Everett Comfort

Optical properties of large-area polycrystalline chemical vapor deposited graphene by spectroscopic ellipsometry

Spectroscopic ellipsometry was used to characterize the complex refractive index of chemical vapor deposition (CVD) graphene grown on copper foils and transferred to glass substrates. Two ellipsometers, with respective wavelength ranges extending into the ultraviolet and infrared (IR), have been used to characterize the CVD graphene optical functions. The optical absorption follows the same relation to the fine structure constant previously observed in the IR region, and displays the exciton-dominated absorption peak at ∼4.5 eV. The optical functions of CVD graphene show some differences when compared to published values for exfoliated graphene.

Angle-Dependent Carrier Transmission in Graphene p–n Junctions

Angle-dependent carrier transmission probability in graphene p-n junctions is investigated. Using electrostatic doping from buried gates, p-n junctions are formed along graphene channels that are patterned to form different angles with the junction. A peak in the junction resistance is observed, which becomes pronounced with angle. This angular dependence is observed for junctions made on both exfoliated and CVD-grown graphene and is consistent with the theoretically predicted dependence of transmission probability on incidence angle.

Reconfigurable p-n junction diodes and the photovoltaic effect in exfoliated MoS2 films

Realizing basic semiconductor devices such as p-n junctions are necessary for developing thin-film and optoelectronic technologies in emerging planar materials such as MoS2. In this work, electrostatic doping by buried gates is used to study the electronic and optoelectronic properties of p-n junctions in exfoliated MoS2 flakes. Creating a controllable doping gradient across the device leads to the observation of the photovoltaic effect in monolayer and bilayer MoS2 flakes. For thicker flakes, strong ambipolar conduction enables realization of fully reconfigurable p-n junction diodes with rectifying current-voltage characteristics, and diode ideality factors as low as 1.6. The spectral response of the photovoltaic effect shows signatures of the predicted band gap transitions. For the first excitonic transition, a shift of >4kBT is observed between monolayer and bulk devices, indicating a thickness-dependence of the excitonic coulomb interaction.

Quantum efficiency and capture cross section of first and second excitonic transitions of single-walled carbon nanotubes measured through photoconductivity.

Comparing photoconductivity measurements, using p-n diodes formed along individual single-walled carbon nanotubes (SWNT), with modeling results, allows determination of the quantum efficiency, optical capture cross section, and oscillator strength of the first (E11) and second (E22) excitonic transitions of SWNTs. This is in the infrared region of the spectrum, where little experimental work on SWNT optical absorption has been reported to date. We estimate quantum efficiency (η) ~1-5% and provide a correlation of η, capture cross section, and oscillator strength for E11 and E22 with nanotube diameter. This study uses the spectral weight of the exciton resonances as the determining parameter in optical measurements.

Measuring carbon nanotube band gaps through leakage current and excitonic transitions of nanotube diodes.

The band gap of a semiconductor is one of its most fundamental properties. It is one of the defining parameters for applications, including nanoelectronic and nanophotonic devices. Measuring the band gap, however, has received little attention for quasi-one-dimensional materials, including single-walled carbon nanotubes. Here we show that the current-voltage characteristics of p-n diodes fabricated with semiconducting carbon nanotubes can be used along with the excitonic transitions of the nanotubes to measure both the fundamental (intrinsic) and renormalized nanotube band-gaps.

Creation of Individual Defects at Extremely High Proton Fluences in Carbon Nanotube

We show that carbon nanotubes are robust under high H2+ ion fluences. We draw this conclusion by analyzing radiation-induced defects in reconfigurable single-walled carbon nanotube p-n diodes with partially suspended nanotubes. Our analysis show that any defects created through radiation is likely the result of interactions between the nanotube and the substrate, whereas the suspended region of the nanotube remains undamaged. In addition, we show that key features in the diode characteristics can be explained by a single radiation-induced defect that enhances the minority carrier generation rate of only one carrier type.

p{-}n

Due to their large surface-to-volume ratio compared to that of a bulk semiconductor, carbon nanotubes are very sensitive to their environment. Their properties can be dramatically changed by simple exposure to air. Here, we show that the electronic and optical properties of p-n diodes fabricated with single-walled carbon nanotubes degrade over time with exposure to ambient conditions, mainly due to adsorption onto the tube’s suspended part, which creates band-gap states. Most importantly, we demonstrate a simple technique—current annealing—that can thermally reverse such degradation.

Diodes

We measured the total ionizing dose response of gate-all-around silicon nanowire n- and pMOSFETs to x-ray doses up to 2Mrad(SiO2). We show that they are radiation hard, with no degradation in threshold voltage, off-state current, or subthreshold slope, even at the highest dose for a wide range of bias conditions. We attribute this to the intrinsically rad-hard feature of the gate-all-around device design, where the channel is no longer in contact with any insulating layers that could form a parasitic channel.

Current-induced cleaning of adsorbates from suspended single-walled carbon nanotube diodes

Graphene p-n junction (GPNJ) can be used to build logic, using the incidence-angle-dependent carrier transmission [1]. This report presents a method to gate Graphene by buried split-gates (SG) which enable precise control of the doping profiles by the SG spacing. The main result of the report is that even at T = 300K and modest graphene mobility, the GPNJ resistance shows a significant variation with incidence angle, indicating carrier chirality effects.