Ultra-thin Film Transistors Research Articles

Sub-5 nm ultrathin IGO film transistor printed by micro-doped InGa liquid alloy

In this study, we introduce a novel perspective that the formation of the atomically ultrathin oxide layer on the surface of liquid alloys is primarily governed by thermodynamic laws and is additionally influenced by the surface tension of the liquid alloys. In this case, we control the competition of Gibbs free energy and surface tension to effectively address the severe segregation problem of Ga2O3 and realize the precise doping of Ga2O3 in the preparation process of two dimensional IGO film through the liquid InGa alloy printing approach. Importantly, the Ga2O3 content in the IGO films can be tailored in the full range and a sub-5 nm ultrathin IGO film transistor is innovatively fabricated. The oxidation law, band gap, Fermi level, field-effect mobility, subthreshold swing, and other properties of IGO films in sub-5 nm scale are systemically investigated in detail. It has been demonstrated that the incorporation of Ga2O3 in the doping process dramatically reduced the device's turn-off current and enhanced its ability to resist ultraviolet interference. This novel doping process opens up a significant opportunity for future developments of atomically ultrathin transparent conducting film.

Ultrathin-Film Transistors Based on Ultrathin Amorphous InZnO Films

Ultrathin amorphous InZnO ( ${a}$ -IZO) films and the corresponding ultrathin-film transistors (UTFTs) have been prepared by pulsed laser deposition (PLD) for the first time here. With ultrathin amorphous ZnSnO ( ${a}$ -ZTO) films and the related UTFTs as a comparison, the properties of ultrathin a-IZO films and behaviors of the corresponding UTFTs were studied in detail. Also, we have taken a new approach to compare the content of oxygen vacancies ( ${V}_{\textit {O}}$ ) in amorphous oxide semiconductors (AOSs) of different systems, which explains their different properties and behaviors well. The thickness of the ultrathin a-IZO film is approximately 3.1 nm. Compared to ultrathin a-ZTO films, ultrathin a-IZO films depict lower ${V}_{O}$ content and lower carrier concentration. In addition, ultrathin ${a}$ -IZO films display very high transmittance over 95% in the visible region. Furthermore, a-IZO UTFTs exhibit better performance than a-ZTO UTFT, such as a lower off-current of 3.3 $\times \,\,10^{-10}$ A, a larger ${I} _{ \mathrm{\scriptscriptstyle ON}}/{I} _{ \mathrm{\scriptscriptstyle OFF}}$ ratio of $1.5\times 10^{7}$ , a larger $\mu _{\text {sat}}$ of 20.7 cm $^{2}\,\,{V} ^{-1}\,\,{s} ^{-1}$ , a smaller ${V} _{\text {th}}$ of 1.2 V (operating in the enhancement mode), a lower subthreshold swing (SS) of 0.303 V/decade, and a lower ${N} _{t}$ of $8.9\times 10^{11}$ cm−2. Meanwhile, the long-term stability of a-IZO UTFT is similar to a-ZTO UTFT. Above all, a-IZO UTFTs exhibit excellent performance and may be a promising candidate for future displays.

Back-End of Line Compatible Transistors for Hybrid CMOS Applications

The low-temperature back-end of line (BEOL) compatible transparent amorphous oxide semiconductor (TAOS) TFTs and poly-Si TFTs are the suitable technology platform for three-dimensional (3D) integration hybrid CMOS technologies. The n-channel amorphous indium tungsten oxide (a-IWO) ultra-thin-film transistors (UTFTs) have been successfully fabricated and demonstrated in the category of indium oxide based thin film transistors (TFTs). We have scaled down thickness of a-IWO channel to 4nm. The proposed a-IWO UTFTs with low operation voltages exhibit good electrical characteristics: near ideal subthreshold swing (S.S.) ~ 63mV/dec., high field-effect mobility (μFE) ~ 25.3 cm2/V-s. In addition, we also have fabricated the p-channel poly-Si TFTs with high-κ gate insulator (GI). The matched electrical characteristics of n-channel and p-channel devices with low operation voltage and low IOFF are exhibiting the promising candidate for future hybrid CMOS applications.

