Si1 − x Ge x /Si heterostructures grown by molecular-beam epitaxy on silicon-on-sapphire substrates

The growth of heterostructures with Si_{1 ? x} Ge _x layers on $\left( {1\bar 102} \right)$ sapphire substrates by molecular-beam epitaxy with a silicon sublimation source and a germanium gas source (GeH₄) is reported. The systematic study of the influence of substrate temperature and thickness of the silicon buffer layer shows that the optimal conditions for growing epitaxial Si_{1 ? x} Ge _x layers are provided at a temperature of T _S = 375–400°C. There are significant differences in the orientations of Si_{1 ? x} Ge _x layers, depending on the thickness d of the Si buffer layer: the preferred orientations are (100) at d ≥ 100 nm and (110) for thinner layers. Heterostructures with thick (～1 μm) Si_{1 ? x} Ge _x layers, doped with erbium atoms, exhibit intense photoluminescence at λ = 1.54 μm.     

Molecular beam epitaxial growth of Si1−xGex/Si pseudomorphic layers using disilane and germanium

Molecular beam epitaxial growth of pseudomorphic Si_1−xGe_x/Si layers using disilane (Si₂H₆) and elemental germanium has been studied for the first time. It is found that at a fixed flow rate of Si₂H₆, the germanium content in the Si_1−xGe_x alloys is a function of the germanium cell temperature. Heterostructures and multi-quantum wells with good surface morphology, excellent crystalline quality, and abrupt interfaces are demonstrated, indicating little or no sourcerelated transient effects.     

Characterization of CMOS sub-65 nm metallic contact by laser scattering: Thermal stability of Ni(Si1−xGex)

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−xGe_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−xGe_x) on strain Si_1−xGe_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−xGe_x) with a segregation phenomena.     

Quantitative calibration and germanium SIMS depth profiling in Ge x Si1 − x /Si heterostructures

Methods for minimizing nonlinear matrix effects in the quantitative determination of germanium concentrations in Ge _x Si_{1 ? x} layers by secondary ion mass spectrometry are discussed. The analysis conditions with positive SiCs⁺, GeCs⁺ and negative Ge^?, Si^? secondary ions produced during sputtering by cesium ions are used in the TOF.SIMS-5 setup with a time-of-flight mass analyzer. In contrast to published works for TOF.SIMS setups, the linear dependence of the ion-concentration ratio Ge^?/Si^? on x/(1 ? x) is shown. Two new linear calibrations for the germanium concentration as a function of the cluster Ge ₂ ^? secondary ion yield are proposed. The calibration factors are determined for all linear calibrations at various energies of sputtered cesium ions and Bi⁺ and probe Bi ₃ ⁺ ions. It is shown for the first time that the best depth resolution among the possible conditions of quantitative germanium depth profiling in Ge _x Si_{1 ? x} /Si multilayer heterostructures is provided by the calibration mode using elemental Ge^? and Si^? negative secondary ions.     

Influence of the deposition parameters on the growth of SiGe nanocrystals embedded in Al2O3 matrix

Si_1−xGe_x nanocrystals (NCs), embedded in Al₂O₃ matrix, were fabricated on Si (100) substrates by RF-magnetron sputtering technique with following annealing procedure at 800 °C, in nitrogen atmosphere. The presence of Si_1−xGe_x NCs was confirmed by grazing incidence X-ray diffraction (GIXRD), grazing incidence small angle X-ray scattering (GISAXS) and Raman spectroscopy. The influence of the growth conditions on the structural properties and composition of Si_1−xGe_x NCs inside the alumina matrix was analyzed. Optimal conditions to grow Si_1−xGe_x (x∼ 0.8) NCs sized between 3 and 4 nm in Al₂O₃ matrix were established.     

