Carrier Transmission Probability Research Articles

A Physical Model of Contact Resistance in Ti-Contacted Graphene-Based Field Effect Transistors

The effect of device fabrication processing conditions is often overlooked in the evaluation of the contact resistance in metal–graphene contact. In this work, we investigate the Ti–graphene contact resistance theoretically and experimentally. A compact model has been proposed to reveal and evaluate the role of different factors in controlling the value of contact resistance. A layer of oxidized Ti or polymer residues at the Ti–graphene interface has been considered to explain the reduced carrier transmission probability through the interface. Experimental results report a low contact resistance as small as 271.45 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\Omega ~\cdot \mu \text{m}$ </tex-math></inline-formula> and our model agrees with the measured values. The proposed model explains the large variation of the contact resistance reported in the literature up to date and help to improve metal–graphene contact for future applications.

Scattering of topological surface-state carriers at steps on surfaces

The resistance across a step on ultrathin films of three different topological insulators, ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, and ${({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Pb}}_{x})}_{2}{\mathrm{Te}}_{3}$, was measured through anisotropy in two-dimensional resistivity by using the in situ square four-point probe method in ultrahigh vacuum. The step resistance was much larger for ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ than for ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ in the range of 1--10 quintuple-layer thickness, due to the smaller critical thickness for isolation of topological surface states in ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$. The transmission probability of carriers across a step is much higher for the bulk-insulating ${({\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Pb}}_{x})}_{2}{\mathrm{Te}}_{3}$ than bulk-metallic ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, due to prevention of scattering of surface-state carriers into the bulk states. We were able to deduce microscopic information concerning the transmission probability at individual steps from the resistance data obtained macroscopically.

The Understanding of SiNR and GNR TFETs for Analog and RF Application With Variation of Drain-Doping Molar Fraction

The 1-D nanoribbon (NR) of monolayer materials has gained immense interest due to their unique properties qualitatively distinct from their bulk properties and the demand for nanoscale applications. In this paper, the quantum transport properties of two most prominent 2-D materials, i.e., silicene NR (SiNR) and a graphene NR (GNR) tunnel field-effect transistor (TFET) with the effect of different dopant molar fractions in the drain region are studied numerically using nonequilibrium Green’s function formalism. In SiNR TFET, higher on-state current ( ${I}_{ \mathrm{\scriptscriptstyle ON}}$ ) is observed due to wider tunneling energy window and high transmission probability of carriers. In order to observe the effect of variation of doping density in the drain region, we have studied analog figures of merit such as the transconductance ( ${g}_{m}$ ), output resistance ( ${r}_{o}$ ), transconductance generation factor ( ${g}_{m}/{I}_{D}$ ), and the intrinsic gain ( ${g}_{m}{r}_{o}$ ) for different molar fractions. Similarly, we have evaluated the RF performance of the SiNR and GNR TFETs as a function of cutoff frequency ( ${f}_{T}$ ), gate capacitance ( ${C}_{G}$ ), and transport delay ( $\tau$ ).

In search of quantum-limited contact resistance: understanding the intrinsic and extrinsic effects on the graphene–metal interface

Owing to its two-dimensional structure, graphene is extremely sensitive to surface contamination. Conventional processing techniques inevitably modify graphene’s intrinsic properties by introducing adsorbents and/or defects which limit device performance and understanding the intrinsic properties of graphene. Here we demonstrate femtosecond laser direct patterning of graphene microstructures, without the aid of resists or other chemicals, that enables us to study both intrinsic and extrinsic effects on the graphene–metal interface. The pulsed femtosecond laser was configured to ablate epitaxial graphene (EG) on a sub-micrometer scale and form a precisely defined region without damaging the surrounding material or substrate. The ablated area was sufficient to electrically isolate transfer length measurement structures and Hall devices for subsequent transport measurements. Using pristine and systematically contaminated surfaces, we found that Ni does not form bonds to EG synthesized on SiC in contrast to the well-known C–Ni bond formation for graphene synthesized on metals; known as end-contacting. Without end-contacting, the contact resistance (RC) of Ni to pristine and resist-contaminated EG are one and two orders of magnitude larger, respectively, than the intrinsic quantum limited contact resistance. The range of reported RC values is explained using carrier transmission probability, as exemplified by the Landauer–Büttiker model, which is dependent on the presence or absence of end-contacts and dopant/work-function mediated conduction. The model predicts the need for both end-contacts and a clean graphene–metal interface as necessary conditions to approach quantum limited contact resistance.

Improvement of Metal-Graphene Ohmic Contact Resistance in Bilayer Epitaxial Graphene Devices**Supported by the National Natural Science Foundation of China under Grant No 61306006.

We report on an improved metal-graphene ohmic contact in bilayer epitaxial graphene on a SiC substrate with contact resistance below 0.1 ω·mm. Monolayer and bilayer epitaxial graphenes are prepared on a 4H-SiC substrate in this work. Their contact resistances are measured by a transfer length method. An improved photoresist-free device fabrication method is used and is compared with the conventional device fabrication method. Compared with the monolayer graphene, the contact resistance Rc of bilayer graphene improves from an average of 0.24 ω·mm to 0.1 ω·mm. Ohmic contact formation mechanism analysis by Landauer's approach reveals that the obtained low ohmic contact resistance in bilayer epitaxial graphene is due to their high carrier density, high carrier transmission probability, and p-type doping introduced by contact metal Au.

