Optical properties of ZnS and ZnCdS/ZnS MQW grown by molecular beam epitaxy on GaAs and CaF2 substrates

ZnS-based II-VI materials were successfully grown by molecular beam epitaxy (MBE). Epitaxial growth was confirmed on GaAs(100) and CaF2(100) substrates by observing reflection high-energy electron diffraction (RHEED) patterns. The low-temperature photoluminescence (PL) spectrum of undoped ZnS grown on both substrates exhibits dominant excitonic emission without any deep emission. Nitrogen (N) doping of ZnS was performed using an RF plasma cell. A new excitonic emission related to N-acceptors was observed around 3.765 eV. A ZnCdS/ZnS multiple quantum well (MQW) exhibits a dominant excitonic PL band, and its quantized energy states up to n = 3 were observed in the photoluminescence excitation (PLE) spectrum by monitoring the lower energy side of the PL emission. Oscillatory structures at equidistant energies (about 42 meV) were also observed in the PLE spectrum by monitoring the higher energy side of the PL emission. These peaks are attributed to a multiple-LO-phonon relaxation process of hot excitons.