J. Tardy

Hole injection and transport in ITO/PEDOT/PVK/Al diodes

Abstract The paper reports on the hole injection in Poly(N-vinylcarbazole) (PVK) films from an indium tin oxide (ITO) anode treated with poly(3,4)ethylenedioxythiophene/polystyrenesulphonate (PEDT/PSS) usually called PEDOT. The influence of the deposition conditions of PEDOT on the onset voltage of the diodes is first, investigated and an optimised procedure for the surface treatment of ITO with PEDOT is given. Second, the analysis of the I–V characteristics of various devices with different PVK thickness in a temperature range 100–330 K shows that the hole conduction is bulk limited rather than injection limited. A trap assisted space charge conduction model with an exponential distribution of traps well describes our results. A trap density of 4×1018 cm−3 and a characteristic energy Et−EHOMO=0.15 eV were deduced. A comparison with a conduction model based on trap free material but with electric field dependant mobility is provided.

ITO-on-top organic light-emitting devices: a correlated study of opto-electronic and structural characteristics

This paper reports on a complementary investigation of the structural and opto-electronic characteristics of top-emitting organic light-emitting diodes (OLEDs). The key point of such a procedure is the deposition of the indium tin oxide (ITO) transparent top electrode onto the organic films structure. This was achieved by RF sputtering under soft enough conditions so as to avoid any deterioration of the underlying organic layers (room temperature deposition, low plasma power density and minimal bombardment effect) as much as possible. The electrical and optical properties of ITO as a function of the deposition conditions are described first. Hall effect measurements show that ITO films can be grown at room temperature with a high mobility (40 cm2 V−1 s−1) and a carrier density exceeding 5 × 1020 cm−3. The surface roughness as a function of the plasma conditions was determined by AFM and can be as low as 1.2 nm. Hole only silicon/Al/poly(N-vinylcarbazole) (PVK)/poly(3,4)ethylenedioxythiophene (PEDOT)/sputtered ITO diodes were processed and characterized both structurally (high resolution TEM in cross section) and electrically. An excellent agreement between the interfacial ITO/PEDOT structure and the hole injection performance was evidenced. Glass/ITO/PEDOT/PVK/Alq/BCP/ITO top-emitting OLED were processed and characterized.

Stability of pentacene top gated thin film transistors

We report on the stability of top gated pentacene field effect transistors processed on Kapton™ with Parylene-C as gate dielectric. The influence of bias stress and ambient atmosphere on device characteristics were investigated. Combined influence of moisture and gate bias stress led to an increase of depletion current and subthreshold slope as well as a drift of onset voltage and threshold voltage. We show that devices stressed in the off state exhibit a high stability.

Controlled spontaneous emission of a tri(8-hydroxyquinoline) aluminum layer in a microcavity

We report the fabrication of all-dielectric microcavities with a tri(8-hydroxyquinoline) aluminum (Alq3) organic layer as the emitting layer. In a first step, we characterized the materials used in the structures by ellipsometry, and ensured nondegradation of the organic material in the fabrication process. Then, by angular-resolved photoluminescence, we investigated changes in the angular emission pattern caused by the cavities and observed a sharply directed emission. We also investigated the influence of the position of the radiative layer in the cavity on normal spontaneous emission. We observed enhancements in spontaneous emission over 20 times higher than that of a single Alq3 layer. These are the highest reported for organic material based microcavities. They are mainly explained by the very small thickness of the Alq3 layer (20 nm≡0.06λ, λ being the resonant wavelength), by high-quality low-loss dielectric mirrors as well as by the narrow collecting angle of our experiment (±3°). This study corrob...

Organic thin film transistors with HfO 2 high-k gate dielectric grown by anodic oxidation or deposited by sol-gel

We report here on pentacene based organic field effect transistors (OFETs) with a high-k HfO 2 gate oxide. HfO 2 layers were prepared by two different methods: anodic oxidation and sol-gel. A comparison of the two processes on the electrical properties of OFETs is given. Ultra thin nanoporous (20 nm) sol-gel deposited oxide films were obtained following an annealing at 450°C. They lead to high mobility and stable devices (μ=0.12 cm2/V.s). On the other hand, devices with anodic HfO 2 revealed a little bit more leaky and show some hysteresis. Anodization, however, presents the advantage of being a fully room temperature process, compatible with plastic substrates.

