Metastable-defect behavior in silicon: Charge-state-controlled reorientation of iron-aluminum pairs

We report the observation of a novel example of defect metastability in silicon. The phenomenon, monitored by deep-level transient spectroscopy, takes place at a well-identified point defect, i.e., the interstitial-iron–substitutional-aluminum pair (${\mathrm{Fe}}_{\left(\mathrm{i}\right)}$ ${\mathrm{Al}}_{\left(\mathrm{s}\right)}$). The charge state of the defect during sample cooldown to low temperature is found to control a reversible transmutation behavior between two defect energy levels, at $E_V$+0.20 eV (H1) and $E_V$+0.13 eV (H2). A kinetic study of the transformation has led to a detailed microscopic description of the phenomenon. It is shown to arise from a charge-state-controlled, electrostatically driven, reorientation of ${\mathrm{Fe}}_{\left(\mathrm{i}\right)}$ ${\mathrm{Al}}_{\left(\mathrm{s}\right)}$ pairs between 〈111〉 and 〈100〉 configurations. Levels H1 and H2 are thus ascribed to (${\mathrm{Fe}}_{\left(\mathrm{i}\right)}$ ${)}^{2+}$-(${\mathrm{Fe}}_{\left(\mathrm{i}\right)}$ ${)}^{+}$ transitions at the nearest and next-nearest tetrahedral sites adjacent to aluminum, respectively. A configuration-coordinate (CC) description of the center, based on the simple ionic model of iron-acceptor pairs, is shown to account for all features of the reaction. No very large lattice relaxation is needed to understand the phenomenon. The CC model of the ${\mathrm{Fe}}_{\left(\mathrm{i}\right)}$ ${\mathrm{Al}}_{\left(\mathrm{s}\right)}$ pair is then extended to non-purely-ionic defect complexes. A complete new class of metastable centers is thus proposed. Metastable phenomena involving other semiconductor defects (A center in silicon, EL2 center in GaAs, M center in InP) are discussed in the light of these new CC models.