Single nanowire spin-valve based infrared photodetctors and equality bit comparators

Virginia Commonwealth University, Richmond VA

Investigators

Abstract

Infrared light is not visible to the human eye. It is usually detected with semiconductor detectors which exhibit a change in their electrical resistance under infrared illumination. The relative change in resistance at room temperature is, however, quite small, which necessitates cooling the detector with liquid nitrogen. In this research, a novel detector will be demonstrated, which relies on light changing the detector's resistance by affecting the quantum mechanical spin properties of the electrons that carry current. With this principle of detection, it is possible to make the resistance change in the detector much larger at room temperature. Room temperature infrared detectors are used in night vision, forensic science, astronomy, missile defense, car-collision avoidance systems and monitoring of global warming, to name a few. Bit comparators are electronic devices that compare two digital (binary) bits of information and render a yes/no decision based on whether the two bits are the same or different. They are important ingredients of electronic circuits and are typically implemented with transistors which cannot remember the decision once the decision has been rendered. A comparator that exploits spin dependent properties and uses magnetic devices instead of transistors can remember the decision and also use less energy. The ability to remember makes it possible to build superior digital electronic circuits that are faster and more error-resilient. In this research, such a comparator will be demonstrated. This project will also integrate research with graduate and undergraduate education, K-12 outreach through the Dean's Early Research Initiative program, and minority enrichment through the Richmond Minorities in Engineering Partnership. This is a proposal to fabricate and demonstrate two novel spintronic devices - an infrared photodetector and a reconfigurable bit comparator - working at room temperature. They will be fabricated with 10- and 50-nm diameter nanowire spin valves with InSb spacer and cobalt contacts. In the InSb spacer, only a single electronic subband is occupied at room temperature. Preliminary experiments in the lab have shown that in these nanowires, the major spin relaxation mechanism of electrons - namely the D'yakonov-Perel' mechanism - is eliminated owing to single subband occupancy, resulting in several-fold increase in the spin relaxation time. It has also been shown that the spin relaxation time can be modulated with infrared light, which will be exploited to build the photodetector. Such a photodetector can, in principle, exhibit near-zero dark current, giant light-to-dark contrast ratio and very high detectivity which always elude conventional photo-detectors because of phonon excitations. Controlling the magnetization state of a nanomagnet with electrically generated strain has been recently demonstrated in our lab. That property will be leveraged to build the reconfigurable bit comparator with unprecedented energy efficiency. These bit comparators are "non-volatile" since they incorporate magnetic elements and hence the result of the comparison can be stored indefinitely in the comparator. The devices will be fabricated with electrochemical self-assembly of nanowires, electron-beam lithography for patterning ferromagnetic contacts, and dielectrophoresis for capturing a single nanowire between a pair of contacts to implement the photodetector and bit comparator.