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friction of these HWCVD a-Si:H thin films provides the opportunity to study the effect of incorporating hydrogen into the amorphous silicon matrix in varying concentrations and under different conditions of preparation. In the work to be reported here, we present the first experimental evidence for the existence of bulk H2 in HWCVD a-Si:H. EXPERIMENTAL

Measurements of internal friction and of sound velocity were performed by using the doublepaddle oscillator technique, described in detail in ref. [8]. The oscillators were made out of high purity, undoped silicon wafers 300 gm thick. Two torsional vibration modes can be excited capacitively along the orientation, the antisymmetric one and the symmetric one with a resonant frequency of approximately 5500 and 420 Hz, respectively. The antisymmetric mode of a bare paddle oscillator has an exceptionally small internal friction, Q-' 2 x 10- at low temperatures, which is reproducible for different oscillators within ± 10%. The results of internal friction and of sound velocity presented in this work were mainly obtained in the antisymmetric mode. The symmetric mode has a larger internal friction making it less accurate in determining the absolute values of the internal friction of the a-Si:H thin films studied here. However, since its resonant frequency is more than one order of magnitude lower, it has been used to search for frequency dependence of the internal friction. Deposition of a thin film onto the double-paddle oscillator will change the internal friction of the paddle, Q-i From the increase above the background, QI, the internal friction of the thin Qpa.ddle*"~u film itself, Qrlm, can be calculated through [8] 595 Mat. Res. Soc. Symp. Proc. Vol. 507 ©1998 Materials Research Society

Gsubtsub Q3Grnlmttfijm(pdlm

-1

QfIrn

-

~l

(1)

where G and t refer to the shear moduli and thicknesses of the paddle (substrate) and the film, respectively. For crystalline silicon along the orientation, Gsub = 6.2 x 10" dyne/cm 2. For the HWCVD a-Si:H, Gfilm is unknown. Since this film has the same mass density as a-Si prepared by ion-implantation [9,10], we assume their shear moduli to be equal, which is Gfilm = 5.54 x 10" dyne/cm 2, based on sound velocity [11 ] and mass density measurements [10] of the latter. The relative variation of sound velocity of a thin film can be measured as follows. If (Af / f0),b and (Af/ fo) are the relative variations in the resonant frequency of a paddle oscillator before and after the deposition of a thin film, respectively, with f0 being the frequency at some reference temperature TO,the relative variation of sound velocity of the thin film is given by: (2)

(G)lmtmlm

The a-Si:H films used in this work were grown by the hot wire technique at the National Renewable Energy Laboratory [12]. The films whose internal frictions are shown in Fig. 1 were deposited at various substrate temperatures and deposition rates to achieve different hydrogen concentrations CH in the films. Film thicknesses were - 2 jim, except for the one with CH = 4 at.% wh