Ni(Pt) 锗硅化合物/Si1-xGex 接触电阻的测试

The electrical properties of Ni0.95Pt0.05-germanosilicide/Si1_xGex contacts on heavily doped p-type strained Sil-xGex layers as a function of composition and doping concentration for a given composition have been investigated. A four-terminal Kelvin-resistor structure has been fabricated by using the conventional com- plementary metal-oxide-semiconductor （CMOS） process to measure contact resistance. The results showed that the contact resistance of the Ni0.95Pt0.05-germanosilicide/Sil-xGex contacts slightly reduced with increasing the Ge fraction. The higher the doping concentration, the lower the contact resistivity. The contact resistance of the samples with doping concentration of 4×10^19 cm^-3 is nearly one order of magnitude lower than that of the sam- ples with doping concentration of 5 × 10^17 cm^-3. In addition, the influence of dopant segregation on the contact resistance for the lower doped samples is larger than that for the higher doped samples.     

Leakage currents and dielectric breakdown of Si1−x−yGexCy thermal oxides

A detailed study of the leakage currents and dielectric wear-out of thermal oxides grown on Si_1−xGe_x, Si_1−yC_y and Si_1−x−yGe_xC_y epilayers to determine their quality and reliability for Si_1−x−yGe_xC_y MOS technology is presented. After applying electrical stress to the samples, we have determined the conduction mechanisms and the dependence of leakage currents upon epilayer composition (Ge and C content). Conduction takes place mainly via Fowler-Nordheim tunneling injection. Ge and C introduce traps in the oxide which assist injection and thus lower the effective height of the tunneling barrier. We have also monitored the oxide reliability, focusing on time-dependent dielectric breakdown (TDDB). The nature of trapped charge in the oxide depends on the initial epilayer composition. We have found that the formation of defects induced by the presence of C leads to extrinsic oxide failure. While the presence of Ge in the oxide does not seem to introduce significant differences with respect to Si breakdown statistics, C in the oxide truly modifies the statistical profile.     

Evaluation of electron irradiated embedded SiGe source/drain diodes by Raman spectroscopy

The stress evolution of Si_0.75Ge_0.25 layers for Si_0.75Ge_0.25 source/drain (S/D) diodes, which have been irradiated at room temperature with 2-MeV electrons, is investigated using Raman spectroscopy measurements. According to Raman results for Si_0.75Ge_0.25/Si hetero-structures before and after irradiation, tensile stress was induced in the silicon substrate under the Si_0.75Ge_0.25 layer. Before irradiation, the range of tensile stress estimated by the peak shift of the Raman spectrum was −110 to −10 MPa for several measurement patterns. After irradiation, the tensile stress was relaxed in all measurement patterns though the compressive stress status in the Si_0.75Ge_0.25 layer was not changed. The stress relaxation in the silicon substrate amounted to about 10 MPa as a maximum. The influence of the irradiation on the stress relaxation in the silicon substrate, could be explained because the electron irradiation at 2-MeV can easily penetrate through the studied 140 nm Si_0.75Ge_0.25 layers.     

Strained‐SiGe Complementary MOSFETs Adopting Different Thicknesses of Silicon Cap Layers for Low Power and High Performance Applications

We introduce a strained‐SiGe technology adopting different thicknesses of Si cap layers towards low power and high performance CMOS applications. By simply adopting 3 and 7 nm thick Si‐cap layers in n‐channel and p‐channel MOSFETs, respectively, the transconductances and driving currents of both devices were enhanced by 7 to 37% and 6 to 72%. These improvements seemed responsible for the formation of a lightly doped retrograde high‐electron‐mobility Si surface channel in nMOSFETs and a compressively strained high‐hole‐mobility Si_0.8Ge_0.2 buried channel in pMOSFETs. In addition, the nMOSFET exhibited greatly reduced subthreshold swing values (that is, reduced standby power consumption), and the pMOSFET revealed greatly suppressed 1/f noise and gate‐leakage levels. Unlike the conventional strained‐Si CMOS employing a relatively thick (typically > 2 µm) Si_xGe_1‐x relaxed buffer layer, the strained‐SiGe CMOS with a very thin (20 nm) Si_0.8Ge_0.2 layer in this study showed a negligible self‐heating problem. Consequently, the proposed strained‐SiGe CMOS design structure should be a good candidate for low power and high performance digital/analog applications.     

