Polycrystalline group-IV alloys, such as Si_1-xGe_x, Ge_1-xSn_x, and Si_1-x-yGe_xSn_y, have attractive benefits for thermoelectric generators which can directly convert heat energy into electricity with less environmental impact. Grain boundaries in the polycrystalline material can successfully reduce thermal conductivity which is essential to operate the thermoelectric devices under temperature difference [1]. Besides, mass differences between component elements of the alloy contribute to modulate phonon characteristics [2].

In terms of the synthesis of the polycrystalline group-IV alloys, it has been challenging to realize crystallization, which typically needs high temperature annealing, simultaneously with controlling composition, especially Sn content which is limited by low solid solubility into Ge and Si as 1 at.% and 0.1 at.%, respectively, under equilibrium condition [3].

As one of the solutions to obtain the polycrystalline group-IV alloys having high Sn content, using crystalline Sn nanodots as nuclei is effective for the growth of crystalline alloys at low temperature. The low temperature growth, which is the growth far away from the equilibrium, enables introduction of Sn more than its solubility limit [4].

Amorphous Sn layer deposited on SiO₂ substrate can be easily transformed into crystalline Sn nanodots by just applying an annealing in vacuum. We demonstrated the formation of polycrystalline Ge_1-xSn_x and Si_1-xSn_x alloys by deposition of Ge and Si on the Sn nanodots at the substrate temperature of 150 °C and 225 °C, respectively [5,6]. Adatoms, namely Ge and Si, were alloyed with the Sn nanodots acting as crystal nuclei. However, we found that Ge preferentially reacts with Sn nanodots when Si and Ge are simultaneously deposited on the Sn nanodots. In this case, Si remains as amorphous, meaning the effect of nucleus for the low temperature crystallization is not effective for Si if Ge exists. This result suggests that Si has to be crystallized prior to Ge.

One can notice that the polycrystalline Si_1-xSn_x can be formed by the Sn nanodots mediated formation process as mentioned above. Therefore, we have proposed the 2-step formation process consisting of poly-Si_1-xSn_x crystallization and Ge alloying with the poly-Si_1-xSn_x which can be considered as a virtual substrate [6]. Using the 2-step formation process, whole deposited region was successfully crystallized with alloying Si, Ge, and Sn. The estimated Si and Sn content in the as-grown polycrystalline Si_1-x-yGe_xSn_y layer were as high as 10.8% and 3.5%, respectively. The impact of introduction of Sn on phonon characteristics will be discussed at the presentation if time allows.

Acknowledgements: This work was supported by JSPS KAKENHI Grant Number JP18K13786 and 21K04137 from the Japan Society for the Promotion of Science.

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