Reduction of vacuum sublimation by ion beam treatment for e-beam deposited SiC films

We report on the low temperature (≤1000 °C) vacuum sublimation behavior of e-beam evaporative deposited SiC film and a method to reduce the vacuum sublimation by an ion beam process. The density of SiC film deposited by e-beam evaporation method was ∼60% of the density of bulk source material. In this sample, we found that sublimation became appreciable above ∼750 °C under 1.5 × 10⁻⁵ torr pressure and the sublimation rate increased with increasing temperature, reaching ∼70 nm/h at 950 °C when the coated sample was heated for 5 h. When the film was irradiated with 70 keV N⁺ ions prior to heating, the sublimation rate decreased down to ∼23 nm/h at a fluence of 1 × 10¹⁷ ions/cm². However, further increase in fluence beyond this value or extended heating period did not change (decrease or increase) the sublimation rate any more.     

Electric characteristics of organic thin-film transistors and logic circuits with a ferroelectric gate insulator

Organic electronic devices using a pentacene have improved importantly in the last several years. We fabricated pentacene organic thin-film transistors (OTFTs) with dielectric SiO₂ and ferroelectric Pb(Zr_0.3,Ti_0.7)O₃ (PZT) gate insulators. The organic devices using SiO₂ and PZT films had the field-effect mobility of approximately 0.1 and 0.004 cm²/V s, respectively. The drain current in the transfer curve of pentacene/PZT transistors showed a hysteresis behavior originated in a ferroelectric polarization switching. In order to investigate the polarization effect of PZT gate dielectrics in a logic circuit, the simple voltage inverter using SiO₂ and PZT films was fabricated and measured by an output-input measurement. The gain of inverter at the poling-down state was approximately 7.2 and it was three times larger than the value measured at the poling-up state.     

Investigation of self-assembled monolayer treatment on SiO2 gate insulator of poly(3-hexylthiophene) thin-film transistors

We have investigated self-assembled monolayer (SAM) treatment on SiO₂ gate insulator of poly(3-hexylthiophene) (P3HT) thin-film transistor (TFT), and demonstrated a correlation between mobility and surface free energy of the insulator. The device with lower surface free energy shows higher mobility. The docosyltrichlorosilane (DCTS)-treated device exhibits the best performance among the various SAM-treated devices examined. Field-effect mobility, on/off ratio and threshold voltage of the DCTS-treated P3HT TFT were 0.015 cm²/Vs, >10⁵ and −14 V, respectively.     

An organic nonvolatile memory using space charge polarization of a gate dielectric

We realize a nonvolatile and rewritable memory effect in an organic field-effect transistor (OFET) structure using polymethylmethacryrate (PMMA) dispersed with 10-methyl-9-phenylacridinium perchlorate (MPA⁺ClO₄⁻) as a gate dielectric. Applying a voltage between a top source-drain electrode and a bottom gate electrode induces electrophoresis of two ions of MPA⁺ and ClO₄⁻ towards the corresponding electrodes in the memory devices. The drain currents of the memory devices markedly increase from 10^− 9 A to 10^− 2 A under no gate voltage condition due to the strong space charge polarization effect. Our memory devices have excellent electrical bistability and retention characteristics, i.e. the memory on/off ratio reached 10⁷ and the drain current maintained 40% of the initial value after 10⁴ s.     

Organic light-emitting diodes with 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole layer inserted between hole-injecting and hole-transporting layers

Organic light-emitting diodes (OLEDs) were fabricated based on copper phthalocyanine (CuPc) (hole-injecting layer), N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB) (hole-transporting layer) and tris(8-hydroxyquinoline) aluminum (Alq₃) (emission and electron-transporting layer). A 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) layer was inserted between CuPc and NPB. The effect of different thickness of PBD layer on the performance of the devices was investigated. The device structure was ITO/CuPc/PBD/NPB/Alq₃/LiF/Al. Optimized PBD thickness was about 1 nm and the electroluminescent (EL) efficiency of the device with 1 nm PBD layer was about 48 percent improvement compared to the device without PBD layer. The inserted PBD layer improved charge carriers balance in the active layer, which resulted in an improved EL efficiency. The performance of devices was also affected by varying the thickness of NPB due to microcavity effect and surface-plasmon loss.     

Thin film transistors by solution-based indium gallium zinc oxide/carbon nanotubes blend

Solution-based indium gallium zinc oxide (IGZO)/single-walled carbon nanotubes (SWNTs) blend have been used to fabricate the channel of thin film transistors (TFTs). The electrical characteristics of the fabricated devices were examined. We found a low leakage current and a higher on/off currents ratio for TFT with SWNTs compared to solution-based TFTs made without SWNTs. The saturation field effect mobility (μ_sat) of about 0.22 cm²/Vs, the current on/off ratio is ~ 10⁵, the subthreshod swing is ~ 2.58 V/decade and the threshold voltage (V_th) is less than − 2.3 V. We demonstrated that the solution-based blend active layer provides the possibility of producing higher performance TFTs for low-cost large area electronic and flexible devices.     

