Electric field gradients at Eu-151 sites in GaN

The electric field gradients at Eu sites in GaN have been investigated in conversion electron Mossbauer spectroscopy (CEMS) in which Eu-151 probe ions were implanted into an undoped GaN layer grown on a sapphire substrate. The sample was implanted with 120 keV Eu-151 ions to a fluence of 1 x 10(15), and annealed at 1,200 K. CEMS spectra of the Eu-151 21.6 keV transition were collected, of the GaN sample as well as of a Si sample implanted with overlapping profiles of Eu-151 and O. The GaN spectra were fitted with two symmetric doublets, D1 and D2, with isomer shifts and quadrupole splittings of delta = -0.27 mm/s (relative to Eu2O3), Delta E-Q = 0.85 (3) mm/s; and delta = - 0.22 mm/s, Delta E-Q = 2.90 (5) mm/s, respectively. D1 is attributed to Eu at substitutional Ga lattice sites; D2 to Eu at or near substitutional sites but with extensive lattice damage. The splittings of D1 and D2 correspond to quadrupole coupling frequency of 15 (2) and 50 (4) MHz, consistent with measurements of Ga-69, Ga-71 and In-111 in GaN.