The irregular habits of crystals

‘Defects’ are the main bugbear in the amorphous materials used as dielectrics in microelectronics – an industry that gives these materials a sharply rising global market value of $66 billion (£51 billion) as estimated in 2023. Yet to what extent these ‘defects’ actually resemble the crystal features from which they get their name remains a point of debate. In the literature on crystals, a ‘defect’ is any feature that breaks the regular order of the repeating crystal lattice – from sites on the crystal lattice with different chemical species swapped in, to sites where atoms are absent altogether. However, amorphous materials have no long-range repeating periodicity.

These gaps in our understanding of amorphous dielectric materials have persisted despite assiduous research over the past 50-year boom in the semiconductor industry. The theory of amorphous dielectrics borrows a lot from the physics of crystals with little clarity as to how valid these ideas may be when where there is no long-range order. And they are not the only materials where concepts from crystallography are applied even though the arrangement of atoms does not follow the periodic order of a regular crystal. Quasicrystals are now not only widely accepted among the scientific community, but accepted as ‘crystals’ thanks to a redefinition of the term by the International Union of Crystallography in 1992.