P. Srivastava

Room temperature transparent ferromagnetism in 200 keV Ni2+ ion implanted pulsed laser deposition grown ZnO/sapphire film

Intrinsic ferromagnetism at room temperature has been observed in ZnO/sapphire films by implantation of 200 keV Ni2+ ions with fluences 6×1015, 8×1015, 1×1016, and 2×1016u2002ions/cm2. Crystalline phases are identified by glancing angle x-ray diffraction, which shows no extra phase in the implanted films. Highest saturation magnetization (Ms) is observed in the film implanted with the fluence of 8×1015u2002ions/cm2 as examined by superconducting quantum interference device magnetometry. This film has almost 80% transmittance across visible wavelength range and hence a potential candidate of transparent ferromagnetic semiconductor. Defectlike oxygen vacancies in the films are studied by x-ray photoelectron spectroscopy. Ferromagnetism of the films is explained on the basis of bound magnetic polaron model.

Phase stabilization by rapid thermal annealing in amorphous hydrogenated silicon nitride film

We have studied the effect of rapid thermal annealing (RTA) in the context of phase evolution and stabilization in hydrogenated amorphous silicon nitride (a-SiN(x):H) thin films having different stoichiometries, deposited by an Hg-sensitized photo-CVD (chemical vapor deposition) technique. RTA-treated films showed substantial densification and increase in refractive index. Our studies indicate that a mere increase in flow of silicon (Si)-containing gas would not result in silicon-rich a-SiN(x):H films. We found that out-diffusion of hydrogen, upon RTA treatment, plays a vital role in the overall structural evolution of the host matrix. It is speculated that less incorporation of hydrogen in as-deposited films with moderate Si content helps in the stabilization of the silicon nitride (Si(3)N(4)) phase and may also enable unreacted Si atoms to cluster after RTA. These studies are of great interest in silicon photonics where the post-treatment of silicon-rich devices is essential.

Coexistence of intrinsic and extrinsic origins of room temperature ferromagnetism in as implanted and thermally annealed ZnO films probed by x-ray absorption spectroscopy

The present work reports the structural and ferromagnetic properties of 200 KeV Ni2+ ion implanted ZnO films before and after annealing at 650u2009°C in air and correlates these properties with their electronic structures. X-ray absorption spectroscopy (XAS) at Zn and Ni K-edges reveal that Zn metal cluster is absent in ZnO matrix, but there is an indication of clustering of Ni metal as a secondary phase. The XAS analysis of O K-edge shows (i) non-stoichiometric NiO segregation in as-implanted film and its absence/reduction in air annealed film, (ii) partial substitution of Ni2+ ions at tetrahedral Zn sites, and (iii) presence of lattice defects such as oxygen vacancies in both films. The observed ferromagnetic behavior of as-implanted and air annealed films has both intrinsic as well as extrinsic origins. The intrinsic and extrinsic origins are discussed in terms of bound magnetic polaron model and presence of antiferromagnetic NiO phase/ferromagnetic Ni metal clusters, respectively.

Probing origin of room temperature ferromagnetism in Ni ion implanted ZnO films with x-ray absorption spectroscopy

We report x-ray absorption at Zn and Ni K-edges in 200u2009keV Ni2+ ion implanted ZnO/sapphire films. The implantation fluences are 6u2009×u20091015 and 2u2009×u20091016 ions/cm2, corresponding to 2% and 7% Ni in a ZnO matrix. The measurements reveal a marginal substitution of Ni in ZnO in both the films and also rule out the presence of ferromagnetic Ni metal clusters. The M-H and field cooled-zero field cooled measurements performed via SQUID magnetometry show that the films are ferromagnetic at room temperature, and the saturation magnetization of 2% Ni film is appreciably higher than that of 7% Ni film. The origin of ferromagnetism is understood on the basis of the oxygen vacancy mediated bound magnetic polaron model.

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.