Intrinsic charge trapping in amorphous oxide films: status and challenges

Abstract

We review the current understanding of intrinsic electron and hole trapping in insulating amorphous oxide films on semiconductor and metal substrates. The experimental and theoretical evidences are provided for the existence of intrinsic deep electron and hole trap states stemming from the disorder of amorphous metal oxide networks. We start from presenting the results for amorphous (a) HfO, chosen due to the availability of highest purity amorphous films, which is vital for studying their intrinsic electronic properties. Exhaustive photo-depopulation spectroscopy measurements and theoretical calculations using density functional theory shed light on the atomic nature of electronic gap states responsible for deep electron trapping observed in a-HfO. We review theoretical methods used for creating models of amorphous structures and electronic structure calculations of amorphous oxides and outline some of the challenges in modeling defects in amorphous materials. We then discuss theoretical models of electron polarons and bi-polarons in a-HfO and demonstrate that these intrinsic states originate from low-coordinated ions and elongated metal-oxygen bonds in the amorphous oxide network. Similarly, holes can be captured at under-coordinated O sites. We then discuss electron and hole trapping in other amorphous oxides, such as a-SiO, a-AlO, a-TiO. We propose that the presence of low-coordinated ions in amorphous oxides with electron states of significant p and d character near the conduction band minimum can lead to electron trapping and that deep hole trapping should be common to all amorphous oxides. Finally, we demonstrate that bi-electron trapping in a-HfO and a-SiO weakens Hf(Si)-O bonds and significantly reduces barriers for forming Frenkel defects, neutral O vacancies and O ions in these materials. These results should be useful for better understanding of electronic properties and structural evolution of thin amorphous films under carrier injection conditions.

Disorder-induced electron and hole trapping in amorphous TiO2

“…The fact that disorder can produce precursor sites for formation of deep localized electron and hole states suggests that balance between the short-ranged phonon-mediated attraction and on-site Hubbard repulsion could be tipped in favor of formation of bipolaronlike states where two electrons or holes coexist on one or several neighboring network sites. The possibility of formation of such states in crystals [57,58] and amorphous solids [59,60] has been predicted by the phenomenological theory and demonstrated in some oxides by DFT simulations [46]. Electron bipolarons have been predicted by DFT calculations in amorphous SiO 2 [48] and HfO 2 [50].…”

“…Deep electron and hole trapping has been predicted in several other amorphous oxides where polarons do not trap or form only shallow states in crystalline phase [46]. In particular, theoretical studies have shown that the precursor sites composed of wide O-Si-O bond angles act as deep electron traps in amorphous SiO 2 [47].…”

“…Holes have been shown to trap at low-coordinated oxygen sites in most amorphous oxides amorphous oxides, including ZnO, Al 2 O 3 , HfO 2 and SiO 2 . [46,56].…”

Black indium oxide a photothermal CO2 hydrogenation catalyst

“…In addition, we know that oxygen vacancies 27,28 . These states will be respectively situated near the oxide-based valence band and indium-based conduction band, with the charge-balancing electrons occupying mid-gap states, as shown in Supplementary Fig.…”

First-principles characterization of single-electron polaron in WO3

“…The self-trapping of a polaron is also known to be more important in strongly distorted crystals than in highly symmetric crystals. This effect is particularly striking in amorphous crystals where the trapping energies are larger than their crystalline counterpart [32] (up to 1.5 eV for a hole polaron in, e.g., amorphous Al 2 O 3 versus 0.13 eV in its crystalline form [33]). It is also known that the crystal structure drives the shape of the polaron (2D polarons are favored in layered structures of HfO 2 or ZrO 2 , for example [34]).…”