Ultrafast optical and magneto-optical studies of III–V ferromagnetic semiconductors

Abstract

We use ultrafast optical techniques to investigate the dynamics of charge and spin carriers and coherent phonons as well as magnetic order in III-V ferromagnetic semiconductors. We observe a rich array of dynamical phenomena that are absent in traditional nonmagnetic semiconductors or metallic ferromagnets. Very short charge and spin lifetimes of the photoinjected carriers (～2ps) and multi-level charge decay dynamics are observed, which are attributed to a large density of mid-bandgap states introduced during low temperature molecular beam epitaxy (LT-MBE) growth and highly p-type Mn doping. During the very short free carrier lifetime, the coercivity of the system is seen to be reduced. We attribute this photo-induced ‘softening’ to the transient modification of carrier-mediated ferromagnetic exchange coupling between Mn spins. After the photogenerated free electrons are trapped by defects, periodic oscillations appear in differential reflectivity due to the coherent generation of acoustic phonon wavepackets.     

Defect distribution near abraded surface of III–V compound semiconductors

Microstructures of damaged layers near the abraded surface of GaP, GaAs_0.6P_0.4 and GaAs single crystals have been observed with a transmission electron microscope. The damage due to 0.3 m Al₂O₃ abrasion consists of dislocations. Dislocation densities in the abrasion band and other regions are larger than 5×10¹¹ cm^–2 and about 10¹¹ cm^–2, respectively. Dislocation density decreases with increasing distance (depth) from the abraded surface. The depths of the damage layers for GaP, GaAs_0.6P_0.4 and GaAs are about 0.4, 0.55, and 0.8 m, respectively. The Burgers vector of dislocations is a/2[1 1 0], and A and B dislocations are observed. During annealing, the dislocations are rearranged and eliminated by the reaction of the dislocations with different Burgers vectors and climbing motion. The rearrangement temperatures for dislocations in GaP and GaAs are about 500 and 450° C, respectively. Electrical resistance changes in the damaged layer of GaP are recognized by electrical resistance measurements.     

Enhancing the physical,optical and shielding properties for ternary Sb2O3–B2O3–K2O glasses

Ternary Sb₂O₃–B₂O₃–K₂O glass system, with general composition of x Sb₂O₃–(70-x) B₂O₃–30 K₂O (where x?=?0, 10, 20, 30, 40, 50) were prepared using melt-quenching technique. Structures of these glasses were investigated using XRD analysis and FT-IR spectroscopy. The optical properties were investigated using the UV–Vis NIR JASCO (Model V-670) Double Beam Spectrophotometer. Different physical parameters, such as density (ρ), molar volume (V_M), oxygen molar volume (V_O) and oxygen packing density values (OPD) have been estimated. Also, the Gamma radiation shielding ability have been characterized for the investigated glasses using Phy-X/PSD software in the photon energy range (0.015–15 MeV). XRD analysis confirm the amorphous nature of the prepared glasses. The optical energy gap of SBK glasses is decreasing from 4 to 2.63 eV with increasing Sb₂O₃ content while refractive index is increasing from 2.17 to 2.5 because of increasing the non-bridging oxygens (NBOs) in the glass matrix. The molar refractivity (R_m), molar polarizability (α_m) and the third-order nonlinear optical susceptibility values χ³ have been calculated, their values are found to increase with increasing Sb₂O₃ content. Glass density and molar volume values for the SBK glasses increase with increasing Sb₂O₃ content. Increasing the Sb₂O₃ concentration enhance the radiation shielding features of the prepared glasses against gamma rays and neutrons. Hence, the mass attenuation coefficient (MAC) increases from 8.6 to 35.4 cm²/gm while the half value layer (HVL) decreases from 0.036 to 0.0046 cm at 0.015 MeV as the Sb₂O₃ concentration increases from 0 to 50 mol%. The fast neutron cross section of the six investigated SBK glasses are (0.087, 0.092, 0.095, 0.091, 0.094 and 0.092 cm^?1), respectively.

     

Chemical thinning of III–V compound semiconductors for transmission electron microscopy

A vapour jet etching method and suitable equipment are tested in order to thin monocrystalline and polycrystalline material of GaAs, InP and InAs for transmission electron microscopy. The reactive vapour jet consists of air, bromine and methanol. The thinning procedure is monitored by an optical microscope. Large thin areas could be obtained using a bubbler temperature of 60° C, 20% bromine in methanol, a distance between nozzle and sample surface of 1.5 mm and a nozzle mouth diameter of 0.7 mm. During pre-etching the vapour flow rate was 0.71 min^–1, and 0.31 min^–1 was realized during final etching up to perforation. The etching times ranged between 1 to 2 min with an initial sample thickness of 350 to 365 m. Large thin areas have only been obtained using an excentric position of the nozzle with respect to the sample centre in combination with a manual rotation of the specimen holder.     

