Auger recombination and photocarrier relaxation in semiconductor quantum dots under intense excitation*

Semiconductor quantum dots (QDs) have high luminescence efficiency, a broad and size-tunable emission spectrum, and they are used in many optoelectronic applications. The relaxation of the photoexcited hot carriers to the equilibrium state shapes the device performance, including the switching speed and the luminescence efficiency. A better understanding of carrier cooling, therefore, has significant technological implications. We conducted a wide range of measurements on the ultrafast lattice heating in lead selenide (PbSe) quantum dots (QDs) using ultrafast electron diffraction (UED). The free-standing PbSe QDs, which are uniformly dispersed in a single layer on a TEM grid, were pumped by 50 fs optical pulses at a wavelength of 800 nm [1]. The structure dynamics were recorded by taking fs snapshots of diffraction patterns (DP) at different time delays relative to the pump optical pulse. The UED experiments were performed on both 3 nm and 6 nm QDs at a pump fluence from 1.5 – 7.0 mJ/cm². The overall time resolution is less than 500 fs. For data analysis, we first convert each 2D DP to a 1D curve displaying the diffraction intensity as a function of diffraction angle. Then, each Bragg peak in the 1D intensity curve was fitted to extract the peak position, height and FWHM. The temperature of the QDs were obtained by converting the changes in the peak position and height (area) to temperature, using the data of static electron diffraction measurement as a reference. Both of them give nearly the same sample temperature. Then, the curves of temperature as a function of pump-probe delay times were fitted with a multiexciton relaxation model, incorporating both the initial electron-phonon (e-ph) coupling rate and the Auger decay rate. The overall fitting reproduces experimental results nicely. We found that at high pump fluence, initial e-ph coupling is slowed down by about an order of magnitude. In addition, the multiexciton Auger recombination mechanism plays a dominant role in the lattice heating process, in contrast to the bulk PbSe sample. It not only increases the amount of energy transferred from the excitons to the lattice, but also accelerates the lattice heating after excitation.