Electric field gradients at 151Eu sites in GaN

The electric field gradients at Eu sites in GaN have been investigated in conversion electron Moessbauer spectroscopy (CEMS) in which 151Eu probe ions were implanted into an undoped GaN layer grown on a sapphire substrate. The sample was implanted with 120 keV 151Eu ions to a fluence of 1 x 1015, and annealed at 1,200 K. CEMS spectra of the 151Eu 21.6 keV transition were collected, of the GaN sample as well as of a Si sample implanted with overlapping profiles of 151Eu and O. The GaN spectra were fitted with two symmetric doublets, D1 and D2, with isomer shifts and quadrupole splittings of δ = -0.27 mm/s (relative to Eu2O3), ΔEQ = 0.85 (3) mm/s; and δ = - 0.22 mm/s, ΔEQ = 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 69Ga, 71Ga and 111In in GaN.