Femtosecond pump-probe reflectivity study of InGaAs/GaAs quantum dots carrier dynamics

Ultrafast carrier dynamics have been studied on a single layer of self-assembled In$_{0.4}$Ga$_{0.6}$As/GaAs quantum dots (QDs) using femtosecond degenerate pump-probe differential reflectivity. The measurements were done with an 800 nm, 28 fs Ti-sapphire oscillator. The growth process of QDs consists of two steps, low temperature growth and high temperature annealing. Specifically, the InGaAs QD structures are fabricated on n-type GaAs(001) using molecular beam epitaxy (MBE). The In$_{0.4}$Ga$_{0.6}$As layer is deposited at 390-400 $^{\circ}$C followed by QDs self assembly at 450-540 $^{\circ}$C. Finally, these QDs are caped with a 10 nm or 100 nm layer of GaAs. Measured width and height of these QDs are typically 33 nm and 6 nm respectively. Dots annealed at higher temperature have larger base area (width and length) and reduced height as compared to those annealed at lower temperature. We have developed a rate equation model to describe the carrier dynamics and fit the reflectivity data. Dynamics depends on the size of the quantum dots: larger QDs have faster dynamics as compared to smaller dots. Additionally, dynamics are slower at higher excitation levels.