Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 2 / Section A(i) / 14 / iii

An atomic layer deposition recipe depends on the surface chemistry during the first cycles. A silicon wafer normally carries native oxide with reactive hydroxyl groups. A pristine graphene basal plane has no comparable dense population of dangling bonds or hydroxyl groups, so precursor adsorption and initial nucleation are much less uniform.

Consequently the nominal number of cycles calibrated to give $2\mathrm{nm}$ on silicon can instead give isolated alumina islands, preferential decoration of edges, grain boundaries and defects, and uncovered graphene between islands. Adsorbed transfer residues may provide additional, irregular nucleation sites. Once alumina has nucleated, later cycles grow on the alumina normally; the problem is delayed, spatially uneven nucleation. The resulting local island height can exceed the film's area-averaged equivalent thickness.^[ref]

Two suitable improvements are:

Ozone or a mild plasma is another possible activation route, but it must be controlled because creating covalent defects can degrade the electrical properties that motivate using graphene.