Material For Thin-film Transistors Research Articles

Controllable film densification and interface flatness for high-performance amorphous indium oxide based thin film transistors

To avoid the problem of air sensitive and wet-etched Zn and/or Ga contained amorphous oxide transistors, we propose an alternative amorphous semiconductor of indium silicon tungsten oxide as the channel material for thin film transistors. In this study, we employ the material to reveal the relation between the active thin film and the transistor performance with aid of x-ray reflectivity study. By adjusting the pre-annealing temperature, we find that the film densification and interface flatness between the film and gate insulator are crucial for achieving controllable high-performance transistors. The material and findings in the study are believed helpful for realizing controllable high-performance stable transistors.

Electron-transporting phenazinothiadiazoles with engineered microstructure

Novel phenazinothiadiazoles were prepared by condensation of ortho-quinones and an alkynylated 5,6-diamino-2,1,3-4 benzothiadiazole and evaluated as transport materials in thin film transistors.

Amorphous zinc-tin oxide thin films fabricated by pulsed laser deposition at room temperature

ABSTRACTFor a cost-efficient fabrication of homogeneous oxide thin films the usage of amorphous materials is favorable. They can be deposited at room temperature (RT) and represent an interesting alternative to amorphous silicon in electronics. Zinc-tin oxide is a promising n-type channel material for thin film transistors and consists of abundant elements, only, in contrast to the well-explored indium gallium zinc oxide. Here, the electrical and optical properties of room temperature deposited ZTO thin films are discussed. These films were fabricated via pulsed-laser deposition on glass substrates by ablating a ceramic target composed of ZnO and SnO2 in a 1:2 ratio. The resistivity has been controlled over seven orders of magnitude via the oxygen growth pressure. Further, the optical transmittance tends to be higher for higher oxygen growth pressures.

Bio-sorbable, liquid electrolyte gated thin-film transistor based on a solution-processed zinc oxide layer.

Among the metal oxide semiconductors, ZnO has been widely investigated as a channel material in thin-film transistors (TFTs) due to its excellent electrical properties, optical transparency and simple fabrication via solution-processed techniques. Herein, we report a solution-processable ZnO-based thin-film transistor gated through a liquid electrolyte with an ionic strength comparable to that of a physiological fluid. The surface morphology and chemical composition of the ZnO films upon exposure to water and phosphate-buffered saline (PBS) are discussed in terms of the operation stability and electrical performance of the ZnO TFT devices. The improved device characteristics upon exposure to PBS are associated with the enhancement of the oxygen vacancies in the ZnO lattice due to Na(+) doping. Moreover, the dissolution kinetics of the ZnO thin film in a liquid electrolyte opens the possible applicability of these devices as an active element in "transient" implantable systems.

ChemInform Abstract: Large N‐Heteroacenes: New Tricks for Very Old Dogs?

AbstractReview: 52 refs.

Sputtering deposition of P-type SnO films using robust Sn/SnO2 mixed target

Although SnO is a promising p-type oxide material for thin-film transistors (TFTs), the development of the TFT-industry-compatible deposition technique and materials for p-type SnO films is still an issue. In this work, we demonstrate the preparation of pure and p-type SnO films using the Sn/SnO2 mixed target and the conventional magnetron sputtering technique. By controlling the sputtering conditions, the deposited films can be tuned from pure n-type SnO2 to pure p-type SnO. Compared to other sputtering target materials (e.g., pure SnO and Sn), the Sn/SnO2 mixed targets can be fabricated by the high-temperature high-pressure pressing/sintering technique and have higher density and robustness more suitable for real uses. The p-type mobility of SnO films obtained here is comparable to results of other approaches, showing the feasibility of this method.

Enhanced Performance of Solution‐Processed TESPE‐ADT Thin‐Film Transistors

A solution-processed anthradithiophene derivative, 5,11-bis(4-triethylsilylphenylethynyl)anthradithiophene (TESPE-ADT), is studied for use as the semiconducting material in thin-film transistors (TFTs). To enhance the electrical performance of the devices, two different kinds of solution processing (spin-coating and drop-casting) on various gate dielectrics as well as additional post-treatment are employed on thin films of TESPE-ADT, and p-channel OTFT transport with hole mobilities as high as ~0.12 cm(2) V(-1) s(-1) are achieved. The film morphologies and formed microstructures of the semiconductor films are characterized in terms of film processing conditions and are correlated with variations in device performance.

