Tanuj Kumar

Role of surface composition in morphological evolution of GaAs nano-dots with low-energy ion irradiation

The surface chemistry of GaAs (100) with 50-keV Ar+ ion beam irradiation at off-normal incidence has been investigated in order to elucidate the surface nano-structuring mechanism(s). Core level and valence band studies of the surface composition were carried out as a function of fluences, which varied from 1 × 1017 to 7 × 1017 ions/cm2. Core-level spectra of samples analyzed by X-ray photoelectron spectroscopy confirmed the Ga enrichment of the surface resulting in bigger sized nano-dots. Formation of such nano-dots is attributed to be due to the interplay between preferential sputtering and surface diffusion processes. Valence band measurement shows that the shift in the Fermi edge is higher for Ga- rich, bigger sized nano-dots due to the partial oxide formation of Ga. ‘One-dimensional power spectral density’ extracted from atomic force micrographs also confirms the significant role of surface diffusion in observed nano-structuring.

Ion beam-generated surface ripples: new insight in the underlying mechanism

A new hydrodynamic mechanism is proposed for the ion beam-induced surface patterning on solid surfaces. Unlike the standard mechanisms based on the ion beam impact-generated erosion and mass redistribution at the free surface (proposed by Bradley-Harper and its extended theories), the new mechanism proposes that the incompressible solid flow in amorphous layer leads to the formation of ripple patterns at the amorphous-crystalline (a/c) interface and hence at the free surface. Ion beam-stimulated solid flow inside the amorphous layer probably controls the wavelength, whereas the amount of material transported and re-deposited at a/c interface control the amplitude of ripples.

An approach to tune the amplitude of surface ripple patterns

An approach is presented to tune the amplitude of ripple patterns using ion beam. By varying the depth location of amorphous/crystalline interface, ripple patterns of different amplitude with similar wavelength were grown on the surface of Si (100) using 50 keV Ar+ beam irradiation. Atomic force microscopy study demonstrates the tuning of amplitude of ripples patterns for wide range. Rutherford backscattering channeling measurement was performed to measure the depth location of amorphous/crystalline interface. It is postulated that the ion beam stimulated solid flow inside the amorphous layer controls the wavelength, whereas mass rearrangement at amorphous/crystalline interface controls the amplitude.

Fabrication of ordered ripple patterns on GaAs(100) surface using 60 keV Ar+ beam irradiation

Abstract Ion beam induced ripples have limitations to be used in widespread nanoscale device applications due to the presence of deformity in surface patterns. Defect free ripples can only be produced on binary materials if the ion species, energy and angle of incidence are appropriately chosen. Fabrication of highly ordered defect free ripples is observed on GaAs(100) surfaces after the irradiation of 60 keV Ar+ ion beam for different fluences. Atomic force microscopy study demonstrates that the ordering as well as coarsening of ripples are increased with increase in fluence. X-ray photoelectron spectroscopy was carried out to study the significant role of altered surface composition for the generation of defect free ripples. The evolution of high degrees of order is explained with the help of existing formalisms of coupling between surface topography and preferential sputtering.

In Situ Investigation of Current Transport Across Pt/n-Si (100) Schottky Junction During 100

In situ current-voltage (<i>I</i>-<i>V</i>) analyses of Pt/n-Si (100) Schottky barrier (SB) diode are carried out during 100 MeV Ni⁺⁷ ion beam irradiation. The effect of MeV ion beam on the electrical parameters like ideality factor (η) and SB height (SBH) (φ_B) of SB diode is investigated. For lower fluences, SBH decreases from its preradiation value, but there is almost no change in SBH for ion fluences ranging from 5×10¹¹ to 1×10¹³ ions/cm². The reverse current is increased by about two orders of magnitude at the fluence of 5×10¹³ ions/cm² which corresponds to an exposure of a few tens of years in low earth orbit. The radiation-induced diffusion of Schottky metal into the semiconductor and creation of trap centers at the metal-semiconductor interface are supposed to be the most plausible mechanisms for these deviations in SB diode characteristics.

