IEEE Transactions on Electron Devices

/IEEE Transactions on Electron Devices

Impact of Phonon Scattering on the Negative-Differential-Resistance Behavior in Graphene Nanoribbon p-n Junctions

2018-06-19T00:34:10+00:00 June 19th, 2018|Categories: Publications|Tags: |

In this paper, the effect of electron–phonon scattering on the negative differential resistance of graphene nanoribbon p-n junctions is investigated. The results show that the optical phonon scattering, due to inelasticity, significantly increases the valley current such that the peak-to-valley ratio decreases several orders of magnitude compared to ballistic approximation. It also shifts up the valley voltage. The former effect manifests itself more in structures with a smaller band gap and better ballistic characteristics. We show that it remains remarkable in different lengths of the transition region and severely affects the performance of the device. The results also show that phonon scattering plays a significant role even in ultrascaled nanodevices.

Published in: "IEEE Transactions on Electron Devices".

Extraction of Intrinsic Electrical Parameters in Partially Depleted MoS<sub>2</sub> Field-Effect Transistors

2018-06-19T00:34:07+00:00 June 19th, 2018|Categories: Publications|Tags: |

Electrical performance and transport mechanisms in 2-D transition-metal dichalcogenide materials should be investigated under a range of electrical parameters for practical application. In this paper, partially depleted (PD) molybdenum disulfide (MoS2) transistors were fabricated with a thick flake mechanically exfoliated from bulk crystals, and their operating mechanism is discussed considering the gate-uncontrollable conduction channel, the maximum depletion width (${D}_{mathrm {text {max}}}$ ), and the impact of series resistance (${R}_{mathrm {text {sd}}}$ ). In addition, the intrinsic mobility of a neutral bulk channel in PD-MoS2 transistors was extracted from the simply separated gate-controllable drain current with a depletion approximation.

Published in: "IEEE Transactions on Electron Devices".

Large-Area, Single-Layer Molybdenum Disulfide Synthesized at BEOL Compatible Temperature as Cu Diffusion Barrier

2018-05-21T23:20:53+00:00 May 21st, 2018|Categories: Publications|Tags: |

The scaling limit of conventional Cu diffusion barriers has become the bottleneck for interconnect technology, which in turn limits the IC performance. Sub-nm diffusion barrier is urgently demanded to maintain the interconnect resistivity for ultra-scaled Cu interconnects. However, with this thickness, the blocking capabilities of conventional Cu diffusion barriers are lost. In this letter, sub-nm Cu diffusion barrier is realized by single-layer molybdenum disulfide (MoS2) grown at 400 °C using metal-organic chemical vapor deposition. MoS2 is directly grown on dielectrics without transfer processes and the continuous coverage in a large area (>1 cm2) is achieved. Its resistance to Cu diffusion is investigated by time-dependent dielectric breakdown (TDDB) measurements. Our results indicate that the MoS2 barrier can efficiently suppress Cu diffusion and enhance dielectric lifetime significantly. Although a few challenges, including Cu adhesion to the MoS2 surface and integration with the Damascene structure, have to be assessed before introducing this novel material to the back-end-of-line technology, our work lays the groundwork for further investigation.

Published in: "IEEE Electron Device Letters".

A New Velocity Saturation Model of MoS<sub>2</sub> Field-Effect Transistors

2018-05-21T23:20:51+00:00 May 21st, 2018|Categories: Publications|Tags: |

In this letter, we present a new velocity saturation model for molybdenum disulfide (MoS2) field-effect transistors (FETs) based on the surface potential. Unlike former models, in addition to modulating the field-effect mobility, our model adjusts the upper limit of current integration to its saturation value. Moreover, the expression of charge density at the drain under the velocity saturation effect is provided. As an example, the model is used to simulate the electrical characteristics of a MoS2 FET under different trap densities, temperatures, and channel lengths. A short channel device is fabricated and evaluated. The excellent agreement between the experimental data and the simulation results validates our model.

