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2014 MRS Spring Meeting


BB5.05 - A Hybrid Functional View of Hydrogen in Functional Oxides


Apr 23, 2014 11:30am ‐ Apr 23, 2014 11:45am

Description

Hydrogen is a ubiquitous impurity in many solids and particularly in oxides. As high K oxides replace SiO2 as the gate dielectric in future CMOS devices, hydrogen is introduced into high K oxide devices during ALD deposition and during the post-deposition anneal. Ionized hydrogen centers are therefore possible sources of positive fixed charge and bias instability. Interaction of H and materials has been extensively studied. Standard DFT approaches with local density functional have predicted the energetically active H induced defect levels and other related behaviours of H interstitial in many host materials. However, LDA is well known for underestimation of band gap and underestimation of Jahn-Teller or polaronic distortions of defect geometries. This failure can be traced back to poor description of exchange in LDA.

In this work, we present HSE06 hybrid functional calculation of the geometry relaxation and charge transition levels of H interstitial in various test-case oxides. H has three charge states, -1, 0, +1. H0 in most cases form amphoteric interstitials except for rutile-GeO2, LaAlO3, TiO2 and SnO2 where H0 act as donors bonding to O. H+ bonds to anion and H- bonds to cation.

This is different from the behaviour of H in tetrahedral semiconductors where H+ or H- bonds to the anion or cation and breaking a host cation-anion bond, leaving a cation or anion dangling bond, respectively. In ionic oxides, however, the oxygen site has four or less neighbours, but the cation site can have more neighbours. Often, H+ or H- can datively bond to the oxygen or metal site, without breaking a metal oxygen bond, it just sits at the side. We find H0 is never the lowest energy state. H acts exclusively as a donor for SnO2, thus SnO2 becomes n-type conductive.

In other cases, +/- charge transition level lies in the upper part of the band gap except for La2O3 and CuAlO2 where +/- charge transition level lies in the lower part of the band gap. The charge transition levels are shown by aligning the band edges of oxides to their respective charge neutrality levels. It is quite interesting that +/- charge transition levels of SnO2, CuAlO2, HfO2 and La2O3 are very close when referred to the vacuum level, regardless of distinctive +/- charge transition levels relative to their respective band gaps: +/- for SnO2 lies above the conduction band, in the lower part of band gap for CuAlO2, in the upper part of band gap for HfO2 and in the midgap for La2O3.

+/- charge transition levels also match among LaAlO3, theta Al2O3 and TiO2, and between quartz GeO2 and beta Ga2O3. There is not the close relationship to dangling bonds or charge neutrality levels as there was in the tetrahedral semiconductors. It is possible that this reference energy happens more generally, but not certain. We also align the band edges to the vacuum level. There is sign of constant +/-charge transition levels below vacuum energy, but not as significant as it is in tetrahedral semiconductors.

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