Relaxed SiGe Buffer Research Articles

Tradeoff between short channel effect and mobility in strained-Si nMOSFETs

A strained-Si CMOS technology with improved immunity to short channel effects (SCE) and reduced defect density was demonstrated at a gate length of 80 nm. The driving current was enhanced by 25% using 20% Ge content relaxed SiGe buffer. The trade-offs between electron mobility enhancement and SCE control for different Si-cap layer thicknesses and various Ge contents in the relaxed-SiGe virtual substrate are investigated. Besides, our work presents the optimum processes window for strained-Si devices in advanced CMOS technology.

From Thin Relaxed SiGe Buffer Layers to Strained Silicon Directly on Oxide

Thin relaxed SiGe buffer layers with Ge contents of 23at% and 26at% and thicknesses between 150 to 180 nm on Si(100) terminated with 6 nm Si were made by He ion implantation and annealing. Subsequent overgrowth of the thin buffers with thin strained Si cap layers with SiGe and strained Si reduced the threading dislocation and pile-up densities significantly. Best results were achieved for high temperature SiGe overgrowth with a thickness of 200nm and Ge contents lower than the effective Ge content of the buffer. Threading dislocation densities as low as 1x105 cm-2 and dislocation pile-up densities of 10 cm-1 were determined. Strained silicon on insulator wafers were produced by wafer bonding. Wafer splitting was done by H ion implantation and annealing. The strain of the layers was measured by Raman spectroscopy revealing a strain of 0.66% in the tensile strained Si on the partially relaxed Si0.77Ge0.23 buffers.

Computation of the preferential nucleation sites for Ge quantum dots on a relaxed SiGe layer

We compared directly the nucleation sites of Ge quantum dots on a relaxed SiGe buffer layer with the sites of a minimum stored energy density of the Ge dots. The computed elastic stored energy of the Ge dots encompasses the effect of both lattice mismatch and misfit dislocations. We computed the strain/stress of the misfit dislocations near a free surface by using the solution presented by Gosling and Willis [J Mech Phys Solids 1994;42:1199]. The study revealed that the intersection of the slip-planes with the free surface is the site of the maximum tensile strain, thus the Ge dots, which have a negative misfit, preferentially nucleate at that site. Our work clearly demonstrates the role of misfit dislocations in the formation of the ordered array of Ge dots. We also discuss the application of misfit dislocations for selective etching and the effects of a free surface on the displacements of the underlying misfit dislocations.

Pure germanium epitaxial growth on thin strained silicon-germanium graded layers on bulk silicon substrate for high-mobility channel metal-oxide-semiconductor field-effect transistors

We demonstrate epitaxially grown high-quality pure germanium (Ge) on bulk silicon (Si) substrates by ultra-high-vacuum chemical vapor deposition (UHVCVD) without involving growth of thick relaxed SiGe buffer layers. The Ge layer is grown on thin compressively strained SiGe layers with rapidly varying Ge mole fraction on Si substrates resulting in several SiGe interfaces between the Si substrate and the pure Ge layer at the surface. The presence of such interfaces between the Si substrate and the Ge layer results in blocking threading dislocation defects, leading to a defect-free pure Ge epitaxial layer on the top. Results from various material characterization techniques on these grown films are shown. In addition, capacitance-voltage (CV) measurements of metal-oxide-semiconductor (MOS) capacitors fabricated on this structure are also presented, showing that the grown structure is ideal for high-mobility metal-oxide-semiconductor field-effect transistor applications.

From Thin Relaxed SiGe Buffer Layers to sSOI: Structural and Electrical Characterisation

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Defect Engineering Considerations for Strained Silicon Substrates

Defect-engineering parameters influence the electrical performance of p-n junctions in strained silicon (SSi) substrates. The thin SSi layers were deposited on thin Strain Relaxed SiGe Buffers (SRBs), whereby strain relaxation is achieved by implementing a thin C-doped SiGe layer. Processing variables studied are the depth dC of this layer and the carbon concentration, determining the threading dislocations (TDs) density. As references, step-graded SRB material and standard Czochralski silicon was used. Combined current-voltage and capacitance-voltage measurements indicate that within the range studied (2-3x105 to 1-2x107 cm-2), the TDD has the strongest impact on the reverse current density and generation lifetime. Additional leakage current is created by defects associated with the carbon-rich layer, when present in the junction depletion region. The electrical activity of the TDs also depends on the well doping type (n- or p). The largest activity is observed in n+/p junctions, which are technologically the more relevant ones.

