05 Fakultät Informatik, Elektrotechnik und Informationstechnik
Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/6
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Item Open Access Ultra-low-noise readout circuits for magnetoresistive sensors(2023) Mohamed, Ayman; Anders, Jens (Prof. Dr.-Ing.)The continuous search for highly sensitive, agile and cost-effective sensors for magnetic biosensing applications has been met with high performance magnetoresistive (MR) sensors. While the MR effect has been discovered 150 years ago, there is a growing trend of improving the sensitivity of MR sensors while keeping their noise performance as low as possible. However, such improvements have to be complemented with high performance frontends that can effectively amplify the minute MR sensor's signals while keeping the system's noise floor unaltered. More importantly, the designed frontends have to be equipped with offset compensation peripheral circuits that can efficiently handle the large spread of the base resistance in MR sensors with high MR ratios such as in tunnel magnetoresistive (TMR) sensors. In this thesis, we developed multiple frontend electronics that successfully interfaced MR sensors while, simultaneously, achieving competitive noise performance compared to state-of-the-art (SoA) designs tailored for MR sensor readout. The first variant of chips are specically designed for high performance and high linearity designs thanks to a novel implementation of an ultra-low-noise current bias achieving SoA current noise floor of 2.2 pA/sqrt(Hz) and chopped voltage-mode amplification stages resulting in a total voltage noise floor of 8 nV/sqrt(Hz), including a TMR sensor and a reference resistor with base resistance of 1 kOhm. In order to integrate an analog-to-digital converter (ADC) without substantial additional power and/or area, we show in this work a continuous-time current-mode Sigma-Delta modulator (CT C-SDM) that can directly interface MR sensors without additional amplifiers. Our proposed design does not only show a competitive noise floor of 8.1 pA/sqrt(Hz), but also features a novel DC servo loop (DSL) around the modulator that maximizes the useful dynamic range (DR) of the modulator while successfully rejecting the undesired DC offsets of MR sensors. Both design variants shown in this thesis, pave the way to designing high performance point-of-care (PoC) systems for in-vitro diagnostics while keeping their costs low compared to alternative bulky and expensive systems.Item Open Access Electrically detected magnetic resonance on a chip (EDMRoC) for analysis of thin-film silicon photovoltaics(2023) Segantini, Michele; Marcozzi, Gianluca; Djekic, Denis; Chu, Anh; Amkreutz, Daniel; Trinh, Cham Thi; Neubert, Sebastian; Stannowski, Bernd; Jacob, Kerstin; Rudolph, Ivo; McPeak, Joseph E.; Anders, Jens; Naydenov, Boris; Lips, KlausElectrically detected magnetic resonance (EDMR) is a spectroscopic technique that provides information about the physical properties of materials through the detection of variations in conductivity induced by spin-dependent processes. EDMR has been widely applied to investigate thin-film semiconductor materials in which the presence of defects can induce the current limiting processes. Conventional EDMR measurements are performed on samples with a special geometry that allows the use of a typical electron paramagnetic resonance (EPR) resonator. For such measurements, it is of utmost importance that the geometry of the sample under assessment does not influence the results of the experiment. Here, we present a single-board EPR spectrometer using a chip-integrated, voltage-controlled oscillator (VCO) array as a planar microwave source, whose geometry optimally matches that of a standard EDMR sample, and which greatly facilitates electrical interfacing to the device under assessment. The probehead combined an ultrasensitive transimpedance amplifier (TIA) with a twelve-coil array, VCO-based, single-board EPR spectrometer to permit EDMR-on-a-Chip (EDMRoC) investigations. EDMRoC measurements were performed at room temperature on a thin-film hydrogenated amorphous silicon (a-Si:H) pin solar cell under dark and forward bias conditions, and the recombination current driven by the a-Si:H dangling bonds (db) was detected. These experiments serve as a proof of concept for a new generation of small and versatile spectrometers that allow in situ and operando EDMR experiments.Item Open Access Silicon vacancy defects in 4H-silicon carbide semiconductor for quantum applications(2019) Nagy, Roland; Anders, Jens (Prof. Dr.)Secure transmission of information is a crucial element nowadays for industry and national security. Today, the only known way to establish provably secure communication is based on quantum key distribution in a quantum network. Currently, transmission rates and communication distances are limited by (unavoidable) optical loss in fibers and the fundamental quantum no-cloning theorem. In analogy to classical communication, improved network performance is obtained in a network based on multiple nodes that are connected by (quantum) repeaters. The information should be transferred with telecom wavelength photons to be compatible with existing classical fiber networks. The nodes and repeaters need to consist of a quantum system with good optical properties and long memory times, e.g. using a spin with high coherence. Additionally, such memories will be useful for computation and entanglement creation. A realistic quantum system should also be scalable and cheap