Nanostructured Silicon Germanium Research Articles

Spark plasma assisted in-situ phase evolution and densification of nano-crystalline magnesium silicide – silicon germanium thermo-electric composite: Pulse current effects and densification mechanisms

The current work reports on the investigation of mechanisms involved during spark plasma assisted in-situ nano-crystalline magnesium silicide (Mg2Si) phase evolution and the simultaneous densification with the substantial increase of nano-structured silicon-germanium alloy (SiGe) inclusion. The anisothermal sintering was carried out up to 650°C with an applied pressure of 50MPa in the vacuum atmosphere (~10Pa). X-ray diffraction and electron microscopy studies conceded the formation of Mg2Si matrix phase and presence of SiGe inclusion. The synthesized thermo-electric composite demonstrated the superior density (>95%). The synthesized TE composites shows the benchmark hardness values >0.97GPa.

Role of Nanoscale Precipitates for Enhancement of Thermoelectric Properties of Heavily P-Doped Si-Ge Alloys

Thermoelectric (TE) devices can save energy by generating electricity from waste heat. For industrial application of TE power generation, TE materials with high conversion efficiency and that are non-toxic and inexpensive are required. The nanostructured silicon-germanium (Si-Ge) alloy is one possible candidate for such TE materials. Nanostructuring is an effective way to improve the conversion efficiency of materials because it dramatically reduces the lattice thermal conductivity (κlat). Here, we experimentally demonstrate effective phonon blocking without carrier mobility deterioration in bulk Si-Ge alloys containing phosphorous-rich nanoscale precipitates connected coherently or semi-coherently with the matrix phase. When the Ge content was less than 5%, the nanoscale precipitates effectively scattered heat-carrying phonons, leading to a sufficient reduction in κlat. However, at higher Ge content compositions, phonon scattering by Ge substitution with Si was more predominant than phonon scattering by nanoscale precipitates for the reduction of κlat. As a result, significant enhancement of zT was achieved at low Ge contents.

Lattice dynamics and thermoelectric properties of nanocrystalline silicon–germanium alloys

AbstractThe lattice dynamics and thermoelectric properties of sintered phosphorus‐doped nanostructured silicon–germanium alloys obtained by gas‐phase synthesis were studied. Measurements of the density of phonon states by inelastic neutron scattering were combined with measurements of the elastic constants and the low‐temperature heat capacity. A strong influence of nanostructuring and alloying on the lattice dynamics was observed. The thermoelectric transport properties of samples with different doping as well as samples sintered at different temperature were characterized between room temperature and C. A peak figure of merit at C is observed and is comparatively insensitive to the aforementioned parameter variations.

SEMICONDUTTORI NANOSTRUTTURATI PER APPLICAZIONI TERMOELETTRICHE: UN CONCETTO INNOVATIVO DIMOSTRATO AL COMPUTER

I will discuss the thermal transport properties of nanostructured silicongermanium alloys for thermoelectric applications. By means of atomistic simulations, I will show that it is in fact possible to design a nanocomposite with optimal thermoelectric performances, i.e. maximizing its figure of merit. In particular, computer experiments provide quantitative evidence that the phonon flux from hot to cold regions (phonons are the microscopic heat carriers) is dramatically affected by the network of grain boundaries present in the nanocomposite

Effects of germanium incorporation on optical performances of silicon germanium passive devices for group-IV photonic integrated circuits