Exploration of entire range of III–V semiconductors and their device applications

Abstract

Superplastic deformation of certain metallic alloys was first observed almost a century ago but simply remained an interesting metallurgical curiosity until the revelation, in 1962, of work in the former Soviet Union. This led to western research and the suggestion by Backofen that the phenomenon could be practically exploited for the manufacture of sheet components. In turn, this resulted in major work to develop new superplastic alloys, forming machinery and forming techniques. Although superplastic behaviour has now been developed in a wide range of materials, its exploitation has been confined to niche applications. It is only the superplastic alloys of titanium and aluminium that have achieved significant commercial markets. The fact that titanium alloys can combine superplastic forming with diffusion bonding to manufacture multisheet structures has assured the material a secure and growing role in aerospace applications. The high basic cost of titanium has precluded significant use in other markets although this could change if current developments in the titanium extraction process are successful. Superplastically formed aluminium components find application in a far wider range of niche applications. It is suggested that the more recent demonstration of superplastic behaviour at much higher strain rates (HSRS) could allow superplastic forming of aluminium alloys to break out of its niche role. However, significant development would be required to combine the existing fundamental knowledge of HSRS with engineering to define a practical, economic manufacturing route for volume production of the new material.     

Describing the heat capacity of III–VI compound semiconductors in a fractal model

The fractal model of heat capacity is shown to be applicable to GaSe, Ga₂Se₃, GaTe, Ga₂Te₃, InSe, and InTe. The parameters of the model are determined: characteristic temperature and temperature-dependent multifractal (fracton) dimension.     

Characterization of SiO2–Na2O–Fe2O 3–CaO–P2O 5–B 2O 3 glass ceramics

Bioactivity and magnetic properties were investigated in glass and glass ceramics based on the SiO₂–Na₂O–Fe₂O₃–CaO–P₂O₅–B₂O₃ system to find their suitability as thermoseed for hyperthermia treatment of cancer. The effect of change in compositions on bioactivity was examined in simulated body fluids. The glass ceramic samples exhibit Na₃CaSi₃O₈ and Na_3-XFe_XPO₄ phases. After dipping the glass ceramic samples in simulated body fluids silica hydrogel first forms, followed by an amorphous calcium phosphate layer. Magnetic and microwave resonance experiments further demonstrate the potential of these glass ceramics for possible use in hyperthermia.     

Synthesis of an integrated biorefinery via the C–H–O ternary diagram

An integrated biorefinery is designed to handle a wide variety of feedstocks (mainly biomass) and can produce a broad range of products (e.g., biofuel, biochemicals, etc.) via multiple conversion pathways and technologies. Gasification is recognized as one of the most promising technologies for initial processing of biomass. It uses thermal energy to convert the biomass feedstock into a gaseous mixture, which is also known as syngas, consisting mainly of carbon dioxide (CO₂), steam (H₂O), methane (CH₄), carbon monoxide (CO) and hydrogen (H₂). It is noted that the composition of syngas, especially the ratio of H₂ to CO, is crucial when the syngas is further converted to liquid fuels and chemicals. In this work, a graphical targeting approach for the evaluation of gas phase equilibrium composition of biomass gasification is proposed. Based on the targeted composition, a conceptual design of an integrated biorefinery can be systematically developed.     

Phase Relations in the Na2O–Al2O3–Nb2O5and CaO–Al2O3–Nb2O5Systems

Phase relations in the Na₂O–Al₂O₃–Nb₂O₅and CaO–Al₂O₃–Nb₂O₅systems were studied. The Na₂O system was found to contain neither ternary compounds nor niobate–aluminate solid solutions. In the CaO system, a ternary compound of composition 4CaO · Al₂O₃·Nb₂O₅was identified (cubic structure, a= 7.628 Å, Z= 2, _meas= _x= 4.43 g/cm³).     

