Quantum confinement: A route to enhance the Curie temperature of Mn doped GaAs

The electronic structure of Mn doped GaAs and GaN have been examined within a multiband Hubbard model. By virtue of the positioning of the Mn $d$ states, Mn doped GaAs is found to belong to the $p-d$ metal regime of the Zaanen-Sawatzky-Allen phase diagram and its variants, while Mn doping in GaN belongs to the covalent insulator regime. Their location in the phase diagram also determines how they would behave under quantum confinement which would increase the charge transfer energy. The ferromagnetic stability of Mn doped GaAs, we find, increases with confinement therefore providing a route to higher ferromagnetic transition temperatures.

Figure 1

Comparison of the ab initio (dashed blue lines) and best fitted tight-binding (solid red lines) band dispersions for nonmagnetic $25\%$ Mn doped GaAs.

Figure 2

Calculated partial density of states for (a) up spin Mn $d$, up and down spin of (b) As $p$ (nearest neighbor of Mn), and (c) As $p$ (far away from Mn) for a Mn doping concentration of $3.125\%$ in GaAs at $\mathrm{\Delta }=0.53$ eV and $U=3.0$ eV in a multiband Hubbard model. The zero of energy represents the Fermi energy. Inset shows Mn $d$ density of states for up spin at $U=4.0$ eV.

Figure 3

Up spin Mn $d$ partial density of states for (a) $\mathrm{\Delta }=0.53$ eV, (b) $\mathrm{\Delta }=1.5$ eV, and (c) $\mathrm{\Delta }=2.7$ eV calculated for a Mn concentration of $3.125\%$ and a U of 3.0 eV within a multiband Hubbard model. The zero of energy represents the Fermi energy. Inset shows the Mn $d$ component of the hole character as $\mathrm{\Delta }$ is varied.

Figure 4

Up and down spin Mn $d$ partial density of states for (a) $\mathrm{\Delta }=0.53$ eV, (b) $\mathrm{\Delta }=0.75$ eV, (c) $\mathrm{\Delta }=1.0$ eV, and (d) $\mathrm{\Delta }=1.27$ eV for a doping concentration of $3.125\%$ of Mn in GaN calculated within a multiband Hubbard model for $U=3.0$ eV. The zero of energy is the Fermi energy. The Mn $d$ character ($n_d$) of the hole state has been indicated.