Papers

AVS1996 Session NS+MI-TuA: Novel Nanoscale Devices and Memories

Tuesday, October 15, 1996 2:00 PM in Room 202 A/B

Tuesday Afternoon

Start	Invited?	Item
2:00 PM		NS+MI-TuA-1 Protein-based Computers and Artificial Retinas R. Birge, C. Martin, J. Stuart, J. Tallent, E. Tan (Syracuse University) This presentation will explore the use of the protein, bacteriorhodopsin, in optical three-dimensional memories, parallel associative processors and artificial retinas. Three-dimensional memories store information in a memory volume element, and provide approximately a 300-fold improvement in memory storage capacity over current technology. The comparative advantages and disadvantages of holographic, two-photon and sequential one-photon volumetric architectures will be discussed. The associative memory operates in a fashion somewhat analogous to the human brain and responds to input data by finding (in a few nanoseconds) the closest match within the data base and feeding this information, and any associated information, to the output. Such a memory is critical to the development of artificial intelligence. The artificial retina relies on the use of a semiconductor charge senitive array to spatially monitor light-induced charge separation in a thin-film of oriented bacteriorhodopsin. After reaching equilibration (~33 ms), the artificial retina is only sensitive to changes in light intensity and provides an excellent motion sensor. A key problem associated with the protein-semiconductor interface has been solved by replacing the native metal ions in the protein with large organic cations. These cations matintain functionality of the protein, but do not poison the semiconductor surface. The use of site directed mutagenesis to improve the properties of the protein for specific applications will also be discussed. Although working prototypes of the above devices have been developed, a number of cost/performance and architectural issues must be resolved prior to commercialization.
2:40 PM		NS+MI-TuA-3 Single-Atomic Point-Contact Devices Fabricated with an AFM E. Snow, P. Campbell (Naval Research Laboratory) We have used AFM anodic oxidation guided by in situ device measurements [1] to fabricate metal/oxide structures in which the current is constricted down to a single, atomic-sized conducting channel [2]. Such structures exhibit a room-temperature conductance which is quantized in discrete units of ~ 2e\super 2\/h. The AFM anodization was first used to define metal nanowires from a thin Al film. In situ electrical measurements were then used as a guide to anodize through their cross section and form atomic-sized metallic point contacts. When the anodization reduced the conductance of these structures below ~ 5 x 10\super -4\ S, it starts to drop in discrete steps of ~ 2e\super 2\/h. In some devices we are able to stabilize the conductance at a value near 2e\super 2\/h which corresponds to a single-atomic conducting channel. Similar experiments on Ti structures result in a continuous decrease of the conductance. This nondiscrete behavior is caused by the large series resistance and the small oxide barrier height of the Ti-TiO\sub x\ system. The resistance of these Ti-TiO\sub x\ lateral tunnel junctions is stable and can be tuned continuously with increased anodization. The intrinsically small capacitance and continuously tunable resistance of such structures make them attractive for lateral tunnel-junction devices such as single-electron transistors. [1] E.S. Snow and P.M. Campbell, Science 270, 1639 (1995). [2] E.S. Snow, D. Park, and P.M. Campbell, Appl. Phys. Lett. (in press).
3:20 PM		NS+MI-TuA-5 Lateral, Nanometer-scale Tunneling Tips with Integrated Microactuators B. Reed, N. MacDonald (Cornell University) Micromachined, 20 nm diameter lateral tips with integrated microactuators have been fabricated and characterized. The lateral tips are formed with a new process involving thermal oxidation of silicon cantilevers. Devices with multiple tips and multiple microactuators can be used for positioning tips within electron tunneling range of each other. The tips can be made singly, in opposing pairs, or in groups of four coming together to a point. The same fabrication process can make silicon "nanothreads:" suspended beams 20 \mu\m or more in length and 50 nm or less in diameter. This technology may be used for a wide variety of experiments in electron tunneling, field emission, surface physics, and nanomechanics. Applications involving multiple, independently moving tips are of particular interest. We form the micromachine components with the SCREAM process for high-aspect-ratio single-crystal-silicon microactuators. The addition of integrated tips involves only minimal design constraints--therefore we can use existing SCREAM mechanical designs and techniques with the new nanostructures.
3:40 PM		NS+MI-TuA-6 GMR Devices J. Daughton (Nonvolatile Electronics, Inc.) Giant Magnetoresistance Ratio (GMR) materials with a broad spectrum of magnetoresistive properties can now be produced uniformly and reproducibly less than 8 years after their discovery. Extremely thin (1-10 nm) films of ferromagnetic and non-ferromagnetic materials form multilayer GMR structures in several forms, including spin valves, symmetric spin valves, unpinned sandwiches, and anti-parallel coupled multilayers. The magnetoresistive properties of these devices is due to the spin polarization of electrons in ferromagnetic conductors. In the past year a new spin dependent structure has been demonstrated by several organizations which uses tunneling from one ferromagnetic layer to another through a thin insulating barrier. The current state of development of these materials is reviewed, and their suitability for applications in thin film read heads, random access memory, magnetic field sensors, and other devices is assessed. Published laboratory demonstrations of these devices are reviewed. Magnetic field sensor products using GMR materials are described.
4:20 PM		NS+MI-TuA-8 Electron Transport Studies of Magnetic Multilayers Using Nanofabricated Point Contacts R. Louie, S. Upadhyay, R. Buhrman (Cornell University) Nanofabrication techniques are used to fabricate three-dimensional thin film "nanocontacts" between a Cu film and a sputtered superlattice consisting of ferromagnetic and noble metal layers, with the minimum contact diameter ranging from 3 to 10 nm. The geometry and scale of these point- contact-like ballistic devices are such that they can be effectively employed for the detailed study of electronic transport within the multilayer as a function of magnetic field, temperature and, in particular, electron energy as determined by the voltage bias. These nanocontact structures have been found to exhibit non-hysteretic giant magnetoresistance (GMR) effects, with the amplitude of this GMR signal decreasing nearly linearly with increasing bias, at low temperatures. We vary layer materials, thicknesses, deposition techniques, and number of bilayers. The non-ohmic aspect of the resistances of these devices appears consistent with a spin flip scattering mechanism within the multilayers which becomes more effective with energy. We will discuss these and related findings obtained from these nanocontact studies.
4:40 PM		NS+MI-TuA-9 Fabrication of Spin-Current Field-Effect Transistor Structures A. Cabbibo, J. Childress, S. Pearton (University of Florida, Gainesville); F. Ren, R. Kopf (Bell Laboratories) It has been theoretically proposed that a narrow gap semiconductor= field-effect transistor with ferromagnetic contacts for injection of= spin-polarized electrons may be used for current modulation.[1] We have= fabricated GaAs/AlGaAs and InGaAs/AlInAs high electron mobility transistor= structures, where the normal AuGeNi source and drain contacts are replaced= by sputter-deposited Fe contacts. Mesa fabrication to allow the Fe to= contact the two-dimensional electron gas formed at the interface between= the doped AlGaAs or AlInAs and the lower bandgap (GaAs or InGaAs) undoped= channel was achieved by either wet chemical etching or angled Ar+ ion= milling. TiPtAu (1 \mu\m channel length) was employed in both materials= systems as a gate contact. The Fe is found to form a rectifying contact to= the GaAs channel devices, preventing current injection, but produces a= nearly ohmic contact to InGaAs. Improved, bilayer magnetic contacts are= currently under investigation for the InGaAs/AlInAs HEMT structures in= order to produce complete ohmic behavior and establish the feasibility of= polarized current injection in these devices. The post-processing magnetic= properties of the contacts will be discussed. [1] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).