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crystalline silicon wafers with extremely small background intemal friction (- 2x108-). The

technique was described in [1] and will not be repeated here. All measurements were performed ×-, with the exception of one shown in Fig. 5. The HWCVD aat a strain-amplitude E = 1xO Si:H film used in this work was one of those reported in [1] (H128, 2 at.% H), where the film preparation also was described. The HWCVD a-Si:D film (H375) was prepared under the same conditions as H128, in an attempt to produce a similar dopant concentration and elastic anomaly. A film of PECVD a-Si:H (T485) was also prepared at NREL, with deposition temperature and rate of 230'C and 1.3 A/s, respectively. This resulted in an optimal device-quality PECVD film with - 9 at.% H. Hydrogen concentrations were measured both by infrared absorption and thermal evolution which resulted in similar values to within 20%, and the values given here are averages. The deuterium concentration in H375 has not been determined, but is expected to be close to 2 at.%. RESULTS Fig. 1 compares internal friction and sound velocity for the HWCVD a-Si:D film (H375) with measurements on HWCVD a-Si:H (H 128) reported previously [1]. In the a-Si:D, the elastic anomalies are very similar, except that they occur at 18.7K, the triple point of bulk D2 [3]. This result provides convincing evidence that the abrupt change of internal friction and sound velocity are closely related to the triple points, i.e. to the onset of freezing in the bulk liquids. We mention only in passing that below ~ 2K the internal friction of the deuterated film is even smaller than that of the a-Si:H film, 4x10-7 vs. 8x10 7. We had previously reported that with the appropriate hydrogen concentration (- 1 at.%) in HWCVD films an anomalously small internal friction was observed in this temperature range, ~ 5

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