Tapas Kumar Chini

Energy dependent wavelength of the ion induced nanoscale ripple

Formation of periodic undulations or ripple like features on various materials with typical wavelength ranging from about 10 nm to 1µm, obtained by obliquely incident ion bombardment, has become an active research subject due to its possible technological applications, as varied as optical devices, templates for liquid crystal orientation and strain-free patterned substrates for heteroepitaxial growth of quantum wires. It is also expected that systematic study of ion beam induced nano ripple formation will help us to understand the basic processes prevalent in formation of sand dune like structures in deserts. Although this ion induced phenomenon was reported first in 1960s [1] and then in 1970s [2, 3], the improvement in experimental conditions such as, better vacuum and ion beam parameters and improved surface characterizing tools, has enabled us to control the growth of these ripple like features [4, 5, 6, 7, 8, 9, 10, 11]. The first widely accepted theoretical approach describing the process of ripple formation due to ion bombardment was developed by Bradley and Harper (BH) [12]. This linear theory [12] predicts the ripple wavelength and orientation in agreement with numerous experimental studies. However, this theory cannot explain a number of experimental observations, such as the saturation of the ripple amplitude [9], the appearance of rotated ripples [11] and kinetic roughening [13]. Moreover, according to the BH theory ripple wavelength should decrease with ion energy but this prediction has not been confirmed experimentally so far [5, 6, 7]. Recently a formalism [14, 15, 16] based on nonlinear continuum theory has been developed to understand these experimental observations not predicted by linear theory. In this new formalism, not only nonlinear and noise terms were included in the equation of height profile for eroded surface but also existence of two different surface diffusion processes were recognised. Based on Sigmund’s theory of sputtering [17], the height evolution h(x,y,t) of an ion eroded surface according to this nonlinear theory [14, 15, 16] can be described by

Evolution of surface morphology of ion sputtered GaAs(1 0 0)

In order to explore possible route to fabricate nano-scale semiconductor dots, a series of ion bombardment experiment on GaAs(1 0 0) was undertaken using a high current isotope separator and ion implanter with 40 Ar þ ions of an energy of 60 keV incident at an angle of 60 with respect to surface normal. Detailed surface topographical features of the bombarded samples were characterised by atomic force microscopy. To observe the growth of topography with time, the samples were bombarded at a number of doses. At a dose of 1 � 10 17 ions/cm 2 , no observable topography was developed. At a dose of 2 � 10 17 ions/cm 2 , the topography started to develop in the form of roughness along with islands or dots formation on the crest of waves or hillocks. Similar kind of topography has been observed up to a dose of 1 � 10 18 ions/cm 2 , remarkable with the formation of nano-dots with the maximum dimension of a few hundred nanometer. At the dose of 3 � 10 18 ions/cm 2 the surface became populated with ripple morphology without formation of any island or dot, in contrast with lower doses. 2002 Elsevier Science B.V. All rights reserved.

Growth and melting of silicon supported silver nanocluster films

Thin films of silver nanoclusters deposited on Si substrates are studied using scanning electron microscopy along with energy dispersive x-ray spectrometry. The nanoclusters are produced by dc magnetron sputtering followed by gas aggregation in a dense buffer gas. The film deposition is performed in a low impact energy regime with mass (size) selected clusters. These clusters were treated with rapid thermal annealing that gives an idea about the melting and evaporation mechanism of silver nanoclusters. Subsequent annealing of the grown silver film allows one to analyse the structure of the film and the character of its evolution. At room temperature, deposited clusters are distributed randomly, and annealing of the film leads to joining of clusters–monomers in non-compact clusters. At high temperatures, evaporation of clusters takes place. Parameters of the processes under consideration are estimated.

Local electron beam excitation and substrate effect on the plasmonic response of single gold nanostars

We performed cathodoluminescence (CL) spectroscopy and imaging in a high-resolution scanning electron microscope to locally and selectively excite and investigate the plasmonic properties of a multi-branched gold nanostar on a silicon substrate. This method allows us to map the local density of optical states from the nanostar with a spatial resolution down to a few nanometers. We resolve, both in the spatial and spectral domain, different plasmon modes associated with the nanostar. Finite-difference time-domain (FDTD) numerical simulations are performed to support the experimental observations. We investigate the effect of the substrate on the plasmonic properties of these complex-shaped nanostars. The powerful CL-FDTD combination helps us to understand the effect of the substrate on the plasmonic response of branched nanoparticles.

