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Geometric models relating the energy of heterophase boundaries to their atomic structure can be based on the static distortion wave concept. This approach allows to separate the energetic contributions of commensurate and incommensurate boundary structures and provides the physical basis for models proposed previously to predict low energy configurations of such interfaces.
The Invariant Line Criteria (ILC) has been used to explain experimentally observed precipitation and epitaxial orientation relationships in a number of metallurgical systems. We propose that ILC may explain the crystallography of misoriented silicide grains in epitaxially grown silicide films. We test our hypothesis on the case of misoriented grains in CoSi2 on Si (001). Our analysis shows that the experimentally observed orientation relationship is predicted by the ILC.
A large softening of the shear modulus has been reported in metallic superlattices composed of insoluble bcc/fcc metals. In an attempt to understand this elastic anomaly, we have studied the microstructure of Fe/Cu bilayers as a function of the Fe thickness with transmission electron microscopy (TEM). Analysis of the moire fringes observed in plan-view TEM images revealed that the fee Fe structure epitaxially grows on the (001) Cu up to a thickness of 2.0 nm. At 2.3 nm, the bec Fe structure nucleates, accompanying lattice rotation around the growth direction with respect to the underlying fee structure. As the Fe thickness further increases, submicron polycrystalline grains formed. Based on these results, the microstructure of the metallic superlattices and its relation to the softening of the shear modulus will be discussed.
An UHV Scanning Transmission Electron Microscope (STEM) has been used to study the nucleation and epitaxial growth of Au and Ag deposited at room temperature on H-passivated thin silicon (111) and (100) substrates. Direct observations of the initial stages of overlayer growth were made using bright field (BF) and annular dark field (ADF) imaging modes. Gold was found to grow on silicon in a quasi-continuous layer mode, while 3D island growth with density as high as 4×1012/cm2 was observed in Ag/Si systems. Electron diffraction studies reveal a good epitaxial relationship for Ag films grown both on silicon (111) and (100) substrates. Diffraction also indicates that Au (111) grows on the silicon (111) surface but the grains may be azimuthally rotated.
A study of the growth of thin Ir silicide films on (111)Si substrates has been undertaken. Thin (2.0nm) ir films deposited onto Si substrates under ultra-high vacuum conditions have been observed to display remarkable film continuity and fine grain structure (lnm). In situ annealing at 1000°C resulted in the formation of large regions (>10µm) of epitaxial IrSi3 islands (∼1µm) with identical epitaxial orientations. By means of annealing an as-deposited (2.0nm) Ir film stepwise to 1000°C within a transmission electron microscope the evolution of Ir silicide phases and morphologies were observed. The epitaxial growth of the semiconducting IrSi1.75 phase is reported along with the formation of Ir silicide islands at temperatures between 700°C and 800°C.
Thin films of Ag were grown on amorphous C and <111= Si substrates with simultaneous Ar+ bombardment at energies ranging from 50–40,000 eV. For deposition of Ag on amorphous C, ion beam bombardment induced no changes in film nucleation behavior relative to evaporation (henceforth referred to as physical vapor deposition, PVD). Film growth was affected at the highest energy (40 keV); the grain size of the Ag films was increased by a factor of three. Rutherford Backscattering (RBS) measurements on Ag films on <111=Si bombarded with Ar+ at 1.5 keV showed that the Ag sputtering yield at film thicknesses <1.5 nm was less than for bulk Ag, in agreement with TRIM calculations. At 40 keV there was evidence for an additional effect of the ion beam due to recoil implantation or ion mixing. Electron diffraction from Ag fdms grown on <111= Si substrates with simultaneous Ar+ bombardment at either 1.5 keV or 40 keV showed evidence for only the expected phases: single crystal Si, polycrystalline Ag, and an amorphous phase that likely resulted from ion damage to the substrate.
We report the significant improvement of GaAs crystal quality on Si grown by metal-organic chemical vapor deposition (MOCVD) with an in situ low temperature hydrogen/arsine plasma cleaning of the Si substrate at 450°C and a consequent controlled two-dimensional-like morphology of the low temperature buffer layer at its early stage. The most critical step that determines the interfacial cleanliness and the early stages of the nucleation and thin film formation of heteroepitaxial GaAs on Si in a non-ultrahigh vacuum MOCVD system is the substitution of hydrogen atoms passivating the Si surface after ex situ HF-dip with pas-sivating As atoms. Reduction of in situ cleaning temperature ensures the very slow kinetics of thermal desorption of the hydrogen atoms and re-oxidation of exposed Si surface from the reactor environment, and provides a fully As-passivated Si surface, leading to a 2D-like buffer layer.
