Resonance transitions between split levels in three-barrier nanostructures and the prospects of using these structures in devices operating in the submillimeter-wave band

A model describing the coherent tunneling of electrons in a three-barrier quantum-well structure in a high-frequency (terahertz) electric field was developed. On the basis of this model, the frequency dependences of negative dynamic conductivity (intensities of quantum transitions) in three-barrier nanostructures with coherent tunneling of electrons via closely spaced split energy levels was studied qualitatively and quantitatively. It is shown that coherent quantum lasers operating in the far-infrared region of the spectrum and emitting at a wavelength of up to 60 µm (5 THz) can be developed on the basis of these structures. The electron lifetime at the lower resonance level is found to be five times shorter than the characteristic scattering time, the space-charge effect is insignificant, and the available power released in the structure exceeds severalfold the losses of power in the optical waveguide.     

Dependence of the resonant conductivity of symmetric double-barrier structures on the amplitude of rf field

An analytical expression for the rf amplitude dependence of the conductivity in a symmetric double-barrier resonance-tunneling structure with high, thin barriers under conditions of collisionless electron transport is found on the basis of the solution of a nonstationary Schrödinger equation describing the resonance interaction of electrons with the rf field. It is shown that under the action of a rf field with frequency ω and amplitude approximately corresponding to triple the width of the resonance level, up to half of the electrons passing through this level can be transferred, emitting or absorbing a quantum of energy ?ω, into a neighboring level.     

Vertical transfer of137Cs in peat soils fallen out as a consequence of the chernobyl accident

On the basis of measurements of the specific radioactivity of soil specimens sampled with a small step along their deposition, depth profiles are plotted for the curves of the vertical change in ¹³⁷ Cs in peat soils not treated since the Chernobyl accident. A theoretical analysis is provided for the plotted profiles that shows that the vertical transfer of ¹³⁷ Cs in these soils is described adequately within the framework of a model that accounts for the processes of diffusion and convection of the radionuclide in the soil solution and its sorption by the solid phase of the soil. The parameters of the model are determined and a prediction is given for the migration character of ¹³⁷ Cs.Institute of Solid State Physics, Academy of Sciences of Belarus, Minsk, Belarus. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 68, No. 1, pp. 33–38, January–February, 1995.     

Alternating space charge and ambiguity of quantum states in double-barrier structures

Summation of a series of perturbation theory was used to obtain a solution to the time-dependent self-consistent Schrödinger and Poisson equations describing the resonance interaction of electrons that tunnel through asymmetric double-barrier structures with a high-frequency electric field. In the case where electrons are uniformly distributed in energy within the width of the quasi-level, the solution is obtained analytically; if the beam is monoenergetic, solution of the equation is reduced to finding the roots of a fifth-degree algebraic polynomial. It is shown that, in a number of cases, the influence of alternating space charge gives rise to an effect that is quite new for the systems under consideration: several different wave functions may correspond to the same amplitude of the high-frequency voltage applied to the structure; consequently, the values obtained for the high-frequency conductivity and the coefficients of transmission and reflection can differ by several times. As a result, instability of the current flow and hysteresis of the current-voltage characteristics can be observed in these structures. Furthermore, the dependence of the coefficients of transmission and reflection of the electrons and high-frequency conductivity on the voltage amplitude are combinations of the N-and S-shaped characteristics for uniform distribution of electrons and are looplike in the case of a monoenergetic beam.     

A laser operating on coherent intersubband transitions with electron transfer to the valence band

We suggest a new approach to creation of a quantum cascade laser with coherent electron transport on intersubband transitions with electron transfer from a quantum well to the valence band.     

A laser operating on the interband transitions in quantum wells with coherent electron transport

A new approach to the creation of a quantum cascade laser is suggested, which employs interband transitions in quantum wells with a coherent electron transport. The coherent electron transport is studied based on the simplest two-band Kane model, in which the interaction between states of the conduction and valence bands is described taking into account only states in the light hole subband of the valence band.     

Specific features of ballistic electron transport in double-level open systems in a large-amplitude high-frequency electric field

Analytical expressions for electron coefficients of transmission and reflection in a large-amplitude high-frequency electric field, with a frequency close to resonance, for asymmetric double- and triple-barrier resonant tunneling structures with thin high barriers forming open two-level systems are reported. The dependences of the width and shape of resonant levels on the high-frequency field amplitude are investigated. It is shown that, in these structures there are almost always conditions at which nonresonant scattering channels near the quantum levels can become absolutely transparent.     

Features of the Upsurge in Drift Velocity of Electrons in DA-pHEMT

A simple phenomenological model for estimating the upsurge in drift velocity of electrons in transistor heterostructures is proposed. This model is based on a self-consistent solution of Schrödinger and Poisson equations and the hydrodynamic system of equations of energy and momentum conservation. It is demonstrated that the conditions in the layer channel of DA-pHEMT structures with additional potential barriers, which are produced by donor–acceptor doping and enhance the localization of hot electrons, are even more conducive to reducing the time of flight of electrons under the gate than those established in heterostructures with deeper quantum wells produced by increasing the conduction band offset at the heterojunction interface.