J. Kovac

Near-UV Irradiation Effects on Pentacene-Based Organic Thin Film Transistors

We irradiated Organic Thin-Film Transistors using light sources with peak wavelength in the near-UV spectrum. Organic Thin-Film Transistors have been found very sensitive to near UV rays. Irradiation generates negative trapped charge due to photogeneration, as well as neutral defects, which degrade the mobility and transconductance. We observed noticeable degradation even with 400-nm wavelength, which lies between visible and UV-light. Stronger variations were observed with the shortest wavelength, with a transconductance drop of -85% after 33000-s irradiation with 310 nm UV. We found that the mobility degradation is permanent after storing the devices in vacuum at least for 50 days.

Low-energy UV effects on Organic Thin-Film-Transistors

We subjected Organic Thin-Film-Transistors with different gate dielectrics interface treatment to visible light and low energy UV irradiation. Devices with silicon nanoparticles only feature a remarkable temporary charge trapping, regardless the irradiation wavelength. On the contrary, devices without silicon nanoparticles feature temporary trapped charge under visible light, and permanent mobility degradation if they are irradiated with a wavelength shorter than 420nm.

Effects of positive and negative constant voltage stress on organic TFTs

We subjected Organic TFTs to positive and negative constant gate voltage stresses. Stress not only induces temporary charge trapping, but also permanent transconductance degradation. The permanent degradation is accelerated if the devices are stressed under illumination. Negative stress degrades the TFT much faster than the positive stress, at the same voltage. Furthermore, hard breakdowns are observed on devices stressed under light, with VGS= -65V. The degradation mainly comes from stress-induced interface trap generation, rather than photochemical reactions as observed with UV irradiations.

Organic Thin Film Transistor degradation under sunlight exposure

We irradiated Organic Thin-Film Transistors with wavelengths ranging in the visible and near ultraviolet parts of the solar spectrum, reporting strong degradation in few hours. We modeled the sunlight-induced permanent degradation, taking into account the mobility reduction. The model closely fits the experimental data, allowing the prediction of the device permanent degradation under sunlight exposure.

Ridge-waveguide InAs/GaAs lasers

We report on the reduction of current spreading of InAs/GaAs lasers by using ridge-waveguide geometry. For the investigation of current spreading, lasers with various ridge widths 5, 10, 20 and 50 /spl mu/m are prepared. The laser properties are experimentally studied by the light output versus current characteristics (L-I) and near field patterns (NFP). Threshold current densities (J/sub th/) as a function of ridge width are compared to oxide-stripe geometry lasers with the same material structure. Finally, lasers with ridge-waveguide geometry show a reduction of total J/sub th/ for all ridge widths. Furthermore, the fundamental zero-order mode TEM/sub 00/ is achieved for 10 /spl mu/m ridge width.

Interface Modification of Pentacene OFET Gate Dielectrics

Pentacene organic field effect transistors (OFETs) electrical and structural properties have already been analysed from the point of view of different gate dielectric and growth conditions utilization. The AFM and micro Raman investigations show that the first organic monolayer at the pentacene/dielectric interface are essential determinants of carrier transport phenomena and achievable drain current of pentacene OFETs.

Light emitting diode with 2D PhC structure in the surface analysed by NSOM

We present light emitting diode with two-dimensional photonic crystal structure prepared by interference lithography in the light emitting diode surface with an emission maximum at 845 nm. Applied two-dimensional photonic crystal structure improved light extraction efficiency for more than 30 %. The photonic crystal light emitting diode surface morphology was analyzed by atomic force microscope. The enhanced extraction efficiency of the photonic crystal diode was documented from L(I) dependencies from the near-field studies.

Polymer based 3D photonic crystals applied on the surface of LEDs and photodiodes

Progress in nanotechnologies accelerated the polymer based photonics, where simple and cheap solutions often bring comparable and sometimes also novel interesting results. Good candidates are polymer photoresists and siloxane materials with unique mechanical and optical properties. We present laser lithography as efficient tool for fabrication of different three-dimensional (3D) structures embedded in polydimethylsiloxane (PDMS) membranes. Presented concept of PDMS based thin membranes with 3D structures works as an effective diffraction element for modification of radiation pattern diagram of light emitting diodes and changes also the angular photoresponse of photodiodes. All these results were demonstrated on two types of 3D structures – spheres arranged in cubic lattice and woodpile structure.

Trap analysis of GaN-based heterostructures using current transients mesurements

This work deals with the trap analysis of the GaN-based structures using current time response on the voltage pulse applied to Schottky and ohmic contacts containing AlGaN and InAlN layers with different composition and thickness. Monitoring of the current time evolution for the investigated samples allows identifying and comparing particular traps. The analysis of the current transients measured at various temperatures via fitting up to four exponentials is used to resolve traps located in the particular layers and their activation energies were determined.

GaAs-based photodetector with applied PDMS membrane with photonic crystal in the surface

In this paper we present GaAs-based photodiode with implemented polydimethylsiloxane (PDMS) membranes with patterned photonic crystal (PhC) structures on surface. Two-dimensional (2D) PhC surface relief structures of square symmetry were analyzed by atomic force microscope and optical effect of 2D PhC structure was investigated from goniophotometer photoresponse measurements. Spatial modulation of light coupling with square symmetry was documented for irradiation the GaAs-based photodiode with red light.