Integrated Charge Sensor Research Articles

An organ-on-chip device with integrated charge sensors and recording microelectrodes

Continuous monitoring of tissue microphysiology is a key enabling feature of the organ-on-chip (OoC) approach for in vitro drug screening and disease modeling. Integrated sensing units are particularly convenient for microenvironmental monitoring. However, sensitive in vitro and real-time measurements are challenging due to the inherently small size of OoC devices, the characteristics of commonly used materials, and external hardware setups required to support the sensing units. Here we propose a silicon-polymer hybrid OoC device that encompasses transparency and biocompatibility of polymers at the sensing area, and has the inherently superior electrical characteristics and ability to house active electronics of silicon. This multi-modal device includes two sensing units. The first unit consists of a floating-gate field-effect transistor (FG-FET), which is used to monitor changes in pH in the sensing area. The threshold voltage of the FG-FET is regulated by a capacitively-coupled gate and by the changes in charge concentration in close proximity to the extension of the floating gate, which functions as the sensing electrode. The second unit uses the extension of the FG as microelectrode, in order to monitor the action potential of electrically active cells. The layout of the chip and its packaging are compatible with multi-electrode array measurement setups, which are commonly used in electrophysiology labs. The multi-functional sensing is demonstrated by monitoring the growth of induced pluripotent stem cell-derived cortical neurons. Our multi-modal sensor is a milestone in combined monitoring of different, physiologically-relevant parameters on the same device for future OoC platforms.

Charge States, Triple Points, and Quadruple Points in an InAs Nanowire Triple Quantum Dot Revealed by an Integrated Charge Sensor

AbstractA serial triple quantum dot (TQD) integrated with a quantum dot (QD) charge sensor is realized from an InAs nanowire via a fine finger‐gate technique. The complex charge states and intriguing properties of the device are studied in the few‐electron regime by direct transport measurements and by charge‐sensor detection measurements. The charge stability diagram for a capacitively coupled, parallel double‐QD formed from a QD in the TQD and the sensor QD show a visible capacitance coupling between the TQD and the sensor QD, indicating a good sensitivity of the charge sensor. The charge stability diagrams of the TQD are measured by the charge sensor and the global features in the charge stability diagrams are well reproduced by the simultaneous direct transport measurements and by the simulation made based on an effective capacitance network model. The complex charge stability diagrams of the TQD are measured in detail with the integrated charge sensor in an energetically degenerate region, where all the three QDs are on or nearly on resonance, and the formations of quadruple points and of all possible eight charge states are observed. In addition, the operation of the TQD as a quantum cellular automata is demonstrated and discussed.

Distinguishing persistent effects in an undoped GaAs/AlGaAs quantum well by top-gate-dependent illumination

Persistent photoconductivity of GaAs/AlGaAs heterostructures has hampered the measurement of charge- and spin-related quantum effects in gate-defined quantum devices and integrated charge sensors due to Si-dopant-related deep donor levels (DX centers). In this study, this effect is overcome by using an undoped GaAs/AlGaAs heterostructure for photonic purposes. We also measure the electron transport before and after LED illumination at low temperatures. In addition to a regular rapid saturation showing the increased carrier density, a slow accumulation of illumination effects appeared when different top-gate voltages were applied during illumination, which indicated the redistribution of charge at the oxide–GaAs interface. This study provides interesting insights into the development of optically stable devices for efficient semiconductor quantum interfaces.

A charge sensor integration to tunable double quantum dots on two neighboring InAs nanowires.

A single quantum dot serving as a charge sensor is integrated to scalable double quantum dots using local top finger-gate techniques on two neighboring pure-phase InAs nanowires. The single dot built on one nanowire capacitively couples one of the double dots constructed on another nanowire via a metal bridge gate. The charge occupation states of double quantum dots can be accurately monitored by the sensor even in a few-electron regime in which transport tunneling current through the double dots vanishes. In the tunneling spectroscopy of double dots, electron inter dot tunneling process is absent; however, it can be illustrated by the sensor in terms of a transconductance line between the two closest triple points. Thus, tunnel coupling strength between the double dots is quantitatively extracted from the detectable charge transition. The highly tunable multiple quantum dots with integrated charge sensors on InAs nanowires could be an essential building block for quantum information processing technology.

