Highly strained metastable structures and selective area epitaxy of Ge-rich Ge1−xSix/Si(100) materials using nanoscale building blocks

Low-temperature heteroepitaxy (330 ^°C–430 ^°C) of Si_0.5Ge_0.5 and Si_0.25Ge_0.75 on Si(100) using single-source silyl-germanes [ SiH₃GeH₃,HSi(GeH₃)₃] produces monocrystalline structures, smooth and continuous surface morphologies and low defect densities. The metastable compressive strain in these films is dramatically enhanced relative to alternative growth methods. At such low temperatures the material grows seamlessly, conformally, and selectively in the “source/drain” regions of prototypical transistors. These results suggest that films grown via silyl-germanes could have applications in optoelectronics and as stressors for mobility enhancement in Si devices.     

Epitaxial growth of fully relaxed Si0.75Ge0.25 on SOI substrate

A fully relaxed Si_0.75Ge_0.25 film with low dislocation densities is fabricated by epitaxial growth on SOI substrate without depositing graded buffers. The relaxation mechanism of the SiGe layer directly grown on SOI substrate is also analyzed. For SiGe grown on SOI with low Ge content, the strain is redistributed between SiGe and the top Si of SOI substrate, and the strain residing in SiGe layer can be fully relaxed by the formation and expansion of dislocation half-loops near the SiGe/Si interface. The surface morphology and crystal quality of all samples are analyzed by optical microscopy and transmission electron microscopy (TEM), respectively. Compared to the Si_0.75Ge_0.25 layer epitaxially grown on graded buffer, the Si_0.75Ge_0.25 directly grown on SOI substrate appears good surface morphology and perfect crystal quality.     

An analytical threshold voltage model for dual-strained channel PMOSFET

Based on the analysis of vertical electric potential distribution across the dual-channel strained p-type Si/strained Si 1-x Ge x /relaxd Si 1-y Ge y (s-Si/s-SiGe/Si 1-y Ge y) metal-oxide-semiconductor field-effect transistor (PMOSFET),an-alytical expressions of the threshold voltages for buried channel and surface channel are presented.And the maximum allowed thickness of s-Si is given,which can ensure that the strong inversion appears earlier in the buried channel (compressive strained SiGe) than in the surface channel (tensile strained Si),because the hole mobility in the buried channel is higher than that in the surface channel.Thus they offer a good accuracy as compared with the results of device simulator ISE.With this model,the variations of threshold voltage and maximum allowed thickness of s-Si with design parameters can be predicted,such as Ge fraction,layer thickness,and doping concentration.This model can serve as a useful tool for p-channel s-Si/s-SiGe/Si 1-y Ge y metal-oxide-semiconductor field-effect transistor (MOSFET) designs.     

Elemental depth profiling of a-Si1−xGex:H films by elastic recoil detection analysis and secondary ion mass spectrometry

The hydrogen content in a-Si_1−xGe_x:H thin films is an important factor deciding the density and the optical band gap. We measured the elemental depth profiles of hydrogen together with Si and Ge by elastic recoil detection analysis (ERDA) combined with Rutherford backscattering (RBS) using MeV He²⁺ ions. In order to determine the hydrogen depth profiles precisely, the energy- and angle-dependent recoil cross-sections were measured in advance for the standard sample of a CH₃⁺-implanted Si substrate. The cross-sections obtained here are reproduced well by a simple expression based on the partial wave analysis assuming a square well potential (width: r₀ = 2.67 × 10⁻¹³ cm, depth: V₀ = −36.9 MeV) within 1%. For the a-Si_1−xGe_x:H films whose elemental compositions were determined by ERDA/RBS, we measured the secondary ions yields of HCs₂⁺, SiCs₂⁺, H⁻, Si⁻ and Ge⁻ as a function of Ge concentration x. As a result, it is found that the useful yield ratios of HCs₂⁺/SiCs₂⁺, H⁻/Si⁻ and Ge⁻/Si⁻ are almost constant and thus the elemental depth profiles of the a-Si_1−xGe_x:H films can be also determined by secondary ion mass spectrometry (SIMS) within 10% free from a matrix effect.     

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     

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.     

Si/Si1−xGex/Si-based quantum wells infrared photodetector operating at 1.55 μm

A quantum well infrared photodetector consisting of self-assembled type II SiGe/Si based quantum wells operating around 1.55 μm at room temperature has been investigated. The Si_1−yGe_y/Si/Si_1−xGe_x/Si/Si_1−yGe_y stack results in a ‘W’ like profiles of the conduction and valence bands strain-compensated in the two low absorption windows of silica fibers infrared photodetectors have been proposed. Such computations have been used for the study of the p-i-n infrared photodetectors operating, around (1.3–1.55 μm) at room temperature. The quantum transport properties of electrons and holes were approved with Schrödinger and kinetic equations resolved self-consistently with the Poisson equation. The theoretical performances of the photodetector were carried out such as the dark current mechanisms, the temperature dependence of normalized dark current and the zero-bias resistance area product (R₀A).     

