Jahn-Teller distortion and ferromagnetism in the dilute magnetic semiconductors GaAs:Mn and cubic GaN:Mn

Using first-principles total-energy methods, we investigate Jahn-Teller distortions in III-V dilute magnetic semiconductors, GaAs:Mn and GaN:Mn in the cubic zinc blended structure. The results for an isolated Mn impurity on a Ga site show that there is no appreciable effect in GaAs, whereas in GaN there is a Jahn-Teller effect in which the symmetry around the impurity changes from $T_d$ to $D_{2d}$ or to $C_{2v}$. The large effect in GaN occurs because of the localized $d^4$ character, which is further enhanced by the distortion. The lower symmetry should be detectable experimentally in cubic GaN with low Mn concentration, and should be affected by charge compensation (reductions of holes and conversion of Mn ions to $d^5$ with no Jahn-Teller effect). The Jahn-Teller effect is greatly reduced because the symmetry at each Mn site is lowered due to the $\mathrm{Mn}-\mathrm{Mn}$ interaction. The tendency toward ferromagnetism is found to be stronger in GaN:Mn than in GaAs:Mn and to be only slightly reduced by charge compensation.

Figure 1

A schematic drawing of three different atomic configurations, which lower symmetries in Jahn-Teller distortion (a) $T_d$ to $D_{2d}$, (b) $T_d$ to $C_{2v}$, and (c) $T_d$ to $C_{3v}$.

Figure 2

The partial density of states and schematic energy level diagrams of Mn $3d$ states in (a) GaAs:Mn and (b) GaN:Mn in $T_d$ symmetry without a Jahn-Teller distortion (no energy level splitting) and (c) with Jahn-Teller distortion of $D_{2d}$ symmetry, which shows the splitting of the $t_{2d}$ $d$ states.