Shallow-acceptor, donor, free-exciton, and bound-exciton states in high-purity zinc telluride

Abstract

Photoluminescence was measured at 1.6 K on ZnTe crystals, both nonintentionally doped high-purity and lightly P- or As-doped samples. Luminescence was excited mostly with a tunable dye laser close to the band gap. Ground states of the shallow-acceptor bound excitons are identified and attributed to the various acceptors by means of excitation spectra taken on two-hole satellites of these bound-exciton lines. Excited bound-exciton states are also obtained from the excitation spectra. The coupling of the electron and holes in the bound-exciton states appears to be very different for the various acceptors even for almost identical exciton localization energy. The excitation spectra of these two-hole luminescence satellites also exhibit negative spectral features attributed to the free-exciton $1S$, $2S$, and $3S$ levels. We derive the free-exciton binding energy ${E}_{\mathrm{FE}}(1S)=13.2\ifmmode\pm\else\textpm\fi{}0.3$ meV and the band-gap energy ${E}_{g}=2.3941\ifmmode\pm\else\textpm\fi{}0.0004$ eV. Excitation spectra were also taken on the donor-acceptor pair bands involving the Li, P, As, and the Cu acceptor. These excitation spectra yield $s$ and $p$ symmetric excited acceptor states and also excited donor levels. The latter yield 18.3 \ifmmode\pm\else\textpm\fi{} 0.3 meV binding energy of the predominant shallow donor. Higher excited $\mathrm{nS}$ states ($n\ensuremath{\ge}4$) for the Cu, Li, Ag, and the still unidentified $k$ acceptor are derived from two-hole series measured on unintentionally doped samples. Our results thus represent the most complete information available to date on excited shallow-acceptor states in ZnTe. The donor-acceptor pair excitation spectra contain features due to the creation of $\mathrm{TO}(\ensuremath{\Gamma})$ and $\mathrm{LO}(\ensuremath{\Gamma})$ phonons and also impurity-dependent lattice vibrations. Two-electron satellite lines are observed for resonant excitation at the shallow-donor bound exciton. The results agree with those from the excitation spectra taken on the various donor-acceptor pair bands.