Nanoelectronics Research Activities
The current stage of development of electronics is characterized by extreme miniaturization with some circuit elements reaching the size of tens of nanometers. In this situation many physical effects that were previously unimportant start to play an adverse role preventing further progress of electronic technology. In the last 5-10 years, devices that use the electron spin (as well as its charge) for information processing have been proposed and recently new techniques for controlling the spin or magnetization using electric fields and currents (as opposed to magnetic fields) have been discovered. Electric control of spins has several advantages: it is easier to create electric fields than magnetic fields, especially in nanosize structures, and it removes many problems of integration with existing semiconductor technology. The term spintronics was coined for this type of technology.
We have ongoing research programs on spin injection from a magnetic material into nanoscale metals and semiconductor nanowires and have developed non-local techniques to produce a spin current with zero charge current. Pure spin currents are a key ingredient in the realization of spin transistors that have the potential advantage of storing more data in less space with less power consumption using less costly materials. A second advantage of a spin transistor is that the spin of an electron is semi-permanent and can be used as means of creating cost-effective non-volatile solid state storage devices that do not require the constant application of current to retain their state. This is one of the technologies being explored for Magnetic Random Access Memory (MRAM). In addition, passing current through a magnetic material can create a spin-polarized current that can be used to transfer spin angular momentum to another nanoscale magnetic material and reversibly switch the orientation of the magnet's moment. This spin-transfer effect may also result in the production of commercially viable MRAM (T. Crawford, Y. Bazaliy, & R. Webb). Switching magnetic materials at high frequencies with DC currents could potentially engender a whole new class of nanoscale spintronic devices, from self-generating GHz nano-oscillators to giant magnetoresistance sensors. In particular, we are studying the coherence of the spin transfer effect in the time domain to ascertain the time scales over which this physics can be employed for device applications.
It has recently become clear that there are numerous genera of bacteria that produce free electrons as a normal byproduct of metabolism. Physics & Astronomy professor S. Crittenden is investigating nanoscale mechanisms of electron transport in these electrogenic bacteria. Multiple researchers have shown experimental data suggesting that pili (hairlike filaments) produced by some of these genera are Ohmic conductors. This is unusual for proteins, the building blocks of the pili, and suggests that nature may have constructed its own class of conductive polymers. Furthermore, S. Crittenden and many other groups across the world are now making fuel cells from electrogenic bacteria (Microbial Fuel Cells, MFCs) and powering them with waste biomass (e.g. grass, crab shells, waste water streams, etc.). MFCs are a novel green alternative energy source that could eventually provide significant power from material that now has to be made to 'go away'. In addition, Xiaodong "Chris" Li has programs on nanoscale imaging and mechanical testing of cells, tissues and biomolecules as well as on fabrication, structural and mechanical characterization of biological nanomaterials and bio-inspired nanomaterials.
The advent of semiconductor devices with nanoscale dimensions creates the potential to integrate nanoelectronic and optoelectronic devices with a great variety of sensor technologies. Moreover, the advances in nanotechnology are opening the way to achieving direct electrical contact of nanoelectronic structures with electrically and electrochemically active structures. We are conducting state-of-the-art research (Simin, Sudarshan, Deng, Koley, and Webb) aimed at developing materials and device technologies for wide band gap semiconductors such as SiC, AlGaN/GaN and AlInGaN metal oxide-semiconductor heterostructure field effect transistors on a variety of substrates. We have programs on growth, fabrication, modeling, and characterization of novel nanoscale devices and sensors that can be used to create new light emitting diodes that span the full visible spectrum, and new high voltage high power devices that work at high frequency and can be used for switches and converters. We have started a program that uses plasma waves in a two dimensional electron gas both as a detector and a generator of terahertz signals (frequencies between 200 GHz and 3,000 GHz, the last research frontier of high frequency electronics). It is believed that terahertz imaging and sensing could have widespread applications in areas such as defense, security, biology, and medicine.
One-dimensional semiconductor nanostructures are attractive building blocks for nanoelectronics since their morphology, size, and electronic properties make them suitable for fabricating both nanoscale devices and interconnects.
