Ambipolar organic field-effect transistors with air stability, high mobility, and balanced transport

Ambipolar organic field-effect transistors (OFETs) based on the organic heterojunction of copper-hexadecafluoro-phthalocyanine (F16CuPc) and 2,5-bis(4-biphenylyl) bithiophene (BP2T) were fabricated. The ambipolar OFETs eliminated the injection barrier for the electrons and holes though symmetrical Au source and drain electrodes were used, and exhibited air stability and balanced ambipolar transport behavior. High field-effect mobilities of 0.04 cm(2)/V s for the holes and 0.036 cm(2)/V s for the electrons were obtained. The capacitance-voltage characteristic of metal-oxide-semiconductor (MOS) diode confirmed that electrons and holes are transported at F16CuPc and BP2T layers, respectively. On this ground, complementary MOS-like inverters comprising two identical ambipolar OFETs were constructed.

Improved n-type organic transistors by introducing organic heterojunction buffer layer under source/drain electrodes

N-type organic thin-film transistors (OTFTs) employing hexadecafluorophthalocyaninatocopper (F16CuPc) as active layer and p-type copper phthalocyanine (CuPc) as buffer layer are demonstrated. The highest field-effect mobility is 7.6x10(-2) cm(2)/V s. The improved performance was attributed to the decrease of contact resistance due to the introduction of highly conductive F16CuPc/CuPc organic heterojunction. Therefore, current method provides an effective path to improve the performance of OTFTs.

Organic heterojunction with reverse rectifying characteristics and its application in field-effect transistors

A diode with a reverse rectifying characteristics was fabricated based on the organic heterojunction of copper phthalocyanine (CuPc) and copper-hexadecafluoro-phthalocyanine (F16CuPc). At the heterojunction interface, HOMO of CuPc is bended upwards and LUMO of F16CuPc is bended downwards, since the charge carriers were accumulated at both side of the interface, electrons in F16CuPc and holes in CuPc. The thickness of holes accumulated at the CuPc layer is about 10 nm. which was determined by fabricating organic field-effect transistors with active layers in series of thickness. By utilizing the heterojunction-effect, the threshold voltage in organic transistors can be modified.

An investigation on air stability of copper phthalocyanine-based organic thin-film transistors and device encapsulation

The device performances of copper phthalocyanine (CuPc)-based organic thin-film transistors (OTFTs) in main components of air were studied. We found that the device stored in O-2 humidified by water exhibited the changes of electric characteristics including positive-shifted threshold voltage and lower I-on/I-off but unchanged mobility, which was similar to the device exposed to room air. These changes are attributed to O-2 doping to copper phthalocyanine thin film assisted by water. Furthermore, a cross-linked polyvinyl alcohol film was used as encapsulation layer to prevent the permeation of O-2 and water, which resulted in excellent stability even when devices were placed in air for over a year. Therefore, current studies will push the development of OTFTs for practical applications.

Organic integrated pixels fabricated by a full evaporation method

An organic integrated pixel consisting of an organic light-emitting diode driven by an organic thin-film field-effect transistor (OTFT) was fabricated by a full evaporation method oil a transparent glass substrate. The OTFT was designed as a top-gate Structure, and the insulator is composed of a double-layer polymer of Nylon 6 and Teflon to lower the operation voltage and the gate-leakage current, and improve the device stability. The field-effect mobility of the OTFT is more than 0.5 cm(2) V-1 s(-1), and the on/off ratio is larger than 10(3). The brightness of the pixel reached as large as 300 cd m(-2) at a driving current of 50 mu A.

