The geometrically confined growth of highly crystalline semiconductor materials like Ge on amorphous layers (without epitaxial template) has been a challenge in materials science. The success of this technology, especially at low fabrication temperature can enable 3D Si photonics by building photonic devices on metal/interconnect layers, and enable inexpensive, high-throughput  fabrication of devices such as tandem solar cell and thin-film transistor.

 

Here we report that, by incorporating  Sn into Ge, we are able to obtain highly crystalline GeSn thin film on amorphous substrates at low crystallization temperatures ranging from 340 to 464 ºC through enhanced crystallization from the Ge-Sn euetectic system, and are able to transform the material toward a direct band-gap material through band structure modification. A highly textured Ge_0.91Sn_0.09 is obtained from the sample with 9.5 at. % as-deposited Sn composition, which shows grain sizes up to tens of microns, as revealed by electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and Raman spectroscopy. EBSD confirms that most grain boundaries are either twin or low angle boundaries, which are benign to optoelectronic properties. Strikingly, the nucleation center distance ranges from 0.1 to 1 mm, orders of magnitude larger than common solid state crystallization, which indicates an exceedingly high grain growth rate vs. a low nucleation rate. As a result of this feature, we are able to fabricate geometrically confined pseudo single crystalline GeSn grain using patterning techniques. Another remarkable result is that 8.7 at.% Sn is incorporated substitutionally into Ge, far exceeding the equilibrium solubility limit of ~1%. The high Sn composition, with the addition of ~0.24% tensile strain in the film, shifts the direct band gap to 0.5 eV (2500nm), right on the verge of indirect-to-direct band gap transition. This transition will greatly enhance the material's optoelectronic performance as a photonic device, and will also broaden the absorption spectrum for a tandem solar cell.​