Analysis of Nanoscale Stress in Strained Silicon Materials and Microelectronics Devices by Energy-Filtered Convergent Beam Electron Diffraction

The convergent beam electron diffraction (CBED) technique of transmission electron microscopy (TEM) has excellent capabilities for strain detection at high spatial resolution. Here we report strain measurements in a bulk s-Si/SiGe/Si wafer and in a strained 35nm PMOS device in which SiGe acts as the source and drain. CBED measurements of the composition of the relaxed SiGe buffer are in quantitative agreement with Raman spectroscopy. For the PMOS device, CBED measured a uniaxial compressive stress of 1.12GPa in the channel. However, it was found that even in the cross-sectional TEM samples with thicknesses greater than 300nm, the intrinsic surface strain relaxation was often so severe that no recognizable high-order Laue zone lines in the CBED patterns could be collected. The amorphorization of both free surfaces of the TEM sample to a range of about 80nm is proposed to minimize the impact of surface strain relaxation for future study.

A semiempirical surface scattering model for quantum corrected full-band Monte-Carlo simulation of biaxially strained Si/SiGe NMOSFETs

A new semiempirical surface scattering model for electrons in strained Si devices including a quantum correction has been developed and implemented into our FBMC simulator. The strain is assumed to be consistent with pseudomorphic growth on a relaxed SiGe buffer. By introducing a few additional terms into the physical scattering rates which depend on the Ge-content in the SiGe buffer, the new surface scattering model can excellently reproduce low-field inversion layer mobility measurements for a wide range of Ge-content (0–30%) and substrate doping levels (10 16–5.5 × 10 18 cm −3). As a device example, an NMOSFET with 23 nm gate length with and without a strained Si channel has been simulated by the new FBMC model.

Special features of the formation of Ge(Si) islands on the relaxed Si1−xGex/Si(001) buffer layers

The results of studying the growth of self-assembled Ge(Si) islands on relaxed Si1−xGex/Si(001) buffer layers (x≈25%), with a low surface roughness are reported. It is shown that the growth of self-assembled islands on the buffer SiGe layers is qualitatively similar to the growth of islands on the Si (001) surface. It is found that a variation in the surface morphology (the transition from dome-to hut-shaped islands) in the case of island growth on the relaxed SiGe buffer layers occurs at a higher temperature than for the Ge(Si)/Si(001) islands. This effect can be caused by both a lesser mismatch between the crystal lattices of an island and the buffer layer and a somewhat higher surface density of islands, when they are grown on an SiGe buffer layer.

Generation of misfit dislocations and stacking faults in supercritical thickness strained-Si∕SiGe heterostructures

Defects in the supercritical thickness strained-Si layer grown on a fully relaxed SiGe buffer layer were investigated by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The AFM observation of chemically etched surfaces showed that etch pits having a crosshatch pattern due to misfit dislocations decrease and segments and deep valley profiles increase with an increase in the strain energy of the Si. We found that the segment corresponds to an extended dislocation, of which a 30° partial is in the strained-Si layer and a 90° partial is in the SiGe layer, and the deep valley corresponds to a stacking fault found by a high-resolution TEM observation. A model of the generation of the stacking fault related to development of the misfit dislocation is also discussed here.

Ge/Si self-assembled Islands for Photonics Applications

ABSTRACTWe first present an analysis of the band line-up in the case of SiGe/Si quantum wells and in the case of SiGe/Si self-assembled islands. The conduction and valence band diagrams are obtained from a 30 band k.p Hamiltonian which allows to describe simultaneously conduction and valence band states. The strain field is obtained from a microscopic valence force field theory. The band edge alignment is strongly dependent on the input parameters for this heterosystem. We determine the average valence band offset from photoluminescence measurements of heterostructures grown on relaxed SiGe buffer layers. A type II band line-up is calculated for all Ge compositions in the case of two-dimensional quantum wells and SiGe/Si self-assembled islands. The 30-band formalism allows the determination of the near-infrared interband recombination energy as a function of the self-assembled island structural parameters. We then present recent results obtained by embedding SiGe/Si self-assembled islands in two-dimensional photonic crystals. The photoluminescence of GeSi islands acts as an internal probe to characterize the optical properties of silicon-based two-dimensional photonic crystals designed for the near-infrared spectral range. Cavities, defect-free photonic crystals operated at the second Bragg order and two-dimensional photonic crystals fabricated on top of one-dimensional Bragg mirrors (2D + 1D) are described. We show that, in the case of 2D +1D structures, we can control the quality factor of optical modes at the second Bragg order by matching the resonance conditions and controlling the thickness of the layers. Photonic crystals with pure Ge layers are finally described.