Electrical conductivity studies of Bi2O3–Li2O–ZnO–B2O3 glasses

Ac conductivity measurements and its analysis has been performed on xBi₂O₃–(65?x)Li₂O–20ZnO–15B₂O₃ (0 ≤ x ≤ 20) glasses in the temperature range 30–300 °C and a frequency range of 100 Hz to 1 MHz. The dc conductivity increased and the activation energy decreased with lithium content. The frequency dependent conductivity has been analyzed employing conductivity and modulus formalisms. The onset of conductivity relaxation shifts towards higher frequencies with temperature. The Almond–West conductivity formalism is used to explain the scaling behavior, and the relaxation mechanism is independent of temperature.     

Characterisation of Ga2O3–Na2O–CaO–ZnO–SiO2 bioactive glasses

The structural role of Gallium (Ga) is investigated when substituted for Zinc (Zn) in a 0.42SiO₂–0.40–xZnO–0.10Na₂O–0.08CaO glass series, (where x = 0.08). Each starting material was amorphous, and the network connectivity (NC) was calculated assuming Ga acts as both a network modifier (1.23), and also as a network former. Assuming a network forming role for Ga the NC increased with increasing Ga concentration throughout the glass series (Control 1.23, TGa-1 2.32 and TGa-2 3.00). X-ray photoelectron spectroscopy confirmed both composition and correlated NC predictions. Raman spectroscopy was employed to investigate Q-structure and found that a shift in wavenumbers occurred as the Ga concentration increased through the glass series, from 933, 951 to 960 cm^?1. Magic angle spinning nuclear magnetic resonance determined a chemical shift from ?73, ?75 to ?77 ppm as the Ga concentration increased, supporting Raman data. These results suggest that Ga acts predominantly as a network former in this particular Zn-silicate system.     

Microwave dielectric properties of 3Li2O–Nb2O5–3TiO2 ceramics with Li2O–V2O5 additions

A new Li₂O–Nb₂O₅–TiO₂ (LNT) ceramic with the Li₂O:Nb₂O₅:TiO₂ mole ratio of 3:1:3 has been investigated. The compound is composed of two phases, the Li₂TiO₃ and “M-phase” solid solution phase. The microwave dielectric ceramic has low sintering temperature (∼1100 °C) and good microwave dielectric properties of a relatively high permittivity (∼51), high Q × f value up to 8700, and small temperature coefficient (∼37 ppm/°C). The low-amount doping of 0.83Li₂O–0.17V₂O₅ (LV) can effectively lower the sintering temperature from 1100 to 900 °C and induce no obvious degradation of the microwave dielectric properties. Typically, the 1 wt.% LV-doped ceramic sintered at 900 °C has better microwave dielectric properties of ε_r = 51.3, Q × f = 7235 GHz, τ _f  = 22 ppm/°C, which suggests that the ceramics can be applied in microwave LTCC devices.     

EPR study of V2O5–P2O5–Li2O glass system

glass system, with 0 < x 50 mol%, was prepared and investigated by EPR method. For low content of V₂O₅ all the spectra present a hyperfine structure typical for isolated V⁴⁺ ions. With the increasing of V₂O₅ content, the EPR absorption signal showing hyperfine structure is superposed by a broad line without hyperfine structure characteristic for clustered ions. At high V₂O₅ content, the vanadium hyperfine structure disappears and only the broad line can be observed in the spectra. Spin Hamiltonian parameters g , g , A , A , dipolar hyperfine coupling parameters, P, and Fermi contact interaction parameters, K, have been calculated.The composition dependence of line widths of the first two absorptions from the parallel band and of the broad line characteristic to the cluster formations was also discussed.     

Phase Relations in the NiFe2O4–NiCr2O4–CuCr2O4System

The phase relations in the NiFe₂O₄–NiCr₂O₄–CuCr₂O₄system were investigated experimentally and theoretically. X-ray diffraction data were used to construct the phase diagram of the system and elucidate the structural mechanisms of the transitions from the cubic spinel structure to the tetragonal (I42d, c/a< 1 and I4₁/amd, c/a> 1) and orthorhombic (Fdd2) structures.     