Trimethylamine biosensor based on pentacene enzymatic organic field effect transistor

This paper reports on an enzymatic organic field effect transistor based biosensor dedicated to the detection of trimethylamine, in view of fish freshness assessment. Flexible devices were processed on Kapton substrate with pentacene as organic semiconductor and Parylene-C as top gate dielectric. Proton sensitive dielectric surface was achieved by depositing a thin hydrogenated silicon nitride layer at moderate temperature on Parylene. An enzymatic membrane made of flavin-containing mono-oxygenase 3 cross-linked with bovine serum albumin deposited on silicon nitride completed the realization of the device. High sensitivity trimethylamine biosensing was demonstrated in the 0–8 ppm range.

Flexible pentacene ion sensitive field effect transistor with a hydrogenated silicon nitride surface treated Parylene top gate insulator

We report on the realization of flexible ion sensitive organic field effect transistors based on pentacene on which Parylene-C was deposited as top gate dielectric. In order to create proton sensitive sites at the insulator/electrolyte interface, Parylene-C surface has been covered with a thin layer of hydrogenated silicon nitride (SiN:H) deposited by photochemical vapor deposition at moderate temperature. The combination of Parylene and SiN:H enables the realization of highly reproducible and good performance transistors as well as ion sensitive sensors with an excellent pH response both in the acidic and alkaline pH range in a nearly all plastic technology.

Electrical and optical characteristics of indium tin oxide thin films deposited by cathodic sputtering for top emitting organic electroluminescent devices

This paper reports on RF cathodic sputtering of indium tin oxide (ITO) films under soft conditions. The aim is to allow for the deposition of the ITO anode onto organic or polymeric materials in inverted (top emitting) organic light emitting diodes (OLED). Electrical, optical and structural properties of ITO deposited at low temperature are first investigated. Then, we report on photoluminescence (PL) experiments in order to assess the influence of the plasma conditions for the deposition of ITO onto organic films. The influence of the discharge power density and of the substrate bias were particularly investigated. Finally, Al/PVK/PEDOT/sputtered ITO inverted diodes were fabricated and characterized in order to validate our processing conditions.

Highly sensitive Hall sensors

High-performance InGaAs/InAlAs/InP Hall sensors with high magnetic sensitivity, good linearity, low temperature coefficient and high resolution are reported. These sensors use the properties of a two-dimensional 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. With the described growth optimization of pseudomorphic heterostructures by molecular beam epitaxy, a high electron mobility of 13 000 at room temperature has been obtained. A physical model of the structure including a self-consistent description of the coupled Schrodinger and Poisson equations has been developed to better understand the influence of the design of the heterostructure on its electronic properties. These results have been used in order to optimize the structure design. A magnetic sensitivity of with a temperature coefficient of between and has been obtained, and high signal-to-noise ratios corresponding to minimal magnetic field of at 100 Hz and at 1 kHz have been measured.

Stability and noise of PdGeAgAu ohmic contacts to InGaAsInAlAs high electron mobility transistors

Abstract This paper discusses the electrical characteristics of PdGeAgAu ohmic contacts for InAlAsInGaAs HEMT structures. A comparison with different AuGe based metallizations is made. Both types of metallization appear to provide low contact resistances. The PdGe system, however, exhibited an excellent thermal stability not achieved with AuGe. No contact degradation was observed following heat treatment at 325°C for more than 100 h plus 425°C for 24 h. The beneficial effect of the Ag barrier layer is evident from the surface morphology and thermal stability of the contact. Low frequency noise measurements on ungated devices with various values of source-to-drain distance enable determination of the channel and contact contributions to the current noise. The contact noise was observed to be essentially controlled by the contact resistance, independently of the nature and process conditions of the metallization.