Characteristics of organic inverters using vertical- and lateral-type organic transistors

The characteristics of vertical-type organic static induction transistors (OSITs) were compared with those of lateral-type organic field effect transistors (OFETs). From these experiments, it was confirmed that the OSITs can operate at a voltage one order less than that required for OFETs. We also fabricated two types of organic inverter based on OSITs and OFETs and investigated their transfer characteristics. These results demonstrate that it is possible to decrease the operational voltage of organic inverters from ± 20 V to ± 2 V by using two OSITs with higher on/off ratios.     

Isothermal and non-isothermal sublimation kinetics of zirconium tetrachloride (ZrCl4) for producing nuclear grade Zr

Sublimation of ZrCl₄ is important for the production of nuclear grade metallic Zr in Kroll's process. The sublimation kinetics of ZrCl₄ was investigated by thermogravimetric analysis under both isothermal and non-isothermal conditions. The sublimation rate of ZrCl₄ increased with increasing temperature under isothermal conditions. ZrCl₄ sublimation was confirmed to be a zero-order process under isothermal conditions, whereas it was first-order kinetics under non-isothermal conditions. The activation energy of ZrCl₄ sublimation under isothermal conditions was 21.7 kJ mol⁻¹. The activation energy for non-isothermal sublimation was 101.4 kJ mol⁻¹ and 108.1 kJ mol⁻¹ with the Kissinger method and Flynn–Wall–Ozawa method, respectively. These non-isothermal activation energies were very close to the heat of sublimation (103.3 kJ mol⁻¹). Sublimation occurs by two elementary steps: surface reaction and desorption. Therefore, the overall activation energy of ZrCl₄ sublimation is 104.8 (±3.4) kJ mol⁻¹. The activation energy of the surface reaction and desorption steps are proposed to be 83.1 kJ mol⁻¹ and 21.7 kJ mol⁻¹, respectively.     

The effect of GdF3 on red emission of Eu3+: LiYF4 single crystals grown by the Bridgman method

The bulk LiYF₄ single crystals with high-quality doped 0.5 mol% Eu³⁺ and various Gd³⁺ from 1.5 to 4.5 mol% in size of about Φ 10?×?65 mm were successfully grown by an improved Bridgman method. The X-ray diffraction (XRD) measurement and Rietveld refinement analysis were conducted to verify the structure of the obtained crystal crystals. The spectroscopic properties of the single crystals as change of GdF₃ concentration were investigated with the help of absorption, excitation, emission spectra, and decay curves of their fluorescence. The characteristic absorption bands of Gd³⁺ at 277 nm and Eu³⁺ at 395 nm were observed in the co-doped samples. Significant enhanced emission intensity of 613 nm was observed as increasing of GdF₃ content into Eu³⁺:LiYF₄ single crystal upon excitations of both 277 nm and 395 nm lights. The former was owing to the energy transfer (ET) between Gd³⁺ and Eu³⁺, while the latter was due to the change of crystal field environment around Eu³⁺ by the increasing of GdF₃ content. The ET from Gd³⁺ to Eu³⁺ ions was further confirmed from the result of the luminescence decay analysis. Besides, the full width half-maximum (FWHM) of 613 nm emission band was estimated to be?~?4.5 nm. The Eu³⁺/Gd³⁺ co-doped LiYF₄ single crystal with excellent optical and physicochemical properties might has significant applications in red laser and display devices.

     

Fabrication and characterization of polyterpenol as an insulating layer and incorporated organic field effect transistor

A non-synthetic polymer material, polyterpenol, was fabricated using a dry polymerization process namely RF plasma polymerization from an environmentally friendly monomer and its surface, optical and electrical properties investigated. Polyterpenol films were found to be transparent over the visible wavelength range, with a smooth surface with an average roughness of less than 0.4 nm and hardness of 0.4 GPa. The dielectric constant of 3.4 for polyterpenol was higher than that of the conventional polymer materials used in the organic electronic devices. The non-synthetic polymer material was then implemented as a surface modification of the gate insulator in field effect transistor (OFET) and the properties of the device were examined. In comparison to the similar device without the polymer insulating layer, the polyterpenol based OFET device showed significant improvements. The addition of the polyterpenol interlayer in the OFET shifted the threshold voltage significantly; + 20 V to −3 V. The presence of trapped charge was not observed in the polyterpenol interlayer. This assisted in the improvement of effective mobility from 0.012 to 0.021 cm²/Vs. The switching property of the polyterpenol based OFET was also improved; 10⁷ compared to 10⁴. The results showed that the non-synthetic polyterpenol polymer film is a promising candidate of insulators in electronic devices.     

