Ultralarge area MOS tunnel devices for electron emission

Abstract

A comparative analysis of metal-oxide-semiconductor (MOS) capacitors by capacitance-voltage $\left(C\text{−}V\right)$ and current-voltage $\left(I\text{−}V\right)$ characteristics has been employed to characterize the thickness variations of the oxide on different length scales. Ultralarge area $\left(1{\mathrm{cm}}^2\right)$ ultrathin ($\sim 5\mathrm{nm}$ 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. $I\text{−}V$ 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 $C\text{−}V$ characteristics. By plotting $I\text{−}V$ 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 $C\text{−}V$ characteristics on the same MOS capacitors, but are systematically lower in value. This offset between the thicknesses obtained by $C\text{−}V$ characteristics and $I\text{−}V$ characteristics is explained by an inherent variation of the oxide thickness. Comparison of MOS capacitors with different oxide areas ranging from $1{\mathrm{cm}}^2\text{to}10\mu {\mathrm{m}}^2$, using the slope from Fowler-Nordheim plots of the $I\text{−}V$ characteristics as a measure of the oxide thickness, points toward two length scales of oxide thickness variations being $\sim 1\mathrm{cm}$ and $\sim 10\mu \mathrm{m}$, respectively.