Amit K. Chakraborty

Leakage current and charge trapping behavior in TiO2/SiO2 high-kappa gate dielectric stack on 4H-SIC substrate

The TiO2∕SiO2 gate dielectric stack on 4H‐SiC substrate has been studied as a high-κ gate dielectric for metal-oxide semiconductor devices. X-ray photoelectron spectroscopy confirmed the formation of stoichiometric TiO2 films. The leakage current through the stack layer was investigated and it has been shown to be a double conduction mechanism. At low fields, the current is governed by properties of the interfacial layer with a hopping like conduction mechanism, while at relatively high electric field, carriers are modulated by a trap assisted tunneling mechanism through traps located below the conduction band of TiO2. The current-voltage characteristics, time evolution of charge transport, and capacitance-voltage behaviors under constant voltage stressing suggest the composite effect of electron trapping and positive charge generation in the dielectric stack layer.

Energy-band alignment of HfO2∕SiO2∕SiC gate dielectric stack

The band alignment of HfO2∕SiO2∕SiC gate dielectric stack has been investigated by x-ray photoelectron spectroscopy and electrical characterization. Two types of valence band offsets are observed in the stack layer; the smaller value of 1.5eV corresponds to the HfO2∕SiC band offset while the larger one of 2.2eV is due to the interfacial SiO2∕SiC. The barrier height is extracted to be 1.5eV from the Schottky emission characteristics and is higher than the reported value for HfO2 on SiC without interfacial SiO2. Thus, presence of an interfacial SiO2 layer increases band offsets to reduce the leakage current characteristics.

Characterization of thermally oxidized Ti/SiO2 gate dielectric stacks on 4H-SiC substrate

The structural and electrical characteristics of thermally oxidized Ti∕SiO2 gate dielectric stacks on 4H–SiC substrates have been investigated. X-ray photoelectron spectroscopy shows a good stoichiometry of TiO2 films formed by thermal oxidation of evaporated Ti. No evidence of the formation of titanium silicide at the surface as well as in the interfacial layer was observed. Electrical measurements show, in particular, no signature of an increase in interface state density towards the conduction band edge of 4H–SiC. The improved leakage current with higher breakdown field of 11MV∕cm makes TiO2∕SiO2 stacks a potential gate insulator for high-power SiC devices.

The electronic fine structure of 4-nitrophenyl functionalized single-walled carbon nanotubes.

Controlling the electronic structure of carbon nanotubes (CNTs) is of great importance to various CNT based applications. Herein the electronic fine structure of single-walled carbon nanotube films modified with 4-nitrophenyl groups, produced following reaction with 4-nitrobenzenediazonium tetrafluoroborate, was investigated for the first time. Various techniques such as x-ray and ultra-violet photoelectron spectroscopy, and near edge x-ray absorption fine structure studies were used to explore the electronic structure, and the results were compared with the measured electrical resistances. A reduction in number of the pi electronic states in the valence band consistent with the increased resistance of the functionalized nanotube films was observed.

TiO2/GeOxNy stacked gate dielectrics for Ge-MOSFETs

In this work, we present the results of physical and electrical characterization of Ti-based high-k gate dielectrics on Ge substrates. Titanium tetrakis iso- propoxide (TTIP) was used as the organometallic source for the deposition of ultra-thin TiO2 films on p-Ge (1 0 0) at low temperature (<200 °C) by plasma enhanced chemical vapor deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of ~65 Pa. The presence of an ultra-thin lossy GeO2 interfacial layer between the deposited high-k film and the substrate, results in frequency-dependent capacitance–voltage (C–V) characteristics in strong accumulation and a high interface state density (~1013 cm−2 eV−1). To improve the electrical properties, nitrogen engineering has been employed to convert the lossy GeO2 interfacial layer to its oxynitride, thus forming TiO2/GeOxNy/Ge stacked-gate structure with improved interface/electrical properties. Different N sources, such as NO, NH3 and NO/NH3, have been used for nitrogen engineering. XPS and Raman spectroscopy analyses have been used for surface morphological study. Electrical properties, such as gate leakage current density, interface state density, charge trapping, flatband voltage shift, etc, have been studied in detail for TiO2/GeOxNy/Ge MIS capacitors using the current–voltage (I–V), capacitance–voltage (C–V), conductance–voltage (G–V) and stress (both constant voltage and current) measurements. Although a significant improvement in electrical characteristics has been observed after nitridation in general, the formation of the interfacial GeOxNy layer, obtained from NO-plasma nitridation, is found to provide the maximum improvement among all the nitridation techniques used in this study. It is shown that the insertion of an ultra-thin oxynitride (GeOxNy) interfacial layer is advantageous for producing gate-quality TiO2 high-k dielectric stacks on Ge substrates.

