Strained SiGe Alloy Research Articles

Limitations of Drude model in determining plasma properties for multivalley semiconductors: a case study with Ge1-xSnx alloy

Classical Drude model has long been employed to study the plasma behavior of metals. The model has also been used for semiconductors to obtain the expressions for plasma frequency and carrier-induced changes in refractive index and absorption in terms of effective mass, transport relaxation time, and mobility of the carriers. However, these expressions remain invalid for semiconductors in which carriers occupy different valleys, as in strained SiGe alloys, GeSn alloys, and even in GaAs or InP under high electric field. We modify Drude equations for occupancy of two valleys by electrons and show deviations from standard textbook expressions. The conditions for recovering known expressions are then established. To illustrate, we consider unstrained GeSn alloys over the range 0 < x < 0.2 in which L and Γ conduction band valleys cross positions at x = 0.08 resulting in a crossover from indirect to direct nature of the band gap. A comparison is made between the present values with values obtained previously.

Modeled optical properties of SiGe and Si layers compared to spectroscopic ellipsometry measurements

The optical response of strained SiGe alloys, as well as thin Si layers, is analyzed using a sp3d5s∗ tight-binding model within the independent particle approximation. The theoretical results are compared to measurements obtained on samples with various Ge content and layer thicknesses. The dielectric function is extracted from spectroscopic ellipsometry allowing a separation of its real and imaginary parts. Theory and simulation show similar trends for the variation of the dielectric function of SiGe with varying Ge content. Variations are also well reproduced for thin Si layers with varying thickness and are attributed to quantum confinement.

Electronic properties of Si/Si-Ge Alloy/Si(100) heterostructures formed by ECR Ar plasma CVD without substrate heating

By using our low-energy Ar plasma enhanced chemical vapor deposition (CVD) at a substrate temperature below 100°C during plasma exposure without substrate heating, modulation of valence band structures and infrared photoluminescence can be observed by change of strain in a Si/strained Si0.4Ge0.6/Si(100) heterostructure. For the strained Si0.5Ge0.5 film, Hall mobility at room temperature was confirmed to be as high as 660cm2V−1s−1 with a carrier concentration of 1.3×1018cm−3 for n-type carrier, although the carrier origin was unclear. Moreover, good rectifying characteristics were obtained for a p+Si/nSi0.5Ge0.5 heterojunction diode. This indicates that the strained Si-Ge alloy and Si films and their heterostructures epitaxially grown by our low-energy Ar plasma enhanced CVD without substrate heating can be applicable effectively for various semiconductor devices utilizing high carrier mobility, built-in potential by doping and band engineering.

Damage accumulation during cryogenic and room temperature implantations in strained SiGe alloys

The amorphization by implantation of strained silicon–germanium epitaxial layers is investigated by experiments and simulation according to the germanium content in the film and the implantation conditions. Experimental results are used to calibrate a numerical model based on a combined approach between a Binary Collision Approximation (BCA) module and a kinetic Monte Carlo (kMC) module. The calibration is implemented in the Synopsys Sentaurus Process simulator and a close agreement between simulations and experiments is shown. Experimental results show that by increasing the germanium concentration, amorphous thickness is reduced for high energy implantations. Germanium content seems to have less impact at low energy implantations.

Formation and Characterization of Strained Si-Ge Alloy Epitaxially Grown on Si(100) by Low-Energy ECR Ar plasma CVD without Substrate Heating

Formation and Characterization of Strained Si-Ge Alloy Epitaxially Grown on Si(100) by Low-Energy ECR Ar plasma CVD without Substrate Heating

Residual order in the thermal oxide of a fully strained SiGe alloy on Si

Introduction Oxidation of SiGe alloy has been a target of research for both fundamental and technological reasons, as in the case of Si oxidation. It has been expected as the fabrication process for the gate oxide of SiGe channel metal oxide semiconductor field effect transistors (MOSFETs), which allows higher carrier mobility than that of Si channel MOSFETs [1]. In recent years, it has also been employed in the fabrication of SiGe-oninsulator and Ge-on-insulator substrates [2]. These substrates are employed to realize high-speed MOSFETs with channel materials of strained Si, SiGe, and Ge. However, the oxidation mechanism of SiGe alloy is still open to question, due to the complexity caused by lattice strain and diffusion of Ge atoms. The most characteristic phenomenon of SiGe oxidation is that Ge atoms are ejected from the interface between the surface oxide and the SiGe layer without the incorporation of Ge atoms into the oxide layer. Ge atoms accumulate at the SiO2/SiGe interface and diffuse into the SiGe layer [1,2]. In the case of a strained SiGe layer epitaxially grown on a Si substrate, the effects of strain relaxation and defect generation during the oxidation process must be taken into account. It is important to investigate the atomic structure of the thermally oxidized layer, because it allows us to understand what really happens at the oxidized interface. In the case of the thermally oxidized layer of Si, the Si atoms in the oxide layer still maintain order, which originates from the diamond structure of the parent Si crystal, although the structure appears to be amorphous at a glance. This provides an actual picture of the oxidation process and explains the almost perfect properties obtained by electric measurements of the SiO2/Si interface. In this study, the residual order in the thermal oxide layer on a fully strained SiGe alloy was investigated.

