Didier Theron

In this paper, a definition of the gain and added noise of impedance matching networks for scanning microwave microscopy is given. This definition can be used to compare different impedance matching techniques independently of the instrument used to measure the S-parameter. As a demonstration, impedance matching devices consisting of a Beatty line, a tuner, and interferometric setups with and without amplifiers have been investigated. Measurement frequencies up to 28 GHz are used, and the maximal resulting gain found was 9504.7 per Siemens.

We report on an accelerometer micro-sensor based on epitaxial gallium nitride and silicon. The device is a vibrating beam accelerometer fabricated with a micro-electro-mechanical-system technology starting from an AlGaN/GaN heterostructure grown on silicon. The vibrating GaN beam has integrated high electron mobility transducers, whereas a high aspect ratio proof mass is engineered in the silicon substrate. The sensor response was investigated for several modes and features a scale factor up to 160 Hz/g, with unconventional dependence vs the mode number. To account for this, we propose an analytical model of the accelerometer scale factor that takes into account the built-in stress during epitaxy. This proof-of-concept device opens perspectives for inertial sensors taking advantage of GaN properties.

The main objectives of this work are the development of fundamental extensions to existing scanning microwave microscopy (SMM) technology to achieve quantitative complex impedance measurements at the nanoscale. We developed a SMM operating up to 67 GHz inside a scanning electron microscope, providing unique advantages to tackle issues commonly found in open-air SMMs. Operating in the millimeter-wave frequency range induces high collimation of the evanescent electrical fields in the vicinity of the probe apex, resulting in high spatial resolution and enhanced sensitivity. Operating in a vacuum allows for eliminating the water meniscus on the tip apex, which remains a critical issue to address modeling and quantitative analysis at the nanoscale. In addition, a microstrip probing structure was developed to ensure a transverse electromagnetic mode as close as possible to the tip apex, drastically reducing radiation effects and parasitic apex-to-ground capacitances with available SMM probes. As a demonstration, we describe a standard operating procedure for instrumentation configuration, measurements and data analysis. Measurement performance is exemplarily shown on a staircase microcapacitor sample at 30 GHz.

Standing at the meeting between solid state physics and optical spectroscopy, microwave characterization methods are efficient methods to probe electronic mechanisms and mesoscopic transport in semiconducting polymers. Scanning microwave microscopy, augmented with a Mach-Zehnder interferometer detection unit to allow for the probing of high impedance structures was applied on poly(3-hexylthiophene-2,5-diy) and exhibited high sensitivity while operating at the nanoscale. Provided a well-defined experiment protocol, S₁₁ phase and amplitude signals are shown to lead simultaneously yet independently to probing the variations of the dielectric properties in the materials, i.e., conductive and capacitive properties, respectively, upon applied DC gate bias. Adjusting the operating microwave frequency can also serve to probe carrier trapping mechanisms.

Extracting simultaneously multimodal nanoscale specimen information, by an integrated microscopy technology, is in the focus of this report. The combination of multiple imaging techniques allows for obtaining complementary and often unique datasets of samples under test. An instrumental setup operating under high-vacuum conditions inside the chamber of a scanning electron microscope (SEM), as a platform fusing various microscopy methods, techniques and processes, illustrates the potential of such multimodal technology. An atomic force microscope based on a compact optical interferometer performs imaging of surface topographies and a scanning microwave microscope records electromagnetic properties in the microwave frequency domain at the same time and spot. An open-source software framework, tailored for vision-based automation by nanorobotics, controls the instrument. The setup allows for simultaneously observing the region-of-interest with SEM resolution, while imaging and characterizing with evanescent microwaves and atomic forces. To validate the approach an analysis of microscale capacitors is included.

The characteristics of piezoelectric micro-resonators based on vibrating beams essentially depend on two basic physical coefficients, an effective Young’s modulus ( Y _e ) and a piezoelectric coupling factor ( e _31e ). An improved analytic model is proposed with newly derived expressions of Y _e and e _31e that account for the anisotropic properties of the III-nitride materials and beam width, W . The analytic model applicable to the only axial stress is completed by finite element (FE) simulations that allow any spatial patterns of pre-stress in wafers to be studied. The value of e _31e for wider beams is analytically demonstrated to be much higher than the usual e ₃₁ , and a strong dependence of e _31e on W is also confirmed by FE simulations. Resonance frequency ( f _r ) and actuation efficiency ( η ) are numerically studied for several pre-stress patterns and beam dimensions. The f _r is found to be sensitive to the beam width only for resonators under 2D pre-stress while the η to the stress magnitude regardless of stress pattern. Compared with measurements published for some fabricated resonators, both analytic and FE approaches agree well quantitatively for the resonance frequency and qualitatively for the dynamic amplitude. The results of this study can help design optimization, such as appropriate electrode length and suitable beam width, to gain better performance for this type of resonators.

The π–π interactions between organic molecules are among the most important parameters for optimizing the transport and optical properties of organic transistors, light-emitting diodes, and (bio-) molecular devices. Despite substantial theoretical progress, direct experimental measurement of the π–π electronic coupling energy parameter t has remained an old challenge due to molecular structural variability and the large number of parameters that affect the charge transport. Here, we propose a study of π–π interactions from electrochemical and current measurements on a large array of ferrocene-thiolated gold nanocrystals. We confirm the theoretical prediction that t can be assessed from a statistical analysis of current histograms. The extracted value of t ≈35 meV is in the expected range based on our density functional theory analysis. Furthermore, the t distribution is not necessarily Gaussian and could be used as an ultrasensitive technique to assess intermolecular distance fluctuation at the subangström level. The present work establishes a direct bridge between quantum chemistry, electrochemistry, organic electronics, and mesoscopic physics, all of which were used to discuss results and perspectives in a quantitative manner.