Electron microscopy techniques have been used to study the effect of deposition conditions on island growth and coalescence in r.f. glow discharge prepared a-Ge:H. Planar views of 500 Å thick films were obtained using transmission electron microscopy (TEM) while scanning electron microscopy (SEM) was used to investigate the cross-sectional microstructure of thicker films. Films were prepared on both electrodes of a capacitively-coupled, diode deposition system. The substrate temperature, level of applied power, externally applied d.c. substrate bias, and the germane to diluent gas ratio were systematically varied. For films prepared on the unpowered electrode, island coalescence was dramatically enhanced with increasing substrate temperature while columnar structure, found for samples prepared at lower substrate temperatures, vanished. Both effects are attributed to the elimination of a coalescence barrier with increased temperature. Increasing negative substrate bias slightly increased island coalescence while increasing the level of applied power had no visible effect on film microstructure. The island size decreased with increasing dilution of the gas plasma by H2. The addition of GeF4 to the gas plasma leads to a low density, porous film. This observation is attributed to either a difference in the plasma chemistry or increased film etching due to the presence of F in the gas plasma. High quality films prepared on the powered electrode were found to be more structurally homogeneous and environmentally stable than films prepared on the unpowered electrode at the same substrate temperature. Enhanced bombardment and/or a different plasma chemistry near the powered electrode are two factors which may contribute to the observed difference in microstructure.
The initial growth of Ag on reconstructed Si(100) has been studied with biassed secondary electron imaging (b-SEI) and Auger electron spectroscopy (AES) in an ultra-high vacuum (UHV) scanning transmission microscope (STEM) with nanometer resolution. Small Ag islands have been observed with strong contrast in b-SE images. Anisotropic growth, correlated with the (2×1) and (1×2) dimer reconstruction, is seen at room temperature and sub-monolayer (ML) coverage. Large Ag islands (∼1 μm) formed at 475 °C substrate temperature have even more dramatic forms with large aspect ratios. The Stranski-Krastanov (SK) mode is confirmed at both temperatures by AES and b-SEI between the islands, with the intermediate layer coverage equal to 0.5 ML or less.
Isothermal-isobaric Molecular Dynamics (MD) simulation of a submonolayer Pb film in c(2×2) ordered structure adsorbed on a Cu(100) substrate showed retention of order to high T. The Embedded Atom Method (EAM) calculated the energy of atoms of overlayer and substrate. The time-averaged squared modulus of the two dimensional structure factor for the Pb overlayer measured the order of the overlayer. The results are for increasing T only, and require verification by simulated cooling.
A theory is presented which describes the capillary-driven aging of discontinuous thin films on a substrate, where the primary transport mechanism among the domains is two-dimensional diffusion of species over the substrate. This theory employs a statistical dynamics formulation, whereby the average growth rate for each domain size class is determined relative to the critical (zero-growth) domain size. The time dependence of the critical size is determined through a global constraint on the individual fields. The effect of fractional area coverage, Aa, is accounted for through a second global constraint over the distribution of island sizes.
This theory yields a self-similar size distribution that is fairly insensitive to Aa. The critical island radius, R*, is found to increase asymptotically as the cube-root of time. The growth rate of R* increases with Aa, which results from the closer proximity of the islands and steeper concentration gradients as Aa increases.
We have studied the initial stages of island formation and coarsening for epitaxial Ge on vicinal Si (100) using in-situ deposition and nanometer resolution biassed secondary electron imaging (b-SEI) in a UHV-STEM. Ge is deposited using MBE techniques on nominally flat Si(100) substrates as well as those misoriented 1° and 5° toward <110>. The temporal evolution of the islanded microstructure can be studied by analysis of computer generated island size distributions. Good statistics can be obtained for islands with radii between 2nm and lOOnm using high resolution b-SE imaging and a large magnification range. Both MBE and Solid phase MBE (SP-MBE) processes have been studied.
We explain the evolution of the islanded microstructure in terms of competition for Ge adatoms among the various available sinks. For the MBE case, control of diffusion distances by varying the substrate temperature has allowed us to identify effects related to coherently strained and highly dislocated Ge islands as well as contaminant particles. In all cases, coherently strained Ge islands appear to be the weakest sinks and contaminant particles the strongest. Metastable growth of the intermediate layer during interrupted depositions at 375°C may be a direct consequence of an energy cost for incorporating adatoms into coherently strained islands. For depositions at higher temperatures, strong adatom sinks influence nucleation densities and size distributions of Ge islands by reducing the effective supersaturation. Island size distributions analyzed for the case of room temperature deposition in the early stages of coarsening also show evidence of effects due to coherently strained islands. These size distributions evolve from an initial distribution to one with increasing number of large islands while the distribution of the smaller islands (< 10nm radius) remains constant.
Coarsening phenomenon is observed among crystalline Si clusters which are deposited over silicon-oxide and -nitride surfaces by chemical vapor deposition using HC1 as an etching gas. As soon as the deposition starts, submicron-sized fine clusters nucleate and increase in number, but do not grow in size. Micron-sized large clusters emerge among the fine ones, gradually increase and rapidly grow, while the preexisting fine ones disappear finally. It is found that the fine clusters must be etched away into vapor again by HCl or reevaporated.
