Ge Mole Fraction Research Articles

Structural roughness and interface strain properties in Si/SiO2/Poly-Si1−xGex tri-layer system with ultrathin oxide

We have explored the microstructure and local interface strain in the poly-Si1−xGe x /SiO2/Si tri-layer system with ultrathin oxides. High-resolution transmission electron microscopy (HRTEM) and high-resolution X-ray diffraction rocking curves (HR-RC) and two-dimensional reciprocal space mapping (2D-RSM) were the main characterization tools. The poly-Si1−xGe x /SiO2/Si structures have x=0, 0.2, and 0.35 for ultrathin oxides (2.0–3.0 nm). The result shows that for the adopted growth process, the poly grain size depends very strongly on the Ge concentration, and it increases with increasing Ge mole fraction. In turn, this increase of the grain size in the poly-Si1−xGe x /SiO2/Si reduces the strain in the film, which then affects the interface strain at the lower SiO2/Si interface. In addition, the presence of defects at the SiO2/Si interface was found to be greater for samples with no local interface strain.

An analytical approach to the modelling of intrinsic base sheet resistance in a SiGe HBT and optimal profile design considerations for its minimization

An analytical model has been developed to calculate the intrinsic base sheet resistance (Rbi), for a given Ge dose and base dopant, with a view to determine the optimal doping profile and Ge profile in the neutral base of a SiGe-base HBT. We have derived an empirical formula for the diffusion constant of holes in a p-type SiGe as a spatial function of mole fraction of Ge as well as base doping concentration. Our model has been verified with experimental and simulated results published in the literature. Our study shows that for a given Ge dose and amount of base dopant, a triangular Ge profile with a retrograde dopant profile yields the lowest Rbi.

Calculation of figures of merit of Si/Si 1− x− yGe xC y/Si HBTs and their optimization

Incorporation of C in the SiGe base of a heterojunction bipolar transistor (HBT) prevents outdiffusion of B across the junctions, improves film stability by reducing strain, and leads to better process integration. This paper investigates theoretically how the values of figure of merit such as current gain ( β), cut-off frequency ( f T) related inversely to the base transit time ( τ b), and the Early voltage ( V A) change with a small amount (<1%) of C incorporation in the base. For this purpose, we have derived generalized expressions for these quantities using a box-type, trapezoidal and triangular material (Ge and C) profile and an exponential doping profile. The variation of saturation-drift velocity with Ge mole fraction has been used to calculate τ b. A comparison between the calculated values, keeping the strain the same in both SiGe and SiGeC bases, indicates that all values of figure of merit are enhanced with C incorporation. The value of V A increases in SiGeC HBTs with small amounts of C because of prevention of B outdiffusion across the collector–base junction and sharp fall off of the C profile at the same junction.

Electrical properties of shallow p+-n junction using boron-doped Si1−xGex layer deposited by ultrahigh vacuum chemical molecular epitaxy

Strained boron-doped Si1−xGex layers with different Ge mole fractions were selectively deposited by ultrahigh vacuum chemical molecular epitaxy to form shallow p+-n junction suitable for raised source/drain metal–oxide–semiconductor field effect transistor applications. Detailed electrical characterizations were performed. Our results show that the reverse leakage current could be optimized by a rapid thermal annealing at 950 °C for 20 s, and a near perfect forward ideality factor (i.e., &amp;lt;1.01) is obtained for the p+-n Si1−xGex/Si junction. By analyzing the periphery and area leakage current components of p+-n Si1−xGex/Si junctions with various perimeter lengths and areas, the degree of misfit dislocations and undercut effect were studied. The specific contact resistance was found to decrease as Ge mole fraction increases. Junction depth measurements also show that the junction depth decreases monotonically with increasing Ge mole fraction. The reduced B diffusion constant is attributed to the increasing Ge gradient in the transition region.

Electrical and Compositional Properties of Co-Silicided Shallow p + ­ n Junction Using Si-Capped/Boron-Doped Si1 − xGex Layer Deposited by UHVCME

Co-silicided, strained boron-doped junction with different Ge mole fractions deposited by ultrahigh vacuum chemical molecular epitaxy (UHVCME), suitable for raised source/drain metal oxide semiconductor field effect transistor applications, is studied. The electrical and compositional characteristics are presented. By using a Co/Si-cap/graded structure, optimum forward and reverse diode characteristics with a near perfect forward ideality factor (i.e., <1.01) can be obtained for the junction. The low area leakage current extracted from Si-capped samples confirms that serious lattice misfit problems during silicidation, typical of Co/pure samples, is largely eliminated by Si capping. In addition, the specific contact resistance is found to decrease as Ge mole fraction increases, and would be even lower if Si capping is used for silicidation. Finally, the composition of and interfacial reaction is studied by high-resolution X-ray spectra. Our results show that even after a second anneal at 850°C, very little lattice distortion is discovered. Five harmonic peaks still remain even after a second anneal at 850°C. Transmission electron micrograph also exhibits good uniformity and interface quality. © 2001 The Electrochemical Society. All rights reserved.