Effect of O2, H2O2, and HNO2 on the stability of Np(III–VII) in aqueous solutions

Published data on reactions of Np ions with O₂, H₂O₂, HNO₂, and HNO₃ in solutions of various compositions in a wide pH range are considered. O₂ oxidizes Np(III) in acid solution and Np(IV) and Np(V) in alkaline solutions. H₂O₂ exhibits dual behavior. In weakly acidic solutions, it converts Np(III) and (IV) to Np(V), in 0.75?C1 M NaHCO₃ it oxidizes Np(V) to Np(VI), whereas in dilute HClO₄ and HNO₃ and in carbonate and alkali solutions it reduces Np(VI), and in alkali solutions it reduces Np(VII). The first step of reduction in most cases is the formation of the Np(VI) peroxide complex, and the next step is the intramolecular charge transfer. In concentrated HNO₃ solutions, H₂O₂ converts Np(V) to Np(IV) and Np(VI) and then reduces Np(VI). Some radiation-, photo-, and sonochemical reactions occur via formation of excimers, i.e., of dimers arising from excited and unexcited Np ions. The excimer decomposes into two ions with higher and lower oxidation states. In reduction reactions, the excimer eliminates H₂O₂ (in addition to the H₂O₂ arising as primary product of water radiolysis). In HNO₃ solutions, oxidation of Np ions occurs only in the presence of HNO₂ arising as reaction product or upon radiolysis, photolysis, or sonolysis. The active species are NO ₂ ^? , NO₂, and NO⁺ present in equilibrium with HNO₂.     

Microwave dielectric properties of CaO–La2O3–Nb2O5–TiO2 ceramics

A series of ceramics with a general formula Ca_1+xLa_4?xNb_xTi_5?xO₁₇ (0 ≤ x ≤ 4) were fabricated using the solid-state ceramic route. The phase, microstructure, and microwave dielectric properties varied distinctly with composition or the value of x. X-ray diffraction results showed that the two end member phases, CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇, crystallized into single phases with orthorhombic and monoclinic crystal structure, respectively. For intermediate compounds with x = 1, 2, and 3, mixture phases CaLa₄Ti₅O₁₇ and Ca₅Nb₄TiO₁₇ coexisted and a trace amount of second phase was detected. The ceramics showed high ε _r in the range of 45–52, relatively high quality factors with Q × f in the range of 9,870–15,680 GHz and τ _f value in the range between ?38 and ?126.4 ppm/°C. τ _f of CaLa₄Ti₅O₁₇ can be tuned to a near-zero value by addition of suitable amount of TiO₂.     

Phase relation of ternary system ZrO2CaOAl2O3

The phase relation in the ternary system ZrO₂CaOAl₂O₃ has been studied at 1,380°C in the composition range below 50 mol% CaO. The mixed oxides with various compositions have been prepared from their acidic solutions through evaporation, calcination, and heat-treatment. Nine regions have been found. In the stabilized zirconia with fluorite structure, 4 mol% Al₂O₃ is soluble. The fluorite phase also exists in equilibrium in two-phase regions (fluorite + ZrO₂, and fluorite + CaO·ZrO₂) and in three-phase regions (fluorite + ZrO₂ + CaO·2Al₂O₃, and fluorite + CaO·ZrO₂ + CaO·2Al₂O₃).     

Phase Relations in the Systems Al2TiO5–Fe2O3 , Al2O3–TiO2–Fe2O3 , and Al2TiO5–Cr2O3

Phase relations in the systems Al₂TiO₅–Fe₂O₃, Al₂TiO₅–Cr₂O₃, and Al₂O₃–TiO₂–Fe₂O₃ are investigated, and the composition ranges of pseudobrookite Al_{2 – 2x} M_2x TiO₅ (M = Fe, Cr) solid solutions are determined.     

Contribution to ternary system Al–Ti–B Part 2 – Study of alloys in Al–AlB2–TiB2 triangle

Abstract

In arc melted and aluminothermically prepared alloys with composition in the triangle Al–AlB₂–TiB₂ four solid phases are observed: α-Al, ρ-AlB₁₂, AlB₂, and TiB₂. The univariant reaction between L, ρ-AlB₁₂, and TiB₂ is the eutectic one L? α-AlB₁₂ + TiB₂. During the reaction TiB₂ platelets are sequentially formed on facets of α-AlB₁₂, but their formation may be completely suppressed by the presence of primary TiB₂. Decomposition of α-AlB₁₂ does not occur on cooling, but only during isothermal annealing under 900°C probably according to the transition reaction L + α-AlB₁₂ → TiB₂ + AlB₂. Dissolution of α-AlB₁₂ and growth of AlB₂ take place in afaceted manner owing to their high entropies of solution. The low temperature AlB₂ phase is not formed at all on cooling of concentrated alloys with an overall composition lying on the AlB₂-TiB₂ tieline, whereas in less concentrated alloys it is usually formed on the primary TiB₂ particles in the regions distant from α-AlB₁₂