Monthly Archives: February 2015

High performance printed organic transistors using a novel scanned thermal annealing technology

Publication date: May 2015 Source:Organic Electronics, Volume 20 Author(s): Gerd Grau , Rungrot Kitsomboonloha , Hongki Kang , Vivek Subramanian Printed organic thin film transistors (OTFTs) are a key component for the realization of low-cost, flexible electronics applications such as printed RFID tags or flexible displays. In recent years, great advances have been made in developing higher performance organic semiconductors. Many of these new materials show strongly process-dependent performance characteristics. The development of novel processing techniques is thus a key step towards utilizing the full potential of these semiconductor materials. Here we demonstrate a novel directional crystallization technique using a scanned thermal gradient to significantly improve the performance of printed OTFTs. A heat source is translated relative to the sample to induce the crystallization of the semiconductor. This scanned annealing creates a moving thermal gradient and thus develops a moving solvent evaporation gradient. Compared with uniform annealing on a hotplate grain size increases markedly and shows a clear directionality due to the separation of grain nucleation and growth. With this technique, mobility is boosted by about one order of magnitude. Mobilities close to 2cm2/Vs can be achieved. Off-state performance is likewise improved as evidenced by a 3× improvement in subthreshold swing. Graphical abstract

Etching mechanisms of (In, Ga, Zn)O thin films in CF4/Ar/O2 inductively coupled plasma

The authors investigated the etching characteristics and mechanisms of (In, Ga, Zn)O (IGZO) thin films in CF4/Ar/O2 inductively coupled plasmas. The etching rates of IGZO as well as the IGZO/SiO2 and IGZO/Al2O3 etching selectivities were measured as functions of O2 content in a feed gas (0%–50%) and gas pressure ( p  = 4–10 mTorr) at fixed input power ( W inp  = 700 W) and bias power ( W dc  = 200 W). It was found that the IGZO etching rate decreases monotonically toward O2 rich plasma but exhibits a maximum under gas pressure conditions. The zero-dimensional plasma model with Langmuir probe diagnostics data provided the information on plasma parameters and densities of plasma active species. The model-based analysis shows the dominance of the ion-flux-limited etching regime at p  ≥ 6 mTorr as well as the noticeable influence of CFx radicals on the overall etching kinetics.

Low temperature and cost-effective growth of vertically aligned carbon nanofibers using spin-coated polymer-stabilized palladium nanocatalysts

We describe a fast and cost-effective process for the growth of carbon nanofibers (CNFs) at a temperature compatible with complementary metal oxide semiconductor technology, using highly stable polymer–Pd nanohybrid colloidal solutions of palladium catalyst nanoparticles (NPs). Two polymer–Pd nanohybrids, namely poly(lauryl methacrylate)-block-poly((2-acetoacetoxy)ethyl methacrylate)/Pd (LauMA x -b-AEMA y /Pd) and polyvinylpyrrolidone/Pd were prepared in organic solvents and spin-coated onto silicon substrates. Subsequently, vertically aligned CNFs were grown on these NPs by plasma enhanced chemical vapor deposition at different temperatures. The electrical properties of the grown CNFs were evaluated using an electrochemical method, commonly used for the characterization of supercapacitors. The results show that the polymer–Pd nanohybrid solutions offer the optimum size range of palladium catalyst NPs enabling the growth of CNFs at temperatures as ...

Depletion Mode Oxide TFT Shift Register for Variable Frame Rate AMOLED Displays

This letter demonstrates a shift register circuit at the backplane of depletion mode oxide thin-film transistors (TFTs), which can cover variable frame rates for low-power active matrix organic light emitting diode (AMOLED) displays. The proposed scheme focuses on the stable gate control of a pull-up TFT by securing the charging and discharging paths at the wide operating frequency range. In a prototype high-definition (HD: $1366 times 768$ ) AMOLED panel, it is ensured that fabricated shift registers operate at the wide frame frequency range from 1 to 240 Hz. The measured current consumptions are 30, 4, 6, and 5.8 mA for four clock signals, VGH of 24 V, VGL1 of −6 V, and VGL2 of −14 V at a frame rate of 120 Hz.

Oxide structure-dependent interfacial layer defects of HfAlO/SiO2/Si stack analyzed by conductance method

This work investigates the interface properties in a metal oxide semiconductor capacitor device with a 3 nm HfAlO/0.5 nm SiO2/Si stacks prepared by various processing conditions. Different Al doping, different postannealing temperatures, and different deposition steps and stacks were considered. Equivalent oxide thickness and flat band voltage (V FB) were obtained from capacitance–voltage measurements. After the measurement, a simple approach was used to correct the error introduced by the series resistance R s associated with the substrate and contact while carefully monitoring the impact of the tunneling current. The interface state density (D it) was calculated by the conductance method, and it was observed that the D it is dependent on the structure of hafnium aluminum oxide film. The amorphous structure has the lowest D it ( 2.76 × 10 11   eV − 1 cm − 2 ) whereas tetragonal HfO2 has the highest D it ( 1.27 × 10 12   eV − 1 cm − 2 ). The D it values of other structures are within the range of observed highest and lowest values.

Effects of high-temperature thermal annealing on the electronic properties of In-Ga-Zn oxide thin films

Indium gallium zinc oxide (IGZO) thin films were deposited by radio-frequency magnetron sputtering at room-temperature. Then, thermal annealing was conducted to improve the structural ordering. X-ray diffraction and high-resolution transmission electron microscopy demonstrated that the as-deposited IGZO thin films were amorphous and crystallization occurred at 800 and 950 °C. As a result of crystallization at high temperature, the carrier concentration and the Hall mobility of IGZO thin films were sharply increased, which could be ascribed to the increased oxygen vacancies and improved structural ordering of the thin films.