We have several programs for growing and characterizing 10-50 nm diameter nanowires of SiC, GaN, and InN (Koley, Webb & Sudarshan) and making new devices for electronics applications. S. Crittenden is working with biologically produced protein nanowires. G. Koley is also investigating the properties of wide band gap materials and devices with special emphasis to their material properties and device behavior. Growth and characterization of semiconducting nanowires is also being performed in an attempt to understand nanoscale electronic properties. In addition, we can fabricate wires from almost any material as small as 25 nm using our electron beam lithography and frequently use this technique to fabricate electrical contacts to our nanowires. In our work on InN nanowires, we have been able to grow high mobility wires that can be gated to provide transistor-like characteristics and to grow a very thin high quality insulating layer on the outside of the wire that can be used in our spin injection and sensor work. We are also working on nanodevices for terahertz signal detection and generation that use nanoscale gates in an effort to develop new semiconductor devices that can generate and detect the radiation signals in the frequency range between microwave and infra-red light (Y. Deng and Webb). These new semiconductor devices could be used in a wide range of potential applications, including wireless communication above 60 GHz, high-performance miniaturized military radars and sensors, far-infrared spectrum analysis, poisonous chemical-compound and biological-agent identification, medical imaging, weather forecasting, plasma diagnostics, and wireless interconnects on future chips. Furthermore, these devices could also be used for airport security checks, weapons inspection, and the prevention of terrorism.
To manufacture magnetic and electronic devices at 10 nm size scales, the metrology used for Six Sigma quality control in manufacturing must have verified spatial resolution in the 50 pm range. Measuring magnetic and electric fields at this size scale is both a technological and fundamental quantum physics challenge. When does Quantum Mechanics apply to nanoscale measurement? Recent observations of single electron spins suggest that, in principle, picoscale field mapping is possible. The Crawford group is exploring new techniques for quantitative metrology with sub-nm resolution. In addition, the Li group focuses on nanocharacterization using scanning force microscopy, as well as nanomechanical characterization and nanomachining of nanomaterials. They study the nanomechanics of carbon nanotube reinforced nanocomposites and are active in the synthesis of multifuctional nanomaterials. Also S. Crittenden is developing novel nanoscale measurement and manipulation techniques and devices, including the ability to measure the complete tip-sample interaction potential under a range of environments and a multi-probe SFM.
Electrogenic bacteria produce free electrons as a natural byproduct of metabolism. Thus, anything that effects the metabolic rate will effect the current (with a time constant on the order of a second) suggesting that one can make biosensors where the bacteria themselves serve as both sensing and signal transduction elements. S. Crittenden is working on sensors based on this idea.
One of the major research focuses of G. Koley's nanoelectronics and sensors laboratory is the trace detection of gases, chemicals and bio-molecules. To perform this detection, he uses SFM-like microcantilevers to sense the change in electrical, mechanical, and chemical properties caused due to adsorption of target molecules. This work includes the fabrication and characterization of novel nanoscale devices.
Victor Giurgiutiu maintains a broad program in sensors for structural health monitoring, damage detection and failure prevention of aircraft, turbomachinery, gearboxes, and civil infrastructure. In some applications, he uses nano-scale thin-film piezoelectric devices for optimal energy transduction in electro-mechanical active material structures. He uses analysis and design of smart/intelligent/adaptronic structures incorporating active materials, smart controls, and embedded intelligence. His research objective is to develop the fabrication, modeling and optimum design of thin-film active sensor arrays for structural health monitoring applications. This interdisciplinary research will cross the engineering and science boundaries and will address the problem in a coordinated approach focused on understanding the fundamentals aspects of fabricating and using thin-film active sensors on typical structural materials.
A superconducting state is formed when electrons in a metal form pairs and condense into a single macroscopic quantum state. Several peculiar properties can arise from the formation of the superconducting state such as electrical conduction without resistance, perfect diamagnetism, and the quantization of magnetic flux into elementary fluxons (also known as flux vortices). The properties of perfect conduction and diamagnetism hold only for low currents and fields, and at higher current densities a superconductor becomes resistive and can dissipate energy intensely. M. Kunchur has programs on the transport behavior of high Tc superconductors at high dissipation and short timescales to be used in technological applications. These basic studies also have implications for technological applications ranging from devices to power generation and transport. T. Datta works on mesoscopic systems that show high temperature superconductivity, mesoscopic quantum transport, deterministic chaos, and the effects of disorder in linear and non-linear systems. He studies the unusual magnetic properties exhibited by these nanoscale systems.
Induced changes in the magnetoresistance of a 60/30nm Au/Co film before and after the chemisorption of alkanethiol due to the increase of the Au magnetic moment as seen in the Planar Hall Effect (left) and the Anisotropic MagnetoResistance (right). By the Crawford group.
The capability to engineer magnetism in traditionally nonmagnetic materials may launch an entirely new paradigm for magnetic data storage at the nanoscale. Gold nanoparticles become ferromagnetic when capped with thiols. The Crawford group is working in collaboration with the Murphy and zur Loye groups in the USC Chemistry and Biochemistry department to understand and extend this chemically-induced magnetism into other nanostructured geometries as well as other chemical species with unique properties.