AVS2004 Session NS1-ThA: Nanoscale Imaging

Thursday, November 18, 2004 2:00 PM in Room 213C

Thursday Afternoon

Start	Invited?	Item
2:00 PM		NS1-ThA-1 Single Electron Charging in Individual InAs Quantum Dot Observed by Nonconctact Atomic Force Microscopy Y. Miyahara, R. Stomp, S. Schaer, Q. Sun, H. Guo (McGill University, Canada); S. Studenikin, P. Poole, A. Sachrajda (National Research Council, Canada); P. Grutter (McGill University, Canada) Understanding the electronic structure of quantum dots (QD) is not only important for application but also of great interest in fundamental physics. Although there have been a number of studies of electronic properties using mainly optical or capacitance spectroscopy techniques, investigating a single QD remains challenging because of the extremely small dot dimensions. Spectroscopic techniques based on scanning probe microscopy have been employed, in particular scanning tunneling spectroscopy (STS). In STS the acquired tunneling spectra feature the Coulomb staircase or/and the discrete energy states of the QD depending on the size of the QD and the tunneling barrier thickness. However, these measurements are limited to substrates with adequate conductivity since a measurable tunneling current of typically 1 pA or more is usually required. Here, we report the first successful observation of the Coulomb blockade effect by electrostatic force measurements. The main experimental features in the electrostatic force vs. the tip-substrate bias voltage curves agree well with a simple theory based on the semi-classical theory of the Coulomb blockade effect. Comparison of the experimental results with the model calculation will be made and the possibility to observe the discrete energy states will also be discussed. One of the important differences to STS is that there is no continuous current flowing in the system. As a consequence, this technique can detect single electron events. Furthermore, this implies that this technique can also be applied to the QDs embedded in other materials.
2:20 PM		NS1-ThA-2 Nanoelectromechanics of Scanning Probe Microscopies of Ferroelectric Surfaces S.V. Kalinin (Oak Ridge National Laboratory); A. Gruverman (North Carolina State University); J. Shin, A.P. Baddorf (Oak Ridge National Laboratory); E. Karapetian, M. Kachanov (Tufts University) Nanoscale applications of ferroelectric materials including MEMS and nonvolatile memory components have motivated a number of studies of ferroelectric behavior on the nanoscale using a wide array of electromechanical Scanning Probe Microscopy techniques including Piezoresponse Force Microscopy, Atomic Force Acoustic Microscopy, Scanning Near-Field Acoustic Microscopy, and Heterodyne Ultrasonic-Electrostatic Force Microscopy. Quantitative interpretation of SPM domain imaging mechanisms and particularly hysteresis loop measurements and tip-induced polarization switching processes requires description of contact mechanics for the ferroelectric surface including electromechanical coupling effects and also the structure of electroelastic fields inside the material. Here, the analytical solution of the coupled electromechanical problem for piezoelectric indentation is used to derive the relationship between indentation force, tip bias and tip displacement. These stiffness relations are utilized for quantitative interpretation of the imaging mechanism of the electromechanical SPM techniques. The structure of the electroelastic fields yields a quantitative measure of the signal generation volume and also provides a quantitative basis for the analysis of tip-induced polarization switching and local hysteresis loop measurements. The early stages of the switching process require the exact structure of the electroelastic fields to be known, while the late stages of switching processes can be adequately described using point charge type models. Tip-induced switching is shown to be possible only above a minimum threshold tip bias, producing a well-defined minimal size of the switched domain. Approaches to reduce minimal written domain size for ferroelectric lithography and data storage are discussed.
2:40 PM	Invited	NS1-ThA-3 High-Resolution Force Microscopy: Observing Atoms at Work R. Bennewitz (McGill University, Canada); L. Nony, E. Gnecco, O. Pfeiffer, A. Socoliuc, S. Maier, A. Wetzel, C. Gerber, E. Meyer, A. Baratoff (University of Basel, Switzerland) Force microscopy has made progress towards quantitative determination of forces with lateral resolution on atomic scale. One example is the observation of enhanceded interactions at the edge atoms of nanoscale pits in KBr surfaces which are able to trap otherwise mobile molecules. Dynamic modes of force microscopy allow to detect dissipative processes with the same lateral resolution. For the molecules trapped in nanoscale pits, a strongly enhanced dissipation is observed compared to the KBr substrate. Dissipation is also the focus of friction force experiments, which recently have revealed new aspects of atomic friction processes, like a regime of ultra-low friction obtained at low loads. New instrumental developments including the combination of a force microscope with mass spectrometer will be discussed.
