K. G. M. Nair

Multiphonon Raman scattering in GaN nanowires

UV Raman scattering studies show longitudinal optical (LO) mode up to fourth order in wurtzite GaN nanowire system. Frohlich interaction of electron with the long range electrostatic field of ionic bonded GaN gives rise to enhancement in LO phonon modes. Good crystalline quality, as indicated by the crystallographic as well as luminescence studies, is thought to be responsible for this significant observation. Calculated size dependence, incorporating size corrected dielectric constants, of electron-phonon interaction energy agrees well with measured values and also predict stronger interaction energy than that of the bulk for diameter below ∼3nm.

Growth of silver nanoclusters embedded in soda glass matrix

Temperature-controlled-growth of silver nanoclusters in soda glass matrix is investigated by low-frequency Raman scattering spectroscopy. Growth of the nanoclusters is ascribed to the diffusion-controlled precipitation of silver atoms due to annealing the silver-exchanged soda glass samples. For the first time, Rutherford backscattering measurements performed in this system to find out activation energy for the diffusion of silver ions in the glass matrix. Activation energy for the diffusion of silver ions in the glass matrix estimated from different experimental results is found to be consistent.

XPS studies on silicide formation in ion beam irradiated Au/Si system

Abstract X-ray photoelectron spectroscopic (XPS) studies were carried out on ion-beam irradiated Au/Si system. Thin films of Au (500 A) were vapour deposited on Si〈111〉 and irradiated with 120 keV Ar + ions at different temperatures. The XPS investigation showed the formation of gold silicide. Even in the case of the sample irradiated at room temperature silicide phase was observed at the top surface indicating the out-diffusion of silicon. Increase in the concentration of silicide phase at the top surface with increasing temperature of irradiation was observed suggesting higher out-diffusion of silicon at elevated irradiation temperature. The paper presents the results of the above study and proposes a simple model to explain the growth of the silicide phase.

Quasiquenching size effects in gold nanoclusters embedded in silica matrix

Abstract The surface plasmon energy of gold clusters formed by Ar + ion beam mixing of Au/silica is investigated for the size effect. The redshift with decreasing cluster size is assigned to the ‘spillout’ effect in small clusters ( 5 nm), assisted with annealing treatment. The competition between ‘spillout’ effect and the frequency dependence of interband dielectric function leads to a quasiquenching of the size effects in the optical response.

Ferromagnetism in cobalt-doped n-GaN

Ferromagnetic ordering is reported in the postannealed samples of Co doped n-GaN formed by Co+ implantation. A maximum Curie temperature ∼250K is recorded for the sample with 8at.% Co. Particle induced x-ray emission–channeling study confirmed the substitutional Co in Ga lattice site. Local atomic arrangement around magnetic impurities is also analyzed using Raman study. A disordered model with carrier mediated coupling of localized magnetic moments is made responsible for the observed ferromagnetic ordering.

Blue luminescence of Au nanoclusters embedded in silica matrix

Photoluminescence study using the 325 nm He-Cd excitation is reported for the Au nanoclusters embedded in SiO(2) matrix. Au clusters are grown by ion beam mixing with 100 KeV Ar(+) irradiation on Au [40 nm]/SiO(2) at various fluences and subsequent annealing at high temperature. The blue bands above approximately 3 eV match closely with reported values for colloidal Au nanoclusters and supported Au nanoislands. Radiative recombination of sp electrons above Fermi level to occupied d-band holes are assigned for observed luminescence peaks. Peaks at 3.1 and 3.4 eV are correlated to energy gaps at the X- and L-symmetry points, respectively, with possible involvement of relaxation mechanism. The blueshift of peak positions at 3.4 eV with decreasing cluster size is reported to be due to the compressive strain in small clusters. A first principle calculation based on density functional theory using the full potential linear augmented plane wave plus local orbitals formalism with generalized gradient approximation for the exchange correlation energy is used to estimate the band gaps at the X- and L-symmetry points by calculating the band structures and joint density of states for different strain values in order to explain the blueshift of approximately 0.1 eV with decreasing cluster size around L-symmetry point.

