Subhendu Sarkar

Towards an understanding of MCsn+ formation mechanism in SIMS

Abstract The present work is aimed towards understanding the formation mechanisms of MCs + and MCs 2 + molecular ions in the SIMS process. Energy distribution studies of MCs + under varying Cs surface concentrations have been made for Au, Cu and Ni. From the peak shifts of the energy distribution curves it is evident that the surface binding of MCs + is stronger at higher Cs surface concentration for elements having greater atomic polarisability. As far as the MCs 2 + formation mechanisms are concerned, we feel that the most probable formation probability could be due to the association of a sputtered M − ion with two self-sputtered Cs + ions. Energy distribution study for MCs + , MCs 2 + and Cs + ions seems to be an effective diagnostic for the plausible explanation of the above mechanism.

Impact of repetitive and random surface morphologies on the ripple formation on ion bombarded SiGe-surfaces

Single (strained and relaxed) and polycrystalline Si1-xGex samples have been bombarded using low-energy oxygen ions to study the impact of strain and relaxation-induced dislocations on their surface morphologies and subsequent formation of ripples. The as-grown relaxed sample surface exhibits cross-hatch patterns due to relaxation along its surface. Compared to Si, all the surfaces of these materials exhibit ripple formation at lower fluences. Moreover, the nature of the induced surface topography depends largely on the original surface morphology. Single crystalline layers show clear ripple formation whereas the polycrystalline surface leads to more isolated features. On a relaxed surface that exhibits the cross-hatch patterns due to relaxation along its surface, very long ripples tend to form along the dislocation ridges which eventually become shorter (and finally break up) with increasing distance from a particular ridge. A detailed temporal study has been done to understand the nature of ripple formation on the relaxed sample. A theoretical estimation of the crossover time indicates that the morphology of the relaxed sample is primarily governed by the nonlinear regime of ripple formation. It is found that the surface follows the scaling laws as dictated by the isotropic Kardar–Parisi–Zhang equation. The growth exponent (n) is found to be ~0.23, which is in agreement with the theoretical value. Ripple coarsening is observed which fits well with the recently predicted model indicating the contribution of nonlinear effects arising out of redeposition of the sputtered atoms and presence of mobile surface adatoms. The subsequent saturation of wavelength takes place as a result of geometrical shadowing.

Interface-dominated Growth of a Metastable Novel Alloy Phase

A new D023 metastable phase of Cu3Au is found to grow at the interfaces of Au/Cu multilayers deposited by magnetron sputtering. The extent of formation of this novel alloy phase depends upon an optimal range of interfacial width primarily governed by the deposition wattage of the dc-magnetron used. Such interfacially confined growth is utilized to grow a ∼ 300 nm thick Au/Cu multilayer with thickness of each layer nearly equal to the optimal interfacial width which was obtained from secondary ion mass spectrometry (SIMS) data. This growth technique is observed to enhance the formation of the novel alloy phase to a considerable extent. SIMS depth profile also indicates that the mass fragment corresponding to Cu3Au occupies the whole film while x-ray diffraction (XRD) shows almost all the strong peaks belonging to the D023 structure. High resolution cross-sectional transmission electron microscopy (HR

Nanoscale Topographical Fluctuations: A Key Factor for Evaporative Colloidal Self-Assembly

This work investigates the role of surface parameters such as the nanoscale roughness, topography, and skewness of smooth and rough Si surfaces in the shape of patterns left by evaporating colloidal droplets of spherical polystyrene particles. The droplet contact angle, colloidal deposition pattern, crack density, and rim growth velocities are experimentally evaluated for varying roughness. The contact angle and rim growth rate are found to be more for rough surfaces in comparison to smooth ones. Roughness also helps in reducing stress in the drying droplets, thereby impeding the process of crack formation as exemplified by the experimental results. The altered Derjaguin-Landau-Verwey-Overbeek (DLVO) interactions emerging from the contribution of nanoscale roughness are theoretically evaluated for each differently rough substrate-particle combination. The forces have been calculated by considering large- and small-scale roughness parameters of the experimental surfaces. The experimental findings have been duly corroborated by theoretical estimates. Finally, it is observed that the skewness of the surface and the small-scale asperity radius bear a correlation with the DLVO forces and subsequently with the ring deposit pattern. The present understanding of the influence of surface fluctuations on evaporative self-assembly would enable one to choose the right topographic surface for particular applications.

