G. Kuri

Shape transition in the epitaxial growth of gold silicide in Au thin films on Si(111)

Growth of epitaxial gold silicide islands has been observed when an Au film deposited on a bromine-passivated vicinal (4\ifmmode^\circ\else\textdegree\fi{} misoriented) Si(111) substrate was annealed around the Au-Si eutectic temperature. The islands grow in the shape of equilateral triangles, reflecting the symmetry of the (111) substrate, up to a critical size beyond which the symmetry of the structure is broken, resulting in a shape transition from triangle to trapezoid. The island edges are aligned along the three equivalent {11\ifmmode\bar\else\textasciimacron\fi{}0} directions of the Si(111) surface. The elongated islands, instead of growing along three equivalent {11\ifmmode\bar\else\textasciimacron\fi{}0} directions, grow only along one preferential direction. This has been attributed to the vicinality of the substrate surface as the steps on this substrate are expected to run along the observed preferential growth direction. We have observed, under various conditions, elongated islands with an aspect ratio as large as 15:1.

An RBS study of interdiffusion across a brominated Si( 111) /Cu interface with and without a barrier layer

Abstract Cu thin films have been deposited on Si(111) substrates with two different initial conditions. In one, the substrates had a top native oxide layer, and in the other the native oxide layer was etched in hydrofluoric acid and the etched surface was treated with a bromine-methanol solution prior to the surface being exposed to atmosphere. This Br-treatment is known to saturate surface dangling bonds on an Si(111) surface. Vacuum annealing of the Cu/Si samples, followed by Rutherford backscattering spectrometry (RBS) measurements showed the onset temperature of interdiffusion to be ∼ 220°C for the Si(111) substrate treated with bromine-methanol solution whereas the onset temperature of interdiffusion was found to be between 600°C and 700°C for Si(111) surfaces with native oxide on top. This shows that the native oxide layer works like a diffusion barrier. The effect of an interposed TiNx layer on the diffusion behaviour has also been investigated. A thin layer ( 600°C) for the Cu/SiO2/Si(111) suggests that SiO2 introduces the highest barrier for Cu-Si interdiffusion among all the barrier combinations studied here. At annealing temperatures above the onset temperature of interdiffusion copper silicides are formed. RBS simulations have been used to study the compositions of the reacted films.

XPS and SIMS analysis of gold silicide grown on a bromine passivated Si(111) substrate

Abstract When a thin film of Au (∼100 nm) deposited under high vacuum conditions on a chemically prepared Br-passivated Si(111) substrate was annealed around 363°C, epitaxial layer-plus-island mode growth of gold silicide was observed along with some unreacted gold in stringy patterns. This unreacted gold was removed by etching the sample in aqua regia. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) measurements were carried out on these samples. SIMS results reveal that the height of the islands is about 1.2 μm and the silicide/Si interface is abrupt. XPS measurements were made after sputtering the sample surface at constant intervals of time. Si 2 p , Au 4 f , C 1 s and O 1 s photoelectrons were detected. XPS spectra of Si 2 p are resolved into three peaks corresponding to bulk Si, Si in silicide and Si in oxide. The Au 4 f 7/2 peak in the silicide is shifted by 1–1.2 eV towards higher binding energy compared to metallic Au. The shift of Si 2 p towards the higher binding energy in the silicide is understood from the higher electronegativity of Au, while the shift of Au 4 f 7/2 peak towards higher binding energy is known to be due to d-electron depletion to form an sd hybrid. The XPS peak intensity profile with sputtering time indicates that the thin uniform layer (∼5.5 nm) of gold silicide is sandwiched between a thin (∼2.8 nm) SiO 2 layer and the Si(111) substrate.

An RBS study on the annealing behaviour of Cu thin films on brominated Si(111) and Si(100) substrates

Abstract The thermal annealing behaviour of Cu thin films deposited on Si(100) and Si(111) substrates has been studied by Rutherford backscatteringspectrometry (RBS). For thin film deposition, two different initial conditions of the substrates have been used. In one, the substrates had a top native oxide layer, and in the other the native oxide layer was etched in hydrofluoric acid and treated with a bromine-methanol solution without the surface being exposed to atmosphere. RBS measurements on the Cu/Si samples showed the onset temperature of interdiffusion to be ≈ 300°C for the Si(111) substrate treated with bromine-methanol solution, whereas the onset temperature of interdiffusion was found to be between 500 and 700°C for both Si(111) and Si(100) surfaces with native oxide on top and Si(100) surface treated with bromine-methanol solution. The difference in behaviour for the bromine treated Si(111) and Si(100) substrates can be explained in terms of preferential saturation of Si dangling bonds by Br on the Si(111) surfaces. The onset temperature of interdiffusion for the Br-treated Si(111) surfaces is comparable to that for the atomically clean Si(111)7 × 7 reconstructed surfaces prepared in ultrahigh-vacuum (UHV) condition.

