Studies on hot-filament chemical vapor deposition grown graphene sheets

Graphene was grown on high purity Cu foils using hot-filament chemical vapor deposition method. The foils were kept directly below the tungsten filament and the whole assembly was kept inside a vacuum chamber. CH4 and H2 were used as precursor gases and were allowed to shower on a hot filament, which was kept at a predetermined temperature. The optimization of the process parameters such as gas flow rates, temperature, durations, etc. was done to grow single layer and multilayer graphene. The graphene was characterized using optical microscopy, field emission scanning electron microscopy and micro-Raman spectroscopy techniques. The graphene layers grown at different methane flow rates are shown in Figure 1. By varying the methane flow rates, graphene domains of different sizes and shapes were achieved and are clearly evident from Figures 1a-c. The curved white lines (Figure 1a) present in the FESEM micrographs correspond to Cu terraces. The graphene grown on Cu foils was successfully transferred to SiO2 substrate and the micrograph of which is shown in Figure 1d. The presence of D, G and G’ bands in the Raman spectrum confirmed the growth of graphene in the Cu foil (Figure 2)

Structural and optical properties of graphene oxide prepared by modified hummers' method

Graphene oxide was synthesized from graphite flakes using modified Hummers' method. The interlayer spacings of graphite, graphite oxide and graphene oxide were measured using X-ray diffraction technique. The C/O atomic ratios of graphite oxide and graphene oxide were calculated from XPS measurements. The transformation of graphite to graphite oxide and finally to graphene oxide was clearly observed from the micro-Raman spectroscopy data and was confirmed from the FESEM micrographs. UV-VIS-NIR spectrophotometer was used to study the absorbance of graphene oxide and reduced graphene oxide samples. Finally, the chemically reduced graphene oxide was heat-treated in air to obtain chemically modified graphene

Fourth Asian meeting on Ferroelectricity organized at the Indian Institute Science, Bangalore, INDIA during December 12-15, 2003. Guest editorial

This Article does not have an abstract

Guest Editoria, Ferroelectrics, 323, 1

The Fourth Asian Meeting on Ferroelectricity (AMF-4)was organized at the Indian Institute of Science, Bangalore, INDIA during December 12–15, 2003. The organization of the conferencewas cosponsored by various agencies which included the Department of Science & Technology (DST), Defense R&D Organization (DRDO), Council of Scientific and Industrial Research (CSIR), Materials Research Society of India (MRSI), Indian Institute of Science (IISC), IEEE UFFC (USA), Taylor and Francis Scientific Publishers, and the international journal FERROELECTRICS

Low threshold voltage ZnO thin film transistor with a Zn0.7Mg0.3OZn_{0.7}Mg_{0.3}O gate dielectric for transparent electronics

A highly transparent all ZnO thin film transistor (ZnO-TFT) with a transmittance of above 80% in the visible part of the spectrum, was fabricated by direct current magnetron sputtering, with a bottom gate configuration. The ZnO-TFT with undoped ZnO channel layers deposited on 300 nm $Zn_{0.7}Mg_{0.3}O$ gate dielectric layers attains an on/off ratio of $10^4$ and mobility of $20 cm^2 /V s$. The capacitance-voltage C−V characteristics of the ZnO-TFT exhibited a transition from depletion to accumulation with a small hysteresis indicating the presence of oxide traps. The trap density was also computed from the Levinson’s plot. The use of $Zn_{0.7}Mg_{0.3}O$ as a dielectric layer adds additional dimension to its applications. The room temperature processing of the device depicts the possibility of the use of flexible substrates such as polymer substrates. The results provide the realization of transparent electronics for next-generation optoelectronics

Dielectric properties of c-axis oriented Zn1-xMgxO thin films grown by multimagnetron sputtering

Zn1−xMgxO (x = 0.3) thin films have been fabricated on Pt/TiO2/SiO2/Si substrates using multimagnetron sputtering technique. The films with wurtzite structure showed a (002) preferred orientation. Ferroelectricity in Zn1−xMgxO films was established from the temperature dependent dielectric constant and the polarization hysteresis loop. The temperature dependent study of dielectric constant at different frequencies exhibited a dielectric anomaly at 110 °C. The resistivity versus temperature characteristics showed an anomalous increase in the vicinity of the dielectric transition temperature. The Zn1−xMgxO thin films exhibit well-defined polarization hysteresis loop, with a remanent polarization of 0.2 μC/cm2 and coercive field of 8 kV/cm at room temperature

Studies on structural and electrical properties of barium strontium titanate thin films developed by metallo-organic decomposition

