Fredrik Carlsson

Spectroscopic ellipsometry studies of the optical properties of doped poly(3,4-ethylenedioxythiophene): an anisotropic metal

Abstract Poly(3,4-ethylenedioxythiophene) (PEDOT) is a conjugated polymer with high electrical conductivity in the doped state, good thermal and chemical stability and fast electrochemical switching. The material is used as an antistatic layer, and has potential for use as a (transparent) electrode material and for electrochromic applications. Thin films ( μ m) of doped PEDOT have been studied with variable angle spectroscopic ellipsometry. Reflectance and transmission measurements have also been performed. From ellipsometry and transmission data, the optical properties and thickness of the PEDOT layers were extracted. By simultaneously fitting optical functions for multiple samples using a recently discussed 4×4 matrix formalism, the analysis reveals that the material is optically anisotropic and of uniaxial character with the optic axis normal to the film surface. The uniaxial anisotropy of PEDOT films reveals an entirely different appearance for the ordinary and extraordinary indices of refraction with the former showing metallic character. The metallic state behavior agrees well with the high electrical conductivity measured for this material at different doping levels. From the measured optical properties and from supplementary optical and structural analysis, a preferential structural orientation of the polymer chains can be deduced.

Luminescence from stacking faults in 4H SiC

A previously unreported photoluminescence spectrum observed in certain 4H SiC bipolar diodes after extended forward voltage operation is reported. We assign this emission to exciton recombination at local potential fluctuations caused by stacking faults, which are created during operation of the diodes. Possible recombination mechanisms responsible for the spectrum are discussed.

Pseudodonor nature of the DI defect in 4H-SiC

We use the recent findings about the pseudodonor character of the DI defect to establish an energy-level scheme in the band gap for the defect, predicting the existence of a hole trap at about 0.35 eV above the valence band. Using minority carrier transient spectroscopy, we prove that the DI defect indeed is correlated to such a hole trap. In addition, we show that the DI defect is not correlated to the Z1/2 electron trap, in contrast to what was previously reported.

Anisotropic optical properties of doped poly(3,4-ethylenedioxythiophene)

Anisotropic optical properties of doped poly(3,4-ethylenedioxythiophene) (PEDOT) thin films have been determined by using variable-angle spectroscopic ellipsometry together with intensity reflectance and transmission spectrophotometry. The optical anisotropy of the PEDOT films are of uniaxial character with the optic axis normal to the film surface. The (ordinary) index of refraction in the plane shows a metallic state behavior while the out of plane (extraordinary) index of refraction shows a near dielectric character. The metallic state behavior in the surface plane agrees well with the high electrical conductivity measured for this material. The anisotropic optical properties of doped PEDOT thin films in terms of the complex indices of refraction together with results from grazing incidence X-ray diffraction studies allow us to deduce a preferential orientation of the polymer chains.

Ion implantation of silicon carbide

Ion implantation is an important technique for a successful implementation of commercial SiC devices. Much effort has also been devoted to optimising implantation and annealing parameters to improve the electrical device characteristics. However, there is a severe lack of understanding of the fundamental implantation process and the generation and annealing kinetics of point defects and defect complexes. Only very few of the most elementary intrinsic point defects have been unambiguously identified so far. To reach a deeper understanding of the basic mechanisms SiC samples have been implanted with a broad range of ions, energies, doses, etc., and the resulting defects and damage produced in the lattice have been studied with a multitude of characterisation techniques. In this contribution we will review some of the results generated recently and also try to indicate where more research is needed. In particular, deep level transient spectroscopy (DLTS) has been used to investigate point defects at very low doses and transmission electron microscopy (TEM) and Rutherford backscattering spectrometry (RBS) are used for studying the damage build-up at high doses.

