Technical: This project is to study the growth of semiconductor quantum wells made of indium antimonide (InSb) and to study charge and spin transport phenomena that are enabled by these materials. It attempts to address the needs of future computer technologies by focusing on InSb materials, which possess high carrier mobilities, small effective masses, and large spin-orbit effects. Such attributes imply higher operating speeds for field-effect transistors, higher operating temperatures for ballistic transport devices, and the opportunity for spintronic devices that could provide new routes for computation and data storage. Both n-type and p-type quantum wells with high carrier mobilities are required for transistors in logic applications. The electrical properties of the proposed metamorphic heterostructures, on GaAs and Si substrates, will be optimized in this project through defect filtering by interlayers and judicious design of strain-balanced barrier and well layers. Maximum strain is particularly important for p-type quantum wells, where a small effective mass, and consequently a high mobility, relies on the degree to which degeneracy is lifted in the valence bands. Magneto-optics experiments are designed to characterize the effects of strain and confinement on the effective mass of holes in InSb. The optimization of the growth of quantum wells will make possible studies of strong spin-orbit coupling effects via weak anti-localization experiments, carrier focusing devices, and spin interferometers.
The project addresses basic research issues in a topical area of materials science with high technological relevance, and is expected to provide new scientific understanding of spin-orbit coupling of not only InSb, but zinc blende semiconductors in general. The research group at the University of Oklahoma is one of the only two groups worldwide who are able to grow high-quality InSb heterostructures needed by many scientists in studying fundamental physics and materials science. This project will also contribute to the preparation of students, including members of underrepresented groups, for employment in areas of technological importance.
The intellectual merit of this project focused on the crystalline growth of very thin layers of a semiconductor, InSb, which has interesting and potentially useful properties. In thick layers of InSb, electric charge carriers (electrons) move easily, as if their mass was only a fraction of the value in free space, and the coupling between the carriers and magnetic fields (via the spin of the electrons) is strongly increased relative to other semiconductors. Over the last two decades, a new field called semiconductor spintronics has emerged with the potential to revolutionize standard electronics. Spintronics requires control over both charge and spin properties of the electrons. When a layer of InSb is made only a few nanometers thick, the confinement brings forth electronic properties that are not evident in thicker layers. For example, our experiments showed that electrical transport in thin InSb layers, called quantum wells, is sensitively dependent upon magnetic field and layer thickness, and that the spin effects are even more pronounced in narrow InSb wires. Spin behavior was different along different crystalline directions which may lead to methods for controlling spin transport in narrow wires. In addition to spin effects, we explored ways to increase the electron velocity in InSb quantum wells by developing methods to reduce the number of crystalline defects created during their growth. Defect propagation was reduced by incorporating mechanical strain into underlying layers. Defect formation within the thin InSb layer was suppressed by balancing the compressive (squeezing) strain in the underlying layer with a nearly equal amount of tensile (stretching) strain in the quantum well. We also maximized the mechanical strain in some structures to enable positively-charged carriers (holes) to travel faster. Ultimately, both fast electrons and holes are required to manufacture high-speed microprocessors from InSb materials. The broader impacts of the project are contributions to technology development, outreach to the general public, and the education and training of students for careers in science. The InSb layers studied in this project have potential application as materials for high-speed transistors that take advantage of high-velocity charge carriers and for spintronic devices. This project supported the Ph.D. research of four students and provided partial support for another five Ph.D. students and five undergraduates. The range of outreach activities included presentations to undergraduates, lectures on nanoscience to the public, laboratory tours to high-school students, and interactive engagement with K-5 students.
