We report the growth and characterization of AlGaN/GaN multiple quantum well structures designed to have intersubband transitions in the mid-infrared region of the spectrum. The samples were nominally undoped but were found to contain a high electron population in the wells induced by the local polarization fields. The sample was characterized by the use of the Raman spectroscopy and photocurrent spectroscopy. The Raman spectroscopy shows electronic Raman scattering from intersubband transitions in the AlGaN/GaN quantum wells. The e_1-e_2 and e_1-e_3 transitions of the confined 2d electron population in the wells can clearly be observed. A sample designed to absorb at 4μm was fabricated into mesa structures and the vertical photocurrent measured under normal incidence illumination from the free-electron laser FELIX. A wavelength and bias dependent photocurrent was observed in the mid-IR region of spectrum. The peak responsivity was of the order of 50μA/W at 4 K, the photocurrent still being measurable at room temperature.
Intraband absorption in n- and p-doped structures with InAs/GaAs quantum dots covered by InGaAs layers is studied both experimentally and theoretically. The absorption cross-section for p-type quantum dots was found to be significantly smaller than that for n-type quantum dots. Interband absorption bleaching under strong interband excitation is found and investigated in undoped quantum dot structures. Structures with artificial molecules were grown. Photoluminescence spectra and transmission electron microscopy images proves the presence of coupled symmetrical quantum dots.
Magnetooptical phenomena in the zero-gap semimagnetic semiconductor Hg(Fe)Se are studied by various techniques in pulsed magnetic fields up to 150 T. Microscopical parameters are estimated in combination with results obtained from transport and magnetization measurements.
Resonant Raman scattering was measured for (Zn,Fe)Se and (Zn,Cr)Se with the magnetic ion mole fraction below 0.02. LO phonons and intra-Fe^{++} transitions were observed. Polarization measurements versus magnetic field allowed to obtain polarization lifetimes (polariton flight times) in the sub-picosecond range.
The low-temperature far-infrared reflectivity spectra of HgSe and HgSeS are measured and analysed. The results suggest the one-mode behavior of phonon excitations in HgSeS mixed crystals.
The influence of the depolarization effect on the spectral characteristics of intraband absorption on bound-free transitions in semiconductor quantum wells is analyzed using the local and nonlocal theories. The results obtained show that the local theory only qualitatively describes the modification of the bound-free absorption spectrum by the depolarization effect.
A ternary semiconductor Ga_{1-x}Al_{x}P has been synthesized for the first time by hot implantation of aluminum in GaP. Two mixed crystals of various compositions have been synthesized when implanting by two different fluencies of aluminum ions. The identification of the above mentioned mixed semiconductors in the amorphous as well as in the crystalline phases has been carried out by the laser Raman spectroscopy. The synthesis of the ternary compositions has been carried out at different depths from the substrate surface by implantation of aluminum ions of various energies. Ga_{1-x}Al_{x}P synthesized by the ion implantation shows the behavior of two-mode mixed semiconductors. The synthesized compounds are defective and the Raman spectra prove the fact. The share of disordered structure of the composition synthesized with high fluencies of aluminum ion implantation, 2.5 × 10^{17} ion/cm^2, is especially big.
The Raman scattering studies of multi-layer graphene obtained by high temperature annealing of carbon terminated face of 4H-SiC(000-1) substrates are presented. Intensity ratio of the D and G bands was used to estimate the average size of the graphene flakes constituting carbon structures. The obtained estimates were compared with flake sizes from atomic force microscopy data. We found that even the smallest structures observed by atomic force microscopy images are much bigger than the estimates obtained from the Raman scattering data. The obtained results are discussed in terms of different average flake sizes inside and on the surface of the multi-layer graphene structure, as well as different type of defects which would be present in the investigated structures apart from edge defects.
The influence of possible presence of Co^{2+} ion pairs in a bulk Zn_{1-x}Co_{x}O mixed crystal on the low-frequency part of the Raman spectrum is discussed. Two effects can be taken into account in the theoretical considerations when analyzing the energy level scheme corresponding to Co ions. The first is a local lattice deformation in the vicinity of Co^{2+} ion due to a presence of the second ion, smaller than the host ZnO lattice cation. Such deformation creates a trigonal field, which can only slightly modify the energy levels of Co^{2+} ion. The second effect, which results from an antiferromagnetic superexchange interaction between two Co^{2+} ions is responsible for a new set of energy levels. The Raman data taken at low temperature on the sample corresponding to the composition x = 0.016 demonstrated the presence of two structures at about 6 cm^{-1} and 13 cm^{-1}. These structures may be interpreted as electronic transitions between the ground state and the first excited state of a single Co^{2+} ion in the substitution site of ZnO lattice and as a similar transition for Co^{2+} ion pair, respectively.