Nanocrystal growth of single-phase Si1−xGex alloys

We present the formation of single-phase Si_1−xGe_x (x=0.2, 0.4, 0.6, and 0.8) alloy nanocrystals dispersed in a SiO₂ matrix. The studied samples were prepared by co-sputtering with excess Si_1−xGe_x in SiO₂ of approximately 33 at%. Upon heat treatment, crystallization of Si_1−xGe_x alloys was examined by using X-ray diffraction and high-resolution transmission electron microscopy measurements. Single structure of face-centered cubic nanocrystals in a space group Fd-3m was concluded. The average nanocrystal size (from 2 nm to 10 nm) and the lattice constant a of the single-phase Si_1−xGe_x nanocrystals were found to increase with the Ge composition parameter x. Density functional theory-generalized gradient approximation calculation showed the replacement of Ge into the Si sites and vice versa.     

Enhanced formation of periodic arrays of low-resistivity NiSi nanocontacts on (0 0 1)Si0.7Ge0.3 by nanosphere lithography with a thin interposing Si layer

In this study, we demonstrated significant enhancement of the formation of low-resistivity NiSi nanocontacts with controlled size on (0 0 1)Si_0.7Ge_0.3 substrates by combining the nanosphere lithography with the use of a new Ni/a-Si bilayer nanodot structure. Low-resistivity NiSi with an average size of 78 nm was observed to be the only silicide phase formed in samples annealed at 350-800 °C. The presence of the interposing Si layer with appropriate thickness was found to effectively prevent Ge segregation and maintain the interface stability in forming NiSi nanocontacts on (0 0 1)Si_0.7Ge_0.3. As the annealing temperature was increased to 900 °C, amorphous SiO_x nanowires were observed to grow from silicide nanocontact regions. The NSL technique in conjunction with a sacrificial Si interlayer process promises to be applicable in fabricating periodic arrays of other low-resistivity silicide nanocontacts on Si_1−xGe_x substrates without complex lithography.     

应变Si电子电导有效质量模型

采用K·P微扰法建立了应变Si导带能谷由纵、横向有效质量表征的E-k关系,并在此基础上,研究分析了(001),(101),(111)晶面应变Si电子的电导有效质量与应力、能谷分裂能及晶向的关系.结果表明,弛豫Si_1-xGe_x材料(001)面生长的应变Si沿[100],[010]晶向的电子电导有效质量和弛豫Si_1-xGe_x材料(101)面生长的应变Si 关键词： 应变Si K·P法 电导有效质量     

Preparation of five-layered Si/SixGe1−x nano-films by RF helicon magnetron sputtering

Five-layered Si/Si_xGe_1−x films on Si(1 0 0) substrate with single-layer thickness of 30 nm, 10 nm and 5 nm, respectively were prepared by RF helicon magnetron sputtering with dual targets of Si and Ge to investigate the feasibility of an industrial fabrication method on multi-stacked superlattice structure for thin-film thermoelectric applications. The fine periodic structure is confirmed in the samples except for the case of 5 nm in single-layer thickness. Fine crystalline Si_xGe_1−x layer is obtained from 700 °C in substrate temperature, while higher than 700 °C is required for Si good layer. The composition ratio (x) in Si_xGe_1−x is varied depending on the applied power to Si and Ge targets. Typical power ratio to obtain x = 0.83 was 7:3, Hall coefficient, p-type carrier concentration, sheet carrier concentration and mobility measured for the sample composed of five layers of Si (10 nm)/Si_0.82Ge_0.18 (10 nm) are 2.55 × 10⁶ /°C, 2.56 × 10¹² cm⁻³, 1.28 × 10⁷ cm⁻², and 15.8 cm⁻²/(V s), respectively.     

应变Si/(001)Si1-xGex空穴有效质量各向异性

基于应变Si/（001）Si_1-xGe_x材料价带E（k）-k关系模型,研究获得了其沿不同晶向的空穴有效质量.结果表明,与弛豫材料相比,应变Si/（001）Si_1-xGe_x材料价带带边（重空穴带）、亚带边（轻空穴带）空穴有效质量在某些k矢方向变化显著,各向异性更加明显.价带空穴有效质量与迁移率密切相关,该研究成 关键词： 应变Si 价带 空穴有效质量     

The study on two-dimensional analytical model for gate stack fully depleted strained Si on silicon-germanium-on-insulator MOSFETs

Based on the exact resultant solution of two-dimensional Poisson's equation in strained Si and Si_{1 - X}Ge_X layer, a simple and accurate two-dimensional analytical model including surface channel potential, surface channel electric field, threshold voltage and subthreshold swing for fully depleted gate stack strained Si on silicon-germanium-on-insulator (SGOI) MOSFETs has been developed. The results show that this novel structure can suppress the short channel effects (SCE), the drain-induced barrier-lowering (DIBL) and improve the subthreshold performance in nanoelectronics application. The model is verified by numerical simulation. The model provides the basic designing guidance of gate stack strained Si on SGOI MOSFETs.     

