Toward a Germanium Laser for Integrated Silicon Photonics

It has been demonstrated theoretically and experimentally that germanium, with proper strain engineering and n-type doping, can be an efficient light emitter and a gain medium at its direct bandgap within the third optical communication window ($sim$1520–1620 nm). In this paper, we systematically discuss the effect of strain, doping, and temperature on the direct-gap optical gain in germanium. For electrically pumped devices, properties and design guidelines of Ge/Si heterojunction are also analyzed and compared with the results from fabricated Ge/Si heterojunction LEDs.      

Germanium: The semiconductor comeback material

In this paper, Ge the comeback semiconductor material is discussed. It is used as an enabling material for the next generation of device technology primarily due to its high carrier mobility. Ge, along with other high-mobility such as III/V compounds, are projected to be the the new range of semiconductor material. The technology for the growth of Ge and (In) GaAs active areas on an Si substrate must be developed     

Numerical simulation of electrical characteristics in nanoscale Si/GaAs MOSFETs

In this paper, electrical characteristics of metal-oxide-semiconductor field effect transistor (MOSFET) with silicon/gallium-arsenic (Si/GaAs) stacked film are numerically studied. By calculating several important device characteristics, such as the on-state current, the subthreshold swing, the drain induced barrier lowering, the threshold voltage, the threshold voltage roll-off, and the output resistance, a 50 nm Si/GaAs MOSFET is simulated with respect to different thicknesses of Si/GaAs film. Compared with the results of pure Si MOSFET, Si/GaAs MOSFET shows promising characteristics after properly selecting the thickness of Si/GaAs film. Among Si, germanium (Ge), and Si/Ge MOSFETs, Si/GaAs MOSFET relatively exhibits a higher driving capability due to higher carrier mobility within the Si/GaAs film. However, quantitatively accurate estimation of device characteristics will depend upon more precise calculation of band structure of the stacked film.     

Advanced electro-optical simulation of nanowire-based solar cells

The improvement of solar cell efficiency requires device optimization, including the careful design of contacts and doping profiles, and the development of light trapping strategies. In this context, electro-optical numerical simulation is essential to analyze the physical mechanisms that limit the cell efficiency and lead to design trade-offs. In this work we discuss the application of advanced electro-optical simulation to the analysis of nanowire-based solar cells. We demonstrate the possibility to combine two numerical tools to perform the electro-optical simulation in order to investigate critical issues and potentialities of nanowires for photovoltaic applications. Thanks to the adopted simulation methodology, requiring relatively low computational resources, analyses involving extended ranges of geometrical and physical parameters are performed. Nanowire-based (NW) solar cells are expected to outperform the thin-film counterparts in terms of optical absorptance. In this theoretical study we optimize the geometry of vertical crystalline–amorphous silicon core-shell NW arrays on doped ZnO:Al glass substrate by means of 3-D optical simulations in order to maximize the photon absorption. The optimized geometry is then analyzed by means of 3-D TCAD electrical simulation in order to calculate the ultimate efficiency and the main figures of merit. We show that optimized crystalline–amorphous silicon core-shell (c-Si/a-Si/AZO/Glass) NWs featuring height in the micrometer range can reach photogenerated current up to 22.94 mA/cm², approximately 40 % larger than that of the planar counterpart with the same amount of absorbing material, and maximum conversion efficiency close to 14 %.     

Study of performance and leakage currents in nanometer-scale bulk,SOI and double-gate MOSFETs

We consider the performance and leakage issues in 80-to-20 nm Ge, Si, and InGaAs bulk, SOI and Double-gate (DG) devices. The performance is studied with DAMOCLES, band-to-band tunneling processes with a nonlocal band-to-band model as a post-processor in DAMOCLES, and gate tunneling by a Green’s function method accounting also for image force effects, so far ignored at ‘internal’ interfaces. The performance is affected by the bottleneck effect in III–Vs, especially for thin channels, but InGaAs and Ge still may be optimized to outperform Si. Zener leakage is high for Ge and tolerable for InGaAs. The effect of image forces led to an order of magnitude increase in gate tunneling currents.     

