All pure metals are crystalline in their most stable forms and many metals can adopt different crystal structures under different conditions. The properties of each metal depend strongly on its crystal structure, also known as its phase. For example, tantalum (Ta) in its stable cubic phase is ductile and is a good conductor of electricity. It is widely used in integrated circuits and thin film capacitors. In 1965, Mildred Read and Carl Altman discovered that, in certain atom-by-atom thin film deposition processes, Ta can also be made in a metastable tetragonal phase which is brittle and has much lower electrical conductivity. Use of this phase has been limited to thin film resistors. However, in 2012, an electronic effect (giant spin Hall effect) was discovered in the metastable phase, which promises to revolutionize information storage by making significant further miniaturization of computer devices possible. Thus, there is now much interest in the ability to reliably produce the metastable phase. As it turns out, all of the elements from group 5 and 6 in the periodic table, which, in addition to Ta, include tungsten (W), chromium (Cr), molybdenum (Mo), vanadium (V), and niobium (Nb), have the same stable crystal structure at all temperatures and pressures, and, with the exception of Nb, metastable phases have been reported for all of them (a metastable Nb phase has been predicted, but not realized). This leads to the prospect that, if the metastable phases can be reliably produced in all of these metals, this may open up a class of new materials with potentially interesting properties. Indeed, it has recently been shown that the metastable phase in W has an even stronger giant spin Hall effect than Ta. However, with the exception of metastable Ta, and now W, the metastable phases of these elements have primarily been laboratory curiosities. Neither their formation mechanisms nor their properties are known. In this research, detailed analyses of the first few atomic layers that form as thin films of Ta, W, Cr, Mo, V, and Nb are deposited, atom by atom, onto a substrate will be used to determine how and why the metastable phases form. In addition, a unique ultra-high vacuum deposition system that is capable of making very pure films under a wide range of conditions will be used to explore the range of conditions under which metastable phase films of these materials can be made. Finally studies of the atomic arrangements and properties such as hardness, electrical conductivity, and electronic effects in those films will provide information about possible new applications for these new materials. In addition to fundamental understanding about why certain phases form, this work should enable technologists to develop new applications using these materials and to determine how to control process parameters to reliably obtain metastable films with desired properties for those applications. In particular, this work has the potential to revolutionize computer random access memory technology, which would enable the development of several generations of higher performance microelectronic devices. In addition, approximately 16 students, including 2 PhD students, 2 MS students, and 12 undergraduate students (6 from Cornell and 6 from Houghton College, a small undergraduate liberal arts college in upstate New York) will engage in this effort. All students will be mentored to come up to speed on the goals, participate in the research, and to present their work in both talks and papers. The net effect will be very high-level training for these future STEM professionals. Finally, the project will involve numerous outreach activities to local area schools and institutions, including presentations, demonstrations, curricular assistance, tutoring (especially for disadvantaged/underrepresented students), and many others. These activities are intended to inform the public about the practice, value, and accomplishments of science, including this project, and to encourage students to pursue STEM fields as they see fit.
The group 5 and 6 elements, Ta, W, Cr, Mo, V, and Nb, are well known for having only one equilibrium crystal structure, the body-centered-cubic phase, at all temperatures and pressures. In addition, it has been shown that in certain atom-by-atom fabrication processes such as sputter deposition, metastable phases can be made in all but Nb (metastable Nb has been predicted, but not realized). However, very little is known about the mechanism by which these phases form, and, with the exception of metastable beta-Ta and beta-W, very little is known about their properties. Recently there has been a spike in interest generated by the discovery of the giant spin Hall effect in both beta-Ta and beta-W. In the present program, phase formation mechanisms and the relationships among film deposition parameters, microstructure, and properties, will be studied for the group 5 and 6 elements. Phase formation will be studied using reflection high energy electron diffraction (RHEED) and angle-resolved photoemission spectroscopy (ARPES) to characterize the initial phases of film growth by molecular beam. The extent to which metastable phases can be produced in W, Cr, Mo, V, and Nb will be determined in an ultra-high vacuum (UHV) sputter deposition system that provides a wide range of deposition parameters (temperature, bias, power, sputter mode) and is capable of reducing oxygen and other impurities to extremely low levels. The effect of various deposition parameters on microstructure will be studied using x-ray diffraction (XRD), electron backscattered diffraction (EBSD) and other methods, and the stability of the metastable phase will be studied by determining stress change (as an indication of phase change) in-situ during heating in the UHV system. Finally, hardness and elastic modulus will be determined using nanoindentation and electrical conductivity will be measured with a four-point probe. This work is expected to provide fundamental details regarding the mechanism of metastable phase formation and the crystal structures of the metastable phases in the group 5 and 6 metals, as well as descriptions of the conditions under which those phases can be grown and the relationships among those deposition conditions and the microstructure and properties of the resulting films.
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.