Strain In Si Research Articles

Ab-initio calculations of the hydrogen–uranium system: Surface phenomena, absorption, transport and trapping

Density functional theory was applied to the initial steps of uranium hydriding: surface phenomena, absorption, bulk transport and trapping. H adsorbs exothermically to the (0 0 1) surface, yet H absorption into the bulk is endothermic, with off-center octahedral absorption having the lowest absorption energy of 0.39 eV, relative to molecular H 2. H absorption in interstitial sites causes a local softening of the bulk modulus. Diffusion of H in unstrained α-U has a barrier of 0.6 eV. The energy of H absorption adjacent to the chemical impurities C, S, Si was lowered by an amount proportional to the size of the impurity atom, and the resulting lattice strain Si > S > C. Thus, impurities may promote hydriding by providing surfaces or prestrained zones for H uptake.

Raman Spectroscopy for the Analysis οf Temperature-Dependent Plastic Relaxation οf SiGe Layers

Novel architectures for electronics and photonics are expected to be developed using the forthcoming Si1−xGex technology. However, in Si1−xGex-based heterostructures, materials and design issues rely on accurate control of strain and composition of the alloy. The Raman spectroscopy has rapidly emerged as a reliable technique for the quantitative determination of such parameters on a sub-micrometric scale. In this work we present an investigation of the effects of the growth conditions of Si1−xGex graded layers on dislocation nucleation and interaction. In particular, we focus on the crucial role the deposition temperature plays in the dislocation kinetics. The analysis of threading dislocation densities is accompanied by a quantitative measurement of the residual strain in Si1−xGex/Si heterostructures, carried out by means of the Raman scattering. Our approach is effective in studying the physical mechanism governing dislocation multiplication and the sharp transition from a state of brittleness to a state of ductility within a narrow temperature window.

The impact of uniaxial stress on subband structure and mobility of strain Si NMOSFETs

An effect of stress distortion on the conduction band structure was derived by k.p method considering a second order perturbation. From k.p conduction band calculations, stress-induced band edge split and the change of effective mass are quantitatively evaluated. The physical reasons of warped subband structure and abnormal mobility enhancement by uniaxial stress are investigated. Variation rates of experimental electron mobility in the silicon n-channel metal-oxide-semiconductor field-effect-transistors under a [110] uniaxial stress as a function of channel direction is theoretically studied.

Impact of High-<I>κ</I> Gate Stacks on Transport and Variability in Nano-CMOS Devices

Scaling of Si MOSFETs beyond the 90 nm technology node requires performance boosters in order to satisfy the International Technology Roadmap for Semiconductors (ITRS) requirements for drive current in high-performance transistors. Amongst the preferred near term solutions are transport enhanced FETs utilising strained Si (SSi) channels. Additionally, high-kappa dielectrics are expected to replace SiO2 around or after the 45 nm node to reduce the associated gate leakage current problem, facilitating further scaling. However, in spite of significant recent technological achievements, in particular, in reducing charge trapping and crystallization of the dielectric, mobility degradation in the devices with the high-kappa gate stacks caused, in part, by the strong soft optical phonon scattering leaves room for further performance improvement. In this work we study the impact of soft optical phonon scattering on the mobility and device performance for conventional and strained Si n-MOSFETs with high-kappa dielectrics using a self-consistent Poisson Ensemble Monte Carlo device simulator, with effective gate lengths of 67 and 35 nm

Evaluation of dry-adapted strains of the predatory mite Neoseiulus californicus for spider mite control on cucumber, strawberry and pepper

The goal of this study was to evaluate spider mite control efficacy of two dry-adapted strains of Neoseiulus californicus. Performance of these strains were compared to a commercial strain of Phytoseiulus persimilis on whole cucumber, pepper and strawberry plants infested with Tetranychus urticae at 50 +/- 5% RH. Under these dry conditions predators' performance was very different on each host plant. On cucumber, spider mite suppression was not attained by any of the three predators, plants 'burnt out' within 4 weeks of spider mite infestation. On strawberry, all predators satisfactorily suppressed spider mites yet they differed in short term efficacy and persistence. Phytoseiulus persimilis suppressed the spider mites more rapidly than did the BOKU and SI N. californicus strains. Both N. californicus strains persisted longer than did P. persimilis. The BOKU strain was superior to SI in population density reached, efficacy in spider mite suppression and persistence. On pepper, in the first 2 weeks of the experiment the BOKU strain was similar to P. persimilis and more efficacious in spider mite suppression than strain SI. Four weeks into the experiment the efficacy of P. persimilis dropped dramatically and was inferior to the SI and BOKU strains. Overall, mean predator density was highest on plants harbouring the BOKU strain, lowest on plants with P. persimilis and intermediate on plants with the SI strain. Implications for biocontrol of spider mites using phytoseiid species under dry conditions are discussed.

