Carbazole and benzimidazole/oxadiazole hybrids as bipolar host materials for sky blue,green, and red PhOLEDs

Two novel bipolar host materials (CBzIm and COxaPh) comprising of a hole-transport (HT) carbazole core functionalized with electron-transport (ET) moieties (benzimidazole/oxadiazole) at C3 and C6 positions have been synthesized. Their thermal, photophysical, electrochemical properties, and carrier mobilities were characterized. Theoretical calculations revealed that the HOMO orbitals were generally delocalized over the hole- and electron-transport moieties for both CBzIm and COxaPh, whereas the LUMO orbitals distribution only involved one benzimidazole moiety in CBzIm instead of fully delocalization over the whole polar moieties for COxaPh, which is consistent with the observation of good hole mobilities for both hosts and better electron mobility for COxaPh over CBzIm. CBzIm with high E_T (2.76 eV) is suitable to serve as a blue phosphor host, where a sky blue phosphor (DFPPM)₂Irpic exhibiting superior properties than those of popular blue emitter FIrpic was used to give highly efficient phosphorescent OLEDs, achieving a maximum external quantum efficiency (η_ext) of 15.7%. The better π-delocalization of COxaPh led to a lower triplet energy (E_T = 2.65 eV), which can be used to accommodate green and red phosphors, providing excellent device performance with η_ext as high as 17.7% for green [(ppy)₂Ir(acac)] and 20.6% for red [Os(bpftz)₂(PPh₂Me)₂], respectively.     

Numerical simulation of a heat sink embedded with a vapor chamber and calculation of effective thermal conductivity of a vapor chamber

This study presents a numerical investigation of a whole set of thermal module, including a plate-fin heat sink embedded with a vapor chamber, subject to the influence of concentrated heat sources. Within the vapor chamber, the internal vapor is assumed as a common heat-transfer interface between the wicks. CFD simulations of the integrated heat sink are carried out with this assumption. The calculated results are in good agreement with the experiments, and show a maximum difference of 6.3% for the hotspot temperature rises. It is found that the area of the heat source has an important influence to the performance of the vapor chamber. The major spreading resistance of the vapor chamber comes from the bottom wall, where a concentrated heat source is applied. In addition, the isotropic and orthotropic approaches are proposed to calculate the effective thermal conductivities of the vapor chamber. By approximating the vapor chamber as a conduction plate, the effective conductivity can be obtained from the analytical solutions of the spreading resistances. The vapor chamber can reduce the spreading resistances sufficiently by its excellent lateral thermal spreading effect, which can be interpreted by the orthotropic approach.     

Method for determining the junction temperature of alternating current light-emitting diodes

A numerical simulation was used to simulate the temperature distribution during AC and DC operations of an alternating current light-emitting diode (AC LED). The relationship between the junction temperature and the temperature at the centre of the bottom surface of the submount of an AC LED was measured under DC operation. This relationship was confirmed by numerical simulation. The numerical results were consistent with the experimental observations in that the temperature at the centre of the bottom surface of the submount was insensitive to the current variations that occur in an AC LED, probably because of the large mass of the submount. However, it was difficult to measure the temperature oscillation at the junctions in an AC LED, although this oscillation can be clearly seen in the numerical results. Thus, the authors propose a formula for predicting the range of the oscillating junction temperature for an AC LED.     

Molecular beam epitaxy growth of TmP/GaAs and transistor action inGaP/TmP/GaAs heterostructures

The growth of thulium phosphide (TmP) by molecular beam epitaxy (MBE) on GaAs substrate is reported. Good epilayer quality was demonstrated through X-ray diffraction (XRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM) analysis. The closely lattice matched TmP layer was n-type with an electron concentration of 1.6×10²¹ cm^-3 and a room temperature mobility of 4.8 cm²V^-1s^-1. The Schottky barrier height determined from 1/capacitance² (1/C²) versus voltage (V) measurements is about 0.81 eV which agrees well with the value obtained through the current-voltage (I-V) measurements. In this work, we also report transistor action in a GaP/TmP/GaAs structure, for which chemical bonding techniques were employed. From I-V measurements, a common base current gain α≈0.55 at V_CB=0 was obtained at room temperature     

Effects of alkali soluble resin on the emulsion polymerization of butyl methacrylate

In this work, alkali soluble resin (ASR) was evaluated as a surfactant in the emulsion polymerization of butyl methacrylate (BMA). Kinetic analysis indicated that the ASR surfactant retarded the reaction rate and reduced the average number of radicals per latex particle. Since the particle nucleation period proceeds until the disappearance of droplets, Interval II does not exist in this system. Experimental results show that the particle number depends on the 0.31 and 0.51 powers of the ASR and KPS concentration, respectively. The particle size distribution of the latex becomes broad with the increase of the ASR concentration in the emulsion polymerization. This phenomenon explains why the period of the particle nucleation is proportional to the ASR concentration used in the reaction.     

