P. M. Campbell

Random networks of carbon nanotubes as an electronic material

We report on the transport properties of random networks of single-wall carbon nanotubes fabricated into thin-film transistors. At low nanotube densities (∼1 μm−2) the networks are electrically continuous and behave like a p-type semiconductor with a field-effect mobility of ∼10 cm2/V s and a transistor on-to-off ratio ∼105. At higher densities (∼10 μm−2) the field-effect mobility can exceed 100 cm2/V s; however, in this case the network behaves like a narrow band gap semiconductor with a high off-state current. The fact that useful device properties are achieved without precision assembly of the nanotubes suggests the random carbon nanotube networks may be a viable material for thin-film transistor applications.

Chemical Vapor Sensing with Monolayer MoS2

Two-dimensional materials such as graphene show great potential for future nanoscale electronic devices. The high surface-to-volume ratio is a natural asset for applications such as chemical sensing, where perturbations to the surface resulting in charge redistribution are readily manifested in the transport characteristics. Here we show that single monolayer MoS(2) functions effectively as a chemical sensor, exhibiting highly selective reactivity to a range of analytes and providing sensitive transduction of transient surface physisorption events to the conductance of the monolayer channel. We find strong response upon exposure to triethylamine, a decomposition product of the V-series nerve gas agents. We discuss these results in the context of analyte/sensor interaction in which the analyte serves as either an electron donor or acceptor, producing a temporary charge perturbation of the sensor material. We find highly selective response to electron donors and little response to electron acceptors, consistent with the weak n-type character of our MoS(2). The MoS(2) sensor exhibits a much higher selectivity than carbon nanotube-based sensors.

Fabrication of Si nanostructures with an atomic force microscope

A method for fabricating Si nanostructures with an air‐operated atomic force microscope (AFM) is presented. An electrically conducting AFM tip is used to oxidize regions of size 10–30 nm of a H‐passivated Si (100) surface at write speeds up to 1 mm/s. This oxide serves as an effective mask for pattern transfer into the substrate by selective liquid etching. The initial oxide growth rate depends exponentially on the applied voltage which produces an effective ‘‘tip sharpening’’ that allows small features to be produced by a relatively large diameter tip.

Epitaxial-Graphene RF Field-Effect Transistors on Si-Face 6H-SiC Substrates

We report dc and the first-ever measured small-signal radio-frequency (RF) performance of epitaxial-graphene RF field-effect transistors (FETs), where the epitaxial-graphene layer is formed by graphitization of 2-in-diameter Si-face semi-insulating 6H-SiC (0001) substrates. The gate is processed with a metal gate on top of a high-k Al2 O3 gate dielectric deposited via an atomic-layer-deposition method. With a gate length (Lg) of 2 mum and an extrinsic transconductance of 148 mS/mm, the extrinsic current-gain cutoff frequency (fT) is measured as 4.4 GHz, yielding an extrinsic fT ldr Lg of 8.8 GHz middot mum. This is comparable to that of Si NMOS. With graphene FETs fabricated in a layout similar to those of Si n-MOSFETs, on-state current density increases dramatically to as high as 1.18 A/mm at Vds = 1 V and 3 A/mm at Vds = 5 V. The current drive level is the highest ever observed in any semiconductor FETs.

High-mobility Carbon-nanotube Thin-film Transistors on a Polymeric Substrate

We report the development of high-mobility carbon-nanotube thin-film transistors fabricated on a polymeric substrate. The active semiconducting channel in the devices is composed of a random two-dimensional network of single-walled carbon nanotubes (SWNTs). The devices exhibit a field-effect mobility of 150cm2∕Vs and a normalized transconductance of 0.5mS∕mm. The ratio of on-current (Ion) to off-current (Ioff) is ∼100 and is limited by metallic SWNTs in the network. With electronic purification of the SWNTs and improved gate capacitance we project that the transconductance can be increased to ∼10–100mS∕mm with a significantly higher value of Ion∕Ioff, thus approaching crystalline semiconductor-like performance on polymeric substrates.

Fabrication of nanometer‐scale side‐gated silicon field effect transistors with an atomic force microscope

The fabrication of nanometer‐scale side‐gated silicon field effect transistors using an atomic force microscope is reported. The probe tip was used to define nanometer‐scale source, gate, and drain patterns by the local anodic oxidation of a passivated silicon (100) surface. These thin oxide patterns were used as etch masks for selective etching of the silicon to form the finished devices. Devices with critical features as small as 30 nm have been fabricated with this technique.

Single‐atom point contact devices fabricated with an atomic force microscope

The fabrication of atomic point contacts by using anodic oxidation of thin Al films with an atomic force microscope is reported. In situ electrical measurements were used as feedback to control the fabrication of Al nanowires that were subsequently anodized through their cross section to form point contacts. When the conductance of a point contact is reduced below ∼5×10−4 S it starts to decrease in discrete steps of ∼2e2/h. In some devices we are able to stabilize the conductance at a value near 2e2/h which corresponds to a single, atomic‐sized conducting channel.

Fabrication of silicon nanostructures with a scanning tunneling microscope

A technique is presented for fabricating Si nanostructures with a scanning tunneling microscope operated in air. The process involves the direct chemical modification of a H‐passivated Si(100) surface and a subsequent liquid etch. The chemically modified portions of the surface can withstand a deep (≳100 nm) liquid etch of the unmodified regions with no etch degradation of the modified surface. At a write speed of 1–10 μm/s, large‐area (50 μm×50 μm) patterns with lateral feature sizes ∼25 nm are reliably fabricated.

Hall effect mobility of epitaxial graphene grown on silicon carbide

Epitaxial graphene (EG) films were grown in vacuo by silicon sublimation from the (0001) and (0001¯) faces of 4H-SiC and 6H-SiC. Hall effect mobilities and sheet carrier densities of the films were measured at 300 and 77 K and the data depended on the growth face. About 40% of the samples exhibited holes as the dominant carrier, independent of face. Generally, mobilities increased with decreasing carrier density, independent of carrier type and substrate polytype. The contributions of scattering mechanisms to the conductivities of the films are discussed. The results suggest that for near-intrinsic carrier densities at 300 K epitaxial graphene mobilities will be ∼150 000 cm2 V−1 s−1 on the (0001¯) face and ∼5800 cm2 V−1 s−1 on the (0001) face.

Top-Gated Epitaxial Graphene FETs on Si-Face SiC Wafers With a Peak Transconductance of 600 mS/mm

In this letter, we present state-of-the-art performance of top-gated graphene n-FETs and p-FETs fabricated with epitaxial graphene layers grown on Si-face 6H-SiC substrates on 50-mm wafers. The current-voltage characteristics of these devices show excellent saturation with on-state current densities (I_on) of 0.59 A/mm at V_ds = 1 V and 1.65 A/mm at V_ds = 3 V. I_on/I_off ratios of 12 and 19 were measured at V_ds = 1 and 0.5 V, respectively. A peak extrinsic g_m as high as 600 mS/mm was measured at V_ds = 3.05 V, with a gate length of 2.94 ¿m. The field-effect mobility versus effective electric field (E_eff) was measured for the first time in epitaxial graphene FETs, where record field-effect mobilities of 6000 cm²/V·s for electrons and 3200 cm²/V·s for holes were obtained at E_eff ~ 0.27 MV/cm .