SiGe System Research Articles

Improvement of 4H–SiC selective epitaxial growth by VLS mechanism using Al and Ge-based melts

Al–Si and Ge–Si systems were studied for selective epitaxial growth (SEG) of 4H–SiC by Vapour–Liquid–Solid mechanism. Al–Si and Ge–Si stacking bilayers were deposited on 8° off, Si face, 4H–SiC substrates. After layer patterning, the samples were heated up to 1000 and 1220 °C respectively for Al–Si and Ge–Si stackings in order to melt the layers. Propane diluted in Ar was introduced either during the initial heating ramp, before melting of the alloy, or after reaching the temperature plateau. In both cases, SEG of SiC was achieved. However, it was found that the introduction of propane before melting was a key parameter in order to improve the homogeneity of the deposit. This improvement was noticeable with Al–Si and spectacular with Ge–Si. This latter case gave the best results in terms of smoothness and uniformity of the epitaxial deposit on all the pattern sizes and shapes. Some 3C–SiC polycrystalline material appears on the pattern edges and also on the layer itself for long time growth. When H 2 was used instead of Ar as the vector gas, this polycrystal was eliminated at the edges. The thickness of the deposits was determined by mechanical profiling and compared to SIMS measurements.

Growth of homogeneous single crystals of GeSi solid solutions using a Ge seed by the modified Bridgman method

A modified technique for the growth of homogeneous Ge-rich GeSi single crystals is developed on the basis of the vertical Bridgman method using a Ge seed and a Si source rod. Single crystals were grown in two stages. In the first stage, a Si source rod was partially immersed in the Ge melt above the seed. A gradual increase in the Si content in the melt leads to constitutional supercooling at the crystallization front and to the growth of an inhomogeneous GeSi buffer single crystal. The first stage ends when the temperature at the crystallization front becomes equal to the liquidus temperature of the specified composition of the Ge-Si system. In the second stage, a homogeneous GeSi single crystal is grown while maintaining a constant growth temperature. The growth temperature is controlled by an appropriate balance between the pulling and feeding rates. Relations determining the optimal process parameters (the pulling and feeding rates and the temperature gradient at the crystallization front) for growing crystals of specified composition are obtained.

Ge–Si system nanoclusters in Si matrix formed by solid-phase epitaxy

Solid-phase epitaxy combined with Ge implantation has been employed to prepare Ge–Si system nanoclusters. The nanoclusters coherently embedded in the single-crystalline Si matrix are, on average, controlled within the size range of a few nanometers. The distribution and the number of these nanoclusters are closely correlated with the implant energy and dose as well as the surface oxidation. Within the nanoclusters, the lattice is modulated with a period of four times of the (111) plane spacing in the regular Si lattice. It is argued that the lattice modulated characteristics should result from Ge ordering instead of moiré interference.

Growth and Ordering of Si-Ge Quantum Dots on Strain Patterned Substrates

Manipulating the strain distribution along the surface of a substrate has been shown experimentally to promote spatial ordering of self-assembled nanostructures in heteroepitaxial film growth without having to resort to expensive nanolithographic techniques. We present here numerical studies of three-dimensional modeling of self-assembly in Si-Ge systems with the aim of understanding the effect of spatially varying mismatch strain-fields on the growth and ordering of quantum dots. We use a continuum model based on the underlying physics of crystallographic surface steps in our calculations. Using appropriate parameters from atomistic studies, the (100) orientation is found to be unstable under compressive strain; the surface energy now develops a new minimum at an orientation that may be interpreted as the (105) facet observed in SiGe∕Si systems. This form of surface energy allows for the nucleationless growth of quantum dots which start off via a surface instability as shallow stepped mounds whose sidewalls evolve continuously toward their low-energy orientations. The interaction of the surface instability with one- and two-dimensional strain modulations is considered in detail as a function of the growth rate. One-dimensional strain modulations lead to the formation of rows of dots in regions of low mismatch—there is some ordering within these rows owing to elastic interactions between dots but this is found to depend strongly upon the kinetics of the growth process. Two-dimensional strain modulations are found to provide excellent ordering within the island array, the growth kinetics being less influential in this case. For purposes of comparison, we also consider self-assembly of dots for an isotropic surface energy. While the results do not differ significantly from those for the anisotropic surface energy with the two-dimensional strain variation, the one-dimensional strain variation produces profoundly different behavior. The surface instability is seen to start off initially as stripes in regions of low mismatch. However, since stripes are less effective at relaxing the mismatch strain they eventually break up into islands. The spacing of these islands is determined by the wavelength of the fastest growing mode of the Asaro-Tiller-Grinfeld instability. However, the fact that such a growth mode is not observed experimentally indicates the importance of accounting for surface energy anisotropy in growth models.

