizes and uniformity, energy-level splittings, and luminescence efficiencies that are now being achieved are discussed. The status of key issues, such as the energy relaxation in quantum dots, is mentioned.