Amorphous germanium III. Optical properties

The optical properties of thick sputtered films (～30μ) of amorphous Ge, grown with different substrate temperatures (0ˇ-T _sˇ-350°C), were obtained between 0·05 and 4·5 eV by a combination of reflectance, transmittance and ellipsometric measurements. The refractive index at 0·15 eV decreases monotonically with increasing T _s, or equivalently, with increasing density, and is 4·13±0·05 eV in the highest density films. The absorption edge is approximately exponential (10²?α?10⁴ cm^?1) but shifts monotonically to higher energy and increases in slope with increasing T _s. Similarly, the peak in ε₂ grows by about 10% and shifts by about 0·15 eV to higher energies, reaching a maximum of about 23 at 2·90±0·05 eV in the high density films. The peak in the transition strength ω²ε₂ occurs at 4·2±0·2 eV in all films, but increases in magnitude with increasing T _s. The sum rules for n _eff(ω) and ε_0,eff(ω) are evaluated for ▄ω?5 eV and vary monotonically with T _s. These trends are neither compatible with Galeener's void resonance theory nor with changes in the oxygen content of the films, determined by the examination of absorption peaks at 0·053 eV and 0·09 eV. An explanation, suggested here and expanded in I, is based on the observed changes in the structure of the network and voids.