Daniel Kälblein

Contact Resistance and Megahertz Operation of Aggressively Scaled Organic Transistors

Bottom-gate, top-contact organic thin-film transistors (TFTs) with excellent static characteristics (on/off ratio: 10(7) ; intrinsic mobility: 3 cm(2) (V s)(-1) ) and fast unipolar ring oscillators (signal delay as short as 230 ns per stage) are fabricated. The significant contribution of the transfer length to the relation between channel length, contact length, contact resistance, effective mobility, and cutoff frequency of the TFTs is theoretically and experimentally analyzed.

Contact Doping and Ultrathin Gate Dielectrics for Nanoscale Organic Thin-Film Transistors

Organic thin-fi lm transistors (TFTs) are of interest for electronic applications on fl exible plastic substrates, such as rollable or foldable active-matrix displays, [ 1 ] conformable sensor arrays, [ 2 ] and fl exible identifi cation tags. [ 3 ] Due to the relatively small intrinsic fi eld-effect mobility in most conjugated organic semiconductors ( 10 MHz) are highly desirable. Such high frequencies are indeed feasible, provided the lateral dimensions of the organic TFTs are suffi ciently small (about 100 nm). However, TFTs with such small lateral dimensions will suffer from a variety of detrimental short-channel effects, unless a number of important scaling requirements are observed in the design and fabrication of the transistors. Here we report on the successful fabrication and detailed analysis of organic TFTs with channel lengths and gate overlaps of about 100 nm in which the short-channel effects are greatly suppressed by area-selective contact doping (using a strong organic dopant) and by aggressive gate-dielectric scaling (using a 5.7 nm-thick, low-temperature-processed gate insulator based on a molecular self-assembled monolayer). As a result, these nanoscale organic TFTs have off-state drain currents below 1 pA, on/ off current ratios near 10 7 , as well as clean linear and saturation characteristics. The transconductance of these transistors reaches 0.4 S m − 1 , which is the largest transconductance reported for organic TFTs with patterned gate electrodes. The gate electrodes and source/drain contacts of organic TFTs are usually defi ned by photolithography, [ 1 , 3 , 4 ]

Probing the electrical properties of highly-doped Al: ZnO nanowire ensembles

The analysis of transparent conducting oxide nanostructures suffers from a lack of high throughput yet quantitatively sensitive set of analytical techniques that can properly assess their electrical properties and serve both as characterization and diagnosis tools. This is addressed by applying a comprehensive set of characterization techniques to study the electrical properties of solution-grown Al-doped ZnO nanowires as a function of composition from 0 to 4 at. % Al:Zn. Carrier mobility and charge density extracted from sensitive optical absorption measurements are in agreement with those extracted from single-wire field-effect transistor devices. The mobility in undoped nanowires is 28 cm2/V s and decreases to ∼14 cm2/V s at the highest doping density, though the carrier density remains approximately constant (1020 cm−3) due to limited dopant activation or the creation of charge-compensating defects. Additionally, the local geometry of the Al dopant is studied by nuclear magnetic resonance, showing the...

Highly Reliable Carbon Nanotube Transistors with Patterned Gates and Molecular Gate Dielectric

The prospect of realizing nanoscale transistors using individual semiconducting carbon nanotubes offers enormous potential, both as an alternative to silicon technology beyond conventional scaling limits and as a way to implement high-speed devices and circuits on flexible substrates. A significant challenge is the realization of low-voltage nanotube transistors with individually addressable gate electrodes that display large transconductance, steep subthreshold swing, and large on/off ratio. Their integration into circuits with large signal gain and good stability still needs to be demonstrated. Here, we demonstrate that these important goals can be achieved with the help of a bottom-gate device structure that combines patterned metal gates with a thin gate dielectric based on a molecular self-assembled monolayer. The obtained transistors operate with a gate-source voltage of 1 V and have a transconductance of 5 microS, a subthreshold swing of 68 mV/decade, and an on/off ratio of 10(7). To verify the excellent operational and shelf life stability, we show that the device performance does not degrade during 10,000 switching cycles and during storage under ambient conditions for more than 300 days. We also demonstrate that the device structure allows the implementation of unipolar logic circuits with good switching characteristics.

