Optical characterization of Ge/Si superlattices with stacked nanoripples

Optical properties of Ge/Si superlattices grown at low temperatures with strain-induced ripple structures were characterized by electroreflectance (ER) and resonance Raman spectroscopy. There were three types of samples under investigation. The growth temperatures of the Ge layers were 250, 300, and 350 °C. The diffusion length of Ge atom at such low temperatures is considerably smaller, and three-dimensional island growth is kinetically delayed or frozen out. The cross-sectional transmission electron microscope (TEM) image of the sample showed that strain-induced Ge/Si intermixing forms stacked SiGe-alloy ripples in the Si layers to release the strain in the Ge layers. When the Ge growth temperature decreases, the spectral feature of the Ge E₁ transition is shifted and enhanced because the three-dimensional island growth is kinetically suppressed, the Ge/Si intermixing formation of strain relieving stacking ripples is in favor, and the optical transition is enhanced. The same optical transition and enhancement is confirmed by the resonance Raman spectroscopy.