Role of hot carriers in the interfacial transport in amorphous silicon/crystalline silicon heterostructure solar cells

AbstractThe transport of photogenerated minority carriers (photocarriers) across the heterointerface of amorphous silicon (a‐Si) and crystalline silicon (c‐Si) in a‐Si/c‐Si heterostructure solar cell is shown in this work to critically depend on the non‐Maxwellian energy distribution function (EDF) of those carriers impinging on the heterointerface. A theoretical model is presented that integrates the effect of the high electric field inversion region upon EDF of the impinging carriers with the transmission probability of those carriers across the heterointerface. The transport of the photocarriers across the high electric field inversion region is simulated by the full solution of the Boltzmann transport equation by Monte Carlo (MC) technique while the transmission probability of carriers across the heterointerface is calculated through the percolation path technique. The results are discussed under two different condition of band bending; strongly inverted and weakly inverted c‐Si surface. The results comparing different conditions of band bending show that the energy distribution of the carriers impinging on the heterointerface is non‐Maxwellian and the integrated photocarrier collection increases with the strength of the inversion field since the carrier population is weighted toward higher energy where the transmission probability through the barrier is higher. Thus, we demonstrate that hot carriers play an important role in heterostructure cell operation.

Angle-Dependent Carrier Transmission in Graphene p–n Junctions

Angle-dependent carrier transmission probability in graphene p-n junctions is investigated. Using electrostatic doping from buried gates, p-n junctions are formed along graphene channels that are patterned to form different angles with the junction. A peak in the junction resistance is observed, which becomes pronounced with angle. This angular dependence is observed for junctions made on both exfoliated and CVD-grown graphene and is consistent with the theoretically predicted dependence of transmission probability on incidence angle.

Reconfigurable Graphene Logic Device Based on Tilted P-N Junctions

ABSTRACTIn this paper, we introduce a novel reconfigurable graphene logic based on graphene p-n junctions. In this logic device, switching is accomplished by using co-planar split gates that modulate the properties that are unique to graphene, including ambipolar conduction, electrostatic doping, and angular dependent carrier reflection. In addition, the use of these control gates can dynamically change the operation of the device, leading to reconfigurable multi-functional logic. A device model is derived from carrier transmission probability across the p-n junction for allowing quantitative comparison to CMOS logic. Based on this model, we show that the proposed graphene logic has significant advantages over CMOS gate in terms of area, delay, power, and signal restoration. Furthermore, the device utilizes a large graphene sheet with minimal patterning, allowing feasible integration with CMOS circuits, for potential CMOS-graphene hybrid circuits.

Reconfigurable Graphene Logic

In this paper, we introduce a novel reconfigurable graphene logic based on graphene p-n junctions. In this logic device, switching is accomplished by using co-planar split gates that modulate the properties that are unique to graphene, including ambipolar conduction, electrostatic doping, and angular dependent carrier reflection. In addition, the use of these control gates can dynamically change the operation of the device, leading to reconfigurable multi-functional logic. A device model is derived from carrier transmission probability across the p-n junction for allowing quantitative comparison to CMOS logic. Based on this model, we show that the proposed graphene logic has significant advantages over CMOS gate in terms of area, delay, power, and signal restoration. Furthermore, the device utilizes a large graphene sheet with minimal patterning, allowing facile integration with CMOS circuits, for potential CMOS-graphene hybrid circuits.

On the Assumption of Detailed Balance in Prediction of Diffusive Transmission Probability During Interfacial Transport

Models intended to predict interfacial transport often rely on the principle of detailed balance when formulating the interfacial carrier transmission probability. However, assumptions invoked significantly impact predictions. Here, we present six derivations of the transmission probability, each subject to a different set of preliminary assumptions regarding the type of scattering at the interface. Application of each case to phonon flux and thermal boundary conductance allows for a final quantitative comparison. Depending on the preliminary assumptions, predictions for thermal boundary conductance span over two orders of magnitude, demonstrating the need for transparency when assessing the accuracy of any predictive model.

In-plane dispersion relations of InAs/AlSb/GaSb/AlSb/InAs interband resonant-tunneling diodes.

We report on a flexible eight-band model, derived from a modified Kane base, generating a transfer-matrix formalism to calculate the transmission probability of carriers through, and the in-plane dispersion relations of, stratified media. We apply this model to study InAs/AlSb/GaSb/AlSb/InAs interband resonant-tunneling diodes. We show that the boundary-condition sets deduced from current continuity and the boundary-condition sets deduced from a Hermitian Hamiltonian defined across the interface are equal. A simplified triple-band model is found to be a good approximation to calculate the deep light-hole energy levels in a resonant interband tunneling diode. However, these simplified triple-band approximations are no longer valid when the carriers have an in-plane momentum.

Current transport mechanisms in atomically abrupt metal-semiconductor interfaces

A comprehensive model for electron transport mechanisms across a fully formed Schottky-barrier junction is proposed in which the metal-semiconductor interface is approximated as an abrupt quantum mechanical transition. Improved formulations of the barrier-lowering mechanisms and carrier tunneling effects are derived where the dipole barrier lowering is modeled as a single exponential decay of the total surface charge density. Quantum calculations follow a two-band model in which the imaginary component of the electron wave vector in the semiconductor energy gap is obtained by including the effect of both conduction and valence states. The energy band profile effects are included in the calculation of tunneling current, and it is shown that the finite negative charge residing at the metal-semiconductor interface considerably modulates the tunneling transmission probability of carriers. Experimental results obtained from atomically clean Al-n/sup +/GaAs-nGaAs interfaces fabricated by in situ molecular-beam epitaxy (MBE) are shown to be in excellent agreement with the transport calculations.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>