Role of the Insulating Fillers in the Encapsulation Material on the Lateral Charge Spreading in HV-ICs

High electric fields and temperatures in high-voltage ICs (HV-ICs) can induce charge transport phenomena in the encapsulation material leading to reliability test failures. In this paper, the resistivity of epoxy-based resins with insulating microfiller weight fraction exceeding 70% has been experimentally and theoretically investigated for the first time. Electrical conductivity has been measured at high temperature (150 °C) using both dielectric spectroscopy analysis on bulk samples and charge-spreading characterizations on a dedicated test chip with integrated charge sensors. The use of a charge sensor close to the internal HV metallization leads to results more pertinent with the active area of HV-ICs. Remarkably, both experiments show an unexpected increase and a significant variability of the electrical conductivity as the microfiller fraction is increased. The strong correlation between bulk and lateral experiments clearly indicates that those features should be attributed to the bulk material. Numerical simulations of diffusion phenomenon in mold structures with random arrangements of spherical microfillers demonstrate that the conductivity increase with filler content can be ascribed to the role of the epoxy/filler interfaces.

Measuring Ligand-Dependent Transport in Nanopatterned PbS Colloidal Quantum Dot Arrays Using Charge Sensing.

Colloidal quantum dot arrays with long organic ligands have better packing order than those with short ligands but are highly resistive, making low-bias conductance measurements impossible with conventional two-probe techniques. We use an integrated charge sensor to study transport in weakly coupled arrays in the low-bias regime, and we nanopattern the arrays to minimize packing disorder. We present the temperature and field dependence of the resistance for nanopatterned oleic-acid and n-butylamine-capped PbS arrays, measuring resistances as high as 10(18) Ω. We find that the conduction mechanism changes from nearest neighbor hopping in oleic-acid-capped PbS dots to Mott's variable range hopping in n-butylamine capped PbS dots. Our results can be understood in terms of a change in the interdot coupling strength or a change in density of trap states and highlight the importance of the capping ligand on charge transport through colloidal quantum dot arrays.

Fabrication and characterization of an undoped GaAs/AlGaAs quantum dot device

We demonstrate the development of a double quantum dot with an integrated charge sensor fabricated in undoped GaAs/AlGaAs heterostructures using a double top-gated design. Based on the evaluation of the integrated charge sensor, the double quantum dot can be tuned to a few-electron region. Additionally, the inter-dot coupling of the double quantum dot can be tuned to a large extent according to the voltage on the middle gate. The quantum dot is shown to be tunable from a single dot to a well-isolated double dot. To assess the stability of such design, the potential fluctuation induced by 1/f noise was measured. Based on the findings herein, the quantum dot design developed in the undoped GaAs/AlGaAs semiconductor shows potential for the future exploitation of nano-devices.

Charge Sensed Pauli Blockade in a Metal–Oxide–Semiconductor Lateral Double Quantum Dot

We report Pauli blockade in a multielectron silicon metal-oxide-semiconductor double quantum dot with an integrated charge sensor. The current is rectified up to a blockade energy of 0.18 ± 0.03 meV. The blockade energy is analogous to singlet-triplet splitting in a two electron double quantum dot. Built-in imbalances of tunnel rates in the MOS DQD obfuscate some edges of the bias triangles. A method to extract the bias triangles is described, and a numeric rate-equation simulation is used to understand the effect of tunneling imbalances and finite temperature on charge stability (honeycomb) diagram, in particular the identification of missing and shifting edges. A bound on relaxation time of the triplet-like state is also obtained from this measurement.