Phase equilibria of the Si-Ge-Ti system relevant to the reactions between SiGe alloys and Ti

The semiconductor-rich region of the Si-Ge-Ti ternary isotherm at 900°C was determined by metallography, x-ray diffraction, and electron microprobe analysis. The sample alloys were prepared by arc-melting. These alloys were brought to equilibrium by annealing at 900°C for 400 h. It was confirmed that at 900°C, TiSi₂ and TiGe₂ form a continuous solid solution Ti(Si_1−yGe_y)₂ with the C54 crystal structure. It was also shown that, other than Ti(Si_1−yGe_y)₂ and Si_1−xGe_x, there is not any binary or ternary phase within the Si-Ge-TiGe₂-TiSi₂ trapezoid region. Between the Ti(Si_1−yGe_y)₂ and Si_1−xGe_x single-phase fields is the Ti(Si_1−yGe_y)₂-Si_1−xGe_x two-phase region. The tie-lines for this two-phase region were determined. The tie-lines tilt slightly toward the TiSi₂ and Ge corners. In other words, at equilibrium, the silicon to germanium atomic ratio is larger in Ti(Si_1−yGe_y)₂ than in Si_1−xGe_x (x>y). This tendency for tie-lines to tilt toward the TiSi₂ and Ge corners had been proposed in the literature as the reason for the interesting microstructure evolution during the reactions between SiGe alloys and Ti. In addition, the possible diffusion paths for the reactions between SiGe alloys and Ti were discussed based on the obtained isotherm. Recognizing Si and Ge have higher mobilities in Ti(Si_1−yGe_y)₂, it is predicted that for SiGe the extent of concentration change is large but occurs over a shorter distance, and for TiSi₂ the extent of concentration change is small but occurs over a longer distance.     

The impact of germanium in strained Si/relaxed Si1-xGex on carrier performance in non-degenerate and degenerate regimes

The impact of the fraction of germanium on the carrier performance of two-dimensional strained silicon, which embraces both the non-degenerate and degenerate regimes,is developed.In this model,the Fermi integral of order zero is employed.The impact of the fraction of germanium on the relaxed Si_1-xGe_x substrate（x）,carrier concentration and temperature is reported.It is revealed that the effect of x on the hole concentration is dominant for a normalized Fermi energy of more than three,or in other words the non-degenerate regime.On the contrary, the x gradient has less influence in the degenerate regime.Furthermore,by increasing x there is an increase in the intrinsic velocity,particularly with high carrier concentration and temperature.     

Characterization of germanium implanted Si1−xGex layer

Characterization of a Si_1−xGe_x layer formed by high-dose germanium implantation and subsequent solid phase epitaxy is reported. Properties of this layer are obtained from electrical measurements on diodes and transistors fabricated in this layer. Results are compared with those of the silicon control devices. It was observed that the germanium implantation created considerable defects that are difficult to eliminate with annealing. These defects result in boron deactivation in the p-type regions of the devices, giving rise to larger resistance. Optimization of the device structure and fabrication process is discussed.     

Experimental and theoretical investigation of defects at (1 0 0) Si1−xGex/oxide interfaces

The identification of a nontrigonal Ge dangling bond at SiO₂/Si_1−xGe_x/SiO₂ heterostructures and its electrical activity are discussed, both from experimental and theoretical points of view. This dangling bond is observed from multifrequency electron-spin resonance experiments performed at 4.2 K, for typical Ge concentrations in the range 0.4 ≤ x ≤ 0.85. The electrical activity of this defect is revealed from capacitance-voltage characteristics measured at 300 and 77 K, and is found to behave like an acceptor defect. First-principles calculations of the electronic properties of this Ge dangling bond indicate that its energy level approaches the valence band edge of the Si_1−xGe_x layer as the Ge content increases, confirming its acceptor-like nature.     

