Scanning Gate Research Articles

Toward utilizing scanning gate microscopy as a high-resolution probe of valley splitting in Si/SiGe heterostructures

Toward utilizing scanning gate microscopy as a high-resolution probe of valley splitting in Si/SiGe heterostructures

P‐8: Distinguished Poster: Realization of Dynamic Local Refresh on LCD with Novel Partial Scan GOA

This paper presents a novel partial scan GOA architecture with gate and cascade scan separated output (GCSS‐GOA). Combined with a newly designed TCON IC and IP, dynamic local refresh at any position and with different refresh rate at different area on LCD panel are achieved. The power consumption of GOA and data drivers can be reduced by up to 50% respectively.

Machine learning methods for background potential estimation in 2DEGs

Two-dimensional electron gases (2DEGs) can show exceptional carrier mobility, making them promising candidates for future quantum technologies. However, impurities and defects can significantly degrade their performance, impacting transport, conductivity, and coherence times. We leverage scanning gate microscopy (SGM) and machine learning approaches to extract the potential landscape of 2DEGs from SGM data. We compare three techniques: image-to-image translation with generative adversarial networks (GANs), cellular neural networks (CNNs), and an evolutionary search algorithm. Notably, the evolutionary approach outperforms both alternatives in defect identification and analysis. This work clarifies the interaction between defects and 2DEG properties, demonstrating the potential of machine learning for understanding and manipulating quantum materials, facilitating advancements in quantum computing and nanoelectronics.

Realization of Dynamic Local Refresh on Liquid Crystal Display with Novel Partial Scan Gate Driver on Array

AbstractIn this paper, a novel partial scan Gate Driver on Array (GOA) architecture with gate and cascade scan separated output (GCSS‐GOA) is proposed. Combined with a newly designed timing controller (TCON) IC and IP, dynamic local refresh at any position and with different refresh rate at different area on liquid crystal display (LCD) panel are achieved. The driving schemes for dynamic local refresh and the display image quality evaluation results are also presented. For power consumption, it can be reduced by up to 50% respectively for GOA and data drivers in dynamic local refresh mode under 60 Hz benchmark refresh rate. This technology is also applicable to OLED, LED, e‐paper and other display products.

Scanning gate microscopy of nonretracing electron-hole trajectories in a normal-superconductor junction

Scanning gate microscopy of nonretracing electron-hole trajectories in a normal-superconductor junction

Self-calibrated subspace reconstruction for multidimensional MR fingerprinting for simultaneous relaxation and diffusion quantification.

To propose a new reconstruction method for multidimensional MR fingerprinting (mdMRF) to address shading artifacts caused by physiological motion-induced measurement errors without navigating or gating. The proposed method comprises two procedures: self-calibration and subspace reconstruction. The first procedure (self-calibration) applies temporally local matrix completion to reconstruct low-resolution images from a subset of under-sampled data extracted from the k-space center. The second procedure (subspace reconstruction) utilizes temporally global subspace reconstruction with pre-estimated temporal subspace from low-resolution images to reconstruct aliasing-free, high-resolution, and time-resolved images. After reconstruction, a customized outlier detection algorithm was employed to automatically detect and remove images corrupted by measurement errors. Feasibility, robustness, and scan efficiency were evaluated through in vivo human brain imaging experiments. The proposed method successfully reconstructed aliasing-free, high-resolution, and time-resolved images, where the measurement errors were accurately represented. The corrupted images were automatically and robustly detected and removed. Artifact-free T1, T2, and ADC maps were generated simultaneously. The proposed reconstruction method demonstrated robustness across different scanners, parameter settings, and subjects. A high scan efficiency of less than 20 s per slice has been achieved. The proposed reconstruction method can effectively alleviate shading artifacts caused by physiological motion-induced measurement errors. It enables simultaneous and artifact-free quantification of T1, T2, and ADC using mdMRF scans without prospective gating, with robustness and high scan efficiency.

Reconstructing the potential configuration in a high-mobility semiconductor heterostructure with scanning gate microscopy

The weak disorder potential seen by the electrons of a two-dimensional electron gas in high-mobility semiconductor heterostructures leads to fluctuations in the physical properties and can be an issue for nanodevices. In this paper, we show that a scanning gate microscopy (SGM) image contains information about the disorder potential, and that a machine learning approach based on SGM data can be used to determine the disorder. We reconstruct the electric potential of a sample from its experimental SGM data and validate the result through an estimate of its accuracy.

