V. A. Kurbatov

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.

Morphological transformation of a germanium layer grown on a silicon surface by molecular-beam epitaxy at low temperatures

Multilayer Si/Ge nanostructures with germanium layers of different thicknesses are grown by molecular-beam epitaxy at low temperatures (<350°C) and studied using photoluminescence and atomic force microscopy. It is found that the germanium layer undergoes a morphological transformation when its thickness becomes equal to approximately five monolayers: an island relief transforms into a smooth undulating relief.

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.

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.

Quasi-hydrodynamic simulation of the conductivity of heavily doped nanosize layered heterostructures in high electric fields

The electrical conductivity of nanosize-layered heterostructures in high electric fields is studied using the methods of mathematical simulation based on the equation of energy balance. It is shown that a characteristic feature of the corresponding current-voltage characteristics is a peak of the differential conductance, whose position and magnitude are determined by the heterobarrier height and steepness, as well as by the doping level of the structure. A physical model is suggested that accounts for the shape of the calculated characteristics by the combined effect of the electrostatic reduction in the heterobarrier height and the increase in the electronic temperature in the vicinity of the injecting heterointerfaces.

A Quasi-Hydrodynamic Simulation of Electrical Conductivity in Selectively Doped Submicrometer-Sized Layered Structures and Island Films in High Electric Fields

Mathematical simulation is used to study special features of the high-field drift of electrons that occurs in submicrometer-sized n+-n-n+ structures with an appreciable impurity-concentration profile in the their high-resistivity region. A quasi-hydrodynamic description of the electron drift is used. In this description, the dependences of the charge-carrier mobility and energy-relaxation time on the electron temperature, the thermodiffusion component of the electron flow, and the divergence of the electron-temperature flux are taken into account. It is shown that sectioning of the high-resistivity carrier-flight n-type region by additional low-resistivity n+-type inclusions with submicrometer thickness appreciably reduces the electron-gas temperature and increases the effective mobility of the charge carriers and, consequently, the high-field electrical conductance of the structure owing to a corresponding increase in the drift velocity.

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.

Room temperature λ=1.3 µm photoluminescence from InGaAs quantum dots on (001) Si substrate

GaAs/InxGa1−xAs quantum dot heterostructures exhibiting high-intensity λ=1.3 µm photoluminescence at room temperature have been grown on (001) Si substrate with a Si1−xGex buffer layer. The growth was done successively on two MBE machines with sample transfer via the atmosphere. The results obtained by the study of the structure growth process by means of high-energy electron diffraction are presented.

Use of magnetic field screening by high-temperature superconducting films to switch microwave signals

The efficiency with which YBCO films screen an alternating magnetic field near the superconducting transition was measured. In the decimeter range measurements were made of the characteristics of a switch whose operating principle was based on the change in the screening of an alternating magnetic field by a superconducting transition.