Octavian Filip

Seeded Growth of AlN on (0001)-Plane 6H-SiC Substrates

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.

Polarization-dependent below band-gap optical absorption of aluminum nitride bulk crystals

The polarization dependence of the below band-gap optical absorption of aluminum nitride (AlN) is investigated in detail using cuts of bulk single crystals grown by physical vapor transport. We show that optical absorption at 445nm (2.8eV) features a polarization-dependent transition which is strongest for P⊥c, while optical absorption in the range of 250–320nm (4–5eV) features a transition which is strongest for P∥c. Such information may aid in understanding the nature of the underlying electronic transitions and subsequently decreasing unwanted blue/UV optical absorption in AlN.

Growth and characterization of high-quality 6H-SiC (0115) bulk crystals

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.

Growth on Rhombohedral (01-1n) Plane: An Alternative for Preparation of High Quality Bulk SiC Crystals

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.

6H-SiC crystals grown in [015] and [001] directions characterized by high energy triple-axis x-ray diffraction

A structural characterisation by high energy x-ray diffraction of [015] and [001] grown 6H-SiC crystals is presented. The [015]-grown crystal shows an improved crystal quality, concerning the curvature of lattice planes and the micro domain structure. We relate this to the lowered level of rotational symmetry for the [015]-growth which reduces the number of active slip systems during the growth process.

LPE of Silicon Carbide Using Diluted Si-Ge Flux

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.