Quest for the perfect dilute magnetic semiconductor: Investigation of chromium-doped gallium(III) selenide on silicon.

摘要

The potential for spin-based electronics (spintronics) to revolutionize silicon-based device structures requires development of new magnetic materials. The optimal room temperature ferromagnetic material should be both impedance and lattice matched to silicon. We have begun studies on a new class of silicon-compatible dilute magnetic semiconductors based on transition metal (TM) doped III-VI materials. These III-VI materials are of particular physics interest due to their intrinsic vacancies, since the resulting multiple incorporation sites for the transition metal may enable separate control of magnetic and electronic doping.;This dissertation demonstrates that the inclusion of the transition metal Cr into the III-VI semiconductor Ga2Se3 leads to room-temperature ferromagnetism, semiconducting electronic states, and epitaxial thin films resembling pure Ga2Se3 on Si(001). This new material is definitely compatible with silicon up to several atomic percent Cr, laying the ground-work for incorporation of Cr-doped Ga2Se3 into the ever-present silicon technology. In this work, we investigate the magnetism, chemical composition, structure, morphology and solubility limit of a possible dilute magnetic semiconductor, namely Cr-doped Ga2Se 3, using magnetometry, x-ray photoemission spectroscopy (XPS), x-ray absorption fine structure (XAFS), scanning tunneling microscopy (STM), and scanning Auger microscopy (SAM).;The ferromagnetism observed in this system could be linked to Cr occupying an octahedral site in a zincblende structure, as revealed by photoemission and XAFS. There is a strong correlation between the magnetism observed, the surface morphology and film thickness. We propose that this ferromagnetism is ultimately mediated by the presence of intrinsic vacancies within the zincblende Ga2Se3 structure, where the presence of a Cr in a locally-octahedral structure only happens because of vacancies, and the Cr-induced states at the top of the valence band overlap the Se lone-pair states lining the vacancy rows, suggesting strong hybridization between the Cr t2 g and Se s,p states.;Through this study, we have laid down the fundamental work needed to make Cr-doped Ga2Se3 a silicon compatible dilute magnetic semiconductor, namely proving room-temperature ferromagnetism, epitaxial growth on silicon, and finding the local valence of the transition metal dopant Cr. This work now opens doors for a broad range of research opportunities among which the investigation of spin-transport properties of Cr-doped Ga2 Se3 on silicon, and the possible applications in spin field effect transistors or photovoltaics.