Carbon nanotube electronics

Design and fabrication of MEMS-actuated devices for on-demand generation of sub-micron diameter fluidic droplets for maskless lithography and spotting applications. The main paradigm is for reservoir fluid to be squeezed through a small orifice by a pressure pulse from the MEMS-based flexure plate actuator, a thin suspended membrane released by a deep etch from the backside of the handle wafer. The active layer is lead zirconate titanate (PZT), a strong piezoelectric material, where advances in sol-gel-derived thin film deposition allow for wafer scale PZT integration. Flexure plate deflection (i.e. the pressure pulse) comes from transverse, or d 31-mode, actuation where lateral contraction of the PZT induces a bending of its suspended dielectric underlayers. The droplet generator will be a monolithic device fabricated with standard thin-film and Si micro-machining technology. As such, thousands of such droplet generators could be fabricated on a single wafer, providing massive parallel droplet dispensing. This, combined with the fact that a droplet can be dispensed every 10 – 100 usec, gives this technique superior throughput capabilities to alternative, high resolution spotting technologies such as dip-pen, electron-beam, and nanoimprint lithography.

Development of magnetic quantum-dot cellular automata (MQCA) for logic operations (with David Carlton). Previous work on thermally-stable (100 – 200 nm lithographic dimension), patterned nanomagnets has shown that basic majority gates are achievable through specific orientation of dipolar interactions between magnetic patterns and global clocking fields [1]. Although MQCA logic is attractive for its low power consumption, the demonstrated logic operations seem to be limited to relatively long magnetic relaxation times (~1 nsec). We extend the design of MQCA to be built from much smaller, sub-20 nm nanomagnets. Although such magnets would typically be labeled as "superparamagnetic", they are actually thermally stable for tens to hundreds of microseconds, much longer than would be necessary for a logic operation to be executed. Logic operations on the magnetic precessional timescale (~50 psec) should be possible if magnetization dynamics, rather than relaxation into energy minima, are employed as the driving functionality. We are simulating such dynamical interactions and exploring the effects of nanomagnet shape, orientation of nanomagnet ensemble, and precessional dynamics timed through local clocking fields through object-oriented micromagnetic code [2].

[1] A. Imre, G. Csaba,L. Ji, A. Orlov, G. H. Bernstein, and W. Porod, Science 311, p.205 (2006).
[2] M. J. Donahue and D. G. Porter, OOMMF User's Guide, Version 1.0 Interagency Report No. 6376, NIST, Gaithersgurg, MD (1999).

Spin-state Readout for Silicon-based quantum computers

Spin states of bounded donor electrons at low temperatures can be utilized as qubits in various silicon-based quantum computation schemes. Two crucial aspects of such schemes are the controlled placement of donor atoms in silicon and reliable electron spin-state read-out.

Single ion implantation [1] or scanning tunneling microscopy (STM) based hydrogen lithography [2] can be used to achieve donor placement with atomic precision. The latter is based on the electron stimulated desorption (ESD) of a hydrogen-terminated silicon surface with a STM tip, and subsequent exposure to PH3 for phosphorous placement. However, the fabrication of electrical contacts for these nano-scale donor patterns is challenging, due to the limited field of view of the STM used for ESD. In this project, pre-implanted As contact arrays are designed and fabricated for the electrical characterization of these 1 and 2D donor patterns.

Spin-state read-outs for silicon-based quantum computers can be achieved by using field-effect transistor (FET) architectures, with the donor atoms positioned in the channel of the FET. The donor electron spin states can be detected by exploiting the difference in singlet and triplet scattering cross sections of the spin-polarized source-injected channel electrons and that of the bounded donor electrons [3]. Accumulation mode field-effect transistors have been fabricated for this purpose, and characterization and measurements are currently in progress.

[1] T. Schenkel et al., J. Vac. Sci. Technol. B, Vol. 20, No. 6.

[2] T.-C. Shen et al., J. Vac. Sci. Technol. B, Vol. 22, No. 6.

[3] R. N. Ghosh and R. H. Silsbee, Physical Review B, Vol. 46, No. 19.