A. V. Novikov

Microscopic and optical investigation of Ge nanoislands on silicon substrates

We investigate self-assembled nanoislands in heteroepitaxial GeSi systems by means of atomic force microscopy and micro-Raman scattering techniques. We show that the surface diffusion of Si atoms from the substrate to the islands is strongly enhanced when the temperature increases, giving rise to a wider stability range of pyramid-shaped volumes.

Self-organization of germanium nanoislands obtained in silicon by molecular-beam epitaxy

Nanometer germanium islands in epitaxial layers of silicon are obtained by molecular-beam epitaxy. The dimensions and shapes of the islands are determined in an atomic-force microscope. The photoluminescence spectra are found to contain lines that can be interpreted as quasidirect optical transitions in the islands. It is concluded on the basis of optical and microprobe measurements and theoretical calculations of the energies of electronic states that silicon is dissolved in the germanium islands. Values of the germanium and silicon contents in the solid solution are presented.

Condensation of excitons and the spectrum of multiparticle states in SiGe/Si quantum wells: The role of the barrier in the conduction band

It has been demonstrated that the barrier in the conduction band represented by the SiGe layer in SiGe/Si quantum wells affects the work function and equilibrium density of the quasi-two-dimensional condensed phase formed in these structures. The existence of a new recombination channel with unconventional characteristics is uncovered in the structures with barrier heights close to the critical value for the formation of the electron-hole liquid.

Optical monitoring of technological parameters during molecular-beam epitaxy

It is shown that one can use low-coherence tandem interferometry to measure the substrate temperature during the course of molecular-beam epitaxy in the case of oblique incidence of the probing light onto the surface. The temperature conditions in the Ob’-M installation for growing heteroepitaxial structures of cadmium and mercury tellurides and in the RIBER SIVA-21 installation for the growth of silicon-germanium structures are investigated. Calibration curves relating the readings of the standard thermocouple fixed within the heater to the true substrate temperature in the range 0–500°C are created.

Gigantic uphill diffusion during self-assembled growth of Ge quantum dots on strained SiGe sublayers

Raman spectroscopy and atomic-force microscopy were applied to study the morphology of nanoislands grown on strained Si1−xGex sublayers. It was shown that the growth of nanoislands on strained Si1−xGex sublayer not only induces the effect of their spatial ordering but also enhances the role of interdiffusion processes. Unusual high island volume increase during the epitaxy is explained by anomalous strong material diffusion from the sublayer into the islands, induced by nonuniform field of elastic strains.

SiGe nanostructures with self-assembled islands for Si-based optoelectronics

The effect of structure parameters on the electroluminescence and photoconductivity of multilayer structures with self-assembled Ge(Si)/Si(0 0 1) islands has been studied. The highest intensity of the room-temperature electroluminescence in the wavelength range of 1.3–1.55 µm has been observed for the islands grown at 600 °C. The same diode structures with Ge(Si)/Si(0 0 1) islands have demonstrated room-temperature photoconductivity signals in the wavelength range of 1.3–1.55 µm. The observed overlap of the electroluminescence and photoconductivity spectra obtained for the same structures with Ge(Si) islands makes these structures a promising material for the fabrication of a Si-based optocoupler. Less degradation after neutron irradiation has been observed for the electroluminescence and photoconductivity signals from multilayer structures with Ge(Si) self-assembled islands in comparison with bulk silicon structures. This result is associated with more effective confinement of charge carriers in the multilayer structures with Ge(Si) islands.

Features of two-dimensional to three-dimensional growth mode transition of Ge in SiGe/Si(001) heterostructures with strained layers

The results of the study of the effect of strained SiGe layers on the critical thickness of two-dimensional growth of Ge layer in different SiGe/Si(001) structures are presented. A significant influence of buried strained SiGe layer on the growth of Ge has been found out, which remains considerable even for SiGe layers capped by unstrained Si layer of thickness up to 3.5 nm. The experimental results are well described by the proposed model, where obtained features are explained by means of introducing a phenomenological parameter called “effective decay length” of the strain energy accumulated in the structure.

Raman Spectroscopy and Electroreflectance Studies of Self-Assembled SiGe Nanoislands Grown at Various Temperatures

SiGe nanoislands grown in a silicon matrix at temperatures of 300 to 600°C are studied using Raman spectroscopy and electroreflectance. For islands grown at relatively low temperatures (300–500°C), phonon bands are observed to have a doublet structure. It is shown that changes in the percentage composition, size, and shape of nanoislands and, hence, in the elastic stresses (depending on the growth temperature of the structures) have a significant effect on the energies of optical electronic interband transitions in the islands. As a consequence, the resonance conditions for Raman scattering also change. It is found that interdiffusion from the silicon substrate and the cover layer (determining the mixed composition of SiGe islands) is of importance even at low growth temperatures of nanostructures (300–400°C).

Usage of antimony segregation for selective doping of Si in molecular beam epitaxy

An original approach to selective doping of Si by antimony (Sb) in molecular beam epitaxy (MBE) is proposed and verified experimentally. This approach is based on controllable utilization of the effect of Sb segregation. In particular, the sharp dependence of Sb segregation on growth temperature in the range of 300–550 °C is exploited. The growth temperature variations between the kinetically limited and maximum segregation regimes are suggested to be utilized in order to obtain selectively doped structures with abrupt doping profiles. It is demonstrated that the proposed technique allows formation of selectively doped Si:Sb layers, including delta (δ-)doped layers in which Sb concentrations can be varied from 5 × 1015 to 1020 cm−3. The obtained doped structures are shown to have a high crystalline quality and the short-term growth interruptions, which are needed to change the substrate temperature, do not lead to any significant accumulation of background impurities in grown samples. Realization of the p...

Electron-hole liquid and excitonic molecules in quasi-two-dimensional SiGe layers of Si/SiGe/Si heterostructures

The electron-hole liquid (EHL) in SiGe layers of Si/Si1 − xGex/Si quantum-confinement heterostructures is discovered. It is composed of quasi-two-dimensional holes in the quantum well formed by the SiGe layer and quasi-three-dimensional electrons, which occupy a wider region of space centered on this layer. The densities of electrons and holes in the EHL are determined to be p0 ≈ 8.5 × 1011 cm−2 and n0 ≈ 4.8 × 1018 cm−3, respectively. It is demonstrated that the gas phase consists of excitons and excitonic molecules. The conditions on the band parameters of the structure under which the formation of the EHL of this kind and biexcitons is possible are formulated.