69,71GaNMR spectra and relaxation in wurtzite GaN

Abstract

Magnetic properties of wurtzite GaN are studied by Ga nuclear magnetic resonance (NMR) in a GaN bulk crystal containing a high carrier concentration, as well as in highly resistive Mg-doped crystals. The quadrupole coupling constant is derived from satellite lines and from the shift of the central line in the quadrupolar perturbed 6 9 , 7 1 Ga NMR spectra. The electric-field gradient is in good agreement with the value calculated by the ah initio full-potential linear-muffin-tin-orbital method, using the local-density approximation to describe exchange and correlation effects. The 6 9 , 7 1 Ga spin-lattice relaxations in the Mg-compensated sample in the range 80-400 K are due to the quadrupolar interaction mechanism, similar to what is observed in GaAs, and the proper scaling factor, given by the square of the ratio of the nuclear quadrupole moments, ( 6 9 Q/ 7 1 Q) 2 , is observed for the two isotopes. In contrast, in the conductive sample, the relaxation mechanism is caused by magnetic interaction with the conduction electrons and one finds a relaxation rate, WT. The isotope ratio 6 9 W/ 7 1 W is close to ( 6 9 γ/ 7 1 γ) 2 , where y is the gyromagnetic factor. Only above 200 K, there is evidence of a relaxation process due to interactions with phonons also in the degenerate sample, with a temperature dependence approximately of the form WT 3 , consistent with a Debye temperature around 600 K.