碳包覆硅/碳复合材料的制备与性能研究

为提高锂离子电池商容量Si/C复合负极材料的电化学性能,采用喷雾干燥法制备了核壳结构的碳包覆Si/C复合材料.碳包覆Si/C复合材料为近球形颗粒,形貌规整,粒度分布均匀,呈正态分布,其物相结构和嵌脱锂的电化学反应与Si/C复合材料保持一致.碳包覆后,减小了充放电过程中复合材料电极的极化,电压滞后现象得到了显著的改善.碳包覆Si/C复合材料的最大放电比容量为512 mAh/g,略低于包覆前的材料,但循环稳定性大大提高,50次循环后的容量保持率为96%.     

Piezoelectricity in Poled Silica Films with Tetravalent Metal Dopants

Piezoelectricity was produced in silica films with tetravalent metal dopants by poling. Poling treatment in germanium-doped silica (Ge:SiO₂) glass films raises their of optical non-linearity and produces, among other things, the Pockels effect. We generated piezoelectricity in poled Ge:SiO₂ glass thin films. Tetravalent-metal-doped SiO₂ (M^{4 +}:SiO₂) films were prepared on Si substrates by RF magnetron sputtering. We used germanium, titanium, and tin as doping materials. The piezoelectricity of the films was compared with the piezoelectricity of quartz. Piezoelectricity of the same order of magnitude as that in quartz was observed in the M^{4 +}:SiO₂ films. However, less than a week later, the piezoelectricity disappeared almost completely in all the samples. To prevent the piezoelectricity from disappearing, we tried to pin the doping ions. We developed a pinning technique based on the structure of a Ge:SiO₂-Ti:SiO₂-Sn:SiO₂-super-lattice. This super-lattice structure was very effective in preventing the piezoelectricity from disappearing.     

Electron mobility in silicon and germanium inversion layers: The role of remote phonon scattering

We calculate the electron mobility in Si and Ge inversion layers in single-gate metal-oxide-semiconductor field effect transistors. Scattering with bulk phonons, surface roughness and remote phonons is included in the mobility calculations. Various high-κ dielectric materials are considered for both Si and Ge substrates. Overall, Ge outperforms Si, but in general Ge is more affected by the use of high-κ dielectrics. HfO₂ degrades the mobility substantially compared to SiO₂ for Si substrates and may prohibitively degrade performance. HfO₂ with Ge yields an improvement over Si with a mobility enhancement ≈3× at an electron sheet density of 1×10¹³ cm⁻³.     

Process modeling of stress and chemical effects in SiGe alloys using kinetic Monte Carlo

In state-of-the-art silicon based process technologies, strained and relaxed SiGe, strained-silicon layers, and process-induced stress are widely present. Based on a literature review, we developed and calibrated continuum and kinetic Monte Carlo process models for chemical and stress effects in SiGe (Zographos et al. in AIP Conf. Proc. 1496:212–216, 2012). In this paper, we explain in full detail the corresponding kinetic Monte Carlo models and calibration. The models take into account the effects on band gap, amorphization, recrystallization, point defect generation and diffusion, extended defect evolution, dopant diffusion and clustering, and dopant segregation. The influence of Ge concentration and strain profile on Si self-interstitials and vacancies properties are deducted from experimental data as well as from ab-initio studies. The {311} interstitial clusters are less stable in the presence of Ge or compressive hydrostatic pressure, and the transformation of {311} defects into dislocation loops is faster. The corresponding parameter adjustments have been calibrated based on experimental data generated within the ATOMICS research project. The effects of Ge and stress on dopant diffusion have been calibrated for boron, arsenic and phosphorus taking into account that in experiments using epitaxial layers of strained SiGe embedded in Si, or strained silicon embedded in relaxed SiGe, boron and phosphorus have been found to segregate at Si/SiGe interfaces.     

Kinetic Monte-Carlo simulations of germanium epitaxial growth on silicon

We present Kinetic Lattice Monte Carlo (KLMC) simulations of Ge deposition onto a reconstructed Si (100) surface. In addition to the anisotropy brought on by surface reconstruction, we take into account the role of the exchange of Ge with Si atoms in the substrate and how it effects the interface between the materials. One method of controlling the resulting structures from the growth process is to use a pre-patterned substrate. We present results where the initial structure is a grid pattern. The KLMC simulations in this case yield Ge-Si stripes, that result largely from the anisotropy generated from the surface reconstruction.     

