Session A21: Focus Session: Advances in Scanned Probe Microscopy I -- Novel Tip and Material Control

Vacuum Phonon Tunneling in Variable Temperature STM

We demonstrate that the temperature of the terminating atom of STM tip can be directly measured by inelastic electron tunneling spectroscopy. A previously unknown mechanism of interfacial thermal transport, field-induced phonon tunneling, has been revealed by ultrahigh vacuum scanning tunneling microscopy. Using thermally broadened Fermi-Dirac distribution in the STM tip as in-situ atomic scale thermometer we found that thermal vibrations of the last tip atom are effectively transmitted to sample surface despite few angstroms wide vacuum gap. We show that phonon tunneling is driven by interfacial electric fields and thermally vibrating image charges, ``thermal mirages''. By comparing experimental data and theory, we show that the thermal energy transmitted through atomically narrow vacuum gap due to thermal vibration of image charges exceeds, by ten orders of magnitude, the Planck's thermal radiation energy. Reference: I. Altfeder, A. A. Voevodin, A. K. Roy, PRL 105, 166101 (2010)   

A Dual Tip STM for Imaging the Superconducting Phase Difference

We have built a dual tipped STM, with each tip capable of independently scanning a sample. We will use the STM at ultra-low (mK) temperatures to study superconducting samples. The two tips along with the superconducting sample constitute a SQUID. This configuration is designed to minimize fluctuations in the Josephson phase of one of the tips, which scans the sample, while the other tip acts as a reference junction. Calculations and separate experiments on test SQUIDs indicate this arrangement will enable us to measure spatial variations of the gauge-invariant phase difference at the atomic scale.   

Nanoelectrical probing with multiprobe SPM Systems compatible with scanning electron microscopes

A scanning electron microscope compatible platform that permits multiprobe atomic force microscopy based nanoelectrical characterization will be described. To achieve such multiple parameter nanocharacterization with scanning electron microscope compatibility involves a number of innovations both in instrument and probe design. This presentation will focus on how these advances were achieved and the results obtained with such instrumentation on electrical nano-characterization and electrical nano-manipulation. The advances include: 1. Specialized scanners; 2. An ultrasensitive feedback mechanism based on tuning forks with no optical feedback interference that can induce carriers in semiconductor devices; and 3. Unique probes compatible with multiprobe geometries in which the probe tips can be brought into physical contact with one another. Experiments will be described with such systems that will include multiprobe electrical measurements with metal and glass coated coaxial nanowires of platinum. This combination of scanning electron microscopes integrated with multiprobe instrumentation allows for important applications not available today in the field of semiconductor processing technology.   

Imaging and manipulation of nanoscale materials with coaxial and triaxial AFM probes

We present coaxial and triaxial Atomic Force Microscope (AFM ) probes and demonstrate their applications to imaging and manipulating nanoscale materials. A coaxial probe with concentric electrodes at its tip creates a highly confined electric field that decays as a dipole field, making the coaxial probe useful for near field imaging of electrical properties. We show nearly an order of magnitude improvement in the step resolution of Kelvin probe force microscopy with coaxial probes. We further demonstrate that coaxial probes can image dielectric materials with the dielectrophoretic force. In addition to imaging, the capacitive structure that makes up the cantilever of a coaxial probe is used to locally mechanically drive the probe, making them self-driving probes. Finally, coaxial probes can create strong forces with dielectrophoresis (DEP) which we combine with the nanometer precision of the AFM to create a nanometer scale pick-and-place tool. We demonstrate 3D assembly of micrometer scale objects with coaxial probes using positive DEP and discuss the assembly of nanometer scale objects with triaxial probes using negative DEP.   