Published in: "IEEE Electron Device Letters".

Role of Biasing and Device Size on Phonon Scattering in Graphene Nanoribbon Transistors

2018-05-18T00:29:18+00:00 May 18th, 2018|Categories: Publications|Tags: , |

We study the noncoherent transport due to electron–phonon (e-ph) interaction in graphene nanoribbon (GNR) field-effect transistors (GNRFETs) using nonequilibrium Green’s function method in mode space. Phonon dispersion calculations in conjunction with the e-ph interaction computations show that in an armchair GNR, only a small number of phonon modes are coupled to the carriers. Our simulation shows that under a threshold gate voltage, the drain–source current is diminished by the low-energy phonons such as acoustic phonon modes and radial-breathing-like phonon modes, while at great biases of the gate, the current drop is essentially due to high-energy optical phonon modes. The effect of drain voltage on scattering process is discussed, which shows that at higher drain voltages, the ballisticity decreases. We also explore the phonon scattering dependence on the dimensions of GNRFET channel. In larger width ribbon, the impact of phonon scattering decreases. The channel with a length of 30 nm is ballistic up to 90%, and only when its length is increased to approximately 180 nm, transport becomes semiballistic, which infers to an effective mean free path of ~200 nm.

Published in: "IEEE Transactions on Electron Devices".

Fractional Fowler–Nordheim Law for Field Emission From Rough Surface With Nonparabolic Energy Dispersion

2018-05-18T00:29:16+00:00 May 18th, 2018|Categories: Publications|Tags: , |

The theories of field electron emission from perfectly planar and smooth canonical surfaces are well understood, but they are not suitable for describing emission from rough, irregular surfaces arising in modern nanoscale electron sources. Moreover, the existing models rely on Sommerfeld’s free-electron theory for the description of electronic distribution, which is not a valid assumption for modern materials with nonparabolic energy dispersion. In this paper, we derive analytically a generalized Fowler–Nordheim (FN)-type equation that considers the reduced space-dimensionality seen by the quantum mechanically tunneling electron at a rough, irregular emission surface. We also consider the effects of nonparabolic energy dispersion on field emission from narrow-gap semiconductors and few-layer graphene using Kane’s band model. The traditional FN equation is shown to be a limiting case of our model in the limit of a perfectly flat surface of a material with parabolic dispersion. The fractional-dimension parameter used in this model can be experimentally calculated from appropriate current–voltage data plot. By applying this model to experimental data, the standard field-emission parameters can be deduced with better accuracy than by using the conventional FN equation.

Published in: "IEEE Transactions on Electron Devices".

Vertical Graphene Nanoribbon Interconnects at the End of the Roadmap

2018-05-18T00:29:14+00:00 May 18th, 2018|Categories: Publications|Tags: |

To solve electrical contact problem of horizontal multilayer graphene nanoribbon (GNR), a vertical GNR (VGNR) interconnect scheme is proposed. Performance of the VGNR interconnect is theoretically evaluated and compared with those of conventional interconnect technologies. It is shown that the performance of VGNR is comparable (and even superior) to that of the horizontal GNR (HGNR). Moreover, VGNRs may provide a promising solution to the thermal challenges faced by HGNRs in nanoscale ICs.

Published in: "IEEE Transactions on Electron Devices".

Optimized Transport Properties in Lithium Doped Black Phosphorus Transistors

2018-04-26T23:20:49+00:00 April 26th, 2018|Categories: Publications|Tags: |

Achieving low contact resistance is one of the main challenges for black phosphorus (BP) transistors for both electronic and optoelectronic applications. Here we demonstrate a novel yet feasible lithium doping technique, which greatly reduces the contact resistance from 2 to 0.85 $text{k}Omega cdot mu text{m}$ and results in more than 2.5 times improvement in output current and ON/OFF ratio. This can be mainly attributed to the high hole doping density by lithium bis(trifluoromethylsulfonyl)-imide, which results in a narrower carrier injection barrier at the source end. The ON/OFF ratios of BP field-effect transistors with contact doping at 300 and 20 K are 432 and $7.2 times 10^{5}$ , respectively. A high drain current of $773~mu text{A}/mu text{m}$ with a $0.56~mu text{m}$ channel length at 20 K is also demonstrated. The doping technique provides a valid way to improve the overall performance of BP transistors.

