ALD growth of ultra-thin Co layers on the topological insulator Sb₂Te₃

Taking the full advantage of the conformal growth characterizing atomic layer deposition (ALD), the possibility to grow Co thin films, with thickness from several tens down to few nanometers on top of a granular topological insulator (TI) Sb₂Te₃ film, exhibiting a quite high surface roughness (2-5 nm), was demonstrated. To study the Co growth on the Sb₂Te₃ substrate, we performed simultaneous Co depositions also on sputtered Pt substrates for comparison. We conducted a thorough chemical-structural characterization of the Co/Sb₂Te₃ and Co/Pt heterostructures, confirming for both cases, not only an excellent conformality, but also the structural continuity of the Co layers. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) analyses evidenced that Co on Sb₂Te₃ grows preferentially oriented along the [00ℓ] direction, following the underlying rhombohedric substrate. Differently, Co crystallizes in a cubic phase oriented along the [111] direction when deposited on Pt. This work shows that, in case of deposition of crystalline materials, the ALD surface selectivity and conformality can be extended to the definition of local epitaxy, where in-plane ordering of the crystal structure and mosaicity of the developed crystallized grains are dictated by the underlying substrate. Moreover, a highly sharp and chemically-pure Co/Sb₂Te₃ interface was evidenced, which is promising for the application of this growth process for spintronics.