Optically-detected magnetic resonance of spin-paired complexes emitting in the $2.3\mathrm{eV}$ spectral region in Mg-doped GaN

Optically-detected magnetic resonance (ODMR) experiments on magnesium-doped GaN produced by metal-organic vapor phase epitaxy show a group of strong signals with large linewidths (in excess of $0.15\text{Tesla}$) obtained when monitoring photoluminescence in the region between $5000\mathrm{\AA }$ and $6200\mathrm{\AA }$ $\left(2.5\mathrm{eV}\text{to}2.0\mathrm{eV}\right)$. The signals depend upon the magnesium concentration and are particularly strong in as-grown material. The behavior of the spectrum for different directions of the applied magnetic field and for three well separated microwave frequencies is well described by the spin Hamiltonian for a coupled pair of centers, each with spins of $S=1∕2$. The large ODMR linewidths are ascribed to hyperfine interactions with gallium nuclei, leading to a model in which this broad green-yellow photoluminescence band is due to a recombination process that involves magnesium acceptors closely paired with donor centers which contain interstitial gallium ions. Annealing is presumed to dissociate these pairs and thus to provide an activation process for the magnesium acceptors.

Figure 1

(a) Part of the PL spectrum and (b) the wavelength dependence of the $13.76\mathrm{GHz}$ optically-detected magnetic resonance at $B=0.45\text{Tesla}$ of an as-grown GaN:Mg sample grown by MOVPE with a magnesium precursor flow rate of $75\mathrm{sccm}$ (sample temperature $1.8\mathrm{K}$; UV argon ion laser excitation). As explained in the text, the noisy response at wavelengths less than $5000\mathrm{\AA }$ in (b) is not in fact due to the ODMR signal.

Figure 2

ODMR spectra from the green-yellow spectral region of unannealed GaN:Mg samples grown by metallo-organic vapor phase epitaxy with magnesium precursor flow rates of (top to bottom) 75, 100, 200, and $300\mathrm{sccm}$ (microwave frequency $13.76\mathrm{GHz}$; sample temperature $1.8\mathrm{K}$). The dotted line marked EM indicates the position of the effective mass donor signal.

Figure 3

Angle dependence of the ODMR spectra from the green-yellow spectral region of an unannealed GaN:Mg sample (Mg precursor flow rate $75\mathrm{sccm}$). The microwave frequency was $34\mathrm{GHz}$ and the sample temperature was $1.5\mathrm{K}$; the angles $\theta$ between the magnetic field and the normal to the sample are indicated on the figure. The spectra have been normalized to similar peak heights. The marker (top left) indicates a weak feature tentatively assigned to a level-crossing.

Figure 4

Peak positions of the $34\mathrm{GHz}$ ODMR spectra of Fig. 3 as a function of the angle $\theta$ between the magnetic field and the sample normal (solid circles). Also shown are fits to the data assuming two spin-coupled $S=1∕2$ centers (solid, dashed, dotted lines) with parameter sets $\left(a\text{−}c\right)$, respectively. The vertical bars indicate the estimated uncertainties in the peak positions. The inset shows the energy levels (for angle $\theta =0°)$ predicted by the model spin Hamiltonian and the vertical arrows represent the microwave quantum energy at the resonance fields (∗ marks the “half-field” line). The levels are labelled in the $(J,m_J)$ notation on the right-hand side of the inset.

Figure 5

Summary of the peak positions of the $13.76\mathrm{GHz}$ ODMR spectra as a function of the angle $\theta$ between the magnetic field and the sample normal (solid circles). As for Fig. 4, the solid, dashed, and dotted lines represent fits to the data with parameter sets $\left(a\text{−}c\right)$, respectively. The vertical bars again indicate the estimated uncertainties in the peak positions.

Figure 6

Peak positions of ODMR signals from GaN:Mg observed in the green-yellow spectral region with different microwave frequencies in the region of $100\mathrm{GHz}$ (with the magnetic field along the crystal $c$ axis). Signals $A$ to $D$ are ascribed to the new center and the sharp signal (EM) at $3.7\mathrm{T}$ to effective mass (EM) donors (dashed-dotted line). As for Figs. 4, 5, the solid, dashed, and dotted lines represent fits to the data with parameter sets $\left(a\right)$ to $\left(c\right)$, respectively. The inset shows a typical spectrum.