Polymer light-emitting diodes based on a bipolar transporting luminescent polymer

A soluble electroluminescent polymer containing hole-deficient triphenylamine and electron-deficient oxadiazole units in the main chains has been designed and studied. The design is based on the consideration that the triphenylamine group possesses good hole-transporting property and the oxadiazole unit is known to be of electron-transporting character. Because of the good bipolar transporting performance, the brightness and electroluminescent efficiency are significantly improved and the turn-on voltage is reduced compared with a similar polymer without the electron-deficient oxadiazole units in the main chains. For a device with configuration ITO/PEDOT/polymer/CsF/Al, a maximum brightness of 3600 cd m(-2) and a maximum luminescent efficiency of 0.65 cd A(-1) (quantum efficiency of 0.3%) were obtained in the polymer with oxadiazole units, about 15 times brighter and 15 times more efficient than the corresponding polymer without oxadiazole units.

Studies of perchlorate triggered ion-gate behavior of sBLM by electrochemiluminescence and its application to a sensor for perchlorate

Ru(bpy)(3)(2+) electrochemiluminescence (ECL) method and electrocatalysis method were first used to study the ion-gate behavior of supported lipid bilayer membrane (sBLM). We found that sBLM, made of dimethyldioctadecylammonium bromide (a kind of synthetic lipid), showed ion-gate behavior for the permeation of Ru(bpy)(3)(2+) in the presence of perchlorate anion. There existed a threshold concentration (0.1 muM) of perchlorate anion for ion-gate opening. Below the threshold the ion-gate was closed. Above the threshold, the number of opened ion-gate sites increased with the increase of perchlorate anion concentration and leveled off at concentrations higher than 1200 muM. Based on it, a new sensor for perchlorate was developed. Furthermore, the opening and closing of the ion-gate behavior was reversible, which means the sensor can repeatedly be used.

Novel bipolar conjugated polymer containing both triphenylamine and oxadizole units

A novel bipolar conjugated polymer containing triphenylamine and 1, 3, 4-oxadiazole units was synthesized by Suzuki reaction. Its structure and properties were characterized by NMR, IR, UV-Vis, PL spectroscopy and electrochemical measurement. The photo luminescent spectroscopy and cyclic voltammograms measurement demonstrated that the resulting polymer shows blue emission (477 nm) and possesses both electron and hole-transporting property.

Synthesis and characterization of novel bipolar PPV-based copolymer containing triazole and carbazole units

Two new blue light-emitting PPV-based conjugated copolymers containing both an electron-withdrawing unit (triazole-TAZ) and electron-rich moieties (carbazole-CAR and bicarbazole-BCAR) were prepared by Wittig condensation polymerization between the triazole diphosphonium salt and the corresponding dialdehyde monomers. Their structures and properties were characterized by FT-IR, TGA, DSC, UV-Vis, PL spectroscopy and electrochemical measurements. The resulting copolymers are soluble in common organic solvents and thermally stable with a T-g of 147degreesC for TAZ-CAR-PPV and of 157degreesC for TAZ-BCAR-PPV. The maximum photoluminescence wavelengths of TAZ-CAR-PPV and TAZ-BCAR-PPV film appear at 460 nm and 480 nm, respectively. Cyclic voltammetry measurement demonstrates that TAZ-BCAR-PPV has good electrochemical reversibility, while TAZ-CAR-PPV exhibits the irreversible redox process. The triazole unit was found to be an effective pi-conjugation interrupter and can play the rigid spacer role in determining the emission colour of the resulting copolymer.

Super insulated aerogel windows: Impact on daylighting and thermal performance.

Window design plays an important role in achieving energy efficient buildings and in providing thermal comfort of building occupants. This paper investigates a newly developed aerogel window and the potential improvement on the comfort factors of an office in relation to daylighting. Improved comfort levels can impact on health and wellbeing of building occupants leading to knock on effects on absenteeism and productivity. A simulation tool was presently created that will easily enable comparison of different façade design and their impact on heat and light transmission and therefore enable optimisation. One of the most important aspects of the present work was comparing the performance of the newly developed aerogel window against the more traditional Argon-filled, coated double-glazing. Whereas the aerogel window provided an extremely low heat-loss index of 0.3 W/m2K, the latter usually offered a centre-glazing U-value of 1.4 W/m2K. On a like-with-like basis the daylight transmission of the aerogel window was significantly lower than double-glazing. However, in view of low thermal loss larger areas of the former can be deployed. This article presents the influence of three key parameters that may lead to an optimum design: daylight, thermal loss and solar gain.