in fabrication. The first demonstration of a solid state quantum repeater has been recently realized with the NV-centre in diamond. This demonstration showed that the NV-centre in diamond can be used in principle as a quantum repeater but it also brings drawbacks. The NV-centre in diamond has a good spin coherence but a low emission of photons inside the zero phonon line which can be used in a quantum network. A crucial challenge for color defects like the NV-centre in diamond is spectral stability. After a certain amount of time, a sanity check needs to be done to measure the wavelength of the resonant absorption lines. Any change in absorption wavelength requires adapting a multitude of experimental parameters, which is a show-stopper for long-term network reliability. These drawbacks decrease the transmission rate within a quantum network. If one would plan to realize a commercial quantum network, a tailored quantum system with all the mentioned desired properties would be needed. My first approach in this thesis was to use a semiconductor material like 4H-SiC with matured industrial fabrication knowledge (Chapter 2). 4H-SiC hosts a large variety of known quantum defects. I choose to analyze the silicon vacancy V1 centre because of the ZPL emission at 861 nm. It is known from the literature that this wavelength can be efficiently converted to commonly used telecom wavelengths (1530 - 1625 nm). I first analyzed the optical and spin properties in ensembles (Chapter 3). The spin measurement showed that one can coherently manipulate silicon vacancy V1 centre. Emission spectra of a single silicon vacancy V1 centres showed that ~ 40 % of the emission is guided into the ZPL (~ 3 % NV centre). I perform resonant excitation studies in Chapter 4 to investigate the optical properties. Surprisingly, the result showed spectrally stable optical transitions, which was not expected. The general opinion in the research community during this time was that only defects with inversion symmetry can show spectrally stable transitions. 4H-SiC is a piezo electric material which, by definition can not host inversion symmetry quantum defects. The physical origin of spectrally stable transitions for the V1 centre in 4H-SiC was found in the symmetry of the ground and excited state. Both states share the same symmetry and, more importantly, nearly the same dipole moment. The symmetry shields the optical transition frequencies of the silicon vacancy V1 centre against electric field fluctuations and causes spectrally stable optical transitions. For the research community, this discovery opened a new approach in identifying spectrally stable quantum defects in various materials. I analyzed additionally the spin coherence properties of a single silicon vacancy V1 centre and measured a spin coherence time up to 1 ms, which is comparable with the NV-centre in diamond. It has been shown for the NV-centre in diamond that nuclear spins can be used as quantum memory. In 4H-SiC, two types of isotopes exist, 29Si and 13C, that can also be exploited as a quantum memory. It turned also out that the symmetries of ground and excited states are responsible for very low inhomogeneous distribution of the V1 centre resonant absorption lines. In Chapter 5, this property is investigated, especially in view of experiments that require multiple indistinguishable single photon emitters. In my thesis I present the physical properties of a quantum system with excellent optical and spin properties. Additionally, the indistinguishable single photon emission of silicon vacancy V1 centre and mature fabrication knowledge in 4H-SiC make the systems scalable. All these properties combined makes the silicon vacancy V1 centre in 4H-SiC an excellent candidate for the realization of a quantum repeater network.Item Open Access Monitoring the state of charge of vanadium redox flow batteries with an EPR-on-a-Chip dipstick sensor(2024) Künstner, Silvio; McPeak, Joseph E.; Chu, Anh; Kern, Michal; Dinse, Klaus-Peter; Naydenov, Boris; Fischer, Peter; Anders, Jens; Lips, KlausThe vanadium redox flow battery (VRFB) is considered a promising candidate for large-scale energy storage in the transition from fossil fuels to renewable energy sources. VRFBs store energy by electrochemical reactions of different electroactive species dissolved in electrolyte solutions. The redox couples of VRFBs are VO2+/VO2+ and V2+/V3+, the ratio of which to the total vanadium content determines the state of charge (SOC). V(iv) and V(ii) are paramagnetic half-integer spin species detectable and quantifiable with electron paramagnetic resonance spectroscopy (EPR). Common commercial EPR spectrometers, however, employ microwave cavity resonators which necessitate the use of large electromagnets, limiting their application to dedicated laboratories. For an SOC monitoring device for VRFBs, a small, cost-effective submersible EPR spectrometer, preferably with a permanent magnet, is desirable. The EPR-on-a-Chip (EPRoC) spectrometer miniaturises the complete EPR spectrometer onto a single microchip by utilising the coil of a voltage-controlled oscillator as both microwave source and detector. It is capable of sweeping the frequency while the magnetic field is held constant enabling the use of small permanent magnets. This drastically reduces the experimental complexity of EPR. Hence, the EPRoC fulfils the requirements for an SOC sensor. We, therefore, evaluate the potential for utilisation of an EPRoC dipstick spectrometer as an operando and continuously online monitor for the SOC of