High performance of rubrene thin film transistor by weak epitaxy growth method

Publication date: May 2015 Source:Organic Electronics, Volume 20 Author(s): Hao Chang , Weili Li , Hongkun Tian , Yanhou Geng , Haibo Wang , Donghang Yan , Tong Wang Rubrene single-crystal transistors have achieved one of the highest carrier mobilities in organic semiconductors. However its thin film transistor usually shows inferior performance due to the poor film quality. Therefore how to obtain large-area and high quality rubrene thin film has become a prominent challenge. This work utilized weak epitaxy growth method with new inducing layer 1,3-di(terphenyl) benzene (m-7P), and lager-area highly ordered terrace rubrene film was obtained. Based on this high quality film, the hole mobility of rubrene polycrystalline thin film transistor has been enhanced to 11.6cm2 V−1 s−1 with VOPc as buffer layer between semiconductor layer and electrodes. This high device performance was attributed to the flat inducing layer and the single orientation of rubrene domains on m-7P layer, which may reduce grain boundaries and improve the film quality. This easy process to prepare large-area high performance rubrene device supplies a good opportunity for large-area electronic device manufacture. Graphical abstract

Non-Arrhenius conduction due to the interface-trap-induced disorder in X-doped amorphous In-X-Zn oxides thin-film transistors

Thin film transistors, with channels composed of In-X-Zn oxides, IXZO, with X dopants: Ga, Sb, Be, Mg, Ag, Ca, Al, Ni, and Cu, were fabricated and their I-V characteristics were taken at selected temperatures in the 77 K μ, and the interface defect density, NST, were extracted from the characteristics for each of the studied IXZOs. At higher T, the mobility follows the Arrhenius law with an upward distortion, increasing as T is lowered, gradually transforming into the exp [-(T0/T)1/4] variation. We showed that μ(T, NST) follows μ 0 exp[-Eaeff(T,NST)/kT], with T-dependent effective activation energy Eaeff(T, NST) accounts for the data, revealing a linear correlation between Eaeff and NST at higher T. Temperature variation of Eaeff(T, NST) was evaluated using a model assuming a random distribution of conduction mobility edge Ec values in the oxides, stemming from spatial fluctuations induced by disorder in the interface traps distribution. For a Gaussian distribution of Ec, the activation energy Eaeff(T, NST) varies linearly with 1/T, which accounts satisfactorily for the data obtained on all the studied IXZOs. The model also shows that Eaeff(T, NST) is a linear function of NST at a fixed T, which explains the exponential decrease of μ with NST.

Plasmon-enhanced photocurrent of Ge-doped InGaO thin film transistors using silver nanoparticles

Germanium-doped indium-gallium oxide (GIGO) thin film transistors (TFTs) decorated with silver (Ag) nanoparticles (NPs) were prepared to study the plasmon effect. GIGO films of various thicknesses were deposited on SiO2/Si substrates, and Ag NPs (∼25 nm in diameter) were formed using a thermal evaporator and a postannealing process. The Ag NPs effectively absorbed light in the wavelength range of 500 and 600 nm, which corresponds to the plasmonic effect. Due to the plasmon resonance of Ag NPs, a significantly enhanced photocurrent was observed on the devices. The current increased by 348% with exposure to light when the Ag NPs were formed at the interface between the 10-nm-thick GIGO film and SiO2 substrate. The increased photocurrent revealed the presence of strong coupling between the localized plasmon and electrical carrier of the devices. The results show that the photocurrent of GIGO TFTs can be greatly enhanced when the plasmonic Ag NPs are located in the channel region of the devices.

Solution processable bilayered gate dielectric towards flexible organic thin film transistors

Publication date: April 2015 Source:Organic Electronics, Volume 19 Author(s): Ranjodh Singh , Jagan Singh Meena , I-Hsin Tsai , Yen-Ting Lin , Cheng-Jyun Wang , Fu-Hsiang Ko In this study, we have successfully explored the potential of a new bilayer gate dielectric material, composed of Polystyrene (PS), Pluronic P123 Block Copolymer Surfactant (P123) composite thin film and Polyacrylonitrile (PAN) through fabrication of metal insulator metal (MIM) capacitor devices and organic thin film transistors (OTFTs). The conditions for fabrication of PAN and PS-P123 as a bilayer dielectric material are optimized before employing it further as a gate dielectric in OTFTs. Simple solution processable techniques are applied to deposit PAN and PS-P123 as a bilayer dielectric layer on Polyimide (PI) substrates. Contact angle study is further performed to explore the surface property of this bilayer polymer gate dielectric material. This new bilayer dielectric having a k value of 3.7 intermediate to that of PS-P123 composite thin film dielectric (k ∼2.8) and PAN dielectric (k ∼5.5) has successfully acted as a buffer layer by preventing the direct contact between the organic semiconducting layer and high k PAN dielectric. The OTFT devices based on α,ω-dihexylquaterthiophene (DH4T) incorporated with this bilayer dielectric, has demonstrated a hole mobility of 1.37×10− 2 and on/off current ratio of 103 which is one of the good values as reported before. Several bending conditions are applied, to explore the charge carrier hopping mechanism involved in deterioration of electrical properties of these OTFTs. Additionally, the electrical performance of OTFTs, which are exposed to open atmosphere for five days, can be interestingly recovered by means of re-baking them respectively at 90°C. Graphical abstract