Probing relaxation times in graphene quantum dots

Graphene quantum dots are attractive candidates for solid-state quantum bits. In fact, the predicted weak spin-orbit and hyperfine interaction promise spin qubits with long coherence times. Graphene quantum dots have been extensively investigated with respect to their excitation spectrum, spin-filling sequence and electron-hole crossover. However, their relaxation dynamics remain largely unexplored. This is mainly due to challenges in device fabrication, in particular concerning the control of carrier confinement and the tunability of the tunnelling barriers, both crucial to experimentally investigate decoherence times. Here we report pulsed-gate transient current spectroscopy and relaxation time measurements of excited states in graphene quantum dots. This is achieved by an advanced device design that allows to individually tune the tunnelling barriers down to the low megahertz regime, while monitoring their asymmetry. Measuring transient currents through electronic excited states, we estimate a lower bound for charge relaxation times on the order of 60–100 ns.

A graphene quantum dot with a single electron transistor as an integrated charge sensor

A quantum dot (QD) with an integrated charge sensor is becoming a common architecture for a spin or charge based solid state qubit. To implement such a structure in graphene, we have fabricated a twin-dot structure in which the larger dot serves as a single electron transistor (SET) to read out the charge state of the nearby gate controlled small QD. A high SET sensitivity of 10−3e/Hz allowed us to probe Coulomb charging as well as excited state spectra of the QD, even in the regime where the current through the QD is too small to be measured by conventional transport means.

Gate-defined double quantum dot with integrated charge sensors realized in InGaAs/InP by incorporating a high-κ dielectric

A gate-defined double quantum dot with two integrated quantum point contact charge sensors is realized in an InGaAs/InP heterostructure by employing a high-κ HfO2 thin film as gate dielectric and a polymer bridge technique. Clear honeycomb patterns are observed in the measured charge stability diagram of the double quantum dot and charge sensing signals of the quantum point contacts. It is also found that the quantum point contact charge sensors can detect the charge states in the double quantum dot even in the condition that the direct transport signal is not visible.

Single-gate accumulation-mode InGaAs quantum dot with a vertically integrated charge sensor

We report on the fabrication and characterization of a few-electron quantum dot controlled by a single gate electrode. Our device has a double-quantum-well design, in which the doping controls the occupancy of the lower well while the upper well remains empty under the free surface. Electrons tunneling between this accumulation-mode dot and the lower well are detected using a quantum point contact, located slightly offset from the dot gate. Addition spectra starting with N=0 were observed as a function of gate voltage. DC sensitivity to single electrons was determined to be as high as 8.6%.

Electron–nuclear interaction in 13C nanotube double quantum dots

Hyperfine coupling to nuclei can be detrimental to the coherence of electron spins, but properly harnessed it can provide a mechanism for manipulation and storage of quantum information. Spin-blockade measurements in 13C carbon nanotubes now show surprisingly strong effects of electron–nuclear interaction, with a hyperfine coupling two orders of magnitude larger than previously anticipated. For coherent electron spins, hyperfine coupling to nuclei in the host material can either be a dominant source of unwanted spin decoherence1,2,3 or, if controlled effectively, a resource enabling storage and retrieval of quantum information4,5,6,7. To investigate the effect of a controllable nuclear environment on the evolution of confined electron spins, we have fabricated and measured gate-defined double quantum dots with integrated charge sensors made from single-walled carbon nanotubes with a variable concentration of 13C (nuclear spin I=1/2) among the majority zero-nuclear-spin 12C atoms. We observe strong isotope effects in spin-blockaded transport, and from the magnetic field dependence estimate the hyperfine coupling in 13C nanotubes to be of the order of 100 μeV, two orders of magnitude larger than anticipated8,9. 13C-enhanced nanotubes are an interesting system for spin-based quantum information processing and memory: the 13C nuclei differ from those in the substrate, are naturally confined to one dimension, lack quadrupolar coupling and have a readily controllable concentration from less than one to 105 per electron.

A Ge/Si heterostructure nanowire-based double quantum dot with integrated charge sensor

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.