Low Temperature Geometrically Confined Growth of Pseudo Single Crystalline GeSn on Amorphous Layers for Advanced Optoelectronics

Abstract

Highly textured Ge0.91Sn0.09 is obtained on both amorphous SiO2/Si and glass substrates at low temperatures <475 oC, which shows grain sizes up to tens of microns. Strikingly, the nucleation center spacing ranges from 0.1 to 1 mm, orders of magnitude larger than common solid state crystallization. This observation indicates an exceedingly high grain growth rate vs. a low nucleation rate. Therefore, we can control nucleation sites and fabricate geometrically confined pseudo single crystalline GeSn grain using patterning techniques, including surface Sn dots/patches, local laser annealing, and nanotaper patterns. Another remarkable result is that ~9 at.% Sn is incorporated substitutionally into Ge, far exceeding the equilibrium solubility limit of ~1 at.%. The high Sn composition, together with ~0.24% thermally induced tensile strain in the film, shifts the GeSn direct band gap to ~0.5 eV (2500nm) and converts it into a direct band gap semiconductor with significantly enhanced optoelectronic properties.

GeSn on Insulators (GeSnOI) Toward Mid-infrared Integrated Photonics

“…These factors favor indirect-to-direct gap transition [21], in contrast to compressive strain in their [001] oriented epitaxial counterparts. Figure 3 further demonstrates that the EEC mechanism of a-GeSn is equally applicable to flexible Willow glass substrate, polyimide substrate, and fused silica substrate alike [26,32]. Large grain sizes of 0.1-1 mm are maintained in all these cases for Ge 0.91 Sn 0.09 thin films.…”

“…Furthermore, because of the lower surface energy of Sn compared to Ge, the Sn-rich liquid phase would segregate at the grain boundaries and on the surface of crystallized GeSn. Subsequently, the excess Sn can be easily and selectively etched away using HCl solution, leaving the crystallized GeSn for integrated photonic device fabrication [3,10,21,[26][27][28][29].…”

“…This phenomenon is in line with the well-defined eutectic temperature in the Ge-Sn equilibrium phase diagram, although the actual crystallization temperature of a-GeSn is beyond the description of the phase diagram due to its nonequilibrium nature. (3) A strong (111) preferred orientation due to minimization of surface energy on amorphous layers [10,21,26,27]. As will be discussed later, this preferred (111) orientation also has a beneficial effect on the strain engineering of GeSn toward direct gap semiconductors.…”

“…This observation indicates a slow nucleation rate vs. an extremely fast lateral growth rate during GeSn crystallization induced by the EEC mechanism. Electron backscatter diffraction (EBSD) analyses show that each grain predominantly consists of micro-twins as a result of lateral growth from the same nucleation center upon crystallization [10,26,27]. Figures 2b,c show the crosssectional SEM image and the corresponding energy dispersion X-ray spectroscopy (EDS) mapping for compositional analyses, respectively.…”

GeSn mid-IR materials and devices for 3D photonic integration

“…If the Sn composition and/or tensile can be further increased, the direct band gap can be reduced to <0.2 eV, covering =3-6 m in the MIR regime. [4,5] (Fig. 2a).…”