Marc Faucher

Site Selective Spectroscopy and Structural Analysis of Yttria-Doped Zirconias

Abstract Yttria-stabilized zirconias Zr 1− x Y x O 2−0.5 x (tetragonal, cubic fluorite, and cubic C Ln 2 O 3 -type forms) are investigated by spectroscopic methods. Optically active europium ions that partially substitute yttrium ions act as a structural microprobe. Both conventional ultraviolet and dye laser (site-selective) excitation are used. The yttria-stabilized zirconias may be well described by a cationic sublattice in which tetravalent and trivalent cations are statistically distributed. The anionic sublattice accommodates vacancies at random on oxygen positions. These vacancies in turn cause complex oxygen displacements giving rise to what may be described as a “glass of anions.”

Gallium Nitride as an Electromechanical Material

Gallium nitride (GaN) is a wide bandgap semiconductor material and is the most popular material after silicon in the semiconductor industry. The prime movers behind this trend are LEDs, microwave, and more recently, power electronics. New areas of research also include spintronics and nanoribbon transistors, which leverage some of the unique properties of GaN. GaN has electron mobility comparable with silicon, but with a bandgap that is three times larger, making it an excellent candidate for high-power applications and high-temperature operation. The ability to form thin-AlGaN/GaN heterostructures, which exhibit the 2-D electron gas phenomenon leads to high-electron mobility transistors, which exhibit high Johnsons figure of merit. Another interesting direction for GaN research, which is largely unexplored, is GaN-based micromechanical devices or GaN microelectromechanical systems (MEMS). To fully unlock the potential of GaN and realize new advanced all-GaN integrated circuits, it is essential to cointegrate passive devices (such as resonators and filters), sensors (such as temperature and gas sensors), and other more than Moore functional devices with GaN active electronics. Therefore, there is a growing interest in the use of GaN as a mechanical material. This paper reviews the electromechanical, thermal, acoustic, and piezoelectric properties of GaN, and describes the working principle of some of the reported high-performance GaN-based microelectromechanical components. It also provides an outlook for possible research directions in GaN MEMS.

Josephson junctions and superconducting quantum interference devices made by local oxidation of niobium ultrathin films

We present a method for fabricating Josephson junctions and superconducting quantum interference devices (SQUIDs) which is based on the local anodization of niobium strip lines 3–6.5 nm thick under the voltage-biased tip of an atomic force microscope. Microbridge junctions and SQUID loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link geometries are fabricated: lateral constriction (Dayem bridges) and variable thickness bridges. SQUIDs based on both geometries show a modulation of the maximum Josephson current with a magnetic flux periodic with respect to the superconducting flux quantum h/2e. They persist up to 4 K. The modulation shape and depth of SQUIDs based on variable thickness bridges indicate that the weak link size becomes comparable to the superconducting film coherence length ξ which is of the order of 10 nm.

Amplified piezoelectric transduction of nanoscale motion in gallium nitride electromechanical resonators

A fully integrated electromechanical resonator is described that is based on high mobility piezoelectric semiconductors for actuation and detection of nanoscale motion. We employ the two-dimensional electron gas present at an AlGaN/GaN interface and the piezoelectric properties of this heterostructure to demonstrate a resonant high-electron-mobility transistor enabling the detection of strain variation. In this device, we take advantage of the polarization field divergence originated by mechanical flexural modes for generating piezoelectric doping. This enables a modulation of carrier density which results in a large current flow and thus constitutes a motion detector with intrinsic amplification.

Searching for THz Gunn oscillations in GaN planar nanodiodes

A detailed study of GaN-based planar asymmetric nanodiodes, promising devices for the fabrication of room temperature THz Gunn oscillators, is reported. By using Monte Carlo simulations, an analysis of the static I-V curves and the time-domain evolution of the current obtained when varying some simulation parameters in the diodes has been made. Oscillation frequencies of hundreds of GHz are predicted by the simulations in diodes with micrometric channel lengths. Following simulation guidelines, a first batch of diodes was fabricated. It was found that surface charge depletion effects are stronger than expected and inhibit the onset of the oscillations. Indeed, a simple standard constant surface charge model is not able to reproduce experimental measurements and a self-consistent model must be included in the simulations. Using a self-consistent model, it was found that to achieve oscillations, wider channels and improved geometries are necessary.

