Heteroepitaxial Islands on Pit-patterned Substrates: Beyond Ordered Positioning

Soon after the first reports on the formation of Stranski-Krastanow (SK) Ge/Si and InGaAs/GaAs nanoislands appeared in the early 90s, it was realized that a major problem had to be tackled in order to make such systems of potential interest for applications. Islands, indeed, tended to nucleate randomly, and displayed poor uniformity in size and shape. Nowadays, extremely ordered arrays can be obtained by exploiting recent nanolithographic techniques. Suitable patterning of the substrate allows one to introduce preferential nucleation sites and, under properly adjusted growth conditions, spectacular ordering in positioning, size and shape has been demonstrated [1]. In the talk, I shall review some recent experimental and theoretical results concerning the prototypical Ge/Si(001) SK system, pointing out some unexpected results, such as anomalous pit-filling [2], delayed onset of plasticity [3], and collective island-shape oscillations [4]. Such phenomena demonstrate that the role played by the pattern is an active one, going well beyond the original idea of "simply" triggering controlled formation of critical nuclei. With respect to growth on standard flat substrates, new possibilities are introduced in terms of controlling islands shape, strain state, and composition. [1] Z. Zhong and G. Bauer, Appl. Phys. Lett. 84, 1922 (2004). [2] R. Bergamaschini, J. Tersoff et al., in preparation [3] Z. Zhong et al., Phys. Rev. Lett. 98, 176102 (2007); F. Boioli et al., Appl. Phys. Lett. 99, 033106 (2011). [4] J.J. Zhang et al., Phys. Rev. Lett. 105, 166102 (2019).