Ultralarge area MOS tunnel devices for electron emission

Abstract

A comparative analysis of metal-oxide-semiconductor (MOS) capacitors by capacitance-voltage $(C\text{\ensuremath{-}}V)$ and current-voltage $(I\text{\ensuremath{-}}V)$ characteristics has been employed to characterize the thickness variations of the oxide on different length scales. Ultralarge area $(1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2})$ ultrathin ($\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ oxide) MOS capacitors have been fabricated to investigate their functionality and the variations in oxide thickness, with the use as future electron emission devices as the goal. $I\text{\ensuremath{-}}V$ characteristics show very low leakage current and excellent agreement to the Fowler-Nordheim expression for the current density. Oxide thicknesses have been extracted by fitting a model based on Fermi-Dirac statistics to the $C\text{\ensuremath{-}}V$ characteristics. By plotting $I\text{\ensuremath{-}}V$ characteristics in a Fowler plot, a measure of the thickness of the oxide can be extracted from the tunnel current. These apparent thicknesses show a high degree of correlation to thicknesses extracted from $C\text{\ensuremath{-}}V$ characteristics on the same MOS capacitors, but are systematically lower in value. This offset between the thicknesses obtained by $C\text{\ensuremath{-}}V$ characteristics and $I\text{\ensuremath{-}}V$ characteristics is explained by an inherent variation of the oxide thickness. Comparison of MOS capacitors with different oxide areas ranging from $1\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}{\mathrm{m}}^{2}$, using the slope from Fowler-Nordheim plots of the $I\text{\ensuremath{-}}V$ characteristics as a measure of the oxide thickness, points toward two length scales of oxide thickness variations being $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{cm}$ and $\ensuremath{\sim}10\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, respectively.