Thin-film transistors fabricated by low-temperature process based on Ga- and Zn-free amorphous oxide semiconductor

We propose the use of indium tungsten oxide (IWO) as a channel material for thin-film transistors (TFTs). In the present study, an IWO film was deposited at room temperature by means of DC magnetron sputtering and then annealed at 100 °C in N2 prior to formation of Au source and drain electrodes. Analysis using X-ray diffraction and transmission electron microscopy revealed that the film remained amorphous even after the post-deposition annealing treatment. TFTs fabricated using a Si substrate as a back-gate electrode showed good performance, with a saturation field-effect mobility of 19.3 cm2 · V−1 · s−1, an on/off current ratio of 8.9 × 109.

Large N‐Heteroacenes: New Tricks for Very Old Dogs?

Azaacenes have been known for a very long time, either as N,N'-dihydro compounds or in their oxidized form as 4n+2π systems, but only recently have processable and charcterizable derivatives been sought. In the last three years synthetic routes to large N-heteroacenes have been developed. In particular, the Pd-catalyzed coupling of aromatic diamines with activated aromatic dihalogenides has enabled simple access to numerous new azaacenes. Since 2010, azapentacene and stabile oligoazahexacene have been synthesized, as well as a symmetrical tetraazapentacene, which acts as an excellent electron-transport material for thin-film transistors.

Comparative study of soluble naphthalene diimide derivatives bearing long alkyl chains as n-type organic thin-film transistor materials

In this study, several naphthalene tetracarboxylic acid diimide (NTCDI) derivatives substituted at the N and N′ positions with long normal alkyl chains of different lengths were evaluated as soluble n-type organic thin-film transistor (TFT) materials. NTCDI derivatives with diundecyl (NTCDI-C11), didodecyl (NTCDI-C12), and ditridecyl (NTCDI-C13) exhibited acceptable solubility in chloroform, and their TFTs showed typical n-type TFT performance with relatively high field effect electron mobility (∼0.2cm2/Vs) after annealing at a workable temperature of 150°C. Although NTCDI with dioctyl (NTCDI-C8) showed good solubility in chloroform, the TFT performance of this material was highly inferior to that of NTCDI-C11, NTCDI-C12, or NTCDI-C13. We could not anneal NTCDI-C8 thin films at workable temperatures in vacuo because of sublimation of the material from the substrates. In contrast, NTCDI with dipentadecyl (NTCDI-C15) and dioctadecyl (NTCDI-C18) exhibited both poor solubility for chloroform and poor TFT performance. In short, these compounds are not suitable as soluble n-type organic TFT materials.

Nanoscale optical and electrical characterization of horizontally aligned single-walled carbon nanotubes

During the recent years, a significant amount of research has been performed on single-walled carbon nanotubes (SWCNTs) as a channel material in thin-film transistors (Pham et al. IEEE Trans Nanotechnol 11:44–50, 2012). This has prompted the application of advanced characterization techniques based on combined atomic force microscopy (AFM) and Raman spectroscopy studies (Mureau et al. Electrophoresis 29:2266–2271, 2008). In this context, we use confocal Raman microscopy and current sensing atomic force microscopy (CS-AFM) to study phonons and the electronic transport in semiconducting SWCNTs, which were aligned between palladium electrodes using dielectrophoresis (Kuzyk Electrophoresis 32:2307–2313, 2011). Raman imaging was performed in the region around the electrodes on the suspended CNTs using several laser excitation wavelengths. Analysis of the G+/G− splitting in the Raman spectra (Sgobba and Guldi Chem Soc Rev 38:165–184, 2009) shows CNT diameters of 2.5 ± 0.3 nm. Neither surface modification nor increase in defect density or stress at the CNT-electrode contact could be detected, but rather a shift in G+ and G− peak positions in regions with high CNT density between the electrodes. Simultaneous topographical and electrical characterization of the CNT transistor by CS-AFM confirms the presence of CNT bundles having a stable electrical contact with the transistor electrodes. For a similar load force, reproducible current–voltage (I/V) curves for the same CNT regions verify the stability of the electrical contact between the nanotube and the electrodes as well as the nanotube and the AFM tip over different experimental sessions using different AFM tips. Strong variations observed in the I/V response at different regions of the CNT transistor are discussed.