Paramagnetic point defects at interfacial layers in biaxial tensile strained (100)Si/SiO2

A comparative electron spin resonance study is reported on (100)Si/SiO2 entities, grown by thermal oxidation of biaxial tensile strained (100)Si (sSi) layers, epitaxially grown on a strain relaxed Si0.8Ge0.2 buffer layer, and standard (100)Si. In the as-oxidized state a significant decrease (∼50%) of inherently incorporated interface defects, Pb0 and Pb1, is observed, i.e., the sSi/SiO2 interface is found to be inherently significantly improved in terms of electrically detrimental interface traps (Pb0). After vacuum-ultraviolet irradiation two more SiO2-associated defects appear, namely Eγ′ (generic entity O3≡Si·) and EX. Interestingly, a decrease (∼50%) of Eγ′ defect density is observed compared to standard Si/SiO2. This reduction in inherent electrically active interface (Pb0) and near-interface (Eγ′) traps would establish sSi/SiO2 as a superior device structure for all electrical properties where (near)-interfacial traps play a detrimental role. For one, the reduction of detrimental (near)-interface defects may be an additional reason for the commonly reported mobility enhancement in sSi/SiO2 based metal–oxide–semiconductor structures over standard Si/SiO2 ones, and at the same time account for the reported reduction of 1/f noise in the former structures. The data also confirm the generally accepted notion that Pb-type defects are mismatch induced defects.

The effects of nickel germanosilicide contacts on the biaxial compressive stress in thin epitaxial silicon-germanium alloys on silicon

When a thin Si1−xGex epitaxial layer is grown on Si, it is under biaxial compression. In this letter, it is shown that a nickel germanosilicide (NiSi1−xGex) layer formed on Si1−xGex can significantly reduce the in-plane compressive strain in Si1−xGex. It is proposed that the observed reduction is due to the biaxial tensile stress applied by the NiSi1−xGex layer. Because the Si1−xGex bandgap is a strong function of the strain, this is expected to have a strong impact on the metal-semiconductor barrier height and the contact resistivity of the interface if the metal Fermi level is pinned near the Si1−xGex midgap.

Microstructural and mechanical characterization of Nb-based in situ composites from Nb–Si–Ti ternary system

This study deals with the Nb–niobium silicide-based composites developed by the hot-pressing of Nb–Si–Ti ternary powder mixtures with a fixed Ti addition (6 at.%) and Si content ranging from hypereutectic (11 at.%) to near-eutectic compositions (18 at.%). The effects of Si content, Ti addition and strain rates on the sample microstructural characterization, flexural strength, fracture toughness, quasi-static compressive deformation and failure processes were investigated. It was revealed that the volume fraction of silicides increased with increasing Si content, and most of the Ti atoms dissolved into the niobium silicides to form (Nb,Ti) 5Si 3 solid solutions instead of binary titanium silicides. The experimental evidence showed that a moderate improvement in the flexural strength, fracture toughness and compressive yield stress of the composites was achieved by the addition of Ti. Higher Si additions produced a much more remarkable enhancement in the compressive yield stress and bulk hardness, whereas both the flexural strength and fracture toughness decreased with increasing Si content owing to the existence of residual porosities in the samples. The composites showed remarkable superiority to the arc-melted Nb–Si alloys and monolithic niobium silicides in fracture toughness ( 8.3 – 13.0 MPa m vs. 4.5 MPa m ), where the toughening effect was attributed mainly to crack bridging and crack deflection by the remaining ductile Nb phase. Moreover, quasi-static uniaxial compression tests at strain rates between 10 –5 and 10 –3 s −1 indicated that the deformation behavior and failure processes were significantly affected by Si content and strain rates. The strain-rate-hardening behavior for all the strain rates was observed in the composite materials and the strain-rate sensitivity decreased with increasing Si content. At a lower strain rate, the composite materials with a hypoeutectic Si composition failed with a pseudoplastic response and at an angle of ∼45° off the compressive loading direction. However, in the case of higher strain rate and Si content, the brittle failure in the samples occurred through vertical splitting and spalling of fragments.

Performance Limitations of Si Bulk CMOS and Alternatives for future ULSI

A channel material with high mobility and therefore high injection velocity can increase on current and reduce delay. Currently, strained-Si is the dominant technology for high performance MOSFETs and increasing the strain provides a viable solution to scaling. However, looking into future scaling of nanoscale MOSFETs it becomes important to look at higher mobility materials, like Ge and III-V materials together with innovative device structures which may perform better than even very highly strained Si. For both Ge and III-V devices problems of leakage need to be solved. Novel heterostructures will be needed to exploit the promised advantages of Ge and III-V based devices.

