xGex Systems Research Articles

Electron-dependent thermoelectric properties in Si/Si1-xGex heterostructures and Si1-xGex alloys from first-principles

Unlike phononic thermal conductivity (which is shown in the literature to be reduced due to alloying and has a nearly constant value over a range of compositional variations), electron-dependent thermoelectric properties are shown here, from first-principles, to vary nonlinearly with composition. Of the Si/Si1−xGex systems considered, the maximum thermopower observed, which is 10% higher than that of crystalline Si, is obtained for a Si0.875Ge0.125 alloy. Also, heterostructuring is shown to reduce thermopower, electrical conductivity, and electron thermal conductivity. Additionally, neither Lorenz number nor Seebeck coefficient shows oscillations for heterostructures, regardless of electron/hole energies, contradicting the conclusions obtained with miniband approximations.

Phase relationships, and structural, magnetic, and magnetocaloric properties in the Ce5Si4–Ce5Ge4 system

The crystallography, phase relationships, and magnetic properties of the Ce5Si4−xGex alloys with 0≤x≤4 have been investigated by using x-ray powder diffraction and isothermal magnetization measurements. There are three different crystal structures in the Ce5Si4−xGex system: the Zr5Si4-type tetragonal structure with space group P41212 exists from 0≤x<2.15, the Gd5Si2Ge2-type monoclinic structure with space group P1121/a exists at x≈2.225, and the Sm5Ge4-type orthorhombic structure with space group Pnma is found for 2.4<x≤4. The magnetic ordering temperature increases when the tetragonal phase changes to the monoclinic phase, and then it remains composition independent throughout the orthorhombic phase, which is the opposite trend compared to that observed in the heavy lanthanide 5:4 compounds when Ge content increases. Another distinct difference is that Ce5Si4 exhibits an antiferromagnetic ground state while Ce5Ge4 phase orders ferromagnetically, which is reverse compared to the R5Si4−xGex systems where R=Gd and Tb. The magnetocaloric effect has been calculated from the magnetization data. The Ce5Ge4 has the maximum magnetic entropy change ΔSM(−11.6 J/kg K) at the Curie temperature of 11.5 K for a field change of 5 T.

Diffusion of beryllium in Ge and Si–Ge alloys

Diffusion of implanted Be7 in Si1−xGex (x=0.20,0.65,1.00) systems has been studied under intrinsic conditions in the temperature range of 460–720°C by the modified radiotracer technique. Arrhenius-type behavior with activation enthalpies of 2.0eV for Ge and 2.5eV for the Si–Ge alloys was noted. Unexpectedly, the diffusivity of beryllium is higher in the Si0.80Ge0.20 material than in Si0.35Ge0.65 which is discussed in terms of possible prevailing diffusion mechanisms. It is proposed that Be diffusion in Si1−xGex systems is dissociative mechanism dominated for germanium rich materials and the kick-out (or interstitialcy) mechanism dominates in silicon rich materials.

Elastic interactions and diffusion of Sn in Si1−xGex systems

Diffusion of Sn in relaxed Si1−xGex alloys has been studied systematically. In Si1−xGex, diffusion of Sn is faster than that of Si and Ge, and the results indicate that Sn diffusion in silicon and germanium is vacancy mediated. As Sn is isovalent, no long range Coulomb interactions exist between Sn atoms and native point defects. Since Sn atoms are larger than Si and Ge atoms, they cause stress in the SiGe lattice. The present findings are explained by attractive elastic interactions caused by pairing of Sn atoms with vacancies relaxing the stress and repulsive elastic interactions between Sn atoms and self-interstitials.

The interstitial CiOi defect in bulk Si and Si1−xGex

The equilibrium geometries, phonon spectra, electronic structures and optical properties of theCiOi defect inbulk Si and Si1−xGex systems are calculated using the ab initio plane wave density-functional method.We find that in a Ge-doped Si crystal it is more energetically favourable for thedefect to stabilize in a configuration with no Ge atoms in the first sphere.Our calculations show that the vibrational properties of the defect in theSi1−xGex alloy are similar to those in the pure Si bulk crystal and only one local vibrationalmode is sensitive to the presence of the Ge substitutions. The effect of C, O, Siand Ge isotopes on the phonon spectra are also investigated and found to bein good agreement with available experimental data. It also follows from ourcalculations of the singlet–triplet splitting and a position of the gap state of thedefect with respect to the top of the valence band that the optical energies of theCiOi defect are expected to increase due to Ge doping, which is in agreement with availableexperimental data.

High-resolution transmission electron microscopy of silicide formation and morphology development of Ni/Si and Ni/Si1−xGex

Nickel-silicide phase formation in the Ni/Si and Ni/Si1−xGex (x=0.20) systems and its correlation with variations in sheet resistance have been studied using high-resolution transmission electron microscopy (HRTEM) and related techniques. Following a 500°C anneal, uniform and low-resistivity NiSi and NiSi1−xGex (x<0.20) crystalline films were formed in the respective systems. Annealed at 900°C, NiSi2, in the form of pyramidal or trapezoidal islands, is found to replace the NiSi in the Ni/Si system. After a 700°C anneal, threading dislocations were observed for the first time in the Ni/Si1−xGex system to serve as heterogeneous nucleation sites for rapid lateral NiSi1−xGex growth.

