arXiv subjects

Redwan N. Sajjad

Publications and source records attributed to Redwan N. Sajjad.

10 recordsLinked to original sources

arXiv2021

Carrier transport and performance limit of semi-transparent photovoltaics: CuIn$_{1-x}$Ga$_x$Se$_2$ as a case study

Semi-transparent photovoltaic devices for building integrated applications have the potential to provide simultaneous power generation and natural light penetration. CuIn$_{1-x}$Ga$_x$Se$_2$ (CIGS) has been established as a mature technology for thin-film photovoltaics, however, its potential for Semi-Transparent Photovoltaics (STPV) is yet to be explored. In this paper, we present its carrier transport physics explaining the trend seen in recently published experiments. STPV requires deposition of films of only a few hundred nanometers to make them transparent and manifests several unique properties compared to a conventional thin-film solar cell. Our analysis shows that the short-circuit current, Jsc is dominated by carriers generated in the depletion region, making it nearly independent of bulk and back-surface recombination. The bulk recombination, which limits the open-circuit voltage Voc, appears to be higher than usual attributable to numerous grain boundaries. When the absorber layer is reduced below 500 nm, grain size reduces resulting in more grain boundaries and higher resistance. This produces an inverse relationship between series resistance and absorber thickness. We also present a thickness-dependent model of shunt resistance showing its impact in these ultra-thin devices. For various scenarios of bulk and interface recombinations, shunt and series resistances, AVT and composition of CuIn$_{1-x}$Ga$_x$Se$_2$, we project the efficiency limit which - for most practical cases - is found to be $\leq$10% for AVT $\geq$25%.

physics.app-ph↗

arXiv2018

Atomic scale characterization of graphene p-n junctions for electron-optical applications

Graphene p-n junctions offer a potentially powerful approach towards controlling electron trajectories via collimation and focusing in ballistic solid-state devices. The ability of p-n junctions to control electron trajectories depends crucially on the doping profile and roughness of the junction. Here, we use four-probe scanning tunneling microscopy and spectroscopy (STM/STS) to characterize two state-of-the-art graphene p-n junction geometries at the atomic scale, one with CMOS polySi gates and another with naturally cleaved graphite gates. Using spectroscopic imaging, we characterize the local doping profile across and along the p-n junctions. We find that realistic junctions exhibit non-ideality both in their geometry as well as in the doping profile across the junction. We show that the geometry of the junction can be improved by using the cleaved edge of van der Waals metals such as graphite to define the junction. We quantify the geometric roughness and doping profiles of junctions experimentally and use these parameters in Nonequilibrium Green's Function based simulations of focusing and collimation in these realistic junctions. We find that for realizing Veselago focusing, it is crucial to minimize lateral interface roughness which only natural graphite gates achieve, and to reduce junction width, in which both devices under investigation underperform. We also find that carrier collimation is currently limited by the non-linearity of the doping profile across the junction. Our work provides benchmarks of the current graphene p-n junction quality and provides guidance for future improvements.

cond-mat.mes-hall↗

arXiv2018

A tunnel FET compact model including non-idealities with verilogy implementation

We present a compact model for Tunnel Field Effect Transistors (TFET), that captures sev- eral non-idealities such as the Trap Assisted Tunneling (TAT) originating from interface traps (Dit), along with Verilog-A implementation. We show that the TAT, together with band edge non-abruptness known as the Urbach tail, sets the lower limit of the sub-threshold swing and the minimum achievable current at a given temperature. Presence of charged trap states also contributes to reduced gate efficiency. We show that we can decouple the contribution of each of these processes and extract the intrinsic sub-threshold swing from a given experimental data. We derive closed form expressions of channel potential, electric field and effective tunnel energy window to accurately capture the essential device physics of TFETs. We test the model against recently published exper- imental data, and simulate simple TFET circuits using the Verilog-A model. The compact model provides a framework for TFET technology projections with improved device metrics such as better electrostatic design, reduced TAT, material with better transport properties etc.

cond-mat.mes-hall↗

arXiv2016

Trap assisted tunneling and its effect on subthreshold swing of tunnel field effect transistors

We provide a detailed study of the interface Trap Assisted Tunneling (TAT) mechanism in tunnel field effect transistors to show how it contributes a major leakage current path before the Band To Band Tunneling (BTBT) is initiated. With a modified Shockley-Read-Hall formalism, we show that at room temperature, the phonon assisted TAT current always dominates and obscures the steep turn ON of the BTBT current for common densities of traps. Our results are applicable to top gate, double gate and gate all around structures where the traps are positioned between the source-channel tunneling region. Since the TAT has strong dependence on electric field, any effort to increase the BTBT current by enhancing local electric field also increases the leakage current. Unless the BTBT current can be increased separately, calculations show that the trap density Dit has to be decreased by 40-100 times compared with the state of the art in order for the steep turn ON (for III-V materials) to be clearly observable at room temperature. We find that the combination of the intrinsic sharpness of the band edges (Urbach tail) and the surface trap density determines the subthreshold swing.

cond-mat.mes-hall↗

arXiv2015

Modified Dirac Hamiltonian for Efficient Quantum Mechanical Simulations of Micron Sized Devices

