(a) Schematic drawing of the instrument configuration. The SEM column is in the center and images the two tips of the double-tip STM and the sample surface. The currents through the STM tips and are measured with two preamplifiers biased with and , respectively. The master PC controls the STM tip 1, the SEM, and the sample-to-ground connection. The slave PC controls the STM tip 2. To assure a synchronous measurement of STM 1 and STM 2, PCs communicate start and stop signals. (b) SEM image of both STM tips in tunneling contact with a Si(111) surface. The visible structures on the surface are bunches of atomic steps formed during heating of the Si sample.
(a) SEM image of a STM tip in tunneling contact with the GaAs substrate close to an array of nine nanocolumns. The side length of a single nanocolumn is 100 nm and the height about 200 nm. (b) characteristics of a 100 nm column measured by STM probing. Characteristic resonant tunneling peaks (at and ) are clearly visible. The PVRs are 2.2 on the left side and 1.6 on the right side, respectively.
spectra for positive voltages of (a) a 45 nm RTD and (b) a 40 nm RTD. The PVR is a measure of the quality of the devices. The PVR in (a) was determined as 2.76 and in (b) 3.08.
Gate effect measurements with the help of the second STM tip. (a) Schematic side view of the measurement setup. The RTD is contacted by tip 1 which can be seen as a source or drain electrode. The second STM tip (gate electrode) is placed at a distance of from the double barrier of the RTD and lifted 30 nm from the substrate surface. (b) Top view of the measurement setup taken with SEM. (c) spectra of a 70-nm-sized RTD with different gate voltages. External electric field shifts the ground energy state of the electrons in the RTD as shown in the inset.
(a) STM image of atomically resolved reconstructed surface. (b) Tip-to-tip tunneling current for three different tip heights: the tips were moved toward the surface by , 3, and for probe distances of .
Voltage drop for a semi-infinite 3D conductor (a) and a 2D sheet (b) in a two-point probe arrangement. To derive the voltage drop between a current source and a current drain, a superposition of and has to be calculated. This is illustrated for the example of the 3D case (c).
A simple scheme to calculate the tip-to-tip tunneling current. (a) Electrical circuit diagram of the two-point probe measurement. The applied voltage is composed of the tunneling voltages and and the voltage drop originating from the sample resistance . (b) Three-layer model of the measured Si sample. The surface is considered as a 2D conductor, whereas the space charge layer and the bulk are treated as a 3D conductor.
The tip-to-tip tunneling current calculated using the 2D two-point probe model in comparison to the measured data points. The free parameters , , and were fitted to obtain the best match with the data points.
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