Muruganathan Manoharan

Silicon-on-Insulator-Based Radio Frequency Single-Electron Transistors Operating at Temperatures above 4.2 K

A radio frequency single-electron transistor (RF-SET) based on a silicon-on-insulator (SOI) substrate is demonstrated to operate successfully at temperatures above 4.2 K. The SOI SET was fabricated by inducing lateral constrictions in doped SOI nanowires. The device structure was optimized to overcome the inherent drawback of high resistance with the SOI SETs. We performed temperature variation measurements after five thermal cyclings of the same sample to 4.2 K and found that the single-dot device transport characteristics are highly stable. The charge sensitivity was measured to be 36 microe(rms) Hz(-1/2) at 4.2 K, and the RF-SET operation was demonstrated up to 12.5 K for the first time. This work is an important prerequisite to realizing operation of RF-SETs at noncryogenic temperatures.

Edge irregularities in extremely down-scaled graphene nanoribbon devices: role of channel width

When it comes to extremely downscaled graphene device research, it is imperative to develop a comprehensive understanding of what kinds of edge irregularities are likely to occur in the realistic graphene nanoribbons (GNRs) as well as their impact on the electronic and transport properties of GNRs. Here we present the first-principle calculations of the formation energy of the edge vacancies and protrusions in the armchair GNRs (AGNRs) with widths ranging from 9 to 12 carbon atoms and zigzag GNRs (ZGNRs). We also examine their influence on the electronic states and transport characteristics of the GNRs. The formation energy calculations show that double vacancy (DV) edge defects and zigzag protrusions are the most likely edge irregularities in the AGNRs. The DV edge defects increase the bandgap in 11-AGNRs and decrease the bandgap in 9, 10, 12-AGNRs. Zigzag protrusions widen the bandgap in 9, 12-AGNRs and reduce the bandgap in 10, 11-AGNRs. Edge defects induced wave function localization leads to the anti-resonant transmission characteristics. Edges of the ZGNRs show a high tendency to be modified by the exothermic effect. However, their current carrying capacity is not compromised by the edge irregularities.

The impacts of electronic state hybridization on the binding energy of single phosphorus donor electrons in extremely downscaled silicon nanostructures

We present the density functional theory calculations of the binding energy of the Phosphorus (P) donor electrons in extremely downscaled single P-doped Silicon (Si) nanorods. In past studies, the binding energy of donor electrons was evaluated for the Si nanostructures as the difference between the ionization energy for the single P-doped Si nanostructures and the electron affinity for the un-doped Si nanostructures. This definition does not take into account the strong interaction of donor electron states and Si electron states explicitly at the conductive states and results in a monotonous increase in the binding energy by reducing the nanostructures dimensions. In this paper, we introduce a new approach to evaluate the binding energy of donor electrons by combining the projected density of states (PDOS) analysis and three-dimensional analysis of associated electron wavefunctions. This enables us to clarify a gradual change of the spatial distribution of the 3D electron wavefunctions (3DWFs) from the ...

Finite element method simulation of graphene nanoelectromechanical contact switches with surface trenches

This article discusses the effect of dynamic contacts on the doubly clamped graphene nanoelectromechanical (NEM) switch with trenches. Trenches with five different depths were used to study their impact on device performance using finite element method simulation. A modified form of a lumped parallel-plate capacitor model is used to describe the NEM switch with trenches. Pull-in and pull-out switching characteristics, contact pressure, and the reaction force of the NEM switch with trenches are discussed in detail. These results provide a good theoretical guideline to design a high-performance NEM switch with high mechanical reliability.

Enhancement of inter-band tunneling due to low-dimensionality of lateral 2D Silicon Esaki diodes

Silicon tunnel devices are important for low power-consumption and fast-switching electronics. Inter-band tunneling current is, however, limited due to the indirect-bandgap nature of Si, but low-dimensionality of the devices may bring new effects into play. In this work, we analyze two-dimensional lateral Si Esaki diodes and first observe that inter-band tunneling is still largely mediated by phonon assistance. More importantly, however, we find that tunnel current is significantly enhanced by resonances involving dopant-atoms with deeper energy levels in the depletion region. These results experimentally illustrate the impact of atomistic effects in low-dimensional tunnel devices.

First-principles study of hydrogen-enhanced phosphorus diffusion in silicon

We present a first-principles study on the interstitial-mediated diffusion process of neutral phosphorus (P) atoms in a silicon crystal with the presence of mono-atomic hydrogen (H). By relaxing initial Si structures containing a P atom and an H atom, we derived four low-energy P-H-Si defect complexes whose formation energies are significantly lower than those of P-Si defect complexes. These four defect complexes are classified into two groups. In group A, an H atom is located near a Si atom, whereas in group B, an H atom is close to a P atom. We found that the H atom pairs with P or Si atom and changes the nature bonding between P and Si atoms from out-of-phase conjugation to in-phase conjugation. This fact results in the lower formation energies compare to the cases without H atom. For the migration of defect complexes, we have found that P-H-Si defect complexes can migrate with low barrier energies if an H atom sticks to either P or Si atom. Group B complexes can migrate from one lattice site to anothe...

Bilayer graphene single carrier transistors

In this work, we study the operation of the bilayer Graphene Single Carrier Transistor (GSCT) by varying the design of tunnel junction structures. Clear Coulomb current oscillation characteristics with the peak-to-valley ratio over 102 are achieved at 1.7 K. In addition, we observed unique double peak structures in the Coulomb oscillation at the bias voltage smaller than 5 mV and at temperature lower than 9 K. By comparing with the bilayer GSET equivalent circuit model, the observed double peak structures are attributable to single carrier tunneling via vertically stacked double charging islands.

Study of quantized-energy effects in Si nanoscale lateral pn junction diodes

In this work, we study SOI nanoscale pn junctions and find that transport characteristics are strongly affected by states of individual dopants and by quantized energy states. For pn diodes with lower doping concentration, we find that individual dopant atoms work as electron traps, inducing RTS in the diode current. On the other hand, for highly-doped pn diodes, quantization effects play critical roles in transport characteristics for both forward and reverse bias regimes.

Control of electron transport regimes via single- and multiple-donors in nano-channel SOI-FETs

In this work, we find systematic transitions of electron transport regimes by controlling the number of strongly-interacting P donors in SOI-FET nano-channels. We found that, as a function of the location and number of dopants, doped using a selective-doping technique, electron transport occurs via individual P donors (in samples with low ND), via “clusters” of a few P donors (for higher ND) and via many-donor QDs, which form complex energy bands (for highly-doped samples).

Impact of Dopant Induced States on Interband Tunneling in Nanoscale pn Junctions

We report on an interband tunneling nanoscale Si pn junction with high doping concentration of ~5.0×10 cm. We find that transport characteristics show step-like structure, indicating that interband tunneling is strongly influenced by dopant-induced states of the depletion region. Also, we find a current peak observed in reverse bias condition at low temperatures, indicating that the dopant states can directly contribute to interband tunneling current. This is different from pn junctions with low doping concentration of ~1.0×10 cm, in which individual dopant atoms work as electron traps.