It was shown employing the functional density approach that interference effects resulting from finite size of the cap layer can modify substantially the absorption line shape connected with bound-to-continuum intraband transitions in quantum wells.
We report FIR laser spectroscopy study of Zn_{1-x}Fe_{x}Se (x < 0.06) Semimagnetic Semiconductor at the temperature range of 2-26 K and magnetic fields up to 18T.
Inelastic scattering of excitons on Fe^{++} ions in Cd_{1-x}Fe_{x}Se was studied by resonant Raman scattering. Polarization measurements were done using a modulation technique, allowing for the first time to determine the full polarization state of the detected light. The obtained results were compared to a simple calculation in an incoherent model of scattering on Fe^{++} ions.
The recent successful growth of single, bulk Pb_{1-x}Cd_{x}Te crystals by self-selecting vapor transport method at the Institute of Physics of the Polish Academy of Sciences in Warsaw opened new opportunities to study the physical properties of this interesting material in detail. In this work we report the preliminary results of X-ray powder diffraction studies performed on a set of Pb_{1-x}Cd_{x}Te solid solutions (where x ≤ 0.056) at high temperatures (295 K ≤ T ≤ 1100 K) and analyzed with the Rietveld refinement. Our results demonstrate the necessity of some correction of the relevant phase diagram and of the solubility limit, known from the literature.
Low-temperature Raman spectra of a bulk mixed crystal Zn_{1-x}Co_xO (x = 0.016) are shown and analyzed. Besides the common modes of the ZnO host lattice, electronic transitions related to the presence of Co^{2+} ions at the Zn sites are observed. In spite of the low Co concentration the presence of collective excitations corresponding to Co and CoO precipitates is also revealed.
The first reflectivity spectra of Hg_{1-x-y}Cd_{x}Mn_{y}Te with 0.03 < x < 0.1 and 0 < y < 0.05 were measured in the spectral region 700-30 cm^{-1} at 300 K and 90 K. The quaternary alloys measured show three mode behaviour. The experimental results are interpreted by using a classical dynamic dielectric function model.
We performed implantation experiments, applying both the chemically active oxygen ions and inactive ions of neon noble gas, to thin epitaxial films of (Ga,Mn)As ferromagnetic semiconductor. Inspection of their magnetic properties by means of a superconducting quantum interference device magnetometer revealed that the implantation with a low dose of either O or Ne ions completely suppressed ferromagnetism in the films. Both the high resolution X-ray diffraction technique and the Raman spectroscopy showed significant changes in the structural and optical properties of the films caused by oxygen implantation and confirmed the preservation of high structural and optical quality of the neon implanted (Ga,Mn)As films.
We review recent magneto-optical investigations performed on HgTe-CdTe semimetallic superlattices. Far infrared magnetotransmission data obtained as a function of temperature, photon energy, and sense of circular polarization are compared with the predictions of a comprehensive new theory which fully incorporates the complexities of type-III superlattice band structure. It is found that the theory accounts for nearly all of the many unusual features which have been observed experimentally. These include the occurrence of two cyclotron resonances due to holes; the coexistence of electron and hole cyclotron resonances in the low temperature limit; the observation of three distinct CRA minima; a step-like change in the temperature dependence of the electron cyclotron mass; and a dramatic increase of the CRI absorption peak intensity with increasing magnetic field.
Nanocrystalline samples of ZnO(Fe) were synthesized by wet chemical method. Samples were characterized by X-ray diffraction to determine the sample composition and the mean crystalline size. Low-frequency Raman modes were measured and assigned according to confined acoustic vibrations of spherical nanoparticles. Frequencies of these vibrational modes were analyzed in elastic continuum aproximation, which considers nanoparticle as homogeneous elastic sphere.
The results of FIR reflectivity measurements for CdMnSe and CdFeSe are presented. These results are described theoretically using Dynamic Dielectric Function in which two phonon modes are included. The composition dependencies of mode parameters and dielectric constant are discussed.
Phonon excitations in (CdTe)_{12}/(MnTe)_{n} (100) superlattices (n=2, 4, 8) were investigated at 295 K and 25 K with the use of Raman scattering. From the "folded" phonon frequencies the elastic constant c_{11} value for MnTe was estimated. The strain arising from lattice mismatch (determined by X-ray diffraction) results in shifts of MnTe and CdTe "confined" LO phonon frequencies. For the precise determination of LO phonon dispersions an additional shift due to Mn diffusion at the CdTe/MnTe interface should be taken into account.
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