Zhihong Chen - Academia.edu (original) (raw)

Papers by Zhihong Chen

The nature of source/drain contacts in graphene field-effect transistors (GFETs) is investigated.... more The nature of source/drain contacts in graphene field-effect transistors (GFETs) is investigated. We argue that the energy dispersion of the graphene portion underneath of the metal electrodes is substantially altered compared to its original. Moreover, different from other materials, the Fermi levels of these graphene segments are not pinned by the metal contacts, but instead can be modulated by the gate. Our experimental findings give important insights into the critical relevance of contacts for the scaling of graphene devices.

After an introduction that explains why graphene is in general considered to be promising for ele... more After an introduction that explains why graphene is in general considered to be promising for electronic applications since a few years, we will discuss several of our own new findings on graphene field-effect transistors. In particular, this article makes three points: 1) The material itself - graphene - offers intrinsic advantages due to reduced scattering in low-dimensional structures. 2) The

2009 IEEE International Electron Devices Meeting (IEDM), 2009

The effects of channel length scaling on carbon nanotube field-effect transistor (CNTFET) perform... more The effects of channel length scaling on carbon nanotube field-effect transistor (CNTFET) performance was investigated by varying device lengths on the same nanotube. Results show that scaling improves performance with substantial increases in on-current, resistances closer to the quantum limit than have ever been reported, and the shortest (~30 nm) well-behaving CNTFETs to date.

Nano Letters, 2015

Highly conductive copper nanowires (CuNWs) are essential for efficient data transfer and heat con... more Highly conductive copper nanowires (CuNWs) are essential for efficient data transfer and heat conduction in wide ranging applications like high-performance semiconductor chips and transparent conductors. However, size scaling of CuNWs causes severe reduction in electrical and thermal conductivity due to substantial inelastic surface scattering of electrons. Here we report a novel scalable technique for low-temperature deposition of graphene around CuNWs and observe strong enhancement of electrical and thermal conductivity for graphene-encapsulated CuNWs compared to uncoated CuNWs. Fitting the experimental data with the theoretical model for conductivity of CuNWs reveals significant reduction in surface scattering of electrons at the oxide-free CuNW surfaces, translating into 15% faster data transfer and 27% lower peak temperature compared to the same CuNW without the graphene coating. Our results provide compelling evidence for improved speed and thermal management by adapting the Cu-graphene hybrid technology in future ultrascaled silicon chips and air-stable flexible electronic applications.

Science, 2006

Single-walled carbon nanotubes (SWCNTs) have been shown to exhibit excellent electrical propertie... more Single-walled carbon nanotubes (SWCNTs) have been shown to exhibit excellent electrical properties, such as ballistic transport over several hundred nanometers at room temperature. Field-effect transistors (FETs) made from individual tubes show dc performance specifications rivaling those of state-of-the-art silicon devices. An important next step is the fabrication of integrated circuits on SWCNTs to study the high-frequency ac capabilities of SWCNTs. We built a five-stage ring oscillator that comprises, in total, 12 FETs side by side along the length of an individual carbon nanotube. A complementary metal-oxide semiconductor‐type architecture was achieved by adjusting the gate work functions of the individual p-type and n-type FETs used.

Physica E: Low-dimensional Systems and Nanostructures, 2007

We investigate electrical transport and noise in semiconducting carbon nanotubes. By studying car... more We investigate electrical transport and noise in semiconducting carbon nanotubes. By studying carbon nanotube devices with various diameters and contact metals, we show that the ON-currents of CNFETs are governed by the heights of the Schottky barriers at the metal/nanotube interfaces. The current fluctuations are dominated by 1/f noise at low-frequencies and correlate with the number of transport carriers in the device regardless of contact metal.

Nano Letters, 2005

Single-wall carbon nanotube field-effect transistors (CNFETs) have been shown to behave as Schott... more Single-wall carbon nanotube field-effect transistors (CNFETs) have been shown to behave as Schottky barrier (SB) devices. It is not clear, however, what factors control the SB size. Here we present the first statistical analysis of this issue. We show that a large data set of more than 100 devices can be consistently accounted by a model that relates the on-current of a CNFET to a tunneling barrier whose height is determined by the nanotube diameter and the nature of the source/drain metal contacts. Our study permits identification of the desired combination of tube diameter and type of metal that provides the optimum performance of a CNFET.