Enhanced and stable electron emission of carbon nanotube emitter arrays by post-growth hydrofluoric acid treatment

Carbon nanotubes (CNT) have been highlighted as possible candidates for field-emission emitters and vacuum nanoelectronic devices. In this article, we studied the effect of acid treatment of CNTs on field emission from carbon nanotube field emitter arrays (FEAs), grown using the resist-assisted patterning process (RAP). The emission current densities of as grown CNT-FEAs and those which were later immersed in hydrofluoric acid (HF) for 20 s, were 19 μA/cm² and 7.0 mA/cm², respectively, when measured at an anode field of 9.2 V/μm. Hence, the emission current densities after HF treatment are 300 times larger than those of as grown CNT-FEAs. Also, it was observed that a very stable electron emission current was obtained after stressing the CNTs with an electric field of 9.2 V/μm for 800 min in dc-mode, where the emission current non-uniformity was 0.13%. The enhancement in electron emission after HF treatment appears to be due to the effect of fluorine bonding. Also, the electron emission characteristics and structural improvement of CNT-FEAs after HF treatment are discussed.     

Gravure printed organic light emitting diodes for lighting applications

A major advantage of polymer based organic light emitting diodes (OLED) is the capability to be manufacturing them with low cost, high-throughput printing techniques. In this paper, we report on double layer gravure printed polymer based OLED light sources with an active area of 0.16 cm² on glass substrate. The devices exhibit brightness of 100 cd/m² and 1000 cd/m² at 4.2 V and 5.4 V, respectively. Furthermore, a large area OLED of 30 cm² in which both polymer layers are gravure printed is demonstrated for lighting applications. Based on the results presented in this paper, the feasibility of the gravure printing technique for the fabrication of large area OLEDs in large-scale production is proved.     

Vanadium-Doped Semi-Insulating 6H-SiC for Microwave Power Device Applications

Two-inch semi-insulating SiC bulk crystals with resistivity higher than 1×106 Ωcm were achieved by vanadium doping during sublimation. Secondary-ion-mass-spectrometry (SIMS) was employed to determine the concen-tration of impurities in the crystals, such as B, Al, V and N. These results indicated that the concentration of nitrogen and aluminum kept on decreasing and the concentration of B and V was almost constant during the whole growth. An inner crucible was used to control the exhausting of vanadium, which made the uniformity of the high resistivity (1×106 Ωcm) in the wafer up to 80%. High-performance AlGaN/GaN high-electron-mobility-transistor (HEMT) materials and devices were grown and fabricated on semi-insulating 6H-SiC sub-strates. The two-dimensional electron gas (2DEG) mobility at room-temperature was 1795 cm2/V·s. The charge carrier concentration of the substrate determined by capacitance-voltage (C-V) test was 7.3 × 1015 cm-3. The device with a gate width of 1 mm exhibits a maximum output power of 5.5 W at 8 GHz, which proves the semi-insulating property of the substrates indirectly.     

Improvement of white organic light emitting diode performances by an annealing process

White organic light emitting diode (OLED) devices with the structure ITO/PHF:rubrene/Al, in which PHF (poly(9,9-di-n-hexylfluorenyl-2,7-diyl)) is used as blue light emitting host and rubrene (5,6,11,12-tetraphenylnapthacene) as an orange dye dopant, have been fabricated. Indium tin oxide (ITO) coated-glass and aluminium were used as anode and cathode, respectively. The devices were fabricated with various rubrene-dopant to obtain a white light emission. The OLED device that composed of several concentrations of rubrene-doped PHF film was prepared in this study. It was found that the concentration of rubrene in the PHF-rubrene thin film matrix plays a key role in producing the white color emission. In a typical result, the device composed of 0.06 wt.% rubrene-dopant produced the white light emission with the Commission Internationale de L'Eclairage (CIE) coordinate of (0.30,0.33). The turn-on voltage and the brightness were found to be as low as 14.0 V and as high as 6540 cd/m², respectively. The annealing technique at relatively low temperature (50 °C, 100 °C, and 150 °C) was then used to optimize the performance of the device. In a typical result, the turn-on voltage of the device could be successfully reduced and the brightness could be increased using the annealing technique. At an optimum condition, for example, annealed at 150 °C, the turn-on voltage as low as 8.0 V and the brightness as high as 9040 cd/m² were obtained. The mechanism for the improvement of the device performance upon annealing will be discussed.     