Electrical characterization and process control of cost-effective high-k aluminum oxide gate dielectrics prepared by anodization followed by furnace annealing

A cost-effective technique was introduced to prepare ultrathin aluminum oxide (Al/sub 2/O/sub 3/) gate dielectrics with equivalent oxide thickness (EOT) down to 14 /spl Aring/. Al/sub 2/O/sub 3/ was fabricated by anodic oxidation (anodization) of ultrathin Al films at room temperature in deionized water and then furnace annealed at 650/spl deg/C in N/sub 2/ ambient. Both dc and dac (dc superimposed with ac) anodization techniques were investigated. Effective dielectric constant of k/spl sim/7.5 and leakage current of 2-3 orders of magnitude lower than SiO/sub 2/ are observed. The conduction mechanism in Al/sub 2/O/sub 3/ gate stack is shown to be Fowler-Nordheim (F-N) tunneling. Saturated current behavior in the inversion region of MOS capacitor is observed. It is found that the saturation current is sensitive to interface state capacitance and can be used as an efficient way to evaluate the Al/sub 2/O/sub 3/ gate stack/Si-substrate interfacial property. An optimal process control for preparing Al/sub 2/O/sub 3/ gate dielectrics with minimized interface state capacitance via monitoring the inversion saturation current is demonstrated.     

Limitations of the back-to-back barrier-intrinsic-n+(BIN) diode frequency tripler

Problems of the back-to-back GaAs barrier-intrinsic-n⁺ (BIN) diode frequency tripler concept along with the associated device physics are presented. The back-to-back GaAs BIN diode structure was originally proposed to have an intrinsic cutoff frequency close to 1 THz and to be a highly efficient millimeter-wave frequency tripler. Frequency limitations will be discussed to explain the failure of the back-to-back GaAs BIN diode as a millimeter-wavelength device. Optimization is also carried out to explore the possibility of improving the high-frequency performance by modifying the BIN diode structure     

Incidence of NIDDM and the effects of gender, obesity and hyperinsulinaemia in Taiwan

Our aim is to determine non-insulin-dependent diabetes mellitus (NIDDM) incidence in Taiwan and examine its relation to obesity and hyperinsulinaemia in Chinese men and women. A total of 995 men and 1195 women aged 35-74 years free from diabetes in two townships in Taiwan were followed up with a second examination. At baseline general and metabolic data were recorded, and detailed anthropometric parameters and plasma glucose and insulin were assessed. World Health Organisation (WHO) criteria of fasting glucose 7.8 mmol/l or greater was utilized for defining diabetes. The age-standardized incidence rate based on the United States population in 1970 was 9.3/1000 (CI 5.8-12.8) in men and 9.3/1000 (CI 6.2-12.4) in women and the based on the WHO population in 1976 was 8.9/1000 (CI .5-12.3) in men and 8.9/1000 (CI 5.9-11.9) in women for the Chinese who had a mean BMI slightly greater than 24 (kg/m2). The predictability of the plasma glucose level was greater than that of the insulin level and the obesity indices. NIDDM incidence increased approximately threefold with each 0.67 mmol/l increase in plasma glucose level in men and women. The present study demonstrated the essential relationship of not only BMI but also central obesity indices (such as subscapular and waist circumference) to the incidence of NIDDM among men and women and a stronger relationship between NIDDM incidence and obesity in women than in men. The predictive effects of obesity indices and fasting plasma insulin values on NIDDM risk were independent of each other in men. Obesity and hyperinsulinaemia each without the presence of the other can lead to an increased risk of NIDDM. In women the NIDDM incidence increased more than additively in those with both obesity and hyperinsulinaemia compared to those with single obesity or hyperinsulinaemia. A slightly higher incidence of NIDDM in Taiwan than in western countries was found. The importance of obesity is indicated for predicting NIDDM in the community. Hyperinsulinaemia was found to play a significant role in predicting NIDDM incidence independent of obesity in men and synergistically with obesity in women.     

Region-based compilation: Introduction, motivation, and initial experience

The most important task of a compiler designed to exploit instruction-level parallelism (ILP) is instruction scheduling. If higher levels of ILP are to be achieved, the compiler must use, as the unit of scheduling, regions consisting of multiple basic blocks—preferably those that frequently execute consecutively, and which capture cycles in the program’s execution. Traditionally, compilers have been built using the function as the unit of compilation. In this framework, function boundaries often act as barriers to the formation of the most suitable scheduling regions. Function inlining may be used to circumvent this problem by assembling strongly coupled functions into the same compilation unit, but at the cost of very large function bodies. Consequently, global optimizations whose compile time and space requirements are superlinear in the size of the compilation unit, may be rendered prohibitively expensive. This paper introduces a new approach, called region-based compilation, wherein the compiler, after inlining, repartitions the program into more desirable compilation units, termed regions. Region-based compilation allows the compiler to control problem size and complexity while exposing inter-procedural scheduling, optimization and code motion opportunities.