Aurimas Čerškus

Investigation of the Soft Mode of SbSBrxI1−x Crystals

The behaviour of the potential energy V(z) of the B 1u (A 1 ) symmetry soft mode vibrations of atoms in the direction of the z(c) axis of SbSBrxI 1−x crystals (x = 0, 0.2, 0.75, 1) upon the normal coordinate (relative displacements of atoms) was investigated in the region of the phase transition temperatures. For this B 1u (A 1 ) symmetry soft mode, the potential energy barrier height ΔV, the potential energy V(z), its harmonic and anharmonic terms were studied as functions of the mixture composition x and temperature T. The dependence of the soft mode frequency ωS upon the composition x and temperature has been established assuming the double-well shape of the potential energy (per atom) V(z). By employing A 1u (A 2 ) symmetry coordinates, the force constants C of interaction between molecular chains have been obtained. The type of the phase transition has been established by calculating the Rhodes–Wohlfarth factor R. To calculate this factor, the ratio /ΔV depending upon the mixture composition was employed. It has been found that for mixed crystals with x < 0.8, the phase transition is intermediate between order–disorder and displacive types, and for crystals with x > 0.8, the phase transition is of displacive type.

Radiative recombination spectra of p-type δ-doped GaAs∕AlAs multiple quantum wells near the Mott transition

Photoluminescence (PL) spectra of beryllium δ-doped GaAs∕AlAs multiple quantum wells are studied over a range of doping concentrations. Possible mechanisms for carrier recombination, both above and below the Mott metal-insulator transition, are discussed. In 15nm width Be δ-doped GaAs∕AlAs quantum wells, it is found that the Mott transition can be observed if the acceptor concentration (NBe)⩾3×1012cm−2. At doping concentrations near the Mott transition band the PL spectra are dominated by excitons-bound-to-acceptor impurity recombinations. Above the Mott transition, the radiative recombination of free electrons with a two-dimensional hole gas is found to be the dominant recombination mechanism.

Splitting of the XPS in Ferroelectric SbSBr Crystals

This paper presents the theoretical investigation of energy of core levels (CL) of the ferroelectric SbSBr single crystals in paraelectric and ferroelectric phases. Since the best approximation for the CL levels is a calculation by the Hartree-Fock method, the molecular model of SbSI crystal was used for calculations. It was found that CL of this semiconductor ferroelectric are sensitive to the small lattice distortion at phase transition, and on ion charges. The experimental splitting of CL obtained by XPS was compared with theoretically calculated one by two different methods. The cluster model calculations showed that the splitting of the CL in SbSBr might be caused by photoelectron emission from the atoms, which have different ion charges at the surface. Communicated by Dr. George W. Taylor

Theoretical investigation of the electronic structure of a ferroelectric SbSI cluster at a phase transition

This paper presents the theoretical investigation of energy levels of valence bands (VB) and core levels (CL) of the ferroelectric SbSl single crystals in antiferroelectric and ferroelectric phases. Since the best approximation for the deep VB levels is a calculation by the Hartree-Fock method, the molecular model of a SbSI crystal was used for calculations. This model of the crystal was also used for calculations of the total density of states. It was found that the VB and CL of this ferroelectric semiconductor are sensitive to the small lattice distortion at the phase transition, and that an average of the total density of states, when all atoms participate in oscillations of all normal modes, are more similar to the experimental X-ray photoelectron spectra (XPS). The experimental splitting of CL obtained by XPS was compared with the theoretically calculated one by two different methods. The cluster model calculations showed that the splitting of the CL in SbSI might be caused by photoelectron emission from the atoms, which have different valence state, at the surface.

Enhanced exciton photoluminescence in the selectively Si-doped GaAs/AlxGa1−xAs heterostructures

Experimental results examining the photoluminescence spectra of selectively Si-doped GaAs/AlxGa1−xAs heterostructures is presented. Possible mechanisms of carrier recombination are discussed with a special emphasis on the peculiarities of excitonic photoluminescence. Strong intensity lines in photoluminescence spectra are associated with the formation and enhancement of free exciton and exciton-polariton emission in the flat band region of an active i-GaAs layer. The excitonic PL intensity is sensitive to the excitation intensity indicating high nonlinear behavior of spectral-integrated photoluminescence intensity and exciton line narrowing. These observed phenomena may be related to the collective interaction of excitons and the interaction of excitons with emitted electromagnetic waves. The gain of the amplification of the excitonic photoluminescence intensity in the heterostructure was found to be more than 1000 times larger than the intensity of i-GaAs active layer. The quality factor of the exciton l...