Creation of a complex butterfly attractor using a novel Lorenz-type system

A novel Lorenz-type system of nonlinear differential equations is proposed. Unlike the original Lorenz system, where the chaotic dynamics remain confined to the positive half-space with respect to the Z state variable due to a limiting threshold effect, the proposed system enables bipolar swing of this state variable. In addition, the classical set of parameters (a, b, c) controlling the behavior of the Lorenz system are reduced to a single parameter, namely a. Two possible modes of operation are admitted by the system; switching between these two modes results in the creation of a complex butterfly chaotic attractor. Numerical simulations and results from an experimental setup are presented

First principles modelling of nucleation and growth during atomic layer deposition onto III-V substrates

Atomic layer deposition (ALD) is now used in semiconductor fabrication lines to deposit nanometre-thin oxide films, and has thus enabled the introduction of high-permittivity dielectrics into the CMOS gate stack. With interest increasing in transistors based on high mobility substrates, such as GaAs, we are investigating the surface treatments that may improve the interface characteristics. We focus on incubation periods of ALD processes on III-V substrates. We have applied first principles Density Functional Theory (DFT) to investigate detailed chemistry of these early stages of growth, specifically substrate and ALD precursor interaction. We have modelled the ‘clean-up’ effect by which organometallic precursors: trimethylaluminium (TMA) or hafnium and titanium amides clean arsenic oxides off the GaAs surface before ALD growth of dielectric commences and similar effect on Si3N4 substrate. Our simulations show that ‘clean-up’ of an oxide film strongly depends on precursor ligand, its affinity to the oxide and the redox character of the oxide. The predominant pathway for a metalloid oxide such as arsenic oxide is reduction, producing volatile molecules or gettering oxygen from less reducible oxides. An alternative pathway is non-redox ligand exchange, which allows non-reducible oxides (e.g. SiO2) to be cleaned-up. First principles study shows also that alkylamides are more susceptible to decomposition rather than migration on the oxide surface. This improved understanding of the chemical principles underlying ‘clean-up’ allows us to rationalize and predict which precursors will perform the reaction. The comparison is made between selection of metal chlorides, methyls and alkylamides precursors.

Development of inversion-mode and junctionless Indium-Gallium-Arsenide MOSFETs

This PhD covers the development of planar inversion-mode and junctionless Al2O3/In0.53Ga0.47As metal-oxidesemiconductor field-effect transistors (MOSFETs). An implant activation anneal was developed for the formation of the source and drain (S/D) of the inversionmode MOSFET. Fabricated inversion-mode devices were used as test vehicles to investigate the impact of forming gas annealing (FGA) on device performance. Following FGA, the devices exhibited a subthreshold swing (SS) of 150mV/dec., an ION/IOFF of 104 and the transconductance, drive current and peak effective mobility increased by 29%, 25% and 15%, respectively. An alternative technique, based on the fitting of the measured full-gate capacitance vs gate voltage using a selfconsistent Poisson-Schrödinger solver, was developed to extract the trap energy profile across the full In0.53Ga0.47As bandgap and beyond. A multi-frequency inversion-charge pumping approach was proposed to (1) study the traps located at energy levels aligned with the In0.53Ga0.47As conduction band and (2) separate the trapped charge and mobile charge contributions. The analysis revealed an effective mobility (μeff) peaking at ~2850cm2/V.s for an inversion-charge density (Ninv) = 7*1011cm2 and rapidly decreasing to ~600cm2/V.s for Ninv = 1*1013 cm2, consistent with a μeff limited by surface roughness scattering. Atomic force microscopy measurements confirmed a large surface roughness of 1.95±0.28nm on the In0.53Ga0.47As channel caused by the S/D activation anneal. In order to circumvent the issue relative to S/D formation, a junctionless In0.53Ga0.47As device was developed. A digital etch was used to thin the In0.53Ga0.47As channel and investigate the impact of channel thickness (tInGaAs) on device performance. Scaling of the SS with tInGaAs was observed for tInGaAs going from 24 to 16nm, yielding a SS of 115mV/dec. for tInGaAs = 16nm. Flat-band μeff values of 2130 and 1975cm2/V.s were extracted on devices with tInGaAs of 24 and 20nm, respectively

