Sub-nanosecond Thermal Spike Induced Nanostructuring of Thin Solid Films Under Swift Heavy Ion (SHI) Irradiation

The interaction between swift heavy ions (SHI) and a solid has been identified as one of the important physical processes to generate or modify nanostructures in thin solid films. The large part of the energy which is deposited in the electronic subsystem of a material by SHI is known as electronic energy loss and gets coupled to the lattice subsystem in a complex way resulting in a transient (picoseconds to sub-nanosecond) thermal spike within a few nanometer diameter region of the thin solid film along the ion path. The temperature of this narrow zone may raise up to 1000 K or more during this time. This transient heating process is known as lattice thermal spike and can be used as a tool to engineer materials down to the nanoscale. Here we address two important consequences of lattice thermal spike; (i) elongation of metal nanoparticles embedded in dielectric thin films and (ii) generation of a-Si/c-Si nanostructures in a silicon nitride matrix.

(i) Metal nanoparticles embedded in a thin film matrix belong to a class of materials that has potential applications as optical and magnetic sensors, storage, memory devices, field emission display etc. The nanoparticle size and shape, orientation, inter-particle separation and dielectric constant of the surrounding matrix are the crucial parameters which control their properties. Thermal spike generated by SHI in these nanoparticles and surrounding matrix can be used as a unique tool to tailor the shape of the embedded nanoparticles, eventually modifying the physical properties of these materials. Metal nanoparticles, which are mostly spherical in shape in as grown films, get elongated along the direction of SHI due to thermal spike induced melting and stress. After a brief introduction of some fundamental aspects and synthesis of these films, a detailed discussion on elongation of nearly spherical Ni nanoparticles embedded in SiO2 thin film matrix under 120 MeV Au ion irradiation is made. Various physical parameters influencing the shape modification of nanoparticles under the framework of thermal spike model are discussed.

(ii) a-Si/c-Si nanostructures embedded in different Si-based dielectric matrix have attracted researchers because of their potential application in low-cost Si-based optoelectronic devices. We report here the response of in-situ formed Si-nanostructures embedded in a Si-rich hydrogenated amorphous silicon nitride matrix to 100 MeV Ni8+ ions irradiation. Si-rich a-SiN x :H films have been prepared by Hg-sensitized Photo Chemical Vapor Deposition. Presence of elemental Si was confirmed from X-ray photoelectron spectroscopy. Irradiation with a fluence of 5×1012 ions/cm2 under normal incidence at room temperature leads to dissolution of these Si-nanostructures. However, irradiation with a relatively higher fluence of 1×1014 ions/cm2 enhances the nucleation and leads to the formation of amorphous Si-nanostructures in the film. In addition, at the surface a novel effect i.e. partial crystallization of Si-nanostructures along the beam direction is observed. The results are explained on the basis of thermal spike model.