Ultra-thin-film transistors based on ultra-thin amorphous ZnSnO films

Ultra-thin amorphous ZnSnO (a-ZTO) films were deposited by pulsed laser deposition at room temperature and annealed at various temperatures for ultra-thin-film transistors (UTFTs). The thicknesses of the ultra-thin a-ZTO films are approximately 3.1 nm. The electrical resistivity of the nanofilms decreases greatly with the annealing temperature initially increasing. All the UTFTs with annealing temperature ranging from 250 to 450 °C exhibit good switching properties operating in the enhancement mode with the field-effect mobility of above 8.2 cm2 V−1 s−1. The annealing treatment exhibits extreme importance for the UTFTs to obtain better performance as oxygen vacancy is controlled easily in the ultra-thin films by annealing. The 350 °C-annealed a-ZTO UTFT depicts the largest field-effect mobility of 20.9 cm2 V−1 s−1, the minimum threshold voltage of 2.3 V, the minimum subthreshold swing of 0.339 V/decade, the minimum density of the interfacial trap states of 1.02 × 1012 cm−2 and a large on/off current ratio of 3.3 × 107. Besides, the UTFT with the annealing temperature of 450 °C depicts excellent long-term stability under bias stress due to the least oxygen vacancies. The observations will offer basic design guideline for improvement and future applications of UTFTs.

Ultrathin amorphous ZnGexSnO films for high performance ultra-thin-film transistors

Ultrathin amorphous ZnSnO (a-ZTO) films and ultrathin amorphous ZnGeSnO (a-ZGTO) films with various Ge contents were deposited by pulsed laser deposition for ultra-thin-film transistors (UTFTs). The thicknesses of the channel layers are approximately 3.2 nm. The properties of these ultrathin films and behaviors of these UTFTs were comparatively studied in detail. The a-ZTO ultrathin film exhibited a low concentration of the oxygen vacancy (VO) compared to a-ZGTO ultrathin films. Among all the UTFTs, the a-ZTO UTFT demonstrated the undoubtedly best performance with an on/off current ratio of more than 107, the largest field-effect mobility of 23.2 cm2 V−1 s−1, a positive threshold voltage of 2.0 V, a very small subthreshold swing of 0.31 V/decade, and the best long-term stability under bias stress, suggesting that the introduction of VO suppressors is dispensable with such a small thickness. Above all, the concentration of the oxygen vacancy is easily controlled in the ultrathin a-ZTO nanofilms, leading to the UTFTs operating in the enhancement mode with a high field-effect mobility of 23.2 cm2 V−1 s−1 and excellent long-term stability. The a-ZTO ultrathin film and ultra-thin-film transistor are very potential for future electrical applications with their excellent properties.

High-current-density indium nitride ultrathin-film transistors on glass substrates

We investigated basic characteristics of InN films grown on glass substrates and fabricated thin-film transistors with them. It was found that highly c-axis oriented InN films can be obtained by the surface modification of glass substrates with amorphous HfO2 layers. The electron mobility of the c-axis-oriented InN on HfO2/glass reached 330 cm2 V−1 s−1. We demonstrated that a field effect transistor based on the ultrathin film of highly c-axis-oriented InN exhibited an on/off current ratio as high as 106; in addition, this InN-on-glass device supported current densities greater than 14 mA mm−1. The results indicate that InN-based ultrathin-film transistors are promising electronic devices that enable high-current densities to be achieved on glass substrates.

Low-Temperature Growth of Indium Oxide Thin Film by Plasma-Enhanced Atomic Layer Deposition Using Liquid Dimethyl(N-ethoxy-2,2-dimethylpropanamido)indium for High-Mobility Thin Film Transistor Application.

Low-temperature growth of In2O3 films was demonstrated at 70-250 °C by plasma-enhanced atomic layer deposition (PEALD) using a newly synthesized liquid indium precursor, dimethyl(N-ethoxy-2,2-dimethylcarboxylicpropanamide)indium (Me2In(EDPA)), and O2 plasma for application to high-mobility thin film transistors. Self-limiting In2O3 PEALD growth was observed with a saturated growth rate of approximately 0.053 nm/cycle in an ALD temperature window of 90-180 °C. As-deposited In2O3 films showed negligible residual impurity, film densities as high as 6.64-7.16 g/cm3, smooth surface morphology with a root-mean-square (RMS) roughness of approximately 0.2 nm, and semiconducting level carrier concentrations of 1017-1018 cm-3. Ultrathin In2O3 channel-based thin film transistors (TFTs) were fabricated in a coplanar bottom gate structure, and their electrical performances were evaluated. Because of the excellent quality of In2O3 films, superior electronic switching performances were achieved with high field effect mobilities of 28-30 and 16-19 cm2/V·s in the linear and saturation regimes, respectively. Furthermore, the fabricated TFTs showed excellent gate control characteristics in terms of subthreshold swing, hysteresis, and on/off current ratio. The low-temperature PEALD process for high-quality In2O3 films using the developed novel In precursor can be widely used in a variety of applications such as microelectronics, displays, energy devices, and sensors, especially at temperatures compatible with organic substrates.

