A. V. Nenashev

Self-assembly of germanium islands under pulsed irradiation by a low-energy ion beam during heteroepitaxy of Ge/Si(100) structures

The effect of pulsed irradiation by a low-energy (50–250 eV) ion beam with a pulse duration of 0.5 s on the nucleation and growth of three-dimensional germanium islands during molecular-beam heteroepitaxy of Ge/Si(100) structures is investigated experimentally. It is revealed that, at specific values of the integrated ion flux (less than 1012 cm−2), pulsed ion irradiation leads to an increase in the density of islands and a decrease in their mean size and size dispersion as compared to those obtained in the case of heteroepitaxy without ion irradiation. The observed phenomena are explained in the framework of the proposed model based on the concept of a change in the diffusion mobility of adatoms due to the instantaneous generation of interstitial atoms and vacancies under pulsed ion irradiation. It is assumed that the vacancies and interstitial atoms give rise to an additional surface strain responsible for the change in the binding energy of the adatoms. Under certain conditions, these processes bring about the formation of centers of preferential nucleation of three-dimensional islands at the places where the ions impinge on the surface. The model accounts for the possibility of annihilating vacancies and interstitial atoms on the surface of the growing layer. It is demonstrated that the results obtained from the Monte Carlo calculations based on the proposed model are in good agreement with the experimental data.

Photoconduction in tunnel-coupled Ge/Si quantum dot arrays

The photoconduction in a tunnel-coupled Ge/Si quantum dot (QD) array has been studied. The photoconductance (PC) sign can be either positive or negative, depending on the initial filling of QDs with holes. The PC kinetics has a long-term character (102−104 s at T = 4.2 K) and is accompanied by persistent photoconduction (PPC), whereby the PC value is not restored on the initial level even after relaxation for several hours. These phenomena are observed upon illumination by light with photon energies both greater and smaller than the silicon bandgap. A threshold light wavelength corresponding to a long-term PC kinetics depends on the QD filling with holes. A model describing the observed PC kinetics is proposed, according to which the main contribution to the PC is related to the degree of QD filling with holes. By applying the proposed model to the analysis of PC kinetics at various excitation levels, it is possible to determine the dependence of the hopping conductance on the number of holes per QD. The rate of the charge carrier density relaxation exponentially depends on the carrier density.

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.

Hole states in artificial molecules formed by vertically coupled Ge/Si quantum dots

In the tight binding approximation, the spatial configuration of the ground state and the binding energy of a hole in a “diatomic” artificial molecule formed by vertically coupled Ge/Si(001) quantum dots are studied. The inhomogeneous spatial distribution of elastic strain arising in the medium due to the lattice mismatch between Ge and Si is taken into account. The strain is calculated using the valence-force-field model with a Keating interatomic potential. The formation of the hole states is shown to be determined by the competition of two processes: the appearance of a common hole due to the overlapping of “atomic” wavefunctions and the appearance of asymmetry in the potential energy of a hole in the two quantum dots because of the superposition of the elastic strain fields from the vertically aligned Ge nanoclusters. When the thickness of the Si layer separating the Ge dots (tSi) is greater than 2.3 nm, the binding energy of a hole in the ground state of the two-dot system proves to be lower than the ionization energy of a single quantum dot because of the partial elastic stress relaxation due to the coupling of the quantum dots and due to the decrease in the depth of the potential well for holes. For the values of the parameter tSi, an intermediate region is revealed, where the covalent molecular bond fails and the hole is localized in one of the two quantum dots, namely, in the dot characterized by the highest strain values.

Binding of electron states in multilayer strained Ge/Si heterostructures with type-II quantum dots

Mechanical strains in a multilayer Ge/Si(001) heterostructure with vertically aligned Ge nanoclusters (quantum dots) are calculated using an interatomic potential based on the Keating valence-force-field model. It is found that the nonuniform spatial elastic strain distribution in this medium gives rise to a three-dimensional potential well for electrons in the strained Si layers near Ge nanoclusters. The depth of the potential well reaches 100 meV, and its spatial dimensions are determined by the diameter of the Ge nanoclusters. For a structure consisting of four Ge islands 23 nm in diameter arranged one above another, the electron binding energies in this well and the spatial electron density distribution are determined. The ground state has an s-like symmetry and is characterized by an electron binding energy of ∼95 and ∼60 meV for the elemental composition of Ge in the nanoclusters c = 1 and c = 0.7, respectively. The existence of bound electron states in the conduction band of strained Si must lead to a relaxation of the selection rules that determine the low efficiency of the radiative recombination in indirect-gap semiconductors. This explains the high value of the oscillator strength observed for the interband transitions in multilayer Ge/Si(001) structures with vertical correlation of the arrangement of Ge nanoclusters.