Representing massless Dirac fermions on a spatial lattice poses a potential challenge known as the Fermion Doubling problem. Addition of a quadratic term to the Dirac Hamiltonian circumvents this problem. We show that the modified Hamiltonian with the additional term results in a very small Hamiltonian matrix when discretized on a real space square lattice. The resulting Hamiltonian matrix is considerably more efficient for numerical simulations without sacrificing on accuracy and is several orders of magnitude faster than the atomistic tight binding model. Using this Hamiltonian and the Non-Equilibrium Green's Function (NEGF) formalism, we show several transport phenomena in graphene, such as magnetic focusing, chiral tunneling in the ballistic limit and conductivity in the diffusive limit in micron sized graphene devices. The modified Hamiltonian can be used for any system with massless Dirac fermions such as Topological Insulators, opening up a simulation domain that is not readily accessible otherwise.

cond-mat.mes-hall↗

arXiv2015

Quantum transport at the Dirac point: Mapping out the minimum conductivity from pristine to disordered graphene

The phase space for graphene's minimum conductivity $σ_\mathrm{min}$ is mapped out using Landauer theory modified for scattering using Fermi's Golden Rule, as well as the Non-Equilibrium Green's Function (NEGF) simulation with a Monte Carlo sampling over impurity distributions. The resulting `fan diagram' spans the range from ballistic to diffusive over varying aspect ratios ($W/L$), and bears several surprises. {The device aspect ratio determines how much tunneling (between contacts) is allowed and becomes the dominant factor for the evolution of $σ_{min}$ from ballistic to diffusive regime. We find an increasing (for $W/L>1$) or decreasing ($W/L<1$) trend in $σ_{min}$ vs. impurity density, all converging around $128q^2/π^3h\sim 4q^2/h$ at the dirty limit}. In the diffusive limit, the {conductivity} quasi-saturates due to the precise cancellation between the increase in conducting modes from charge puddles vs the reduction in average transmission from scattering at the Dirac Point. In the clean ballistic limit, the calculated conductivity of the lowest mode shows a surprising absence of Fabry-Pérot oscillations, unlike other materials including bilayer graphene. We argue that the lack of oscillations even at low temperature is a signature of Klein tunneling.

cond-mat.mes-hall↗

arXiv2015

Chiral tunneling of topological states: towards the efficient generation of spin current using spin-momentum locking

We show that the interplay between chiral tunneling and spin-momentum locking of helical surface states leads to spin amplification and filtering in a 3D Topological Insulator (TI). Chiral tunneling across a TI pn junction allows normally incident electrons to transmit, while the rest are reflected with their spins flipped due to spin-momentum locking. The net result is that the spin current is enhanced while the dissipative charge current is simultaneously suppressed, leading to an extremely large, gate tunable spin to charge current ratio (~20) at the reflected end. At the transmitted end, the ratio stays close to one and the electrons are completely spin polarized.

cond-mat.mes-hall↗

arXiv2013

Manipulating chiral transmission by gate geometry: switching in graphene with transmission gaps

We explore the chiral transmission of electrons across graphene heterojunctions for electronic switching using gate geometry alone. A sequence of gates is used to collimate and orthogonalize the chiral transmission lobes across multiple junctions, resulting in negligible overall current. The resistance of the device is enhanced by several orders of magnitude by biasing the gates into the bipolar $npn$ doping regime, as the ON state in the near homogeneous $nn^-n$ regime remains highly conductive. The mobility is preserved because the switching involves a transmission gap instead of a structural band-gap that would reduce the number of available channels of conduction. Under certain conditions this transmission gap is highly gate tunable, allowing a subthermal turn-on that beats the Landauer bound on switching energy limiting present day digital electronics.

cond-mat.mes-hall↗

arXiv2013

Atomistic deconstruction of current flow in graphene based hetero-junctions

We describe the numerical modeling of current flow in graphene heterojunctions, within the Keldysh Landauer Non-equilibrium Green's function (NEGF) formalism. By implementing a $k$-space approach along the transverse modes, coupled with partial matrix inversion using the Recursive Green's function Algorithm (RGFA), we can simulate on an atomistic scale current flow across devices approaching experimental dimensions. We use the numerical platform to deconstruct current flow in graphene, compare with experimental results on conductance, conductivity and quantum Hall, and deconstruct the physics of electron `optics' and pseudospintronics in graphene $p-n$ junctions. We also demonstrate how to impose exact open boundary conditions along the edges to minimize spurious edge reflections.

cond-mat.mes-hall↗

arXiv2012

Manifestation of chiral tunneling at a tilted graphene pn junction

Electrons in graphene follow unconventional trajectories at PN junctions, driven by their pseudospintronic degree of freedom. Significant is the prominent angular dependence of transmission, capturing the chiral nature of the electrons and culminating in unit transmission at normal incidence (Klein tunneling). We theoretically show that such chiral tunneling can be directly observed from the junction resistance of a tilted interface probed with separate split gates. The junction resistance is shown to increase with tilt in agreement with recent experimental evidence. The tilt dependence arises because of the misalignment between modal density and the anisotropic transmission lobe oriented perpendicular to the tilt. A critical determinant is the presence of edge scattering events that can completely reverse the angle-dependence. The absence of such reversals in the experiments indicates that these edge effects are not overwhelmingly deleterious, making the premise of transport governed by electron `optics' in graphene an exciting possibility.

cond-mat.mes-hall↗