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.     

Type III - Type I transition and strain-effect in Hg1−xCdxTeCdTe and Hg1−xZnxTeCdTe superlattices

In this paper, the results of Hg_1−xZn_xTeCdTe strained layer superlattices grown by MBE are reported, and compared to Hg_1−xCd_xTeCdTe superlattices. Both Type III and Type I Hg_1−xZn_xTeCdTe superlattices with different strain have been grown on CdTe(111)B/GaAs(100) and CdTe(100)/GaAs(100) substrates and characterized by electron, X-ray diffraction, infrared transmission and Hall measurements. The values of hole mobility between 5×10³ up to 2×10⁴cm²v⁻¹s⁻¹ at T = 23K along (111)B growth orientation and up to 4.9×10⁴cm²v⁻¹s⁻¹ at T = 5K along (100) growth orientation are obtained for Type III superlattices whereas in Type I superlattices, the hole mobility is between 200–300cm²v⁻¹s⁻¹. This drastic change in the hole mobility between Type III and Type I superlattices along with the role of the strain are discussed in this paper.     

Strain-balanced Si<Subscript>1-x</Subscript>Ge<Subscript>x</Subscript>/Si type II quantum wells for 1.55 μm detection and emission

This work deals with the optoelectronic properties of heterostructures built on type II Si_1-xGe_x/Si strained quantum wells grown on relaxed Si_1-yGe_y/Si (001) pseudo-substrates. To limit the intrinsic problem due to the real-space indirect nature of the interface, we propose and model three heterostructures having three different potential profiles of the valence and conduction bands which consist in various arrangements of Si and Si_1-xGe_x barriers of different Ge contents. The proposed stacks are designed in a pragmatic way for a pseudomorphic growth on relaxed Si_1-yGe_y assuming individual layer thickness being smaller than the known critical thickness and an overall compensation of the strain. Variation of thickness and compositions (x>y) permits to optimize i) the quantum confinement of electrons and heavy-hole levels and ii) the wave function's overlap and the out-of-plane oscillator strength. The optimum parameters satisfy a fundamental emission at a key 1.55 μm wavelength below the absorption edge of each layer constitutive of the stacks. A comparison between the characteristics of the three heterostructures brings out the superior advantages of the W architecture.     

Ge metal oxide semiconductor field effect transistors with optimized Si cap and HfSiO2 high-k metal gate stacks

High mobility metal-oxide-semiconductor-field-effect-transistors (MOSFETs) are demonstrated on high quality epitaxial Si_0.75Ge_0.25 films selectively grown on Si (100) substrates. With a Si cap processed on Si_0.75Ge_0.25 channels, HfSiO₂ high-k gate dielectrics exhibited low C–V hysteresis (<10 mV), interface trap density (7.5 × 10¹⁰), and gate leakage current (∼10⁻²A/cm² at an EOT of 13.4 Å), which are comparable to gate stack on Si channels. The mobility enhancement afforded intrinsically by the Si_0.75Ge_0.25 channel (60%) is further increased by a Si cap (40%) process, resulting in a combined ∼100% enhancement over Si channels. The Si cap process also mitigates the low potential barrier issues of Si_0.75Ge_0.25 channels, which are major causes of the high off-state current of small band gap energy Si_0.75Ge_0.25 pMOSFETs, by improving gate control over the channel.     

Synthesis and ionic conduction of C3A-type nasicon Na3+3x−yY1−xSi3−yPyO9

In the solid solutions of Na_3+3x−yY_1−xSi_3−yP_yO₉, new fast Na⁺ -conductors were obtained. The ionic conductivity, σ, and the activation energy, E, of the most conductive material, Na_4.425Y_0.375Si_2.55P_0.45O₉ were 5 × 10⁻³ S/cm at 300°C and 10 kcal/mol, respectively. Those values were strongly dependent upon the composition. With an increase of Na concentration, [Na], σ increased and E decreased. Accommodation of higher [Na] in the structure was madefeasible by decreasing [Y] and increasing [P]. Based on the crystal structure assumed for Na_3+3x−yY_1−xSi_3−yP_yO₉ and on the above results, a conduction model was presented. The structural consideration revealed that a conduction path is formed along the 〈1 1 1 〉 direction, where Y³⁺ ions locate in the way of Na⁺ ions.     

