Physical mechanisms of photoluminescence of chlorine-doped ZnSe epilayers grown by molecular beam epitaxy

Abstract

Reported here are some temperature-dependent and excitation-dependent photoluminescence (PL) results from chlorine-doped ZnSe layers grown by molecular beam epitaxy. The PL spectrum is characterized by an overwhelming exciton recombination emission at 2.797 eV (10 K) near the band edge, and no other PL features are found. This overwhelming neutral donor-bound exciton (Cl0X) emission line at 2.797 eV (10 K) with a full width at half maximum (FWHM) of ∼13 meV reveals the high crystalline quality of the samples. The quick quenching of this exciton line above 200 K is due to the presence of a nonradiative center with a thermal activation energy of ∼90 meV. The decrease of the Cl0X line over the temperature range from 10 to 200 K is due to the thermal activation of Cl0X bound excitons to free excitons with an activation energy of ∼9.0 meV. The change of the emission nature occurs at about ∼200 K from Cl0X bound exciton recombination to free exciton recombination; accordingly, a kink appears on the temperature-dependent FWHM curve at ∼190 K. The broadening behavior of the Cl0X line over the temperature range 10–200 K can be understood within the theoretical framework of the phonon scattering model, while the impurity scattering mechanisms are responsible for the broadening of the exciton line above 200 K. This PL peak does not shift with the change in excitation power, while the integrated intensity increases exponentially with the increase in excitation power, i.e., IPL∝Iex1.21, indicating the competition between the radical recombination process and the nonradical recombination process of photogenerated carriers. The FWHM remains unchanged when the excitation power was varied from 0.5 to 50 W cm−2, and then increases exponentially when the excitation power exceeds 50 W cm−2. However, the PL peak consists of four exciton subpeaks, and these subpeaks do not broaden. The PL peak is found to broaden due to recombination competition among the four kinds of excitons.