Ion scattering studies of strained Si/Si1−xGex superlattices

Helium ion channeling and backscattering experiments have been performed on MBE-grown Si/Si_1−xGe_x superlattices to investigate interdiffusion, defect densities and tetragonal lattice distortion. Rapid Thermal Annealing (RTA) of these structures leads to substantial interdiffusion and strain relaxation. In some cases also relaxation by dislocation formation is observed after RTA, depending on the alloy composition x. Grazing incidence Rutherford backscattering spectroscopy was employed to observe the modulation of the backscattering yield of the superlattices. The modulation decreases due to interdiffusion after RTA. Interdiffusion coefficients were deduced for Ge concentrations between x=0.20 and x=0.70 in the temperature range between 900°C and 1125°C. The diffusivity increases with larger Ge concentrations. The activation energy for interdiffusion is 3.8 ± 0.2 eV.     

Metastable impurity bands: Conduction in Si1−xCox and Si1−xNix alloy films

Conductivity data for silicon-nickel and silicon-cobalt films prepared by electron beam evaporation shows a hopping conduction at metal concentrations in the range of 1 to 6 at.%. A semiconductor-to-metal transition was observed at metal concentrations of 15 at.% Co for SiCo and 13 at.% Ni for SiNi films.     

Itinerant metamagnetic transition in Y(Co1−xFex)2

High field magnetization measurements have been performed to examine the existence of itinerant metamagnetism in exchange-enhanced systems related to YCo₂ together with Fe_1−x Co_xSi. In the Y(Co_−xFe_x)₂ system, the meta magnetism inherent in YCo₂ has been observed in 0.04 x0.07. The transition is not as sharp as in the Y(Co_1−xAl_x)₂ system. Other exchange-enhanced paramagnets Y(Co_−xCu_x)₂ and Y_1−xLa_xCo_x2 and weakly itinerant ferromagnet Fe_1−xCo_xSi exhibit no metamagnetic transition up to 430 kOe.     

Spin fluctuations of Y(Mn1−xAlx)2 and Y1−xScxMn2

The thermal expansion and low temperature and low temperature specific heat were measured for Y_1−xScxMn2. The results are discussed in terms of spin fluctuations and compared with those of Y(Mn_1−xAl_x)₂, which show al local moment character. It is revealed that Y_1−xSc_xMn₂ is a typical nearly antiferromagnet in which giant spin fluctuations are thermally excited.     

Hyperfine investigations of the magnetic state in ordered Pt3−xFe1+x

Mössbauer effect studies of Pt_3−xFe_1+x fcc ordered alloys in the range 4.2 K <T<300 K, in zero and in external magnetic field, for samples with x = 0.16 to 0.28, are reported. The low-temperature spectra show several satellite lines which are related to different excess-Fe nearest-neighbor configurations. Experiments in external fields give information on the different local spin structure at different concentrations.     

Magnetic properties of amorphous sputtered CoxSi1−x films

Magnetic properties have been measured by transverse magneto-optic Kerr effect (MOKE) in amorphous sputtered Co_xSi_1?x films which are ferromagnetic and amorphous at room temperature with 0.55<x<0.80. These as-sputtered films show an in-plane easy axis with the ratio of coercitive force in the hard axis H_c (HA) to easy axis H_c (EA) and the value of the blocking field H_B almost constant with 0.60<x<0.80. Transverse magnetic susceptibilities with bias magnetic field parallel (χ_t0) and perpendicular $\text{(χ}{}_{\mathrm{<mtext>t\pi </mtext><mtext>2</mtext>}}\text{)}$ to the easy axis has been measured. These measurements show a deviation of Hoffmann's micromagnetic law for the susceptibilities.     

Linear and nonlinear optical properties of CdSexS1−x microcrystallites

Microcrystals of CdSe_xS_1−x (x≈0.6) with nanometer dimensions have been investigated experimentally by a range of optical techniques. This system of ‘quantum dot’ consists of nanometer sized semiconductor particles embedded within an insulating glass matrix. The existence of microscopic CdSeS crystals within the glass matrix is demonstrated by the observation of Raman scattering from the ‘CdSe-like’ and ‘CdS-like’ LO phonons. The nature of the electronic states within these three dimensionally confined systems is investigated by linear absorption and photoluminescence spectroscopy. The photoluminescence spectra show features which are attributed to direct electron-hole recombination and recombination via states within the ‘blue shifted’ energy gap (which are possibly surface related). Carrier relaxation is also investigated by a pump-probe experiment whereby the absorption is partially bleached by a short pump pulse then probed some variable time later by a delayed probe pulse of much lower intensity. Fast and slow components in the carrier relaxation process are identified, and a relationship is suggested between the carrier relaxation and the features observed in the photoluminescence spectra. The well known ‘photodarkening’ properties of such materials are also investigated by the above techniques.     

