Germanium Segregation Research Articles

Kinetics and Mechanism of Nanostructures in Oxidation ofSi1-xGex Alloys

We investigate the oxidation behaviour of Si1-xGex alloys(x = 0.05, 0.15, and 0.25). The oxidation of SiGe films withdifferent compositions was carried out in O2 (dry) atmosphere at800, 900 and 1000°C, respectively, for variouslengths of time. The thickness and property of the nanoparticle andnanolayer in oxide films and germanium segregation in oxidation of SiGealloys are measured by using a high precision ellipsometer. Theresults are in good agreement with the Rutherford backscatteringspectrometry, profile dektak instrument andhigh-resolution scanning transmission electron microscopy. Wefound that the Ge content in the oxide layer increases with the Gecontent in SiGe alloys, and that the Ge content in the oxide film decreases withthe increasing oxidation temperature and time. Rejection of Ge resultsin piling up of Ge at the interface between the growing SiO2 andthe remaining SiGe, which forms a nanometre Ge-rich layer. Substantialinterdiffusion of Si and Ge takes place in the remaining SiGe, whichleads to the complicated distribution of Ge segregation. We find ananometre cap layer over the oxide film after fast oxidation, in whichthere are many Ge nanoparticles. We analyse the kinetics and mechanismof the nanostructure of the oxide and Ge segregation in oxidation ofSi1-xGex alloys.

Interference Fringes in the Plane Wave Topographic Images of Growth Bands in Si:Ge

An Si:Ge crystal with approximately 3% of germanium was studied with strongly collimated short-wavelength monochromatic synchrotron beam (beamline E2 at HASYLAB). The topographs obtained in the asymmetric 224 reflection revealed the presence of interference fringes related to growth bands caused by segregation of germanium. The fringes, observed for the first time, were strongly dependent on the angular setting and it was possible to distinguish at least three systems of fringes. A number of features of the existing strain field, which may be important for the formation of the fringes, was determined using other topographic methods, especially the Bragg-case section topography.

Local Bonding States of Titanium and Germanium-doped Tetragonal Zirconia Polycrystal and Their Correlation to High Temperature Ductility

bond with oxygen ions rather than Zr‐O bond. Covalency of TZP is more increased by solution of germanium ions than that of titanium ones. In superplastic deformation of TZP, an addition of GeO2 or TiO2 enhances tensile ductility of TZP. Germanium ion is more effective to improve ductility than titanium. The increment of covalency is in a good agreement with the improvement of elongation to failure in doped TZP. Dopant cations segregate at grain boundaries and form no secondary phase. Assuming that a dopant effect on chemical bonding states in grain boundaries is similar to that in grain interior, segregation of germanium or titanium ion increases covalent bonding strength nearby grain boundaries. Such increasing of covalency is likely to enhance cohesion of grain boundaries. The enhancement of grain boundary cohesion suppresses intergranular failure during tensile deformation at elevated temperatures. This must be the reason why an addition of GeO 2 and TiO2 is effective to improve the high temperature ductility of TZP. Our calculation suggests that the covalency nearby grain boundaries have a critical role in the tensile ductility of TZP.

Terahertz-Emitting Silicon-Germanium Devices

ABSTRACTIn this paper we report on far-infrared emission in the 1–12 THz frequency range from strained SiGe structures. Pseudomorphic superlattices were grown by Molecular Beam Epitaxy (MBE) at the relatively low substrate temperature of 400°C to prevent germanium segregation. Layer thicknesses, composition, and crystallinity were confirmed by high-resolution X-ray diffraction. Devices were designed to produce confined hole states with various energy separations. Mesa devices were etched in a reactive-ion etching system and tested for edge emission over a wide range of drive currents using an FTIR spectrometer in step-scan mode. THz emission was observed in pulsed mode at current densities as low as 50 A/cm2 and at temperatures as high as 50 K, using a liquid-helium-cooled silicon bolometer detector with a lock-in amplifier. Emission spectral peaks occurred at 7.9 and 9.36 THz for two different samples, in good agreement with k·p calculations.

Segregation of phosphorus and germanium to grain boundaries in chemical vapor deposited silicon-germanium films determined by scanning transmission electron microscopy

The segregation of phosphorus and germanium to grain boundaries in P implanted, Si0.87Ge0.13 films deposited by chemical vapor deposition (CVD), was investigated using energy dispersive x-ray (EDX) micro-analysis. A quantitative analysis of the x-ray spectra obtained at grain boundaries showed that the excess amount of P varied with the crystallography of the boundary but that the segregation always followed an equilibrium process with an activation energy of 0.28 eV. On the other hand, Ge did not segregate to grain boundaries in either P implanted Si0.87Ge0.13 films nor in intrinsic Si1−xGex films, containing 2, 13 and 31 at.% Ge. Possible reasons for the absence of Ge segregation are discussed.

