Investigating the spectra of electron spin resonance in SiGe/Si heterolayers doped with phosphorous

Studies of the spin resonance spectra of electrons localized on donors and the conduction electrons in Si_{1 ? x} Ge _x layers grown on silicon show that the phosphorous atoms in a SiGe layer can have two positions in the lattice. The line with g ～ 1.998 refers to the electron localized in the phosphorous atom with a predominantly silicon environment; the line with g ～ 1.994 is observed when there is a substantial concentration of germanium in the phosphorous environment.     

Shallow-impurity-assisted transitions in the course of submillimeter magnetoabsorption of strained Ge/GeSi(111) quantum-well heterostructures

The submillimeter (f=130–1250 GHz) magnetoabsorption spectra of strained Ge/GeSi(111) multilayer heterostructures with quantum wells are investigated at T=4.2 K upon band-gap optical excitation. It is found that the magnetoabsorption spectra contain lines associated with the excitation of residual shallow acceptors. The resonance absorption observed can be initiated by optical transitions between the impurity states belonging to two pairs of Landau levels of holes in germanium quantum-well layers.     

Effect of the composition of the solid solution on the high-temperature microhardness of SiGe heteroepitaxial layers grown on Ge and Si substrates

The effect of the composition of epitaxial layers (ELs) of the Si_xGe_1?x solid solution grown on Ge and Si substrates on their microhardness and the length of dislocation rosettes forming around indentations is studied at a homologous temperature 0.5T_melt for each composition. For the Si_xGe_1?x/Ge (0≤x<0.15) and Si_xGe_1?x/Si (0.85<x≤1) ELs, the dependences of the microhardness and the length of dislocation rosettes on the solid-solution composition are nonmonotonic. The nonmonotonic change in the plasticity of the ELs is most likely caused by hardening of the solid solutions in a certain composition range due to their spinodal decomposition with the formation of clusters and disperse precipitates.     

Electron gas induced in SrTiO<Subscript>3</Subscript>

This mini-review is dedicated to the 85th birthday of Prof. L.V. Keldysh, from whom we have learned so much. In this paper, we study the potential and electron density depth profiles in surface accumulation layers in crystals with a large and nonlinear dielectric response such as SrTiO₃ (STO) in the cases of planar, spherical, and cylindrical geometries. The electron gas can be created by applying an induction D₀ to the STO surface. We describe the lattice dielectric response of STO using the Landau–Ginzburg free energy expansion and employ the Thomas–Fermi (TF) approximation for the electron gas. For the planar geometry, we arrive at the electron density profile n(x) ∝ (x + d)^–12/7, where d ∝ D₀^–12/7. We extend our results to overlapping electron gases in GTO/STO/GTO heterojunctions and electron gases created by spill-out from NSTO (heavily n-type doped STO) layers into STO. Generalization of our approach to a spherical donor cluster creating a big TF atom with electrons in STO brings us to the problem of supercharged nuclei. It is known that for an atom with a nuclear charge Ze where Z > 170, electrons collapse onto the nucleus, resulting in a net charge Zn < Z. Here, instead of relativistic physics, the collapse is caused by the nonlinear dielectric response. Electrons collapse into the charged spherical donor cluster with radius R when its total charge number Z exceeds the critical value Z_c ≈ R/a, where a is the lattice constant. The net charge eZ_n grows with Z until Z exceeds Z* ≈ (R/a)^9/7. After this point, the charge number of the compact core Z_n remains ≈ Z*, with the rest Z* electrons forming a sparse TF atom with it. We extend our studies of collapse to the case of long cylindrical clusters as well.     

Cyclotron resonance of holes in silicon in quantizing magnetic fields

The cyclotron resonance spectra of holes in bulk silicon in quantizing magnetic fields are investigated in the low-temperature range. The data obtained agree well with the results of the numerical calculation performed earlier by Owner-Petersen and Samuelsen for effective cyclotron masses m*/m₀ and matrix elements M upon transitions between different Landau levels of holes in silicon with a magnetic-field orientation H ∥ [001].     

Multicomponent Electron–Hole Liquid in Si/SiGe Quantum Wells

The density functional theory is used to calculate the energy of an electron–hole liquid in Si/Si_1–xGe_x/Si quantum wells. Three one-dimensional nonlinear Schrödinger equations for electrons and light and heavy holes are solved numerically. It is shown that, in shallow quantum wells (small x), both light and heavy holes exist in the electron–hole liquid. Upon an increase in the Ge content, a transition to a state with one type of holes occurs, with the equilibrium density of electron–hole pairs decreasing by more than a factor of 2.     

