Electrical and Optical Properties of Amorphous Germanium

Measurements of density, optical absorption, resistivity, photoconductivity, and Hall effect have been made on amorphous Ge films ranging in thickness from 1 to 23 \ensuremath{\mu}. The density was found to be 3.9\ifmmode\pm\else\textpm\fi{}0.4g/${\mathrm{cm}}^{3}$. The absorption coefficient (measured at room temperature) was found to be exponential from 0.6 to 1.1 eV with a slope of 0.14 eV. Resistivity was determined to be 300 \ensuremath{\Omega} cm at room temperature with an activation energy of 0.15 eV. The activation energy at 30\ifmmode^\circ\else\textdegree\fi{}K was about 0.02 eV. The spectral response of dc photoconductivity (measured at 4.2\ifmmode^\circ\else\textdegree\fi{}K) showed a maximum at 1.5 eV, which may correspond to intrinsic transitions. The Hall measurement gave a value for carrier concentration (at room temperature) of ${10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and a Hall mobility of ${10}^{\ensuremath{-}2}$ ${\mathrm{cm}}^{2}$/V sec. The sign of the Hall constant was negative. The optical measurements (both in this study and the work of Glass and Tauc et al.) suggest that much of the band structure is retained in the amorphous phase. There is an apparent contradiction, however, in that the resistivity and Hall measurements suggest transport via localized states, whereas the absorption data of Tauc et al. indicate bands of nonlocalized states.