Evolution of SiGe nanoclusters and micro defects in the Si1−xGex layer fabricated by two-step ion implantation and subsequent thermal annealing

The Si_1−xGe_x thin layer is fabricated by two-step Ge ion implantation into (0 0 1) silicon. The embedded SiGe nanoclusters are produced in the Si_1−xGe_x layer upon further annealing. The number and size of the nanoclusters changed due to the Ge diffusion during annealing. Micro defects around the nanoclusters are illustrated. It is revealed that the change of Si-Si phonon mode is causing by the nanoclusters and micro defects.     

Si–Si optical phonon behavior in localized Si clusters of Si<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript> alloy nanocrystals

Raman spectra acquired from Si _x Ge_1−x -nanocrystal-embedded SiO₂ films show dependence of the Si–Si optical phonon frequency on Si content. The frequency upshifts, and peak intensity increases as the silicon concentration increases. For a given Si content, the frequency remains unchanged with annealing temperature. Spectral analysis and density functional theory calculation reveal that the optical Si–Si phonon is related to the formation of localized Si clusters surrounded by Si/Ge atomic layers in the Si _x Ge_1−x nanocrystals and the intensity enhancement arises from the larger cluster size. The synergetic effect of surface tensile stress and phonon confinement determines the Si–Si optical phonon behavior.     

Si/SiGe量子级联激光器的能带设计

详细论述Si/SiGe量子级联激光器的工作原理，通过对比找到一组合适的Si，Ge和SiGe合金的能带参数，进而应用6×6 k·p方法计算了不同阱宽、不同Ge组分Si/Si_1-xGe_x/Si量子阱价带量子化的空穴能级本征值及其色散关系，分析Si/Si_1-xGe_x/Si量子阱空穴态能级间距随阱宽和组分的变化规律，最后应用计算结果讨论了Si/SiGe量子级联激光器有源区的能带设计，有益于优化Si /SiGe量子级联激光器结构. 关键词： 硅锗材料 量子级联激光器 子带跃迁 k·p方法')" href="#">k·p方法     

Subsurface localization of charge carriers in Si/SiO2/Si x Ge1 − x nanostructures

Analysis of the capacitance-voltage (C-V) characteristics reveals an elevated concentration of charge carriers under the surface of a silicon substrate due to the formation of elastically stressed regions induced in the substrate by Si _x Ge_{1 ? x} nanoislands grown on the preliminarily oxidized Si surface. The C-V characteristics exhibit charge density peaks at a depth from 700 to 1000 nm for Si/SiO₂/Si _x Ge_{1 ? x} structures with various thicknesses of the SiO₂ layer. The results of theoretical calculations of the electron density distribution in the bulk of the silicon substrate correspond on the whole to the C-V characteristics. The state of the interfaces in the structures with different thicknesses of the oxide layer, which determines the effects of surface and interfacial recombination as well as charge carrier scattering, is studied by analyzing the kinetics of decay of a photo-emf signal and the photo-emf distribution over the surface of the structure. The results can be used in the development of various devices based on SiGe with inclusions of oxide layers.     

四方晶系应变Si空穴散射机制

基于Fermi黄金法则及Boltzmann方程碰撞项近似理论, 推导建立了(001)弛豫Si_1-xGe_x衬底外延四方晶系应变Si空穴散射几率与应力及能量的理论关系模型, 包括离化杂质、声学声子、非极性光学声子及总散射概率(能量40 meV时)模型. 结果表明: 当Ge组分(x)低于0.2时, 应变Si/(001)Si_1-xGe_x材料空穴总散射概率随应力显著减小. 之后, 其随应力的变化趋于平缓. 与立方晶系未应变Si材料相比, 四方晶系应变Si材料空穴总散射概率最多可减小66%. 应变Si材料空穴迁移率增强与其散射概率的减小密切相关, 本文所得量化模型可为应变Si空穴迁移率及PMOS器件的研究与设计提供理论参考.     