Hole Transport in Orthorhombically Strained Silicon

Linear and nonlinear transport of holes in orthorhombically strained Si to be used in vertical p-MOSFETs is theoretically analyzed. Strong mobility enhancements compared to unstrained Si by up to a factor of three is found at a Ge content of 40% in the SiGe pillar. The anisotropy in the three Cartesian directions is rather small and the saturation velocity remains unchanged. The enhanced material properties make orthorhombically strained Si attractive for device applications, although the improvements are not as strong as for biaxial tensile strain.     

Simulations of Scaled Sub-100 nm Strained Si/SiGe p-Channel MOSFETs

This paper investigates scaled sub-100 nm strained Si channel p-type MOSFETs. For a 30–40% Ge content SiGe buffer, 1D Poisson-Schrödinger analysis indicates that the parasitic effects of the SiGe buffer are negligible in small devices with high n-type channel doping (>10¹⁷ cm^?3). The device published by IBM and calibrated by us has been scaled down to a 35 nm physical gate length and shows notable performance enhancement over the Si control MOSFET. Well-tempered MOSFET designs have also been adopted to study potential performance improvement associated with the introduction of a strained Si channel. These provide a performance improvement comparable with the scaled versions of the IBM devices for effective gate length down to 25 nm. Improved well engineering is required to suppress short channel effects during the scaling process.     

Simulation of inhomogeneous strain in Ge-Si core-shell nanowires

This paper studies the elastic deformation field in lattice-mismatched Ge-Si core-shell nanowires (NWs). Infinite wires with a cylindrical cross section under the assumption of translational symmetry are considered. The strain distributions are found by minimizing the elastic energy per unit cell using finite element method. This paper finds that the trace of the strain is discontinuous with a simple, almost piecewise variation between core and shell, whereas the individual components of the strain can exhibit complex variations. The simulation results are prerequisite of strained band structure calculation, and pave a way for further investigation of strain effect on the related transport property simulation.     

RF Performance of Strained SiGe pMOSFETs: Linearity and Gain

The RF performance of strained-SiGe pMOSFETs on SOI substrates has been investigated through the use of TCAD simulations. To optimize RF performance of strained-SiGe pMOSFETs, including intrinsic gain, linearity and g_m/I_d, we propose to vary the Ge concentration in the channel, shrink the SOI thickness and adopt an asymmetric doping profile along the channel. We find that neither strain nor the asymmetric doping approach is able to unlock the trade-off between intrinsic gain and linearity found in bulk and SOI relaxed Si MOSFETs. Instead, SOI layer thickness control provides an alternative approach to improving gain without sacrificing linearity. For optimized RF performance, the strained-SiGe pMOSFETs with high Ge concentrations (0.3 ≤ x ≤ 0.7) in the channel and thin SOI layers (< 20 nm) are preferred.     

Growth characteristics and film properties of gallium doped zinc oxide prepared by atomic layer deposition

We carried out comprehensive studies on structural, optical, and electrical properties of gallium-doped zinc oxide (Ga:ZnO) films deposited by atomic layer deposition (ALD). The gallium(III) isopropoxide (GTIP) was used as a Ga precursor, which showed pure Ga₂O₃ thin film with high growth rate. Using this precursor, conductive Ga doped ZnO thin film can be successfully deposited. The electrical, structural and optical properties were systematically investigated as functions of the Ga doping contents and deposition temperature. The best carrier concentration and transmittance (7.2?×?10²⁰ cm^?3 and 83.5 %) with low resistivity (≈3.5?×?10^?3?Ωcm) were observed at 5 at.% Ga doping concentration deposited at 250 °C. Also, low correlation of deposition temperature with the carrier concentration and film structure was observed. This can be explained by the almost same atomic radius of Ga and Zn atom.     

