Robust Fabrication Techniques for Si/SiGe Quantum Dots
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Electrons and holes confined in quantum dots define an excellent building block for quantum emergence, simulation, and computation. In order for quantum electronics to become practical, large numbers of quantum dots will be required, necessitating the fabrication of scaled structures such as linear and 2D arrays. Group IV semiconductors contain stable isotopes with zero nuclear spin and can thereby serve as excellent host for spins with long quantum coherence. Here we demonstrate group IV quantum dot arrays in silicon metal-oxide-semiconductor (SiMOS), strained silicon (Si/SiGe) and strained germanium (Ge/SiGe). We fabricate using a multi-layer technique to achieve tightly confined quantum dots and compare integration processes. While SiMOS can benefit from a larger temperature budget and Ge/SiGe can make ohmic contact to metals, the overlapping gate structure to define the quantum dots can be based on a nearly identical integration. We realize charge sensing in each platform, for the first time in Ge/SiGe, and demonstrate fully functional linear and two-dimensional arrays where all quantum dots can be depleted to the last charge state. In Si/SiGe, we tune a quintuple quantum dot using the N+1 method to simultaneously reach the few electron regime for each quantum dot. We compare capacitive cross talk and find it to be the smallest in SiMOS, relevant for the tuning of quantum dot arrays. These results constitute an excellent base for quantum computation with quantum dots and provide opportunities for each platform to be integrated with standard semiconductor manufacturing.
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Indirect energy band structure can be transformed to a direct structure in nano-scale Ge quantum dots materials self-assembled on a Si substrate, as a result of the three dimensional quantum confinement effect. Enhancement of the exciton behavior and radiative transition via the energy band gap present a possible way for development of effective Si-based active photonic devices and the realization of ordered and uniform Ge quantum dot arrays. Their controllable fabrication, will be helpful for the development of a new generation of Si-based electron-wave quantum devices. A review is presented on the recent development of self-assembled Ge/Si quantum dots and possible wide applications. With reference for our recent research results, emphasis is given to the morphological evolution of Ge grown on a Si (001) substrate and their dynamical process, derivation of the type-Ⅱenergy band diagram of Ge/Si quantum dots through photo-luminescence studies, and our efforts to fabricate ordered and uniform Ge/Si quantum dot arrays with a Si pattern substrate.
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As CMOS device scaling approaches to sub 10-nm node, quantum size effects become significant even at room temperature. Recent progress in the fabrication technology of silicon nanostructures has made possible observations of novel electrical and optical properties of silicon quantum dots, such as single electron tunneling, ballistic transport, visible photoluminescence and electron emission. Possible applications in silicon quantum dots include high-efficiency light emitters and photovoltaic devices, and quantum information processing by spin manipulation. Silicon quantum dots are fabricated either by bottom-up or top-down processes. Fabrication and electrical characterization of silicon nanocrystals by plasma processes and coupled silicon quantum dots by electron beam lithography processes are discussed.
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