Resonance acoustic-phonon spectroscopy for studying elasticity of ultrathin films

Abstract

Hopping conduction and piezoelectricity in Fe-doped GaN studied by non-contacting resonant ultrasound spectroscopy

Acoustic Waves: A Probe for the Elastic Properties of Films

“…Bryner et al (2006) estimate that with an aluminium surface, a near infrared laser (800 nm), and a pulse width of 70 fs, the absorption depth and therefore the dominant acoustic wavelength are of the order of 10 nm. Quite consistently, for a similar laser pulse but for Pt and Fe ultra thin films, Ogi et al (2007) estimate at several THz the upper bound of the frequencies excited by the laser pulse. The thermal and mechanical fields are thus (almost perfectly) laterally uniform, and essentially one dimensional: the absorbed pulse has a depth of the order of nanometres, and propagates like an acoustic wave with a plane wavefront which travels perpendicular to the surface, towards the specimen depth.…”

“…The lowest limit for layer detection is also estimated at about 10 nm thickness. In a different configuration, namely a single Pt or Fe layer on a silicon substrate or a borosilicate glass substrate, Ogi et al (2007) were able to characterize metallic films of thickness down to 5 nm. They found, at nanometric thicknesses, a dependence of the elastic moduli on thickness.…”

Laser-Induced Coherent Acoustic Phonons for Measuring Elastic Constants of Ultra-Thin Films

“…Detail of our optical setup is described elsewhere. (14), (18) Figure 1 shows the typical reflectivity change signal from an Fe film deposited on Si substrate. The large step at 8 ps indicates the time when the pump and probe beams arrive at the same time, meaning that acoustic phonons are generated at this moment.…”

“…(12), (13) When the film thickness is smaller than the pulse width, the acousticpulse wave overlaps, and standing phonon vibrations occur. (14), (15) Because C 33 governs their resonance frequencies, we can determine the C 33 from the resonance frequency. Thus, we have established the high-frequency coherent phonon method to determine the C 33 of thin films whose thickness is 10-500 nm.…”

“…(14) In determining the elastic constants, discussion about the measurement accuracy of resultant elastic constants is indispensable. Before determining the C 33 , we discuss the effects of measurement errors in the film thickness, mass density, and temperature increment by the laser irradiation on the resultant C 33 .…”