Au In Amorphous Si Research Articles

Solid-state amorphization of gold and silicon bilayer films by annealing

ABSTRACT Interfacial reactions in vacuum-deposited gold and silicon bilayer films were investigated by transmission electron microscopy and electron diffraction. By heating to 453 K, an amorphous Au–Si structure formed in gold films deposited at room temperature on both crystalline and amorphous silicon films, and the amorphization strongly depended on the grain size of the gold films. The driving force for the formation of the amorphous Au–Si structure was analyzed based on the free energy increase due to defects, such as grain boundaries, in the gold films on silicon films. The amorphization was concluded to be a transformation from the crystalline phase to the undercooled liquid Au–Si phase.

On the Nature and Origin of Si Surface Segregation in Amorphous AuSi Alloys

We examined the surface segregation behavior of Si in amorphous AuSi alloys using ab initio molecular dynamics simulations within density functional theory. For a thin Au70Si30 film, our simulations predict Si surface enrichment that leads to a stoichiometry close to Au60Si40 in the surface layer. The surface structure exhibits a rather ordered Au3Si2 phase, which somewhat differs from the bulk Au60Si40 structure that exhibits a tendency of random hard-sphere packing. We also discuss the origin of the Si surface segregation based on analysis of segregation-induced changes in the atomic and electronic structure of the AuSi alloy surface.

Structure, energetics, and bonding of amorphous Au–Si alloys

First principles periodic calculations based on gradient-corrected density functional theory have been performed to examine the structure, energetics, and bonding of amorphous Au-Si alloys with varying Au:Si composition ratios. Our results predict that the Au-Si alloy forms the most stable structure when the Si content is around 40-50 at. %, with an energy gain of about 0.15 eV/atom. In addition, the volume change per atom in the alloy exhibits a distinctive nonlinear trend, with the minimum value around 60 at. % Si. The occurrence of the minimum in the Au-Si mixing energy and volume is attributed to strong hybridization of the Au 5d-Si 3p states. We also present variations in the radial distribution function and atomic coordination number as a function of Au:Si composition ratio, with discussion of the nature of local packing and chemical bonding in the Au-Si alloy system.

Diffusion of gold and platinum in amorphous silicon and germanium

195Au radiotracer atoms produced via the radioactive decay of implanted 195Hg ions have been used to investigate systematically the diffusivities of Au in amorphous Si and Ge by means of serial Ar +-beam sectioning as a function of the diffusion temperature, doping with Au and/or charging with hydrogen. Less extensive studies of the same type have been carried out for the diffusion of Pt in Pt-doped amorphous silicon. The Au diffusion data are interpreted in terms of direct diffusion in which the diffusing 195Au atoms are temporarily trapped by different kind of vacancy-like defects. These traps can be saturated with Au or hydrogen. Their nature and concentrations are found to change as a result of annealing-induced structural relaxation of the amorphous specimens.

SANS investigation of internal interfaces in nanostructured Si [sbnd]Au alloys

Contrast variation of SANS by immersion in liquids has been shown to be a powerful tool to identify the scattering contributions in nanostructured amorphous Si Au samples. The fractal nature of the internal surface ( D s = 2.63) between a channel system of open pores and the amorphous material has been verified, as well as the microstructural concept of dense nanosized amorphous particles embedded in a less dense “matrix” formed by grain boundaries and free volumes. Using C 6D 12, CS 2 and D 2O, a molecular-size effect of different access to smaller pores was observed.

Nanostructured amorphous SiAu alloys — Structure and atomic correlations

Nanostructured amorphous Si 1− x Au x alloys ( 0.1 ⪕ x Au ⪕ 0.35 ) were produced by an inert gas condensation and in situ consolidation technique. Small-angle neutron scattering (SANS), density measurements and wide-angle neutron scattering (WANS) were employed to characterize the microstructure and local atomic order. The SANS results are interpreted in terms of a microstructure which consists of nanometre-sized amorphous grains embedded in a matrix of lower density, formed by grain boundaries and free volumes. The relative densities depend on the average particle size, but not on the alloy composition. In all samples, the SANS intensity decreases at low Q according to a power law − Q −3.37, which is attributed to a surface fractal structure of dimension D s = 2.63. WANS reveals two contributions: (a) a continuously decreasing part resulting from the presence of about 10 at.% hydrogen, and (b) a modulation of the intensity as a function of the scattering angle, characteristic of amorphous material. The total pair correlation function g( R) presents maxima corresponding to the first and second atomic neighbours. Nanostructured amorphous SiAu alloys exhibit a reduced short-range order compared with systems with dense random packing of hard spheres, e.g. metallic glasses, or continuous random network structures, e.g. covalent systems.