Growth and alignment of gold silicide islands on Br-passivated vicinal Si(111) surfaces

Abstract Growth of gold silicide islands on bromine-passivated Si(111) substrates have been studied by Rutherford backscattering spectrometry, optical microscopy, scanning electron microscopy and energy dispersive X-ray analysis. Formation of epitaxial gold silicide islands were observed when Au thin films, deposited on bromine-passivated Si(111) substrates, were annealed at temperatures close to the AuSi eutectic temperature and above. The silicide islands mainly grow in equilateral triangular or trapezoidal (or rod-like) shape. The rod-like islands of uniform width have an aspect ratio as large as 15:1. The island edges are aligned along the substrate [110], [011] and [101] directions. The large trapezoid (or rod)-shaped islands have been found to be aligned along one of the three equivalent {110} directions. The substrate surface normal was 4° misoriented from the [111] direction towards the [112] azimuthal direction. The aligned growth of large islands is attributed to the vicinality of the substrate. The Br-passivated substrates were prepared by a chemical method and Au film deposition and annealing were performed under high vacuum. The composition and the general features of growth of the silicide islands are comparable to those obtained under UHV conditions.

Ion-irradiation-induced mixing, interface broadening and period dilation in Pt/C multilayers

Pt/C multilayers of nanometric dimension have been irradiated with 2-MeV-Au ions to a fluence of 1×1014 ions/cm2 and analyzed by x-ray reflectivity and x-ray standing wave measurements. The multilayer period has expanded by ∼9%, the expansion of the Pt layers being larger than that of the C layers. Ion-induced displacement of atoms across the interfaces led to an increased interface roughness and an increase of 2 at. % Pt in C layers. Monte Carlo simulations for ion-induced atomic displacement have been used to explain the observed effects.

MeV As and Au ion implantation in Si, GaP, GaAs, InSb and LiNbO3: Study of range and lattice location

Abstract MeV As+ ions into Si and Au2+ ions into Si, GaP, GaAs, InSb and LiNbO3 crystal substrates have been implanted at room temperature and tilted geometry. Using Rutherford backscattering spectrometry and channeling techniques, measurements of the range profile and lattice location of As atoms (dose: 1.0×1015/cm2) were made. It appears that 95% As atoms are substitutional. For Au implantation, range parameters have been determined for ion energies 1.37-2.03 MeV for the Si substrate. Measurements were also made on other substrates implanted with 1.43 MeV Au ions. The measured range values have been compared with TRIM and the so called universal range-energy theories. Our experimental results show that the theoretically predicted values are underestimated in all the cases. We also present a new approach for the determination of the projected range and straggling—both longitudinal and transverse—from a single measurement.

Epitaxy driven fractal growth

Abstract Fractal structures of Au with an average dimension of 1.69±0.06 with concomitant growth of epitaxial gold silicide islands have been observed in Au thin films prepared by vacuum evaporation of Au on bromine-passivated Si(111) substrates and subsequent annealing around the AuSi eutectic temperature. When a thin (native) silicon oxide layer is present at the interface, neither the epitaxial islands nor the Au fractals are observed. Results of simulations based on diffusion-limited-aggregation using Witten-Sander model agree with the measured dimension. The observed fractal structures are not isotropic. The directional growth of the fractal patterns is apparently related to the shape and alignment of the silicide islands.

Ion dechanneling studies of defects in an ion-beam-synthesized epilayer sandwich system: Si(111)/CoSi2/Si

Rutherford backscattering and channeling studies have been performed on an ion-beam-synthesized heteroepitaxial Si(111)/CoSi 2 (68 nm)/Si(88 nm) sample. The dependence of dechanneling probability on the incident ion energy has been determined to characterize the defects at the buried interfaces and in the epilayers. While the defects at the bulk Si/CoSi 2 interface have been identified to be misfit dislocations, the scattering behavior from the top Si/CoSi 2 interface and the Si epilayer appears to be that of stacking faults. The incident ion energy dependence of the direct backscattering yield confirms these results.

Microstructural evolution, atomic migration, and FePt nanoparticle formation in ion-irradiated Pt(Fe)/C(Fe) multilayers

Ion-irradiation-induced microstructural modifications, atomic migration, and nanoparticle formation in nanoscale periodic Pt/C multilayer systems containing about 15 at. % Fe have been analyzed by x-ray reflectivity, x-ray standing wave (XSW), transmission electron microscopy, and grazing incidence x-ray diffraction (GIXRD) studies. The multilayer stack with 15 Pt(Fe)/C(Fe) layer pairs (period 4.2 nm) was irradiated with a 2 MeV Au2+ ion beam, which was rastered on the sample to obtain uniformly irradiated strips with fluences (ϕ) from 1×1014 to 1×1015 ions/cm2. Up to a certain threshold ion fluence (ϕt) we have observed mixing between Pt and C atoms. At higher fluences (ϕ>ϕt) demixing effect is pronounced. This phenomenon has been explained in terms of two competitive processes including ballistic and chemically guided atomic movements. As revealed by XSW measurements, ion irradiation causes preferential migration of Fe toward Pt layers from C layers. Consequently Fe concentration in the Pt layers increa...