Semiconductor-based quantum registers require scalable quantum-dots (QDs) to be accurately located in close proximity to and independently addressable by external electrodes. Si-based QD qubits have been realized in various lithographically-defined Si/SiGe heterostructures and validated only for milli-Kelvin temperature operation. QD qubits have recently been explored in germanium (Ge) materials systems that are envisaged to operate at higher temperatures, relax lithographic-fabrication requirements, and scale up to large quantum systems. We report the unique scalability and tunability of Ge spherical-shaped QDs that are controllably located, closely coupled between each another, and self-aligned with control electrodes, using a coordinated combination of lithographic patterning and self-assembled growth. The core experimental design is based on the thermal oxidation of poly-SiGe spacer islands located at each sidewall corner or included-angle location of Si3N4/Si-ridges with specially designed fanout structures. Multiple Ge QDs with good tunability in QD sizes and self-aligned electrodes were controllably achieved. Spherical-shaped Ge QDs are closely coupled to each other via coupling barriers of Si3N4 spacer layers/c-Si that are electrically tunable via self-aligned poly-Si or polycide electrodes. Our ability to place size-tunable spherical Ge QDs at any desired location, therefore, offers a large parameter space within which to design novel quantum electronic devices.
Beginning with our exciting discovery of germanium (Ge) spherical quantum-dot (QD) formation via the peculiar and symbiotic interactions of Si, Ge, and O interstitials, we have embarked on a journey of vigorous exploration, creating unique configurations of self-organized Ge-QDs/Si-containing layers. Our aim is to generate advanced Ge-QD photonic devices, while using standard, mainstream Si processing techniques. This paper summarizes our portfolio of innovative Ge-QD configurations. With emphasis on both controllability and repeatability, we have fabricated size-tunable, spherical Ge-QDs that are placed at predetermined spatial locations within Si-containing layers (SiO2, Si3N4, and Si) using a coordinated combination of lithographic patterning and self-assembled growth. We have successfully exploited the multi-dimensional, parameter spaces of process conditions in combination with layout designs to achieve exquisite control available through the thermal oxidation of lithographically patterned, poly-Si1 − xGex structures in close proximity with Si3N4/Si layers. In so doing, we have gained insight into the growth kinetics and formation mechanisms of self-organized, Ge spherical QDs embedded within SiO2, Si3N4, and Si layers, respectively. Our Ge-QD configurations have opened up a myriad of process/integration possibilities including top-to-bottom evanescent-wave coupling structures for SiN-waveguided Ge-QD photodetectors and Ge-QD light emitters for Si photonics within Si3N4 integrated photonics platforms for on-chip interconnects and sensing.
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