Showing papers by "Samit K. Ray published in 2008"

Structural and electrical properties of metal–ferroelectric–insulator–semiconductor structure of Al/SrBi2Ta2O9/HfO2/Si using HfO2 as buffer layer

Abstract: Ferroelectric SrBi2Ta2O9 (SBT) thin films have been deposited by the radio-frequency magnetron sputtering technique on bare p-Si as well as on HfO2 insulating buffer p-Si. XRD patterns revealed the formation of a well-crystallized SBT perovskite thin film on the HfO2 buffer layer. The electrical properties of the metal–ferroelectric–insulator–semiconductor (MFIS) structure were characterized by varying thicknesses of the HfO2 layer. The MFIS structure exhibits a maximum clockwise C–V memory window of 1.60 V when the thickness of the HfO2 layer was 12 nm with a lower leakage current density of 6.20 × 10−7 A cm−2 at a positive applied voltage of 7 V. However, the memory window reaches a maximum value of 0.7 V at a bias voltage of ±5 and then decreases due to charge injection in the case of the insulating buffer layer thickness of 3 nm. The density of oxide trapped charges at/near the buffer layer–ferroelectric interface is studied by the voltage stress method. Capacitance–voltage (C–V) and leakage current density (J–V) characteristics of the Al/SBT/HfO2/Si(1 0 0) capacitor indicate that the introduction of the HfO2 buffer layer prevents interfacial diffusion between the SBT thin film and the Si substrate effectively and improves the interface quality. Furthermore, the Al/SBT/HfO2/Si structures exhibit excellent retention characteristics, the high and low capacitance values clearly distinguishable for over 1 h and 30 min. This shows that the proposed Al/SrBi2Ta2O9/HfO2/Si structure is ideally suitable for high performance ferroelectric memories.

Memory effect in a junction-like CdS nanocomposite/conducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)1,4-phenylene-vinylene] heterostructure

Abstract: The operation of a nonvolatile memory device is demonstrated using junction-like CdS nanocomposites embedded in a polymer matrix. The capacitance-voltage characteristics of Al/conducting polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene]/CdS nanocomposites in a polyvinyl alcohol matrix/indium tin oxide device exhibit hysteresis, which is attributed to the trapping, storage, and emission of holes in the quantized valence band energy levels of isolated CdS nanoneedles. The characteristics at different operating frequencies show that the hysteresis is due to trapping of charge carriers in CdS nanocomposites rather than in the interfacial states. The memory behavior in the inorganic/organic heterostructure is explained on the basis of a simple energy band diagram.

Effect of Al concentration in grain and grain boundary region of Al-doped ZnO films: a dielectric approach

Abstract: Aluminium-doped zinc oxide (ZnO : Al) films have been deposited by RF magnetron sputtering on Pyrex glass substrates. The structural, electrical and optical properties of films as a function of Al concentration have been studied. Al-doped films exhibit high optical transparency and a reduction of resistivity with increasing Al concentration. The minimum resistivity value is found to be ρ = 2.2 × 10−3 Ω cm as compared with ~104 Ω cm, for undoped ZnO. The optical band gap is observed to follow a Burstein–Moss shift with the increase in the concentration of Al doping and shows the maximum band gap of 3.53 eV. Relative variation of the grain and grain boundary scattering with doping concentration has been investigated with impedance spectroscopy.

Evolution of strain and composition of Ge islands on Si (001) grown by molecular beam epitaxy during postgrowth annealing

Abstract: Self-assembled Ge islands have been grown using a Stranski–Krastanov growth mechanism on Si (001) substrates by solid source molecular beam epitaxy. We performed time varying annealing experiments at a representative temperature of 650°C to study the shape and size evolution of islands for a relatively high Ge coverage. Islands are found to coarsen due to heat treatment via structural and compositional changes through continuous strain relaxation. Different island morphologies, namely, “pyramids,” “transitional domes,” and “domes” are found during the annealing sequence. The dominant coarsening mechanisms for the temporal evolution of islands of as-grown and annealed samples are explained by the comprehensive analysis of Rutherford back scattering, Raman spectroscopy, high-resolution x-ray diffraction, and atomic force microscopy. A correlation of the morphological evolution with the composition and strain relaxation of grown islands is presented.