Specific features of dielectric anomalies in Pb<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Te(Ga) near a ferroelectric phase transition

The dielectric properties of Pb_{1 ? x} Ge _x Te(Ga) (x = 0.02, 0.03, 0.05) were studied in the temperature range 77–150 K at frequencies of 10⁴–10⁶ Hz. It is revealed that the ferroelectric phase transition temperature T _c and the permittivity ? of Pb_{1 ? x} Ge _x Te(Ga) increase substantially with the Ge content. The temperature dependence of the permittivity of Pb_{1 ? x} Ge _x Te shows two peaks; the main peak is at the ferroelectric phase transition temperature T _c , and an additional peak is at T ₁ > T _c .     

Fabrication of composite based on GeSi with Ag nanoparticles using ion implantation

Comparative analysis of the structural and optical properties of composite layers fabricated with the aid of implantation of single-crystalline silicon (c-Si) using Ge⁺ (40 keV/1 × 10¹⁷ ions/cm²) and Ag⁺ (30 keV/1.5 × 10¹⁷ ions/cm²) ions and sequential irradiation using Ge⁺ and Ag⁺ ions is presented. The implantation of the Ge⁺ ions leads to the formation of Ge: Si fine-grain amorphous surface layer with a thickness of 60 nm and a grain size of 20–40 nm. The implantation of c-Si using Ag⁺ ions results in the formation of submicron porous amorphous a-Si structure with a thickness of about 50 nm containing ion-synthesized Ag nanoparticles. The penetration of the Ag⁺ ions in the Ge: Si layer stimulates the formation of pores with Ag nanoparticles with more uniform size distribution. The reflection spectra of the implanted Ag: Si and Ag: GeSi layers exhibit a sharp decrease in the intensity in the UV (220–420 nm) spectral interval relative to the intensity of c-Si by more than 50% owing to the amorphization and structuring of surface. The formation of Ag nanoparticles in the implanted layers gives rise to a selective band of the plasmon resonance at a wavelength of about 820 nm in the optical spectra. Technological methods for fabrication of a composite based on GeSi with Ag nanoparticles are demonstrated in practice.     

Computation of the electronic structure and direct-gap absorption spectra in Ge-rich Si<Subscript>1−x</Subscript> Ge<Subscript>x</Subscript>/Ge/Si<Subscript>1−x</Subscript>Ge<Subscript>x</Subscript> type-I quantum wells

This paper presents the results of conduction band discontinuities calculation for strained/relaxed Si_1?x Ge _x /Si_1?y Ge _y heterointerfaces in Γ _15C , Γ _2′C and L upper bands minima, as well as the room-temperature strained (vs. relaxed) band gaps deduced from the classical model-solid theory. Based upon the obtained data, we propose a type-I W-like Si_1?y Ge _y /Si_1?x Ge _x /Ge/Si_1?x Ge _x /Si_1?y Ge _y quantum wells heterostructure optimized in terms of compositions and thicknesses. Electronic states and wave functions are found by solving Schrödinger equation without and under applied bias voltage. An accurate investigation of the optical properties of this heterostructure is done by calculating the energies of the interband transitions and their oscillator strengths. Moreover, a detailed computation of the bias-voltage evolution of the absorption spectra is presented. These calculations prove the existence of type-I band alignment at Γ _2′C point in compressively strained Ge quantum wells grown on relaxed Ge-rich Si_1?y Ge _y buffers. The strong absorption coefficient (> 8 × 10³ cm^-1) and the large Stark effect (0.1 eV @ 2 V) of the Γ _2′C transitions thresholds open up perspectives for application of these heterostructures for near-infrared optical modulators.     

Structure and physical properties of titanium diselenide intercalated with nickel

The structure, electrical resistivity, thermopower, and magnetic susceptibility of titanium diselenide intercalated with nickel (N_xTiSe₂) are studied systematically in the nickel concentration range x=0–0.5. In accordance with a model proposed earlier, strong hybridization of the Ni3d/Ti3d states is observed, giving rise to suppression of the magnetic moment because of delocalization of the nickel d electrons. It is shown that the strain caused by the Ni3d/Ti3d hybridization does not change the local coordination of a titanium atom.     

