We report on MBE growth and study of optical and structural properties of (ZnSe/MgS)/ZnCdSe distributed Bragg reflectors with λ=520 nm and R_{max}=97%. The samples were grown pseudomorphically on GaAs substrate using ZnS as a sulphur source. Scanning electron microscopy, X-ray diffraction, and optical measurements showed good optical and structural characteristics of the Bragg reflectors.
Three kinds of precursor thin films with stacking orders of Cu/Sn/ZnS, Cu/Sn/ZnS/Cu/Sn/ZnS and Cu/Sn/ZnS/Cu/Sn/ZnS/Cu/Sn/ZnS were prepared by magnetron sputtering and annealed with sulfur powder. The microstructure, morphology and optical properties of thin films were investigated by X-ray diffraction, the Raman scattering, scanning electron microscopy and UV-visible spectrophotometer. The increase of cycle number decreases the sulfurizing temperature of the formation of Cu₂ZnSnS₄ phase. Chemical composition can be controlled by cycle sputtering deposition. After sulfurizing at 500°C, the particle size and the band gap increase with increasing cycle number.
The liquid nitrogen temperature reflectivity spectra of the tetrahedral diluted magnetic semiconductors (DMS's) Zn_{1-y}Fe_{y}Se and Zn_{1-x}Mn_{x}Se crystallizing in the zinc-blende structure were investigated. The reflectivity measurements for different concentration of Fe (y = 0.01, 0.05, and 0.10) and Mn (x = 0.10 and 0.30) ions were taken out in a wide energy range between 4 and 30 eV using synchrotron radiation from ADONE storage ring in Frascati. The average resolution ΔE/E used for these measurements was better than 1 × 10^{-3} over the entire spectral range. The single crystals were cleaved, before being mounted inside the reflectometer, from the samples grown by the Bridgman method. The comparison between the reflectivity spectra of ternary systems and host crystal ZnSe is made. The changes of the structures of reflectivity spectra of host crystal ZnSe caused by the influence of Fe and Mn ions are discussed.
Methodology of impedance measurements and ferroelectric hysteresis loops observed in temperature range 292-475 K for antimony sulfoiodide (SbSI) grown from vapour phase are discussed. Temperature dependences of spontaneous polarization and coercive field of SbSI crystals are presented.
The coherent population trapping effect in double tunnel-coupled quantum wells is analyzed. One of two quantum wells interacts with the two-frequency laser radiation and low-frequency field, thus forming a closed contour of excitation. It is possible to control the excited level population in such a scheme of excitation by changing relative phases of the fields in the coherent population trapping state. The quantum well is bound to the other quantum well by tunnel coupling of the excited levels, therefore the population and optical properties of the other quantum well depend on the coherent population trapping state in the first quantum well and can be controlled.
Complementary X-ray photoelectron spectroscopy and optical reflectivity studies of crystalline Si(111) surfaces prepared by two different wet chemical etching processes were performed. These included aqueous HF solution etch or diluted CP-4 bath. Optical reflectivity spectra of Si surfaces, measured in the range 3.7-11 eV, were found strongly dependent on the applied etching process. Analysis of the core level X-ray photoelectron spectroscopy data has shown similarity of the surface structure, irrespectively of the etching procedure. Finally, comparison of optical reflectivity and valence band X-ray photoelectron spectra revealed a qualitative correlation between them indicating dominant influence of the bulk (here, the subsurface region containing polishing-induced defects) in the case studied. This paper is the first one which presents correlations between optical reflectivity and X-ray photoelectron spectroscopy data for Si and thus illustrates a bulk sensitivity of both techniques considered.
We present a theoretical study of excitons in GaN/Al_{x}Ga_{1 - x}N wurtzite (0001) quantum wells subjected to hydrostatic pressure. Our results show that the combined effect of pressure induced changes in band structure and piezoelectric field leads to reduction of the exciton binding energy. This subtle effect is described quite accurately by our multiband model of excitons in quantum wells.
