Engineered coalescence of three-dimensional Ge microcrystals into high-quality suspended layers on Si pillars

The move from dimensional to functional scaling in microelectronics has recently revamped the interest towards integration of high-quality Ge on Si [1]. However, due to the 4% lattice misfit, this is quite a technological challenge. In ref. [2] we showed the possibility to grow vertically-aligned micrometric Ge crystals, separated by nanometric gaps, on top of Si pillar patterned substrates, without dislocations at the top region. Herein we show by simulation-driven experiments, how these structures can evolve into a suspended Ge film by simply exploiting their temperature-driven coalescence. By closely comparing experimental data and simulation results based on a phase-field model of surface diffusion [3], we demonstrate that the massive morphological evolution is essentially driven by the lowering of surface-curvature gradients. Scanning Tunneling Electron Microscopy (STEM) and High Resolution - Transmission Electron Microscopy (HR-TEM) measurements indicate that the resulting film has very high crystal quality. In particular, dislocations are not observed to nucleate when the crystals start to coalesce, as confirmed by etch-pit analysis. Our approach thus provides a viable path for the integration on Si of Ge layers with very low defect density. [1] R. Pillarisetty, Nature 479, 324 (2011); [2] C.V. Falub et al., Science 335, 1330 (2012); [3] M. Salvalaglio, R. Backofen, R. Bergamaschini, F. Montalenti, A. Voigt, Cryst. Growth Des, in press (doi: 10.1021/acs.cgd.5b00165)