Oxygen Engineered Hafnium Oxide Thin Films grown by Reactive Molecular Beam Epitaxy

Abstract

This study applies RMBE to grow thin films of hafnium oxide, a widely studied material which has found its way into commercialisation as a replacement of SiO2 in a field effect transistor. After investigating different substrates and probing various deposition conditions, RMBE-grown films of hafnium oxide yielded to epitaxial films of hafnia on c-cut sapphire. Having the ability to grow high-quality thin films of hafnium oxide allows studying the influence of defined oxygen deficiency on its physical properties, as the next step of this work. The optical properties changed dramatically from colourless and transparent for stoichiometric HfO2 to dark black and opaque for highly deficient films of HfO2-x. The optical band gap could be tuned within more than one eV, visualising the introduction of defects (oxygen vacancies) in situ during growth. In fact, Hafnia showed a metal to insulator transition as a function of the oxygen content, conductive HfO2-x exhibited electrical p-type conductivity with resistivities of 300 µWcm, charge carrier concentrations of 6 times 10 to the power of 21 cm-3 at mobilities of 2 cm²/(Vs). The observed conductivity seems to be intrinsic to oxygen deficient hafnia and not due to a percolation of a conducting phase in an insulating matrix, as evident from various characterisations. A simple defect band structure model has been developed based on the observations, covering the formation of defect bands within the band gap being responsible for electrical conductivity and absorption of radiation within the visible range. With respect to reports on high-Tc ferromagnetism, no evidence for d0-ferromagnetism and room temperature ferromagnetism in Ni-doped HfO2-x could be found.