Experimental demonstration of direct terahertz detection at room-temperature in AlGaN/GaN asymmetric nanochannels

The potentialities of AlGaN/GaN nanodevices as THz detectors are analyzed. Nanochannels with broken symmetry (so called self switching diodes) have been fabricated for the first time in this material system using both recess-etching and ion implantation technologies. The responsivities of both types of devices have been measured and explained using Monte Carlo simulations and non linear analysis. Sensitivities up to 100 V/W are obtained at 0.3 THz with a 280 pW/Hz1/2 noise equivalent power.

Electromechanical Transconductance Properties of a GaN MEMS Resonator With Fully Integrated HEMT Transducers

We investigate the response of a GaN microelectromechanical resonator where the strain detection is performed by a resonant high-electron mobility transistor (R-HEMT). The R-HEMT gate located above the 2-DEG (two-dimensional electron gas) appears to enable a strong control of the electromechanical response with a gate voltage dependence close to a transconductance pattern. A quantitative approach based on the mobility of the carriers induced in the device by the piezoelectric response of the GaN buffer is proposed. These results show for the first time the electromechanical transconductance dependence versus external biasing and confirm that active piezoelectric transduction is governed by the AlGaN/GaN 2-DEG transport properties.

Design and operation of a silicon ring resonator for force sensing applications above 1 MHz

We present an integrated force probe based on a silicon bulk-mode MEMS resonator. This device uses a silicon ring with symmetrical tips vibrating in the elliptic vibration mode. The tips enable us to make mechanical interactions with surfaces or external objects. Both excitation and detection of the resonator are integrated thanks to electrostatic actuation and capacitive detection. Apart from optical and electrical characterizations of the fabricated device, we report for the first time on the interaction between the resonator tip and a hydrodynamic force applied thanks to a water droplet. This demonstrates a first step toward high frequency atomic force probes for liquid medium applications.

Niobium and niobium nitride SQUIDs based on anodized nanobridges made with an Atomic Force Microscope

Abstract We present a fabrication method of superconducting quantum interference devices (SQUIDs) based on direct write lithography with an atomic force microscope (AFM). This technique involves maskless local anodization of Nb or NbN ultrathin films using the voltage biased tip of the AFM. The SQUIDs are of weak-link type, for which two geometries have been tested: Dayem and variable thickness nanobridges. The magnetic field dependence of the maximum supercurrent Ic(Φ) in resulting SQUIDs is thoroughly measured for different weak link geometries and for both tested materials. It is found that the modulation shape and depth of Ic(Φ) curves are greatly dependent on the weak link size. We analyze the results taking into account the kinetic inductance of nanobridges and using the Likharev–Yakobson model. Finally we show that the present resolution reached by this technique (20 nm) enables us to fabricate Nb weak-links which behavior approaches those of ideal Josephson junctions.

Analysis of mechanical properties of single wall carbon nanotubes fixed at a tip apex by atomic force microscopy

An investigation of the mechanical properties of single wall carbon nanotubes (SWNT) fixed at a tip apex was performed using a frequency modulation-atomic force microscope (FM-AFM). The FM-AFM method allows the measurement of conservative and non-conservative forces separately and unambiguously. The FM-AFM analysis provides information that aids the understanding of the effects of the interaction between the free SWNT end and the surface: the resonant frequency shifts provide information on the effective SWNT spring constant, while the damping signal gives information on the type of contact between the tube and the surface. The variation of the damping signal as a function of the tip surface distance shows that the additional energy loss produced by the interaction between the tube and the surface is mostly due to an adhesion hysteresis. As a result, the increase of the damping signal is correlated to the existence of intermittent contact situations. The whole variations show how the contact between the free SWNT end and the surface modifies the elastic response of the tube.