A. F. Zinovieva

Spin relaxation in inhomogeneous quantum dot arrays studied by electron spin resonance

Electron states in an inhomogeneous Ge/Si quantum dot array with groups of closely spaced quantum dots were studied by the conventional continuous-wave electron spin resonance and spin-echo techniques. We have found that the existence of quantum dot groups allows increasing the spin relaxation time in the system. The created structures permit us to change the effective localization radius of electrons by an external magnetic field. With the localization radius being close to the size of a quantum dot group, we obtain a fourfold increase in the spin relaxation time T1 as compared to conventional homogeneous quantum dot arrays. This effect is attributed to an averaging of the local magnetic fields produced by 29 Si nuclear spins and a stabilization of the Sz polarization during the electron back-and-forth motion within a quantum dot group.

ESR Study of Electron States in Ge/Si Heterostructures with Nanodisc Shaped Quantum Dots

Abstract Electron states in Ge/Si heterostructures with nanodisc shaped quantum dots (QDs) were studied using the electron spin resonance (ESR) method. Three ESR signals were observed under light illumination of the samples. The analysis of the angular dependences of the ESR spectra allows one performing signal assignment. Two ESR signals observed in the dark, as well as under illumination, are related to the electrons localized due to the strain at the top edges and base edges of the nanodiscs, while the third ESR signal observed only under illumination is related to the electrons localized in the Si layer above (or under) the central parts of the nanodiscs due to the Coulomb interaction with photoholes. A comparison with ESR results obtained for structures with lens-shaped QDs was performed. It was found that the nanodisc shape is crucial for the simultaneous localization of three electrons with different g values on the same QD. In the case of lens shaped QDs, only two ESR signals corresponding to the electrons localized at the QD top and base edges of QDs were observed.

Photoluminescence enhancement in double Ge/Si quantum dot structures

The luminescence properties of double Ge/Si quantum dot structures are studied at liquid helium temperature depending on the Si spacer thickness d in QD molecules. A seven-fold increase in the integrated photoluminescence intensity is obtained for the structures with optimal thickness d = 2 nm. This enhancement is explained by increasing the overlap integral of electron and hole wavefunctions. Two main factors promote this increasing. The first one is that the electrons are localized at the QD base edges and their wavefunctions are the linear combinations of the states of in-plane Δ valleys, which are perpendicular in k-space to the growth direction [001]. This results in the increasing probability of electron penetration into Ge barriers. The second factor is the arrangement of Ge nanoclusters in closely spaced QD groups. The strong tunnel coupling of QDs within these groups increases the probability of hole finding at the QD base edge, that also promotes the increase in the radiative recombination probability.

Electron spins in an array of tunnel-coupled quantum dots

Mechanisms of electron spin relaxation in semiconductor arrays with tunnel-coupled quantum dots are reviewed. The contribution for anisotropic exchange interaction is shown for asymmetrical quantum dots having no inversion axis relative to their plane. The configuration of vertically coupled double Ge/Si quantum dots is found where anisotropic exchange coupling does not contribute to spin decoherence. It could be a basic configuration of spin-based quantum computation schemes.

Energy Spectrum of Charge Carriers in Elastically Strained Assemblies of Ge/Si Quantum Dots

The results of studying the energy spectrum of electrons and holes localized in second-type Ge/Si heterostructures with Ge quantum dots are presented. In such structures, holes are localized at Ge quantum dots, and electrons, in three-dimensional quantum wells, which form in Si at the Ge—Si interface because of inhomogeneous deformations that appear as a result of the difference between the Ge and Si lattice constants. It is shown that changes in the deformations in the assembly of quantum dots as a result of a variation in their spatial arrangement significantly changes the binding energy of electrons, the position of their localization at quantum dots, the binding energy and wave-function symmetry of holes at double quantum dots (artificial molecules), and the exchange interaction of electrons and holes in the exciton composition. A practically important result of the presented data is the development of approaches to increase the luminescence quantum efficiency and the absorption coefficient in assemblies of quantum dots.

Simultaneous Localization of Electrons in Different Δ-Valleys in Ge/Si Quantum Dot Structures

In the present work the possibility of simultaneous localization of two electrons in Δ100 and Δ001 valleys in ordered structures with Ge/Si quantum dots (QD) was verified experimentally by electron spin resonance (ESR) method. ESR spectra obtained for the ordered ten-layered QD structure in the dark shows the signal corresponding to electron localization in Si at the Ge QD base edges, in Δ100, Δ010 valleys (gzz=1.9985, gin-plane=1.999). Light illumination causes the appearance of a new ESR line (gzz=1.999) attributed to the electrons in Δ001 valley localized at the QD apexes. Observed effect is explained by enhancement of electron confienment near QD apex by Coloumb attraction to the photogenerated hole trapped in Ge QD.

Electron localization in self-assembled Si quantum dots grown on Ge(111)

Electron localization in a Si/Ge heterosystem with Si quantum dots (QDs) was studied by transport and electron spin resonance (ESR) measurements. For Si QD structures grown on Ge(111) substrates, the ESR signal with a g-factor and ESR line width Oe was observed and attributed to the electrons localized in QDs. The g-factor value was explained taking into account the energy band modification due to both strain and quantum confinement. The transport behavior confirms the efficient electron localization in QDs for a Si/Ge(111) system. A strong Ge–Si intermixing in QD structures grown on Ge(001) is assumed to be the main reason for an unobserved ESR signal from the QDs.Electron localization in the Si/Ge heterosystem with Si quantum dots (QDs) was studied by transport and electron spin resonance (ESR) measurements. For Si QD structures grown on Ge(111) substrates, the ESR signal with g-factor

Spin Dynamics in Two-Dimensional Arrays of Quantum Dots with Local Ordering of Nanoclusters

g=2.0022\pm0.0001

Spin dynamics in two-dimensional arrays of quantum dots with different shapes

and ESR line width

Spin Transport And Spin Relaxation In Ge/Si Quantum Dots

\Delta H\approx1.2