Deterministic Single Atom STM Tip Technology for Atomically Precise Manufacturing

Deterministic tip fabrication for Scanning Tunneling Microscopy (STM) has long been an elusive goal, where the primary method of tip preparation usually includes significant ``tip conditioning'' once the tip has been incorporated into the STM. We have developed a process for generating reproducible single atom tips (SATs) with a small radius of curvature (r.o.c.) of less than 10nm. First, W(111) or W(110) tips are sputter sharpened using a self-limiting process to yield with r.o.c. of $<$3nm; the consistent r.o.c. greatly improves the reliability of the process. Next, we use a Field Ion Microscope (FIM) to perform field evaporation and analysis of the tips. Once a clear crystal structure is determined, an SAT is formed. Transmission Electron Microscopy is used to verify that after field evaporation the r.o.c. remains small. Correlations between FIM and tip performance in STM are determined, and long term STM stability is discussed.   

Atmospheric Stability of Tungsten STM Tips for Atomically Precise Manufacturing (APM)

In APM, STM tungsten tips are used to selectively remove or add surface atoms to build atomically precise 3D structures. Therefore, the development of stable atomically sharp tips is crucial for long term tip performance and process efficiency. These tips have been shown to be extremely sensitive to electrostatic discharge (ESD) events and some environmental conditions. However, recent work has demonstrated that tungsten tips with three to eight atoms at their apex can be stable structurally and chemically after days of ambient exposure with ESD-safe practices. Whereas macroscale W surfaces will oxidize under atmospheric oxygen, HRTEM and 3-D atom probe measurements confirm that no oxide is formed on these tips with an extremely stable surface structure; however, some oxygen does diffuse into the material. In addition to the description of the chemical and structural characterization employed in this work, several possible explanations for the stability of these tips will be offered.   

STM manipulation and measurement of charged species in semiconductors

The scaling of transistors to nanometer dimensions requires more precise control of individual dopants in semiconductor nanostructures, as statistical fluctuations in dopant distributions can significantly impact device performance. Proposals for next-generation quantum- and spin-based electronics also rely on the tuning of the charge, spin and interactions of dopant atoms with local electric fields. Using a scanning tunneling microscope (STM), we demonstrate how to control the binding energy and ionization state of individual acceptors in p- GaAs [1]. Charged species such as native dopants, vacancies and adatoms directly influence the acceptor binding energy via the Coulomb interaction. In addition, a combination of defect- and tip-induced band bending can be used to remotely tune the acceptors' ionization state. We find that by applying voltage pulses with the STM tip, charged vacancies and adatoms can be positioned on the surface. These experiments suggest a new and direct method for quantifying the charge of adsorbates (e.g. adatoms or molecules) as well as defects (e.g. vacancies, antisites, interstitials) at semiconductor surfaces. \\[4pt] [1] D.H. Lee and J.A. Gupta (submitted)   

Dirac Fermions in Nanoassembled Artificial Graphene

In condensed matter, electronic properties derive from the energy band structure created by a periodic potential formed by the atoms that constitute a particular material. The power to design unique electronic states is ultimately tied to the power to design the atomic lattice. Utilizing the technique of atomic manipulation with a scanning tunneling microscope, we create an artificial lattice potential that reshapes the band structure of a normal 2D electron gas---found in the surface states of a normal metal---into a unique and distinct 2D gas of massless Dirac fermions. We present scanning tunneling spectroscopic measurements of nanoassembled honeycomb electron lattices, and we characterize their band structure through Fourier transform analysis of impurity scattering maps. The control of every atomic position in the lattice provides unprecedented control over physical parameters elusive in natural graphene systems. These abilities include atomically sharp doping configurations and the power to embed topological singularities, resulting in unique electronic states rarely encountered in natural systems.   

Topological properties of artificial graphene assembled by atom manipulation

Graphene exhibits special electronic properties stemming from its two-dimensional (2D) structure and embedded relativistic Dirac cones. However, many proposed topologically ordered ground states remain elusive in conventional measurements due to the difficulty in arranging the necessary quantum textures into natural graphene. By exploiting atomic manipulation with a custom-built ultrastable scanning tunneling microscope, we have constructed graphene-like structures by arranging molecules to create a honeycomb lattice of electrons drawn from normal 2D surface states. Spectroscopy reveals a spectacular transformation of nonrelativistic massive 2D electrons into massless Dirac fermions carrying a chiral pseudospin symmetry. We demonstrate the tailoring of this new class of graphene to reveal signature topological properties: an energy gap and emergent mass created by breaking the pseudospin symmetry or changing the hopping term non-uniformly with a Kekul\'{e} bond distortion; gauge fields generated by applying atomically engineered strains; and the condensation of electrons into quantum Hall-like states and topologically confined phases.   

