Luminescence Quantum Efficiency in CDSSE Mixed Crystals

Abstract

There are basically two different ways to determine absolute photoluminescence (PL) quantum efficiencies. With the calorimetric absorption spectroscopy (CAS) the heating of a sample under illumination due to nonradiative processes of excited carriers is measured. In combination with calorimetric transmission spectroscopy (CTS) it is then possible to calculate PL quantum efficiencies. In this indirect method it is useful to go to the mK regime to reach high sensitivity. We use the other, more direct method with an integrating or Ulbricht sphere fixed into a cryostat. With this setup PL quantum efficiencies can be determined from 5 K up to room temperature or even above. At low temperatures we found in the binary systems CdS and CdSe efficiencies less than 25 % depending on the crystal quality within the main luminescence bands (arising from donor and acceptor bound excitons and donator acceptor pair recombination and their phonon replica). In contrast, CdSSe mixed crystals show an efficiency up to 70 % within the luminescence out of localized states originating from composition fluctuations of the alloy. Electron-hole pairs created above the mobility edge relax into localized states from where they recombine radiatively with low nonradiative losses. With increasing temperature the efficiency is firstly unaffected, but decreases drastically when the temperature exceeds the equivalent of a certain thermal activation energy. Taking into account three parallel recombination channels, namely radiative, nonradiative and thermal activated nonradiative recombination, we can fit the efficiency as a function of temperature and deduce an activation energy which coincides with the localization depth of the excitons below the mobility edge. This work was done in collaboration with C. Klingshirn.