CuCl nanocrystals were elaborated in a NaCl ionic matrix by doping the latter with copper powder during growth. Optical absorption measurements revealed nanocrystals with a mean size of order 32 Å. This is consolidated by the Raman scattering measurements which showed nanodomains of similar size. X-ray diffraction measurements indicate a good crystallinity of the matrix and confirm the presence of CuCl nanocrystals within our samples. The annealing effect at 300˚C showed an increase in CuCl nanocrystal size with annealing time and demonstrated clearly the existence of a compound containing copper within our samples.
By using the complete diagonalization (of energy matrix) method based on the cluster approach (where the admixture between the d orbitals of d^{n} ion and p orbitals of ligands due to covalence effect is considered), the optical and electron paramagnetic resonance data (three optical band positions and four EPR parameters g_{∥}, g_{⊥}, A_{∥}, A_{⊥}) are calculated for the tetragonal Cu^{2+} center in Cu^{2+}-doped poly vinyl alcohol (PVA) passivated ZnSe nanocrystals with the defect model of Cu^{2+} in the interstitial octahedral site in ZnSe. The calculated results are near the experimental values. The tetragonal elongation of the Cu^{2+} octahedral cluster in ZnSe nanocrystals is obtained. The results are discussed.
Semiconducting ferroelectric antimony sulfoiodide (SbSI) photonic crystals were fabricated. The SiO_{2} nanospheres were synthesized and gravity sedimented to obtain opal matrices. These opals were infiltrated with melted SbSI and etched in HF acid to produce inverted SbSI opals.
This paper reviews the background of modulation spectroscopy, particularly electromodulation, presents some recent room temperature results having both fundamental and technological significance, including two-dimensional electron gas effects in modulation-doped, pseudomorphic GaAlAs/InGaAs/GaAs single quantum wells (HΕΜΤ structures), quantum well laser structures and process-induced damage in quantum dot arrays fabricated by reactive ion etching.
We have investigated the properties of structures incorporating graded index materials with parabolic permittivity profile. Surface-plasmon-polaritons at the interface of graded index material and semiconductor are studied by means of numerical simulations. We analyze the dependence of the dispersion characteristics on the graded index material profile as well as on the semiconductor concentration via the finite-difference time-domain simulations. Effects of the structure on dielectric and magnetic properties are taken into account by introducing the Drude model in the semiconductor dispersion.
The temperature dependence of the energy gap of MBE grown Cd_{1-x}Mn_{x}Te (0.6 < x ≤ 1.0) was measured for 2 K ≤ T ≤ 200 K and B ≤ 5 T. The results are interpreted in the frames of the model predicting that the exchange contribution to the band edge shift is proportional to the product of the magnetic susceptibility and the temperature.
We present a study of time-dependent transmission spectra of a modulation-doped Cd_{1-x}Mn_xTe/Cd_{1-y-z}Zn_yMg_zTe quantum well with variable hole gas concentration. We study the influence of pump pulses on excitonic absorption in subpicosecond time scale. A spectrally broad probe pulse of duration of 40 femtoseconds was used to record the absorption spectra at controlled delay. Studies of temporal evolution of exciton energies revealed coherence decay of linearly polarized excitons and thermalization of non-equilibrium exciton states. We found that a characteristic timescale for thermalization of non-equilibrium populations of photocreated excitons is between 0.8 and 3.6 ps. The timescale of this process depends on the hole concentration in quantum well: for higher hole concentration the decay is faster. Long-lived photo-induced magnetization accompanied by heating of the magnetic system was also observed.