Flexible Electronics Based on InGaZnO Transparent Thin Film Transistors

There is increasing interest in use of conducting oxide materials in new forms of transparent, flexible or wearable electronics on cheap substrates, including paper. While Si-based thin film transistors (TFTs) are widely used in displays, there are some drawbacks such as light sensitivity and light degradation and low field effect mobility (<1 cm2/Vs). For example, virtually all liquid crystal displays (LCDs) use TFTs imbedded in the panel itself. One of the promising alternatives to use of Si TFTs involves amorphous or nanocrystalline n-type oxide semiconductors. There have been promising results with zinc oxide, indium gallium oxide and zinc tin oxide channels. In this paper, recent progress in these new materials for TFTs on substrates such as paper is reviewed. In addition, InGaZnO transistor arrays show promise for driving laminar electroluminescent, organic light-emitting diode (OLED) and LCD displays. These transistors may potentially operate at up to an order of magnitude faster than Si TFTs. We have fabricated bottomgate amorphous (α-) indium-gallium-zinc-oxide (InGaZnO4) thin film transistors (TFTs) on both paper and glass substrates at low processing temperature (≤100°C). As a water and solvent barrier layer, cyclotene (BCB 3022-35 from Dow Chemical) was spin-coated on the entire paper substrate. TFTs on the paper substrates exhibited saturation mobility (μsat) of 1.2 cm2.V-1.s-1, threshold voltage (VTH) of 1.9V, subthreshold gate-voltage swing (S) of 0.65V.decade-1, and drain current onto- off ratio (ION/IOFFSubscript text) of ~104. These values were only slightly inferior to those obtained from devices on glass substrates (μsat~2.1 cm2.V-1.s-1, VTH ~0 V, S~0.74 V.decade-1, and ION/IOFF=105- 106). The uneven surface of the paper sheet led to relatively poor contact resistance between source-drain electrodes and channel layer. Future areas for development are identified.

Low temperature deposition of low stress silicon nitride by reactive magnetron sputtering

Silicon nitride thin film is a well-know kind of material in semiconductor industries. These films are used in important applications such as protective layers; insulators for optical devices, as dielectric material for thin film transistors (TFTs), etc. In this work, silicon nitride thin films were deposited at room temperature through of the reaction of sputtered silicon by argon ions and nitrogen gas with relatively high deposition rates in sputtering reactive processes. We have obtained silicon nitride thin films with low stress for MEMS (Micro Electro Mechanical Systems) fabrication and in post-processing of sensors and actuators devices. Silicon nitride thin films obtained in this work showed stoichiometric (Si3N4) and/or rich in silicon with low mechanical stress and good dielectric properties. The films were analyzed by scanning electron microscope, micro-Raman spectroscopy, profilometry, current-voltage (I-V) and capacitance-voltage (C-V) measurements, laser-based measurement for determination the internal stress and Rutherford Backscattering.

Thin-film transistors with a channel composed of semiconducting metal oxide nanoparticles deposited from the gas phase

The fabrication of semiconducting functional layers using low-temperature processes is of high interest for flexible printable electronics applications. Here, the one-step deposition of semiconducting nanoparticles from the gas phase for an active layer within a thin-film transistor is described. Layers of semiconducting nanoparticles with a particle size between 10 and 25 nm were prepared by the use of a simple aerosol deposition system, excluding potentially unwanted technological procedures like substrate heating or the use of solvents. The nanoparticles were deposited directly onto standard thin-film transistor test devices, using thermally grown silicon oxide as gate dielectric. Proof-of-principle experiments were done deploying two different wide-band gap semiconducting oxides, tin oxide, SnO x , and indium oxide, In2O3. The tin oxide spots prepared from the gas phase were too conducting to be used as channel material in thin-film transistors, most probably due to a high concentration of oxygen defects. Using indium oxide nanoparticles, thin-film transistor devices with significant field effect were obtained. Even though the electron mobility of the investigated devices was only in the range of 10−6 cm2 V−1 s−1, the operability of this method for the fabrication of transistors was demonstrated. With respect to the possibilities to control the particle size and layer morphology in situ during deposition, improvements are expected.

Schottky Barrier Thin Film Transistors Using Solution-Processed n-ZnO

Solution-processed ZnO thin films are attractive as active materials in thin film transistors (TFTs) for low-cost electronic device applications. However, the lack of true enhancement mode operation, low mobility, and unreliability in transistor characteristics due to the high density of traps and other defects present challenges in using such TFTs in circuits. We demonstrate in this report that the electrical characteristics of such TFTs can be improved by source injection barriers. Asymmetrical Schottky source metal-oxide-semiconductor field-effect transistors (MOSFETs) have been fabricated by utilizing heavily doped solution-processed ZnO as the active layer. n(+)-ZnO was obtained by using triethylamine as the stabilizer in the solution process instead of the more commonly used monoethanolamine. Au was chosen for source metallization to create a Schottky contact to the ZnO and an Al ohmic contact was chosen as the drain. Voltage applied to the gate induced field emission through the Schottky barrier and allowed modulation of the drain current by varying the width of the barrier. By operating the asymmetrical MOSFET when the Schottky contact is reverse biased, effective control over the transistor characteristics was obtained.