Advances in remote plasma-enhanced chemical vapor deposition for low temperature In situ hydrogen plasma clean and Si and Si1-xGex epitaxy

Remote plasma-enhanced chemical vapor deposition (RPCVD) is a low temperature growth technique which has been successfully employed inin situ remote hydrogen plasma clean of Si(100) surfaces, silicon homoepitaxy and Si_{1- x}Ge_x heteroepitaxy in the temperature range of 150–450° C. The epitaxial process employs anex situ wet chemical clean, anin situ remote hydrogen plasma clean, followed by a remote argon plasma dissociation of silane and germane to generate the precursors for epitaxial growth. Boron doping concentrations as high as 10²¹ cm^?3 have been achieved in the low temperature epitaxial films by introducing B₂H₆/He during the growth. The growth rate of epitaxial Si can be varied from 0.4Å/min to 50Å/min by controlling therf power. The wide range of controllable growth rates makes RPCVD an excellent tool for applications ranging from superlattice structures to more conventional Si epitaxy. Auger electron spectroscopy analysis has been employed to confirm the efficacy of this remote hydrogen plasma clean in terms of removing surface contaminants. Reflection high energy electron diffraction and transmission electron microscopy have been utilized to investigate the surface structure in terms of crystallinity and defect generation. Epitaxial Si and Si_1-xGe_x films have been grown by RPCVD with defect densities below the detection limits of TEM (~10⁵ cm^-2 or less). The RPCVD process also exploits the hydrogen passivation effect at temperatures below 500° C to minimize the adsorption of C and 0 during growth. Epitaxial Si and Si_1-xGe_x films with low oxygen content (~3 × 10₁₈ cm^-3) have been achieved by RPCVD. Silicon and Si/Si_1-xGe_x mesa diodes with boron concentrations ranging from 10¹⁷ to 10¹⁹ cm^-3 in the epitaxial films grown by RPCVD show reasonably good current-voltage characteristics with ideality factors of 1.2-1.3. A Si/Si_1-xGe_x superlattice structure with sharp Ge transitions has been demonstrated by exploiting the low temperature capability of RPCVD.In situ plasma diagnostics using single and double Langmuir probes has been performed to reveal the nature of the RPCVD process.     

Room temperature oxidation of Cu3Ge and Cu3(Si1−xGex) on Si1−xGex

Room temperature oxidation of Cu₃(Si_1−xGe_x) and Cu₃Ge films grown on Si_1−xGe_x at a temperature of 200–400°C was studied using transmission electron microscopy (TEM) in conjunction with energy dispersive spectrometry (EDS). For Cu₃(Si_1−xGe_x) and Cu₃Ge films grown at 200°C and then exposed to air, room temperature oxidation occurred. The Cu₃Ge film was superior to the Cu₃(Si_1−xGe_x) film in reducing the oxidation rate because of its higher Ge concentration. Annealing at higher temperatures such as 400°C resulted in Ge segregation out of the Cu₃(Si_1−xGe_x) film or Si diffusion from the Si_1−xGe_x substrate into the Cu₃Ge overlayer, and hence enhanced the oxidation rate of Cu₃(Si_1−xGe_x) and Cu₃Ge films. The present study shows that upon exposure to air even the Cu₃Ge film grown on Si_1−xGe_x is subject to room-temperature oxidation, revealing that the use of Cu₃Ge contacts on Si_1−xGe_x may be limited by some strict conditions.     

Role of surface segregation in formation of abrupt interfaces in Si/Si1−x Gex heterocompositions grown by molecular-beam epitaxy with combined sources

The coefficients of segregation of germanium atoms were measured for the Si_1?x Ge_x system grown by molecular-beam epitaxy with combined Si-GeH₄ sources under the conditions of efficient filling of surface bonds by the products of the decomposition of hydrides. In their turn, these measurements made it possible to determine for the first time the ratio between the coefficients of the incorporation of Si and Ge atoms into the growing Si_1?x Ge_x layer using the developed kinetic model of growth. For the Si-Si_1?x Ge_x structures grown by molecular-beam epitaxy with the Si-GeH₄ combined sources, the role of various mechanisms (pyrolysis, segregation, etc.) in the formation of the profile of metallurgical layer interfaces was compared for a wide range of technological parameters.     

