Nondestructive imaging of atomically thin nanostructures buried in silicon

EUV-induced hydrogen desorption as a step towards large-scale silicon quantum device patterning

Atomically precise hydrogen desorption lithography using scanning tunnelling microscopy (STM) has enabled the development of single-atom, quantum-electronic devices on a laboratory scale. Scaling up this technology to mass-produce these devices requires bridging the gap between the precision of STM and the processes used in next-generation semiconductor manufacturing. Here, we demonstrate the ability to remove hydrogen from a monohydride Si(001):H surface using extreme ultraviolet (EUV) light. We quantify the desorption characteristics using various techniques, including STM, X-ray photoelectron spectroscopy (XPS), and photoemission electron microscopy (XPEEM). Our results show that desorption is induced by secondary electrons from valence band excitations, consistent with an exactly solvable non-linear differential equation and compatible with the current 13.5 nm (~92 eV) EUV standard for photolithography; the data imply useful exposure times of order minutes for the 300 W sources characteristic of EUV infrastructure. This is an important step towards the EUV patterning of silicon surfaces without traditional resists, by offering the possibility for parallel processing in the fabrication of classical and quantum devices through deterministic doping.

Imaging beyond the surface region: Probing hidden materials via atomic force microscopy

Probing material properties at surfaces down to the single-particle scale of atoms and molecules has been achieved, but high-resolution subsurface imaging remains a nanometrology challenge due to electromagnetic and acoustic dispersion and diffraction. The atomically sharp probe used in scanning probe microscopy (SPM) has broken these limits at surfaces. Subsurface imaging is possible under certain physical, chemical, electrical, and thermal gradients present in the material. Of all the SPM techniques, atomic force microscopy has entertained unique opportunities for nondestructive and label-free measurements. Here, we explore the physics of the subsurface imaging problem and the emerging solutions that offer exceptional potential for visualization. We discuss materials science, electronics, biology, polymer and composite sciences, and emerging quantum sensing and quantum bio-imaging applications. The perspectives and prospects of subsurface techniques are presented to stimulate further work toward enabling noninvasive high spatial and spectral resolution investigation of materials including meta- and quantum materials.

Experimental realization of an extended Fermi-Hubbard model using a 2D lattice of dopant-based quantum dots

The Hubbard model is an essential tool for understanding many-body physics in condensed matter systems. Artificial lattices of dopants in silicon are a promising method for the analog quantum simulation of extended Fermi-Hubbard Hamiltonians in the strong interaction regime. However, complex atom-based device fabrication requirements have meant emulating a tunable two-dimensional Fermi-Hubbard Hamiltonian in silicon has not been achieved. Here, we fabricate 3 × 3 arrays of single/few-dopant quantum dots with finite disorder and demonstrate tuning of the electron ensemble using gates and probe the many-body states using quantum transport measurements. By controlling the lattice constants, we tune the hopping amplitude and long-range interactions and observe the finite-size analogue of a transition from metallic to Mott insulating behavior. We simulate thermally activated hopping and Hubbard band formation using increased temperatures. As atomically precise fabrication continues to improve, these results enable a new class of engineered artificial lattices to simulate interactive fermionic models.

Atomically precise artificial lattices of dopant-based quantum dots offer a tunable platform for simulations of interacting fermionic models. By leveraging advances in fabrication and atomic-state control, Wang et al. report quantum simulations of the 2D Fermi-Hubbard model on a 3 × 3 few-dopant quantum dot array.

Electric current paths in a Si:P delta-doped device imaged by nitrogen-vacancy diamond magnetic microscopy

