Growth of AlN bulk crystals on SiC seeds: Chemical analysis and crystal properties

AlN single crystals are grown by physical vapor transport (PVT) in sintered TaC crucibles in the presence of SiC, i.e. by seeding on 6H–SiC or by adding SiC to the AlN source material. Different growth conditions, i.e. growth temperatures T=1900–2050 °C and off-orientation angles in respect to [0001] φ=0–42°, are applied in order to investigate to which extent the incorporation of Si and/or C can be controlled during the PVT growth. Chemical analysis is performed by energy-dispersive X-ray analysis (EDX), secondary ion mass spectrometry (SIMS), and Rutherford backscattering spectroscopy (RBS). The grown crystals are significantly contaminated with Si and C in the range of several atomic percents, but they do not represent pseudo-binary AlN–SiC solid solutions. Samples cut from these crystals are probed by Raman micro-spectroscopy, optical absorption, cathodoluminescence (CL), and near band-gap photoluminescence (PL) in order to evaluate the crystal properties. All crystals exhibit phonon bands at the positions of AlN phonon energies, and a low-temperature band-gap exceeding 5.8 eV, which is only slightly lower than the band-gap of pure AlN. On the other hand, the samples show very different optical properties below the band-gap, including coloration. As the changes in optical properties do not correlate to chemical analysis, we suggest that self-compensation of silicon and carbon as well as formation of intrinsic defects lead to a complex compensation scenario in the samples. The grown crystals are thus to be considered as highly compensated, Si/C co-doped AlN.

► Different growth conditions used to control SiC incorporation in AlN bulk growth.
► Phonon energies and band-gap of the crystals close to ‘pure AlN’.
► Deep-UV opacity regardless of silicon and carbon content.
► Two types of samples with very different absorption and luminescence.
► No pseudo-binary alloy, but highly compensated, Si/C co-doped AlN.