The effect of adsorbed atomic hydrogen on the growth of ultrathin silicon films on Ge(100) studied by positron-annihilation-induced Auger electron spectroscopy (PAES)

The effects of adsorbed atomic hydrogen on the stability of silicon films grown on a Ge(100) substrate were studied by using positron-annihilation-induced Auger electron spectroscopy (PAES) and electron-induced Auger electron spectroscopy (EAES). PAES is almost exclusively sensitive to the topmost atomic layer due to the trapping of positrons in an image potential well just outside the surface before annihilation. This surface specificity was exploited in the study of film stability and interfacial mixing during the growth of silicon on Ge(100). The PAES results show that the prior adsorption of hydrogen prevented the segregation of germanium on top of the deposited silicon, and that the hydrogen adsorption was useful in growing a thermally stable structure.

RHEED and SIMS studies of germanium segregation during growth of SiGe/Si heterostructures; a two-site exchange model with growth rate dependence

Segregation of Ge during the formation of SiGe/Si heterojunctions in thin film epitaxial growth is perhaps the largest obstacle to the formation of the desirable atomically sharp interfaces in device structures. This process has been studied experimentally using reflection high-energy electron diffraction (RHEED), secondary ion mass spectrometry (SIMS) and numerically using a two-site exchange model. In this paper, the current two-site exchange model is modified to incorporate the growth process explicitly in the rate equation. This allows the extraction of analytical solutions in the dilute regime and provides a better insight into the influence of temperature and other parameters on the eventual composition profiles across the Si/SiGe heterojunction. Using numerical methods, the rate equation is solved for practical conditions and compared with experimental measurements by RHEED and SIMS. The dependence of composition profiles on the growth rate is demonstrated by the model and good agreements with experimental data without modifying the Gibbs energy of segregation.

Non-equilibrium primary crystallisation in silver–germanium alloy under microgravity conditions

The influence of convection on the morphology of primary dendrites and on the distribution of germanium in primary crystals of hypoeutectic Ag–Ge alloy was studied. The experiment in microgravity provided conditions for the suppression of melt flow. Both the morphology of the primary dendrites and their chemical composition profiles were found to vary considerably depending on the intensity of convection. Primary crystals in specimens solidified in the space laboratory exhibited strong supersaturation with germanium. This supersaturation is associated with a decrease of solute in the eutectic. The overall segregation of germanium between crystals and the melt during the growth of primary dendrites is strongly promoted by convection. The influence of convection on the non-equilibrium primary crystallisation, particularly on the solute trapping process, the occurrence of which is likely in the initial stage of primary crystallisation (solute rich core), is discussed. A higher solute gradient close to the core in the space sample implies a higher melt stability below the liquidus when convection is suppressed.

Germanium segregation in the Co/SiGe/Si(001) thin film system

Cobalt disilicide contacts to silicon–germanium alloys were formed by direct deposition of pure cobalt metal onto silicon–germanium films on Si(001) substrates. Segregation of germanium was observed during the reaction of the cobalt with the silicon–germanium alloy. The nature of the Ge segregation was studied by transmission electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. In the case of cobalt films deposited onto strained silicon–germanium films, the Ge segregation was discovered to be in the form of Ge-enriched Si1−xGex regions found at the surface of the film surrounding CoSi and CoSi2 grains. In the case of cobalt films deposited onto relaxed silicon–germanium films, the Ge segregation was dependent on formation of CoSi2. In samples annealed below 800 °C, where CoSi was the dominant silicide phase, the Ge segregation was similar in form to the strained Si1−xGex case. In samples annealed above 800 °C, where CoSi2 was the dominant silicide phase, the Ge segregation was also in the form of tetrahedron-shaped, Ge-enriched, silicon–germanium precipitates, which formed at the substrate/silicon– germanium film interface and grew into the Si substrate. A possible mechanism for the formation of these precipitates is presented based on vacancy generation during the silicidation reaction coupled with an increased driving force for Ge diffusion due to silicon depletion in the alloy layer.

Theoretical investigation of the equilibrium surface structure of Si 1− x− yGe xC y alloys

We investigate the surface structure of SiGeC alloys at typical growth temperatures using Monte Carlo simulations within the empirical potential formalism. We find a strong tendency of carbon to segregate to the top layer. As a consequence, germanium segregation is drastically reduced. The carbon composition profile oscillates in the subsurface layers, with a marked enhancement in the third layer. Si–C dimers are the favored surface bonding geometry, and are mostly found in (2×2) and c(4×2) arrangements.

