Measurement of the Sound Velocity in AlAs by Picosecond Ultrasonics
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MEASUREMENT OF THE SOUND VELOCITY IN AlAs BY PICOSECOND ULTRASONICS D. A. Young,* H. T. Grahn,* H. J. Maris,* .1.Tauc,* .. M. Hong,** and T. P. Smith, III** *Brown University, Providence, RI 02912 **IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598
ABSTRACT Picosecond ultrasonics have been used to measure the sound velocity and index of refraction in a thin film of AlAs. The AlAs layer was grown by molecular-beam epitaxy on a buffer layer of GaAs which was deposited on' a semi-insulating GaAs wafer. The AlAs film was capped by a thin layer of GaAs, and a very thin film of InSb. To generate a sound wave effectively a picosecond light pulse was absorbed in the InSb film. As the sound wave propagates through the microstructure it changes the optical properties in the various films, and this produces a change in the optical reflectivity, which is measured. From the round trip time of the acoustic wave we have determined the sound velocity in the AlAs film. EXPERIMENTAL We have measured the sound velocity and index of refraction of AlAs using a novel technique based on picosecond optics.' This method is ideal for ultrasonics in nanometer-scale films, and epitaxial films in particular. The samples measured in this work were grown using molecular-beam epitaxy. The substrate was semi-insulating GaAs aligned to within 0.5' of the < 100 > axis. To ensure high-quality AlAs epitaxy, a 600 nm GaAs buffer layer was grown prior to the growth of the 800 nm AlAs film. A 22 nm GaAs cap layer was grown over the AlAs to smooth the surface and reduce oxidation. Picosecond optical ultrasonics depends on the interaction of light with an absorbing material. Optical energy is deposited in an absorbing material and this generates a stress which in turn launches a strain pulse into the structure. This strain pulse is then tracked through the sample optically. In our experiments the energy gap of the AlAs (2.2 eV) is larger than the photon energy (2.0 eV) and the absorption is negligible. In addition, the absorption length of the GaAs is too long (about 230 nm) to yield significant stress in the cap layer. For these reasons a very thin strongly Mat. Res. Soc. Symp. Proc. Vol. 130. 01989 Materials Research Society
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absorbing layer (transducer) of InSb was sputtered onto the sample surface. We can safely assume that the stress generated by the light is only in the transducer. The strain pulse is tracked through the structure by measuring the changes it produces in the index of refraction and absorption coefficient of the layers and the attendant changes in the optical reflectivity of the sample. These changes are measured in a pump-probe technique where the time delay between a pump pulse (used to create the strain pulse) and a probe pulse (used to measure the optical reflectivity) is varied. RESULTS AND DISCUSSION
Results for room temperature measurements of the AlAs film described above are shown in Fig. I. There are a number of oscillatory components to the spectra, and we have performed computer simulations of the opti
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