Summary of Post Doctoral Research Work (April 2009- November 2010)
Nanosystems Integration Laboratory, Research Institute of Electronics, Shizuoka University.
Group leader:- Prof. Hiroshi Inokawa (view staff members here)
Since the early discovery tunneling of single electron and Coulomb Blockade phenomena in the 1980s, it was envisioned by the scientists worldwide that if the size of the quantum dots is reduced to several nanometers, it is highly possible to produce single electron transistor (SET) that works above liquid nitrogen temperature, and thus will bring the revolution in the VLSI industry. Since then SET has been a hot area of research. The breakthroughs in nanotech and their combination with the already established state of the art VLSI industry gives a lot of hope to SET.
Usually electrons move continuously in common MOS Transistors. However, as the size of the system goes down to nanoscale, the energy of the system becomes quantized, that is, the process of charging and discharging becomes discontinuous. The energy for one electron to move into the system is Ec = e^2/2C. Where C denotes the capacitance of the system. This Ec is called Coulomb Blockade energy, which is the repelling energy of the previous electron to the next electron. This energy can be considerably high for a tiny system, as a QD. Thus the electrons in such a system cannot move simultaneously, but pass through one by one. If two QDs are joined at a point and form a channel, it is possible for an electron to pass from one dot over the energy barrier to move over to the other dot, this phenomenon is called quantum mechanical tunneling. In order to overcome the energy barrier Ec, the applied voltage on the quantum dots (V/2) should be V > e/C. In order to observe Coulomb Blockade and tunneling the energy that an electron assumes must be higher than the scattering thermal energy, i.e. e^2/2C > kT. (Where k denotes Boltzmann constant).
This equation clearly shows that if the capacitance is low enough, and if the size of QDs can be reduced to several nm, Coulomb blockade and tunneling can be observed at room temperature (RT) as well. In general an SET consist of two tunnel junctions which are connected by a Coulomb island, whose potential can be controlled by the gate capacitance Cg. A voltage Vg applied to the gate controls the opening and closing of the SET as shown in the Figure below. Rs and Cs and Rd and Cd denote the resistances and capacitances of the two tunneling junctions respectively.
It is due to this unique structure that SET has many prospective potential applications, viz. Low power consumption circuits, high sensitivity, high switching speed, high packet density, etc. Also, It is due to these, that SET has attracted considerable attention towards its fabrication and industrial realization.
