X-ray photoelectron spectroscopy experiments on PbTe:Yb, PbS:Yb, PbTe, and PbS crystals have been performed using the monochromatized Al K_{α} radiation. The valence band spectrum of PbTe:Yb exhibits one set of peaks associated with divalent Yb initial states, but two sets associated with divalent and trivalent Yb are observed for PbS:Yb. The valency of Yb in PbTe:Yb is 2 (within an accuracy of the experiment) but in PbS:Yb the mixed valency of Yb is seen. These conclusions are confirmed by an analysis of Yb 4d spectra in PbTe:Yb and PbS:Yb crystals.
The electronic band structure of GaAs_{1-x}N_x (x=0.016 and 0.031) and Ga_{1-x}B_xAs (x= 0.031) is studied by ab initio calculations using a supercell approach. Based on ab initio calculations and group theory we present a comprehensive analysis of the electronic structure of GaAs:N and GaAs:B alloys. In particular, we study the effective mass of conduction electrons in GaAs:N as a function of pressure and the Fermi energy. We find that the lowest conduction band is strongly non-parabolic, which leads to an increase in the effective mass with the electron energy. The rate of the increase is enhanced by the hydrostatic pressure. Theoretical results are compared to experimental data, and a qualitative agreement is found.
Non-stoichiometric Sn_{1-y}Te is a strongly degenerated n-type semiconductor. This is important for understanding unusual features of magnetic behaviour of Sn_{1-x}Gd_{x}Te where the relative positions of the Fermi energy and the atomic d-level of Gd govern the exchange coupling. The influence of the Sn vacancies on the band structure cannot be neglected if their concentration reaches a few atomic percent. We address this problem by using a tight-binding coherent potential approach and show that although the character of the bands remains unchanged, they are modified so that ε_{d} can come out above the heavy-hole band.
The temperature dependence of the electrical conductivity σ , the Hall coefficient R and the thermopower coefficient α in the solid solutions TlIn_{1-x}Yb_xTe₂ (0 ≤ x ≤0.10) have been investigated in the temperature range of 80-1000 K. The effective masses of electrons and holes have been determined on the basis of the kinetic parameters. It was established that starting from x = 0.05 the solid solutions of TlIn_{1-x}Yb_xTe₂ belong to narrow-gap semiconductors having high values of the interaction matrix element. It is shown that the samples of TlIn_{1-x}Yb_xTe₂ in the region of 0.10 ≥ x ≥ 0.05 are promising materials for the energy converters operating at high temperatures.
Boron doped MnO films were prepared by spray pyrolysis technique at 375°C substrate temperature, which is a low cost and large area technique to be well-suited for the manufacture of solar cells, using boric acid (H₃BO₃) as dopant source, and their properties were investigated as a function of doping concentration. Boron doping was achieved by adding 0.1 M, 0.2 M, 0.3 M, and 0.4 M H₃BO₃ to the starting solution. X-ray analysis showed that the films were polycrystalline fitting well with a cubic structure and have preferred orientation in (111), (220) and (311) directions. Optical band gap of the undoped and B-doped MnO films were found to vary from 2.25 to 2.54 eV. The changes observed in the energy band gap and structural properties of the films related to the boric acid concentration are discussed in detail.
Resonant photoemission spectroscopy was applied to determine the Mn 3d derived contribution to the valence band density of states of Mn_{0.44} Mg_{0.56}Te grown by molecular beam epitaxy on a GaAs(001) substrate. The modifications of the valence band density-of-states distribution are discussed as a consequence of the substitution of Mg ions for Mn ions.
The synchrotron radiation in the energy range between 15 and 70 eV was used to investigate the electronic structure of the crystalline Sn_{0.9}Mn_{0.1}Te by means of the resonant photoemission spectroscopy. Fano-type resonance has been observed in the obtained constant initial energy curves with the resonant energy 50.6 eV and antiresonant energy 49.0 eV. The energy distribution curves taken at photon energies close to the Mn 3p-3d transitions allow us to conclude that Mn atoms contribute to the valence band mainly at energies of 4.0 eV and 7.8 eV below the valence band edge.
