Browsing by Author "Sarkar, Niladri"

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Application of a self-consistent NEGF procedure to study the coherent transport with phase breaking scattering in low dimensional systems
(AIP, 2016-04) Sarkar, Niladri
We have studied Quantum Transport with dephasing in Low Dimensional systems. Here, we apply a self-consistent NEGF procedure to study the transport mechanism in low-dimensional systems with phase breaking scatterers. Under this we have determined the transmission coefficient of a very small Multi-Moded Nanowire which is under a small bias potential of few meV. We have calculated the transmission of this device first with no scatterers. Then we have introduced scatterers in the device and calculated the transmission for the device.
Application of the Density Matrix Formalism for Obtaining the Channel Density of a Dual Gate Nanoscale Ultra-Thin MOSFET and its Comparison with the Semi-Classical Approach
(World Scientific, 2020) Sarkar, Niladri
Density Matrix Formalism using quantum methods has been used for determining the channel density of dual gate ultra-thin MOSFETs. The results obtained from the quantum methods have been compared with the semi-classical methods. This paper discusses in detail the simulation methods using self-consistent schemes and the discretization procedures for constructing the Hamiltonian Matrix for a dual gate MOSFET consisting of oxide/semiconductor/oxide interface and the self-consistent procedure involving the discretization of Poisson’s equation for satisfying the charge neutrality condition in the channel of different thicknesses. Under quantum methods, the channel densities are determined from the diagonal elements of the density matrix. This successfully simulates the size quantization effect for thin channels. For semi-classical methods, the Fermi–Dirac Integral function is used for the determination of the channel density. For thin channels, the channel density strongly varies with the values of the effective masses. This variation is simulated when we use Quantum methods. The channel density also varies with the asymmetric gate bias and this variation is more for thicker channels where the electrons get accumulated near the oxide/semiconductor interface. All the calculations are performed at room temperature (300K).
Application of the self-consistent quantum method for simulating the size quantization effect in the channel of a nano-scale dual gate MOSFET
(AIP, 2015-06) Sarkar, Niladri
Self-Consistent Quantum Method using Schrodinger-Poisson equations have been used for determining the Channel electron density of Nano-Scale MOSFETs for 6nm and 9nm thick channels. The 6nm thick MOSFET show the peak of the electron density at the middle where as the 9nm thick MOSFET shows the accumulation of the electrons at the oxide/semiconductor interface. The electron density in the channel is obtained from the diagonal elements of the density matrix; [ρ]=[1/(1+exp(β(H − μ)))] A Tridiagonal Hamiltonian Matrix [H] is constructed for the oxide/channel/oxide 1D structure for the dual gate MOSFET. This structure is discretized and Finite-Difference method is used for constructing the matrix equation. The comparison of these results which are obtained by Quantum methods are done with Semi-Classical methods.
Development of PIC-FDTD code for beam-wave interaction study in ‘PASOTRON’
(IEEE, 2015-01) Sarkar, Niladri
Plasma-assisted high power microwave source `PASOTRON' is being developed at a few places internationally to utilize plasma channel transport of the beam through Slow Wave Structure (SWS) to significantly reduce the size and weight in conventional linear high power microwave (HPM) sources by eliminating the need for the applied axial magnetic field [1-2]. In this device, very strong non-linear interaction between electron beam and electromagnetic wave can occur, thus making analytical design very cumbersome. Consequently, a particle simulation code is very much required to make its design simpler. We have made an efforts to study beam wave interaction in plasma filled SWS, which is a backward wave oscillator (BWO). The aim is to develop a particle-in-cell finite-difference-time-domain (PIC-FDTD) code [3] using MATLAB for the simulation of beam-wave interaction in the PASOTRON. The updating equations for electromagnetic fields are formulated using FDTD in cylindrical coordinate system since the SWS geometry is axially symmetric. In order to examine the field configuration in 3D, a field solver is implemented using the BOR-FDTD (Body of revolution FDTD) [4]. The results are being compared with MAGIC [5] tool software to validate the analysis. The results of this analysis will be presented.
Effect of Line Defects on the Band Structures, Local Density of States, and the Landau Levels for Armchair Graphene Nanoribbons in the Quantum Hall Effect Regime
(Springer, 2023-11) Sarkar, Niladri
The effects of one and two line defects are investigated with respect to the band structures and local density of states (LDOS) in armchair graphene nanoribbons (AGNRs) under the quantum Hall effect (QHE) regime. The E–k diagrams for these systems with multiple line defects are compared with those of pristine systems. The Landau levels and the edge states are affected as the number of line defects increases, corroborated by the reduction in the transmission function. This is also reflected in the change observed in the local density of states (LDOS) and the Landau levels as the number of line defects increases. This work offers a strategy for controlling the magnetoresistance of AGNRs with intentionally invoked line defects for device applications.
