Ultrafast carrier dynamics in semiconductor nanowires

Abstract

The novel properties of semiconductor nanowires, along with their potential for device applications in areas including nanoscale lasers and thermoelectrics, have led to a resurgence of interest in their growth and characterization over the past decade. However, the further development and optimization of nanowire-based devices will depend critically on an understanding of carrier relaxation in these nanostructures. For example, the operation of GaN-based photonic devices is often influenced by the presence of a large defect state concentration. Ultrafast optical spectroscopy can address this problem by measuring carrier transfer into and out of these states, which will be important in optimizing device performance. In this work, we use ultrafast wavelength-tunable optical spectroscopy to temporally resolve carrier dynamics in semiconductor nanowires. Wavelength-tunable optical pump-probe measurements enable us to independently measure electron and hole dynamics in Ge nanowires, revealing that the lifetime of both electrons and holes decreases with decreasing nanowire diameter. Similar measurements on CdSe nanostructures reveal that the surface-to-volume ratio strongly influences carrier relaxation. Finally, ultrafast optical experiments on GaN nanowires probe carrier dynamics in the defect states that influence device operation. These experiments provide fundamental insight into carrier relaxation in these nanosystems and reveal information critical to optimizing their performance for applications.