Schecker, Olivier
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Mode shape and dispersion relation of bending waves in thin silicon membranes
We study the vibrational behavior of silicon membranes with a thickness of a few hundred nanometers and macroscopic lateral size. A piezo is used to couple in transverse vibrations, which we monitor with a phase-shift interferometer using stroboscopic light. The observed wave pattern of the membrane deflection is a superposition of the mode corresponding to the excitation frequency and several higher harmonics. Using a Fourier transformation in time, we separate these contributions and image up to the eighth harmonic of the excitation frequency. With this method we determine the dispersion relation of membrane oscillations in a frequency range up to 12 MHz. We develop a simple analytical model combining stress of a membrane and bending of a thin plate that describes both the experimental data and finite-elements simulations very well. We derive correction terms to account for a finite curvature of the membrane and for the inertia of the surrounding atmosphere. A simple criterion for the transition between stressed membrane and thin plate behavior is presented.
Optical transmission and laser structuring of silicon membranes
The optical linear and nonlinear properties of » 340-nm thick Si membranes were investigated. The investigation included both experiments in which the reflection and transmission from the membranes were measured, and finite differences time domain simulations. The linear optical transmission of the Si membranes can be controlled by changing the thickness of a thermally grown oxide on the membrane. Illumination of the membranes with high levels of irradiation leads to optical modifications that are consistent with the formation of amorphous silicon and dielectric breakdown. When irradiated under conditions where dielectric breakdown occurs, the membranes can be ablated in a well-controlled manner. Laser micro-structuring of the membranes by ablation was carried out to make micrometer-sized holes by focused fs-pulses. Ns-pulses were also used to fabricate arrays of holes by proximity-ablation of a closely-packed pattern of colloidal particles.
Subharmonic Resonant Optical Excitation of Confined Acoustic Modes in a Free-Standing Semiconductor Membrane at GHz Frequencies with a High-Repetition-Rate Femtosecond Laser
We propose subharmonic resonant optical excitation with femtosecond lasers as a new method for the characterization of phononic and nanomechanical systems in the gigahertz to terahertz frequency range. This method is applied for the investigation of confined acoustic modes in a free-standing semiconductor membrane. By tuning the repetition rate of a femtosecond laser through a subharmonic of a mechanical resonance we amplify the mechanical amplitude, directly measure the linewidth with megahertz resolution, infer the lifetime of the coherently excited vibrational states, accurately determine the system’s quality factor, and determine the amplitude of the mechanical motion with femtometer resolution.
Nanofabricated adjustable multicontact devices on membranes
Adjustable atomic size contacts realized by break junctions have become a standard tool during the last decade. Although nanofabricated break junctions may in principle be incorporated onto complex electronic circuits, a fundamental drawback of the standard break junction technique is its limitation to a single adjustable junction per device. We have fabricated single break junctions as well as devices containing two break junctions on a silicon membrane. The junctions are adjusted by positioning a fine tip via piezocontrol on the rear side of the membrane. We describe the fabrication process of the membranes and the devices and present results obtained on circuits made of gold and platinum. We show that the junctions can be addressed independently by a suitable choice of the tip position. Single-atom contacts, vacuum tunneling contacts as well as larger contacts can be stabilized.
Confined longitudinal acoustic phonon modes in free-standing Si membranes coherently excited by femtosecond laser pulses
In this Rapid Communication we report the first time-resolved measurements of confined acoustic phonon modes in free-standing Si membranes excited by fs laser pulses. Pump-probe experiments using asynchronous optical sampling reveal the impulsive excitation of discrete acoustic modes up to the 19th harmonic order for membranes of two different thicknesses. The modulation of the membrane thickness is measured with fm resolution. The experimental results are compared with a theoretical model including the electronic deformation potential and thermal stress for the generation mechanism. The detection is modeled by the photoelastic effect and the thickness modulation of the membrane, which is shown to dominate the detection process. The lifetime of the acoustic modes is found to be at least a factor of 4 larger than that expected for bulk Si.
Von Punktkontakten zu Nano-Elektro-Mechanischen Systemen (NEMS) : Herstellung und Charakterisierung von Ein-Atom-Kontakten
The study of adjustable atomic contacts is made possible by the use of break-junctions. A break-junction is constituted of a metallic conductor, aluminum or gold for example, which has been deposited on a substrate. This conductor locally possesses an underetched constriction forming a suspended bridge. When bending the substrate this constriction can be stretched in a controlled way. By this means, the stretching of the conductor can be adjusted in such a way, that a contact made of single atoms is stabilized.
This research work is subdivided into two parts. On the one hand, single aluminum break-junctions made on bronze substrates were studied at very low temperatures. Using the phenomenon of multiple Andreev reflections we conclude that aluminum, in contrast to gold, does not form monoatomic chains longer than a dimer. A single electron transistor (SET) structure was also characterized at very low temperatures. On the other hand, a system composed of one or two break-junctions on a monocrystalline silicon membrane was developed and characterized at room temperature.
This system forms a nano-electro-mechanical-system, named NEMS, integrable in silicon technology. Membranes of a thickness of just 340 nm, were fabricated out of SOI substrates. The mechanical static and dynamic properties of these membranes were studied. Several stable mechanics based on the use of a tip mounted on a piezo were developed. These allow us to address each break-junction on a common membrane individually, both at room temperature and at very low temperature. The effect of laser light on the conductance of a break-junction, which leads to a conductance increase, was studied. The influence of the substrate can be excluded through the use of silicon membranes. The conductance variation is associated with the phenomenon of photoassisted transport. In this work break-junctions initially designed for fundamental physics were integrated on silicon membranes allowing them to be used as electromagnetic sensors.