The effect of quantum confinement and the role of electron-phonon interaction on the band gap shrinkage of some II-VI semiconductors
(Elsevier, 2024-11) Sarkar, Niladri
The role of electron-phonon interaction in band gap shrinkage for some II-VI bulk and low-dimensional semiconductors is investigated in this work. The variation of the energy band gap is studied as a function of temperature using Varshni's, Vina's, and Passler's relations. It is observed that the change in the energy band gap is affected due to the quantum confinement as the dimensionality of these semiconductors is decreased.
Effect of size quantization and quantum capacitance on the threshold voltage of a 2D nanoscale dual gate MOSFET
(IOP, 2020-09) Sarkar, Niladri
The size quantization effect in the channel of a 2D nanoscale MOSFET is studied using a self-consistent quantum method. Under this, Schrodinger-Poisson equations are solved for determining the electron density for 2D device channels from 3 nm × 3 nm to 100 nm × 100 nm. The lower dimension channels show a peak of the electron density at the middle whereas higher dimension channels show the accumulation of the electrons at the oxide/semiconductor interface. Also, the role of quantum capacitance on the threshold voltages of these nanoscale devices is investigated as a function of channel dimensions and electron effective masses. It is observed that not only the size but the electron effective masses dominate the conductivity of the channel for such nanoscale devices. Here, the channel electron densities are obtained using density matrix formalism. A block diagonal Hamiltonian Matrix [H] is constructed for this oxide/channel/oxide 2D structure and the channel is discretized by using the finite-difference method. This analysis is important for understanding the physics of the size quantization and its effect on the threshold voltage.
Efficiency Enhancement of Multi-Stage Depressed Collector Using an External Magnet in TWTs
(IEEE, 2022-11) Sarkar, Niladri
The dependence of overall traveling-wave tube (TWT) efficiency on the collector efficiency is of paramount importance. The objective of achieving high overall efficiency drives us to design a multi-stage depressed collector (MDC) with the theoretically highest possible efficiency with appropriate depressed potentials. Various design parameters, namely electrode geometry, depressed potentials, material parameters, etc., can be optimized to attain the desired theoretical maximum collector efficiency. But in a practical situation, the measured MDC efficiency is relatively less than the designed efficiency, which may be attributed to approximations involved in the nonlinear interaction of the electron beam and RF wave, fabrication inaccuracies, process limitations, simulation approximations, etc. This inexplicable practical decrease in MDC efficiency is typically accompanied by an increase in the backstreaming of electrons into the interaction structure. To compensate for this decrease, a permanent magnet has been suitably positioned on the base plate of the MDC, which results in an increase in collector efficiency. Simulation studies have been carried out to optimize the parameters of the magnet in terms of the geometrical dimensions, position, and magnetic field value, and has been validated experimentally. This technique of collector efficiency enhancement using an external magnet has proven to be very effective, contributing to a significant collector efficiency enhancement of > 5% from ~78% to ~83%, which in turn corresponds to an overall tube efficiency improvement of ~5.8% from 50.5% to 56.3%.
Exploiting hyperbolic metamaterial as a substrate for graphene surface plasmonic Cherenkov THz radiation source
(Springer, 2020-10) Sarkar, Niladri
In this work, a graphene-dielectric multilayer hyperbolic metamaterial (GHMM) has been analyzed for the generation of THz Cherenkov radiation (CR) from graphene surface plasmonics (GSP), induced by a moving electron bunch. The structure under analysis consists of a graphene-dielectric multilayer HMM in which the top graphene layer is considered as an interface between air and the underlying multilayer as the substrate. The dispersion analysis of the structure shows the existence of bulk-HMM (BH) and non-bulk HMM (NBH)-region in the – space. The structure is found to generate intense CR from the GSP, when the operating point of the electron bunch lies in the BH-region, as the bulk states of the HMM substrate allows the efficient transformation of high-k GSP into THz-CR. The proposed structure can be exploited as a potential candidate for the development of low-voltage, electron beam-driven, ultra-tunable, compact graphene plasmonic Cherenkov THz radiation source. Moreover, the application of the principle of utilizing bulk states of HMM for CR generation has also been discussed for black phosphorous (BP), an emerging 2Dmaterial for the THz to infrared (IR) region.
Grain-boundary-controlled current transport in copper phthalocyanine
(AIP, 2006-04) Sarkar, Niladri
Anomalous temperature dependence of resistivity at low temperature is observed in copper-phthallocyanine thin film. A model based on grain-boundary-controlled transport has been developed for the explanation of the observed anomaly. The prediction is based on the assumption that the thin film beyond a certain thickness is mainly polycrystalline, consisting of grains. The transport is expected to be limited by potential barriers at grain boundaries.
