T. Som

Quantum Confined Atoms of Doped ZnO Nanocrystals

The luminescent properties of ZnO nanocrystals doped with divalent manganese and europium ions show efficient broad-band emission at room temperature. These ZnO based quantum confined atoms potentially offer efficient nanophosphors that can be excited in the near UV range for applications in the solid state lighting, displays and optically pumped lasers.

Implantation-assisted Co-doped CdS thin films: Structural, optical, and vibrational properties

This paper reports on structural, optical, vibrational, and morphological properties of cobalt-doped CdS thin films, prepared by 90 keV Co+ implantation at room temperature. In this work, we have used cobalt concentration in the range of 0.34–10.8 at. %. Cobalt doping does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. However, with increasing cobalt concentration a decrease in the optical band gap, from 2.39 to 2.26 eV, is observed. This reduction is addressed on the basis of band tailing due to the creation of localized energy states in association with Urbach energy calculations. In addition, implantation gives rise to grain growth and increase in the surface roughness. Size and shape fluctuations of individual CdS grains, at higher fluences, give rise to inhomogeneity in strain. The results are discussed in the light of ion-matter interaction in the keV regime.

Swift heavy ion induced modification of the Co/Si interface; cobalt silicide formation

Electron beam evaporated 60-nm Co films on Si (100) substrates were subjected to 120 MeV Au beam irradiation and subsequent thermal annealing to induce interdiffusion at the interface. Although low energy (keV/nucleon) ion beam induced mixing is known to produce cobalt silicides, the effect of MeV/nucleon radiation or swift heavy ions (SHIs), on the Co/Si system, has not been reported earlier. The irradiated and ex situ annealed samples were analyzed by Rutherford backscattering spectroscopy measurements. Respective crystalline cobalt silicide phases were identified by grazing incidence X-ray diffraction. Complete intermixing of Co and Si in the interfacial region to form CoSi was achieved following thermal annealing (at 400 °C) of the SHI irradiated Co/Si system. The radiation enhanced diffusion mechanism has been invoked to explain the SHI induced intermixing in the Co/Si system after annealing. Thermally assisted (ion generated) defect migration across the Co/Si interface enhanced defect mediated atomic mobility, which led to the formation of cobalt silicides, in accordance with the thermodynamically favored route. The post SHI irradiation annealing temperature required for the formation of crystalline phases (400 °C), is lower than that reported for low energy ion beam mixing cases where post irradiation annealing temperatures in excess of 700 °C are required for the occurrence of phases. Due to lower processing temperatures, SHI induced mixing may be considered as a promising silicidation technique in solid state technology.

Transition from ripples to faceted structures under low-energy argon ion bombardment of silicon: understanding the role of shadowing and sputtering

In this study, we have investigated temporal evolution of silicon surface topography under 500-eV argon ion bombardment for two angles of incidence, namely 70° and 72.5°. For both angles, parallel-mode ripples are observed at low fluences (up to 2 × 1017 ions cm-2) which undergo a transition to faceted structures at a higher fluence of 5 × 1017 ions cm-2. Facet coarsening takes place at further higher fluences. This transition from ripples to faceted structures is attributed to the shadowing effect due to a height difference between peaks and valleys of the ripples. The observed facet coarsening is attributed to a mechanism based on reflection of primary ions from the facets. In addition, the role of sputtering is investigated (for both the angles) by computing the fractional change in sputtering yield and the evolution of surface roughness.PACS81.05.Cy, 81.16.Rf, 61.80.Jh, 87.64.Dz

Formation of Au0.6Ge0.4 alloy induced by Au-ion irradiation of Au/Ge bilayer

We report on the formation of a-axis oriented Au0.6Ge0.4 alloy on a Si(100) substrate on 120 MeV Au-ion irradiation of a Au/Ge bilayer and subsequent vacuum annealing at 360 °C. Irradiation-induced changes occurring across the Au/Ge interface were studied using Rutherford backscattering spectrometry. Phase identification was done by x-ray diffraction and the surface morphology of the samples was studied by scanning electron microscopy. Formation of oriented Au0.6Ge0.4 alloy was confirmed by transmission electron microscopy and discussed on the basis of swift heavy ion induced effects followed by thermal annealing.