The recently-developed ability to control phosphorous-doping of silicon at an atomic level using scanning tunneling microscopy, a technique known as atomic precision advanced manufacturing (APAM), has allowed us to tailor electronic devices with atomic precision, and thus has emerged as a way to explore new possibilities in Si electronics. In these applications, critical questions include where current flow is actually occurring in or near APAM structures as well as whether leakage currents are present. In general, detection and mapping of current flow in APAM structures are valuable diagnostic tools to obtain reliable devices in digital-enhanced applications. In this paper, we used nitrogen-vacancy (NV) centers in diamond for wide-field magnetic imaging (with a few-mm field of view and micron-scale resolution) of magnetic fields from surface currents flowing in an APAM test device made of a P delta-doped layer on a Si substrate, a standard APAM witness material. We integrated a diamond having a surface NV ensemble with the device (patterned in two parallel mm-sized ribbons), then mapped the magnetic field from the DC current injected in the APAM device in a home-built NV wide-field microscope. The 2D magnetic field maps were used to reconstruct the surface current densities, allowing us to obtain information on current paths, device failures such as choke points where current flow is impeded, and current leakages outside the APAM-defined P-doped regions. Analysis on the current density reconstructed map showed a projected sensitivity of ∼0.03 A m^-1, corresponding to a smallest-detectable current in the 200μm wide APAM ribbon of ∼6μA. These results demonstrate the failure analysis capability of NV wide-field magnetometry for APAM materials, opening the possibility to investigate other cutting-edge microelectronic devices.

In operando charge transport imaging of atomically thin dopant nanostructures in silicon

Novel approaches to materials design, fabrication processes and device architectures have accelerated next-generation electronics component production, pushing device dimensions down to the nano- and atomic-scale. For device metrology methods to keep up with these developments, they should not only measure the relevant electrical parameters at these length-scales, but ideally do so during active operation of the device. Here, we demonstrate such a capability using the full functionality of an advanced scanning microwave/scanning capacitance/kelvin probe atomic force microscope to inspect the charge transport and performance of an atomically thin buried phosphorus wire device during electrical operation. By interrogation of the contact potential, carrier density and transport properties, we demonstrate the capability to distinguish between the different material components and device imperfections, and assess their contributions to the overall electric characteristics of the device in operando. Our experimental methodology will facilitate rapid feedback for the fabrication of patterned nanoscale dopant device components in silicon, now important for the emerging field of silicon quantum information technology. More generally, the versatile setup, with its advanced inspection capabilities, delivers a comprehensive method to determine the performance of nanoscale devices while they function, in a broad range of material systems.

3D visualization of microwave electric and magnetic fields by using a metasurface-based indicator

Visualizations of the microwave electric and magnetic near-field distributions of radio-frequency (RF) filters were performed using the technique of thermoelastic optical indicator microscopy (TEOIM). New optical indicators based on periodic dielectric-metal structures were designed for electric field visualization. Depending on the structure orientation, such metasurface-based indicators allow separately visualization of the E_x and E_y components of the in-plane electric field. Numerical simulations were conducted to examine the working principle of the designed indicator structures, and the results were compared to the experimental, showing good agreement. In addition, the 3D visualization of the microwave near-field distribution was built, to show the field intensity and distribution dependencies on the distance from the RF filter.

An approach to determine solution properties in micro pipes by near-field microwave microscopy

In this article, we propose a quantitative, non-destructive and noninvasive approach to obtain electromagnetic properties of liquid specimens utilizing a home-designed near-field microwave microscopy. The responses of aqueous solutions can be acquired with varying concentrations, types (CaCl₂, MgCl₂, KCl and NaCl) and tip-sample distances. An electromagnetic simulation model also successfully predicts the behaviors of saline samples. For a certain type of solutions with varying concentrations, the results are concaves with different bottoms, and the symmetric graphs of concave extractions can clearly identify different specimens. Moreover, we obtain electromagnetic images of capillaries with various saline solutions, as well as a Photinia × fraseri Dress leaf.

Depth mapping of metallic nanowire polymer nanocomposites by scanning dielectric microscopy

Polymer nanocomposite materials based on metallic nanowires are widely investigated as transparent and flexible electrodes or as stretchable conductors and dielectrics for biosensing. Here we show that Scanning Dielectric Microscopy (SDM) can map the depth distribution of metallic nanowires within the nanocomposites in a non-destructive way. This is achieved by a quantitative analysis of sub-surface electrostatic force microscopy measurements with finite-element numerical calculations. As an application we determined the three-dimensional spatial distribution of ∼50 nm diameter silver nanowires in ∼100 nm-250 nm thick gelatin films. The characterization is done both under dry ambient conditions, where gelatin shows a relatively low dielectric constant, ε_r∼ 5, and under humid ambient conditions, where its dielectric constant increases up to ε_r∼ 14. The present results show that SDM can be a valuable non-destructive subsurface characterization technique for nanowire-based nanocomposite materials, which can contribute to the optimization of these materials for applications in fields such as wearable electronics, solar cell technologies or printable electronics.

