Joshua T. Smith

Carbon Nanotube Complementary Wrap-Gate Transistors

Among the challenges hindering the integration of carbon nanotube (CNT) transistors in digital technology are the lack of a scalable self-aligned gate and complementary n- and p-type devices. We report CNT transistors with self-aligned gates scaled down to 20 nm in the ideal gate-all-around geometry. Uniformity of the gate wrapping the nanotube channels is confirmed, and the process is shown not to damage the CNTs. Further, both n- and p-type transistors were realized by using the appropriate gate dielectric-HfO2 yielded n-type and Al2O3 yielded p-type-with quantum simulations used to explore the impact of important device parameters on performance. These discoveries not only provide a promising platform for further research into gate-all-around CNT devices but also demonstrate that scalable digital switches with realistic technological potential can be achieved with carbon nanotubes.

Silicon Nanowire Tunneling Field-Effect Transistor Arrays: Improving Subthreshold Performance Using Excimer Laser Annealing

We have experimentally established that the inverse subthreshold slope S of a Si nanowire tunneling field-effect transistor (NW-TFET) array can be within 9% of the theoretical limit when the doping profile along the channel is properly engineered. In particular, we have demonstrated that combining excimer laser annealing with a low-temperature rapid thermal anneal results in an abrupt doping profile at the source/channel interface as evidenced by the electrical characteristics. Gate-controlled tunneling has been confirmed by evaluating S as a function of temperature. The good agreement between our experimental data and simulation allows performance predictions for more aggressively scaled TFETs. We find that Si NW-TFETs can be indeed expected to deliver S-values below 60 mV/dec for optimized device structures.

Fixed-Gap Tunnel Junction for Reading DNA Nucleotides

Previous measurements of the electronic conductance of DNA nucleotides or amino acids have used tunnel junctions in which the gap is mechanically adjusted, such as scanning tunneling microscopes or mechanically controllable break junctions. Fixed-junction devices have, at best, detected the passage of whole DNA molecules without yielding chemical information. Here, we report on a layered tunnel junction in which the tunnel gap is defined by a dielectric layer, deposited by atomic layer deposition. Reactive ion etching is used to drill a hole through the layers so that the tunnel junction can be exposed to molecules in solution. When the metal electrodes are functionalized with recognition molecules that capture DNA nucleotides via hydrogen bonds, the identities of the individual nucleotides are revealed by characteristic features of the fluctuating tunnel current associated with single-molecule binding events.

Highly ordered diamond and hybrid triangle-diamond patterns in porous anodic alumina thin films

Utilizing nonequilibrium formation kinetics in porous anodic alumina (PAA) thin films, diamond and hybrid triangle-diamond pore patterns are achieved. During anodization, the self-compensation abilities of PAA allow diamond-shaped pores to form by omitting certain sites in the surface prepatterning process. The effects of tessellation on cell formation in these arrangements yield elongated, regular, and partially compressed hexagonal cell structures leading to diamond, circular, and triangular pores, respectively. The diamond-shaped porous templates provide a low-cost option for the preparation of scalable nanostructures with diamond-shaped cross sections with utility in a range of nanoscale applications, including enhanced sensing and field emission.

Transport Modulation in Ge/Si Core/Shell Nanowires through Controlled Synthesis of Doped Si Shells

Appropriately controlling the properties of the Si shell in Ge/Si core/shell nanowires permits not only passivation of the Ge surface states, but also introduces new interface phenomena, thereby enabling novel nanoelectronics concepts. Here, we report a rational synthesis of Ge/Si core/shell nanowires with doped Si shells. We demonstrate that the morphology and thickness of Si shells can be controlled for different dopant types by tuning the growth parameters during synthesis. We also present distinctly different electrical characteristics that arise from nanowire field-effect transistors fabricated using the synthesized Ge/Si core/shell nanowires with different shell morphologies. Furthermore, a clear transition in the modification of device characteristics is observed for crystalline shell nanowires following removal of the shell using a unique trimming process of successive native oxide formation/etching. Our results demonstrate that the preferred transport path through the nanowire structure can be modulated by appropriately tuning the growth conditions.