应变Si/(001)Si1-xGex电子迁移率

依据离化杂质散射、声学声子散射和谷间散射的散射模型,在考虑电子谷间占有率的基础上,通过求解玻尔兹曼方程计算了不同锗组分下,不同杂质浓度时应变Si/(001)Si_1-xGe_x的电子迁移率.结果表明:当锗组分达到0.2时,电子几乎全部占据Δ₂能谷;低掺杂时,锗组分为0.4的应变Si电子迁移率与体硅相比增加约64%;对于张应变Si NMOS器件,从电子迁移率角度来考虑不适合做垂直沟道.选择相应的参数,该方 关键词： 电子谷间占有率 散射模型 锗组分 电子迁移率     

The progress towards terahertz quantum cascade lasers on silicon substrates

A review is presented of work over the last 10 years which has been aimed at trying to produce a Si‐based THz quantum cascade laser. Potential THz applications and present THz sources will be briefly discussed before the materials issues with the Si/SiGe system is discussed. Waveguide designs and waveguide losses will be presented. Experimental measurements of the non‐radiative lifetimes for intersubband transitions in Si_1‐xGe_x quantum wells will be presented along with theory explaining the important scattering mechanisms which determine the lifetimes. Examples of p‐type Si/SiGe quantum cascade designs with the experimental electroluminescence will be reviewed and examples of n‐type Si‐based designs will be presented. In the conclusion designs and structures will be discussed with the greatest potential to achieve an electrically pumped Si‐based THz laser.     

Probing of the multilayer Si/SiGe structure with a flux of nonequilibrium acoustic phonons

The Si/Si_0.8Ge_0.2/Si double quantum wells grown on a silicon substrate were probed in the reflection geometry with a flux of terahertz nonequilibrium acoustic phonons produced by the heat pulse technique. A comparison of the detected phonon arrival signals with those obtained under similar conditions from a silicon sample without a quantum well structure permits one to isolate the component due to the phonon reflection from quantum wells. This reflected signal was found to be strongly anisotropic.     

Evidence for a thermally reversing window in bulk Ge–Te–Si glasses revealed by alternating differential scanning calorimetry

Experiments on Ge₁₅Te_{85? x} Si _x glasses (2 ≤ x ≤ 12) using alternating differential scanning calorimetry (ADSC) indicate that these glasses exhibit one glass transition and two crystallization reactions upon heating. The glass transition temperature has been found to increase almost linearly with silicon content, in the entire composition tie-line. The first crystallization temperature (T _c1) exhibits an increase with silicon content for x < 5; T _c1 remains almost a constant in the composition range 5 < x ≤ 10 and it increases comparatively more sharply with silicon content thereafter. The specific heat change (ΔC _p) is found to decrease with an increase in silicon content, exhibiting a minimum at x = 5 (average coordination number, ?r? = 2.4); a continuous increase is seen in ΔC _p with silicon concentration above x = 5. The effects seen in the variation with composition of T _c1 and ΔC _p at x = 5, are the specific signatures of the mean-field stiffness threshold at ?r? = 2.4. Furthermore, a broad trough is seen in the enthalpy change (ΔH ^NR), which is indicative of a thermally reversing window in Ge₁₅Te_{85? x} Si _x glasses in the composition range 2 ≤ x ≤ 6 (2.34 ≤ ?r? ≤ 2.42).     

First-principles studies on Ti3Si0.5Ge0.5C2 under pressure

The structural, electronic and elastic properties of Ti₃Si_0.5Ge_0.5C₂ have been investigated by using the pseudopotential plane-wave method within the density-functional theory. Our calculated equation of state (EOS) is consistent with the experimental results. The density of states (DOS) indicates that Ti₃Si_xGe_1−xC₂ (x=0, 0.5, 1.0) are metallic, and these compounds have nearly the same electrical conductivity. The elastic constants for Ti₃Si_0.5Ge_0.5C₂ are obtained at zero pressure, which is compared to Ti₃SiC₂ and Ti₃GeC₂. We can conclude that Ti₃Si_0.5Ge_0.5C₂ is brittle in nature by analyzing the ratio between bulk and shear moduli. There appears to be little effect on the electronic and elastic properties with the Ge substitution to Si atoms in Ti₃SiC₂.     

