Temperature-dependent midinfrared photoluminescence of epitaxial PbTe/CdTe quantum dots and calculation of the corresponding transition energy

We present the temperature dependence of continuous-wave midinfrared photoluminescence of PbTe quantum dots in a CdTe matrix. The quantum dots are formed by epitaxial precipitation of a two-dimensional PbTe layer upon thermal annealing. A strong shift of the emission to longer wavelengths with decreasing temperature is found. This shift is not only caused by the strong temperature dependence of the band gap of PbTe but also by the strain in the dot as well as by the change in quantization energies, both being temperature dependent due to the thermal-expansion mismatch between PbTe and CdTe, and via the effective masses of PbTe. At low temperatures we observed an increase in the emission intensity with rising temperature, depending on dot size. This is attributed to the presence of a dark ground state, as was also observed for lead salt nanocrystals. The influence of the excitation power on the emission spectra at various temperatures indicates carrier redistribution between the dots. Furthermore, an analytical calculation of the ground-state transition energy in the dots is performed using a spherical dot shape, and including the temperature-dependent strain in the dots and the matrix as well as the temperature dependence of the effective masses of PbTe. From these model calculations, a good agreement to the experimental data is obtained over the whole temperature range from 20 to 300 K.