Assessment of the Thermal Conductivity of BN-C Nanostructures

Chemical and structural diversity present in hexagonal boron nitride ((h-BN) and graphene hybrid nanostructures provide new avenues for tuning various properties for their technological applications. In this paper we investigate the variation of thermal conductivity ($\kappa$) of hybrid graphene/h-BN nanostructures: stripe superlattices and BN (graphene) dots embedded in graphene (BN) are investigated using equilibrium molecular dynamics. To simulate these systems, we have parameterized a Tersoff type interaction potential to reproduce the ab initio energetics of the B-C and N-C bonds for studying the various interfaces that emerge in these hybrid nanostructures. We demonstrate that both the details of the interface, including energetic stability and shape, as well as the spacing of the interfaces in the material exert strong control on the thermal conductivity of these systems. For stripe superlattices, we find that zigzag configured interfaces produce a higher $\kappa$ in the direction parallel to the interface than the armchair configuration, while the perpendicular conductivity is less prone to the details of the interface and is limited by the $\kappa$ of h-BN. Additionally, the embedded dot structures, having mixed zigzag and armchair interfaces, affects the thermal transport properties more strongly than superlattices. Though dot radius appears to have little effect on the magnitude of reduction, we find that dot concentration (50% yielding the greatest reduction) and composition (embedded graphene dots showing larger reduction that h-BN dot) have a significant effect

High field transport phenomena in wide bandgap semiconductors

Cataloged from PDF version of article.The Ensemble Monte Carlo (EMC) method is widely used in the field of computational electronics related to the simulation of the state of the art devices. Using this technique our specific intention is to scrutinize the high-field transport phenomena in wide bandgap semiconductors (Such as GaN, AlGaN and AlN). For this purpose, we have developed an EMC-based computer code. After a brief introduction to our methodology, we present detailed analysis of three different types of devices, operating under high-field conditions, namely, unipolar n-type structures, avalanche photodiodes (APD) and finally the Gunn diodes. As a testbed for understanding impact ionization and hot electron effects in sub-micron sized GaN, AlN and their ternary alloys, an n +−n−n + channel device is employed having a 0.1 µm-thick n region. The time evolution of the electron density along the device is seen to display oscillations in the unintentionally doped n-region, until steady state is established. The fermionic degeneracy effects are observed to be operational especially at high fields within the anode n +-region. For AlxGa1−xNbased systems, it can be noted that due to alloy scattering, carriers cannot acquire the velocities attained by the GaN and AlN counterparts. Next, multiplication and temporal response characteristics under a picosecond pulsed optical illumination of p +-n-n + GaN and n-type Schottky Al0.4Ga0.6N APDs are analyzed. For the GaN APD, our simulations can reasonably reproduce the available measured data without any fitting parameters. In the case of AlGaN, the choice of a Schottky contact APD is seen to improve drastically the field confinement resulting in satisfactory gain characteristics. Moreover, alloy scattering is seen to further slow down the temporal response while displacing the gain threshold to higher fields. Finally, the dynamics of large-amplitude Gunn domain oscillations from 120 GHz to 650 GHz are studied in detail by means of extensive EMC simulations. The basic operation is checked under both impressed single-tone sinusoidal bias and external tank circuit conditions. The width of the doping-notch is observed to enhance higher harmonic efficiency at the expense of the fundamental frequency up to a critical value, beyond which sustained Gunn oscillations are ceased. The degeneracy effects due to the Pauli Exclusion principle and the impact ionization are also considered but observed to have negligible effect within the realistic operational bounds. Finally, the effects of lattice temperature, channel doping and DC bias on the RF conversion efficiency are investigatedSevik, CemM.S