Published in: "IEEE Electron Device Letters".

A Physics-Based Compact Model for Transition-Metal Dichalcogenides Transistors With the Band-Tail Effect

2018-04-26T23:20:48+00:00 April 26th, 2018|Categories: Publications|Tags: |

Due to structural disorder effects, variable range hopping (VRH) transport via band-tail states has been widely observed in the transition-metal dichalcogenide field-effect transistor (TMD FET). However, this significant mechanism has not been incorporated into existing compact models. In this letter, a continuous physics-based compact model considering VRH in TMD FET is developed. Key parameters are extracted by calibration to experimental molybdenum disulfide FET. The voltage dependent carrier density and temperature dependent current characteristics are physically predicted by utilizing the general percolation theory and generalized Einstein relation. Our model is validated by the good agreement between the simulation and experimental results. Furthermore, the relationship between the disorder effects and circuit-level performances are presented. This letter is significant for material engineering and device optimization of TMD FET.

Published in: "IEEE Electron Device Letters".

Current Carrying Capacity of Quasi-1D ZrTe<sub>3</sub> Van Der Waals Nanoribbons

2018-04-26T23:20:45+00:00 April 26th, 2018|Categories: Publications|

Quasi-1D van der Waals materials, such as transition metal trichalcogenides, have strong covalent bonds in one direction and weaker bonds in cross-plane directions. They can be prepared as crystalline nanowires or nanoribbons consisting of 1D atomic threads, i.e., chains. We have examined the current carrying capacity of ZrTe3 nanoribbons using a set of structures fabricated by the shadow mask method. The bulk crystals were synthesized by the chemical vapor transport method and exfoliated onto Si/SiO2 substrates. It was found that ZrTe3 nanoribbons reveal an exceptionally high current density, on the order of ~100 MA/cm2, at the peak of the stressing DC current. The low-frequency noise was of 1/ ${f}$ type near room temperature ( ${f}$ is the frequency). The noise amplitude scaled with the resistance, following the trend established for other low-dimensional materials. The high current density in ZrTe3 can be attributed to the single-crystal nature of quasi-1D van der Waals materials.

Published in: "IEEE Electron Device Letters".

Origin of Nonideal Graphene-Silicon Schottky Junction

2018-04-20T00:29:29+00:00 April 20th, 2018|Categories: Publications|Tags: |

In this paper, we investigate the origin of the nonideal turn- ON characteristics of the graphene-silicon Schottky junction. Native oxide (SiO2) is proved to play a critical role in determining the behavior of a graphene-Si junction. Within the metal–oxide–semiconductor structure, the effective voltage drop across the junction degrades due to the capacitor network, which contributes to an increased ideality factor. Residual metal catalysts are detected, which act as recombination centers in the silicon and further degrade the ideality factor by enhancing the recombination current. The recombination current is found to be the dominant factor in making the junction nonideal. Forming gas annealing and the insertion of an interfacial dielectric restore the ideality of a graphene-Si junction by reducing the interface states and bulk recombination centers.

Published in: "IEEE Transactions on Electron Devices".

Highly Sensitive and Optically Transparent Resistive Pressure Sensors Based on a Graphene/Polyaniline-Embedded PVB Film

2018-04-20T00:29:27+00:00 April 20th, 2018|Categories: Publications|Tags: |

The development of a facile, low-temperature, and low-cost method to fabricate highly sensitive and optically transparent resistive pressure sensors remains a challenge. In this paper, a graphene/polyaniline-embedded polyvinyl butyral (GPANI-PVB) composite film is employed as the active layer in a resistive pressure sensor; this film is ultrathin, highly sensitive, optically transparent, anisotropically conductive, highly durable, highly flexible, and bending insensitive. A flexible touch panel based on the GPANI-PVB composite film is fabricated using a facile process, which supports multitouch and multilevel pressure detection. The GPANI-PVB composite film offers great potential for applications in flexible and transparent interactive electronic devices.