Response enhancement mechanism of NO2 gas sensing in ultrathin pentacene field-effect transistors

By optimization of structural and physical–chemical properties at a precision level of molecules, organic sensing devices seek to realize the state of the art in monolayer based device applications. In our work, pentacene ultrathin film transistor has been integrated into the implement of a gas sensor based on an increase in its mobility and a shift of its threshold voltage. A limit of detection of pentacene monolayer field-effect transistor was found to be at sub ppm level when it is applied for detection of NO2. Compared with a thick layer sensor device, the pentacene monolayer NO2 sensor has boosted up sensing response with three orders of magnitude. An enhancement of high selectivity and response mechanism can be understood with a reasonable description emphasizing on the 2D transport characters and a series of gauzy interplay for monolayer pentacene film with NO2 analyte.

Organic ultra-thin film transistors with a liquid gate for extracellular stimulation and recording of electric activity of stem cell-derived neuronal networks

Electronic transducers of neuronal cellular activity are important devices in neuroscience and neurology. Organic field-effect transistors (OFETs) offer tailored surface chemistry, mechanical flexibility, and high sensitivity to electrostatic potential changes at device interfaces. These properties make them attractive for interfacing electronics with neural cells and performing extracellular recordings and stimulation of neuronal network activity. In this work we operate pentacene ultra-thin film (9 nm thick) transistors with a liquid gate both as transducers and electrical stimulators of neuronal network activity. These devices are highly sensitive to small potential changes in cell medium and exhibit sufficient stability under standard cell culture conditions for nine days. We show that murine neural stem cells can be adhered on top of functional devices without the need for an additional layer of cell-adhesive molecules, and then differentiated into neuronal networks. OFET response is monitored during the different phases of the neuronal differentiation process up to nine days. Only when stem cells are differentiated into neurons, it is possible to measure electrical signals in the OFET current following the stimulation. Due to the large sensing area of our device, which accommodates from hundreds to thousands of interconnected neurons, the OFET electrical signals arise from the collective electrophysiological response of the neuronal population. The maximum extracellular potential change in the cleft region adjacent to the transistor surface amounts to 350 μV. This demonstrates that pentacene ultra-thin film OFETs enable good cellular adhesion and efficient coupling of the ionic currents at the biological-organic semiconductor interface with the OFET current.

Self-Assembled Monolayers Made of 6-(5-((6-((5-hexylthiophen-2-yl)ethynyl)-9,10-bis(phenylethynyl)anthracen-2-yl)ethynyl)thiophen-2-yl)hexyl 3-(Triethoxysilyl)Propylcarbamate for Ultrathin Film Transistors

A new functionalized triethoxysilane bearing an X-shaped, anthracene-based semiconducting molecule on one arm was designed and synthesized as a precursor for the preparation of a self-assembled monolayer (SAM) on a SiO(2) substrate. 3-Isocyanatopropyl triethoxysilane was reacted with a monohydroxyl-terminated X-shaped, anthracene-based semiconducting molecule in the presence of tin catalyst. The 6-(5-((6-((5-hexylthiophen-2-yl)ethynyl)-9,10-bis(phenylethynyl)anthracen-2-yl)ethynyl)thiophen-2-yl)hexyl 3-(triethoxysilyl)propylcarbamate (BATHT-TEOS) was found to be stable and sufficiently reactive to form organic monolayers on hydroxylated SiO(2) surfaces. The structures and properties of these SAMs were investigated using X-ray photoelectron spectroscopy, UV-vis absorption spectroscopy, photoluminescence (PL) spectroscopy, laser scanning confocal microscopy-PL spectrometry, and spectroscopic ellipsometry. In this work, BATHT-SAM was employed as an interfacial layer on SiO(2) to fabricate ultrathin film transistors (UTFTs, active layer thickness ∼ 16.09 nm). The device UTFT-I, made of 0.06 wt % 5,5'-(9,10-bis(phenylethynyl)anthracene-2,6-diyl)bis(ethyne-2,1-diyl)bis(2-hexylthiophene) (BATHT) solution on an n-octyltrichlorosilane-SAM/SiO(2) layer, showed no gate effect for the carrier transport behavior; however, the device UTFT-II, fabricated on BATHT-SAM/SiO(2), exhibited field effect mobilities of 0.04 cm(2) V(-1) s(-1) (I(on/off) ∼ 6.3 × 10(3) to 1.0 × 10(4)). This can be attributed to the effect of BATHT-SAM inducing uniform coverage and ordering of BATHT molecules as an upper layer.

