Coulomb blockade is based on the charging energy of a small capacitor and allows the transport of single electrons. If a charge Q is brought on a capacitor with capacitance , the stored electrostatic energy of the capacitor is given by:

If this charge is a single electron, the charging energy is normally very small compared to the thermal energy. This energy will only be significant in extremely small capacitors. One implementation of such a small capacitor is a small island connected with two electron reservoirs by tunnel barriers. Electrons can only be transferred from one reservoir (source) to the other (drain) by tunneling processes. If electrons are to tunnel onto the island, the capacitor must be charged. Therefore a threshold bias voltage is needed for electron transport. Below this voltage, electron transport is suppressed, as shown in fig. 1, and no current is observed. Only if a larger voltage is applied electrons can tunnel onto the island and further to the other reservoir. In this case only single electron transport occurs. The suppression of the current at low bias is called "Coulomb blockade" and the region below the threshold voltage is called the "Coulomb blockade region". Figure 1: Electrical characteristics of the Coulomb blockade

The potential of the island can be changed by applying a voltage to an external gate electrode. When the voltage on the electrode is varied, periodic oscillations in the current through the island, so called Coulomb oscillations with a period are observed as shown in figure 2. In this way, a switch is realized which has large potential for applications in future electronics. Figure 2: Drain current as a function of the voltage applied to an external gate: characteristic Coulomb blockade oscillations

During the last years we have fabricated several silicon single and multiple dot structures, in both mono- and polycrystalline silicon on insulator (SOI) material. The technology is based on electron beam lithography, dry etching and a subsequent size reduction by controlled thermal oxidation [1,2]. Electrical characterization of single dot SET-structures at 4.2 K showed the distinct single electron charging effects, as shown in figure 1. [3,4,5,]

Figure 1,2: Experimental Coulomb blockade characteristics in source-drain current and Coulomb blockade oscillations

Double dot structures, e.g. as shown in the top half of figure 3, are suitable for use as a single electron pump. At a proper back gate voltage the dependence of the source-drain current on both side gate voltages, I(Vg1,Vg2), shows a clear pattern which is expected from a 2 dot system [6]. In this pattern, the charging diagram I(Vg1,Vg2), the principle of pumping electrons through the device can be demonstrated and the capacitance of the structures can be estimated. From these values it is concluded, that the geometry of the electrically active structure comes close to the actually fabricated double dot structure.
Figure 3: A pair of adjacent Si-double dots on SiO2 after dry etching. The structure consists of 4 dots with one side gate per dot to adjust the energy levels in each dot

A pair of adjacent double dots forms a basic building block for a quantum cellular automata (QCA) [7]. Such a device is shown in figure 3. The devices were electrically characterized at the appropriate backgate voltage where both doped structures independently show clear double dot characteristics. Figure 4 shows the drain current of the top and the bottom double dots as a function of the voltage applied to one of the bottom gates. The bottom double dot shows clear Coulomb blockade oscillations. The current through the top double dot exhibits two distinct features [8]. First, it shows broad peaks with a large period, which are Coulomb blockade oscillations caused by the small capacitive coupling between the top double dot and the bottom gate. Second, a change in conductivity correlated to the transport in the bottom double dot can be observed: each time the number of electrons on the bottom structure is changed, indicated by a peak in the bottom current, the potential of the top double dot changes, resulting in a change in its conductivity. The demonstration of this correlated electron transport [8,9] is an important step towards possible applications of such structures in future electronic devices. Ongoing research in this area includes the investigation of the influence of back ground charges on the characteristics of the single electron devices using a low temperature electron microscope. Future work will also focus on a further reduction of the size of the structures in order to observe new effects due to the stronger coupling between different dots and the gates. One way to obtain smaller and denser structures would be the use of calixarene resists.
Figure 4: Drain current through the top and the bottom double dot as a function of the voltage applied to one of the bottom gates. Between the Coulomb blockade peaks of the number of electrons on the double dots is fixed

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[6] C. Single, R. Augke, F.E. Prins, and D.P. Kern, Semicond. Sci. Technol. 14, 1165 (1999)
[7] P.D. Tougaw, C.S. Lent and W. Porod, J. Appl. Phys. 74, 3558 (1993)
[8] C. Single, R. Augke, F.E. Prins, D.A. Wharam, D.P. Kern, accepted for publication in Superlattices and Microstructures
[9] A.O. Orlov, I. Amlani, I.G. Toth, C.S. Lent, G.H. Bernstein, G.L. Snider, Appl. Phys. Lett. 73, 2787 (1998)