First-principles identification of optically efficient erbium centers in GaAs

Abstract

Gallium arsenide (GaAs) doped with erbium (Er), a material of interest for optoelectronics and quantum information, has been studied for decades. Yet the formation of Er luminescence centers in the semiconductor host and their properties are still not well understood. Here we present a systematic investigation of Er-related defects in GaAs, including defect complexes consisting of Er and native point defects or oxygen impurities, using first-principles hybrid-functional defect calculations. We find that these defects have electronic structure and energetics that are generally asymmetric with respect to n- and p-type doping and tend to favor electron trapping. On the basis of the calculated defect levels, formation energies, and nonradiative carrier capture coefficients, we identify Er-related defect centers that are efficient as trap-assisted nonradiative recombination centers for Er$^{3+}$ excitation under host photoexcitation or via minority carrier injection. Our results provide an understanding for why a particular center with Er coupled to two oxygen atoms, often referred to as Er-2O, is most efficient and for the effects of n- and p-type doping and of the Er/O ratio on the formation of optically active Er centers and on the Er luminescence observed in experiments.

Figures (9)

• Figure 1: Schematic illustration of indirect Er$^{3+}$ excitation. Following band-to-band excitation of the semiconductor host, an electron is excited from the valence band to the conduction band. The excited electron is then trapped at a defect center D before recombining nonradiatively with a hole and the recombination energy is transferred into the Er$^{3+}$$4f$-electron core. The same mechanism, mutatis mutandis, is applied to host photoexcitation involving hole trapping or when host photoexcitation is replaced with minority carrier injection.
• Figure 2: Formation energies of isolated Er defects in GaAs, plotted as a function of the Fermi level from the VBM ($E_{\mathrm{v}}$, at 0 eV) to the CBM ($E_{\mathrm{c}}$, at 1.51 eV), under the extreme Ga-rich and As-rich conditions. For each defect, only segments corresponding to the lowest-energy charge states are shown. The slope indicates the charge state ($q$): positively (negatively) charged defects have positive (negative) slopes. Large solid dots connecting two segments mark the defect levels. The $2+$ and $+$ charge states of Er$_{i,{\rm As}}$ are unstable.
• Figure 3: Formation energies of complexes of Er and native defects under the extreme Ga-rich and As-rich conditions.
• Figure 4: Formation energies of various (Er,O)-related defect complexes in GaAs, under the extreme Ga-rich and As-rich conditions. The dashed segments represent energetically metastable (but electronically stable) charge states.
• Figure 5: Configuration coordinate diagrams for transitions involving select Er-related defect complexes in GaAs. $\Delta E$ is the ionization energy (trap depth); $\Delta Q$ is the mass-weighted difference between the geometries of the excited and ground states; and $\Delta E_b$ is the (semiclassical) electron capture barrier.