Donor compensation in the depletion layer of CdF<Subscript>2</Subscript> crystals with a Schottky barrier

RF response and capacitance-voltage and current-voltage characteristics of n-type semiconductor crystals CdF₂:In, CdF₂:Ga, and CdF₂:Y with a Schottky barrier were studied. Specific features of these characteristics are accounted for based on the assumption that the charge transport from the metal to the depletion layer is due to the formation of Cd⁰ excitations in the contact layer, which occurs because of the supply of electron pairs from the metal (Au). These excitations compensate donors in the space-charge region of ∼1 μm thickness, adjacent to the contact. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 1, 2004, pp. 72–78. Original Russian Text Copyright ? 2004 by Shcheulin, Kupchikov, Angervaks, Ryskin.     

Physical foundations of metastable impurity center reconstruction in semiconductors

The manner in which the electronic structure and chemical bonding of a perfect crystal change upon doping and vary with the charge state of a defect is analyzed. The obtained results serve as a basis for proposing a general pattern of reconstruction of metastable impurity centers, of both donor and acceptor types, in various semiconductors: “classical” III-V and II-VI semiconductor compounds, mostly ionic CdF₂ crystal, and narrow-gap IV-VI compounds. Reasons are revealed for center reconstruction; general tendencies of the process and the specificity of their manifestation in some classes of crystals and types of impurity centers are established.     

Structure of the metastable centers of the group-III atoms in IV-VI crystals

A comparative analysis of reconstruction of the metastable impurity centers in tetrahedral (III-V and II-VI) and cubic (IV-VI) crystals is performed for the case where the charge of the center changes. The influence of the chemical structure of the crystal matrix on the nature of defect reconstruction is studied on the basis of cluster approximation. For the IV-VI:III centers, the fundamental significance of the three-center crystal bonds and the specific role of the antibonding (weakly bonding) 3a _1g state, which has the properties of both the impurity and perturbed crystal states simultaneously, are emphasized.     

Quantum spin Hall effect in nanostructures based on cadmium fluoride

Tunneling current-voltage (I-V) characteristics and temperature dependences of static magnetic susceptibility and specific heat of the CdB _x F_{2 − x} /p-CdF₂-QW/CdB _x F_{2 − x} planar sandwich structures formed on the surface of an n-CdF₂ crystal have been studied in order to identify superconducting properties of the CdB _x F_{2 − x} δ barriers confining the p-type CdF₂ ultranarrow quantum well. Comparative analysis of current-voltage (I-V) characteristics and conductance-voltage dependences (measured at the temperatures, respectively, below and above the critical temperature of superconducting transition) indicates that there is an interrelation between quantization of supercurrent and dimensional quantization of holes in the p-CdF₂ ultranarrow quantum well. It is noteworthy that detection of the Josephson peak of current in each hole subband is accompanied by the appearance of the spectrum of the multiple Andreev reflection (MAR). A high degree of spin polarization of holes in the edge channels along the perimeter of the p-CdF₂ ultranarrow quantum well appears as a result of MAR and makes it possible to identify the quantum spin Hall effect I-V characteristics; this effect becomes pronounced in the case of detection of nonzero conductance at the zero voltage applied to the vertical gate in the Hall geometry of the experiment. Within the energy range of superconducting gap, the I-V characteristics of the spin transistor and quantum spin Hall effect are controlled by the MAR spectrum appearing as the voltage applied to the vertical gate is varied. Beyond the range of the superconducting gap, the observed I-V characteristic of the quantum spin Hall effect is represented by a quantum conductance staircase with a height of the steps equal to e ₂/h; this height is interrelated with the Aharonov-Casher oscillations of longitudinal and depends on the voltage applied to the vertical gate.     

A change in the character of modulation instability in the vicinity of a critical frequency

Modulation instability of a quasi-harmonic wave propagating in a cubic nonlinear medium is analyzed in the vicinity of a critical frequency. As the wave amplitude increases, the character of instability changes from convective to absolute. This is explained by expansion of the range of unstable perturbations to the region of counterpropagating waves possessing a negative group velocity. The proposed theory is confirmed by the results of computer simulations.     

Nucleation of submicroscopic cracks in stressed solids

The study of very fine cracks formed in stressed solids was carried out using different types of solids, such as polymers, ionic crystals, and metals. Such methods as small-angle X-ray scattering, light scattering, and electron microscopy were used. Size, form, orientation and concentration of submicrocracks have been estimated. A study of the connection between submicrocracks and the structure of solids and of the kinetics of crack accumulation has been started. The development of the fracture process in solids on the basis of nucleation and successive coalescence of submicrocracks is discussed.     

Self-similar modes of amplification and compression of electromagnetic pulses in their interaction with electron flows

Self-similar solutions describing the amplification and compression of ultrashort electromagnetic pulses in their interaction with quasi-stationary electron flows are constructed on the basis of analogy with the coherent amplification of short optical pulses by inverted laser media. It is shown that the effects of amplification of pulses with their simultaneous compression are universal and must be observed for different mechanisms of induced radiation, including the Cherenkov and cyclotron mechanisms.     

Multistability and Complex Oscillatory Regimes in a Microwave Generator with Delayed Reflection from the Load

Technical Physics Letters - Peculiarities of the formation of multistability in a microwave generator with delayed reflection from the load have been studied. Characteristic scenarios of the...     

A Subterahertz Multiplier Based on a Traveling Wave Tube with a Sheet Electron Beam

Technical Physics Letters - In this paper, we present the results of the study of a multiplier based on a traveling wave tube with a sheet electron beam and slow-wave double-grating structures....     

Hyperbolic chaos generator based on coupled drift klystrons

A scheme of the generator of chaotic oscillations in the microwave band is proposed that is based on two drift klystrons coupled in a ring circuit. The first klystron doubles the frequency of the input signal and the second klystron mixes the second harmonic signal and the reference signal representing a periodic sequence of radio pulses filled with the third harmonic, followed by separation of the differential first harmonic signal. As a result, the transformation of the signal phase during the period of pulse repetition is described by a stretching map of the circle (Bernoulli map) and demonstrates chaotic behavior. Results of numerical calculations are presented that confirm the implementation of the proposed mechanism to ensure the generation of robust, structurally stable chaos in the system under consideration.