Atomic layer deposition of interface control layers for the fabrication of III/V MOS devices

The continued advancement of metal oxide semiconductor field effect transistor (MOSFET) technology has shifted the focus from Si/SiO2 transistors towards high-κ/III-V transistors for high performance, faster devices. This has been necessary due to the limitations associated with the scaling of the SiO2 thickness below ~1 nm and the associated increased leakage current due to direct electron tunnelling through the gate oxide. The use of these materials exhibiting lower effective charge carrier mass in conjunction with the use of a high-κ gate oxide allows for the continuation of device scaling and increases in the associated MOSFET device performance. The high-κ/III-V interface is a critical challenge to the integration of high-κ dielectrics on III-V channels. The interfacial chemistry of the high-κ/III-V system is more complex than Si, due to the nature of the multitude of potential native oxide chemistries at the surface with the resultant interfacial layer showing poor electrical insulating properties when high-κ dielectrics are deposited directly on these oxides. It is necessary to ensure that a good quality interface is formed in order to reduce leakage and interface state defect density to maximise channel mobility and reduce variability and power dissipation. In this work, the ALD growth of aluminium oxide (Al2O3) and hafnium oxide (HfO2) after various surface pre-treatments was carried out, with the aim of improving the high-κ/III-V interface by reducing the Dit – the density of interface defects caused by imperfections such as dangling bonds, dimers and other unsatisfied bonds at the interfaces of materials. A brief investigation was performed into the structural and electrical properties of Al2O3 films deposited on In0.53Ga0.47As at 200 and 300oC via a novel amidinate precursor. Samples were determined to experience a severe nucleation delay when deposited directly on native oxides, leading to diminished functionality as a gate insulator due to largely reduced growth per cycle. Aluminium oxide MOS capacitors were prepared by ALD and the electrical characteristics of GaAs, In0.53Ga0.47As and InP capacitors which had been exposed to pre-pulse treatments from triethyl gallium and trimethyl indium were examined, to determine if self-cleaning reactions similar to those of trimethyl aluminium occur for other alkyl precursors. An improved C-V characteristic was observed for GaAs devices indicating an improved interface possibly indicating an improvement of the surface upon pre-pulsing with TEG, conversely degraded electrical characteristics observed for In0.53Ga0.47As and InP MOS devices after pre-treatment with triethyl gallium and trimethyl indium respectively. The electrical characteristics of Al2O3/In0.53Ga0.47As MOS capacitors after in-situ H2/Ar plasma treatment or in-situ ammonium sulphide passivation were investigated and estimates of interface Dit calculated. The use of plasma reduced the amount of interface defects as evidenced in the improved C-V characteristics. Samples treated with ammonium sulphide in the ALD chamber were found to display no significant improvement of the high-κ/III-V interface. HfO2 MOS capacitors were fabricated using two different precursors comparing the industry standard hafnium chloride process with deposition from amide precursors incorporating a ~1nm interface control layer of aluminium oxide and the structural and electrical properties investigated. Capacitors furnished from the chloride process exhibited lower hysteresis and improved C-V characteristics as compared to that of hafnium dioxide grown from an amide precursor, an indication that no etching of the film takes place using the chloride precursor in conjunction with a 1nm interlayer. Optimisation of the amide process was carried out and scaled samples electrically characterised in order to determine if reduced bilayer structures display improved electrical characteristics. Samples were determined to exhibit good electrical characteristics with a low midgap Dit indicative of an unpinned Fermi level