Rectennas Revisited

In the late 1960s, a new concept was proposed for an infrared absorbing device called a “rectenna” that, combining an antenna and a nanoscale metal-insulator-metal diode rectifier, collects electromagnetic radiation in the terahertz regime, with applications as detectors and energy harvesters. Previous theories hold that the diode rectifies the induced terahertz currents. Our results, however, demonstrate that the Seebeck thermal effect is the actual dominant rectifying mechanism. This new realization that the underlying mechanism is thermal-based, rather than tunneling-based, can open the way to important new developments in the field, since the fabrication process of rectennas based on the Seebeck effect is far simpler than existing processes that require delicate tunnel junctions. We demonstrate for the first time the fabrication of a rectenna array using an efficient parallel transfer printing process featuring nearly one million elements.

Logic circuits based on individual semiconducting and metallic carbon-nanotube devices

Nanoscale transistors employing an individual semiconducting carbon nanotube as the channel hold great potential for logic circuits with large integration densities that can be manufactured on glass or plastic substrates. Carbon nanotubes are usually produced as a mixture of semiconducting and metallic nanotubes. Since only semiconducting nanotubes yield transistors, the metallic nanotubes are typically not utilized. However, integrated circuits often require not only transistors, but also resistive load devices. Here we show that many of the metallic carbon nanotubes that are deposited on the substrate along with the semiconducting nanotubes can be conveniently utilized as load resistors with favorable characteristics for the design of integrated circuits. We also demonstrate the fabrication of arrays of transistors and resistors, each based on an individual semiconducting or metallic carbon nanotube, and their integration on glass substrates into logic circuits with switching frequencies of up to 500 kHz using a custom-designed metal interconnect layer.

Ultra-thin titanium oxide

We demonstrate the fabrication of ultra-thin titanium oxide films by plasma-induced surface oxidation. Ellipsometry measurements indicate an oxide thickness of about 2 nm. Electrical characterization was performed on microscale and nanoscale metal-insulator-metal tunneling diodes. Electrical fields up to 22 MV/cm were applied without destroying the titanium oxide films. The current-voltage-characteristic of the diodes are found to be asymmetric with respect to zero bias when employing electrodes with different work functions. The permittivity of the ultra-thin titanium oxide was determined to be less than 6, which is the smallest permittivity that has been reported for titanium oxide.

Unipolar sequential circuits based on individual-carbon-nanotube transistors and thin-film carbon resistors.

A fabrication process for the monolithic integration of field-effect transistors based on individual carbon nanotubes and load resistors based on vacuum-evaporated carbon films into fast unipolar logic circuits on glass substrates is reported for the first time. The individual-carbon-nanotube transistors operate with relatively small gate-source and drain-source voltages of 1 V and combine large transconductance (up to 6 μS), large ON/OFF ratio (>10(4)), and short switching delay time constants (12 ns). The thin-film carbon load resistors provide linear current-voltage characteristics and resistances between 300 kΩ and 100 MΩ, depending on the layout of the resistors and the thickness of the vacuum-evaporated carbon films. Various combinational circuits (NAND, NOR, AND, OR gates) as well as a sequential circuit ( ̅S ̅R NAND latch) have been fabricated and characterized. Although these unipolar circuits cannot compete with optimized complementary circuits in terms of integration density and static power consumption, they offer the possibility of realizing air-stable, low-voltage integrated circuits with promising static and dynamic performance on unconventional substrates for large-area electronics applications, such as displays or sensors.

Low-voltage metal-gate top-contact organic thin-film transistors and complementary inverters with submicron channel length

Since the mobility of organic semiconductors cannot be increased indefinitely , improvements in the dynamic performance of organic thin-film transistors (TFTs) require reductions in the TFT dimensions. Assuming a mobility of 0.1 cm2/Vs and aiming for a cutoff frequency of 10 MHz at 3 V, channel length and gate-to-contact overlap have to be reduced well below 1 ¿m. Although no cost-effective methods to manufacture high-mobility submicron organic TFTs on large-area flexible substrates currently exist, such methods may become available in the near future. To ensure that the carrier density in short-channel TFTs is controlled by the gate, rather than the drain, the gate dielectric thickness must also be reduced, ideally using a low-temperature-processable dielectric and without introducing large gate leakage.

Nano-transfer printing of functioning MIM tunnel diodes

Nano diodes show great potential for applications in detectors, communications and energy harvesting. In this work, we focus on nano transfer printing (nTP) to fabricate nm-scale diodes over extensive areas. Using a temperature-enhanced process, several millions of diodes were transfer-printed in one single step. We show the reliable transfer of functioning MIM diodes, which were electrically characterized by conductive Atomic Force Microscopy (c-AFM) measurements. Quantum-mechanical tunneling was determined to be the main conduction mechanism across the metal-oxide-metal junction.