Publication Date:
2015-07-03
Description:
Vitreous silica (v-SiO 2 ) shows anomalous temperature dependence of velocity, including positive temperature coefficient of velocity (TCV) and velocity minimum around 70 K. The former characteristic allows its application in acoustic-resonator devices as a temperature compensating material. In this paper, we study the temperature dependence of velocity of fluorine-doped v-SiO 2 (v-SiO 2– x F x ) thin films using picosecond ultrasonic spectroscopy. To correct the temperature increase caused by irradiation with light pulses, we calculated the steady temperature increase in the measuring volume with a finite volume method, considering the temperature dependence of thermal conductivity, and find that temperature in the measurement region remains high even when the back surface is cryogenically cooled. Using the corrected temperature, we determine TCV of v-SiO 2– x F x thin films for 0 〈 x 〈 0.264 , which increases as x increases and is smaller than reported bulk values by a factor of 0.5 − 0.7 . The velocity minimum is absent for the film with x = 0, but it is clearly observed at 70 K for the film with the highest fluorine concentration of x = 0.264. These temperature behaviors are attributed to the change in the Si-O-Si bond angle caused by the fluorine doping.
Print ISSN:
0021-8979
Electronic ISSN:
1089-7550
Topics:
Physics
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