The origin of negative charging in amorphous Al2O3 films: the role of native defects

Amorphous aluminum oxide Al$_2$O$_3$ (a-Al$_2$O$_3$) layers grown by various deposition techniques contain a significant density of negative charges. In spite of several experimental and theoretical studies, the origin of these charges still remains unclear. We report the results of extensive Density Functional Theory (DFT) calculations of native defects - O and Al vacancies and interstitials, as well as H interstitial centers - in different charge states in both crystalline $\alpha$-Al$_2$O$_3$ and in a-Al$_2$O$_3$. The results demonstrate that both the charging process and the energy distribution of traps responsible for negative charging of a-Al$_2$O$_3$ films [M. B. Zahid et al., IEEE Trans. Electron Devices 57, 2907 (2010)] can be understood assuming that the negatively charged O$_{\textrm{i}}$ and V$_{\textrm{Al}}$ defects are nearly compensated by the positively charged H$_{\textrm{i}}$, V$_{\textrm{O}}$ and Al$_{\textrm{i}}$ defects in as prepared samples. Following electron injection, the states of Al$_{\textrm{i}}$, V$_{\textrm{O}}$ or H$_{\textrm{i}}$ in the band gap become occupied by electrons and sample becomes negatively charged. The optical excitation energies from these states into the oxide conduction band agree with the results of exhaustive photo-depopulation spectroscopy (EPDS) measurements [M. B. Zahid et al., IEEE Trans. Electron Devices 57, 2907 (2010)]. This new understanding of the origin of negative charging of a-Al$_2$O$_3$ films is important for further development of nanoelectronic devices and solar cells.