In this work, we report on the study of pentacene field effect transistors with a polymer (PMMA) on a high-k oxide (Ta2O5) two-layer gate dielectric, in order to combine low voltage operating devices and stability. Two-layer dielectric devices were compared to single layer (Ta2O5 or PMMA) gate insulators. To assess the importance of the structural quality of the Ta2O5, two deposition processes (e-beam evaporation and anodization) are studied. In order to evidence the relation between the electrical performances and behavior and the structural quality of the organic thin film, X-ray diffraction and AFM measurements were carried out to determine the pentacene morphology and ordering on the different dielectrics. Devices with bilayer gate dielectric present the best mobility, up to 0,6cm²/V/s, the pentacene film being well ordered in large grains on PMMA with or without high-k under layer. The transistors operate at 15V and show reduced polarisation effect compared to devices with only Ta2O5.
This paper reports on Organic Light Emitting Diodes (OLED) the structure of which has been inverted in order to have a light emission through the top electrode instead of through the substrate. This technology is of prime importance for high resolution active matrix displays and microdisplays as well. The key point of such a procedure is the deposition of the Indium Tin Oxide (ITO) transparent top anode. This was carried out onto the organic films structure by RF sputtering under soft enough conditions so as to avoid as much as possible any deterioration of the underlying organic layers (room temperature deposition, low plasma power density and minimal bombardment effect). The electrical and optical properties of ITO as a function of the deposition conditions are first described. Hall effect measurements show that ITO films can be grown at room temperature with a high mobility (40 cm2/V.s) and a carrier density exceeding 5 1020 cm-3. The surface roughness as a function of the plasma conditions was determined by the AFM and can be as low as 12 Angstrom. The non invasiveness of the proposed ITO deposition conditions on organic layers was assessed by measuring the photoluminescence (PL) degradation of (8-(hydroquinoline) Aluminum (Alq3). It was shown that under optimized deposition conditions, no appreciable PL degradation of the organic layers was observed. At last hole only Silicon/Al/Poly(N-vinylcarbazole)(PVK)/Poly(3,4)ethylenedioxythiophene / Polystyrenesulphonate (PEDT/PSS). ITO diodes and Silicon/Al/Alq3/NPB/CuPc/ITO diodes were processed, characterized and compared to standard “through the substrate” emitting diodes.
This paper reports on the potentialities of sol-gel deposited Sb doped SnO2(T)) as a new transparent conducting oxide (TCO) for anode in organic light emitting diodes (OLED). Multilayered films with transparency over 85% and resistivity lower than 5 103(Omega) -cm were obtained. Structural observations by Transmission Electron Microscopy (TEM) show that the films are nanocrystallized. Smaller and more uniform grains are obtained upon rapid thermal annealing. Atomic Force Microscopy (AFM) imaging shows the surface roughness does not exceed 20 A. TO films are very stable and cannot be chemically etched. Anode patterning by reactive ion etching (RIE) in a Methane- Hydrogen plasma has been experienced and is described. Typical etching around 250 A/min were obtained. TO/PEDOT/PVK/Al hole only diodes were realized to assess sol gel TO films as hole injection electrodes. Devices with threshold voltages of 6 volts were obtained. A comparison with ITO deposited by low temperature cathodic sputtering is given.
In this paper we report on Organic Light Emitting Diodes (OLEDs) based on heterojunctions between a polymer, Poly(N- vinylcarbazole) (PVK) as hole transport layer (HTL) and an evaporated layer of Tris (8-Hydroxy)quinoline) Aluminum (Alq3) as electron transport layer (ETL) and emissive layer (EM). The electrical properties of ITO/PVK/Alq3/Mg-Ag were thoroughly investigated and Trapped Charges Limited (TCL) currents were shown to be the main transport process in these devices. In order to assess the pertinence of PVK as HTL, a comparison with other HTL deposited by vacuum evaporation was carried out. The color of the emitted light then could have been tuned by adding DCM in PVK. Light emitted by Alq3 was partly absorbed by DCM thus exciting the photoluminescence of DCM. Exiting light was thus a combination of green-yellow characteristic of Alq3 and of red-orange due to DCM. Wavelengths spanning the range 530-620 nm were obtained for DCM concentrations in PVK up to 20% wt. without significant degradation of the external quantum yield.
This work reports on color controlled light converters fabricated from organic dyes (coumarin, DCM) and luminescent organic (Alq3) and inorganic (SrGa2S4:Eu++) molecules dispersed in a transparent polymer host matrix. These blends were pumped with a commercial GaN blue LED having a peak emission at 430 nm. First are reported the photoluminescence spectra of Alq3- DCM and coumarin-SrGa2S4:Eu++-DCM blends. By photoexcitation of these blends with GaN blue LEDs (which is partially absorbed by the dispersed dyes), it has been observed that the color can be tuned from blue to red depending on the composition. In particular, very pure white emitting hybridLEDs could have been obtained with organic- inorganic blends. Observed luminescence is due to cascade light absorption and emission from the various dyes in the blend. The influence of the polymer host matrix on the emitted color was also evidenced. Our results show how multicolor and white emitters could be fabricated with commercially available organic materials and rare earth doped inorganic dyes.
In this work, we propose solutions based on engineering of III-V heterostructures to develop new types of semiconductor magnetic sensors. These micro-Hall sensors use the properties of a 2D electron gas and the benefit of pseudomorphic material, in which both the alloy composition and the built-in strain offer additional degrees of freedom for band structure tailoring, to exhibit high magnetic sensitivity, good linearity, low temperature coefficient and high resolution. With the growth optimization which is described, two pseudomorphic In0.75Ga0.25As/In0.52Al0.48As heterostructures were grown on a semi- insulating InP substrate by molecular beam epitaxy. To understand better the influence of the heterostructure design on its electronic properties, a model involving the self-consistent solution of the Poisson and Schrodinger equations using the Fermi-Dirac statistics has been developed. These results have been used to optimize the structure design. A magnetic sensitivity of 346 V/AT with a temperature coefficient of -230 ppm/ degree(s)C between -80 degree(s)C and 85 degree(s)C has been obtained. The device show good linearity against magnetic field and also against the supply current. High signal-to-noise ratios corresponding to minimal magnetic field of 350 nT/Hz1/2 at 100 Hz and 120 nT/Hz1/2 at 1 kHz have been measured.
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