P.C. Srivastava

Ion transport studies in PEO∶NH4I polymer electrolytes with dispersed Al2O3

Ion conducting polymer electrolyte films having high salt concentrations have been studied. The system studied is PEO∶NH4I, dispersed with α-Al2O3. Mechanically stable films with NH4+/EO ratio ≥0.13 have been obtained by dispersal of Al2O3. The films have been characterized using various techniques such as X-ray diffraction (XRD), differential thermal analysis (DTA), polarization and complex impedance spectroscopy. Intercorrelation between polymer matrix crystallite size, conductivity, and α-Al2O3 particle size is established.

AFM study of swift gold ion irradiated silicon

Abstract AFM studies of swift (∼100 MeV) heavy ion (Au7+) irradiated crystalline p- and n-type silicon surfaces have been performed. The irradiation fluence has been varied from 1010 to 10 12 ions cm −2 . The studies have shown the formation of clear craters surrounding a hill for the irradiated p-type only whereas a hill on one side of the crater for the irradiated p- and n-type silicon surfaces has been observed. The surface roughness has been observed to be increased after the irradiation and the increase for n-type is significantly larger as compared to that for p-type. The quantitative estimate of the volume of the craters and hills has shown a larger volume (2.0–4.0 times) for the craters as compared to the hills. Moreover a larger volume for craters and hills for n-type as compared to p-type has been found. The observed features have been discussed as the radiation induced mass transfer from within the substrate to its surface and a fraction sputtered out due to the explosive evaporation as a result of heating due to electronic energy loss. The distinctly different features for n-type and p-type seem to be due to dopant dependent fault growth and shrinkage rates.

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.

Transient capacitance spectroscopy in polycrystalline silicon

We have performed deep level transient spectroscopy (DLTS) on 5 Ω cm p‐type polycrystalline silicon. Typical DLTS spectra exhibit three peaks centered at 250, 300, and 340 K. The density of states which is estimated from these spectra is found to extend from 0.18 eV of the valence band with a density ∼ 2×1015 eV−1 cm−2. Measurements of capture rates have shown a characteristic trap filling time of ∼10−3 s corresponding to an apparent capture cross section of the order of 10−21 cm2. Studies of the passivation (in a deuterium plasma) of these boundary states have also been made. In that case, the DLTS spectra present only one significant peak at 245 K; the density of states of the unpassivated material is reduced to localized states centered at 0.32 eV from the valence band with a concentration of 5.6×1014 cm−2. The capture cross section of these states is 3.7×10−20 cm2.

Ultrasonic attenuation due to phonon–phonon interaction, thermoelastic loss and dislocation damping in transition metal carbides

Temperature dependent ultrasonic attenuation due to phonon–phonon (p–p) interaction, thermoelastic loss and dislocation damping due to screw and edge dislocations have been investigated in fcc (NaCl B1 type) structured group IVb and Vb monocarbides (transition metal carbides, namely TiC, ZrC, HfC, VC, NbC and TaC) in the temperature range 50–500 K, along the three crystallographic directions of propagation, namely [100], [110] and [111] for longitudinal and shear modes of propagation. The second- and third-order elastic moduli (SOEM and TOEM) obtained at different temperatures using the electrostatic and Born repulsive potentials and taking interactions up to next-nearest neighbours, have been used to obtain Gruneisen numbers, acoustic coupling constants and their ratios along different directions of propagation and polarization for longitudinal and shear modes of wave propagation. Temperature variation of the phonon relaxation time shows exponential decay. The results have been discussed and compared with available data.

Hydrogen in semiconductors

Hydrogen in crystalline semiconductors has become a recent curiosity because of its high diffusivity and strong chemical activity in such materials. In contrast to the proton motion in ionic materials which gives rise to an enhanced conductivity, hydrogen in electronic materials interact with structural disorders and chemical impurities to control the electronic flow. Deep gap states in crystalline semiconductors due to various disorders such as surface/interface, grain boundaries, dislocations, irradiation and implantation damage etc. have been removed due to hydrogen bondings.Hydrogen incorporation is done by plasma and direct ion beam hydrogenation methods, implantation technique and by a novel technique of damage free introduction. The most studied materials are silicon and gallium arsenide.I - V,C - V, DLTS and IR studies have been carried out on hydrogenated semiconductors to characterize the electronic flow, gap states and the nature of chemical bonds. Improvement in ideality factors of diodes, reduction in free carrier concentration, removal or reduction of deep states and appearance of new bondings such as Si-H, P-H, B-H etc. have been observed from various techniques.The present paper reviews the various features of hydrogenation studies in crystalline silicon and gallium arsenide and highlights our results of hydrogenation studies on Pd/semiconductor devices.

AFM studies of swift heavy ion-irradiated surface modification in Si and GaAs

Abstract Atomic force microscopy (AFM) studies of Swift Heavy Ions (SHI ∼100 MeV Si 7+ and Au7+) irradiated Si and GaAs surfaces have been performed for a variable fluence in the range of 1010– 10 13 ions cm −2 . The craters with piled up material, which is called hill, are clearly seen in the micrographs. A significant direct observation of amorphization (or melting due to SHI irradiation damage), plastic flow and subsequent recrystallization in the form of platelets has been made. The quantitative estimation of the features revealed that the volume of the craters for silicon ion irradiation is smaller than the gold ion irradiation. However, surface roughness has been found to be enhanced after the irradiation. Moreover, the GaAs surfaces were found to be less rough than the Si surface. The features are related to the difference in electronic energy loss of incident ions, thermal diffusivity, thermal conductivity and density of the target materials.

HIGH ENERGY HEAVY ION IRRADIATION IN SEMICONDUCTORS

Abstract Pd/n-Si and Pd/n-GaAs devices have been irradiated from high energy (∼100 MeV) heavy ions of Au7+ (gold) and Si7+ (silicon) to study the irradiation effects in these junction devices on semiconductor substrates. The devices have been characterized from I–V and C–V studies for electronic flow characterization. It has been found that the devices become high resistive on the irradiation and the substrates change the conductivity type from n- to p- on the irradiation of fluence of ∼1012–1013 ions/cm2. The change in conductivity type has been understood as a result of creation of deep acceptors on the irradiation.

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.

PdSi device characteristics on 100 MeV gold ions irradiation

Pdp-Si and Pdn-Si devices were irradiated from 100 MeV gold (7+) ions for varying doses (∼ 1011−1013 ions cm−2). The devices were characterized from I-V and C-V studies. It has been found that there is a change of conductivity type i.e. from n to p at a probed depth of ∼ 8 μm which is approximately the stopping range of the gold ions in silicon. A deep acceptor state (∼ 0.61 eV above V.B. edge) with a peak density of ∼ 109 cm−2 is observed for p-type irradiated devices at ∼ 3 μm which is attributed to the displacement damage caused by the high energy heavy ion irradiation.