CAREER: Spintronic Devices Using Screw Dislocations in Single-Crystalline Semiconductors

“This project is jointly funded by the Electronics, Photonics, and Magnetic Devices (EPMD) Program of the Electrical, Communications and Cyber Systems (ECCS) Division, and the Established Program to Stimulate Competitive Research (EPSCoR)”.

This synergistic research and education program aims to investigate the device physics of a novel spin field-effect transistor (spin FET) while expanding the participation of students in nanoscience and engineering. The work will be the first experimental study to focus on the fabrication and characterization of spin FETs that leverage screw dislocations in single srystal-crystalline semiconductors.

A screw dislocation is a type of crystal defect that promises to act as a suitable channel for a spin transistor that operates at variable temperatures (i.e., room temperature and above) for commercial, industrial, and military applications. The improved knowledge and technological advances generated in this project will potentially accelerate very-large-scale integration of high-performance spintronic devices, which is required for the practical realization of hybrid and entirely spin-based classical and quantum computers.

This program will also benefit society by impacting science, technology, engineering, and math (STEM) education. Experiential exhibits that will target K-12 students and the public will be established to increase engagement in STEM. As such, this work will have a wide-ranging impact by communicating the value of nanoscience, nanotechnology, and spintronics to a broad audience.

The research effort encompasses three strongly overlapping intellectual threads: (i) fabrication and structural characterization of spin FETs embedded screw dislocations(SDs); (ii) characterization of gate-tunable spin transport; (iii) investigation of process-structure-property relationships in the fabricated spin FETs. The operation of the proposed spin FET is based on controlling the spin lifetime in screw dislocations channels using a gate voltage.

Two opposite aligned spin-selective ferromagnetic/semiconductor contacts will be placed at the two ends of a screw dislocation. As the spin lifetime in the dislocation core is expected to be long, the transistor will be in an OFF state in this condition. Under an applied gate voltage, spin-polarization can be tailored to allow transport from one ferromagnetic contact to another, putting the transistor in an ON state.

The ratio between the spin lifetime and the relaxation time will determine the ON/OFF ratio of the device. Spin FETs designs that rely on controlling spin lifetimes have been proposed before. However, these devices have not found practical uses due to the requirement of large (>100 ms) spin relaxation times at room temperature.

Screw dislocation channels can overcome this limitation owing to a unique form of spin-orbit coupling (SOC) that promises to increase spin lifetimes even at high temperatures, thereby enabling the practical application of the proposed spin FET design. Briefly, the interplay between Rashba and Dresselhaus SOC in screw dislocations of materials with medium ionicity makes spin polarization scattering resistant.

The technical plan relies on the controlled synthesis of a screw dislocations network by annealing ultra-compliant twisted bicrystals or two single-crystalline nanomembranes (NMs) that have been previously overlayed with a controlled twist angle. NMs will provide more uniform interfacial bonding than their bulk counterpart, expand the palette of semiconductor hosts of screw dislocations to include epitaxially grown alloys, and increase the ON/OFF ratio of the transistor by reducing the transit time of the carriers.

Dislocated NMs will be processed in spin FETs, and the process-structure property relationships of the devices will be evaluated. The focus will be on SDs in SiC NMs as theoretical predictions have shown that a fixed spin orientation, which is robust against all forms of electron scattering, can be obtained in this material.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.