Ge Epitaxy at Ultralow Growth Temperatures Enabled by a Pristine Growth Environment

Germanium (Ge), the next-in-line group-IV material, bears great potential to add functionality and performance to next-generation nanoelectronics and solid-state quantum transport based on silicon (Si) technology. Here, we investigate the direct epitaxial growth of two-dimensional high-quality crystalline Ge layers on Si deposited at ultralow growth temperatures (<i>T</i> _Ge = 100-350 °C) and pristine growth pressures (≲10^-10 mbar). First, we show that a decreasing <i>T</i> _Ge does not degrade the crystal quality of homoepitaxial Ge/Ge(001) by comparing the point defect density using positron annihilation lifetime spectroscopy. Subsequently, we present a systematic investigation of the Ge/Si(001) heteroepitaxy, varying the Ge coverage (Θ_Ge, 1, 2, 4, 8, 12, and 16 nm) and <i>T</i> _Ge (100-300 °C, in increments of 50 °C) to assess the influence of these parameters on the layer's structural quality. Atomic force microscopy revealed a rippled surface topography with superimposed grainy features and the absence of three-dimensional structures, such as quantum dots. Transmission electron microscopy unveiled pseudomorphic grains of highly crystalline growth separated by defective domains. Thanks to nanobeam scanning X-ray diffraction measurements, we were able to evidence the lattice strain fluctuations due to the ripple-like structure of the layers. We conclude that the heteroepitaxial strain contributes to the formation of the ripples, which originate from the kinetic limitations of the ultralow temperatures.