High Thermal Stability of Tensile Strained Direct Gap GeSn Crystallized on Amorphous Layers

Abstract

In this paper, we systematically research the thermal evolution of substitutional Sn composition, tensile strain, and the direct bandgap of highly (111) textured, direct-gap Ge1-xSnx (0.075<x<0.085) thin films crystallized on amorphous SiO2 layers. We are able to demonstrate highly effective strain induced band-engineering in Ge1-xSnx as high performance optoelectronic materials for monolithic 3D electronic-photonic integration. For a Ge0.913Sn0.087 thin film, xSn stays at 8.7 at. % without any Sn segregation even after 2 hours of annealing at the crystallization temperature of 464 oC, indicating that the material is thermally stable in the entire back-end-of-the-line processing temperature range (<450 oC). We then sequentially anneal the Ge0.913Sn0.087 sample from 500 oC to 700 oC at a step of 50 oC, for 15 min each step, and selectively etch away any surface segregated Sn after each step. After analyzing the materials using a combination of X-ray Diffraction (XRD) and Raman spectroscopy, we determine that the crystallinity improves with the increase of annealing temperature, as evidenced by decreased full width at half maximum of both Raman and XRD peaks. The GeSn film maintained its strong (111) texture, as evidenced by the >270 ratio of the strongest to second strongest XRD peak. The xSn only decreases slightly from 8.3 at. % at 500 oC to 7.4 at. % at 700 oC, still ~7 times higher than the equilibrium solubility limit of ~1 at. % whereas the in-plane tensile strain e|| increases by nearly 4 times from 0.12% to 0.44%. By measuring and fitting the absorption spectra, we find that remarkably, the 0.44% thermally induced biaxial tensile strain reduces the direct bandgap by as much as 0.125 eV, twice as effective as the tensile strain in Ge(100) films. Due to a smaller Poisson’s ratio under biaxial tensile stress, the (111) oriented films experience larger volume dilatation than their (100) oriented counterparts under the same in-plane tensile strain, thereby enhancing the beneficial effects of tensile strain on the indirect-to-direct gap transition. By fitting the data with deformation potential theory, we derive a dilatational deformation potential of -12.8+/-0.8 eV for Ge1-xSnx thin films with xSn=8 at.%. Despite of the slight decrease in Sn composition at high annealing temperatures, the tensile-strain-induced direct-gap shrinkage, not only extends the optical response to λ = 2.8 um for MIR applications but also leads to stronger direct-gap semiconductor behavior. These results indicate that tensile strained GeSn crystallized on amorphous layers offers both excellent direct-gap optoelectronic properties and fabrication/operation robustness for integrated photonics. The same concept can also be readily applied to epitaxial GeSn on (111) substrates to achieve more effective strain-induced band engineering. Figure 1