We use the coherent potential method based on the density-functional theory to compare the electronic structures and magnetic characteristics of mixed crystals (Ga,Mn)As, (Zn,Mn)Se, and Li(Zn,Mn)As. We show that, with the same degree of doping, the magnetic behavior of these materials is very similar, reflecting the similarity of their electronic structures. In particular, the superexchange in (Ga,Mn)As is as strong as in (Zn,Mn)Se and the ferromagnetic coupling in all these dilute magnetic semiconductors becomes dominant only at sufficient concentration of the holes. Remarkably, we find that ferromagnetic state appears not only in p-type, but also in n-type materials at a comparable level of doping. Although obtaining strongly n-type doped III-V dilute magnetic semiconductors is improbable, Li(Zn,Mn)As seems to be a promising candidate for a ferromagnetic n-type semiconductor.
Recently new effects that are not characteristic of undoped lead telluride, such as the Fermi level pinning, giant negative magnetoresistance, were observed in Pb_{1-x}Mn_xTe alloys doped with transition and rare earth elements - Cr, Mo, Yb. We have studied transport and magnetic properties of Pb_{1-x}Mn_xTe doped with another transition element - vanadium. A series of Pb_{1-x}Mn_xTe(V) samples of different composition and degree of doping was investigated. It was observed that the resistivity demonstrates activation behavior at low temperatures for the samples with considerable amount of vanadium as well as for the samples without vanadium. The activation energy is proportional to the Mn content. In some of the samples, photoconductivity was observed at low temperatures. The results are discussed in terms of a model assuming formation of the impurity level by the vanadium impurity and the effect of the Fermi level pinning by this level.
Two deep traps associated with lattice-mismatch induced defects in n-type In_{0.042}Ga_{0.958}As/GaAs heterostructures and three deep point traps were observed by means of DLTS method. In order to determine the overlapping DLTS-line peaks parameters precisely, high resolution Laplace DLTS studies werw performed. A simple procedure of distinguishing between point and extended defects in DLTS measurements was used.
A series of In_{x}Tl_{1-x}I (x=0.4-0.9) single crystalline solid state alloys were successfully synthesized by the vertical Bridgman method. For the first time the density functional theory based band structure calculations are performed and features of the band energy dispersion are discussed. Comparison with the experimental energy gap E_{g} obtained from the optical photoconductivity is discussed. An essential role of the localized defects and excitons in formation of the edge photoconductivity is analyzed. Relation between the crystal structure and photoconductivity of the materials studied is discussed. Also the origin of the principal valence and conduction bands is explored. Experimental measurements of the band gap E_{g} by the optical and photoconductivity methods have given opposite dependences with respect to the indium content x. Features of the photoconductivity spectra for different crystallographic directions have been discussed with taking into account the chemical bonds anisotropy.
Photoluminescence spectra of Tl_2In_2S_3Se layered single crystals have been studied in the wavelength region of 535-725 nm and in the temperature range of 22-58 K. Two photoluminescence bands centered at 564 (2.20 eV, A-band) and 642 nm (1.93 eV, B-band) were observed at T = 22 K. Variations of both bands have been investigated as a function of excitation laser intensity in the range from 16 to 516 mW cm^{-2}. These bands are attributed to recombination of charge carriers through donor-acceptor pairs located in the band gap. Radiative transitions from shallow donor levels located 0.02 and 0.01 eV below the bottom of conduction band to acceptor levels located 0.05 and 0.44 eV above the top of the valence band are suggested to be responsible for the observed A- and B-bands in the photoluminescence spectra, respectively.
We present first-principles studies of the zero field spin splitting of energy bands in typical III-V semiconductors. Our calculations reveal that the strain induces linear-k spin splitting of the conduction band in theΓ point, which is linear in strain, and determine the magnitude of the so-called acoustic phonon constant that characterizes the magnitude of the spin splitting. In addition, we show that optical phonons lead to spin-flip processes and we present quantitative results for the spin-phonon deformation potentials in GaAs. Most importantly, the calculations show that the linear-k spin splitting can be resonantly enhanced when bands cross in a particular point of the Brillouin zone. This resonant enhancement of the bulk inversion asymmetry coupling constant by more than one order of magnitude was observed in both valence and conduction bands and can be steered by the application of the external stress. This allows tailoring of the spin relaxation and spin precession of conduction electrons in nanostructures to a much larger extent than was hitherto assumed.
