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Current-induced local oxidation |
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A novel route is introduced for oxidizing thin metal films with nanometer-scale resolution in air. By locally subjecting Ti films to high in-plane current densities, metal-oxide tunneling
barriers are formed in a self-limiting fashion. The oxidation is triggered by current-induced atomic rearrangements and local heating. At the final stages of the barrier formation, when only atomic-scale
channels remain unoxidized, the oxidation rate decreases drastically while the conductance drops in steps of about the conductance quantum. This behavior gives evidence of ballistic transport and a superior
stability of such metallic nanowires against current-induced forces compared with the bulk metal. We used the current-induced local oxidation process to fabricate a single-electron transistor, which
exhibits a Coulomb staircase at room temperature. |
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The current-induced local oxidation (CILO) process |
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The CILO process is based on thin Ti films as illustrated in the sketch and the AFM picture above. Subsequently, oxide notches are via AFM oxidation such that a narrow metallic gap is formed. |
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Upon applying a constant bias voltage across the gap under ambient conditions (in air), the measured current starts to decrease rapidly as a function of time, stabilizing seconds later at a two
orders of magnitude lower value. In the meantime, the gap has been closed by a current-induced nanometer-scale oxide barrier. After the barrier formation the current-voltage curve became strongly nonlinear
(red, top right), while it was linear before (blue). The CILO process occurs at current densities of 107 A/cm2
and is triggered by current-induced atomic rearrangements and local heating. |
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Quantum-size effects |
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The conductance-time curve provides a measure for the oxidation rate during the CILO process. At the final stages of the barrier formation, when only nanometer-scale channels within the gap
remain metallic, the oxidation rate decreases drastically and the conductance drops in steps of about 2e2/h (black, top left and right). These steps give evidence of conductance quantization as a
consequence of ballistic transport. The reduced oxidation rate indicates a superior stability of such metallic nanowires against current-induced forces compared with the bulk metal. Thus, the CILO process
provides a striking example, where quantum transport influences a chemical reaction. |
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Fabrication of a single-electron transistor |
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By using the CILO process in combination with AFM oxidation, we fabricated a Coulomb island separated by two tunneling barriers from large-area contacts. Using the doped Si substrate
underneath the metal film as gate electrode, this device can be employed as a single-electron transistor. |
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The current-voltage curve of a smaller device with an island diameter of less than 20 nm (see inset) exhibits several equidistant steps as a consequence of Coulomb blockade at room temperature. |
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