Optical characterization of GaN by N+ implantation into GaAs at elevated temperature

Both hexagonal wurtzite and cubic zinc blend GaN phases were synthesized in GaAs by 50keV N+ implantation at 400°C and subsequent annealing at 900°C for 15min in N2 ambient. The crystallographic structural and Raman scattering studies revealed that GaN phases were grown for fluence above 2×1017cm−2. Temperature-dependent photoluminescence study showed a sharp direct band-to-band transition peak ∼3.32eV at temperature ⩽200K. The intermediate band-gap value, with respect to ∼3.4eV for hexagonal and ∼3.27eV for cubic phases of GaN, is indicative of the formation of mixed hexagonal and cubic phases.

Flux dependent MeV self-ion-induced effects on Au nanostructures: dramatic mass transport and nanosilicide formation

We report a direct observation of dramatic mass transport due to 1.5xa0MeV Au(2+) ion impact on isolated Au nanostructures of average size ≈7.6xa0nm and height ≈6.9xa0nm that are deposited on Si(111) substrate under high flux (3.2 × 10(10)-6.3 × 10(12)xa0ionsxa0cm(-2)xa0s(-1)) conditions. The mass transport from nanostructures was found to extend up to a distance of about 60xa0nm into the substrate, much beyond their size. This forward mass transport is compared with the recoil implantation profiles using SRIM simulation. The observed anomalies with theory and simulations are discussed. At a given energy, the incident flux plays a major role in mass transport and its redistribution. The mass transport is explained on the basis of thermal effects and the creation of rapid diffusion paths in the nanoscale regime during the course of ion irradiation. The unusual mass transport is found to be associated with the formation of gold silicide nano-alloys at subsurfaces. The complexity of the ion-nanostructure interaction process is discussed with a direct observation of melting (in the form of spherical fragments on the surface) phenomena. Transmission electron microscopy, scanning transmission electron microscopy, and Rutherford backscattering spectroscopy methods have been used.

Ion-beam-induced enhanced diffusion from gold thin films in silicon

We report enhanced diffusion of gold atoms from gold films of various thicknesses (that are deposited on Si) due to 1.5 MeV Au2+ ion impacts under high flux conditions. The maximum depths of mass transport have been found to be 95, 160 and 13 nm for the cases of 5.3, 10.9 and 27.5 nm thick gold films, respectively, at a fluence of 1 × 1014 ions cm−2. Interestingly, at a higher fluence of 1 × 1015 ions cm−2, gold atoms from the 27.5 thick films are transported to a maximum depth of 265 nm in the substrate. The enhanced diffusion for various film thicknesses is consistent with the recoil profiles of Au atoms into Si, which are obtained using Monte Carlo simulations (TRIM). These results have been explained on the basis of the ion-beam-induced flux-dependent amorphous nature of the substrate, and transient beam-induced temperature effects. This work also confirms the absence of ion-induced spike confinement effects that might arise from the morphological nature of the isolated nanostructures.

Flux dependent 1.5 MeV self-ion beam-induced sputtering from gold nanostructured thin films

We discuss four important aspects of 1.5 MeV Au2+ ion-induced flux dependent sputtering from gold nanostructures (of an average size of ≈7.6 nm and height ≈6.9 nm) that are deposited on silicon substrates: (a) the Au sputtering yield at the ion flux of 6.3 × 1012 ions cm−2 s−1 is found to be ≈312 atoms ion−1, which is about five times the sputtering yield reported earlier under identical irradiation conditions at a lower beam flux of ≈109 ions cm−2 s−1, (b) the sputtered yield increases with increasing flux at a lower fluence and reduces at a higher fluence (1.0 × 1015 ions cm−2) for nanostructured thin films while the sputtering yield increases with increasing flux and fluence for thick films (27.5 nm Au deposited on Si), (c) the size distribution of sputtered particles has been found to vary with the incident beam flux showing a bimodal distribution at a higher flux and (d) the decay exponent (δ) obtained from the size distributions of the sputtered particles showed an inverse power-law dependence ranging from 1.5 to 2.5 as a function of the incident beam flux. The exponent values have been compared with existing theoretical models to understand the underlying mechanism. The role of wafer temperature associated with the beam flux has been invoked for a qualitative understanding of the sputtering results in both the nanostructured thin films and thick films.