Because of the 4% lattice mismatch between Ge and Si, threading dislocations (TDs) are generated in Ge epilayers on Si, deteriorating the performance of Ge devices on Si platform. We recently modeled the reduction of TD density in heteroepitaxial coalesced layers in terms of the bending of TDs induced by image forces at non-planar selective epitaxial growth (SEG) surfaces before the coalescence. The reduction of TD density was quantitatively verified for Ge layers on (001) Si with line-and-space SiO2 masks. In the present paper, detailed theoretical calculation and experimental results are presented. Numerical calculation shows that the image force is large enough to bend/move dislocations considering the Peierls stress or the mobility of TDs in Ge. Transmission electron microscope observations show that the TD bending is certainly induced. The TD density is lower above the SiO2 masks, as confirmed by the etch pit method. Such spatial distribution is well explained by the image-force-induced dislocation bending.
The bandgap tuning of sub-micron wide Germanium (Ge) waveguides by selective epitaxial growth (SEG)
method with a SiO2 mask has been demonstrated. SEG-grown Ge waveguides on Si substrate are designed to show
various compressive strain depending on the growth parameters, such as the width and thickness of Ge waveguides and
SiO2 masks. X-Ray Diffraction (XRD) verifies that -0.25% (compressive) strain is induced in a 0.6μm-wide Ge
waveguide with SiO2 mask of 20μm width and 1.0μm thickness. The strained Ge waveguide should show the absorption
edge wavelength of ~1.55μm. Furthermore, compressive strain can be tuned between -0.03% and -0.25% by changing
the lateral structure of the device, which correspond to the absorption edge wavelength of 1.548~1.568μm. It means that
only one epitaxial growth with specific lateral design of the electro-absorption modulator can modulate light in the
wavelength range.
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