Published in: "IEEE Transactions on Electron Devices".

On Low-Resistance Contacts to 2-D MoTe<sub>2</sub> by Crystalline Phase Junctions

2018-04-10T02:29:36+00:00 April 10th, 2018|Categories: Publications|Tags: |

Low contact resistance to 2-D semiconductor materials plays a critical role on their device applications. It has been experimentally demonstrated recently that the crystalline phase homojunctions between the 1T’ metallic and 2H semiconducting phases of 2-D transition metal dichacolgenide (TMDC) materials can be formed by a variety of fabrication techniques. A multiscale simulation approach that integrates atomistic ab initio simulations with quantum transport calculations based on the nonequilibrium Green’s function formalism is used to examine the contact properties of the crystalline phase junctions of monolayer MoTe2. It is shown that the following mechanisms can contribute to the low contact resistance of crystalline phase metal-semiconductor junctions of 2-D materials. First, the electric field is significantly enhanced at the 2-D phase junction interface due to the extremely thin body, which results in a thin Schottky barrier. Second, the coupling of electron wave functions cross the 1T’-2H junction interface is strong. Third, different from 3-D bulk metal-semiconductor junctions, metal-induced band gap states (MIGS) do not pin the Fermi level in the 2-D material junctions due to low dimensionality of the MIGS charge. The results provide insights into the possibility and limits of achieving low-contact-resistance contacts to 2-D TMDC semiconductors by using crystalline phase metal-semiconductor junctions.

Published in: "IEEE Transactions on Electron Devices".

Complementary Resistive Switching Observed in Graphene Oxide-Based Memory Device

2018-03-22T23:20:43+00:00 March 22nd, 2018|Categories: Publications|Tags: , |

In this letter, complementary resistive switching (CRS) was demonstrated in a single-stack graphene oxide (GO) memory cell for the first time, where the high resistance state can be distinguished into ”0” and ”1” states by different bias polarities. The high switching uniformity ensures reliable reading/writing operations. By changing the compliance currents in the forming and switching processes, the quantity of oxygen defects required for the conducting-filament (CF) formation and supplied by the GO layer ( ${Q} _{R}$ and ${Q} _{S}$ ) was adjusted to determine their influence on the CRS. It was found that the CRS only occurred at the condition of ${Q} _{R} > {Q}_{S}$ and its mechanism is due to the inversion of CF geometry.

Published in: "IEEE Electron Device Letters".

All-Two-Dimensional-Material Hot Electron Transistor

2018-03-22T23:20:41+00:00 March 22nd, 2018|Categories: Publications|Tags: , |

In this letter, we report the first experimental realization of purely two-dimensional-material-based hot electron transistor (2D-HET) by the van der Waals stacking. We used ultra-thin graphene as the base, and WSe2 or ${h}$ -BN as the emitter-base or base-collector barriers. We quantitatively determined that the transport mechanism through the 2D barrier changes from the Fowler–Nordheim tunneling to the thermionic emission with the increase of temperature. In our 2D-HET, the dangling-bond-free 2D materials provide atomically sharp interfaces to suppress the hot electron scattering, which along with the optimization of the barriers, gives a relatively large collection efficiency of 99.95% and a relatively high current density of 233 A/cm2 in the family of graphene-base HETs.

Published in: "IEEE Electron Device Letters".