Dual erosion phases in HNA etched Si surfaces

Morphological studies were done on Si (100) surfaces after etching with HNA (HF, HNO3 and CH3COOH) for seven different time intervals till 600s. The resulting morphology was studied using atomic force microscopy (AFM). The images obtained were analyzed using scaling theory. Large number of images from different regions of the same surface were used to find the average behavior of each scaling parameter. The roughness at different length scales was extracted and quantified from AFM measurements. Results indicated two erosion phases of the evolving surface which became evident from power spectral density (PSD) and interface width analysis of the etched surfaces.

Dual growth modes in ion bombarded Si surfaces

Morphological studies were done on Si (001) surfaces after rastering them with a 1 keV O2+ ion beam at an angle. The resulting mounded morphology was studied using atomic force microscopy (AFM) measurements. The roughness at different length scales were further extracted and quantified from AFM measurements using scaling analysis. Results indicate two growth regimes of the evolving surface which were evident from the power spectral density (PSD) and interface width analysis of the eroded surfaces. Initially the growth is unstable followed by a stable regime of the nanostructures evolved after about 35 minutes of erosion. Temporal studies done on these surfaces show the onset of shadowing at higher sputtering times thus indicating the breakdown of the growth model at these times.

Morphology of Au nanoparticles formed by magnetron sputtering: ellipsoids and rings

Abstract Ultraviolet (UV) visible spectroscopy, atomic force microscopy and grazing incidence X-ray reflectivity have been used to study morphology of Au nanoparticles grown by direct current (DC) magnetron sputter deposition on hard (glass) and soft amorphous (polystyrene films on quartz) substrates. Au nanoparticles are found to be ellipsoidal showing an increase in ellipticity ϵ [≡a/b, a(b)=semimajor (semiminor) axis] with decrease in polystyrene film thickness from 250 to 20 nm, where b remains almost invariant around 3 nm. They sit on top of the film with the semimajor axes roughly parallel to film surface. On glass, the Au film was probed at different stages of growth. After an initial period (1 min) of spheroid nanoparticle formation by dewetting, the coverage was complete (as observed from Au optical spectra) and partially wetting islands appeared after 2 min on the Au covered glass surface. After 5 min, these islands formed rings resembling quantum rings. The rings broke up again into islands after 10 min.

FORMATION OF NOVEL Cu3Au ALLOY PHASE UNDER CONFINEMENT

Our present work deals with the formation and thermal behavior of a nonbulk alloy phase confined within about 8 nm across the interfaces of Au/Cu multilayer systems. These multilayers deposited on silicon and float glass by DC magnetron sputtering have been studied by secondary ion mass spectrometry (SIMS), X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM). Along with the highly oriented growth of the Cu and Au layers along [111], Cu3Au alloy was found to be present only at the Cu/Au interfaces in the nonbulk tetragonal D023 phase. Co-sputtering of Au and Cu under similar conditions produces only conventional fccCu3Au alloy phases, suggesting that interfacial confinement plays a significant role in producing the novel Cu3Au alloy phase in gold/copper multilayers. This novel phase is found to form only when the interfacial width is less than 10 nm. The D023 alloy phase tends to stabilize, rather than transforming to the bulk L12 phase, when the multilayer is vacuum-annealed at 150°C. As alloy formation spreads out of the interfaces (on vacuum annealing at 200°C), the dominant alloy is CuAu, consistent with the Cu:Au atomic ratio averaged over the multilayer.