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Atomic Scale Group-IV Materials for Beyond-CMOS Applications

Final Report Summary - ATOMIC SCALE GFM (Atomic Scale Group-IV Materials for Beyond-CMOS Applications)

Primary objective of this project is to find out fundamental properties of group-IV materials in an atomic scale. It is well-known that graphene, monolayer of a carbon sheet, has completely different characteristics from bulk counterpart of graphite; the mass of electron becomes zero in graphene. Graphene is the same group-IV material with silicon, which is indispensable for modern era of Information Communication Technology (ICT). Thus, graphene has a potential to be used for future electronic devices. Another important group-IV material is germanium, which is compatible with existing infrastructures to fabricate silicon based electronic and photonic devices. The properties of silicon and germanium can be different in an atomic scale, as is true for graphene. For example, the nano-structured silicon can emit lights efficiently, regardless of the indirect band gap structure in the bulk, which is a clear signature for showing the electronic band structure can be completely changed in the atomic scale. However, it is very difficult to fabricate these nano-structures in a well-controlled way, especially using bottom up chemical reactions. Now, silicon based process technologies are ready to manufacture sub-20-nm features by top-down processes. The idea of this project is to utilize these state-of-the-art silicon based patterning capabilities to explore fundamental properties of atomic scale group-IV materials, such as
light-emissions and quantum confinements, to expand new frontiers of nano-electronics.

Work Packages (WPs):
Following WPs are set through the project.
WP1: Investigate the light-emission mechanisms from atomic-scale group-IV materials.
WP2: Developments of refined patterning technologies for silicon.
WP3: Applications of atomic scale group-IV materials.

Main results:
WP1: The fellow developed a germanium micro-disk on a free-standing oxide beam, and found a whispering gallery mode for the first time in a structure, where a strong tensile-strain engineering is compatible with an optical confinement. This structure will enable the fellow to investigate the light-emission mechanism towards a realization of the full monolithic light source on a silicon photonic chip. He also succeeded to dope donors into germanium by spin-on-doping techniques.

The fellow also developed a process to make Germanium (Ge) fins by selective oxidation of Silicon (Si) in SiGe fins. This enabled to condensate Ge inside fins, and he could fabricate a Ge LED in collaboration with Hitachi. The paper published JJAP was selected as Spotlights 2017. He also made a suspended Ge membrane with improved homogeneity in tensile strain in collaboration with the University of Leeds. The strong tensile strain was enough to observe the direct recombination from Ge. The developed processes will be useful to realise the Ge laser monolithically integrated on Si. The fellow published 6 invited review papers and contributed 3 invited talks in this subject. He led the world leading activities in this field towards the grand challenge.

WP2: The fellow successfully patterned silicon down to 20-nm. The surface of silicon was atomically flat by using highly anisotropic etching techniques. This nano-wire will be used for a single electron pump. The technique was also applied to making silicon waveguide, and low-loss propagation of 1.4 dB/cm was achieved.
The fellow successfully refined his patterning technologies using silicon.
In 2017, the minimum feature size below 5nm is routinely patterned in his group. As one of the most out-standing application, he developed a Si-based single electron pump in collaboration with NPL. The number of process steps exceeded more than 400, and it took more than 1 year for completion. He organised a briefing everyday with a postdoc and PhD students, to check the status of fabrication and make a decision for the next process step. Finally, the device was successfully operated and more than 100 devices implementing many ideas are working. We have already confirmed various exotic phenomena including single electron charging, Random-Telegraph-Noise, single-electron-pumping, and so on. We will characterise these devices in collaboration with NPL and Hitachi Cambridge Laboratory. There are a lot of opportunities in the EU for the future development of Quantum Technologies, and this project will act as a leverage to extend further projects.

WP3: Various applications of atomic scale group-IV materials have been identified. Single electron pump will be applied to realize the new definition of SI unit by using the elementary charge for the current standard. The silicon waveguide will be useful for various silicon photonic devices, including
high speed optical modulators. Silicon based single photon source will also be useful for quantum technologies.
Applications of group-IV materials are also important for Photonics. We have developed a patterning process to make a waveguide with atomically flat interface using Si (111) plane. We confirmed the propagation loss of less than 1dB/cm in the clean room at the University, which is comparable to the industry grade. Moreover, we have also achieved the world record of the loss in a slot waveguide. This will be useful for future sensing applications. We have also established a process flow to fabricate a slot waveguide with arbitrary narrow oxide layer, which is useful for the high speed optical modulator. We have attracted interests from industries and follow-up projects are funded by EPSRC.