First-principles characterization of native defects and oxygen impurities in GaAs

Abstract

We present a systematic investigation of native point defects and oxygen impurities in GaAs using hybrid functional calculations. Defects are characterized by their structural, electronic, and optical properties. Under thermodynamic equilibrium, dominant native defects are Ga antisites (Ga$_{\rm As}$), As antisites (As$_{\rm Ga}$), and/or Ga vacancies ($V_{\rm Ga}$) in which As$_{\rm Ga}$ and $V_{\rm Ga}$ are charge-compensating defects under As-rich conditions. On the basis of the defect transition levels, the isolated As$_{\rm Ga}$ can be identified with the $EL2$ center reported in experiments. The defect, however, has a negligible nonradiative electron capture cross section and thus cannot be the ``main electron trap'' as commonly believed. We find that GaAs can have multiple O-related defect centers, especially when prepared under As-rich conditions. The quasi-substitutional O impurity (O$_{\rm As}$) and its complex with two As$_{\rm Ga}$ defects (O$_{\rm As}$-2As$_{\rm Ga}$) both have a metastable and paramagnetic middle (neutral) charge state; however, only the latter can be identified with the experimentally observed Ga--O--Ga or ``OX'' center. These two defects have large nonradiative electron capture cross sections and can be effective carrier traps or recombination centers, which has important implications for materials design.

Figures (9)

• Figure 1: Formation energies of native defects in GaAs, plotted as a function of the Fermi level from the VBM (at 0 eV) to the CBM (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 level$\epsilon(q_1/q_2)$.
• Figure 2: Formation energies of native defect complexes in GaAs under the extreme Ga-rich and As-rich conditions.
• Figure 3: Formation energies of O-related defects in GaAs under the extreme Ga-rich and As-rich conditions. The formation energies of O$_{\rm As}^0$ and (O$_{\rm As}$-2As$_{\rm Ga}$)$^0$ are represented by dotted blue and orange horizontal segments, respectively.
• Figure 4: Configuration coordinate diagrams for relevant optical transitions in GaAs. As illustrated in the case of the ${\rm O}_{\rm As}^+ + e^- \rightleftharpoons {\rm O}_{\rm As}^0$ transitions, $\Delta E$ is the ionization energy (i.e., ZPL), $E_{\rm abs}$ ($E_{\rm em}$) is the peak absorption (emission) energy, $E_{\rm FC}$ is the Franck-Condon shift, $\Delta Q$ is the mass-weighted difference between the geometries of the excited and ground states, and $\Delta E_{\rm b}$ is the electron capture barrier.
• Figure 5: Nonradiative electron capture cross sections (in Å$^2$ and cm$^2$) of relevant defect centers in GaAs.