V. A. Zinovyev

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.

Nucleation and growth of ordered groups of SiGe quantum dots

An approach for the formation of ordered groups of Ge nanoislands (quantum dots, QDs) upon epitaxial growth on the surface of a heterostructure constituted by a Si (100) substrate having preliminarily formed seeds in the form of disk-like SiGe nanomounds is developed. It is found that the observed arrangement of QDs within a group is due to the anisotropic elastic-strain energy distribution on the surface of a SiGe nanomound, namely, to the existence of four local energy minima arranged in an ordered manner along the [100] and [010] directions with respect to the seed center. Multilayer structures with vertically aligned QD groups are grown using the suggested approach. The crystal structure and the elemental composition of the spatially ordered nanostructures are examined by transmission electron microscopy, X-ray diffraction analysis, and Raman spectroscopy.

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.

Nucleation sites of Ge nanoislands grown on pit-patterned Si substrate prepared by electron-beam lithography

Regular pit-patterned Si(001) substrates were prepared by electron-beam lithography followed by plasma chemical etching. The geometry of the pits was controlled by varying the etching conditions and the electron-beam exposure duration. It was shown that the location of three-dimensional (3D) Ge nanoislands subsequently grown on the pit-patterned Si substrates depends on the shape of the pit bottom. In the case of pits having a sharp bottom, 3D Ge islands nucleate inside the pits. For pits with a wide flat bottom, the 3D Ge island nucleation takes place at the pit periphery. This effect is attributed to the strain relaxation depending not only on the initial pit shape, but also on its evolution during the Ge wetting layer deposition. It was shown by Monte Carlo simulations that in the case of a pit with a pointed bottom, the relaxation is most effective inside the pit, while for a pit with a wide bottom, the most relaxed area migrates during Ge deposition from the pit bottom to its edges, where 3D Ge islands nucleate.Regular pit-patterned Si(001) substrates were prepared by electron-beam lithography followed by plasma chemical etching. The geometry of the pits was controlled by varying the etching conditions and the electron-beam exposure duration. It was shown that the location of three-dimensional (3D) Ge nanoislands subsequently grown on the pit-patterned Si substrates depends on the shape of the pit bottom. In the case of pits having a sharp bottom, 3D Ge islands nucleate inside the pits. For pits with a wide flat bottom, the 3D Ge island nucleation takes place at the pit periphery. This effect is attributed to the strain relaxation depending not only on the initial pit shape, but also on its evolution during the Ge wetting layer deposition. It was shown by Monte Carlo simulations that in the case of a pit with a pointed bottom, the relaxation is most effective inside the pit, while for a pit with a wide bottom, the most relaxed area migrates during Ge deposition from the pit bottom to its edges, where 3D Ge islan...

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.

Surface morphology transitions induced by ion beam action during Ge/Si MBE

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) experiments were performed to study growth modes induced by hyperthermal Ge+ ion action during molecular beam epitaxy (MBE) of Ge on Si(100). The continuous and pulsed ion beams were used. These studies have shown that ion-beam action during heteroepitaxy leads to decrease in critical film thickness for transition from two-dimensional (2D) to three-dimensional (3D) growth modes, enhancement of 3D island density and narrowing of island size distribution, as compared with conventional MBE experiments. Moreover, it was found that ion beam assists the transition from hut to dome shaped Ge islands on Si(100).

Study of the Structural and Emission Properties of Ge(Si) Quantum Dots Ordered on the Si(001) Surface

A method for creation of Ge/Si structures with space-arranged nanoislands by heteroepitaxy on the pre-patterned Si(001) substrates with a square grid of the etched pits is developed. The influence of depth and inter-pit spacing on the nucleation and growth of Ge(Si) nanoislands is studied. It is shown, that the nanoislands are formed either inside pits or at their periphery depending on the pit depth. It is found that the size of the nanoislands grown inside the pits goes up with the increase of the inter-pit distance from 1 to 4 μm. The pronounced photoluminescence signal related with the space-arranged arrays of quantum dots with a period of 1 μm is observed in the range of energies from 0.9 to 1.0 eV.

Self-organized low density SiGe quantum dot molecules

Strain feld distribution at the surface of SiGe nanomounds formed by heteroepitaxy is exploited to obtain a positional ordering of the closely spaced Ge quantum dots (quantum dot molecules). We demonstrated, that a low density of the lateral quantum dot molecules (up to 107 cm-2) can be achieved by tuning of the growth conditions. We present a growth model that provide physical insights into possible mechanisms underlying the formation of lateral SiGe quantum dot molecules. The electronic band structure of the molecules was calculated by 6-band kp method. The results of theoretical study are in a good agreement with experimental measurements of photoluminescence spectra from the samples with quantum dot molecules.

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

SiGe quantum rings on the Si(100) surface

g=2.0022\pm0.0001