Ultraviolet and blue photoluminescence from sputter deposited Ge nanocrystals embedded in SiO2 matrix

Abstract: Ge nanocrystals (NCs) embedded in silicon dioxide (SiO2) matrix are grown by radio-frequency magnetron sputtering and studied in order to understand the origin of ultraviolet (UV) and blue photoluminescence (PL) from the NC-SiO2 system. Ge NCs of diameter 7–8 nm are formed after postdeposition annealing, as confirmed by transmission electron microscopy and Raman scattering studies. Optical Raman studies indicate the presence of strain in the embedded Ge NCs. Polarization dependent low frequency Raman studies reveal surface symmetrical and surface quadrupolar acoustic phonon modes of Ge NCs. PL studies with 488 nm excitation shows a broad emission band peaked at ∼545 nm, which is attributed to oxygen deficient defects in the SiO2 matrix. PL studies with 325 nm excitation show additional strong peaks in the 377–400 nm region. Time resolved PL studies in the UV-blue range show double exponential decay dynamics in the nanosecond time scale, irrespective of the NC size. Comparative studies of PL emission from ...

Schottky Barrier Characteristics of Cobalt–Nickel Silicide/n-Si Junctions for Scaled-Si CMOS Applications

Abstract: Ternary cobalt-nickel silicide/n-Si Schottky diodes have been fabricated by sputtering using an equiatomic cobalt- nickel alloy target. A minimum sheet resistivity of the ternary silicide is found to be 5-7 Omega/sq. Grazing-incidence X-ray diffraction shows the formation of ternary silicide phases. Cross-sectional TEM micrograph shows a fairly uniform diffusion of metals into Si with the formation of fully silicided film. Selected-area electron diffraction pattern exhibits the crystalline nature of the silicide layer. Temperature-dependent electrical current-voltage measurements have been used to characterize an optimized Schottky diode formed by annealing at 450degC. The room-temperature barrier height and ideality factor are found to be 0.656 eV and 1.6, respectively, from the I-V characteristics. The series resistance of the diode has been calculated and is found to be 1-11.8 kOmega. The variation of barrier height has been attributed to the inhomogeneity in Schottky junction.

Interfacial and electrical properties of SrBi2Ta2O9/ZrO2/Si heterostructures for ferroelectric memory devices

Abstract: We have investigated the interfacial and frequency dependent electrical properties of metal-ferroelectric-insulator-semiconductor capacitors with SrBi2Ta2O9 (SBT) ferroelectric films grown on ZrO2 buffer layer coated Si. Heterostructure SBT and ZrO2 thin films were deposited using rf magnetron sputtering. Interfacial and surface roughness parameters of heterostructures were extracted from the simulation of specular x-ray reflectivity data. The structure exhibited clockwise capacitance-voltage hysteresis with a maximum memory window of 2.0 V at a bias voltage of ±7 V. Frequency dependent (5 kHz–1 MHz) measurements at room temperature indicated that the clockwise hysteresis originates from the ferroelectric domain reversal. A minimum leakage current density of 4×10−8 A/cm2 of fabricated capacitors at an applied voltage of ±5 V revealed that the ZrO2 buffer layer prevents the interfacial diffusion between SBT thin film and the substrate, resulting in an improvement of interface quality. The charge retention ...

Self-Assembled Growth of Hexagonal ZnO Nanoprisms Exhibiting Good Photoluminescence Property

Abstract: Hexagonal-shaped nanoprisms of ZnO on indium tin oxide substrates are grown using an electrochemical process. The surface morphology and crystallinity of the ZnO nanostructures are characterized by field-emission scanning electron microscopy and X-ray diffraction, respectively. Perfectly hexagonal-shaped ZnO nanoprisms can be obtained by varying the current density and potential. On increasing the current density, initial nucleation of isolated and uniform nanoprisms are achieved at a certain current density. The bandgap energy of the ZnO nanoprisms calculated from optical data is found to be 3.308 eV. Photoluminescence data at low temperatures demonstrate a high optical quality of these structures, with intense excitonic peaks. A high-energy excitonic emission at 3.387 eV is observed at 10 K, close to the bandedge emission, which is due to the A-exciton in ZnO nanostructures.

