Ge Epitaxy at Ultralow Growth Temperatures Enabled\nby a Pristine Growth Environment

Germanium (Ge), the\nnext-in-line group-IV material, bears great\npotential to add functionality and performance to next-generation\nnanoelectronics and solid-state quantum transport based on silicon\n(Si) technology. Here, we investigate the direct epitaxial growth\nof two-dimensional high-quality crystalline Ge layers on Si deposited\nat ultralow growth temperatures (<i>T</i><sub>Ge</sub> =\n100–350 °C) and pristine growth pressures (≲10<sup>–10</sup> mbar). First, we show that a decreasing <i>T</i><sub>Ge</sub> does not degrade the crystal quality of homoepitaxial\nGe/Ge(001) by comparing the point defect density using positron annihilation\nlifetime spectroscopy. Subsequently, we present a systematic investigation\nof the Ge/Si(001) heteroepitaxy, varying the Ge coverage (Θ<sub>Ge,</sub> 1, 2, 4, 8, 12, and 16 nm) and <i>T</i><sub>Ge</sub> (100–300 °C, in increments of 50 °C) to assess\nthe influence of these parameters on the layer’s structural\nquality. Atomic force microscopy revealed a rippled surface topography\nwith superimposed grainy features and the absence of three-dimensional\nstructures, such as quantum dots. Transmission electron microscopy\nunveiled pseudomorphic grains of highly crystalline growth separated\nby defective domains. Thanks to nanobeam scanning X-ray diffraction\nmeasurements, we were able to evidence the lattice strain fluctuations\ndue to the ripple-like structure of the layers. We conclude that the\nheteroepitaxial strain contributes to the formation of the ripples,\nwhich originate from the kinetic limitations of the ultralow temperatures.