J.K. Tripathi

Structural and optical properties of Mn-doped CdS thin films prepared by ion implantation

We report on structural and optical properties of Mn-doped CdS thin films prepared by 190 keV Mn-ion implantation at different temperatures. Mn-ion implantation in the fluence range of 1×1013–1×1016 ions cm−2 does not lead to the formation of any secondary phase. However, it induces structural disorder, causing a decrease in the optical band gap. This is addressed on the basis of band tailing due to creation of localized energy states and Urbach energy calculations. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d (T41→A61) transition of tetrahedrally coordinated Mn2+ ions.

Giant magnetoresistance (GMR) in swift heavy ion irradiated Fe films on c-silicon (Fe/c-Si)

Fe/c-Si devices have been irradiated with 100 MeV swift heavy ions of Fe7+ at a dose of 1014 ions cm−2. The devices have been studied using XRD and SEM. Electronic transport across the interface of the devices (i.e. in current perpendicular to the plane (CPP) of the device) has been measured from room temperature to liquid N2 temperature. The CPP current has also been studied in a magnetic field (up to 10 kG which has been applied along the plane of the device). Unirradiated devices do not show any effect of the magnetic field whereas large magnetoresistance (MR) up to 2400% giant magnetoresistance (GMR) has been observed for the irradiated devices. An M–H study of the irradiated devices shows a behaviour of coupled magnetic nanograins. The results have been understood by considering the formation of a nanogranular magnetic silicide phase (of Fe5Si3) due to intermixing at the interface (as evidenced from XRD and SEM features). The electronic and magnetotransport characteristics of the irradiated devices show that the interface becomes intimate enough (due to the irradiation induced strong intermixing) to result in a tunnel transmission of carriers. A tunnelling barrier seems to form (for the irradiated ones) between Fe5Si3 magnetic nanocrystals separated by a nanometre scaled silicon tunnelling barrier. The observed very large (strong) GMR could be due to the spin dependent interface scattering in the presence of the strong AF coupling across the tunnelling barrier.

The effect of Fe-coverage on the structure, morphology and magnetic properties of α-FeSi2 nanoislands.

Self-assembled α-FeSi(2) nanoislands were formed using solid-phase epitaxy of low (~1.2 ML) and high (~21 ML) Fe coverages onto vicinal Si(111) surfaces followed by thermal annealing. At a resulting low Fe-covered Si(111) surface, we observed in situ, by real-time scanning tunneling microscopy and surface electron diffraction, the entire sequence of Fe-silicide formation and transformation from the initially two-dimensional (2 × 2)-reconstructed layer at 300 °C into (2 × 2)-reconstructed nanoislands decorating the vicinal step-bunch edges in a self-ordered fashion at higher temperatures. In contrast, the silicide nanoislands at a high Fe-covered surface were noticeably larger, more three-dimensional, and randomly distributed all over the surface. Ex situ x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy indicated the formation of an α-FeSi(2) island phase, in an α-FeSi(2){112} // Si{111} orientation. Superconducting quantum interference device magnetometry showed considerable superparamagnetism, with ~1.9 μ(B)/Fe atom at 4 K for the low Fe-coverage, indicating stronger ferromagnetic coupling of individual magnetic moments, as compared to high Fe-coverage, where the calculated moments were only ~0.8 μ(B)/Fe atom. Such anomalous magnetic behavior, particularly for the low Fe-coverage case, is radically different from the non-magnetic bulk α-FeSi(2) phase, and may open new pathways to high-density magnetic memory storage devices.

Low energy Ar+ ion irradiation induced surface modification in cadmium zinc telluride (CdZnTe)

In this paper, we report on modifications in structural, stoichiometry, and optical properties of cadmium zinc telluride (CdZnTe) crystals due to 1 keV Ar+ ion irradiation as a function of ion fluence, using ion flux of 1.7 × 1017 ions cm−2 s−1. The CdZnTe crystals were irradiated at normal incidence, using fluence range of 8 × 1017–3 × 1019 ions cm−2. Atomic force microscopy studies show sequential change in surface structure as a function of ion fluence, from homogeneously populated nano-hole to micron sized holes on the entire CZT crystal surface. These holes are well geometrically defined and most of them are rectangular in shape. X-ray photoelectron spectroscopy studies show a reduction in Zn at % while Raman and photoluminescence studies show almost complete depletion of Te inclusions and slight red shifts, respectively, due to ion irradiations. Schottky diode radiation detectors fabricated from such defect free CZT crystals will show significantly higher energy resolution.