Effects of composition on the formation temperatures and electrical resistivities of C54 titanium germanosilicide in Ti–Si1−xGex systems

The effects of alloy composition on the formation temperature and electrical resistivities of C54 titanium germanosilicide formed during the Ti/Si1−xGex (x=0, 0.3, 0.4, 0.7, 1) solid state reaction have been investigated. Ti5(Si1−yGey)3, C49– and C54–Ti(Si1−zGez)2 were observed to form in the Ti/Si1−xGex (x⩾0.4) systems. On the other hand, Ti6(Si1−yGey)5 and C54–Ti(Si1−zGez)2 were found in the Ti/Si1−xGex (x≧0.7) systems. For both cases, the relationship of x&amp;gt;y&amp;gt;z was found. The appearance and agglomeration temperature of low-resistivity C54–Ti(Si1−zGez)2 were both found to decrease with the Ge concentration. The resistivities of C54–Ti(Si1−zGez)2 were measured to be 15–20 μΩ/cm. The segregation of Si1−wGew (w&amp;gt;x) was found in all samples annealed above 800 °C. The effects of thermodynamic driving force, kinetic factor, and composition of the micro-area are discussed.

Modeling and characterization of a strained Si/Si1−xGex transistor with δ-doped layers

A two-dimensional model of a strained Si/Si1−xGex transistor with δ-doped layers was developed. A semiclassical drift diffusion model is used to study the effects of different conduction-band offsets and variation of the distance between the Si channel and an n-type δ-doped layer as well as the thickness of this δ-doped layer at room temperature. We found that a large conduction-band offset, or a large Ge concentration, confines electrons more strongly to the Si channel. These factors raise the drain current when the doping level per unit area is constant. The area between the Si channel and the δ-doped layer and the δ-doped layer itself forms a barrier to electrons donated by the donor atoms in the δ-doped layer. Hence, the smaller the distance between the Si channel and the δ-doped layer and the thinner the δ-doped layer, the larger the number of electrons in the Si channel. Through the present analysis, an optimum design concept is clarified for device applications of Si/Si1−xGex systems.

Compositonal dependence of sound velocity and elastic moduli of amorphous Se1−xGex systems: comparative study with percolation theory

Abstract A comparative study of thermal and elastic properties has been made on amorphous Se 1− x Ge x alloy by employing a dynamical treatment. It has been observed that all these properties show an increasing linear trend with the increase of germanium content in Se 1− x Ge x amorphous system and an excellent agreement with the experimental results. Further, the linear variation shows no anomaly in the elastic moduli–coordination number relation in the studied rigidity percolation threshold region ( m =2.4–2.6).

Kinetics of processes in the Ti–Si1−xGex systems

The kinetics of processes related to the formation of C49 and C54 Ti(Si1−yGey)2 germanosilicide phases in the two relaxed and strained Ti/Si1−xGex systems (x1=0.35 and x2=0.20) in the temperature range 600–800 °C are considered. These processes have been studied through Auger electron spectroscopy, secondary ion mass spectroscopy, x-ray diffraction, and Raman scattering spectroscopy supported by ion beam etching techniques. Si/Ge ‘‘intergrain’’ alloy has been found between the grains of the C49 or/and C54 phases, with a Ge-rich part Si1−zGez of z=2x–3x in the upper region. At higher temperatures, the Ge concentration in the Ge-rich alloy decreases and its volume increases. The temperature required for obtaining similar changes are higher when x2&amp;lt;x1. A kinetic electron-related model is proposed to explain the observed phenomena.

Theoretical investigation of CoSi2/Si1−xGex detectors: Influence of a Si tunneling barrier on the electro-optical characteristics

We present a theoretical investigation of the influence of a non-reacted Si layer on the transport and optical properties of CoSi2/Si1−xGex Schottky barrier diodes grown from Co/Si/Si1−xGex systems. The presence of this layer reduces the effect of the lowering of the Schottky barrier height which would be expected in a CoSi2/Si1−xGex. However, due to the small thickness of this Si layer, the charge carriers are able to tunnel through it. This tunneling process allows for a significant lowering of the Schottky barrier height and therefore an extension of the detection regime into the infrared.

Lattice dynamics and specific heat of Al-Si and Al-Ge solid solutions

A simplified treatment has been proposed to study quantitatively the lattice dynamics of Al-Si and Al-Ge alloy systems by solid-solutioning under pressure. The volume and electron density effects on the lattice dynamics of the pure constituents aluminium, silicon or germanium are considered, and the phonon dispersion relations of the local and band modes were obtained. Then, the concentration dependence of the local and band mode frequencies were calculated for the Al1−xSix and Al1−xGex systems. Using the local and band mode frequencies, the lattice specific heat at constant volume was determined theoretically, and results obtained for the temperature-dependent specific heat of matrix aluminium were found to be in good agreement with the experimental data. The concentration dependence of the specific heat could then be predicted quantitatively for Al1−xSix and Al1−xGex alloy systems.

The electronic theory of SiGe solid solutions

AbstractThe pseudopotential theory of covalent crystals is applied to Si1−xGex systems using the virtual crystal approximation, the pseudo alloy atom model, and the ordered zinc‐blende‐type lattice (Si0.5Ge0.5). Calculated values of the equilibrium point are in good agreement with the experimental lattice constants. But, at the low temperatures, the phase mixture Si1−x + Gex is favourable compared with the disordered solid solution.