M. M. Rzaev

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.

Electron-hole liquid in strained SiGe layers of silicon heterostructures

The electron-hole liquid has been found in strained SiGe thin films of Si/Si1−xGex/Si heterostructures. The density and binding energy of the electron-hole liquid have been determined. Owing to the presence of internal strains in the SiGe layer, the density and binding energy are significantly smaller than the respective quantities for the electron-hole liquid in a bulk single crystal of the solid solution of the same composition. The critical temperature of the transition from the exciton gas to the electron-hole liquid is estimated using the experimental data. The Mott transition (from the exciton gas to electron-hole plasma) occurs above the critical temperatures for high excitation intensities.

Misfit Dislocations in SiGe/Si Heterostructures: Nucleation - Propagation - Multiplication

We present experimental data on the effect of low-temperature buffer layers on the dislocation structure formation in SiGe/Si strained-layer heterostructures under thermal annealing. Specific subjects include mechanisms of misfit dislocation nucleation, propagation and multiplication as well as the role of intrinsic point defects in these processes. Samples with lowtemperature Si (400°C) and SiGe (250°C) buffer layers were grown by MBE. In general, the processes of MD generation occur similarly in the heterostructures studied independently of the alloy composition (Ge content: 0.15, 0.30) and kind of buffer layer. Intrinsic point defects related to the low-temperature epitaxial growth influence mainly the rate of misfit dislocation nucleation.

Very first relaxation steps in low temperature buffer layers SiGe/Si heterostructures studied by X-ray Topography

First relaxation stages in Si1-x Gex layers on Si substrates are induced by annealing of metastable, low-temperature buffer layer samples and observed by X-ray topography (XRT). This method allows observing large area (several square millimetres) of a sample and reveals very low densities of defects, located in the layer as well as in the substrate. It allow to follow the evolution of the very first steps of the relaxation, starting with dislocation crosses which were characterized and evolving to misfit dislocation network by very low increases of thermal budget. It is proposed a nucleation mechanism of these crosses based on Frank loops due to point defects condensation which can transform locally in glide dislocations under the influence of the biaxial stress in the film.

Photoluminescence from germanium quantum wells and quantum dots in silicon grown by MBE at low temperature

The low-temperature (T=2 K) photoluminescence (PL) has been studied in Si/Ge structures grown by MBE at a low (250–350°C) temperature of Ge deposition. The luminescence spectra change dramatically when the average thickness of the Ge layer exceeds six monolayers. In this case, the PL line from the pseudomorphic layer (quantum well) retains its spectral position and increases in intensity at the expense of the luminescence line from islands (quantum dots), which then totally fades. The data obtained indicate a considerable difference between the epitaxial growth modes dominating in low and conventional (500–700°C) temperature ranges.

Structural and optical features of InGaAs quantum dots grown on Si(001) substrates

A multilayer GaAs/SiGe/Si heterostructure with InGaAs quantum dots (QDs) embedded in a GaAs layer was grown by molecular beam epitaxy (MBE) on a Si(001) substrate. A step-graded Si1−xGex (0 ≤ x ≤ 1) buffer layer and a GaAs layer with InyGa1 −yAs (y ~ 0.5) QDs were deposited consecutively in two different MBE systems. The heterostructure exhibits intense photoluminescence in the region of 1.3 μm at room temperature. Perfect crystal InGaAs islands with height less than 10 nm are the sources of this radiation.

Phase transitions in nonequilibrium electron-hole systems of Si/SiGe/Si nanoheterostructures

The exciton condensation in a Si1−xGex solid solution layer of Si/Si1−xGex/Si heterostructures with the formation of electron-hole liquid has been investigated by low-temperature photoluminescence spectroscopy. In the temperature range above the critical temperature of the transition from an exciton gas to electron-hole liquid, a Mott transition from an exciton gas to electron-hole plasma has been found and investigated.

Lateral conductivity of p-type doped Si/Ge island structures

Numerical calculations of the conductance in structures with doping modulated along the current-flow direction are carried out taking into account band offsets at the interfaces between high-and low-resistivity regions. It is found that such structures exhibit S-shaped current-voltage characteristics; in the limiting case, there should be a negative-conductance region, with the abruptness of the heterojunction between the narrow-and wide-gap sections of the structure and the doping level being the critically important parameters in the theory. p-type Si/Si1−xGex island structures with different sizes of islands and different band offsets were grown by molecular-beam epitaxy. Theoretical results are compared with the data on lateral conductance of the grown structures.

Electron‐Hole Liquid in Si/SiGe Heterostructures

In thin SiGe layers of Si/Si1−xGex/Si heterostructures electron‐hole liquid (EHL) photoluminescence is found out. The threshold density of excitation necessary for the EHL formation is considerably lower and the threshold temperature is noticeably higher than in bulk alloy of the same composition owing to accumulation of the photoexcited charge carriers in the potential well of the SiGe layer. The analysis of the form of luminescence lines in the SiGe layer allows us to estimate the EHL density (n0 = 2.6⋅1018 cm−3) and the binding energy with respect to gas of excitons (φ = 5.2 meV). At a high level of excitation and temperature exceeding 30 K the line of radiation of electron‐hole plasma (EHP) in SiGe layer is observed in spectra.In thin SiGe layers of Si/Si1−xGex/Si heterostructures electron‐hole liquid (EHL) photoluminescence is found out. The threshold density of excitation necessary for the EHL formation is considerably lower and the threshold temperature is noticeably higher than in bulk alloy of the same composition owing to accumulation of the photoexcited charge carriers in the potential well of the SiGe layer. The analysis of the form of luminescence lines in the SiGe layer allows us to estimate the EHL density (n0 = 2.6⋅1018 cm−3) and the binding energy with respect to gas of excitons (φ = 5.2 meV). At a high level of excitation and temperature exceeding 30 K the line of radiation of electron‐hole plasma (EHP) in SiGe layer is observed in spectra.

Photoluminescence of Si/Ge nanostructures grown by molecular-beam epitaxy at low temperatures

Multilayer Si/Ge nanostructures grown by molecular-beam epitaxy at low temperatures (250–300°C) of germanium deposition are studied using photoluminescence and atomic-force microscopy (AFM). It is assumed that, upon low-temperature epitaxy, the wetting layer is formed through the intergrowth of two-dimensional (2D) and three-dimensional (3D) nanoislands.