Crystallization of amorphous Fe100−xPx (13x24) alloys

A study of the structure change with temperature in amorphous Fe₁₀₀−xP_x (13 x 24) alloys was carried out by measuring magnetization and thermal expansion and also by structural analysis using X-ray diffraction and differential thermal analysis (DTA). The structure of the amorphous alloys relaxes (the decrease of excess free volume) at temperatures 100–150 K below the crystallization temperatures. The alloys with x 15 transform into (α-Fe + amorphous) at about 600 K. The alloys with x15 transform into (α-Fe+amorphous+Fe₃P) at about 600 K. With further heating, the alloys transform into (α-Fe+Fe₃P) both of which are stable phases from the equilibrium phase diagram.     

Spectroscopic study of BixEu1−xVO4 and BiyGd1−yVO4 mixed oxides

The mixed oxides Bi_xEu_1−xVO₄ and Bi_yGd_1−yVO₄ crystallize in a zircon-type structure, for 0 <x < 0.6 and 0 < y < 0.64, and in a fergusonite-type structure, for 0.94 < x < 1 and 0.93 < y < 1. A process of competition between the dominant and the constrained effects of the lone-pair 6s² of Bi³⁺ is discussed. The diffuse reflectance spectroscopic studies of these mixed oxides are presented. The observed broad bands are attributed to charge transfer processes and the sharp peaks in the Bi_xEu_1−xVO₄ spectra are ascribed to intra-configurational 4f – 4ftransitions of the Eu³⁺ ion. The broad absorption shift in BiLnVO₄ (Ln : Eu and Gd) compounds to the longer wavelengths range, when Bi is introduced in the LnVO₄ lattice, is ascribed to charge transfer processes in a Bi-VO₄ center and are interpreted assuming a Jahn-Teller effect in the excited state of Bi³⁺. The concept of an internal pressure of Bi³⁺ ions is also used to explain the broad A-band shifts.     

Thermoelectric power of Y(Mn1−xAlx)2

Electrical resistivity and thermoelectric power of Y(Mn_1−xAl_x2 and (Y_1−xSc_x)Mn₂ were measured from 2 to room temperature. Anomalous behavior of the thermoelectric power of YMn₂ disappears rapidly with the increase of Al content in Y(Mn_1−xAl_x)₂. Based upon spin fluctuations of the itinerant antiferromagnetism of the samples, the anomaly of the thermoelectric power is discussed.     

Luminescence from Si-Si1−xGex/Si1−yCy-Si structures

Near band edge photoluminescence has been obtained from Si_1−yC_y quantum well (QW) and neighboring Si_1−xGe_x/Si_1−yC_y double QW (DQW) structures. Enhanced no-phonon recombination is observed from the DQW structures and it is attributed to a breaking of the k-selection rule in the presence of the heterointerface. The luminescence persists for measurement temperatures up to 30–50 K and the intensity exhibits a quenching behavior with an activation energy equal to 8–20 meV. In electroluminescence only recombination in the Si_1−xGe_x layer has been observed from neighboring Si_1−xGe_x and Si_1−yC_y DQW structures.     

The Hall effect in selectively doped strained-layer Ge-Ge1−xSix superlattices superlattices

For the first time, research on the unique galvanomagnetic properties of the hole gas in the channels of selectively doped CVD Ge-Ge_1−XSi_X (X≤0, 2) superlattices with strained Ge layers was carried out. We have obtained a high value of the hole mobility 1.5 × 10⁴ cm²/V s (T = 4, 2 K) at a hole concentrations of (1–5) × 10¹⁷ cm⁻³ in SLs channels. It is shown that the main contribution into the longitudinal conductivity of strained Ge-Ge_1−XSi_X SL due to light hole band splitting under the strains in Ge layers.     