\hbox{MeV Ni}^{+7}

Ag:TiO2 nanocomposite films have been synthesized by sol-gel method followed by electron beam physical vapour deposition. Targets for this deposition were prepared by a hydraulic press using a powder containing Ag and TiO2 prepared by sol-gel technique. Microstructure, surface, and plasmonic properties of nanocomposite films were studied using glancing angle X-ray diffractometer, atomic force microscopy, field emission secondary electron microscopy, and UV-Vis spectroscopy. Microstructural study reveals that Ag nanoparticles are embedded in TiO2 matrix consisting of mixed phases of anatase and rutile. Size estimation using Scherrer formula reveals that average crystallite size of Ag nanoparticles is 23 nm. Surface morphological studies indicate that deposited films are uniform and intact to the substrate and have very low value of root mean square roughness. Optical studies exhibit a surface plasmon resonance induced absorption band in visible region, which is the characteristic feature of Ag nanoparticles. The intensity of this absorption band is found to increase with the increase in deposition time. Multiple peaks observed in absorption band were explained using the concepts of extended Mie scattering. Preliminary experiments also suggested that these nanocomposite films exhibit promising photocatalytic properties, which can be used for water treatment.

Ion Irradiation

The effect of electron–phonon coupling strength in structural manipulation of different degrees of pre-damaged Ge by swift heavy ions (SHI) has been investigated. Starting with c-Ge (100), three sets of samples were prepared with various degrees of damage, i.e. isolated amorphous pockets, connected amorphous zones and continuous amorphous layer by 100 keV Ar ion irradiation. Structural changes in these samples induced by 100 MeV Ag ions were investigated by RBS/C, micro-Raman spectroscopy and XTEM studies. Results show that the sample having isolated amorphous pockets undergoes re-crystallization whereas the sample having uniform amorphous layer shows volume expansion with formation of nanorod-like structures, after SHI irradiation. In this work, it is emphasized that the different degrees of pre-damage have a prominent role against SHI-induced structural manipulation under different laws or equations.

Fabrication of Ag:TiO2 Nanocomposite Thin Films by Sol-Gel Followed by Electron Beam Physical Vapour Deposition Technique

Ni/GaN Schottky barrier diodes were irradiated with 200 MeV Ag ions up to fluence of 1 × 1011 ions/cm2 at the substrate temperature of 80 K. Post-irradiation current-voltage measurements showed that the ideality factor, n increased and the reverse leakage current, IR decreased with increase in fluence. But Schottky barrier height, ϕb increased only marginally with increase in ion fluence. In situ resistivity measurements showed orders of magnitude increase in resistivity of GaN epitaxial film with irradiation fluence. Cross-sectional transmission electron microscopy images revealed the presence of defect clusters in bulk GaN after irradiation.

Structural manipulation in Ge by swift heavy ions governed by electron–phonon coupling strength

In this report, the recrystallization of pre-damaged Ge samples is extensively investigated under steady-state thermal annealing and ultrafast thermal spike-assisted annealing generated by high-energy ions. The (100) single-crystal Ge samples were pre-damaged using 100 keV Ar ion implantation. Three sets of pre-damaged Ge samples with sub-threshold (set A), threshold (set B) and above-threshold (set C) doses of amorphization, as estimated by Rutherford backscattering spectrometry in channeling mode (RBS/C), were suitably selected. Cross-sectional transmission electron microscopy (XTEM) images show distributed damaged pockets surrounded by crystalline material in the case of the as-damaged set A sample and completely damaged layer in the set C sample. These samples were used to study the regrowth of damage by (i) vacuum annealing at temperatures ranging from 373 K to 873 K for 30 minutes each and (ii) 100 MeV Ag ion irradiation-assisted annealing at four different temperatures: 100 K, 300 K, 373 K and 473 K. After 100 MeV Ag ion irradiation, set A samples have undergone complete recrystallization at 473 K. Similar recrystallization, but with lower magnitude, is also observed in the set B sample with increase in temperature. In set C samples, interestingly, nanowire formation was observed instead of recrystallization after irradiation at 100 K and 300 K, but recrystallization is observed at high-temperature irradiation, though it is much lower than those of set A and set B samples. The Arrhenius plot of the recrystallized fraction reveals a reduced activation energy of recrystallization by a substantial factor due to thermal spike-assisted recrystallization.