Small-angle neutron scattering studies of nanosized amorphous SiAu alloys

Small-angle neutron scattering (SANS) was employed to characterize the microstructure of nanosized amorphous Si 1− x alloys (0.1 ⩽ x ⩽ 0.3), produced by an inert gas condensation and in situ consolidation technique. Amorphous grains are embedded in a matrix of lower density, formed by grain boundaries and free volumes. The relative densities were found to be dependent on the average particle sizes and volume fractions, but independent of the alloy composition. In all samples, the SANS intensity decreases according to a power law Q −3.37 at low Q, which is attributed to a surface fractal structure of dimension D s = 2.63. This has been supported by a contrast variation experiment.

Electron-irradiation-induced gold atom implantation into a silicon substrate and AES analysis of the gold-implanted silicon

Electron-irradiation-induced gold atom implantation into a silicon substrate has been examined by a combination of HVEM and AES. Bilayer films of Au(target)/Si(substrate) were irradiated with 2.0–2.4 MeV electrons in a HVEM, with the electron beam incident on the gold layer. Gold atoms which have been knocked-off from the gold layer by the collision with 2.0–2.4 MeV electrons are able to intrude into the silicon substrate. With increasing total dose of electrons, an amorphous Si-Au alloy is produced at the gold-enriched region. In the amorphous Si-Au alloy, the sp 3 hybrid of silicon decays and a new bonding state is formed between silicon and gold. The repetition of these displacements of gold atoms and the subsequent alloy-formation results in the implantation of gold into the silicon substrate.

Interface-state-controlled segregation of gold during ion-beam-induced epitaxy of amorphous silicon.

Segregation coefficients and velocity enhancements for Au in amorphous Si during ion-beam-induced epitaxial crystallization were measured. At 320 {degree}C, velocity enhancements of up to 2.5 times were observed, and interface breakdown occurred at interfacial Au concentrations of 11 at. %. Although qualitatively similar to thermal solid-phase epitaxy, these velocity enhancements are substantially reduced in magnitude while the interface breakdown occurs at much higher concentrations. Between 250 {degree}C and 400 {degree}C, Au is trapped at the moving interface with segregation coefficients {ital k}, which are approximately velocity and concentration independent and vary between 0.001 and 0.012. In contrast with classical segregation, however, {ital k} increases linearly with interface position during the initial stages of growth to a temperature-dependent steady-state value. At 250 {degree}C, for example, {ital k} increases by at least a factor of 4 to 0.012 during growth. These results suggest an evolving interface structure from the initial thermal configuration to an irradiation-induced steady-state configuration with a higher trapping efficiency.

Influence of thermal relaxation and ion bombardment on the electronic properties of amorphous SiAu films

Effects of thermal relaxation on amorphous SiAu films are investigated by means of photoelectron spectroscopy and measurements of the electrical resistivity and specific heat. Annealing of the films in the amorphous state increases the resistivity and reduces the electronic density of states at the Fermi energy. These relaxation effects can be reversed by irradiation with 5 keV argon ions at low temperature. The results are discussed in the framework of structural relaxation, clustering and a scaling theory given by McMillan (1981).

Diffusivity of gold in amorphous silicon measured by the artificial multilayer technique

Artificial multilayers of amorphous Si and amorphous Si containing 0.7 at. % Au, with repeat lengths between 44 and 48A, were fabricated by ion beam sputtering. The change, with annealing time in the intensity of the first-order x-ray diffraction peak resulting from the composition modulation, is used to determine the diffusivity of Au in amorphous Si. Diffusion lengths on the order of an interatomic distance have been measured. The diffusivities over the temperature range 200–260 °C have an Arrhenius-type temperature dependence with an activation enthalpy of about 1.3 eV, and are in agreement with the extrapolation of published higher temperature data.