Spin-wave resonance in Ge<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> films exhibiting percolation ferromagnetism

Magnetic properties of Ge_{1 ? x} Mn _x (x = 0.02, 0.04, 0.08) thin films obtained by ion-implanting Mn⁺ ions into single-crystal Ge plates are investigated. The contributions of the subsystem of dispersed Mn²⁺ ions, Ge³Mn⁵ ferromagnetic clusters, and Mn-enriched ferromagnetic domains of Ge to the magnetic moment of Ge_{1 ? x} Mn _x films are distinguished. In the subsystem of dispersed Mn²⁺ ions in Ge_{1 ? x} Mn _x films at temperatures below 10 K, a spin-wave resonance is observed in the magnetically ordered state due to percolation ferromagnetism. It is established that, in the films with percolation ferromagnetism, the exchange integrals determined from static measurements correspond to those determined by dynamic measurements.     

Parameters of the potential well of an off-center Ge atom in a GeTe-SnTe solid solution

An method is proposed for determining the shape of the three-dimensional multiwell potential of an off-center atom from EXAFS data. The parameters of the potential well of a Ge atom in GeTe and Sn_1?x Ge _x Te (x ≥ 0.4) are determined in the classical and quantum-mechanical approximations. The potential-well depth is varied in the interval 20–40 meV depending on the Ge content, which indicates that the phase transition in these crystals is intermediate in character between the displacement and order-disorder transitions. From analyzing the conditions for the applicability of the classical approximation, it follows that quantum effects must be taken into account in determining the parameters of the potential well of an off-center Ge atom in Sn_1?x Ge _x Te. Quantum-mechanical calculations show that the energy of the lower level in the vibration spectrum of the Ge atom coincides with the maximum energy in the potential well to within several millielectronvolts. The high probability of tunneling or an over-barrier transition of the off-center atom between the potential-well minima prevents dipole reorientations from being frozen at low temperatures.     

An optical study of vacuum evaporated amorphous Ge<Subscript>20</Subscript>Te<Subscript>80−<Emphasis Type="Italic">x</Emphasis></Subscript>Bi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> thin films using transmission and reflection spectra

The present paper reports the effect of Bi addition on the optical behavior (optical band gap and refractive index) of Ge₂₀Te_80?x Bi _x (where x=0, 1.5, 2.5, 5.0) glassy alloys by analyzing the transmission and reflection spectra of their thin films in the 900–2400 nm range. Films are deposited on glass substrate using a thermal evaporation technique under vacuum. Various optical parameters viz. refractive index, extinction coefficient, absorption coefficient, optical band gap, etc. are determined and the effect of Bi incorporation on these parameters is studied. The refractive index has been found to increase with increasing Bi content over the entire spectral range and this behavior is due to the increased polarizability of the larger Bi atomic radius (1.46 Å) compared to Te atomic radius (1.36 Å). Dispersion energy, E _d , average energy gap, E ₀ and static refractive index, n ₀ is calculated using Wemple–DiDomenico model. Optical band gap is estimated using Tauc’s extrapolation and is found to decrease from 0.86 to 0.73 eV with the Bi addition. This behavior of the optical band gap is interpreted in terms of the electronegativity difference of the atoms involved and the cohesive energy of the system.     

Molecular Beam Epitaxy of Si–Ge–Sn Heterostructures for Monolithically Integrated Optoelectronic Devices Based on Silicon

Elastically strained metastable Ge_1–xSn _x layers with molar fractions of tin of up to 0.15 are grown on Si (001) substrates. To analyze the optical properties of the samples, photoluminescence (PL) spectra are measured at room temperature and IR transmission spectra are measured at the liquid helium temperature. The room temperature direct intrinsic absorption edge at 0.71–0.72 eV is visible in the spectra of the studied structures with tin contents of 12–13%.     

Shallow acceptor levels in Ge/GeSi heterostructures with quantum wells in a magnetic field

Shallow acceptors in Ge/GeSi heterostructures with quantum wells are studied theoretically and experimentally in the presence of a magnetic field. It is shown that, in addition to the cyclotron resonance lines, magnetoabsorption spectra reveal transitions from the acceptor ground state to excited states related to Landau levels from the first and second confinement subbands, as well as the resonances caused by ionization of A⁺ centers.     

Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript>/Si(001) relaxed buffer layers grown by chemical vapor deposition at atmospheric pressure

Relaxed step-graded buffer layers of Si_1?xGe_x/Si(001) heterostructures with a low density of threading dislocations are grown through chemical vapor deposition at atmospheric pressure. The surface of the Si_1?xGe_x/Si(001) (x ～ 25%) buffer layers is subjected to chemical mechanical polishing. As a result, the surface roughness of the layers is decreased to values comparable to the surface roughness of the Si(001) initial substrates. It is demonstrated that Si_1?xGe_x/Si(001) buffer layers with a low density of threading dislocations and a small surface roughness can be used as artificial substrates for growing SiGe/Si heterostructures of different types through molecular-beam epitaxy.     

