R. X. Wang

Flexible organic light-emitting diodes with a polymeric nanocomposite anode.

Flexible organic light-emitting diodes (FOLEDs) are facing mechanical issues arising from failure of the indium-tin oxide (ITO) films fabricated on flexible substrates. Polymeric nanocomposite anodes were fabricated by including single-wall carbon nanotubes (SWCNTs) in aqueous poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The conductivity, transmittance and flexibility of the polymeric nanocomposite anode were characterized. The polymeric nanocomposite anodes fabricated on a poly(ethylene terephthalate) (PET) substrate exhibited superior bending properties to ITO anodes on PET. The FOLEDs fabricated on the polymeric nanocomposite anodes had a low turn-on voltage and higher luminous intensity than those fabricated on ITO/PET anodes. This flexible nanocomposite polymeric anode is a very promising for fully FOLEDs and other optoelectronics.

Influence of gaseous annealing environment on the properties of indium-tin-oxide thin films

The influence of postannealing in different gaseous environments on the optical properties of indiu-tin-oxide (ITO) thin films deposited on glass substrates using e-beam evaporation has been systematically investigated. It is found that the annealing conditions affect the optical and electrical properties of the films. Atomic force microscopy, x-ray diffraction, and x-ray photoemission spectroscopy (XPS) were employed to obtain information on the chemical state and crystallization of the films. These data suggest that the chemical states and surface morphology of the ITO film are strongly influenced by the gaseous environment during the annealing process. The XPS data indicate that the observed variations in the optical transmittance can be explained by oxygen incorporation into the film, decomposition of the indium oxide phases, as well as the removal of metallic In.

Micro-Raman and photoluminescence studies of neutron-irradiated gallium nitride epilayers

GaN epilayers grown on sapphire substrate were irradiated with various dosages of neutrons and were characterized using Micro-Raman and photoluminescence. It was found that the A1(LO) peak in the Raman spectra clearly shifted with neutron irradiation dosage. Careful curve fitting of the Raman data was carried out to obtain the carrier concentration which was found to vary with the neutron irradiation dosage. The variation of the full width at half maximum height of the photoluminescence was consistent with the Raman results. The neutron irradiation-induced structural defects (likely to be GeGa) give rise to carrier trap centers which are responsible for the observed reduction in carrier concentration of the irradiated GaN.

Influence of annealing temperature and environment on the properties of indium tin oxide thin films

Indium tin oxide (ITO) thin films were deposited on glass substrates using the e-beam evaporating technique. The influence of deposition rate and post-deposition annealing on the optical properties of the films was investigated in detail. It is found that the deposition rate and annealing conditions strongly affect the optical properties of the films. The transmittance of films greatly increases with increasing annealing temperature below 300°C but drastically drops at 400°C when they are annealed in forming gas (mixed N2 and H2 gas). An interesting phenomenon observed is that the transmittance of the darkened film can recover under further 400°C annealing in air. Atomic force microscopy, x-ray diffraction and x-ray photoemission spectroscopy were employed to obtain information on the chemical state and crystallization of the films. Analysis of these data suggests that the loss and re-incorporating of oxygen are responsible for the reversible behaviour of the ITO thin films.

Properties of ITO thin films deposited on amorphous and crystalline substrates with e-beam evaporation

Indium tin oxide (ITO) thin films were deposited using the e-beam evaporation method on amorphous and crystalline substrates under identical conditions. The properties of the films were investigated using optical transmittance, XRD and XPS techniques. It was found that the properties of the films depend strongly on the nature of the substrate surface. Analysis suggests that changes in chemical composition and microstructure of the ITO films deposited on crystalline and amorphous substrates are responsible for the differences in optical properties.

Probing deep level centers in GaN epilayers with variable-frequency capacitance-voltage characteristics of Au∕GaN Schottky contacts

Under identical preparation conditions, Au/GaN Schottky contacts were prepared on two kinds of GaN epilayers with significantly different background electron concentrations and mobility as well as yellow emission intensities. Current-voltage (I-V) and variable-frequency capacitance-voltage (C-V) characteristics show that the Schottky contacts on the GaN epilayer with a higher background carrier concentration and strong yellow emission exhibit anomalous reverse-bias I-V and C-V characteristics. This is attributed to the presence of deep level centers. Theoretical simulation of the low-frequency C-V curves leads to a determination of the density and energy level position of the deep centers. (c) 2006 American Institute of Physics.

Influence of indium-tin-oxide thin-film quality on reverse leakage current of indium-tin-oxide/n-GaN Schottky contacts

Indium-tin-oxide (ITO)/n-GaN Schottky contacts were prepared by e-beam evaporation at 200 degrees C under various partial pressures of oxygen. X-ray photoemission spectroscopy and positron beam measurements were employed to obtain chemical and structural information of the deposited ITO films. The results indicated that the observed variation in the reverse leakage current of the Schottky contact and the optical transmittance of the ITO films were strongly dependent on the quality of the ITO film. The high concentration of point defects at the ITO-GaN interface is suggested to be responsible for the large observed leakage current of the ITO/n-GaN Schottky contacts. (c) 2006 American Institute of Physics.

Frequency response of TiNi shape memory alloy thin film micro-actuators

Relatively independent of the physical dimensions of micro-actuators based on shape memory alloys (SMA), the reported frequency response typically is capped at a few tens of Hz. The slow response agrees well with that of the rotating micro-actuators fabricated in this work. On the other hand, based on heat transfer analyses, a theoretical response time of the order of a few milli-seconds should be possible for scaled microactuators, thus implying a frequency performance of at least a few hundred Hz. Therefore it is concluded that the response of SMA micro-actuators may not be limited by heat transfer, but by the slow rate of phase transformation between the austenitic and the martensitic phases. This is consistent with the slow phase growth rate of about 0.3 /spl mu/m/s observed using in-situ transmission electron microscopy.

Impurity scattering induced excitonic polariton damping and its influence on the reflectance spectra of GaN epilayers

The low-temperature reflectance spectra of a number of GaN epilayers with various impurity/defect concentrations and Hall mobility are systematically investigated in this paper. The interacting excitonic polariton model, considering both excitons A and B, is employed to reproduce the measured reflectance spectra of the GaN films, leading to the determination of the damping coefficients of the propagating polaritons in the films. It is shown that the impurity scattering plays a dominant role in the damping of the excitonic polaritons in the GaN epilayers with higher impurity concentrations at low temperatures, which is additionally supported by the photoluminescence and Hall measurements.

The effects of process-induced stress on the microstructures and the phase transformation characteristics of sputtered titanium-nickel thin-film shape-memory alloys

The microstructures and the phase transformation temperatures of sputtered titanium-nickel (TiNi) thin films, both free-standing and attached on different underlying multi-layer substrates, have been studied. Differences in the microstructures, such as the lattice constants and relative concentrations of TiNi, Ti2Ni and TiNi3 phases, have been observed among the free-standing and the attached films, among the films attached on different underlying multi-layers and among the films with different relative orders of ageing and release. Not surprisingly, the corresponding phase transformation temperatures are also different. It is proposed that both process- and substrate-induced stresses affect the microstructures, hence the phase transformation characteristics, of the resulting shape-memory thin films.