Welcome to the home page of RPI MBE/STM group

We are doing research on electron transport in thin-film metal-semiconductor and metal-insulator-semiconductor structures.

We grow these structures by Molecular Beam Epitaxy (MBE), with in-situ structural characterization by Reflection High Energy Electron Diffraction (RHEED).  Scanning Tunneling Microscopy (STM) and Ballistic Electron Emission Microscopy (BEEM) measurements are performed in the same vacuum chamber. We also have the DI-Dimension-3100 Atomic Force Microscope (AFM) to characterise surface morphology of the insulating films.

Our current interest is to study the ballistic electron transport in such technologically important metals as Al and Cu.

According to the National Technology Roadmap for Semiconductors, the minimum feature size in 2012 will be 50nm for dense lines (DRAM half-pitch) and 35nm for isolated lines (MPU gates). For such narrow lines, scattering at various boundaries will become important. And as the length of shortest interconnects (such as those connecting two transistors in CMOS inverter) goes into sub-micron range, the effects of ballistic electron transport may also become noticeable or even dominant. This motivates studies of electron scattering at boundaries and ballistic electron transport in general. And thin films are a very natural object for such studies, since they are easy to fabricate, and both film thickness and surface roughness may be well controlled and characterized.

The two approaches adopted by us are: BEEM measurements of vertical electron transport through the metal films, and measurements of the size effect in film resistivity. In both cases, it is beneficial to have single-crystalline metal layers, so we are working with epitaxial metal/CaF2/Si(111) structures.

The picture of our MBE-STM/BEEM system:


The system is located in class-100 clean room  managed by CIEEM. Complex post-growth processing of our samples is enabled by presence of many other pieces of useful equipment, such as oxidation furnaces, hexode plasma etcher, silicon-oriented CVD systems, photolithography stepper etc. The equipment is now being upgraded to handle 6" diameter wafers.

Two UHV chambers for Si-based and for GaAs-based growth are coupled together. Each is equipped with Reflection high-energy electron diffraction (RHEED) system. On the left side, you can see part of the Arsenide-growth chamber. The Si chamber is the fartherst one, into which a person looks.

Closer to the Si chamber, between two big flat gate valves, you can see a high vertical cilinder. That is our STM/BEEM tool, built by Dr. Carl Ventrice Jr. Atomic-resolution image of Si 7x7 reconstruction below certifies its quality.
The Ballistic Electron Emission Microscopy (BEEM) is an STM extention capable of probing transport of hot electrons through thin layers and at buried interfaces.  Its idea is sketched out below:

Hot electrons are injected into the thin metal layer from STM tip. If an injected electron is scattered inside the metal, it loses energy and is unable to cross the interface between metal and insulator or semiconductor, whichever lies below.  If an electron is not scattered inside the metal and has sufficient injection energy, it is able to cross the interface, gets collected in the back contact, and counts as BEEM current.

By studying several samples with different metal thicknesses and by measuring the dependence of the BEEM current on thickness, it is possible to extract information about the mean free path (MFP) of electrons in the metal. Moreover, since the energy of  the injected electrons may be varied simply by appropriately biasing the STM tip, the energy dependence of the MFP may also be obtained.

 
The group consists of:
 
  • Professor Leo J. Schowalter
  • Dr. Nikolay Yakovlev
  • graduate student Yuriy Shusterman
    • Former group members:


    We belong to the Condensed Matter Physics subdivision of the Department of Physics, Applied Physics, and Astronomy

    Here are some MBE-related links (more for intrinsic use, no comments supplied)