The proximity effect in an Fe-Cr-V-Cr-Fe system

The proximity effect was studied in a thin-film Fe-Cr-V-Cr-Fe layered system. As the chromium layer thickness (d_Cr) increases at a fixed thickness of iron layers (d_Fe), the dependence of the superconducting transition temperature (T_c) on d_Cr exhibits a maximum at d_Cr ? 40 Å followed by a sharp decrease. Investigation of the dependence of T_c on d_Fe at a fixed d_Cr showed that the depth of penetration of the Cooper pairs into a chromium layer does not exceed 40 Å. Analysis of the results obtained suggests that, at d_Cr ? 40 Å, chromium layers exhibit the transition from a nonmagnetic state to an incommensurate spin density wave state.     

High-frequency susceptibility and ferromagnetic resonance in multidomain thin films of the yttrium-iron garnet type

The field dependence of the high-frequency susceptibility and the ferromagnetic resonance were experimentally studied in a thin (d≈0.1 µm) (111)-oriented single-crystal film of substituted yttrium-iron garnet with the factor q?1. It was shown that the anomaly in the high-frequency susceptibility observed in a magnetic field H parallel to the normal to the film surface in the magnetization saturation region (H≈H_s) has a dual nature; more specifically, this anomaly is associated with an abrupt collapse of the stripe domain structure and a ferromagnetic resonance in the experimental configuration H ∥ [111] and h ⊥ H. In this case, the film transition from the inhomogeneous multidomain state to the homogeneous (single-domain) state at the point H≈H_s has no indications of a second-order phase transition. The experimental frequency-field dependence of ferromagnetic resonance (FMR) in the sample under study, having a characteristic minimum at the point ω₀=5 MHz and H_FMR=H_s, agrees qualitatively and quantitatively with calculations. The influence of the cubic magnetic anisotropy and the film thickness on the FMR spectrum and the orientation of the spontaneous magnetization in domains with respect to the film plane in the zero field H was theoretically studied.     

Magnetic-field-induced slowing-down of molecular rotation in C<Subscript>60</Subscript> crystals

The molecular dynamics of C₆₀ crystals was studied by inelastic neutron scattering at T=290 K, i.e., above the first-order phase transition temperature (T_C≈260 K), in the region of free C₆₀-spheroid rotation in the lattice. The energy broadening of the original neutron spectrum 2Γ₀≈0.1 meV for a momentum transfer q=2 Å^?1 is in agreement with NMR data on the rotational relaxation time of the molecule τ～10^?11 s～ ?Γ₀. This effect was observed to decrease in magnetic fields H=2.5–4.5 kOe applied along the scattering vector: Γ_H=0.7Γ₀. The slowing-down of the molecular rotation is discussed in connection with the interaction of a magnetic field with the molecular currents, which fluctuate when the C₆₀ cage rotates.     

Thermopower in the region of hopping conduction in TlNiS<Subscript>2</Subscript>

Samples of the composition TlNiS₂ in the hexagonal system with the unit cell parameters a=12.28 Å, c=19.32 Å, and ρ=6.90 g/cm³ are synthesized. The results of the investigation into the electrical and thermoelectrical properties of TlNiS₂ samples in the temperature range 80–300 K indicate that TlNiS₂ is a p-type semiconductor. It is found that, at temperatures ranging from 110 to 240 K, TlNiS₂ samples in a dc electric field possess variable-range-hopping conduction at the states localized in the vicinity of the Fermi level. The density of localized states near the Fermi level is determined to be N_F=9×10²⁰ eV^?1 cm^?3, and the scatter of the states is estimated as J≈2×10^?2 eV. In the temperature range 80–110 K, TlNiS₂ exhibits activationless hopping conduction. At low temperatures (80–240 K), the thermopower of TlNiS₂ is adequately described by the relationship α(T)=A+BT, which is characteristic of the hopping mechanism of charge transfer. In the case when the temperature increases to the temperature of the onset of intrinsic conduction with the activation energy ΔE=1.0 eV, there arise majority intrinsic charge carriers of both signs. This leads to an increase in the electrical conductivity σ and, at the same time, to a drastic decrease in the thermopower α; in this case, the thermopower is virtually independent of the temperature.