(Invited) Epitaxial Growth of Nanodots on Si Substrates with Controlled Interfaces and Their Application to Electronics and Thermoelectronics

Fe-based materials (Fe, its oxide, its silicide and germanide) have been widely studied for device applications: spintronic or magnetic devices and resistance random access memory using resistive switching properties in iron oxide. Skyrmions recently-observed in ε-FeGe(Si) also led to a large interest in Fe-based materials. In terms of device application, Fe-based epitaxial films on Si substrates, which are low-cost and ecofriendly materials, have drawn much attention.  However, epitaxial growth of the films on Si with sharp interfaces is difficult because the interfaces are easily disordered. On the other hand, we have developed an ultrathin SiO₂ film technique for epitaxial growth of Si and Fe silicide nanodots (NDs) [1-3].  In this paper, by applying the ultrathin SiO₂film technique, we develop the epitaxial growth technique of Fe-based NDs on Si with crystal structure controllability. We also demonstrate the application of these NDs.

 Clean Si surfaces were oxidized at 600 °C at an oxygen pressure of 2×10^-4 Pa to form SiO₂ films with a thickness of ~0.3 nm. A few monolayers (ML) of Si or Ge were predeposited on the ultrathin SiO₂ films to form ultrahigh density (>10¹² cm^-2) nanovoids in the ultrathin SiO₂films. Excessive Si or Ge atoms deposited subsequently formed epitaxial ultrasmall nuclei on nanovoids that are referred to be Si or Ge nanowindows (NWs). Fe was then deposited on the surfaces to form Fe-based NDs.

 Fe NDs were epitaxially grown on Si substrates by Fe deposition on the ultrathin SiO₂ films with Si NWs at room temperature (RT) [4].  In order to form Fe oxide NDs, we exposed the Fe NDs to 1 atmosphere of dry oxygen [4]. Then, oxidation and silicidation were enhanced at the interfaces between Fe-based NDs and Si substrates, leading to the destruction of prior ND structures. This reaction was due to the existence of Fe(ND)/Si(substrates) interfaces. We also formed epitaxial NDs by deposition of Fe on Ge NWs at RT instead of Si NWs. In this case,  the above reaction did not occur during O₂exposure due to the lack of Fe/Si interfaces, and Fe oxide NDs were then formed while maintaining prior ND structures [4], demonstrating that reaction between gas and NDs were strongly influenced by the nanometer-sized interface between NDs and substrates.

 On the other hand, at higher temperature of Fe deposition iron germanide NDs were formed. We formed two kinds of Ge nuclei in NWs by tuning temperature of Ge deposition: flattened strained and spherical strain-relaxed ones. By Fe deposition on these Ge nuclei, ultrahigh density iron germanide NDs were epitaxially grown on Si substrates with sharp interfaces [5]. Their formed crystal structures were determined by the shape and strain status of Ge nuclei at the interface between NDs and substrates. These results demonstrated that intermixing and crystal structures in the Fe-Si-Ge system were well-controlled in this technique.

 We also demonstrate the application of Fe- and Si- based NDs formed by the above technique to electronics or thermoelectronics.

 This work was supported in part by a JST-PRESTO program (Y. Nakamura). It was also supported by a Grant-in-Aid for Scientific Research B (25286026) and Exploratory Research (25600016), and by TEPCO Memorial Foundation, Research Grant. HRTEM observations were carried out at the Research Center for UHVEM, Osaka Univ. and AIST Tech.–Kansai.

[1] Y. Nakamura, et al., Phys. Rev. B 72, 075404 (2005). [2] Y. Nakamura, et al., J. Appl. Phys. 100, 044313 (2006). [3] S. Amari, et al., J. Appl. Phys. 115, 084306 (2014). [4] H. Hamanaka, et al., J. Appl. Phys. 114, 114309 (2013). [5] Y. Nakamura, et al., J. Appl. Phys. 115 044301 (2014).