Si<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript>x</Subscript>/Si(001) relaxed buffer layers grown by chemical vapor deposition at atmospheric pressure

Relaxed step-graded buffer layers of Si_1?xGe_x/Si(001) heterostructures with a low density of threading dislocations are grown through chemical vapor deposition at atmospheric pressure. The surface of the Si_1?xGe_x/Si(001) (x ～ 25%) buffer layers is subjected to chemical mechanical polishing. As a result, the surface roughness of the layers is decreased to values comparable to the surface roughness of the Si(001) initial substrates. It is demonstrated that Si_1?xGe_x/Si(001) buffer layers with a low density of threading dislocations and a small surface roughness can be used as artificial substrates for growing SiGe/Si heterostructures of different types through molecular-beam epitaxy.     

高能电子衍射研究H钝化偏角Si衬底上Si,GexSi1-x的分子束外延生长模式

用高能电子衍射(RHEED)研究H钝化偏角Si衬底上Si,Ge_xSi_1-x材料的分子束外延(MBE)生长模式,发现经低温处理的H钝化Si衬底上要经过10nm左右的Si生长才能获得比较平整的表面.Si,Ge_xSi_1-x外延时的稳定表面均以双原子台阶为主,双原子台阶与单原子台阶并存的结构.Si双原子台阶上的Si二聚体列(dimerrow)取向垂直于台阶边缘,而Ge_xSi_1-x双原 关键词：     

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.     

Atomic structure and lattice dynamics of strain-compensated Si1-x-yGexCy layers

The local atomic structure and lattice dynamics are studied for strain compensated Si_1-x-yGe_xC_y layers grown by molecular beam epitaxy on Si (001) substrates. The layers were characterized by transmission electron microscopy, x-ray diffraction, and Raman scattering and modeled using a valence-force field model. For a [Ge]/[C] ratio of approximately ten, the lattice constant in the growth direction is equal to that of the substrate, indicating an absence of macroscopic strain. Experimental and theoretical results are compatible with Vegard's rule. To handle the large bond length distortions near C atoms properly, the valence-force field model used includes anharmonic effects via bond-length-dependent interatomic force constants which were determined from ab initio density-functional calculations. The dependence of the Raman spectra on strain and composition of Si_1-x-yGe_xC_y layers can be explained by the model calculations.     

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.     

Surface kinetics driven annealing and phase segregation in non-equilibrium crystal growth

We simulate the epitaxial growth and annealing of Ge_xSi_1−x alloys on a Si(001) substrate using a stochastic Monte Carlo technique. Initiation of phase segregation into Si(001) and Ge(001) layers is obtained in our simulation. This results from the kinetically more active nature of Ge atoms as compared to Si atoms, together with an effect arising from the compressive strain in the alloy layers. We discuss our results in the light of the existing experimental data.     

Raman spectra of silicon monoxide

The stoichiometric thin films of SiO have been studied by Raman scattering. The spectra indicate three main peaks at 444 cm⁻¹, 454 cm⁻¹ and 502 cm⁻¹ which may correspond to an atomic arrangement of tetrahedra of the type SiSi_yO_4−y where y ≠ 0 and y ≠ 1. The results contradict the mixture model (Si+SiO₂) and show that the SiO oxide cannot be considered as a mixture of Si and SiO₂.     

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.     

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     

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.     

