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Rishabh Upadhyay- ​ESR 5,  November 2018 - November 2021

AALTO, Helsinki, Finland

Master thesis: "High mobility ITZO (Indium Tin Zinc Oxide) thin film transistors"

In this work, we report on high-performance coplanar self-aligned (SA) amorphous-Indium-Tin-Zinc-Oxide (a-ITZO) thin-film transistors (TFTs) on various architectures. The a-ITZO films are first optimized with respect to the oxygen ratio, thickness and final anneal conditions with common-gate TFTs structure on Si/SiO2substrate. Optimized TFTs show mobility (μlin) between 20.0–25.0 cm2/(V.s). Material characterization revealed some degree of order compared to a truly amorphous film like a-IGZO but no grain boundaries or crystalline domains were observed. The a-ITZO films were integrated in coplanar SA TFT architecture on polyimide using hydrogen rich plasma (SiH4 based chemistry) as dopant for the source/drain (S/D) regions resulting in field-effect mobility (μFE) of 27.0 cm2/ (V.s), sub-threshold slope (SS−1) of 0.40 V/decade and ION /IOFF ratio of >108. The threshold voltage shifts of the TFTs under both positive and negative gate bias stress of 1MV/cm for 104 seconds were less than 1.5 V. We have also investigated the applicability of the SA a-ITZO TFTs in logic circuitry such as 19-stage ring-oscillators (ROs). Supervisor: Prof. Francky Catthoor, IMEC, KU Leuven, Belgium

Personal Training Committee

Main supervisor: Jukka Pekola, AALTO

Co-supervisor: Thilo Bauch, Chalmers

Mentor: Arttu Luukanen, ASQELLA

Planned secondments

At ASQELLA, (October/November 2020) to get trained on software and programming techniques,

At CNRS, (remotely) to get trained in design and fabrication of mesoscopic devices for quantum heat transport,

At UKON, (cancelled due to Covid-19) to get trained on non-equilibrium Green’s functions.

PhD Project

Quantum heat engines and refrigerators in mesoscopic circuits

Objectives: Heat engines using coherent quantum systems, such as superconducting circuits, as working substance are of considerable interest currently. Yet realisations of the theoretical concepts of such machines are still lacking. In this project, I will work on heat engines based on known superconducting qubit technology, now being harnessed at AALTO for thermodynamics experiments. An archetypal device is the so called quantum Otto engine, where the four-stroke cycle consists of adiabatic expansion, rejection of heat at constant volume, adiabatic compression, and heat extraction at constant volume. This cycle can be realised by employing two non-identical LCR-resonators coupled to a qubit whose level spacing (« volume ») is varied by magnetic flux. The resistors of the LCR-resonators serve as the cold and hot bath of the Otto engine, and their temperatures are monitored by tunnel junction probes. I will be responsible for designing, realisation and measurement of such heat engines. In the first place, I plan to use a superconducting qubit as a heat switch between baths coupled through it only at their resonance frequency. Following this DC experiment, my focus will be on RF-cycles of the qubit realizing a four-stroke quantum engine/refrigerator. One of the key questions to be addressed by me is whether quantum coherence can be used into the benefit in terms of power and efficiency of a heat engine.

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