Impurity-related photoluminescence line shape asymmetry in GaAs/AlAs multiple quantum wells: Fractional-dimensional space approach

The optical transitions in 20 nm wide silicon and beryllium δ-doped GaAs/AlAs multiple quantum wells with various doping levels were investigated at different excitation intensities. A fractional dimensionality model was used to describe the free hole-donor and free electron-acceptor transitions in the quantum wells. The measured photoluminescence spectra from samples of different doping level related to donor-impurity or acceptor-impurity induced effects in the photoluminescence lineshape, were compared within the framework of these model calculations. Both experimentally and theoretically it was shown that acceptor and donor related optical transitions and photoluminescence line shapes were related to the difference in the effective masses of holes and electrons. This effect also leads to a difference in the photoluminescence spectra in which the luminescence band for the donor related spectrum is narrower in comparison to the acceptor related spectrum.

Theoretical investigation of the electronic structure of ferroelectric SbSBr molecular cluster

This paper presents the theoretical calculation of energy levels of the valence bands and bond orders of the SbSBr single crystals using the molecular cluster model consisting twenty SbSBr molecules. The theoretical calculation revealed that the ferroelectric phase transition changes the bond orders and shift valence bands. Results of theoretical calculations of averaged total density of states of SbSBr molecular cluster are compared with the experimentally results of X-ray photoelectron spectroscopy (XPS) of SbSI crystals, because that SbSI and SbSBr crystals have isomorphic electronic structure.

Light emission lifetimes in p-type δ-doped GaAs/AlAs multiple quantum wells near the Mott transition

The time resolved photoluminescence of beryllium δ-doped GaAs/AlAs multiple quantum wells have been studied over a range of doping concentrations, in order to investigate possible mechanisms for the carrier radiative recombination, both above and below the Mott metal-insulator transition. It was found that at doping concentrations near the Mott transition (NBe ∼ 3 × 1012 cm−2), the radiative recombination of excitons-bound-to-acceptor impurities as well as free electrons with acceptor impurities, dominated in the Be δ-doped GaAs/AlAs MQWs (LW = 15 nm) that were used in this study. Above the Mott transition, the major contribution was from radiative recombination of free electrons with a two-dimensional hole gas. The radiative lifetime would therefore exhibit different behavior with doping. In lightly doped GaAs/AlAs MQWs, this changed from 0.3–1 ns at 3.6 K to 8 ns at 300 K, whilst in quantum wells above the Mott transition, it changed from ∼0.36 ns at 3.6 K to ∼1 ns at 300 K, and was also weakly dependen...

Investigation of Vibrational Spectra of SbOxS1–xI Crystals in the Phase Transition Region

SbOxS1 - xI (x=0–0.5) crystals have been grown from the vapour phase. Reflection spectra of the SbOxS1 - xI (x=0.2) crystals were studied by a modernized Fourier spectrometer. Using an improved Karmers–Kronig technique with two confining spectral limits, the spectra of optical parameters and optical functions were calculated. The vibrational frequenciesωLandωThave been evaluated. The vibrational frequencies of SbSI and SbOxS1 - xI (x=0.5) chains in different phases have been calculated in the harmonic approximation. The theoretical results are compared with experimental data. Taking into account the anharmonicity of the electronic structure and lattice, caused mainly by the phonon–phonon interaction, the phase transition temperature TCas a function of the mixture compositionxhas been evaluated. The dependence of the total potential energy function of the soft B1umode upon temperature determines variation of its frequencyωs2in the phase transition region.

The nature of soft mode and ferroelectric phase transition in SbOxS1-xI and SbSexS1-xI mixed crystals

The ferroelectric phase transition temperature has different dependence upon the mixture composition x in mixed SbO x S1−x I and SbSe x S1−x I crystals. From dependence of potential energy V(z) upon normal coordinates of the B 1u soft mode at a Sb site of SbO x S1−x I and SbSe x S1−x I crystals we proved that main reason for the growth of T C in SbO x S1−x I crystals is that atomic formfactor VI decrease with increasing x and decrease of T C in SbSe x S1−x I crystals is caused by the increase of VI with increasing x.