Claudio Balocco

Influence of processing conditions on the stability of poly(3-hexylthiophene)-based field-effect transistors

Bottom-contact organic field-effect transistors (OFETs) based on poly(3-hexylthiophene)-2,5-diyl were fabricated under different process conditions. The devices displayed drastic differences in their ambient-air stability. Whereas it took only about 10min in air for the off current to increase by one order of magnitude in OFETs prepared with chloroform and hexamethyldisilazane, a 120min exposure to air caused only a slight degradation of OFETs prepared using 1,2,4-trichlorobenzene, n-octadecyltrichlorosilane, and a heat treatment. The differences in the film surface morphology were analyzed and possible mechanisms for the enhanced stability are discussed.

Room-temperature operation of a unipolar nanodiode at terahertz frequencies

We report on the room-temperature electrical rectification at 1.5 THz of a unipolar nanodiode based on symmetry breaking in a nanochannel. The exploitation of its nonlinear diodelike characteristic and intrinsically low parasitic capacitance enables rectification at ultrahigh speed. The zero-voltage threshold and unique planar layout make the nanodiode suitable for building large arrays. This is the highest speed reported in nanorectifiers to date.

Room-temperature operations of memory devices based on self-assembled InAs quantum dot structures

Memory devices have been fabricated in high-electron-mobility transistors with embedded InAs quantum dots (QDs). We show that memory operations can be fully controlled by gate biases at room temperature, without the need for light excitations to erase memory states. Real-time measurements indicate a charge retention time of a few minutes. Neither such retention time nor the self-consistent simulations can justify the picture that the memory effect is due to charging/discharging of intrinsic QD states. Experiments at a series of gate biases point to the presence of deep levels coexisting in the QD layer(s), which are responsible for the memory effect.

Low-frequency noise of unipolar nanorectifiers

Unipolar nanodiodes, also known as self-switching devices, have recently been demonstrated as terahertz detectors at room temperature. Here, we study their low-frequency noise spectra and noise equivalent power and show that both performance parameters are comparable to those reported for state-of-the-art Schottky diodes. The truly planar nanodiode layout enables building structures with thousands of devices connected in parallel, which reduce low-frequency noise without affecting sensitivity. The observed 1/f noise can be described by Hooge’s mobility fluctuation theory.

Zinc-oxide-based planar nanodiodes operating at 50 MHz

Nanometer-scale self-switching devices (SSDs) fabricated in polycrystalline zinc oxide have been demonstrated up to at least 51.5 MHz, functioning as rectifiers to generate DC voltage. The SSDs require only a single nanolithography step and hence are of interest to low-cost printed electronics. The devices showed stable performance within the frequency range tested. The as-fabricated devices possessed strongly nonlinear current-voltage characteristics, resembling those of conventional diodes. After coating the devices with poly methyl methacrylate and poly vinylidene fluoride to enhance the electric field coupling, the nonlinear behavior was maintained while the device current increased dramatically.

Highly Reproducible Nanolithography by Dynamic Plough of an Atomic-Force Microscope Tip and Thermal-Annealing Treatment

An approach has been developed to use atomic-force microscope (AFM) to pattern materials at the nanoscale in a controlled manner. By introducing a thermal-annealing process above the glass-transition temperature of poly (methylmethacrylate) (PMMA), the profile of indented nanopatterns has been dramatically improved by abatement of the tip-induced debris. This eliminates the main problem of the previous AFM-based tip-ploughing lithography method, namely the debris formation during the nanoplough and trench refilling by debris. We are able to reproducibly fabricate nanopatterns down to 40 nm. Meanwhile, the AFM-tip lifetime has been increased substantially. In particular, the adhesion between the PMMA layer on the edge of trenches and the substrate is significantly improved to enable reliable pattern transfer into GaAs/AlGaAs heterostructures by wet-chemical etching. Functional nanodevices with a lateral feature size of 100 nm to an etching depth of 70 nm are demonstrated using the method.

