S. R. Bhattacharyya

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.

Development of metal nanocluster ion source based on dc magnetron plasma sputtering at room temperature

A simple and cost effective nanocluster ion source for the deposition of size selected metal nanocluster has been developed based on the dc magnetron discharge (including pulsed dc discharge). The most important and interesting feature of this cluster source is that it is working at room temperature, cooled by chilled water during the experiment. There is no extraction unit in this device and the cluster streams flow only due to the pressure gradient from source chamber to substrate via quadrupole mass filter. It has provision of multiple substrate holders in the deposition chamber, which can be controlled manually. The facility consists of quadrupole mass filter (QMF 200), which can select masses in the range of 2-125 000 atoms depending on the target materials, with a constant mass resolution (M/DeltaM approximately 25). The dc magnetron discharge at a power of about 130 W with Ar as feed/buffer gas was used to produce the Cu nanocluster in an aggregation tube and deposited on Si (100) wafer temperature.

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.

Structural interpretation, growth mechanism and optical properties of ZnO nanorods synthesized by a simple wet chemical route†

ZnO nanorods are synthesized at room temperature through a simple chemical process without using any template or capping agent. ZnO nanopowders used in this synthesis are synthesized by mechanically alloying the ZnO powder. Here, we report primarily the crystal structure and microstructure interpretations of ZnO nanorods by analyzing X-ray diffraction patterns employing Rietveld refinement, field emission scanning electron microscopy (FESEM) and high resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray (EDX) spectroscopy techniques. Uniformly shaped pure ZnO nanorods with different lengths and diameters are synthesized within 5 h of reaction time. The Rietveld refinement and HRTEM images ascertain the growth of ZnO nanorods along the plane. STEM-HAADF images and EDX spectra and imaging of nanorods confirm the chemical composition and reveal the uniform elemental distributions of Zn and O over the entire nanorod. UV-visible spectra analyses of ZnO nanopowder and nanorods reveal a small decrease in optical band gap of nanorods due to morphological change. Photoluminescence (PL) spectra of both powder and rod-shaped ZnO reveal the presence of excess of oxygen in nanorods. Rietveld analysis corroborates the findings of PL and quantifies the content of oxygen in ZnO nanorods.

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.

Preparation of manganese-doped ZnO thin films and their characterization

In this study, pure and manganese-doped zinc oxide (Mn:ZnO) thin films were deposited on quartz substrate following successive ion layer adsorption and reaction (SILAR) technique. The film growth rate was found to increase linearly with number of dipping cycle. Characterization techniques of XRD, SEM with EDX and UV–visible spectra measurement were done to investigate the effect of Mn doping on the structural and optical properties of Mn:ZnO thin films. Structural characterization by X-ray diffraction reveals that polycrystalline nature of the films increases with increasing manganese incorporation. Particle size evaluated using X-ray line broadening analysis shows decreasing trend with increasing manganese impurification. The average particle size for pure ZnO is 29·71 nm and it reduces to 23·76 nm for 5% Mn-doped ZnO. The strong preferred c-axis orientation is lost due to manganese (Mn) doping. The degree of polycrystallinity increases and the average microstrain in the films decreases with increasing Mn incorporation. Incorporation of Mn was confirmed from elemental analysis using EDX. As the Mn doping concentration increases the optical bandgap of the films decreases for the range of Mn doping reported here. The value of fundamental absorption edge is 3·22 eV for pure ZnO and it decreases to 3·06 eV for 5% Mn:ZnO.

Mass and energy dependence of the sputtering yield of gallium arsenide

Abstract The sputtering yields of GaAs(100) bombarded at normal incidence by mass analyzed 40Ar+, 84Kr+ and 132Xe+ ions obtained from an electromagnetic isotope separator are determined in the energy range 15–35 keV. The experimental values of the sputtering yield obtained by the method of weighing the collected sputtered material are compared with the total sputtering yield calculated theoretically. The surface topography of a Xe+ bombarded sample is examined under SEM as an associated part of the multicomponent physical sputtering.

Self-organized titanium oxide nano-channels for resistive memory application

Towards developing next generation scalable TiO2-based resistive switching (RS) memory devices, the efficacy of 50 keV Ar+-ion irradiation to achieve self-organized nano-channel based structures at a threshold fluence of 5 × 1016 ions/cm2 at ambient temperature is presented. Although x-ray diffraction results suggest the amorphization of as-grown TiO2 layers, detailed transmission electron microscopy study reveals fluence-dependent evolution of voids and eventual formation of self-organized nano-channels between them. Moreover, gradual increase of TiO/Ti2O3 in the near surface region, as monitored by x-ray photoelectron spectroscopy, establishes the upsurge in oxygen deficient centers. The impact of structural and chemical modification on local RS behavior has also been investigated by current-voltage measurements in conductive atomic force microscopy, while memory application is manifested by fabricating Pt/TiO2/Pt/Ti/SiO2/Si devices. Finally, the underlying mechanism of our experimental results has been...

Performance of a size-selected nanocluster deposition facility and in situ characterization of grown films by x-ray photoelectron spectroscopy.

We report here on a newly installed gas aggregation type nanocluster deposition unit based on magnetron sputtering ion source with mass selection of the clusters by quadrupole mass filter. The system is ultra high vacuum compatible and is equipped with an in situ X-ray Photoelectron Spectroscopy facility, giving compositional information of the films formed by nanoclusters deposition on a substrate. Detailed descriptions and working of the components of the system are presented. For the characterization of the nanocluster source and associated mass filter for size selected clusters, the dependence of output performance as a function of aggregation length, sputter gas flow and magnetron power of the cluster source have been studied. Copper nanoclusters deposited on Silicon (100) surface and on transmission electron microscope grids are, respectively, studied with scanning electron microscopy and transmission electron microscopy for the morphology.