P. Hess

Application of acoustic resonators in photoacoustic trace gas analysis and metrology

The application of different types of acoustic resonators such as pipes, cylinders, and spheres in photoacoustics is considered. This includes a discussion of the fundamental properties of these resonant cavities. Modulated and pulsed laser excitation of acoustic modes is discussed. The theoretical and practical aspects of high-Q and low-Q resonators and their integration into complete photoacoustic detection systems for trace gas monitoring and metrology are covered in detail. The characteristics of the available laser sources and the performance of the photoacoustic resonators, such as signal amplification, are discussed. Setup properties and noise features are considered in detail. This review is intended to give newcomers the information needed to design and construct state-of-the-art photoacoustic detectors for specific purposes such as trace gas analysis, spectroscopy, and metrology.

Elastic, mechanical, and thermal properties of nanocrystalline diamond films

Nanocrystalline columnar-structured diamond films with column diameters less than 100 nm and thicknesses in the range of 1–5 μm were grown on silicon substrates by chemical vapor deposition (CVD) in a microwave plasma reactor with purified methane and hydrogen used as the reactants. Uniform conformal nucleation densities in excess of 1012 cm−2 were accomplished prior to growth by seeding with explosively formed nanodiamonds, which resulted in good optical quality films. The film thickness was measured in situ by the laser reflectometry method. The grain size and optical quality of the films were characterized by scanning electron microscopy and Raman measurements. Broadband surface acoustic wave pulses were used to measure the anomalous dispersion in the layered systems. The experimental dispersion curves were fitted by theory, assuming the diamond film as an isotropic layer on an anisotropic silicon substrate, to determine mean values of the density and Young’s modulus of the diamond films. The density w...

Laser generation and detection of surface acoustic waves: Elastic properties of surface layers

A noncontact all‐optical method for surface photoacoustics is described. The surface acoustic waves (SAWs) were excited employing a KrF laser and detected with a Michelson interferometer using a 633‐nm HeNe laser. Due to an active stabilization scheme developed for the interferometer a surface displacement of 0.2 A could be detected. The materials investigated included pure materials such as polycrystalline aluminum, and crystalline silicon; films of gold, silver, aluminum, iron, and nickel on fused silica; and a‐Si:H on Si(100). In the case of pure materials the shape of the acoustic pulse and the phase velocity were determined. The dispersion of the SAW phase velocity observed for the film systems was used to extract information on the film thickness, density, and transverse and longitudinal sound velocity. Models for the theoretical treatment of film systems and the calculation of dispersion curves are presented.

Laser diagnostics of mechanical and elastic properties of silicon and carbon films

Abstract A novel method has been developed for the determination of mechanical and elastic properties of thin films such as film thickness, density, Youngs modulus and Poissons ratio. In this technique short laser pulses (ns-ps) are used to excite a broad-band surface acoustic wave pulse, and a cw laser (Michelson interferometer, probe beam deflection) or piezoelectric foil detector is employed for time-resolved detection of the resulting surface displacements. In a hydrogen-terminated ideal silicon crystal the surface wave pulse shows no dispersion effect. However, a thin native oxide layer, normally present on the surface, leads to a linear decrease of the phase velocity with frequency. Partially this dispersion effect may be due to damage caused by lapping. A quantitative analysis of the shape of the surface wave pulse as a function of energy of the exciting laser pulse yields the threshold fluences for the melting and ablation of silicon. Doping of silicon leads to nonlinear dispersion, which was used to characterize the doping profile and elastic properties of the doped region. For amorphous hydrogenated silicon films, used in photovoltaics, the density and elastic constants were measured. Different carbon films with widely varying mechanical and elastic properties were studied. For thin fullerite films (C60, C70) the density and elastic constants were determined for the first time, showing that this is the softest form of carbon. The quality of amorphous diamondlike and polycrystalline diamond films was investigated by comparing the density and elastic constants with those of single-crystal diamond. Due to its high information content the method allows a reliable characterization of these films with a thickness in the micrometer range.

