AVS2001 Session NS-TuA: Novel Surface Nanoprobes

Tuesday, October 30, 2001 2:00 PM in Room 133

Tuesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	NS-TuA-1 Nano-scale Science by Means of UHV Electron Microscopy K. Takayanagi (Tokyo Institute of Technology, Japan) Nano-scale materials attract interest in fundamental science and technology. Because nano-meter scale materials behave as a new matter, differently from the condenced matter or liquid. As seen from magic number of clusters and carbon nanotubes, nano-scale matter presents degitized and/or quatized characteristics. These characteristics relate with nature of their surface characteristics. We developed an UHV high-resolution electron microscope combined with a miniaturized STM , which enabled us to study nanowires extending our previous analysis on Si(111)7x7 reconstruction. The gold nanowires were found to have multi-tube structure, like carbon nanotubes. In gold tube, each tube consits of gold atomic rows which coil the axis of the tube. The number of atomic rows in the outer tube increases by seven from that of the inner tube, so that the shell closing number is seven. The present STM combined UHV-TEM allow us to open new sciece of the nano-scale matter.
2:40 PM		NS-TuA-3 Tunneling Spectroscopy of Superconducting Quasiparticles by Scanning Tunneling Microscope with a Bulk NbN Tip H. Bando, Y. Aiura, K. Mitsugi (National Institute of Advanced Industrial Science and Technology, Japan); K. Oguchi, Y. Nishihara (Ibaraki University, Japan); Y. Kumashiro (Yokohama National University, Japan) Use of superconducting tip is expected to extend the capability of scanning tunneling microsopy by allowing detection of superconducting quasiparticles in the atomic resolution. A few groups have succeeded in the measurements with supercunducting tips made of Nb or Ag/Pb coated PtIr, however, technically there remain challenges as for the cleaning of tip apex etc. We employed bulk NbN crystals, whose surface is chemically stable, and prepared tips by fracuture. Results of topographic measurements, SIN tunneling on Au, and SIS' tunneling on NbN films with the tunneling resistance varied for orders of magnitute are presented.
3:00 PM		NS-TuA-4 Demonstration of a Tunable Bistable Tunnel Device with a Low Temperature STM and a Self-organized Ga Array on Si(111)¹ I.B. Altfeder (Harvard University); D. Chen (Rowland Institute for Science) Self-organized nano-structures on Si substrates have drawn much current research effort. The usefulness of these structures critically depends on physical properties they can offer for potential nanoscale device applications. We report here an unusual bistable transport behavior of a tunnel junction form between an STM tip and a two dimensional (2D) Ga array self-assembled on a Si(111) substrate. At 77K, large hysteresis loops appear in the I-V spectra when electrons are injected from the tip to the 2D Ga array, characteristic of the switching between ON - OFF conductance states. The turn-on bias varies from -3.1 V to -4.0 V and shows an inverse dependence on the tip-sample distance, indicating a strong field effect. The turn-off bias, however, is essentially pinned at a conductance threshold of -2.7 V. These observations demonstrate the basis of a nanoscale tunable bistable tunnel device with the potential for digital and storage applications. ¹I. B. Altfeder and D. M. Chen, Phys. Rev. Lett. 84, 1284 (2000).
3:20 PM	Invited	NS-TuA-5 Theory and Perspective of the Spin-Polarized STM S. Blügel (Forschungszentrum Jülich, Germany) In the frontier field of nano-magnetism understanding complex magnetic structures is crucial. We demonstrate that the spin-polarized scanning tunneling microscope (SP-STM) offers a great potential to unravel complex magnetic superstructures on different length scales. Different operating modes of a SP-STM¹ will be discussed: It is shown that the spectroscopy mode (SP-STS) and the modulating tip-magnetization mode (SP-STMTM) are ideal to analyze complex magnetic structures on a mesoscopic length scale as created by atomic-scale ferromagnetism. Further we propose the application of the constant-current mode of a SP-STM for the investigation of surfaces of complex atomic-scale magnetic structures of otherwise chemically equivalent atoms. A recent application² gave evidence of the capabilities of the SP-STM in terms of the first unambigious proof of two-dimensional antiferromagnetism in magnetic monolayer films on non-magnetic substrates predicted already in 1988.³ We argue, that in general any magnetic superstructure leads to an SP-STM image characteristic of the superstructure and is not superimposed on the image of the chemical lattice. This opens the door to study even more complex magnetic structures such as non-collinear spin-structures or spin-density waves. We present calculated SP-STM images of frustrated antiferromagnets on triangular lattices: a coplanar non-collinear periodic Néel state for Cr/Ag(111) and a multiple spin-density wave state of a Mn-monolayer on Cu(111), which we determined in Ref.⁴ to be the magnetic ground states of these systems. All calculations are based on the vector-spindensity formulation of the density functional theory. ¹ D. Wortmann et al., Phys. Rev. Lett. 86, accepted (2001). ² S. Heinze et al., Science 288, 1805 (2000). ³ S. Blügel et al., Phys. Rev. Lett. 60, 1077 (1988). ⁴ Ph. Kurz et al., Phys. Rev. Lett. 86, 1106 (2001).
