Dynamics of a nanowire superlattice in an ac electric field

With a one-band envelope function theory, we investigate the dynamics of a finite nanowire superlattice driven by an ac electric field by solving numerically the time-dependent Schroedinger equation. We find that for an ac electric field resonant with two energy levels located in two different minibands, the coherent dynamics in nanowire superlattices is much more complex as compared to the standard two-level description. Depending on the energy levels involved in the transitions, the coherent oscillations exhibit different patterns. A signature of barrier-well inversion phenomenon in nanowire superlattices is also obtained.Comment: 14 pages, 4 figure

Effects of the electrostatic environment on superlattice Majorana nanowires

Finding ways of creating, measuring, and manipulating Majorana bound states (MBSs) in superconducting-semiconducting nanowires is a highly pursued goal in condensed matter physics. It was recently proposed that a periodic covering of the semiconducting nanowire with superconductor fingers would allow both gating and tuning the system into a topological phase while leaving room for a local detection of the MBS wave function. We perform a detailed, self-consistent numerical study of a three-dimensional (3D) model for a finite-length nanowire with a superconductor superlattice including the effect of the surrounding electrostatic environment, and taking into account the surface charge created at the semiconductor surface. We consider different experimental scenarios where the superlattice is on top or at the bottom of the nanowire with respect to a back gate. The analysis of the 3D electrostatic profile, the charge density, the low-energy spectrum, and the formation of MBSs reveals a rich phenomenology that depends on the nanowire parameters as well as on the superlattice dimensions and the external back-gate potential. The 3D environment turns out to be essential to correctly capture and understand the phase diagram of the system and the parameter regions where topological superconductivity is establishedWe thank E. J. H. Lee, H. Beidenkopf, E. G. Michel, N. Avraham, H. Shtrikman, and J. Nygård for valuable discussions. Research supported by the Spanish MINECO through Grants No. FIS2016-80434-P, No. BES-2017-080374, and No. FIS2017-84860-R (AEI/FEDER, EU), the European Union's Horizon 2020 research and innovation programme under the FETOPEN Grant Agreement No. 828948 and Grant Agreement LEGOTOP No. 788715, the Ramón y Cajal programme RYC-2011-09345, the María de Maeztu Programme for Units of Excellence in R&D (MDM-2014-0377), the DFG (CRC/Transregio 183, EI 519/7- 1), the Israel Science Foundation (ISF), and the Binational Science Foundation (BSF

kp Theory of Semiconductor Nanostructures

The objective of this project was to extend fundamentally the current kp theory by applying the Burt-Foreman formalism, rather than the conventional Luttinger-Kohn formalism, to a number of novel nanostructure geometries. The theory itself was extended in two ways. First in the application of the Burt-Foreman theory to computing the momentum matrix elements. Second in the development of a new formulation of the multiband kp Hamiltonian describing cylindrical quantum dots. A number of new and interesting results have been obtained. The computational implementation using the finite difference method of the Burt-Foreman theory for two dimensional nanostructures has confirmed that a non-uniform grid is much more efficient, as had been obtained by others in one dimensional nanostructures. In addition we have demonstrated that the multiband problem can be very effectively and efficiently solved with commercial software (FEMLAB). Two of the most important physical results obtained and discussed in the dissertation are the following. One is the first ab initio demonstration of possible electron localization in a nanowire superlattice in a barrier material, using a full numerical solution to the one band kp equation. The second is the demonstration of the exactness of the Sercel-Vahala transformation for cylindrical wurtzite nanostructures. Comparison of the subsequent calculations to experimental data on CdSe nanorods revealed the important role of the linear spin splitting term in the wurtzite valence band

Fabrication, Characterization, Modeling and Testing of a Nanostructured Bulk Thermoelectric Cooler