THE TWO-FLUX COMPOSITE FERMION SERIES OF FRACTIONAL QUANTUM HALL STATES IN STRAINED (100) Si

Magnetotransport properties are studied in a high-mobility 2DES in the strained Si quantum well. We observe around ν=1/2 the two-flux composite fermion ( CF 2) series of the FQHE states at ν=2/3, 3/5, 4/7, and at ν=4/9, 2/5, 1/3. Of the CF series, the ν=3/5 state is weaker than the nearby 4/7 state and the 3/7 state is missing, resembling the observation that the ν=3 is weaker than the ν=4 state. Our data indicate that the CF model still applies for the multivalley Si / SiGe system when taking into account the two-fold valley degeneracy.

Effect of a Ge interlayer on the high-temperature behavior of NiSi films

The effect of a thin Ge interlayer on the formation of Ni silicides on (100)Si substrates has been investigated. X-ray diffraction shows a remarkable increase of the nucleation temperature of NiSi2 in the presence of the Ge interlayer. Four-probe measurements show that the sheet resistance of silicide formed in Ni∕Ge∕Si system remains stable up to 850 °C, while the sheet resistance of silicide formed in Ni∕Si system presents a significant increase at 750 °C. Scanning electron microscopy indicates that island formation is not observed in the NiSi film grown on Ge∕Si substrate annealing at 800 °C. The classical nucleation theory is employed to explain the increased temperature of the nucleation of NiSi2 in the Ni∕Ge-Si system.

Impact of the Ge concentration on the Ge-ionisation probability and the matrix sputter yield for a SiGe matrix under oxygen irradiation

As SiGe becomes a building block of advanced devices, depth profiling of SiGe gains in importance, requiring an understanding of its basic behavior under oxygen bombardment. In this study, the sputter yield and ionization probability of Si and Ge sputtered from SiGe-substrates are studied as a function of the incidence angle. The altered layer formation and the element redistribution are studied with in situ sputter/RBS and the oxidation of the bombarded SiGe surface is analyzed with XPS. The results show that the altered layer formation in the SiGe system is much more complex than in the Si case and strongly dependent of the incident angle/primary energy. In addition to sputter related effects, thermodynamics and volatility of the Ge-oxide are important mechanisms.

Impact of the Ge concentration on the Ge-ionisation probability and the matrix sputter yield for a SiGe matrix under oxygen irradiation

As SiGe becomes a building block of advanced devices, depth profiling of SiGe gains in importance, requiring an understanding of its basic behavior under oxygen bombardment. In this study, the sputter yield and ionization probability of Si and Ge sputtered from SiGe-substrates are studied as a function of the incidence angle. The altered layer formation and the element redistribution are studied with in situ sputter/RBS and the oxidation of the bombarded SiGe surface is analyzed with XPS. The results show that the altered layer formation in the SiGe system is much more complex than in the Si case and strongly dependent of the incident angle/primary energy. In addition to sputter related effects, thermodynamics and volatility of the Ge-oxide are important mechanisms.

