Ge epitaxy at ultra-low 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 ultra-low growth temperatures ($T_{Ge} = 100^{\circ}\mathrm{C}-350^{\circ}\mathrm{C}$) and pristine growth pressures ($\lesssim 10^{-10}\,\mathrm{mbar}$). First, we show that $T_{Ge}$ 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 $T_{Ge}$ ($100^{\circ}\mathrm{C}$ to $300^{\circ}\mathrm{C}$, in increments of $50^{\circ}\mathrm{C}$) 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 ultra-low temperatures.