Effect of surface roughness on thermal conductivity of VLS-grown rough Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanowires

The recent demonstration of thermal conductivity of rough electrolessly etched Si nanowire (Hochbaum et al., Nature, 451:163, 2008) attracted a lot of interest, because it could not be explained by the existing theory; thermal conductivity of rough Si nanowires falls below the boundary scattering of the thermal conductivity. However, nanoscale pores presented in the nanowires (Hochbaum et al., Nano Letters, 9:3550–3554, 2009) hinder one to be fully convinced that the surface roughness solely made a contribution to the significant reduction in thermal conductivity. In this study, we synthesized vapor–liquid–solid (VLS) grown rough Si_1−x Ge _x nanowire and measured and theoretically simulated thermal conductivity of the nanowire. The thermal conductivity of rough Si_0.96Ge_0.04 nanowire is an order of magnitude lower than that of bulk Si_0.96Ge_0.04 and around a factor of four times lower than that of smooth Si_0.96Ge_0.04 nanowire. This significant reduction could be explained by the fact that the surface roughness scatters medium-wavelength phonons, whereas the long-wavelength phonons are scattered by phonon boundary scattering, and the short-wavelength phonons are scattered by alloy scattering.     

Vibrational modes and phase transition of Si-Ge solid solution

We present the simplified treatment where the lattice vibrations of Si or Ge atoms in the Si-Ge solid solution are replaced with that of pure Si or Ge crystal at lattice constants of the alloy. Considering the volume effect on the force constants of the pure constituent, we obtain the phonon dispersion curves of the local and band modes for Si_0.91Ge_0.09 and Si_0.11Ge_0.89 systems and the concentration x-dependence of the local and band modes frequencies in the Si_1?xGe_x solid solutions. Then, from the calculation of the effective mode Grüneisen parameter γ_i for the average phonon modes in the Si_1?xGe_x systems, we obtain the predominant correlation between TA mode Grüneisen parameter γ^X_TA at the point X and the phase transition pressure P_t, and the softening of TA modes is related to the pressure-induced phase transition of the Si-Ge solid solution.     

Poly(vinylpyrrolidone)‐stabilized silver nanoparticles for strained‐silicon surface enhanced Raman spectroscopy

Poly(vinylpyrrolidone)‐stabilized silver nanoparticles deposited onto strained‐silicon layers grown on graded Si_1−xGe_x virtual substrates are utilized for selective amplification of the Si–Si vibration mode of strained silicon via surface‐enhanced Raman scattering spectroscopy. This solution‐based technique allows rapid, highly sensitive and accurate characterization of strained silicon whose Raman signal would usually be overshadowed by the underlying bulk SiGe Raman spectra. The analysis was performed on strained silicon samples of thickness 9, 17.5 and 42 nm using a 488 nm Ar⁺ micro‐Raman excitation source. The quantitative determination of strained‐silicon enhancement factors was also made. Copyright © 2011 John Wiley & Sons, Ltd.     

Potential of Si1−xGex alloys for Auger generation in highly efficient solar cells

Recent investigation on Si solar cells demonstrated the utility of Auger generation for the creation of more than merely one electron/hole pair per absorbed photon. The semiconductor Si requires a minimum photon energy of about 3.4 eV for this internal carrier multiplication. The current of a Si cell is therefore not significantly increased by Auger generation when the cell is illuminated by an air mass 1.5 spectrum, which contains only few photons with energies above 3.4 eV. Use of Si_1–x Ge _x alloys promises a lower onset energy. Unfortunately, incomplete data on band structures ofrandom Si_1–x Ge _x alloys preclude a detailed quantitative discussion of the full potential for these materials. Nevertheless, (i) analogies to our own quantum efficiency data from pure Si, (ii) the calculated band structure of the hypothetical,ordered zincblende type Si_0.5Ge_0.5 crystal, and (iii) optical data for Si_1–x Ge _x alloys indicate an optimum Ge content ofx=0.6 tox=0.7.     

