Using quantum-well nano-apertures to probe hot-electron motion in metal films Jonathan Pelz, Ohio State University, DMR-0505165 Unique cleaved-quantum well (QW) nanoapertures (with width down to 1 nanometer) are used in combination with Scanning Tunneling Microscopy (STM) to visualize and quantify how hot electrons move and scatter in thin metal films. Understanding hot electron motion in metal interconnects is increasingly important as the spacing between transistors in integrated circuits (such as PC processors) shrinks below 100 nanometers, and will become even more critical if circuits made from nanotubes and nanowires become commercially viable. In the experiment, an STM tip injects a cloud of hot electrons into the metal (whose size and shape distribution is not known), and is scanned over a QW aperture [see Fig. (a)]. By measuring how the hot electron current entering a QW changes with tip position [data points in Fig. (b)] the hot-electron distribution was determined to have a simple exponential shape [blue curve in (c)] with a lateral size this increases with metal film thickness. This rules out several common assumptions about hot electron transport in metals, and will lead to better predictions of how hot electrons enter and scatter in ultra-small metal interconnects. Manuscript submitted to Applied Physics Letters. See also Tivarus, Pelz, Hudait, and Ringel, Appl. Phys. Lett. 87, 182105 (2005).

Imaging electron transport at nanometer-sized pinholes in molecular monolayers There is great world-wide effort to incorporate thin films of electronically-active organic molecular materials with semiconductor-based circuits, to combine synthetic chemistrys power to design molecular functionality with the scalability of existing semiconductor fabrication technology. However, the uniformity and electronic properties of these new molecule/semiconductor and molecule/metal interfaces are not yet well understood or optimized. The PIs group (in collaboration with the Weizmann Institute in Israel) used nm-resolution Ballistic Electron Emission Microscopy (BEEM) to image the spatial integrity and electronic behavior of metal/dC-X/GaAs stacks, where dC-X is a discontinuous monolayer of dicarboxylic acids with different functional end group. BEEM imaged the metal grains in the top metal film [Fig. (b)] simultaneously with the nm-sized pinholes in the buried molecular film [white spots in Fig. (c)]. It also showed that the molecular film surrounding individual pinholes significantly increased the local Schottky barrier height (SBH), which is the energy needed by electrons to enter the GaAs through the pinholes. Collaborations with other groups are currently underway to investigate how molecular type and film processing affect electron energy levels and magnetic behavior in different molecular films. H. Haick, et al., Phys. Stat. Sol. (A) 203, 3438 (2006).Jonathan Pelz, Ohio State University, DMR-0505165

Interdisciplinary Training:With NSF support, two undergraduate REU students (Corey Campbel and Steven Stone) and three graduate students (Camelia Marginean, Cristian Tivarus, and Yulu Che) received highly interdisciplinary training using advanced techniques in technologically important topics in nanoscience.Collaboration with Undergraduate Institutions:Professor Pelz collaborates with Professor Susan Lehman and her students of the College of Wooster to study nanometer scale electronic behavior of cleaved quantum dots (top figure) resulting so far in a March Meeting presentation for Prof. Lehman and a manuscript in preparation.Outreach:The PI regularly presents physics demonstrations in elementary school classrooms (1st 6th Grade), and has worked with elementary school teachers to help prepare for the Ohio State Proficiency Test. This year, the PI is also advising a local middle school teacher and student team for the National Engineers Week Future City Competition (see http://www.futurecity.org). This years essay topic is Keep our Future Citys Infrastructure Healthy: Using Nanotechnology To Monitor City Structures and Systems. Interdisciplinary Training, Collaborations, and Outreach Jonathan Pelz, Ohio State University, DMR-0505165

Who: Professor Jonathan Pelz (Ohio State University), with graduate students Camelia Marginean and Cristian Tivarus, and in collaboration with Professor Steven Ringel (also at Ohio State University).

Acronyms/background info: - Scanning Tunneling Microscopy (STM). The technique of Ballistic Electron Emission Microscopy (BEEM) is based on STM. It uses an STM tip to inject hot electrons into a thin metal film as shown in Fig. (a).- Quantum Well (QWs) are made by sandwiching a thin film of a material with small bandgap [such as gallium arsenide (GaAs)] between larger bandgap barrier layers [such as aluminum gallium arsenide (AlGaAs)].- Nano-apertures. We make these by cleaving a QW heterostructure to expose the cleaved QW openings, then depositing a thin metal film [which in our case is gold (Au)].

What/So-what: Over the past 20 years, there have been a number of conflicting reports about how a nanometer-sized metal tip tunnel-injects hot electrons into metal films, and how hot electrons scatter and cool (lose energy) as they move from the top surface to the buried metal/substrate interface. A measurement of how the electrons spread laterally away from the injection point would help resolve some to the conflicts, in particular related whether the electrons enter the metal as a forward focused beam, and how quickly their directions are randomized by elastic scattering. The PIs group and collaborators have pioneered the use of Schottky contacts on cleaved quantum well apertures in combination with applied the STM-related technique of Ballistic Electron Emission Microscopy (BEEM). In this highlight, BEEM profiles [Fig. (b)] measured by scanning the tip over the QW apertures were analyzed to make the first quantitative estimates of the shape and later extent of the hot electron distribution. It was found that the profiles could be explained very well by a hot-electrons distribution (at the buried metal/semiconductor interface) that decayed exponentially with distance from the location directly below the tip, with a decay length that varied from ~6 nm for a 4 nm-thick Au film to ~11 nm for a 15 nm-thick Au film. By measuring how the electrons spread sideways AND how they attenuate when the metal film thickness is varied, stringent tests can be made of proposed models. By comparing with Monte Carlo simulations of electron motion, it was concluded that the injected electron beam could not be forward focused, but rather must enter the metal with a wide range of angles, or quickly scatter to large angles before reaching the buried interface. This was not the prevailing belief. This has potential relevance for device simulations of how electrons move between nm-sized field effect transistors (FETs) through ultra-small metal interconnects. Future studies will address how the scattering and spreading is affected by temperature (scattering is expected to reduce at lower temperature), metal type, and metal film structure.

Who: Professor Jonathan Pelz (Ohio State University), with graduate students Camelia Marginean and Cristian Tivarus, and in collaboration with Professor Steven Ringel (also at Ohio State University).

Acronyms/background info: - Scanning Tunneling Microscopy (STM). The technique of Ballistic Electron Emission Microscopy (BEEM) is based on STM. It uses an STM tip (held at a certain voltage) to inject hot electrons (with an energy that depends on the tip voltage) into a thin metal film as shown in Fig. (a). BEEM works by measuring how many of these electrons can cross the metal film and enter the substrate, as the tip voltage and/or the tip position is varied. The local conduction band energy of the substrate can be determined from the minimum threshold voltage the tip must have to see current in the substrate, and by scanning the tip regions with higher transmittance (i.e. over a pinhole) can be identified.

What/So-what: A great many things are still not well understood at all about how electrons (and holes) move across molecule/metal and molecule semiconductor interfaces. Two extremely important issues are (1) how the energy levels in the molecular materials line up with the energy bands in the inorganic materials, and (2) are the molecular films continuous and uniform. BEEM can give valuable nm-scale information about these issues that is not possible to measure with other techniques.

Both the 4th grade and 6th grade Ohio proficiency tests have (or had) questions about Newtons laws. So in discussions with the elementary school teachers, we decided some demonstrations and explanations would be the most useful.