****NON-TECHNICAL ABSTRACT**** There is currently an intense worldwide movement in contemporary electronics to explore the role of the electron spin, a quantum mechanical magnetic property of the electron, - in addition to the electron's charge - with an eye on increasing the functionality of electronic microchip devices, particularly in the realm of computation. This award supports a project to address the issue by employing state-of-the-art techniques to fabricate and characterize a series of novel semiconductor nano-scale structures in which the role of the electron spin is magnified by incorporating magnetic ions. By training graduate and undergraduate students in cutting-edge semiconductor fabrication techniques as well as in designing multi-functional materials, the project is expected to have a broad impact far beyond its immediate goals. Skills in these areas of materials science are in broad demand in U.S. Industry, National Laboratories, and Academia. Additionally, the Notre Dame team collaborates with many scientists (currently with more than thirty-five other institutions) either by providing research samples or by carrying out joint experiments. This activity of dissemination and sharing of results (which has the added benefit of exposing students to inter-institutional and inter-disciplinary collaborations) is expected to further expand as new spin-based electronic materials are developed in the course of the investigation.
This grant supports a project that focuses on two new complementary areas involving spin phenomena in low dimensional magnetic semiconductor systems. First, by using molecular beam epitaxy (MBE), dendritic nanowires based on magnetic semiconductors, including both II-Mn-VI and III-Mn-V alloys will be fabricated and studied. The second area will involve MBE growth and study of GaMnAs/Ge systems, a lattice-matched combination characterized by a special band alignment that is expected to lead to new spin effects and new insights both in multilayer and in nanowire geometries. The project is expected to have a broad impact far beyond its explicit scientific goals by contributing to the arsenal of spin-electronics materials generally; and by training graduate and undergraduate students in cutting-edge semiconductor fabrication techniques and in designing multi-functional materials, thus contributing to U.S. manpower skills in areas which are in wide demand in U.S. Industry, National Laboratories, and Academia. Additionally, the Notre Dame team already collaborates with many scientists (currently with more than thirty-five other institutions) either by providing research samples or by carrying out joint experiments. This activity of dissemination and sharing of results is expected to further intensify as new spin-electronic materials are developed during this investigation.
General objectives: Advances in contemporary technology depend critically on development of new materials, and on discoveries of new effects which occur in these materials. Leading these advances are discoveries relevant to storage and manipulation of information; and strategies for miniaturization of device architectures in which these processes are implemented. Traditionally storage of information involves magnetic systems (i.e., systems involving electronic spin); and manipulation of information involves semiconductor materials (e.g., the semiconductor chip). However, typical magnetic materials are distinct from semiconductors, so that one requires two different systems to achieve both storage and manipulation of information. The primary objective of the present project was to develop materials which combine both semiconductor and magnetic properties, which can then be used to achieve both these functionalities – information storage and manipulation – in a single chip, thus leading to enormous advantages in device size, processing speed, and storage capacity. Specific outcomes and their intellectual merit: To achieve the goals outlined in the preceding paragraph, a systematic program was launched in this grant project to design, develop, and study semiconductors such as GaAs, InAs, and InSb, which can be made magnetic by introducing magnetic ions (in our case Mn ions) into their crystal lattice. The program consisted of three main thrusts: of exploring the crystal growth of these combinatorial materials by the process of molecular beam epitaxy, whereby the desired magnetic semiconductor alloy (e.g., GaMnAs) is achieved by atomic layer-by-layer deposition; of systematic studies of the resulting materials using structural, electrical, magnetic and optical techniques; and of combining materials developed in these investigations into a wide range of device architectures intended to serve as prototypes for future devices based on the semiconductor and magnetic properties identified in this program. One of the bottlenecks of achieving practical devices based on such materials is the fact that magnetism in semiconductors of this type so far occurs only below room temperature. In this project significant advances were made to raise this limiting temperature closer to room temperature, both by gaining theoretical understanding of the process by which these systems become magnetic; and, based on this understanding, by developing strategies to optimize crystal growth and device design aimed at higher temperature operation. Major progress has also been made in understanding physical phenomena that govern the phenomenon of spin injection, one of the key processes which are expected to govern the operation of future spin-electronic ("spintronic") devices. A number of prototype devices were constructed based on the magnetic semiconducting alloys in this program, including magnetic tunnel diodes and devices designed to operate beyond the standard binary logic. In parallel with the efforts aimed at optimizing the properties of magnetic semiconductors that were already known when this program began, new magnetic semiconductor materials had also been successfully obtained as part of this project, including the alloy InMnSb, which is attractive for its mid-infrared optical properties. Additionally, it was recognized that attractive spin-electronic phenomena can be obtained by combining semiconductors with magnetic metals into hybrid architectures, and considerable effort has been made in this project to develop such hybrid combinations. It was discovered that in certain cases structures of excellent crystal quality can be formed in this way. Based on this, several novel device structures have already been realized, displaying very promising spin-injection functionalities. It is expected that such metal/semiconductor hybridization will serve as a new approach for achieving spintronictronic devices. Broader impacts: Apart from contributing to current understanding of the interplay of semiconductor physics and magnetism and of its potential for spin-electronic devices, as argued in the preceding paragraphs, this project has had impacts in three areas that go significantly beyond these specific achievements, as follows: 1. All personnel involved in this program (post-doctoral scholars, graduate students, and undergraduates) were systematically exposed to the process of crystal growth, as well as to fabrication of so-called "designer materials". Such man-made materials are at the heart of modern technology, and training in this area constitutes important preparation of students and post-doctoral scholars for the national needs in materials science. 2. Fabrication of specimens (including the "designer materials" mentioned above) involves instrumentation that is not commonly available. This program has therefore served as an important resource by providing research specimens to other institutions. It is estimated that in the period of this project some fifty researchers from outside universities and national laboratories have benefited from these collaborative interactions. 3. This program has also been very actively involved in international collaborations, both through specimen exchange and through sharing of information. Specifically, during the past four-year period this program has had collaborative projects with universities in Brazil, Canada, China, Germany, Hungary, Poland, Russia, and South Korea. Maintaining such two-way scientific exchanges contributes not only to valuable exchange of information, but also to generating good will between the collaborating sides.