4:00 PM		NS-TuA-7 Charge Imaging of Electrons and Holes Trapped in Gate Oxides and at the Oxide-silicon Interface R. Ludeke, E. Cartier (IBM T.J. Watson Research Center) We have observed individual electrons and positive charge both in bulk SiO₂ and Al₂O₃ gate oxides, and trapped on dangling-bond derived states (P_b centers) at the SiO₂-Si(111) interface. The charge is observed as sharp virtual holes in the surface topography measured with an Atomic Force Microscope operating in the non-contact mode in ultra high vacuum (UHV). The polarity of the charge was ascertained from contrast differences in Kelvin images that were simultaneously acquired with the topographs. The trapped charges in the bulk of the oxides exhibited a tendency to cluster. The P_b centers were created by the removal of H from the H-Si interfacial bonds by annealing the Si(111) samples to 600°C. Their densities in the mid 10¹² cm^-2 range were measured with the AFM and confirmed by capacitance-voltage experiments. The distributions of the negatively and positively charged (holes) traps are peaked, respectively, in the upper and lower halves of the band gap. This allows the selective capture of electrons or holes that depends on the position of the local Fermi level E_F. E_F is controlled by the bias applied to the metallic AFM tip. We have modeled the band bending beneath the tip, which, depending on the bias and the binding energy of the trap state, can lead to temporary filling of the trap that leads to unusually sharp features in the images. Thus the trap's physical location and, less directly, the trap level relative to the band edges can be ascertained from these experiments.
4:20 PM		NS-TuA-8 'Artifact-free' Electrostatic Force Measurement using Noncontact Atomic Force Microscopy K. Okamoto, Y. Sugawara, S. Morita (Osaka University, Japan) Noncontact atomic force microscopes (NC-AFM) using frequency modulation (FM) method have the potential ability to obtain surface structure of any materials, such as metals, semiconductors and insulators with atomic resolution. NC-AFMs simultaneously detect several kinds of forces acting between the tip and the sample surface; the van der Waals force, the chemical bonding force and the electrostatic force (ESF) et al. Each of the forces contains the different physical information about the surface. Although each of them is desirable to be separately detected, no method to completely separate them has not been realized. This lack results in the deviation of the NC-AFM images, namely the 'artifacts.' In this presentation, we propose the novel method to completely separate the ESF from other forces. The Kelvin probe force microscopy (KPFM) is often used to measure and separate the ESF with AFM. However, the KPFMs still involve an extra deviation, which is due to an AC electric field applied for KPFM measurement. Our new method proposed here can completely eliminate the influences due to the ESF on the topography with keeping all merits of KPFM by introducing the artifact eliminator circuit, which adjust the signal for height regulation of the tip. Our method can measure the topography and the spatial distribution of the ESF, which reflects the spatial distribution of work function or charges on the sample surface, with the atomic resolution without the 'artifact.' This system has high possibility to identify the kind of each atom.
5:00 PM		NS-TuA-10 Scanning Impedance Microscopy: From Impedance Spectra to Impedance Images S.V. Kalinin, D.A. Bonnell (University of Pennsylvania) Electrostatic force sensitive scanning probe microscopy (SPM) for quantitative imaging of dc and ac transport behavior of electrically active interfaces is presented. SPM is used to study transport properties of a metal-semiconductor junction and SrTiO₃ bicrystal grain boundary. Scanning Surface Potential Microscopy (SSPM) of laterally biased sample is used to quantify potential drops at the interface. Varying the lateral bias allows the voltage and I-V characteristics of the interface to be reconstructed. A novel scanning probe technique based on phase change of cantilever oscillations induced by a lateral bias applied to the sample is presented. This technique, further referred to as Scanning Impedance Microscopy (SIM), allows mapping of the local voltage phase angle and voltage oscillation amplitude in complex systems. The frequency dependence of the voltage phase angle shift across the interface allows interface capacitance and resistance to be determined locally. Quantitative agreement between metal-semiconductor junction capacitance obtained from SIM measurements and macroscopic impedance spectroscopy is demonstrated. Variation of the dc component of lateral bias in SIM allows reconstruction of the C-V characteristics of the junction. SSPM and SIM imaging of a SrTiO3 grain boundary has demonstrated the non-linear transport properties of the interface and identified a large density of interface states at the boundary. The combination of SSPM and SIM provides an approach for the quantitative analysis of local dc and ac transport properties from SPM data and provides spatially resolved impedance spectra of complex microstructures. Finally, the applicability of SIM to characterize complex polycrystalline materials will be demonstrated.