Optical interconnect in integrated optoelectronic circuits is one of the promising next-generation technologies for replacing metalized interconnect. Efforts have been made to use silicon (Si)-compatible materials such as germanium (Ge) and Ge-buffered III–V compound semiconductors, along with Si, as optical sources for Si and group-IV integrated optoelectronic systems. This opens the possibility that higher fraction of Ge with its high refractive index (n) can be incorporated in Si waveguide for optical interconnect and the graftability between Si and group-IV or III–V materials would be improved in silicon photonics. In this work, advantageous features of nano-structured silicon germanium (Si1−xGex) optical waveguide with different Ge fraction (x) were evaluated by both optical simulations and theoretical calculations, which are mainly found in the enhanced optical confinement and better interfacing capability. Along with the SiGe waveguide, performance of Si1−xGex microring resonator under material loss in the effect of extinction coefficient (k) has been investigated to suggest the necessity of optimizing the Ge content in Si1−xGex passive devices. While carrying out the establish design criteria, n and k have been modelled in closed-form functions of Ge fraction at 1550nm. Furthermore, by examining high-resolution transmission electron microscopy (HR-TEM) images, process compatibility of Ge with either group-IV alloys or III–V compound semiconductors is confirmed for the monolithically integrated photonic circuits.

The effect of phase heterogeneity on thermoelectric properties of nanostructured silicon germanium alloy

Detailed examination of the nanostructured bulk Si0.80Ge0.20 alloy synthesized by mechanical alloying and hot-press methods revealed that the alloy composition can unintentionally deviate from its nominal value. The phase deviation is difficult to be detected with x-ray diffraction due to the continuous solid solution characteristics of the Si-Ge alloy. Differential thermal analysis, in particular, showed that the synthesized nanostructured bulk Si0.80Ge0.20 alloy was a composition of two unintentional phases. The dominant phase was Si0.88Ge0.12 with admixture of Si0.58Ge0.42 in a much lower concentration. The two-phase structure is difficult to be detected in X-ray diffraction analysis and is often neglected. Thermoelectric properties of Si1−xGex significantly depend on the Ge content in the synthesized alloy. The thermoelectric properties of the synthesized material were studied experimentally and theoretically. The comparison of the data of the mixed phase nanostructured alloy with those of the single phase Si0.80Ge0.20 alloy showed enhancement in Seebeck coefficient and reduction in thermal conductivity of the former material. It was found using model calculations that these differences are due to the existence of the Si0.88Ge0.12 phase in the two-phase structure that results in the reduction of the bipolar diffusion part of the thermal conductivity and the bipolar effect in the Seebeck coefficient at high temperature. The results can stimulate a new route for enhancing the thermoelectric properties of silicon germanium alloy based on multicomponent material design.

Comparison of boron precipitation in p-type bulk nanostructured and polycrystalline silicon germanium alloy

Boron precipitation process and its effect on electronic properties of p-type bulk nanostructured silicon germanium (Si0.8Ge0.2) compared with large grain polycrystalline Si0.8Ge0.2 have been studied. The structures were synthesized and their thermoelectric properties were measured versus temperature during heating and cooling cycles. The experimental data showed stronger temperature variation of Seebeck coefficient, carrier concentration, and conductivity in the nanostructured Si0.8Ge0.2 compared with the polycrystalline form indicating stronger boron precipitation in this structure. The electrical properties of both samples were calculated using a multi-band semi-classical model. The theoretical calculations confirm that the increase of boron precipitation in the nanostructured Si0.8Ge0.2 is responsible for its higher thermal instability. Since the thermoelectric properties of the nanostructured sample degrade as a result of thermal cycling, the material is appropriate only for continuous operation at high temperature without cooling.

High-temperature elastic moduli of bulk nanostructuredn- andp-type silicon germanium

Resonant ultrasound spectroscopy (RUS) has been used to measure the elastic moduli of $n$- and $p$-type doped polycrystalline bulk nanostructured silicon germanium alloys at elevated temperatures. A direct contact RUS transducer system with a working temperature range up to 900 K was successfully constructed for these measurements. For higher temperatures (up to 1300 K), we employed a traditional buffer rod RUS system. Experimental results show the Young's and shear moduli of $p$-type SiGe alloys monotonically decrease with increasing temperatures in the 300--1200 K range. The $n$-type samples show a marked stiffening beginning at 675 K which does not repeat upon cooling or subsequent reheating. We attribute the stiffening of the $n$-type samples to the thermally activated precipitation of the phosphorous dopant. Electrical resistivity and Seebeck coefficient data are also presented for both types of SiGe which support this conclusion.