Carrier dynamics in silicon and Germanium nanocrystals

Ankara : The Department of Physics and the Institute of Engineering and Sciences of Bilkent University, 2008.Thesis (Ph.D.) -- Bilkent University, 2008.Includes bibliographical references leaves 65-75.This is a computational work on the Si and Ge nanocrystals (NCs) embedded in wide band gap host matrices. As the initial task, extensive ab initio work on the structural and electronic properties of various NC host matrices, namely, SiO2, GeO2, Si3N4, and Al2O3 are preformed. The structural parameters, elastic constants, static and optical dielectric constants are obtained in close agreement with the available results. Furthermore, recently reported high density cubic phase of SiO2 together with GeO2 and SnO2 are studied and their stable highdielectric constant alloys are identified. Based on the ab initio study of host matrices, two related high field phenomena, vital especially for the electroluminescence in Si and Ge NCs, are examined. These are the hot carrier transport through the SiO2 matrix and the subsequent quantum-confined impact ionization (QCII) process which is responsible for the creation of electron-hole pairs within the NCs. First, the utility and the validity of the ab initio density of states results are demonstrated by studying the high field carrier transport in bulk SiO2 up to fields of 12 MV/cm using the ensemble Monte Carlo technique. Next, a theoretical modeling of the impact ionization of NCs due to hot carriers of the bulk SiO2 matrix is undertaken. An original expression governing the QCII probability as a function of the energy of the hot carriers is derived. Next, using an atomistic pseudopotential approach the electronic structures for embedded Si and Ge NCs in wide band-gap matrices containing several thousand atoms are employed. Effective band-gap values as a function of NC diameter reproduce very well the available experimental and theoretical data. To further check the validity of the electronic structure on radiative processes, direct photon emission rates are computed. The results for Si and Ge NCs as a function of diameter are in excellent agreement with the available ab initio calculations for small NCs. In the final part, non-radiative channels, the Auger recombination (AR) and carrier multiplication (CM) in Si and Ge NCs are investigated again based on the atomistic pseudopotential Hamiltonian. The excited electron and excited hole type AR and CM and biexciton type AR lifetimes are calculated for different sized and shaped NCs embedded in SiO2 and Al2O3. Asphericity is also observed to increase the AR and CM rates. An almost monotonous size-scaling and satisfactory agreement with experiment for AR lifetime is obtained considering a realistic interface region between the NC core and the host matrix. It is further shown that the size-scaling of AR can simply be described by slightly decreasing the established bulk Auger constant for Si to 1.0×10−30cm6 s −1 . The same value for germanium is extracted as 1.5×10−30cm6 s −1 which is very close to the established bulk value. It is further shown that both Si and Ge NCs are ideal for photovoltaic efficiency improvement via CM due to the fact that under an optical excitation exceeding twice the band gap energy, the electrons gain lion’s share from the total excess energy and can cause a CM. Finally, the electron-initiated CM is predicted to be enhanced by couple orders of magnitude with a 1 eV of excess energy beyond the CM threshold leading to subpicosecond CM lifetimes.Sevik, CemPh.D

Gate induced monolayer behavior in twisted bilayer black phosphorus

Optical and electronic properties of black phosphorus strongly depend on the number of layers and type of stacking. Using first-principles calculations within the framework of density functional theory, we investigate the electronic properties of bilayer black phosphorus with an interlayer twist angle of 90$^\circ$. These calculations are complemented with a simple $\vec{k}\cdot\vec{p}$ model which is able to capture most of the low energy features and is valid for arbitrary twist angles. The electronic spectrum of 90$^\circ$ twisted bilayer black phosphorus is found to be x-y isotropic in contrast to the monolayer. However x-y anisotropy, and a partial return to monolayer-like behavior, particularly in the valence band, can be induced by an external out-of-plane electric field. Moreover, the preferred hole effective mass can be rotated by 90$^\circ$ simply by changing the direction of the applied electric field. In particular, a +0.4 (-0.4) V/{\AA} out-of-plane electric field results in a $\sim$60\% increase in the hole effective mass along the y (x) axis and enhances the $m^*_{y}/m^*_{x}$ ($m^*_{x}/m^*_{y}$) ratio as much as by a factor of 40. Our DFT and $\vec{k}\cdot\vec{p}$ simulations clearly indicate that the twist angle in combination with an appropriate gate voltage is a novel way to tune the electronic and optical properties of bilayer phosphorus and it gives us a new degree of freedom to engineer the properties of black phosphorus based devices.Comment: 8 pages, 8 figure

Enhancing superconductivity in MXenes through hydrogenation

Two-dimensional transition metal carbides and nitrides (MXenes) are an emerging class of atomically-thin superconductors, whose characteristics are highly prone to tailoring by surface functionalization. Here we explore the use of hydrogen adatoms to enhance phonon-mediated superconductivity in MXenes, based on first-principles calculations combined with Eliashberg theory. We first demonstrate the stability of three different structural models of hydrogenated Mo- and W-based MXenes. Particularly high critical temperatures of over 30 K are obtained for hydrogenated Mo$_2$N and W$_2$N. Several mechanisms responsible for the enhanced electron-phonon coupling are uncovered, namely (i) hydrogen-induced changes in the phonon spectrum of the host MXene, (ii) emerging hydrogen-based phonon modes, and (iii) charge transfer from hydrogen to the MXene layer, boosting the density of states at the Fermi level. Finally, we demonstrate that hydrogen adatoms are moreover able to induce superconductivity in MXenes that are not superconducting in pristine form, such as Nb$_2$C

Superconductivity in functionalized niobium-carbide MXenes

We show the effect of Cl and S functionalization on the superconducting properties of layered (bulk) and monolayer niobium carbide (Nb$_2$C) MXene crystals, based on first-principles calculations combined with Eliashberg theory. For the bulk layered Nb$_2$CCl$_2$, the calculated superconducting transition temperature ($T_c$) is in very good agreement with the recently measured value of 6 K. We show that $T_c$ is enhanced to 10 K for monolayer Nb$_2$CCl$_2$, due to an increase in the density of states at the Fermi level, and the corresponding electron-phonon coupling. We further demonstrate a feasible gate-induced enhancement of $T_c$ up to 40 K for both bulk-layered and monolayer Nb$_2$CCl$_2$ crystals. For the S-functionalized cases our calculations reveal the importance of phonon softening in understanding their superconducting properties. Finally, we predict that Nb$_3$C$_2$S$_2$ in bulk-layered and monolayer form is potentially superconducting, with a $T_c$ around 30 K. Considering that Nb$_2$C is not superconducting in pristine form, our findings promote functionalization as a pathway towards robust superconductivity in MXenes