Shallow versus Deep Nature of Mg Acceptors in Nitride Semiconductors

We investigate the properties of Mg acceptors in nitride semiconductors with hybrid functional calculations. We find that although the thermodynamic transition level is relatively close to the valence band in GaN (260 meV), ${\mathrm{Mg}}_{\mathrm{Ga}}$ exhibits key features of a deep acceptor: the hole is localized on a N atom neighboring the Mg impurity, inducing a large local lattice distortion and giving rise to broad blue luminescence. We show that the ultraviolet photoluminescence peak attributed to Mg acceptors in GaN is likely related to Mg-H complexes, explaining the results of photoluminescence and electron paramagnetic resonance experiments. Predictions for Mg acceptors in AlN and InN are also presented.

Figure 1

(a) Local structure showing the distortion around a neutral Mg acceptor in GaN. The localization of the hole on the axial nitrogen neighbor is illustrated by the spin-density isosurface (yellow), set to 5% of the maximum. (b) Structure of the Mg-H complex in GaN in the positive charge state. The spin density shows the more delocalized nature of the complex. Large green spheres denote Ga atoms, medium light blue spheres N atoms, and the orange (darker) sphere denotes the ${\mathrm{Mg}}_{\mathrm{Ga}}$.

Figure 2

(a) Formation energy versus Fermi level for the Mg acceptor (${\mathrm{Mg}}_{\mathrm{Ga}}$), hydrogen interstitial (${\mathrm{H}}_i$), and their complex (Mg-H) in GaN. (b) Configuration-coordinate diagram illustrating optical processes related to Mg in GaN. Recombination of an electron at the CBM with ${\mathrm{Mg}}_{\mathrm{Ga}}^0$ leads to emission peaking at 2.70 eV, explaining BL in GaN.

Figure 3

(a) Formation energy versus Fermi level for the Mg acceptor, H interstitial, and their complex in AlN. (b) Structure and spin density associated with the localized hole states ($S=1$) of ${\mathrm{Mg}}_{\mathrm{Al}}^{+}$ (isosurface at 5% of the maximum in yellow). Large blue spheres denote Al atoms, medium light blue spheres N atoms, and the orange (darker) sphere ${\mathrm{Mg}}_{\mathrm{Al}}$.