Hole Spin Relaxation in Ge Quantum Dots

Hole spin relaxation in an isolated Ge quantum dot due to interaction with phonons is investigated. Spin relaxation in this case occurs through the mechanism of the modulation of the spin-orbit interaction by lattice vibrations. According to the calculations performed, the spin relaxation time due to direct single-phonon processes for the hole ground state equals 1.4 ms in the magnetic field H = 1 T at the temperature T = 4 K. The dependence of the relaxation time on the magnetic field is described by the power function H−5. At higher temperatures, a substantial contribution to spin relaxation is made by two-phonon (Raman) processes. Because of this, the spin relaxation time decreases to nanoseconds as the temperature is raised to T = 20 K. Analysis of transition probabilities shows that the third and twelfth excited hole states, which are intermediate in two-step relaxation processes, play the main part in Raman processes.

Selective oxidation of poly-Si with embedded Ge nanocrystals in Si/SiO2/Ge(NCs)/poly-Si structure for memory device fabrication

Selective oxidation of poly-Si in Si/SiO2/Ge(NCs)/poly-Si structure was proposed as the method of tunable synthesis of a control insulator in a memory device with Ge nanocrystals (NCs). Different behavior of oxidation resulting in partial and total oxidation of poly-Si was investigated using spectral ellipsometry and capacitance?voltage (CV) techniques. The ~2.1 V memory window corresponding to the electron and hole charging/discharging processes in NCs was obtained in Si/SiO2/Ge(NCs)/SiO2 structure. To compare the experimental and calculated CV characteristics the energies of the electron and hole ground states were determined.

Ge/Si quantum dots in external electric and magnetic fields

Electric field-induced splitting of the lines of exciton optical transitions into two peaks is observed for Ge/Si structures with quantum dots (QDs). With increasing field, one of the peaks is displaced to higher optical transition energies (blue shift), whereas the other peack is shifted to lower energies (red shift). The results are explained in terms of the formation of electron-hole dipoles of two types differing in the direction of the dipole moment; these dipoles arise due to the localization of one electron at the apex of the Ge pyramid and of the other electron under the base of the pyramid. By using the tight-binding method, the principal values of the g factor for the hole states in Ge/Si quantum dots are determined. It is shown that the g factor is strongly anisotropic, with the anisotropy becoming smaller with decreasing QD size. The physical reason for the dependence of the g factor on quantum-dot size is the fact that the contributions from the states with different angular-momentum projections to the total wave function change with the QD size. Calculations show that, with decreasing QD size, the contribution from heavy-hole states with the angular-momentum projections ±3/2 decreases, while the contributions from light-hole states and from states of the spin-split-off band with the angular-momentum projections ±1/2 increase.

Zeeman Effect for Holes in a Ge/Si System with Quantum Dots

The tight binding approximation is employed to study the Zeeman effect for the hole ground state in a quantum dot. A method is proposed for calculating the g factor for localized states in a quantum dot. This method can be used both for hole states and for electron states. Calculations made for a Ge/Si system with quantum dots show that the g factor of a hole in the ground state is strongly anisotropic. The dependence of the g factor on the size of a germanium island is analyzed and it is shown that anisotropy of the g factor increases with the island size. It is shown that the value of the g factor is mainly determined by the contribution of the state with the angular momentum component Jz=±3/2 along the symmetry axis of the germanium island.

Linear chains of Ge/Si quantum dots grown on a prepatterned surface formed by ion irradiation

The growth of Ge nanoclusters on a prepatterned Si (100) surface formed by imprint lithography in combination with subsequent irradiation with Ge+ ions is studied. The prepatterned surface presents a system of parallel 10-nm-wide grooves repeating with a period of 180 nm. Irradiation of the substrate was conducted at two temperatures, room temperature (cold irradiation) and 400°C (hot irradiation). It is shown that, during epitaxy (550–700°C), the residual radiation defects located in the bulk under the grooves suppress the nucleation of Ge nanoclusters in the grooves. In the case of prepatterned substrates, from which imperfect regions are completely removed, nanoclusters grow in the grooves.