The Emission of Terahertz Radiation from Doped Silicon Devices

ABSTRACTRecent interest in biological imaging, remote sensing, and biochemical spectroscopy has made nanoscale Terahertz (THz) emitters based on semiconductors become extremely attractive. THz lasers with good performance (emitting over 1 milliWatt CW at 10 K) have been fabricated from the group III-V compounds by molecular beam epitaxy, but such devices are complex with over 500 quantum wells and barriers and are incompatible with low cost Si processing. Similar devices from the SiGe system have had difficulties because the strain limits the total number of active layers, and have produced relatively poor performance (peak powers near 5 nW), even using strain symmetric active regions on virtual substrates of relaxed SiGe buffers. We report here on the characteristics of a new type of THz emitting device with higher powers (above 0.1 milliWatt) based on radiative impurity transitions in doped silicon, which can be simply fabricated without Ge alloying.The THz emitters were fabricated from both p-type and n-type doped silicon wafers with typical resistivities of 5 Ω-cm, using conventional photolithography and metal contact lift off. Samples were electrically pulsed and the electroluminescence was measured by Fourier Transform Infrared spectrometry. At 4.2 K, the emission spectra showed several peaks centered around 8.1 THz for the boron doped device, 7 to 13 THz for the gallium doped device, and 6.6 THz for the phosphorus doped device. The electroluminescence was attributed to radiative transitions from excited p-like to s-like hydrogenic dopant states, with emission energies in remarkable agreement with the known dopant absorption levels. Current-voltage measurement suggested an excitation mechanism based on impact ionization of neutral dopants by hot carriers. The net quantum efficiency (emitted photons per injected electron) was estimated to be up to 2 × 10-3. The temperature dependence will be reported with operation above 30 K.

Morphology of Small Particles of Binary Systems Forming Complete Solid Solutions Prepared by Gas-Evaporation Technique

The crystal morphology of small particles of the Cu–Au system, six binary alkali halide systems (KCl–RbCl, KCl–KBr, KCl–RbBr, KBr–RbCl, KBr–RbBr and RbCl–RbBr) and Si–Ge system prepared by gas-evaporation technique was investigated by electron microscopy. The experimental evidence that the mixing of constituent atoms or ions as well as the impurities contained in both the starting material and the atmospheric gas affect the crystal morphology of the small particles to let the external shape spherical for the Cu–Au and Si–Ge systems and the aspect ratio of external shape larger for the binary alkali halide systems was obtained in this study.

Imaging, Manipulating, and Analyzing with Nanometer Precision: Application of the Nanoworkbench

ABSTRACTWe report the development of novel nanometer-scale manipulative and analytical devices for imaging, chemically analyzing and manipulating nanometer-scaled materials. Two different versions of the nanoworkbench are operating currently at the Surface Science Center of the University of Pittsburgh and at the Seagate Research Center in Pittsburgh. The instrument at Seagate consists of a modified commercially available high resolution scanning electron microscope (lateral resolution ∼1 nm at 10 kV) in combination with a set of four unique nano-manipulators, operating in the pressure range from 102 to 10−7 mbar. At the University of Pittsburgh a home-built UHV version of the nanoworkbench is in operation. In this UHV-instrument, several inter-connected UHV chambers allow in-situ deposition of thin-films and conventional surface analysis. The resolution of the SEM of the UHV system is limited to about 50 nm. We report the first results obtained by using both versions of the nanoworkbench, where we succeeded in writing patterns of ultra-small carbon-containing dots (8nm in diameter) with high position accuracy (<5nm) by electron-beam-induced deposition of carbon-containing background gases. Additionally, four-point probe measurements were performed on a SiGe system.

3D atomic imaging of SiGe system by X-ray fluorescence holography

X-ray fluorescence holography (XFH) provides three-dimensional atomic images around specific elements without any assumption of the structural model. Six X-ray holograms of Si0.8Ge0.2/Si at different energies were measured at the synchrotron radiation facility of SPring-8. Si and/or Ge atoms within 0.7 nm of a radius were clearly visible in the atomic images reconstructed from the holograms. From these images, slight displacements of the images at each shell in between Si0.8Ge0.2/Si and the Ge bulk were distinctly revealed. This demonstrated that the XFH method has a great potential to quantitatively analyze a three-dimensional local lattice structure in epitaxial crystals.

