Souri Banerjee

Electron trapping, storing, and emission in nanocrystalline Si dots by capacitance–voltage and conductance–voltage measurements

Temperature and frequency dependent electrical properties of SiO2/nanocrystalline Si (nc-Si)/SiO2 sandwich structures have been studied. A clear shift of the capacitance–voltage and conductance–voltage characteristics toward positive gate voltage suggests electron trapping in an nc-Si dot. The role of interface states and deep traps in our devices has also been thoroughly examined and shown to be unimportant on the overall device performance. The discharging process is found to be logarithmic with time and weakly temperature dependent. The long memory retention time and the logarithmic time dependence of charge loss in the dots are explained by a buildup of opposing electric field in the tunnel oxide, which hinders the discharge of electrons remaining in the dots.

Quantum confinement energy in nanocrystalline silicon dots from high-frequency conductance measurement

Electron charging and discharging processes in floating gate metal–oxide–semiconductor memory based on nanocrystalline silicon (nc-Si) dots were investigated at room temperature using capacitance–voltage and conductance–voltage (G–V) measurements. From charged nc-Si dots, a sequential electron discharging processes was clearly observed in G–V spectroscopy. The fine structure in the observed conductance peaks has been interpreted in terms of the Coulomb blockade and quantum confinement effects of nc-Si dots, which allowed the electron-addition energy to be estimated at 50 meV. Taking the electron-charging energy between the silicon substrate and the floating dot (30 meV) into account, the quantum confinement energy was found to be as significant as the electron charging energy for nc-Si dots, with ∼8 nm in diameter, embedded in silicon oxide.

Conducting-tip atomic force microscopy for injection and probing of localized charges in silicon nanocrystals

A conducting-tip atomic force microscopy (AFM) is utilized to inject localized charges in an ensemble of closely packed nanocrystalline Si dots prepared by plasma decomposition of SiH4. A noncontact-mode topography imaging carried out to probe the charging effect indicates an increase in the apparent height of the Si nanocrystal. A generalized tip-sample force interaction model is also developed to quantitatively evaluate the deposited charge. The study prescribes that the presence of surface charges might result in an overestimation of the actual height of an object measured by AFM, which could be nontrivial for a nanomaterial in particular.

Coulomb-blockade effect observed at room temperature in Ge nanocrystalline films deposited by the cluster-beam evaporation technique

The temperature-dependent current–voltage (I–V) characteristics have been studied across the thickness of Ge nanocrystalline films prepared by the cluster beam evaporation technique. It is found that a film with a thickness of 30 nm, deposited on a substrate kept at 77 K, does not exhibit any distinct step-like feature in the I–V characteristics at room temperature. However, with the lowering of the temperature, a “Coulomb gap” is observed and a pronounced step-like feature appear in the I–V characteristics suggesting the Coulomb blockade (CB) effect. It is hypothesized that the observed CB effect in these nanocrystalline thin films results from a very highly selective electron transport process where the electron transport is dominated by one local well-defined current path with the largest conductance. The result of similar measurements on a photo-oxidized sample, which shows a signature of a step-like feature in the I–V characteristics, even at room temperature, supports this hypothesis.

Non-ohmic hopping conduction in Ge nanocrystalline film

Abstract Temperature-dependent transport properties of Ge nanocrystalline films (nanofilms) prepared by the cluster beam evaporation technique have been studied. The nanofilms of thicknesses about 20 nm , deposited on the substrates at room temperature, exhibit non-linear current–voltage characteristics in the low bias range with decreasing temperature. In order to understand the conduction via the grain boundaries between two adjacent Ge nanocrystals, the temperature-dependent conductivity of the nanofilms has also been investigated, which could be explained by Motts variable range hopping mechanism between 100 and 300 K . Below 100 K , the conductivity is limited by ordinary tunneling of carriers giving rise to temperature-independent behavior.

Operation of nanocrystalline-silicon-based few-electron memory devices in the light of electron storage, ejection, and lifetime characteristics

A metal-oxide-semiconductor field-effect transistor memory device using nanocrystalline Si (nc-Si) dots as a floating gate over a short and narrow channel has been fabricated. Its operation at 77 K presents experimental evidence of storing and ejection of electrons associated with the nc-Si dot in the active area of the device. Though the lifetime of a single electron is apparently longer than the case when it is associated with another electron in the same nc-Si dot, a distribution in lifetime has been generally observed for the stored electrons in the nc-Si dots with the present memory devices.

C-V and G-V Measurements Showing Single Electron Trapping in Nanocrystalline Silicon Dot Embedded in MOS Memory Structure

We prepared a SiO2/nanocrystalline Si (nc-Si)/SiO2 sandwich structure. A clear positive shift in C-V and G-V curves due to electrons trapped in nc-Si dots has been observed at room temperature. The peak in conductance around flat band condition indicates that a trap event had occurred where an electron is stored per nc-Si dot. A logarithmic charge loss function is found and this discharging process is independent of the thermal activation mechanism. The longer memory retention time and logarithmic charge loss in the dots are explained by a “built-in” electric field through the tunnel oxide, which varies with time, resulting in a variable tunneling probability. The electric repulsion induced by the built-in electric field hinders the discharging of electrons remained in the dots.

A photo-oxidation generated low- k dielectric film deposited by reactive evaporation of SiO

A non-stoichiometric silicon oxide film has been deposited by evaporating SiO as a source material in Ar and O2 mixed gas. The film is composed of SiO and SiO2, and has a porous structure. The SiO2 results from some part of SiO reacting with O2 and its amount depends on the pressure in the chamber. The residual SiO in the film can be photo-oxidized into SiO2 by ultraviolet radiation with a Hg lamp. The dielectric constant of the film after photo-oxidation is ∼ 1.89±0.04 (at frequency of 1 MHz), which shows that this porous structure film is promising for potential application as a low-k dielectric.

Electron transport in Ge nanocrystalline films deposited using the cluster beam evaporation technique

The carrier transport properties across Ge nanocrystalline films (hereafter referred to as nanofilms) deposited by the cluster beam evaporation technique have been thoroughly studied. A thin nanofilm deposited at liquid nitrogen substrate temperature (Ge-LNT) exhibits the Coulomb blockade characteristic at low temperatures, while that deposited at room temperature (Ge-RT) does not show any evidence of single electron tunneling. This is explained by the difference in the nature of the overall electron transport due to a structural difference between the two types of nanofilms. The electron transport between two adjacent nanocrystals in both types of nanofilms was also studied in detail. It was found that, under relatively low electric fields, a Ge-LNT nanofilm exhibited T−1/2 conductivity dependence, which could be explained by the variable range hopping model of Efros and Shklovskii [A. L. Efros and B. I. Shklovskii, J. Phys. C 8, L49 (1975)], while in the Ge-RT nanofilms, the conductivity-temperature dep...

Investigation of localization of DNA molecules using triangular metal electrodes with varying separation

In this paper we investigate the effect of separation of triangular metal electrodes with both convex and concave geometries, on the localization of suspended DNA molecules under the combined effect of dielectrophoresis and AC electro-osmosis through simulations using COMSOL Multiphysics. Trapping points are realized within the electrodes which are found to vary with the separation of the electrodes.