Strained Silicon on Ultrathin Silicon-Germanium Virtual Substrates

Molecular beam epitaxy is employed for the growth of strained-Si layers on top of virtual substrates with highly-relaxed ultrathin SiGe buffers in a continuous procedure. An initial growth stage at a temperature-ramp down to below 200°C causes misfit-dislocation generation by nucleation from point defects and provides an early relaxation in the SiGe buffers. In situ monitoring is used for the growth control. Layer thicknesses and composition are proved by ex situ spectroscopic ellipsometry. %Raman investigations on the layer stacks reveal high degrees of relaxation (70-100%) in sub-100nm SiGe buffer layers containing from 12 to 42 % Ge. Stress in strained Si layers estimated by means of Raman-spectra shift is adjustable from 0.92 to 6.84 GPa by the Ge-content in virtual substrates. Surface morphology of strained Si and of relaxed SiGe buffers is smooth and crosshatch-free. Device test structures show substantial increase of carrier mobilities in nMOSFETs fabricated on these strained-Si layers.

Compressive strain dependence of hole mobility in strained Ge channels

The strain dependence of the hole mobility was systematically investigated in the compressively strained Ge channel modulation-doped structure. It was clearly observed that the mobility increases with increasing compressive strain until the strain as high as 1.9%. The highest mobility of 20 800 and 2000cm2∕Vs at 8 K and room temperature, respectively, was obtained for the Ge channel structure grown on the relaxed SiGe buffer layers with Ge composition of 53%. The origins of this mobility increase are speculated to be the reduction of effective mass, suppression of interband phonon scattering, and the increased confinement of the holes in the channel layer.

Surface roughness and dislocation distribution in compositionally graded relaxed SiGe buffer layer with inserted-strained Si layers

We investigate the surface roughness and dislocation distribution of compositionally graded relaxed SiGe buffer layers by inserting two tensile-strained Si layers. The 20nm thick strained Si layers, less than the critical thickness for dislocation formation, are inserted at 10 and 20% Ge content regions of the 1μm thick graded SiGe layer with a final Ge content of 30%. The surface immediately after growing the second strained Si layer on SiGe with 20% Ge content is found to be flat with about 1.1nm root-mean-square roughness. However, the crosshatched surface with the 7.8nm roughness develops during subsequent SiGe growths, which is slightly less than the 10.3nm value for SiGe without inserted Si layers. Another important issue of consideration is that inserting the strained Si layers leads to increased interaction among dislocations as shown by cross-sectional transmission electron microscopy. This study explores the possibility of using strained layers for achieving flat surfaces and illustrates the need for optimization when using this approach.

Formation of high-quality and relaxed SiGe buffer layer with H-implantation and subsequent thermal annealing

High-quality, thin relaxed Si0.8Ge0.2 layers grown on Si(100) by ultrahigh-vacuum chemical vapor deposition (UHVCVD) have been formed with hydrogen (H)-implantation and subsequent thermal annealing. H-implantation was used to introduce a layer with a high density of defects (cavities) below a 200-nm-thick strained Si0.8Ge0.2. The peak of the implanted profile was located just ∼50nm below the Si0.8Ge0.2/Si interface. The dependence of residual strain in pseudomorphic Si0.8Ge0.2 layer on the annealing temperature has been investigated. By adjusting the dose of H-implantation and the subsequent annealing conditions, almost relaxed (∼95%) Si0.8Ge0.2 layers with a smooth surface were achieved. The method provides a simple approach for the formation of thin relaxed Si0.8Ge0.2 with reduction in surface roughness for advanced complementary metal-oxide-semiconductor electronic devices.

Characterization of strained Si structures using SIMS and visible Raman

Novel strained Si structures, such as Si/SiGe, are being evaluated for the next generation of high carrier mobility devices. Innovative metrology applications are required to assist the semiconductor industry in development and process monitoring as these technologies are evaluated. Crucial factors such as layer intermixing, stoichiometry, contamination and strain inhomogeneities must be accurately and reliably monitored. This paper focuses on the characterization by SIMS and Raman spectroscopy to meet the metrological requirements of strained Si on relaxed SiGe buffer structures. The latest benchmarks for SIMS depth resolution and quantification accuracy are discussed along with Raman characterization of lateral strain variations.

