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


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Microwave amplifier. Microwave switch. Microwave source. Signal. Function generator. Lock-in amplifier. Helium 1.8K. Superconducting magnet. Photo-detector. Filters , analysers. PL. Spectrometer. Laser. [N O 3- ] -. [Zn i ] +. c -axis.

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slide1

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