Index of content:
Volume 91, Issue 2, 15 January 2002
- DEVICE PHYSICS (PACS 85)
91(2002); http://dx.doi.org/10.1063/1.1421217View Description Hide Description
Quantum-dot cellular automata (QCA) may provide a novel way to bypass the transistor paradigm to form molecular-scale computing elements. In the QCA paradigm information is represented by the charge configuration of a QCA cell. We develop a theoretical approach, based on the density matrix formalism, which permits examination of energy flow in QCA devices. Using a simple two-state model to describe the cell, and an energy relaxation time to describe the coupling to the environment, we arrive at an equation of motion well suited to the quasi-adiabatically switched regime. We use this to examine the role of power gain and power dissipation in QCA cells. We show that QCA cells can exhibit true signal power gain. The energy lost to dissipative processes is restored by the clock. We calculate the power dissipated to the environment in QCA circuits and show that it is possible to achieve the ultralow levels of power dissipation required at molecular densities.
91(2002); http://dx.doi.org/10.1063/1.1427145View Description Hide Description
Electrostatic force sensitive scanning probe microscopy is used to quantify dc and ac transport properties of an active Schottky barrierdiode. Scanning surface potentialmicroscopy (SSPM) of the laterally biased device is used to quantify the potential drop at the metal–semiconductor interface. Ramping the lateral bias allows the local voltage and characteristics of the diode to be reconstructed. Scanning impedance microscopy (SIM) demonstrates the phase and amplitude change of voltage oscillations across the interface. The frequency dependence of voltage phase shifts across the interface defines the appropriate equivalent circuit for the reverse biased junction. Excellent agreement between junction capacitance obtained from SIM measurements and impedance spectroscopy is demonstrated. Variation of the dc component of lateral bias in SIM yields the local capacitance–voltage characteristics of the junction. SIM contrast of grain boundaries in p-doped silicon was interpreted in terms of minority carrier generation in the interface region. The combination of SSPM and SIM provides an approach for the quantitative analysis of local dc and ac transport properties which were demonstrated for a Schottky diode but can be applied to any semiconductor device.