3:20 PM		NS1-ThA-5 Imaging of Radiation Effects on an Active Silicon-On-Insulator (SOI) Device using Scanning Capacitance Microscopy (SCM) C.Y. Nakakura, M.R. Shaneyfelt, R.A. Jones (Sandia National Laboratories) Two-dimensional (2D) imaging of electrical properties using atomic force microscopy (AFM)-based techniques has attracted considerable attention in the semiconductor industry, primarily for 2D-dopant profiling of cross-sectioned device junctions. Scanning capacitance microscopy (SCM) has been most widely used for this purpose by acquiring nanometer-scale, 2D free carrier images, from which dopant information can be extracted. The bulk of reported dopant profiling studies using SCM, however, have been performed on shorted, inoperable devices that only show the static device as fabricated. In this study, we expand on conventional, cross-sectional carrier profiling: first, the images were acquired using a modified SCM to permit carrier profiling of active devices and, second, the devices were measured before and after radiation exposure to visualize the effects on device operation. Understanding radiation effects in semiconductor devices is critical to the development of radiation-hardened integrated circuits used in harsh environments, such as Earth-orbit and outer space. We will demonstrate that SCM is a promising tool for directly imaging the effects of radiation in cross-sectioned, silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs). Following exposure to radiation, the impact of the radiation-induced charge buildup at the Si/SiO₂ interface of the SOI substrate is readily observed in the SCM images. The methodology of the active device measurements, as well as the implications of radiation exposure on the operation of these devices, will be discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
3:40 PM		NS1-ThA-6 High Resolution Structural and Chemical Information: A Combined STM, SEM and SAM Analysis of Ag Nanocrystallites on Si J. Westermann, M. Maier, J. Poppensieker, G. Schaefer (OMICRON NanoTechnology GmbH, Germany) For the analysis of nanostructured materials it is of crucial importance to be able to characterise not only structural properties of the material, but also determine chemical composition, as well as electronic and magnetic properties of individual nanostructures. To accomplish this, we have combined a high resolution SEM column with an STM under true UHV conditions with extension possibilities for AFM, SEMPA, FIB, and EBL. This enables in situ SEM and STM imaging to study the sample topography from mm down to 10 pm scale on the same sample spot. Using the electron column as an excitation source for Auger, chemical analysis is possible with sub 10 nm resolution. STM related techniques like STS allow the characterisation of the electronic structure and magnetic imaging using spin polarised STM. Besides the observation of sample properties, the STM+SEM combination also offers unique possibilities for sample modification on the nanoscale, offering both electron beam lithography and STM manipulation under SEM control. Using Ag nanocrystals on Si(111), we demonstrate the capabilities of this approach. The samples were prepared by evaporation of Ag on a clean Si (111) substrate at elevated temperatures. We show SEM and SAM images with a resolution down to 3 nm (SEM), and 10 nm (SAM) respecively allowing for chemical analysis with ultimate resolution. We demonstrate the importance of SEM to select areas of interest, and subsequently position the STM tip to image these areas. STM images with atomic resolution show the surface structure and reconstructions on both the silver crystals and the silicon substrate. The latter shows a √3x√3 reconstruction induced by the silver.
4:00 PM		NS1-ThA-7 Atomic Level Analysis of Polythiophene by the Scanning Atom Probe O. Nishikawa, M. Taniguchi, M. Ihara (Kanazawa Institute of Technology, Japan); H. Kato, S. Takemura (Kanto Gakuin University, Japan) Thin films of conductive polythiophene grown by the electrochemical process on silicon and ITO substrates are investigated by utilizing unique capability of the scanning atom probe (SAP). The thickness of the films is 10 to 30 Î¼m and the dopant is BF4-. Since the variation of the field emission current I with the applied voltage V to the polythiophene is related with the work function of the specimen surface, the I-V curves, the F-N plot, were obtained. The slope and intercept of the F-N plots (S-I chart) indicates that the work function does not vary with the synthesizing process and the substrate. The comparison of the S-I chart of CNT, W and Si shows that the work function of the polythiophene is close to that of Si. The variation of the S-I chart with temperature also suggests that the polythiophene films are semi-conductive because the emission current varies with temperature as Si. The SAP analysis was conducted by applying voltage pulses or laser pulses. The mass spectra of the detected ions are closely related with the structure of the polythiophene. The Most abundant ions have the masses 80 to 83 which indicates that these ions are SC4Hn, the basic unit of the polythiophene. The ratio of the number of sulphur atoms to that of carbon atoms is 1 to 4 as expected. The detection of various ions such as C+, C2+ and SC3Hn+ suggests that some areas not polymerized and stay in the graphite like state. All sulphur atoms were found as a clustering atom with carbon and hydrogen such as SC4Hn2+ and SC3Hn+. Since the evaporation field of doubly charged ions is usually higher than that of singly charged ions, the detection of doubly charged cluster ions indicates that the SC4Hn2+ clusters are very stable and the atoms forming the clusters are fairly strongly bound. No oxidation of the polythiophene was noticed.