Advanced calibration kit for scanning microwave microscope: Design, fabrication, and measurement

We present in this paper a new design of a capacitive calibration kit for scanning microwave microscopy (SMM). As demonstrated by finite element modelings, the produced devices are highly independent of material parameters due to their lateral configuration. The fabrication of these gold-based structures is realized by using well established clean-room techniques. SMM measurements are performed under different conditions, and all capacitive structures exhibit a strong contrast with respect to the non-capacitive background. The obtained experimental data are employed to calibrate the used SMM tips and to extract the capacitance of produced devices following a method based on the short-open-load calibration algorithm for one-port vector network analyzers. The comparison of experimental capacitance and nominal values provided by our models proves the applicability of the used calibration approach for a wide frequency range.

Nanoscale Charge Accumulation and Its Effect on Carrier Dynamics in Tri-cation Perovskite Structures

Nanoscale investigations by scanning probe microscopy have provided major contributions to the rapid development of organic-inorganic halide perovskites (OIHP) as optoelectronic devices. Further improvement of device level properties requires a deeper understanding of the performance-limiting mechanisms such as ion migration, phase segregation, and their effects on charge extraction both at the nano- and macroscale. Here, we have studied the dynamic electrical response of Cs0.05(FA0.83MA0.17)0.95PbI3-xBrx perovskite structures by employing conventional and microsecond time-resolved open-loop Kelvin probe force microscopy (KPFM). Our results indicate strong negative charge carrier trapping upon illumination and very slow (>1 s) relaxation of charges at the grain boundaries. The fast electronic recombination and transport dynamics on the microsecond scale probed by time-resolved open-loop KPFM show diffusion of charge carriers toward grain boundaries and indicate locally higher recombination rates because of intrinsic compositional heterogeneity. The nanoscale electrostatic effects revealed are summarized in a collective model for mixed-halide CsFAMA. Results on multilayer solar cell structures draw direct relations between nanoscale ionic transport, charge accumulation, recombination properties, and the final device performance. Our findings extend the current understanding of complex charge carrier dynamics in stable multication OIHP structures.

An ultra-high bandwidth nano-electronic interface to the interior of living cells with integrated fluorescence readout of metabolic activity

We present the first ever broadband, calibrated electrical connection to the inside of a cell. The interior of a vital, living cell contains multiple dynamic and electrically active organelles such as mitochondria, chloroplasts, lysosomes, and the endoplasmic reticulum. However, little is known about the detailed electrical activity inside the cell. Here we show an ultra-high bandwidth nano-electronic interface to the interior of living cells with integrated fluorescence readout of metabolic activity. On-chip/on-petri dish nanoscale capacitance calibration standards are used to quantify the electronic coupling from bench to cell from DC to 26 GHz (with cell images at 22 GHz). The interaction of static to high frequency electromagnetic fields with the cell constituents induce currents of free charges and local reorganization of linked charges. As such, this enables a direct, calibrated, quantitative, nanoscale electronic interface to the interior of living cells. The interface could have a variety of applications in interfacing life sciences to nano-electronics, including electronic assays of membrane potential dynamics, nano-electronic actuation of cellular activity, and tomographic, nano-radar imaging of the morphology of vital organelles in the cytoplasm, during all phases of the cell life cycle (from development to senescence), under a variety of physiological environments, and under a broad suite of pharmacological manipulations.

Atomic fluctuations in electronic materials revealed by dephasing

The microscopic origin and timescale of the fluctuations of the energies of electronic states has a significant impact on the properties of interest of electronic materials, with implication in fields ranging from photovoltaic devices to quantum information processing. Spectroscopic investigations of coherent dynamics provide a direct measurement of electronic fluctuations. Modern multidimensional spectroscopy techniques allow the mapping of coherent processes along multiple time or frequency axes and thus allow unprecedented discrimination between different sources of electronic dephasing. Exploiting modern abilities in coherence mapping in both amplitude and phase, we unravel dissipative processes of electronic coherences in the model system of CdSe quantum dots (QDs). The method allows the assignment of the nature of the observed coherence as vibrational or electronic. The expected coherence maps are obtained for the coherent longitudinal optical (LO) phonon, which serves as an internal standard and confirms the sensitivity of the technique. Fast dephasing is observed between the first two exciton states, despite their shared electron state and common environment. This result is contrary to predictions of the standard effective mass model for these materials, in which the exciton levels are strongly correlated through a common size dependence. In contrast, the experiment is in agreement with ab initio molecular dynamics of a single QD. Electronic dephasing in these materials is thus dominated by the realistic electronic structure arising from fluctuations at the atomic level rather than static size distribution. The analysis of electronic dephasing thereby uniquely enables the study of electronic fluctuations in complex materials.