The crystal structure, electronic, and mechanical properties of antifluorite Be_2X (X = C, Si) are calculated using the first-principles method based on density functional theory. Our calculated lattice parameters at equilibrium volume are in good agreement with the experimental data and other theoretical calculations. In order to obtain further information, the mechanical properties including bulk modulus, shear modulus, Young's modulus, and Poisson's ratio are deduced from calculated elastic constants. Meanwhile, the sound velocity and Debye temperature are also predicted. The bonding nature in Be_2X (X = C, Si) is a complex mixture of covalent and ionic characters.
The investigation of the ab initio band structure and the resulting spatial electron density distribution of the In-Se system in the framework of the density functional theory and the elementary energy bands concept is presented. It gives us reliable information about the valence band structure and peculiarities of the chemical bonding in these crystals. Some regularities in the evolution of the elementary energy bands topology is established, together with the choice of the actual Wyckoff position that is responsible for this topology and, at the same time, for the valence band formation in the InSe, In_2Se_3, In_4Se_3, and In_6Se_7 crystals. The calculated Mulliken charges and the degree of ionicity allow to estimate the character of chemical bonding in these crystals.
We investigate theoretically the possibility of n-type DMS based on III-V materials with Mn impurities in interstitial instead of substitutional positions, and discuss some situations when this can happen. We show that the d-states at interstitial Mn atoms in (Ga,Mn)As hybridize with both valence and conduction bands. The hybridization is strong enough to establish an indirect ferromagnetic coupling of the Mn magnetic moments mediated either by holes or by conduction electrons. Moreover, the Curie temperatures estimated within the mean-field theory are comparable with T_c obtained for conventional materials with the same concentration of Mn_{Ga}.
Based upon the ab initio band structure calculations results the dispersion law parameters of charge carriers of the orthorhombic In_4 Se_3 semiconductor as well as of its Sn- and Te-doped compounds were calculated. This allowed to estimate parameters of the electron condenson states in those compounds.
Zn_{0.95}Co_{0.05}O and Zn_{0.97}Ni_{0.03}O nanorods, prepared by a solvothermal method, show intriguing morphology and magnetic properties when co-doped with Li. At low and moderate Li incorporation (below 10 and 3 at.% Li in the Co- and Ni-doped samples, respectively) the rod aspect ratio is increased and room temperature ferromagnetic properties are enhanced, whereas the ferromagnetic coupling in Zn_{0.97}Ni_{0.03}O is decreased for Li concentrations < 3 at.%. First-principles theoretical analyses demonstrate that Li co-doping has primarily two effects in bulk Zn_{1- x}M_{x}O (with M = Co or Ni). First, the Li-on-Zn acceptors increase the local magnetic moment by depopulating the M 3d minority spin-states. The magnetic coupling is Ruderman-Kittel-Kasuya-Yosida-like both without and with Li co-doping. Second, Li-on-Zn prefer to be close to the M atoms to compensate the M-O bonds and to locally depopulate the 3d states, and this will help forming high aspect nanostructures. The observed room temperature ferromagnetism in Li co-doped Zn_{1- x}M_{x}O nanorods can therefore be explained by the better rod morphology in combination with ionizing the magnetic M atoms.
We present results of first-principles calculations for Bi₅GdFe₆O₁₈ compound in idealized the rhombohedral R3c structure for a variety of magnetic ordering. Within DFT+U approach it is found that the insulating ground state with the G-type antiferromagnetic arrangement of Fe sublattice gives a minimal total energy for BiFeO₃ substituted by magnetically active Gd³⁺. The Bi₅GdFe₆O₁₈ compound has nonzero total magnetic moment, which arises from antiparallel spin moments on Fe sites and reduced spin moment on Gd. Chemical bonding of the studied compound is analyzed using partial density of states, electron localization function and charge density distribution.
We investigate the compositional dependence of the total energy of the mixed crystals (Ga,Mn)As co-doped with As, Sn, and Zn. Using the ab initio linear muffin-tin orbital coherent potential approximation method we find a correlation between the incorporation of acceptors (Mn, Zn) and donors (Sn, antisite As). In particular, the formation energy of As_{Ga} is reduced by approximately 0.1 eV in the presence of Mn, and vice versa. This leads to the self-compensating behavior of (Ga,Mn)As.
JavaScript is turned off in your web browser. Turn it on to take full advantage of this site, then refresh the page.