The bonding configuration in a partially relaxed pseudomorphic epilayer of SiGe: evidence of the BC-8 phase of silicon

Abstract: The bonding configuration of a silicon host lattice in a carbon-induced partially relaxed pseudomorphic epilayer of SiGe is studied using x-ray photoelectron spectroscopy, high-resolution XRD and Raman spectroscopic techniques. In-plane and out-of-plane strains are estimated using high-resolution x-ray diffraction. Careful analysis of the x-ray photoelectron spectra (XPS) suggests that the bonding environment of the silicon host lattice is modified, and is attributed to the presence of graphite-like-sp2 bonding in addition to the tetrahedral-sp3 bonding. These results are further supported by our Raman studies. Our Raman studies indicate the presence of the BC-8 phase; a high-pressure phase of silicon. The modified configuration of the silicon host lattice at high pressure is responsible for the observed changes in the XPS and Raman spectra. These results are also compared with the carbon incorporated silicon epilayers (Si1−xCx) having negligible strain relaxation. We attribute these effects to the strain-induced effects and not to the compositional effects of Ge.

Synthesis and characterization of nickel titanium melt-spun ribbon for micro-actuator device application

Abstract: Nickel titanium shape memory alloy ribbons of different nickel ratio with refined grain size have been fabricated by rapidly quenched melt spinning technique. The nickel-titanium alloy buttons are produced by arc melting, before the ribbon fabrication. XRD has been used to study phase formation of the alloy buttons and ribbons. Microstructure, composition and surface topography of melt-spun ribbons are studied using scanning electron microscopy, energy dispersive X-ray measurements and atomic force microscopy. The phase transformation of the ribbons has been studied using resistivity measurement as a function of temperature.

Synthesis of CdS nanowires using nanoporous alumina template

Abstract: Hexagonal Porous Alumina (PA) nanostructures with ordered pores have been fabricated using two-step anodisation of aluminium. CdS nanowires of diameters between 50?60 and 100?110 nm have been grown on PA templates by two different chemical and electrochemical techniques. Microstructural properties of the nanowires have been investigated using Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). Selected area electron and X-ray diffraction patterns show the growth of polycrystalline nanowires with hexagonal CdS phase. Optical properties of CdS nanowires grown by two different methods have been studied using optical absorption, Photoluminescence (PL) and Raman spectroscopy.

Temperature-dependent texture, stress and resistivity in melt spun Cu0.95Co0.05 ribbon

Abstract: Ribbon samples of Cu0.95Co0.05 were prepared by melt spinning method to perform systematic investigations on structure and transport properties as a function of annealing temperature. X-ray diffraction study shows that the ribbon is polycrystalline with a strong 〈2 0 0〉 texture along the surface normal of the as-quenched Cu0.95Co0.05 ribbon and the degree of texture is enhanced upon annealing. The compressive stress, which relaxes upon annealing, is observed in as-quenched ribbon. The resistivity, which is higher in as-quenched ribbon, decreases toward the bulk value of Cu upon annealing. The compressive stress and higher resistivity in as-quenched ribbon are attributed to the incorporation of Co atoms/particles in Cu matrix. The decrement of the stress and resistivity upon annealing is due to the precipitation of Co atoms from the Cu matrix, segregating as Co or Co-rich Cu grains as observed from the transmission electron microscopy measurements.

Optical properties of self-assembled Ge(Si) quantum dots grown on Si(001) by molecular beam epitaxy

Abstract: We report the Raman, continuous-wave (CW) photoluminescence (PL) measurements of Ge(Si)/Si samples with an islands like morphology. The values of composition and elastic deformation of the self-assembled Ge(Si) nanoislands formed at 550degC were determined by Raman scattering spectroscopy. Due to larger islands size, and intermixing of Si and Ge, the PL peak is red shifted for longer time deposition of Ge. The observed broad PL signal from Ge(Si) islands is associated with radiative recombination of holes confined in the Ge(Si) islands and electrons localized in the Si layer.