First-principles calculations of the effect of Ge content on the electronic,mechanical and acoustic properties of Li17Si4-xGex

The electronic, mechanical and acoustic properties of Li₁₇Si_4-xGe_x (x = 0, 2.3, 3.08, 3.53, and 4) have been investigated by using first-principles calculations based on the density functional theory (DFT). The research shows that the bulk modulus B, Young's modulus E, shear modulus G, and hardness H_v gradually decrease with the increasing Ge content. Li₁₇Si_4-xGe_x have the brittle nature from the analysis of B/G ratio and Cauchy pressure. The maximum Young's moduli are all along [1 1 0] plane, and the sequence of degree of anisotropic property is Li₁₇Ge₄ > Li₁₇Si_0.48Ge_3.52 > Li₁₇Si_0.92Ge_3.08 > Li₁₇Si_1.7Ge_2.3 > Li₁₇Si₄. The analysis of acoustic velocity shows that all the sound velocities decrease with the increasing Ge content for Li₁₇Si_4-xGe_x (x = 0, 2.3, 3.08, 3.53, and 4), and the longitudinal wave along [111] direction is fastest for the studied compounds. Debye temperature Θ_D, v_t and v_l decrease with the increasing Ge content. The minimum thermal conductivity decreases with the increasing Ge content, and Li₁₇Si_4-xGe_x have low thermal conductivities and are not potential thermal conductors. The analysis of electronic properties indicates that Li₁₇Si_4-xGe_x have the metal nature and anisotropic electrical conductivity. The electric conduction is improved with the increasing Ge content.     

Local and quasi-local vibrational modes of Si in Ge

We report far infrared absorption spectra of crystalline Ge_0.89 Si_0.11 and Ge_0.84Si_0.16 at 300 and 77 K in the range 40–430 cm^-1. We have observed in these data several mass defect quasi-bound vibrational states and impurity induced one-phonon peaks. The results are interpreted with a mass defect Green's function calculation based on the density of phonon states of pure c-Ge.     

Oxidation process of SiGe on SOI substrates

The oxidation of SiGe film epitaxial grown on top of SOI wafers has been studied. These SiGe/SOI samples were oxidized at 700, 900, 1100 °C. Germanium atoms were rejected from SiGe film to SOI layer. A new Si_1−xGe_x (x is minimal) layer formed at SiGe/Si interface. As the germanium atoms diffused, the new Si_1−xGe_x (x is minimal) layer moved to Si/SiO₂ interface. Propagation of threading dislocation in SiGe film to SOI substrate was hindered by the new SiGe/Si interface. Strain in SOI substrate transferred from SiGe film was released through dislocation nucleation and propagation inner. The relaxation of SiGe film could be described as: strain relaxed through strain equalization and transfer process between SiGe film and SOI substrates. Raman spectroscopy was used to characterize the strain of SiGe film. Microstructure of SiGe/SOI was observed by transmission electron microscope (TEM).     

An effective compliant substrate for low-dislocation relaxed Si1-xGex growth

An effective compliant substrate for Si_1-xGe_x growth is presented. A silicon-on-insulator substrate was implanted with B and O forming 20 wt % borosilicate glass within the SiO₂. The addition of the borosilicate glass to the buried oxide acted to reduce the viscosity at the growth temperature of Si_1-xGe_x, promoting the in situ elastic deformation of the thin Si (∼20 nm) layer on the insulator. The sharing of the misfit between the Si and the Si_1-xGe_x layers was observed and quantified by double-axis X-ray diffraction. In addition, the material quality was assessed using cross-sectional transmission electron microscopy, photoluminescence and etch pit density measurements. No misfit dislocations were observed in the partially relaxed 150-nm Si_0.75Ge_0.25 sample as-grown on a 20% borosilicate glass substrate. The threading dislocation density was estimated at 2×10⁴ cm^-2 for 500-nm Si_0.75Ge_0.25 grown on the 20% borosilicate glass substrate. This method may be used to prepare compliant substrates for the growth of low-dislocation relaxed SiGe layers. Received: 4 January 2001 / Accepted: 30 May 2001 / Published online: 17 October 2001     

Influence of a predeposited Si1−x Gex layer on the growth of self-assembled SiGe/Si(001) islands

The growth of self-assembled Ge(Si) islands on a strained Si_1?xGe_x layer (0% < x < 20%) is studied. The size and the surface density of islands are found to increase with Ge content in the Si_1?xGe_x layer. The increased surface density is related to augmentation of the surface roughness after deposition of the SiGe layer. The enlargement of islands is accounted for by the decrease of the wetting layer in thickness due to the additional elastic energy accumulated in the SiGe layer and to enhanced Si diffusion from the Si_1?xGe_x layer into the islands. The increase in the fraction of the surface occupied by islands leads to a greater order in the island arrangement.