Synthesis and optoelectronic properties of oxadiazole-functionalized iridium complexes in the poly(vinylcarbazole)-hosted devices

A class of oxadiazole-functionalized iridium complexes was used as phosphor emitters in poly (vinylcarbazole)-hosted devices. Efficient green electrophosphorescences were achieved in the devices with a maximum luminance efficiency of 9.3 cd/A at 10.6 mA/cm² and brightness of 3882 cd/m² at 92.1 mA/cm². More importantly, the iridium complexes-doped devices exhibited a low turn-on voltage of 7.0 V and an applied voltage of 9.2 V at 500 cd/m². The good optoelectronic properties of the complexes were attributed to the enhanced electron-injection and transport properties resulting from the effect of oxadiazole ligands in the complexes.     

Effect of substituents on electronic properties, thin film structure and device performance of dithienothiophene-phenylene cooligomers

Dithienothiophene-phenylene cooligomers with n-hexyloxy or n-dodecyloxy substituents have been synthesized and compared to the previously reported unsubstituted parent compound. The effect of substituents on the thermal, electronic, optical, thin film structure and field-effect transistor (OFET) properties was investigated. Structural phase transitions from highly-ordered nanocrystalline to liquid crystalline were observed at 241 and 213 °C for n-hexyloxy- and n-dodecyloxy-substituted compounds respectively, different from the parent compound. For the alkoxy-substituted compounds, the absorption spectra in thin film blue shift 50 nm, while the fluorescence spectra in thin film red shift 88-100 nm compared to those in solution. The OFET devices based on the alkoxy-substituted compounds exhibit mobilities as high as ca 0.02 cm²V^− 1s^− 1 and their performance is sensitive to the alkoxy substituents and substrate temperatures.     

Linear field emission cathode with ZnO grown in aqueous solutions

A new type of linear field emission cathode with ZnO nanostructure grown on nickel wires was prepared by hydrothermal approach. The obtained ZnO nanotapers were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results indicated that the ZnO nanotapers with sharp tips were high quality single crystals, and grow along (002) direction. The field emission properties were investigated by ZnO nanotapers on nickel wire as the cathode in the centre of a cylindrical ITO anode. The field enhancement factor β was about 2.23 × 10⁴ cm⁻¹, which improved greatly for the cylindrical configuration and sharp geometry of the ZnO nanotapers tip.     

High efficiency red organic light-emitting diodes using a phosphorescent iridium complex doped into a hole-blocking material

We demonstrate high-efficiency red electrophosphorescent organic light-emitting devices (OLEDs) by doping a red-emitting iridium complex, Bis[7-methyl-1-p- tolyisoquinolinato-N,C²′]-iridium(III)(acetylacetonate) [(7-mtiq)₂Ir(acac)], into a hole-blocking material, 4-biphenyloxolato aluminum(III)bis(2-methyl-8- quinolinato)4-phenylphenolate. Both the phosphorescent characteristics of (7-mtiq)₂Ir(acac) and the electroluminescence mechanisms of OLEDs are investigated in this study. The Commission Internationale de L'Eclairage coordinates of (0.66, 0.34) is very close to the National Television System Committee standard red point (0.66, 0.33). With a dopant concentration of about 4%, a maximum luminance of 31317 cd/m² and a luminous efficiency of 21.6 cd/A have been obtained.     

Controllable synthesis of nickel dendritic crystals induced by magnetic field

We demonstrated in this paper a simple and easy method for the preparation of dendritic nickel crystals in an external magnetic field in boiling ethylene glycol (EG) solution. The structural features and morphology of the sample were investigated using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The values of saturation magnetization (M_s) and coercivity (H_c) of the dendritic crystals characterized by using a vibrating sample magnetometer (VSM) are 170.3 emu g^?1 and 50.7 Oe, respectively. It was clear that the external magnetic field was the most important factor for controlling the morphology of the product.     

Effect of fabrication process on characteristics of phosphorescence organic light emitting diodes with methoxy-substituted starburst low-molecule as a host

The effect of dry process and wet process on the characteristics of phosphorescence organic light-emitting devices (OLEDs) employing a phosphorescent dye fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)₃) doped into a methoxy-substituted starburst low-molecule material methoxy-substituted 1,3,5-tris[4-(diphenylamino) phenyl]benzene (TDAPB) are investigated. The FT-IR and absorption spectra of TDAPB films fabricated by a dry process, and a wet process are almost same, and the PL spectra of those films are different. The carrier transport capability of TDAPB by a dry process is lower than that by a wet process. The photoluminescence intensity of Ir(ppy)₃ doped in TDAPB fabricated by a wet process is higher than that by a dry process. A maximum external current efficiency of more than 20 cd/A and luminance of more than 10,000 cd/m² were obtained. Maximum luminance of devices monotonously decreases with increasing the thickness of a dry-processed emitting layer. The main emission zone of the OLED was located in almost at the center of the emitting layer. The improvement of device performance in the OLED fabricated by a wet process was achieved due to the high efficient energy transfer from TDAPB to Ir(ppy)₃, high carrier transporting capability and the formation of homogeneous film, compared with that fabricated by a dry process.