2021 Fiscal Year Annual Research Report
Fabrication, characterization and application of GeSn core-shell nanowires
| Project/Area Number |
21J11537
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| Research Institution | University of Tsukuba |
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Principal Investigator |
孫 永烈 筑波大学, 数理物質科学研究科, 特別研究員(DC2)
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| Project Period (FY) |
2021-04-28 – 2023-03-31
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| Keywords | nanowire / core-shell / germanium / nanoimprint lithography |
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| Outline of Annual Research Achievements |
For application to GeSn core-shell nanowires, the top-down fabrication of Ge nanowire array cores and their core-shell heterostructure were investigated. Ge nanowire arrays with smooth surfaces and high aspect ratio were successfully fabricated by using nanoimprint lithography (NIL) and the Bosch process. By varying the number of cycles in the Bosch process, different lengths from 500 nm to 2140 nm were achieved. The diameter of nanowires can be adjusted from 220 nm to 30 nm by varying the wet chemical etching time. On the other hand, doped shell layers were formed on Ge nanowires by chemical vapor deposition (CVD) to fabricate core-shell nanowire structures. The effects of growth time, temperature, precursor gas flux, and annealing temperature on shell morphology and doping concentration were studied. Raman scattering analysis showed that hole gas accumulated in the Ge region, and the concentration of hole gas was higher in the small core than in the large core. Defects formed during the Bosch process and core-shell formation were investigated by electron spin resonance (ESR), and the results show that the defects can be removed by thermal annealing with different types of atmospheric gases. A journal manuscript on this top-down method is currently in preparation. Moreover, Ge nanotube structures with much higher surface-to-volume ratios were also fabricated for future applications in high-mobility electronics, and the results are summarized in one journal paper.
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| Current Status of Research Progress |
Current Status of Research Progress
3: Progress in research has been slightly delayed.
Reason
Previously, we had planned to use nanowires fabricated by vapor－liquid－solid (VLS) growth as the core of GeSn core-shell nanowires. However, we found that this method will degrade the electrical properties of the nanowires due to metal contamination and also resulted in different growth directions. Therefore, we changed to the nanoimprint lithography (NIL) method, a top-down approach that can solve these problems. On the other hand, the molecular beam epitaxy (MBE) method was shown to be unsuitable for nanowire shell formation due to the non-uniform morphology and thickness. Therefore, we switched to the chemical vapor deposition (CVD) method to form uniform shell layers with smooth surfaces. The top-down fabrication of Ge nanowire cores by NIL and the studies of core-shell formation by CVD are progressing well, as described in the research achievement part.
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| Strategy for Future Research Activity |
In this fiscal year, we plan to fabricate vertical GeSn core-shell nanowire MOSFETs. Their mobility and other characteristics will be evaluated for demonstrating ultra-fast switching and high controllability. The fabrication processes will be optimized by considering the deposition conditions such as temperature, rate, and thickness of the dielectric oxide layer, gate, and electrode. All the above processes can be performed using the shared facilities of NIMS. Finally, we will measure the electrical properties of the completed MOSFETs and investigate the factors (lattice scattering, impurity scattering, etc.) that determine carrier mobility in relation to channel current, gate voltage, and temperature and study their dependence on Sn concentration and core diameter.
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