Kinetic lattice Monte Carlo simulations of interdiffusion in strained silicon germanium alloys

Point-defect-mediated diffusion processes are investigated in strained SiGe alloys using kinetic lattice Monte Carlo (KLMC) simulation technique. The KLMC simulator incorporates an augmented lattice domain and includes defect structures, atomistic hopping mechanisms, and the stress dependence of transition rates obtained from density functional theory calculation results. Vacancy-mediated interdiffusion in strained SiGe alloys is analyzed, and the stress effect caused by the induced strain of germanium is quantified separately from that due to germanium-vacancy binding. The results indicate that both effects have substantial impact on interdiffusion.

Measurement of phonon pressure coefficients for a precise determination of deformation potentials in SiGe alloys

AbstractFor an effective use of Raman scattering as strain characterization tool in SiGe nanostructures a precise knowledge of the phonon deformation potentials (DPs) is strictly necessary. The optical phonon DPs can be determined by means of Raman scattering measurements from the cleaved edge of a biaxially strained SiGe alloy layer grown pseudomorphically on silicon and subsequently capped. Due to uncertainties in the literature values of the unstrained phonon frequencies it turns out that the desired degree of accuracy is only attained by complementing the Raman measurements from the edge with that of the hydrostatic pressure coefficient of the optical phonons. For that purpose we have grown by molecular beam epitaxy up to seven partially strained Si1–x Gex alloys on Si spanning the entire compositional range and measured the dependence on hydrostatic pressure of the frequency of the Ge‐like, Si‐like and mixed Si–Ge optical modes. After correcting for the pressure dependent biaxial stress induced by the Si substrate on the alloy layer and taking into account the dependence on alloy composition of the bulk modulus we obtain a fairly constant value of the Grüneisen parameter around 1.0 for all three optical modes in the whole range of Ge contents. We also determined the strain shift coefficients for the three modes, which are essentially independent of Ge content between 0.4 and 1. Our results are in very good agreement with recentcalculations of the SiGe phonon deformation potentials using a modified Keating model, which settles the longstanding issue about the large discrepancies between results from different experiments. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Strained Si films grown by chemical vapor deposition of trisilane on Ge buffered Si(100)

Relaxed Ge buffer layers were grown on Si substrates using a recently developed chemical vapor deposition (CVD) approach that combines digermylmethane and digermane precursors. Ultrathin Si films were deposited on these buffer layers at 420 °C by CVD using trisilane as a precursor. The Si films were studied with a variety of experimental techniques, with emphasis on Raman spectroscopy. The analysis of the Raman results shows that a thin (< 1 nm), fully strained Si–Ge alloy layer is formed at the Si–Ge interface. Pure Si grows on this transitional alloy with a strain that approximately follows the predictions from a simple equilibrium strain theory. These results are significant for Ge-based Metal-Oxide-Semiconductor applications that require a thin Si-layer to isolate the Ge channel from the high permittivity oxide.

Nanovoids in MBE-grown SiGe alloys implantedin situwithGe+ions

Spherically shaped voids, of nanometer size, are observed in molecular-beam epitaxially grown SiGe alloy layers implanted in-situ at elevated temperature with low-energy Ge ions, followed by thermal treatments. The voids are exclusively assembled in the narrow, implanted band. The voids only appear in the layers after a heat treatment at a temperature higher than 700 \ifmmode^\circ\else\textdegree\fi{}C, and they are stable up to 900 \ifmmode^\circ\else\textdegree\fi{}C. Arsenic ion implantation at similar conditions does not give rise to void formation but to regular interstitial dislocation loops. The nucleation stage of the voids is accompanied by a strong photoluminescence-yield enhancement in the range of 1.4--1.55 \ensuremath{\mu}m, originating from the strained SiGe alloy layer which contain vacancy clusters or small voids.

Si/SiGe Nanostructures Fabricated by Atomic Force Microscopy Oxidation

AbstractIn this work, local AFM oxidation technique in a controlled humidity environment has been used to create small features in strained SiGe alloys. When directly oxidizing SiGe alloys, minimum line widths of 20nm were achieved by adjusting parameters such as the bias voltage on the microscope tip and the tip writing speed. It was found that when bias voltage increases, and/or when the tip writing speed decreases, the oxidation height of silicon-germanium increases. In contrast to conventional thermal oxidation, the oxide height on SiGe alloys is slightly less than that on Si. Finally, this method was used to successfully cut conducting SiGe quantum well lines with high resolution.

