Visible-light electroluminescence in Mn-doped GaAs light-emitting diodes

We demonstrate visible-light electroluminescence (EL) due to $d$-$d$ transitions in GaAs:Mn based LEDs. We design p$^{+}$-n junctions with a p$^{+}$ GaAs:Mn layer, in which at a reverse bias voltage (-3 to -6 V), an intense electric field builds up in the depletion layers of the p$^{+}$-n junctions. Holes are injected to the depletion layer by Zener tunneling from the conduction band or by diffusion of minority holes from the valence band of the n-type layer. These holes are accelerated by the intense electric field in the depletion layer, and excite the $d$ electrons of Mn in the p$^{+}$ GaAs:Mn layer by impact excitations. We observe visible-light emission at $E_{1}=$ 1.89 eV and $E_{2}=$ 2.16 eV, which are exactly the same as the $^{4}T_{1}\to^{6}A_{1}$ and $^{4}A_{2}\to^{4}T_{1}$ transition energy of Mn. The threshold voltage for observation of visible-light EL is -4 V, corresponding to -($E_{1}+E_{2})$/$e$. This indicates that the impact excitation is most effective for the one step excitation from the ground state $^{6}A_{1}$ to the highest excited state $^{4}A_{2}$.