Self-navigation of STM tip toward a micron sized sample

Scanning probe microscopy (SPM) of small samples on insulating substrates, for example graphene devices, is of significant current interest as it can provide invaluable information on the electronic, structural chemical and optical properties of these materials. Accessing such samples with SPM often requires locating a micron sized area within a much larger region of several mm. This is a very difficult task because SPM is intrinsically nearsighted and in many cases combining it with larger scan probes such as optical microscopy is not practical. Here we report a simple capacitance-based method to navigate a STM tip operating at low temperatures in strong magnetic field which allows to find such small samples quickly and efficiently. The method consists of back-gate compensation, refocusing during the search, and distinguishing edges of conducting electrodes and the sample.   

Magnetic Particle Imagining with a Cantilever Detector

We present a novel detection scheme for magnetic nano and micro particles using a magnetic force microscope (MFM) that allows for the local measurement of AC magnetic susceptibility. The method makes use of the nonlinearities in the magnetic response of a particle that come from its intrinsic magnetic susceptibility as well as its interaction with the surrounding environment. We excite the particle at subharmonic frequencies of the resonator detector to minimize cross talk similar to Magnetic Particle Imagining (MPI) (Gleich B, Weizenecker, J. Nature 435, 1214 2005) although here a cantilever acts as a detector instead of a tuned coil. This allows for the detection and characterization of magnetic particles with high signal to noise and low distortion making it ideal for characterizing magnetic nanoparticles over larger distances compared to typical scanned probe tip-sample separation. It also allows for the reconstruction of the local susceptibility curve.   

The importance of cantilever mechanics in the quantitative interpretation of Kelvin Probe Force Microscopy

A realistic interpretation of the measured contact potential difference (CPD) in Kelvin Probe Force Microscopy (KPFM) is crucial in order to extract quantitative information. Thus far, simulations of KPFM have treated the cantilever as a rigid object. We present a technique to simulate KPFM measurements by simulating a realistic three dimensional probe above a planar sample. We study three methods of weighing the probe-sample interactions to include cantilever mechanics. (1) The commonly-used force method treats the probe-sample interaction from all parts of the probe equally. This method only allows for translation of the probe. (2) The torque method allows for rotation of the probe, taking into account the fixed cantilever end. (3) The bending method acknowledges the flexibility of the cantilever by modeling it as an Euler-Bernoulli beam. We compare simulated step responses from each method to experimental data. We find the force and torque methods overestimate the effect of the cantilever and that the bending method produces the best agreement with experiment.   

Nm-Scale Surface Potential Transient Measurements of the E$_{C}$-0.57eV Trap in an AlGaN/GaN High Electron Mobility Transistor

AlGaN/GaN high electron mobility transistors (HEMTs) are intrinsically ideal for high frequency and high power applications, but have degraded performance due to charge trapping. A suspected virtual gate-related trap at E$_{C}$-0.57eV (with $\sim $30 ms emission time constant at 300 K) has been shown to have a significant impact on HEMT performance and reliability [1]. Using scanning Kelvin probe microscopy, we report on nm-scale measurements of surface potential transients consistent with the E$_{C}$-0.57 eV level at different locations across the surface of an AlGaN/GaN HEMT immediately after bias switching. We find that the amplitude of this surface potential transient is largest at locations close to the drain side of the gate, consistent with the ``virtual gate model'' where charge leaks and is stored near the gate edge in the drain --gate access region. Comparison of nm-scale measurements and electrostatic simulations will be discussed, to quantify the spatial distribution of this $\sim $30ms trap as a function of gate- and drain-biasing. Work supported by ONR-DRIFT (P. Maki). \\[4pt] [1] A.R. Arehart, S.A. Ringel, et al., IEEE International Electron Devices Meeting(IEDM), 2010, 20.1.