Broken-Gap Tunnel MOSFET: A Constant-Slope Sub-60-mV/decade Transistor

We propose a novel low-power transistor device, called the broken-gap tunnel MOSFET (BG-TMOS), which is capable of achieving constant sub-60-mV/decade inverse subthreshold slopes S at room temperature. Structurally, the device resembles an ungated broken-gap heterostructure Esaki region in series with a conventional MOSFET. The gate voltage independence of the energy spacing between the conduction and valence bands at the heterojunction is the key to producing a constant S <; 60 mV/decade, which can be tuned by properly engineering the material composition at this interface. In contrast to the tunneling field-effect transistor, the tunnel junction in the BG-TMOS is independent of the electrostatics in the channel region, enabling the use of 2-D architectures for improved current drive without degradation of S -attractive features from a circuit design perspective. Simulations show that the BG-TMOS can exceed MOSFET performance at low supply voltages.

Scalable and fully self-aligned n-type carbon nanotube transistors with gate-all-around

While proven to provide high performance at sub-10 nm lengths, carbon nanotube (CNT) field-effect transistors (FETs) typically employ impractical gate geometries. Here we demonstrate fully self-aligned CNTFETs that include a gate-all-around (GAA) the nanotube channels - the ideal gate geometry for a 1D CNT. These GAA-CNTFETs have 30 nm channel lengths and exhibit n-type operation with high on-currents and good switching behavior that is explained by quantum transport (NEGF) simulations. This work is an important milestone showing that a technologically relevant self-aligned device can be realized with nanotubes.

Hydrodynamics of Diamond-Shaped Gradient Nanopillar Arrays for Effective DNA Translocation into Nanochannels

Effective DNA translocation into nanochannels is critical for advancing genome mapping and future single-molecule DNA sequencing technologies. We present the design and hydrodynamic study of a diamond-shaped gradient pillar array connected to nanochannels for enhancing the success of DNA translocation events. Single-molecule fluorescence imaging is utilized to interrogate the hydrodynamic interactions of the DNA with this unique structure, evaluate key DNA translocation parameters, including speed, extension, and translocation time, and provide a detailed mapping of the translocation events in nanopillar arrays coupled with 10 and 50 μm long channels. Our analysis reveals the important roles of diamond-shaped nanopillars in guiding DNA into as small as 30 nm channels with minimized clogging, stretching DNA to nearly 100% of their dyed contour length, inducing location-specific straddling of DNA at nanopillar interfaces, and modulating DNA speeds by pillar geometries. Importantly, all critical features down to 30 nm wide nanochannels are defined using standard photolithography and fabrication processes, a feat aligned with the requirement of high-volume, low-cost production.

Broken flow symmetry explains the dynamics of small particles in deterministic lateral displacement arrays

Significance Deterministic lateral displacement (DLD) is a technique for size fractionation of particles in continuous flow that has shown great potential for biological and clinical applications. Several theoretical models have been proposed to explain the trajectories of different-sized particles in relation to the geometry of the pillar array, but experimental evidence has demonstrated that a rich class of intermediate migration behavior exists, which is not predicted by models. In this work, we present a unified theoretical framework to infer the trajectory of particles in the whole array on the basis of trajectories in the unit cell. This framework explains many of the unexpected particle trajectories reported in literature and can be used to design arrays for the fractionation of particles, even at the smallest scales reaching the molecular realm. We also performed experiments that verified our predictions, even at the nanoscales. Using our model as a set of design rules, we developed a condenser structure that achieves full particle separation with a single fluidic input. Deterministic lateral displacement (DLD) is a technique for size fractionation of particles in continuous flow that has shown great potential for biological applications. Several theoretical models have been proposed, but experimental evidence has demonstrated that a rich class of intermediate migration behavior exists, which is not predicted. We present a unified theoretical framework to infer the path of particles in the whole array on the basis of trajectories in a unit cell. This framework explains many of the unexpected particle trajectories reported and can be used to design arrays for even nanoscale particle fractionation. We performed experiments that verify these predictions and used our model to develop a condenser array that achieves full particle separation with a single fluidic input.

Effects of nanoscale contacts to silicon nanowires on contact resistance: Characterization and Modeling

We have shown the first experimental data related to the contact resistance associated with nanoscale contact lengths along NWs to address scaling issues. We have implemented a new model that appropriately accounts for the finite depth and depletion width of a given semiconductor that preserves resistivity values in the short contact regime. Our model permits accurate predictions for the contact resistance over the entire range of Lcontact values knowing just two experimental points, one in both the short and long contact limit.