Electrical characterization of ultra-thin gate oxides on Si/Si1-x-yGexCy/Si quantum wellheterostructures

Ultra-thin gate oxides (<100 Å) have been grown on partiallystrain-compensated Si/Si1-x-yGexCy/Si heterolayers(with different carbon concentrations) using microwave O2-plasma. AMOS capacitor has been used for the characterization of grown oxides.Capacitance-voltage (C-V) profiling has been used to measure theapparent doping profile and thickness of the unconsumed Si-cap layer. Asignificant improvement in charge trapping properties, under Fowler-Nordheim(F-N) constant current stressing, has been observed for low carbon containingfilms. The valence band offsets (ΔEv) of Si/Si0.69Ge0.3C0.01 and Si/Si0.685Ge0.3C0.015 heterostructures have been extracted using the hole confinementcharacteristics. Incorporation of C lowers the valence band offset of theternary alloy compared to those in Si1-xGex with the same Gemole fraction.

Simulation and optimization of strained Si 1−xGe x buried channel p-MOSFETs

Deep submicron (0.35 μm) strained Si 1− x Ge x buried channel p-MOSFETs with a Ge concentration up to 50% were simulated using the MEDICI device simulator. A buried channel structure offers several benefits over a surface channel structure without a Si cap. Simulation results show that the maximum drain current increases monotonically with the Ge mole fraction. The drive current enhancement is more than 300% for Si 0.5 Ge 0.5 over Si. Subthreshold characteristics were analyzed for different Ge mole fractions in this study. The effects of Si cap layer thickness and Si 1− x Ge x channel thickness on drive current and gate voltage operating window were analyzed. The simulation results show that the drive current is the highest when the Si 1− x Ge x layer thickness is between 100 and 300 Å and that Si 1− x Ge x layer thickness can be as low as 50 Å with less than 10% penalty in the drive current, for structures with a 50 Å Si cap layer.

High-performance deep submicron CMOS technologies with polycrystalline-SiGe gates

The use of polycrystalline SiGe as the gate material for deep submicron CMOS has been investigated. A complete compatibility to standard CMOS processing is demonstrated when polycrystalline Si is substituted with SiGe (for Ge fractions below 0.5) to form the gate electrode of the transistors. Performance improvements are achieved for PMOS transistors by careful optimization of both transistor channel profile and p-type gate workfunction, the latter by changing Ge mole fraction in the gate. For the 0.18 /spl mu/m CMOS generation we record up to 20% increase in the current drive, a 10% increase in the channel transconductance and subthreshold swing improvement from 82 mV/dec to 75 mV/dec resulting in excellent on/off currents ratio. At the same time, NMOS transistor performance is not affected by gate material substitution.

A surfactant-mediated relaxed Si0.5Ge0.5 graded layer with a very low threading dislocation density and smooth surface

A method to grow a relaxed Si0.5Ge0.5 graded layer with a very smooth surface and a very low threading dislocation density using solid-source molecular-beam epitaxy is reported. This method included the use of Sb as a surfactant for the growth of a 2 μm compositionally graded SiGe buffer with the Ge concentration linearly graded from 0% to 50% followed by a 0.3 μm constant Si0.5Ge0.5 layer. The substrate temperature was kept at 510 °C during the growth. Both Raman scattering and x-ray diffraction were used to determine the Ge mole fraction and the degree of strain relaxation. Both x-ray reflectivity and atomic force microscopy measurements show a surface root mean square roughness of only 20 Å. The threading dislocation density was determined to be as low as 1.5×104 cm−2 as obtained by the Schimmel etch method. This study shows that the use of a Sb surfactant and low temperature growth is an effective method to fabricate high-quality graded buffer layers.