Shielded cantilever with on-chip interferometer circuit for THz scanning probe impedance microscopy

We have realized a microstrip based terahertz (THz) near field cantilever that enables quantitative measurements of the impedance of the probe tip at THz frequencies (0.3 THz). A key feature is the on-chip balanced hybrid coupler that serves as an interferometer for passive signal cancellation to increase the readout circuit sensitivity despite extreme impedance mismatch at the tip. We observe distinct changes in the reflection coefficient of the tip when brought into contact with different dielectric (Si, SrTiO₃) and metallic samples (Au). By comparing finite element simulations, we determine the sensitivity of our THz probe to be well below 0.25 fF. The cantilever further allows for topography imaging in a conventional atomic force microscope mode. Our THz cantilever removes several critical technology challenges and thus enables a shielded cantilever based THz near field microscope.

Nanoscale dipole dynamics of protein membranes studied by broadband dielectric microscopy

We investigate the nearfield dipole mobility of protein membranes in a wide frequency range from 3 kHz to 10 GHz. The results of our nanoscale dielectric images and spectra of bacteriorhodopsin (bR) reveal Debye relaxations with time constants of τ ∼ 2 ns and τ ∼ 100 ns being characteristic of the dipole moments of the bR retinal and α-helices, respectively. However, the dipole mobility and therefore the protein biophysical function depend critically on the amount of surface water surrounding the protein, and the characteristic mobility in the secondary structure is only observed for humidity levels <30%. Our results have been achieved by adding the frequency as a second fundamental dimension to quantitative dielectric microscopy. The key elements for the success of this advanced technique are the employed heterodyne detection scheme, the broadband electrical signal source, a high frequency optimized cabling, development of calibration procedures and precise finite element modelling. Our study demonstrates the exciting possibilities of broadband dielectric microscopy for the investigation of dynamic processes in cell bioelectricity at the individual molecular level. Furthermore, the technique may shed light on local dynamic processes in related materials science applications like semiconductor research or nano-electronics.

Imaging Carrier Inhomogeneities in Ambipolar Tellurene Field Effect Transistors

The development of van der Waals (vdW) homojunction devices requires materials with narrow bandgaps and simultaneously high hole and electron mobilities for bipolar transport, as well as methods to image and study spatial variations in carrier type and associated conductivity with nanometer spatial resolution. Here, we demonstrate the general capability of near-field scanning microwave microscopy (SMM) to image and study the local carrier type and associated conductivity in operando by studying ambiploar field-effect transistors (FETs) of the 1D vdW material tellurium in 2D form. To quantitatively understand electronic variations across the device, we produce nanometer-resolved maps of the local carrier equivalence backgate voltage. We show that the global device conductivity minimum determined from transport measurements does not arise from uniform carrier neutrality but rather from the continued coexistence of p-type regions at the device edge and n-type regions in the interior of our micrometer-scale devices. This work both underscores and addresses the need to image and understand spatial variations in the electronic properties of nanoscale devices.

Induced quantum dot probe for material characterization

We propose a non-destructive means of characterizing a semiconductor wafer via measuring parameters of an induced quantum dot on the material system of interest with a separate probe chip that can also house the measurement circuitry. We show that a single wire can create the dot, determine if an electron is present, and be used to measure critical device parameters. Adding more wires enables more complicated (potentially multi-dot) systems and measurements. As one application for this concept we consider silicon metal-oxide-semiconductor (MOS) and silicon/silicon-germanium quantum dot qubits relevant to quantum computing and show how to measure low-lying excited states (so-called "valley" states). This approach provides an alternative method for characterization of parameters that are critical for various semiconductor-based quantum dot devices without fabricating such devices.

