Ultrafast scanning tunneling microscopy
Abstract
This thesis presents the ongoing efforts to combine ultrafast optical techniques with scanning tunneling microscopy into a single instrument, so called ultrafast scanning tunneling microscope (USTM), which is capable of characterization of ultrafast dynamical events occurring on the nanoscales. In such USTM experiments, a femtosecond laser pulses initiate ultrafast electronic processes on the structured surface, and transients in the tunneling current induced by such processes are monitored as a function of time and position. Since the coupling between the laser light and STM can be achieved in many ways, several practical USTM designs are discussed here and a versatile experimental setup is developed to allow realization of different USTM schemes without the need for significant elements rearrangement. We applied the technique of photoconductively gated USTM to study carrier dynamics in InAs/GaAs self-assembled quantum dot samples (SAQD). The results obtained with this method are compared to standard ensemble-averaging ultrafast optoelectronic techniques such as femtosecond optical pump/probe reflectivity measurements and time-resolved terahertz spectroscopy. These measurements reveal a unified picture of the relaxation dynamics in InAs/GaAs SAQDs at T = 300 K. The initial carrier relaxation proceeds via Auger carrier capture from the InAs wetting layer (WL) on an timescale of 1--2 ps, followed by recombination of carriers in the WL, GaAs substrate and in quantum dots on timescales of 350 ps, 2.3 ns and 900 ps respectively. The consistency of these three experimental techniques demonstrates USTM as a reliable tool for probing the local dynamics of semiconductor nanostructures. We also demonstrate ultrafast dynamical imaging of surfaces using a junction mixing USTM. We detect picosecond transient voltage pulses on a microstrip transmission line and demonstrate a temporal resolution (full-width at half max imum) of 8 ps. We show that the time resolution achieved in these experiments is limited mostly by the microstrip geometry and can be significantly improved by reducing the value of the transmission line impedance. By dynamically imaging the microstrip line and detecting picosecond voltage pulses propagating on a patterned metal-on-metal (Ti/Pt) structure, we demonstrate that 1-nm spatial resolution is achievable for a 13 ps (full width at half maximum) transient correlated signal.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- 2004
- Bibcode:
- 2004PhDT.......505Y