Photoresponse of Si 1− xGe x heteroepitaxial p–i–n photodiodes

A simulation study of spectral response, dark current and capacitance of Si 1− x Ge x heteroepitaxial p–i–n photodiodes as a function of thickness of the i-layer and Ge mole fraction has been performed. It has been found that the cut-off wavelength of the diodes increases from 1.12 to 1.50 μm as the Ge mole fraction ( x) is increased from 0.0 to 0.75 and the photoresponse increases with the thickness of the absorbing i-layer. The simulated photoresponse of p–i–n diodes is in good agreement with reported experimental data. The dark current is found to increase with Ge concentration and remains in the nA range for upto 30% Ge which corresponds to a cut-off wavelength of ≈1.3 μm, extending the possibility of these devices in fiberoptic communication applications.

Properties of Si1−xGex three-dimensional islands

Thin, coherently strained films of SiGe were deposited on Si(001) to form faceted, dislocation-free, three-dimensional (3D) islands via the Stranski–Krastanov (SK) growth mode. Deposition was interrupted to determine the saturation island number density as a function of alloy composition, substrate temperature during growth, and growth rate. To control the shape of buried islands during encapsulation with Si, the 3D islands were embedded and overgrown at various temperatures. The temperature dependence of the island number density yields an approximate activation energy of 0.7 eV for diffusion of Ge dimers on a Ge covered Si(001) surface. The dependence of the 3D-island number density on growth rate cannot be understood without modifying the classical model of nucleation and growth to account for the wetting layer present in SK systems. To explain the island number density as a function of alloy composition, a simple linear elastic model is developed in which the island number density is proportional to the inverse square of the Ge mole fraction in the alloy plus a constant. Finally, cross-sectional transmission electron microscopy reveals that the island shape changes dramatically during encapsulation, but the morphology can be kinetically preserved.

Mechanisms determining three-dimensional SiGe lsland density on Si(001)

Thin, coherently strained, films of SiGe were deposited on Si(001) in the Stranski-Krastanow (SK) growth mode to form small, faceted, dislocation-free-three-dimensional (3D) islands. The number density of these islands was determined as functions of SiGe alloy composition, growth rate, and substrate temperature during growth. From these experiments, the classical model of 3D island nucleation and growth yields an approximate activation energy for diffusion of Ge dimers on a Ge covered Si(001) surface of 0.70 eV. The dependence of the 3D-island number density on growth rate cannot be understood without modifying the classical model to account for the wetting layer present in SK systems. Heteroepitaxial strain is not included in the classical model of island nucleation and growth. A simple linear elastic model that fits the data is developed that predicts the island number density is proportional to the inverse square of the Ge mole fraction in the alloy plus a constant.

Dependence of the Structural and Optical Properties of SixGe1-x/Si Heterostructures on the Si Substrate Temperature and the Ge Mole Fraction

Dependence of the Structural and Optical Properties of SixGe1-x/Si Heterostructures on the Si Substrate Temperature and the Ge Mole Fraction

Growth and characterization of strained Si 1− xGe x multi-quantum-well waveguide photodetectors on (1 1 0) Si for 1.3 and 1.55 μm

Pseudomorphic Si 1− x Ge x /Si multi-quantum well p–i–n photodiodes have been grown on (1 1 0) Si by molecular beam epitaxy. Using waveguide geometry we have obtained an external quantum efficiency of 17% at 1.32 μm (TE-polarisation) with a Ge mole fraction of x=0.5. At λ=1.52 μm (TE-polarisation) an external quantum efficiency of 7% has been achieved. From the dependence of photocurrent on device length effective interband absorption coefficients for x=0.5 and 0.37 have been determined quantitatively.

Investigation of poly-Si/sub 1-x/Ge/sub x/ for dual-gate CMOS technology

Poly-Si/sub 1-x/Ge/sub x/-gated MOS capacitors were fabricated with x varying from 0 to 0.5. NMOS and PMOS C-V characteristics were measured. Reduced poly-gate depletion effect (PDE) was observed in PMOS devices with increasing Ge mole fraction; while for NMOS, devices with a Ge content /spl sim/20% exhibit the least PDE. Higher active dopant concentration and reduced gate-depletion width for devices featuring less PDE were confirmed. Work function difference (/spl Phi//sub MS/) was found to decrease slightly in N/sup +/ films and significantly in P/sup +/ films as Ge content increases. The shift in /spl Phi//sub MS/ for N/sup +/ poly-Si/sub 1-x/Ge/sub x/ is negligible while it is -0.13 V for P/sup +/Si/sub 0.8/Ge/sub 0.2/ and -0.32 V for P/sup +/Si/sub 0.5/Ge/sub 0.5/. The reduction in energy bandgap (/spl Delta/E/sub g/) was also determined to increase from 0 to 0.26 eV as Ge content increases from 0 to 50%. For deep submicron dual-gate CMOS application, the shift in /spl Phi//sub MS/ should be minimized for low and symmetrical V/sub th/ as well as improved short-channel effect (SCE). A Ge content of /spl sim/20% therefore seems to offer the best tradeoff between SCE and PDE.

