Saskia Schimmel

Ammonothermal Synthesis of Earth-Abundant Nitride Semiconductors ZnSiN2 and ZnGeN2 and Dissolution Monitoring by In Situ X-ray Imaging

In this contribution, first synthesis of semiconducting ZnSiN2 and ZnGeN2 from solution is reported with supercritical ammonia as solvent and KNH2 as ammonobasic mineralizer. The reactions were conducted in custom-built high-pressure autoclaves made of nickel-based superalloy. The nitrides were characterized by powder X-ray diffraction and their crystal structures were refined by the Rietveld method. ZnSiN2 (a=5.24637(4), b=6.28025(5), c=5.02228(4) Å, Z=4, Rwp =0.0556) and isotypic ZnGeN2 (a=5.46677(10), b=6.44640(12), c=5.19080(10) Å, Z=4, Rwp =0.0494) crystallize in the orthorhombic space group Pna21 (no. 33). The morphology and elemental composition of the nitrides were examined by electron microscopy and energy-dispersive X-ray spectroscopy (EDX). Well-defined single crystals with a diameter up to 7 μm were grown by ammonothermal synthesis at temperatures between 870 and 1070 K and pressures up to 230 MPa. Optical properties have been analyzed with diffuse reflectance measurements. The band gaps of ZnSiN2 and ZnGeN2 were determined to be 3.7 and 3.2 eV at room temperature, respectively. In situ X-ray measurements were performed to exemplarily investigate the crystallization mechanism of ZnGeN2 . Dissolution in ammonobasic supercritical ammonia between 570 and 670 K was observed which is quite promising for the crystal growth of ternary nitrides under ammonothermal conditions.

Polycrystalline SiC as Source Material for the Growth of Fluorescent SiC Layers

Polycrystalline doped SiC act as source for fluorescent SiC. We have studied the growth of individual grains with different polytypes in the source material. We show an evolution and orientation of grains of different polytypes in polycrystalline SiC ingots grown by the Physical Vapor Transport method. The grain influence on the growth rate of fluorescent SiC layers grown by a sublimation epitaxial process is discussed in respect of surface kinetics.

Nucleation and growth of polycrystalline SiC

The nucleation and bulk growth of polycrystalline SiC in a 2 inch PVT setup using isostatic and pyrolytic graphite as substrates was studied. Textured nucleation occurs under near-thermal equilibrium conditions at the initial growth stage with hexagonal platelet shaped crystallites of 4H, 6H and 15R polytypes. It is found that pyrolytic graphite results in enhanced texturing of the nucleating gas species. Reducing the pressure leads to growth of the crystallites until a closed polycrystalline SiC layer containing voids with a rough surface is developed. Bulk growth was conducted at 35 mbar Ar pressure at 2250°C in diffusion limited mass transport regime generating a convex shaped growth form of the solid-gas interface leading to lateral expansion of virtually [001] oriented crystallites. Growth at 2350°C led to the stabilization of 6H polytypic grains. The micropipe density in the bulk strongly depends on the substrate used.

The role of defects in fluorescent silicon carbide layers grown by sublimation epitaxy

Donor-acceptor co-doped SiC is a promising light converter for novel monolithic all-semiconductor white LEDs due to its broad-band donor-acceptor pair luminescence and potentially high internal quantum efficiency. Besides sufficiently high doping concentrations in an appropriate ratio yielding short radiative lifetimes, long nonradiative lifetimes are crucial for efficient light conversion. The impact of different types of defects is studied by characterizing fluorescent silicon carbide layers with regard to photoluminescence intensity, homogeneity and efficiency taking into account dislocation density and distribution. Different doping concentrations and variations in gas phase composition and pressure are investigated.

Step-flow growth of fluorescent 4H-SiC layers on 4 degree off-axis substrates

Homoepitaxial layers of fluorescent 4H-SiC were grown on 4 degree off-axis substrates by sublimation epitaxy. Luminescence in the green spectral range was obtained by co-doping with nitrogen and boron utilizing donor-acceptor pair luminescence. This concept opens possibilities to explore green light emitting diodes using a new materials platform.

Towards Bulk-Like 3C-SiC Growth Using Low Off-Axis Substrates

Bulk-like 3C-SiC was grown on 1.2 degrees low off-axis 6H-SiC substrates using a sublimation epitaxy technique. The effects of temperature ramp-up and increase in layer thickness on the 3C-SiC domain formation were explored. The temperature ramp-up had no significant effect on the domain size. The domain size was considerably increased and the crystal quality was significantly improved by increasing the thickness of the layer towards bulk-like material. Average full width at half maximum values of 149 arcsec and 65 arcsec were measured in samples with thicknesses of 305 µm and 1080 µm, respectively, at a footprint of 1x3 mm2. This result implies that heteropeitaxial growth of 3C-SiC on low off-axis 6H-SiC substrates by a sublimation method can be used to prepare 3C-SiC seeds or can be further developed for growth of bulk 3C-SiC material.