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Surface Electronic Characterization with SPM

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Surface Electronic Characterization with SPM

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  1. This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items during your presentation • In Slide Show, click on the right mouse button • Select “Meeting Minder” • Select the “Action Items” tab • Type in action items as they come up • Click OK to dismiss this box • This will automatically create an Action Item slide at the end of your presentation with your points entered. Surface Electronic Characterization with SPM Sidney Cohen Franco-Israel Conference on Nanocharacterization

  2. Preview • Introduction to modes of SPM electronic characterization - • current/voltage spectroscopy (I/V), scanning spreading resistance microscopy (SSRM), scanning capacitance microscopy (SCM), scanning Kelvin microscopy (SKPM). • Examples: • Study of electronic states in Quantum dots • Study of electron transport in thin organic films • Investigation of transport at grain boundaries Franco-Israel Conference on Nanocharacterization

  3. Why use these techniques? • Combination of high resolution imaging with electronic characterization • Possible to identify, characterize, modify, and characterize again with same probe. • BUT !!…Need to consider interaction of probe with sample. Franco-Israel Conference on Nanocharacterization

  4. Current-Voltage Spectroscopy I • dI/dV gives directly local density of electronic states. • Possible influence of measurement (band bending, charging) • Difference between I/V in STM/SFM Franco-Israel Conference on Nanocharacterization

  5. I/V spectroscopy Fermi Level DOS eV =Bias voltage = energy Franco-Israel Conference on Nanocharacterization

  6. Contact Resistance (a=contact radius, =electron mean free path Contact resistance) 1. Spreading resistance, 2. Sharvin (Ballistic) transport, Note: Franco-Israel Conference on Nanocharacterization

  7. For typical experimental values, metals: But measure (Contaminants, oxidation, etc) Franco-Israel Conference on Nanocharacterization

  8. Modes based on capacitative force • Scanning Capacitance Microscopy • Scanning Kelvin Probe Microscopy • Forces are long-range. Finite size of tip causes broadening of features. Franco-Israel Conference on Nanocharacterization

  9. Scanned Probe Measurements of CdSe Quantum dot Structures* • Want to correlate size of dot with electronic properties • Due to confinement, gap varies inversely with size: • bulk Eg • *Alperson, Cohen,Rubinstein, Hodes, Phys. Rev. B 52 Localization energy Franco-Israel Conference on Nanocharacterization

  10. I/V spectroscopy on CdSe Q. Dot Gap 1 0.15 eV Gap 2, 0.2 eV Eg=2.1 V Franco-Israel Conference on Nanocharacterization

  11. Double capacitor configuration 4 nm gaps in parallel gives C = 6e-19. This translates to charging energy of 0.15 eV Supports premise that each peak corresponds to addition of electron to quantum dot “Coulomb Charging” Franco-Israel Conference on Nanocharacterization

  12. Size Distribution vs. Msd. Energy Gap TEM This Exp.,with Calculated Gap Franco-Israel Conference on Nanocharacterization

  13. Work Function Variations on thin film surfaces* • May be expected due to microscopic domain structure • SKPM can be used to detect domains with different work function down to 50 nm size. • Evidence supports domain existence: • Macroscopic Kelvin Msmts. Cannot give the spatial resolution • * Cohen, Efimov, Dimitrov, Trakhtenberg, Naaman, • submitted Franco-Israel Conference on Nanocharacterization

  14. Microscopic Domain Structure in Mixed Film Franco-Israel Conference on Nanocharacterization

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  16. Results show NO variation of signal across surface Topography Raw SKPM Contrast < 5 mV Corrected SKPM Franco-Israel Conference on Nanocharacterization

  17. Contact Potential Differences on Different Surfaces Monolayer types CN MIX3 MIX2 MIX 1 RL858 OM Mstm. 1 -600 -260 210 20 410 450 (mV) Msmt. 2 -640 -300 190 -40 360 500 (mV) Monolayer = Lewis Acid Lewis Base CPD is of tip relative to surface. More negative CPD therefore corresponds to higher work function because monolayers have extracted electrons from the gold substrate. Franco-Israel Conference on Nanocharacterization

  18. Electron Transport at grain boundaries in semiconductors* For polycrystalline semiconductors, the electron transport properties across grain boundaries play a significant role in solar cell function, and particularly in their degradation. Crystallites can be a fraction of a micron in size, making it difficult to determine these transport properties by conventional means. Scanning Spreading Resistance, I/V spectroscopy, and SKPM can give this information *I. Visoly-Fisher, D. Cahen, S. Cohen (samples from C. Farakadis Franco-Israel Conference on Nanocharacterization

  19. Electronic properties of Grain Boundaries can be measured by: 1. Comparing I/V curves across the grain boundary 2. Monitoring change in surface potential across boundary with SKPM Franco-Israel Conference on Nanocharacterization

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  21. Spatially-resolved I/V spectroscopy on CdTe film Using conducting SFM 2 1 3 Forward-biased currents are highest near grain boundary. May be due to lower gap energy or higher carrier concentration Franco-Israel Conference on Nanocharacterization

  22. SKPM - Contrast in CPD image CdTe with Molecular Layer contrast = 15 meV Uncoated CdTe contrast = 30 meV Franco-Israel Conference on Nanocharacterization

  23. Conclusions • SPM can give useful information on the nanoscale surface electronic properties • Correlation can be made between topography and electronic characteristic • Knowledge of the effect of measurement on the system is required to interpret results • Many possibilities untouched here (photo-effects, direct capacitance msmt., STM UHV work) Franco-Israel Conference on Nanocharacterization

  24. Acknowledgements Quantum Dot Work - I. Rubinstein, G. Hodes, B. Alperson Organic Films - R. Naaman, D. Dimitrov Photovoltaics - D. Cahen, I. Visoli-Fisher All work performed at: Franco-Israel Conference on Nanocharacterization

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