Optical study of strongly coupled CdSe quantum dots

Abstract

Electronic coupling of vertically stacked CdSe quantum dots is investigated by photoluminescence (PL), time-resolved photoluminescence (t-PL) techniques and Raman techniques. Five samples with each containing 30 layers of self-assembled CdSe quantum dots were grown over InP substrate by molecular beam epitaxy method. The nominal deposition thickness of CdSe layers is 3 ML (monolayer) for all samples. The barrier layers, ZnCdMgSe with MgSe fraction of 59%, were grown with different deposition times to produce different thicknesses of 14, 15, 20, 22, and 25 Å. Strong scattering from lomgitudinal-optical phonons of CdSe quantum dots was observed at 228 cm-1 in Ramen spectroscopy. Photoluminescence spectra and time-resolved photoluminescence spectra were measured at 77 K. With decreasing of barrier thickness, the PL peak shifts to low energy side. This indicates that thinner spacing barrier results in stronger interdot coupling. The lifetime of electrons for each sample was determined by fitting the t-PL data. With the reduction in barrier thickness, the lifetime of electrons increases accordingly. The strong coupling of dots hinders carriers from escaping from quantum dots, thus prolonging carrier lifetime.