Origin of an absorption band peaked at 5560 cm−1 and related to divacancies in Si1−x Gex

It is found that two types of centers are formed in Si_1?x Ge_x single crystals as a result of irradiation with fast electrons: divacancies (V ₂) characteristic of silicon and the V ₂ ^* centers; the latter are complexes of divacancies V ₂ with germanium atoms (V ₂Ge). It is shown that an absorption band peaked at about 5560 cm^?1 is a superposition of two absorption bands that correspond to the above centers. The V ₂ divacancies diffuse during isochronous heat treatment and interact with germanium atoms, thus giving rise to additional V ₂ ^* centers. The latter have a higher thermal stability than the V ₂ centers do, and their annealing temperature increases with increasing content of germanium.     

Features of electron transport in relaxed Si/Si1 − x Ge x transistor heterostructures with a high doping level

The low-temperature electrical and magnetotransport characteristics of partially relaxed Si/Si_{1 ? x} Ge _x heterostructures with an electron conduction channel in an elastically strained nanoscale silicon layer are investigated. It is demonstrated that the electron gas in the system exhibits 2D properties. A dependence of the conductivity along layers in the system on the degree of elastic-stress relaxation in it is observed. To understand the observed regularities, the potential and the electron distribution over the structure layers are calculated in detail for samples with different layer strains and doping levels. For the structure with x = 0.25, the parameters of the potential barrier and characteristics of the quantum well formed in the Si layer are estimated. It is established that the characteristics of the potential formed near interfaces strongly depend on the initial parameters of the system, in particular, on the degree of the plastic relaxation of elastic stresses and on the doping level. The formation of a thin tunneling-transparent barrier near the upper interface can lead to the redistribution of electrons between the 2D and 3D conduction channels in the structure, which ensures the spread of the measured transport characteristics of the samples during the measurements. The interlayer tunneling transitions of carriers from the 2D state in the Si transport channel to the 3D state of the Si_{1 ? x} Ge _x crystal matrix, which are separated by a tunneling-transparent potential barrier near the heterointerface, were observed for the first time during transport in the direction transverse to the layer plane.     

The Hakoniwa method,an approach to predict material properties based on statistical thermodynamics and ab initio calculations

An approach based on statistical thermodynamics and ab initio calculations to predict properties of materials composed of different types of atoms is presented. The key point of what the authors called the “Hakoniwa” method, is to take into account all possible structural supercells constructed by the fixed number of atoms of each species according to the composition of the target material. The conservation of the total number of atoms enables calculating the average value of a material property for a given temperature by applying statistical thermodynamics to the material property values obtained for each of the possible supercells. The application of the Hakoniwa method is illustrated by calculating the average energy gain by mixing Sn and Si atoms in a Ge matrix, as function of the Ge_1−x−ySn_xSi_y composition. The relative stability of each composition is compared allowing predicting the impact of Si doping on the stability of Ge_1−x−ySn_xSi_y films epitaxially grown on a Ge substrate. In addition, the average bandgap is calculated for a given Ge_1−x−ySn_xSi_y composition as a function of the temperature.     

Segregation of phosphorus and germanium to grain boundaries in chemical vapor deposited silicon-germanium films determined by scanning transmission electron microscopy

The segregation of phosphorus and germanium to grain boundaries in P implanted, Si_0.87Ge_0.13 films deposited by chemical vapor deposition (CVD), was investigated using energy dispersive x-ray (EDX) micro-analysis. A quantitative analysis of the x-ray spectra obtained at grain boundaries showed that the excess amount of P varied with the crystallography of the boundary but that the segregation always followed an equilibrium process with an activation energy of 0.28 eV. On the other hand, Ge did not segregate to grain boundaries in either P implanted Si_0.87Ge_0.13 films nor in intrinsic Si_1−xGe_x films, containing 2, 13 and 31 at.% Ge. Possible reasons for the absence of Ge segregation are discussed.