Probing the Crystallization Process and Morphology of Thin Films of Yttrium Iron Garnet on Non-Garnet Substrates with in situ TEM Methods

University of Minnesota Ph.D. dissertation. October 2018. Major: Material Science and Engineering. Advisors: David Flannigan, Bethanie Stadler. 1 computer file (PDF); vii, 142 pages.Thin films of yttrium iron garnet (YIG) are of high interest for promising photonics and spintronics applications. Integration challenges with current silicon processing technology have limited device geometries and caused reduced performance largely arising from crystallization issues of as-deposited films. In order to gain understanding of the amorphous to crystalline phase transformation of YIG thin films on non-garnet substrates, plan-view TEM and in situ laser annealing TEM methods were utilized. Thin YIG films were sputtered onto SiO2 TEM window membranes. These films were initially annealed ex situ using standard RTA annealing methods. A nanocrystalline matrix phase between YIG crystallites was discovered where previous studies had reported uncrystallized material. Preliminary in situ laser annealing led to the serendipitous discovery of a 2-step rapid thermal anneal which improved garnet phase formation in the films. To investigate YIG crystallization kinetics on SiO2, temperature dependent in situ laser annealing TEM diffraction experiments were conducted. Avrami constants and apparent activation energy for the nanocrystalline phase formation is reported. In situ bright-field TEM was also used to investigate the growth of the YIG crystallites and indicated they enter a stress limited growth phase after reaching a critical dimension. Additionally, considerable effort was put into instrument development for in situ TEM methods, including optimization of single-shot pump probe capability. A range for optimized cathode to Wehnelt aperture distance and photoelectron inducing laser fluence are reported. Demonstrations of single-shot capabilities in both diffraction and imaging modes with current equipment are shown