基于低温硅技术的赝晶SiGe应变弛豫机理

基于能量平衡条件,结合低温硅(LT-Si)剪切模量小于SiGe的实验结果,从螺位错形成模型出发,给出了基于LT-Si技术的赝晶SiGe应变弛豫机理.该机理指出,赝晶SiGe薄膜厚度小于位错形成临界厚度,可通过LT-Si缓冲层中形成位错释放应变;等于与大于临界厚度,位错在LT-Si层中优先形成,和文献报道中已观察到的实验结果相符合.同时,实验制备了基于LT-Si技术的弛豫Si_0.8Ge_0.2虚拟衬底材料.结果显示,位错被限制在LT-Si缓冲层中,弛豫度达到了85.09%,且在Si_0.8Ge_0.2中未观察到穿透位错,实验结果证实了赝晶Si_0.8Ge_0.2是通过在LT-Si缓冲层形成位错来释放应变的弛豫机理. 关键词： 低温硅 赝晶锗硅 弛豫机理 位错理论     

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).     

Practical aspects of strain measurement in thin SiGe layers by (0 0 4) dark-field electron holography in Lorentz mode

Dark-field electron holography (DFEH) is a powerful transmission electron microscopy technique for mapping strain with nanometer resolution and high precision. However the technique can be difficult to set up if some practical steps are not respected. In this article, several measurements were performed on thin Si_(1−x)Ge_x layers using (0 0 4) DFEH in Lorentz mode. Different practical aspects are discussed such as sample preparation, reconstruction of the holograms and interpretation of the strain maps in terms of sensitivity and accuracy. It was shown that the measurements are not significantly dependent on the preparation tool. Good results can be obtained using both FIB and mechanical polishing. Usually the most important aspect is a precise control of the thickness of the sample. A problem when reconstructing (0 0 4) dark-field holograms is the relatively high phase gradient that characterises the strained regions. It can be difficult to perform reconstructions with high sensitivity in both strained and unstrained regions. Here we introduce simple methods to minimise the noise in the different regions using a specific mask shape in Fourier space or by combining several reconstructions. As a test, DFEH was applied to the characterization of eight Si_(1−x)Ge_x samples with different Ge concentrations. The sensitivity of the strain measured in the layers varies between 0.08% and 0.03% for spatial resolutions of 3.5–7 nm. The results were also compared to finite element mechanical simulations. A good accuracy of ±0.1% between experiment and simulation was obtained for strains up to 1.5% and ±0.25% for strains up to 2.5%.     

Intermixing-controlled epitaxy for Si–Ge heterostructure devices

To fabricate high quality SiGe/Si heterostructures, control of intermixing between Si and Ge is essential during crystal growth. This paper describes the recent progress of ‘intermixing-controlled epitaxy’. A combined method of MBE (molecular beam epitaxy) and SPE (solid-phase epitaxy) was developed and used to fabricate a new heterostructure (n-Si_0.8Ge_0.2/Si channel/Si_1 − xGe_xbuffer layer/Si substrate). Observation by TEM demonstrated that the hetero-interface obtained by SPE was atomically flat. This interface provides the ultrahigh mobility of a two-dimensional electron gas (2DEG). In addition, the influence of atomic-hydrogen irradiation during MBE on Ge dispersion in the SiGe mixed crystal is examined. Results indicate that the number of Ge–Ge pairings was decreased by hydrogen irradiation. Such a decrease deformed the local symmetry of the Si–Ge bond from tetrahedral symmetry. As a result, photoluminescence intensity was sucessfully increased.     

Induced electrostatic confinement of electron gas in W-designed strain-compensated Si/Si1-xGex/Si type-II quantum wells

 We present a numerical modeling of the conduction- and the valence-band diagrams of W designed Si/Si_0.4Ge_0.6/Si type II quantum wells. These W structures, strain-compensated on relaxed Si_0.75Ge_0.25 pseudo-substrates, are potentially interesting for emission and photo-detection around a 1.55μm wavelength. Two main features have been extrapolated by solving self-consistently Schrödinger and Poisson equations, taking into account the electrostatic attraction induced by carrier injection: (i) Coulomb attraction strongly modifies the band profiles and increases the electron probability density at the quantum well interfaces. (ii) The injected carrier concentration enhances the in-plane oscillator strength and the electron-hole wave-function overlap.     

应变Si价带色散关系模型

基于K.P理论框架,通过引入应变哈密顿微扰项,详细推导并建立了应变Si的价带色散关系模型.所得模型适用于任意晶向弛豫Si_1－xGe_x(0≤x≤0.6)衬底上生长的应变Si,并且,通过该模型可以获取任意K矢方向的应变Si价带结构及空穴有效质量,对器件研究设计可提供有价值的参考. 关键词： 应变Si K.P理论 色散关系