In situHREM irradiation study of point-defect clustering in MBE-grown strainedSi1−xGex/(001)Sistructures

We present a detailed analysis of the point-defect clustering in strained ${\mathrm{S}\mathrm{i}/\mathrm{S}\mathrm{i}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/(001)\mathrm{Si}$ structures, including the interaction of the point defects with the strained interfaces and the sample surface during 400 kV electron irradiation at room temperature. Point-defect cluster formation is very sensitive to the type and magnitude of the strain in the Si and ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}$ layers. A small compressive strain $(\ensuremath{-}0.3%)$ in the SiGe alloy causes an aggregation of vacancies in the form of metastable [110]-oriented chains. They are located on {113} planes and further recombine with interstitials. Tensile strain in the Si layer causes an aggregation of interstitial atoms in the forms of additional [110] rows which are inserted on {113} planes with [001]-split configurations. The chainlike configurations are characterized by a large outward lattice relaxation for interstitial rows $(0.13\ifmmode\pm\else\textpm\fi{}0.01\mathrm{nm})$ and a very small inward relaxation for vacancy chains $(0.02\ifmmode\pm\else\textpm\fi{}0.01\mathrm{nm}).$ A compressive strain higher than $\ensuremath{-}0.5%$ strongly decreases point-defect generation inside the strained SiGe alloy due to the large positive value of the formation volume of a Frenkel pair. This leads to the suppression of point-defect clustering in a strained SiGe alloy so that SiGe relaxes via a diffusion of vacancies from the Si layer, giving rise to an intermixing at the Si/SiGe interface. In material with a 0.9% misfit a strongly increased flow of vacancies from the Si layer to the SiGe layer and an increased biaxial strain in SiGe both promote the preferential aggregation of vacancies in the (001) plane, which relaxes to form intrinsic $60\ifmmode^\circ\else\textdegree\fi{}$ dislocation loops.

Compressive and Tensile Strain Effects on Atomic Distribution in Strained Si–Ge Alloys

The effects of compressive and extensive strain on the atomic distribution have been studied by Raman scattering. Compressive strain will make the atoms distribute inhomogeneously and nonuniformly. This effect can be suppressed by increasing the substrate temperature or increasing the Ge content in the Si-Ge alloy. The atoms are distributed uniformly and randomly in extensively strained Ge-Si alloys.

Low-frequency noise characteristics of UHV/CVD epitaxial Si- and SiGe-base bipolar transistors

We report the first measurements of low-frequency noise in high-performance, UHV/CVD epitaxial Si- and SiGe-base bipolar transistors. The magnitude of the noise power spectral density at fixed frequency for both Si and SiGe devices is comparable for similar bias, geometry, and doping conditions, indicating that the use of strained SiGe alloys does not degrade transistor noise performance. The best recorded values of noise corner frequency were 480 Hz and 373 Hz for the Si and SiGe transistors, respectively, for multi-stripe devices with an emitter area of 0.5/spl times/10.0/spl times/3 /spl mu/m/sup 2/. A functional dependence of the noise power spectral density on base current for both device types of I/sub B//sup 1.90/ was observed, and noise measurements as a function of device geometry suggest that the contributing noise sources are uniformly distributed across the emitter of the transistors, not at the emitter periphery.

Theory and observation of enhanced, high field hole transport in Si/sub 1-x/Ge/sub x/ quantum well p-MOSFETs

We report on the observation of enhanced high field hole velocity in strained Si/Si/sub 1-x/Ge/sub x//Si quantum wells. This effect manifests itself in the drive current capability of nanometer scale p-channel Quantum Well Metal-Oxide-Semiconductor-Field-Effect-Transistors (p-QWMOSFETs). The high-field transport of a two-dimensional hole gas confined in a Si/Si/sub 1-x/Ge/sub x//Si quantum well is formulated and solved. The results indicate an increase in the hole saturated drift velocity in strained SiGe quantum wells with increasing Ge mole fractions up to x=0.5. This is a consequence of the optical phonon spectrum of the strained SiGe alloy remaining Si-like (i.e., high energy) while the carrier transverse effective mass decreases with higher Ge content. To investigate the theoretical prediction of increased high-field drift velocity, p-QWMOSFETs were fabricated with Si/Si/sub i-x/Ge/sub x//Si quantum well heterostructures grown by Molecular Beam Epitaxy (MBE) with varying Ge mole fractions, x. The fabrication sequence maintained a low thermal budget to prevent strain relaxation in the SiGe layer and involved a mixed optical/electron beam lithography scheme to define junction-isolated transistors with a minimum drawn gate lengths of 200 nm. The measured saturated transconductance, g/sub msat/, of the p-QWMOSFETs were 20-50% higher than that of a reference Si pMOSFET under equivalent biasing conditions. The importance of this g/sub msat/ increase for high-speed, low-power VLSI applications is discussed.

