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In this Letter, a comprehensive analysis of how the metallic behavior transition to hopping was presented by studying the transport mechanisms of low resistance states (LRS) in Ta2O5−x/TaOy resistive switching devices at very low temperatures. Three types of conduction behaviors were reported through temperature-dependent measurements ranging from 5 K to 250 K. Memory cells at low LRS show metallic behavior due to the formation of metallic filament. The temperature dependence of resistance at medium LRS exhibits an interesting phenomenon that a positive temperature coefficient transfers into a negative one at temperature of 20 K. Detailed analysis reveals that this phenomenon is caused by the coexistence of extended and localized states, with metallic conduction at higher temperatures and variable-range hopping at lower temperatures. Carrier transport at high LRS is dominated by electrons hopping conduction with nearest-neighboring hopping conduction changing into variable-range hopping as temperature dec...

In this Letter, the origins of current fluctuations of Al/AlOx/WOy/W bilayer resistive random access memory (RRAM) devices are investigated through detailed noise analysis. Random telegraph noise (RTN) measurements were performed on RRAMs with three different resistance states. An obvious RTN signal with 40.7% amplitude difference was found at high resistance state, and the trapping/de-trapping process leading to the RTN signal was studied in detail by extracting the trap energy from energy diagram. For median and low resistance states, the resistance fluctuations were 34.0% and 0.3%, respectively. To further study the RTN characteristics, the normalized power spectral density (PSD) was analyzed. It is found that, for one dominant-trap caused RTN phenomena, the normalized noise PSD behaves as 1/f 2 on the high resistance state; while for median and low resistance states, the noise follows 1/f rule, suggesting that the current fluctuations are associated with the envelop of multiple RTNs caused by traps located near/in the conductive filament. Based on the noise analyses in time and frequency domains, a conduction mechanism is proposed to describe the trap effects on the current fluctuations of different resistance states.

In this letter, the conduction and switching mechanisms of Al/AlOx/WOx/W bilayer resistive random access memory devices are investigated. Five stable resistance states were achieved through current compliance control. For each resistance state, I-V characteristics at different temperatures were measured. Conduction mechanisms are found to vary with resistance states. At low resistance levels, devices show ohmic conduction with metallic behavior. Conduction at medium resistance levels is due to electron hopping. The carrier transport at high resistance levels is governed by Schottky emission. Based on the resistance-dependent transport characteristics, an oxygen migration model is proposed to explain the switching mechanism between different resistance states.

We report the design and fabrication of W:AlOx/WOx bilayer based resistive switching cells in a standard 0.18 μm CMOS process with only one extra mask. The devices show excellent performance with low power consumption. Low operation voltages (SET voltage

The channel mobility and device performance of graphene field-effect transistors (GFETs) were investigated using a theoretical model. Surface polarized phonon scattering and charged impurity Coulomb scattering are two dominant scattering mechanisms in carrier mobility calculation. Mobilities are used to calculate the drain current and transconductance of GFETs. Adding a polymer buffer layer (PBL) with low permittivity between graphene and gate dielectric can effectively improve GFETs performance at the expense of decreasing gate controllability. PBL thickness was optimized to achieve best device transconductance. Experimental results and the model calculations of both channel mobility and device transconductance are in agreement.

In this paper, we perform a modeling and simulation study on strained armchair graphene nanoribbon (AGNR). Two uniaxial strain models based on a tight binding method are compared with results from first-principles calculation. Tunneling field effect transistors (TFETs) with channels made of strained AGNR of different widths are modeled and simulated by a ballistic quantum transport model based on nonequilibrium Green’s function and nonparabolic effective mass approximation. Compared with TFETs with narrow AGNR, those with strained wide AGNR can achieve better device performance.

This paper studies the coating geometries of metals on single-walled carbon nanotubes (SWNTs) on the basis of the nucleation theory and wetting theory. The metal surface energy, cohesion energy, diffusion barrier, and metal-SWNT interfacial energy are calculated using first-principles calculation. Metals including Fe, Al, Au, Pd, Ni, and Ti are considered. For Ti, Ni, and Pd, low metal-SWNT interfacial energies and high diffusion barriers are responsible for forming continuous or quasicontinuous layers on the SWNT surface. In contrast, Al and Au have small diffusion barriers and poor SWNT surface wetting, thus they tend to aggregate and form large clusters. Although the binding energy between Fe and SWNTs is large, due to the large cohesion energy and poor wetting, Fe may form isolated clusters. All results are in good agreement with experimental observations.

Metal-graphene contacts play a critical role in graphene-based electronics. It is found through first-principles calculation of contacts between graphene and 12 different metals that there exist two types of contacts depending on the strength of interaction between d-orbitals in metals and pz-orbitals in graphene. Fermi level shift in the contacted graphene from the freestanding one is investigated, and the electronic structure and electrostatic potential are calculated. The carrier transport through these contacts is calculated using the extended Huckel theory-based non-equilibrium Green’s function formalism, and one type of contact is shown to have less contact resistance than the other.

Schottky barrier (SB) formation at the contact interface between metal and semiconducting carbon nanotubes (CNTs) is of great importance in determining the transport characteristics of a CNT field effect transistor. In this paper, we studied the SB height (SBH) between different metals and CNT contacts using first-principles calculation. A method to calculate SBH is proposed based on the interface dipole effect, which will induce an electrical potential variation at the metal and CNT interface. The SBH of a metal and CNT contact could then be determined by the work function difference between the metal and CNT and the electrical potential variation. We extensively investigated different contacts between Sc, Al, Pd, (8,0) CNT, and (11,0) CNT. The calculated SBHs for these contacts are all in good agreement with experimental results.