Optical properties and recombination processes in (Zn,Cd)Se Graded Index Separate Confinement Heterostructures

Graded Index Separate Confinement Heterostructures have been grown by MOVPE using (Zn,Cd)Se and ZnSe wide bandgap semiconductors. These structures are composed of a deep Zn_1-xCd_xSe (x<0.23%) central well embedded between two thick (Zn,Cd)Se graded layers within which the cadmium composition varies from 0 up to 10% on one side of the well, and from 10 down to 0% on the other side. Both carriers and photon confinement occur in such structures making them suitable for blue-green stimulated emission.Reflectance and photoreflectance data, taken at 2K on a series of samples of different designs allowed us to observe excitonic transitions up to the third quantum number. The structures exhibit a strong photoluminescence line due to the recombination of the carriers in the quantum well. The photoluminescence intensity is analyzed as a function of the temperature and displays a strong thermal quenching. This quenching is due to an increase of the non-radiative recombinations through defects at low temperature and to the thermal escape of the carriers above 40-60K. The value of the activation energy for thermal escape shows that this mechanism is unipolar and concerns the heavy holes.