(Invited) Formation and Characterization of Fe-Silicide Nanodots for Optoelectronic Application
Authors: Miyazaki Seiichi , Makihara Katsunori 
Published: February 2024

Light emission from group-IV based nanostructures has been attracting much attention in the field of Si-based photonics because of its potential to combine photonic processing with electronic processing on a single chip [1]. Among various Fe-silicides, semiconducting β-FeSi2 shows light emission in the near-infrared region [2, 3] and is one of promising materials in optoelectronic application from viewpoints of tuning emission energy and improving luminous efficiency by means of quantized structures. In this work, we focus on low temperature formation of β-FeSi2 nanodots (NDs) over ~1011 cm-2 in areal density on SiO2 and the characterization of light emission properties of β–FeSi2 NDs.
After conventional wet-chemical cleaning steps of p-type Si(100) substrates, a ~300-nm-thick SiO2 layer was grown at 1000°C in dry O2 and followed by dipping shortly in a dilute HF solution to make uniform surface termination with Si-OH bonds. After that, Fe films in the thickness range from 1.0 to 3.0 nm were formed on the OH-terminate SiO2 layer by electron beam evaporation without any extra heating. And then, the Fe–films were exposed to remote plasma of pure H2 (H2-RP) at 400°C to form highly-dense Fe-NDs [4]. Subsequently, so-prepared Fe-NDs were exposed to pure SiH4 at 400°C while controlling the amount of SiH4 exposure by duration and pressure.
AFM images taken after exposure of Fe-films to H2-RP confirm the high-density formation of well-defined NDs. In the cases of H2-RP exposure of 1.3-nm-thick and 3.0-nm-thick Fe-films, the areal dot densities as high as ~1.1 × 1011 and ~1.3 × 1011 cm−2 were obtained, respectively, in which the average dot-heights of self-assembled Fe-NDs were determined to be ~5.3 and ~10.9 nm, respectively, by fitting a log-normal function to each measured size distribution. Notice that AFM images taken after SiH4 exposure show almost no change in the areal dot density while a slight increase in the average dot height. From cross-sectional transmission electron microscope images, we have also verified the formation of distorted-spherical shape NDs with a lattice spacing of ~0.18 nm associated with β-FeSi2. And the result is well consistent with the chemical composition of NDs after SiH4 exposure as evaluated by x-ray photoelectron spectroscopy. In addition, with 976 nm light excitation using a semiconductor laser, the NDs after the SiH4 exposure show stable photoluminescence (PL) signals in the energy region from ~0.7 to ~0.9 eV even at room temperature. On the other hand, no PL signals were detected from the NDs before the SiH4 exposure and after Ar anneal at 400°C. Also, when the size of the Fe-silicide NDs was increased from ~5.3 to ~10.9 nm in average dot height by increasing the initial Fe-film thickness, a distinct red-shift in PL was clearly observed. Thus, the observed PL can be interpreted in terms of radiative recombination of photogenerated carriers through quantized states of β-FeSi2 NDs. These results indicate that SiH4 exposure to Fe-NDs formed on SiO2 is a very promising technique for the low-temperature fabrication of luminescent FeSi2 NDs.
Acknowledgement
This work was supported in part by Grant-in-Aid for Scientific Research (A) 21H04559 of MEXT Japan, and the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University.
References
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