Papers by Author: Albrecht Winnacker

Abstract: Amorphous pseudobinary (SiC)1 x(AlN)x thin films have been produced by radio frequency dual magnetron sputtering from bulk SiC and AlN targets. For each target the emission characteristic, i.e. the spatial variation of the deposition rate was determined, in order to predict thickness distribution and spatial composition variation for the (SiC) (AlN) alloy. Impedance spectroscopy shows a high resistivity of the films in the SiC rich region, decreasing significantly towards the AlN rich region.

Abstract: Amorphous wide bandgap semiconductor thin films of the pseudobinary compound (SiC)1-x(AlN)x were grown by radio frequency dual magnetron sputtering on CaF2, MgO and glass substrates. We performed isochronical annealing steps up to 500°C. The optical bandgap is determined for each composition from spectroscopic transmission measurement in two different ways: according to Tauc and using the (αhν)2 plot. The dependence of the optical bandgap on the composition x can be described by Vegard’s empirical law for alloys.

Abstract: We produce amorphous terbium doped wide bandgap semiconductor thin films of the pseudobinary compound (SiC)1 x(AlN)x by rf triple magnetron sputtering. Cathodoluminescence measurements performed at samples having different compositions x show pronounced intra 4f shell transition peaks of the trivalent terbium. Thermal activation of the terbium emission by isochronal annealing of the films leads to a strong increase in emission intensity.

Abstract: Aluminum nitride (AlN) is a promising substrate material for epitaxy of Al-rich III-nitrides to be employed, e.g., in deep-UV optoelectronic and high-power microwave devices. In this context, preparation of bulk AlN crystals by physical vapor transport (PVT) appears to be of most importance. In this work, seeded growth of AlN on (0001)-plane 6H-SiC substrates was investigated. SiC substrates with a diameter of 15 mm were used. AlN layers with thicknesses up to 3 mm were deposited at growth rates in the range of 10 to 40 μm/hour. Such templates provide large-area seeds, but they are often cracked, especially at thicknesses below 1mm. Besides cracks, other defects from the SiC seed propagate into the AlN layer and subsequently into the bulk AlN crystal. That is why, the aim of this work is to assess structural quality and defect content in thick AlN templates grown on (0001) plane SiC substrates. An optimum thickness-quality, the most appropriate growth stage for further use of the AlN template as a seed for subsequent PVT growth of bulk AlN growth, will be provided. We found that low growth rates mitigate crack propagation; slow cooling as well as optimization of the thermal field inside the crucible can prevent formation of new cracks after growth.

Abstract: Results on bulk growth of SiC crystals along rhombohedral [01-1n] directions are presented. 6H- and 4H-crystals were grown on rhombohedral planes, which make angles of about 45o with the (0001) plane. Etching features on three differently oriented planes cut from characteristic crystals were compared. Utmost care was concentrated on defect development in the case of non-conventional growth orientation using the seed cut from a “standard” (0001) crystal, containing a typical (standard for [0001] growth) set of crystal defects. We clearly distinguished between a transient layer adjacent to the seed and the main crystal body grown at latter stages. The defect selection and/or transformation in the transient layer appeared strongly depending on the SiC polytype and growth direction. This study brings directly the information on stability of particular defects in the chosen crystal orientation and allows us to distinguish between defects characteristic for [0001] and rhombohedral growth.

Abstract: New results on bulk growth of 6H-SiC crystals along the [01-15] direction are presented. The aim of our work is to improve the quality of the crystal grown by classical PVT method by employing alternative growth directions, other than conventional [0001]. Using a specially designed graphite crucible, crystals with an expansion angle of 30 degrees and diameters up to 40 mm have been grown. No polycrystalline rim develops at the contact with the graphite wall. Concerning specific defect content in the [01-15]-oriented crystals, they appear completely free of micropipes and screw or threading edge ([0001]-oriented) dislocations. The [01-15] crystal relaxes adopting a network of in-plane (0001) dislocations. They are not uniformly distributed reaching the maximum density of about 106 cm-2.

Abstract: We present comprehensive cathodoluminescence measurements from thin amorphous a- SiC films doped with rare earths. The a-SiC films were prepared by rf magnetron sputtering using a high purity SiC wafer in high purity argon atmosphere (5N, pressure approx. 0.2 mbar). The rare earth doping (Tb, Dy and Eu concentrations were below 2%) was performed by placing respective rare earth metal pieces of appropriate size onto the Silicon Carbide wafer. The rare earth ion emissions cover the colors green (Tb), yellow (Dy) and red (Eu). The optical and related structural properties of the films are correlated by means of high resolution transmission electron microscopy in combination with cathodoluminescence measurements in a scanning electron microscope. In addition, the corresponding compositions are determined by energy-dispersive x-ray analysis. The cathodoluminescence spectra of the rare earth 3+ ions are recorded in the visible at 20°C in the asgrown condition and after annealing treatments in the temperature range from 300°C to 1050°C by steps of 150°C. The anneal-related changes in the cathodoluminescence emission spectra and in the microstructure of the films are addressed. The SiC films show amorphous structure almost independent of the annealing treatment. Optimal annealing temperature for emissions of Tb3+ doped a-SiC were derived to be 600°C whereas Dy3+ and Eu3+ emissions increase at least up to 1050°C.

Abstract: Epitaxial growth of 4H-SiC has been carried out at temperatures up to 1650 oC on 4HSiC substrates dipped in strongly diluted Si-based solutions. Liquid Phase Epitaxy (LPE) in conditions of low supersaturation was shown to be an effective technique to overgrow micropipe defects (MPs) in SiC wafers prepared by the Physical Vapour Transport (PVT) technique. The aim of this work was to investigate the SiC growth morphology and the dependence of MP elimination efficiency on Si-Ge flux composition. Macroscopically flat, single crystalline SiC layers of a thickness up to 10 µm were grown with a growth rate of about 0.5 µm/h. Stepped growth morphology was observed independent of the melt composition. Micropipes with the diameter below 5 µm were closed with an efficiency of about 80%. SEM investigations as well as inspection under reflected/transmitted light did not show any specific distortion of the growth morphology at the micropipe healing place.

Abstract: Several SiC bulk crystals were grown with erbium and ytterbium as doping materials. Erbium contents determined by secondary ion mass spectroscopy (SIMS) ranged from 1.2 · 1014 cm-3 to 1.04 · 1015 cm-3, while ytterbium contents were below SIMS detection limit. Photoluminescence (PL) investigations of the characteristic 4f-4f-transition lines revealed a reduced luminescence yield in highly nitrogen and aluminum co-doped samples. Also, samples without intentional co-doping grown on the C-face showed less luminescence intensity than those grown on the Si-face. A stabilizing effect of erbium doping on the 4H polytype was observed.