Formation and stability of radiation defect complexes in Si and Si:Ge: Composition and pressure effects

Vacancy-type defects, e.q. divacancies (V 2) and E-centers (P–V, Sb–V pairs), are known to be the most common radiation defects in n-type Si and SiGe after ion implantation. It is known that V 2 and P–V exhibit preferential configurations in SiGe system in respect with the amount of Ge atoms surrounding the defect. In this work stability of vacancy-type defect complexes, specifically V 2 and P–V pair, are studied as a function of pressure and amount of Ge atoms in the vicinity of a point defect in Si. The methods used are a molecular static method with Tersoff’s many-body interatomic potential as well as an ab initio approach. It is found that the formation energies for vacancy and divacancy, having a Ge atom in the nearest neighbourhood, are lowered by 1.42 and 1.76 eV with respect to those in pure Si. An introduction of one more Ge atom as the neighbour of a vacancy or divacancy does not cause any change in the formation energy. For all types of the considered defects, the formation energy has a tendency to decrease with increasing pressure.

Automating first-principles phase diagram calculations

Devising a computational tool that assesses the thermodynamic stability of materials is among the most important steps required to build a “virtual laboratory,” where materials could be designed from first principles without relying on experimental input. Although the formalism that allows the calculation of solid-state phase diagrams from first principles is well established, its practical implementation remains a tedious process. The development of a fully automated algorithm to perform such calculations serves two purposes. First, it will make this powerful tool available to a large number of researchers. Second, it frees the calculation process from arbitrary parameters, guaranteeing that the results obtained are truly derived from the underlying first-principles calculations. The proposed algorithm formalizes the most difficult step of phase diagram calculations, namely the determination of the “cluster expanison,” which is a compact representation of the configurational dependence of the alloy’s energy. This is traditionally achieved by a fit of the unknown interaction parameters of the cluster expansion to a set of structural energies calculated from first principles. We present a formal statistical basis for the selection of both the interaction parameters to include in the cluster expansion and the structures to use to determine them. The proposed method relies on the concepts of cross-validation and variance minimization. An application to the calculation of the phase diagram of the Si-Ge, CaO-MgO, Ti-Al, and Cu-Au systems is presented.

A signal transduction system in Streptomyces coelicolor that activates the expression of a putative cell wall glycan operon in response to vancomycin and other cell wall-specific antibiotics.

We have investigated a signal transduction system proposed to allow Streptomyces coelicolor to sense and respond to changes in the integrity of its cell envelope. The system consists of four proteins, encoded in an operon: sigmaE, an RNA polymerase factor; CseA (formerly ORF202), a protein of unknown function; CseB, a response regulator; and CseC, a sensor histidine protein kinase with two predicted transmembrane helices (Cse stands for control of sigma E). To develop a sensitive bioassay for inducers of the sigE system, the promoter of the sigE operon (sigEp) was fused to a reporter gene conferring resistance to kanamycin. Antibiotics that acted as inducers of the sigE signal transduction system were all inhibitors of intermediate and late steps in peptidoglycan biosynthesis, including ramoplanin, moenomycin A, bacitracin, several glycopeptides and some beta-lactams. The cell wall hydrolytic enzyme lysozyme also acted as an inducer. These data suggest that the CseB-CseC signal transduction system may be activated by the accumulation of an intermediate in peptidoglycan biosynthesis or degradationa. A computer-based searching method was used to identify a sigmaE target operon of 12 genes (the cwg operon), predicted to specify the biosynthesis of a cell wall glycan. In low-Mg(2+) medium, transcription of the cwg operon was induced by vancomycin in a sigE-dependent manner but, in high-Mg(2+) medium, there was substantial cwg transcription in a sigE null mutant, and this sigE-independent activity was also induced by vancomycin. Based on these data, we propose a model for the regulation and function of the sigmaE signal transduction system.