Metal Reaction Doping and Ohmic Contact with Cu-Mn Electrode on Amorphous In-Ga-Zn-O Semiconductor

We investigated the microstructure and electrical properties of Cu and Cu-Mn alloy on amorphous In-Ga-Zn-O (a-IGZO) oxide semiconductor in order to explore a high performance electrode material for thin film transistors (TFTs) in advanced flat panel displays. Current-voltage measurements of metal/semiconductor contact structure showed a non-linear behavior with Cu, while a good ohmic behavior [ρC = (1.2−2.9) × 10−4 Ω·cm2] with the Cu-Mn alloy after annealing at 250°C for 1 h. Transfer and output characteristics of TFT structure also showed excellent performance with the Cu-Mn alloy. The good electrical property was due to the formation of a highly doped n+ a-IGZO layer with the carrier density of 1.4 × 1020 cm−3. The donor doping could be achieved simply by heat treatment to promote the oxidation of Mn and the reduction of a-IGZO.

Solution-processed zinc–tin oxide thin-film transistors with low interfacial trap density and improved performance

Solution-processed amorphous oxide semiconductors are attractive channel materials in thin-film transistors (TFTs) for low-cost electronics. We demonstrate improved performance and uniformity of solution-processed zinc–tin oxide (ZTO) TFTs by optimizing the prebake process for the ZTO precursor film. ZTO prebake process prearranges the dielectric/semiconductor interface and minimizes the performance variation caused by the uneven thermal distribution during annealing process. Prearranging the interface also reduces interfacial trap density and results in improved performance. A mobility of 27.3 cm2/V s, an on/off ratio of ∼107, and a subthreshold swing of 122 mV/decade have been obtained. Significant improvement in operational stability has also been observed.

Origin of definite Hall voltage and positive slope in mobility-donor density relation in disordered oxide semiconductors

Amorphous oxide semiconductors (AOSs) are expected for alternative channel materials in thin-film transistors owing to their large electron mobilities. While, it is known that AOSs exhibit peculiar electron transport properties. Definite Hall voltages are observed even for mobilities <0.2 cm2/V s, which correspond to a very short mean free path (MFP) of 0.008 nm. Furthermore, Hall mobility increases with increasing the donor density. This paper reports that a percolation conduction model explains them; quantitative analyses based on the Boltzmann’s transport theory prove that carriers within the potential barriers have large MFPs of 0.5–1 nm. The percolation model also explains variable-range-hoppinglike and weak-localizationlike behaviors.

An investigation of contact resistance between metal electrodes and amorphous gallium–indium–zinc oxide (a-GIZO) thin-film transistors

This paper reports our investigation of different source/drain (S/D) electrode materials in thin-film transistors (TFTs) based on an indium–gallium–zinc oxide (IGZO) semiconductor. Transfer length, contact resistance, channel conductance, and effective resistances between S/D electrodes and amorphous IGZO thin-film transistors were examined. Intrinsic TFT parameters were extracted by the transmission line method (TLM) using a series of TFTs with different channel lengths measured at a low drain voltage. The TFTs fabricated with Cu S/D electrodes showed the lowest contact resistance and transfer length indicating good ohmic characteristics, and good transfer characteristics with intrinsic field-effect mobility ( μ FE- i ) of 10.0 cm 2/Vs.

Aligned Tin Oxide Nanonets for High-Performance Transistors

Highly oriented tin oxide (SnO2) nanowire network (nanonets) based devices fabricated through photolithography-free techniques are studied. These nanowire networks are studied at submillimeter scales for their utilization as the active material in thin film transistors for macroelectronics. The SnO2 nanowire network transistors show excellent device characteristics and possess electron mobilities of ∼7.5 cm2 V−1 s−1 and on/off ratios between 106 and 108 with channel lengths ranging from 75 to 175 μm. Exposure of the SnO2 nanonet transistors to the ambient results in positive threshold voltage shifts due to electron trapping by oxygen at the nanowire surface. On the contrary, the electrical properties of the devices remained unchanged upon passivation by a polystyrene (PS) layer, which demonstrates a practical way to enhance the device performance in air. These results suggest that SnO2 nanonets that offer fault tolerance, flexibility, and high transparency due to low areal coverage could be a suitable can...