Effect of grain-boundaries on electrical properties of n-ZnO:Al/p-Si heterojunction diodes

We report on room temperature diode characteristics of ZnO:Al (AZO)/Si heterostructures by current-voltage measurements. In this study, with increasing AZO film thickness, systematic reduction in the turn-on potential (from 3.16 to 1.80 V) and the film stress are observed. Complementary capacitance-voltage studies reveal a decreasing trend in barrier height at the junction with increasing AZO film thickness. A gradual decrease in resistivity takes place with increasing AZO film thickness. Above observations are explained in the framework of AZO thickness dependent variation in grain size and in turn trap density at the grain boundaries influencing carrier transport across the adjacent grains.

Swift heavy-ion-induced epitaxial crystallization of buried Si3N4 layer

We report on swift heavy-ion-beam-induced epitaxial crystallization of a buried Si3N4 layer. We observe good epitaxial crystallization at 200 °C, which is a much lower temperature than that required for the conventional solid phase epitaxial growth. High-resolution transmission electron microscopy and selected area diffraction patterns have been used to study the 100-MeV Ag8+-ion-beam-induced epitaxial growth of the Si3N4 layer. A possible mechanism of recrystallization is discussed on the basis of synergetic effects of electronic and nuclear energy-loss processes along the trajectory of the swift heavy ions at elevated temperatures.

Room‐temperature synthesis of copper germanide phase by ion beam mixing

This letter reports room‐temperature synthesis by ion beam mixing of the e1‐Cu3Ge phase which is a promising candidate for interconnect and contact material in very large scale integrated circuit technology. The resistivity of the mixed sample was found to be nearly the same as the one obtained from thermally prepared films. We briefly discuss the likely mechanisms of phase formation and conclude that reaction kinetics dominates over thermodynamic forces during phase formation. The sequence of phase formation is explained by effective heat of formation rule.

Study of sputtered particles from gold nano-islands due to MeV self-ion irradiation

Abstract Very thin films of gold deposited on silicon substrates form isolated nano-island structures due to the non-wetting nature of gold. Thick films are more homogeneous and do not have the isolated island structures. Thin gold films of various thicknesses (≈0.4–21.4 nm) are deposited under high vacuum condition and irradiated with 1.5 MeV Au2+ ions. The sputtered particles are collected on catcher grids (carbon coated) during the irradiation and are analyzed with transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS). The average sputtered particle size is determined from TEM measurements, whereas the amount of gold on the catcher grid is found by RBS. The average sputtered particle size from thin (up to a thickness of ≈2 nm) discontinuous films is larger compared to the average particle size from thick continuous films. The coverage of the sputtered particles on the catcher grids is also discussed. Energy spike and its distribution in the nano-islands is proposed to be the main reason for the variation in the particle size and the coverage of the sputtered particles on the catcher grid.

Swift heavy ion-based materials science research at NSC

Abstract Effects of large electronic excitation in different types of materials, viz. metals, semiconductors, superconductors, polymers and organic crystals are being investigated using the swift heavy ions (SHI) of energies from 10 to 270 MeV available from 15 UD NSC Pelletron. Defect production in semiconductors, structural strain on electronic transport properties of materials having colossal magneto resistance (CMR), flux pinning in high T c superconductors (HTS), desorption of hydrogen from polymers, ion beam mixing in metal/Si interface and optical waveguide formation are some of our research activities. Resistivity and 1/ f noise measurements from room temperature down to 77 K are used for the studies of SHI-induced defects into the materials. Quadrupole mass spectrometer (QMS) in the materials science beam line is used for the studies of radiolysis of polymers induced by SHI. For polymers, ion track diameters have been estimated by on-line monitoring of the loss of hydrogen (using ERD and QMS) during ion irradiation. An ultra high vacuum scanning tunneling microscope (UHV STM) is being installed at the materials science beam line to study the individual damage created by SHI on the solid surfaces. A three-axes goniometer has been installed for RBS-channeling studies of various materials. Experimental facilities and the research work carried out at Nuclear Science Centre are briefed.