Investigation of the effect of scattering centers on low dimensional nanowire channel
(AIP, 2018-05) Sarkar, Niladri
In this work, we studied the effect of scattering centers on the electron density profiles of a one dimensional Nanowire channel. Density Matrix Formalism is used for calculating the local electron densities at room temperature. Various scattering centers have been simulated in the channel. The nearest neighbor tight binding method is applied to construct the Hamiltonian of nanoscale devices. We invoke scattering centers by adding local scattering potentials to the Hamiltonian. This analysis could give an insight into the understanding and utilization of defects for device engineering.
Investigation of the role of defects on channel density profiles and their effect on the output characteristics of a nanowire FET
(IOP, 2021-12) Sarkar, Niladri
This work investigates the effect of defects on the electron density profiles of nanowire FETs with a rectangular cross-section. It also presents a framework for the discretization of the nanowire channels with defects. A self-consistent procedure using Schrodinger-Poisson solver with density matrix formalism calculates the local electron density profiles. The local electron density decreases due to defect-induced scattering potentials. The electron density profiles vary according to the nature of the intrinsic defects. The effect of defect-induced potentials on the output characteristics of the nanowire FET device is studied using the non-equilibrium Green's function (NEGF) methodology. An increase in scattering potential in the nanowire channel causes a considerable decrease in the saturation voltage and current. This results in a faster saturation which changes the overall device performance. Hence, defect-controlled channels can be utilized to fabricate FETs with desired characteristics.
An investigation of the role of line defects on the transport properties of armchair graphene nanoribbons
(Springer, 2022-04) Sarkar, Niladri
Graphene nanoribbons (GNRs) are narrow strips of graphene which show interesting electronic properties. The occurrence of defects (vacancy, line, impurity) in GNR can alter these properties. This work investigates the role of line defects on transmission probability for AGNR (armchair) for Ny = 3n, Ny = 3n + 1, and Ny = 3n + 2 systems. It is observed that the bandgap of these AGNRs can be tailored by inducing one and multiple line defects. The effect of defect position of the line defects in the above-mentioned AGNR systems is studied within a framework of a non-interacting tight-binding approach. The energy band diagrams (E–k diagrams) for these systems with one and multiple line defects are plotted and analyzed. Also, the two-terminal electron transport of these AGNRs with one and multiple line defects is obtained by using the non-equilibrium Green’s function (NEGF) methodology. This work opens up the possibility of controlling the bandgaps of AGNRs through multiple line defects for device applications.
Photoinduced Suppression of Superconductivity in Bi2Sr2Ca2Cu3O10+δ
(ARXIV, 2006-04) Sarkar, Niladri
Superconductivity in high temperature superconductors is achieved by introducing charge carriers into cuprate insulators containing CuO2 planes. Perturbation in these CuO2 planes suppresses superconductivity by strongly affecting the electronic environment. Here we have use photoinduced electronic modification of Cu ion in CuO2 plane to investigate the effect of local perturbation in Bi2Sr2Ca2Cu3O10+δ. This method has been used to suppress superconducting transition temperature Tc. We show that our results on photoinduced suppression of superconductivity are consistent with a scenario based on pinning of fluctuating stripes in Bi2Sr2Ca2Cu3O10+δ.
Photoluminescence spectroscopy of many-body effects in heavily doped AlxGa1−xAs
(APS, 2005-06) Sarkar, Niladri
We present an experimental investigation of heavy doping-induced many-body effects such as band gap narrowing and Fermi-edge singularity (FES) in AlxGa1−xAs using photoluminescence (PL) spectroscopy. The band-to-band transition energy shifts to lower energies and the FES feature in PL spectra grows with increasing electron concentration. We show that the FES feature is a nonmonotonic function of temperature and excitation intensity. Our data lead us to suggest that a small concentration of nonequilibrated holes is required to enhance the FES feature in the PL spectra.
PIC-FDTD code for beam-wave interaction analysis in rippled wall slow wave structure
(IEEE, 2015) Sarkar, Niladri
Summary form only given. A plasma-assisted high power microwave (HPM) source `PASOTRON' is being developed at CSIR-CEERI, where a rippled wall waveguide is used as slow wave structure (SWS) 1 . In this device, very strong non-linear interaction between electron beam and electromagnetic wave can occur, making analytical design very cumbersome. So, a particle simulation code is highly required to make its design simpler. In this work an effort has been made and a particle-in-cell finite-difference-time-domain (PIC-FDTD) code is developed. In designing the field solver for this code, the FDTD algorithm2 is taken up as the method of choice as it allows having frequency domain analysis by applying a fast Fourier transform. The time component in the FDTD code allows the particle wave interaction to be solved self-consistently wherein the Yee's grid2 scheme for the field arrangement ensures that the divergence free nature of the electromagnetic waves is maintained even after the curl equations are discretized. The charge densities due to the particles are interpolated to the grids using the PIC method3. The particle push is implemented using the Boris push method which takes care of the translational motion due to the electric field as well as the rotational motion due to the magnetic field. The current due to particle movement is calculated on the grid using a charge conservation scheme. The phase space patterns produced by the code are then compared with the published results and with the MAGIC simulation software. The axial phase space plots are found to be similar to those which are published elsewhere4 and also to those which are obtained using MAGIC software tool. The results of these efforts will be presented.