Lysosomal membrane permeabilization causes oxidative stress and ferritin induction in macrophages

Moderate lysosomal membrane permeabilization (LMP) is an important inducer of apoptosis. Macrophages are professional scavengers and are rich in hydrolytic enzymes and iron. In the present study, we found that LMP by lysosomotropic detergent MSDH resulted in early up‐regulation of lysosomal cathepsins, oxidative stress and ferritin up‐regulation, and cell death. Lysosomotropic base NH4Cl reduced the ferritin induction and oxidative stress in apoptotic cells induced by MSDH. Cysteine cathepsin inhibitors significantly protected cell death and oxidative stress, but had less effect on ferritin induction. We conclude that oxidative stress induced by lysosomal rupture causes ferritin induction with concomitant mitochondrial damage, which are the potential target for prevention of cellular oxidative stress and cell death induced by typical lysosomotropic substances in different disorders.

Observation of recombination enhanced defect annealing in 4H–SiC

We report observation of recombination enhanced defect annealing in 4H–SiC detected by capacitance transient spectroscopy and low temperature photoluminescence (PL). Intrinsic defect centers, created by 160keV electron irradiation, reduce in concentration after illumination at temperatures much lower than previously reported annealing temperatures of 400 and 800°C. The effect is observed after both external intense above band gap laser excitation, and with recombination in a forward biased pin diode. PL measurements show that several lines, normally detected after electron irradiation, have almost or entirely disappeared by recombination enhanced annealing at room temperature. From capacitance transient measurements, the annealing enhancement is found to be largest for the HS2 hole trap, while the EH1 and EH3 electron traps also anneal out by recombination enhanced reaction but at a lower rate.

Defects in 4H silicon carbide

We present experimental results related to several different intrinsic defects that in different ways influence the material properties and are therefore technologically important defects. This inc ...

Dll PL Intensity Dependence on Dose, Implantation Temperature and Implanted Species in 4H- and 6H-SiC

In most semi-conductor processing ion implantation is a key technology. The drawback of ion implantation is that a great deal of lattice defects, such as vacancies, interstitials, anti sites and complexes, are introduced. The annealing behaviour of these defects is important for the viability of ion implantation as a commonly used method. In SiC a defect that is only seen after ion implantation and not after irradiation with neutrons or electrons is the D-II defect. The use of Si or C as implanted species have made it possible to investigate the D-II photoluminescence (PL) intensity dependence on an excess of either of the two constituents in SiC. The effect of performing a hot implant at 600degreesC compared to a room temperature implant was also looked into. The D-II PL intensity was measured after a 1500degreesC anneal. When the implantation was performed at room temperature the C implanted samples showed a significantly higher D-II luminescence than the Si implanted. This makes it tempting to assume that a surplus of C and likely C interstitials are involved in the defect formation. However, when the implantation is done at 600degreesC the difference between Si and C implanted samples almost disappears and a slightly higher D-II intensity can be seen in the Si implanted samples. This effect may be due to the mobility of C interstitials at temperatures above 500degreesC. This clearly demonstrates the effect of hot implantation that there is a major change in D-II PL intensity even after a 1500degreesC anneal.

Trapped carrier electroluminescence (TraCE)—A novel method for correlating electrical and optical measurements

Abstract SiC is a semiconductor with very good material properties for high power, high frequency and high temperature applications. During device fabrication irradiation with particles is often used, e.g., ion-implantation, which creates intrinsic defects. The most persistent defect in SiC is DI that appears after irradiation and subsequent high temperature annealing. A direct method called Trapped Carrier Electroluminescence (TraCE) for correlating minority carrier traps with luminescence measurements is presented. A semi-transparent Schottky diode under reverse bias is illuminated with a laser pulse of above band gap light to create minority carriers that are captured to traps in the space charge region. Majority carriers are introduced when the reverse bias is removed and the space charge region is reduced. The majority carriers recombine with the trapped minority carriers and the emitted light from the recombination is detected. TraCE has been used to study and correlate the DI bound exciton luminescence from intrinsic defects in SiC with an electrically observed hole trap HS1.