Observation of piezoelectric-like behaviour in coherently strained B-doped [formula omitted] heterostructures

In the present work the hybrid acoustic spectroscopy technique has been used to demonstrate the conversion of a high frequency (HF) electric field into acoustic waves and to provide the first direct observation of the piezoelectric effect in the SiGe Si strained layer system. The sample was a p-type modulation doped Si 0.88 Ge 0.12 Si heterostructure containing a two-dimensional hole gas with carrier sheet density 2 × 10 11 cm −2 and a 4.2 K mobility of 10500 cm 2 V −1 s −1. It was excited with a 225.7 MHz high frequency pulse of 1 μs duration and 0.5 W peak power. The amplitude of the output acoustic signal was about 110 dB below the electromagnetic input signal at 77 K. We believe that the observed electric field-acoustic conversion is associated with the non-centrosymmetric structure of the ordered unit cell of the strained SiGe alloy. Additionally we report on phonon Raman scattering at T = 300 K and hot hole Shubnikov-de Haas and zero magnetic field resistivity behaviour in the same heterostructures in the temperature range of 0.35 to 1.4 K. A broad band near 255 cm −1 and a peak near 435 cm −1 have been attributed to a particular SiGe ordering within the alloy layer. The energy relaxation of the carriers has been measured and is found to be dominated by a weakly screened piezoelectric coupled acoustic-phonon mechanism, thereby providing further evidence of ordering in the SiGe alloy.

Investigation of the high frequency noise figure reduction of SiGe heterojunction bipolar transistors using actualised physical models

High frequency noise figure reduction is one of the major requirements for analog circuits. The introduction of strained SiGe alloys as the base layer material in heterojunction bipolar transistors opens new possibilities to improve the (base resistance) −1 × current gain product. Hence, the use of SiGe as the base layer material instead of silicon can result in a major improvement of the high frequency noise. In this paper, the minimum noise figure reduction as a function of the germanium fraction is addressed, using actualized physical models for the bandgap and the mobility. Two limiting cases will be discussed: a heterojunction bipolar transistor with very low base resistance and still acceptable minimum noise figure and current gain, and a high gain heterojunction transistor with superior noise figure for acceptable base resistance values.

Initial stages of SiGe epitaxy on Si(001) studied by scanning tunneling microscopy

We have studied the initial stages of strained SiGe alloy growth on the Si(001)-(2 × 1) surface by scanning tunneling microscopy. The Si 0.36Ge 0.64 alloy was grown on the silicon substrate at various coverages (0.13–3.6 ML) and at different temperatures (∼ 310–470°C). The growth was one dimensional, preferring the direction perpendicular to the underlying silicon dimer rows at low coverages and low temperatures. Anti-phase boundaries were observed to lead multi-layer growth. Strong interaction between the overlayer and the substrate was found to buckle the substrate as well as SiGe dimers. Different growth mechanisms, island formation and step flow, were identified at low and high substrate temperatures. (2 × n) ordering of the strained overlayer was only observed at an intermediate growth temperature (∼ 390°C).

Substrate temperature and Ge concentration dependence of the microstructure of strained Si-Ge alloys

Raman spectroscopy has been employed to investigate the microstructural features of strained Si-Ge layers deposited by the rapid-thermal-process very-low-pressure chemical vapour deposition method. Auger electron spectroscopy was used to determine the Ge concentrations in the Si-Ge layers. The Ge atoms in the high-Ge-concentration Si-Ge alloy are distributed more homogeneously and randomly than in the low-Ge-concentration alloy. The substrate temperature has a great influence on the microstructure of the strained Si-Ge alloy. A high Ge content needs a relatively low substrate temperature to deposit high-quality Si-Ge alloy, but a low Ge content requires a higher growth temperature during the deposition of the Si-Ge alloy.

Band-edge emission from strained SiGe alloy layers on Ge(100) substrates

We have fabricated strained SixGe1−x/SiyGe1−y multiple quantum wells on Ge(100) substrates and measured the photoluminescence (PL) spectra, observing band-edge emission from the SiGe alloy layers. The emission is due to the recombination of both bound excitons and free excitons in the quantum wells. From the positions of the observed PL lines, we have evaluated the band-gap energies of the strained SiGe alloy layers, and found them to be smaller than those of bulk SiGe alloys. The band-gap energy increases with the Si content of the alloy, reaching a maximum at about 15% Si, and subsequently decreases. These results agree well with the theoretical calculations for strained layers, and suggest a type II band alignment in some cases for SixGe1−x/SiyGe1−y heterostructures on Ge(100).