Electrical and structural properties of zirconium germanosilicide formed by a bilayer solid state reaction of Zr with strained Si1−xGex alloys

The effects of alloy composition on the electrical and structural properties of zirconium germanosilicide (Zr–Si–Ge) films formed during the Zr/Si1−xGex solid state reaction were investigated. Thin films of Zr(Si1−yGey) and C49 Zr(Si1−yGey)2 were formed from the solid phase reaction of Zr and Si1−xGex bilayer structures. The thicknesses of the Zr and Si1−xGex layers were 100 and 500 Å, respectively. It was observed that Zr reacts uniformly with the Si1−xGex alloy and that C49 Zr(Si1−yGey)2 with y=x is the final phase of the Zr/Si1−xGex solid phase reaction for all compositions examined. The sheet resistance of the Zr(Si1−yGey)2 thin films was higher than the sheet resistance of similarly prepared ZrSi2 films. The stability of Zr(Si1−yGey)2 in contact with Si1−xGex was investigated and compared to the stability of Ti(Si1−yGey)2 in contact with Si1−xGex. The Ti(Si1−yGey)2/Si1−xGex structure is unstable when annealed for 10 min at 700 °C, with Ge segregating from Ti(Si1−yGey)2 and forming Ge-rich Si1−zGez precipitates at grain boundaries. In contrast, no Ge segregation was detected in the Zr(Si1−yGey)2/Si1−xGex structures. We attribute the stability of the Zr-based structure to a smaller thermodynamic driving force for germanium segregation and stronger atomic bonding in C49 Zr(Si1−yGey)2. Classical thermodynamics were used to calculate Zr(Si1−yGey)2–Si1−xGex tie lines in the Zr–Si–Ge ternary phase diagram. The calculations were compared with previously calculated Ti(Si1−yGey)2–Si1−xGex tie lines.

Film thickness effects in the Ti–Si1−xGex solid phase reaction

The effects of film thickness on the Ti–Si1−xGex solid phase reaction were investigated. Thin C49 TiM2 (M=Si1−yGey) films were formed from the solid phase reaction of 400 Å Ti or 100 Å Ti with Si1−xGex alloys. It was determined that for films formed from 400 Å Ti, the nucleation barrier of the C49-to-C54 transformation decreases with increasing germanium content, for alloy compositions with up to ≊40 at. % germanium (i.e., x≤0.40). It was also observed that germanium segregates out of the TiM2 lattice, for both the C49 and C54 phases, and is replaced on the TiM2 lattice with Si from the substrate. The germanium segregation changes the Ge index y of the Ti(Si1−yGey)2. For films formed from a 100 Å Ti layer it was observed that the C54 TiSi2 nucleation temperature was increased by ≥125 °C. The addition of germanium to the silicon increased the agglomeration of the C49 phase and caused the C54 TiM2 nucleation barrier to increase further. The results also indicate that the increased temperature required for the transition to the C54 phase, for the 100 Å films, leads to an increased rate of germanium segregation.

Characterization of Zirconium Germanosilicide Formed by Solid State Reaction of ZR With Sil−xGex Alloys

AbstractWe have investigated the electrical and structural properties of zirconium germanosilicide (Zr-Si-Ge) films formed during the Zr-Sil−xGex solid state reaction. Thin films of C49 Zr(Si1−xGex)2 were formed from the solid phase reaction of Zr and Si1−xGex bilayer structures. It was observed that Zr reacts uniformly with the Sil−xGex alloy and that C49 Zr(Si1−x Gex)2 is the final phase of the Zr-Si1−xGex, solid phase reaction (such tht y = x) for all compositions examined (x = 0.20, 0.33, and 0.50). The sheet resistance of the Zr(Si1−xGex)2 thin films were higher than the sheet resistance measured for ZrSi2 films. The stability of Zr(Sil−x Gex)2 in contact with Si1−Gex was investigated and no germanium segregation was detected in the Zr(Si1−xGex)2/Si1−Gex structures.

Interface Stability of Ti(Sil−yGey)2 and Si1−x Gex Alloys

AbstractThe stability of C54 Ti(Si1−yGey)2 films in contact with Si1−xGex substrates was investigated. The titanium germanosilicide films were formed from the Ti − Si1−xGex solid phase metallization reaction. It was observed that Ti(Si1−yGey) 2 initially forms with the same germanium content as the Si1−x Gex substrate (i.e., y = x). Following the initial formation of TiM2 (M = Sil−yGey), silicon and germanium from the substrate diffuse into the TiM2 layer, the composition of the TiM2 changes, and Si1−z Gez precipitates form along the TiM2 grain boundaries. The germanium content of the Ti(Sil−y Gey)2 decreases, and the Sil−z Gez precipitates are germanium rich such that y < x < z. This instability of the TiM2 film and the dynamics of the germanium segregation were examined using the Ti-Si-Ge ternary equilibrium diagram. The relevant region of the ternary diagram is the two phase domain limited by a Si-Ge solid solution and a TiSi2 − TiGe2 solid solution. In this study first approximation Ti(Sil−y Gey)2 -to- Sil−xGex tie lines were calculated on the basis of classical thermodynamics. The tie line calculations indicate that for C54 Ti(Sil−yGey)2 to be stable in contact with Sil−xGex, the compositions of the two phases in equilibrium must be such that y < x. The specific compositions of the two phases in equilibrium depend on the temperature and the relative quantities of the two phases. The dynamic processes by which the Ti(Si1−yGey)2/Si1−x. Gex, system progresses from the as-formed state (y = x) to the equilibrium state (y < x) can be predicted using the tie line calculations.

