MBE-grown heavily Sm-doped ZnTe studied by optical spectroscopy

Sm-doped ZnTe films with thicknesses of several hundred nm were grown by molecular beam epitaxy (MBE) on GaAs substrates at temperatures between 300 and 400 °C. Total Sm concentrations between 1018 and about 1021 cm−3 as determined by X-ray microprobe analysis and secondary ion mass spectrometry (SIMS) were incorporated by varying the temperature of the Sm source between 400 and 650 °C. Studies of optical transmission spectra between 0.6 and 2.3 eV, and photoluminescence spectra in the excitonic range, are reported. The transmission spectra were measured after removing the substrates by etching, and fixing the films on glass platelets. These spectra were used to evaluate absorption spectra by correcting for Fabry-Perot interferences. Two prominent absorption bands peaking at about 1.45 and 2.1 eV are seen, and these are attributed, by comparison with SmTe data, to Sm2+ 4f6 → 4f5 5d transitions. We argue that the Sm2+ is incorporated on sites surrounded by an octahedron of Te atoms, which are formed from zincblende interstitial sites during MBE growth. The coalescence of these centers occurring above approximately 1020 cm−3 is thought to be the reason for the observation of rocksalt-type reflections in the reflection high-energy electron diffraction (RHEED) patterns in this concentration range.