Relaxation of electronic excitations in K2GeF6 studied by means of time-resolved luminescence spectroscopy under VUV and pulsed electron beam excitation

Abstract

Highly crystalline K2GeF6 micropowders were prepared by co-precipitation in the hydrofluoric acid-water-alcohol solution and studied by means of the cathodoluminescence and time-resolved luminescence spectroscopy under synchrotron radiation excitation at 7 K. The nature of various intrinsic emissions was revealed. The luminescence band at 510 nm detected under excitation by photons with the energy above 9 eV is related to the radiative decay of self-trapped excitons. Fast emission bands with ~400 ps decay time found in the spectral range from VUV to visible (8–2.4 eV) are assigned to the cross-luminescence transitions from K 3p hole states and intraband luminescence transitions due to the presence of the Ge 4s, 4p valence states. The creation of cation excitons was detected in the energy range 18–20 eV. Their non-radiative decay results in the formation of hole states in the Ge energy bands, which finally recombine with electrons from the above lying valence states and provide fast intraband luminescence. Important electronic band structure parameters including the energy gap width of 11 eV, cation exciton formation energy 18.1 eV and ionization energy of cation states 20.0 eV were determined from the luminescence excitation spectra.