N+/P and P+/N Junctions in Strained Si on Strain Relaxed SiGe Buffers: the Effect of Defect Density and Layer Structure

AbstractThe electrical performance of junctions in SiGe Strain Relaxed Buffers (SRB's) with a strained Si top layer is investigated. Most of the SRB's grown in this experiment use a thin C-doped SiGe layer, which allows to fabricate thin (∼250nm) SRB's with a high relaxation degree. The effects of Threading Dislocation Density (TDD) and C-rich layer depth on the electrical behaviour of n+/p and p+/n junctions are studied. The C atoms in the junction's Space Charge Region (SCR) give rise to defects and induce a noticeable increase in the leakage. The effect of the TDD on the leakage in n+/p junctions is linear over the complete voltage range applied, while for p+/n junctions, only a small effect on leakage is measured at V=1V reverse for TDD's below 1×107cm-2. For low reverse voltages, the current varies more linearly with TDD.

Strain Relaxed SiGe Buffer Prepared by Means of Thermally Driven Relaxation and CMP

Strain Relaxed SiGe Buffer Prepared by Means of Thermally Driven Relaxation and CMP

Chemical vapour etching of Si, SiGe and Ge with HCl; applications to the formation of thin relaxed SiGe buffers and to the revelation of threading dislocations

We have studied the etching of silicon, SiGe and germanium layers with gaseous HCl in reduced pressure-chemical vapour deposition (RP-CVD). We have observed the occurrence of two etch regimes depending on the etching temperature. The first regime takes place at high temperatures and is characterized by low activation energies (∼7 kcal mol−1), this whatever the germanium content of the etched layer. The other regime occurs at low temperatures and has associated high activation energies (which strongly depend upon the germanium concentration of the etched layer: 86 kcal mol−1 for pure Si versus 28 kcal mol−1 for pure Ge). Modifying the HCl partial pressure has different effects depending on the regime. In the high temperature regime, increasing the HCl partial pressure will almost quadratically increase the etch rate (ER ∝ PHCl1.76), this both for Si and Si0.67Ge0.33. Meanwhile, the dependence is sub-linear in the low temperature regime (Si ER ∝ PHCl0.53 and Si0.67Ge0.33 ER ∝ PHCl0.82). The temperature where the regime shifts from one to the other decreases when the Ge concentration increases. To illustrate the added value of the chemical vapour etching, we have demonstrated two possible applications. The first one is the realization of SiGe thin strain relaxed buffers (TSRBs) in the active areas of shallow trench isolation (STI) patterned wafers after etching away the silicon with HCl. We have observed the occurrence of some etching loading effects when moving from a blanket to a patterned wafer. The SiGe TSRBs exhibit some good structural properties (rms roughness of 0.12 nm, no defects observed in cross-sectional transmission electron microscopy). However, they are not fully relaxed and facets are present at the STI/epitaxial stack boundary, signifying they are still not mature enough to be integrated in a metal oxide semiconductor technology. Another possible application is to decorate through some in situ HCl etching the dislocations threading through SiGe relaxed thick layers, with some significant advantages over commonly used wet etching solutions such as the Secco and the Schimmel ones.

On the formation mechanism of epitaxial Ge islands on partially relaxed SiGe buffer layers

We describe the morphological evolution of Ge epitaxial films grown on partially relaxed SiGe buffer layers. Three different types of surface sites exist on the buffer layer surface when the spacing between the buried dislocations is significantly larger than the surface diffusion length of Ge adatoms. The three types of nucleation sites include those over dislocation intersections, those over single dislocation lines, and those far away from dislocations. Epitaxial growth of Ge on such a relaxed buffer under near equilibrium condition exhibits three-stage nucleation and growth of Ge islands, also known as self-assembled quantum dots (SAQD). Denuded zones exist next to the buried dislocation lines. The temperature dependence of the denuded zone width together with the average spacing between SAQDs provides sufficient information for determining both the activation energy and the pre-exponential factor of surface diffusion of Ge adatoms. SAQDs located at the three sites are strained differently as indicated by the distinct difference in the critical dot size for pyramid-to-dome transition. The experimental observation can be explained by assuming the energy barrier for the surface diffusion of Ge adatoms increases with decreasing misfit strain as predicted by theory [van de Walle et al., Phys Rev. B 67, R41308 (2003)]. Calculations of Ge adatom concentration profile based on Fick’s second law of diffusion does predict the presence of denuded zones with their widths being dependent on the diffusion coefficient; i.e., on the substrate temperature. The observed morphological evolution of Ge epitaxial films supports Ge island nucleation and growth as the SAQD formation mechanism as opposed to strain-induced surface instability.