Two-dimensional (2D) materials such as GaSe, InSe, and WSe2 in form of bulk crystal and few-layer sheets have been prepared by mechanical exfoliation method. Atomic force microscopy (AFM) analysis was used for the study of surface morphology and estimation of thicknesses of the exfoliated thin semiconductor layers. Optical absorption and magneto-optical Faraday rotation spectra of 2D materials at different temperatures have been studied. In spectral region near long-wavelength absorption edge for all the studied layered crystals at low temperatures the exciton series have been observed. It was shown that in GaSe and InSe crystals these series consist of three lines, whereas in WSe2 two exciton lines were revealed. The observed exciton series were interpreted in framework of 3D exciton model. In the absorption spectra of GaSe and InSe crystals at higher photon energies than band gap Eg additional exciton structure also was observed. In case of thin exfoliated WSe2 flakes with thickness less than 80 nm shift of ground exciton band to shorter wavelengths has been revealed, which is associated with quantum size effect. Faraday rotation spectra of the 2D crystals have been confirme INTRODd dominant role of excitons near absorption edge.
The phonon and electron subsystems were studied in quaternary solid solutions of ZnxCdyHg1-x-yTe by means of Raman scattering and Magnetophonon Resonance. The Raman spectra of several compositions confirm the three-mode behavior of phonon spectra. The cluster mode has also been observed. Four kinds of LO-phonons participate in the electron-phonon interaction. Four types of one-phonon Magnetophonon Resonances and two types of Magnetophonon Resonances on the difference of phonon frequencies have been observed.
The modulation of composition and electro-physical properties of Hg0.8Cd0.2Te by laser annealing without melting is investigated. Concentration of Hg, Cd, Te as a function of the depth from the irradiated surface, for samples with the same composition, but with different types of electrical conductivity as well as with different initial concentrations of intrinsic defects (vacation and interstitial mercury atoms) were measured before and after laser annealing. The possible explain the unexpected oscillations regularly repeated in each sample, basing upon the thermodynamic theory of decomposition of solid solution having an unbalancing thermodynamic equilibrium and basing on change the electrical properties after treatment is presented.
Magnetophonon Resonance in parallel transport of three types of Multiple Quantum Wells was studied. They consisted of ten QW of GaAs and ten AlGaAs barriers, and were obtained by the Metal Organic Vapor Deposition an semi-insulating GaAs. The MPR research were performed in pulsed magnetic fields up to 30 T. The transverse magneto resistance was measured between 77K and 340K and the MPR oscillations extracted by subtracting a voltage linear in magnetic field. The oscillating part of magneto resistance (Delta) (rho) xx was recorded. A fine structure of MPR peaks was observed. This effect could be attributed to two phenomena: contribution of barrier phonons and influence of thermostresses.
Transport phenomena such as conductivity, Hall effect and Magnetophonon Resonance (MPR) are studied in quaternary solid solutions ZnxCdyHg1-x-yTe. The three-mode phonon spectrum as well as the presence of ZnTe- and HgTe-clusters deducible from Raman spectra have been confirmed by MPR-data. The comparison of the dependence of Eg(T) obtained by temperature shift of the MPR peaks with one calculated by means of empirical formulae for the energy gap dependences on the component concentration and temperature is performed. The discrepancies between experimental and calculated data are probably caused by the difference in the concentration of components within those regions where the electron transport occurs and the averaged compound concentration measured by X-ray micro analysis (the space resolution for those measurements exceeds 1 micrometers ). Unbounded HgTe- and ZnTe-clusters are formed due to the escape of Hg- and Zn-atoms out of this region. It can be suggested that the electron transport occurs in a part of the crystal where a stochastically homogeneous distribution of cation atoms takes place.
Two geometries are essential in the Magnetophonon Resonance (MPR) experiments in the layered semiconductor structures. First, the magnetic field and current are perpendicular to layers--MPR in vertical transport. Second, magnetic field is perpendicular to layers, whereas current occurs in layers-- MPR in parallel transport in superlattices and Multiple- Quantum-Well-Structures (MQWS). This anisotropy is connected mainly with anisotropy of the electron effective mass. Simultaneously, other important deferences are placed in the electron transport in superlattices and MQWS for these main directions. The peculiarities caused by motion in minibands (vertical transport) and influence of the thermal stress (parallel transport) as well as contribution of the confident phonons was discussed. The present paper discusses the MPR works in the superlattices and MQWS which were conducted during the last decade.
A laser method of segregation of impurities or interstitial mercury atoms (IMA) in solid phase of Hg0.8Cd0.2Te (MCT) is presented. A theoretical model for this process also has been proposed. Equation for diffusion of impurities or IMA was completed by a term describing the influence of phonon flux on diffusion processes. Computer simulations of the laser annealing process point out possibilities of obtaining a sharp maximum of Hg concentration for appropriately chosen parameters of laser pulse. This was experimentally verified with MCT specimens annealed by a YAG:Nd3+ laser.
A high photo-sensitive area has been created in solid solution of Hg1-xCdxTe (x approximately equals 0.2) (MCT) without melting its surface. That idea of formation of heterojunction, which was indicated by computer modelling of mass transportation processes under laser treatment of MCT, is realized experimentally. MCT samples were irradiated with a Nd:YAG laser beam having an energy density 0.7 J/cm2. The presence of a heterojunction not far below the upper surface has been verified by X-ray microanalysis.
It was demonstrated by the simulation of laser beam annealing process that one can make a great local change in concentration of impurities or intrinsic defects in a Hg1-xCdxTe crystals by long (moreover only by such) pulses of laser radiation. The diffusion processes of defects in semiconductors in the presence of high power laser beam can be described using Schottky's thermodiffusion theory. The results of calculations of time- spatial distribution of the concentration of interstitial mercury for pulse lengths 8 ns, 100 ns, and 250 microsecond(s) are presented. These computer simulated results are compared with distribution of Hg concentration in specimens subjected to laser annealing under the same condition.
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