Abstract
The characteristics of intrinsic electron and hole trapping in crystalline and amorphous Al2O3 have been studied using density functional theory (DFT). Special attention was paid to enforcing the piece-wise linearity of the total energy with respect to electron number through the use of a range separated, hybrid functional PBE0-TC-LRC (Guidon et al 2009 J. Chem. Theory Comput. 5 3010) in order to accurately model the behaviour of localized states. The tuned functional is shown to reproduce the geometric and electronic structures of the perfect crystal as well as the spectroscopic characteristics of MgAl hole centre in corundum α-Al2O3. An ensemble of ten amorphous Al2O3 structures was generated using classical molecular dynamics and a melt and quench method and their structural characteristics compared with the experimental data. The electronic structure of amorphous systems was characterized using the inverse participation ratio method. Electrons and holes were then introduced into both crystalline and amorphous alumina structures and their properties calculated. Holes are shown to trap spontaneously in both crystalline and amorphous alumina. In the crystalline phase they localize on single O ion with the trapping energy of 0.38 eV. In amorphous phase, holes localize on two nearest neighbour oxygen sites with an average trapping energy of 1.26 eV, with hole trapping sites separated on average by about 8.0 Å. No electron trapping is observed in the material. Our results suggest that trapping of positive charge can be much more severe and stable in amorphous alumina rather than in crystalline samples.
Highlights
Thin films of amorphous alumina (a-Al2O3) play a key role in the development of a wide range of applications, notably non-volatile memory [2, 3] and amorphous indium gallium zinc oxide (a-IGZO) thin film transistors [4]
Due to low trapping energies, it is difficult to measure the properties of self-trapped hole polarons in α-Al2O3 experimentally
It has originally been assumed that Mg acts as an acceptor [55], with the hole localizing on the nearest neighbour oxygen, becoming O−, which makes it a good test system for hole trapping
Summary
Thin films of amorphous alumina (a-Al2O3) play a key role in the development of a wide range of applications, notably non-volatile memory [2, 3] and amorphous indium gallium zinc oxide (a-IGZO) thin film transistors [4]. With its wide band gap and dielectric constant double that of SiO2, Al2O3 is a suitable replacement as the blocking dielectric in these devices, and has been investigated as a charge trapping layer [3] in charge trap flash memory. Surprisingly little is still known about intrinsic electron and hole trapping in amorphous oxides. The aim of this work is to investigate the electron and hole trapping in the bulk of crystalline and amorphous alumina
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