Thin films of barium strontium titanate (BST) including BaTiO3 and SrTiO3 end members were deposited using the metallo-organic decomposition (MOD) technique. Processing parameters such as nonstoichiometry, annealing temperature and time, film thickness and doping concentration were correlated with the structural and electrical properties of the films. A random polycrystalline structure was observed for all MOD films under the processing conditions in this study. The microstructures of the films showed multi-grains structure through the film thickness. A dielectric constant of 563 was observed for (Ba0.7Sr0.3)TiO3 films rapid thermal annealed at 750 degrees C for 60 s. The dielectric constant increased with annealing temperature and film thickness, while the dielectric constant could reach the bulk values for thicknesses as thin as similar to 0.3 mu m. Nonstoichiometry and doping in the films resulted in a lowering of the dielectric constant. For near-stoichiometric films, a small dielectric dispersion obeying the Curie-von Schweidler type dielectric response was observed. This behavior may be attributed to the presence of the high density of disordered grain boundaries. All MOD processed films showed trap-distributed space-charge limited conduction (SCLC) behavior with slope of similar to 7.5-10 regardless of the chemistry and processing parameter due to the presence of main boundaries through the film thickness. The grain boundaries masked the effect of donor-doping, so that all films showed distributed-trap SCLC behavior without discrete-traps. Donor-doping could significantly improve the time-dependent dielectric breakdown behavior of BST thin films, mostly likely due to the lower oxygen vacancy concentration resulted from donor-doping. From the results of charge storage density, leakage current and time-dependent dielectric breakdown behavior, BST thin films are found to be promising candidates for 64 and 256Mb ULSI DRAM applications. (C) 1997 Elsevier Science S.A

Low threshold voltage ZnO thin film transistor with a Zn0.7Mg0.3O gate dielectric for transparent electronics

A highly transparent all ZnO thin film transistor (ZnO-TFT) with a transmittance of above 80% in the visible part of the spectrum, was fabricated by direct current magnetron sputtering, with a bottom gate configuration. The ZnO-TFT with undoped ZnO channel layers deposited on 300 nm Zn0.7Mg0.3O gate dielectric layers attains an on/off ratio of 104 and mobility of 20 cm2/V s. The capacitance-voltage (C−V) characteristics of the ZnO-TFT exhibited a transition from depletion to accumulation with a small hysteresis indicating the presence of oxide traps. The trap density was also computed from the Levinson’s plot. The use of Zn0.7Mg0.3O as a dielectric layer adds additional dimension to its applications. The room temperature processing of the device depicts the possibility of the use of flexible substrates such as polymer substrates. The results provide the realization of transparent electronics for next-generation optoelectronics

Dielectric properties of c-axis oriented Zn1−xMgxOZn_{1-x}Mg_xO thin films grown by multimagnetron sputtering

$Zn_{1-x}Mg_xO$ (x=0.3) thin films have been fabricated on $Pt/TiO_2 /SiO_2 /Si$ substrates using multimagnetron sputtering technique. The films with wurtzite structure showed a (002) preferred orientation. Ferroelectricity in $Zn_{1-x}Mg_xO$ films was established from the temperature dependent dielectric constant and the polarization hysteresis loop. The temperature dependent study of dielectric constant at different frequencies exhibited a dielectric anomaly at 110 °C. The resistivity versus temperature characteristics showed an anomalous increase in the vicinity of the dielectric transition temperature. The $Zn_{1-x}Mg_xO$ thin films exhibit well-defined polarization hysteresis loop, with a remanent polarization of $0.2\mu C/cm^2$ and coercive field of 8 kV/cm at room temperature

Antiferroelectric lead zirconate thin films by pulsed laser ablation

Lead Zirconate (PbZrO3) thin films were deposited by pulsed laser ablation method. Pseudocubic (110) oriented in-situ films were grown at low pressure. The field enforced anti-ferroelectric (AFE) to ferroelectric (FE) phase transformation behaviour was investigated by means of a modified Sawyer Tower circuit as well as capacitance versus applied voltage measurements. The maximum polarisation obtained was 36 mu C cm(-2) and the critical field to induce ferroelectric state and to reverse the antiferroelectric slates were 65 and 90 kV cm(-1) respectively. The dielectric properties were investigated as a function of frequency and temperature. The dielectric constant of the AFE lead zirconate thin him was 190 at 100 kHz which is more than the bulk ceramic value (120) with a dissipation factor of less than 0.07. The polarisation switching kinetics of the antiferroelectric PbZrO3 thin films showed that the switching time to be around 275 ns between antipolar state to polar states. (C) 1999 Elsevier Science S.A. All rights reserved