Scanning gate microscopy response for local tip potentials beyond perturbation theory

Scanning gate microscopy response for local tip potentials beyond perturbation theory

Study on annealing effect of bipolar transistors at different temperatures after total dose irradiation

The article investigates the annealing process of bipolar transistors after total dose irradiation, using the GLPNP transistor as the research object. After total dose irradiation, room temperature and high temperature annealing tests were carried out. Gummel curve, GS curve and SS curve of bipolar transistor are obtained by I/V scanning, gate scanning and sub-threshold scanning. The evolution behavior of radiation-induced oxide trap charges and interface state damage defects during annealing of bipolar transistors at different temperatures was quantitatively analyzed by different microscopic defect characterization methods. Generally speaking, this study is of great significance to better understand the recovery mechanism of electrical properties of bipolar transistors during annealing after total dose irradiation, and can provide reference for the design, application and radiation-hard of transistors.

Scanning gate microscopy in graphene nanostructures

The conductance of graphene nanoribbons and nanoconstrictions under the effect of a scanning gate microscopy tip is systematically studied. Using a scattering approach for noninvasive probes, the first- and second-order conductance corrections caused by the tip potential disturbance are expressed explicitly in terms of the scattering states of the unperturbed structure. Numerical calculations confirm the perturbative results, showing that the second-order term prevails in the conductance plateaus, exhibiting a universal scaling law for armchair graphene strips. For stronger tips, at specific probe potential widths and strengths beyond the perturbative regime, the conductance corrections reveal the appearance of resonances originated from states trapped below the tip. The zero-transverse-energy mode of an armchair metallic strip is shown to be insensitive to the long-range electrostatic potential of the probe. For nanoconstrictions defined on a strip, scanning gate microscopy allows to get insight into the breakdown of conductance quantization. The first-order correction generically dominates at low tip strength, while for Fermi energies associated with faint conductance plateaus, the second-order correction becomes dominant for relatively small potential tip strengths. In accordance with the spatial dependence of the partial local density of states, the largest tip effect occurs in the central part of the constriction, close to the edges. Nanoribbons and nanoconstrictions with zigzag edges exhibit a similar response as in the case of armchair nanostructures, except when the intervalley coupling induced by the tip potential destroys the chiral edge states.

An investigation of the background potential in quantum constrictions using scanning gate microscopy and a swarming algorithm

Scanning gate microscopy (SGM) is a valuable scanning probe technique for characterizing electronic transport in mesoscopic systems. However, the interpretation of the method is often limited by many experimental challenges. In this work, we propose an empirically-constrained optimization approach based on swarm search and Green’s functions to extract more information from SGM measurements. The approach is applied to a quantum point contact fabricated on an InAlAs/InGaAs/InAlAs quantum well, and the results indicate that the corresponding SGM could be generated, in a weak approximation, by a fluctuating background potential with features with radii in the order of 20 to 30 nm and a correlation length of 5.7 nm. Our method represents a data-driven tool for estimating solutions for inverse problems in mesoscopic physics, and can be used to generate estimates for the potential landscape experienced by free electrons in mesoscopic systems.

Theory of scanning gate microscopy imaging of the supercurrent distribution in a planar Josephson junction

We theoretically investigate the mapping of the supercurrent distribution in a planar superconductor-normal-superconductor junction in the presence of a perpendicular magnetic field via the scanning gate microscopy technique. We find that the distribution of counterpropagating supercurrents aligned in Josephson vortices can be mapped by the change in the critical current induced by the tip of the scanning probe if the flux in the junction is set close to the maxima of the Fraunhofer pattern. Instead, when the magnetic field drives the junction to a supercurrent minimum in the Fraunhofer pattern, the superconducting phase adapts, and the tip always increases the supercurrent. The perpendicular magnetic field leads to the formation of Josephson vortices, whose extension for highly transparent junctions depends on the current circulation direction. We show that this leads to an asymmetric supercurrent distribution in the junction and that this can be revealed by scanning gate microscopy. We explain our findings on the basis of numerical calculations for both short- and long-junction limits and provide a phenomenological model for the observed phenomena.