Ge surface segregation at low temperature during SiGe growth by molecular beam epitaxy

The temperature dependence of germanium surface segregation during growth by solid source SiGe molecular beam epitaxy (MBE) was studied by x-ray photoelectron spectroscopy and kinetic Monte Carlo (KMC) modeling. Germanium segregation persisted at temperatures 60 °C below that predicted by a two-state exchange model. KMC simulations, where film growth, surface diffusion, and surface segregation are modeled consistently, successfully describe the low temperature segregation of germanium. Realistic descriptions of MBE must follow the physical rates of the growth, surface diffusion, and surface segregation processes.

Kinetic Monte Carlo Simulations of Ge Segregation During Epitaxial Growth of Si-SixGel-x Heterostructures

ABSTRACTThe temperature dependence of germanium surface segregation during growth by solid source SiGe molecular beam epitaxy was studied by x-ray photoelectron spectroscopy and kinetic Monte Carlo (KMC) modeling. Germanium segregation persisted at temperatures 60ºC below that predicted by a two-state exchange model. KMC simulations, where film growth, surface diffusion, and surface segregation are modeled consistently, successfully describe the low temperature segregation of germanium. Realistic descriptions of MBE must follow the physical rates of the growth, surface diffusion, and surface segregation processes. A specific, step mediated exchange mechanism is also considered and shown to lead to surface segregation. While this model of Ge segregation seems possible, more work is necessary to obtain a consistent set of energy barriers.

Growth of SiGe/Si quantum well structures by atmospheric pressure chemical vapor deposition

First structural and electrical data are reported for SiGe/Si quantum well structures grown by a new ultra clean low temperature epitaxial deposition process at atmospheric pressure. It is found that the process suppresses the segregation of germanium, possibly by a chemical termination of the surface during the growth. Mutiple-quantum-well structures with controllable well widths and abrupt interfaces have been prepared at temperatures ranging from 550 to 650°C. Magneto-transport measurements of modulation doped quantum wells reveal hole mobilities of 2000 cm2/Vs at 4.2 K at a carrier density of 1.7*1012 cm−2 and a germanium concentration of 18% in the SiGe channel. Resonant tunneling diodes grown by this technique exhibit well resolved regions of negative differential resistance within a very symmetric I-V characteristic.

Germanium segregation induced reconstruction of SiGe layers deposited on Si(100)

The surface structure of SiGe layers deposited on Si(100) has been studied by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM). AES spectra reveal a strong germanium segregation. The 2 × 1 surface reconstruction of Si(100) observed by LEED is changed into a 2 × n ( n > 8) structure on deposition of SiGe layers. Corresponding STM pictures reveal a surface built up from terraces with alternately oriented dimer rows. After n - 1 subsequent dimers, however, one dimer is missing. Those missing dimers form lines which give rise to the 2 × n LEED pattern. The density of missing dimers depends on the germanium content and on the layer thickness. The evaluation of the LEED intensities can explain the structure by a small expansion of the topmost SiGe layer enriched by germanium.

A comparative study of heavy boron doping in silicon and Si 1− x Ge x layers grown by molecular beam epitaxy

Different aspects of boron doping have been studied for silicon and Si 1− x Ge x layers grown by molecular beam epitaxy. Boron segregation at high doping levels was measured in situ in both cases and it was found to be relatively suppressed in the alloy samples. Some possible mechanisms for this effect are discussed. Germanium segregation was also measured during the growth of the first monolayers in Si 1− x Ge x structures. The limits for boron incorporation and activation as dopant in such structures are discussed on the basis of electrical measurements.

Surface and bulk equilibrium structures of silicon-germanium alloys from Monte Carlo simulations

The technologically important alloy of silicon and germanium is known to have no long-range order in the bulk. Whether there is any short-range order in the bulk and the nature of the structure of the surface and near-surface regions remains the subject of some debate. We present a method of Monte Carlo simulation that is effective for studying the equilibrium structural properties of the bulk and strained-superlattice alloys. The simulations of the bulk alloy indicate that there is no short-range order and that the Si-Ge alloy is a true random alloy at room temperature and above. The surfaces of these alloys experience segregation of germanium to the surface, with no long-range order observed in the slightly silicon-rich subsurface layers, in agreement with recent experiments.