Electroreflectance of Disordered Germanium Films

The transition from the crystalline to the amorphous state has been studied in germanium thin films by observing the electroreflectance spectra related to critical points at $\Gamma (E_0, E_0+{\Delta }_0)$, along the $\Lambda$ direction ($E_1, E_1+{\Delta }_1$), and at areas with $\Delta$ and $\Sigma$ symmetry ($E_2$). A small shift of $E_0$ to lower energy with increasing disorder in the crystalline state, and weak structure in the amorphous state, have been observed. The spectral positions of $E_1$, $E_1+{\Delta }_1$ and $E_2$ do not change with disorder in the crystalline state but these responses are completely absent in the amorphous state.