Diffusion of Au in Amorphous Si Measured by the Artificial Multilayer Technique

AbstractThe diffusivity of Au in amorphous Si (a-Si) has been determined in the temperature range 200-260°C using and Ar-ion sputter deposited artificial multilayered films of a-Si and a-Si(0.7at%Au) with repeat lengths between 44 and 48 Å. Diffusion on lengths scales of the order of interatomic spacings have been investigated. The Au diffusivity exhibits an Arrhenius temperature dependence with an activation enthalpy of 1.2eV and shows good agreement with extrapolations of higher temperature Au diffusion data obtained by Rutherford backscattering spectrometry (RBS) in ion-implanted, CVD, and sputter deposited a-Si. The measured Au diffusivity also shows a significant time dependence.

Radiation-enhanced diffusion of Au in amorphous Si

The radiation-enhanced diffusion of implanted Au markers in amorphous Si has been measured in the temperature range 77–693 K. Samples were irradiated with 2.5 MeV Ar ions. The diffusion coefficients show three well-defined regions. For temperatures <400 K, diffusion is athermal and due to ballistic mixing. For temperatures in the range 400–700 K diffusion is Arrhenius-type with an activation energy of 0.37 eV and is considerably enhanced over the normal thermal diffusion. The defects that cause the enhanced diffusion come from nuclear energy loss processes. Thermal diffusion, with an activation energy of 1.42 eV, dominates at temperatures greater than 750 K.

Diffusion of Au in Amorphous Si During Ion-Beam Irradiation

ABSTRACTWe have measured the radiation-enhanced diffusion of Au in amorphous Si in the temperature range 77–700 K. Gold was implanted to depths of 500Å at concentrations of an atomic %. The samples were than amorphized to depths of -2μm using MeV Ar implants at liquid nitrogen temperature. Radiation-enhanced diffusion was induced by a 2.5 MeV Ar beam at doses of 2×1016−2×1017/cm2 and dose rates of 7×1012−7×1013/cm2sec. The diffusion coefficients show three well defined regions. At temperatures <400K diffusion is essentially athermal and due to ballistic mixing. At temperatures between 400K and 700K the diffusion, which is considerably enhanced over the usual thermal values, has an Arrhenius behavior with an activation energy of 0.37 eV. At higher temperatures thermal diffusion, with an activation energy of 1.42 eV, dominates.

High-resolution EM study of the gold silicon interface reaction

Gold occupies a key role in silicon solid state device technology, for contacts and carrier lifetime control. Gold is added to silicon by evaporation onto HF cleaned Si wafers, followed by annealing at high temperature. The Au-Si binary system is a simple eutectic, with very limited terminal solid solubility and no reported stable intermediate compounds. When Si wafers are HF cleaned and exposed to air, even for very short times, a native oxide film a few nm thickness will form, and be interposed between the Si substrate and the evaporated Au layer. The effect of this film on the interface reaction is of interest. Alternatively, conventional surface research on Au/Si interfaces formed by UHV in-situ cleaning, evaporation and annealing provided evidence for an amorphous Au-Si layer and metastable crystalline Au silicides at the position of the original Au/Si interface.

Diffusion, Solubility and Segregation of Implanted Cu, Ag and Au in Amorphous Si

AbstractThe diffusion of Cu, Ag and Au has been measured in implanted, amorphous Si, over the range 150–600° C. The diffusion coefficients are characterized by Arrhenius relationships with activation energies for Cu, Ag and Au of 1.25, 1.6 and 1.4 eV respectively. The solubility of Au in amorphous Si was measured to be 6 orders of magnitude greater than crystalline Si at a temperature of 515°C. The Cu, Ag and Au are segregated ahead of the moving amorphous-crystalline interface. The presence of Au can increase the velocity of the interface.

Formation and crystallization of amorphous silicides at the interface between thin metal and amorphous silicon films