X-ray diffraction pattern of 4-aminoantipyrine was studied and it is a single phase with a polycrystalline structure. 4-aminoantipyrine has hexagonal structure with space group P6/mcc. The electrical properties of 4-aminoantipyrine were studied in the temperature range (303-373 K) below the melting point of the studied compound and in the frequency range (100 Hz-100 kHz). The obtained results of dc conductivity showed a positive temperature coefficient at the lower temperatures and a negative temperature coefficient at the higher temperatures. The ac conductivity obeys the power law. Ac conductivity can be reasonably interpreted in terms of overlapping-large polaron tunneling model and the correlated barrier hopping model. 4-aminoantipyrine is a good candidate for electronic device due to its electrical conductivity and capacitance.
Optical spectra analysis provides a wealth of information on physical properties of various semiconductor materials. Fractional derivative spectrum technique is especially interesting when the limitations of the standard treatment occur. In this paper we present the fractional derivative spectrum method for analysis of the optical spectra for both Si and GaAs. The significant changes in critical point parameters in each treated Si and GaAs samples in comparison to that before treatment have been observed. Our investigation illustrates that fractional derivative spectrum is a very good technique to extract basic information on relevant physical quantities from the observed optical spectra, and it has the advantages of flexibility, directness, and sensitivity, which give possibility to obtain the Van Hove singularities (critical point parameters) efficiently with one consent.
We study theoretically the influence of the anisotropic biaxial strain originating from the lattice mismatch between the m-plane GaN/AlGaN quantum wells structure and the substrate on the optical anisotropy of such systems. It is demonstrated that the oscillator strengths for optical transitions with polarization of light parallel and perpendicular to the crystal axis c strongly depend on strain to such an extent that, by increasing the concentration of Al in the substrate from x = 0 to x = 0.5 one can change the polarization of the emitted light with respect to the c-axis by 90 degrees.
The optical properties of Tl_4InGa_3S_8 layered single crystals have been studied by means of transmission and reflection measurements in the wavelength region between 400 and 1100 nm. The analysis of the room temperature absorption data revealed the presence of both optical indirect and direct transitions with band gap energies of 2.40 and 2.61 eV, respectively. Transmission measurements carried out in the temperature range of 10-300 K revealed the rate of change of the indirect band gap with temperature asγ=-6.0×10^{-4} eV/K. The absolute zero value of the band gap energy was obtained as E_{gi}(0)= 2.52 eV. The dispersion of the refractive index is discussed in terms of the Wemple-DiDomenico single-effective-oscillator model. The refractive index dispersion parameters: oscillator energy, dispersion energy, oscillator strength, and zero-frequency refractive index were found to be 5.07 eV, 26.67 eV, 8.82×10^{13} m^{-2}, and 2.50, respectively.
We show theoretically that for narrow GaN/AlGaN quantum wells, lattice matched to GaN substrate/buffer and grown along the (0001) crystallographic direction the topmost valence subband symmetry depends critically on such parameters as quantum well thickness and barrier composition. This effect determines polarization of the emitted light. It is noted that the symmetry of the topmost valence band level is sensitive to the values of the D_3 and D_4 deformation potentials and can be employed in verification of existing literature values of these parameters.
A review on the experimental determination of the dielectric function for hexagonal nitride semiconductors is presented. The peculiarities of nitride samples such as surface roughness and extended interface layers alter in comparison to an ideal film spectroscopic ellipsometry or reflectance spectra in a characteristic manner. It requires the application of multi-layer models for data analysis in order to determine reliable dielectric functions. Results of such an analysis for GaN covering a broad spectral range are given. Below the band gap, both ordinary and extraordinary components of the dielectric function tensor are determined for GaN as well as for AlN. The dielectric functions of MBE-grown InN characterised by a band gap of around 0.75 eV and a sputtered film exhibiting an absorption edge of around 1.9 eV are compared with results of first-principles calculations. Good agreement between theory and experiment is only found for the MBE-grown material providing further evidence that InN is a "narrow" band gap semiconductor. Finally, photocurrent measurements of a GaN Schottky-diode reveal the influence of electric fields on the shape of the excitonic absorption edge. The interpretation is supported by results of dielectric function calculations.
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