K. Dybko

Band structure of β-HgS from Shubnikov–de Haas effect

Abstract The Shubnikov–de Haas oscillations were measured in zinc–blende Hg 0.98 TM 0.02 S (TM= Co, Mn, Fe) at temperatures ranging from 1.6 to 30 K, in magnetic fields up to 13 T. From standard harmonic analysis the values of the effective masses at the Fermi level versus electron concentration were determined. The analysis of this dependence within the frame of Kane model for narrow-band semiconductors yielded a low temperature value for energy gap E 0 =−0.110 eV ±0.040 eV and momentum matrix element P=7.1×10 −8 eV cm ±0.2×10 −8 eV cm .

Light- and environment-sensitive electrospun ZnO nanofibers

One-dimensional (1D) ZnO nanostructures have been widely studied because of their electronic and optoelectronic applications. This report discusses the morphology, optical, electrical and sensory properties of polycrystalline ZnO nanofibers (NFs). We observed that the electrospun ceramic NFs interband emission increases with the nanocrystal size, consistent with decreasing of the surface-to-volume ratio. The observation is novel for the electrospun ceramic NFs. The chemical composition and structural characterization reveal that the NFs consist of ZnO wurzite nanocrystals, whose mean diameters increase from 7 to 22 nm with calcination temperature. Emission properties are studied by cathodo- and photoluminescence. The NFs are applied to construct light, gas and liquid sensors. We find an increase of the NFs conductivity by three orders of magnitude under UV illumination as a result of desorption of molecular oxygen from the nanocrystal surface. We study the influence of oxygen on NF conductivity by purging the NFs with air or nitrogen. We show that the flow of nitrogen removes the oxygen resulting in an important increase of the conductivity. Also, we study the dynamics of this process with and without UV illumination. We show sensitivity of the NFs to liquid environment by studying the conductivity of NFs immersed in water and ethanol and find an increased conductivity with respect to a dry air environment. These light- and environmental-sensitive ZnO NFs have useful optical and electronic properties for building high-performance sensors.

Annealing-induced changes in electrical, optical, and magnetic properties of phosphorus doped bulk Zn1-xMnxTe

We report on successful growth of heavily p-type doped bulk Zn 1-x Mn x Te crystals. Free hole density as high as 10 19 cm -3 has been achieved by use of Zn 3 P 2 as a source of phosphorus acceptors and by post-growth high pressure annealing. The latter proved to be of a vital importance, removing a strong compensation present in as-grown materials. As a result, the annealed crystals exhibit a very bright green photoluminescence (λ 520 nm) at temperatures as high as 160 K. The annealed samples with hole concentration p > 2 × 10 18 cm -3 show a positive Curie-Weiss temperature and open hysteresis curves, characteristic of a system with ferromagnetic correlations.

NEGATIVE MAGNETORESISTANCE AND IMPURITY BAND CONDUCTION IN AN IN0.53GA0.47AS/INP HETEROSTRUCTURE

The electrical conduction in an n-type In0.53Ga0.47As/InP sample grown by molecular beam epitaxy has been analyzed in the magnetic field up to 1.5 T, at temperatures from 15 to 295 K. The electrical conduction has been ascribed to the impurity band (IB), located in the interface between the epilayer InGaAs and the substrate InP. The contribution of the conduction band electrons in bulk InGaAs layer to the electrical conduction was negligible. The IB conduction was almost metallic. We observed within the IB two conducting channels which give positive and negative contribution to the Hall voltage. The magnetoresistance in the IB at low temperatures was negative, and at high temperatures was positive. The negative magnetoresistance in the IB has been phenomenologicaly described by carriers with the constant concentration and the mobility increasing proportionally to the square of the applied magnetic field.

Ferromagnetic Transition in Ge_{1-x}Mn_{x}Te Layers

Ferromagnetic transition temperature in thin layers of diluted magnetic (semimagnetic) semiconductor Ge1−xMnxTe was studied experimentally by SQUID magnetometry method and analyzed theoretically for a model Ising-type diluted magnetic system with Ruderman–Kittel–Kasuya–Yosida indirect exchange interaction. The key features of the experimentally observed dependence of the Curie temperature on Mn content (x ≤ 0.12) and conducting hole concentration p = (1–10)× 10 cm−3 were reproduced theoretically for realistic valence band and crystal lattice parameters of p-Ge1−xMnxTe taking into account short carrier mean free path encountered in this material and Ruderman–Kittel–Kasuya–Yosida mechanism with both delta-like and diffused character of spatial dependence of the exchange coupling between magnetic ions and free carriers.