In situ HCl etching of Si for the elaboration of locally misorientated surfaces

We have studied the in situ HCl etching of Si active areas on patterned wafers. After some in situ HCl etching at 20 Torr of Si(1 0 0), we have locally obtained 2 μm long areas with misorientation angles around 4.5° towards 〈1 1 0〉. Furthermore, we have evidenced a recess shape transition from convex ( T ≤ 865 °C) to concave ( T ≥ 895 °C) as the etch temperature increases, with a nearly flat surface with no facets at T = 880 °C. The morphology of the etched structures at a given time, temperature and P HCl / P H 2 ratio will be a function of the slope lengths and the pattern dimensions. Different kinds of surfaces (rounded areas, facets) were obtained in 3.5 μm × 3.5 μm Si windows after HCl etching at 850 °C during 300 s, depending on the stress within. Thermal oxidations can indeed be used to increase by 65 MPa up to 110 MPa the compressive stress in those Si windows which are bordered by SiO 2 shallow trench isolation. An increase of the misorientation angle from 4.5° up to 6° occurred after the above-mentioned HCl etch when switching from conventional to highly strained Si windows. For the shortest etching times studied here (150 s), a selective etching of 3.5 μm × 3.5 μm Si windows edges is responsible for the misorientation. The etch is then more uniform. Stress gradients might consequently be one of the main misorientation causes. We have also probed the influence of the shallow trench isolation (STI) thickness on the misorientation. A morphological difference before HCl etching has been shown to be responsible for the transition from sloped to rounded areas. A local loading effect may prevail in this case.

Characterization of CMOS sub-65 nm metallic contact by laser scattering: Thermal stability of Ni(Si 1− xGe x)

Very efficient in particles detection, light scattering also offers fast non-invasive full-mapping wafer surface state. This sensitivity was used in the case of germano-silicide process development. As a matter of fact, we report on haze measurement performances, compared to the usual methods used to investigate thermal stability of Ni(Si 1− x Ge x ), such as sheet resistance (SR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). We observed defectivity related to thermal agglomeration and Ge-segregation of Ni(Si 1− x Ge x ) on strain Si 1− x Ge x ( x ⩽ 30%) by haze measurement (like SEM observations) earlier than SR measurement. Moreover, we noticed that a high Ge content affects at lower temperature the stability of Ni(Si 1− x Ge x ) with a segregation phenomena.

Si(001) Surface Layer-by-Layer Oxidation Studied by Real-Time Photoelectron Spectroscopy using Synchrotron Radiation

High-resolution O 1s and Si 2p photoelectron spectroscopy using synchrotron radiation was employed to clarify a layer-by-layer oxidation reaction mechanism on a Si(001) surface from the viewpoint of point defect generation due to an oxidation-induced strain at a SiO2/Si interface. The Siβ and Siα components in Si 2p3/2 spectra, which are assigned to the first and second strained Si layers, respectively, below the transition layer composed of suboxides, Si1+, Si2+, and Si3+, significantly decrease during the step-by-step temperature increase-enhanced growth of the second oxide layer. Because of the corresponding band bending changes measured using the O 1s peak position, which are caused by defect-related band gap states, the observed decreases in Siβ and Siα components, indicating a decrease in interfacial strain, are induced not only by the structural relaxation of a SiO2 network due to a thermal annealing effect, but also due to the generation of point defects at the SiO2/Si interface. Continuous band bending changes with the growth of the third oxide layer also suggest that the point defects are generated during oxide growth, whereas the Siβ and Siα components are maintained almost constant. On the basis of the observed interfacial strain and point defect generation changes, the layer-by-layer growth kinetics of the first, second and third oxide layers is discussed using a unified Si oxidation reaction model mediated by point defect generation at the SiO2/Si interface [S. Ogawa and Y. Takakuwa: Jpn. J. Appl. Phys. 45 (2006) 7063].

An analytical model for electrostatics of strained-Si n-type metal-oxide-semiconductor capacitors

We propose a semiclassically based analytical model for electrostatic properties of strained-Si (SS) n-type metal-oxide-semiconductor (MOS) devices. Our model explicitly incorporates the discontinuity of the dielectric constant across the SS/SiGe interface. The model is applied to calculate the threshold voltage reduction in SS devices. Comparison with results from an accurate, self-consistent, quantum-mechanical model, as well as with published experimental data, demonstrates that the predictions from the proposed analytical model are accurate provided that the SS layer is not too thin, and the Ge content in the SiGe buffer layer is not too high.

A new method of fabricating strained Silicon materials

Strain-relaxed SiGe virtual substrates are of great importance for fabricating strained Si materials. Instead of using graded buffer method to obtain fully relaxed SiGe film, in this study a new method to obtain relaxed SiGe film and strained Si film with much thinner SiGe film was proposed. Almost fully relaxed thin SiGe buffer layer was obtained by Si/SiGe/Si multi-structure oxidation and the SiO 2 layer removing before SiGe regrowth. Raman spectroscopy analysis indicates that the regrown SiGe film has a strain relaxation ratio of about 93% while the Si cap layer has a strain of 0.63%. AFM shows good surface roughness. This new method is proved to be a useful approach to fabricate thin relaxed epilayers and strain Si films.