Microwave-Frequency Scanning Gate Microscopy of a Si/SiGe Double Quantum Dot.

Conventional transport methods provide quantitative information on spin, orbital, and valley states in quantum dots but lack spatial resolution. Scanning tunneling microscopy, on the other hand, provides exquisite spatial resolution at the expense of speed. Working to combine the spatial resolution and energy sensitivity of scanning probe microscopy with the speed of microwave measurements, we couple a metallic tip to a Si/SiGe double quantum dot (DQD) that is integrated with a charge detector. We first demonstrate that the dc-biased tip can be used to change the occupancy of the DQD. We then apply microwaves through the tip to drive photon-assisted tunneling (PAT). We infer the DQD level diagram from the frequency and detuning dependence of the tunneling resonances. These measurements allow the resolution of ∼65 μeV excited states, an energy consistent with valley splittings in Si/SiGe. This work demonstrates the feasibility of scanning gate experiments with Si/SiGe devices.

A method for finding the background potential of quantum devices from scanning gate microscopy data using machine learning

The inverse problem of estimating the background potential from measurements of the local density of states is a challenging issue in quantum mechanics. Even more difficult is to do this estimation using approximate methods such as scanning gate microscopy (SGM). Here, we propose a machine-learning-based solution by exploiting adaptive cellular neural networks (CNNs). In the paradigmatic setting of a quantum point contact, the training data consist of potential-SGM functional relations represented by image pairs. These are generated by the recursive Green’s function method. We demonstrate that the CNN-based machine learning framework can predict the background potential corresponding to the experimental image data. This is confirmed by analyzing the estimated potential with image processing techniques based on the comparison between the charge densities and those obtained using different techniques. Correlation analysis of the images suggests the possibility of estimating different contributions to the background potential. In particular, our results indicate that both charge puddles and fixed impurities contribute to the spatial patterns found in the SGM data. Our work represents a timely contribution to the rapidly evolving field of exploiting machine learning to solve difficult problems in physics.

Revisiting Coulomb diamond signatures in quantum Hall interferometers

Coulomb diamonds are the archetypal signatures of Coulomb blockade, a well-known charging effect mainly observed in nanometer-sized "electronic islands" tunnel-coupled with charge reservoirs. Here, we identify apparent Coulomb diamond features in the scanning gate spectroscopy of a quantum point contact carved out of a semiconductor heterostructure, in the quantum Hall regime. Varying the scanning gate parameters and the magnetic field, the diamonds are found to smoothly evolve to checkerboard patterns. To explain this surprising behavior, we put forward a model which relies on the presence of a nanometer-sized Fabry-P\'erot quantum Hall interferometer at the center of the constriction with tunable tunneling paths coupling the central part of the interferometer to the quantum Hall channels running along the device edges. Both types of signatures, diamonds and checkerboards, and the observed transition, are reproduced by simply varying the interferometer size and the transmission probabilities at the tunneling paths. The new proposed interpretation of diamond phenomenology will likely lead to revisit previous data, and opens the way towards engineering more complex interferometric devices with nanoscale dimensions.

Graphene Whisperitronics: Transducing Whispering Gallery Modes into Electronic Transport.

When confined in circular cavities, graphene relativistic charge carriers occupy whispering gallery modes (WGMs) in analogy to classical acoustic and optical fields. The rich geometrical patterns of the WGMs decorating the local density of states offer promising perspectives to devise new disruptive quantum devices. However, exploiting these highly sensitive resonances requires the transduction of the WGMs to the outside world through source and drain electrodes, a yet unreported configuration. Here, we create a circular p-n island in a graphene device using a polarized scanning gate microscope tip and probe the resulting WGM signatures in in-plane electronic transport through the p-n island. Combining tight-binding simulations and the exact solution of the Dirac equation, we assign the measured device conductance features to WGMs and demonstrate mode selectivity by displacing the p-n island with respect to a constriction. This work therefore constitutes a proof of concept for graphene whisperitronic devices.

