Browsing by Subject "Scanning tunneling microscopy."

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Open Access
Atomic scale investigation of clean and epi-grown Si(001) surfaces using scanning tunneling microscopy
(1996) Özer, H. Özgür
In this thesis, clean and epi-grown Si(001)(2x1)surfaces are analyzed by Scanning Tunneling Microscopy (STM). The STM and Ultra High Vacuum System (UHV) in which the microscope is installed, are described. A brief history of the studies on the reconstruction and fundamental features of the Si(001) surface is also given. First, the sample and tip preparation techniques were optimized. Sample preparation method, which includes both ex situ chemical and in situ heating cleaning procedures, was found not to give routinely the clean and atomically flat surfaces, because of the criticality of the temperature values used during heat treatments. The monoatomic steps, dimer rows, defects such as missing dimer and dimer groups, were observed on clean Si(001) surfaces. Double height step formation due to contamination was also detected on a few samples. Buckling of dimers, which is believed to be due mainly to either the high defect density or tip-surface interaction, was observed on one sample. Si and Ge were grown epitaxially on the silicon substrate, with 0.11 ML and 3.2 ML coverages, respectively. The Si growth on Si(001) was found to occur as island formation because of the low substrate temperature (ca. 300 degrees C). Strong shape anisotropy and diffusional anistropy in the growth have been observed. On the other hand, the large coverage of Ge on Si(001) at a relatively high substrate temperature (ca. 500 degrees C) resulted in step flow growth rather than individual island formation on the terraces.
Open Access
Atomic theory of the scanning tunneling microscope
(1988) Tekman, Ahmet Erkan
The Scanning Tunneling Microscope is proven to be one of the most powerful tools for surface structure determination. Present theories are able to explain the operation of the microscope when the tip is far from the surface. For the small tip height case the atomic-scale interaction of the tip and the surface has to be included in the theory. The electronic structure of the combined system of the tip and the surface is calculated with an Empirical Tight Binding approach for graphite. It is found that in the vicinity of the tip some Tip Induced Localized States are formed. These states play an important role in the tunneling phenomenon. The contribution of these states to the tunneling current is calculated.
Open Access
Ballistic transport and tunneling in small systems
(1990) Tekman, A Erkan
Ballistic transport and tunneling of electrons in mesoscopic systems have become one of the most important subjects of condensed matter physics. The quantum point contacts and scanning tunneling microscope form the basic experimental tools in this area and have been used for understanding many features of small systems. In this work ballistic transport and tunneling in small systems are investigated theoretically. Ballistic transport through narrow constrictions is investigated for a variety of configurations. It is found that for a uniform constriction the conductance is quantized in units of the quantum of conductance (2e^/A) for long channels. The interference of waves in the constriction gives rise to the resonance structure superimposed on the quantized steps. The lack of the resonance structure in the experimental results are attributed to temperature effects and/or adiabatic transport due to tapering of the constriction. It is shown that elastic scattering by an impurity distorts the quantization of conductance. Novel resonant tunneling effects due to formation of bound states are predicted for an attractive impurity or a local widening at the center of the constriction. It is shown that the probing in scanning tunneling microscopy have very much in common with narrow constrictions. The transition from tunneling to point contact regime is explained by the vanishing effective potential barrier as a result of tip-sample interaction. For noble and simple metals it is conjectured that lateral position dependent interaction between the tip and sample leads to corrugation of the potential barrier and in turn to atomic corrugation observed by scanning tunneling microscopy. The focused field emission of electrons from point sources is analyzed in a systematical way. The effective barrier due to the lateral confinement and nonadiabatic transport through the horn-like opening are found to be responsible for focusing. The nonequilibrium nature of transport is investigated by use of Keldysh Green’s function technique. The effects of elastic and inelastic scattering are analyzed in a strictly one-dimensional geometry. The features of voltage and current probes are studied and the Landauer formulae are examined for multiprobe measurements.
Open Access
Construction of a scanning tunneling microscope and first results
(1990) Oral, Ahmet
In this thesis, construction of a Scanning Tunneling Microscope in air is explained. A step motor sample approach mechanism and a tripod scanner are used in the construction. Atomic resolution images of graphite samples are obtained in both constant current and constant height modes. Loss of trigonal symmetry in some Graphite images are also observed. This anomaly is attributed to the multiple atom tip or slipped top layer of Graphite.
Open Access
Investigation of Si1-xGex alloy formation by using STM
(1994) Oral, Ahmet
Open Access
Noise analysis of interdigital cantilevers for atomic force microscopy
(1998) Yaralıoğlu, G. Göksenin
Atomic force microscoiDe (AFM) is proved to be a powerful tool for atomic resolution surface imaging. The most crucial parts of an AFM system are the cantilever with an integrated tip and the deflection detection sensor. AFM systems measure deflections that are comparable to atomic dimensions using technicpies such as tunneling, interferometry, piezoresistive sensing and optical lever detection. Interdigital (ID) cantilevers are the most recently introduced method which makes use of its interferometric nature to improve deflection detection sensitivity. Basicallj^ ID cantilever is composed of two sets of interleaving fingers which create an optical phase grating. In this thesis, a detailed analysis of ID cantilevers will be presented. The theory underlying the o[)eration of the phase gratings with the response curves curd confirming e.xperimental results will be formulated. The noise performance of the ID cantilever will be compared to the optical lever detection method. We will present a new method for the mechaniccd noise calculation by using the analogy between electrical circuits and mechanical structures. This new method will be applied to the AFM cantilevers to calculate the noise correlation on the cantilever surface. We will also present the signal to noise ratio (SNR) calculation method on the cantilever. One of the basic problem of the all AF'M systems is the speed limitation due to single AF'M tip scanning at relatively low frequencies yielding low throughput. A direct approach to this problem is the operation of cantilever arrays instead of one cantilever. In this thesis, we will also present the electronics for cantilever arrays which increases the throughput of the AFM systems.
Open Access
Scanning probe microscopy for optoelectronic characterization at the nanoscale
(2010) Ürel, Mustafa
In this work, we propose methods for electrical characterization of nanostructured surfaces using electrostatic force and tunneling current measurements in scanning probe microscopy. Resolution smaller than 10 nm in electrostatic force microscopy (EFM) is attained and reasons for this attainment is explained in terms of the tip-sample capacitance and mechanical vibrations of tip design. Dynamic measurements are done in EFM using a lumped model for tip-sample electrostatic interaction instead of a simple tip-sample capacitance model. Surface photovoltage measurements are done and assured in EFM using frequency response techniques. Also, combining tunneling current measurements by EFM measurements, optoelectonic properties of graphene/graphene oxide samples are characterized.