Nanocrystal formation by thermal treatment of Ge/Sn/TaZrO_x superlattice structures

Ge nanocrystals embedded in amorphous TaZrO_x matrices were synthesized by a magnetron co-sputtering process in a size-controlled manner, separated from each other by a superlattice approach of alternating TaZrO_x-Ge/TaZrO_x layers. Ge offers, in contrast to silicon, a large Exciton-Bohr radius, which allows the regulation of the bandgap by the diameter.

After annealing at 725 °C, Ge nanostructures were found to phase-separate and crystallize [1]. The addition of Sn to Ge promises a way towards a direct bandgap Ge-based semiconductor material [2] in a wide concentration range between 6 at.-% and 22 at.-% of Sn [3]. A direct bandgap material, which is fully compatible to common Si-CMOS processes would enable the integration in future devices, like light emitters or detectors.

The fabrication of GeSn alloys is challenging, due to the low solubility of Sn in Ge [4] but it is known that Ge nanocrystals after annealing contain a higher concentration of foreign atoms than expected in thermodynamic equilibrium [1].

Different structural measurements like EDX or Raman scattering monitor the structural changes within the Ge nanostructures by addition of Sn to the superlattice structure under annealing temperatures between 600 °C and 800 °C. TEM EDX measurements revealed that the TaZrO_x, which acts as an efficient diffusion barrier for Ge, does not prohibit the diffusion of the Sn completely. However, the formation of pure Sn nanocrystals was not observed. In addition, crystallization of the matrix material can be avoided for appropriate annealing temperatures.

1. D. Lehninger, F. Honeit, D. Rafaja, V. Klemm, C. Röder, L. Khomenkova, F. Schneider, J. von Borany, and J. Heitmann, MRS Bull. 511–512, 654 (2022).
2. A. Slav, C. Palade, C. Logofatu, I. Dascalescu, A. M. Lepadatu, I. Stavarache, F. Comanescu, S. Iftimie, S. Antohe, S. Lazanu, V. S. Teodorescu, D. Buca, M. L. Ciurea, M. Braic, and T. Stoica, ACS Appl. Nano Mater. 2, 3626 (2019).
3. W.-J. Yin, X.-G. Gong, and S.-H. Wei, Phys. Rev. B 78, 161203(4) (2008).
4. H. Li, J. Brouillet, A. Salas, X. Wang, and J. Liu, Opt. Mater. Express 3, 1385 (2013).