Optically-detected magnetic resonance studies of MBE ZnO:N

1
Optically-detected magnetic resonance studies of
MBE ZnON 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 resonance
involves 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 gt 2) 4
• But the perpendicular g factor of the Z centre is
  less than the parallel g factor, g?lt 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 ZnO is 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 term s.a.S
(equation 3). If a is large, then S and s form
singlet and triplet states S0,1. In the triplet
state S1, 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 lt g?