On the Adequacy of the Transmission Line Model to Describe the Graphene–Metal Contact Resistance

2018-03-20T00:30:19+00:00 March 19th, 2018|Categories: Publications|Tags: |

The contact-end-resistance (CER) method is applied to transfer length method structures to characterize in-depth the graphene–metal contact and its dependence on the back-gate bias. Parameters describing the graphene–metal stack resistance are extracted through the widely used transmission line model. The results show inconsistencies which highlight application limits of the model underlying the extraction method. These limits are attributed to the additional resistance associated with the p-p+ junction located at the contact edge, that is not part of the conventional transmission line model. Useful guidelines for a correct application of the extraction technique are provided, identifying the bias range in which this additional resistance is negligible. Finally, the CER method and the transmission line model are exploited to characterize the graphene–metal contacts featuring different metals.

Published in: "IEEE Transactions on Electron Devices".

Modeling Thermal Performance of Nano-GNRFET Transistors Using Ballistic-Diffusive Equation

2018-03-20T00:30:15+00:00 March 19th, 2018|Categories: Publications|Tags: |

This paper inquires the phonon transport and temperature distribution in graphene nanoribbon field-effect transistor (GNRFET). We propose a modified Ballistic-diffusive equation model (BDE) to explore the phonon transport in GNRFET transistors. In addition, we have studied the effect of the channel length on thermal characteristics. Finite-element approximation used to study the temperature distribution in nano-GNRFET. The proposed model has been validated on the basis of available results from the literature. Our results shows that phonon transport predicted by the proposed BDE model is close to that obtained by Boltzmann transport equation. We found that the proposed model is able to estimate the thermal performance of GNRFET. The results prove also that the characteristic length of the GNR has no important role in the increasing of the temperature.

Published in: "IEEE Transactions on Electron Devices".

Flexible IZO Homojunction TFTs With Graphene Oxide/Chitosan Composite Gate Dielectrics on Paper Substrates

2018-03-02T01:20:42+00:00 March 2nd, 2018|Categories: Publications|Tags: , |

Solution-processed graphene oxide/chitosan composite electrolyte film showed a large specific gate electric-double-layer capacitance of $sim 3.16~mu text{F}$ /cm $^{{mathsf {2}}}$ at 1 Hz. Indium–zinc-oxide (IZO) homojunction thin-film transistors (TFTs) on paper substrates using such composite electrolytes as gate dielectrics showed a good electrical performance and a high stability. Flexible IZO-based homojunction TFTs showed a high drain current ON/OFF ratio of $sim textsf {1.8}times textsf {10}^{{textsf {7}}}$ , a large field-effect mobility of >30 cm $^{{textsf {2}}}text{V}^{{-textsf {1}}}text{s}^{{-textsf {1}}}$ , and a low subthreshold swing of 90 mV/decade. At last, a resistor-loaded inverter with a maximal conversion factor of 6.7, and a dual in-plane gate NAND logic operation were also demonstrated on such flexible IZO-based TFTs. Such oxide-based homojunction TFTs on paper substrates have potential applications in next-generation low-cost and portable new-concept electronics.

Published in: "IEEE Electron Device Letters".

Graphene-Based Thermionic Solar Cells

2018-03-02T01:20:40+00:00 March 2nd, 2018|Categories: Publications|Tags: |

A model of the graphene-based thermionic solar cell (TSC) consisting of a concentrator, an absorber, and a thermionic emission device configured with graphene-based cathode is proposed, where the radiation and reflection losses from the absorber to the environment, the thermal radiation between the cathode and the anode electrodes, and the heat losses from the anode to the environment are considered. The performance characteristics of the TSC are analyzed by numerical calculations. It is found that the maximum efficiency can reach 21% when the area ratio is 0.24 and the voltage output is 2.01 V. In addition, the maximum efficiencies of the TSC under different concentrations and the optimal values of some key parameters are determined, and consequently, the corresponding optimally operating conditions are obtained. The results obtained here may provide guidance for the appropriate selection of electrode materials and the optimum design of practical TSC devices.

Published in: "IEEE Electron Device Letters".

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