4:20 PM		NS1-ThA-8 Probing Ion Transport at the Nanoscale: Time-Domain Electrostatic Force Spectroscopy on Glassy Electrolytes A. Schirmeisen, A. Taskiran, H. Fuchs, B. Roling, S. Murugavel, H. Bracht, F. Natrup (University of Muenster, Germany) Ion conducting solid materials play an important role as electrolytes in energy conversion systems, such as batteries and fuel cells, and also in electrochemical sensors. Of particular interest are so-called fast ion conductors with conductivities that are comparable to liquid electrolytes. Currently, a lot of research work is being done in order to find new materials with improved conductivities. For instance, nanostructured materials become more and more technologically relevant. An important prerequisite for further progress in this field is a better understanding of the ion transport mechanisms on microscopic length scales. Up to now, the experimental techniques used for probing the ion transport are mainly macroscopic in nature, e.g. conductivity spectroscopy, tracer diffusion measurements and NMR relaxation techniques. The macroscopic averaging over the motions of all ions in a sample leads to a loss of information making it desirable to develop techniques that are capable of probing the ion transport on nanoscopic length scales. In this contribution, we demonstrate that electrical atomic force microscopy (AFM) techniques yield information about the dynamics of mobile ions in small subvolumes of a sample. In dynamic mode AFM, a voltage is applied between the tip and the sample, at typical tip-sample distances of about 10 nm. In this case, the voltage drop in the sample occurs mainly in a nanoscopic subvolume below the surface. Ionic motions in this subvolume influence the electrostatic forces acting between tip and sample. We record the time dependent evolution of the forces at sample temperatures from 100 K to 600 K, which allows us to extract the activation energy of the ion conduction process. The comparison of macroscopic with our nanoscopic measurements on different solid electrolytes shows that time-domain electrical AFM is capable of probing the ion dynamics and transport in nanoscopic subvolumes of the samples.
5:00 PM		NS1-ThA-10 Novel MEMS Devices for Accurate Lateral and Normal Force Measurement in AFM P.J. Cumpson (National Physical Laboratory, UK); J. Hedley (Newcastle University, UK) The uncertainty in the spring constants of AFM cantilevers is a limiting factor in a wide range of measurements of nanoscale quantities (chemical, electrical, mechanical) at a lateral resolution of 30 nm or below. The tip/cantilever fabrication process suggests that manufacturing cantilevers to the exquisite dimensional accuracy required to produce sufficiently repeatable spring constants will always be a challenge; an easy and accurate calibration method is needed. A number of methods have been suggested for calibrating AFM cantilevers¹, all of which have limited accuracy or are difficult to perform. We have developed a novel microfabricated device capable of calibrating AFM cantilevers for normal spring constant² much more easily. The device consists of a surface micromachined silicon resonator, which can be set into resonance at an amplitude of around 10 nm in vacuum. By applying Doppler interferometry and electrical measurement simultaneously, the spring-constant of the MEMS device can be deduced, to an uncertainty of around ±2%, without physical contact. Here we present a new MEMS device for the first time, specifically designed to make lateral force constant calibration easy and accurate. This presented challenging fabrication and interferometry problems. Nevertheless, it is particularly valuable since few other options exist for quantitative measurement of lateral force. ¹ N A Burnham et al, Nanotechnology 14 (2003) 1-6. ² P J Cumpson and J Hedley, Nanotechnology 14 (2003) 1279-1288.