Practical Thermal and Structural Simulations for Performance Improvement of Electron Gun in TWTs
(IEEE, 2022-12) Sarkar, Niladri
Low-perveance electron guns are highly desired in high-efficiency space TWTs mainly due to the low operating beam current. In a low-perveance electron gun, the distance between the cathode and beam-forming electrode (BFE) plays a critical role in achieving laminar electron beam flow with the required specifications. It is an extremely sensitive parameter that gets affected by minor variations in the dimensions occurring due to thermal expansion. Hence, thermal and structural simulations are carried out to back-calculate the distance at which the cathode must be placed from the BFE during fabrication under cold conditions. In this article, the authors have presented the development of two electron guns–one developed based on the ideal simulation results and for the other one, the practical brazing conditions were considered in the simulations. Both the guns were integrated with TWTs, and their performances were evaluated under the same operating voltages and magnetic focusing conditions. It has been observed that the electron gun fabricated based on practical simulation conditions performed well with a low helix interception current. In contrast, the other gun had poor performance with a high helix interception current. Hence, it experimentally validates the fact that simulation considering the practical brazing scenario has resulted in precise back-computation of the cathode-BFE distance and hence better beam transmission characteristics of the tube.
Purpose-led Publishing logo. Effect of disorder on the electrical and superconducting properties in Ln1.2Ba1.2Ca0.6Cu3O7+δ (Ln = La, Nd, Sm) and La1.2−xNdxBa1.2Ca0.6Cu3O7+δ
(IOP, 2008-05) Sarkar, Niladri
The effect of disorder on the suppression of superconductivity has been studied in two series of high temperature superconducting oxides of the type Ln1.2Ba1.2Ca0.6Cu3O7+δ (Ln = La, Nd, Sm) and La1.2−xNdxBa1.2Ca0.6Cu3O7+δ (x = 0.3, 0.6, 0.9). These oxides crystallize in the tetragonal structure related to the 1113-type superconductors. It has been shown that considerable cation disorder exists at the Ln and Ba crystallographic sites in these oxides. Furthermore, it has been shown that Ln and Ca ions occupy both the 1d- and the 2h-crystallographic sites whereas Ba ions only occupy the 2h-crystallographic site. La1.2Ba1.2Ca0.6Cu3O7+δ shows a superconducting transition with Tc of 65 K while the Sm analog shows semiconducting behavior. A broad metal–semiconductor transition is observed in Nd1.2Ba1.2Ca0.6Cu3O7+δ. It has also been shown that the gradual suppression of Tc in La1.2−xNdxBa1.2Ca0.6Cu3O7+δ, as x increases, is related to disorder which could be related to the variance (σ2) of the average ionic radii of the ions at the 2h site.
Purpose-led Publishing logo. Zinc oxalate nanorods: a convenient precursor to uniform nanoparticles of ZnO
(IOP, 2006-02) Sarkar, Niladri
Nanorods of zinc oxalate dihydrate have been synthesized using the reverse micellar route. These nanorods were decomposed at 450 °C in air to obtain nanoparticles of zinc oxide. Transmission electron microscopy shows the nanorods to be 120 nm in diameter and 600 nm in length. The ZnO nanoparticles are 55 nm in diameter. The photoluminescence studies show two peaks at 370 and 403 nm which can be ascribed to free excitonic transition and donor–acceptor pair transition respectively. The temperature dependent PL intensity shows an anomalous non-monotonous temperature dependence probably due to two different optical processes.
The role of the grain boundary on persistent photoconductivity in GaN
(IOP, 2003-10) Sarkar, Niladri
We present an experimental investigation on temperature, excitation intensity and spectral dependence of persistent photoconductivity (PPC) in GaN. A grain boundary induced potential barrier is predicted to be responsible for PPC. The non-exponential nature of the PPC decay has been explained by a Gaussian distribution of capture barriers, arising from the trapped charges at the grain boundary interface. The spectral dependence of the PPC suggests the origin of PPC and the yellow luminescence band may arise from the same intrinsic defect.