Numerical study of the thermoelectric power factor in ultra-thin Si nanowires

Low dimensional structures have demonstrated improved thermoelectric (TE) performance because of a drastic reduction in their thermal conductivity, ?? _l . This has been observed for a variety of materials, even for traditionally poor thermoelectrics such as silicon. Other than the reduction in ?? _l , further improvements in the TE figure of merit ZT could potentially originate from the thermoelectric power factor. In this work, we couple the ballistic (Landauer) and diffusive linearized Boltzmann electron transport theory to the atomistic sp³d⁵s*-spin-orbit-coupled tight-binding (TB) electronic structure model. We calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires (NWs). We describe the numerical formulation of coupling TB to those transport formalisms, the approximations involved, and explain the differences in the conclusions obtained from each model. We investigate the effects of cross section size, transport orientation and confinement orientation, and the influence of the different scattering mechanisms. We show that such methodology can provide robust results for structures including thousands of atoms in the simulation domain and extending to length scales beyond 10?nm, and point towards insightful design directions using the length scale and geometry as a design degree of freedom. We find that the effect of low dimensionality on the thermoelectric power factor of Si NWs can be observed at diameters below ??7?nm, and that quantum confinement and different transport orientations offer the possibility for power factor optimization.     

Synthesis of copper nanowires via a liquid phase reduction process

Copper nanowires (Cu NWs) were successfully synthesized via a method of liquid phase reduction by using hydrazine hydrate as reducing agent, ethylenediamine (EDA) as structure-directing agent. The impact of various factors on Cu NWs was analyzed by orthogonal experiment. Cu NWs were characterized by SEM, XRD, and TEM techniques. The results show that: the Cu NWs were free of oxidation, and were highly oriented along the (111) direction. The sequence of factors which impact the length and diameter of Cu NWs is: hydrazine hydrate - NaOH - EDA - reaction temperature. Considering all the factors, to synthesize Cu NWs, the optimal conditions are as follows: 4mL EDA, 50mL NaOH, 10 mL1% hydrazine hydrate, 10 mL 10% hydrazine hydrate, 80°C.     

Improvement of magnetic properties in Fe-based nanocrystalline alloys by addition of Si,Ge, C,Ga, P,Al elements and their applications

Magnetic properties of Fe-Cu-Nb-X-B (X Si, Ge, C, Ga) and Fe-Cu-Nb-Si-B-X (X: Ge, C, P, Ga, Al) nanocrystalline alloys were studied to improve soft magnetic properties and they were evaluated for some applications. Si was the best element for the Fe-Cu-Nb-X-B system. Substitution of P for B made the coercive force small in the Fe-Cu-Nb-Si-B-X system having high saturation flux density. The cores using the Fe-based nanocrystalline alloys showed low core loss, excellent dc superimposed characteristics and good noise attenuation characteristics. Hence, Fc-based nanocrystalline alloys have high performance for such magnetic components as transformers and choke coils.     

Quantum-Confined Stark Effect in Ge/SiGe Quantum Wells on Si

In this paper, we present observations of quantum confinement and quantum-confined Stark effect electroabsorption in Ge quantum wells with SiGe barriers grown on Si substrates. Though Ge is an indirect gap semiconductor, the resulting effects are at least as clear and strong as seen in typical III–V quantum well structures at similar wavelengths. We also designed and fabricated a coplanar high-speed modulator, and demonstrated modulation at 10 GHz and a 3.125-GHz eye diagram for 30-$mu$m-sized modulators.      

Modeling of the core-shell microstructure of temperature-stable BaTiO3 based dielectrics for multilayer ceramic capacitors

A simplified model was proposed for the core-shell microstructure, which existed in the temperature-stable BaTiO₃ (BT) based dielectrics for multilayer ceramic capacitors (MLCCs) and was regarded as the reason resulting in the temperature-stable characteristics. In this model the capacitance of a core-shell grain can be regarded as the parallel combination of the capacitance of the grain core and the grain shell. To verify the validity of the model, core material, shell material and core-shell material were prepared. Based on our previous work, BT with a grain size of 400 nm was chosen as the core material; X7R material milled for 1 hour was chosen as the core-shell material; doped BT milled for 36 h was chosen as the shell material. The calculated results showed good agreement with the measured experimental results of the core-shell material, which proved the validity of the model.