(Digital Presentation) Low Temperature Formation of Binary and Ternary Group-IV Poly-Layers Via Sn-Nanodots Nuclei
Authors: Shimura Yosuke , Okado Masaki , Utsumi Junya , Tatsuoka Hirokazu 
Published: November 2022

Polycrystalline group-IV alloys, such as Si1-xGex, Ge1-xSnx, and Si1-x-yGexSny, 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 Ge1-xSnx and Si1-xSnx 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 Si1-xSnx 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-Si1-xSnx crystallization and Ge alloying with the poly-Si1-xSnx 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 Si1-x-yGexSny 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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