The formation and growth of islands on GaAs substrates has been observed during high temperature (550°C) heat treatment of GaAs substrates in the presence of H2Se. We have used SEM, TEM and XPS to characterize these islands and we have proposed a mechanism for island formation based on coarsening of Ga on the substrate surface followed by reaction with H2Se to form Ga2Se3.
An equilibrium model for agglomeration based upon the mechanism of grain boundary grooving in polycrystalline thin films is suggested. It involves an energy balance between surface, interface, and grain boundary energies, and predicts parameters which will influence the onset of agglomeration. It has been determined that small grain size, low grain boundary energy, high film surface and interface energies, and growth of single crystal epitaxial layers should promote resistance to agglomeration. Polycrystalline TiSi2 thin films deposited on Si substrates have been observed using cross-section TEM. The micrographs provide evidence that, for these films, the grain boundary grooving mechanism is dominant and most of the modeling assumptions are valid.
The agglomeration of Co silicide films formed on Si substrates processed with different Co film thickness was investigated by TEM, XRD, and four-point-probe measurements. It was found that thermal grooving always accompanies the film formation, while islanding can occur during high temperature thermal stability testing, or during formation of very thin films at moderate temperatures. In addition to whole film agglomeration, partial agglomeration on the top of the film has been observed, which is prominent and important for thin films. A theoretical model of agglomeration for silicide films is presented, which shows that when the ratio of grain size to film thickness is smaller than a critical value, the film will not lose its continuity. Also, grain boundary migration was found to have a suppressing effect on thermal grooving. Both a small grain size and a low grain boundary energy are shown to be favorable for improving the thermal stability.
Polysilicon/silicon interfacial oxides are shown by cross-sectional Transmission Electron Microscopy studies to agglomerate upon annealing. In addition to presenting highlights of microscopy results, we report on electrical characterization data obtained from Cross-Bridge Kelvin Resistors. Resistor data not only support a model for agglomeration proven on microscopy data, but also allow for a quantitative macroscopic understanding of the agglomeration of polysilicon/silicon interfacial oxides over a wide range of times and temperatures.
This paper will review the topic of computer simulation of the evolution of grain structure in polycrystalline thin films, with particular attention to the modelling of the grain growth process. If the grain size is small compared to the film thickness, then the grain structure is three-dimensional. As the grains grow to become larger than the film thickness, so that most grains traverse the entire thickness of the film, the microstructure may approach the conditions for a two-dimensional grain structure. Both two- and three-dimensional grain growth have been simulated by various authors.
When the grains become large enough for the microstructure to be two-dimensional, the surface energy associated with the two free surfaces of the film becomes comparable to the surface energy of the grain boundaries. In this condition, the free surface may profoundly effect the grain growth. One effect is that grooves may develop along the lines where the grain boundaries meet the free surfaces. This grooving may pin the boundaries against further migration and lead to grain-growth stagnation. Another possible effect is that differences in the free surface energy for grains with different crystallographic orientation may provide a driving force for the migration of the boundaries which is additional to that provided by grain boundary capillarity. Grains with favorable orientations will grow at the expense of grains with unfavorable orientations. The coupling of grain-growth stagnation with an additional driving force can produce abnormal or secondary grain growth in which a few grains grow very large by consuming the normal grains.
Thin Sb films have been prepared on glass substrates by rapid thermal evaporation. Films with thicknesses varied from 260 Å to 1300Å were used for the study. X-ray diffraction data showed that for films deposited at room substrate temperature, an almost random grain orientation was observed for films of 1300 Å thick and a tendency for preferred grain orientation was observed as films got thinner. For films of 260 Å thick, only two x-ray diffraction peaks--(003) and (006) were observed. After thermal annealing, secondary grains grew to show preferred orientation in all the films. This phenomenon was explained by surface-energy-driven secondary grain growth. This paper reports the effects of annealing time and film thickness on the secondary grain growth and the evolution of thin Sb film microstmctures. Transmission electron microscopy (TEM) and x-ray diffraction were used to characterize the films.
Titanium films of 0.5 µm thickness were sputter deposited on silicon substrates. After rapid thermal annealing at temperatures ranging from 600°C to 850°C for times up to 45 seconds in nitrogen, transmission electron microscope (TEM) cross section specimens were made from the wafers. Grain sizes of the resulting titanium disilicide were measured from TEM cross section micrographs. The results show that C49-TiSi2 has a different grain growth rate than C54-TiSi2- Under our experimental conditions, C54-TiSi2 has a much higher growth rate. Titanium silicide on arsenic implanted silicon substrates shows a lower grain growth rate than that on unimplanted substrates under the same conditions. The thickness of the silicide layer was also measured for each specimen. The relationship of thickness and grain size will be discussed.