In situ I–V study of swift (~100 MeV) O6+ ion-irradiated Pd/n-Si devices

An in situ I–V study of Pd/n-Si devices irradiated to swift (~100 MeV) O6+ ions for a fluence of 1011–1013 cm−2 has been carried out. The devices have been irradiated at room and LN2 temperatures. The irradiated devices have been hydrogenated in ex situ condition by molecular hydrogen. It has been observed that resistivity increases after the irradiation and there is a progressive increase with the increase of irradiation fluence. On hydrogenation, the devices irradiated at LN2 temperature show that the irradiation-induced increased resistivity decreases back to the pre-irradiated condition, whereas the devices irradiated at room temperature do not show any change. The results have been understood in the realm of irradiation-induced defects.

Magnetic and semiconducting nanostructures by swift heavy ion irradiation of Fe20Ni80 films on Si substrates

Fe20Ni80/Si interface devices have been fabricated and irradiated from swift (100 MeV) heavy (Fe7+) ions with a dose of 1014 cm−2. The current measured across the irradiated devices from room temperature (RT) to liquid nitrogen (LN2) temperature shows a positive temperature coefficient. The results were understood by considering the formation of a semiconducting and magnetic silicide nanophase as a result of ion beam mixing (which was shown from x-ray diffraction (XRD) data). The particle size was estimated to be ~25 nm from the XRD data. The induced magnetization at the interface was studied from the magnetization versus magnetic field (M–H) variation at room temperature. The magnetization shows superparamagnetic behaviour characterizing the magnetic nanoparticles.

Electronic flow across swift (~100 MeV) heavy ion irradiated Fe/Si interfaces

The current flow across Fe/Si interface devices has been studied after swift (∼100 MeV) heavy ion irradiation. The current flow has been also studied in a low magnetic field of <1 KG. It has been observed that the current flow in such devices increases substantially (by two orders of magnitude) after irradiation and shows a strong effect in the magnetic field. The current flow through the devices has been found to be temperature-independent from liquid nitrogen temperature to room temperature. The scanning electron microscopy and x-ray diffraction features show a mixed phase of iron silicide (FexSiy) with an average grain size of ∼200 nm. It seems that a Fe/FexSiy/Fe tunnel junction is formed at the irradiated Fe/Si interface resulting in the observed features. Moreover, the magnetic interlayer coupling at the interface seems to control the effect of the magnetic field on current flow through the reacted (or mixed) interface.

Ion erosion induced nanostructured semiconductor surfaces

We consider nanostructures formed on semiconductor surfaces of Si(100), InP(100) and GaAs using medium energy (50–100 keV) Ar+–ion beam sputtering. The issues of dependence of nanostructure formation on these semiconductor substrates on ion–energy, –fluence, –flux, angle of incidence, and crystallographic orientation are addressed. The threshold fluence for formation of nano–islands on Si(100) implanted with normally incident 50 keV Ar+–ions was found to be 2 × 1017 ions/cm². For InP(100) implanted with 100 keV Ar+–ions an increase in angle of incidence results in decrease of surface roughness. Surfaces of GaAs(100) and GaAs(111) implanted with normally incident 50 keV Ar+–ions show nanopits of density 3–4 × 109/cm². The existing theories are applied to explain the formation of observed surface nanostructures.

MAGNETIC BEHAVIOR OF NANOGRANULAR SILICIDE PHASE (FORMED DUE TO SWIFT HEAVY Fe7+ ION IRRADIATION-INDUCED INTERMIXING)

Fe films (of ~50 nm) on p-silicon substrates have been deposited by electron beam evaporation technique to realize Fe/pSi devices. The devices have been irradiated from 100 MeV swift heavy ions of Fe7+ with a fluence of 1014 ions-cm-2. The morphological and structural characterization have been done from SEM, AFM and XRD facilities. Magnetic behavior has been studied from vibrating sample magnetometer (VSM) and SQUID facilities. SEM and AFM studies show the formation of a nanogranular structure. Further, XRD data has shown the formation of intermixed magnetic nanogranular silicide phase (Fe5Si3) having average grain size of 25 nm. M–H and ZFC (zero field-cooled)/FC (field-cooled) studies show the magnetic behavior of interacting magnetic particles. The observed results have been understood as a typical behavior of interacting magnetic nanograins.