Deep impurity states in Hg1−xCdxTe

The chemical trends for the energy level of substitutional defects in narrow-band-gap semiconductor alloy Hg_1-xCd_xTe is studied. A minimal basis set in the empirical tight binding formalism is employed. The calculated trends seem to agree well with previous studies but there are some quantitative differences.     

Oxygen isotope effect of superconducting (Nd1−xCex)2CuO4−δ system

The effect of oxygen isotope substitution on the transition temperature T_c of a superconducting (Nd_1−xCe_x)₂CuO_4−δ system was studied, where the special synthetic method was taken to minimize ambiguous factors on the oxygen concentration and to guarantee the complete substitution of ¹⁶O by ¹⁸O. The isotope exponent in the relationship of T_c∝1/M was estimated to be less than 0.15 by magnetic susceptibility measurements.     

Superconductivity and metal-insulator transition in amorphous Si1−xAux system

Electrical conductivity of metallic amorphous Si_1?xAu_x films was measured down to 10 mK. Superconducting transition was observed in a certain range of Au content. In the normal state, the temperature variation of electrical conductivity showed √T-dependence at higher temperatures and ln T-dependence at lower temperatures. This crossover is interpreted as being due to a change of sample dimensionality.     

Epitaxial growth of MgO on lattice-matched CrxMO1−x(001)

We describe the MBE growth and structural characterization of ultrathin layers of epitaxial MgO on Cr_xMO_1−x (x ≈ 0.65 ). The metal alloy was grown as an epitaxial film on high-quality MgO(001) substrates. This kind of epitaxy produces unstrained MgO surfaces of very high structural quality on a conducting substrate. Such materials will enable definitive investigations of surface structure and reactivity using charged-particle spectroscopy, diffraction, and imaging. The role of lattice matching is discussed and shown to be very important.     

Morphology of GaAs1−xPx superlattices grown by MOCVD and chloride VPE

Distorted layer growth manifested by nonuniform etching may occur in GaAs_1−xP_x superlattices grown by MOCVD and VPE. The distortion was found to be more severe for MOCVD growth than for VPE. The distortion is decreased by decreasing the magnitude of the interlayer strain in the superlattice but does not depend upon the layer thickness. The rate of crystal growth, the temperature of growth and the strain rate in the layer supporting the superlattice also influence the distortion. Several possible causes of the effect are discussed, including nonuniform elastic stress and/or compositional nonuniformity.     

NMR study of nearly itinerant antiferromagnetic Y1−xScxMn2

The NMR investigation of Y_0.97Sc_0.03Mn₂ has revealed that this compound remains paramagnetic down to 4.2 K. The nuclear spin-lattice relaxation rates, 1/T₁, of ⁵⁵Mn and ⁴⁵Sc in Y_0.97Sc_0.03Mn₂ show the T dependence as predicted from the self-consistent renormalization (SCR) theory of spin fluctuations for nearly antiferromagnetic metals.     

Effects of structure on exchange interactions in crystalline and amorphous alloys GdxY50−xAg50

Magnetic interactions and effects of dilution with nonmagnetic Y on the magnetic properties of crystalline and amorphous alloys Gd_xY_50−xAg₅₀ (10 ≤ x ≤ 50) have been investigated by measurements of bulk magnetization and susceptibility and by Mössbauer spectroscopy with ¹⁵⁵Gd. The crystalline alloys order antiferromagnetically for all Gd concentrations with a noncollinear arrangement of Gd moments induced by negative biquadratic exchange interactions. In amorphous alloys, ferromagnetic order is found for large Gd concentrations (x ≥ 40). Below the critical concentration x_cr, in the range 30<x_cr<40, properties typical for magnetic cluster glasses are observed. Magnetic hyperfine fields B_hf at ¹⁵⁵Gd nuclei vary with x in opposite directions in amorphous and in crystalline alloys. In crystalline alloys, the variation is due to a positive transferred hyperfine field. In amorphous alloys, a reduction of |B_hf| with decreasing Gd concentration is caused by a reduction of the frozen Gd moments in the cluster glass phase.     

NMR study of Laves phase (Mg1−xRx)Co2 compounds

Nuclear magnetic resonances of ⁵⁹Co nuclei in (Mg_1−xR_x)Co₂ (R = Y, Gd) compounds at 4.2 K have been observed for 0x1. By replacing Mg with R, the ⁵⁹Co hyperfine field decreases or is nearly constant in the C14 range and increases in the C15 range. These results are explained by the difference of the contribution from R atoms.