Analysis of scattering mechanisms in zinc oxide films grown by the atomic layer deposition technique

In this work, the analysis of the temperature-dependent electrical conductivity of highly crystalline zinc oxide (ZnO) thin films obtained by the Atomic Layer Deposition (ALD) method is performed. It is deduced that the most important scattering mechanisms are: scattering by ionized defects (at low temperatures) as well as by phonons (mainly optical ones) at higher temperatures. Nevertheless, the role of grain boundaries in the carrier mobility limitation ought to be included as well. These conclusions are based on theoretical analysis and temperature-dependent Hall mobility measurements. The presented results prove that existing models can explain the mobility behavior in the ALD-ZnO films, being helpful for understanding their transport properties, which are strongly related both to the crystalline quality of deposited ZnO material and defects in its lattice.

Epitaxial Growth and Characterization of PbGeEuTe Layers

The structural and electrical properties of Pb1−yGeyTe and Pb1−x−yGeyEuxTe (0≤y≤0.4 and x≤0.05) monocrystalline layers grown by molecular beam epitaxy technique on BaF2 (111) substrate were studied by X-ray diffraction, Hall effect, and electrical conductivity measurements. Based on the temperature dependence of the lattice parameter the structural (ferroelectric) transition temperature was found in the temperature range before 100 to 250 K in layers with varying Ge and Eu content. Electrical measurements indicates that incorporation of Eu ions in the PbGeTe crystal matrix decreases the electrical conductivity in p-type PbGeEuTe layers by 1–2 orders of magnitude.

Local modes of transition metal ions (Mn, Fe, Co) in narrow-gap II-VI semiconductors

Abstract Optical phonons in mercury chalcogenides doped with selected transition metal ions (TMI) are investigated by infrared reflectivity and Raman scattering measurements performed at temperatures from 5 to 295 K. The results obtained for Hg 1− x Fe x Se, Hg 1− x Co x Se, Hg 1− x Fe x S and Hg 1− x Co x S mixed crystals confirm the chemical trends in the dependence of the local mode frequency on the number of 3d electrons of the TMI, reported previously for the wide-gap, zinc-blende II–VI semiconductors.

Sn-doped Bi 2 Te 2 Se as a broadband saturable absorber for Q-switched fiber lasers

Passively Q-switched fiber lasers based on low-dimensional saturable absorbers have been extensively developed due to their efficiency, robustness and great simplicity [1]. However, an environmentally stable, industrial class lasers are still lacking. Here, we present a new material for the application as a broadband saturable absorber for large-energy Q-switched ytterbium- and erbium-doped fiber lasers. Bi2Te2Se:Sn is a small band gap semiconductor and ternary topological insulator [2]. It is characterized by broadband saturable absorption [3].

Exploiting nonlinear properties of pure and Sn-doped Bi 2 Te 2 Se for passive Q-switching of all-polarization maintaining ytterbium- and erbium-doped fiber lasers

Due to their broadband nonlinear optical properties, low-dimensional materials are widely used for pulse generation in fiber and solid-state lasers. Here we demonstrate novel materials, Bi2Te2Se (BTS) and Sn-doped Bi2Te2Se (BSTS), which can be used as a universal saturable absorbers for distinct spectral regimes. The material was mechanically exfoliated from a bulk single-crystal and deposited onto a side-polished fiber. We have performed characterization of the fabricated devices and employed them in polarization-maintaining ytterbium- and erbium-doped fiber lasers. This enabled us to obtain self-starting passively Q-switched regime at 1 µm and 1.56 µm. The oscillators emitted stable, linearly polarized radiation with the highest single pulse energy approaching 692 nJ. Both lasers are characterized by the best performance observed in all-polarization maintaining Q-switched fiber lasers with recently investigated new saturable absorbers, which was enabled by a very high damage threshold of the devices. This demonstrates the great potential of the investigated materials for the ultrafast photonics community.