Local Electronic Structure in AlN Studied by Single-Crystal Al-27 and N-14 NMR and DFT Calculations

Both the chemical shift and quadrupole coupling tensors for N-14 and Al-27 in the wurtzite structure of aluminum nitride have been determined to high precision by single-crystal NMR spectroscopy. A homoepitaxially grown AlN single crystal with known morphology was used, which allowed for optical alignment of the crystal on the goniometer axis. From the analysis of the rotation patterns of N-14 (I=1) and Al-27 (I=5/2), the quadrupolar coupling constants were determined to chi(N-14)=(8.19 +/- 0.02) kHz, and chi(Al-27)=(1.914 +/- 0.001) MHz. The chemical shift parameters obtained from the data fit were delta iso=-(292.6 +/- 0.6) ppm and delta Delta=-(1.9 +/- 1.1) ppm for 14N, and (after correcting for the second-order quadrupolar shift) delta iso=(113.6 +/- 0.3) ppm and delta Delta=(12.7 +/- 0.6) ppm for Al-27. DFT calculations of the NMR parameters for non-optimized crystal geometries of AlN generally did not match the experimental values, whereas optimized geometries came close for Al-27 with (chi) over bar (calc) = (1.791 +/- 0.003) MHz, but not for N-14 with (chi) over bar (calc) = -(19.5 +/- 3.3) kHz.