Double layer capacitance measured by organic field effect transistor operated in water

Pentacene ultra thin film transistors were exposed to water and operated with a conventional silicon/silicon oxide bottom gate and an electrolyte top gate controlled by a working electrode. The transistors are highly sensible (µV) to the electrochemical potential of the aqueous electrolyte. We show that dual gate operation permits the measurement of the double layer capacitance, CDL = 14.6 µF/cm2. The device exhibits a fast (4.6 ms) and stable response, without bias stress as opposed to conventional bottom gate operations, when controlled with the electrolyte gate. These features make the device a promising candidate for potentiometric transducers required for non-invasive electrophysiology.

Performance and analytical modeling of Metal–Insulator-Metal Field Controlled Tunnel Transistors

The authors investigate the performance of a Metal–Insulator-Metal Field Controlled Tunneling Transistor (MIMT). In this structure, the flow of electrons between the source and the drain is due to a Fowler–Nordheim like tunneling mechanism through a dielectric whose potential barrier is modulated by the gate bias. This silicon-free device is immune to dopant fluctuations or band to band tunneling and its performance is solely limited by the tunneling mechanism. We have evaluated the theoretical device performance and proposed an analytical model capturing the underlying physics. The impact of material properties (dielectric permittivity, bandoffset) and geometrical parameters (gate oxide and tunneling oxide thickness, channel length) is discussed and guidelines for the fabrication of an optimized device are provided. We show that, with appropriate materials and architecture, a well tempered MIMT can demonstrate OFF current as low as 1 nA/um and ON current on the order of 1 mA/um for a voltage swing of 0.5 V, making it an excellent candidate for ultra low power applications and thin film transistors. We also demonstrate that although its operation is based on tunneling, this device does not exhibit sub-60 mV/dec subthreshold slope, but maintains nearly 60 mV/dec between the OFF and ON-states.

Electron spin resonance observation of field-induced charge carriers in ultrathin-film transistors of regioregular poly(3-hexylthiophene) with controlled in-plane chain orientation

Ultrathin-film polymeric transistors with controlled in-plane chain orientation have been fabricated based on Langmuir–Blodgett technique by cospreading liquid crystal molecule with regioregular poly(3-hexylthiophene). The mobilities parallel to the chain direction reached 0.001–0.01 cm2/V s for 1 to 5 monolayer thick transistors. Mobility ratio was about 2 for the parallel and perpendicular directions. Electron spin resonance (ESR) signals of the field-induced polarons exhibited clear in-plane anisotropies due to unpaired π-electrons. The anisotropic ESR spectra together with the optical dichroism determine the detailed molecular orientations in the channel of such ultrathin transistors.

Sensing Biomolecules with Ultra-Thin Film Organic Field Effect Transistors

Organic field effect transistors (OFET), where ordered conjugated molecules act as charge transport material, are low-dimensional devices. Charge carriers are transported within the first few mono-layers in contact with the gate dielectric. The structural/morphological and electronic control of the first few monolayers of the organic semiconductor (e.g. pentacene) allows us to fabricate ultra-thin film transistors with the accumulation layer directly exposed to the outer environment. We developed OFETs as label-free biological transducers and sensors of biological systems. Unconventional patterning techniques and microfluidics have been adapted to proteins and nucleic acids to dose the molecules on the OFET channel with a high control of the concentration. Patterning is useful to impart conformations and architectures which are otherwise not accessible spontaneously by the biological systems.Operations an ultra-thin film OFETs under water will be shown, together with the response of OFET parameters to different biomolecules (linear DNA, beta-amyloid 1-40 peptide) upon a systematic change of concentration, secondary structure, aggregation state. The sensitivity issues will be addressed. These results show a quantitative approach for the detection of biological molecules in vitro and monitoring their slow dynamics. Finally, first experiments using OFETs transducers of cell signaling at the molecular level will be presented in the presence of diffentiated murine stem cells NE4C forming neural networks.

