Scanning Tunneling Microscopy with a Frequency Comb

There is a growing need for new SPM techniques that provide additional information about the sample with higher data rates. In microwave harmonic scanning tunneling microscopy (MHSTM) a microwave signal is coupled to the STM, and several harmonics that are generated by the nonlinearity of the tunneling junction are measured to provide spectroscopic information about the sample such as identifying molecular adsorbates and the high-resolution dopant profiling of semiconductors. The PI has used a passively mode-locked Kerr-lens Ti:sapphire laser to generate a frequency comb in an STM by intermode mixing, and measured the first 12 harmonics in the comb. The decay of the successive harmonics is much less than that in MHSTM so measurements of the magnitude and phase of a large number (perhaps 100) harmonics can be used to characterize the sample and the impedance of the tunneling junction. In Phase I efficient active and passive devices will be developed to couple to a larger number of the harmonics to determine if it is feasible to construct a multi-channel system for collecting and processing the data from the frequency comb in each scan of the STM for Phase II and Phase III. Commercial Applications and Other Benefits: The International Technology Roadmap for Semiconductors (ITRS) for 2009 concurs with earlier editions in stating that there are continued difficult challenges in materials characterization at nanoscale. The rapid shrinking in the size of semiconductors now requires a means for determining 2-D dopant profiles with a spatial resolution of 3 nm or less. NIST Special Publication 1048 addresses the economic significance of the need for innovative instrumentation to measure dopant distribution in sub-22 nm technologies. As already noted, MHSTM can provide high-resolution dopant profiling, and the vastly greater amount of data from the frequency comb will extend this capability. The possible benefits of the new technology to basic research include establishing precise frequency and timing standards at nanoscale, determining local temperature from the width of each peak, and providing a new means for probing other processes within the tunneling junction.