Stable trapping of electrons and holes in deposited insulating oxides: Al2O3, ZrO2, and HfO2

Abstract

Charge trapping in high-permittivity metal oxides $({\mathrm{Al}}_2{\mathrm{O}}_3,$ ${\mathrm{ZrO}}_2,$ and ${\mathrm{HfO}}_2)$ grown on (100)Si using various types of chemical vapor deposition (CVD) was studied using generation of electron-hole pairs in the oxide by 10 eV photons. For most of the CVD methods, thin (≈5 nm) oxide films exhibit positive charging suggesting hole trapping as most efficient charge trapping process. Negative charge is observed only in as-deposited nitrogen-containing films grown from ${\mathrm{Hf(NO}}_3{)}_4.$ The trapped positive charge depends only weakly on the ${\mathrm{HfO}}_2$ thickness indicating that holes are trapped in a silicon oxide interlayer grown between the Si and ${\mathrm{HfO}}_2$ during deposition, which is further affirmed by enhanced positive charging after additional oxidation of the samples at high temperatures. The work function of the metal electrode material has a large influence on hole trapping in thin oxides, indicating electron exchange between the metal and defect states in the oxide. In addition, trapping of positive charge correlates with liberation of atomic hydrogen during electron-hole pair generation pointing to a considerable contribution of protonic species to the charge. Electron trapping increases with thickness of oxide layers and appears strongly sensitive to both the method of oxide deposition and post-deposition processing. This suggests the electron traps to be related to defects and/or impurities incorporated into the oxide film during deposition.