Cryogen-free scanning gate microscope for the characterization of Si/Si0.7Ge0.3 quantum devices at milli-Kelvin temperatures

Silicon can be isotopically enriched, allowing for the fabrication of highly coherent semiconductor spin qubits. However, the conduction band of bulk Si exhibits a six-fold valley degeneracy which may adversely impact the performance of silicon quantum devices. To date, the spatial characterization of valley states in Si has remained limited. Moreover, techniques for probing valley states in functional electronic devices are needed. Here, we describe a cryogen-free scanning gate microscope for the characterization of Si/Si0.7Ge0.3 quantum devices at mK temperatures. The newly built instrument is the first cryogen-free scanning gate microscope capable of forming and measuring a quantum dot on a Si/SiGe device with an overlapping gate structure without compromising the ability to host multiple DC and microwave lines for quantum control experiments. The microscope is based on the Pan-walker design, with coarse positioning piezostacks and a fine scanning piezotube. A tungsten microscope tip is attached to a tuning fork for active control of the tip-to-sample distance. To reduce vibration noise from the pulse tube cooler, we utilize both active and passive vibration isolation mechanisms and achieve a root-mean-square noise in z of ∼2 nm. Our microscope is designed to characterize fully functioning Si/Si0.7Ge0.3 quantum devices. As a proof of concept, we use the microscope to manipulate the charge occupation of a Si quantum dot, opening up a range of possibilities for the exploration of quantum devices and materials.

Upstream modes and antidots poison graphene quantum Hall effect

The quantum Hall effect is the seminal example of topological protection, as charge carriers are transmitted through one-dimensional edge channels where backscattering is prohibited. Graphene has made its marks as an exceptional platform to reveal new facets of this remarkable property. However, in conventional Hall bar geometries, topological protection of graphene edge channels is found regrettably less robust than in high mobility semi-conductors. Here, we explore graphene quantum Hall regime at the local scale, using a scanning gate microscope. We reveal the detrimental influence of antidots along the graphene edges, mediating backscattering towards upstream edge channels, hence triggering topological breakdown. Combined with simulations, our experimental results provide further insights into graphene quantum Hall channels vulnerability. In turn, this may ease future developments towards precise manipulation of topologically protected edge channels hosted in various types of two-dimensional crystals.

Imaging signatures of the local density of states in an electronic cavity

We use Scanning Gate Microscopy to study electron transport through an open, gate-defined resonator in a Ga(Al)As heterostructure. Raster-scanning the voltage-biased metallic tip above the resonator, we observe distinct conductance modulations as a function of the tip-position and voltage. Quantum mechanical simulations reproduce these conductance modulations and reveal their relation to the partial local density of states in the resonator. Our measurements illustrate the current frontier between possibilities and limitations in imaging the local density of states in buried electron systems using scanning gate microscopy.

(Invited) Scanning Gate Imaging of Charge Transport in Graphene Nanodevices

Quantum transport in nanodevices is usually probed thanks to measurements of the electrical resistance or conductance, which lack the spatial resolution necessary to probe local-scale electron behaviour. Here, we will discuss how to get real-space local information on peculiar quantum transport phenomena inside graphene constrictions. The results were obtained using low temperature scanning gate microscopy (SGM), which consists in mapping the electrical conductance of a device as an electrically-biased sharp metallic tip scans in its vicinity [1].If the tip-induced perturbation is relatively small, SGM is an imaging technique giving access to the local density of states inside mesoscopic devices [2]. Here, we will focus on the regime of large tip-induced perturbation. This regime is particularly interesting in the case of graphene: depending on tip voltage and tip-sample distance, one can tune the shape, size, and smoothness of the moving scattering region associated with the tip, and eventually create a moving circular pn-junction within the device. We use this moving pn-junction to probe peculiar aspects of relativistic charge carrier dynamics within graphene devices: Klein tunnelling and current focusing and defocusing [3], Fabry-Pérot interferometry [4], signatures of whispering gallery modes [5] and tunneling between counterpropagating quantum Hall edge states [6]. This work unlocks new possibilities in the field of electron optics in graphene.[1] M.A. Eriksson et al., Appl. Phys. Lett. 69, 671 (1996); M. Topinka et al., Science 289, 671 (2000).[2] F. Martins et al., Phys. Rev. Lett. 99, 136807 (2007) ; M. Pala et al., Phys. Rev. B 77, 125310 (2008).[3] B. Brun et al., Phys. Rev. B 100, 041401 (2019).[4] B. Brun et al., 2D Materials, 7, 025037 (2020).[5] B. Brun et al., submitted.[6] N. Moreau et al., submitted.