Optically-detected magnetic resonance studies of MBE ZnO:N GN Aliev , SJ Bingham, D Wolverson, JJ Davies, H Makino*, T Y

1. Microwave amplifier Microwave switch Microwave source Signal Function generator Lock-in amplifier Helium 1.8K Superconducting magnet Photo-detector Filters, analysers PL Spectrometer Laser [NO3-]- [Zni]+ c-axis Optically-detected magnetic resonance studies of MBE ZnO:N GN Aliev, SJ Bingham, D Wolverson, JJ Davies, H Makino*, T Yao* Department of Physics, University of Bath, Bath BA2 7AY UK *Institute for Materials Research, Tohoku University, Sendai 980-85777, Japan ZnO is a candidate material for UV photonic applications ODMR: optically detected magnetic resonanceinvolves the modulation of (long-lived) PL signals by a chopped microwave field in the presence of a slowly-swept magnetic field. Centre Z Zn ~20º Rotation of the magnetic field in two perpendicular planes The axis of the Z centre 3 is tilted by q ~ 20º to the crystal c-axis. There are six equivalent planes in which it can be tilted. • Model for centre Z: a zinc interstitial? • Magnitude of the g-factors suggests a hole centre (g > 2) [4]; • But the perpendicular g factor of the Z centre is less than the parallel g factor, g< g||, unlike the A centre; • Can a zinc interstitial be the answer? The hole may then be in a d orbital or an s-d hybrid. • For a hole in a pure 3d orbital, the octahedral crystal field would split the t2 and e states by an amount Dd, with the e states lowest in energy; • A further tetragonal distortion (via the Jahn-Teller effect) of the appropriate sign would split the e states; • The g-factors are then g|| = [2.0023 - (8d/d)] and g = [2.0023 - (2d/d)]; • If the hole wavefunction is hybridised, y = af4s + bf3d, then the g-factors are: • With reasonable values of the parameters Dd, ld, and b, the experimentally observed g-factors are obtained. p-type doping of ZnOis essential and has recently been achieved using a nitrogen plasma source [2], but there is a compensation mechanism [3] that is not fully understood. • Experimental details • ZnO grown by MBE, doped with N (expected 1019 cm-3); • Broad PL band peaking in the green region at 10K; • ODMR studies carried out at 13.7 and 34 GHz, monitoring the PL intensity. Review of model for the g factors of an A centre: The A centre: a hole is trapped in, for example, an oxygen 2p orbit directed towards a Li ion or a zinc vacancy. px,y • Other discussions of Zn interstitials • often proposed in the context of, e.g, diffusion studies of bulk single crystals [5] • VO and VZn are predicted to be the the two most stable intrinsic point defects in ZnO; however, formation energy of Zni is also low [6,7]. • [5] DG Thomas, J. Phys. Chem. Sols. 3 229 (1957) • [6] AF Kohan, Phys. Rev. B, 61 15019 (2000) • [7] SB Zhang, Phys. Rev. B, 63 075205 (2001) Model for centre T Shallow donor (s) and acceptor (S) are coupled by an exchange terms.a.S(equation3). Ifais large, thenSandsform singlet and triplet states S=0,1. In the triplet state S=1, the effective Hamiltonian is HT, as in equation 4, where D and E are axial and rhombic terms. Perturbation theory gives an anisotropic g (l is spin-orbit coupling constant): References: (first authors only) [1] Y. Chen, Mat. Sci. and Eng. B75 (2000) 190 [2] RM Park, Appl. Phys. Letts. 57 (1990) 2127 [3] E-C Lee, Physica B 308 (2001) 912 [4] A Abragam, Electron Paramagnetic Resonance of Transition Ions, Dover 1970 Dp pz g|| < g