Young’s modulus of silicon nitride used in scanning force microscope cantilevers

The Young’s modulus and Poisson’s ratio of high-quality silicon nitride films with 800 nm thickness, grown on silicon substrates by low-pressure chemical vapor deposition, were determined by measuring the dispersion of laser-induced surface acoustic waves. The Young’s modulus was also measured by mechanical tuning of commercially available silicon nitride cantilevers, manufactured from the same material, using the tapping mode of a scanning force microscope. For this experiment, an expression for the oscillation frequencies of two-media beam systems is derived. Both methods yield a Young’s modulus of 280–290 GPa for amorphous silicon nitride, which is substantially higher than previously reported (E=146 GPa). For Poisson’s ratio, a value of ν=0.20 was obtained. These values are relevant for the determination of the spring constant of the cantilever and the effective tip–sample stiffness.

Structural behavior of thin BaTiO3 film grown at different conditions by pulsed laser deposition

Thin epitaxial BaTiO3 (BTO) films having a high crystallinity and uniformity of grains were deposited on the SrTiO3 (STO) substrates. When the oxygen pressure is from 0.04 to 70Pa the lattice constant decreases from 4.1100 to 3.9972 Angstrom and the orientation normal to the substrate changes from (001) to (100). The surface topography changes from Bat to hilly with a surface roughness (rms) changing between 1.2 nm at 0.7 Pa, 3.4 nm at 7 Pa and 6.4 nm at 70 Pa. At the substrate temperature of 550, 750 and 850 degrees C the surface topography of the films varies from corrugated structure to rectangular grain and the surface roughness increases from 1.2, 3.4 to 3.4 nm. The epitaxial BTO films were also deposited on the YBCO/LaAlO3 (YBCO/LAO) substrates at 750 degrees C and 7 Pa. The films have a rough surface (rms=29.1 nm), but a good uniformity of grains. The c-axis oriented BTO films with a poor crystallinity were deposited on the MgO substrates at 750 degrees C and 7 Pa. The films have a smooth surface (rms=1.0 nm), but a poor uniformity of grains. The interfaces between the BTO films and the substrates were determined by transmission electron microscopy (TEM).

Surface Acoustic Waves in Materials Science

Laser-generated surface waves provide new tools for studying material properties, from linear elastic behavior to fracture.

Detection of ammonia by photoacoustic spectroscopy with semiconductor lasers

Sensitive photoacoustic detection of ammonia with near-infrared diode lasers (1.53 microns) and a novel differential acoustic resonator is described; a sensitivity of 0.2 parts per million volume (signal-to-noise ratio = 1) is attained. To eliminate adsorption-desorption processes of the polar NH3 molecules, a relatively high gas flow of 300 SCCM was used for the ammonia-nitrogen mixture. The results are compared with recent ammonia measurements with a NIR diode and absorption spectroscopy used for detection and photoacoustic experiments performed with an infrared quantum-cascade laser. The performance of the much simpler and more compact setup introduced here was comparable with these previous state-of-the-art measurements.

Broadband detection of laser‐excited surface acoustic waves by a novel transducer employing ferroelectric polymers

Broad-band surface acoustic wave transients were excited by ps laser pulses. A novel ferroelectric polymer foil transducer with a bandwidth of 100MHz was employed for their detection. The observed SAW pulse width was limited by materials properties. Phase velocities of surface acoustic waves are given for polycrystalline aluminum, copper, steel and brass samples.

Stress-driven nucleation of coherent islands: theory and experiment

In order to determine the driving force of coherent island nucleation and growth, the formation of Ge islands on H-terminated Si(100) surfaces has been investigated by spectroscopic ellipsometry in real time. The process of island nucleation is found to be accompanied by a strong reduction of the wetting layer thickness. The results indicate that, in this case, islands are nucleated mainly due to the relaxation of the stress energy in the wetting layer. Thermodynamic and kinetic models have been developed for this process. It is shown that the free energy of stress-driven formation of islands contains a saddle point. The value of free energy at this point determines the nucleation process. The time evolution of the thickness of the wetting layer is described and the time dependencies of the nucleation rate and surface density of islands are calculated. Good agreement between this theory and real-time ellipsometric analysis of the growth process is found.