Junction mixing scanning tunneling microscopy.

摘要

Research in the fields of nanotechnology and nanoelectronics is burgeoning. As nanodevices shrink in size and subsequently electronic operations on these length scales accelerate, new techniques will be required to study and characterize these devices. Techniques with atomic scale spatial resolution and femtosecond time resolution will soon be necessary. Looking to conventional scanning probe microscopy, the scanning tunneling microscope (STM) already possesses sufficient spatial resolution to image any feature current nanotechnologies can produce (STMs are used to build the worlds smallest nano-devices). Similarly ultrafast pump/probe optical techniques exist, which can resolve femtosecond dynamics. The goal of this research is to wed ultrafast optical techniques with the scanning tunneling microscope to produce an aggregate probe with the ability to image nanoscale dynamics. This goal has been achieved using a technique known as junction mixing STM (JM-STM).; Initial work using the junction mixing technique was performed to demonstrate the detection of a time-resolved signal using a home built scanning tunneling microscope. This work demonstrated an order of magnitude improvement in time resolution over previous experiments by utilizing ion implanted gallium arsenide substrates to generate fast electrical pulses. This work decisively demonstrated the STM tunnel junction as the origin of the measured time resolved signal, a crucial requirement in maintaining STM spatial resolution.; Following these experiments, new test structures were designed to demonstrate combined STM spatial resolution with picosecond time resolution. Measurements were made on small titanium dots patterned onto a gold transmission line. The titanium provided electronic contrast to the gold, so that our time resolved signal was modulated as we scanned our STM across the titanium/gold interface. Combined 20 ns-20 ps spatio-temporal resolution was achieved, the first direct confirmation that time-resolved STM was possible.; These experiments were numerically reproduced using a lumped element circuit model of the non-linear tunnel junction in parallel with STM geometrical capacitance. Results from this model suggest the junction mixing technique should be able to yield time resolution in the hundreds of femtoseconds while maintaining atomic spatial resolution.; To demonstrate the operation of this junction mixing technique efforts were directed to designing and building a low temperature high vacuum STM and a home built Ti/Sapph laser system. These systems are being incorporated into a new low temperature high vacuum time resolved scanning tunneling microscope.