Paul Heimann

Orientation-dependent phonon observation in single-crystalline aluminum nitride

In this study, we present a microspectroscopic investigation performed on different facets of a self-nucleated aluminum nitride (AlN) single crystal. We show that, apart from evaluating crystalline quality, Raman and Fourier transform infrared spectroscopy can provide means to detect the orientation of any AlN facet. Such local, nondestructive technique is very useful for selecting and evaluating samples of single crystalline AlN.

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.