Room temperature photoluminescence from the amorphous Si structure generated under keV Ar-ion-induced surface rippling condition

We observe room temperature (RT) visible and infrared (IR) photoluminescence (PL) bands peaked around 680 and 1020nm, respectively, from a silicon (Si) surface amorphized and patterned with ripples by 60keV Ar+ bombardment at 60° angle of ion incidence. However, the Si surface amorphized but not patterned under normal bombardment (0° angle of ion incidence) condition shows a drastic reduction in the intensity of the visible PL along with the complete suppression of IR emission. The present work demonstrates that Ar ion irradiation at rippling condition may yield a porouslike light emitting amorphous silicon (a-Si) nanostructure.

Angular distribution of sputtered Ge atoms by low keV Ar+ and Ne+ ion bombardment

Abstract We report the measurement on the angular distribution of material sputtered from Ge target bombarded with normally incident Ar+ and Ne+ ions at room temperature. The energy of the ions was varied from 0.6 keV to 4.0 keV. Sputter deposited material was collected on Al foils and subsequently analyzed by an electron probe micro analyzer (EPMA) to obtain the angular distribution. All the results were well-fitted by distributions of the form cosnθ with n varying from 1.25 to 1.63. In the present experiment Ne sputtering of germanium gives rise to strong over-cosine angular distribution of sputtered material in comparison to Ar sputtering.

Nanostructuring with a high current isotope separator and ion implanter

The oblique angle Ar bombardment with a high current isotope separator and ion implanter gives rise to nanoscale (400-900 nm) ripple formation on Si(1 0 0) at 80 and 100 keV for the dose of 10 18 ions/cm 2 . The most important aspect of our preliminary investigation regarding the beam influencec on ripple wavelength indicates that the meaningful data comparable to the theoretical models can be obtained with homogeneous irradiation via beam sweeping. At 60 keV, Ar bombarded GaAs surface also shows nanoparticle decorated ripples for the dose of 5×10 17 ions/cm 2 and at higher dose ripples without nanoparticles.

Angular distribution of In and P particles sputtered from InP by inert-gas ion bombardment

Abstract An InP(100) surface was bombarded with 1 and 3 keV Ar + and Xe + ions at normal incidence in ultra-high vacuum at 153 and 293 K. We determined the angular distributions of sputtered In and P particles using a collection method. The surfaces sputtered at 153 K were free of cones, irrespective of ion species and energy. The angular distribution of In was almost identical with that of P at 153 K. This strongly suggests that no radiation-enhanced Gibbsian segregation occurred at this temperature in the surface region, where the in-depth composition is almost homogeneous. In contrast, a preferential ejection of In in the oblique direction was observed for samples bombarded with 3 keV ions at 293 K, where the surface was flatly eroded. We thus conclude that the outermost layer of the sample sputtered at 293 K is enriched with In.

A preliminary study on the nature of particulate matters in vehicle fuel wastes

Powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and tunneling electron microscopy (TEM) studies of two solid vehicle wastes (pollutants) from petrol- and diesel-fueled engines of Kolkata (India) have detected a significant amount of ultrafine particles in the nanometer scale in these wastes. Both powder XRD and selected area electron diffraction from TEM have confirmed the existence of inhomogeneous distribution of nanocrystallites in these pollutants. Energy dispersive X-ray spectrometry shows that these wastes contain mainly carbon and oxygen as the constituent components. These pollutants are magnetic in nature as seen with SQUID magnetometry, and the presence of a high amount of carbon presumably is likely the origin of the magnetic property.

Probing Ar ion induced nanocavities/bubbles in silicon by small-angle x-ray scattering

Small-angle x-ray scattering (SAXS) measurements have been performed to investigate the nanocavities/bubbles and the amorphous silicon surrounding the cavities/bubbles generated after high fluence medium-energy (60 keV) Ar ion implantation in single crystalline Si as a function of incidence angle (with respect to the surface normal of the sample). The measurements were carried out using a high flux/high transmission laboratory scale SAXS set up with Mo-Kα radiation in transmission geometry. The scattering data have been used to calculate the average size (Dave), number density (dN), and volume fraction (Vf) of cavities/bubbles in ion induced amorphous layer of the crystalline Si substrate. The novelty of the SAXS technique applied in the present case lies on its ability to detect ultrafine defect features of size even less than 1 nm, which is otherwise impossible from the transmission electron microscopy measurements usually employed for inert gas ion induced cavities/bubbles in amorphous silicon.