Structural properties and stability of Zr and Ti germanosilicides formed by rapid thermal annealing

Because of their good ohmic and rectifying properties, silicides are routinely used in Si technology. This approach has been recently extended to the novel devices produced using Si1−xGex alloys. Here, we study the Zr and Ti germanosilicides produced in the low thermal budget contact formation during Si/Si1−xGex heterodevice processing. Phase formation was monitored by combining a range of spectrometries with electron microscopy and x-ray diffraction techniques, while sheet resistance measurements allowed correlation of phase formation with film conductance. After completion of the reaction, the final crystalline phase was either C49–Zr(Si1−yGey)2 in the entire Ge composition (x) range, or C54–Ti(Si1−yGey)2 in the Ge composition range 0–0.47. In the Zr–Si–Ge system, the C49–Zr(Si1−yGey)2 formation temperature (Tf) decreases as x increases, and films formed at this temperature are continuous. Excess heating (above Tf) produces islanded films with embedded grains. A most significant feature of the results was that no Ge segregation was detected at any annealing temperature and that the Ge content in the C49 phase (y) remained equal to x for all x. This is in contrast to results on the C54–Ti(Si1−yGey)2 films, which were discontinuous when x>0.10, and in which Ge segregation occurred in the form of Ge-rich SiGe decorations separating the germanosilicide grains. The Ge content in the final C54 phase (y) was always lower than the value of x in the initial SiGe alloy, and the measured sheet resistance of the corresponding contacts was large. Our results indicate that the alloys formed between Zr and Si1−xGex are good candidates as stable contacts on Si1−xGex, and hence that Zr should be preferred for contacting in Ge-rich SiGe-based applications.

Microscopic and optical investigation ofGe nanoislands on silicon substrates

We investigate self-assembled nanoislands in heteroepitaxial GeSi systems by means of atomic force microscopyand micro-Raman scattering techniques. We show that the surfacediffusion of Si atoms from the substrate to the islands isstrongly enhanced when the temperature increases, giving riseto a wider stability range of pyramid-shaped volumes.

High-resolution transmission electron microscopy of phase formation and growth in metal–Si–Ge systems

High-resolution transmission electron microscopy (HRTEM) in conjunction with nano-beam (NB) analysis as well as energy dispersive spectrometry analysis have been fruitfully utilized to study the interfacial reactions in the metal–Si–Ge systems. In this paper we report the results of TEM study of the phase formation and growth in Ti–Si–Ge, Cu–Si–Ge and Ni–Si–Ge systems.

Lattice parameter and density of Ge–Si solid solutions

Solid solutions of the Ge–Si system have been prepared by reaction under vacuum and gravity of the constituent elements followed by different quenching procedures. Least-squares refinement to the lattice parameters of the Ge–Si phase has been established in the composition range 0.5–0.9 at% Si. Almost homogeneous alloys were obtained by rapid quenching in liquid nitrogen, whereas pseudo-homogeneous alloys occur as an intermediate stage when quenched in water. The refined lattice parameters, although computed with very high precision, did not predict the formation rate of the solid solution. However, careful analysis of the X-ray diffraction pattern of residual Ge and Si revealed that the lattice parameter of each component is modified in accordance with the atomic percentage of either element present in the lattice of the other.

Ab-Initio Modeling of C-B Interactions In Si

ABSTRACTWe present the results of ab-initio calculations for the structure and energetics of small boron-carbon (BCI) as well as carbon-carbon (C2I) clusters in Si, a continuum model for the nucleation, growth, and dissolution of the clusters, and experimental investigation by SIMS. The modeling results suggest that these clusters may play a role in controlling B diffusion in Si and SiGe systems and the experimental results seem to support the modeling findings.