VRFBs. Herein, we present quantitative proof-of-principle submersible EPRoC experiments on variably charged vanadium electrolyte solutions. EPR data obtained with a commercial EPR spectrometer are in good agreement with the EPRoC data.Item Open Access Compact electron paramagnetic resonance on a chip spectrometer using a single sided permanent magnet(2024) Segantini, Michele; Marcozzi, Gianluca; Elrifai, Tarek; Shabratova, Ekaterina; Höflich, Katja; Deaconeasa, Mihaela; Niemann, Volker; Pietig, Rainer; McPeak, Joseph E.; Anders, Jens; Naydenov, Boris; Lips, KlausElectron paramagnetic resonance (EPR) spectroscopy provides information about the physical and chemical properties of materials by detecting paramagnetic states. Conventional EPR measurements are performed in high Q resonator using large electromagnets which limits the available space for operando experiments. Here we present a solution toward a portable EPR sensor based on the combination of the EPR-on-a-Chip (EPRoC) and a single-sided permanent magnet. This device can be placed directly into the sample environment (i.e., catalytic reaction vessels, ultrahigh vacuum deposition chambers, aqueous environments, etc.) to conduct in situ and operando measurements. The EPRoC reported herein is comprised of an array of 14 voltage-controlled oscillator (VCO) coils oscillating at 7 GHz. By using a single grain of crystalline BDPA, EPR measurements at different positions of the magnet with respect to the VCO array were performed. It was possible to create a 2D spatial map of a 1.5 mm × 5 mm region of the magnetic field with 50 μm resolution. This allowed for the determination of the magnetic field intensity and homogeneity, which are found to be 254.69 mT and 700 ppm, respectively. The magnetic field was mapped also along the vertical direction using a thin film a-Si layer. The EPRoC and permanent magnet were combined to form a miniaturized EPR spectrometer to perform experiments on tempol (4-hydroxy-2,2,6,6-teramethylpiperidin-1-oxyl) dissolved in an 80% glycerol and 20% water solution. It was possible to determine the molecular tumbling correlation time and to establish a calibration procedure to quantify the number of spins within the sample.Item Open Access Current trends in VCO-based EPR(2024) Kern, Michal; Chu, Anh; Anders, JensIn this article we provide an overview of chip-integrated voltage-controlled oscillator (VCO)-based EPR detection as a new paradigm in EPR sensing. After a brief motivation for this alternative detection method, we provide a self-contained overview of the detection principle, both for continuous-wave and pulsed detection. Based on this introduction, we will highlight the advantages and disadvantages of VCO-based detection compared to conventional resonator-based detection. This is followed by an overview of the current state of the art in VCO-based EPR and interesting emerging applications of the technology. The paper concludes with a brief summary and outlook on future research directions.Item Open Access Frontend and backend electronics achieving flexibility and scalability for tomographic tactile sensing(2024) Sánchez-Delgado, Alberto; Garg, Keshav; Scherjon, Cor; Lee, HyosangTactile sensing is essential for robots to adequately interact with the physical world, but creating tactile sensors for the robot’s soft and flexible body surface has been a challenge. The resistance tomography-based tactile sensors have been introduced as a promising approach to creating soft tactile skins because the sensor fabrication can be greatly simplified with the aid of a computation model. This article introduces an electronic design strategy dividing frontend and backend electronics for the resistance tomography-based tactile sensors. In this scheme, the frontend is made of the piezoresistive structure and electrodes that can be changed depending on the required geometry. The backend is the electronic circuit for resistance tomography, which can be used for various frontend geometries. To evaluate the use of a unified backend for different frontend geometries, two frontend specimens with a square shape and a circular shape are tested. The minimum detectable contact force and the minimum discernible contact distance are calculated as 0.83×10-4 N/mm 2, 2.51 mm for the square-shaped frontend and 1.19×10-4 N/mm 2, 3.42 mm for the circular-shaped frontend. The results indicated that the proposed electronic design strategy can be used to create tactile skins with different scales and geometries while keeping the same backend design.Item Open Access Dead time-free detection of NMR signals using voltage-controlled oscillators(2023) Kern, Michal; Klotz, Tobias; Spiess, Maximilian; Mavridis, Petros; Blümich, Bernhard; Anders, JensIn this paper, we introduce voltage-controlled oscillators (VCOs) as a new type of nuclear magnetic resonance (NMR) detector, enabling dead time-free detection of NMR signals after an excitation pulse as well as the real-time inductive detection of Rabi oscillations during the pulse. Together with the theory of operation, we present the details of a custom-designed prototype implementation of a VCO-based NMR detector with an operating frequency around 62 MHz. The proof-of-concept measurements obtained with this prototype clearly demonstrate the possibility of performing dead time-free NMR experiments with coherent spin manipulation. Moreover, we also experimentally verified the capability of VCO-based detectors for performing real-time inductive detection of Rabi oscillations during the excitation pulse.