Highly tunable, high-throughput nanolithography based on strained regioregular conducting polymer films

Atomic force microscope (AFM) is now a standard imaging tool in laboratories but has displayed limited capability of nanolithography. We discover that an internal tensile strain exists in poly(3-hexylthiophene-2,5-diyl) (P3HT) films, and the physical effect is utilized to achieve highly tunable and high-throughput nanolithography. Trenches with widths spanning nearly two orders of magnitude from 40nmto2.3μm are fabricated. We show that P3HT is also excellent for pattern transfer to inorganic materials. Furthermore, a lithography speed of 0.5mm∕s is achieved, which is a few orders of magnitude higher than other known methods of AFM-based nanolithography.

Micro rectennas: Brownian ratchets for thermal-energy harvesting

We experimentally demonstrated the operation of a rectenna for harvesting thermal (blackbody) radiation and converting it into dc electric power. The device integrates an ultrafast rectifier, the self-switching nanodiode, with a wideband log-periodic spiral microantenna. The radiation from the thermal source drives the rectenna out of thermal equilibrium, permitting the rectification of the excess thermal fluctuations from the antenna. The power conversion efficiency increases with the source temperatures up to 0.02% at 973 K. The low efficiency is attributed mainly to the impedance mismatch between antenna and rectifier, and partially to the large field of view of the antenna. Our device not only opens a potential solution for harvesting thermal energy but also provides a platform for experimenting with Brownian ratchets.

Scanning probe microscope based nanolithography on conducting polymer films

We demonstrate successful nanolithography to pattern organic semiconductor films by using the oscillating tip of an atomic force microscope (AFM). The nano-indentation technique, which was previously used to create trenches on materials including thin photoresist and metal films, has now been systematically studied and applied to the most commonly used semiconducting polymer, regioregular poly(3-hexylthiophenc) (P3HT). We discover that the internal tensile strain in the films and the long P3HT molecules have actually enabled us to eliminate all major common problems of the nano-indentation method to date, namely the refilling of the trenches by debris, tip contamination by debris, and the short AFM tip life time, which have so far seriously hampered and limited the practical applications of the nano-indentation technique. The trenches that we have created are generally formed through the entire P3HT film, with a clean bottom Virtually free from debris. Successful pattern transfer to the underneath inorganic semiconductor has been achieved by a wet chemical etch with the created organic nanostructures as the etching mask. Furthermore, no obvious degradation of the AFM tip either by debris contamination or mechanical wearing is observed after many days of nanolithography. This allows nanostructures over tens of microns in length to be reproducibly fabricated in large numbers.

Free-Space Permittivity Measurement at Terahertz Frequencies With a Vector Network Analyzer

A simple system, based on a vector network analyzer, has been used with new numerical de-embedding and parameter inversion techniques to determine the relative permittivity (dielectric properties) of materials within the frequency range 750-1100 GHz. Free-space (noncontact), nondestructive testing has been performed on various planar dielectric and semiconducting samples. This system topology is well suited for quality control testing in an industrial setting requiring high throughput. Scattering parameters, measured in the absence of a sample, were used to computationally move the measurement plane to the surface of the samples being characterized. This de-embedding process can be completed much faster than a traditional calibration process and does not require exact knowledge of system geometric lengths. An iterative method was developed for simultaneously determining both sample geometric thickness and electric permittivity, through calculation of theoretical scattering parameters at material boundaries. A constrained nonlinear optimization process was employed to minimize the discrepancy between measured transmission and reflection data with this simulated data, in lieu of a closed-form parameter inversion algorithm. Monte Carlo simulations of parameter retrieval in the presence of artificial noise have demonstrated our methods robustness and superior noise rejection compared with a noniterative method. The precision of derived results has been improved by a factor of almost 50, compared to a closed-form extraction technique with identical input.