Device parameter optimization of strained Si channel SiGe/Si n-MODFET's using a one-dimensional charge control model

We have simulated the strained Si channel SiGe n-MODFET structure using a one-dimensional (1-D) self-consistent Schroedinger-Poisson charge control model. The quantum confinement effect has been investigated and key transistor parameters have been optimized for maximum f/sub T/. It has been found that the doping concentration into the donor layer and the Ge mole fraction of the SiGe layers should be as high as possible, provided that the doping diffusion and the avalanche breakdown are under control and the crystalline quality of the epilayers is not significantly degraded. The optimum channel thickness was found to be between 5 and 7.5 nm. In addition, it has been shown that the thickness of the donor layer should be used for threshold voltage adjustment rather than for f/sub T/ improvement.

Solid-state interdiffusion mechanism in strained Si 1-x Ge x /Si heterostructures

The interdiffusion in a low-strained Si0.93Ge0.07/Si epilayer was analyzed by double-crystal X-ray diffraction. The interdiffusion was characterized by a low diffusion barrier of 1.81 eV with a diffusion constant of 4.3 × 10−5 cm2/sec, which indicates correlation with the stacking fault generated by the homoepitaxial growth of the Si layer prior to the growth of the strained SiGe layer. At the very low-strained layer, the driving force causing the interdiffusion is the concentration gradient, and the mechanism is self-diffusion of Si. Furthermore, the interdiffusion mechanisms were classified into three groups, depending on the Ge mole fraction x. For x 0.4, Si atoms can be diffused into the alloy.

Si/Si 1−x Ge x heterostructures: Electron transport and field-effect transistor operation using Monte Carlo simulation

Using an ensemble Monte Carlo simulation, we study the electron transport properties in tensile strained Si grown on a Si1−xGex substrate and in an n-channel Si/Si1−xGex modulation-doped field-effect transistor (MODFET). Owing to the strain-induced modification of the conduction-band structure, the in-plane drift mobility in undoped material reaches 3250 cm2/V s at 300 K (for x⩾0.2) and 31000 cm2/V s at 77 K (for x⩾0.05). The two-dimensional Monte Carlo modeling of 0.18 μm gate Si/SiGe MODFETs reveals a high velocity overshoot of 2.5×107 cm/s related to a significant ballistic transport through the gated part of the strained Si channel. For a 0.18 μm gate and a Ge mole fraction x=0.3, intrinsic transconductance and transition frequency as high as 460 mS/mm and 85 GHz are obtained.

Dislocation-related photoluminescence peak shift due to atomic interdiffusion in SiGe/Si

Low-temperature photoluminescence (PL) spectroscopy was used to study electronic states associated with threading dislocations (D lines) in strain-relaxed Si1−xGex layers. The structures investigated were grown by ultrahigh vacuum chemical vapor deposition at 550 °C and consist of a Si(001) substrate, followed by a stepwise graded buffer layer, followed by a thick uniform composition Si1−xGex layer. The PL peak positions of the four D lines after isochronal annealing at temperatures between 600 and 800 °C were measured. We show that the large energy shift of the D1 line is due to a change in the local band gap energy at the dislocation core due to strain-driven diffusion of Ge atoms away from the dislocation core with an activation energy Ea, which varies with Ge mole fraction x.

Study of uniform and graded SiGe channel heterojunction p-MOSFETs using Monte Carlo simulation

We report on a Monte Carlo analysis of a SiGe p-channel 0.18 μm MOSFET. This work emphasizes the influence of the Ge channel profile on device performances. A uniform Ge profile with 0.3 Ge mole fraction provides a good hole confinement in the channel, with a reduction of the surface scattering at oxide interface and an improvement of the hole transport properties in SiGe. A graded Ge profile (0% at the bottom of the channel to 30% at the top) provides a further improvement due to the hole confinement closer to the gate. A better gate-control of the drain current is achieved leading to an increased transconductance, a reduced drain-induced-barrier-lowering, a reduced subthreshold slope, and a better saturation behavior.