Nano Letters, 2013

Spin-based devices are widely discussed for post-complementary metal−oxide−semiconductor (CMOS) a... more Spin-based devices are widely discussed for post-complementary metal−oxide−semiconductor (CMOS) applications. A number of spin device ideas propose using spin current to carry information coherently through a spin channel and transfering it to an output magnet by spin transfer torque. Graphene is an ideal channel material in this context due to its long spin diffusion length, gate-tunable carrier density, and high carrier mobility. However, spin transfer torque has not been demonstrated in graphene or any other semiconductor material as of yet. Here, we report the first experimental measurement of spin transfer torque in graphene lateral nonlocal spin valve devices. Assisted by an external magnetic field, the magnetization reversal of the ferromagnetic receiving magnet is induced by pure spin diffusion currents from the input magnet. The magnetization switching is reversible between parallel and antiparallel configurations, depending on the polarity of the applied charged current. The presented results are an important step toward developing graphene-based spin logic and understanding spin-transfer torque in systems with tunneling barriers.

Nano Letters, 2003

A postsynthesis method of separating metallic from semiconducting single-walled carbon nanotubes ... more A postsynthesis method of separating metallic from semiconducting single-walled carbon nanotubes and a method based on absorption spectroscopy for assay of the separation efficiency are described. The separation method relies on chemical discrimination in the chargetransfer complex formation between bromine and the metallic versus semiconducting nanotubes and takes advantage of the resulting density difference to effect a centrifugation-based separation. Calculations support the proposed separation mechanism.

Nano Letters, 2008

The problem of separating single-wall carbon nanotubes (CNTs) by diameter and/or chirality is one... more The problem of separating single-wall carbon nanotubes (CNTs) by diameter and/or chirality is one of the greatest impediments toward the widespread application of these promising materials in nanoelectronics. In this paper, we describe a novel physical−chemical method for diameter-selective CNT separation that is both simple and effective and that allows up-scaling to large volumes at modest cost. Separation is based on size-selective noncovalent matching of an appropriate anchor molecule to the wall of the CNT, enabling suspension of the CNTs in solvents in which they would otherwise not be soluble. We demonstrate size-selective separation in the 1−2 nm diameter range using easily synthesized oligo-acene adducts as a diameter-selective molecular anchor. CNT field effect transistors fabricated from diameter-selected CNTs show markedly improved electrical properties as compared to nonselected CNTs.

IEEE Transactions On Nanotechnology, 2007

IEEE Electron Device Letters, 2008

ACS Nano, 2014

A graphene lateral spin valve structure with asymmetric contacts is presented for the first time,... more A graphene lateral spin valve structure with asymmetric contacts is presented for the first time, with enhancement of spin angular momentum absorption in its receiving magnet. The asymmetric device with tunneling barrier only at the injector magnet shows a comparable spin valve signal but lower electrical noises compared to the device with two tunneling barriers. We also report experimental measurements of spin transfer torque. Assisted by an external magnetic field of 2.5 mT, spin diffusion currentinduced magnetization reversal occurs at a nonlocal charge current density of 33 mA/μm 2 , smaller than that needed in devices with two tunneling barriers.

IEEE Transactions on Nanotechnology, 2012

We present a study on the metal-graphene contact properties. Utilizing a dual-gate field-effect t... more We present a study on the metal-graphene contact properties. Utilizing a dual-gate field-effect transistor device, an energetic separation between the Fermi level and the Dirac point in the contact areas can be adjusted deliberately by applying an appropriate front-gate voltage that acts only on the channel. This front-gate voltage is compensated by an opposite large-area backgate voltage, thereby mimicking the metal induced doping effect. A back-gate voltage sweep enables identifying two distinct resistance peaks-a result of the combined impact of the graphene cones in the contact and in the channel region. Comparing our experimental data with simulations allows extracting the coupling strength between metal and graphene and also estimating the magnitude of the metal-induced doping concentration in the case of palladium contacts. In contrast to conventional metal-semiconductor contacts, our simulations predict a decreased on-current for increased coupling strength in graphene field-effect transistors.

AIP Conference Proceedings, 2004

Perhaps the most wonderful feature of carbon nanotubes is that they are synthesized in both metal... more Perhaps the most wonderful feature of carbon nanotubes is that they are synthesized in both metallic and semiconducting variants, and perhaps the most problematic feature is that they are synthesized in both metallic and semiconducting variants. The intimate mixture hampers numerous envisioned applications and separation of the nanotubes into their respective electronic transport classes has emerged (after high yield synthesis