Carbon doping effect on strain relaxation during Si1−x−yGexCy epitaxial growth on Si(1 0 0) at 500 °C

On strain relaxation during Si1−x−yGexCy epitaxial growth on Si(1 0 0) using an ultraclean hot-wall low-pressure chemical vapour deposition system at 500 °C, the carbon doping effect has been investigated. The depth profile of the strain in Si1−x−yGexCy films is measured by x-ray diffraction and Raman scattering spectroscopy combined with a wet etching technique. It is confirmed that the strain relaxation of Si1−xGex films (x = 0.45) occurs uniformly at every depth from the surface to the heterointerface. On the other hand, strain relaxation occurs partially in the surface part of Si1−x−yGexCy films (x = 0.45, y = 0.016), although the bottom part near the heterointerface is still strained.

Epitaxial growth of highly strained Si on relaxed Ge/Si(1 0 0) using ECR plasma CVD without substrate heating

By using electron-cyclotron resonance (ECR) plasma chemical vapour deposition (CVD), epitaxial growth of highly strained Si on 84%-relaxed Ge/Si(1 0 0) without substrate heating has been investigated. For a Si thickness of 1.7 nm, the deposited surface is atomically flat, and the strain (ratio of change in the lattice constant to strain-free lattice constant) is about 4%. This value indicates that the strained Si lattice is matched to the relaxed Ge lattice. Furthermore, it is found that the heterostructure of strained Si/Ge is thermally stable up to 600 °C.

Highly-Strained Silicon-On-Insulator Development

Bi-axially highly-strained Silicon-On-Insulator (sSOI) substrates with a tensile stress up to 2.5 GPa have been obtained by Smart CutTM technology. Thin strained silicon (sSi) layers epitaxially grown on relaxed Si0.6Ge0.4 virtual substrates (VS) were used as starting materials. The threading dislocation density in those sSi layers was in the low 105 cm-2. Some stacking faults were also present in those highly strained Si films. The evolution of this linear defect density was characterized as a function of the sSi thickness by Secco etch. 2.5 GPa sSOI wafers have been demonstrated in 200 mm diameter. Stress uniformity σ equal to 1.14% and 2 nm thickness range has been obtained for 8 nm thick sSi layers.

Inherent density of point defects in thermal tensile strained (100)Si∕SiO2 entities probed by electron spin resonance

An electron spin resonance analysis has been carried out of the intrinsic point defects in (100)Si∕SiO2 entities thermally grown at 800°C on biaxial tensile strained Si (s-Si). As compared to coprocessed standard (100)Si∕SiO2, a significant reduction (>50%) is observed in the inherent density of the trivalent Pb-type interface defects (Pb0,Pb1). With the Pb0’s established as detrimental fast interface traps, this result may adduce one more reason for the observed enhancement of device channel carrier mobility with increasing Si substrate tensile strain as well as reduction in 1∕f noise. The s-Si∕SiO2 interface exhibits a generally superior device grade quality.

Fabrication of well-defined individual dislocations in SiGe as a novel one-dimensional system

We show a new way to fabricate well-defined individual dislocations in SiGe. We started with a fully pseudomorphic but metastable SiGe layer grown on Si(0 0 1) by molecular beam epitaxy. Next, elongated (∼1 mm) mesa stripes with various widths (0.5–3 μm) were fabricated by a combination of isotropic and anisotropic etching. For smaller stripes, elastic relaxation of the strained SiGe layer can occur, transforming the originally biaxial strained layer into uniaxial strained subsystems. Subsequent strain relaxation caused by high temperature treatments leads to the formation of individual dislocation along the mesa stripes. The number of parallel dislocation can be adjusted by the original strain (Si:Ge ratio and layer thickness) and the mesa widths. We were able to fabricate structures with exactly one dislocation. Finally, contact pads were added to the stripes enabling the electrical characterization of individual dislocation.

Analysis of boron strain compensation in silicon-germanium alloys by Raman spectroscopy

The impact of heavy boron doping on the biaxial compressive strain in Si1−xGex layers grown on Si has been investigated using Raman spectroscopy and theoretical calculations. It is shown that one boron atom is sufficient to compensate the strain due to approximately 6.9 Ge atoms. This effect is appreciably large for boron concentrations as low as 1%, typical for applications, which employ heavily boron doped layers. Using strain compensation, the Ge content can be substantially increased without increasing the stored strain energy. This phenomenon can be useful in applications, which require low-resistivity p-type strained Si1−xGex layers with high Ge content.