ac conductivity and optical properties of amorphous Si(H) sputtered at 77 K

The ac conductivity of amorphous Si(H) films deposited at 77 and 300 K by dc sputtering in an argon-hydrogen mixture was measured from 300 to 4.2 K. The fact that ${\sigma }_{\mathrm{ac}}\propto T$ over a restricted temperature range, as was previously observed on glow discharge samples, was taken as evidence for the quantum-mechanical tunneling theory. The discovery that at low temperatures (<30 K) ${\sigma }_{\mathrm{ac}}$ becomes essentially temperature independent, as do the chalcogenide glasses, can be explained by Elliott's correlated barrier hopping theory. While the optical properties of Si(H) films sputtered at 300 K are quite similar to those reported for rf-sputtered films at 500 K, those for films deposited at 77 K show interesting differences. The optical properties of films deposited at 77 K were measured at 123 K without annealing, and remained unchanged at room temperature. The vibrational mode at 850 ${\mathrm{cm}}^{-1\mathrm{}}$ for films deposited at 77 K increases with respect to the 900-${\mathrm{cm}}^{-1\mathrm{}}$ mode as a function of increasing hydrogen content and eventually becomes strongest. This would imply, if we judge from the existing literature, a high concentration of ${\left(\mathrm{Si}{\mathrm{H}}_2\right)}_n$. This strong concentration of Si—H bonds decreases the Si oscillator strength homogeneously as is shown by the decrease and shift to higher energy of the peak in the imaginary part of the dielectric constant. The values of the refractive index are appreciably smaller in films deposited at 77 K than in films deposited at 300 K. On the other hand, the structure of both films as observed in transmission electron microscopy is quite similar and also close to the structure previously observed in pure $a-\mathrm{S}\mathrm{i}$.