Growth features at competing nucleation of quantum dots and nanopits in Si-Ge-C ternary system

The continuum elasticity model is applied to quantitatively investigate the growth features and nucleation mechanism of quantum dots (QDs), nanopits, and joint QDs-nanopits structures in SiGeC ternary systems. We determined the critical concentrations of carbon, x = 0.08 for Si_1?x C _x and y = 0.0033 for Si_1?x?y Ge _x C _y at the concentration of germanium x = 0.467, when the growth mode changes from, respectively, nucleation of QDs to formation of nanopits. We have shown that the increase in the carbon concentration in a SiC binary system and of germanium concentration in a SiGeC ternary system leads to a decrease in the critical size of QD. The free energy of mixing for Si_1?x?y Ge _x C _y ternary system was calculated and studied and its 3D sketch plotted. It is demonstrated that incorporation of carbon into Si and Ge up to background impurity concentration is energetically preferable, which explains the experimentally detected ??contamination?? of silicon and germanium crystals by carbon during the growth in the graphite crucible.     

Effect of the addition of Cr,Ta and Nb on structural and magnetic properties of Fe–Si alloys

High-resolution X-ray diffraction patterns of Fe_78.5Si₂₀Ta_1.5 and Fe_80−xSi₂₀Nb_x (x=0.5, 3.0, 10) show the existence of a metastable hexagonal phase (NbFe_2−xSi_x and TaFe_2−xSi_x) together with bcc-Fe_100−ySi_y solid solution and an amorphous matrix. The grain size of the bcc-Fe_100−ySi_y phase decreases with increasing Nb content in Fe–Si compounds resulting in nanocrystalline materials. On the other side, the micro-strain, the lattice constant, coercivity and the Curie temperature increase with Nb concentration. Enhancements of the local strain and local magnetic anisotropy were also observed by means of NMR measurements. On the contrary, the addition of Cr causes a decrease in the coercivity and Curie temperature.     

Electrical characterization studies of p-type Ge,Ge1−ySny,and Si0.09Ge0.882Sn0.028 grown on n-Si substrates

The electrical properties of p-type Ge, Ge_1−ySn_y, and Si_0.09Ge_0.882Sn_0.028 samples grown on n-type Si substrates using ultra-high vacuum chemical vapor deposition have been investigated as a function of temperature. Degenerate parallel conducting layers were found in all Ge/Si, Ge_1−ySn_y/Si, and Si_0.09Ge_0.882Sn_0.028/Si samples, which are believed to be associated with dislocation defects at the interface produced by the lattice mismatch between the two materials. These degenerate conducting layers affect the electrical properties of all the thin epitaxial films. Additionally, temperature dependent Hall-effect measurements show that these materials exhibit a conductivity type change from p to n at around 370–435 K. The mobilities of these samples are generally lower than that of bulk Ge due to carrier scattering near the interface between the epitaxial layer and the Si substrate and also due to alloy scattering. Detailed behavior of temperature-dependent conductivity of these samples is also discussed.     

On optimum designs of a RCE Si/SiGe/Si MQW photodetector for long wavelength applications

In the present paper, performance analysis of a resonant-cavity-enhanced Si/Si_1–y Ge _y /Si multiple quantum well photodetector has been carried out. The effects of material parameters of active Si_1–y Ge _y layers and carrier trapping by the potential barriers at the heterointerfaces of Si/Si_1–y Ge _y /Si quantum wells on the bandwidth (BW) and Responsivity of the photodetector have been investigated using numerical simulation. Results on possible optimum designs of the photodetector have been suggested to maximize BW-quantum efficiency product.     

Real-time observation of the diffusive motion of 2×2 adlayer formed on Ag(100) nanoparticle facet by nitric acid adsorption

1-x-y Ge_xC_y have been grown on Si(100) substrates by rapid thermal chemical vapor deposition (RTCVD) using C₂H₄ as C source. The composition and microstructure of Si_1-x-yGe_xC_y films were characterized by Auger electron spectroscopy, Raman spectra and Fourier-transform infrared spectroscopy. The results show that lower temperature and higher SiH₄/C₂H₄ flow ratio are helpful in forming the substitutional C and improving the crystal quality. A possible mechanism for C incorporation in Si_1-x-yGe_xC_y layers grown by RTCVD using C₂H₄ is proposed. Received: 26 April 1998 / Accepted: 28 September 1998 / Published online: 24 February 1999