Localized exciton bound to an isoelectronic trap in silicon

Abstract

A photo- and electroluminescence study is reported on the recombination radiation from excitons bound to an isoelectronic trap in silicon. This type of luminescence is novel in silicon. The luminescence which was obtained from only few samples consists of three long-lived emission lines $A$, $B$, and $C$, and a number of associated weak phonon wings. The decay characteristics and the temperature dependence of the fluorescence along with the "forbidden" $B$ line exhibited under perturbations of a magnetic or elastic strain field give rise to our interpretation in terms of an isoelectronic trap binding the electron-hole pair. These features are highly reminiscent of familiar isoelectronic systems which can bind an exciton such as GaP:N, GaP:Bi, or ZnTe:O. Orientational Zeeman measurements show that the trap has axial symmetry in the direction. The $g$ factors are $g_e=1.6\mathrm{}\pm 0.1$ and $g_h=1.1\mathrm{}\pm 0.1$. The $g_e$ value is largely reduced in comparison with known bound-electron values and suggests that an electron is tightly bound to the trap capturing the hole by its Coulomb field. This conception also explains the observed cw-excitation dependence of the fluorescence. The samples showing the $A$, $B$, and $C$ fluorescence are $n$ and $p$ type ($\rho \ge 2$ Ω cm) and are grown by different techniques. The strongest fluorescence is observed from an aluminumdoped sample (2.5 Ω cm) for which the trap density is estimated to be no more than some ${10}^{13}$ ${\mathrm{cm}}^{-3\mathrm{}}$. No correlation of luminescence with shallow-donor or -acceptor doping could be established, and the nature of the trap remains unidentified. A correlation of luminescence with carbon concentration in a number of samples and the annealing behavior of the exciton luminescence may hint that carbon is one of the trap constituents.