AVS2001 Session NS-ThA: Quantum Dots & Single Electronics

Thursday, November 1, 2001 2:00 PM in Room 133

Thursday Afternoon

Start	Invited?	Item
2:00 PM	Invited	NS-ThA-1 Self-Organized Growth of Semiconductor Nanostructures G. Springholz (University of Linz, Austria) Strained-layer heteroepitaxy has evolved as a novel method for direct synthesis of self-assembled quantum dots based on the Stranski-Krastanow growth mode where nano-scale 3D islands spontaneously form on the surface of a thin wetting layer. In multilayers, the buried dots tend to influence the dot nucleation in the subsequent layers due to the existence of long range elastic interactions. As a result, vertical and lateral correlations within the dot ensembles are formed, which can lead to a lateral ordering and size homogenization of the dots. Here, it is shown that for various materials systems, different correlated structures are formed depending on the elastic anisotropy of the materials and depending on the growth orientation. As a most prominent example, in IV-VI semiconductor dot superlattices a nearly perfect lateral ordering and a fcc-like ABCABC... vertical stacking of the dots is obtained, and the spacing between the dots can be tuned continuously just by changing of the superlattice period. The basic mechanisms and the limits of the ordering process is discussed on the basis of theoretical calculations and Monte Carlo growth simulations.
2:40 PM		NS-ThA-3 Applications of Quantum Dots in Photonic and Resonant Tunneling Devices L. Samuelson, M. Borgstrom, T. Bryllert, B. Gustafson, S. Jeppesen, M.-E. Pistol, W. Seifert, V. Zwiller (Lund University, Sweden)
3:00 PM		NS-ThA-4 Single-electron Transistor Based on a 7 nm Gold Particle with Carbon Nanotube Leads C. Thelander, M.H. Magnusson, K. Deppert, L. Samuelson (Lund University, Sweden); P.R. Poulsen, J. Nygaard, J. Borggreen (Niels Bohr Institute, Denmark) We have used CVD grown carbon nanotubes to electrically contact an individual 7 nm gold particle by scanning probe manipulation. The result was a single-electron transistor showing a periodic modulation of the current as a function of gate voltage for temperatures up to ~ 200 K. Based on a theoretical fit we conclude that the particle was responsible for the main features of the electron transport, whereas charging effects in the nanotube leads only appeared as a fine-structure. This interpretation could later be verified when the gold particle was removed and the two nanotubes were moved into electrical contact.
3:20 PM	Invited	NS-ThA-5 Control Over Spin Effects in Quantum Dot Structures S. Tarucha (University of Tokyo, Japan) Experimental studies on a few-electron spin state in semiconductor quantum dots and double quantum dots will be presented. Control and electronic properties of a few-electron spin states and a technical approach for tuning an exchange coupling between two different spin states will be discussed.
4:00 PM	Invited	NS-ThA-7 Probing the Dependence of the Spin Splitting in Quantum Dots on Residual Disorder M. Morgenstern (Hamburg University, Germany); V. Gudmundsson (Science-Institute, Iceland); R. Wiesendanger (Hamburg University, Germany) A scanning tunneling microscope is used to induce a quantum dot into the InAs(110) surface. This quantum dot provides the unique possibility to be moved in a controlled manner across the surface. Thus charged impurities of the substrate are positionable in the quantum dot area (diameter 100 nm). Working at low temperatures (6 K) in magnetic field (6 T) allows to detect the energies of the spin split states corresponding to different Landau indices. While the state energies closely follow the disorder potential in the quantum dot, the energy difference between spin split states does not. From comparison with detailed Hartree-Fock calculations we conclude that this behaviour is directly guided by the non-local character of the exchange-interaction.
4:40 PM	Invited	NS-ThA-9 Electronic Devices Using Single Electron Effects S. Tiwari (Cornell University) The single electron effect is a consequence of reduced capacitance in confined islands when dimensions are reduced, usually to the sub-30 nm range. Small capacitances, of the order of aF’s, result in a measurable discreteness in the transfer of electrons through the islands because of the large electrostatic energy needed for transfer of charge. In semiconductors, single electron effects occur together with strong quantum-confinement effects due to the smaller density of states. In a single electron transistor, the discrete transfer of the charge is modulated by a gate voltage, and circuits analogous to CMOS can be fabricated. In most single electron memories, the effect of the single electron charge influences transport in a field-effect channel through screening, i.e., discreteness effects are coupled to the traditional field-effect of the transistor. While powerful demonstrations of room temperature operation of single electron transistors, single charge transfer devices, and simple gates with gain have been made, the use of the devices in general purpose electronics is limited by large impedance, low currents, and fluctuation effects. One particularly unique use of single electron transistor has been in charge profiling due to the strong intrinsic charge sensitivity. Memories based on single electron effects, however, are finding wider appeal because of large improvement in power, speed, voltage, and reliability characteristics over traditional non-volatile memory alternatives and their strong compatibility with present-day practice of silicon microelectronics. Such memories have been demonstrated at large dimensions (100’s of nm) where numerous discrete nanocrystal islands are employed as well as in the ultimate limits of field-effect when device dimensions reduce to nearly 10 nm in dimension. We will discuss the properties of the single electron device structures and relate them to the underlying physics.