We investigated the interaction of extremely thin (less than 10 nm) crystalline gold and rhodium films with amorphous silicon by transmission electron microscope in situ annealing. In thin Au/Si bilayers an amorphous phase with a diffraction peak at d ≈ 0.226 nm is formed by thermal annealing between 150 and 200 °C. Depending on the thickness and composition, silicon sputtered onto thin gold films leads to the formation of a layer of amorphous silicon and a partially amorphous AuSi layer during deposition. The silicon layer crystallizes by itself at temperatures as low as 150 °C, and at 300 °C the amorphous AuSi layer crystallizes into a metastable gold silicide (for silicon-rich compositions). In Rh/Si bilayers an amorphous RhSi phase is formed by annealing to 300 °C and can be detected by electron diffraction for a rhodium thickness of less than 5 nm and compositions with more than 50% Si if completely reacted. Above 300 °C the amorphous RhSi crystallizes preferentially in the cubic form of RhSi for intermediate silicon compositions and in the orthorhombic form of RhSi for high silicon compositions. Excess amorphous silicon is not found to have a lowered crystallization temperature when in contact with the amorphous RhSi alloy, and crystalline silicon is only observed above 730 °C together with the cubic and/or orthorhombic RhSi. In Rh/Si bilayers with a thicker rhodium layer, no formation of an amorphous phase was observed on annealing; instead crystalline Rh2Si forms during annealing above 300 °C.

Angle-resolved photoemission of the initial stages of Au growth on Si(111) 7 × 7

The initial formation of the Au-Si(111) 7 × 7 interface has been examined by ARUPS, LEED and AES techniques. At room temperature, strong interactions between Au and Si atoms occur for coverages as low as 0.2 monolayer. This is related to the existence of a 1 × 1 LEED structure, to the suppression of the intrinsic silicon surface states and to the appearance of prominent Au 5d bands. The energy position of the latter structures is compatible with a significant gold-silicon interdiffusion. For increasing coverages, an amorphous Au-Si layer with a more metallic character gradually grows. This is supported by the removal of the 1 × 1 LEED pattern, the lack of influence of polar angle on the photoemission features and the substantial increase of the density of states near the Fermi level, EF. At the same time, the shift of the Au 5d emission towards EF is clearly related to the existence of a gold-rich Si alloy. The effects of annealing on one gold monolayer are presented. A sharp 31/2 × 31/2 R(30 degrees ) LEED pattern is obtained, whereas normal photoemission spectra reveal the existence of electronic states at -1 and -2.6 eV below EF. Their insensitivity to polarisation and their strong dispersion along the Gamma K'M' direction of the surface Brillouin zone show that they correspond to new interface states. These results are discussed in terms of structural models proposed for 31/2 × 31/2 ordered surfaces.

Formation and Crystallization of Amorphous Silicides at the Interface Between Thin Metal and Amorphous Silicon Films

We investigated the interaction of extremely thin (less than 10 nm) crystalline gold and rhodium films with amorphous silicon by transmission electron microscope in situ annealing. In thin Au/Si bilayers an amorphous phase with a diffraction peak at d ≂ 0.226 nm is formed by thermal annealing between 150 and 200 °C. Depending on the thickness and composition, silicon sputtered onto thin gold films leads to the formation of a layer of amorphous silicon and a partially amorphous Au-Si layer during deposition. The silicon layer crystallizes by itself at temperatures as low as 150 °C, and at 300 °C the amorphous Au–Si layer crystallizes into a metastable gold silicide (for silicon-rich compositions). In Rh/Si bilayers an amorphous Rh–Si phase is formed by annealing to 300 °C and can be detected by electron diffraction for a rhodium thickness of less than 5 nm and compositions with more than 50% Si if completely reacted. Above 300 °C the amorphous Rh-Si crystallizes preferentially in the cubic form of RhSi for intermediate silicon compositions and in the orthorhombic form of RhSi for high silicon compositions. Excess amorphous silicon is not found to have a lowered crystallization temperature when in contact with the amorphous Rh-Si alloy, and crystalline silicon is only observed above 730 °C together with the cubic and/or orthorhombic RhSi. In Rh/Si bilayers with a thicker rhodium layer, no formation of an amorphous phase was observed on annealing; instead crystalline Rh2Si forms during annealing above 300 °C.

Conductivity Change Due to Electron Spin Resonance in Amorphous Si-Au System

Conductivity-enhancement effect induced by electron spin resonance was investigated in amorphous Si 1- x Au x films prepared by electron-beam evaporation. Conductivity, conventional ESR and conductivity-change at ESR were measured from 77 K to 300 K for various Au concentrations. The ESR signal was detected in Au concentration region of \(x \lesssim 10\)%. The linewidth of ESR showed temperature variation and the spin density was reduced by doping of Au atoms. The conductivity-increase was detected at Au concentrations of \(x \lesssim 1.4\)% and showed characteristic variations with temperature and Au concentration. These results are discussed in terms of variable-range hopping conduction.