Effects of interface engineering for HfO2 gate dielectric stack on 4H-SiC

HfO2 films were grown on SiO2∕4H-SiC and SiON∕4H-SiC layers by deposition of metallic Hf in an electron beam evaporation system followed by thermal oxidation. X-ray photoelectron spectroscopy confirmed the formation of stoichiometric HfO2 films. There is no evidence of formation of hafnium silicide or carbon pileup at the surface as well as at the interfacial layer. Electrical measurements show the presence of fewer trapped charges in the HfO2∕SiON gate dielectric stack compared to HfO2∕SiO2 stack with a comparable interface state density. The HfO2∕SiON stack layer improves leakage current characteristics with a higher breakdown field and has smaller flatband voltage shift under electrical stress, indicating improved reliability.

Temperature dependent shape transformation of Ge nanostructures by the vapor-liquid-solid method

A vapor-liquid-solid method has been used to study the temperature dependent growth mechanism of Ge nanostructures on Au-coated Si (100) substrates. The formation of Ge nanodots, nanorods, and nanowires has been observed at different growth temperatures. The diameter of grown nanowires is found to be varying from 40 to 80 nm and that of nanorods from 70 to 90 nm, respectively. A comparative study has been done on three types of samples using x-ray diffraction and Raman spectroscopy. Photoluminescence spectra of grown nanostructures exhibit a broad emission band around 2.6 eV due to oxide related defect states.

High yield synthesis of amine functionalized graphene oxide and its surface properties

Graphene and its derivatives have attracted great research interest due to their many exciting properties leading to a number of potential applications. However, for many practical applications including reinforcement in epoxy resin matrices, chemical functionalization of graphene is often a necessary requirement. Herein we report a simple temperature-assisted reflux method to synthesize graphene oxide (GO) functionalized with n-butylamine in high yield without the use of toxic chemicals. X-ray photoelectron spectroscopy and energy dispersive analysis by X-rays showed successful attachment of amine groups onto GO to form (GO-ButA). The functionalized GO was further characterized using Raman, nuclear magnetic resonance and Fourier transform infrared spectroscopies. The surface morphology and particle size, etc. were characterized using scanning electron microscopy. The solution properties of the GO-ButA were investigated by dispersing in water and common organic solvents which indicated an increased hydrophobic nature of the product which was also confirmed by contact angle measurements. Further, the interaction of GO-ButA with epoxy resin was tested by dispersing it in an epoxy resin which indicated an improved and more stable dispersion as compared to that of GO which shows the potential application of GO-ButA as a reinforcement (filler) in epoxy based composites.

A New Route to the Production and Nanoscale Patterning of Highly Smooth, Ultrathin Zirconium Oxide Films

Metal-stabilized bilayers, prepared by the self-assembly of octadecyltrichorosilane on an oxidized silicon surface followed by the Langmuir-Blodgett deposition of a monolayer of octadecylphosphonic acid, have been used to generate 1.6 nanometer thick, highly uniform, zirconium oxide films following annealing. Patterning of the thin films on the nanometre scale was achieved using nanodisplacement methodology, by careful control of an atomic force microscope (AFM) probe, which allowed the selective removal of the upper leaflet of the bilayer.

Rapid thermal oxidation of Ge-rich Si1−xGex heterolayers

Rapid thermal oxidation (RTO) of the Ge-rich (x=0.7)Si1−xGex heterolayer is reported. In particular, the structural modifications of SiGe films during oxidation process and the dependence of the oxidation kinetics on Ge content, oxidation temperature, and oxide thickness have been studied. The segregation mechanism of Ge at the oxide∕SiGe interface is discussed. Interface properties of the RTO-grown oxides studied using high-frequency capacitance-voltage (C-V) characteristics of metal-oxide-semiconductor capacitors are also reported.