New generation micro/nano devices are emerging to monitor, control and act on living systems. Particularly, in the field of cryobiology, there is a need to monitor and control temperature at the cellular level. An important step towards achieving this aim is to fabricate a novel bulk nanostructured thermoelectric cooler (TEC). As a first step towards achieving efficient localized control of temperature in biological systems, Bismuth-telluride (Bi2Te3) and Antimony-Telluride (Sb2Te3) arrays of nanowires and nanotubes were fabricated, characterized and modeled. A thermal conductivity model originally developed by Dames and Chen for superlattice nanowires was extended to nanotubes. Based on this model thermal conductivity of Bi2Te3 and Sb2Te3 nanowire or nanotube is determined. Lumped parameter model was also used to determine the performance of a device composed of nanowires or nanotubes. The modeling results suggest that nanotubes would yield higher reduction in thermal conductivity compared to nanowires. Bi2Te3 and Sb2Te3 arrays of nanowires and nanotubes were electrodeposited into the nanochannels of the polycarbonate template as n-type and p-type thermoelectric leg elements of the bulk thermoelectric cooler, respectively. SEM, XRD and WDS were employed to characterize the fabricated Bi2Te3 and Sb2Te3 nanowire or nanotube arrays. A custom built device is developed to characterize the Seebeck coefficient of the electrodeposited nanowires or nanotubes. The Seebeck coefficient values of Sb2Te3 nanowire and nanotube arrays were found to be +359 µV K-1 and +332 µV K-1, respectively. The positive Seebeck coefficient values indicated that electrodeposited Sb2Te3 nanowires and nanotubes were p-type. The Seebeck coefficient values of Bi2Te3 nanowire and nanotube arrays were found to be -118 µV K-1 and -143 µV K-1, respectively. The negative Seebeck coefficient values indicated that electrodeposited Bi2Te3 nanowire and nanotube arrays were n-type. The electrical resistance measurements confirmed that Bi2Te3 and Sb2Te3 nanowire or nanotube arrays resistance were semiconductors. A bulk nanostructured TEC is assembled using the best Bi2Te3 (n-type) and Sb2Te3 (p-type) nanowire or nanotube arrays. The ZT of the thus assembled device is determined by “Harmans Technique”. It is found that a combination of Bi2Te3 nanowires and Sb2Te3 nanotubes yielded highest ZT of around 0.4 at room temperature. Results suggest that there is clearly a need to significantly improve the performance of the nanostructured bulk TEC to compete with commercially available vapor compression coolers

ELECTROSTATIC AND ELECTRICAL TRANSPORT ANALYSIS OF NANOMATERIALS AND NUMERICAL METHODS DEVELOPMENT

The nanotechnology today is continuously boosting the application of nanostructured materials in the development and innovation of electronic devices, such as Nano &ndash Electromechanical Systems (NEMS), electrical transistors, thermoelectric devices, and solar cells. Due to the size miniaturization, quantum mechanical effects play important roles in the performance of such devices. To correctly capture the quantum mechanical effects and understand how these effects influence the electrostatic and electrical transport properties of nanomaterials, efficient and accurate computational models are highly desirable. Currently, the commonly used model for electrostatic analysis of nanoscale devices is based on self &ndash consistent solution of the effective &ndash mass Schroedinger equation coupled with the Poisson equation. However, a major drawback of this model is its inefficiency to simulate systems with large Degrees of Freedom (DOFs). To reduce the computational cost, in this thesis, two Component Mode Synthesis (CMS) approaches, namely the fixed &ndash interface CMS and the free &ndash interface CMS, are incorporated into the Schroedinger &ndash Poisson model to speed up the electrostatic analysis in nanostructures. The new model is employed to analyze the quantum electrostatics in both nanowires and FinFETs. Numerical results demonstrate the superior computational performance in terms of efficiency and accuracy. In addition to the electrostatic analysis, carrier transport in nanostructures with perturbation from quantum effects also merits careful consideration. Among the computational models developed for quantum mechanical carrier transport analysis, the Non &ndash Equilibrium Green &rsquo s Function (NEGF) coupled with Poisson equation has gained vast application in both ballistic and diffusive transport analysis of nanodevices. In this thesis, the NEGF model is expanded to include mechanical strain and carrier scattering effects. Two important multiphysics systems are investigated in this work. We first study the effect of mechanical strain on the electrical conductivity of Si/Si 1 &minus x Ge x nanocomposite thin films. The strain effect on the bandstructures of nano &ndash thin films is modeled by a degenerate two &ndash band k · p theory. The strain induced bandstructure variation is then incorporated in the NEGF &ndash Poisson model. The results introduce new perspectives on electrical transport in strained nano &ndash thin films, which provides useful guidance in the design of flexible electronics. Secondly, nanoporous Si as an efficient thermoelectric material is studied. The Seebeck coefficient and electrical conductivity of nanoporous Si are computed by using the NEGF &ndash Poisson model with scatterings modeled by Buttiker probes. The phonon thermal conductivity is obtained by using a Boltzmann Transport Equation (BTE) model while the electron thermal conductivity is captured by the Wiedemann &ndash Franz law. The thermoelectric figure of merit of nanoporous Si is computed for different doping density, porosities, temperature and pore size. An optimal combination of the material design parameters is explored and the result proves that nanoporous Si has better thermoelectric properties than its bulk counterpart. In the electrical transport analysis of nanomaterials, we found that the standard NEGF &ndash Poisson model using the Finite Difference (FD) method has a high computational cost, and is inapplicable to devices with irregular geometries. To overcome these difficulties, an accelerated Finite Element Contact Block Reduction (FECBR) method is developed in this thesis. The performance of the accelerated FECBR is evaluated through the simulation of two types of electronic devices: taper &ndash shaped DG &ndash MOSFETs and DG &ndash MOSFETs with Si/SiO 2 interface roughness. Numerical results show that the accelerated FECBR can be applied to model ballistic carrier transport in devices with multiple leads, arbitrary geometry and complex potential profile. The accelerated FECBR significantly improves the flexibility and efficiency of electrical transport analysis of nanomaterials and nanodevices