High-Resolution Mapping of the Electrostatic Potential in Organic Thin-Film Transistors by Phase Electrostatic Force Microscopy

We investigate by a scanning probe technique termed phase-electrostatic force microscopy the local electrostatic potential and its correlation to the morphology of the organic semiconductor layer in operating ultra-thin film pentacene field effect transistors. This technique yields a lateral resolution of about 60 nm, allowing us to visualize that the voltage drop across the transistor channel is step-wise. Spatially localized voltage drops, adding up to about 75% of the potential difference between source and drain, are clearly correlated to the morphological domain boundaries in the pentacene film. This strongly supports and gives a direct evidence that in pentacene ultra-thin film transistors charge transport inside the channel is ultimately governed by domain boundaries.

Low-Temperature Polysilicon History and A CPU with an Operating Frequency in the GHz Range

Research on LTPS (low-temperature polysilicon) technology proceeded from the early 1980s together with the development of laser devices. At present, in 2007, technology for manufacturing a high-performance semiconductor film on glass is already being put into practice. In this paper, I would like to discuss the history of LTPS and the development of CGS (continuous-grain silicon) technology which enabled the realization of system-on-glass technology. In addition, I will compare the characteristics of ULSI (ultra-large-scale integration) on silicon and UTFTs (ultra-thin film transistors) on glass and talk about their potential for application to various devices.

Increase in carrier mobility of organic ultrathin-film transistor with increasing molecular layers investigated by Kelvin probe force microscopy

Organic ultrathin transistors with a 3-μm-length channel were fabricated using dimethylquinquethiophene molecules. The local structures of the films and electric potential profiles across the channels were investigated by Kelvin probe force microscopy with the gate electrode grounded. We found that the local carrier mobility was much higher for the second-layer regions than for the monolayer regions from potential profiles measured while the gate electrode was electrically grounded. However, the macroscopic carrier mobility increased only slightly by increasing the number of molecular layers. One of the possible reasons for this contradiction was the large contact resistance at the source∕channel interface, which was apparently observed in the potential profiles.

A Poly-Silicon TFT With a Sub-5-nm Thick Channel for Low-Power Gain Cell Memory in Mobile Applications

This work presents a gain-cell solution in which a novel ultrathin polysilicon film transistor provides the basis for dense and low-power embedded random-access memory (RAM). This is made possible by the new transistor's 2-nm-thick channel, which realizes a quantum-confinement effect that produces a low leakage current value of only 10/sup -19/ A at room temperature. The memory has the potential to solve the power and stability problems that static RAM (SRAM) is going to face in the very near future.

Thermal annealing effects on morphology and electrical response in ultrathin film organic transistors

Ultra-thin organic film transistors were annealed in vacuum at constant temperature and for different time intervals. This treatment eliminates unintentional doping and reduces hysteresis in I/ V curves, both for pentacene and α-sexithienyl, without substantially improving the charge mobility. Devices that originally exhibited high and non-ohmic contact resistance never matched the characteristics of devices with well grown electrode/semiconductor interfaces, even after annealing. Prolonging the annealing time resulted in film rupture due to either re-crystallization or molecular desorption. This process started before healing of molecular disorder at the relevant interfaces could take place.

Fabrication and characterization of ultra-thin film transistor using TMPD-C nTCNQ LB films

Charge–Transfer (CT) complexes are stabilized by partial transfer of electric charges from donor molecules to acceptor molecules. The conductivity of the film of CT-complexes can be controlled if the degree of charge transfer is varied by applying an external field, for example an electric field. In this study, we fabricated field-effect transistors (FETs) using CT complex Langmuir–Blodgett (LB) films (TMPD-C nTCNQ), and have attempted to control the conductivity of the CT-complex layers by gate voltages. The temperature dependence of conductivity shows that semiconducting and metallic parts coexist in parallel between the electrodes.