CMOS platform for atomic-scale device fabrication

Controlled atomic scale fabrication based on scanning probe patterning or surface assembly typically involves a complex process flow, stringent requirements for an ultra-high vacuum environment, long fabrication times and, consequently, limited throughput and device yield. We demonstrate a device platform that overcomes these limitations by integrating scanning-probe based dopant device fabrication with a CMOS-compatible process flow. Silicon on insulator substrates are used featuring a reconstructed Si(001):H surface that is protected by a capping chip and has pre-implanted contacts ready for scanning tunneling microscope (STM) patterning. Processing in ultra-high vacuum is thereby reduced to a few critical steps. Subsequent reintegration of the samples into the CMOS process flow opens the door to successful application of STM fabricated dopant devices in more complex device architectures. Full functionality of this approach is demonstrated with magnetotransport measurements on degenerately doped STM patterned Si:P nanowires up to room temperature.

Interface Characterization and Control of 2D Materials and Heterostructures

2D materials and heterostructures have attracted significant attention for a variety of nanoelectronic and optoelectronic applications. At the atomically thin limit, the material characteristics and functionalities are dominated by surface chemistry and interface coupling. Therefore, methods for comprehensively characterizing and precisely controlling surfaces and interfaces are required to realize the full technological potential of 2D materials. Here, the surface and interface properties that govern the performance of 2D materials are introduced. Then the experimental approaches that resolve surface and interface phenomena down to the atomic scale, as well as strategies that allow tuning and optimization of interfacial interactions in van der Waals heterostructures, are systematically reviewed. Finally, a future outlook that delineates the remaining challenges and opportunities for 2D material interface characterization and control is presented.

Probing quantum coherence in single-atom electron spin resonance

Spin resonance of individual spin centers allows applications ranging from quantum information technology to atomic-scale magnetometry. To protect the quantum properties of a spin, control over its local environment, including energy relaxation and decoherence processes, is crucial. However, in most existing architectures, the environment remains fixed by the crystal structure and electrical contacts. Recently, spin-polarized scanning tunneling microscopy (STM), in combination with electron spin resonance (ESR), allowed the study of single adatoms and inter-atomic coupling with an unprecedented combination of spatial and energy resolution. We elucidate and control the interplay of an Fe single spin with its atomic-scale environment by precisely tuning the phase coherence time T₂ using the STM tip as a variable electrode. We find that the decoherence rate is the sum of two main contributions. The first scales linearly with tunnel current and shows that, on average, every tunneling electron causes one dephasing event. The second, effective even without current, arises from thermally activated spin-flip processes of tip spins. Understanding these interactions allows us to maximize T₂ and improve the energy resolution. It also allows us to maximize the amplitude of the ESR signal, which supports measurements even at elevated temperatures as high as 4 K. Thus, ESR-STM allows control of quantum coherence in individual, electrically accessible spins.

In-situ Near-Field Probe Microscopy of Plasma Processing

There exists a great necessity for in situ nanoscale characterization of surfaces and thin films during plasma treatments. To address this need, the current approaches rely on either 'post mortem' sample microscopy, or in situ optical methods. The latter, however, lack the required nanoscale spatial resolution. In this paper, we propose scanning near-field microwave microscopy to monitor plasma-assisted processes with a submicron spatial resolution. In our approach, a plasma environment with an object of interest is separated from the near-field probe and the rest of the microscope by a SiN membrane of a few-10s nm thickness, and the imaging is performed through this membrane. As a proof of concept, we were able to monitor gradual transformations of carbon nanotube films upon plasma-induced oxidation by a low-pressure air plasma. In the implemented approach with the near-field probe in contact with the membrane, the plasma processing should be interrupted during imaging to preserve the membrane integrity. Possible solutions to achieve in situ real-time imaging during plasma conditions are discussed.

Full-wave modeling of broadband near field scanning microwave microscopy

A three-dimensional finite element numerical modeling for the scanning microwave microscopy (SMM) setup is applied to study the full-wave quantification of the local material properties of samples. The modeling takes into account the radiation and scattering losses of the nano-sized probe neglected in previous models based on low-frequency assumptions. The scanning techniques of approach curves and constant height are implemented. In addition, we conclude that the SMM has the potential for use as a broadband dielectric spectroscopy operating at higher frequencies up to THz. The results demonstrate the accuracy of previous models. We draw conclusions in light of the experimental results.