Thermal conductivity $\kappa (T)$ of single crystal aluminum nitride grown by physical vapor transport has been measured at temperatures $T$ from 5 to 410 K. The samples exhibit high thermal conductivity with a value of up to 316 W m${}^{-1}$ K${}^{-1}$ at room temperature and about 2800 W m${}^{-1}$ K${}^{-1}$ at a peak of 66 K. At lowest temperatures, $\kappa (T)$ approaches the conductivity limited by the diffuse phonon scattering from sample surfaces. The peculiarities in measured $\kappa (T)$ suggest that the phonon scattering from point defects contributes essentially to the total phonon scattering in samples under investigation at low temperatures. The phonon interaction with electrons and holes bound to neutral donor and acceptor centers is suggested, adding substantially to thermal resistivity near and below the peak in $\kappa (T)$ .

Electron paramagnetic resonance (EPR) at 9.4 and 35 GHz were studied on two types of AlN single crystals, grown by a sublimation sandwich method. These investigations revealed the presence of transition metals impurities in the first sample: Fe2+ (S=2) and some paramagnetic centers with S=3/2, we suggest Cr3+ or Ni3+ as the possible candidates. The EPR spectra of Fe2+ were observed up to the room temperature. After sample illumination at 5 K with light (wavelength shorter 700 nm) strong EPR signal with a g factor of shallow donors (SDs) and slightly anisotropic linewidth appears. This light-induced EPR signal, once excited at low temperature, still persists after switching off the light and is about constant up to 30 K then it drops quickly. SDs show a negative correlation energy U and oxygen in the N position (ON) is the most probable model. EPR spectra of deep-donor center which was assumed to be the nitrogen vacancy VN have been observed in the second sample. The x-ray irradiation leads to considerable enhancement of deep donor’s (VN) signals intensity. The annealing resulted in recombination thermoluminescence and the deep donor (VN) energy level was estimated to be about 0.5 eV. The models of shallow (ON) and deep (VN) donor centers were supported by comprehensive hyperfine structure analysis.

We report the influence of static mechanical deformation on the zero-field spin splitting of silicon vacancies in silicon carbide at room temperature. We use AlN/6H-SiC heterostructures deformed by growth conditions and monitor the stress distribution as a function of distance from the heterointerface with spatially resolved confocal Raman spectroscopy. The zero-field spin splitting of the V1/V3 and V2 centers in 6H-SiC, measured by optically detected magnetic resonance, reveals significant changes at the heterointerface compared to the bulk value. This approach allows unambiguous determination of the spin-deformation interaction constant, which is 0.75 GHz/strain for the V1/V3 centers and 0.5 GHz/strain for the V2 centers. Provided piezoelectricity of AlN, our results offer a strategy to realize fine tuning of spin transition energies in SiC by deformation.

We grow AlN/4H-SiC and AlN/6H-SiC heterostructures by physical vapor deposition and characterize the heterointerface with micro-scale resolution. We investigate the spatial stress and strain distribution in these heterostructures using confocal Raman spectroscopy. We measure the spectral shifts of various vibrational Raman modes across the heterointerface and along the entire depth of the 4H- and 6H-SiC layers. Using the earlier experimental prediction for the phonon-deformation potential constants, we determine the stress tensor components in SiC as a function of the distance from the AlN/SiC heterointerface. Despite the fact that the lattice parameter of SiC is smaller than that of AlN, the SiC layers are compressively strained at the heterointerface. This counterintuitive behavior is explained by different coefficients of thermal expansion of SiC and AlN when the heterostructures are cooled from growth to room temperature. The compressive stress values are maximum at the heterointerface, approaching 1 GPa, and relax to the equilibrium value on the scale of several tens of micrometers from the heterointerface.