Constancy of the quadrupolar interaction product in nanocrystalline gallium nitride revealed by 71Ga MAS NMR shift distribution

The question of whether the broad 71,69Ga nuclear magnetic resonance (NMR) signal of hexagonal gallium nitride (h-GaN) at 530–330 ppm is related to the Knight shift (caused by the presence of carriers in semiconductors) is the subject of intense debate. The intensity increase observed for the narrower 71Ga magic angle spinning (MAS) NMR signals above 1050 C suggests that the broader signals do not reflect the decomposition of h-GaN. Herein, we utilized 71Ga multi-quantum (MQ) MAS NMR spectroscopy to reveal that the quadrupolar interaction products for the broad signal of nanocrystalline h-GaN are almost constant in the entire shift range that we investigated, equaling 1.7 0.1 MHz or similar values. Since the above parameter is sensitive to the local chemical symmetry around the Ga atom, the NMR shift distribution is considered not to be related to that of the chemical environment. Consistent with the most recent reports, including those on double-resonance 15N{71Ga} measurements, the Knight shift may be ascribed to defects serving as shallow donors and populating the conduction band. Thus, MQMAS measurements performed using a low-field NMR instrument or by choosing half-integer quadrupole nuclei with a large quadrupole constant such as 69Ga are expected to provide important information for each Knight shift value and for analyzing the nature of semiconductors other than GaN.