Identification of iron transition group trace impurities in GaN bulk crystals by electron paramagnetic resonance

We report on the observation of electron paramagnetic resonance of iron, manganese and nickel trace impurities in bulk GaN crystals grown by the sublimation sandwich method. The resolved hyperfine structure due to interaction with Mn-55 (I=5/2) nuclei has been observed in GaN, allowing unambiguous identification of the impurity. Manganese and nickel exist in Mn2+ (3d(5)) and Ni3+ (3d(7)) charge states with electron spin S=5/2 and S=3/2, respectively, and occupy gallium sites in the GaN lattice. For Mn2+ we found g=1.999, hyperfine structure constant A=70.10(-4) cm(-1) and fine structure parameter /D/ = 240.10(-4) cm(-1). The EPR spectrum of Ni3+ in GaN had the characteristic anisotropy of an S = 3/2 system in a strong axial crystalline field. The effective g-factor values were found to be g(parallel to)' = 2.10 and g(perpendicular to)' congruent to 4.20 for a system with an effective spin S' = 1/2. An analogy was revealed between the parameters of Mn2+ and Ni3+ in GaN and ZnO crystals. The zero-phonon line at 1.047 eV seems to belong to transition T-4(2)(F)-(4)A(2)(F) within 3d levels of Ni3+ ion with a 3d(7) electronic configuration. In addition, EPR spectra of two new axial centres, labelled as ill and ii2 centres, have been observed.