Phonon Reflection in Silicon.

An experiment using heat pulses has been performed to study phonon reflection at a silicon-vacuum interface. The phonons are generated by sending a voltage pulse of 40 to 160 ns duration through a thin film constantan heater. The resultant power densities in the heater range from about 25 to 40000 mW/mm('2), corresponding to phonon frequencies in the hundreds of gigahertz. The phonons propagate ballistically across the silicon single crystal and reflect or scatter at the opposite surface. Upon their return to the surface where they were generated, some of them are detected by a superconducting tin bolometer, producing what is called a time of flight spectrum. This spectrum can be analyzed by comparing it to linear combinations of the spectra predicted by two models; one assuming that the phonons are specularly reflected with conservation of the component of the wave vector lying parallel to the surface, the other assuming that they are scattered randomly back into the crystal. The particular linear combination that most closely approximates the experimental spectrum defines the percentage of the incident energy said to be specularly reflected. The reflecting surface was polished with 1(mu) diamond paste or with a chemi-mechanical silica sol (Syton). In addition, the oxide layer that forms on silicon on contact with air was altered by stripping and reformation in basic and acidic peroxide solutions and by HF etching. In some cases it was possible to retain the same thin film devices while varying the reflecting surface preparation, thus allowing absolute comparison of the spectra for the differently prepared surfaces. Results support the applicability of the weighted model described above. In addition, it has been found that the character of the oxide layer strongly affects the reflecting properties of the surface. For example, cleaning with peroxide solutions decreases the fraction of phonons which are specularly reflected, whereas HF etching increases it.