Ismet I. Kaya

Characterization of spinel iron-oxide nanocrystals grown on Fe whiskers

Passive iron-oxide nanocrystals are grown on Fe(100) and Fe(110) facets of single-crystal Fe whiskers. Transmission electron microscopy and electron diffraction characterize the oxide spinel structure and their epitaxial growth on Fe whiskers. Iron-oxide nanocrystals grown on Fe(100) facets have sizes close to that of the single magnetic domain Fe3O4 particles, which is supported by our preliminary magnetic force microscopy measurement at room temperature.

Scanning Hall probe microscopy of ferromagnetic structures

Abstract A state-of-the-art scanning Hall probe microscope (magnetic field sensitivity : spatial resolution: 30 : 0.8 μm—300 : 0.25 μm) has been used to image a range of ferromagnetic media. The technique is non-invasive and yields quantitative profiles of the stray fields near the sample surface as illustrated by images of a standard magnetic reference sample, permalloy nanostructures and colossal magnetoresistive perovskites. Rapid scanning (∼ 1 frame/12 sec) has also been used to study the domain reversal in a thin Ni film.

The effect of a SiC cap on the growth of epitaxial graphene on SiC in ultra high vacuum

We demonstrate a technique to produce thin graphene layers on C-face of SiC under ultra high vacuum conditions. A stack of two SiC substrates comprising a half open cavity at the interface is used to partially confine the depleted Si atoms from the sample surface during the growth. We observe that this configuration significantly slows the graphene growth to easily controllable rates on C-face SiC in UHV environment. Results of low-energy electron diffractometry and Raman spectroscopy measurements on the samples grown with stacking configuration are compared to those of the samples grown by using bare UHV sublimation process.

Dual-probe scanning tunneling microscope for study of nanoscale metal-semiconductor interfaces

Using a dual-probe scanning tunneling microscope, we have performed three-terminal ballistic electron emission spectroscopy on Au∕GaAs(100) by contacting the patterned metallic thin film with one tip and injecting ballistic electrons with another tip. The collector current spectra agree with a Monte-Carlo simulation based on modified planar tunneling theory. Our results suggest that it is possible to study nanoscale metal-semiconductor interfaces without the requirement of an externally-contacted continuous metal thin film.

Spatial evolution of the generation and relaxation of excited carriers near the breakdown of the quantum Hall effect

Abstract We have measured the generation and relaxation of excited carriers along their drift direction near the breakdown of the quantum Hall effect (QHE). The dissipative resistivity ρ xx ( x ) at current densities close to the critical value for the QHE breakdown was measured as a function of the distance x from the electron injection at x =0. By injecting “cold” electrons into constrictions at supercritical current levels, the evolution of the breakdown along the drift direction was monitored. After a smooth increase of the resistivity with the drifting distance, an avalanche-like rise towards a saturation value occurs. Drastic changes of the resistivity profiles with the applied current were found in a narrow range around the critical current. The observed behavior is attributed to impurity-assisted tunneling between Landau levels. By injecting hot electrons (excited in a periodic set of constrictions) into a region with subcritical current density, the relaxation process was analyzed. Inelastic relaxation lengths with typical values in the range from 0.3 to 4 μm were found, which agree within 10% with the elastic mean free path determined from the Hall mobility at zero magnetic field. We conclude that the energy relaxation process is triggered by scattering at impurity potentials.

Dynamics of nonequilibrium electrons at the breakdown of the quantum Hall effect

Abstract We have analysed the energy excitation and relaxation of hot electrons close to the breakdown of the quantum Hall effect. Hot electrons were generated by a periodic set of constrictions (parallel microscopic trenches or antidot arrays). For the relaxation experiments, hot electrons are injected into two-dimensional electron systems (2DES) of various mobilities, with Hall fields below the breakdown value. The resistivity of the 2DES is measured as a function of the distance from the injection front. The characteristic decay length of the resistivity was found to strongly increase with current in a rather narrow range of currents until the dissipation persists over the entire sample at the breakdown. The results are compared with calculations on the basis of a nonequilibrium between the excitation and relaxation of hot electrons. Energy relaxation lengths from 0.3 to 3 μm were deduced, which are comparable to the mean free path. Thus, the inelastic scattering, which is responsible for the breakdown of the QHE, is strongly related to elastic Coulomb scattering. Spatially resolved measurements of the electron heating were performed in samples with antidot and wire arrays, and with macroscopic constrictions. An avalanche-like heating of electrons could be observed in the antidot array, but the data have not yet been reconciled with a quantitative picture.

Nonequilibrium Quantum Transport in Antidot Arrays

We have studied the onset of dissipation in the quantum Hall effect (QHE) in different antidot arrays (periodic or aperiodic antidot arrays, and single lines of antidots), patterned on a two-dimensional electron system. In periodic arrays, the breakdown current is systematically reduced with increasing antidot density. In comparable aperiodic arrays, the breakdown current is markedly lower, and the electron temperature starts rising at lower currents than in periodic arrays. Single lines of antidots across the current channel cause only a small reduction of the breakdown current, indicating the relevance of avalanche electron heating. In arrays of very small antidots, a suppression of the electron heating due to additional scattering and a complete absence of hot-electron-induced hysteresis in the current–voltage characteristics were observed.

In-situ focused ion beam implantation for the fabrication of a hot electron transistor oscillator structure

Recent advances using in situ focused ion beam implantation during an MBE growth interruption have been exploited to fabricate planar GaAs hot electron structures without the need for shallow ohmic contacts. This novel fabrication route shows a very high yield and has been used to demonstrate a prototype high-frequency oscillator structure based on electron multiplication in the base layer. Existing devices show transfer factors in excess of unity as well as reversal of the base current at high injection levels, which are the prerequisites for oscillator action. Future improvements in device design are discussed.

Foamlike 3D Graphene Coatings for Cooling Systems Involving Phase Change

Boiling is an efficient heat-transfer mechanism because of the utilization of latent heat of vaporization and has the potential to be used for cooling high-power electronic devices. Surface enhancement is one of the widely used techniques for heat-transfer augmentation in boiling systems. Here, an experimental investigation was conducted on chemical vapor deposition-grown three-dimensional (3D) foamlike graphene-coated silicon surfaces to investigate the effect of pore structures on pool boiling heat transfer and corresponding heat-transfer enhancement mechanisms. 3D graphene-coated samples with four graphene thicknesses were utilized along with a plain surface to investigate boiling heat-transfer characteristics and enhancement mechanisms. A high-speed camera was used to provide a deeper understanding of the bubble dynamics upon departure of emerging bubbles and visualize vapor columns in different boiling regimes. On the basis of the obtained results, in addition to interfacial evaporation, mechanical resonance of the 3D structure had also a considerable effect on vapor column formation. The results indicated that there is an optimum thickness, which exhibits the best performance in terms of boiling heat transfer.

Tuning of nanogap size in high tensile stress silicon nitride thin films

High tensile stress suspended structures are demanded for high mechanical quality factor applications. However, high tensile stress causes distortion of the original shapes by contracting, buckling, and bending the suspended structures. We demonstrate a method to compensate for the shape deformation of suspended structures due to intrinsic tensile stress after they are released. With a new design, the distance between two suspended structures after wet etch can easily be tuned by a single fabrication beyond the lithographic resolution limits. The technique is simulated by finite element analysis and experimentally implemented to demonstrate a gap tuning capability with 2.4 nm standard error.